# MIT/GNU Scheme 11.2

# MIT/GNU Scheme

This manual documents MIT/GNU Scheme 11.2.

Copyright © 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Massachusetts Institute of Technology

> Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License.”

## Table of Contents

- Acknowledgements
- 1 Overview
  - 1.1 Notational Conventions
    - 1.1.1 Errors
    - 1.1.2 Examples
    - 1.1.3 Entry Format
  - 1.2 Scheme Concepts
    - 1.2.1 Variable Bindings
    - 1.2.2 Environment Concepts
    - 1.2.3 Initial and Current Environments
    - 1.2.4 Static Scoping
    - 1.2.5 True and False
    - 1.2.6 External Representations
    - 1.2.7 Disjointness of Types
    - 1.2.8 Storage Model
  - 1.3 Lexical Conventions
    - 1.3.1 Whitespace
    - 1.3.2 Delimiters
    - 1.3.3 Identifiers
    - 1.3.4 Uppercase and Lowercase
    - 1.3.5 Naming Conventions
    - 1.3.6 Comments
    - 1.3.7 Additional Notations
  - 1.4 Expressions
    - 1.4.1 Literal Expressions
    - 1.4.2 Variable References
    - 1.4.3 Special Form Syntax
    - 1.4.4 Procedure Call Syntax
- 2 Special Forms
  - 2.1 Lambda Expressions
  - 2.2 Lexical Binding
  - 2.3 Dynamic Binding
    - 2.3.1 Fluid-Let
  - 2.4 Definitions
    - 2.4.1 Top-Level Definitions
    - 2.4.2 Internal Definitions
  - 2.5 Assignments
  - 2.6 Quoting
  - 2.7 Conditionals
  - 2.8 Sequencing
  - 2.9 Iteration
  - 2.10 Structure Definitions
  - 2.11 Macros
    - 2.11.1 Binding Constructs for Syntactic Keywords
    - 2.11.2 Pattern Language
    - 2.11.3 Syntactic Closures
      - 2.11.3.1 Syntax Terminology
      - 2.11.3.2 Transformer Definition
      - 2.11.3.3 Identifiers
    - 2.11.4 Explicit Renaming
  - 2.12 SRFI syntax
    - 2.12.1 cond-expand (SRFI 0)
    - 2.12.2 receive (SRFI 8)
    - 2.12.3 and-let* (SRFI 2)
    - 2.12.4 define-record-type (SRFI 9)
- 3 Equivalence Predicates
- 4 Numbers
  - 4.1 Numerical types
  - 4.2 Exactness
  - 4.3 Implementation restrictions
  - 4.4 Syntax of numerical constants
  - 4.5 Numerical operations
  - 4.6 Numerical input and output
  - 4.7 Fixnum and Flonum Operations
    - 4.7.1 Fixnum Operations
    - 4.7.2 Flonum Operations
    - 4.7.3 Floating-Point Environment
    - 4.7.4 Floating-Point Exceptions
    - 4.7.5 Floating-Point Rounding Mode
  - 4.8 Random Number Generation
- 5 Characters
  - 5.1 Character implementation
  - 5.2 Unicode
  - 5.3 Character Sets
- 6 Strings
  - 6.1 Searching and Matching Strings
  - 6.2 Regular Expressions
    - 6.2.1 Regular S-Expressions
    - 6.2.2 Regsexp Procedures
- 7 Lists
  - 7.1 Pairs
  - 7.2 Construction of Lists
  - 7.3 Selecting List Components
  - 7.4 Cutting and Pasting Lists
  - 7.5 Filtering Lists
  - 7.6 Searching Lists
  - 7.7 Mapping of Lists
  - 7.8 Folding of Lists
  - 7.9 Miscellaneous List Operations
- 8 Vectors
  - 8.1 Construction of Vectors
  - 8.2 Selecting Vector Components
  - 8.3 Cutting Vectors
  - 8.4 Modifying Vectors
- 9 Bit Strings
  - 9.1 Construction of Bit Strings
  - 9.2 Selecting Bit String Components
  - 9.3 Cutting and Pasting Bit Strings
  - 9.4 Bitwise Operations on Bit Strings
  - 9.5 Modification of Bit Strings
  - 9.6 Integer Conversions of Bit Strings
- 10 Miscellaneous Datatypes
  - 10.1 Booleans
  - 10.2 Symbols
  - 10.3 Parameters
    - 10.3.1 Cells
  - 10.4 Records
  - 10.5 Promises
  - 10.6 Streams
  - 10.7 Weak References
    - 10.7.1 Weak Pairs
    - 10.7.2 Ephemerons
    - 10.7.3 Reference barriers
- 11 Associations
  - 11.1 Association Lists
  - 11.2 1D Tables
  - 11.3 The Association Table
  - 11.4 Hash Tables
    - 11.4.1 Construction of Hash Tables
    - 11.4.2 Basic Hash Table Operations
    - 11.4.3 Resizing of Hash Tables
    - 11.4.4 Address Hashing
  - 11.5 Object Hashing
  - 11.6 Red-Black Trees
  - 11.7 Weight-Balanced Trees
    - 11.7.1 Construction of Weight-Balanced Trees
    - 11.7.2 Basic Operations on Weight-Balanced Trees
    - 11.7.3 Advanced Operations on Weight-Balanced Trees
    - 11.7.4 Indexing Operations on Weight-Balanced Trees
  - 11.8 Associative Maps
    - 11.8.1 Amap constructors
    - 11.8.2 Amap predicates
    - 11.8.3 Amap accessors
    - 11.8.4 Amap mutators
    - 11.8.5 Amap mapping and folding
    - 11.8.6 Amap contents
    - 11.8.7 Amap copying and conversion
    - 11.8.8 Amaps as sets
- 12 Procedures
  - 12.1 Procedure Operations
  - 12.2 Arity
  - 12.3 Primitive Procedures
  - 12.4 Continuations
  - 12.5 Application Hooks
- 13 Environments
  - 13.1 Environment Operations
  - 13.2 Environment Variables
  - 13.3 REPL Environment
  - 13.4 Top-level Environments
- 14 Input/Output
  - 14.1 Ports
  - 14.2 File Ports
  - 14.3 String Ports
  - 14.4 Bytevector Ports
  - 14.5 Input Procedures
    - 14.5.1 Reader Controls
  - 14.6 Output Procedures
  - 14.7 Blocking Mode
  - 14.8 Terminal Mode
  - 14.9 Format
  - 14.10 Custom Output
  - 14.11 Prompting
  - 14.12 Textual Port Primitives
    - 14.12.1 Textual Port Types
    - 14.12.2 Constructors and Accessors for Textual Ports
    - 14.12.3 Textual Input Port Operations
    - 14.12.4 Textual Output Port Operations
  - 14.13 Parser Buffers
  - 14.14 Parser Language
    - 14.14.1 *Matcher
    - 14.14.2 *Parser
    - 14.14.3 Parser-language Macros
  - 14.15 XML Support
    - 14.15.1 XML Input
    - 14.15.2 XML Output
    - 14.15.3 XML Names
    - 14.15.4 XML Structure
- 15 Operating-System Interface
  - 15.1 Pathnames
    - 15.1.1 Filenames and Pathnames
    - 15.1.2 Components of Pathnames
    - 15.1.3 Operations on Pathnames
    - 15.1.4 Miscellaneous Pathname Procedures
  - 15.2 Working Directory
  - 15.3 File Manipulation
  - 15.4 Directory Reader
  - 15.5 Date and Time
    - 15.5.1 Universal Time
    - 15.5.2 Decoded Time
    - 15.5.3 File Time
    - 15.5.4 Time-Format Conversion
    - 15.5.5 External Representation of Time
  - 15.6 Machine Time
  - 15.7 Subprocesses
    - 15.7.1 Subprocess Procedures
    - 15.7.2 Subprocess Conditions
    - 15.7.3 Subprocess Options
  - 15.8 TCP Sockets
  - 15.9 Miscellaneous OS Facilities
- 16 Error System
  - 16.1 Condition Signalling
  - 16.2 Error Messages
  - 16.3 Condition Handling
  - 16.4 Restarts
    - 16.4.1 Establishing Restart Code
    - 16.4.2 Invoking Standard Restart Code
    - 16.4.3 Finding and Invoking General Restart Code
    - 16.4.4 The Named Restart Abstraction
  - 16.5 Condition Instances
    - 16.5.1 Generating Operations on Conditions
    - 16.5.2 Condition Abstraction
    - 16.5.3 Simple Operations on Condition Instances
  - 16.6 Condition Types
  - 16.7 Condition-Type Taxonomy
- 17 Graphics
  - 17.1 Opening and Closing of Graphics Devices
  - 17.2 Coordinates for Graphics
  - 17.3 Drawing Graphics
  - 17.4 Characteristics of Graphics Output
  - 17.5 Buffering of Graphics Output
  - 17.6 Clipping of Graphics Output
  - 17.7 Custom Graphics Operations
  - 17.8 Images
  - 17.9 X Graphics
    - 17.9.1 X Graphics Type
    - 17.9.2 Utilities for X Graphics
    - 17.9.3 Custom Operations on X Graphics Devices
  - 17.10 Win32 Graphics
    - 17.10.1 Win32 Graphics Type
    - 17.10.2 Custom Operations for Win32 Graphics
- 18 Win32 Package Reference
  - 18.1 Overview
  - 18.2 Foreign Function Interface
    - 18.2.1 Windows Types
    - 18.2.2 Windows Foreign Procedures
    - 18.2.3 Win32 API names and procedures
  - 18.3 Device Independent Bitmap Utilities
    - 18.3.1 DIB procedures
    - 18.3.2 Other parts of the DIB Utilities implementation
- 19 Standards Support
  - 19.1 Revised7 Report on the Algorithmic Language Scheme
  - 19.2 SRFI 1: List Library
  - 19.3 SRFI 2: `and-let*`
  - 19.4 SRFI 8: `receive`
  - 19.5 SRFI 9: Record Types
  - 19.6 SRFI 14: Character-set Library
  - 19.7 SRFI 23: Error Reporting Mechanism
  - 19.8 SRFI 27: Sources of Random Bits
  - 19.9 SRFI 39: Parameter Objects
  - 19.10 SRFI 69: Basic Hash Tables
  - 19.11 SRFI 115: Scheme Regular Expressions
  - 19.12 SRFI 124: Ephemerons
  - 19.13 SRFI 125: Intermediate Hash Tables
  - 19.14 SRFI 128: Comparators (reduced)
  - 19.15 SRFI 131: ERRR5RS Record Syntax (reduced)
  - 19.16 SRFI 133: Vector Library (R7RS-compatible)
  - 19.17 SRFI 140: Immutable Strings
  - 19.18 SRFI 143: Fixnums
  - 19.19 SRFI 158: Generators and Accumulators
- Appendix A GNU Free Documentation License
  - A.1 ADDENDUM: How to use this License for your documents
- Appendix B Binding Index
- Appendix C Concept Index

## Acknowledgements

While "a cast of thousands" may be an overstatement, it is certainly the case that this document represents the work of many people. First and foremost, thanks go to the authors of the Revised^4 Report on the Algorithmic Language Scheme, from which much of this document is derived. Thanks also to BBN Advanced Computers Inc. for the use of parts of their Butterfly Scheme Reference, and to Margaret O’Connell for translating it from BBN’s text-formatting language to ours.

Special thanks to Richard Stallman, Bob Chassell, and Brian Fox, all of the Free Software Foundation, for creating and maintaining the Texinfo formatting language in which this document is written.

This report describes research done at the Artificial Intelligence Laboratory and the Laboratory for Computer Science, both of the Massachusetts Institute of Technology. Support for this research is provided in part by the Advanced Research Projects Agency of the Department of Defense and by the National Science Foundation.

## 1 Overview

This manual is a detailed description of the MIT/GNU Scheme runtime system. It is intended to be a reference document for programmers. It does not describe how to run Scheme or how to interact with it — that is the subject of the MIT/GNU Scheme User’s Manual.

This chapter summarizes the semantics of Scheme, briefly describes the MIT/GNU Scheme programming environment, and explains the syntactic and lexical conventions of the language. Subsequent chapters describe special forms, numerous data abstractions, and facilities for input and output.

Throughout this manual, we will make frequent references to standard Scheme, which is the language defined by the document Revised^4 Report on the Algorithmic Language Scheme, by William Clinger, Jonathan Rees, et al., or by IEEE Std. 1178-1990, IEEE Standard for the Scheme Programming Language (in fact, several parts of this document are copied from the Revised Report). MIT/GNU Scheme is an extension of standard Scheme.

These are the significant semantic characteristics of the Scheme language:

Variables are statically scoped ¶

  Scheme is a statically scoped programming language, which means that each use of a variable is associated with a lexically apparent binding of that variable. Algol is another statically scoped language.

Types are latent ¶

  Scheme has latent types as opposed to manifest types, which means that Scheme associates types with values (or objects) rather than with variables. Other languages with latent types (also referred to as weakly typed or dynamically typed languages) include APL, Snobol, and other dialects of Lisp. Languages with manifest types (sometimes referred to as strongly typed or statically typed languages) include Algol 60, Pascal, and C.

Objects have unlimited extent ¶

  All objects created during a Scheme computation, including procedures and continuations, have unlimited extent; no Scheme object is ever destroyed. The system doesn’t run out of memory because the garbage collector reclaims the storage occupied by an object when the object cannot possibly be needed by a future computation. Other languages in which most objects have unlimited extent include APL and other Lisp dialects.

Proper tail recursion ¶

  Scheme is properly tail-recursive, which means that iterative computation can occur in constant space, even if the iterative computation is described by a syntactically recursive procedure. With a tail-recursive implementation, you can express iteration using the ordinary procedure-call mechanics; special iteration expressions are provided only for syntactic convenience.

Procedures are objects

  Scheme procedures are objects, which means that you can create them dynamically, store them in data structures, return them as the results of other procedures, and so on. Other languages with such procedure objects include Common Lisp and ML.

Continuations are explicit

  In most other languages, continuations operate behind the scenes. In Scheme, continuations are objects; you can use continuations for implementing a variety of advanced control constructs, including non-local exits, backtracking, and coroutines.

Arguments are passed by value

  Arguments to Scheme procedures are passed by value, which means that Scheme evaluates the argument expressions before the procedure gains control, whether or not the procedure needs the result of the evaluations. ML, C, and APL are three other languages that pass arguments by value. In languages such as SASL and Algol 60, argument expressions are not evaluated unless the values are needed by the procedure.

Scheme uses a parenthesized-list Polish notation to describe programs and (other) data. The syntax of Scheme, like that of most Lisp dialects, provides for great expressive power, largely due to its simplicity. An important consequence of this simplicity is the susceptibility of Scheme programs and data to uniform treatment by other Scheme programs. As with other Lisp dialects, the `read` primitive parses its input; that is, it performs syntactic as well as lexical decomposition of what it reads.

### 1.1 Notational Conventions

This section details the notational conventions used throughout the rest of this document.

#### 1.1.1 Errors

When this manual uses the phrase “an error will be signalled,” it means that Scheme will call `error`, which normally halts execution of the program and prints an error message.

When this manual uses the phrase “it is an error,” it means that the specified action is not valid in Scheme, but the system may or may not signal the error. When this manual says that something “must be,” it means that violating the requirement is an error.

#### 1.1.2 Examples

This manual gives many examples showing the evaluation of expressions. The examples have a common format that shows the expression being evaluated on the left hand side, an “arrow” in the middle, and the value of the expression written on the right. For example:

    (+ 1 2)          ⇒  3

Sometimes the arrow and value will be moved under the expression, due to lack of space. Occasionally we will not care what the value is, in which case both the arrow and the value are omitted.

If an example shows an evaluation that results in an error, an error message is shown, prefaced by ‘`error→`’:

    (+ 1 'foo)                      error→ Illegal datum

An example that shows printed output marks it with ‘`-|`’:

    (begin (write 'foo) 'bar)
         -| foo
         ⇒ bar

When this manual indicates that the value returned by some expression is unspecified, it means that the expression will evaluate to some object without signalling an error, but that programs should not depend on the value in any way.

#### 1.1.3 Entry Format

Each description of an MIT/GNU Scheme variable, special form, or procedure begins with one or more header lines in this format:

category: template ¶

where category specifies the kind of item (“variable”, “special form”, or “procedure”). The form of template is interpreted depending on category.

Variable

  Template consists of the variable’s name.

Parameter

  Template consists of the parameter’s name. See Dynamic Binding and Parameters for more information.

Special Form

  Template starts with the syntactic keyword of the special form, followed by a description of the special form’s syntax. The description is written using the following conventions.

  Named components are italicized in the printed manual, and uppercase in the Info file. “Noise” keywords, such as the `else` keyword in the `cond` special form, are set in a fixed width font in the printed manual; in the Info file they are not distinguished. Parentheses indicate themselves.

  A horizontal ellipsis (…) is describes repeated components. Specifically,

      thing …

  indicates zero or more occurrences of thing, while

      thing thing …

  indicates one or more occurrences of thing.

  Brackets, `[ ]`, enclose optional components.

  Several special forms (e.g. `lambda`) have an internal component consisting of a series of expressions; usually these expressions are evaluated sequentially under conditions that are specified in the description of the special form. This sequence of expressions is commonly referred to as the body of the special form.

Procedure

  Template starts with the name of the variable to which the procedure is bound, followed by a description of the procedure’s arguments. The arguments are described using “lambda list” notation (see Lambda Expressions), except that brackets are used to denote optional arguments, and ellipses are used to denote “rest” arguments.

  The names of the procedure’s arguments are italicized in the printed manual, and uppercase in the Info file.

  When an argument names a Scheme data type, it indicates that the argument must be that type of data object. For example,

  procedure: cdr pair ¶

  indicates that the standard Scheme procedure `cdr` takes one argument, which must be a pair.

  Many procedures signal an error when an argument is of the wrong type; usually this error is a condition of type `condition-type:wrong-type-argument`.

  In addition to the standard data-type names (pair, list, boolean, string, etc.), the following names as arguments also imply type restrictions:

  - object: any object
  - thunk: a procedure of no arguments
  - x, y: real numbers
  - q, n: integers
  - k: an exact non-negative integer

Some examples:

procedure: list object … ¶

indicates that the standard Scheme procedure `list` takes zero or more arguments, each of which may be any Scheme object.

procedure: write-char char [output-port] ¶

indicates that the standard Scheme procedure `write-char` must be called with a character, char, and may also be called with a character and an output port.

### 1.2 Scheme Concepts

#### 1.2.1 Variable Bindings

Any identifier that is not a syntactic keyword may be used as a variable (see Identifiers). A variable may name a location where a value can be stored. A variable that does so is said to be bound to the location. The value stored in the location to which a variable is bound is called the variable’s value. (The variable is sometimes said to name the value or to be bound to the value.)

A variable may be bound but still not have a value; such a variable is said to be unassigned. Referencing an unassigned variable is an error. When this error is signalled, it is a condition of type `condition-type:unassigned-variable`; sometimes the compiler does not generate code to signal the error. Unassigned variables are useful only in combination with side effects (see Assignments).

#### 1.2.2 Environment Concepts

An environment is a set of variable bindings. If an environment has no binding for a variable, that variable is said to be unbound in that environment. Referencing an unbound variable signals a condition of type `condition-type:unbound-variable`.

A new environment can be created by extending an existing environment with a set of new bindings. Note that “extending an environment” does not modify the environment; rather, it creates a new environment that contains the new bindings and the old ones. The new bindings shadow the old ones; that is, if an environment that contains a binding for `x` is extended with a new binding for `x`, then only the new binding is seen when `x` is looked up in the extended environment. Sometimes we say that the original environment is the parent of the new one, or that the new environment is a child of the old one, or that the new environment inherits the bindings in the old one.

Procedure calls extend an environment, as do `let`, `let*`, `letrec`, and `do` expressions. Internal definitions (see Internal Definitions) also extend an environment. (Actually, all the constructs that extend environments can be expressed in terms of procedure calls, so there is really just one fundamental mechanism for environment extension.) A top-level definition (see Top-Level Definitions) may add a binding to an existing environment.

#### 1.2.3 Initial and Current Environments

MIT/GNU Scheme provides an initial environment that contains all of the variable bindings described in this manual. Most environments are ultimately extensions of this initial environment. In Scheme, the environment in which your programs execute is actually a child (extension) of the environment containing the system’s bindings. Thus, system names are visible to your programs, but your names do not interfere with system programs.

The environment in effect at some point in a program is called the current environment at that point. In particular, every REP loop has a current environment. (REP stands for “read-eval-print”; the REP loop is the Scheme program that reads your input, evaluates it, and prints the result.) The environment of the top-level REP loop (the one you are in when Scheme starts up) starts as `user-initial-environment`, although it can be changed by the `ge` procedure. When a new REP loop is created, its environment is determined by the program that creates it.

#### 1.2.4 Static Scoping

Scheme is a statically scoped language with block structure. In this respect, it is like Algol and Pascal, and unlike most other dialects of Lisp except for Common Lisp.

The fact that Scheme is statically scoped (rather than dynamically bound) means that the environment that is extended (and becomes current) when a procedure is called is the environment in which the procedure was created (i.e. in which the procedure’s defining lambda expression was evaluated), not the environment in which the procedure is called. Because all the other Scheme binding expressions can be expressed in terms of procedures, this determines how all bindings behave.

Consider the following definitions, made at the top-level REP loop (in the initial environment):

    (define x 1)
    (define (f x) (g 2))
    (define (g y) (+ x y))
    (f 5)                                       ⇒  3 ; not 7

Here `f` and `g` are bound to procedures created in the initial environment. Because Scheme is statically scoped, the call to `g` from `f` extends the initial environment (the one in which `g` was created) with a binding of `y` to `2`. In this extended environment, `y` is `2` and `x` is `1`. (In a dynamically bound Lisp, the call to `g` would extend the environment in effect during the call to `f`, in which `x` is bound to `5` by the call to `f`, and the answer would be `7`.)

Note that with static scoping, you can tell what binding a variable reference refers to just from looking at the text of the program; the referenced binding cannot depend on how the program is used. That is, the nesting of environments (their parent-child relationship) corresponds to the nesting of binding expressions in program text. (Because of this connection to the text of the program, static scoping is also called lexical scoping.) For each place where a variable is bound in a program there is a corresponding region of the program text within which the binding is effective. For example, the region of a binding established by a `lambda` expression is the entire body of the `lambda` expression. The documentation of each binding expression explains what the region of the bindings it makes is. A use of a variable (that is, a reference to or assignment of a variable) refers to the innermost binding of that variable whose region contains the variable use. If there is no such region, the use refers to the binding of the variable in the global environment (which is an ancestor of all other environments, and can be thought of as a region in which all your programs are contained).

#### 1.2.5 True and False

In Scheme, the boolean values true and false are denoted by `#t` and `#f`. However, any Scheme value can be treated as a boolean for the purpose of a conditional test. This manual uses the word true to refer to any Scheme value that counts as true, and the word false to refer to any Scheme value that counts as false. In conditional tests, all values count as true except for `#f`, which counts as false (see Conditionals).

#### 1.2.6 External Representations

An important concept in Scheme is that of the external representation of an object as a sequence of characters. For example, an external representation of the integer 28 is the sequence of characters ‘`28`’, and an external representation of a list consisting of the integers 8 and 13 is the sequence of characters ‘`(8 13)`’.

The external representation of an object is not necessarily unique. The integer 28 also has representations ‘`#e28.000`’ and ‘`#x1c`’, and the list in the previous paragraph also has the representations ‘`( 08 13 )`’ and ‘`(8 . (13 . ( )))`’.

Many objects have standard external representations, but some, such as procedures and circular data structures, do not have standard representations (although particular implementations may define representations for them).

An external representation may be written in a program to obtain the corresponding object (see Quoting).

External representations can also be used for input and output. The procedure `read` parses external representations, and the procedure `write` generates them. Together, they provide an elegant and powerful input/output facility.

Note that the sequence of characters ‘`(+ 2 6)`’ is not an external representation of the integer 8, even though it is an expression that evaluates to the integer 8; rather, it is an external representation of a three-element list, the elements of which are the symbol `+` and the integers `2` and `6`. Scheme’s syntax has the property that any sequence of characters that is an expression is also the external representation of some object. This can lead to confusion, since it may not be obvious out of context whether a given sequence of characters is intended to denote data or program, but it is also a source of power, since it facilitates writing programs such as interpreters and compilers that treat programs as data or data as programs.

#### 1.2.7 Disjointness of Types

Every object satisfies at most one of the following predicates (but see True and False, for an exception):

    bit-string?     environment?    port?           symbol?
    boolean?        null?           procedure?      vector?
    cell?           number?         promise?        weak-pair?
    char?           pair?           string?
    condition?

#### 1.2.8 Storage Model

This section describes a model that can be used to understand Scheme’s use of storage.

Variables and objects such as pairs, vectors, and strings implicitly denote locations or sequences of locations. A string, for example, denotes as many locations as there are characters in the string. (These locations need not correspond to a full machine word.) A new value may be stored into one of these locations using the `string-set!` procedure, but the string continues to denote the same locations as before.

An object fetched from a location, by a variable reference or by a procedure such as `car`, `vector-ref`, or `string-ref`, is equivalent in the sense of `eqv?` to the object last stored in the location before the fetch.

Every location is marked to show whether it is in use. No variable or object ever refers to a location that is not in use. Whenever this document speaks of storage being allocated for a variable or object, what is meant is that an appropriate number of locations are chosen from the set of locations that are not in use, and the chosen locations are marked to indicate that they are now in use before the variable or object is made to denote them.

In many systems it is desirable for constants (i.e. the values of literal expressions) to reside in read-only memory. To express this, it is convenient to imagine that every object that denotes locations is associated with a flag telling whether that object is mutable or immutable. The constants and the strings returned by `symbol->string` are then the immutable objects, while all objects created by other procedures are mutable. It is an error to attempt to store a new value into a location that is denoted by an immutable object. Note that the MIT/GNU Scheme compiler takes advantage of this property to share constants, but that these constants are not immutable. Instead, two constants that are `equal?` may be `eq?` in compiled code.

### 1.3 Lexical Conventions

This section describes Scheme’s lexical conventions.

#### 1.3.1 Whitespace

Whitespace characters are spaces, newlines, tabs, and page breaks. Whitespace is used to improve the readability of your programs and to separate tokens from each other, when necessary. (A token is an indivisible lexical unit such as an identifier or number.) Whitespace is otherwise insignificant. Whitespace may occur between any two tokens, but not within a token. Whitespace may also occur inside a string, where it is significant.

#### 1.3.2 Delimiters

All whitespace characters are delimiters. In addition, the following characters act as delimiters:

    (  )  ;  "  '  `  |

Finally, these next characters act as delimiters, despite the fact that Scheme does not define any special meaning for them:

    [  ]  {  }

For example, if the value of the variable `name` is `"max"`:

    (list"Hi"name(+ 1 2))                   ⇒  ("Hi" "max" 3)

#### 1.3.3 Identifiers

An identifier is a sequence of one or more non-delimiter characters. Identifiers are used in several ways in Scheme programs:

- An identifier can be used as a variable or as a syntactic keyword.
- When an identifier appears as a literal or within a literal, it denotes a symbol.

Scheme accepts most of the identifiers that other programming languages allow. MIT/GNU Scheme allows all of the identifiers that standard Scheme does, plus many more.

MIT/GNU Scheme defines a potential identifier to be a sequence of non-delimiter characters that does not begin with either of the characters ‘`#`’ or ‘`,`’. Any such sequence of characters that is not a syntactically valid number (see Numbers) is considered to be a valid identifier. Note that, although it is legal for ‘`#`’ and ‘`,`’ to appear in an identifier (other than in the first character position), it is poor programming practice.

Here are some examples of identifiers:

    lambda             q
    list->vector       soup
    +                  V17a
    <=?                a34kTMNs
    the-word-recursion-has-many-meanings

#### 1.3.4 Uppercase and Lowercase

Scheme doesn’t distinguish uppercase and lowercase forms of a letter except within character and string constants; in other words, Scheme is case-insensitive. For example, ‘`Foo`’ is the same identifier as ‘`FOO`’, and ‘`#x1AB`’ is the same number as ‘`#X1ab`’. But ‘`#\a`’ and ‘`#\A`’ are different characters.

#### 1.3.5 Naming Conventions

A predicate is a procedure that always returns a boolean value (`#t` or `#f`). By convention, predicates usually have names that end in ‘`?`’.

A mutation procedure is a procedure that alters a data structure. By convention, mutation procedures usually have names that end in ‘`!`’.

#### 1.3.6 Comments

The beginning of a comment is indicated with a semicolon (`;`). Scheme ignores everything on a line in which a semicolon appears, from the semicolon until the end of the line. The entire comment, including the newline character that terminates it, is treated as whitespace.

An alternative form of comment (sometimes called an extended comment) begins with the characters ‘`#|`’ and ends with the characters ‘`|#`’. This alternative form is an MIT/GNU Scheme extension. As with ordinary comments, all of the characters in an extended comment, including the leading ‘`#|`’ and trailing ‘`|#`’, are treated as whitespace. Comments of this form may extend over multiple lines, and additionally may be nested (unlike the comments of the programming language C, which have a similar syntax).

    ;;; This is a comment about the FACT procedure.  Scheme
    ;;; ignores all of this comment.  The FACT procedure computes
    ;;; the factorial of a non-negative integer.

    #|
    This is an extended comment.
    Such comments are useful for commenting out code fragments.
    |#

    (define fact
      (lambda (n)
        (if (= n 0)                      ;This is another comment:
            1                            ;Base case: return 1
            (* n (fact (- n 1))))))

#### 1.3.7 Additional Notations

The following list describes additional notations used in Scheme. See Numbers, for a description of the notations used for numbers.

`+ - .`

  The plus sign, minus sign, and period are used in numbers, and may also occur in an identifier. A delimited period (not occurring within a number or identifier) is used in the notation for pairs and to indicate a “rest” parameter in a formal parameter list (see Lambda Expressions).

`( )`

  Parentheses are used for grouping and to notate lists (see Lists).

`"`

  The double quote delimits strings (see Strings).

`\`

  The backslash is used in the syntax for character constants (see Characters) and as an escape character within string constants (see Strings).

`;`

  The semicolon starts a comment.

`'`

  The single quote indicates literal data; it suppresses evaluation (see Quoting).

```

  The backquote indicates almost-constant data (see Quoting).

`,`

  The comma is used in conjunction with the backquote (see Quoting).

`,@`

  A comma followed by an at-sign is used in conjunction with the backquote (see Quoting).

`#`

  The sharp (or pound) sign has different uses, depending on the character that immediately follows it:

`#t #f`

  These character sequences denote the boolean constants (see Booleans).

`#\`

  This character sequence introduces a character constant (see Characters).

`#(`

  This character sequence introduces a vector constant (see Vectors). A close parenthesis, ‘`)`’, terminates a vector constant.

`#e #i #b #o #d #l #s #x`

  These character sequences are used in the notation for numbers (see Numbers).

`#|`

  This character sequence introduces an extended comment. The comment is terminated by the sequence ‘`|#`’. This notation is an MIT/GNU Scheme extension.

`#!`

  This character sequence is used to denote a small set of named constants. Currently there are only two of these, `#!optional` and `#!rest`, both of which are used in the `lambda` special form to mark certain parameters as being “optional” or “rest” parameters. This notation is an MIT/GNU Scheme extension.

`#*`

  This character sequence introduces a bit string (see Bit Strings). This notation is an MIT/GNU Scheme extension.

`#[`

  This character sequence is used to denote objects that do not have a readable external representation (see Custom Output). A close bracket, ‘`]`’, terminates the object’s notation. This notation is an MIT/GNU Scheme extension.

`#@`

  This character sequence is a convenient shorthand used to refer to objects by their hash number (see Custom Output). This notation is an MIT/GNU Scheme extension.

`#=`
`##`

  These character sequences introduce a notation used to show circular structures in printed output, or to denote them in input. The notation works much like that in Common Lisp, and is an MIT/GNU Scheme extension.

### 1.4 Expressions

A Scheme expression is a construct that returns a value. An expression may be a literal, a variable reference, a special form, or a procedure call.

#### 1.4.1 Literal Expressions

Literal constants may be written by using an external representation of the data. In general, the external representation must be quoted (see Quoting); but some external representations can be used without quotation.

    "abc"                                   ⇒  "abc"
    145932                                  ⇒  145932
    #t                                      ⇒  #t
    #\a                                     ⇒  #\a

The external representation of numeric constants, string constants, character constants, and boolean constants evaluate to the constants themselves. Symbols, pairs, lists, and vectors require quoting.

#### 1.4.2 Variable References

An expression consisting of an identifier (see Identifiers) is a variable reference; the identifier is the name of the variable being referenced. The value of the variable reference is the value stored in the location to which the variable is bound. An error is signalled if the referenced variable is unbound or unassigned.

    (define x 28)
    x                                       ⇒  28

#### 1.4.3 Special Form Syntax

    (keyword component …)

A parenthesized expression that starts with a syntactic keyword is a special form. Each special form has its own syntax, which is described later in the manual.

Note that syntactic keywords and variable bindings share the same namespace. A local variable binding may shadow a syntactic keyword, and a local syntactic-keyword definition may shadow a variable binding.

The following list contains all of the syntactic keywords that are defined when MIT/GNU Scheme is initialized:

access | and | begin
case | cond | cons-stream
declare | define
define-integrable | define-structure | define-syntax
delay | do | er-macro-transformer
fluid-let | if | lambda
let | let* | let*-syntax
let-syntax | letrec | letrec-syntax
local-declare | named-lambda | non-hygienic-macro-transformer
or | quasiquote | quote
rsc-macro-transformer | sc-macro-transformer | set!
syntax-rules | the-environment

#### 1.4.4 Procedure Call Syntax

    (operator operand …)

A procedure call is written by simply enclosing in parentheses expressions for the procedure to be called (the operator) and the arguments to be passed to it (the operands). The operator and operand expressions are evaluated and the resulting procedure is passed the resulting arguments. See Lambda Expressions, for a more complete description of this.

Another name for the procedure call expression is combination. This word is more specific in that it always refers to the expression; “procedure call” sometimes refers to the process of calling a procedure.

Unlike some other dialects of Lisp, Scheme always evaluates the operator expression and the operand expressions with the same evaluation rules, and the order of evaluation is unspecified.

    (+ 3 4)                                 ⇒  7
    ((if #f = *) 3 4)                       ⇒  12

A number of procedures are available as the values of variables in the initial environment; for example, the addition and multiplication procedures in the above examples are the values of the variables `+` and `*`. New procedures are created by evaluating `lambda` expressions.

If the operator is a syntactic keyword, then the expression is not treated as a procedure call: it is a special form.

## 2 Special Forms

A special form is an expression that follows special evaluation rules. This chapter describes the basic Scheme special forms.

### 2.1 Lambda Expressions

extended standard special form: lambda formals expr expr … ¶

  A `lambda` expression evaluates to a procedure. The environment in effect when the `lambda` expression is evaluated is remembered as part of the procedure; it is called the closing environment. When the procedure is later called with some arguments, the closing environment is extended by binding the variables in the formal parameter list to fresh locations, and the locations are filled with the arguments according to rules about to be given. The new environment created by this process is referred to as the invocation environment.

  Once the invocation environment has been constructed, the exprs in the body of the `lambda` expression are evaluated sequentially in it. This means that the region of the variables bound by the `lambda` expression is all of the exprs in the body. The result of evaluating the last expr in the body is returned as the result of the procedure call.

  Formals, the formal parameter list, is often referred to as a lambda list.

  The process of matching up formal parameters with arguments is somewhat involved. There are three types of parameters, and the matching treats each in sequence:

  Required

    All of the required parameters are matched against the arguments first. If there are fewer arguments than required parameters, an error of type `condition-type:wrong-number-of-arguments` is signalled; this error is also signalled if there are more arguments than required parameters and there are no further parameters.

  Optional

    Once the required parameters have all been matched, the optional parameters are matched against the remaining arguments. If there are fewer arguments than optional parameters, the unmatched parameters are bound to special objects called default objects. If there are more arguments than optional parameters, and there are no further parameters, an error of type `condition-type:wrong-number-of-arguments` is signalled.

    The predicate `default-object?`, which is true only of default objects, can be used to determine which optional parameters were supplied, and which were defaulted.

  Rest

    Finally, if there is a rest parameter (there can only be one), any remaining arguments are made into a list, and the list is bound to the rest parameter. (If there are no remaining arguments, the rest parameter is bound to the empty list.)

    In Scheme, unlike some other Lisp implementations, the list to which a rest parameter is bound is always freshly allocated. It has infinite extent and may be modified without affecting the procedure’s caller.

  Specially recognized keywords divide the formals parameters into these three classes. The keywords used here are ‘`#!optional`’, ‘`.`’, and ‘`#!rest`’. Note that only ‘`.`’ is defined by standard Scheme — the other keywords are MIT/GNU Scheme extensions. ‘`#!rest`’ has the same meaning as ‘`.`’ in formals.

  The use of these keywords is best explained by means of examples. The following are typical lambda lists, followed by descriptions of which parameters are required, optional, and rest. We will use ‘`#!rest`’ in these examples, but anywhere it appears ‘`.`’ could be used instead.

  `(a b c)`

    `a`, `b`, and `c` are all required. The procedure must be passed exactly three arguments.

  `(a b #!optional c)`

    `a` and `b` are required, `c` is optional. The procedure may be passed either two or three arguments.

  `(#!optional a b c)`

    `a`, `b`, and `c` are all optional. The procedure may be passed any number of arguments between zero and three, inclusive.

  `a`
  `(#!rest a)`

    These two examples are equivalent. `a` is a rest parameter. The procedure may be passed any number of arguments. Note: this is the only case in which ‘`.`’ cannot be used in place of ‘`#!rest`’.

  `(a b #!optional c d #!rest e)`

    `a` and `b` are required, `c` and `d` are optional, and `e` is rest. The procedure may be passed two or more arguments.

  Some examples of `lambda` expressions:

      (lambda (x) (+ x x))            ⇒  #[compound-procedure 53]

      ((lambda (x) (+ x x)) 4)                ⇒  8

      (define reverse-subtract
        (lambda (x y)
          (- y x)))
      (reverse-subtract 7 10)                 ⇒  3

      (define foo
        (let ((x 4))
          (lambda (y) (+ x y))))
      (foo 6)                                 ⇒  10

special form: named-lambda formals expression expression … ¶

  The `named-lambda` special form is similar to `lambda`, except that the first “required parameter” in formals is not a parameter but the name of the resulting procedure; thus formals must have at least one required parameter. This name has no semantic meaning, but is included in the external representation of the procedure, making it useful for debugging. In MIT/GNU Scheme, `lambda` is implemented as `named-lambda`, with a special name that means “unnamed”.

      (named-lambda (f x) (+ x x))    ⇒  #[compound-procedure 53 f]
      ((named-lambda (f x) (+ x x)) 4)        ⇒  8

### 2.2 Lexical Binding

The binding constructs `let`, `let*`, `letrec`, `letrec*`, `let-values`, and `let*-values` give Scheme block structure, like Algol 60. The syntax of the first four constructs is identical, but they differ in the regions they establish for their variable bindings. In a `let` expression, the initial values are computed before any of the variables become bound; in a `let*` expression, the bindings and evaluations are performed sequentially; while in `letrec` and `letrec*` expressions, all the bindings are in effect while their initial values are being computed, thus allowing mutually recursive definitions. The `let-values` and `let*-values` constructs are analogous to `let` and `let*` respectively, but are designed to handle multiple-valued expressions, binding different identifiers to the returned values.

extended standard special form: let ((variable init) …) expr expr … ¶

  The inits are evaluated in the current environment (in some unspecified order), the variables are bound to fresh locations holding the results, the exprs are evaluated sequentially in the extended environment, and the value of the last expr is returned. Each binding of a variable has the exprs as its region.

  MIT/GNU Scheme allows any of the inits to be omitted, in which case the corresponding variables are unassigned.

  Note that the following are equivalent:

      (let ((variable init) …) expr expr …)
      ((lambda (variable …) expr expr …) init …)

  Some examples:

      (let ((x 2) (y 3))
        (* x y))                              ⇒  6

      (let ((x 2) (y 3))
        (let ((foo (lambda (z) (+ x y z)))
              (x 7))
          (foo 4)))                           ⇒  9

  See Iteration, for information on “named `let`”.

extended standard special form: let* ((variable init) …) expr expr … ¶

  `let*` is similar to `let`, but the bindings are performed sequentially from left to right, and the region of a binding is that part of the `let*` expression to the right of the binding. Thus the second binding is done in an environment in which the first binding is visible, and so on.

  Note that the following are equivalent:

      (let* ((variable1 init1)
             (variable2 init2)
             …
             (variableN initN))
         expr
         expr …)

      (let ((variable1 init1))
        (let ((variable2 init2))
          …
            (let ((variableN initN))
              expr
              expr …)
          …))

  An example:

      (let ((x 2) (y 3))
        (let* ((x 7)
               (z (+ x y)))
          (* z x)))                           ⇒  70

extended standard special form: letrec ((variable init) …) expr expr … ¶

  The variables are bound to fresh locations holding unassigned values, the inits are evaluated in the extended environment (in some unspecified order), each variable is assigned to the result of the corresponding init, the exprs are evaluated sequentially in the extended environment, and the value of the last expr is returned. Each binding of a variable has the entire `letrec` expression as its region, making it possible to define mutually recursive procedures.

  MIT/GNU Scheme allows any of the inits to be omitted, in which case the corresponding variables are unassigned.

      (letrec ((even?
                (lambda (n)
                  (if (zero? n)
                      #t
                      (odd? (- n 1)))))
               (odd?
                (lambda (n)
                  (if (zero? n)
                      #f
                      (even? (- n 1))))))
        (even? 88))                           ⇒  #t

  One restriction on `letrec` is very important: it shall be possible to evaluated each init without assigning or referring to the value of any variable. If this restriction is violated, then it is an error. The restriction is necessary because Scheme passes arguments by value rather than by name. In the most common uses of `letrec`, all the inits are `lambda` or `delay` expressions and the restriction is satisfied automatically.

extended standard special form: letrec* ((variable init) …) expr expr … ¶

  The variables are bound to fresh locations, each variable is assigned in left-to-right order to the result of evaluating the corresponding init (interleaving evaluations and assignments), the exprs are evaluated in the resulting environment, and the values of the last expr are returned. Despite the left-to-right evaluation and assignment order, each binding of a variable has the entire `letrec*` expression as its region, making it possible to define mutually recursive procedures.

  If it is not possible to evaluate each init without assigning or referring to the value of the corresponding variable or the variable of any of the bindings that follow it in bindings, it is an error. Another restriction is that it is an error to invoke the continuation of an init more than once.

      ;; Returns the arithmetic, geometric, and
      ;; harmonic means of a nested list of numbers
      (define (means ton)
        (letrec*
           ((mean
              (lambda (f g)
                (f (/ (sum g ton) n))))
            (sum
              (lambda (g ton)
                (if (null? ton)
                  (+)
                  (if (number? ton)
                      (g ton)
                      (+ (sum g (car ton))
                         (sum g (cdr ton)))))))
            (n (sum (lambda (x) 1) ton)))
          (values (mean values values)
                  (mean exp log)
                  (mean / /))))

  Evaluating `(means '(3 (1 4)))` returns three values: `8/3`, `2.28942848510666` (approximately), and `36/19`.

standard special form: let-values ((formals init) …) expr expr … ¶

  The inits are evaluated in the current environment (in some unspecified order) as if by invoking `call-with-values`, and the variables occurring in the formals are bound to fresh locations holding the values returned by the inits, where the formals are matched to the return values in the same way that the formals in a `lambda` expression are matched to the arguments in a procedure call. Then, the exprs are evaluated in the extended environment, and the values of the last expr are returned. Each binding of a variable has the exprs as its region.

  It is an error if the formals do not match the number of values returned by the corresponding init.

      (let-values (((root rem) (exact-integer-sqrt 32)))
        (* root rem))         ⇒  35

standard special form: let*-values ((formals init) …) expr expr … ¶

  The `let*-values` construct is similar to `let-values`, but the inits are evaluated and bindings created sequentially from left to right, with the region of the bindings of each formals including the inits to its right as well as body. Thus the second init is evaluated in an environment in which the first set of bindings is visible and initialized, and so on.

      (let ((a 'a) (b 'b) (x 'x) (y 'y))
        (let*-values (((a b) (values x y))
                      ((x y) (values a b)))
          (list a b x y)))    ⇒  (x y x y)

### 2.3 Dynamic Binding

standard special form: parameterize ((parameter value) …) expr expr … ¶

  Note that both parameter and value are expressions. It is an error if the value of any parameter expression is not a parameter object.

  A `parameterize` expression is used to change the values of specified parameter objects during the evaluation of the body expressions.

  The parameter and value expressions are evaluated in an unspecified order. The body is evaluated in a dynamic environment in which each parameter is bound to the converted value—the result of passing value to the conversion procedure specified when the parameter was created. Then the previous value of parameter is restored without passing it to the conversion procedure. The value of the parameterize expression is the value of the last body expr.

  The `parameterize` special form is standardized by SRFI 39 and by R7RS.

Parameter objects can be used to specify configurable settings for a computation without the need to pass the value to every procedure in the call chain explicitly.

    (define radix
      (make-parameter
       10
       (lambda (x)
         (if (and (exact-integer?  x) (<= 2 x 16))
             x
             (error "invalid radix")))))

    (define (f n) (number->string n (radix)))

    (f 12)                                  ⇒ "12"
    (parameterize ((radix 2))
      (f 12))                               ⇒ "1100"
    (f 12)                                  ⇒ "12"
    (radix 16)                              error→ Wrong number of arguments
    (parameterize ((radix 0))
      (f 12))                               error→ invalid radix

A dynamic binding changes the value of a parameter (see Parameters) object temporarily, for a dynamic extent. The set of all dynamic bindings at a given time is called the dynamic environment. The new values are only accessible to the thread that constructed the dynamic environment, and any threads created within that environment.

The extent of a dynamic binding is defined to be the time period during which calling the parameter returns the new value. Normally this time period begins when the body is entered and ends when it is exited, a contiguous time period. However Scheme has first-class continuations by which it is possible to leave the body and reenter it many times. In this situation, the extent is non-contiguous.

When the body is exited by invoking a continuation, the current dynamic environment is unwound until it can be re-wound to the environment captured by the continuation. When the continuation returns, the process is reversed, restoring the original dynamic environment.

The following example shows the interaction between dynamic binding and continuations. Side effects to the binding that occur both inside and outside of the body are preserved, even if continuations are used to jump in and out of the body repeatedly.

    (define (complicated-dynamic-parameter)
      (let ((variable (make-settable-parameter 1))
            (inside-continuation))
        (write-line (variable))
        (call-with-current-continuation
         (lambda (outside-continuation)
           (parameterize ((variable 2))
             (write-line (variable))
             (variable 3)
             (call-with-current-continuation
              (lambda (k)
                (set! inside-continuation k)
                (outside-continuation #t)))
             (write-line (variable))
             (set! inside-continuation #f))))
        (write-line (variable))
        (if inside-continuation
            (begin
              (variable 4)
              (inside-continuation #f)))))

Evaluating ‘`(complicated-dynamic-binding)`’ writes the following on the console:

    1
    2
    1
    3
    4

Commentary: the first two values written are the initial binding of `variable` and its new binding inside `parameterize`’s body. Immediately after they are written, the binding visible in the body is set to ‘`3`’, and `outside-continuation` is invoked, exiting the body. At this point, ‘`1`’ is written, demonstrating that the original binding of `variable` is still visible outside the body. Then we set `variable` to ‘`4`’ and reenter the body by invoking `inside-continuation`. At this point, ‘`3`’ is written, indicating that the binding modified in the body is still the binding visible in the body. Finally, we exit the body normally, and write ‘`4`’, demonstrating that the binding modified outside of the body was also preserved.

#### 2.3.1 Fluid-Let

The `fluid-let` special form can change the value of any variable for a dynamic extent, but it is difficult to implement in a multi-processing (SMP) world. It and the cell object type (see Cells) are now deprecated. They are still available and functional in a uni-processing (non-SMP) world, but will signal an error when used in an SMP world. The `parameterize` special form (see parameterize) should be used instead.

special form: fluid-let ((variable init) …) expression expression … ¶

  The inits are evaluated in the current environment (in some unspecified order), the current values of the variables are saved, the results are assigned to the variables, the expressions are evaluated sequentially in the current environment, the variables are restored to their original values, and the value of the last expression is returned.

  The syntax of this special form is similar to that of `let`, but `fluid-let` temporarily rebinds existing variables. Unlike `let`, `fluid-let` creates no new bindings; instead it assigns the value of each init to the binding (determined by the rules of lexical scoping) of its corresponding variable.

  MIT/GNU Scheme allows any of the inits to be omitted, in which case the corresponding variables are temporarily unassigned.

  An error of type `condition-type:unbound-variable` is signalled if any of the variables are unbound. However, because `fluid-let` operates by means of side effects, it is valid for any variable to be unassigned when the form is entered.

### 2.4 Definitions

extended standard special form: define variable [expression] ¶
standard special form: define formals expression expression … ¶

  Definitions are valid in some but not all contexts where expressions are allowed. Definitions may only occur at the top level of a program and at the beginning of a lambda body (that is, the body of a `lambda`, `let`, `let*`, `letrec`, `letrec*`, `let-values`, `let*-values`, `parameterize`, or “procedure `define`” expression). A definition that occurs at the top level of a program is called a top-level definition, and a definition that occurs at the beginning of a body is called an internal definition.

  In the second form of `define` (called “procedure `define`”), the component formals is identical to the component of the same name in a `named-lambda` expression. In fact, these two expressions are equivalent:

      (define (name1 name2 …)
        expression
        expression …)

      (define name1
        (named-lambda (name1 name2 …)
          expression
          expression …))

#### 2.4.1 Top-Level Definitions

A top-level definition,

    (define variable expression)

has essentially the same effect as this assignment expression, if variable is bound:

    (set! variable expression)

If variable is not bound, however, `define` binds variable to a new location in the current environment before performing the assignment (it is an error to perform a `set!` on an unbound variable). If you omit expression, the variable becomes unassigned; an attempt to reference such a variable is an error.

    (define add3
       (lambda (x) (+ x 3)))                ⇒  unspecified
    (add3 3)                                ⇒  6

    (define first car)                      ⇒  unspecified
    (first '(1 2))                          ⇒  1

    (define bar)                            ⇒  unspecified
    bar                                     error→ Unassigned variable

#### 2.4.2 Internal Definitions

An internal definition is a definition that occurs at the beginning of a body (that is, the body of a `lambda`, `let`, `let*`, `letrec`, `letrec*`, `let-values`, `let*-values`, `parameterize`, or “procedure `define`” expression), rather than at the top level of a program. The variable defined by an internal definition is local to the body. That is, variable is bound rather than assigned, and the region of the binding is the entire body. For example,

    (let ((x 5))
      (define foo (lambda (y) (bar x y)))
      (define bar (lambda (a b) (+ (* a b) a)))
      (foo (+ x 3)))                        ⇒  45

A body containing internal definitions can always be converted into a completely equivalent `letrec*` expression. For example, the `let` expression in the above example is equivalent to

    (let ((x 5))
      (letrec* ((foo (lambda (y) (bar x y)))
                (bar (lambda (a b) (+ (* a b) a))))
        (foo (+ x 3))))

### 2.5 Assignments

extended standard special form: set! variable [expression] ¶

  If expression is specified, evaluates expression and stores the resulting value in the location to which variable is bound. If expression is omitted, variable is altered to be unassigned; a subsequent reference to such a variable is an error. In either case, the value of the `set!` expression is unspecified.

  Variable must be bound either in some region enclosing the `set!` expression, or at the top level. However, variable is permitted to be unassigned when the `set!` form is entered.

      (define x 2)                            ⇒  unspecified
      (+ x 1)                                 ⇒  3
      (set! x 4)                              ⇒  unspecified
      (+ x 1)                                 ⇒  5

  Variable may be an `access` expression (see Environments). This allows you to assign variables in an arbitrary environment. For example,

      (define x (let ((y 0)) (the-environment)))
      (define y 'a)
      y                                       ⇒  a
      (access y x)                            ⇒  0
      (set! (access y x) 1)                   ⇒  unspecified
      y                                       ⇒  a
      (access y x)                            ⇒  1

### 2.6 Quoting

This section describes the expressions that are used to modify or prevent the evaluation of objects.

standard special form: quote datum ¶

  `(quote datum)` evaluates to datum. Datum may be any external representation of a Scheme object (see External Representations). Use `quote` to include literal constants in Scheme code.

      (quote a)                               ⇒  a
      (quote #(a b c))                        ⇒  #(a b c)
      (quote (+ 1 2))                         ⇒  (+ 1 2)

  `(quote datum)` may be abbreviated as `'datum`. The two notations are equivalent in all respects.

      'a                                      ⇒  a
      '#(a b c)                               ⇒  #(a b c)
      '(+ 1 2)                                ⇒  (+ 1 2)
      '(quote a)                              ⇒  (quote a)
      ''a                                     ⇒  (quote a)

  Numeric constants, string constants, character constants, and boolean constants evaluate to themselves, so they don’t need to be quoted.

      '"abc"                                  ⇒  "abc"
      "abc"                                   ⇒  "abc"
      '145932                                 ⇒  145932
      145932                                  ⇒  145932
      '#t                                     ⇒  #t
      #t                                      ⇒  #t
      '#\a                                    ⇒  #\a
      #\a                                     ⇒  #\a

standard special form: quasiquote template ¶

  “Backquote” or “quasiquote” expressions are useful for constructing a list or vector structure when most but not all of the desired structure is known in advance. If no commas appear within the template, the result of evaluating ``template` is equivalent (in the sense of `equal?`) to the result of evaluating `'template`. If a comma appears within the template, however, the expression following the comma is evaluated (“unquoted”) and its result is inserted into the structure instead of the comma and the expression. If a comma appears followed immediately by an at-sign (@), then the following expression shall evaluate to a list; the opening and closing parentheses of the list are then “stripped away” and the elements of the list are inserted in place of the comma at-sign expression sequence.

      `(list ,(+ 1 2) 4)                       ⇒  (list 3 4)

      (let ((name 'a)) `(list ,name ',name))   ⇒  (list a 'a)

      `(a ,(+ 1 2) ,@(map abs '(4 -5 6)) b)    ⇒  (a 3 4 5 6 b)

      `((foo ,(- 10 3)) ,@(cdr '(c)) . ,(car '(cons)))
                                               ⇒  ((foo 7) . cons)

      `#(10 5 ,(sqrt 4) ,@(map sqrt '(16 9)) 8)
                                               ⇒  #(10 5 2 4 3 8)

      `,(+ 2 3)                                ⇒  5

  Quasiquote forms may be nested. Substitutions are made only for unquoted components appearing at the same nesting level as the outermost backquote. The nesting level increases by one inside each successive quasiquotation, and decreases by one inside each unquotation.

      `(a `(b ,(+ 1 2) ,(foo ,(+ 1 3) d) e) f)
           ⇒  (a `(b ,(+ 1 2) ,(foo 4 d) e) f)

      (let ((name1 'x)
            (name2 'y))
         `(a `(b ,,name1 ,',name2 d) e))
           ⇒  (a `(b ,x ,'y d) e)

  The notations ``template` and (`quasiquote template`) are identical in all respects. `,expression` is identical to `(unquote expression)` and `,@expression` is identical to `(unquote-splicing expression)`.

      (quasiquote (list (unquote (+ 1 2)) 4))
           ⇒  (list 3 4)

      '(quasiquote (list (unquote (+ 1 2)) 4))
           ⇒  `(list ,(+ 1 2) 4)
           i.e., (quasiquote (list (unquote (+ 1 2)) 4))

  Unpredictable behavior can result if any of the symbols `quasiquote`, `unquote`, or `unquote-splicing` appear in a template in ways otherwise than as described above.

### 2.7 Conditionals

The behavior of the conditional expressions is determined by whether objects are true or false. The conditional expressions count only `#f` as false. They count everything else, including `#t`, pairs, symbols, numbers, strings, vectors, and procedures as true (but see True and False).

In the descriptions that follow, we say that an object has “a true value” or “is true” when the conditional expressions treat it as true, and we say that an object has “a false value” or “is false” when the conditional expressions treat it as false.

standard special form: if predicate consequent [alternative] ¶

  Predicate, consequent, and alternative are expressions. An `if` expression is evaluated as follows: first, predicate is evaluated. If it yields a true value, then consequent is evaluated and its value is returned. Otherwise alternative is evaluated and its value is returned. If predicate yields a false value and no alternative is specified, then the result of the expression is unspecified.

  An `if` expression evaluates either consequent or alternative, never both. Programs should not depend on the value of an `if` expression that has no alternative.

      (if (> 3 2) 'yes 'no)                   ⇒  yes
      (if (> 2 3) 'yes 'no)                   ⇒  no
      (if (> 3 2)
          (- 3 2)
          (+ 3 2))                            ⇒  1

standard special form: cond clause clause … ¶

  Each clause has this form:

      (predicate expression …)

  where predicate is any expression. The last clause may be an `else` clause, which has the form:

      (else expression expression …)

  A `cond` expression does the following:

  1. Evaluates the predicate expressions of successive clauses in order, until one of the predicates evaluates to a true value.
  2. When a predicate evaluates to a true value, `cond` evaluates the expressions in the associated clause in left to right order, and returns the result of evaluating the last expression in the clause as the result of the entire `cond` expression.

     If the selected clause contains only the predicate and no expressions, `cond` returns the value of the predicate as the result.

  3. If all predicates evaluate to false values, and there is no `else` clause, the result of the conditional expression is unspecified; if there is an `else` clause, `cond` evaluates its expressions (left to right) and returns the value of the last one.

      (cond ((> 3 2) 'greater)
            ((< 3 2) 'less))                  ⇒  greater

      (cond ((> 3 3) 'greater)
            ((< 3 3) 'less)
            (else 'equal))                    ⇒  equal

  Normally, programs should not depend on the value of a `cond` expression that has no `else` clause. However, some Scheme programmers prefer to write `cond` expressions in which at least one of the predicates is always true. In this style, the final clause is equivalent to an `else` clause.

  Scheme supports an alternative clause syntax:

      (predicate => recipient)

  where recipient is an expression. If predicate evaluates to a true value, then recipient is evaluated. Its value must be a procedure of one argument; this procedure is then invoked on the value of the predicate.

      (cond ((assv 'b '((a 1) (b 2))) => cadr)
            (else #f))                        ⇒  2

standard special form: case key clause clause … ¶

  Key may be any expression. Each clause has this form:

      ((object …) expression expression …)

  No object is evaluated, and all the objects must be distinct. The last clause may be an `else` clause, which has the form:

      (else expression expression …)

  A `case` expression does the following:

  1. Evaluates key and compares the result with each object.
  2. If the result of evaluating key is equivalent (in the sense of `eqv?`; see Equivalence Predicates) to an object, `case` evaluates the expressions in the corresponding clause from left to right and returns the result of evaluating the last expression in the clause as the result of the `case` expression.
  3. If the result of evaluating key is different from every object, and if there’s an `else` clause, `case` evaluates its expressions and returns the result of the last one as the result of the `case` expression. If there’s no `else` clause, `case` returns an unspecified result. Programs should not depend on the value of a `case` expression that has no `else` clause.

  For example,

      (case (* 2 3)
         ((2 3 5 7) 'prime)
         ((1 4 6 8 9) 'composite))            ⇒  composite

      (case (car '(c d))
         ((a) 'a)
         ((b) 'b))                            ⇒  unspecified

      (case (car '(c d))
         ((a e i o u) 'vowel)
         ((w y) 'semivowel)
         (else 'consonant))                   ⇒  consonant

standard special form: and expression … ¶

  The expressions are evaluated from left to right, and the value of the first expression that evaluates to a false value is returned. Any remaining expressions are not evaluated. If all the expressions evaluate to true values, the value of the last expression is returned. If there are no expressions then `#t` is returned.

      (and (= 2 2) (> 2 1))                   ⇒  #t
      (and (= 2 2) (< 2 1))                   ⇒  #f
      (and 1 2 'c '(f g))                     ⇒  (f g)
      (and)                                   ⇒  #t

standard special form: or expression … ¶

  The expressions are evaluated from left to right, and the value of the first expression that evaluates to a true value is returned. Any remaining expressions are not evaluated. If all expressions evaluate to false values, the value of the last expression is returned. If there are no expressions then `#f` is returned.

      (or (= 2 2) (> 2 1))                    ⇒  #t
      (or (= 2 2) (< 2 1))                    ⇒  #t
      (or #f #f #f)                           ⇒  #f
      (or (memq 'b '(a b c)) (/ 3 0))         ⇒  (b c)

### 2.8 Sequencing

The `begin` special form is used to evaluate expressions in a particular order.

standard special form: begin expression expression … ¶

  The expressions are evaluated sequentially from left to right, and the value of the last expression is returned. This expression type is used to sequence side effects such as input and output.

      (define x 0)
      (begin (set! x 5)
             (+ x 1))                 ⇒  6

      (begin (display "4 plus 1 equals ")
             (display (+ 4 1)))
                                      -|  4 plus 1 equals 5
                                      ⇒  unspecified

  Often the use of `begin` is unnecessary, because many special forms already support sequences of expressions (that is, they have an implicit `begin`). Some of these special forms are:

      case
      cond
      define          ;“procedure define” only
      do
      lambda
      let
      let*
      letrec
      letrec*
      let-values
      let*-values
      named-lambda
      parameterize

### 2.9 Iteration

The iteration expressions are: “named `let`” and `do`. They are also binding expressions, but are more commonly referred to as iteration expressions. Because Scheme is properly tail-recursive, you don’t need to use these special forms to express iteration; you can simply use appropriately written “recursive” procedure calls.

extended standard special form: let name ((variable init) …) expr expr … ¶

  MIT/GNU Scheme permits a variant on the syntax of `let` called “named `let`” which provides a more general looping construct than `do`, and may also be used to express recursions.

  Named `let` has the same syntax and semantics as ordinary `let` except that name is bound within the exprs to a procedure whose formal arguments are the variables and whose body is the exprs. Thus the execution of the exprs may be repeated by invoking the procedure named by name.

  MIT/GNU Scheme allows any of the inits to be omitted, in which case the corresponding variables are unassigned.

  Note: the following expressions are equivalent:

      (let name ((variable init) …)
        expr
        expr …)

      ((letrec ((name
                 (named-lambda (name variable …)
                   expr
                   expr …)))
         name)
       init …)

  Here is an example:

      (let loop
           ((numbers '(3 -2 1 6 -5))
            (nonneg '())
            (neg '()))
        (cond ((null? numbers)
               (list nonneg neg))
              ((>= (car numbers) 0)
               (loop (cdr numbers)
                     (cons (car numbers) nonneg)
                     neg))
              (else
               (loop (cdr numbers)
                     nonneg
                     (cons (car numbers) neg)))))

           ⇒  ((6 1 3) (-5 -2))

extended standard special form: do ((variable init step) …) (test expression …) command … ¶

  `do` is an iteration construct. It specifies a set of variables to be bound, how they are to be initialized at the start, and how they are to be updated on each iteration. When a termination condition is met, the loop exits with a specified result value.

  `do` expressions are evaluated as follows: The init expressions are evaluated (in some unspecified order), the variables are bound to fresh locations, the results of the init expressions are stored in the bindings of the variables, and then the iteration phase begins.

  Each iteration begins by evaluating test; if the result is false, then the command expressions are evaluated in order for effect, the step expressions are evaluated in some unspecified order, the variables are bound to fresh locations, the results of the steps are stored in the bindings of the variables, and the next iteration begins.

  If test evaluates to a true value, then the expressions are evaluated from left to right and the value of the last expression is returned as the value of the `do` expression. If no expressions are present, then the value of the `do` expression is unspecified in standard Scheme; in MIT/GNU Scheme, the value of test is returned.

  The region of the binding of a variable consists of the entire `do` expression except for the inits. It is an error for a variable to appear more than once in the list of `do` variables.

  A step may be omitted, in which case the effect is the same as if `(variable init variable)` had been written instead of `(variable init)`.

      (do ((vec (make-vector 5))
            (i 0 (+ i 1)))
          ((= i 5) vec)
         (vector-set! vec i i))               ⇒  #(0 1 2 3 4)

      (let ((x '(1 3 5 7 9)))
         (do ((x x (cdr x))
              (sum 0 (+ sum (car x))))
             ((null? x) sum)))                ⇒  25

### 2.10 Structure Definitions

This section provides examples and describes the options and syntax of `define-structure`, an MIT/GNU Scheme macro that is very similar to `defstruct` in Common Lisp. The differences between them are summarized at the end of this section. For more information, see Steele’s Common Lisp book.

special form: define-structure (name structure-option …) slot-description … ¶

  Each slot-description takes one of the following forms:

      slot-name
      (slot-name default-init [slot-option value]*)

  The fields name and slot-name must both be symbols. The field default-init is an expression for the initial value of the slot. It is evaluated each time a new instance is constructed. If it is not specified, the initial content of the slot is undefined. Default values are only useful with a BOA constructor with argument list or a keyword constructor (see below).

  Evaluation of a `define-structure` expression defines a structure descriptor and a set of procedures to manipulate instances of the structure. These instances are represented as records by default (see Records) but may alternately be lists or vectors. The accessors and modifiers are marked with compiler declarations so that calls to them are automatically transformed into appropriate references. Often, no options are required, so a simple call to `define-structure` looks like:

      (define-structure foo a b c)

  This defines a type descriptor `rtd:foo`, a constructor `make-foo`, a predicate `foo?`, accessors `foo-a`, `foo-b`, and `foo-c`, and modifiers `set-foo-a!`, `set-foo-b!`, and `set-foo-c!`.

  In general, if no options are specified, `define-structure` defines the following (using the simple call above as an example):

  type descriptor

    The name of the type descriptor is `"rtd:"` followed by the name of the structure, e.g. ‘`rtd:foo`’. The type descriptor satisfies the predicate `record-type?`.

  constructor

    The name of the constructor is `"make-"` followed by the name of the structure, e.g. ‘`make-foo`’. The number of arguments accepted by the constructor is the same as the number of slots; the arguments are the initial values for the slots, and the order of the arguments matches the order of the slot definitions.

  predicate

    The name of the predicate is the name of the structure followed by `"?"`, e.g. ‘`foo?`’. The predicate is a procedure of one argument, which returns `#t` if its argument is a record of the type defined by this structure definition, and `#f` otherwise.

  accessors

    For each slot, an accessor is defined. The name of the accessor is formed by appending the name of the structure, a hyphen, and the name of the slot, e.g. ‘`foo-a`’. The accessor is a procedure of one argument, which must be a record of the type defined by this structure definition. The accessor extracts the contents of the corresponding slot in that record and returns it.

  modifiers

    For each slot, a modifier is defined. The name of the modifier is formed by appending `"set-"`, the name of the accessor, and `"!"`, e.g. ‘`set-foo-a!`’. The modifier is a procedure of two arguments, the first of which must be a record of the type defined by this structure definition, and the second of which may be any object. The modifier modifies the contents of the corresponding slot in that record to be that object, and returns an unspecified value.

  When options are not supplied, `(name)` may be abbreviated to name. This convention holds equally for structure-options and slot-options. Hence, these are equivalent:

      (define-structure foo a b c)
      (define-structure (foo) (a) b (c))

  as are

      (define-structure (foo keyword-constructor) a b c)
      (define-structure (foo (keyword-constructor)) a b c)

  When specified as option values, `false` and `nil` are equivalent to `#f`, and `true` and `t` are equivalent to `#t`.

Possible slot-options are:

slot option: read-only value ¶

  When given a value other than `#f`, this specifies that no modifier should be created for the slot.

slot option: type type-descriptor ¶

  This is accepted but not presently used.

Possible structure-options are:

structure option: predicate [name] ¶

  This option controls the definition of a predicate procedure for the structure. If name is not given, the predicate is defined with the default name (see above). If name is `#f`, the predicate is not defined at all. Otherwise, name must be a symbol, and the predicate is defined with that symbol as its name.

structure option: copier [name] ¶

  This option controls the definition of a procedure to copy instances of the structure. This is a procedure of one argument, a structure instance, that makes a newly allocated copy of the structure and returns it. If name is not given, the copier is defined, and the name of the copier is `"copy-"` followed by the structure name (e.g. ‘`copy-foo`’). If name is `#f`, the copier is not defined. Otherwise, name must be a symbol, and the copier is defined with that symbol as its name.

structure option: print-procedure expression ¶

  Evaluating expression must yield a procedure of two arguments, which is used to print instances of the structure. The procedure is a print method (see Custom Output).

structure option: constructor [name [argument-list]] ¶

  This option controls the definition of constructor procedures. These constructor procedures are called “BOA constructors”, for “By Order of Arguments”, because the arguments to the constructor specify the initial contents of the structure’s slots by the order in which they are given. This is as opposed to “keyword constructors”, which specify the initial contents using keywords, and in which the order of arguments is irrelevant.

  If name is not given, a constructor is defined with the default name and arguments (see above). If name is `#f`, no constructor is defined; argument-list may not be specified in this case. Otherwise, name must be a symbol, and a constructor is defined with that symbol as its name. If name is a symbol, argument-list is optionally allowed; if it is omitted, the constructor accepts one argument for each slot in the structure definition, in the same order in which the slots appear in the definition. Otherwise, argument-list must be a lambda list (see Lambda Expressions), and each of the parameters of the lambda list must be the name of a slot in the structure. The arguments accepted by the constructor are defined by this lambda list. Any slot that is not specified by the lambda list is initialized to the default-init as specified above; likewise for any slot specified as an optional parameter when the corresponding argument is not supplied.

  If the `constructor` option is specified, the default constructor is not defined. Additionally, the `constructor` option may be specified multiple times to define multiple constructors with different names and argument lists.

      (define-structure (foo
                         (constructor make-foo (#!optional a b)))
        (a 6 read-only #t)
        (b 9))

structure option: keyword-constructor [name] ¶

  This option controls the definition of keyword constructor procedures. A keyword constructor is a procedure that accepts arguments that are alternating slot names and values. If name is omitted, a keyword constructor is defined, and the name of the constructor is `"make-"` followed by the name of the structure (e.g. ‘`make-foo`’). Otherwise, name must be a symbol, and a keyword constructor is defined with this symbol as its name.

  If the `keyword-constructor` option is specified, the default constructor is not defined. Additionally, the `keyword-constructor` option may be specified multiple times to define multiple keyword constructors; this is usually not done since such constructors would all be equivalent.

      (define-structure (foo (keyword-constructor make-bar)) a b)
      (foo-a (make-bar 'b 20 'a 19))         ⇒ 19

structure option: type-descriptor name ¶

  This option cannot be used with the `type` or `named` options.

  By default, structures are implemented as records. The name of the structure is defined to hold the type descriptor of the record defined by the structure. The `type-descriptor` option specifies a different name to hold the type descriptor.

      (define-structure foo a b)
      foo             ⇒ #[record-type 18]

      (define-structure (bar (type-descriptor <bar>)) a b)
      bar             error→ Unbound variable: bar
      <bar>         ⇒ #[record-type 19]

structure option: conc-name [name] ¶

  By default, the prefix for naming accessors and modifiers is the name of the structure followed by a hyphen. The `conc-name` option can be used to specify an alternative. If name is not given, the prefix is the name of the structure followed by a hyphen (the default). If name is `#f`, the slot names are used directly, without prefix. Otherwise, name must a symbol, and that symbol is used as the prefix.

      (define-structure (foo (conc-name moby/)) a b)

  defines accessors `moby/a` and `moby/b`, and modifiers `set-moby/a!` and `set-moby/b!`.

      (define-structure (foo (conc-name #f)) a b)

  defines accessors `a` and `b`, and modifiers `set-a!` and `set-b!`.

structure option: type representation-type ¶

  This option cannot be used with the `type-descriptor` option.

  By default, structures are implemented as records. The `type` option overrides this default, allowing the programmer to specify that the structure be implemented using another data type. The option value representation-type specifies the alternate data type; it is allowed to be one of the symbols `vector` or `list`, and the data type used is the one corresponding to the symbol.

  If this option is given, and the `named` option is not specified, the representation will not be tagged, and neither a predicate nor a type descriptor will be defined; also, the `print-procedure` option may not be given.

      (define-structure (foo (type list)) a b)
      (make-foo 1 2)                          ⇒ (1 2)

structure option: named [expression] ¶

  This is valid only in conjunction with the `type` option and specifies that the structure instances be tagged to make them identifiable as instances of this structure type. This option cannot be used with the `type-descriptor` option.

  In the usual case, where expression is not given, the `named` option causes a type descriptor and predicate to be defined for the structure (recall that the `type` option without `named` suppresses their definition), and also defines a default print method for the structure instances (which can be overridden by the `print-procedure` option). If the default print method is not wanted then the `print-procedure` option should be specified as `#f`. This causes the structure to be printed in its native representation, as a list or vector, which includes the type descriptor. The type descriptor is a unique object, not a record type, that describes the structure instances and is additionally stored in the structure instances to identify them: if the representation type is `vector`, the type descriptor is stored in the zero-th slot of the vector, and if the representation type is `list`, it is stored as the first element of the list.

      (define-structure (foo (type vector) named) a b c)
      (vector-ref (make-foo 1 2 3) 0) ⇒ #[structure-type 52]

  If expression is specified, it is an expression that is evaluated to yield a tag object. The expression is evaluated once when the structure definition is evaluated (to specify the print method), and again whenever a predicate or constructor is called. Because of this, expression is normally a variable reference or a constant. The value yielded by expression may be any object at all. That object is stored in the structure instances in the same place that the type descriptor is normally stored, as described above. If expression is specified, no type descriptor is defined, only a predicate.

      (define-structure (foo (type vector) (named 'foo)) a b c)
      (vector-ref (make-foo 1 2 3) 0) ⇒ foo

structure option: safe-accessors [boolean] ¶

  This option allows the programmer to have some control over the safety of the slot accessors (and modifiers) generated by `define-structure`. If `safe-accessors` is not specified, or if boolean is `#f`, then the accessors are optimized for speed at the expense of safety; when compiled, the accessors will turn into very fast inline sequences, usually one to three machine instructions in length. However, if `safe-accessors` is specified and boolean is either omitted or `#t`, then the accessors are optimized for safety, will check the type and structure of their argument, and will be close-coded.

      (define-structure (foo safe-accessors) a b c)

structure option: initial-offset offset ¶

  This is valid only in conjunction with the `type` option. Offset must be an exact non-negative integer and specifies the number of slots to leave open at the beginning of the structure instance before the specified slots are allocated. Specifying an offset of zero is equivalent to omitting the `initial-offset` option.

  If the `named` option is specified, the structure tag appears in the first slot, followed by the “offset” slots, and then the regular slots. Otherwise, the “offset” slots come first, followed by the regular slots.

      (define-structure (foo (type vector) (initial-offset 3))
        a b c)
      (make-foo 1 2 3)                ⇒ #(() () () 1 2 3)

The essential differences between MIT/GNU Scheme’s `define-structure` and Common Lisp’s `defstruct` are:

- The default constructor procedure takes positional arguments, in the same order as specified in the definition of the structure. A keyword constructor may be specified by giving the option `keyword-constructor`.
- BOA constructors are described using Scheme lambda lists. Since there is nothing corresponding to `&aux` in Scheme lambda lists, this functionality is not implemented.
- By default, no `copier` procedure is defined.
- The side-effect procedure corresponding to the accessor `foo` is given the name `set-foo!`.
- Keywords are ordinary symbols – use `foo` instead of `:foo`.
- The option values `false`, `nil`, `true`, and `t` are treated as if the appropriate boolean constant had been specified instead.
- The `print-function` option is named `print-procedure`. Its argument is a procedure of two arguments (the structure instance and a textual output port) rather than three as in Common Lisp.
- By default, named structures are tagged with a unique object of some kind. In Common Lisp, the structures are tagged with symbols. This depends on the Common Lisp package system to help generate unique tags; MIT/GNU Scheme has no such way to generate unique symbols.
- The `named` option may optionally take an argument, which is normally the name of a variable (any expression may be used, but it is evaluated whenever the tag name is needed). If used, structure instances will be tagged with that variable’s value. The variable must be defined when `define-structure` is evaluated.
- The `type` option is restricted to the values `vector` and `list`.
- The `include` option is not implemented.

### 2.11 Macros

(This section is largely taken from the Revised^4 Report on the Algorithmic Language Scheme. The section on Syntactic Closures is derived from a document written by Chris Hanson. The section on Explicit Renaming is derived from a document written by William Clinger.)

Scheme programs can define and use new derived expression types, called macros. Program-defined expression types have the syntax

    (keyword datum …)

where keyword is an identifier that uniquely determines the expression type. This identifier is called the syntactic keyword, or simply keyword, of the macro. The number of the datums, and their syntax, depends on the expression type.

Each instance of a macro is called a use of the macro. The set of rules that specifies how a use of a macro is transcribed into a more primitive expression is called the transformer of the macro.

MIT/GNU Scheme also supports anonymous syntactic keywords. This means that it’s not necessary to bind a macro transformer to a syntactic keyword before it is used. Instead, any macro-transformer expression can appear as the first element of a form, and the form will be expanded by the transformer.

The macro definition facility consists of these parts:

- A set of expressions used to establish that certain identifiers are macro keywords, associate them with macro transformers, and control the scope within which a macro is defined.
- A standard high-level pattern language for specifying macro transformers, introduced by the `syntax-rules` special form.
- Two non-standard low-level languages for specifying macro transformers, syntactic closures and explicit renaming.

The syntactic keyword of a macro may shadow variable bindings, and local variable bindings may shadow keyword bindings. All macros defined using the pattern language are “hygienic” and “referentially transparent” and thus preserve Scheme’s lexical scoping:

- If a macro transformer inserts a binding for an identifier (variable or keyword), the identifier will in effect be renamed throughout its scope to avoid conflicts with other identifiers.
- If a macro transformer inserts a free reference to an identifier, the reference refers to the binding that was visible where the transformer was specified, regardless of any local bindings that may surround the use of the macro.

#### 2.11.1 Binding Constructs for Syntactic Keywords

`let-syntax`, `letrec-syntax`, `let*-syntax` and `define-syntax` are analogous to `let`, `letrec`, `let*` and `define`, but they bind syntactic keywords to macro transformers instead of binding variables to locations that contain values.

Any argument named transformer-spec must be a macro-transformer expression, which is one of the following:

- A macro transformer defined by the pattern language and denoted by the syntactic keyword `syntax-rules`.
- A macro transformer defined by one of the low-level mechanisms and denoted by one of the syntactic keywords `sc-macro-transformer`, `rsc-macro-transformer`, or `er-macro-transformer`.
- A syntactic keyword bound in the enclosing environment. This is used to bind another name to an existing macro transformer.

standard special form: let-syntax bindings expression expression … ¶

  Bindings should have the form

      ((keyword transformer-spec) …)

  Each keyword is an identifier, each transformer-spec is a a macro-transformer expression, and the body is a sequence of one or more expressions. It is an error for a keyword to appear more than once in the list of keywords being bound.

  The expressions are expanded in the syntactic environment obtained by extending the syntactic environment of the `let-syntax` expression with macros whose keywords are the keywords, bound to the specified transformers. Each binding of a keyword has the expressions as its region.

      (let-syntax ((when (syntax-rules ()
                           ((when test stmt1 stmt2 ...)
                            (if test
                                (begin stmt1
                                       stmt2 ...))))))
        (let ((if #t))
          (when if (set! if 'now))
          if))                           ⇒  now

      (let ((x 'outer))
        (let-syntax ((m (syntax-rules () ((m) x))))
          (let ((x 'inner))
            (m))))                       ⇒  outer

standard special form: letrec-syntax bindings expression expression … ¶

  The syntax of `letrec-syntax` is the same as for `let-syntax`.

  The expressions are expanded in the syntactic environment obtained by extending the syntactic environment of the `letrec-syntax` expression with macros whose keywords are the keywords, bound to the specified transformers. Each binding of a keyword has the bindings as well as the expressions within its region, so the transformers can transcribe expressions into uses of the macros introduced by the `letrec-syntax` expression.

      (letrec-syntax
        ((my-or (syntax-rules ()
                  ((my-or) #f)
                  ((my-or e) e)
                  ((my-or e1 e2 ...)
                   (let ((temp e1))
                     (if temp
                         temp
                         (my-or e2 ...)))))))
        (let ((x #f)
              (y 7)
              (temp 8)
              (let odd?)
              (if even?))
          (my-or x
                 (let temp)
                 (if y)
                 y)))        ⇒  7

standard special form: let*-syntax bindings expression expression … ¶

  The syntax of `let*-syntax` is the same as for `let-syntax`.

  The expressions are expanded in the syntactic environment obtained by extending the syntactic environment of the `letrec-syntax` expression with macros whose keywords are the keywords, bound to the specified transformers. Each binding of a keyword has the subsequent bindings as well as the expressions within its region. Thus

      (let*-syntax
         ((a (syntax-rules …))
          (b (syntax-rules …)))
        …)

  is equivalent to

      (let-syntax ((a (syntax-rules …)))
        (let-syntax ((b (syntax-rules …)))
          …))

standard special form: define-syntax keyword transformer-spec ¶

  Keyword is an identifier, and transformer-spec is a macro transformer expression. The syntactic environment is extended by binding the keyword to the specified transformer.

  The region of the binding introduced by `define-syntax` is the entire block in which it appears. However, the keyword may only be used after it has been defined.

  MIT/GNU Scheme permits `define-syntax` to appear both at top level and within `lambda` bodies. The Revised^4 Report permits only top-level uses of `define-syntax`.

  When compiling a program, a top-level instance of `define-syntax` both defines the syntactic keyword and generates code that will redefine the keyword when the program is loaded. This means that the same syntax can be used for defining macros that will be used during compilation and for defining macros to be used at run time.

  Although macros may expand into definitions and syntax definitions in any context that permits them, it is an error for a definition or syntax definition to shadow a syntactic keyword whose meaning is needed to determine whether some form in the group of forms that contains the shadowing definition is in fact a definition, or, for internal definitions, is needed to determine the boundary between the group and the expressions that follow the group. For example, the following are errors:

      (define define 3)

      (begin (define begin list))

      (let-syntax
        ((foo (syntax-rules ()
                ((foo (proc args ...) body ...)
                 (define proc
                   (lambda (args ...)
                     body ...))))))
        (let ((x 3))
          (foo (plus x y) (+ x y))
          (define foo x)
          (plus foo x)))

#### 2.11.2 Pattern Language

MIT/GNU Scheme supports a high-level pattern language for specifying macro transformers. This pattern language is defined by the Revised^4 Report and is portable to other conforming Scheme implementations. To use the pattern language, specify a transformer-spec as a `syntax-rules` form:

standard special form: syntax-rules [ellipsis] literals syntax-rule … ¶

  Ellipsis is an identifier, and if omitted defaults to `...`. Literals is a list of identifiers and each syntax-rule should be of the form

      (pattern template)

  The pattern in a syntax-rule is a list pattern that begins with the keyword for the macro.

  A pattern is either an identifier, a constant, or one of the following

      (pattern …)
      (pattern pattern … . pattern)
      (pattern … pattern ellipsis)

  and a template is either an identifier, a constant, or one of the following

      (element …)
      (element element … . template)

  where an element is a template optionally followed by an ellipsis and an ellipsis is the identifier ‘`...`’ (which cannot be used as an identifier in either a template or a pattern).

  An instance of `syntax-rules` produces a new macro transformer by specifying a sequence of hygienic rewrite rules. A use of a macro whose keyword is associated with a transformer specified by `syntax-rules` is matched against the patterns contained in the syntax-rules, beginning with the leftmost syntax-rule. When a match is found, the macro use is transcribed hygienically according to the template.

  An identifier that appears in the pattern of a syntax-rule is a pattern-variable, unless it is the keyword that begins the pattern, is listed in literals, or is the identifier ‘`...`’. Pattern variables match arbitrary input elements and are used to refer to elements of the input in the template. It is an error for the same pattern variable to appear more than once in a pattern.

  The keyword at the beginning of the pattern in a syntax-rule is not involved in the matching and is not considered a pattern variable or literal identifier.

  Identifiers that appear in literals are interpreted as literal identifiers to be matched against corresponding subforms of the input. A subform in the input matches a literal identifier if and only if it is an identifier and either both its occurrence in the macro expression and its occurrence in the macro definition have the same lexical binding, or the two identifiers are equal and both have no lexical binding.

  A subpattern followed by ‘`...`’ can match zero or more elements of the input. It is an error for ‘`...`’ to appear in literals. Within a pattern the identifier ‘`...`’ must follow the last element of a nonempty sequence of subpatterns.

  More formally, an input form F matches a pattern P if and only if:

  - P is a non-literal identifier; or
  - P is a literal identifier and F is an identifier with the same binding; or
  - P is a list `(P_1 … P_n)` and F is a list of n forms that match P_1 through P_n, respectively; or
  - P is an improper list `(P_1 P_2 … P_n . P_n+1)` and F is a list or improper list of n or more forms that match P_1 through P_n, respectively, and whose nth “cdr” matches P_n+1; or
  - P is of the form `(P_1 … P_n P_n+1 ellipsis)` where ellipsis is the identifier ‘`...`’ and F is a proper list of at least n forms, the first n of which match P_1 through P_n, respectively, and each remaining element of F matches P_n+1; or
  - P is a datum and F is equal to P in the sense of the `equal?` procedure.

  It is an error to use a macro keyword, within the scope of its binding, in an expression that does not match any of the patterns.

  When a macro use is transcribed according to the template of the matching syntax rule, pattern variables that occur in the template are replaced by the subforms they match in the input. Pattern variables that occur in subpatterns followed by one or more instances of the identifier ‘`...`’ are allowed only in subtemplates that are followed by as many instances of ‘`...`’. They are replaced in the output by all of the subforms they match in the input, distributed as indicated. It is an error if the output cannot be built up as specified.

  Identifiers that appear in the template but are not pattern variables or the identifier ‘`...`’ are inserted into the output as literal identifiers. If a literal identifier is inserted as a free identifier then it refers to the binding of that identifier within whose scope the instance of `syntax-rules` appears. If a literal identifier is inserted as a bound identifier then it is in effect renamed to prevent inadvertent captures of free identifiers.

      (let ((=> #f))
        (cond (#t => 'ok)))           ⇒ ok

  The macro transformer for `cond` recognizes `=>` as a local variable, and hence an expression, and not as the top-level identifier `=>`, which the macro transformer treats as a syntactic keyword. Thus the example expands into

      (let ((=> #f))
        (if #t (begin => 'ok)))

  instead of

      (let ((=> #f))
        (let ((temp #t))
          (if temp
              ('ok temp))))

  which would result in an invalid procedure call.

#### 2.11.3 Syntactic Closures

MIT/GNU Scheme’s syntax-transformation engine is an implementation of syntactic closures, a mechanism invented by Alan Bawden and Jonathan Rees. The main feature of the syntactic-closures mechanism is its simplicity and its close relationship to the environment models commonly used with Scheme. Using the mechanism to write macro transformers is somewhat cumbersome and can be confusing for the newly initiated, but it is easily mastered.

#### 2.11.3.1 Syntax Terminology

This section defines the concepts and data types used by the syntactic closures facility.

- Forms are the syntactic entities out of which programs are recursively constructed. A form is any expression, any definition, any syntactic keyword, or any syntactic closure. The variable name that appears in a `set!` special form is also a form. Examples of forms:

      17
      #t
      car
      (+ x 4)
      (lambda (x) x)
      (define pi 3.14159)
      if
      define

- An alias is an alternate name for a given symbol. It can appear anywhere in a form that the symbol could be used, and when quoted it is replaced by the symbol; however, it does not satisfy the predicate `symbol?`. Macro transformers rarely distinguish symbols from aliases, referring to both as identifiers. Another name for an alias is synthetic identifier; this document uses both names.
- A syntactic environment maps identifiers to their meanings. More precisely, it determines whether an identifier is a syntactic keyword or a variable. If it is a keyword, the meaning is an interpretation for the form in which that keyword appears. If it is a variable, the meaning identifies which binding of that variable is referenced. In short, syntactic environments contain all of the contextual information necessary for interpreting the meaning of a particular form.
- A syntactic closure consists of a form, a syntactic environment, and a list of identifiers. All identifiers in the form take their meaning from the syntactic environment, except those in the given list. The identifiers in the list are to have their meanings determined later.

  A syntactic closure may be used in any context in which its form could have been used. Since a syntactic closure is also a form, it may not be used in contexts where a form would be illegal. For example, a form may not appear as a clause in the `cond` special form.

  A syntactic closure appearing in a quoted structure is replaced by its form.

#### 2.11.3.2 Transformer Definition

This section describes the special forms for defining syntactic-closures macro transformers, and the associated procedures for manipulating syntactic closures and syntactic environments.

special form: sc-macro-transformer expression ¶

  The expression is expanded in the syntactic environment of the `sc-macro-transformer` expression, and the expanded expression is evaluated in the transformer environment to yield a macro transformer as described below. This macro transformer is bound to a macro keyword by the special form in which the `transformer` expression appears (for example, `let-syntax`).

  In the syntactic closures facility, a macro transformer is a procedure that takes two arguments, a form and a syntactic environment, and returns a new form. The first argument, the input form, is the form in which the macro keyword occurred. The second argument, the usage environment, is the syntactic environment in which the input form occurred. The result of the transformer, the output form, is automatically closed in the transformer environment, which is the syntactic environment in which the `transformer` expression occurred.

  For example, here is a definition of a `push` macro using `syntax-rules`:

      (define-syntax push
        (syntax-rules ()
          ((push item list)
           (set! list (cons item list)))))

  Here is an equivalent definition using `sc-macro-transformer`:

      (define-syntax push
        (sc-macro-transformer
         (lambda (exp env)
           (let ((item (make-syntactic-closure env '() (cadr exp)))
                 (list (make-syntactic-closure env '() (caddr exp))))
             `(set! ,list (cons ,item ,list))))))

  In this example, the identifiers `set!` and `cons` are closed in the transformer environment, and thus will not be affected by the meanings of those identifiers in the usage environment `env`.

  Some macros may be non-hygienic by design. For example, the following defines a `loop` macro that implicitly binds `exit` to an escape procedure. The binding of `exit` is intended to capture free references to `exit` in the body of the loop, so `exit` must be left free when the body is closed:

      (define-syntax loop
        (sc-macro-transformer
         (lambda (exp env)
           (let ((body (cdr exp)))
             `(call-with-current-continuation
               (lambda (exit)
                 (let f ()
                   ,@(map (lambda (exp)
                            (make-syntactic-closure env '(exit)
                              exp))
                          body)
                   (f))))))))

special form: rsc-macro-transformer expression ¶

  This form is an alternative way to define a syntactic-closures macro transformer. Its syntax and usage are identical to `sc-macro-transformer`, except that the roles of the usage environment and transformer environment are reversed. (Hence RSC stands for Reversed Syntactic Closures.) In other words, the procedure specified by expression still accepts two arguments, but its second argument will be the transformer environment rather than the usage environment, and the returned expression is closed in the usage environment rather than the transformer environment.

  The advantage of this arrangement is that it allows a simpler definition style in some situations. For example, here is the `push` macro from above, rewritten in this style:

      (define-syntax push
        (rsc-macro-transformer
         (lambda (exp env)
           `(,(make-syntactic-closure env '() 'SET!)
             ,(caddr exp)
             (,(make-syntactic-closure env '() 'CONS)
              ,(cadr exp)
              ,(caddr exp))))))

  In this style only the introduced keywords are closed, while everything else remains open.

  Note that `rsc-macro-transformer` and `sc-macro-transformer` are easily interchangeable. Here is how to emulate `rsc-macro-transformer` using `sc-macro-transformer`. (This technique can be used to effect the opposite emulation as well.)

      (define-syntax push
        (sc-macro-transformer
         (lambda (exp usage-env)
           (capture-syntactic-environment
            (lambda (env)
              (make-syntactic-closure usage-env '()
                `(,(make-syntactic-closure env '() 'SET!)
                  ,(caddr exp)
                  (,(make-syntactic-closure env '() 'CONS)
                   ,(cadr exp)
                   ,(caddr exp)))))))))

To assign meanings to the identifiers in a form, use `make-syntactic-closure` to close the form in a syntactic environment.

procedure: make-syntactic-closure environment free-names form ¶

  Environment must be a syntactic environment, free-names must be a list of identifiers, and form must be a form. `make-syntactic-closure` constructs and returns a syntactic closure of form in environment, which can be used anywhere that form could have been used. All the identifiers used in form, except those explicitly excepted by free-names, obtain their meanings from environment.

  Here is an example where free-names is something other than the empty list. It is instructive to compare the use of free-names in this example with its use in the `loop` example above: the examples are similar except for the source of the identifier being left free.

      (define-syntax let1
        (sc-macro-transformer
         (lambda (exp env)
           (let ((id (cadr exp))
                 (init (caddr exp))
                 (exp (cadddr exp)))
             `((lambda (,id)
                 ,(make-syntactic-closure env (list id) exp))
               ,(make-syntactic-closure env '() init))))))

  `let1` is a simplified version of `let` that only binds a single identifier, and whose body consists of a single expression. When the body expression is syntactically closed in its original syntactic environment, the identifier that is to be bound by `let1` must be left free, so that it can be properly captured by the `lambda` in the output form.

In most situations, the free-names argument to `make-syntactic-closure` is the empty list. In those cases, the more succinct `close-syntax` can be used:

procedure: close-syntax form environment ¶

  Environment must be a syntactic environment and form must be a form. Returns a new syntactic closure of form in environment, with no free names. Entirely equivalent to

      (make-syntactic-closure environment '() form)

To obtain a syntactic environment other than the usage environment, use `capture-syntactic-environment`.

procedure: capture-syntactic-environment procedure ¶

  `capture-syntactic-environment` returns a form that will, when transformed, call procedure on the current syntactic environment. Procedure should compute and return a new form to be transformed, in that same syntactic environment, in place of the form.

  An example will make this clear. Suppose we wanted to define a simple `loop-until` keyword equivalent to

      (define-syntax loop-until
        (syntax-rules ()
          ((loop-until id init test return step)
           (letrec ((loop
                     (lambda (id)
                       (if test return (loop step)))))
             (loop init)))))

  The following attempt at defining `loop-until` has a subtle bug:

      (define-syntax loop-until
        (sc-macro-transformer
         (lambda (exp env)
           (let ((id (cadr exp))
                 (init (caddr exp))
                 (test (cadddr exp))
                 (return (cadddr (cdr exp)))
                 (step (cadddr (cddr exp)))
                 (close
                  (lambda (exp free)
                    (make-syntactic-closure env free exp))))
             `(letrec ((loop
                        (lambda (,id)
                          (if ,(close test (list id))
                              ,(close return (list id))
                              (loop ,(close step (list id)))))))
                (loop ,(close init '())))))))

  This definition appears to take all of the proper precautions to prevent unintended captures. It carefully closes the subexpressions in their original syntactic environment and it leaves the `id` identifier free in the `test`, `return`, and `step` expressions, so that it will be captured by the binding introduced by the `lambda` expression. Unfortunately it uses the identifiers `if` and `loop` within that `lambda` expression, so if the user of `loop-until` just happens to use, say, `if` for the identifier, it will be inadvertently captured.

  The syntactic environment that `if` and `loop` want to be exposed to is the one just outside the `lambda` expression: before the user’s identifier is added to the syntactic environment, but after the identifier `loop` has been added. `capture-syntactic-environment` captures exactly that environment as follows:

      (define-syntax loop-until
        (sc-macro-transformer
         (lambda (exp env)
           (let ((id (cadr exp))
                 (init (caddr exp))
                 (test (cadddr exp))
                 (return (cadddr (cdr exp)))
                 (step (cadddr (cddr exp)))
                 (close
                  (lambda (exp free)
                    (make-syntactic-closure env free exp))))
             `(letrec ((loop
                        ,(capture-syntactic-environment
                          (lambda (env)
                            `(lambda (,id)
                               (,(make-syntactic-closure env '() `if)
                                ,(close test (list id))
                                ,(close return (list id))
                                (,(make-syntactic-closure env '() `loop)
                                 ,(close step (list id)))))))))
                (loop ,(close init '())))))))

  In this case, having captured the desired syntactic environment, it is convenient to construct syntactic closures of the identifiers `if` and the `loop` and use them in the body of the `lambda`.

  A common use of `capture-syntactic-environment` is to get the transformer environment of a macro transformer:

      (sc-macro-transformer
       (lambda (exp env)
         (capture-syntactic-environment
          (lambda (transformer-env)
            …))))

#### 2.11.3.3 Identifiers

This section describes the procedures that create and manipulate identifiers. The identifier data type extends the syntactic closures facility to be compatible with the high-level `syntax-rules` facility.

As discussed earlier, an identifier is either a symbol or an alias. An alias is implemented as a syntactic closure whose form is an identifier:

    (make-syntactic-closure env '() 'a) ⇒ an alias

Aliases are implemented as syntactic closures because they behave just like syntactic closures most of the time. The difference is that an alias may be bound to a new value (for example by `lambda` or `let-syntax`); other syntactic closures may not be used this way. If an alias is bound, then within the scope of that binding it is looked up in the syntactic environment just like any other identifier.

Aliases are used in the implementation of the high-level facility `syntax-rules`. A macro transformer created by `syntax-rules` uses a template to generate its output form, substituting subforms of the input form into the template. In a syntactic closures implementation, all of the symbols in the template are replaced by aliases closed in the transformer environment, while the output form itself is closed in the usage environment. This guarantees that the macro transformation is hygienic, without requiring the transformer to know the syntactic roles of the substituted input subforms.

procedure: identifier? object ¶

  Returns `#t` if object is an identifier, otherwise returns `#f`. Examples:

      (identifier? 'a)        ⇒ #t
      (identifier? (make-syntactic-closure env '() 'a))
                              ⇒ #t

      (identifier? "a")       ⇒ #f
      (identifier? #\a)       ⇒ #f
      (identifier? 97)        ⇒ #f
      (identifier? #f)        ⇒ #f
      (identifier? '(a))      ⇒ #f
      (identifier? '#(a))     ⇒ #f

The predicate `eq?` is used to determine if two identifers are “the same”. Thus `eq?` can be used to compare identifiers exactly as it would be used to compare symbols. Often, though, it is useful to know whether two identifiers “mean the same thing”. For example, the `cond` macro uses the symbol `else` to identify the final clause in the conditional. A macro transformer for `cond` cannot just look for the symbol `else`, because the `cond` form might be the output of another macro transformer that replaced the symbol `else` with an alias. Instead the transformer must look for an identifier that “means the same thing” in the usage environment as the symbol `else` means in the transformer environment.

procedure: identifier=? environment1 identifier1 environment2 identifier2 ¶

  Environment1 and environment2 must be syntactic environments, and identifier1 and identifier2 must be identifiers. `identifier=?` returns `#t` if the meaning of identifier1 in environment1 is the same as that of identifier2 in environment2, otherwise it returns `#f`. Examples:

      (let-syntax
          ((foo
            (sc-macro-transformer
             (lambda (form env)
               (capture-syntactic-environment
                (lambda (transformer-env)
                  (identifier=? transformer-env 'x env 'x)))))))
        (list (foo)
              (let ((x 3))
                (foo))))
                              ⇒ (#t #f)

      (let-syntax ((bar foo))
        (let-syntax
            ((foo
              (sc-macro-transformer
               (lambda (form env)
                 (capture-syntactic-environment
                  (lambda (transformer-env)
                    (identifier=? transformer-env 'foo
                                  env (cadr form))))))))
          (list (foo foo)
                (foo bar))))
                              ⇒ (#f #t)

Sometimes it is useful to be able to introduce a new identifier that is guaranteed to be different from any existing identifier, similarly to the way that `generate-uninterned-symbol` is used.

procedure: make-synthetic-identifier identifier ¶

  Creates and returns and new synthetic identifier (alias) that is guaranteed to be different from all existing identifiers. Identifier is any existing identifier, which is used in deriving the name of the new identifier.

#### 2.11.4 Explicit Renaming

Explicit renaming is an alternative facility for defining macro transformers. In the MIT/GNU Scheme implementation, explicit-renaming transformers are implemented as an abstraction layer on top of syntactic closures. An explicit-renaming macro transformer is defined by an instance of the `er-macro-transformer` keyword:

special form: er-macro-transformer expression ¶

  The expression is expanded in the syntactic environment of the `er-macro-transformer` expression, and the expanded expression is evaluated in the transformer environment to yield a macro transformer as described below. This macro transformer is bound to a macro keyword by the special form in which the `transformer` expression appears (for example, `let-syntax`).

  In the explicit-renaming facility, a macro transformer is a procedure that takes three arguments, a form, a renaming procedure, and a comparison predicate, and returns a new form. The first argument, the input form, is the form in which the macro keyword occurred.

  The second argument to a transformation procedure is a renaming procedure that takes the representation of an identifier as its argument and returns the representation of a fresh identifier that occurs nowhere else in the program. For example, the transformation procedure for a simplified version of the `let` macro might be written as

      (lambda (exp rename compare)
        (let ((vars (map car (cadr exp)))
              (inits (map cadr (cadr exp)))
              (body (cddr exp)))
          `((lambda ,vars ,@body)
            ,@inits)))

  This would not be hygienic, however. A hygienic `let` macro must rename the identifier `lambda` to protect it from being captured by a local binding. The renaming effectively creates an fresh alias for `lambda`, one that cannot be captured by any subsequent binding:

      (lambda (exp rename compare)
        (let ((vars (map car (cadr exp)))
              (inits (map cadr (cadr exp)))
              (body (cddr exp)))
          `((,(rename 'lambda) ,vars ,@body)
            ,@inits)))

  The expression returned by the transformation procedure will be expanded in the syntactic environment obtained from the syntactic environment of the macro application by binding any fresh identifiers generated by the renaming procedure to the denotations of the original identifiers in the syntactic environment in which the macro was defined. This means that a renamed identifier will denote the same thing as the original identifier unless the transformation procedure that renamed the identifier placed an occurrence of it in a binding position.

  The renaming procedure acts as a mathematical function in the sense that the identifiers obtained from any two calls with the same argument will be the same in the sense of `eqv?`. It is an error if the renaming procedure is called after the transformation procedure has returned.

  The third argument to a transformation procedure is a comparison predicate that takes the representations of two identifiers as its arguments and returns true if and only if they denote the same thing in the syntactic environment that will be used to expand the transformed macro application. For example, the transformation procedure for a simplified version of the `cond` macro can be written as

      (lambda (exp rename compare)
        (let ((clauses (cdr exp)))
          (if (null? clauses)
              `(,(rename 'quote) unspecified)
              (let* ((first (car clauses))
                     (rest (cdr clauses))
                     (test (car first)))
                (cond ((and (identifier? test)
                            (compare test (rename 'else)))
                       `(,(rename 'begin) ,@(cdr first)))
                      (else `(,(rename 'if)
                              ,test
                               (,(rename 'begin) ,@(cdr first))
                               (cond ,@rest))))))))))

  In this example the identifier `else` is renamed before being passed to the comparison predicate, so the comparison will be true if and only if the test expression is an identifier that denotes the same thing in the syntactic environment of the expression being transformed as `else` denotes in the syntactic environment in which the `cond` macro was defined. If `else` were not renamed before being passed to the comparison predicate, then it would match a local variable that happened to be named `else`, and the macro would not be hygienic.

  Some macros are non-hygienic by design. For example, the following defines a `loop` macro that implicitly binds `exit` to an escape procedure. The binding of `exit` is intended to capture free references to `exit` in the body of the loop, so `exit` is not renamed.

      (define-syntax loop
        (er-macro-transformer
         (lambda (x r c)
           (let ((body (cdr x)))
             `(,(r 'call-with-current-continuation)
               (,(r 'lambda) (exit)
                (,(r 'let) ,(r 'f) () ,@body (,(r 'f)))))))))

  Suppose a `while` macro is implemented using `loop`, with the intent that `exit` may be used to escape from the `while` loop. The `while` macro cannot be written as

      (define-syntax while
        (syntax-rules ()
          ((while test body ...)
           (loop (if (not test) (exit #f))
                 body ...))))

  because the reference to `exit` that is inserted by the `while` macro is intended to be captured by the binding of `exit` that will be inserted by the `loop` macro. In other words, this `while` macro is not hygienic. Like `loop`, it must be written using the `er-macro-transformer` syntax:

      (define-syntax while
        (er-macro-transformer
         (lambda (x r c)
           (let ((test (cadr x))
                 (body (cddr x)))
             `(,(r 'loop)
               (,(r 'if) (,(r 'not) ,test) (exit #f))
               ,@body)))))

### 2.12 SRFI syntax

Several special forms have been introduced to support some of the Scheme Requests for Implementation (SRFI). Note that MIT/GNU Scheme has for some time supported SRFI 23 (error-reporting mechanism) and SRFI 30 (nested multi-line comments), since these SRFIs reflect existing practice rather than introducing new functionality.

#### 2.12.1 cond-expand (SRFI 0)

SRFI 0 is a mechanism for portably determining the availability of SRFI features. The `cond-expand` special form conditionally expands according to the features available.

standard special form: cond-expand clause clause dots ¶

  Each clause has the form

      (feature-requirement expression …)

  where feature-requirement can have one of the following forms:

      feature-identifier
      (and feature-requirement …)
      (or feature-requirement …)
      (not feature-requirement)
      else

  (Note that at most one `else` clause may be present, and it must always be the last clause.)

  The `cond-expand` special form tests for the existence of features at macro-expansion time. It either expands into the body of one of its clauses or signals an error during syntactic processing. `cond-expand` expands into the body of the first clause whose feature-requirement is currently satisfied (an `else` clause, if present, is selected if none of the previous clauses is selected).

  A feature-requirement has an obvious interpretation as a logical formula, where the feature-identifier variables have meaning true if the feature corresponding to the feature-identifier, as specified in the SRFI registry, is in effect at the location of the `cond-expand` form, and false otherwise. A feature-requirement is satisfied if its formula is true under this interpretation.

      (cond-expand
        ((and srfi-1 srfi-10)
         (write 1))
        ((or srfi-1 srfi-10)
         (write 2))
        (else))

      (cond-expand
        (command-line
         (define (program-name) (car (argv)))))

  The second example assumes that `command-line` is an alias for some feature which gives access to command line arguments. Note that an error will be signaled at macro-expansion time if this feature is not present.

  Note that MIT/GNU Scheme allows `cond-expand` in any context where a special form is allowed. This is an extension of the semantics defined by SRFI 0, which only allows `cond-expand` at top level.

#### 2.12.2 receive (SRFI 8)

SRFI 8 defines a convenient syntax to bind an identifier to each of the values of a multiple-valued expression and then evaluate an expression in the scope of the bindings. As an instance of this pattern, consider the following excerpt from a ‘`quicksort`’ procedure:

    (call-with-values
      (lambda ()
        (partition (precedes pivot) others))
      (lambda (fore aft)
        (append (qsort fore) (cons pivot (qsort aft)))))

Here ‘`partition`’ is a multiple-valued procedure that takes two arguments, a predicate and a list, and returns two lists, one comprising the list elements that satisfy the predicate, the other those that do not. The purpose of the expression shown is to partition the list ‘`others`’, sort each of the sublists, and recombine the results into a sorted list.

For our purposes, the important step is the binding of the identifiers ‘`fore`’ and ‘`aft`’ to the values returned by ‘`partition`’. Expressing the construction and use of these bindings with the call-by-values primitive is cumbersome: One must explicitly embed the expression that provides the values for the bindings in a parameterless procedure, and one must explicitly embed the expression to be evaluated in the scope of those bindings in another procedure, writing as its parameters the identifiers that are to be bound to the values received.

These embeddings are boilerplate, exposing the underlying binding mechanism but not revealing anything relevant to the particular program in which it occurs. So the use of a syntactic abstraction that exposes only the interesting parts – the identifiers to be bound, the multiple-valued expression that supplies the values, and the body of the receiving procedure – makes the code more concise and more readable:

    (receive (fore aft) (partition (precedes pivot) others)
      (append (qsort fore) (cons pivot (qsort aft))))

The advantages are similar to those of a ‘`let`’ expression over a procedure call with a ‘`lambda`’ expression as its operator. In both cases, cleanly separating a “header” in which the bindings are established from a “body” in which they are used makes it easier to follow the code.

special form: receive formals expression body ¶

  Formals and body are defined as for ‘`lambda`’ (see Lambda Expressions). Specifically, formals can have the following forms (the use of ‘`#!optional`’ and ‘`#!rest`’ is also allowed in formals but is omitted for brevity):

  ‘`(ident1 … identN)`’

    The environment in which the ‘`receive`’ expression is evaluated is extended by binding ident1, …, identN to fresh locations. The expression is evaluated, and its values are stored into those locations. (It is an error if expression does not have exactly N values.)

  ‘`ident`’

    The environment in which the ‘`receive`’ expression is evaluated is extended by binding ident to a fresh location. The expression is evaluated, its values are converted into a newly allocated list, and the list is stored in the location bound to ident.

  ‘`(ident1 … identN . identN+1)`’

    The environment in which the ‘`receive`’ expression is evaluated is extended by binding ident1, …, identN+1 to fresh locations. The expression is evaluated. Its first N values are stored into the locations bound to ident1 … identN. Any remaining values are converted into a newly allocated list, which is stored into the location bound to identN+1. (It is an error if expression does not have at least N values.)

  In any case, the expressions in body are evaluated sequentially in the extended environment. The results of the last expression in the body are the values of the ‘`receive`’ expression.

#### 2.12.3 and-let* (SRFI 2)

SRFI 2 provides a form that combines ‘`and`’ and ‘`let*`’ for a logically short-circuiting sequential binding operator.

special form: and-let* (clause …) body ¶

  Runs through each of the clauses left-to-right, short-circuiting like ‘`and`’ in that the first false clause will result in the whole ‘`and-let*`’ form returning false. If a body is supplied, and all of the clauses evaluate true, then the body is evaluated sequentially as if in a ‘`begin`’ form, and the value of the ‘`and-let*`’ expression is the value of the last body form, evaluated in a tail position with respect to the ‘`and-let*`’ expression. If no body is supplied, the value of the last clause, also evaluated in a tail position with respect to the ‘`and-let*`’ expression, is used instead.

  Each clause should have one of the following forms:

  ‘`identifier`’

    in which case identifier’s value is tested.

  ‘`(expression)`’

    in which case the value of expression is tested.

  ‘`(identifier expression)`’

    in which case expression is evaluated, and, if its value is not false, identifier is bound to that value for the remainder of the clauses and the optional body.

Example:

    (and-let* ((list (compute-list))
               ((pair? list))
               (item (car list))
               ((integer? item)))
      (sqrt item))

#### 2.12.4 define-record-type (SRFI 9)

The ‘`define-record-type`’ syntax described in SRFI 9 is a slight simplification of one written for Scheme 48 by Jonathan Rees. Unlike many record-defining special forms, it does not create any new identifiers. Instead, the names of the record type, predicate, constructor, and so on are all listed explicitly in the source. This has the following advantages:

- It can be defined using a simple macro in Scheme implementations that provide a procedural interface for creating record types.
- It does not restrict users to a particular naming convention.
- Tools like `grep` and the GNU Emacs tag facility will see the defining occurrence of each identifier.

extended standard special form: define-record-type type-name (constructor-name field-tag …) predicate-name field-spec … ¶

  Type-name, contructor-name, field-tag, and predicate-name are identifiers. Field-spec has one of these two forms:

      (field-tag accessor-name)
      (field-tag accessor-name modifier-name)

  where field-tag, accessor-name, and modifier-name are each identifiers.

  `define-record-type` is generative: each use creates a new record type that is distinct from all existing types, including other record types and Scheme’s predefined types. Record-type definitions may only occur at top-level (there are two possible semantics for “internal” record-type definitions, generative and nongenerative, and no consensus as to which is better).

  An instance of `define-record-type` is equivalent to the following definitions:

  - Type-name is bound to a representation of the record type itself. Operations on record types, such as defining print methods, reflection, etc. are left to other SRFIs.
  - constructor-name is bound to a procedure that takes as many arguments as there are field-tags in the (constructor-name …) subform and returns a new type-name record. Fields whose tags are listed with constructor-name have the corresponding argument as their initial value. The initial values of all other fields are unspecified.
  - predicate-name is a predicate that returns `#t` when given a value returned by constructor-name and `#f` for everything else.
  - Each accessor-name is a procedure that takes a record of type type-name and returns the current value of the corresponding field. It is an error to pass an accessor a value which is not a record of the appropriate type.
  - Each modifier-name is a procedure that takes a record of type type-name and a value which becomes the new value of the corresponding field; an unspecified value is returned. It is an error to pass a modifier a first argument which is not a record of the appropriate type.

  Assigning the value of any of these identifiers has no effect on the behavior of any of their original values.

The following

    (define-record-type :pare
      (kons x y)
      pare?
      (x kar set-kar!)
      (y kdr))

defines ‘`kons`’ to be a constructor, ‘`kar`’ and ‘`kdr`’ to be accessors, ‘`set-kar!`’ to be a modifier, and ‘`pare?`’ to be a predicate for objects of type ‘`:pare`’.

    (pare? (kons 1 2))        ⇒ #t
    (pare? (cons 1 2))        ⇒ #f
    (kar (kons 1 2))          ⇒ 1
    (kdr (kons 1 2))          ⇒ 2
    (let ((k (kons 1 2)))
      (set-kar! k 3)
      (kar k))                ⇒ 3

## 3 Equivalence Predicates

A predicate is a procedure that always returns a boolean value (`#t` or `#f`). An equivalence predicate is the computational analogue of a mathematical equivalence relation (it is symmetric, reflexive, and transitive). Of the equivalence predicates described in this section, `eq?` is the finest or most discriminating, and `equal?` is the coarsest. `eqv?` is slightly less discriminating than `eq?`.

procedure: eqv? obj1 obj2 ¶

  The `eqv?` procedure defines a useful equivalence relation on objects. Briefly, it returns `#t` if obj1 and obj2 should normally be regarded as the same object.

  The `eqv?` procedure returns `#t` if:

  - obj1 and obj2 are both `#t` or both `#f`.
  - obj1 and obj2 are both interned symbols and

        (string=? (symbol->string obj1)
                  (symbol->string obj2))
             ⇒ #t

  - obj1 and obj2 are both numbers, are numerically equal according to the `=` procedure, and are either both exact or both inexact (see Numbers).
  - obj1 and obj2 are both characters and are the same character according to the `char=?` procedure (see Characters).
  - both obj1 and obj2 are the empty list.
  - obj1 and obj2 are procedures whose location tags are equal.
  - obj1 and obj2 are pairs, vectors, strings, bit strings, records, cells, or weak pairs that denote the same locations in the store.

  The `eqv?` procedure returns `#f` if:

  - obj1 and obj2 are of different types.
  - one of obj1 and obj2 is `#t` but the other is `#f`.
  - obj1 and obj2 are symbols but

        (string=? (symbol->string obj1)
                  (symbol->string obj2))
             ⇒ #f

  - one of obj1 and obj2 is an exact number but the other is an inexact number.
  - obj1 and obj2 are numbers for which the `=` procedure returns `#f`.
  - obj1 and obj2 are characters for which the `char=?` procedure returns `#f`.
  - one of obj1 and obj2 is the empty list but the other is not.
  - obj1 and obj2 are procedures that would behave differently (return a different value or have different side effects) for some arguments.
  - obj1 and obj2 are pairs, vectors, strings, bit strings, records, cells, or weak pairs that denote distinct locations.

  Some examples:

      (eqv? 'a 'a)                    ⇒  #t
      (eqv? 'a 'b)                    ⇒  #f
      (eqv? 2 2)                      ⇒  #t
      (eqv? '() '())                  ⇒  #t
      (eqv? 100000000 100000000)      ⇒  #t
      (eqv? (cons 1 2) (cons 1 2))    ⇒  #f
      (eqv? (lambda () 1)
            (lambda () 2))            ⇒  #f
      (eqv? #f 'nil)                  ⇒  #f
      (let ((p (lambda (x) x)))
        (eqv? p p))                   ⇒  #t

  The following examples illustrate cases in which the above rules do not fully specify the behavior of `eqv?`. All that can be said about such cases is that the value returned by `eqv?` must be a boolean.

      (eqv? "" "")                    ⇒  unspecified
      (eqv? '#() '#())                ⇒  unspecified
      (eqv? (lambda (x) x)
            (lambda (x) x))           ⇒  unspecified
      (eqv? (lambda (x) x)
            (lambda (y) y))           ⇒  unspecified

  The next set of examples shows the use of `eqv?` with procedures that have local state. `gen-counter` must return a distinct procedure every time, since each procedure has its own internal counter. `gen-loser`, however, returns equivalent procedures each time, since the local state does not affect the value or side effects of the procedures.

      (define gen-counter
        (lambda ()
          (let ((n 0))
            (lambda () (set! n (+ n 1)) n))))
      (let ((g (gen-counter)))
        (eqv? g g))                   ⇒  #t
      (eqv? (gen-counter) (gen-counter))
                                      ⇒  #f

      (define gen-loser
        (lambda ()
          (let ((n 0))
            (lambda () (set! n (+ n 1)) 27))))
      (let ((g (gen-loser)))
        (eqv? g g))                   ⇒  #t
      (eqv? (gen-loser) (gen-loser))
                                      ⇒  unspecified

      (letrec ((f (lambda () (if (eqv? f g) 'both 'f)))
               (g (lambda () (if (eqv? f g) 'both 'g)))
        (eqv? f g))
                                      ⇒  unspecified

      (letrec ((f (lambda () (if (eqv? f g) 'f 'both)))
               (g (lambda () (if (eqv? f g) 'g 'both)))
        (eqv? f g))
                                      ⇒  #f

  Objects of distinct types must never be regarded as the same object.

  Since it is an error to modify constant objects (those returned by literal expressions), the implementation may share structure between constants where appropriate. Thus the value of `eqv?` on constants is sometimes unspecified.

      (let ((x '(a)))
        (eqv? x x))                    ⇒  #t
      (eqv? '(a) '(a))                 ⇒  unspecified
      (eqv? "a" "a")                   ⇒  unspecified
      (eqv? '(b) (cdr '(a b)))         ⇒  unspecified

  Rationale: The above definition of `eqv?` allows implementations latitude in their treatment of procedures and literals: implementations are free either to detect or to fail to detect that two procedures or two literals are equivalent to each other, and can decide whether or not to merge representations of equivalent objects by using the same pointer or bit pattern to represent both.

procedure: eq? obj1 obj2 ¶

  `eq?` is similar to `eqv?` except that in some cases it is capable of discerning distinctions finer than those detectable by `eqv?`.

  `eq?` and `eqv?` are guaranteed to have the same behavior on symbols, booleans, the empty list, pairs, records, and non-empty strings and vectors. `eq?`’s behavior on numbers and characters is implementation-dependent, but it will always return either true or false, and will return true only when `eqv?` would also return true. `eq?` may also behave differently from `eqv?` on empty vectors and empty strings.

      (eq? 'a 'a)                     ⇒  #t
      (eq? '(a) '(a))                 ⇒  unspecified
      (eq? (list 'a) (list 'a))       ⇒  #f
      (eq? "a" "a")                   ⇒  unspecified
      (eq? "" "")                     ⇒  unspecified
      (eq? '() '())                   ⇒  #t
      (eq? 2 2)                       ⇒  unspecified
      (eq? #\A #\A)                   ⇒  unspecified
      (eq? car car)                   ⇒  #t
      (let ((n (+ 2 3)))
        (eq? n n))                    ⇒  unspecified
      (let ((x '(a)))
        (eq? x x))                    ⇒  #t
      (let ((x '#()))
        (eq? x x))                    ⇒  #t
      (let ((p (lambda (x) x)))
        (eq? p p))                    ⇒  #t

  Rationale: It will usually be possible to implement `eq?` much more efficiently than `eqv?`, for example, as a simple pointer comparison instead of as some more complicated operation. One reason is that it may not be possible to compute `eqv?` of two numbers in constant time, whereas `eq?` implemented as pointer comparison will always finish in constant time. `eq?` may be used like `eqv?` in applications using procedures to implement objects with state since it obeys the same constraints as `eqv?`.

procedure: equal? obj1 obj2 ¶

  `equal?` recursively compares the contents of pairs, vectors, and strings, applying `eqv?` on other objects such as numbers, symbols, and records. A rule of thumb is that objects are generally `equal?` if they print the same. `equal?` may fail to terminate if its arguments are circular data structures.

      (equal? 'a 'a)                  ⇒  #t
      (equal? '(a) '(a))              ⇒  #t
      (equal? '(a (b) c)
              '(a (b) c))             ⇒  #t
      (equal? "abc" "abc")            ⇒  #t
      (equal? 2 2)                    ⇒  #t
      (equal? (make-vector 5 'a)
              (make-vector 5 'a))     ⇒  #t
      (equal? (lambda (x) x)
              (lambda (y) y))         ⇒  unspecified

## 4 Numbers

(This section is largely taken from the Revised^4 Report on the Algorithmic Language Scheme.)

Numerical computation has traditionally been neglected by the Lisp community. Until Common Lisp there was no carefully thought out strategy for organizing numerical computation, and with the exception of the MacLisp system little effort was made to execute numerical code efficiently. This report recognizes the excellent work of the Common Lisp committee and accepts many of their recommendations. In some ways this report simplifies and generalizes their proposals in a manner consistent with the purposes of Scheme.

It is important to distinguish between the mathematical numbers, the Scheme numbers that attempt to model them, the machine representations used to implement the Scheme numbers, and notations used to write numbers. This report uses the types number, complex, real, rational, and integer to refer to both mathematical numbers and Scheme numbers. Machine representations such as fixed point and floating point are referred to by names such as fixnum and flonum.

### 4.1 Numerical types

Mathematically, numbers may be arranged into a tower of subtypes in which each level is a subset of the level above it:

    number
    complex
    real
    rational
    integer

For example, 3 is an integer. Therefore 3 is also a rational, a real, and a complex. The same is true of the Scheme numbers that model 3. For Scheme numbers, these types are defined by the predicates `number?`, `complex?`, `real?`, `rational?`, and `integer?`.

There is no simple relationship between a number’s type and its representation inside a computer. Although most implementations of Scheme will offer at least two different representations of 3, these different representations denote the same integer.

Scheme’s numerical operations treat numbers as abstract data, as independent of their representation as possible. Although an implementation of Scheme may use fixnum, flonum, and perhaps other representations for numbers, this should not be apparent to a casual programmer writing simple programs.

It is necessary, however, to distinguish between numbers that are represented exactly and those that may not be. For example, indexes into data structures must be known exactly, as must some polynomial coefficients in a symbolic algebra system. On the other hand, the results of measurements are inherently inexact, and irrational numbers may be approximated by rational and therefore inexact approximations. In order to catch uses of inexact numbers where exact numbers are required, Scheme explicitly distinguishes exact from inexact numbers. This distinction is orthogonal to the dimension of type.

### 4.2 Exactness

Scheme numbers are either exact or inexact. A number is exact if it was written as an exact constant or was derived from exact numbers using only exact operations. A number is inexact if it was written as an inexact constant, if it was derived using inexact ingredients, or if it was derived using inexact operations. Thus inexactness is a contagious property of a number.

If two implementations produce exact results for a computation that did not involve inexact intermediate results, the two ultimate results will be mathematically equivalent. This is generally not true of computations involving inexact numbers since approximate methods such as floating point arithmetic may be used, but it is the duty of each implementation to make the result as close as practical to the mathematically ideal result.

Rational operations such as `+` should always produce exact results when given exact arguments. If the operation is unable to produce an exact result, then it may either report the violation of an implementation restriction or it may silently coerce its result to an inexact value. See Implementation restrictions.

With the exception of `exact`, the operations described in this section must generally return inexact results when given any inexact arguments. An operation may, however, return an exact result if it can prove that the value of the result is unaffected by the inexactness of its arguments. For example, multiplication of any number by an exact zero may produce an exact zero result, even if the other argument is inexact.

### 4.3 Implementation restrictions

Implementations of Scheme are not required to implement the whole tower of subtypes (see Numerical types), but they must implement a coherent subset consistent with both the purposes of the implementation and the spirit of the Scheme language. For example, an implementation in which all numbers are real may still be quite useful.1

Implementations may also support only a limited range of numbers of any type, subject to the requirements of this section. The supported range for exact numbers of any type may be different from the supported range for inexact numbers of that type. For example, an implementation that uses flonums to represent all its inexact real numbers may support a practically unbounded range of exact integers and rationals while limiting the range of inexact reals (and therefore the range of inexact integers and rationals) to the dynamic range of the flonum format. Furthermore the gaps between the representable inexact integers and rationals are likely to be very large in such an implementation as the limits of this range are approached.

An implementation of Scheme must support exact integers throughout the range of numbers that may be used for indexes of lists, vectors, and strings or that may result from computing the length of a list, vector, or string. The `length`, `vector-length`, and `string-length` procedures must return an exact integer, and it is an error to use anything but an exact integer as an index. Furthermore any integer constant within the index range, if expressed by an exact integer syntax, will indeed be read as an exact integer, regardless of any implementation restrictions that may apply outside this range. Finally, the procedures listed below will always return an exact integer result provided all their arguments are exact integers and the mathematically expected result is representable as an exact integer within the implementation:

    *                gcd                modulo
    +                imag-part          numerator
    -                exact              quotient
    abs              lcm                rationalize
    angle            magnitude          real-part
    ceiling          make-polar         remainder
    denominator      make-rectangular   round
    expt             max                truncate
    floor            min

Implementations are encouraged, but not required, to support exact integers and exact rationals of practically unlimited size and precision, and to implement the above procedures and the `/` procedure in such a way that they always return exact results when given exact arguments. If one of these procedures is unable to deliver an exact result when given exact arguments, then it may either report a violation of an implementation restriction or it may silently coerce its result to an inexact number. Such a coercion may cause an error later.

An implementation may use floating point and other approximate representation strategies for inexact numbers. This report recommends, but does not require, that the IEEE 32-bit and 64-bit floating point standards be followed by implementations that use flonum representations, and that implementations using other representations should match or exceed the precision achievable using these floating point standards.

In particular, implementations that use flonum representations must follow these rules: A flonum result must be represented with at least as much precision as is used to express any of the inexact arguments to that operation. It is desirable (but not required) for potentially inexact operations such as `sqrt`, when applied to exact arguments, to produce exact answers whenever possible (for example the square root of an exact 4 ought to be an exact 2). If, however, an exact number is operated upon so as to produce an inexact result (as by `sqrt`), and if the result is represented as a flonum, then the most precise flonum format available must be used; but if the result is represented in some other way then the representation must have at least as much precision as the most precise flonum format available.

Although Scheme allows a variety of written notations for numbers, any particular implementation may support only some of them.2 For example, an implementation in which all numbers are real need not support the rectangular and polar notations for complex numbers. If an implementation encounters an exact numerical constant that it cannot represent as an exact number, then it may either report a violation of an implementation restriction or it may silently represent the constant by an inexact number.

### 4.4 Syntax of numerical constants

A number may be written in binary, octal, decimal, or hexadecimal by the use of a radix prefix. The radix prefixes are `#b` (binary), `#o` (octal), `#d` (decimal), and `#x` (hexadecimal). With no radix prefix, a number is assumed to be expressed in decimal.

A numerical constant may be specified to be either exact or inexact by a prefix. The prefixes are `#e` for exact, and `#i` for inexact. An exactness prefix may appear before or after any radix prefix that is used. If the written representation of a number has no exactness prefix, the constant may be either inexact or exact. It is inexact if it contains a decimal point, an exponent, or a `#` character in the place of a digit, otherwise it is exact.

In systems with inexact numbers of varying precisions it may be useful to specify the precision of a constant. For this purpose, numerical constants may be written with an exponent marker that indicates the desired precision of the inexact representation. The letters `s`, `f`, `d`, and `l` specify the use of short, single, double, and long precision, respectively. (When fewer than four internal inexact representations exist, the four size specifications are mapped onto those available. For example, an implementation with two internal representations may map short and single together and long and double together.) In addition, the exponent marker `e` specifies the default precision for the implementation. The default precision has at least as much precision as double, but implementations may wish to allow this default to be set by the user.

    3.14159265358979F0
           Round to single — 3.141593
    0.6L0
           Extend to long — .600000000000000

### 4.5 Numerical operations

See Entry Format, for a summary of the naming conventions used to specify restrictions on the types of arguments to numerical routines. The examples used in this section assume that any numerical constant written using an exact notation is indeed represented as an exact number. Some examples also assume that certain numerical constants written using an inexact notation can be represented without loss of accuracy; the inexact constants were chosen so that this is likely to be true in implementations that use flonums to represent inexact numbers.

procedure: number? object ¶
procedure: complex? object ¶
procedure: real? object ¶
procedure: rational? object ¶
procedure: integer? object ¶

  These numerical type predicates can be applied to any kind of argument, including non-numbers. They return `#t` if the object is of the named type, and otherwise they return `#f`. In general, if a type predicate is true of a number then all higher type predicates are also true of that number. Consequently, if a type predicate is false of a number, then all lower type predicates are also false of that number.3

  If z is an inexact complex number, then `(real? z)` is true if and only if `(zero? (imag-part z))` is true. If x is an inexact real number, then `(integer? x)` is true if and only if `(= x (round x))`.

      (complex? 3+4i)         ⇒  #t
      (complex? 3)            ⇒  #t
      (real? 3)               ⇒  #t
      (real? -2.5+0.0i)       ⇒  #t
      (real? #e1e10)          ⇒  #t
      (rational? 6/10)        ⇒  #t
      (rational? 6/3)         ⇒  #t
      (integer? 3+0i)         ⇒  #t
      (integer? 3.0)          ⇒  #t
      (integer? 8/4)          ⇒  #t

  Note: The behavior of these type predicates on inexact numbers is unreliable, since any inaccuracy may affect the result.

procedure: exact? z ¶
procedure: inexact? z ¶

  These numerical predicates provide tests for the exactness of a quantity. For any Scheme number, precisely one of these predicates is true.

procedure: exact-integer? object ¶
procedure: exact-nonnegative-integer? object ¶
procedure: exact-rational? object ¶

  These procedures test for some very common types of numbers. These tests could be written in terms of simpler predicates, but are more efficient.

procedure: = z1 z2 z3 … ¶
procedure: < x1 x2 x3 … ¶
procedure: > x1 x2 x3 … ¶
procedure: <= x1 x2 x3 … ¶
procedure: >= x1 x2 x3 … ¶

  These procedures return `#t` if their arguments are (respectively): equal, monotonically increasing, monotonically decreasing, monotonically nondecreasing, or monotonically nonincreasing.

  These predicates are transitive. Note that the traditional implementations of these predicates in Lisp-like languages are not transitive.

  Note: While it is not an error to compare inexact numbers using these predicates, the results may be unreliable because a small inaccuracy may affect the result; this is especially true of `=` and `zero?`. When in doubt, consult a numerical analyst.

procedure: zero? z ¶
procedure: positive? x ¶
procedure: negative? x ¶
procedure: odd? x ¶
procedure: even? x ¶

  These numerical predicates test a number for a particular property, returning `#t` or `#f`. See note above regarding inexact numbers.

procedure: max x1 x2 … ¶
procedure: min x1 x2 … ¶

  These procedures return the maximum or minimum of their arguments.

      (max 3 4)              ⇒  4    ; exact
      (max 3.9 4)            ⇒  4.0  ; inexact

  Note: If any argument is inexact, then the result will also be inexact (unless the procedure can prove that the inaccuracy is not large enough to affect the result, which is possible only in unusual implementations). If `min` or `max` is used to compare numbers of mixed exactness, and the numerical value of the result cannot be represented as an inexact number without loss of accuracy, then the procedure may report a violation of an implementation restriction.4

procedure: + z1 … ¶
procedure: * z1 … ¶

  These procedures return the sum or product of their arguments.

      (+ 3 4)                 ⇒  7
      (+ 3)                   ⇒  3
      (+)                     ⇒  0
      (* 4)                   ⇒  4
      (*)                     ⇒  1

procedure: - z1 z2 … ¶
procedure: / z1 z2 … ¶

  With two or more arguments, these procedures return the difference or quotient of their arguments, associating to the left. With one argument, however, they return the additive or multiplicative inverse of their argument.

      (- 3 4)                 ⇒  -1
      (- 3 4 5)               ⇒  -6
      (- 3)                   ⇒  -3
      (/ 3 4 5)               ⇒  3/20
      (/ 3)                   ⇒  1/3

procedure: 1+ z ¶
procedure: -1+ z ¶

  `(1+ z)` is equivalent to `(+ z 1)`; `(-1+ z)` is equivalent to `(- z 1)`.

procedure: abs x ¶

  `abs` returns the magnitude of its argument.

      (abs -7)                ⇒  7

procedure: quotient n1 n2 ¶
procedure: remainder n1 n2 ¶
procedure: modulo n1 n2 ¶

  These procedures implement number-theoretic (integer) division: for positive integers n1 and n2, if n3 and n4 are integers such that then

      (quotient n1 n2)        ⇒  n3
      (remainder n1 n2)       ⇒  n4
      (modulo n1 n2)          ⇒  n4

  For integers n1 and n2 with n2 not equal to 0,

      (= n1
         (+ (* n2 (quotient n1 n2))
            (remainder n1 n2)))
                                          ⇒  #t

  provided all numbers involved in that computation are exact.

  The value returned by `quotient` always has the sign of the product of its arguments. `remainder` and `modulo` differ on negative arguments — the `remainder` always has the sign of the dividend, the `modulo` always has the sign of the divisor:

      (modulo 13 4)           ⇒  1
      (remainder 13 4)        ⇒  1

      (modulo -13 4)          ⇒  3
      (remainder -13 4)       ⇒  -1

      (modulo 13 -4)          ⇒  -3
      (remainder 13 -4)       ⇒  1

      (modulo -13 -4)         ⇒  -1
      (remainder -13 -4)      ⇒  -1

      (remainder -13 -4.0)    ⇒  -1.0  ; inexact

  Note that `quotient` is the same as `integer-truncate`.

procedure: integer-floor n1 n2 ¶
procedure: integer-ceiling n1 n2 ¶
procedure: integer-truncate n1 n2 ¶
procedure: integer-round n1 n2 ¶

  These procedures combine integer division with rounding. For example, the following are equivalent:

      (integer-floor n1 n2)
      (floor (/ n1 n2))

  However, the former is faster and does not produce an intermediate result.

  Note that `integer-truncate` is the same as `quotient`.

procedure: integer-divide n1 n2 ¶
procedure: integer-divide-quotient qr ¶
procedure: integer-divide-remainder qr ¶

  `integer-divide` is equivalent to performing both `quotient` and `remainder` at once. The result of `integer-divide` is an object with two components; the procedures `integer-divide-quotient` and `integer-divide-remainder` select those components. These procedures are useful when both the quotient and remainder are needed; often computing both of these numbers simultaneously is much faster than computing them separately.

  For example, the following are equivalent:

      (lambda (n d)
        (cons (quotient n d)
              (remainder n d)))

      (lambda (n d)
        (let ((qr (integer-divide n d)))
          (cons (integer-divide-quotient qr)
                (integer-divide-remainder qr))))

procedure: gcd n1 … ¶
procedure: lcm n1 … ¶

  These procedures return the greatest common divisor or least common multiple of their arguments. The result is always non-negative.

      (gcd 32 -36)            ⇒  4
      (gcd)                   ⇒  0

      (lcm 32 -36)            ⇒  288
      (lcm 32.0 -36)          ⇒  288.0  ; inexact
      (lcm)                   ⇒  1

procedure: modexp b e m ¶

  Modular exponentiation. Returns b^e mod m. b, e, and m must be exact integers; m must be nonzero. Mathematically equivalent to `(modulo (expt b e) m)`, but works even for large inputs for which the intermediate `(expt b e)` would overflow memory.

      (modexp 1234 5678 90)   ⇒ 46
      (modexp 2 (expt 2 1024) 299)
                              ⇒ 55

procedure: numerator q ¶
procedure: denominator q ¶

  These procedures return the numerator or denominator of their argument; the result is computed as if the argument was represented as a fraction in lowest terms. The denominator is always positive. The denominator of 0 is defined to be 1.

      (numerator (/ 6 4))  ⇒  3
      (denominator (/ 6 4))  ⇒  2
      (denominator (inexact (/ 6 4))) ⇒ 2.0

procedure: floor x ¶
procedure: ceiling x ¶
procedure: truncate x ¶
procedure: round x ¶

  These procedures return integers. `floor` returns the largest integer not larger than x. `ceiling` returns the smallest integer not smaller than x. `truncate` returns the integer closest to x whose absolute value is not larger than the absolute value of x. `round` returns the closest integer to x, rounding to even when x is halfway between two integers.

  Rationale: `round` rounds to even for consistency with the rounding modes required by the IEEE floating point standard.

  Note: If the argument to one of these procedures is inexact, then the result will also be inexact. If an exact value is needed, the result should be passed to the `exact` procedure (or use one of the procedures below).

      (floor -4.3)          ⇒  -5.0
      (ceiling -4.3)        ⇒  -4.0
      (truncate -4.3)       ⇒  -4.0
      (round -4.3)          ⇒  -4.0

      (floor 3.5)           ⇒  3.0
      (ceiling 3.5)         ⇒  4.0
      (truncate 3.5)        ⇒  3.0
      (round 3.5)           ⇒  4.0  ; inexact

      (round 7/2)           ⇒  4    ; exact
      (round 7)             ⇒  7

procedure: floor->exact x ¶
procedure: ceiling->exact x ¶
procedure: truncate->exact x ¶
procedure: round->exact x ¶

  These procedures are similar to the preceding procedures except that they always return an exact result. For example, the following are equivalent

      (floor->exact x)
      (exact (floor x))

  except that the former is faster and has fewer range restrictions.

procedure: rationalize x y ¶
procedure: rationalize->exact x y ¶

  `rationalize` returns the simplest rational number differing from x by no more than y. A rational number r1 is simpler than another rational number r2 if `r1=p1/q1` and `r2=p2/q2` (both in lowest terms) and `|p1|<=|p2|` and `|q1|<=|q2|`. Thus `3/5` is simpler than `4/7`. Although not all rationals are comparable in this ordering (consider `2/7` and `3/5`) any interval contains a rational number that is simpler than every other rational number in that interval (the simpler `2/5` lies between `2/7` and `3/5`). Note that `0=0/1` is the simplest rational of all.

      (rationalize (exact .3) 1/10)  ⇒ 1/3    ; exact
      (rationalize .3 1/10)          ⇒ #i1/3  ; inexact

  `rationalize->exact` is similar to `rationalize` except that it always returns an exact result.

procedure: simplest-rational x y ¶
procedure: simplest-exact-rational x y ¶

  `simplest-rational` returns the simplest rational number between x and y inclusive; `simplest-exact-rational` is similar except that it always returns an exact result.

  These procedures implement the same functionality as `rationalize` and `rationalize->exact`, except that they specify the input range by its endpoints; `rationalize` specifies the range by its center point and its (half-) width.

procedure: exp z ¶
procedure: log z ¶
procedure: sin z ¶
procedure: cos z ¶
procedure: tan z ¶
procedure: asin z ¶
procedure: acos z ¶
procedure: atan z ¶
procedure: atan y x ¶

  These procedures compute the usual transcendental functions. `log` computes the natural logarithm of z (not the base ten logarithm). `asin`, `acos`, and `atan` compute arcsine, arccosine, and arctangent, respectively. The two-argument variant of `atan` computes `(angle (make-rectangular x y))` (see below).

  In general, the mathematical functions log, arcsine, arccosine, and arctangent are multiply defined. For nonzero real x, the value of log x is defined to be the one whose imaginary part lies in the range minus pi (exclusive) to pi (inclusive). log 0 is undefined. The value of log z when z is complex is defined according to the formula With log defined this way, the values of arcsine, arccosine, and arctangent are according to the following formulae: The above specification follows Common Lisp: the Language, which in turn cites Principal Values and Branch Cuts in Complex APL; refer to these sources for more detailed discussion of branch cuts, boundary conditions, and implementation of these functions. When it is possible these procedures produce a real result from a real argument.

procedure: log1p z ¶
procedure: expm1 z ¶

  Equivalent to:

      log1p z = log(1 + z).
      expm1 z = exp(z) - 1,

  However, for real numbers close to zero, these provide better approximations than `(log (+ 1 z))` or `(- (exp z) 1)`:

  - Floating-point numbers have much higher density around 0 than around 1, so naive translation from near 0 to near 1 loses precision, and naive computation of a number near 1 loses precision even if it is followed by translation to near 0.
  - The condition number of log near 1 is unbounded, while the condition number of log1p near 0 is near 1:

        x f'(x)/f(x) = [x/(1 + x)]/log(1 + x).

    (Conversely, the condition number of log near 0 approaches 0, while the condition number of log1p near -1 is unbounded, so for inputs near 0 it is better to compute them via log rather than via log1p.)

  - Similarly, although the condition number of exp near 0 is near 0, its value near 0 is near 1, and the condition number of y - 1 is unbounded for y near 1, so the intermediate error introduced by `(exp z)` may be amplified arbitrarily by then computing `(- (exp z) 1)`. In contrast, the condition number of expm1 itself near 0 is near 1, so it does not inherently amplify errors:

        x f'(x)/f(x) = x e^x/(e^x - 1).

  The forward relative error of this implementation is determined by the system’s math library, usually below 1ulp.

procedure: log1mexp x ¶
procedure: log1pexp x ¶

  Equivalent to:

      log1mexp x = log (1 - e^x),
      log1pexp x = log (1 + e^x).

  Like log1p and expm1, these avoid numerical pathologies with the intermediate quantities 1 - e^x and 1 + e^x and inputs to log near 1.

  - log1mexp computes the complement of a probability p in log-space \log p, and as such is a self-inverse. It is finite when x < 0; negative infinity when x = 0; and invalid otherwise.
  - log1pexp is related to the logistic function 1/(1 + e^{-x}) — specifically, it differs from the logarithm of the logistic function only by the sign of the input and the output.

  This implementation gives forward relative error bounded by ten times the forward relative error bound of the system math library’s log and exp, which is usually below 1ulp.

  Beware that although the forward relative error of the MIT/GNU Scheme implementations of these functions is bounded, these functions are ill-conditioned for large negative inputs:

      x f'(x)/f(x) = (+/- x exp(x))/((1 +/- e^x) log(1 +/- e^x)),
        --> x,  for x << 0.

procedure: logistic x ¶
procedure: logit x ¶

  Logistic and logit functions. Equivalent to:

      logistic x = exp(x)/[1 + exp(x)] = 1/[1 + exp(-x)],
      logit p = log p/(1 - p).

  These functions are inverses of one another. The logit function maps a probablity p in [0, 1] into log-odds x in the extended real line, and the logistic function maps back from log-odds to probabilities.

  - The logistic function is defined on the entire real line, but is ill-conditioned for large negative x, with condition number

        x f'(x)/f(x) = x exp(-x)/[1 + exp(-x)].

    The identity

        logistic(-x) = 1 - logistic(x)

    may help to rearrange a computation, along with the logistic-1/2 function which ranges from -1/2 to +1/2 and centered at zero rather than from 0 to 1 and centered at 1/2.

    This implementation gives forward relative error bounded by at most seven times the forward relative error bound of the system math library’s exp, which is usually below 1ulp.

  - The logit function is defined on the closed unit interval [0, 1], but is ill-conditioned near 1/2 and 1, with condition number

        x f'(x)/f(x) = 1/[(1 - p) log(p/(1 - p))].

    The identity

        logit(1 - p) = -logit(p)

    may help to rearrange a computation, along with the logit1/2+ function which is defined on -1/2 to +1/2 and centered at zero rather than on 0 to 1 and centered at 1/2.

    This implementation gives forward relative error bounded by at most ten times the forward relative error bound of the system math library’s log, which is usually below 1ulp.

procedure: logistic-1/2 x ¶
procedure: logit1/2+ x ¶

  Equivalent to:

      logistic-1/2 x = logistic(x) - 1/2,
      logit1/2+ p = logit(1/2 + p).

  Like `logistic` and `logit`, these functions are inverses of one another; unlike `logistic` and `logit`, their domains and codomains are both centered at zero.

  - The logistic-1/2 function is well-conditioned on the entire real line, with maximum condition number 1 at 0:

        x f'(x)/f(x) = 2 x e^-x / (1 - (e^-x)^2).

    This implementation gives forward relative error bounded by 5 times the forward relative error bound of the system math library’s exp, which is usually below 1ulp.

  - The logit1/2+ function is defined on [-1/2, +1/2], and is ill-conditioned near -1/2 and +1/2:

        x f'(x)/f(x) = x/[(1 - 4 x^2) logit(1/2 + x)].

    For points near -1/2 or +1/2, it may be better to compute logit of a point near 0 instead. This implementation gives forward relative error bounded by 34 times the forward relative error bound of the system math library’s log, which is usually below 1ulp.

procedure: log-logistic x ¶
procedure: logit-exp x ¶

  Equivalent to:

      log-logistic x = log(logistic(x)) = log [1/(1 + exp(-x))]
      logit-exp x = logit(exp(x)) = log [exp(x)/(1 - exp(x))]

  Like `logistic` and `logit`, these functions are inverses of one another.

  - The loglogistic function maps log-odds on the extended real line to logprobability on the nonpositive half of the extended real line, and is illconditioned for large positive x:

        x f'(x)/f(x) = (-x exp(-x))/[(1 + exp(-x)) log(1 + exp(-x))]

  - The logitexp function maps log-probability on the nonpositive half of the extended real line to log-odds on the extended real line, and is illconditioned near \log(1/2):

        x f'(x)/f(x) = x/[(1 - exp(x)) log(exp(x)/(1 - exp(x)))]

  This implementation gives forward relative error bounded by ten times the forward relative error bound of the system math library’s log and exp, which is usually below 1ulp.

procedure: logsumexp list ¶

  List must be a list of real numbers x1, x2, …, xn. Returns an approximation to:

      log(exp(x1) + exp(x2) + … + exp(xn)).

  The computation avoids intermediate overflow; `logsumexp` returns `+inf.0` if and only if one of the inputs is `+inf.0`.

procedure: sqrt z ¶

  Returns the principal square root of z. The result will have either positive real part, or zero real part and non-negative imaginary part.

procedure: expt z1 z2 ¶

  Returns z1 raised to the power z2:

procedure: make-rectangular x1 x2 ¶
procedure: make-polar x3 x4 ¶
procedure: real-part z ¶
procedure: imag-part z ¶
procedure: magnitude z ¶
procedure: angle z ¶
procedure: conjugate z ¶

  Suppose x1, x2, x3, and x4 are real numbers and z is a complex number such that Then `make-rectangular` and `make-polar` return z, `real-part` returns x1, `imag-part` returns x2, `magnitude` returns x3, and `angle` returns x4. In the case of `angle`, whose value is not uniquely determined by the preceding rule, the value returned will be the one in the range minus pi (exclusive) to pi (inclusive).

  `conjugate` returns the complex conjugate of z.

The procedures `exact` and `inexact` implement the natural one-to-one correspondence between exact and inexact integers throughout an implementation-dependent range.

procedure: inexact z ¶
procedure: exact->inexact z ¶

  `inexact` returns an inexact representation of z. The value returned is the inexact number that is numerically closest to the argument. For inexact arguments, the result is the same as the argument. For exact complex numbers, the result is a complex number whose real and imaginary parts are the result of applying `inexact` to the real and imaginary parts of the argument, respectively. If an exact argument has no reasonably close inexact equivalent (in the sense of `=`), then a violation of an implementation restriction may be reported.

  The procedure `exact->inexact` has been deprecated by R7RS.

procedure: exact z ¶
procedure: inexact->exact z ¶

  `exact` returns an exact representation of z. The value returned is the exact number that is numerically closest to the argument. For exact arguments, the result is the same as the argument. For inexact non-integral real arguments, the implementation may return a rational approximation, or may report an implementation violation. For inexact complex arguments, the result is a complex number whose real and imaginary parts are the result of applying `exact` to the real and imaginary parts of the argument, respectively. If an inexact argument has no reasonably close exact equivalent (in the sense of `=`), then a violation of an implementation restriction may be reported.

  The procedure `inexact->exact` has been deprecated by R7RS.

procedure: copysign x1 x2 ¶

  Returns a real number with the magnitude of x1 and the sign of x2.

      (copysign 123 -1)              ⇒ -123
      (copysign 0. -1)               ⇒ -0.
      (copysign -0. 0.)              ⇒ 0.
      (copysign -nan.123 0.)         ⇒ +nan.123

### 4.6 Numerical input and output

procedure: number->string number [radix] ¶

  Radix must be an exact integer, either 2, 8, 10, or 16. If omitted, radix defaults to 10. The procedure `number->string` takes a number and a radix and returns as a string an external representation of the given number in the given radix such that

      (let ((number number)
            (radix radix))
        (eqv? number
              (string->number (number->string number radix)
                              radix)))

  is true. It is an error if no possible result makes this expression true.

  If number is inexact, the radix is 10, and the above expression can be satisfied by a result that contains a decimal point, then the result contains a decimal point and is expressed using the minimum number of digits (exclusive of exponent and trailing zeroes) needed to make the above expression true; otherwise the format of the result is unspecified.

  The result returned by `number->string` never contains an explicit radix prefix.

  Note: The error case can occur only when number is not a complex number or is a complex number with an non-rational real or imaginary part.

  Rationale: If number is an inexact number represented using flonums, and the radix is 10, then the above expression is normally satisfied by a result containing a decimal point. The unspecified case allows for infinities, NaNs, and non-flonum representations.

variable: flonum-parser-fast? ¶

  This variable controls the behavior of `string->number` when parsing inexact numbers. Specifically, it allows the user to trade off accuracy against speed.

  When set to its default value, `#f`, the parser provides maximal accuracy, as required by the Scheme standard. If set to `#t`, the parser uses faster algorithms that will sometimes introduce small errors in the result. The errors affect a few of the least-significant bits of the result, and consequently can be tolerated by many applications.

variable: flonum-unparser-cutoff ¶

  This variable is deprecated; use `param:flonum-printer-cutoff` instead.

parameter: param:flonum-printer-cutoff ¶

  This parameter controls the action of `number->string` when number is a flonum (and consequently controls all printing of flonums). This parameter may be called with an argument to set its value.

  The value of this parameter is normally a list of three items:

  rounding-type

    One of the following symbols: `normal`, `relative`, or `absolute`. The symbol `normal` means that the number should be printed with full precision. The symbol `relative` means that the number should be rounded to a specific number of digits. The symbol `absolute` means that the number should be rounded so that there are a specific number of digits to the right of the decimal point.

  precision

    An exact integer. If rounding-type is `normal`, precision is ignored. If rounding-type is `relative`, precision must be positive, and it specifies the number of digits to which the printed representation will be rounded. If rounding-type is `absolute`, the printed representation will be rounded precision digits to the right of the decimal point; if precision is negative, the representation is rounded `(- precision)` digits to the left of the decimal point.

  format-type

    One of the symbols: `normal`, `scientific`, or `engineering`. This specifies the format in which the number will be printed.
    `scientific` specifies that the number will be printed using scientific notation: `x.xxxeyyy`. In other words, the number is printed as a significand between zero inclusive and ten exclusive, and an exponent. `engineering` is like `scientific`, except that the exponent is always a power of three, and the significand is constrained to be between zero inclusive and 1000 exclusive. If `normal` is specified, the number will be printed in positional notation if it is “small enough”, otherwise it is printed in scientific notation. A number is “small enough” when the number of digits that would be printed using positional notation does not exceed the number of digits of precision in the underlying floating-point number representation; IEEE 754-2008 binary64 floating-point numbers have 17 digits of precision.

  This three-element list may be abbreviated in two ways. First, the symbol `normal` may be used, which is equivalent to the list `(normal 0 normal)`. Second, the third element of the list, format-type, may be omitted, in which case it defaults to `normal`.

  The default value for `param:flonum-printer-cutoff` is `normal`. If it is bound to a value different from those described here, `number->string` issues a warning and acts as though the value had been `normal`.

Some examples of `param:flonum-printer-cutoff`:

    (number->string (* 4 (atan 1 1)))
                                        ⇒ "3.141592653589793"
    (parameterize ((param:flonum-printer-cutoff '(relative 5)))
      (number->string (* 4 (atan 1 1))))
                                        ⇒ "3.1416"
    (parameterize ((param:flonum-printer-cutoff '(relative 5)))
      (number->string (* 4000 (atan 1 1))))
                                        ⇒ "3141.6"
    (parameterize ((param:flonum-printer-cutoff '(relative 5 scientific)))
      (number->string (* 4000 (atan 1 1))))
                                        ⇒ "3.1416e3"
    (parameterize ((param:flonum-printer-cutoff '(relative 5 scientific)))
      (number->string (* 40000 (atan 1 1))))
                                        ⇒ "3.1416e4"
    (parameterize ((param:flonum-printer-cutoff '(relative 5 engineering)))
      (number->string (* 40000 (atan 1 1))))
                                        ⇒ "31.416e3"
    (parameterize ((param:flonum-printer-cutoff '(absolute 5)))
      (number->string (* 4 (atan 1 1))))
                                        ⇒ "3.14159"
    (parameterize ((param:flonum-printer-cutoff '(absolute 5)))
      (number->string (* 4000 (atan 1 1))))
                                        ⇒ "3141.59265"
    (parameterize ((param:flonum-printer-cutoff '(absolute -4)))
      (number->string (* 4e10 (atan 1 1))))
                                        ⇒ "31415930000."
    (parameterize ((param:flonum-printer-cutoff '(absolute -4 scientific)))
      (number->string (* 4e10 (atan 1 1))))
                                        ⇒ "3.141593e10"
    (parameterize ((param:flonum-printer-cutoff '(absolute -4 engineering)))
      (number->string (* 4e10 (atan 1 1))))
                                        ⇒ "31.41593e9"
    (parameterize ((param:flonum-printer-cutoff '(absolute -5)))
      (number->string (* 4e10 (atan 1 1))))
                                        ⇒ "31415900000."

procedure: string->number string [radix] ¶

  Returns a number of the maximally precise representation expressed by the given string. Radix must be an exact integer, either 2, 8, 10, or 16. If supplied, radix is a default radix that may be overridden by an explicit radix prefix in string (e.g. `"#o177"`). If radix is not supplied, then the default radix is 10. If string is not a syntactically valid notation for a number, then `string->number` returns `#f`.

      (string->number "100")        ⇒  100
      (string->number "100" 16)     ⇒  256
      (string->number "1e2")        ⇒  100.0
      (string->number "15##")       ⇒  1500.0

  Note that a numeric representation using a decimal point or an exponent marker is not recognized unless radix is `10`.

### 4.7 Fixnum and Flonum Operations

This section describes numerical operations that are restricted forms of the operations described above. These operations are useful because they compile very efficiently. However, care should be exercised: if used improperly, these operations can return incorrect answers, or even malformed objects that confuse the garbage collector.

#### 4.7.1 Fixnum Operations

A fixnum is an exact integer that is small enough to fit in a machine word. In MIT/GNU Scheme, fixnums are typically 24 or 26 bits, depending on the machine; it is reasonable to assume that fixnums are at least 24 bits. Fixnums are signed; they are encoded using 2’s complement.

All exact integers that are small enough to be encoded as fixnums are always encoded as fixnums — in other words, any exact integer that is not a fixnum is too big to be encoded as such. For this reason, small constants such as `0` or `1` are guaranteed to be fixnums.

procedure: fix:fixnum? object ¶

  Returns `#t` if object is a fixnum; otherwise returns `#f`.

Here is an expression that determines the largest fixnum:

    (let loop ((n 1))
      (if (fix:fixnum? n)
          (loop (* n 2))
          (- n 1)))

A similar expression determines the smallest fixnum.

procedure: fix:= fixnum fixnum ¶
procedure: fix:< fixnum fixnum ¶
procedure: fix:> fixnum fixnum ¶
procedure: fix:<= fixnum fixnum ¶
procedure: fix:>= fixnum fixnum ¶

  These are the standard order and equality predicates on fixnums. When compiled, they do not check the types of their arguments.

procedure: fix:zero? fixnum ¶
procedure: fix:positive? fixnum ¶
procedure: fix:negative? fixnum ¶

  These procedures compare their argument to zero. When compiled, they do not check the type of their argument. The code produced by the following expressions is identical:

      (fix:zero? fixnum)
      (fix:= fixnum 0)

  Similarly, `fix:positive?` and `fix:negative?` produce code identical to equivalent expressions using `fix:>` and `fix:<`.

procedure: fix:+ fixnum fixnum ¶
procedure: fix:- fixnum fixnum ¶
procedure: fix:* fixnum fixnum ¶
procedure: fix:quotient fixnum fixnum ¶
procedure: fix:remainder fixnum fixnum ¶
procedure: fix:gcd fixnum fixnum ¶
procedure: fix:1+ fixnum ¶
procedure: fix:-1+ fixnum ¶

  These procedures are the standard arithmetic operations on fixnums. When compiled, they do not check the types of their arguments. Furthermore, they do not check to see if the result can be encoded as a fixnum. If the result is too large to be encoded as a fixnum, a malformed object is returned, with potentially disastrous effect on the garbage collector.

procedure: fix:divide fixnum fixnum ¶

  This procedure is like `integer-divide`, except that its arguments and its results must be fixnums. It should be used in conjunction with `integer-divide-quotient` and `integer-divide-remainder`.

The following are bitwise-logical operations on fixnums.

procedure: fix:not fixnum ¶

  This returns the bitwise-logical inverse of its argument. When compiled, it does not check the type of its argument.

      (fix:not 0)                             ⇒  -1
      (fix:not -1)                            ⇒  0
      (fix:not 1)                             ⇒  -2
      (fix:not -34)                           ⇒  33

procedure: fix:and fixnum fixnum ¶

  This returns the bitwise-logical “and” of its arguments. When compiled, it does not check the types of its arguments.

      (fix:and #x43 #x0f)                     ⇒  3
      (fix:and #x43 #xf0)                     ⇒  #x40

procedure: fix:andc fixnum fixnum ¶

  Returns the bitwise-logical “and” of the first argument with the bitwise-logical inverse of the second argument. When compiled, it does not check the types of its arguments.

      (fix:andc #x43 #x0f)                    ⇒  #x40
      (fix:andc #x43 #xf0)                    ⇒  3

procedure: fix:or fixnum fixnum ¶

  This returns the bitwise-logical “inclusive or” of its arguments. When compiled, it does not check the types of its arguments.

      (fix:or #x40 3)                         ⇒ #x43
      (fix:or #x41 3)                         ⇒ #x43

procedure: fix:xor fixnum fixnum ¶

  This returns the bitwise-logical “exclusive or” of its arguments. When compiled, it does not check the types of its arguments.

      (fix:xor #x40 3)                        ⇒ #x43
      (fix:xor #x41 3)                        ⇒ #x42

procedure: fix:lsh fixnum1 fixnum2 ¶

  This procedure returns the result of logically shifting fixnum1 by fixnum2 bits. If fixnum2 is positive, fixnum1 is shifted left; if negative, it is shifted right. When compiled, it does not check the types of its arguments, nor the validity of its result.

      (fix:lsh 1 10)                          ⇒  #x400
      (fix:lsh #x432 -10)                     ⇒  1
      (fix:lsh -1 3)                          ⇒  -8
      (fix:lsh -128 -4)                       ⇒  #x3FFFF8

#### 4.7.2 Flonum Operations

A flonum is an inexact real number that is implemented as a floating-point number. In MIT/GNU Scheme, all inexact real numbers are flonums. For this reason, constants such as `0.` and `2.3` are guaranteed to be flonums.

MIT/GNU Scheme follows the IEEE 754-2008 floating-point standard, using binary64 arithmetic for flonums. All floating-point values are classified into:

normal ¶

  Numbers of the form

      r^e (1 + f/r^p)

  where r, the radix, is a positive integer, here always 2; p, the precision, is a positive integer, here always 53; e, the exponent, is an integer within a limited range, here always -1022 to 1023 (inclusive); and f, the fractional part of the significand, is a (p-1)-bit unsigned integer,

subnormal ¶

  Fixed-point numbers near zero that allow for gradual underflow. Every subnormal number is an integer multiple of the smallest subnormal number. Subnormals were also historically called “denormal”.

zero ¶

  There are two distinguished zero values, one with “negative” sign bit and one with “positive” sign bit.

  The two zero values are considered numerically equal, but serve to distinguish paths converging to zero along different branch cuts and so some operations yield different results for differently signed zero values.

infinity ¶

  There are two distinguished infinity values, negative infinity or `-inf.0` and positive infinity or `+inf.0`, representing overflow on the real line.

NaN ¶

  There are 4 r^{p-2} - 2 distinguished not-a-number values, representing invalid operations or uninitialized data, distinguished by their negative/positive sign bit, a quiet/signalling bit, and a (p-2)-digit unsigned integer payload which must not be zero for signalling NaNs.

  Arithmetic on quiet NaNs propagates them without raising any floating-point exceptions. In contrast, arithmetic on signalling NaNs raises the floating-point invalid-operation exception. Quiet NaNs are written `+nan.123`, `-nan.0`, etc. Signalling NaNs are written `+snan.123`, `-snan.1`, etc. The notation `+snan.0` and `-snan.0` is not allowed: what would be the encoding for them actually means `+inf.0` and `-inf.0`.

procedure: flo:flonum? object ¶

  Returns `#t` if object is a flonum; otherwise returns `#f`.

procedure: flo:= flonum1 flonum2 ¶
procedure: flo:< flonum1 flonum2 ¶
procedure: flo:<= flonum1 flonum2 ¶
procedure: flo:> flonum1 flonum2 ¶
procedure: flo:>= flonum1 flonum2 ¶
procedure: flo:<> flonum1 flonum2 ¶

  These procedures are the standard order and equality predicates on flonums. When compiled, they do not check the types of their arguments. These predicates raise floating-point invalid-operation exceptions on NaN arguments; in other words, they are “ordered comparisons”. When floating-point exception traps are disabled, they return false when any argument is NaN.

  Every pair of floating-point numbers — excluding NaN — exhibits ordered trichotomy: they are related either by `flo:=`, `flo:<`, or `flo:>`.

procedure: flo:safe= flonum1 flonum2 ¶
procedure: flo:safe< flonum1 flonum2 ¶
procedure: flo:safe<= flonum1 flonum2 ¶
procedure: flo:safe> flonum1 flonum2 ¶
procedure: flo:safe>= flonum1 flonum2 ¶
procedure: flo:safe<> flonum1 flonum2 ¶
procedure: flo:unordered? flonum1 flonum2 ¶

  These procedures are the standard order and equality predicates on flonums. When compiled, they do not check the types of their arguments. These predicates do not raise floating-point exceptions, and simply return false on NaN arguments, except `flo:unordered?` which returns true iff at least one argument is NaN; in other words, they are “unordered comparisons”.

  Every pair of floating-point values — including NaN — exhibits unordered tetrachotomy: they are related either by `flo:safe=`, `flo:safe<`, `flo:safe>`, or `flo:unordered?`.

procedure: flo:zero? flonum ¶
procedure: flo:positive? flonum ¶
procedure: flo:negative? flonum ¶

  Each of these procedures compares its argument to zero. When compiled, they do not check the type of their argument. These predicates raise floating-point invalid-operation exceptions on NaN arguments; in other words, they are “ordered comparisons”.

      (flo:zero? -0.)                ⇒ #t
      (flo:negative? -0.)            ⇒ #f
      (flo:negative? -1.)            ⇒ #t

      (flo:zero? 0.)                 ⇒ #t
      (flo:positive? 0.)             ⇒ #f
      (flo:positive? 1.)             ⇒ #f

      (flo:zero? +nan.123)           ⇒ #f  ; (raises invalid-operation)

procedure: flo:normal? flonum ¶
procedure: flo:subnormal? flonum ¶
procedure: flo:safe-zero? flonum ¶
procedure: flo:infinite? flonum ¶
procedure: flo:nan? flonum ¶

  Floating-point classification predicates. For any flonum, exactly one of these predicates returns true. These predicates never raise floating-point exceptions.

      (flo:normal? 1.23)             ⇒ #t
      (flo:subnormal? 4e-124)        ⇒ #t
      (flo:safe-zero? -0.)           ⇒ #t
      (flo:infinite? +inf.0)         ⇒ #t
      (flo:nan? -nan.123)            ⇒ #t

procedure: flo:finite? flonum ¶

  Equivalent to:

      (or (flo:safe-zero? flonum)
          (flo:subnormal? flonum)
          (flo:normal? flonum))
      ; or
      (and (not (flo:infinite? flonum))
           (not (flo:nan? flonum)))

  True for normal, subnormal, and zero floating-point values; false for infinity and NaN.

procedure: flo:classify flonum ¶

  Returns a symbol representing the classification of the flonum, one of `normal`, `subnormal`, `zero`, `infinity`, or `nan`.

procedure: flo:sign-negative? flonum ¶

  Returns true if the sign bit of flonum is negative, and false otherwise. Never raises a floating-point exception.

      (flo:sign-negative? +0.)       ⇒ #f
      (flo:sign-negative? -0.)       ⇒ #t
      (flo:sign-negative? -1.)       ⇒ #t
      (flo:sign-negative? +inf.0)    ⇒ #f
      (flo:sign-negative? +nan.123)  ⇒ #f

      (flo:negative? -0.)            ⇒ #f
      (flo:negative? +nan.123)       ⇒ #f  ; (raises invalid-operation)

procedure: flo:+ flonum1 flonum2 ¶
procedure: flo:- flonum1 flonum2 ¶
procedure: flo:* flonum1 flonum2 ¶
procedure: flo:/ flonum1 flonum2 ¶

  These procedures are the standard arithmetic operations on flonums. When compiled, they do not check the types of their arguments.

procedure: flo:*+ flonum1 flonum2 flonum3 ¶
procedure: flo:fma flonum1 flonum2 flonum3 ¶
procedure: flo:fast-fma? ¶

  Fused multiply-add: `(flo:*+ u v a)` computes uv+a correctly rounded, with no intermediate overflow or underflow arising from uv. In contrast, `(flo:+ (flo:* u v) a)` may have two rounding errors, and can overflow or underflow if uv is too large or too small even if uv + a is normal. `Flo:fma` is an alias for `flo:*+` with the more familiar name used in other languages like C.

  `Flo:fast-fma?` returns true if the implementation of fused multiply-add is supported by fast hardware, and false if it is emulated using Dekker’s double-precision algorithm in software.

      (flo:+ (flo:* 1.2e100 2e208) -1.4e308)
                                     ⇒ +inf.0  ; (raises overflow)
      (flo:*+ 1.2e100 2e208  -1.4e308)
                                     ⇒ 1e308

procedure: flo:negate flonum ¶

  This procedure returns the negation of its argument. When compiled, it does not check the type of its argument.

  This is not equivalent to `(flo:- 0. flonum)`:

      (flo:negate 1.2)               ⇒ -1.2
      (flo:negate -nan.123)          ⇒ +nan.123
      (flo:negate +inf.0)            ⇒ -inf.0
      (flo:negate 0.)                ⇒ -0.
      (flo:negate -0.)               ⇒ 0.

      (flo:- 0. 1.2)                 ⇒ -1.2
      (flo:- 0. -nan.123)            ⇒ -nan.123
      (flo:- 0. +inf.0)              ⇒ -inf.0
      (flo:- 0. 0.)                  ⇒ 0.
      (flo:- 0. -0.)                 ⇒ 0.

procedure: flo:abs flonum ¶
procedure: flo:exp flonum ¶
procedure: flo:log flonum ¶
procedure: flo:sin flonum ¶
procedure: flo:cos flonum ¶
procedure: flo:tan flonum ¶
procedure: flo:asin flonum ¶
procedure: flo:acos flonum ¶
procedure: flo:atan flonum ¶
procedure: flo:sinh flonum ¶
procedure: flo:cosh flonum ¶
procedure: flo:tanh flonum ¶
procedure: flo:asinh flonum ¶
procedure: flo:acosh flonum ¶
procedure: flo:atanh flonum ¶
procedure: flo:sqrt flonum ¶
procedure: flo:cbrt flonum ¶
procedure: flo:expt flonum1 flonum2 ¶
procedure: flo:erf flonum ¶
procedure: flo:erfc flonum ¶
procedure: flo:hypot flonum1 flonum2 ¶
procedure: flo:j0 flonum ¶
procedure: flo:j1 flonum ¶
procedure: flo:jn flonum ¶
procedure: flo:y0 flonum ¶
procedure: flo:y1 flonum ¶
procedure: flo:yn flonum ¶
procedure: flo:gamma flonum ¶
procedure: flo:lgamma flonum ¶
procedure: flo:floor flonum ¶
procedure: flo:ceiling flonum ¶
procedure: flo:truncate flonum ¶
procedure: flo:round flonum ¶
procedure: flo:floor->exact flonum ¶
procedure: flo:ceiling->exact flonum ¶
procedure: flo:truncate->exact flonum ¶
procedure: flo:round->exact flonum ¶

  These procedures are flonum versions of the corresponding procedures. When compiled, they do not check the types of their arguments.

procedure: flo:expm1 flonum ¶
procedure: flo:log1p flonum ¶

  Flonum versions of `expm1` and `log1p` with restricted domains: `flo:expm1` is defined only on inputs bounded below log(2) in magnitude, and `flo:log1p` is defined only on inputs bounded below 1 - sqrt(1/2) in magnitude. Callers must use `(- (flo:exp x) 1)` or `(flo:log (+ 1 x))` outside these ranges.

procedure: flo:atan2 flonum1 flonum2 ¶

  This is the flonum version of `atan` with two arguments. When compiled, it does not check the types of its arguments.

procedure: flo:signed-lgamma x ¶

  Returns two values,

      m = log(|Gamma(x)|)

      and

      s = sign(Gamma(x)),

  respectively a flonum and an exact integer either `-1` or `1`, so that

      Gamma(x) = s * e^m.

procedure: flo:min x1 x2 ¶
procedure: flo:max x1 x2 ¶

  Returns the min or max of two floating-point numbers. If either argument is NaN, raises the floating-point invalid-operation exception and returns the other one if it is not NaN, or the first argument if they are both NaN.

procedure: flo:min-mag x1 x2 ¶
procedure: flo:max-mag x1 x2 ¶

  Returns the argument that has the smallest or largest magnitude, as in minNumMag or maxNumMag of IEEE 754-2008. If either argument is NaN, raises the floating-point invalid-operation exception and returns the other one if it is not NaN, or the first argument if they are both NaN.

procedure: flo:ldexp x1 x2 ¶
procedure: flo:scalbn x1 x2 ¶

  `Flo:ldexp` scales by a power of two; `flo:scalbn` scales by a power of the floating-point radix.

      ldexp x e := x * 2^e,
      scalbn x e := x * r^e.

  In MIT/GNU Scheme, these procedures are the same; they are both provided to make it clearer which operation is meant.

procedure: flo:logb x ¶

  For nonzero finite x, returns floor(log(x)/log(r)) as an exact integer, where r is the floating-point radix.

  For all other inputs, raises invalid-operation and returns `#f`.

procedure: flo:nextafter x1 x2 ¶

  Returns the next floating-point number after x1 in the direction of x2.

      (flo:nextafter 0. -1.)         ⇒ -4.9406564584124654e-324

procedure: flo:copysign x1 x2 ¶

  Returns a floating-point number with the magnitude of x1 and the sign of x2.

      (flo:copysign 123. 456.)       ⇒ 123.
      (flo:copysign +inf.0 -1)       ⇒ -inf.0
      (flo:copysign 0. -1)           ⇒ -0.
      (flo:copysign -0. 0.)          ⇒ 0.
      (flo:copysign -nan.123 0.)     ⇒ +nan.123

constant: flo:radix ¶
constant: flo:radix. ¶
constant: flo:precision ¶

  Floating-point system parameters. `Flo:radix` is the floating-point radix as an integer, and `flo:precision` is the floating-point precision as an integer; `flo:radix.` is the flotaing-point radix as a flonum.

constant: flo:error-bound ¶
constant: flo:log-error-bound ¶
constant: flo:ulp-of-one ¶
constant: flo:log-ulp-of-one ¶

  `Flo:error-bound`, sometimes called the machine epsilon, is the maximum relative error of rounding to nearest:

      max |x - fl(x)|/|x| = 1/(2 r^(p-1)),

  where r is the floating-point radix and p is the floating-point precision.

  `Flo:ulp-of-one` is the distance from 1 to the next larger floating-point number, and is equal to 1/r^{p-1}.

  `Flo:error-bound` is half `flo:ulp-of-one`.

  `Flo:log-error-bound` is the logarithm of `flo:error-bound`, and `flo:log-ulp-of-one` is the logarithm of `flo:log-ulp-of-one`.

procedure: flo:ulp flonum ¶

  Returns the distance from flonum to the next floating-point number larger in magnitude with the same sign. For zero, this returns the smallest subnormal. For infinities, this returns positive infinity. For NaN, this returns the same NaN.

      (flo:ulp 1.)                    ⇒ 2.220446049250313e-16
      (= (flo:ulp 1.) flo:ulp-of-one) ⇒ #t

constant: flo:normal-exponent-max ¶
constant: flo:normal-exponent-min ¶
constant: flo:subnormal-exponent-min ¶

  Largest and smallest positive integer exponents of the radix in normal and subnormal floating-point numbers.

  - `Flo:normal-exponent-max` is the largest positive integer such that `(expt flo:radix. flo:normal-exponent-max)` does not overflow.
  - `Flo:normal-exponent-min` is the smallest positive integer such that `(expt flo:radix. flo:normal-exponent-min)` is a normal floating-point number.
  - `Flo:subnormal-exponent-min` is the smallest positive integer such that `(expt flo:radix. flo:subnormal-exponent-min)` is nonzero; this is also the smallest positive floating-point number.

constant: flo:largest-positive-normal ¶
constant: flo:smallest-positive-normal ¶
constant: flo:smallest-positive-subnormal ¶

  Smallest and largest normal and subnormal numbers in magnitude.

constant: flo:greatest-normal-exponent-base-e ¶
constant: flo:greatest-normal-exponent-base-2 ¶
constant: flo:greatest-normal-exponent-base-10 ¶
constant: flo:least-normal-exponent-base-e ¶
constant: flo:least-normal-exponent-base-2 ¶
constant: flo:least-normal-exponent-base-10 ¶
constant: flo:least-subnormal-exponent-base-e ¶
constant: flo:least-subnormal-exponent-base-2 ¶
constant: flo:least-subnormal-exponent-base-10 ¶

  Least and greatest exponents of normal and subnormal floating-point numbers, as floating-point numbers. For example, `flo:greatest-normal-exponent-base-2` is the greatest floating-point number such that `(expt 2. flo:greatest-normal-exponent-base-2)` does not overflow and is a normal floating-point number.

procedure: flo:total< x1 x2 ¶
procedure: flo:total-mag< x1 x2 ¶
procedure: flo:total-order x1 x2 ¶
procedure: flo:total-order-mag x1 x2 ¶

  These procedures implement the IEEE 754-2008 total ordering on floating-point values and their magnitudes. Here the “magnitude” of a floating-point value is a floating-point value with positive sign bit and everything else the same; e.g., `+nan.123` is the “magnitude” of `-nan.123` and `0.0` is the “magnitude” of `-0.0`.

  The total ordering has little to no numerical meaning and should be used only when an arbitrary choice of total ordering is required for some non-numerical reason.

  - `Flo:total<` returns true if x1 precedes x2.
  - `Flo:total-mag<` returns true if the magnitude of x1 precedes the magnitude of x2.
  - `Flo:total-order` returns -1 if x1 precedes x2, 0 if they are the same floating-point value (including sign of zero, or sign and payload of NaN), and +1 if x1 follows x2.
  - `Flo:total-order-mag` returns -1 if the magnitude of x1 precedes the magnitude of x2, etc.

procedure: flo:make-nan negative? quiet? payload ¶
procedure: flo:nan-quiet? nan ¶
procedure: flo:nan-payload nan ¶

  `Flo:make-nan` creates a NaN given the sign bit, quiet bit, and payload. Negative? and quiet? must be booleans, and payload must be an unsigned (p-2)-bit integer, where p is the floating-point precision. If quiet? is false, payload must be nonzero.

      (flo:sign-negative? (flo:make-nan negative? quiet? payload))
                                     ⇒ negative?
      (flo:nan-quiet? (flo:make-nan negative? quiet? payload))
                                     ⇒ quiet?
      (flo:nan-payload (flo:make-nan negative? quiet? payload))
                                     ⇒ payload

      (flo:make-nan #t #f 42)        ⇒ -snan.42
      (flo:sign-negative? +nan.123)  ⇒ #f
      (flo:quiet? +nan.123)          ⇒ #t
      (flo:payload +nan.123)         ⇒ 123

#### 4.7.3 Floating-Point Environment

The IEEE 754-2008 computation model includes a persistent rounding mode, exception flags, and exception-handling modes. In MIT/GNU Scheme, the floating-point environment is per-thread. However, because saving and restoring the floating-point environment is expensive, it is maintained only for those threads that have touched the floating-point environment explicitly, either:

- during a procedure such as `flo:with-exceptions-trapped` that establishes a change to the floating-point environment for a dynamic extent, or
- after `flo:set-environment!` to a non-default environment (but not after `flo:set-environment!` to the default environment), or
- after various other procedures such as `flo:clear-exceptions!` that explicitly change the floating-point environment.

The default environment is as in IEEE 754-2008: no exceptions are trapped, and rounding is to nearest with ties broken to even. The set of exception flags in the default environment is indeterminate — callers must enter a per-thread environment, e.g. by calling `flo:clear-exceptions!`, before acting on the exception flags. Like the default environment, a per-thread environment initially has no exceptions trapped and rounds to nearest with ties to even.

A floating-point environment descriptor is a machine-dependent object representing the IEEE 754-2008 floating-point rounding mode, exception flags, and exception-handling mode. Users should not inspect a floating-point environment descriptor other than to use it with the procedures here; its representation may vary from system to system.

procedure: flo:default-environment ¶

  Returns a descriptor for the default environment, with no exceptions trapped and round-to-nearest/ties-to-even.

procedure: flo:environment ¶
procedure: flo:set-environment! floenv ¶
procedure: flo:update-environment! floenv ¶

  `Flo:environment` returns a descriptor for the current floating-point environment. `Flo:set-environment!` replaces the current floating-point environment by floenv. `Flo:update-environment!` does likewise, but re-raises any exceptions that were already raised in the current floating-point environment, which may cause a trap if floenv also traps them.

  `Flo:update-environment!` is usually used together with `flo:defer-exception-traps!` to defer potentially trapping on exceptions in a large intermediate computation until the end.

procedure: flo:preserving-environment thunk ¶

  Saves the current floating-point environment if any and calls thunk. On exit from thunk, including non-local exit, saves thunk’s floating-point environment and restores the original floating-point environment as if with `flo:set-environment!`. On re-entry into thunk, restores thunk’s floating-point environment.

  Note: `Flo:preserving-environment` does not enter a per-thread environment. If the current thread is in the default environment, the exception flags are indeterminate, and remain so inside `flo:preserving-environment`. Callers interested in using the exception flags should start inside `flo:preserving-environment` by clearing them with `flo:clear-exceptions!`.

#### 4.7.4 Floating-Point Exceptions

In IEEE 754-2008, floating-point operations such as arithmetic may raise exceptions. This sets a flag in the floating-point environment that is maintained until it is cleared. Many machines can also be configured to trap on exceptions, which in Scheme leads to signalling a condition. (Not all CPUs support trapping exceptions — for example, most ARMv8 CPUs do not.) In the default environment, no exceptions are trapped.

Floating-point exceptions and sets of floating-point exceptions are represented by small integers, whose interpretation is machine-dependent — for example, the invalid-operation exception may be represented differently on PowerPC and AMD x86-64 CPUs. The number for a floating-point exception is the same as the number for a set of exceptions containing only that one; the bitwise-AND of two sets is their intersection, the bitwise-IOR is their union, etc. The procedures `flo:exceptions->names` and `flo:names->exceptions` convert between machine-dependent integer representations and machine-independent lists of human-readable symbols.

The following exceptions are recognized by MIT/GNU Scheme:

inexact-result ¶

  Raised when the result of a floating-point computation is not a floating-point number and therefore must be rounded.

  The inexact-result exception is never trappable in MIT/GNU Scheme.

underflow ¶

  Raised when the result of a floating-point computation is too small in magnitude to be represented by a normal floating-point number, and is therefore rounded to a subnormal or zero.

overflow ¶

  Raised when the result of a floating-point computation is too large in magnitude to be represented by a floating-point number, and is therefore rounded to infinity.

divide-by-zero ¶

  Raised on division of a nonzero finite real number by a zero real number, or logarithm of zero, or other operation that has an unbounded limit at a point like division by a divisor approaching zero.

invalid-operation ¶

  Raised when the input to a floating-point computation is nonsensical, such as division of zero by zero, or real logarithm of a negative number. The result of an invalid-operation is a NaN. Also raised when the input to a floating-point operation is a signalling NaN, but not for a quiet NaN.

subnormal-operand ¶

  Raised when an operand in a floating-point operation is subnormal.

  (This is not a standard IEEE 754-2008 exception. It is supported by Intel CPUs.)

procedure: flo:supported-exceptions ¶

  Returns the set of exceptions that are supported on the current machine.

procedure: flo:exception:divide-by-zero ¶
procedure: flo:exception:inexact-result ¶
procedure: flo:exception:invalid-operation ¶
procedure: flo:exception:overflow ¶
procedure: flo:exception:subnormal-operand ¶
procedure: flo:exception:underflow ¶

  Returns the specified floating-point exception number. On machines that do not support a particular exception, the corresponding procedure simply returns `0`.

procedure: flo:exceptions->names excepts ¶
procedure: flo:names->exceptions list ¶

  These procedures convert between a machine-dependent small integer representation of a set of exceptions, and a representation of a set of exceptions by a list of human-readable symbols naming them.

      (flo:preserving-environment
       (lambda ()
         (flo:clear-exceptions! (flo:supported-exceptions))
         (flo:/ (identity-procedure 1.) 0.)
         (flo:exceptions->names
          (flo:test-exceptions (flo:supported-exceptions)))))
                                     ⇒ (divide-by-zero)

procedure: flo:test-exceptions excepts ¶

  Returns the set of exceptions in excepts that are currently raised.

  In the default environment, the result is indeterminate, and may be affected by floating-point operations in other threads.

procedure: flo:clear-exceptions! excepts ¶
procedure: flo:raise-exceptions! excepts ¶

  Clears or raises the exceptions in excepts, entering a per-thread environment. Other exceptions are unaffected.

procedure: flo:save-exception-flags ¶
procedure: flo:restore-exception-flags! exceptflags ¶
procedure: flo:test-exception-flags exceptflags excepts ¶

  `Flo:save-exception-flags` returns a machine-dependent representation of the currently trapped and raised exceptions. `Flo:restore-exception-flags!` restores it, entering a per-thread environment. `Flo:test-exception-flags` returns the set of exceptions in excepts that are raised in exceptflags.

  Exceptflags is not the same as a set of exceptions. It is opaque and machine-dependent and should not be used except with `flo:restore-exception-flags!` and `flo:test-exception-flags`.

  Bug: `Flo:test-exception-flags` is unimplemented.

procedure: flo:have-trap-enable/disable? ¶

  Returns true if trapping floating-point exceptions is supported on this machine.

procedure: flo:default-trapped-exceptions ¶

  Returns the set of exceptions that are trapped in the default floating-point environment. Equivalent to `(flo:names->exceptions '())`, or simply `0`, since by default, no exceptions are trapped.

procedure: flo:trapped-exceptions ¶

  Returns the set of exceptions that are currently trapped.

procedure: flo:trap-exceptions! excepts ¶
procedure: flo:untrap-exceptions! excepts ¶
procedure: flo:set-trapped-exceptions! excepts ¶

  `Flo:trap-exceptions!` requests that any exceptions in the set excepts be trapped, in addition to all of the ones that are currently trapped. `Flo:untrap-exceptions!` requests that any exceptions in the set excepts not be trapped. `Flo:set-trapped-exceptions!` replaces the set of trapped exceptions altogether by excepts. All three procedures enter a per-thread environment.

      (define (flo:trap-exceptions! excepts)
        (flo:set-trapped-exceptions!
         (fix:or (flo:trapped-exceptions) excepts)))

      (define (flo:untrap-exceptions! excepts)
        (flo:set-trapped-exceptions!
         (fix:andc (flo:trapped-exceptions) excepts)))

      (define (flo:set-trapped-exceptions! excepts)
        (flo:trap-exceptions! excepts)
        (flo:untrap-exceptions!
         (fix:andc (flo:supported-exceptions) excepts)))

procedure: flo:with-exceptions-trapped excepts thunk ¶
procedure: flo:with-exceptions-untrapped excepts thunk ¶
procedure: flo:with-trapped-exceptions excepts thunk ¶

  Dynamic-extent analogues of `flo:trap-exceptions!`, `flo:untrap-exceptions!`, and `flo:set-trapped-exceptions!`. These call thunk with their respective changes to the set of trapped exceptions in a per-thread environment, and restore the environment on return or non-local exit.

procedure: flo:defer-exception-traps! ¶

  Saves the current floating-point environment, clears all raised exceptions, disables all exception traps, and returns a descriptor for the saved floating-point environment.

  `Flo:defer-exception-traps!` is typically used together with `flo:update-environment!`, to trap any exceptions that the caller had wanted trapped only after a long intermediate computation. This pattern is captured in `flo:deferring-exception-traps`.

procedure: flo:deferring-exception-traps thunk ¶

  Calls thunk, but defers trapping on any exceptions it raises until it returns. Equivalent to:

      (flo:preserving-environment
       (lambda ()
         (let ((environment (flo:defer-exception-traps!)))
           (begin0 (thunk)
             (flo:update-environment! environment)))))

procedure: flo:ignoring-exception-traps thunk ¶

  Calls thunk with all exceptions untrapped and unraised. Equivalent to:

      (flo:preserving-environment
       (lambda ()
         (flo:defer-exception-traps!)
         (thunk)))

#### 4.7.5 Floating-Point Rounding Mode

IEEE 754-2008 supports four rounding modes, which determine the answer given by a floating-point computation when the exact result lies between two floating-point numbers but is not a floating-point number itself:

`to-nearest`

  Round to the nearest floating-point number. If there are two equidistant ones, choose the one whose least significant digit is even. Also known as “round-to-nearest/ties-to-even”.

`toward-zero`

  Round to the floating-point number closest to zero.

`downward`

  Round to the greatest floating-point number below.

`upward`

  Round to the least floating-point number above.

Warning: Not all procedures in MIT/GNU Scheme respect the rounding mode. Only the basic arithmetic operations — +, -, *, /, and sqrt — will reliably respect it. The main purpose of changing the rounding mode is to diagnose numerical instability by injecting small perturbations throughout the computation.

Bug: It would be nice if we had “round-to-odd”, where any inexact result is rounded to the nearest odd floating-point number, for implementing “doubled”-precision algorithms. But we don’t. Sorry.

procedure: flo:default-rounding-mode ¶

  Returns a symbol for the default rounding mode, which is always `to-nearest`.

procedure: flo:rounding-modes ¶

  Returns a list of the supported rounding modes as symbols.

procedure: flo:rounding-mode ¶
procedure: flo:set-rounding-mode! mode ¶

  Gets or sets the current rounding mode as a symbol, entering a per-thread environment.

procedure: flo:with-rounding-mode mode thunk ¶

  Call thunk in a per-thread environment with the rounding mode set to mode. On return, the floating-point environment, including rounding mode, is restored to what it was before.

  Non-local exit from and re-entrance to thunk behaves as if the call is surrounded by `flo:preserving-environment` (see Floating-Point Environment).

### 4.8 Random Number Generation

MIT/GNU Scheme provides a facility for random number generation. The current implementation uses the ChaCha stream cipher, reseeding itself at each request so that past outputs cannot be distinguished from uniform random even if the state of memory is compromised in the future.

The interface described here is a mixture of the Common Lisp and SRFI 27 systems.

procedure: random m [state] ¶

  The argument m must be either an exact positive integer, or an inexact positive real.

  - If m is an exact positive integer, then `random` returns an exact nonnegative integer below m with uniform distribution.
  - If m is an inexact positive real, then `random` returns an inexact real in the interval [0, m) with uniform distribution.

  If state is given and not `#f`, it must be a random-state object; otherwise, it defaults to the `default-random-source`. This object is used to maintain the state of the pseudorandom number generator and is altered as a side effect of the `random` procedure.

  Use of the default random state requires synchronization between threads, so it is better for multithreaded programs to use explicit states.

      (random 1.0)    ⇒ .32744744667719056
      (random 1.0)    ⇒ .01668326768172354
      (random 10)     ⇒ 3
      (random 10)     ⇒ 8
      (random 100)    ⇒ 38
      (random 100)    ⇒ 63

procedure: flo:random-unit-closed state ¶
procedure: flo:random-unit-open state ¶

  State must be a random-state object. `Flo:random-unit-closed` returns a flonum in the closed interval [0,1] with uniform distribution. In practical terms, the result is in the half-closed interval (0,1] because the probability of returning 0 is 2^{-1075}, far below the standard probability 2^{-128} that means “never” in cryptographic engineering terms.

  `Flo:random-unit-open` is like `flo:random-unit-closed`, but it explicitly rejects `0.0` and `1.0` as outputs, so that the result is a floating-point number in the open interval (0,1). `(flo:random-unit-open)` is equivalent `(random 1.)`, except that it is faster.

  Callers should generally use `flo:random-unit-closed`, because for the uniform distribution on the interval [0,1] of real numbers, the probability of a real number that is rounded to the floating-point `1.0` is the small but nonnegligible 2^{-54}, and arithmetic downstream should be prepared to handle results that are rounded to `1.0` much more readily than results that are rounded to `0.0` — in other words, a requirement to use `flo:random-unit-open` is evidence of bad numerics downstream.

procedure: flo:random-unit state ¶

  Deprecated alias for `flo:random-unit-open`.

procedure: random-bytevector n [state] ¶

  Returns a bytevector of n bytes drawn independently uniformly at random from state.

procedure: random-bytevector! bytevector [start end state] ¶

  Replaces the bytes in bytevector from start to end by bytes drawn independently uniformly at random from state.

The next three definitions concern random-state objects. In addition to these definitions, it is important to know that random-state objects are specifically designed so that they can be saved to disk using the `fasdump` procedure, and later restored using the `fasload` procedure. This allows a particular random-state object to be saved in order to replay a particular pseudorandom sequence.

variable: *random-state* ¶

  This variable is deprecated; pass an explicit state instead.

procedure: make-random-state [state] ¶

  This procedure returns a new random-state object, suitable for use as as the state argument to `random`. If state is not given or `#f`, `make-random-state` returns a copy of `default-random-source`. If state is a random-state object, a copy of that object is returned. If state is `#t`, then a new random-state object is returned that has been “randomly” initialized by some means (such as by a time-of-day clock).

procedure: random-state? object ¶

  Returns `#t` if object is a random-state object, otherwise returns `#f`.

procedure: export-random-state state ¶
procedure: import-random-state state ¶

  `Export-random-state` returns an external representation of a random state — an object that can be safely read and written with `read` and `write`, consisting only of nested lists, vectors, symbols, and small exact integers. `Import-random-state` creates a random state from its external representation.

In the MIT/GNU Scheme implementation of the SRFI 27 API, a “random source” happens to be the same as a random state, but users should not rely on this.

procedure: make-random-source ¶

  [SRFI 27] Returns a random source. Every random source created by `make-random-source` returns the same sequence of outputs unless modified by `random-source-state-set!`, `random-source-randomize!`, or `random-source-pseudo-randomize!`.

procedure: random-source? object ¶

  [SRFI 27] Returns `#t` if object is a random source, otherwise returns `#f`.

constant: default-random-source ¶

  [SRFI 27] The default random source, used by the various random procedures if no explicit state is specified and `*random-state*` is false.

procedure: random-source-state-ref source ¶
procedure: random-source-state-set! source exported-state ¶

  [SRFI 27] `Random-source-state-ref` returns an external representation of a random source — an object that can be safely read and written with `read` and `write`, consisting only of nested lists, vectors, symbols, and small exact integers. `Random-source-state-set!` replaces the innards of source by the source represented by exported-state from `random-source-state-ref`.

procedure: random-source-randomize! source ¶

  [SRFI 27] Loads entropy from the environment into source so that its subsequent outputs are nondeterministic.

  Warning: Most implementations of SRFI 27 do not make subsequent outputs unpredictable with cryptography, so don’t rely on this.

procedure: random-source-pseudo-randomize! source i j ¶

  [SRFI 27] The arguments i and j must be exact nonnegative integers below 2^{128}. This procedure sets source to generate one of 2^{256} distinct possible streams of output, so that if i and j are chosen uniformly at random, it is hard to distinguish the outputs of the source from uniform random.

  Warning: Most implementations of SRFI 27 do not make it hard to distinguish the outputs of the source from uniform random even if the indices i and j are uniform random, so don’t rely on this.

procedure: random-integer n ¶

  [SRFI 27] Returns an exact nonnegative integer below n chosen uniformly at random.

  Equivalent to:

      ((random-source-make-integers default-random-source) n)

procedure: random-real ¶

  [SRFI 27] Returns an inexact real in the open interval (0, 1) with uniform distribution.

  Equivalent to:

      ((random-source-make-reals default-random-source))

procedure: random-source-make-integers source ¶

  [SRFI 27] Returns a procedure of one argument, n, that deterministically draws from source a exact nonnegative integer below n with uniform distribution.

procedure: random-source-make-reals source [unit] ¶

  [SRFI 27] Returns a procedure of zero arguments that deterministically draws from source an inexact real in the interval (0,1) with uniform distribution. If unit is specified, the results are instead uniform random integral multiples of unit in (0,1) and of the same exactness as unit.

## 5 Characters

Characters are objects that represent printed characters such as letters and digits. MIT/GNU Scheme supports the full Unicode character repertoire.

Characters are written using the notation `#\character` or `#\character-name` or `#\xhex-scalar-value`.

The following standard character names are supported:

    #\alarm                 ; U+0007
    #\backspace             ; U+0008
    #\delete                ; U+007F
    #\escape                ; U+001B
    #\newline               ; the linefeed character, U+000A
    #\null                  ; the null character, U+0000
    #\return                ; the return character, U+000D
    #\space                 ; the preferred way to write a space, U+0020
    #\tab                   ; the tab character, U+0009

Here are some additional examples:

    #\a                     ; lowercase letter
    #\A                     ; uppercase letter
    #\(                     ; left parenthesis
    #\                      ; the space character

Case is significant in `#\character`, and in `#\character-name`, but not in `#\xhex-scalar-value`. If character in `#\character` is alphabetic, then any character immediately following character cannot be one that can appear in an identifier. This rule resolves the ambiguous case where, for example, the sequence of characters ‘`#\space`’ could be taken to be either a representation of the space character or a representation of the character ‘`#\s`’ followed by a representation of the symbol ‘`pace`’.

Characters written in the `#\` notation are self-evaluating. That is, they do not have to be quoted in programs.

Some of the procedures that operate on characters ignore the difference between upper case and lower case. The procedures that ignore case have ‘`-ci`’ (for “case insensitive”) embedded in their names.

MIT/GNU Scheme allows a character name to include one or more bucky bit prefixes to indicate that the character includes one or more of the keyboard shift keys Control, Meta, Super, or Hyper (note that the Control bucky bit prefix is not the same as the ASCII control key). The bucky bit prefixes and their meanings are as follows (case is not significant):

    Key             Bucky bit prefix        Bucky bit
    ---             ----------------        ---------

    Meta            M- or Meta-                 1
    Control         C- or Control-              2
    Super           S- or Super-                4
    Hyper           H- or Hyper-                8

For example,

    #\c-a                   ; Control-a
    #\meta-b                ; Meta-b
    #\c-s-m-h-A             ; Control-Meta-Super-Hyper-A

procedure: char->name char ¶

  Returns a string corresponding to the printed representation of char. This is the character, character-name, or `xhex-scalar-value` component of the external representation, combined with the appropriate bucky bit prefixes.

      (char->name #\a)                        ⇒  "a"
      (char->name #\space)                    ⇒  "space"
      (char->name #\c-a)                      ⇒  "C-a"
      (char->name #\control-a)                ⇒  "C-a"

procedure: name->char string ¶

  Converts a string that names a character into the character specified. If string does not name any character, `name->char` signals an error.

      (name->char "a")                        ⇒  #\a
      (name->char "space")                    ⇒  #\space
      (name->char "SPACE")                    ⇒  #\space
      (name->char "c-a")                      ⇒  #\C-a
      (name->char "control-a")                ⇒  #\C-a

standard procedure: char? object ¶

  Returns `#t` if object is a character, otherwise returns `#f`.

standard procedure: char=? char1 char2 char3 … ¶
standard procedure: char<? char1 char2 char3 … ¶
standard procedure: char>? char1 char2 char3 … ¶
standard procedure: char<=? char1 char2 char3 … ¶
standard procedure: char>=? char1 char2 char3 … ¶

  These procedures return `#t` if the results of passing their arguments to `char->integer` are respectively equal, monotonically increasing, monotonically decreasing, monotonically non-decreasing, or monotonically non-increasing.

  These predicates are transitive.

char library procedure: char-ci=? char1 char2 char3 … ¶
char library procedure: char-ci<? char1 char2 char3 … ¶
char library procedure: char-ci>? char1 char2 char3 … ¶
char library procedure: char-ci<=? char1 char2 char3 … ¶
char library procedure: char-ci>=? char1 char2 char3 … ¶

  These procedures are similar to `char=?` et cetera, but they treat upper case and lower case letters as the same. For example, `(char-ci=? #\A #\a)` returns `#t`.

  Specifically, these procedures behave as if `char-foldcase` were applied to their arguments before they were compared.

char library procedure: char-alphabetic? char ¶
char library procedure: char-numeric? char ¶
char library procedure: char-whitespace? char ¶
char library procedure: char-upper-case? char ¶
char library procedure: char-lower-case? char ¶

  These procedures return `#t` if their arguments are alphabetic, numeric, whitespace, upper case, or lower case characters respectively, otherwise they return `#f`.

  Specifically, they return `#t` when applied to characters with the Unicode properties Alphabetic, Numeric_Decimal, White_Space, Uppercase, or Lowercase respectively, and `#f` when applied to any other Unicode characters. Note that many Unicode characters are alphabetic but neither upper nor lower case.

procedure: char-alphanumeric? char ¶

  Returns `#t` if char is either alphabetic or numeric, otherwise it returns `#f`.

char library procedure: digit-value char ¶

  This procedure returns the numeric value (0 to 9) of its argument if it is a numeric digit (that is, if `char-numeric?` returns `#t`), or `#f` on any other character.

      (digit-value #\3) ⇒ 3
      (digit-value #\x0664) ⇒ 4
      (digit-value #\x0AE6) ⇒ 0
      (digit-value #\x0EA6) ⇒ #f

standard procedure: char->integer char ¶
standard procedure: integer->char n ¶

  Given a Unicode character, `char->integer` returns an exact integer between `0` and `#xD7FF` or between `#xE000` and `#x10FFFF` which is equal to the Unicode scalar value of that character. Given a non-Unicode character, it returns an exact integer greater than `#x10FFFF`.

  Given an exact integer that is the value returned by a character when `char->integer` is applied to it, `integer->char` returns that character.

  Implementation note: MIT/GNU Scheme allows any Unicode code point, not just scalar values.

  Implementation note: If the argument to `char->integer` or `integer->char` is a constant, the MIT/GNU Scheme compiler will constant-fold the call, replacing it with the corresponding result. This is a very useful way to denote unusual character constants or ASCII codes.

char library procedure: char-upcase char ¶
char library procedure: char-downcase char ¶
char library procedure: char-foldcase char ¶

  The `char-upcase` procedure, given an argument that is the lowercase part of a Unicode casing pair, returns the uppercase member of the pair. Note that language-sensitive casing pairs are not used. If the argument is not the lowercase member of such a pair, it is returned.

  The `char-downcase` procedure, given an argument that is the uppercase part of a Unicode casing pair, returns the lowercase member of the pair. Note that language-sensitive casing pairs are not used. If the argument is not the uppercase member of such a pair, it is returned.

  The `char-foldcase` procedure applies the Unicode simple case-folding algorithm to its argument and returns the result. Note that language-sensitive folding is not used. See UAX #44 (part of the Unicode Standard) for details.

  Note that many Unicode lowercase characters do not have uppercase equivalents.

procedure: char->digit char [radix] ¶

  If char is a character representing a digit in the given radix, returns the corresponding integer value. If radix is specified (which must be an exact integer between 2 and 36 inclusive), the conversion is done in that base, otherwise it is done in base 10. If char doesn’t represent a digit in base radix, `char->digit` returns `#f`.

  Note that this procedure is insensitive to the alphabetic case of char.

      (char->digit #\8)                       ⇒  8
      (char->digit #\e 16)                    ⇒  14
      (char->digit #\e)                       ⇒  #f

procedure: digit->char digit [radix] ¶

  Returns a character that represents digit in the radix given by radix. The radix argument, if given, must be an exact integer between 2 and 36 (inclusive); it defaults to 10. The digit argument must be an exact non-negative integer strictly less than radix.

      (digit->char 8)                         ⇒  #\8
      (digit->char 14 16)                     ⇒  #\E

### 5.1 Character implementation

An MIT/GNU Scheme character consists of a code part and a bucky bits part. The code part is a Unicode code point, while the bucky bits are an additional set of bits representing shift keys available on some keyboards.

There are 4 bucky bits, named control, meta, super, and hyper. On GNU/Linux systems running a graphical desktop, the control bit corresponds to the `CTRL` key; the meta bit corresponds to the `ALT` key; and the super bit corresponds to the “windows” key. On macOS, these are the `CONTROL`, `OPTION`, and `COMMAND` keys respectively.

Characters with bucky bits are not used much outside of graphical user interfaces (e.g. Edwin). They cannot be stored in strings or character sets, and aren’t read or written by textual I/O ports.

procedure: make-char code bucky-bits ¶

  Builds a character from code and bucky-bits. The value of code must be a Unicode code point; the value of bucky-bits must be an exact non-negative integer strictly less than `16`. If `0` is specified for bucky-bits, `make-char` produces an ordinary character; otherwise, the appropriate bits are set as follows:

      1               meta
      2               control
      4               super
      8               hyper

  For example,

      (make-char 97 0)                        ⇒  #\a
      (make-char 97 1)                        ⇒  #\M-a
      (make-char 97 2)                        ⇒  #\C-a
      (make-char 97 3)                        ⇒  #\C-M-a

procedure: char-code char ¶

  Returns the Unicode code point of char. Note that if char has no bucky bits set, then this is the same value returned by `char->integer`.

  For example,

      (char-code #\a)                         ⇒  97
      (char-code #\c-a)                       ⇒  97

procedure: char-bits char ¶

  Returns the exact integer representation of char’s bucky bits. For example,

      (char-bits #\a)                         ⇒  0
      (char-bits #\m-a)                       ⇒  1
      (char-bits #\c-a)                       ⇒  2
      (char-bits #\c-m-a)                     ⇒  3

constant: char-code-limit ¶

  This constant is the strict upper limit on a character’s code value. It is `#x110000` unless some future version of Unicode increases the range of code points.

constant: char-bits-limit ¶

  This constant is the strict upper limit on a character’s bucky-bits value. It is currently `#x10` and unlikely to change in the future.

procedure: bitless-char? object ¶

  Returns `#t` if object is a character with no bucky bits set, otherwise it returns `#f` .

procedure: char->bitless-char char ¶

  Returns char with any bucky bits removed. The result is guaranteed to satisfy `bitless-char?`.

procedure: char-predicate char ¶

  Returns a procedure of one argument that returns `#t` if its argument is a character `char=?` to char, otherwise it returns `#f`.

procedure: char-ci-predicate char ¶

  Returns a procedure of one argument that returns `#t` if its argument is a character `char-ci=?` to char, otherwise it returns `#f`.

### 5.2 Unicode

MIT/GNU Scheme implements the full Unicode character repertoire, defining predicates for Unicode characters and their associated integer values. A Unicode code point is an exact non-negative integer strictly less than `#x110000`. A Unicode scalar value is a Unicode code point that doesn’t fall between `#xD800` inclusive and `#xE000` exclusive; in other words, any Unicode code point except for the surrogate code points.

procedure: unicode-code-point? object ¶

  Returns `#t` if object is a Unicode code point, otherwise it returns `#f`.

procedure: unicode-scalar-value? object ¶

  Returns `#t` if object is a Unicode scalar value, otherwise it returns `#f`.

procedure: unicode-char? object ¶

  Returns `#t` if object is any character corresponding to a Unicode code point, except for those with general category `other:surrogate` or `other:not-assigned`.

procedure: char-general-category char ¶
procedure: code-point-general-category code-point ¶

  Returns the Unicode general category of char (or code-point) as a descriptive symbol:

  Category | Symbol
  --- | ---
  Lu | `letter:uppercase`
  Ll | `letter:lowercase`
  Lt | `letter:titlecase`
  Lm | `letter:modifier`
  Lo | `letter:other`
  Mn | `mark:nonspacing`
  Mc | `mark:spacing-combining`
  Me | `mark:enclosing`
  Nd | `number:decimal-digit`
  Nl | `number:letter`
  No | `number:other`
  Pc | `punctuation:connector`
  Pd | `punctuation:dash`
  Ps | `punctuation:open`
  Pe | `punctuation:close`
  Pi | `punctuation:initial-quote`
  Pf | `punctuation:final-quote`
  Po | `punctuation:other`
  Sm | `symbol:math`
  Sc | `symbol:currency`
  Sk | `symbol:modifier`
  So | `symbol:other`
  Zs | `separator:space`
  Zl | `separator:line`
  Zp | `separator:paragraph`
  Cc | `other:control`
  Cf | `other:format`
  Cs | `other:surrogate`
  Co | `other:private-use`
  Cn | `other:not-assigned`

### 5.3 Character Sets

MIT/GNU Scheme’s character-set abstraction is used to represent groups of characters, such as the letters or digits. A character set may contain any character. Alternatively, a character set can be treated as a set of code points.

Implementation note: MIT/GNU Scheme allows any “bitless” character to be stored in a character set; operations that accept characters automatically strip their bucky bits.

procedure: char-set? object ¶

  Returns `#t` if object is a character set, otherwise it returns `#f`.

procedure: char-in-set? char char-set ¶

  Returns `#t` if char is in char-set, otherwise it returns `#f`.

procedure: code-point-in-set? code-point char-set ¶

  Returns `#t` if code-point is in char-set, otherwise it returns `#f`.

procedure: char-set-predicate char-set ¶

  Returns a procedure of one argument that returns `#t` if its argument is a character in char-set, otherwise it returns `#f`.

procedure: compute-char-set predicate ¶

  Calls predicate once on each Unicode code point, and returns a character set containing exactly the code points for which predicate returns a true value.

The next procedures represent a character set as a code-point list, which is a list of code-point range elements. A code-point range is either a Unicode code point, or a pair `(start . end)` that specifies a contiguous range of code points. Both start and end must be exact nonnegative integers less than or equal to `#x110000`, and start must be less than or equal to end. The range specifies all of the code points greater than or equal to start and strictly less than end.

procedure: char-set element … ¶
procedure: char-set* elements ¶

  Returns a new character set consisting of the characters specified by elements. The procedure `char-set` takes these elements as multiple arguments, while `char-set*` takes them as a single list-valued argument; in all other respects these procedures are identical.

  An element can take several forms, each of which specifies one or more characters to include in the resulting character set: a character includes itself; a string includes all of the characters it contains; a character set includes its members; or a code-point range includes the corresponding characters.

  In addition, an element may be a symbol from the following table, which represents the characters as shown:

  Name | Unicode character specification
  --- | ---
  `alphabetic` | Alphabetic = True
  `alphanumeric` | Alphabetic = True | Numeric_Type = Decimal
  `cased` | Cased = True
  `lower-case` | Lowercase = True
  `numeric` | Numeric_Type = Decimal
  `unicode` | General_Category != (Cs | Cn)
  `upper-case` | Uppercase = True
  `whitespace` | White_Space = True

procedure: char-set->code-points char-set ¶

  Returns a code-point list specifying the contents of char-set. The returned list consists of numerically sorted, disjoint, and non-abutting code-point ranges.

procedure: char-set=? char-set-1 char-set-2 ¶

  Returns `#t` if char-set-1 and char-set-2 contain exactly the same characters, otherwise it returns `#f`.

procedure: char-set-invert char-set ¶

  Returns a character set that’s the inverse of char-set. That is, the returned character set contains exactly those characters that aren’t in char-set.

procedure: char-set-union char-set … ¶
procedure: char-set-intersection char-set … ¶
procedure: char-set-difference char-set-1 char-set … ¶

  These procedures compute the respective set union, set intersection, and set difference of their arguments.

procedure: char-set-union* char-sets ¶
procedure: char-set-intersection* char-sets ¶

  These procedures correspond to `char-set-union` and `char-set-intersection` but take a single argument that’s a list of character sets rather than multiple character-set arguments.

constant: char-set:alphabetic ¶
constant: char-set:numeric ¶
constant: char-set:whitespace ¶
constant: char-set:upper-case ¶
constant: char-set:lower-case ¶
constant: char-set:alphanumeric ¶

  These constants are the character sets corresponding to `char-alphabetic?`, `char-numeric?`, `char-whitespace?`, `char-upper-case?`, `char-lower-case?`, and `char-alphanumeric?` respectively.

procedure: 8-bit-char-set? char-set ¶

  Returns `#t` if char-set contains only 8-bit code points (i.e.. ISO 8859-1 characters), otherwise it returns `#f`.

## 6 Strings

Strings are sequences of characters. Strings are written as sequences of characters enclosed within quotation marks (`"`). Within a string literal, various escape sequences represent characters other than themselves. Escape sequences always start with a backslash (`\`):

    \a : alarm, U+0007
    \b : backspace, U+0008
    \t : character tabulation, U+0009
    \n : linefeed, U+000A
    \r : return, U+000D
    \" : double quote, U+0022
    \\ : backslash, U+005C
    \| : vertical line, U+007C
    \intraline-whitespace* line-ending intraline-whitespace*
         : nothing
    \xhex-scalar-value;
         : specified character (note the terminating semi-colon).

The result is unspecified if any other character in a string occurs after a backslash.

Except for a line ending, any character outside of an escape sequence stands for itself in the string literal. A line ending which is preceded by `\`intraline-whitespace expands to nothing (along with any trailing intraline whitespace), and can be used to indent strings for improved legibility. Any other line ending has the same effect as inserting a `\n` character into the string.

Examples:

    "The word \"recursion\" has many meanings."
    "Another example:\ntwo lines of text"
    "Here's text \
       containing just one line"
    "\x03B1; is named GREEK SMALL LETTER ALPHA."

The length of a string is the number of characters that it contains. This number is an exact, non-negative integer that is fixed when the string is created. The valid indexes of a string are the exact non-negative integers less than the length of the string. The first character of a string has index 0, the second has index 1, and so on.

Some of the procedures that operate on strings ignore the difference between upper and lower case. The names of the versions that ignore case end with ‘`-ci`’ (for “case insensitive”).

Implementations may forbid certain characters from appearing in strings. However, with the exception of `#\null`, ASCII characters must not be forbidden. For example, an implementation might support the entire Unicode repertoire, but only allow characters U+0001 to U+00FF (the Latin-1 repertoire without `#\null`) in strings.

Implementation note: MIT/GNU Scheme allows any “bitless” character to be stored in a string. In effect this means any character with a Unicode code point, including surrogates. String operations that accept characters automatically strip their bucky bits.

It is an error to pass such a forbidden character to `make-string`, `string`, `string-set!`, or `string-fill!`, as part of the list passed to `list->string`, or as part of the vector passed to `vector->string`, or in UTF-8 encoded form within a bytevector passed to `utf8->string`. It is also an error for a procedure passed to `string-map` to return a forbidden character, or for `read-string` to attempt to read one.

MIT/GNU Scheme supports both mutable and immutable strings. Procedures that mutate strings, in particular `string-set!` and `string-fill!`, will signal an error if given an immutable string. Nearly all procedures that return strings return immutable strings; notable exceptions are `make-string` and `string-copy`, which always return mutable strings, and `string-builder` which gives the programmer the ability to choose mutable or immutable results.

standard procedure: string? obj ¶

  Returns `#t` if obj is a string, otherwise returns `#f`.

standard procedure: make-string k [char] ¶

  The `make-string` procedure returns a newly allocated mutable string of length k. If char is given, then all the characters of the string are initialized to char, otherwise the contents of the string are unspecified.

extended standard procedure: string object … ¶
procedure: string* objects ¶

  Returns an immutable string whose characters are the concatenation of the characters from the given objects. Each object is converted to characters as if passed to the `display` procedure.

  This is an MIT/GNU Scheme extension to the standard `string` that accepts only characters as arguments.

  The procedure `string*` is identical to `string` but takes a single argument that’s a list of objects, rather than multiple object arguments.

standard procedure: string-length string ¶

  Returns the number of characters in the given string.

standard procedure: string-ref string k ¶

  It is an error if k is not a valid index of string.

  The `string-ref` procedure returns character k of string using zero-origin indexing. There is no requirement for this procedure to execute in constant time.

standard procedure: string-set! string k char ¶

  It is an error if `string` is not a mutable string or if k is not a valid index of string.

  The `string-set!` procedure stores char in element k of string. There is no requirement for this procedure to execute in constant time.

      (define (f) (make-string 3 #\*))
      (define (g) "***")
      (string-set! (f) 0 #\?)  ⇒  unspecified
      (string-set! (g) 0 #\?)  ⇒  error
      (string-set! (symbol->string 'immutable) 0 #\?)  ⇒  error

standard procedure: string=? string1 string2 string … ¶

  Returns `#t` if all the strings are the same length and contain exactly the same characters in the same positions, otherwise returns `#f`.

char library procedure: string-ci=? string1 string2 string … ¶

  Returns `#t` if, after case-folding, all the strings are the same length and contain the same characters in the same positions, otherwise returns `#f`. Specifically, these procedures behave as if `string-foldcase` were applied to their arguments before comparing them.

standard procedure: string<? string1 string2 string … ¶
char library procedure: string-ci<? string1 string2 string … ¶
standard procedure: string>? string1 string2 string … ¶
char library procedure: string-ci>? string1 string2 string … ¶
standard procedure: string<=? string1 string2 string … ¶
char library procedure: string-ci<=? string1 string2 string … ¶
standard procedure: string>=? string1 string2 string … ¶
char library procedure: string-ci>=? string1 string2 string … ¶

  These procedures return `#t` if their arguments are (respectively): monotonically increasing, monotonically decreasing, monotonically non-decreasing, or monotonically non-increasing.

  These predicates are required to be transitive.

  These procedures compare strings in an implementation-defined way. One approach is to make them the lexicographic extensions to strings of the corresponding orderings on characters. In that case, `string<?` would be the lexicographic ordering on strings induced by the ordering `char<?` on characters, and if the two strings differ in length but are the same up to the length of the shorter string, the shorter string would be considered to be lexicographically less than the longer string. However, it is also permitted to use the natural ordering imposed by the implementation’s internal representation of strings, or a more complex locale-specific ordering.

  In all cases, a pair of strings must satisfy exactly one of `string<?`, `string=?`, and `string>?`, and must satisfy `string<=?` if and only if they do not satisfy `string>?` and `string>=?` if and only if they do not satisfy `string<?`.

  The ‘`-ci`’ procedures behave as if they applied `string-foldcase` to their arguments before invoking the corresponding procedures without ‘`-ci`’.

procedure: string-compare string1 string2 if-eq if-lt if-gt ¶
procedure: string-compare-ci string1 string2 if-eq if-lt if-gt ¶

  If-eq, if-lt, and if-gt are procedures of no arguments (thunks). The two strings are compared; if they are equal, if-eq is applied, if string1 is less than string2, if-lt is applied, else if string1 is greater than string2, if-gt is applied. The value of the procedure is the value of the thunk that is applied.

  `string-compare` distinguishes uppercase and lowercase letters;
  `string-compare-ci` does not.

      (define (cheer) (display "Hooray!"))
      (define (boo)   (display "Boo-hiss!"))
      (string-compare "a" "b"  cheer  (lambda() 'ignore)  boo)
              -|  Hooray!
              ⇒  unspecified

char library procedure: string-upcase string ¶
char library procedure: string-downcase string ¶
procedure: string-titlecase string ¶
char library procedure: string-foldcase string ¶

  These procedures apply the Unicode full string uppercasing, lowercasing, titlecasing, and case-folding algorithms to their arguments and return the result. In certain cases, the result differs in length from the argument. If the result is equal to the argument in the sense of `string=?`, the argument may be returned. Note that language-sensitive mappings and foldings are not used.

  The Unicode Standard prescribes special treatment of the Greek letter \Sigma, whose normal lower-case form is \sigma but which becomes \varsigma at the end of a word. See UAX #44 (part of the Unicode Standard) for details. However, implementations of `string-downcase` are not required to provide this behavior, and may choose to change \Sigma to \sigma in all cases.

procedure: string-upper-case? string ¶
procedure: string-lower-case? string ¶

  These procedures return `#t` if all the letters in the string are lower case or upper case, otherwise they return `#f`. The string must contain at least one letter or the procedures return `#f`.

      (map string-upper-case? '(""    "A"    "art"  "Art"  "ART"))
                             ⇒ (#f    #t     #f     #f     #t)

standard procedure: substring string [start [end]] ¶

  Returns an immutable copy of the part of the given string between start and end.

procedure: string-slice string [start [end]] ¶

  Returns a slice of string, restricted to the range of characters specified by start and end. The returned slice will be mutable if `string` is mutable, or immutable if `string` is immutable.

  A slice is a kind of string that provides a view into another string. The slice behaves like any other string, but changes to a mutable slice are reflected in the original string and vice versa.

      (define foo (string-copy "abcde"))
      foo ⇒ "abcde"

      (define bar (string-slice foo 1 4))
      bar ⇒ "bcd"

      (string-set! foo 2 #\z)
      foo ⇒ "abzde"
      bar ⇒ "bzd"

      (string-set! bar 1 #\y)
      bar ⇒ "byd"
      foo ⇒ "abyde"

standard procedure: string-append string … ¶
procedure: string-append* strings ¶

  Returns an immutable string whose characters are the concatenation of the characters in the given strings.

  The non-standard procedure `string-append*` is identical to `string-append` but takes a single argument that’s a list of strings, rather than multiple string arguments.

standard procedure: string->list string [start [end]] ¶
standard procedure: list->string list ¶

  It is an error if any element of list is not a character.

  The `string->list` procedure returns a newly allocated list of the characters of string between start and end. `list->string` returns an immutable string formed from the elements in the list list. In both procedures, order is preserved. `string->list` and `list->string` are inverses so far as `equal?` is concerned.

standard procedure: string-copy string [start [end]] ¶

  Returns a newly allocated mutable copy of the part of the given string between start and end.

standard procedure: string-copy! to at from [start [end]] ¶

  It is an error if to is not a mutable string or if at is less than zero or greater than the length of to. It is also an error if `(- (string-length to) at)` is less than `(- end start)`.

  Copies the characters of string from between start and end to string to, starting at at. The order in which characters are copied is unspecified, except that if the source and destination overlap, copying takes place as if the source is first copied into a temporary string and then into the destination. This can be achieved without allocating storage by making sure to copy in the correct direction in such circumstances.

      (define a "12345")
      (define b (string-copy "abcde"))
      (string-copy! b 1 a 0 2) ⇒ 3
      b ⇒ "a12de"%

  Implementation note: in MIT/GNU Scheme `string-copy!` returns the value `(+ at (- end start))`.

standard procedure: string-fill! string fill [start [end]] ¶

  It is an error if string is not a mutable string or if fill is not a character.

  The `string-fill!` procedure stores fill in the elements of string between start and end.

The next two procedures treat a given string as a sequence of grapheme clusters, a concept defined by the Unicode standard in UAX #29:

> It is important to recognize that what the user thinks of as a “character”—a basic unit of a writing system for a language—may not be just a single Unicode code point. Instead, that basic unit may be made up of multiple Unicode code points. To avoid ambiguity with the computer use of the term character, this is called a user-perceived character. For example, “G” + acute-accent is a user-perceived character: users think of it as a single character, yet is actually represented by two Unicode code points. These user-perceived characters are approximated by what is called a grapheme cluster, which can be determined programmatically.

procedure: grapheme-cluster-length string ¶

  This procedure returns the number of grapheme clusters in string.

  For ASCII strings, this is identical to `string-length`.

procedure: grapheme-cluster-slice string start end ¶

  This procedure slices string at the grapheme-cluster boundaries specified by the start and end indices. These indices are grapheme-cluster indices, not normal string indices.

  For ASCII strings, this is identical to `string-slice`.

procedure: string-word-breaks string ¶

  This procedure returns a list of word break indices for string, ordered from smallest index to largest. Word breaks are defined by the Unicode standard in UAX #29, and generally coincide with what we think of as the boundaries of words in written text.

MIT/GNU Scheme supports the Unicode canonical normalization forms NFC (Normalization Form C) and NFD (Normalization Form D). The reason for these forms is that there can be multiple different Unicode sequences for a given text; these sequences are semantically identical and should be treated equivalently for all purposes. If two such sequences are normalized to the same form, the resulting normalized sequences will be identical.

By default, most procedures that return strings return them in NFC. Notable exceptions are `list->string`, `vector->string`, and the `utfX->string` procedures, which do no normalization, and of course `string->nfd`.

Generally speaking, NFC is preferred for most purposes, as it is the minimal-length sequence for the variants. Consult the Unicode standard for the details and for information about why one normalization form is preferable for a specific purpose.

When doing search and match operations, it is recommended that the argument strings be in NFC. Without normalization, strings that should match may not, if they have inconsistent encodings for one or more characters.

procedure: string-in-nfc? string ¶
procedure: string-in-nfd? string ¶

  These procedures return `#t` if string is in Unicode Normalization Form C or D respectively. Otherwise they return `#f`.

  Note that if string consists only of code points strictly less than `#xC0`, then `string-in-nfd?` returns `#t`. If string consists only of code points strictly less than `#x300`, then `string-in-nfc?` returns `#t`. Consequently both of these procedures will return `#t` for an ASCII string argument.

procedure: string->nfc string ¶
procedure: string->nfd string ¶

  The procedures convert string into Unicode Normalization Form C or D respectively. If string is already in the correct form, they return string itself, or an immutable copy if string is mutable.

standard procedure: string-map proc string string … ¶

  It is an error if proc does not accept as many arguments as there are strings and return a single character.

  The `string-map` procedure applies proc element-wise to the elements of the strings and returns an immutable string of the results, in order. If more than one string is given and not all strings have the same length, `string-map` terminates when the shortest string runs out. The dynamic order in which proc is applied to the elements of the strings is unspecified. If multiple returns occur from `string-map`, the values returned by earlier returns are not mutated.

      (string-map char-foldcase "AbdEgH")  ⇒  "abdegh"

      (string-map
       (lambda (c)
         (integer->char (+ 1 (char->integer c))))
       "HAL")                 ⇒  "IBM"

      (string-map
       (lambda (c k)
         ((if (eqv? k #\u) char-upcase char-downcase) c))
       "studlycaps xxx"
       "ululululul")          ⇒  "StUdLyCaPs"

standard procedure: string-for-each proc string string … ¶

  It is an error if proc does not accept as many arguments as there are strings.

  The arguments to `string-for-each` are like the arguments to `string-map`, but `string-for-each` calls proc for its side effects rather than for its values. Unlike `string-map`, `string-for-each` is guaranteed to call proc on the elements of the lists in order from the first element(s) to the last, and the value returned by `string-for-each` is unspecified. If more than one string is given and not all strings have the same length, `string-for-each` terminates when the shortest string runs out. It is an error for proc to mutate any of the strings.

      (let ((v '()))
        (string-for-each
         (lambda (c) (set! v (cons (char->integer c) v)))
         "abcde")
        v)                    ⇒  (101 100 99 98 97)

procedure: string-count proc string string … ¶

  It is an error if proc does not accept as many arguments as there are strings.

  The `string-count` procedure applies proc element-wise to the elements of the strings and returns a count of the number of true values it returns. If more than one string is given and not all strings have the same length, `string-count` terminates when the shortest string runs out. The dynamic order in which proc is applied to the elements of the strings is unspecified.

procedure: string-any proc string string … ¶

  It is an error if proc does not accept as many arguments as there are strings.

  The `string-any` procedure applies proc element-wise to the elements of the strings and returns `#t` if it returns a true value. If proc doesn’t return a true value, `string-any` returns `#f`.

  If more than one string is given and not all strings have the same length, `string-any` terminates when the shortest string runs out. The dynamic order in which proc is applied to the elements of the strings is unspecified.

procedure: string-every proc string string … ¶

  It is an error if proc does not accept as many arguments as there are strings.

  The `string-every` procedure applies proc element-wise to the elements of the strings and returns `#f` if it returns a false value. If proc doesn’t return a false value, `string-every` returns `#t`.

  If more than one string is given and not all strings have the same length, `string-every` terminates when the shortest string runs out. The dynamic order in which proc is applied to the elements of the strings is unspecified.

procedure: string-null? string ¶

  Returns `#t` if string has zero length; otherwise returns `#f`.

      (string-null? "")       ⇒  #t
      (string-null? "Hi")     ⇒  #f

procedure: string-hash string [modulus] ¶
procedure: string-hash-ci string [modulus] ¶

  These SRFI 69 procedures return an exact non-negative integer that can be used for storing the specified string in a hash table. Equal strings (in the sense of `string=?` and `string-ci=?` respectively) return equal (`=`) hash codes, and non-equal but similar strings are usually mapped to distinct hash codes.

procedure: string-head string end ¶

  Equivalent to `(substring string 0 end)`.

procedure: string-tail string start ¶

  Equivalent to `(substring string start)`.

  If the optional argument modulus is specified, it must be an exact positive integer, and the result of the hash computation is restricted to be less than that value. This is equivalent to calling `modulo` on the result, but may be faster.

procedure: string-builder [buffer-length] ¶

  This procedure returns a string builder that can be used to incrementally collect characters and later convert that collection to a string. This is similar to a string output port, but is less general and significantly faster.

  The optional buffer-length argument, if given, must be an exact positive integer. It controls the size of the internal buffers that are used to accumulate characters. Larger values make the builder somewhat faster but use more space. The default value of this argument is `16`.

  The returned string builder is a procedure that accepts zero or one arguments as follows:

  - Given a character argument, the string builder appends that character to the string being built and returns an unspecified value.
  - Given a string argument, the string builder appends that string to the string being built and returns an unspecified value.
  - Given no arguments, or one of the “result” arguments (see below), the string builder returns a copy of the string being built. Note that this does not affect the string being built, so immediately calling the builder with no arguments a second time returns a new copy of the same string.
  - Given the argument `empty?`, the string builder returns `#t` if the string being built is empty and `#f` otherwise.
  - Given the argument `count`, the string builder returns the size of the string being built.
  - Given the argument `reset!`, the string builder discards the string being built and returns to the state it was in when initially created.

  The “result” arguments control the form of the returned string. The arguments `immutable` and `mutable` are straightforward, specifying the mutability of the returned string. For these arguments, the returned string contains exactly the same characters, in the same order, as were appended to the builder.

  However, calling with the argument `nfc`, or with no arguments, returns an immutable string in Unicode Normalization Form C, exactly as if `string->nfc` were called on one of the other two result strings.

procedure: string-joiner infix prefix suffix ¶
procedure: string-joiner* infix prefix suffix ¶

  This procedure’s arguments are keyword arguments; that is, each argument is a symbol of the same name followed by its value. The order of the arguments doesn’t matter, but each argument may appear only once.

  These procedures return a joiner procedure that takes multiple strings and joins them together into an immutable string. The joiner returned by `string-joiner` accepts these strings as multiple string arguments, while `string-joiner*` accepts the strings as a single list-valued argument.

  The joiner produces a result by adding prefix before, suffix after, and infix between each input string, then concatenating everything together into a single string. Each of the prefix, suffix, and infix arguments is optional and defaults to an empty string, so normally at least one is specified.

  Some examples:

      ((string-joiner) "a" "b" "c")
        ⇒  "abc"

      ((string-joiner 'infix " ") "a" "b" "c")
        ⇒  "a b c"

      ((string-joiner 'infix ", ") "a" "b" "c")
        ⇒  "a, b, c"

      ((string-joiner* 'infix ", " 'prefix "<" 'suffix ">")
       '("a" "b" "c"))
        ⇒  "<a>, <b>, <c>"

procedure: string-splitter delimiter allow-runs? copier copy? ¶

  This procedure’s arguments are keyword arguments; that is, each argument is a symbol of the same name followed by its value. The order of the arguments doesn’t matter, but each argument may appear only once.

  This procedure returns a splitter procedure that splits a given string into parts, returning a list of the parts. This is done by identifying delimiter characters and breaking the string at those delimiters. The splitting process is controlled by the arguments:

  - delimiter is either a character, a character set, or more generally a procedure that accepts a single character argument and returns a boolean value. The splitter uses this to identify delimiters in the string. The default value of this argument is `char-whitespace?`.
  - allow-runs? is a boolean that controls what happens when two or more adjacent delimiters are found. If allow-runs? is `#t`, then all of the adjacent delimiters are treated as if they were a single delimiter, and the string is split at the beginning and end of the delimiters. If allow-runs? is `#f`, then adjacent delimiters are treated as if they were separate with an empty string between them. The default value of this argument is `#t`.
  - copier is a procedure that accepts three arguments: a string, a start index, and an end index, returning the specified substring as a string. It defaults to `string-slice`.
  - copy? is a boolean, for backwards compatibility; instead use copier. A value of `#t` is equivalent to a copier of `substring`, while a value of `#f` is equivalent to a copier of `string-slice`.

  Some examples:

      ((string-splitter) "a b c")
        ⇒  ("a" "b" "c")

      ((string-splitter) "a\tb\tc")
        ⇒  ("a" "b" "c")

      ((string-splitter 'delimiter #\space) "a\tb\tc")
        ⇒  ("a\tb\tc")

      ((string-splitter) " a  b  c ")
        ⇒  ("a" "b" "c")

      ((string-splitter 'allow-runs? #f) " a  b  c ")
        ⇒  ("" "a" "" "b" "" "c" "")

procedure: string-padder where fill-with clip? ¶

  This procedure’s arguments are keyword arguments; that is, each argument is a symbol of the same name followed by its value. The order of the arguments doesn’t matter, but each argument may appear only once.

  This procedure returns a padder procedure that takes a string and a grapheme-cluster length as its arguments and returns a new string that has been padded to that length. The padder adds grapheme clusters to the string until it has the specified length. If the string’s grapheme-cluster length is greater than the given length, the string may, depending on the arguments, be reduced to the specified length.

  The padding process is controlled by the arguments:

  - where is a symbol: either `leading` or `trailing`, which directs the padder to add/remove leading or trailing grapheme clusters. The default value of this argument is `leading`.
  - fill-with is a string that contains exactly one grapheme cluster, which is used as the padding to increase the size of the string. The default value of this argument is `" "` (a single space character).
  - clip? is a boolean that controls what happens if the given string has a longer grapheme-cluster length than the given length. If `clip?` is `#t`, grapheme clusters are removed (by slicing) from the string until it is the correct length; if it is `#f` then the string is returned unchanged. The grapheme clusters are removed from the beginning of the string if `where` is `leading`, otherwise from the end of the string. The default value of this argument is `#t`.

  Some examples:

      ((string-padder) "abc def" 10)
        ⇒  "   abc def"

      ((string-padder 'where 'trailing) "abc def" 10)
        ⇒  "abc def   "

      ((string-padder 'fill-with "X") "abc def" 10)
        ⇒  "XXXabc def"

      ((string-padder) "abc def" 5)
        ⇒  "c def"

      ((string-padder 'where 'trailing) "abc def" 5)
        ⇒  "abc d"

      ((string-padder 'clip? #f) "abc def" 5)
        ⇒  "abc def"

obsolete procedure: string-pad-left string k [char] ¶
obsolete procedure: string-pad-right string k [char] ¶

  These procedures are deprecated and should be replaced by use of `string-padder` which is more flexible.

  These procedures return an immutable string created by padding string out to length k, using char. If char is not given, it defaults to `#\space`. If k is less than the length of string, the resulting string is a truncated form of string. `string-pad-left` adds padding characters or truncates from the beginning of the string (lowest indices), while `string-pad-right` does so at the end of the string (highest indices).

      (string-pad-left "hello" 4)             ⇒  "ello"
      (string-pad-left "hello" 8)             ⇒  "   hello"
      (string-pad-left "hello" 8 #\*)         ⇒  "***hello"
      (string-pad-right "hello" 4)            ⇒  "hell"
      (string-pad-right "hello" 8)            ⇒  "hello   "

procedure: string-trimmer where to-trim copier copy? ¶

  This procedure’s arguments are keyword arguments; that is, each argument is a symbol of the same name followed by its value. The order of the arguments doesn’t matter, but each argument may appear only once.

  This procedure returns a trimmer procedure that takes a string as its argument and trims that string, returning the trimmed result. The trimming process is controlled by the arguments:

  - where is a symbol: either `leading`, `trailing`, or `both`, which directs the trimmer to trim leading characters, trailing characters, or both. The default value of this argument is `both`.
  - to-trim is either a character, a character set, or more generally a procedure that accepts a single character argument and returns a boolean value. The trimmer uses this to identify characters to remove. The default value of this argument is `char-whitespace?`.
  - copier is a procedure that accepts three arguments: a string, a start index, and an end index, returning the specified substring as a string. It defaults to `string-slice`.
  - copy? is a boolean, for backwards compatibility; instead use copier. A value of `#t` is equivalent to a copier of `substring`, while a value of `#f` is equivalent to a copier of `string-slice`.

  Some examples:

      ((string-trimmer 'where 'leading) "    ABC   DEF    ")
        ⇒  "ABC   DEF    "

      ((string-trimmer 'where 'trailing) "    ABC   DEF    ")
        ⇒  "    ABC   DEF"

      ((string-trimmer 'where 'both) "    ABC   DEF    ")
        ⇒  "ABC   DEF"

      ((string-trimmer) "    ABC   DEF    ")
        ⇒  "ABC   DEF"

      ((string-trimmer 'to-trim char-numeric? 'where 'leading)
       "21 East 21st Street #3")
        ⇒  " East 21st Street #3"

      ((string-trimmer 'to-trim char-numeric? 'where 'trailing)
       "21 East 21st Street #3")
        ⇒  "21 East 21st Street #"

      ((string-trimmer 'to-trim char-numeric?)
       "21 East 21st Street #3")
        ⇒  " East 21st Street #"

obsolete procedure: string-trim string [char-set] ¶
obsolete procedure: string-trim-left string [char-set] ¶
obsolete procedure: string-trim-right string [char-set] ¶

  These procedures are deprecated and should be replaced by use of `string-trimmer` which is more flexible.

  Returns an immutable string created by removing all characters that are not in char-set from: (`string-trim`) both ends of string; (`string-trim-left`) the beginning of string; or (`string-trim-right`) the end of string. Char-set defaults to `char-set:not-whitespace`.

      (string-trim "  in the end  ")          ⇒  "in the end"
      (string-trim "              ")          ⇒  ""
      (string-trim "100th" char-set:numeric)  ⇒  "100"
      (string-trim-left "-.-+-=-" (char-set #\+))
                                              ⇒  "+-=-"
      (string-trim "but (+ x y) is" (char-set #\( #\)))
                                              ⇒  "(+ x y)"

procedure: string-replace string char1 char2 ¶

  Returns an immutable string containing the same characters as string except that all instances of char1 have been replaced by char2.

### 6.1 Searching and Matching Strings

This section describes procedures for searching a string, either for a character or a substring, and matching two strings to one another.

procedure: string-search-forward pattern string [start [end]] ¶

  Searches string for the leftmost occurrence of the substring pattern. If successful, the index of the first character of the matched substring is returned; otherwise, `#f` is returned.

      (string-search-forward "rat" "pirate")
          ⇒ 2
      (string-search-forward "rat" "pirate rating")
          ⇒ 2
      (string-search-forward "rat" "pirate rating" 4 13)
          ⇒ 7
      (string-search-forward "rat" "pirate rating" 9 13)
          ⇒ #f

procedure: string-search-backward pattern string [start [end]] ¶

  Searches string for the rightmost occurrence of the substring pattern. If successful, the index to the right of the last character of the matched substring is returned; otherwise, `#f` is returned.

      (string-search-backward "rat" "pirate")
          ⇒ 5
      (string-search-backward "rat" "pirate rating")
          ⇒ 10
      (string-search-backward "rat" "pirate rating" 1 8)
          ⇒ 5
      (string-search-backward "rat" "pirate rating" 9 13)
          ⇒ #f

procedure: string-search-all pattern string [start [end]] ¶

  Searches string to find all occurrences of the substring pattern. Returns a list of the occurrences; each element of the list is an index pointing to the first character of an occurrence.

      (string-search-all "rat" "pirate")
          ⇒ (2)
      (string-search-all "rat" "pirate rating")
          ⇒ (2 7)
      (string-search-all "rat" "pirate rating" 4 13)
          ⇒ (7)
      (string-search-all "rat" "pirate rating" 9 13)
          ⇒ ()

procedure: substring? pattern string ¶

  Searches string to see if it contains the substring pattern. Returns `#t` if pattern is a substring of string, otherwise returns `#f`.

      (substring? "rat" "pirate")             ⇒  #t
      (substring? "rat" "outrage")            ⇒  #f
      (substring? "" any-string)              ⇒  #t
      (if (substring? "moon" text)
          (process-lunar text)
          'no-moon)

procedure: string-find-first-index proc string string … ¶
procedure: string-find-last-index proc string string … ¶

  The proc argument must accept as many arguments as there are strings.

  These procedures apply proc element-wise to the elements of the strings and return the first or last index for which proc returns a true value. If there is no such index, then `#f` is returned.

  If more than one string is given and not all strings have the same length, then only the indexes of the shortest string are tested.

procedure: string-find-next-char string char [start [end]] ¶
procedure: string-find-next-char-ci string char [start [end]] ¶
procedure: string-find-next-char-in-set string char-set [start [end]] ¶

  These procedures search string for a matching character, starting from start and moving forwards to end. If there is a matching character, the procedures stop the search and return the index of that character. If there is no matching character, the procedures return `#f`.

  The procedures differ only in how they match characters: `string-find-next-char` matches a character that is `char=?` to char; `string-find-next-char-ci` matches a character that is `char-ci=?` to char; and `string-find-next-char-in-set` matches a character that’s a member of char-set.

      (string-find-next-char "Adam" #\A)           ⇒  0
      (string-find-next-char "Adam" #\A 1 4)       ⇒  #f
      (string-find-next-char-ci "Adam" #\A 1 4)    ⇒  2
      (string-find-next-char-in-set my-string char-set:alphabetic)
          ⇒  start position of the first word in my-string
      ; Can be used as a predicate:
      (if (string-find-next-char-in-set my-string
                                        (char-set #\( #\) ))
          'contains-parentheses
          'no-parentheses)

procedure: string-find-previous-char string char [start [end]] ¶
procedure: string-find-previous-char-ci string char [start [end]] ¶
procedure: string-find-previous-char-in-set string char-set [start [end]] ¶

  These procedures search string for a matching character, starting from end and moving backwards to start. If there is a matching character, the procedures stop the search and return the index of that character. If there is no matching character, the procedures return `#f`.

  The procedures differ only in how they match characters: `string-find-previous-char` matches a character that is `char=?` to char; `string-find-previous-char-ci` matches a character that is `char-ci=?` to char; and `string-find-previous-char-in-set` matches a character that’s a member of char-set.

procedure: string-match-forward string1 string2 ¶

  Compares the two strings, starting from the beginning, and returns the number of characters that are the same. If the two strings start differently, returns 0.

      (string-match-forward "mirror" "micro") ⇒  2  ; matches "mi"
      (string-match-forward "a" "b")          ⇒  0  ; no match

procedure: string-match-backward string1 string2 ¶

  Compares the two strings, starting from the end and matching toward the front, returning the number of characters that are the same. If the two strings end differently, returns 0.

      (string-match-backward "bulbous" "fractious")
                                              ⇒  3  ; matches "ous"

procedure: string-prefix? string1 string2 ¶
procedure: string-prefix-ci? string1 string2 ¶

  These procedures return `#t` if the first string forms the prefix of the second; otherwise returns `#f`. The `-ci` procedures don’t distinguish uppercase and lowercase letters.

      (string-prefix? "abc" "abcdef")         ⇒  #t
      (string-prefix? "" any-string)          ⇒  #t

procedure: string-suffix? string1 string2 ¶
procedure: string-suffix-ci? string1 string2 ¶

  These procedures return `#t` if the first string forms the suffix of the second; otherwise returns `#f`. The `-ci` procedures don’t distinguish uppercase and lowercase letters.

      (string-suffix? "ous" "bulbous")        ⇒  #t
      (string-suffix? "" any-string)          ⇒  #t

### 6.2 Regular Expressions

MIT/GNU Scheme provides support for matching and searching strings against regular expressions. This is considerably more flexible than ordinary string matching and searching, but potentially much slower. On the other hand it is less powerful than the mechanism described in Parser Language.

Traditional regular expressions are defined with string patterns in which characters like ‘`[`’ and ‘`*`’ have special meanings. Unfortunately, the syntax of these patterns is not only baroque but also comes in many different and mutually-incompatible varieties. As a consequence we have chosen to specify regular expressions using an s-expression syntax, which we call a regular s-expression, abbreviated as regsexp.

Previous releases of MIT/GNU Scheme provided a regular-expression implementation nearly identical to that of GNU Emacs version 18. This implementation supported only 8-bit strings, which made it unsuitable for use with Unicode strings. This implementation still exists but is deprecated and will be removed in a future release.

#### 6.2.1 Regular S-Expressions

A regular s-expression is either a character or a string, which matches itself, or one of the following forms.

Examples in this section use the following definitions for brevity:

    (define (try-match pattern string)
      (regsexp-match-string (compile-regsexp pattern) string))

    (define (try-search pattern string)
      (regsexp-search-string-forward (compile-regsexp pattern) string))

These forms match one or more characters literally:

regsexp: char-ci char ¶

  Matches char without considering case.

regsexp: string-ci string ¶

  Matches string without considering case.

regsexp: any-char ¶

  Matches one character other than `#\newline`.

      (try-match '(any-char) "") ⇒ #f
      (try-match '(any-char) "a") ⇒ (0 1)
      (try-match '(any-char) "\n") ⇒ #f
      (try-search '(any-char) "") ⇒ #f
      (try-search '(any-char) "ab") ⇒ (0 1)
      (try-search '(any-char) "\na") ⇒ (1 2)

regsexp: char-in datum … ¶
regsexp: char-not-in datum … ¶

  Matches one character in (not in) the character set specified by `(char-set datum …)`.

      (try-match '(seq "a" (char-in "ab") "c") "abc") ⇒ (0 3)
      (try-match '(seq "a" (char-not-in "ab") "c") "abc") ⇒ #f
      (try-match '(seq "a" (char-not-in "ab") "c") "adc") ⇒ (0 3)
      (try-match '(seq "a" (+ (char-in numeric)) "c") "a019c") ⇒ (0 5)

These forms match no characters, but only at specific locations in the input string:

regsexp: line-start ¶
regsexp: line-end ¶

  Matches no characters at the start (end) of a line.

      (try-match '(seq (line-start)
                       (* (any-char))
                       (line-end))
                 "abc") ⇒ (0 3)

      (try-match '(seq (line-start)
                       (* (any-char))
                       (line-end))
                 "ab\nc") ⇒ (0 2)

      (try-search '(seq (line-start)
                        (* (char-in alphabetic))
                        (line-end))
                  "1abc") ⇒ #f

      (try-search '(seq (line-start)
                        (* (char-in alphabetic))
                        (line-end))
                  "1\nabc") ⇒ (2 5)

regsexp: string-start ¶
regsexp: string-end ¶

  Matches no characters at the start (end) of the string.

      (try-match '(seq (string-start)
                       (* (any-char))
                       (string-end))
                 "abc") ⇒ (0 3)

      (try-match '(seq (string-start)
                       (* (any-char))
                       (string-end))
                 "ab\nc") ⇒ #f

      (try-search '(seq (string-start)
                        (* (char-in alphabetic))
                        (string-end))
                  "1abc") ⇒ #f

      (try-search '(seq (string-start)
                        (* (char-in alphabetic))
                        (string-end))
                  "1\nabc") ⇒ #f

These forms match repetitions of a given regsexp. Most of them come in two forms, one of which is greedy and the other shy. The greedy form matches as many repetitions as it can, then uses failure backtracking to reduce the number of repetitions one at a time. The shy form matches the minimum number of repetitions, then uses failure backtracking to increase the number of repetitions one at a time. The shy form is similar to the greedy form except that a `?` is added at the end of the form’s keyword.

regsexp: ? regsexp ¶
regsexp: ?? regsexp ¶

  Matches regsexp zero or one time.

      (try-search '(seq (char-in alphabetic)
                        (? (char-in numeric)))
                  "a") ⇒ (0 1)

      (try-search '(seq (char-in alphabetic)
                        (?? (char-in numeric)))
                  "a") ⇒ (0 1)

      (try-search '(seq (char-in alphabetic)
                        (? (char-in numeric)))
                  "a1") ⇒ (0 2)

      (try-search '(seq (char-in alphabetic)
                        (?? (char-in numeric)))
                  "a1") ⇒ (0 1)

      (try-search '(seq (char-in alphabetic)
                        (? (char-in numeric)))
                  "1a2") ⇒ (1 3)

      (try-search '(seq (char-in alphabetic)
                        (?? (char-in numeric)))
                  "1a2") ⇒ (1 2)

regsexp: * regsexp ¶
regsexp: *? regsexp ¶

  Matches regsexp zero or more times.

      (try-match '(seq (char-in alphabetic)
                       (* (char-in numeric))
                       (any-char))
                 "aa") ⇒ (0 2)

      (try-match '(seq (char-in alphabetic)
                       (*? (char-in numeric))
                       (any-char))
                 "aa") ⇒ (0 2)

      (try-match '(seq (char-in alphabetic)
                       (* (char-in numeric))
                       (any-char))
                 "a123a") ⇒ (0 5)

      (try-match '(seq (char-in alphabetic)
                       (*? (char-in numeric))
                       (any-char))
                 "a123a") ⇒ (0 2)

regsexp: + regsexp ¶
regsexp: +? regsexp ¶

  Matches regsexp one or more times.

      (try-match '(seq (char-in alphabetic)
                       (+ (char-in numeric))
                       (any-char))
                 "aa") ⇒ #f

      (try-match '(seq (char-in alphabetic)
                       (+? (char-in numeric))
                       (any-char))
                 "aa") ⇒ #f

      (try-match '(seq (char-in alphabetic)
                       (+ (char-in numeric))
                       (any-char))
                 "a123a") ⇒ (0 5)

      (try-match '(seq (char-in alphabetic)
                       (+? (char-in numeric))
                       (any-char))
                 "a123a") ⇒ (0 3)

regsexp: ** n m regsexp ¶
regsexp: **? n m regsexp ¶

  The n argument must be an exact nonnegative integer. The m argument must be either an exact integer greater than or equal to n, or else `#f`.

  Matches regsexp at least n times and at most m times; if m is `#f` then there is no upper limit.

      (try-match '(seq (char-in alphabetic)
                       (** 0 2 (char-in numeric))
                       (any-char))
                 "aa") ⇒ (0 2)

      (try-match '(seq (char-in alphabetic)
                       (**? 0 2 (char-in numeric))
                       (any-char))
                 "aa") ⇒ (0 2)

      (try-match '(seq (char-in alphabetic)
                       (** 0 2 (char-in numeric))
                       (any-char))
                 "a123a") ⇒ (0 4)

      (try-match '(seq (char-in alphabetic)
                       (**? 0 2 (char-in numeric))
                       (any-char))
                 "a123a") ⇒ (0 2)

regsexp: ** n regsexp ¶

  This is an abbreviation for `(** n n regsexp)`. This matcher is neither greedy nor shy since it matches a fixed number of repetitions.

These forms implement alternatives and sequencing:

regsexp: alt regsexp … ¶

  Matches one of the regsexp arguments, trying each in order from left to right.

      (try-match '(alt #\a (char-in numeric)) "a") ⇒ (0 1)
      (try-match '(alt #\a (char-in numeric)) "b") ⇒ #f
      (try-match '(alt #\a (char-in numeric)) "1") ⇒ (0 1)

regsexp: seq regsexp … ¶

  Matches the first regsexp, then continues the match with the next regsexp, and so on until all of the arguments are matched.

      (try-match '(seq #\a #\b) "a") ⇒ #f
      (try-match '(seq #\a #\b) "aa") ⇒ #f
      (try-match '(seq #\a #\b) "ab") ⇒ (0 2)

These forms implement named registers, which store matched segments of the input string:

regsexp: group key regsexp ¶

  The key argument must be a fixnum, a character, or a symbol.

  Matches regsexp. If the match succeeds, the matched segment is stored in the register named key.

      (try-match '(seq (group a (any-char))
                       (group b (any-char))
                       (any-char))
                 "radar") ⇒ (0 3 (a . "r") (b . "a"))

regsexp: group-ref key ¶

  The key argument must be a fixnum, a character, or a symbol.

  Matches the characters stored in the register named key. It is an error if that register has not been initialized with a corresponding `group` expression.

      (try-match '(seq (group a (any-char))
                       (group b (any-char))
                       (any-char)
                       (group-ref b)
                       (group-ref a))
                 "radar") ⇒ (0 5 (a . "r") (b . "a"))

#### 6.2.2 Regsexp Procedures

The regular s-expression implementation has two parts, like many other regular-expression implementations: a compiler that translates the pattern into an efficient form, and one or more procedures that use that pattern to match or search inputs.

procedure: compile-regsexp regsexp ¶

  Compiles regsexp by translating it into a procedure that implements the specified matcher.

The match and search procedures each return a list when they are successful, and `#f` when they fail. The returned list is of the form `(s e register …)`, where s is the index at which the match starts, e is the index at which the match ends, and each register is a pair `(key . contents)` where key is the register’s name and contents is the contents of that register as a string.

In order to get reliable results, the string arguments to these procedures must be in Unicode Normalization Form C. The string implementation keeps most strings in this form by default; in other cases the caller must convert the string using `string->nfc`.

procedure: regsexp-match-string crse string [start [end]] ¶

  The crse argument must be a value returned by `compile-regsexp`.

  Matches string against crse and returns the result.

procedure: regsexp-search-string-forward crse string [start [end]] ¶

  The crse argument must be a value returned by `compile-regsexp`.

  Searches string from left to right for a match against crse and returns the result.

## 7 Lists

A pair (sometimes called a dotted pair) is a data structure with two fields called the car and cdr fields (for historical reasons). Pairs are created by the procedure `cons`. The car and cdr fields are accessed by the procedures `car` and `cdr`. The car and cdr fields are assigned by the procedures `set-car!` and `set-cdr!`.

Pairs are used primarily to represent lists. A list can be defined recursively as either the empty list or a pair whose cdr is a list. More precisely, the set of lists is defined as the smallest set X such that

- The empty list is in X.
- If list is in X, then any pair whose cdr field contains list is also in X.

The objects in the car fields of successive pairs of a list are the elements of the list. For example, a two-element list is a pair whose car is the first element and whose cdr is a pair whose car is the second element and whose cdr is the empty list. The length of a list is the number of elements, which is the same as the number of pairs. The empty list is a special object of its own type (it is not a pair); it has no elements and its length is zero.5

The most general notation (external representation) for Scheme pairs is the “dotted” notation `(c1 . c2)` where c1 is the value of the car field and c2 is the value of the cdr field. For example, `(4 . 5)` is a pair whose car is `4` and whose cdr is `5`. Note that `(4 . 5)` is the external representation of a pair, not an expression that evaluates to a pair.

A more streamlined notation can be used for lists: the elements of the list are simply enclosed in parentheses and separated by spaces. The empty list is written `()`. For example, the following are equivalent notations for a list of symbols:

    (a b c d e)
    (a . (b . (c . (d . (e . ())))))

Whether a given pair is a list depends upon what is stored in the cdr field. When the `set-cdr!` procedure is used, an object can be a list one moment and not the next:

    (define x (list 'a 'b 'c))
    (define y x)
    y                                       ⇒ (a b c)
    (list? y)                               ⇒ #t
    (set-cdr! x 4)                          ⇒ unspecified
    x                                       ⇒ (a . 4)
    (eqv? x y)                              ⇒ #t
    y                                       ⇒ (a . 4)
    (list? y)                               ⇒ #f
    (set-cdr! x x)                          ⇒ unspecified
    (list? y)                               ⇒ #f

A chain of pairs that doesn’t end in the empty list is called an improper list. Note that an improper list is not a list. The list and dotted notations can be combined to represent improper lists, as the following equivalent notations show:

    (a b c . d)
    (a . (b . (c . d)))

Within literal expressions and representations of objects read by the `read` procedure, the forms `'datum`, ``datum`, `,datum`, and `,@datum` denote two-element lists whose first elements are the symbols `quote`, `quasiquote`, `unquote`, and `unquote-splicing`, respectively. The second element in each case is datum. This convention is supported so that arbitrary Scheme programs may be represented as lists. Among other things, this permits the use of the `read` procedure to parse Scheme programs.

### 7.1 Pairs

This section describes the simple operations that are available for constructing and manipulating arbitrary graphs constructed from pairs.

standard procedure: pair? object ¶

  Returns `#t` if object is a pair; otherwise returns `#f`.

      (pair? '(a . b))                        ⇒ #t
      (pair? '(a b c))                        ⇒ #t
      (pair? '())                             ⇒ #f
      (pair? '#(a b))                         ⇒ #f

standard procedure: cons obj1 obj2 ¶

  Returns a newly allocated pair whose car is obj1 and whose cdr is obj2. The pair is guaranteed to be different (in the sense of `eqv?`) from every previously existing object.

      (cons 'a '())                           ⇒ (a)
      (cons '(a) '(b c d))                    ⇒ ((a) b c d)
      (cons "a" '(b c))                       ⇒ ("a" b c)
      (cons 'a 3)                             ⇒ (a . 3)
      (cons '(a b) 'c)                        ⇒ ((a b) . c)

SRFI 1 procedure: xcons obj1 obj2 ¶

  Returns a newly allocated pair whose car is obj2 and whose cdr is obj1.

      (xcons '(b c) 'a)                       ⇒ (a b c)

standard procedure: car pair ¶

  Returns the contents of the car field of pair. Note that it is an error to take the `car` of the empty list.

      (car '(a b c))                          ⇒ a
      (car '((a) b c d))                      ⇒ (a)
      (car '(1 . 2))                          ⇒ 1
      (car '())                               error→ Illegal datum

standard procedure: cdr pair ¶

  Returns the contents of the cdr field of pair. Note that it is an error to take the `cdr` of the empty list.

      (cdr '((a) b c d))                      ⇒ (b c d)
      (cdr '(1 . 2))                          ⇒ 2
      (cdr '())                               error→ Illegal datum

SRFI 1 procedure: car+cdr pair ¶

  The fundamental pair deconstructor:

      (lambda (p) (values (car p) (cdr p)))

      (receive (a b) (car+cdr (cons 1 2))
        (write-line a)
        (write-line b))
      -| 1
      -| 2

standard procedure: set-car! pair object ¶

  Stores object in the car field of pair. The value returned by `set-car!` is unspecified.

      (define (f) (list 'not-a-constant-list))
      (define (g) '(constant-list))
      (set-car! (f) 3)                        ⇒ unspecified
      (set-car! (g) 3)                        error→ Illegal datum

standard procedure: set-cdr! pair object ¶

  Stores object in the cdr field of pair. The value returned by `set-cdr!` is unspecified.

standard procedure: caar pair ¶
standard procedure: cadr pair ¶
standard procedure: cdar pair ¶
standard procedure: cddr pair ¶
standard procedure: caaar pair ¶
standard procedure: caadr pair ¶
standard procedure: cadar pair ¶
standard procedure: caddr pair ¶
standard procedure: cdaar pair ¶
standard procedure: cdadr pair ¶
standard procedure: cddar pair ¶
standard procedure: cdddr pair ¶
standard procedure: caaaar pair ¶
standard procedure: caaadr pair ¶
standard procedure: caadar pair ¶
standard procedure: caaddr pair ¶
standard procedure: cadaar pair ¶
standard procedure: cadadr pair ¶
standard procedure: caddar pair ¶
standard procedure: cadddr pair ¶
standard procedure: cdaaar pair ¶
standard procedure: cdaadr pair ¶
standard procedure: cdadar pair ¶
standard procedure: cdaddr pair ¶
standard procedure: cddaar pair ¶
standard procedure: cddadr pair ¶
standard procedure: cdddar pair ¶
standard procedure: cddddr pair ¶

  These procedures are compositions of `car` and `cdr`; for example, `caddr` could be defined by

      (define caddr (lambda (x) (car (cdr (cdr x)))))

procedure: general-car-cdr object path ¶

  This procedure is a generalization of `car` and `cdr`. Path encodes a particular sequence of `car` and `cdr` operations, which `general-car-cdr` executes on object. Path is an exact non-negative integer that encodes the operations in a bitwise fashion: a zero bit represents a `cdr` operation, and a one bit represents a `car`. The bits are executed LSB to MSB, and the most significant one bit, rather than being interpreted as an operation, signals the end of the sequence.6

  For example, the following are equivalent:

      (general-car-cdr object #b1011)
      (cdr (car (car object)))

  Here is a partial table of path/operation equivalents:

      #b10    cdr
      #b11    car
      #b100   cddr
      #b101   cdar
      #b110   cadr
      #b111   caar
      #b1000  cdddr

SRFI 1 procedure: tree-copy tree ¶

  This copies an arbitrary tree constructed from pairs, copying both the car and cdr elements of every pair. This could have been defined by

      (define (tree-copy tree)
        (let loop ((tree tree))
          (if (pair? tree)
              (cons (loop (car tree)) (loop (cdr tree)))
              tree)))

### 7.2 Construction of Lists

standard procedure: list object … ¶

  Returns a list of its arguments.

      (list 'a (+ 3 4) 'c)                    ⇒ (a 7 c)
      (list)                                  ⇒ ()

  These expressions are equivalent:

      (list obj1 obj2 … objN)
      (cons obj1 (cons obj2 … (cons objN '()) …))

SRFI 1 procedure: make-list n [fill] ¶

  Returns an n-element list, whose elements are all the value fill. If the fill argument is not given, the elements of the list may be arbitrary values.

      (make-list 4 'c)                        ⇒ (c c c c)

SRFI 1 procedure: cons* object object … ¶

  `cons*` is similar to `list`, except that `cons*` conses together the last two arguments rather than consing the last argument with the empty list. If the last argument is not a list the result is an improper list. If the last argument is a list, the result is a list consisting of the initial arguments and all of the items in the final argument. If there is only one argument, the result is the argument.

      (cons* 'a 'b 'c)                        ⇒ (a b . c)
      (cons* 'a 'b '(c d))                    ⇒ (a b c d)
      (cons* 'a)                              ⇒ a

  These expressions are equivalent:

      (cons* obj1 obj2 … objN-1 objN)
      (cons obj1 (cons obj2 … (cons objN-1 objN) …))

SRFI 1 procedure: list-tabulate k init-proc ¶
obsolete procedure: make-initialized-list k init-proc ¶

  Returns a k-element list. Element i of the list, where 0 <= i < k, is produced by (init-proc i). No guarantee is made about the dynamic order in which init-proc is applied to these indices.

      (list-tabulate 4 values) => (0 1 2 3)

SRFI 1 procedure: list-copy list ¶

  Returns a newly allocated copy of list. This copies each of the pairs comprising list. This could have been defined by

      (define (list-copy list)
        (if (null? list)
            '()
            (cons (car list)
                  (list-copy (cdr list)))))

SRFI 1 procedure: iota count [start [step]] ¶

  Returns a list containing the elements

      (start start+step … start+(count-1)*step)

  Count must be an exact non-negative integer, while start and step can be any numbers. The start and step parameters default to 0 and 1, respectively.

      (iota 5) ⇒ (0 1 2 3 4)
      (iota 5 0 -0.1) ⇒ (0 -0.1 -0.2 -0.3 -0.4)

standard procedure: vector->list vector [start [end]] ¶
obsolete procedure: subvector->list vector start end ¶

  Returns a newly allocated list of the elements of vector between start inclusive and end exclusive. The inverse of `vector->list` is `list->vector`.

      (vector->list '#(dah dah didah))        ⇒ (dah dah didah)

### 7.3 Selecting List Components

standard procedure: list? object ¶
SRFI 1 procedure: proper-list? object ¶

  Returns `#t` if object is a proper list, otherwise returns `#f`. By definition, all proper lists have finite length and are terminated by the empty list. If object is a circular list, returns `#f`.

  Any object satisfying this predicate will also satisfy exactly one of `pair?` or `null?`.

      (list? (list 'a 'b 'c))                 ⇒ #t
      (list? (cons* 'a 'b 'c))                error→
      (list? (circular-list 'a 'b 'c))        ⇒ #f

SRFI 1 procedure: circular-list? object ¶

  Returns `#t` if object is a circular list, otherwise returns `#f`.

      (circular-list? (list 'a 'b 'c))        ⇒ #f
      (circular-list? (cons* 'a 'b 'c))       ⇒ #f
      (circular-list? (circular-list 'a 'b 'c)) ⇒ #t

SRFI 1 procedure: dotted-list? object ¶

  Returns `#t` if object is an improper list, otherwise returns `#f`.

      (dotted-list? (list 'a 'b 'c))          ⇒ #f
      (dotted-list? (cons* 'a 'b 'c))         ⇒ #t
      (dotted-list? (circular-list 'a 'b 'c)) ⇒ #f

standard procedure: length list ¶

  Returns the length of list. Signals an error if list isn’t a proper list.

      (length (list 'a 'b 'c))                ⇒ 3
      (length (cons* 'a 'b 'c))               error→
      (length (circular-list 'a 'b 'c))       error→

SRFI 1 procedure: length+ clist ¶

  Clist must be a proper, dotted, or circular list. If clist is a circular list, returns `#f`, otherwise returns the number of pairs comprising the list (which is the same as the length for a proper list).

      (length+ (list 'a 'b 'c))               ⇒ 3
      (length+ (cons* 'a 'b 'c))              ⇒ 2
      (length+ (circular-list 'a 'b 'c))      ⇒ #f

standard procedure: null? object ¶

  Returns `#t` if object is the empty list; otherwise returns `#f`.

      (null? '())                             ⇒ #t
      (null? (list 'a 'b 'c))                 ⇒ #f
      (null? (cons* 'a 'b 'c))                ⇒ #f
      (null? (circular-list 'a 'b 'c))        ⇒ #f

SRFI 1 procedure: null-list? list ¶

  List is a proper or circular list. This procedure returns `#t` if the argument is the empty list `()`, and `#f` if the argument is a pair. It is an error to pass this procedure any other value. This procedure is recommended as the termination condition for list-processing procedures that are not defined on dotted lists.

standard procedure: list-ref list k ¶

  Returns the kth element of list, using zero-origin indexing. The valid indexes of a list are the exact non-negative integers less than the length of the list. The first element of a list has index `0`, the second has index `1`, and so on.

      (list-ref '(a b c d) 2)                 ⇒ c
      (list-ref '(a b c d)
                (exact (round 1.8)))
           ⇒ c

  `(list-ref list k)` is equivalent to `(car (drop list k))`.

SRFI 1 procedure: first list ¶
SRFI 1 procedure: second list ¶
SRFI 1 procedure: third list ¶
SRFI 1 procedure: fourth list ¶
SRFI 1 procedure: fifth list ¶
SRFI 1 procedure: sixth list ¶
SRFI 1 procedure: seventh list ¶
SRFI 1 procedure: eighth list ¶
SRFI 1 procedure: ninth list ¶
SRFI 1 procedure: tenth list ¶

  Returns the specified element of list. It is an error if list is not long enough to contain the specified element (for example, if the argument to `seventh` is a list that contains only six elements).

### 7.4 Cutting and Pasting Lists

SRFI 1 procedure: take x i ¶
SRFI 1 procedure: drop x i ¶

  `take` returns the first i elements of list x. `drop` returns all but the first i elements of list x.

      (take '(a b c d e)  2) => (a b)
      (drop '(a b c d e)  2) => (c d e)

  x may be any value—a proper, circular, or dotted list:

      (take '(1 2 3 . d) 2) => (1 2)
      (drop '(1 2 3 . d) 2) => (3 . d)
      (take '(1 2 3 . d) 3) => (1 2 3)
      (drop '(1 2 3 . d) 3) => d

  For a legal i, `take` and `drop` partition the list in a manner which can be inverted with `append`:

      (append (take x i) (drop x i)) = x

  `drop` is exactly equivalent to performing i `cdr` operations on x; the returned value shares a common tail with x. If the argument is a list of non-zero length, `take` is guaranteed to return a freshly-allocated list, even in the case where the entire list is taken, e.g. `(take lis (length lis))`.

obsolete procedure: list-head x i ¶
standard procedure: list-tail x i ¶

  Equivalent to `take` and `drop`, respectively. `list-head` is deprecated and should not be used. `list-tail` is defined by R7RS.

procedure: sublist list start end ¶

  Start and end must be exact integers satisfying

      0 <= start <= end <= (length list)

  `sublist` returns a newly allocated list formed from the elements of list beginning at index start (inclusive) and ending at end (exclusive).

standard procedure: append list … ¶

  Returns a list consisting of the elements of the first list followed by the elements of the other lists.

      (append '(x) '(y))                      ⇒ (x y)
      (append '(a) '(b c d))                  ⇒ (a b c d)
      (append '(a (b)) '((c)))                ⇒ (a (b) (c))
      (append)                                ⇒ ()

  The resulting list is always newly allocated, except that it shares structure with the last list argument. The last argument may actually be any object; an improper list results if the last argument is not a proper list.

      (append '(a b) '(c . d))                ⇒ (a b c . d)
      (append '() 'a)                         ⇒ a

SRFI 1 procedure: append! list … ¶

  Returns a list that is the argument lists concatenated together. The arguments are changed rather than copied. (Compare this with `append`, which copies arguments rather than destroying them.) For example:

      (define x (list 'a 'b 'c))
      (define y (list 'd 'e 'f))
      (define z (list 'g 'h))
      (append! x y z)                         ⇒ (a b c d e f g h)
      x                                       ⇒ (a b c d e f g h)
      y                                       ⇒ (d e f g h)
      z                                       ⇒ (g h)

SRFI 1 procedure: last pair ¶
SRFI 1 procedure: last-pair pair ¶

  `last` returns the last element of the non-empty, finite list pair. `last-pair` returns the last pair in the non-empty, finite list pair.

      (last '(a b c)) => c
      (last-pair '(a b c)) => (c)

obsolete procedure: except-last-pair list ¶
obsolete procedure: except-last-pair! list ¶

  These procedures are deprecated. Instead use `drop-right` or `drop-right!`, respectively, with a second argument of `1`.

### 7.5 Filtering Lists

SRFI 1 procedure: filter predicate list ¶

  Returns a newly allocated copy of list containing only the elements satisfying predicate. Predicate must be a procedure of one argument.

      (filter odd? '(1 2 3 4 5)) ⇒ (1 3 5)

SRFI 1 procedure: remove predicate list ¶

  Like `filter`, except that the returned list contains only those elements not satisfying predicate.

      (remove odd? '(1 2 3 4 5)) ⇒ (2 4)

SRFI 1 procedure: partition predicate list ¶

  Partitions the elements of list with predicate, and returns two values: the list of in-elements and the list of out-elements. The list is not disordered—elements occur in the result lists in the same order as they occur in the argument list. The dynamic order in which the various applications of `predicate` are made is not specified. One of the returned lists may share a common tail with the argument list.

      (partition symbol? '(one 2 3 four five 6)) =>
          (one four five)
          (2 3 6)

SRFI 1 procedure: filter! predicate list ¶
SRFI 1 procedure: remove! predicate list ¶
SRFI 1 procedure: partition! predicate list ¶

  Linear-update variants of `filter`, `remove` and `partition`. These procedures are allowed, but not required, to alter the cons cells in the argument list to construct the result lists.

SRFI 1 procedure: delete x list [compare] ¶
SRFI 1 procedure: delete! x list [compare] ¶

  `delete` uses the comparison procedure compare, which defaults to `equal?`, to find all elements of list that are equal to x, and deletes them from list. The dynamic order in which the various applications of compare are made is not specified.

  The list is not disordered—elements that appear in the result list occur in the same order as they occur in the argument list. The result may share a common tail with the argument list.

  Note that fully general element deletion can be performed with the `remove` and `remove!` procedures, e.g.:

      ;; Delete all the even elements from LIS:
      (remove even? lis)

  The comparison procedure is used in this way: `(compare x ei)`. That is, x is always the first argument, and a list element is always the second argument. The comparison procedure will be used to compare each element of list exactly once; the order in which it is applied to the various ei is not specified. Thus, one can reliably remove all the numbers greater than five from a list with `(delete 5 list <)`.

  `delete!` is the linear-update variant of `delete`. It is allowed, but not required, to alter the cons cells in its argument list to construct the result.

procedure: delq x list ¶
procedure: delq! x list ¶
procedure: delv x list ¶
procedure: delv! x list ¶

  Equivalent to `(delete x list eq?)`, `(delete! x list eq?)`, `(delete x list eqv?)`, and `(delete! x list eqv?)`, respectively.

procedure: delete-member-procedure deletor predicate ¶

  Returns a deletion procedure similar to `delv` or `delete!`. Deletor should be one of the procedures `list-deletor` or `list-deletor!`. Predicate must be an equivalence predicate. The returned procedure accepts exactly two arguments: first, an object to be deleted, and second, a list of objects from which it is to be deleted. If deletor is `list-deletor`, the procedure returns a newly allocated copy of the given list in which all entries equal to the given object have been removed. If deletor is `list-deletor!`, the procedure returns a list consisting of the top-level elements of the given list with all entries equal to the given object removed; the given list is destructively modified to produce the result. In either case predicate is used to compare the given object to the elements of the given list.

  Here are some examples that demonstrate how `delete-member-procedure` could have been used to implement `delv` and `delete!`:

      (define delv
        (delete-member-procedure list-deletor eqv?))
      (define delete!
        (delete-member-procedure list-deletor! equal?))

procedure: list-deletor predicate ¶
procedure: list-deletor! predicate ¶

  These procedures each return a procedure that deletes elements from lists. Predicate must be a procedure of one argument. The returned procedure accepts exactly one argument, which must be a proper list, and applies predicate to each of the elements of the argument, deleting those for which it is true.

  The procedure returned by `list-deletor` deletes elements non-destructively, by returning a newly allocated copy of the argument with the appropriate elements removed. The procedure returned by `list-deletor!` performs a destructive deletion.

### 7.6 Searching Lists

SRFI 1 procedure: find predicate list ¶

  Returns the first element in list for which predicate is true; returns `#f` if it doesn’t find such an element. Predicate must be a procedure of one argument.

      (find even? '(3 1 4 1 5 9)) => 4

  Note that `find` has an ambiguity in its lookup semantics—if `find` returns `#f`, you cannot tell (in general) if it found a `#f` element that satisfied predicate, or if it did not find any element at all. In many situations, this ambiguity cannot arise—either the list being searched is known not to contain any `#f` elements, or the list is guaranteed to have an element satisfying predicate. However, in cases where this ambiguity can arise, you should use `find-tail` instead of `find`—`find-tail` has no such ambiguity:

      (cond ((find-tail pred lis)
              => (lambda (pair) …)) ; Handle (CAR PAIR)
            (else …)) ; Search failed.

SRFI 1 procedure: find-tail predicate list ¶

  Returns the first pair of list whose car satisfies predicate; returns `#f` if there’s no such pair. `find-tail` can be viewed as a general-predicate variant of memv.

standard procedure: memq object list ¶
standard procedure: memv object list ¶
standard procedure: member object list [compare] ¶

  These procedures return the first pair of list whose car is object; the returned pair is always one from which list is composed. If object does not occur in list, `#f` (n.b.: not the empty list) is returned. `memq` uses `eq?` to compare object with the elements of list, while `memv` uses `eqv?` and `member` uses compare, or `equal?` if compare is not supplied.7

      (memq 'a '(a b c))                      ⇒ (a b c)
      (memq 'b '(a b c))                      ⇒ (b c)
      (memq 'a '(b c d))                      ⇒ #f
      (memq (list 'a) '(b (a) c))             ⇒ #f
      (member (list 'a) '(b (a) c))           ⇒ ((a) c)
      (memq 101 '(100 101 102))               ⇒ unspecified
      (memv 101 '(100 101 102))               ⇒ (101 102)

procedure: member-procedure predicate ¶

  Returns a procedure similar to `memq`, except that predicate, which must be an equivalence predicate, is used instead of `eq?`. This could be used to define `memv` as follows:

      (define memv (member-procedure eqv?))

### 7.7 Mapping of Lists

standard procedure: map procedure list list … ¶

  Procedure must be a procedure taking as many arguments as there are lists. If more than one list is given, then they must all be the same length. `map` applies procedure element-wise to the elements of the lists and returns a list of the results, in order from left to right. The dynamic order in which procedure is applied to the elements of the lists is unspecified; use `for-each` to sequence side effects.

      (map cadr '((a b) (d e) (g h)))           ⇒ (b e h)
      (map (lambda (n) (expt n n)) '(1 2 3 4))  ⇒ (1 4 27 256)
      (map + '(1 2 3) '(4 5 6))                 ⇒ (5 7 9)
      (let ((count 0))
        (map (lambda (ignored)
               (set! count (+ count 1))
               count)
             '(a b c)))                         ⇒ unspecified

obsolete procedure: map* knil proc list1 list2 … ¶

  Deprecated, use `fold-right` instead. Equivalent to

      (fold-right (lambda (e1 e2 … acc)
                    (cons* (proc e1)
                           (proc e2)
                           …
                           acc))
                  knil
                  list1
                  list2
                  …)

SRFI 1 procedure: append-map procedure list list … ¶

  Similar to `map` except that the results of applying procedure to the elements of lists are concatenated together by `append` rather than by `cons`. The following are equivalent, except that the former is more efficient:

      (append-map procedure list1 list2 …)
      (apply append (map procedure list1 list2 …))

obsolete procedure: append-map* knil proc list1 list2 … ¶

  Deprecated, use `fold-right` instead. Equivalent to

      (fold-right (lambda (e1 e2 … acc)
                    (append (proc e1)
                            (proc e2)
                            …
                            acc))
                  knil
                  list1
                  list2
                  …)

SRFI 1 procedure: append-map! proc list list … ¶

  Similar to `map` except that the results of applying proc to the elements of lists are concatenated together by `append!` rather than by `cons`. The following are equivalent, except that the former is more efficient:

      (append-map! proc list list …)
      (apply append! (map proc list list …))

obsolete procedure: append-map*! knil proc list1 list2 … ¶

  Deprecated, use `fold-right` instead. Equivalent to

      (fold-right (lambda (e1 e2 … acc)
                    (append! (proc e1)
                             (proc e2)
                             …
                             acc))
                  knil
                  list1
                  list2
                  …)

standard procedure: for-each procedure list list … ¶

  The arguments to `for-each` are like the arguments to `map`, but `for-each` calls procedure for its side effects rather than for its values. Unlike `map`, `for-each` is guaranteed to call procedure on the elements of the lists in order from the first element to the last, and the value returned by `for-each` is unspecified.

      (let ((v (make-vector 5)))
        (for-each (lambda (i)
                    (vector-set! v i (* i i)))
                  '(0 1 2 3 4))
        v)                            ⇒ #(0 1 4 9 16)

SRFI 1 procedure: any predicate list list … ¶

  Applies predicate across the lists, returning true if predicate returns true on any application.

  If there are n list arguments list1 … listn, then predicate must be a procedure taking n arguments and returning a boolean result.

  `any` applies predicate to the first elements of the list parameters. If this application returns a true value, `any` immediately returns that value. Otherwise, it iterates, applying predicate to the second elements of the list parameters, then the third, and so forth. The iteration stops when a true value is produced or one of the lists runs out of values; in the latter case, `any` returns `#f`. The application of predicate to the last element of the lists is a tail call.

  Note the difference between `find` and `any`—`find` returns the element that satisfied the predicate; `any` returns the true value that the predicate produced.

  Like `every`, `any`’s name does not end with a question mark—this is to indicate that it does not return a simple boolean (`#t` or `#f`), but a general value.

      (any integer? '(a 3 b 2.7))   => #t
      (any integer? '(a 3.1 b 2.7)) => #f
      (any < '(3 1 4 1 5)
             '(2 7 1 8 2)) => #t

SRFI 1 procedure: every predicate list list … ¶

  Applies predicate across the lists, returning true if predicate returns true on every application.

  If there are n list arguments list1 … listn, then predicate must be a procedure taking n arguments and returning a boolean result.

  `every` applies predicate to the first elements of the list parameters. If this application returns false, `every` immediately returns false. Otherwise, it iterates, applying predicate to the second elements of the list parameters, then the third, and so forth. The iteration stops when a false value is produced or one of the lists runs out of values. In the latter case, `every` returns the true value produced by its final application of predicate. The application of predicate to the last element of the lists is a tail call.

  If one of the lists has no elements, `every` simply returns `#t`.

  Like `any`, `every`’s name does not end with a question mark—this is to indicate that it does not return a simple boolean (`#t` or `#f`), but a general value.

### 7.8 Folding of Lists

SRFI 1 procedure: fold kons knil clist1 clist2 … ¶

  The fundamental list iterator.

  First, consider the single list-parameter case. If clist1 = `(e1 e2 … en)`, then this procedure returns

      (kons en … (kons e2 (kons e1 knil)) …)

  That is, it obeys the (tail) recursion

      (fold kons knil lis) = (fold kons (kons (car lis) knil) (cdr lis))
      (fold kons knil '()) = knil

  Examples:

      (fold + 0 lis)                  ; Add up the elements of LIS.

      (fold cons '() lis)             ; Reverse LIS.

      (fold cons tail rev-head)       ; See APPEND-REVERSE.

      ;; How many symbols in LIS?
      (fold (lambda (x count) (if (symbol? x) (+ count 1) count))
            0
            lis)

      ;; Length of the longest string in LIS:
      (fold (lambda (s max-len) (max max-len (string-length s)))
            0
            lis)

  If n list arguments are provided, then the kons procedure must take n+1 parameters: one element from each list, and the "seed" or fold state, which is initially knil. The fold operation terminates when the shortest list runs out of values:

      (fold cons* '() '(a b c) '(1 2 3 4 5)) => (c 3 b 2 a 1)

  At least one of the list arguments must be finite.

SRFI 1 procedure: fold-right kons knil clist1 clist2 … ¶

  The fundamental list recursion operator.

  First, consider the single list-parameter case. If clist1 = `(e1 e2 … en)`, then this procedure returns

      (kons e1 (kons e2 … (kons en knil)))

  That is, it obeys the recursion

      (fold-right kons knil lis) = (kons (car lis) (fold-right kons knil (cdr lis)))
      (fold-right kons knil '()) = knil

  Examples:

      (fold-right cons '() lis)               ; Copy LIS.

      ;; Filter the even numbers out of LIS.
      (fold-right (lambda (x l) (if (even? x) (cons x l) l)) '() lis))

  If n list arguments are provided, then the kons function must take n+1 parameters: one element from each list, and the "seed" or fold state, which is initially knil. The fold operation terminates when the shortest list runs out of values:

      (fold-right cons* '() '(a b c) '(1 2 3 4 5)) => (a 1 b 2 c 3)

  At least one of the list arguments must be finite.

obsolete procedure: fold-left proc knil list ¶

  Deprecated, use `fold` instead. Equivalent to

      (fold (lambda (acc elt) (proc elt acc)) knil list)

SRFI 1 procedure: reduce f ridentity list ¶

  `reduce` is a variant of `fold`.

  ridentity should be a "right identity" of the procedure f—that is, for any value x acceptable to f,

      (f x ridentity) = x

  `reduce` has the following definition:

      If list = (), return ridentity;
      Otherwise, return (fold f (car list) (cdr list)).

  ...in other words, we compute `(fold f ridentity list)`.

  Note that ridentity is used only in the empty-list case. You typically use `reduce` when applying f is expensive and you’d like to avoid the extra application incurred when `fold` applies f to the head of list and the identity value, redundantly producing the same value passed in to f. For example, if f involves searching a file directory or performing a database query, this can be significant. In general, however, `fold` is useful in many contexts where `reduce` is not (consider the examples given in the `fold` definition—only one of the five `fold`s uses a function with a right identity. The other four may not be performed with `reduce`).

      ;; Take the max of a list of non-negative integers.
      (reduce max 0 nums) ; i.e., (apply max 0 nums)

SRFI 1 procedure: reduce-right kons knil list ¶

  `reduce-right` is the `fold-right` variant of `reduce`. It obeys the following definition:

      (reduce-right f ridentity '()) = ridentity
      (reduce-right f ridentity '(e1)) = (f e1 ridentity) = e1
      (reduce-right f ridentity '(e1 e2 …)) =
          (f e1 (reduce f ridentity '(e2 …)))

  ...in other words, we compute `(fold-right f ridentity list)`.

      ;; Append a bunch of lists together.
      ;; I.e., (apply append list-of-lists)
      (reduce-right append '() list-of-lists)

obsolete procedure: reduce-left f ridentity list ¶

  Deprecated, use `reduce` instead. Equivalent to

      (reduce (lambda (acc elt) (f elt acc)) ridentity list)

### 7.9 Miscellaneous List Operations

SRFI 1 procedure: circular-list object … ¶
procedure: make-circular-list k [element] ¶

  `circular-list` returns a circular list containing the given objects. `make-circular-list` returns a circular list of length k; if element is given, the returned list is filled with it, otherwise the elements are unspecified.

  This procedure is like `list` except that the returned list is circular. `circular-list` could have been defined like this:

      (define (circular-list . objects)
        (append! objects objects))

  `circular-list` is compatible with SRFI 1, but extended so that it can be called with no arguments.

standard procedure: reverse list ¶

  Returns a newly allocated list consisting of the top-level elements of list in reverse order.

      (reverse '(a b c))                  ⇒ (c b a)
      (reverse '(a (b c) d (e (f))))      ⇒ ((e (f)) d (b c) a)

SRFI 1 procedure: reverse! list ¶

  Returns a list consisting of the top-level elements of list in reverse order. `reverse!` is like `reverse`, except that it destructively modifies list. Because the result may not be `eqv?` to list, it is desirable to do something like `(set! x (reverse! x))`.

procedure: sort sequence procedure ¶
procedure: merge-sort sequence procedure ¶
procedure: quick-sort sequence procedure ¶

  Sequence must be either a list or a vector. Procedure must be a procedure of two arguments that defines a total ordering on the elements of sequence. In other words, if x and y are two distinct elements of sequence, then it must be the case that

      (and (procedure x y)
           (procedure y x))
           ⇒ #f

  If sequence is a list (vector), `sort` returns a newly allocated list (vector) whose elements are those of sequence, except that they are rearranged to be sorted in the order defined by procedure. So, for example, if the elements of sequence are numbers, and procedure is `<`, then the resulting elements are sorted in monotonically nondecreasing order. Likewise, if procedure is `>`, the resulting elements are sorted in monotonically nonincreasing order. To be precise, if x and y are any two adjacent elements in the result, where x precedes y, it is the case that

      (procedure y x)
           ⇒ #f

  Two sorting algorithms are implemented: `merge-sort` and `quick-sort`. The procedure `sort` is an alias for `merge-sort`.

  See also the definition of `sort!`.

## 8 Vectors

Vectors are heterogenous structures whose elements are indexed by exact non-negative integers. A vector typically occupies less space than a list of the same length, and the average time required to access a randomly chosen element is typically less for the vector than for the list.

The length of a vector is the number of elements that it contains. This number is an exact non-negative integer that is fixed when the vector is created. The valid indexes of a vector are the exact non-negative integers less than the length of the vector. The first element in a vector is indexed by zero, and the last element is indexed by one less than the length of the vector.

Vectors are written using the notation `#(object …)`. For example, a vector of length 3 containing the number zero in element 0, the list `(2 2 2 2)` in element 1, and the string `"Anna"` in element 2 can be written as

    #(0 (2 2 2 2) "Anna")

Note that this is the external representation of a vector, not an expression evaluating to a vector. Like list constants, vector constants must be quoted:

    '#(0 (2 2 2 2) "Anna")          ⇒  #(0 (2 2 2 2) "Anna")

A number of the vector procedures operate on subvectors. A subvector is a segment of a vector that is specified by two exact non-negative integers, start and end. Start is the index of the first element that is included in the subvector, and end is one greater than the index of the last element that is included in the subvector. Thus if start and end are the same, they refer to a null subvector, and if start is zero and end is the length of the vector, they refer to the entire vector. The valid indexes of a subvector are the exact integers between start inclusive and end exclusive.

### 8.1 Construction of Vectors

procedure: make-vector k [object] ¶

  Returns a newly allocated vector of k elements. If object is specified, `make-vector` initializes each element of the vector to object. Otherwise the initial elements of the result are unspecified.

procedure: vector object … ¶

  Returns a newly allocated vector whose elements are the given arguments. `vector` is analogous to `list`.

      (vector 'a 'b 'c)                       ⇒  #(a b c)

procedure: vector-copy vector ¶

  Returns a newly allocated vector that is a copy of vector.

procedure: list->vector list ¶

  Returns a newly allocated vector initialized to the elements of list. The inverse of `list->vector` is `vector->list`.

      (list->vector '(dididit dah))           ⇒  #(dididit dah)

standard procedure: string->vector string [start [end]] ¶
standard procedure: vector->string vector [start [end]] ¶

  It is an error if any element of vector is not a character.

  The `vector->string` procedure returns a newly allocated string of the objects contained in the elements of vector between start and end. The `string->vector` procedure returns a newly created vector initialized to the elements of the string string between start and end.

  In both procedures, order is preserved.

      (string->vector "ABC")                  ⇒  #(#\A #\B #\C)
      (vector->string #(#\1 #\2 #\3)          ⇒  "123"

procedure: make-initialized-vector k initialization ¶

  Similar to `make-vector`, except that the elements of the result are determined by calling the procedure initialization on the indices. For example:

      (make-initialized-vector 5 (lambda (x) (* x x)))
           ⇒  #(0 1 4 9 16)

procedure: vector-grow vector k ¶

  K must be greater than or equal to the length of vector. Returns a newly allocated vector of length k. The first `(vector-length vector)` elements of the result are initialized from the corresponding elements of vector. The remaining elements of the result are unspecified.

procedure: vector-map procedure vector ¶

  Procedure must be a procedure of one argument. `vector-map` applies procedure element-wise to the elements of vector and returns a newly allocated vector of the results, in order from left to right. The dynamic order in which procedure is applied to the elements of vector is unspecified.

      (vector-map cadr '#((a b) (d e) (g h)))     ⇒  #(b e h)
      (vector-map (lambda (n) (expt n n)) '#(1 2 3 4))
                                                  ⇒  #(1 4 27 256)
      (vector-map + '#(5 7 9))                    ⇒  #(5 7 9)

### 8.2 Selecting Vector Components

procedure: vector? object ¶

  Returns `#t` if object is a vector; otherwise returns `#f`.

procedure: vector-length vector ¶

  Returns the number of elements in vector.

procedure: vector-ref vector k ¶

  Returns the contents of element k of vector. K must be a valid index of vector.

      (vector-ref '#(1 1 2 3 5 8 13 21) 5)    ⇒  8

procedure: vector-set! vector k object ¶

  Stores object in element k of vector and returns an unspecified value. K must be a valid index of vector.

      (let ((vec (vector 0 '(2 2 2 2) "Anna")))
        (vector-set! vec 1 '("Sue" "Sue"))
        vec)
           ⇒  #(0 ("Sue" "Sue") "Anna")

procedure: vector-first vector ¶
procedure: vector-second vector ¶
procedure: vector-third vector ¶
procedure: vector-fourth vector ¶
procedure: vector-fifth vector ¶
procedure: vector-sixth vector ¶
procedure: vector-seventh vector ¶
procedure: vector-eighth vector ¶

  These procedures access the first several elements of vector in the obvious way. It is an error if the implicit index of one of these procedurs is not a valid index of vector.

procedure: vector-binary-search vector key<? unwrap-key key ¶

  Searches vector for an element with a key matching key, returning the element if one is found or `#f` if none. The search operation takes time proportional to the logarithm of the length of vector. Unwrap-key must be a procedure that maps each element of vector to a key. Key<? must be a procedure that implements a total ordering on the keys of the elements.

      (define (translate number)
        (vector-binary-search '#((1 . i)
                                 (2 . ii)
                                 (3 . iii)
                                 (6 . vi))
                              < car number))
      (translate 2)  ⇒  (2 . ii)
      (translate 4)  ⇒  #F

### 8.3 Cutting Vectors

procedure: subvector vector start end ¶

  Returns a newly allocated vector that contains the elements of vector between index start (inclusive) and end (exclusive).

procedure: vector-head vector end ¶

  Equivalent to

      (subvector vector 0 end)

procedure: vector-tail vector start ¶

  Equivalent to

      (subvector vector start (vector-length vector))

### 8.4 Modifying Vectors

procedure: vector-fill! vector object ¶
procedure: subvector-fill! vector start end object ¶

  Stores object in every element of the vector (subvector) and returns an unspecified value.

procedure: subvector-move-left! vector1 start1 end1 vector2 start2 ¶
procedure: subvector-move-right! vector1 start1 end1 vector2 start2 ¶

  Destructively copies the elements of vector1, starting with index start1 (inclusive) and ending with end1 (exclusive), into vector2 starting at index start2 (inclusive). Vector1, start1, and end1 must specify a valid subvector, and start2 must be a valid index for vector2. The length of the source subvector must not exceed the length of vector2 minus the index start2.

  The elements are copied as follows (note that this is only important when vector1 and vector2 are `eqv?`):

  `subvector-move-left!`

    The copy starts at the left end and moves toward the right (from smaller indices to larger). Thus if vector1 and vector2 are the same, this procedure moves the elements toward the left inside the vector.

  `subvector-move-right!`

    The copy starts at the right end and moves toward the left (from larger indices to smaller). Thus if vector1 and vector2 are the same, this procedure moves the elements toward the right inside the vector.

procedure: sort! vector procedure ¶
procedure: merge-sort! vector procedure ¶
procedure: quick-sort! vector procedure ¶

  Procedure must be a procedure of two arguments that defines a total ordering on the elements of vector. The elements of vector are rearranged so that they are sorted in the order defined by procedure. The elements are rearranged in place, that is, vector is destructively modified so that its elements are in the new order.

  `sort!` returns vector as its value.

  Two sorting algorithms are implemented: `merge-sort!` and `quick-sort!`. The procedure `sort!` is an alias for `merge-sort!`.

  See also the definition of `sort`.

## 9 Bit Strings

A bit string is a sequence of bits. Bit strings can be used to represent sets or to manipulate binary data. The elements of a bit string are numbered from zero up to the number of bits in the string less one, in right to left order, (the rightmost bit is numbered zero). When you convert from a bit string to an integer, the zero-th bit is associated with the zero-th power of two, the first bit is associated with the first power, and so on.

Bit strings are encoded very densely in memory. Each bit occupies exactly one bit of storage, and the overhead for the entire bit string is bounded by a small constant. However, accessing a bit in a bit string is slow compared to accessing an element of a vector or character string. If performance is of overriding concern, it is better to use character strings to store sets of boolean values even though they occupy more space.

The length of a bit string is the number of bits that it contains. This number is an exact non-negative integer that is fixed when the bit string is created. The valid indexes of a bit string are the exact non-negative integers less than the length of the bit string.

Bit strings may contain zero or more bits. They are not limited by the length of a machine word. In the printed representation of a bit string, the contents of the bit string are preceded by ‘`#*`’. The contents are printed starting with the most significant bit (highest index).

Note that the external representation of bit strings uses a bit ordering that is the reverse of the representation for bit strings in Common Lisp. It is likely that MIT/GNU Scheme’s representation will be changed in the future, to be compatible with Common Lisp. For the time being this representation should be considered a convenience for viewing bit strings rather than a means of entering them as data.

    #*11111
    #*1010
    #*00000000
    #*

All of the bit-string procedures are MIT/GNU Scheme extensions.

### 9.1 Construction of Bit Strings

procedure: make-bit-string k initialization ¶

  Returns a newly allocated bit string of length k. If initialization is `#f`, the bit string is filled with 0 bits; otherwise, the bit string is filled with 1 bits.

      (make-bit-string 7 #f)                  ⇒  #*0000000

procedure: bit-string-allocate k ¶

  Returns a newly allocated bit string of length k, but does not initialize it.

procedure: bit-string-copy bit-string ¶

  Returns a newly allocated copy of bit-string.

### 9.2 Selecting Bit String Components

procedure: bit-string? object ¶

  Returns `#t` if object is a bit string; otherwise returns `#f`.

procedure: bit-string-length bit-string ¶

  Returns the length of bit-string.

procedure: bit-string-ref bit-string k ¶

  Returns `#t` if the kth bit is 1; otherwise returns `#f`. K must be a valid index of bit-string.

procedure: bit-string-set! bit-string k ¶

  Sets the kth bit in bit-string to 1 and returns an unspecified value. K must be a valid index of bit-string.

procedure: bit-string-clear! bit-string k ¶

  Sets the kth bit in bit-string to 0 and returns an unspecified value. K must be a valid index of bit-string.

procedure: bit-substring-find-next-set-bit bit-string start end ¶

  Returns the index of the first occurrence of a set bit in the substring of bit-string from start (inclusive) to end (exclusive). If none of the bits in the substring are set `#f` is returned. The index returned is relative to the whole bit string, not substring.

  The following procedure uses `bit-substring-find-next-set-bit` to find all the set bits and display their indexes:

      (define (scan-bitstring bs)
        (let ((end (bit-string-length bs)))
          (let loop ((start 0))
            (let ((next
                   (bit-substring-find-next-set-bit bs start end)))
              (if next
                  (begin
                    (write-line next)
                    (if (< next end)
                        (loop (+ next 1)))))))))

### 9.3 Cutting and Pasting Bit Strings

procedure: bit-string-append bit-string-1 bit-string-2 ¶

  Appends the two bit string arguments, returning a newly allocated bit string as its result. In the result, the bits copied from bit-string-1 are less significant (smaller indices) than those copied from bit-string-2.

procedure: bit-substring bit-string start end ¶

  Returns a newly allocated bit string whose bits are copied from bit-string, starting at index start (inclusive) and ending at end (exclusive).

### 9.4 Bitwise Operations on Bit Strings

procedure: bit-string-zero? bit-string ¶

  Returns `#t` if bit-string contains only 0 bits; otherwise returns `#f`.

procedure: bit-string=? bit-string-1 bit-string-2 ¶

  Compares the two bit string arguments and returns `#t` if they are the same length and contain the same bits; otherwise returns `#f`.

procedure: bit-string-not bit-string ¶

  Returns a newly allocated bit string that is the bitwise-logical negation of bit-string.

procedure: bit-string-movec! target-bit-string bit-string ¶

  The destructive version of `bit-string-not`. The arguments target-bit-string and bit-string must be bit strings of the same length. The bitwise-logical negation of bit-string is computed and the result placed in target-bit-string. The value of this procedure is unspecified.

procedure: bit-string-and bit-string-1 bit-string-2 ¶

  Returns a newly allocated bit string that is the bitwise-logical “and” of the arguments. The arguments must be bit strings of identical length.

procedure: bit-string-andc bit-string-1 bit-string-2 ¶

  Returns a newly allocated bit string that is the bitwise-logical “and” of bit-string-1 with the bitwise-logical negation of bit-string-2. The arguments must be bit strings of identical length.

procedure: bit-string-or bit-string-1 bit-string-2 ¶

  Returns a newly allocated bit string that is the bitwise-logical “inclusive or” of the arguments. The arguments must be bit strings of identical length.

procedure: bit-string-xor bit-string-1 bit-string-2 ¶

  Returns a newly allocated bit string that is the bitwise-logical “exclusive or” of the arguments. The arguments must be bit strings of identical length.

procedure: bit-string-and! target-bit-string bit-string ¶
procedure: bit-string-or! target-bit-string bit-string ¶
procedure: bit-string-xor! target-bit-string bit-string ¶
procedure: bit-string-andc! target-bit-string bit-string ¶

  These are destructive versions of the above operations. The arguments target-bit-string and bit-string must be bit strings of the same length. Each of these procedures performs the corresponding bitwise-logical operation on its arguments, places the result into target-bit-string, and returns an unspecified result.

### 9.5 Modification of Bit Strings

procedure: bit-string-fill! bit-string initialization ¶

  Fills bit-string with zeroes if initialization is `#f`; otherwise fills bit-string with ones. Returns an unspecified value.

procedure: bit-string-move! target-bit-string bit-string ¶

  Moves the contents of bit-string into target-bit-string. Both arguments must be bit strings of the same length. The results of the operation are undefined if the arguments are the same bit string.

procedure: bit-substring-move-right! bit-string-1 start1 end1 bit-string-2 start2 ¶

  Destructively copies the bits of bit-string-1, starting at index start1 (inclusive) and ending at end1 (exclusive), into bit-string-2 starting at index start2 (inclusive). Start1 and end1 must be valid substring indices for bit-string-1, and start2 must be a valid index for bit-string-2. The length of the source substring must not exceed the length of bit-string-2 minus the index start2.

  The bits are copied starting from the MSB and working towards the LSB; the direction of copying only matters when bit-string-1 and bit-string-2 are `eqv?`.

### 9.6 Integer Conversions of Bit Strings

procedure: unsigned-integer->bit-string length integer ¶

  Both length and integer must be exact non-negative integers. Converts integer into a newly allocated bit string of length bits. Signals an error of type `condition-type:bad-range-argument` if integer is too large to be represented in length bits.

procedure: signed-integer->bit-string length integer ¶

  Length must be an exact non-negative integer, and integer may be any exact integer. Converts integer into a newly allocated bit string of length bits, using two’s complement encoding for negative numbers. Signals an error of type `condition-type:bad-range-argument` if integer is too large to be represented in length bits.

procedure: bit-string->unsigned-integer bit-string ¶
procedure: bit-string->signed-integer bit-string ¶

  Converts bit-string into an exact integer. `bit-string->signed-integer` regards bit-string as a two’s complement representation of a signed integer, and produces an integer of like sign and absolute value. `bit-string->unsigned-integer` regards bit-string as an unsigned quantity and converts to an integer accordingly.

## 10 Miscellaneous Datatypes

### 10.1 Booleans

The boolean objects are true and false. The boolean constant true is written as ‘`#t`’, and the boolean constant false is written as ‘`#f`’.

The primary use for boolean objects is in the conditional expressions `if`, `cond`, `and`, and `or`; the behavior of these expressions is determined by whether objects are true or false. These expressions count only `#f` as false. They count everything else, including `#t`, pairs, symbols, numbers, strings, vectors, and procedures as true (but see True and False).

Programmers accustomed to other dialects of Lisp should note that Scheme distinguishes `#f` and the empty list from the symbol `nil`. Similarly, `#t` is distinguished from the symbol `t`. In fact, the boolean objects (and the empty list) are not symbols at all.

Boolean constants evaluate to themselves, so you don’t need to quote them.

    #t                                      ⇒  #t
    #f                                      ⇒  #f
    '#f                                     ⇒  #f
    t                                       error→ Unbound variable

variable: false ¶
variable: true ¶

  These variables are bound to the objects `#f` and `#t` respectively. The compiler, given the `usual-integrations` declaration, replaces references to these variables with their respective values.

  Note that the symbol `true` is not equivalent to `#t`, and the symbol `false` is not equivalent to `#f`.

standard procedure: boolean? object ¶

  Returns `#t` if object is either `#t` or `#f`; otherwise returns `#f`.

      (boolean? #f)                           ⇒  #t
      (boolean? 0)                            ⇒  #f

standard procedure: not object ¶
procedure: false? object ¶

  These procedures return `#t` if object is false; otherwise they return `#f`. In other words they invert boolean values. These two procedures have identical semantics; their names are different to give different connotations to the test.

      (not #t)                                ⇒  #f
      (not 3)                                 ⇒  #f
      (not (list 3))                          ⇒  #f
      (not #f)                                ⇒  #t

extended standard procedure: procedure boolean=? boolean1 boolean2 boolean3 … ¶

  This predicate is true iff the boolean args are either all true or all false.

  Implementation note: The standard requires this procedure’s arguments to satisfy `boolean?`, but MIT/GNU Scheme allows any object to be an argument.

procedure: boolean/and object … ¶

  This procedure returns `#t` if none of its arguments are `#f`. Otherwise it returns `#f`.

procedure: boolean/or object … ¶

  This procedure returns `#f` if all of its arguments are `#f`. Otherwise it returns `#t`.

### 10.2 Symbols

MIT/GNU Scheme provides two types of symbols: interned and uninterned. Interned symbols are far more common than uninterned symbols, and there are more ways to create them. Interned symbols have an external representation that is recognized by the procedure `read`; uninterned symbols do not.8

Interned symbols have an extremely useful property: any two interned symbols whose names are the same, in the sense of `string=?`, are the same object (i.e. they are `eq?` to one another). The term interned refers to the process of interning by which this is accomplished. Uninterned symbols do not share this property.

The names of interned symbols are not distinguished by their alphabetic case. Because of this, MIT/GNU Scheme converts all alphabetic characters in the name of an interned symbol to a specific case (lower case) when the symbol is created. When the name of an interned symbol is referenced (using `symbol->string`) or written (using `write`) it appears in this case. It is a bad idea to depend on the name being lower case. In fact, it is preferable to take this one step further: don’t depend on the name of a symbol being in a uniform case.

The rules for writing an interned symbol are the same as the rules for writing an identifier (see Identifiers). Any interned symbol that has been returned as part of a literal expression, or read using the `read` procedure and subsequently written out using the `write` procedure, will read back in as the identical symbol (in the sense of `eq?`).

Usually it is also true that reading in an interned symbol that was previously written out produces the same symbol. An exception are symbols created by the procedures `string->symbol` and `intern`; they can create symbols for which this write/read invariance may not hold because the symbols’ names contain special characters or letters in the non-standard case.9

The external representation for uninterned symbols is special, to distinguish them from interned symbols and prevent them from being recognized by the `read` procedure:

    (string->uninterned-symbol "foo")
         ⇒  #[uninterned-symbol 30 foo]

In this section, the procedures that return symbols as values will either always return interned symbols, or always return uninterned symbols. The procedures that accept symbols as arguments will always accept either interned or uninterned symbols, and do not distinguish the two.

procedure: symbol? object ¶

  Returns `#t` if object is a symbol, otherwise returns `#f`.

      (symbol? 'foo)                                  ⇒  #t
      (symbol? (car '(a b)))                          ⇒  #t
      (symbol? "bar")                                 ⇒  #f

procedure: symbol->string symbol ¶

  Returns the name of symbol as a string. If symbol was returned by `string->symbol`, the value of this procedure will be identical (in the sense of `string=?`) to the string that was passed to `string->symbol`. It is an error to apply mutation procedures such as `string-set!` to strings returned by this procedure.

      (symbol->string 'flying-fish)           ⇒  "flying-fish"
      (symbol->string 'Martin)                ⇒  "martin"
      (symbol->string (string->symbol "Malvina"))
                                              ⇒  "Malvina"

  Note that two distinct uninterned symbols can have the same name.

procedure: intern string ¶

  Returns the interned symbol whose name is string. Converts string to the standard alphabetic case before generating the symbol. This is the preferred way to create interned symbols, as it guarantees the following independent of which case the implementation uses for symbols’ names:

      (eq? 'bitBlt (intern "bitBlt")) ⇒     #t

  The user should take care that string obeys the rules for identifiers (see Identifiers), otherwise the resulting symbol cannot be read as itself.

procedure: intern-soft string ¶

  Returns the interned symbol whose name is string. Converts string to the standard alphabetic case before generating the symbol. If no such interned symbol exists, returns `#f`.

  This is exactly like `intern`, except that it will not create an interned symbol, but only returns symbols that already exist.

procedure: string->symbol string ¶

  Returns the interned symbol whose name is string. Although you can use this procedure to create symbols with names containing special characters or lowercase letters, it’s usually a bad idea to create such symbols because they cannot be read as themselves. See `symbol->string`.

      (eq? 'mISSISSIppi 'mississippi)         ⇒  #t
      (string->symbol "mISSISSIppi")
           ⇒  the symbol with the name "mISSISSIppi"
      (eq? 'bitBlt (string->symbol "bitBlt")) ⇒  #f
      (eq? 'JollyWog
            (string->symbol
              (symbol->string 'JollyWog)))    ⇒  #t
      (string=? "K. Harper, M.D."
                 (symbol->string
                   (string->symbol
                     "K. Harper, M.D.")))     ⇒  #t

procedure: string->uninterned-symbol string ¶

  Returns a newly allocated uninterned symbol whose name is string. It is unimportant what case or characters are used in string.

  Note: this is the fastest way to make a symbol.

procedure: generate-uninterned-symbol [object] ¶

  Returns a newly allocated uninterned symbol that is guaranteed to be different from any other object. The symbol’s name consists of a prefix string followed by the (exact non-negative integer) value of an internal counter. The counter is initially zero, and is incremented after each call to this procedure.

  The optional argument object is used to control how the symbol is generated. It may take one of the following values:

  - If object is omitted or `#f`, the prefix is `"G"`.
  - If object is an exact non-negative integer, the internal counter is set to that integer prior to generating the result.
  - If object is a string, it is used as the prefix.
  - If object is a symbol, its name is used as the prefix.

      (generate-uninterned-symbol)
           ⇒  #[uninterned-symbol 31 G0]
      (generate-uninterned-symbol)
           ⇒  #[uninterned-symbol 32 G1]
      (generate-uninterned-symbol 'this)
           ⇒  #[uninterned-symbol 33 this2]
      (generate-uninterned-symbol)
           ⇒  #[uninterned-symbol 34 G3]
      (generate-uninterned-symbol 100)
           ⇒  #[uninterned-symbol 35 G100]
      (generate-uninterned-symbol)
           ⇒  #[uninterned-symbol 36 G101]

procedure: symbol-append symbol … ¶

  Returns the interned symbol whose name is formed by concatenating the names of the given symbols. This procedure preserves the case of the names of its arguments, so if one or more of the arguments’ names has non-standard case, the result will also have non-standard case.

      (symbol-append 'foo- 'bar)              ⇒  foo-bar
      ;; the arguments may be uninterned:
      (symbol-append 'foo- (string->uninterned-symbol "baz"))
                                              ⇒  foo-baz
      ;; the result has the same case as the arguments:
      (symbol-append 'foo- (string->symbol "BAZ"))    ⇒  foo-BAZ

procedure: symbol-hash symbol ¶

  Returns a hash number for symbol, which is computed by calling `string-hash` on symbol’s name. The hash number is an exact non-negative integer.

procedure: symbol-hash-mod symbol modulus ¶

  Modulus must be an exact positive integer. Equivalent to

      (modulo (symbol-hash symbol) modulus)

  This procedure is provided for convenience in constructing hash tables. However, it is normally preferable to use `make-strong-eq-hash-table` to build hash tables keyed by symbols, because `eq?` hash tables are much faster.

procedure: symbol<? symbol1 symbol2 ¶

  This procedure computes a total order on symbols. It is equivalent to

      (string<? (symbol->string symbol1)
                (symbol->string symbol2))

### 10.3 Parameters

Parameters are objects that can be bound to new values for the duration of a dynamic extent. See Dynamic Binding.

procedure: make-parameter init [converter] ¶
procedure: make-unsettable-parameter init [converter] ¶

  Returns a newly allocated parameter object, which is a procedure that accepts zero arguments and returns the value associated with the parameter object. Initially this value is the value of `(converter init)`, or of init if the conversion procedure converter is not specified. The associated value can be temporarily changed using the `parameterize` special form (see parameterize).

  The `make-parameter` procedure is standardized by SRFI 39 and by R7RS, while `make-unsettable-parameter` is an MIT/GNU Scheme extension.

procedure: make-settable-parameter init [converter] ¶

  This procedure is like `make-parameter`, except that the returned parameter object may also be assigned by passing it an argument. Note that an assignment to a settable parameter affects only the extent of its current binding.

  `make-settable-parameter` is an MIT/GNU Scheme extension.

procedure: parameterize* bindings thunk ¶

  Bindings should be an alist associating parameter objects with new values. Returns the value of thunk while the parameters are dynamically bound to the values.

  Note that the `parameterize` special form expands into a call to this procedure. `parameterize*` is an MIT/GNU Scheme extension.

#### 10.3.1 Cells

A cell object is very similar to a parameter but is not implemented in multi-processing worlds and thus is deprecated. Parameters should be used instead.

procedure: cell? object ¶

  Returns `#t` if object is a cell; otherwise returns `#f`.

procedure: make-cell object ¶

  Returns a newly allocated cell whose contents is object.

procedure: cell-contents cell ¶

  Returns the current contents of cell.

procedure: set-cell-contents! cell object ¶

  Alters the contents of cell to be object. Returns an unspecified value.

procedure: bind-cell-contents! cell object thunk ¶

  Alters the contents of cell to be object, calls thunk with no arguments, then restores the original contents of cell and returns the value returned by thunk. This is completely equivalent to dynamic binding of a variable, including the behavior when continuations are used (see Dynamic Binding).

### 10.4 Records

MIT/GNU Scheme provides a record abstraction, which is a simple and flexible mechanism for building structures with named components. Records can be defined and accessed using the procedures defined in this section. A less flexible but more concise way to manipulate records is to use the `define-structure` special form (see Structure Definitions).

procedure: make-record-type type-name field-names ¶

  Returns a record-type descriptor, a value representing a new data type, disjoint from all others. The type-name argument must be a string, but is only used for debugging purposes (such as the printed representation of a record of the new type). The field-names argument is a list of symbols naming the fields of a record of the new type. It is an error if the list contains any duplicates. It is unspecified how record-type descriptors are represented.

procedure: record-constructor record-type [field-names] ¶

  Returns a procedure for constructing new members of the type represented by record-type. The returned procedure accepts exactly as many arguments as there are symbols in the given list, field-names; these are used, in order, as the initial values of those fields in a new record, which is returned by the constructor procedure. The values of any fields not named in the list of field-names are unspecified. The field-names argument defaults to the list of field-names in the call to `make-record-type` that created the type represented by record-type; if the field-names argument is provided, it is an error if it contains any duplicates or any symbols not in the default list.

procedure: record-keyword-constructor record-type ¶

  Returns a procedure for constructing new members of the type represented by record-type. The returned procedure accepts arguments in a keyword list, which is an alternating sequence of names and values. In other words, the number of arguments must be a multiple of two, and every other argument, starting with the first argument, must be a symbol that is one of the field names for record-type.

  The returned procedure may be called with a keyword list that contains multiple instances of the same keyword. In this case, the leftmost instance is used and the other instances are ignored. This allows keyword lists to be accumulated using `cons` or `cons*`, and new bindings added to the front of the list override old bindings at the end.

procedure: record-predicate record-type ¶

  Returns a procedure for testing membership in the type represented by record-type. The returned procedure accepts exactly one argument and returns `#t` if the argument is a member of the indicated record type; it returns `#f` otherwise.

procedure: record-accessor record-type field-name ¶

  Returns a procedure for reading the value of a particular field of a member of the type represented by record-type. The returned procedure accepts exactly one argument which must be a record of the appropriate type; it returns the current value of the field named by the symbol field-name in that record. The symbol field-name must be a member of the list of field names in the call to `make-record-type` that created the type represented by record-type.

procedure: record-modifier record-type field-name ¶

  Returns a procedure for writing the value of a particular field of a member of the type represented by record-type. The returned procedure accepts exactly two arguments: first, a record of the appropriate type, and second, an arbitrary Scheme value; it modifies the field named by the symbol field-name in that record to contain the given value. The returned value of the modifier procedure is unspecified. The symbol field-name must be a member of the list of field names in the call to `make-record-type` that created the type represented by record-type.

procedure: record? object ¶

  Returns `#t` if object is a record of any type and `#f` otherwise. Note that `record?` may be true of any Scheme value; of course, if it returns `#t` for some particular value, then `record-type-descriptor` is applicable to that value and returns an appropriate descriptor.

procedure: record-type-descriptor record ¶

  Returns the record-type descriptor representing the type of record. That is, for example, if the returned descriptor were passed to `record-predicate`, the resulting predicate would return `#t` when passed record. Note that it is not necessarily the case that the returned descriptor is the one that was passed to `record-constructor` in the call that created the constructor procedure that created record.

procedure: record-type? object ¶

  Returns `#t` if object is a record-type descriptor; otherwise returns `#f`.

procedure: record-type-name record-type ¶

  Returns the type name associated with the type represented by record-type. The returned value is `eqv?` to the type-name argument given in the call to `make-record-type` that created the type represented by record-type.

procedure: record-type-field-names record-type ¶

  Returns a list of the symbols naming the fields in members of the type represented by record-type. The returned value is `equal?` to the field-names argument given in the call to `make-record-type` that created the type represented by record-type.10

### 10.5 Promises

special form: delay expression ¶

  The `delay` construct is used together with the procedure `force` to implement lazy evaluation or call by need. `(delay expression)` returns an object called a promise which at some point in the future may be asked (by the `force` procedure) to evaluate expression and deliver the resulting value.

procedure: force promise ¶

  Forces the value of promise. If no value has been computed for the promise, then a value is computed and returned. The value of the promise is cached (or “memoized”) so that if it is forced a second time, the previously computed value is returned without any recomputation.

      (force (delay (+ 1 2)))                 ⇒  3

      (let ((p (delay (+ 1 2))))
        (list (force p) (force p)))           ⇒  (3 3)

      (define head car)

      (define tail
        (lambda (stream)
          (force (cdr stream))))

      (define a-stream
        (letrec ((next
                  (lambda (n)
                    (cons n (delay (next (+ n 1)))))))
          (next 0)))

      (head (tail (tail a-stream)))           ⇒  2

procedure: promise? object ¶

  Returns `#t` if object is a promise; otherwise returns `#f`.

procedure: promise-forced? promise ¶

  Returns `#t` if promise has been forced and its value cached; otherwise returns `#f`.

procedure: promise-value promise ¶

  If promise has been forced and its value cached, this procedure returns the cached value. Otherwise, an error is signalled.

`force` and `delay` are mainly intended for programs written in functional style. The following examples should not be considered to illustrate good programming style, but they illustrate the property that the value of a promise is computed at most once.

    (define count 0)

    (define p
      (delay
       (begin
         (set! count (+ count 1))
         (* x 3))))

    (define x 5)

    count                                   ⇒  0
    p                                       ⇒  #[promise 54]
    (force p)                               ⇒  15
    p                                       ⇒  #[promise 54]
    count                                   ⇒  1
    (force p)                               ⇒  15
    count                                   ⇒  1

Here is a possible implementation of `delay` and `force`. We define the expression

    (delay expression)

to have the same meaning as the procedure call

    (make-promise (lambda () expression))

where `make-promise` is defined as follows:

    (define make-promise
      (lambda (proc)
        (let ((already-run? #f)
              (result #f))
          (lambda ()
            (cond ((not already-run?)
                   (set! result (proc))
                   (set! already-run? #t)))
            result))))

Promises are implemented here as procedures of no arguments, and `force` simply calls its argument.

    (define force
      (lambda (promise)
        (promise)))

Various extensions to this semantics of `delay` and `force` are supported in some implementations (none of these are currently supported in MIT/GNU Scheme):

- Calling `force` on an object that is not a promise may simply return the object.
- It may be the case that there is no means by which a promise can be operationally distinguished from its forced value. That is, expressions like the following may evaluate to either `#t` or `#f`, depending on the implementation:

      (eqv? (delay 1) 1)              ⇒  unspecified
      (pair? (delay (cons 1 2)))      ⇒  unspecified

- Some implementations will implement “implicit forcing”, where the value of a promise is forced by primitive procedures like `car` and `+`:

      (+ (delay (* 3 7)) 13)          ⇒  34

### 10.6 Streams

In addition to promises, MIT/GNU Scheme supports a higher-level abstraction called streams. Streams are similar to lists, except that the tail of a stream is not computed until it is referred to. This allows streams to be used to represent infinitely long lists.

procedure: stream object … ¶

  Returns a newly allocated stream whose elements are the arguments. Note that the expression `(stream)` returns the empty stream, or end-of-stream marker.

procedure: list->stream list ¶

  Returns a newly allocated stream whose elements are the elements of list. Equivalent to `(apply stream list)`.

procedure: stream->list stream ¶

  Returns a newly allocated list whose elements are the elements of stream. If stream has infinite length this procedure will not terminate. This could have been defined by

      (define (stream->list stream)
        (if (stream-null? stream)
            '()
            (cons (stream-car stream)
                  (stream->list (stream-cdr stream)))))

special form: cons-stream object expression ¶

  Returns a newly allocated stream pair. Equivalent to `(cons object (delay expression))`.

procedure: stream-pair? object ¶

  Returns `#t` if object is a pair whose cdr contains a promise. Otherwise returns `#f`. This could have been defined by

      (define (stream-pair? object)
        (and (pair? object)
             (promise? (cdr object))))

procedure: stream-car stream ¶
procedure: stream-first stream ¶

  Returns the first element in stream. `stream-car` is equivalent to `car`. `stream-first` is a synonym for `stream-car`.

procedure: stream-cdr stream ¶
procedure: stream-rest stream ¶

  Returns the first tail of stream. Equivalent to `(force (cdr stream))`. `stream-rest` is a synonym for `stream-cdr`.

procedure: stream-null? stream ¶

  Returns `#t` if stream is the end-of-stream marker; otherwise returns `#f`. This is equivalent to `null?`, but should be used whenever testing for the end of a stream.

procedure: stream-length stream ¶

  Returns the number of elements in stream. If stream has an infinite number of elements this procedure will not terminate. Note that this procedure forces all of the promises that comprise stream.

procedure: stream-ref stream k ¶

  Returns the element of stream that is indexed by k; that is, the kth element. K must be an exact non-negative integer strictly less than the length of stream.

procedure: stream-head stream k ¶

  Returns the first k elements of stream as a list. K must be an exact non-negative integer strictly less than the length of stream.

procedure: stream-tail stream k ¶

  Returns the tail of stream that is indexed by k; that is, the kth tail. This is equivalent to performing `stream-cdr` k times. K must be an exact non-negative integer strictly less than the length of stream.

procedure: stream-map procedure stream stream … ¶

  Returns a newly allocated stream, each element being the result of invoking procedure with the corresponding elements of the streams as its arguments.

### 10.7 Weak References

Weak references are a mechanism for building data structures that point at objects without protecting them from garbage collection. An example of such a data structure might be an entry in a lookup table that should be removed if the rest of the program does not reference its key. Such an entry must still point at its key to carry out comparisons, but should not in itself prevent its key from being garbage collected.

A weak reference is a reference that points at an object without preventing it from being garbage collected. The term strong reference is used to distinguish normal references from weak ones. If there is no path of strong references to some object, the garbage collector will reclaim that object and mark any weak references to it to indicate that it has been reclaimed.

If there is a path of strong references from an object A to an object B, A is said to hold B strongly. If there is a path of references from an object A to an object B, but every such path traverses at least one weak reference, A is said to hold B weakly.

MIT Scheme provides two mechanisms for using weak references. Weak pairs are like normal pairs, except that their car slot is a weak reference (but the cdr is still strong). The heavier-weight ephemerons additionally arrange that the ephemeron does not count as holding the object in its key field strongly even if the object in its datum field does.

Warning: Working with weak references is subtle and requires careful analysis; most programs should avoid working with them directly. The most common use cases for weak references ought to be served by hash tables (see Hash Tables), which can employ various flavors of weak entry types, 1d tables (see 1D Tables), which hold their keys weakly, and the association table (see The Association Table), which also holds its keys weakly.

#### 10.7.1 Weak Pairs

The car of a weak pair holds its pointer weakly, while the cdr holds its pointer strongly. If the object in the car of a weak pair is not held strongly by any other data structure, it will be garbage-collected, and the original value replaced with a unique reclaimed object.

Note: weak pairs can be defeated by cross references among their slots. Consider a weak pair P holding an object A in its car and an object D in its cdr. P points to A weakly and to D strongly. If D holds A strongly, however, then P ends up holding A strongly after all. If avoiding this is worth a heavier-weight structure, See Ephemerons.

Note: weak pairs are not pairs; that is, they do not satisfy the predicate `pair?`.

procedure: weak-pair? object ¶

  Returns `#t` if object is a weak pair; otherwise returns `#f`.

procedure: weak-cons car cdr ¶

  Allocates and returns a new weak pair, with components car and cdr. The car component is held weakly.

procedure: gc-reclaimed-object? object ¶

  Returns `#t` if object is the reclaimed object, and `#f` otherwise.

procedure: gc-reclaimed-object ¶

  Returns the reclaimed object.

obsolete procedure: weak-pair/car? weak-pair ¶

  This predicate returns `#f` if the car of weak-pair has been garbage-collected; otherwise returns `#t`. In other words, it is true if weak-pair has a valid car component.

  This is equivalent to

      (not (gc-reclaimed-object? (weak-car weak-pair)))

  This predicate has been deprecated; instead use `gc-reclaimed-object?`. Please note that the previously recommended way to use `weak-pair/car?` will no longer work, so any code using it should be rewritten.

procedure: weak-car weak-pair ¶

  Returns the car component of weak-pair. If the car component has been garbage-collected, this operation returns the reclaimed object.

procedure: weak-set-car! weak-pair object ¶

  Sets the car component of weak-pair to object and returns an unspecified result.

procedure: weak-cdr weak-pair ¶

  Returns the cdr component of weak-pair.

procedure: weak-set-cdr! weak-pair object ¶

  Sets the cdr component of weak-pair to object and returns an unspecified result.

#### 10.7.2 Ephemerons

An ephemeron is an object with two weakly referenced components called its key and datum. The garbage collector drops an ephemeron’s references to both key and datum, rendering the ephemeron broken, if and only if the garbage collector can prove that there are no strong references to the key. In other words, an ephemeron is broken when nobody else cares about its key. In particular, the datum holding a reference to the key will not in itself prevent the ephemeron from becoming broken; in contrast, See Weak Pairs. Once broken, ephemerons never cease to be broken; setting the key or datum of a broken ephemeron with `set-ephemeron-key!` or `set-ephemeron-datum!` has no effect. Note that an ephemeron’s reference to its datum may be dropped even if the datum is still reachable; all that matters is whether the key is reachable.

Ephemerons are considerably heavier-weight than weak pairs, because garbage-collecting ephemerons is more complicated than garbage-collecting weak pairs. Each ephemeron needs five words of storage, rather than the two words needed by a weak pair. However, while the garbage collector spends more time on ephemerons than on other objects, the amount of time it spends on ephemerons scales linearly with the number of live ephemerons, which is how its running time scales with the total number of live objects anyway.

procedure: ephemeron? object ¶

  Returns `#t` if object is a ephemeron; otherwise returns `#f`.

procedure: make-ephemeron key datum ¶

  Allocates and returns a new ephemeron, with components key and datum.

procedure: ephemeron-broken? ephemeron ¶

  Returns `#t` if the garbage collector has dropped ephemeron’s references to its key and datum; otherwise returns `#f`.

procedure: ephemeron-key ephemeron ¶
procedure: ephemeron-datum ephemeron ¶

  These return the key or datum component, respectively, of ephemeron. If ephemeron has been broken, these operations return `#f`, but they can also return `#f` if that is the value that was stored in the key or value component.

procedure: set-ephemeron-key! ephemeron object ¶
procedure: set-ephemeron-datum! ephemeron object ¶

  These set the key or datum component, respectively, of ephemeron to object and return an unspecified result. If ephemeron is broken, neither of these operations has any effect.

Like `weak-pair/car?`, `ephemeron-broken?` must be used with care. If `(ephemeron-broken? ephemeron)` yields false, it guarantees only that prior evaluations of `(ephemeron-key ephemeron)` or `(ephemeron-datum ephemeron)` yielded the key or datum that was stored in the ephemeron, but it makes no guarantees about subsequent calls to `ephemeron-key` or `ephemeron-datum`: the garbage collector may run and break the ephemeron immediately after `ephemeron-broken?` returns. Thus, the correct idiom to fetch an ephemeron’s key and datum and use them if the ephemeron is not broken is

    (let ((key (ephemeron-key ephemeron))
          (datum (ephemeron-datum ephemeron)))
      (if (ephemeron-broken? ephemeron)
          … broken case …
          … code using key and datum …))

#### 10.7.3 Reference barriers

The garbage collector may break an ephemeron if it can prove that the key is not strongly reachable. To ensure that it does not do so before a certain point in a program, the program can invoke a reference barrier on the key by calling the `reference-barrier` procedure, which guarantees that even if the program does not use the key, it will be considered strongly reachable until after `reference-barrier` returns.

procedure: reference-barrier object ¶

  Guarantee that object is strongly reachable until after `reference-barrier` returns.

## 11 Associations

MIT/GNU Scheme provides several mechanisms for associating objects with one another. Each of these mechanisms creates a link between one or more objects, called keys, and some other object, called a datum. Beyond this common idea, however, each of the mechanisms has various different properties that make it appropriate in different situations:

- Association lists are one of Lisp’s oldest association mechanisms. Because they are made from ordinary pairs, they are easy to build and manipulate, and very flexible in use. However, the average lookup time for an association list is linear in the number of associations.
- 1D tables have a very simple interface, making them easy to use, and offer the feature that they do not prevent their keys from being reclaimed by the garbage collector. Like association lists, their average lookup time is linear in the number of associations; but 1D tables aren’t as flexible.
- The association table is MIT/GNU Scheme’s equivalent to the property lists of Lisp. It has the advantages that the keys may be any type of object and that it does not prevent the keys from being reclaimed by the garbage collector. However, two linear-time lookups must be performed, one for each key, whereas for traditional property lists only one lookup is required for both keys.
- Hash tables are a powerful mechanism with constant-time access to large amounts of data. Hash tables are not as flexible as association lists, but because their access times are independent of the number of associations in the table, for most applications they are the mechanism of choice.
- Balanced binary trees are another association mechanism that is useful for applications in which the keys are ordered. Binary trees have access times that are proportional to the logarithm of the number of associations in the tree. While they aren’t as fast as hash tables, they offer the advantage that the contents of the tree can be converted to a sorted alist in linear time. Additionally, two trees can be compared for equality in worst-case linear time.
- Red-Black trees are a kind of balanced binary tree. The implementation supports destructive insertion and deletion operations with a good constant factor.
- Weight-Balanced trees are a kind of balanced binary tree. The implementation provides non-destructive operations. There is a comprehensive set of operations, including: a constant-time size operation; many high-level operations such as the set operations union, intersection and difference; and indexing of elements by position.

### 11.1 Association Lists

An association list, or alist, is a data structure used very frequently in Scheme. An alist is a list of pairs, each of which is called an association. The car of an association is called the key.

An advantage of the alist representation is that an alist can be incrementally augmented simply by adding new entries to the front. Moreover, because the searching procedures `assv` et al. search the alist in order, new entries can “shadow” old entries. If an alist is viewed as a mapping from keys to data, then the mapping can be not only augmented but also altered in a non-destructive manner by adding new entries to the front of the alist.11

procedure: alist? object ¶

  Returns `#t` if object is an association list (including the empty list); otherwise returns `#f`. Any object satisfying this predicate also satisfies `list?`.

procedure: assq object alist ¶
procedure: assv object alist ¶
procedure: assoc object alist ¶

  These procedures find the first pair in alist whose car field is object, and return that pair; the returned pair is always an element of alist, not one of the pairs from which alist is composed. If no pair in alist has object as its car, `#f` (n.b.: not the empty list) is returned. `assq` uses `eq?` to compare object with the car fields of the pairs in alist, while `assv` uses `eqv?` and `assoc` uses `equal?`.12

      (define e '((a 1) (b 2) (c 3)))
      (assq 'a e)                             ⇒  (a 1)
      (assq 'b e)                             ⇒  (b 2)
      (assq 'd e)                             ⇒  #f
      (assq (list 'a) '(((a)) ((b)) ((c))))   ⇒  #f
      (assoc (list 'a) '(((a)) ((b)) ((c))))  ⇒  ((a))
      (assq 5 '((2 3) (5 7) (11 13)))         ⇒  unspecified
      (assv 5 '((2 3) (5 7) (11 13)))         ⇒  (5 7)

procedure: association-procedure predicate selector ¶

  Returns an association procedure that is similar to `assv`, except that selector (a procedure of one argument) is used to select the key from the association, and predicate (an equivalence predicate) is used to compare the key to the given item. This can be used to make association lists whose elements are, say, vectors instead of pairs (also see Searching Lists).

  For example, here is how `assv` could be implemented:

      (define assv (association-procedure eqv? car))

  Another example is a “reverse association” procedure:

      (define rassv (association-procedure eqv? cdr))

procedure: del-assq object alist ¶
procedure: del-assv object alist ¶
procedure: del-assoc object alist ¶

  These procedures return a newly allocated copy of alist in which all associations with keys equal to object have been removed. Note that while the returned copy is a newly allocated list, the association pairs that are the elements of the list are shared with alist, not copied. `del-assq` uses `eq?` to compare object with the keys, while `del-assv` uses `eqv?` and `del-assoc` uses `equal?`.

      (define a
        '((butcher . "231 e22nd St.")
          (baker . "515 w23rd St.")
          (hardware . "988 Lexington Ave.")))

      (del-assq 'baker a)
           ⇒
           ((butcher . "231 e22nd St.")
            (hardware . "988 Lexington Ave."))

procedure: del-assq! object alist ¶
procedure: del-assv! object alist ¶
procedure: del-assoc! object alist ¶

  These procedures remove from alist all associations with keys equal to object. They return the resulting list. `del-assq!` uses `eq?` to compare object with the keys, while `del-assv!` uses `eqv?` and `del-assoc!` uses `equal?`. These procedures are like `del-assq`, `del-assv`, and `del-assoc`, respectively, except that they destructively modify alist.

procedure: delete-association-procedure deletor predicate selector ¶

  This returns a deletion procedure similar to `del-assv` or `del-assq!`. The predicate and selector arguments are the same as those for `association-procedure`, while the deletor argument should be either the procedure `list-deletor` (for non-destructive deletions), or the procedure `list-deletor!` (for destructive deletions).

  For example, here is a possible implementation of `del-assv`:

      (define del-assv
        (delete-association-procedure list-deletor eqv? car))

procedure: alist-copy alist ¶

  Returns a newly allocated copy of alist. This is similar to `list-copy` except that the “association” pairs, i.e. the elements of the list alist, are also copied. `alist-copy` could have been implemented like this:

      (define (alist-copy alist)
        (if (null? alist)
            '()
            (cons (cons (car (car alist)) (cdr (car alist)))
                  (alist-copy (cdr alist)))))

### 11.2 1D Tables

1D tables (“one-dimensional” tables) are similar to association lists. In a 1D table, unlike an association list, the keys of the table are held weakly: if a key is garbage-collected, its associated value in the table is removed. 1D tables compare their keys for equality using `eq?`.

1D tables can often be used as a higher-performance alternative to the two-dimensional association table (see The Association Table). If one of the keys being associated is a compound object such as a vector, a 1D table can be stored in one of the vector’s slots. Under these circumstances, accessing items in a 1D table will be comparable in performance to using a property list in a conventional Lisp.

procedure: make-1d-table ¶

  Returns a newly allocated empty 1D table.

procedure: 1d-table? object ¶

  Returns `#t` if object is a 1D table, otherwise returns `#f`. Any object that satisfies this predicate also satisfies `list?`.

procedure: 1d-table/put! 1d-table key datum ¶

  Creates an association between key and datum in 1d-table. Returns an unspecified value.

procedure: 1d-table/remove! 1d-table key ¶

  Removes any association for key in 1d-table and returns an unspecified value.

procedure: 1d-table/get 1d-table key default ¶

  Returns the datum associated with key in 1d-table. If there is no association for key, default is returned.

procedure: 1d-table/lookup 1d-table key if-found if-not-found ¶

  If-found must be a procedure of one argument, and if-not-found must be a procedure of no arguments. If 1d-table contains an association for key, if-found is invoked on the datum of the association. Otherwise, if-not-found is invoked with no arguments. In either case, the result of the invoked procedure is returned as the result of `1d-table/lookup`.

procedure: 1d-table/alist 1d-table ¶

  Returns a newly allocated association list that contains the same information as 1d-table.

### 11.3 The Association Table

MIT/GNU Scheme provides a generalization of the property-list mechanism found in most other implementations of Lisp: a global two-dimensional association table. This table is indexed by two keys, called x-key and y-key in the following procedure descriptions. These keys and the datum associated with them can be arbitrary objects. `eq?` is used to discriminate keys.

Think of the association table as a matrix: a single datum can be accessed using both keys, a column using x-key only, and a row using y-key only.

procedure: 2d-put! x-key y-key datum ¶

  Makes an entry in the association table that associates datum with x-key and y-key. Returns an unspecified result.

procedure: 2d-remove! x-key y-key ¶

  If the association table has an entry for x-key and y-key, it is removed. Returns an unspecified result.

procedure: 2d-get x-key y-key ¶

  Returns the datum associated with x-key and y-key. Returns `#f` if no such association exists.

procedure: 2d-get-alist-x x-key ¶

  Returns an association list of all entries in the association table that are associated with x-key. The result is a list of `(y-key . datum)` pairs. Returns the empty list if no entries for x-key exist.

      (2d-put! 'foo 'bar 5)
      (2d-put! 'foo 'baz 6)
      (2d-get-alist-x 'foo)           ⇒  ((baz . 6) (bar . 5))

procedure: 2d-get-alist-y y-key ¶

  Returns an association list of all entries in the association table that are associated with y-key. The result is a list of `(x-key . datum)` pairs. Returns the empty list if no entries for y-key exist.

      (2d-put! 'bar 'foo 5)
      (2d-put! 'baz 'foo 6)
      (2d-get-alist-y 'foo)           ⇒  ((baz . 6) (bar . 5))

### 11.4 Hash Tables

Hash tables are a fast, powerful mechanism for storing large numbers of associations. MIT/GNU Scheme’s hash tables feature automatic resizing, customizable growth parameters, customizable hash procedures, and many options for weak references to keys or data.

The average times for the insertion, deletion, and lookup operations on a hash table are bounded by a constant. The space required by the table is proportional to the number of associations in the table; the constant of proportionality is described below (see Resizing of Hash Tables).

The hash table interface described below is a superset of SRFI 69: “Basic hash tables”. The reason for supporting the extra functionality is that SRFI 69 fails to specify certain optimization-enabling exceptions to its semantics, forcing a correct implementation to pay the non-negligible performance cost of completely safe behavior. 13 The MIT/GNU Scheme native hash table interface, in contrast, specifies the minor exceptions it needs, and is therefore implemented more efficiently.

We do not describe the SRFI 69-compliant interface here, as that would be redundant with the SRFI document.

#### 11.4.1 Construction of Hash Tables

The next few procedures are hash-table constructors. All hash table constructors are procedures that accept one optional argument, initial-size, and return a newly allocated hash table. If initial-size is given, it must be an exact non-negative integer or `#f`. The meaning of initial-size is discussed below (see Resizing of Hash Tables).

Hash tables are normally characterized by two things: the equivalence predicate that is used to compare keys, and how the table allows its keys and data to be reclaimed by the garbage collector. If a table prevents its keys and data from being reclaimed by the garbage collector, it is said to hold its keys and data strongly; other arrangements are possible, where a table may hold keys or data weakly or ephemerally (see Weak References).

procedure: make-strong-eq-hash-table [initial-size] ¶
obsolete procedure: make-symbol-hash-table [initial-size] ¶

  Returns a newly allocated hash table that accepts arbitrary objects as keys, and compares those keys with `eq?`. The keys and data are held strongly. These are the fastest of the standard hash tables.

procedure: make-key-weak-eq-hash-table [initial-size] ¶
obsolete procedure: make-weak-eq-hash-table [initial-size] ¶
obsolete procedure: make-eq-hash-table [initial-size] ¶

  Returns a newly allocated hash table that accepts arbitrary objects as keys, and compares those keys with `eq?`. The keys are held weakly and the data are held strongly. Note that if a datum holds a key strongly, the table will effectively hold that key strongly.

procedure: make-datum-weak-eq-hash-table [initial-size] ¶

  Returns a newly allocated hash table that accepts arbitrary objects as keys, and compares those keys with `eq?`. The keys are held strongly and the data are held weakly. Note that if a key holds a datum strongly, the table will effectively hold that datum strongly.

procedure: make-key-ephemeral-eq-hash-table [initial-size] ¶

  Returns a newly allocated hash table that accepts arbitrary objects as keys, and compares those keys with `eq?`. The keys are held weakly, even if some of the data should hold some of the keys strongly.

procedure: make-strong-eqv-hash-table [initial-size] ¶

  Returns a newly allocated hash table that accepts arbitrary objects as keys, and compares those keys with `eqv?`. The keys and data are held strongly. These hash tables are a little slower than those made by `make-strong-eq-hash-table`.

procedure: make-key-weak-eqv-hash-table [initial-size] ¶
obsolete procedure: make-weak-eqv-hash-table [initial-size] ¶
obsolete procedure: make-eqv-hash-table [initial-size] ¶
obsolete procedure: make-object-hash-table [initial-size] ¶

  Returns a newly allocated hash table that accepts arbitrary objects as keys, and compares those keys with `eqv?`. The keys are held weakly, except that booleans, characters, numbers, and interned symbols are held strongly. The data are held strongly. Note that if a datum holds a key strongly, the table will effectively hold that key strongly.

procedure: make-datum-weak-eqv-hash-table [initial-size] ¶

  Returns a newly allocated hash table that accepts arbitrary objects as keys, and compares those keys with `eqv?`. The keys are held strongly and the data are held weakly. Note that if a key holds a datum strongly, the table will effectively hold that datum strongly.

procedure: make-key-ephemeral-eqv-hash-table [initial-size] ¶

  Returns a newly allocated hash table that accepts arbitrary objects as keys, and compares those keys with `eqv?`. The keys are held weakly, except that booleans, characters, numbers, and interned symbols are held strongly. The keys are effectively held weakly even if some of the data should hold some of the keys strongly.

procedure: make-equal-hash-table [initial-size] ¶

  Returns a newly allocated hash table that accepts arbitrary objects as keys, and compares those keys with `equal?`. The keys and data are held strongly. These hash tables are quite a bit slower than those made by `make-strong-eq-hash-table`.

procedure: make-string-hash-table [initial-size] ¶

  Returns a newly allocated hash table that accepts character strings as keys, and compares them with `string=?`. The keys and data are held strongly.

All of the above are highly optimized table implementations. Next are some general constructors that allow for more flexible table definitions.

procedure: make-hash-table comparator arg … ¶
procedure: make-hash-table [key=? [hash-function arg …]] ¶
procedure: alist->hash-table alist comparator arg … ¶
procedure: alist->hash-table alist [key=? [hash-function arg …]] ¶

  These are the standard constructors for making hash tables. The behavior of each differs depending on its arguments: if the first argument is a comparator, then it behaves like a SRFI 125 procedure, otherwise it behaves like a SRFI 69 procedure.

  For SRFI 125 behavior the comparator must be a comparator that satisfies `comparator-hashable?`. The remaining args are optional, and may include the following symbols:

  `weak-keys`

    Specifies that the table will have weak keys.

  `weak-values`

    Specifies that the table will have weak values.

  `ephemeral-keys`

    Specifies that the table will have ephemeral keys.

  `ephemeral-values`

    Specifies that the table will have ephemeral values.

  The symbols `weak-keys` and `weak-values` can be specified together or separately, likewise for `ephemeral-keys` and `ephemeral-values`. But weak and ephemeral symbols can’t be mixed. If none of these symbols are present, then the keys and values are strongly held.

  Additionally args may contain an exact non-negative integer, which specifies an initial size for the table; otherwise a default size is used.

  For SRFI 69 behavior the key=? argument specifies how keys are compared and defaults to `equal?`. The hash-function argument specifies the hash function to use. If hash-function is not specified, it defaults to a standard value that depends on key=?; an error is signaled if there’s no standard value. The arg arguments are allowed but are implementation dependent; do not provide them.

  The procedure `alist->hash-table` creates a new hash table, as with `make-hash-table`, and then fills it with the contents of alist.

The remaining constructors use hash-table types to encapsulate the hashing parameters.

obsolete procedure: make-hash-table* type [initial-size] ¶

  Constructs a new hash table using the hashing parameters in type.

procedure: hash-table-constructor comparator arg … ¶
obsolete procedure: hash-table-constructor type ¶

  Returns a procedure that, when called, constructs a new hash table using the specified parameters. The returned procedure accepts an optional initial-size.

  If its first argument is a comparator, it uses the comparator and args as in `make-hash-table`, except that any initial size specified in args can be overridden by the initial-size argument to the returned procedure.

  If its first argument is a type, returns a procedure that, when called, constructs a new hash table using the hashing parameters in type. This is equivalent to

      (lambda (#!optional initial-size)
        (make-hash-table* type initial-size))

The next two procedures are used to create hash-table types. The procedures are equivalent in power; they differ only in how the types are described.

obsolete procedure: make-hash-table-type hash-function key=? rehash-after-gc? entry-type ¶

  This procedure accepts four arguments and returns a hash-table type, which can be used to make hash tables of that type. The key=? argument is an equivalence predicate for the keys of the hash table. The hash-function argument is a procedure that computes a hash number. Specifically, hash-function accepts two arguments, a key and an exact positive integer (the modulus), and returns an exact non-negative integer that is less than the modulus.

  The argument rehash-after-gc?, if true, says that the values returned by hash-function might change after a garbage collection. If so, the hash-table implementation arranges for the table to be rehashed when necessary. (See Address Hashing, for information about hash procedures that have this property.) Otherwise, it is assumed that hash-function always returns the same value for the same arguments.

  The argument entry-type determines the strength with which the hash table will hold its keys and values. It must be one of the entry-type variables described below, which all start with `hash-table-entry-type:`.

obsolete procedure: make-hash-table-type* key=? hash-function rehash-after-gc? entry-type ¶

  This procedure’s arguments, except for key=?, are keyword arguments; that is, each argument is a symbol of the same name followed by its value. Aside from how they are passed, the arguments have the same meaning as those for `make-hash-table-type`. Note that all of the keyword arguments are optional, while key=? is required.

  The argument entry-type specifies the name of an entry type. It must be a symbol corresponding to one of the entry-type variables described below. The name of an entry type is the symbol composed of the suffix of the corresponding variable; for example the type `hash-table-entry-type:key-weak` has the name `key-weak`.

  The default values for the keyword arguments are as follows. The arguments hash-function and rehash-after-gc? default to standard values that depend on key=?; an error is signaled if key=? has no standard values. The argument entry-type defaults to strong.

obsolete variable: hash-table-entry-type:strong ¶

  The entry type for hash tables that hold both keys and data strongly.

obsolete variable: hash-table-entry-type:key-weak ¶

  An entry type for hash tables that hold keys weakly and data strongly. An entry of this type is a weak pair (see Weak Pairs) whose weak (car) slot holds the key of the entry and whose strong (cdr) slot holds the datum of the entry. If a key of such a hash table is garbage collected, the corresponding entry will be removed. Note that if some datum holds some key strongly, the table will effectively hold that key strongly.

obsolete variable: hash-table-entry-type:datum-weak ¶

  An entry type for hash tables that hold keys strongly and data weakly. An entry of this type is a weak pair (see Weak Pairs) whose weak (car) slot holds the datum of the entry and whose strong (cdr) slot holds the key of the entry. If a datum of such a hash table is garbage collected, all corresponding entries will be removed. Note that if some key holds some datum strongly, the table will effectively hold that datum strongly.

obsolete variable: hash-table-entry-type:key&datum-weak ¶
obsolete variable: hash-table-entry-type:key/datum-weak ¶

  The entry type for hash tables that hold both keys and data weakly. An entry of this type is a weak list, holding both the key and the datum in the weak (car) slot of weak pairs (see Weak Pairs). If either a key or datum of such a hash table is garbage collected, all corresponding entries will be removed.

obsolete variable: hash-table-entry-type:key-ephemeral ¶

  An entry type for hash tables that hold data ephemerally, keyed by the keys. An entry of this type is an ephemeron (see Ephemerons) whose key is the key of the entry and whose datum is the datum of the entry. If a key of such a hash table is garbage collected, the corresponding entry will be removed. Note that the table holds all its keys weakly even if some data should hold some keys strongly.

obsolete variable: hash-table-entry-type:datum-ephemeral ¶

  An entry type for hash tables that hold keys ephemerally, keyed by the data. An entry of this type is an ephemeron (see Ephemerons) whose key is the datum of the entry and whose datum is the key of the entry. If a datum of such a hash table is garbage collected, all corresponding entries will be removed. Note that the table holds all its data weakly even if some keys should hold some data strongly.

obsolete variable: hash-table-entry-type:key&datum-ephemeral ¶

  The entry type for hash tables that hold both keys and data ephemerally keyed on each other. An entry of this type is a pair of ephemerons (see Ephemerons), one holding the datum keyed by the key and the other holding the key keyed by the datum. If both the key and the datum of any entry of such a hash table are garbage collected, the entry will be removed. The table holds all its keys and data weakly itself, but will prevent any key or datum from being garbage collected if there are strong references to its datum or key, respectively.

Some examples showing how some standard hash-table constructors could have been defined:

    (define make-weak-eq-hash-table
      (hash-table-constructor
        (make-hash-table-type eq-hash eq? #t
          hash-table-entry-type:key-weak)))

    (define make-equal-hash-table
      (hash-table-constructor
        (make-hash-table-type equal-hash equal? #t
          hash-table-entry-type:strong)))

    (define make-string-hash-table
      (hash-table-constructor
        (make-hash-table-type string-hash string=? #f
          hash-table-entry-type:strong)))

The following procedures are provided only for backward compatibility. They should be considered deprecated and should not be used in new programs.

obsolete procedure: hash-table/constructor hash-function key=? rehash-after-gc? entry-type ¶

  This procedure is deprecated. Instead use the equivalent

      (hash-table-constructor
        (make-hash-table-type hash-function key=? rehash-after-gc?
                              entry-type))

obsolete procedure: strong-hash-table/constructor hash-function key=? [rehash-after-gc?] ¶

  Like `hash-table/constructor` but always uses `hash-table-entry-type:strong`. If rehash-after-gc? is omitted, it defaults to `#f`.

obsolete procedure: weak-hash-table/constructor hash-function key=? [rehash-after-gc?] ¶

  Like `hash-table/constructor` but always uses `hash-table-entry-type:key-weak`. If rehash-after-gc? is omitted, it defaults to `#f`.

#### 11.4.2 Basic Hash Table Operations

The procedures described in this section are the basic operations on hash tables. They provide the functionality most often needed by programmers. Subsequent sections describe other operations that provide additional functionality needed by some applications.

procedure: hash-table? object ¶

  Returns `#t` if object is a hash table, otherwise returns `#f`.

procedure: hash-table-set! hash-table key datum ¶
obsolete procedure: hash-table/put! hash-table key datum ¶

  Associates datum with key in hash-table and returns an unspecified result.

  The average time required by this operation is bounded by a constant.

procedure: hash-table-ref hash-table key [get-default] ¶

  Returns the datum associated with key in hash-table. If there is no association for key, and get-default is provided, it is called with no arguments and the value it yields is returned; if get-default is not provided, an error is signaled.

  The average time required by this operation is bounded by a constant.

procedure: hash-table-ref/default hash-table key default ¶
obsolete procedure: hash-table/get hash-table key default ¶

  Equivalent to

      (hash-table-ref hash-table key (lambda () default))

procedure: hash-table-delete! hash-table key ¶
obsolete procedure: hash-table/remove! hash-table key ¶

  If hash-table has an association for key, removes it. Returns an unspecified result.

  The average time required by this operation is bounded by a constant.

procedure: hash-table-clear! hash-table ¶
obsolete procedure: hash-table/clear! hash-table ¶

  Removes all associations in hash-table and returns an unspecified result.

  The average and worst-case times required by this operation are bounded by a constant.

procedure: hash-table-size hash-table ¶
obsolete procedure: hash-table/count hash-table ¶

  Returns the number of associations in hash-table as an exact non-negative integer. If hash-table does not hold its keys and data strongly, this is a conservative upper bound that may count some associations whose keys or data have recently been reclaimed by the garbage collector.

  The average and worst-case times required by this operation are bounded by a constant.

procedure: hash-table->alist hash-table ¶

  Returns the contents of hash-table as a newly allocated alist. Each element of the alist is a pair `(key . datum)` where key is one of the keys of hash-table, and datum is its associated datum.

  The average and worst-case times required by this operation are linear in the number of associations in the table.

procedure: hash-table-keys hash-table ¶
obsolete procedure: hash-table/key-list hash-table ¶

  Returns a newly allocated list of the keys in hash-table.

  The average and worst-case times required by this operation are linear in the number of associations in the table.

procedure: hash-table-values hash-table ¶
obsolete procedure: hash-table/datum-list hash-table ¶

  Returns a newly allocated list of the datums in hash-table. Each element of the list corresponds to one of the associations in hash-table; if the table contains multiple associations with the same datum, so will this list.

  The average and worst-case times required by this operation are linear in the number of associations in the table.

procedure: hash-table-walk hash-table procedure ¶
obsolete procedure: hash-table/for-each hash-table procedure ¶

  Procedure must be a procedure of two arguments. Invokes procedure once for each association in hash-table, passing the association’s key and datum as arguments, in that order. Returns an unspecified result. Procedure must not modify hash-table, with one exception: it is permitted to call `hash-table-delete!` to remove the association being processed.

The following procedure is useful when there is no sensible default value for `hash-table-ref` and the caller must choose between different actions depending on whether there is a datum associated with the key.

obsolete procedure: hash-table/lookup hash-table key if-found if-not-found ¶

  If-found must be a procedure of one argument, and if-not-found must be a procedure of no arguments. If hash-table contains an association for key, if-found is invoked on the datum of the association. Otherwise, if-not-found is invoked with no arguments. In either case, the result yielded by the invoked procedure is returned as the result of `hash-table/lookup` (`hash-table/lookup` reduces into the invoked procedure, i.e. calls it tail-recursively).

  The average time required by this operation is bounded by a constant.

procedure: hash-table-update! hash-table key procedure [get-default] ¶

  Procedure must be a procedure of one argument and get-default, if supplied, must be a procedure of zero arguments. Applies procedure to the datum associated with key in hash-table or to the value of calling get-default if there is no association for key, associates the result with key, and returns an unspecified value. If get-default is not supplied and there’s no association for key, an error is signaled.

  The average time required by this operation is bounded by a constant.

procedure: hash-table-update!/default hash-table key procedure default ¶
obsolete procedure: hash-table/modify! hash-table key default procedure ¶

  Equivalent to

      (hash-table-update! hash-table key procedure (lambda () default))

procedure: hash-table-intern! hash-table key get-default ¶
obsolete procedure: hash-table/intern! hash-table key get-default ¶

  Get-default must be a procedure of zero arguments. Ensures that hash-table has an association for key and returns the associated datum. If hash-table did not have a datum associated with key, get-default is called and its value is used to create a new association for key.

  The average time required by this operation is bounded by a constant.

The following procedure is sometimes useful in conjunction with weak and ephemeral hash tables. Normally it is not needed, because such hash tables clean themselves automatically as they are used.

procedure: hash-table-clean! hash-table ¶
obsolete procedure: hash-table/clean! hash-table ¶

  If hash-table is a type of hash table that holds its keys or data weakly or ephemerally, this procedure recovers any space that was being used to record associations for objects that have been reclaimed by the garbage collector. Otherwise, this procedure does nothing. In either case, it returns an unspecified result.

#### 11.4.3 Resizing of Hash Tables

Normally, hash tables automatically resize themselves according to need. Because of this, the programmer need not be concerned with management of the table’s size. However, some limited control over the table’s size is provided, which will be discussed below. This discussion involves two concepts, usable size and physical size, which we will now define.

The usable size of a hash table is the number of associations that the table can hold at a given time. If the number of associations in the table exceeds the usable size, the table will automatically grow, increasing the usable size to a new value that is sufficient to hold the associations.

The physical size is an abstract measure of a hash table that specifies how much space is allocated to hold the associations of the table. The physical size is always greater than or equal to the usable size. The physical size is not interesting in itself; it is interesting only for its effect on the performance of the hash table. While the average performance of a hash-table lookup is bounded by a constant, the worst-case performance is not. For a table containing a given number of associations, increasing the physical size of the table decreases the probability that worse-than-average performance will occur.

The physical size of a hash table is statistically related to the number of associations. However, it is possible to place bounds on the physical size, and from this to estimate the amount of space used by the table:

    (define (hash-table-space-bounds count rehash-size rehash-threshold)
      (let ((tf (/ 1 rehash-threshold)))
        (values (if (exact-integer? rehash-size)
                    (- (* count (+ 4 tf))
                       (* tf (+ rehash-size rehash-size)))
                    (* count (+ 4 (/ tf (* rehash-size rehash-size)))))
                (* count (+ 4 tf)))))

What this formula shows is that, for a “normal” rehash size (that is, not an exact integer), the amount of space used by the hash table is proportional to the number of associations in the table. The constant of proportionality varies statistically, with the low bound being

    (+ 4 (/ (/ 1 rehash-threshold) (* rehash-size rehash-size)))

and the high bound being

    (+ 4 (/ 1 rehash-threshold))

which, for the default values of these parameters, are `4.25` and `5`, respectively. Reducing the rehash size will tighten these bounds, but increases the amount of time spent resizing, so you can see that the rehash size gives some control over the time-space tradeoff of the table.

The programmer can control the size of a hash table by means of three parameters:

- Each table’s initial-size may be specified when the table is created.
- Each table has a rehash size that specifies how the size of the table is changed when it is necessary to grow or shrink the table.
- Each table has a rehash threshold that specifies the relationship of the table’s physical size to its usable size.

If the programmer knows that the table will initially contain a specific number of items, initial-size can be given when the table is created. If initial-size is an exact non-negative integer, it specifies the initial usable size of the hash table; the table will not change size until the number of items in the table exceeds initial-size, after which automatic resizing is enabled and initial-size no longer has any effect. Otherwise, if initial-size is not given or is `#f`, the table is initialized to an unspecified size and automatic resizing is immediately enabled.

The rehash size specifies how much to increase the usable size of the hash table when it becomes full. It is either an exact positive integer, or a real number greater than one. If it is an integer, the new size is the sum of the old size and the rehash size. Otherwise, it is a real number, and the new size is the product of the old size and the rehash size. Increasing the rehash size decreases the average cost of an insertion, but increases the average amount of space used by the table. The rehash size of a table may be altered dynamically by the application in order to optimize the resizing of the table; for example, if the table will grow quickly for a known period and afterwards will not change size, performance might be improved by using a large rehash size during the growth phase and a small one during the static phase. The default rehash size of a newly constructed hash table is `2.0`.

Warning: The use of an exact positive integer for a rehash size is almost always undesirable; this option is provided solely for compatibility with the Common Lisp hash-table mechanism. The reason for this has to do with the time penalty for resizing the hash table. The time needed to resize a hash table is proportional to the number of associations in the table. This resizing cost is amortized across the insertions required to fill the table to the point where it needs to grow again. If the table grows by an amount proportional to the number of associations, then the cost of resizing and the increase in size are both proportional to the number of associations, so the amortized cost of an insertion operation is still bounded by a constant. However, if the table grows by a constant amount, this is not true: the amortized cost of an insertion is not bounded by a constant. Thus, using a constant rehash size means that the average cost of an insertion increases proportionally to the number of associations in the hash table.

The rehash threshold is a real number, between zero exclusive and one inclusive, that specifies the ratio between a hash table’s usable size and its physical size. Decreasing the rehash threshold decreases the probability of worse-than-average insertion, deletion, and lookup times, but increases the physical size of the table for a given usable size. The default rehash threshold of a newly constructed hash table is `1`.

procedure: hash-table-grow-size hash-table ¶
obsolete procedure: hash-table/size hash-table ¶

  Returns the usable size of hash-table as an exact positive integer. This is the maximum number of associations that hash-table can hold before it will grow.

procedure: hash-table-shrink-size hash-table ¶

  Returns the minimum number of associations that hash-table can hold before it will shrink.

procedure: hash-table-rehash-size hash-table ¶
obsolete procedure: hash-table/rehash-size hash-table ¶

  Returns the rehash size of hash-table.

procedure: set-hash-table-rehash-size! hash-table x ¶
obsolete procedure: set-hash-table/rehash-size! hash-table x ¶

  X must be either an exact positive integer, or a real number that is greater than one. Sets the rehash size of hash-table to x and returns an unspecified result. This operation adjusts the “shrink threshold” of the table; the table might shrink if the number of associations is less than the new threshold.

procedure: hash-table-rehash-threshold hash-table ¶
obsolete procedure: hash-table/rehash-threshold hash-table ¶

  Returns the rehash threshold of hash-table.

procedure: set-hash-table-rehash-threshold! hash-table x ¶
obsolete procedure: set-hash-table/rehash-threshold! hash-table x ¶

  X must be a real number between zero exclusive and one inclusive. Sets the rehash threshold of hash-table to x and returns an unspecified result. This operation does not change the usable size of the table, but it usually changes the physical size of the table, which causes the table to be rehashed.

#### 11.4.4 Address Hashing

The procedures described in this section may be used to make very efficient key-hashing procedures for arbitrary objects. All of these procedures are based on address hashing, which uses the address of an object as its hash number. The great advantage of address hashing is that converting an arbitrary object to a hash number is extremely fast and takes the same amount of time for any object.

The disadvantage of address hashing is that the garbage collector changes the addresses of most objects. The hash-table implementation compensates for this disadvantage by automatically rehashing tables that use address hashing when garbage collections occur. Thus, in order to use these procedures for key hashing, it is necessary to tell the hash-table implementation (by means of the rehash-after-gc? argument to the hash-table type constructors) that the hash numbers computed by your key-hashing procedure must be recomputed after a garbage collection.

procedure: eq-hash object ¶
procedure: eqv-hash object ¶
procedure: equal-hash object ¶

  These procedures return a hash number for object. The result is always a non-negative integer, and in the case of `eq-hash`, a non-negative fixnum. Two objects that are equivalent according to `eq?`, `eqv?`, or `equal?`, respectively, will produce the same hash number when passed as arguments to these procedures, provided that the garbage collector does not run during or between the two calls.

procedure: hash-by-identity key [modulus] ¶

  This SRFI 69 procedure returns the same value as `eq-hash`, optionally limited by modulus.

procedure: hash key [modulus] ¶

  This SRFI 69 procedure returns the same value as `equal-hash`, optionally limited by modulus.

obsolete procedure: hash-by-eqv key [modulus] ¶

  This procedure returns the same value as `eqv-hash`, optionally limited by modulus.

obsolete procedure: eq-hash-mod object modulus ¶

  This procedure is the key-hashing procedure used by `make-strong-eq-hash-table`.

obsolete procedure: eqv-hash-mod object modulus ¶

  This procedure is the key-hashing procedure used by `make-strong-eqv-hash-table`.

obsolete procedure: equal-hash-mod object modulus ¶

  This procedure is the key-hashing procedure used by `make-equal-hash-table`.

### 11.5 Object Hashing

The MIT/GNU Scheme object-hashing facility provides a mechanism for generating a unique hash number for an arbitrary object. This hash number, unlike an object’s address, is unchanged by garbage collection. The object-hashing facility is used in the generation of the written representation for many objects (see Custom Output), but it can be used for anything that needs a stable identifier for an arbitrary object.

All of these procedures accept an optional argument called hasher which contains the object-integer associations. If given, this argument must be an object hasher as constructed by `make-object-hasher` (see below). If not given, a default hasher is used.

procedure: hash-object object [hasher] ¶
obsolete procedure: hash object [hasher] ¶
obsolete procedure: object-hash object [hasher] ¶

  `hash-object` associates an exact non-negative integer with object and returns that integer. If `hash-object` was previously called with object as its argument, the integer returned is the same as was returned by the previous call. `hash-object` guarantees that distinct objects (in the sense of `eqv?`) are associated with distinct integers.

procedure: unhash-object k [hasher] ¶
obsolete procedure: unhash k [hasher] ¶
obsolete procedure: object-unhash k [hasher] ¶

  `unhash-object` takes an exact non-negative integer k and returns the object associated with that integer. If there is no object associated with k, or if the object previously associated with k has been reclaimed by the garbage collector, an error of type `condition-type:bad-range-argument` is signalled. In other words, if `hash-object` previously returned k for some object, and that object has not been reclaimed, it is the value of the call to `unhash-object`.

An object that is passed to `hash-object` as an argument is not protected from being reclaimed by the garbage collector. If all other references to that object are eliminated, the object will be reclaimed. Subsequently calling `unhash-object` with the hash number of the (now reclaimed) object will signal an error.

    (define x (cons 0 0))           ⇒  unspecified
    (hash-object x)                 ⇒  77
    (eqv? (hash-object x)
          (hash-object x))          ⇒  #t
    (define x 0)                    ⇒  unspecified
    (gc-flip)                       ;force a garbage collection
    (unhash-object 77)              error→

procedure: object-hashed? object [hasher] ¶

  This predicate is true iff object has an associated hash number.

procedure: valid-object-hash? k [hasher] ¶
obsolete procedure: valid-hash-number? k [hasher] ¶

  This predicate is true iff k is the hash number associated with some object.

Finally, this procedure makes new object hashers:

procedure: make-object-hasher ¶
obsolete procedure: hash-table/make ¶

  This procedure creates and returns a new, empty object hasher that is suitable for use as the optional hasher argument to the above procedures. The returned hasher contains no associations.

### 11.6 Red-Black Trees

Balanced binary trees are a useful data structure for maintaining large sets of associations whose keys are ordered. While most applications involving large association sets should use hash tables, some applications can benefit from the use of binary trees. Binary trees have two advantages over hash tables:

- The contents of a binary tree can be converted to an alist, sorted by key, in time proportional to the number of associations in the tree. A hash table can be converted into an unsorted alist in linear time; sorting it requires additional time.
- Two binary trees can be compared for equality in linear time. Hash tables, on the other hand, cannot be compared at all; they must be converted to alists before comparison can be done, and alist comparison is quadratic unless the alists are sorted.

MIT/GNU Scheme provides an implementation of red-black trees. The red-black tree-balancing algorithm provides generally good performance because it doesn’t try to keep the tree very closely balanced. At any given node in the tree, one side of the node can be twice as high as the other in the worst case. With typical data the tree will remain fairly well balanced anyway.

A red-black tree takes space that is proportional to the number of associations in the tree. For the current implementation, the constant of proportionality is eight words per association.

Red-black trees hold their keys strongly. In other words, if a red-black tree contains an association for a given key, that key cannot be reclaimed by the garbage collector.

procedure: make-rb-tree key=? key<? ¶

  This procedure creates and returns a newly allocated red-black tree. The tree contains no associations. Key=? and key<? are predicates that compare two keys and determine whether they are equal to or less than one another, respectively. For any two keys, at most one of these predicates is true.

procedure: rb-tree? object ¶

  Returns `#t` if object is a red-black tree, otherwise returns `#f`.

procedure: rb-tree/insert! rb-tree key datum ¶

  Associates datum with key in rb-tree and returns an unspecified value. If rb-tree already has an association for key, that association is replaced. The average and worst-case times required by this operation are proportional to the logarithm of the number of assocations in rb-tree.

procedure: rb-tree/lookup rb-tree key default ¶

  Returns the datum associated with key in rb-tree. If rb-tree doesn’t contain an association for key, default is returned. The average and worst-case times required by this operation are proportional to the logarithm of the number of assocations in rb-tree.

procedure: rb-tree/delete! rb-tree key ¶

  If rb-tree contains an association for key, removes it. Returns an unspecified value. The average and worst-case times required by this operation are proportional to the logarithm of the number of assocations in rb-tree.

procedure: rb-tree->alist rb-tree ¶

  Returns the contents of rb-tree as a newly allocated alist. Each element of the alist is a pair `(key . datum)` where key is one of the keys of rb-tree, and datum is its associated datum. The alist is sorted by key according to the key<? argument used to construct rb-tree. The time required by this operation is proportional to the number of associations in the tree.

procedure: rb-tree/key-list rb-tree ¶

  Returns a newly allocated list of the keys in rb-tree. The list is sorted by key according to the key<? argument used to construct rb-tree. The time required by this operation is proportional to the number of associations in the tree.

procedure: rb-tree/datum-list rb-tree ¶

  Returns a newly allocated list of the datums in rb-tree. Each element of the list corresponds to one of the associations in rb-tree, so if the tree contains multiple associations with the same datum, so will this list. The list is sorted by the keys of the associations, even though they do not appear in the result. The time required by this operation is proportional to the number of associations in the tree.

  This procedure is equivalent to:

      (lambda (rb-tree) (map cdr (rb-tree->alist rb-tree)))

procedure: rb-tree/equal? rb-tree-1 rb-tree-2 datum=? ¶

  Compares rb-tree-1 and rb-tree-2 for equality, returning `#t` iff they are equal and `#f` otherwise. The trees must have been constructed with the same equality and order predicates (same in the sense of `eq?`). The keys of the trees are compared using the key=? predicate used to build the trees, while the datums of the trees are compared using the equivalence predicate datum=?. The worst-case time required by this operation is proportional to the number of associations in the tree.

procedure: rb-tree/empty? rb-tree ¶

  Returns `#t` iff rb-tree contains no associations. Otherwise returns `#f`.

procedure: rb-tree/size rb-tree ¶

  Returns the number of associations in rb-tree, an exact non-negative integer. The average and worst-case times required by this operation are proportional to the number of associations in the tree.

procedure: rb-tree/height rb-tree ¶

  Returns the height of rb-tree, an exact non-negative integer. This is the length of the longest path from a leaf of the tree to the root. The average and worst-case times required by this operation are proportional to the number of associations in the tree.

  The returned value satisfies the following:

      (lambda (rb-tree)
        (let ((size (rb-tree/size rb-tree))
              (lg (lambda (x) (/ (log x) (log 2)))))
          (<= (lg size)
              (rb-tree/height rb-tree)
              (* 2 (lg (+ size 1))))))

procedure: rb-tree/copy rb-tree ¶

  Returns a newly allocated copy of rb-tree. The copy is identical to rb-tree in all respects, except that changes to rb-tree do not affect the copy, and vice versa. The time required by this operation is proportional to the number of associations in the tree.

procedure: alist->rb-tree alist key=? key<? ¶

  Returns a newly allocated red-black tree that contains the same associations as alist. This procedure is equivalent to:

      (lambda (alist key=? key<?)
        (let ((tree (make-rb-tree key=? key<?)))
          (for-each (lambda (association)
                      (rb-tree/insert! tree
                                       (car association)
                                       (cdr association)))
                    alist)
          tree))

The following operations provide access to the smallest and largest members in a red/black tree. They are useful for implementing priority queues.

procedure: rb-tree/min rb-tree default ¶

  Returns the smallest key in rb-tree, or default if the tree is empty.

procedure: rb-tree/min-datum rb-tree default ¶

  Returns the datum associated with the smallest key in rb-tree, or default if the tree is empty.

procedure: rb-tree/min-pair rb-tree ¶

  Finds the smallest key in rb-tree and returns a pair containing that key and its associated datum. If the tree is empty, returns `#f`.

procedure: rb-tree/max rb-tree default ¶

  Returns the largest key in rb-tree, or default if the tree is empty.

procedure: rb-tree/max-datum rb-tree default ¶

  Returns the datum associated with the largest key in rb-tree, or default if the tree is empty.

procedure: rb-tree/max-pair rb-tree ¶

  Finds the largest key in rb-tree and returns a pair containing that key and its associated datum. If the tree is empty, returns `#f`.

procedure: rb-tree/delete-min! rb-tree default ¶
procedure: rb-tree/delete-min-datum! rb-tree default ¶
procedure: rb-tree/delete-min-pair! rb-tree ¶
procedure: rb-tree/delete-max! rb-tree default ¶
procedure: rb-tree/delete-max-datum! rb-tree default ¶
procedure: rb-tree/delete-max-pair! rb-tree ¶

  These operations are exactly like the accessors above, in that they return information associated with the smallest or largest key, except that they simultaneously delete that key.

### 11.7 Weight-Balanced Trees

Balanced binary trees are a useful data structure for maintaining large sets of ordered objects or sets of associations whose keys are ordered. MIT/GNU Scheme has a comprehensive implementation of weight-balanced binary trees which has several advantages over the other data structures for large aggregates:

- In addition to the usual element-level operations like insertion, deletion and lookup, there is a full complement of collection-level operations, like set intersection, set union and subset test, all of which are implemented with good orders of growth in time and space. This makes weight-balanced trees ideal for rapid prototyping of functionally derived specifications.
- An element in a tree may be indexed by its position under the ordering of the keys, and the ordinal position of an element may be determined, both with reasonable efficiency.
- Operations to find and remove minimum element make weight-balanced trees simple to use for priority queues.
- The implementation is functional rather than imperative. This means that operations like ‘inserting’ an association in a tree do not destroy the old tree, in much the same way that `(+ 1 x)` modifies neither the constant 1 nor the value bound to `x`. The trees are referentially transparent thus the programmer need not worry about copying the trees. Referential transparency allows space efficiency to be achieved by sharing subtrees.

These features make weight-balanced trees suitable for a wide range of applications, especially those that require large numbers of sets or discrete maps. Applications that have a few global databases and/or concentrate on element-level operations like insertion and lookup are probably better off using hash tables or red-black trees.

The size of a tree is the number of associations that it contains. Weight-balanced binary trees are balanced to keep the sizes of the subtrees of each node within a constant factor of each other. This ensures logarithmic times for single-path operations (like lookup and insertion). A weight-balanced tree takes space that is proportional to the number of associations in the tree. For the current implementation, the constant of proportionality is six words per association.

Weight-balanced trees can be used as an implementation for either discrete sets or discrete maps (associations). Sets are implemented by ignoring the datum that is associated with the key. Under this scheme if an association exists in the tree this indicates that the key of the association is a member of the set. Typically a value such as `()`, `#t` or `#f` is associated with the key.

Many operations can be viewed as computing a result that, depending on whether the tree arguments are thought of as sets or maps, is known by two different names. An example is `wt-tree/member?`, which, when regarding the tree argument as a set, computes the set membership operation, but, when regarding the tree as a discrete map, `wt-tree/member?` is the predicate testing if the map is defined at an element in its domain. Most names in this package have been chosen based on interpreting the trees as sets, hence the name `wt-tree/member?` rather than `wt-tree/defined-at?`.

#### 11.7.1 Construction of Weight-Balanced Trees

Binary trees require there to be a total order on the keys used to arrange the elements in the tree. Weight-balanced trees are organized by types, where the type is an object encapsulating the ordering relation. Creating a tree is a two-stage process. First a tree type must be created from the predicate that gives the ordering. The tree type is then used for making trees, either empty or singleton trees or trees from other aggregate structures like association lists. Once created, a tree ‘knows’ its type and the type is used to test compatibility between trees in operations taking two trees. Usually a small number of tree types are created at the beginning of a program and used many times throughout the program’s execution.

procedure: make-wt-tree-type key<? ¶

  This procedure creates and returns a new tree type based on the ordering predicate key<?. Key<? must be a total ordering, having the property that for all key values `a`, `b` and `c`:

      (key<? a a)                         ⇒ #f
      (and (key<? a b) (key<? b a))       ⇒ #f
      (if (and (key<? a b) (key<? b c))
          (key<? a c)
          #t)                             ⇒ #t

  Two key values are assumed to be equal if neither is less than the other by key<?.

  Each call to `make-wt-tree-type` returns a distinct value, and trees are only compatible if their tree types are `eq?`. A consequence is that trees that are intended to be used in binary-tree operations must all be created with a tree type originating from the same call to `make-wt-tree-type`.

variable: number-wt-type ¶

  A standard tree type for trees with numeric keys. `Number-wt-type` could have been defined by

      (define number-wt-type (make-wt-tree-type  <))

variable: string-wt-type ¶

  A standard tree type for trees with string keys. `String-wt-type` could have been defined by

      (define string-wt-type (make-wt-tree-type  string<?))

procedure: make-wt-tree wt-tree-type ¶

  This procedure creates and returns a newly allocated weight-balanced tree. The tree is empty, i.e. it contains no associations. Wt-tree-type is a weight-balanced tree type obtained by calling `make-wt-tree-type`; the returned tree has this type.

procedure: singleton-wt-tree wt-tree-type key datum ¶

  This procedure creates and returns a newly allocated weight-balanced tree. The tree contains a single association, that of datum with key. Wt-tree-type is a weight-balanced tree type obtained by calling `make-wt-tree-type`; the returned tree has this type.

procedure: alist->wt-tree tree-type alist ¶

  Returns a newly allocated weight-balanced tree that contains the same associations as alist. This procedure is equivalent to:

      (lambda (type alist)
        (let ((tree (make-wt-tree type)))
          (for-each (lambda (association)
                      (wt-tree/add! tree
                                    (car association)
                                    (cdr association)))
                    alist)
          tree))

#### 11.7.2 Basic Operations on Weight-Balanced Trees

This section describes the basic tree operations on weight-balanced trees. These operations are the usual tree operations for insertion, deletion and lookup, some predicates and a procedure for determining the number of associations in a tree.

procedure: wt-tree? object ¶

  Returns `#t` if object is a weight-balanced tree, otherwise returns `#f`.

procedure: wt-tree/empty? wt-tree ¶

  Returns `#t` if wt-tree contains no associations, otherwise returns `#f`.

procedure: wt-tree/size wt-tree ¶

  Returns the number of associations in wt-tree, an exact non-negative integer. This operation takes constant time.

procedure: wt-tree/add wt-tree key datum ¶

  Returns a new tree containing all the associations in wt-tree and the association of datum with key. If wt-tree already had an association for key, the new association overrides the old. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in wt-tree.

procedure: wt-tree/add! wt-tree key datum ¶

  Associates datum with key in wt-tree and returns an unspecified value. If wt-tree already has an association for key, that association is replaced. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in wt-tree.

procedure: wt-tree/member? key wt-tree ¶

  Returns `#t` if wt-tree contains an association for key, otherwise returns `#f`. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in wt-tree.

procedure: wt-tree/lookup wt-tree key default ¶

  Returns the datum associated with key in wt-tree. If wt-tree doesn’t contain an association for key, default is returned. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in wt-tree.

procedure: wt-tree/delete wt-tree key ¶

  Returns a new tree containing all the associations in wt-tree, except that if wt-tree contains an association for key, it is removed from the result. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in wt-tree.

procedure: wt-tree/delete! wt-tree key ¶

  If wt-tree contains an association for key the association is removed. Returns an unspecified value. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in wt-tree.

#### 11.7.3 Advanced Operations on Weight-Balanced Trees

In the following the size of a tree is the number of associations that the tree contains, and a smaller tree contains fewer associations.

procedure: wt-tree/split< wt-tree bound ¶

  Returns a new tree containing all and only the associations in wt-tree that have a key that is less than bound in the ordering relation of the tree type of wt-tree. The average and worst-case times required by this operation are proportional to the logarithm of the size of wt-tree.

procedure: wt-tree/split> wt-tree bound ¶

  Returns a new tree containing all and only the associations in wt-tree that have a key that is greater than bound in the ordering relation of the tree type of wt-tree. The average and worst-case times required by this operation are proportional to the logarithm of the size of wt-tree.

procedure: wt-tree/union wt-tree-1 wt-tree-2 ¶

  Returns a new tree containing all the associations from both trees. This operation is asymmetric: when both trees have an association for the same key, the returned tree associates the datum from wt-tree-2 with the key. Thus if the trees are viewed as discrete maps then `wt-tree/union` computes the map override of wt-tree-1 by wt-tree-2. If the trees are viewed as sets the result is the set union of the arguments. The worst-case time required by this operation is proportional to the sum of the sizes of both trees. If the minimum key of one tree is greater than the maximum key of the other tree then the worst-case time required is proportional to the logarithm of the size of the larger tree.

procedure: wt-tree/intersection wt-tree-1 wt-tree-2 ¶

  Returns a new tree containing all and only those associations from wt-tree-1 that have keys appearing as the key of an association in wt-tree-2. Thus the associated data in the result are those from wt-tree-1. If the trees are being used as sets the result is the set intersection of the arguments. As a discrete map operation, `wt-tree/intersection` computes the domain restriction of wt-tree-1 to (the domain of) wt-tree-2. The worst-case time required by this operation is proportional to the sum of the sizes of the trees.

procedure: wt-tree/difference wt-tree-1 wt-tree-2 ¶

  Returns a new tree containing all and only those associations from wt-tree-1 that have keys that do not appear as the key of an association in wt-tree-2. If the trees are viewed as sets the result is the asymmetric set difference of the arguments. As a discrete map operation, it computes the domain restriction of wt-tree-1 to the complement of (the domain of) wt-tree-2. The worst-case time required by this operation is proportional to the sum of the sizes of the trees.

procedure: wt-tree/subset? wt-tree-1 wt-tree-2 ¶

  Returns `#t` iff the key of each association in wt-tree-1 is the key of some association in wt-tree-2, otherwise returns `#f`. Viewed as a set operation, `wt-tree/subset?` is the improper subset predicate. A proper subset predicate can be constructed:

      (define (proper-subset? s1 s2)
        (and (wt-tree/subset? s1 s2)
             (< (wt-tree/size s1) (wt-tree/size s2))))

  As a discrete map operation, `wt-tree/subset?` is the subset test on the domain(s) of the map(s). In the worst-case the time required by this operation is proportional to the size of wt-tree-1.

procedure: wt-tree/set-equal? wt-tree-1 wt-tree-2 ¶

  Returns `#t` iff for every association in wt-tree-1 there is an association in wt-tree-2 that has the same key, and vice versa.

  Viewing the arguments as sets, `wt-tree/set-equal?` is the set equality predicate. As a map operation it determines if two maps are defined on the same domain.

  This procedure is equivalent to

      (lambda (wt-tree-1 wt-tree-2)
        (and (wt-tree/subset? wt-tree-1 wt-tree-2
             (wt-tree/subset? wt-tree-2 wt-tree-1)))

  In the worst case the time required by this operation is proportional to the size of the smaller tree.

procedure: wt-tree/fold combiner initial wt-tree ¶

  This procedure reduces wt-tree by combining all the associations, using an reverse in-order traversal, so the associations are visited in reverse order. Combiner is a procedure of three arguments: a key, a datum and the accumulated result so far. Provided combiner takes time bounded by a constant, `wt-tree/fold` takes time proportional to the size of wt-tree.

  A sorted association list can be derived simply:

      (wt-tree/fold (lambda (key datum list)
                      (cons (cons key datum) list))
                    '()
                    wt-tree))

  The data in the associations can be summed like this:

      (wt-tree/fold (lambda (key datum sum) (+ sum datum))
                    0
                    wt-tree)

procedure: wt-tree/for-each action wt-tree ¶

  This procedure traverses wt-tree in order, applying action to each association. The associations are processed in increasing order of their keys. Action is a procedure of two arguments that takes the key and datum respectively of the association. Provided action takes time bounded by a constant, `wt-tree/for-each` takes time proportional to the size of wt-tree. The example prints the tree:

      (wt-tree/for-each (lambda (key value)
                          (display (list key value)))
                        wt-tree))

procedure: wt-tree/union-merge wt-tree-1 wt-tree-2 merge ¶

  Returns a new tree containing all the associations from both trees. If both trees have an association for the same key, the datum associated with that key in the result tree is computed by applying the procedure merge to the key, the value from wt-tree-1 and the value from wt-tree-2. Merge is of the form

      (lambda (key datum-1 datum-2) …)

  If some key occurs only in one tree, that association will appear in the result tree without being processed by merge, so for this operation to make sense, either merge must have both a right and left identity that correspond to the association being absent in one of the trees, or some guarantee must be made, for example, all the keys in one tree are known to occur in the other.

  These are all reasonable procedures for merge

      (lambda (key val1 val2) (+ val1 val2))
      (lambda (key val1 val2) (append val1 val2))
      (lambda (key val1 val2) (wt-tree/union val1 val2))

  However, a procedure like

      (lambda (key val1 val2) (- val1 val2))

  would result in a subtraction of the data for all associations with keys occuring in both trees but associations with keys occuring in only the second tree would be copied, not negated, as is presumably be intent. The programmer might ensure that this never happens.

  This procedure has the same time behavior as `wt-tree/union` but with a slightly worse constant factor. Indeed, `wt-tree/union` might have been defined like this:

      (define (wt-tree/union tree1 tree2)
        (wt-tree/union-merge tree1 tree2
                             (lambda (key val1 val2) val2)))

The merge procedure takes the key as a parameter in case the data are not independent of the key.

#### 11.7.4 Indexing Operations on Weight-Balanced Trees

Weight-balanced trees support operations that view the tree as sorted sequence of associations. Elements of the sequence can be accessed by position, and the position of an element in the sequence can be determined, both in logarthmic time.

procedure: wt-tree/index wt-tree index ¶
procedure: wt-tree/index-datum wt-tree index ¶
procedure: wt-tree/index-pair wt-tree index ¶

  Returns the 0-based indexth association of wt-tree in the sorted sequence under the tree’s ordering relation on the keys. `wt-tree/index` returns the indexth key, `wt-tree/index-datum` returns the datum associated with the indexth key and `wt-tree/index-pair` returns a new pair `(key . datum)` which is the `cons` of the indexth key and its datum. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in the tree.

  These operations signal a condition of type `condition-type:bad-range-argument` if index`<0` or if index is greater than or equal to the number of associations in the tree. If the tree is empty, they signal an anonymous error.

  Indexing can be used to find the median and maximum keys in the tree as follows:

      median:   (wt-tree/index wt-tree
                               (quotient (wt-tree/size wt-tree)
                                         2))
      maximum:  (wt-tree/index wt-tree
                               (- (wt-tree/size wt-tree)
                                  1))

procedure: wt-tree/rank wt-tree key ¶

  Determines the 0-based position of key in the sorted sequence of the keys under the tree’s ordering relation, or `#f` if the tree has no association with for key. This procedure returns either an exact non-negative integer or `#f`. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in the tree.

procedure: wt-tree/min wt-tree ¶
procedure: wt-tree/min-datum wt-tree ¶
procedure: wt-tree/min-pair wt-tree ¶

  Returns the association of wt-tree that has the least key under the tree’s ordering relation. `wt-tree/min` returns the least key, `wt-tree/min-datum` returns the datum associated with the least key and `wt-tree/min-pair` returns a new pair `(key . datum)` which is the `cons` of the minimum key and its datum. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in the tree.

  These operations signal an error if the tree is empty. They could have been written

      (define (wt-tree/min tree)
        (wt-tree/index tree 0))
      (define (wt-tree/min-datum tree)
        (wt-tree/index-datum tree 0))
      (define (wt-tree/min-pair tree)
        (wt-tree/index-pair tree 0))

procedure: wt-tree/delete-min wt-tree ¶

  Returns a new tree containing all of the associations in wt-tree except the association with the least key under the wt-tree’s ordering relation. An error is signalled if the tree is empty. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in the tree. This operation is equivalent to

      (wt-tree/delete wt-tree (wt-tree/min wt-tree))

procedure: wt-tree/delete-min! wt-tree ¶

  Removes the association with the least key under the wt-tree’s ordering relation. An error is signalled if the tree is empty. The average and worst-case times required by this operation are proportional to the logarithm of the number of associations in the tree. This operation is equivalent to

      (wt-tree/delete! wt-tree (wt-tree/min wt-tree))

### 11.8 Associative Maps

Starting with version 11.1, MIT/GNU Scheme provides an abstract associative-map interface that can be backed by any kind of association mechanism. The interface is similar to that of SRFI 125. Associative maps can be mutable or immutable, depending on the backing implementation. As of this writing we support the following implementations: alists, weak alists, red/black trees, hash tables, and tries, all of which are mutable.

#### 11.8.1 Amap constructors

All associative-map constructors take a comparator argument followed by a list of additional specification arguments. In all cases, the comparator is used to compare keys in the map, while the additional arguments specify the implementation and/or features of the map being created.

procedure: make-amap comparator arg … ¶

  Creates a new mutable associative map as specified by comparator and args.

procedure: alist->amap alist comparator arg … ¶

  Creates a new associative map as specified by comparator and args, and which contains the associations in alist.

procedure: amap-unfold stop? mapper successor seed comparator arg … ¶

  Creates a new associative map as specified by comparator and args. The associations in the resulting map are generated using the additional arguments.

  The stop? argument is a unary predicate that takes a state value and returns `#t` if generation is complete, otherwise `#f`. The mapper argument is a unary procedure that takes a state value and returns two values: a key and a value. The successor argument is a unary procedure that takes a state value and returns a new state value. And seed is the initial state value.

  The process by which the generator adds associations to the map is this:

      (let loop ((state seed))
        (if (stop? state)
            result
            (let-values (((key value) (mapper state)))
              (amap-set! result key value)
              (loop (successor state)))))

The arguments passed to a constructor can be divided into categories:

- Each backing implementation has an associated name, which can be passed as one of the arguments. At present this includes `alist`, `hash-table`, `red/black-tree`, etc.

  There must be at most one such name; it is an error to pass two or more implementation names. If an implementation name is given, then the remaining arguments must be supported by that implementation.

- The arguments `weak-keys`, `weak-values`, `ephemeral-keys`, and `ephemeral-values` specify how associations interact with garbage collection. Not all implementations support these arguments, and those that do may support only some of them. Some combinations are not allowed: for example, `weak-keys` and `ephemeral-keys` are mutually exclusive.
- Additional arguments are used to guide the choice of implementation when it is not explicitly specified. The desired time complexity can be specified: `linear-time`, `log-time`, `amortized-constant-time`, and `sublinear-time`. The argument `ordered-by-key` specifies that the map keeps its associations ordered.
- The argument `thread-safe` specifies that the implementation is safe to use in a multi-threaded environment. At present none of the implementations are thread-safe.
- Finally, an exact non-negative integer argument specifies the initial size of the map.

This is a complex set of possible arguments. In order to help explore what arguments can be used, and in what combinations, we provide some utility procedures:

procedure: amap-implementation-names ¶

  Returns a list of the supported implementation names.

procedure: amap-implementation-supported-args name ¶

  Returns a list of the arguments supported by the implementation specified by name. This list can include the procedure `exact-nonnegative-integer?` if the implementation supports an initial size.

procedure: amap-implementation-supports-args? name args ¶

  Returns `#t` if the implementation specified by name supports args, otherwise returns `#f`.

An implementation may support a limited set of comparators. For example, a hash table requires a comparator that satisfies `comparator-hashable?`, while a binary tree requires one satisfying `comparator-ordered?`.

procedure: amap-implementation-supports-comparator? name comparator ¶

  Returns `#t` if the implementation specified by name supports comparator, otherwise returns `#f`.

#### 11.8.2 Amap predicates

procedure: amap? object ¶

  True iff object is an associative map.

procedure: amap-contains? amap key ¶

  True iff amap contains an association for key.

procedure: amap-empty? amap ¶

  True iff amap has no associations.

procedure: amap=? value-comparator amap1 amap2 ¶

  True iff the associations in amap1 are the same as the associations in amap2. This means that amap1 and amap2 have the same keys (in the sense of their shared equality predicate), and that for each key they have the same value (in the sense of value-comparator).

procedure: amap-mutable? amap ¶

  True iff amap is mutable.

#### 11.8.3 Amap accessors

procedure: amap-ref amap key [fail [succeed]] ¶

  Looks up the value associated to key in amap, invokes the procedure succeed on it, and returns its result; if succeed is not provided, then the value itself is returned. If key is not contained in amap and fail is supplied, then fail is invoked on no arguments and its result is returned. Otherwise an error is signaled.

procedure: amap-ref/default amap key default ¶

  Semantically equivalent to, but may be more efficient than, the following code:

      (amap-ref amap key (lambda () default))

procedure: amap-comparator amap ¶

  Returns the comparator that was used to create amap.

procedure: amap-args amap ¶

  Returns the extra arguments that were used to create amap.

procedure: amap-implementation-name amap ¶

  Returns the name of amap’s backing implementation.

#### 11.8.4 Amap mutators

procedure: amap-set! amap [key value] … ¶

  Repeatedly mutates amap, creating new associations in it by processing the arguments from left to right. The args alternate between keys and values. Whenever there is a previous association for a key, it is deleted. It is an error if a key does not satisfy the type check procedure of the comparator of amap. Likewise, it is an error if a key is not a valid argument to the equality predicate of amap. Returns an unspecified value.

procedure: amap-delete! amap key … ¶

  Deletes any association to each key in amap and returns the number of keys that had associations.

procedure: amap-intern! amap key fail ¶

  Effectively invokes `amap-ref` with the given arguments and returns what it returns. If key was not found in amap, its value is set to the result of calling fail.

procedure: amap-update! amap key updater [fail [succeed]] ¶

  Semantically equivalent to, but may be more efficient than, the following code:

      (amap-set! amap key (updater (amap-ref amap key fail succeed)))

procedure: amap-update!/default amap key updater default ¶

  Semantically equivalent to, but may be more efficient than, the following code:

      (amap-set! amap key (updater (amap-ref/default amap key default)))

procedure: amap-pop! amap ¶

  Chooses an arbitrary association from amap and removes it, returning the key and value as two values. Signals an error if amap is empty.

procedure: amap-clear! amap ¶

  Delete all the associations from amap.

procedure: amap-clean! amap ¶

  If amap has weak or ephemeral associations, cleans up any storage for associations whose key and/or value has been reclaimed by the garbage collector. Otherwise does nothing.

  This procedure does not have any visible effect, since the associations it cleans up are ignored by all other parts of the interface. Additionally, most implementations clean up their storage incrementally as they are used. But this procedure provides for edge cases where the reclaimed storage might matter.

#### 11.8.5 Amap mapping and folding

procedure: amap-map procedure amap [comparator arg …] ¶

  Returns a newly allocated associative map as if by `(make-amap comparator arg …)`. Calls procedure for every association in amap with the value of the association. The key of the association and the result of invoking procedure are entered into the new map.

  If comparator recognizes multiple keys in amap as equivalent, any one of such associations is taken.

procedure: amap-for-each procedure amap ¶

  Calls procedure for every association in amap with two arguments: the key of the association and the value of the association. The value returned by procedure is discarded. Returns an unspecified value.

procedure: amap-map! procedure amap ¶

  Calls procedure for every association in amap with two arguments: the key of the association and the value of the association. The value returned by procedure is used to update the value of the association. Returns an unspecified value.

procedure: amap-map->list procedure amap ¶

  Calls procedure for every association in amap with two arguments: the key of the association and the value of the association. The values returned by the invocations of procedure are accumulated into a list, which is returned.

procedure: amap-fold kons knil amap ¶

  Calls kons for every association in amap with three arguments: the key of the association, the value of the association, and an accumulated value val. The argument knil is seed for the first invocation of kons, and for subsequent invocations of kons, the returned value of the previous invocation. The value returned by `amap-fold` is the return value of the last invocation of kons.

procedure: amap-prune! predicate amap ¶

  Calls predicate for every association in amap with two arguments, the key and the value of the association, and removes all associations from amap for which predicate returns true. Returns an unspecified value.

#### 11.8.6 Amap contents

procedure: amap-size amap ¶

  Returns the number of associations in amap as an exact integer.

procedure: amap-keys amap ¶

  Returns a newly allocated list of all the keys in amap.

procedure: amap-values amap ¶

  Returns a newly allocated list of all the values in amap.

procedure: amap-entries amap ¶

  Returns two values, a newly allocated list of all the keys in amap and a newly allocated list of all the values in amap in the corresponding order.

procedure: amap-find procedure amap fail ¶

  For each association of amap, invoke procedure on its key and value. If procedure returns true, then `amap-find` returns what procedure returns. If all the calls to procedure return `#f`, returns the result of invoking the thunk fail.

procedure: amap-count predicate amap ¶

  For each association of amap, invoke predicate on its key and value. Returns the number of calls to predicate that returned true.

#### 11.8.7 Amap copying and conversion

procedure: amap-copy amap [mutable?] ¶

  Returns a newly allocated associative map with the same properties and associations as amap. If mutable? is given and is true, the new associative map is mutable. Otherwise it is immutable provided that the implementation supports immutable maps.

procedure: amap-empty-copy amap ¶

  Returns a newly allocated mutable associative map with the same properties as amap, but with no associations.

procedure: amap->alist amap ¶

  Returns an alist with the same associations as amap in an unspecified order.

#### 11.8.8 Amaps as sets

procedure: amap-union! amap1 amap2 ¶

  Adds the associations of amap2 to amap1 and returns amap1. If a key appears in both maps, its value is set to the value appearing in amap1.

procedure: amap-intersection! amap1 amap2 ¶

  Deletes the associations from amap1 whose keys don’t also appear in amap2 and returns amap1.

procedure: amap-difference! amap1 amap2 ¶

  Deletes the associations of amap1 whose keys are also present in amap2 and returns amap1.

procedure: amap-xor! amap1 amap2 ¶

  Deletes the associations of amap1 whose keys are also present in amap2, and then adds the associations of amap2 whose keys are not present in amap1 to amap1. Returns amap1.

## 12 Procedures

Procedures are created by evaluating `lambda` expressions (see Lambda Expressions); the `lambda` may either be explicit or may be implicit as in a “procedure `define`” (see Definitions). Also there are special built-in procedures, called primitive procedures, such as `car`; these procedures are not written in Scheme but in the language used to implement the Scheme system. MIT/GNU Scheme also provides application hooks, which support the construction of data structures that act like procedures.

In MIT/GNU Scheme, the written representation of a procedure tells you the type of the procedure (compiled, interpreted, or primitive):

    pp
         ⇒  #[compiled-procedure 56 ("pp" #x2) #x10 #x307578]
    (lambda (x) x)
         ⇒  #[compound-procedure 57]
    (define (foo x) x)
    foo
         ⇒  #[compound-procedure 58 foo]
    car
         ⇒  #[primitive-procedure car]
    (call-with-current-continuation (lambda (x) x))
         ⇒  #[continuation 59]

Note that interpreted procedures are called “compound” procedures (strictly speaking, compiled procedures are also compound procedures). The written representation makes this distinction for historical reasons, and may eventually change.

### 12.1 Procedure Operations

procedure: apply procedure object object … ¶

  Calls procedure with the elements of the following list as arguments:

      (cons* object object …)

  The initial objects may be any objects, but the last object (there must be at least one object) must be a list.

      (apply + (list 3 4 5 6))                ⇒  18
      (apply + 3 4 '(5 6))                    ⇒  18

      (define compose
        (lambda (f g)
          (lambda args
            (f (apply g args)))))
      ((compose sqrt *) 12 75)                ⇒  30

procedure: procedure? object ¶

  Returns `#t` if object is a procedure; otherwise returns `#f`. If `#t` is returned, exactly one of the following predicates is satisfied by object: `compiled-procedure?`, `compound-procedure?`, or `primitive-procedure?`.

procedure: compiled-procedure? object ¶

  Returns `#t` if object is a compiled procedure; otherwise returns `#f`.

procedure: compound-procedure? object ¶

  Returns `#t` if object is a compound (i.e. interpreted) procedure; otherwise returns `#f`.

procedure: primitive-procedure? object ¶

  Returns `#t` if object is a primitive procedure; otherwise returns `#f`.

procedure: procedure-environment procedure ¶

  Returns the closing environment of procedure. Signals an error if procedure is a primitive procedure, or if procedure is a compiled procedure for which the debugging information is unavailable.

### 12.2 Arity

Each procedure has an arity, which is the minimum and (optionally) maximum number of arguments that it will accept. MIT/GNU Scheme provides an abstraction that represents arity, and tests for the apparent arity of a procedure.

Arity objects come in two forms: the simple form, an exact non-negative integer, represents a fixed number of arguments. The general form is a pair whose `car` represents the minimum number of arguments and whose `cdr` is the maximum number of arguments.

procedure: make-procedure-arity min [max [simple-ok?]] ¶

  Returns an arity object made from min and max. Min must be an exact non-negative integer. Max must be an exact non-negative integer at least as large as min. Alternatively, max may be omitted or given as ‘`#f`’, which represents an arity with no upper bound.

  If simple-ok? is true, the returned arity is in the simple form (an exact non-negative integer) when possible, and otherwise is always in the general form. Simple-ok? defaults to ‘`#f`’.

procedure: procedure-arity? object ¶

  Returns ‘`#t`’ if object is an arity object, and ‘`#f`’ otherwise.

procedure: procedure-arity-min arity ¶
procedure: procedure-arity-max arity ¶

  Return the lower and upper bounds of arity, respectively.

The following procedures test for the apparent arity of a procedure. The results of the test may be less restrictive than the effect of calling the procedure. In other words, these procedures may indicate that the procedure will accept a given number of arguments, but if you call the procedure it may signal a `condition-type:wrong-number-of-arguments` error. For example, here is a procedure that appears to accept any number of arguments, but when called will signal an error if the number of arguments is not one:

    (lambda arguments (apply car arguments))

procedure: procedure-arity procedure ¶

  Returns the arity that procedure accepts. The result may be in either simple or general form.

      (procedure-arity (lambda () 3))         ⇒  (0 . 0)
      (procedure-arity (lambda (x) x))        ⇒  (1 . 1)
      (procedure-arity car)                   ⇒  (1 . 1)
      (procedure-arity (lambda x x))          ⇒  (0 . #f)
      (procedure-arity (lambda (x . y) x))    ⇒  (1 . #f)
      (procedure-arity (lambda (x #!optional y) x))
                                              ⇒  (1 . 2)

procedure: procedure-arity-valid? procedure arity ¶

  Returns ‘`#t`’ if procedure accepts arity, and ‘`#f`’ otherwise.

procedure: procedure-of-arity? object arity ¶

  Returns ‘`#t`’ if object is a procedure that accepts arity, and ‘`#f`’ otherwise. Equivalent to:

      (and (procedure? object)
           (procedure-arity-valid? object arity))

procedure: guarantee-procedure-of-arity object arity caller ¶

  Signals an error if object is not a procedure accepting arity. Caller is a symbol that is printed as part of the error message and is intended to be the name of the procedure where the error occurs.

procedure: thunk? object ¶

  Returns ‘`#t`’ if object is a procedure that accepts zero arguments, and ‘`#f`’ otherwise. Equivalent to:

      (procedure-of-arity? object 0)

### 12.3 Primitive Procedures

procedure: make-primitive-procedure name [arity] ¶

  Name must be a symbol. Arity must be an exact non-negative integer, `-1`, `#f`, or `#t`; if not supplied it defaults to `#f`. Returns the primitive procedure called name. May perform further actions depending on arity:

  `#f`

    If the primitive procedure is not implemented, signals an error.

  `#t`

    If the primitive procedure is not implemented, returns `#f`.

  integer

    If the primitive procedure is implemented, signals an error if its arity is not equal to arity. If the primitive procedure is not implemented, returns an unimplemented primitive procedure object that accepts arity arguments. An arity of `-1` means it accepts any number of arguments.

procedure: primitive-procedure-name primitive-procedure ¶

  Returns the name of primitive-procedure, a symbol.

      (primitive-procedure-name car)          ⇒  car

procedure: implemented-primitive-procedure? primitive-procedure ¶

  Returns `#t` if primitive-procedure is implemented; otherwise returns `#f`. Useful because the code that implements a particular primitive procedure is not necessarily linked into the executable Scheme program.

### 12.4 Continuations

procedure: call-with-current-continuation procedure ¶

  Procedure must be a procedure of one argument. Packages up the current continuation (see below) as an escape procedure and passes it as an argument to procedure. The escape procedure is a Scheme procedure of one argument that, if it is later passed a value, will ignore whatever continuation is in effect at that later time and will give the value instead to the continuation that was in effect when the escape procedure was created. The escape procedure created by `call-with-current-continuation` has unlimited extent just like any other procedure in Scheme. It may be stored in variables or data structures and may be called as many times as desired.

  The following examples show only the most common uses of this procedure. If all real programs were as simple as these examples, there would be no need for a procedure with the power of `call-with-current-continuation`.

      (call-with-current-continuation
        (lambda (exit)
          (for-each (lambda (x)
                      (if (negative? x)
                          (exit x)))
                    '(54 0 37 -3 245 19))
          #t))                                ⇒  -3

      (define list-length
        (lambda (obj)
          (call-with-current-continuation
            (lambda (return)
              (letrec ((r
                        (lambda (obj)
                          (cond ((null? obj) 0)
                                ((pair? obj) (+ (r (cdr obj)) 1))
                                (else (return #f))))))
                (r obj))))))
      (list-length '(1 2 3 4))                ⇒  4
      (list-length '(a b . c))                ⇒  #f

  A common use of `call-with-current-continuation` is for structured, non-local exits from loops or procedure bodies, but in fact `call-with-current-continuation` is quite useful for implementing a wide variety of advanced control structures.

  Whenever a Scheme expression is evaluated a continuation exists that wants the result of the expression. The continuation represents an entire (default) future for the computation. If the expression is evaluated at top level, for example, the continuation will take the result, print it on the screen, prompt for the next input, evaluate it, and so on forever. Most of the time the continuation includes actions specified by user code, as in a continuation that will take the result, multiply it by the value stored in a local variable, add seven, and give the answer to the top-level continuation to be printed. Normally these ubiquitous continuations are hidden behind the scenes and programmers don’t think much about them. On the rare occasions that you may need to deal explicitly with continuations, `call-with-current-continuation` lets you do so by creating a procedure that acts just like the current continuation.

procedure: continuation? object ¶

  Returns `#t` if object is a continuation; otherwise returns `#f`.

procedure: within-continuation continuation thunk ¶

  Thunk must be a procedure of no arguments. Conceptually,
  `within-continuation` invokes continuation on the result of invoking thunk, but thunk is executed in the dynamic state of continuation. In other words, the “current” continuation is abandoned before thunk is invoked.

procedure: dynamic-wind before thunk after ¶

  Calls thunk without arguments, returning the result(s) of this call. Before and after are called, also without arguments, as required by the following rules. Note that in the absence of calls to continuations captured using `call-with-current-continuation` the three arguments are called once each, in order. Before is called whenever execution enters the dynamic extent of the call to thunk and after is called whenever it exits that dynamic extent. The dynamic extent of a procedure call is the period between when the call is initiated and when it returns. In Scheme, because of `call-with-current-continuation`, the dynamic extent of a call may not be a single, connected time period. It is defined as follows:

  - The dynamic extent is entered when execution of the body of the called procedure begins.
  - The dynamic extent is also entered when execution is not within the dynamic extent and a continuation is invoked that was captured (using `call-with-current-continuation`) during the dynamic extent.
  - It is exited when the called procedure returns.
  - It is also exited when execution is within the dynamic extent and a continuation is invoked that was captured while not within the dynamic extent.

  If a second call to `dynamic-wind` occurs within the dynamic extent of the call to thunk and then a continuation is invoked in such a way that the afters from these two invocations of `dynamic-wind` are both to be called, then the after associated with the second (inner) call to `dynamic-wind` is called first.

  If a second call to `dynamic-wind` occurs within the dynamic extent of the call to thunk and then a continuation is invoked in such a way that the befores from these two invocations of `dynamic-wind` are both to be called, then the before associated with the first (outer) call to `dynamic-wind` is called first.

  If invoking a continuation requires calling the before from one call to `dynamic-wind` and the after from another, then the after is called first.

  The effect of using a captured continuation to enter or exit the dynamic extent of a call to before or after is undefined.

      (let ((path '())
            (c #f))
        (let ((add (lambda (s)
                     (set! path (cons s path)))))
          (dynamic-wind
            (lambda () (add 'connect))
            (lambda ()
              (add (call-with-current-continuation
                     (lambda (c0)
                       (set! c c0)
                       'talk1))))
            (lambda () (add 'disconnect)))
          (if (< (length path) 4)
              (c 'talk2)
              (reverse path))))

      ⇒ (connect talk1 disconnect connect talk2 disconnect)

The following two procedures support multiple values.

procedure: call-with-values thunk procedure ¶

  Thunk must be a procedure of no arguments, and procedure must be a procedure. Thunk is invoked with a continuation that expects to receive multiple values; specifically, the continuation expects to receive the same number of values that procedure accepts as arguments. Thunk must return multiple values using the `values` procedure. Then procedure is called with the multiple values as its arguments. The result yielded by procedure is returned as the result of `call-with-values`.

procedure: values object … ¶

  Returns multiple values. The continuation in effect when this procedure is called must be a multiple-value continuation that was created by `call-with-values`. Furthermore it must accept as many values as there are objects.

### 12.5 Application Hooks

Application hooks are objects that can be applied like procedures. Each application hook has two parts: a procedure that specifies what to do when the application hook is applied, and an arbitrary object, called extra. Often the procedure uses the extra object to determine what to do.

There are two kinds of application hooks, which differ in what arguments are passed to the procedure. When an apply hook is applied, the procedure is passed exactly the same arguments that were passed to the apply hook. When an entity is applied, the entity itself is passed as the first argument, followed by the other arguments that were passed to the entity.

Both apply hooks and entities satisfy the predicate `procedure?`. Each satisfies either `compiled-procedure?`, `compound-procedure?`, or `primitive-procedure?`, depending on its procedure component. An apply hook is considered to accept the same number of arguments as its procedure, while an entity is considered to accept one less argument than its procedure.

procedure: make-apply-hook procedure object ¶

  Returns a newly allocated apply hook with a procedure component of procedure and an extra component of object.

procedure: apply-hook? object ¶

  Returns `#t` if object is an apply hook; otherwise returns `#f`.

procedure: apply-hook-procedure apply-hook ¶

  Returns the procedure component of apply-hook.

procedure: set-apply-hook-procedure! apply-hook procedure ¶

  Changes the procedure component of apply-hook to be procedure. Returns an unspecified value.

procedure: apply-hook-extra apply-hook ¶

  Returns the extra component of apply-hook.

procedure: set-apply-hook-extra! apply-hook object ¶

  Changes the extra component of apply-hook to be object. Returns an unspecified value.

procedure: make-entity procedure object ¶

  Returns a newly allocated entity with a procedure component of procedure and an extra component of object.

procedure: entity? object ¶

  Returns `#t` if object is an entity; otherwise returns `#f`.

procedure: entity-procedure entity ¶

  Returns the procedure component of entity.

procedure: set-entity-procedure! entity procedure ¶

  Changes the procedure component of entity to be procedure. Returns an unspecified value.

procedure: entity-extra entity ¶

  Returns the extra component of entity.

procedure: set-entity-extra! entity object ¶

  Changes the extra component of entity to be object. Returns an unspecified value.

## 13 Environments

### 13.1 Environment Operations

Environments are first-class objects in MIT/GNU Scheme. An environment consists of some bindings and possibly a parent environment, from which other bindings are inherited. The operations in this section reveal the frame-like structure of environments by permitting you to examine the bindings of a particular environment separately from those of its parent.

There are several types of bindings that can occur in an environment. The most common is the simple variable binding, which associates a value (any Scheme object) with an identifier (a symbol). A variable binding can also be unassigned, which means that it has no value. An unassigned variable is bound, in that is will shadow other bindings of the same name in ancestor environments, but a reference to that variable will signal an error of type `condition-type:unassigned-variable`. An unassigned variable can be assigned (using `set!` or `environment-assign!`) to give it a value.

In addition to variable bindings, an environment can also have keyword bindings. A keyword binding associates a syntactic keyword (usually a macro transformer) with an identifier. Keyword bindings are special in that they are considered “bound”, but ordinary variable references don’t work on them. So an attempt to reference or assign a keyword binding results in an error of type `condition-type:macro-binding`. However, keyword bindings can be redefined using `define` or `environment-define`.

procedure: environment? object ¶

  Returns `#t` if object is an environment; otherwise returns `#f`.

procedure: environment-has-parent? environment ¶

  Returns `#t` if environment has a parent environment; otherwise returns `#f`.

procedure: environment-parent environment ¶

  Returns the parent environment of environment. It is an error if environment has no parent.

procedure: environment-bound-names environment ¶

  Returns a newly allocated list of the names (symbols) that are bound by environment. This does not include the names that are bound by the parent environment of environment. It does include names that are unassigned or keywords in environment.

procedure: environment-macro-names environment ¶

  Returns a newly allocated list of the names (symbols) that are bound to syntactic keywords in environment.

procedure: environment-bindings environment ¶

  Returns a newly allocated list of the bindings of environment; does not include the bindings of the parent environment. Each element of this list takes one of two forms: `(symbol)` indicates that symbol is bound but unassigned, while `(symbol object)` indicates that symbol is bound, and its value is object.

procedure: environment-reference-type environment symbol ¶

  Returns a symbol describing the reference type of symbol in environment or one of its ancestor environments. The result is one of the following:

  `normal`

    means symbol is a variable binding with a normal value.

  `unassigned`

    means symbol is a variable binding with no value.

  `macro`

    means symbol is a keyword binding.

  `unbound`

    means symbol has no associated binding.

procedure: environment-bound? environment symbol ¶

  Returns `#t` if symbol is bound in environment or one of its ancestor environments; otherwise returns `#f`. This is equivalent to

      (not (eq? 'unbound
                (environment-reference-type environment symbol)))

procedure: environment-assigned? environment symbol ¶

  Returns `#t` if symbol is bound in environment or one of its ancestor environments, and has a normal value. Returns `#f` if it is bound but unassigned. Signals an error if it is unbound or is bound to a keyword.

procedure: environment-lookup environment symbol ¶

  Symbol must be bound to a normal value in environment or one of its ancestor environments. Returns the value to which it is bound. Signals an error if unbound, unassigned, or a keyword.

procedure: environment-lookup-macro environment symbol ¶

  If symbol is a keyword binding in environment or one of its ancestor environments, returns the value of the binding. Otherwise, returns `#f`. Does not signal any errors other than argument-type errors.

procedure: environment-assignable? environment symbol ¶

  Symbol must be bound in environment or one of its ancestor environments. Returns `#t` if the binding may be modified by side effect.

procedure: environment-assign! environment symbol object ¶

  Symbol must be bound in environment or one of its ancestor environments, and must be assignable. Modifies the binding to have object as its value, and returns an unspecified result.

procedure: environment-definable? environment symbol ¶

  Returns `#t` if symbol is definable in environment, and `#f` otherwise. At present, this is false for environments generated by application of compiled procedures, and true for all other environments.

procedure: environment-define environment symbol object ¶

  Defines symbol to be bound to object in environment, and returns an unspecified value. Signals an error if symbol isn’t definable in environment.

procedure: environment-define-macro environment symbol transformer ¶

  Defines symbol to be a keyword bound to transformer in environment, and returns an unspecified value. Signals an error if symbol isn’t definable in environment. The type of transformer is defined by the syntax engine and is not checked by this procedure. If the type is incorrect this will subsequently signal an error during syntax expansion.

procedure: eval expression environment ¶

  Evaluates expression, a list-structure representation (sometimes called s-expression representation) of a Scheme expression, in environment. You rarely need `eval` in ordinary programs; it is useful mostly for evaluating expressions that have been created “on the fly” by a program. `eval` is relatively expensive because it must convert expression to an internal form before it is executed.

      (define foo (list '+ 1 2))
      (eval foo (the-environment))            ⇒  3

### 13.2 Environment Variables

The `user-initial-environment` is where the top-level read-eval-print (REP) loop evaluates expressions and binds definitions. It is a child of `system-global-environment`, which is where all of the Scheme system definitions are bound. All of the bindings in `system-global-environment` are available when the current environment is `user-initial-environment`. However, any new bindings that you create in the REP loop (with `define` forms or by loading files containing `define` forms) occur in `user-initial-environment`.

variable: system-global-environment ¶

  The variable `system-global-environment` is bound to the distinguished environment that’s the ancestor of most other environments (except for those created by `make-root-top-level-environment`). It is the parent environment of `user-initial-environment`. Primitives, system procedures, and most syntactic keywords are bound (and sometimes closed) in this environment.

variable: user-initial-environment ¶

  The variable `user-initial-environment` is bound to the default environment in which typed expressions are evaluated by the top-level REP loop.

  Although all bindings in `system-global-environment` are visible to the REP loop, definitions that are typed at, or loaded by, the REP loop occur in the `user-initial-environment`. This is partly a safety measure: if you enter a definition that happens to have the same name as a critical system procedure, your definition will be visible only to the procedures you define in the `user-initial-environment`; the MIT/GNU Scheme system procedures, which are defined in `system-global-environment`, will continue to see the original definition.

### 13.3 REPL Environment

procedure: nearest-repl/environment ¶

  Returns the current REP loop environment (i.e. the current environment of the closest enclosing REP loop). When Scheme first starts up, this is the same as `user-initial-environment`.

procedure: ge environment ¶

  Changes the current REP loop environment to environment. Environment can be either an environment or a procedure object. If it’s a procedure, the environment in which that procedure was closed is the new environment.

### 13.4 Top-level Environments

The operations in this section manipulate top-level environments, as opposed to environments created by the application of procedures. For historical reasons, top-level environments are referred to as interpreter environments.

special form: the-environment ¶

  Returns the current environment. This form may only be evaluated in a top-level environment. An error is signalled if it appears elsewhere.

procedure: top-level-environment? object ¶
procedure: interpreter-environment? object ¶

  Returns `#t` if object is an top-level environment; otherwise returns `#f`.

  `interpreter-environment?` is an alias for `top-level-environment?`.

procedure: extend-top-level-environment environment [names [values]] ¶
procedure: make-top-level-environment [names [values]] ¶
procedure: make-root-top-level-environment [names [values]] ¶

  Returns a newly allocated top-level environment. `extend-top-level-environment` creates an environment that has parent environment, `make-top-level-environment` creates an environment that has parent `system-global-environment`, and `make-root-top-level-environment` creates an environment that has no parent.

  The optional arguments names and values are used to specify initial bindings in the new environment. If specified, names must be a list of symbols, and values must be a list of objects. If only names is specified, each name in names will be bound in the environment, but unassigned. If names and values are both specified, they must be the same length, and each name in names will be bound to the corresponding value in values. If neither names nor values is specified, the environment will have no initial bindings.

procedure: link-variables environment1 symbol1 environment2 symbol2 ¶

  Defines symbol1 in environment1 to have the same binding as symbol2 in environment2, and returns an unspecified value. Prior to the call, symbol2 must be bound in environment2, but the type of binding is irrelevant; it may be a normal binding, an unassigned binding, or a keyword binding. Signals an error if symbol1 isn’t definable in environment1, or if symbol2 is unbound in environment2.

  By “the same binding”, we mean that the value cell is shared between the two environments. If a value is assigned to symbol1 in environment1, a subsequent reference to symbol2 in environment2 will see that value, and vice versa.

procedure: unbind-variable environment symbol ¶

  If symbol is bound in environment or one of its ancestor environments, removes the binding, so that subsequent accesses to that symbol behave as if the binding never existed. Returns `#t` if there was a binding prior to the call, and `#f` if there wasn’t.

## 14 Input/Output

This chapter describes the procedures that are used for input and output (I/O). The chapter first describes ports and how they are manipulated, then describes the I/O operations. Finally, some low-level procedures are described that permit the implementation of custom ports and high-performance I/O.

### 14.1 Ports

Ports represent input and output devices. To Scheme, an input port is a Scheme object that can deliver data upon command, while an output port is a Scheme object that can accept data. Whether the input and output port types are disjoint is implementation-dependent. (In MIT/GNU Scheme, there are input ports, output ports, and input/output ports.)

Different port types operate on different data. Scheme implementations are required to support textual ports and binary ports, but may also provide other port types.

A textual port supports reading or writing of individual characters from or to a backing store containing characters using `read-char` and `write-char` below, and it supports operations defined in terms of characters, such as `read` and `write`.

A binary port supports reading or writing of individual bytes from or to a backing store containing bytes using `read-u8` and `write-u8` below, as well as operations defined in terms of bytes. Whether the textual and binary port types are disjoint is implementation-dependent. (In MIT/GNU Scheme, textual ports and binary ports are distinct.)

Ports can be used to access files, devices, and similar things on the host system on which the Scheme program is running.

standard procedure: call-with-port port procedure ¶

  It is an error if procedure does not accept one argument.

  The `call-with-port` procedure calls procedure with port as an argument. If procedure returns, then the port is closed automatically and the values yielded by procedure are returned. If procedure does not return, then the port must not be closed automatically unless it is possible to prove that the port will never again be used for a read or write operation.

  Rationale: Because Scheme’s escape procedures have unlimited extent, it is possible to escape from the current continuation but later to resume it. If implementations were permitted to close the port on any escape from the current continuation, then it would be impossible to write portable code using both `call-with-current-continuation` and `call-with-port`.

procedure: call-with-truncated-output-port limit output-port procedure ¶

  The limit argument must be a nonnegative integer. It is an error if procedure does not accept one argument.

  This procedure uses a continuation to escape from procedure if it tries to write more than limit characters.

  It calls procedure with a special output port as an argument. Up to limit characters may be written to that output port, and those characters are transparently written through to output-port.

  If the number of characters written to that port exceeds limit, then the escape continuation is invoked and `#t` is returned. Otherwise, procedure returns normally and `#f` is returned.

  Note that if procedure writes exactly limit characters, then the escape continuation is not invoked, and `#f` is returned.

  In no case does `call-with-truncated-output-port` close output-port.

standard procedure: input-port? object ¶
standard procedure: output-port? object ¶
procedure: i/o-port? object ¶
standard procedure: textual-port? object ¶
standard procedure: binary-port? object ¶
standard procedure: port? object ¶

  These procedures return `#t` if object is an input port, output port, input/output port, textual port, binary port, or any kind of port, respectively. Otherwise they return `#f`.

standard procedure: input-port-open? port ¶
standard procedure: output-port-open? port ¶

  Returns `#t` if port is still open and capable of performing input or output, respectively, and `#f` otherwise.

standard parameter: current-input-port [input-port] ¶
standard parameter: current-output-port [output-port] ¶
standard parameter: current-error-port [output-port] ¶

  Returns the current default input port, output port, or error port (an output port), respectively. These procedures are parameter objects, which can be overridden with `parameterize`. The initial bindings for these are implementation-defined textual ports.

parameter: notification-output-port [output-port] ¶

  Returns an output port suitable for generating “notifications”, that is, messages to the user that supply interesting information about the execution of a program. For example, the `load` procedure writes messages to this port informing the user that a file is being loaded.

  This procedure is a parameter object, which can be overridden with `parameterize`.

parameter: trace-output-port [output-port] ¶

  Returns an output port suitable for generating “tracing” information about a program’s execution. The output generated by the `trace` procedure is sent to this port.

  This procedure is a parameter object, which can be overridden with `parameterize`.

parameter: interaction-i/o-port [i/o-port] ¶

  Returns an I/O port suitable for querying or prompting the user. The standard prompting procedures use this port by default (see Prompting).

  This procedure is a parameter object, which can be overridden with `parameterize`.

standard procedure: close-port port ¶
standard procedure: close-input-port port ¶
standard procedure: close-output-port port ¶

  Closes the resource associated with port, rendering the port incapable of delivering or accepting data. It is an error to apply the last two procedures to a port which is not an input or output port, respectively. Scheme implementations may provide ports which are simultaneously input and output ports, such as sockets; the close-input-port and close-output-port procedures can then be used to close the input and output sides of the port independently.

  These routines have no effect if the port has already been closed.

obsolete procedure: set-current-input-port! input-port ¶
obsolete procedure: set-current-output-port! output-port ¶
obsolete procedure: set-notification-output-port! output-port ¶
obsolete procedure: set-trace-output-port! output-port ¶
obsolete procedure: set-interaction-i/o-port! i/o-port ¶

  These procedures are deprecated; instead call the corresponding parameters with an argument.

obsolete procedure: with-input-from-port input-port thunk ¶
obsolete procedure: with-output-to-port output-port thunk ¶
obsolete procedure: with-notification-output-port output-port thunk ¶
obsolete procedure: with-trace-output-port output-port thunk ¶
obsolete procedure: with-interaction-i/o-port i/o-port thunk ¶

  These procedures are deprecated; instead use `parameterize` on the corresponding parameters.

variable: console-i/o-port ¶

  `console-i/o-port` is an I/O port that communicates with the “console”. Under unix, the console is the controlling terminal of the Scheme process. Under Windows, the console is the window that is created when Scheme starts up.

  This variable is rarely used; instead programs should use one of the standard ports defined above. This variable should not be modified.

### 14.2 File Ports

Before Scheme can access a file for reading or writing, it is necessary to open a port to the file. This section describes procedures used to open ports to files. Such ports are closed (like any other port) by `close-port`. File ports are automatically closed if and when they are reclaimed by the garbage collector.

Before opening a file for input or output, by whatever method, the filename argument is converted to canonical form by calling the procedure `merge-pathnames` with filename as its sole argument. Thus, filename can be either a string or a pathname, and it is merged with the current pathname defaults to produce the pathname that is then opened.

standard procedure: call-with-input-file filename procedure ¶
standard procedure: call-with-output-file filename procedure ¶

  It is an error if procedure does not accept one argument.

  These procedures obtain a textual port obtained by opening the named file for input or output as if by `open-input-file` or `open-output-file`. The port and procedure are then passed to a procedure equivalent to `call-with-port`.

procedure: call-with-binary-input-file filename procedure ¶
procedure: call-with-binary-output-file filename procedure ¶

  It is an error if procedure does not accept one argument.

  These procedures obtain a binary port obtained by opening the named file for input or output as if by `open-binary-input-file` or `open-binary-output-file`. The port and procedure are then passed to a procedure equivalent to `call-with-port`.

standard procedure: with-input-from-file filename thunk ¶
standard procedure: with-output-to-file filename thunk ¶

  The file named by filename is opened for input or output as if by `open-input-file` or `open-output-file`, and the new port is made to be the value returned by `current-input-port` or `current-output-port` (as used by `(read)`, `(write obj)`, and so forth). The thunk is then called with no arguments. When the thunk returns, the port is closed and the previous default is restored. It is an error if thunk does not accept zero arguments. Both procedures return the values yielded by thunk. If an escape procedure is used to escape from the continuation of these procedures, they behave exactly as if the current input or output port had been bound dynamically with `parameterize`.

obsolete procedure: with-input-from-binary-file filename thunk ¶
obsolete procedure: with-output-to-binary-file filename thunk ¶

  These procedures are deprecated; instead use `parameterize` along with `call-with-binary-input-file` or `call-with-binary-output-file`.

procedure: open-input-file filename ¶
procedure: open-binary-input-file filename ¶

  Takes a filename for an existing file and returns a textual input port or binary input port that is capable of delivering data from the file. If the file does not exist or cannot be opened, an error an error that satisfies `file-error?` is signaled.

standard procedure: open-output-file filename [append?] ¶
standard procedure: open-binary-output-file filename [append?] ¶

  Takes a filename naming an output file to be created and returns a textual output port or binary output port that is capable of writing data to a new file by that name. If a file with the given name already exists, the effect is unspecified. (In that case, MIT/GNU Scheme overwrites an existing file.) If the file cannot be opened, an error that satisfies `file-error?` is signalled.

  The optional argument append? is an MIT/GNU Scheme extension. If append? is given and not `#f`, the file is opened in append mode. In this mode, the contents of the file are not overwritten; instead any characters written to the file are appended to the end of the existing contents. If the file does not exist, append mode creates the file and writes to it in the normal way.

procedure: open-i/o-file filename ¶
procedure: open-binary-i/o-file filename ¶

  Takes a filename referring to an existing file and returns an I/O port that is capable of both reading from and writing to the file. If the file cannot be opened, an error that satisfies `file-error?` is signalled.

  This procedure is often used to open special files. For example, under unix this procedure can be used to open terminal device files, PTY device files, and named pipes.

procedure: close-all-open-files ¶

  This procedure closes all file ports that are open at the time that it is called, and returns an unspecified value.

### 14.3 String Ports

This section describes textual input ports that read their input from given strings, and textual output ports that accumulate their output and return it as a string.

standard procedure: open-input-string string [start [end]] ¶

  Takes a string and returns a textual input port that delivers characters from the string. If the string is modified, the effect is unspecified.

  The optional arguments start and end may be used to specify that the string port delivers characters from a substring of string; if not given, start defaults to `0` and end defaults to `(string-length string)`.

standard procedure: open-output-string ¶

  Returns a textual output port that will accumulate characters for retrieval by `get-output-string`.

standard procedure: get-output-string port ¶

  It is an error if port was not created with open-output-string.

  Returns a string consisting of the characters that have been output to the port so far in the order they were output. If the result string is modified, the effect is unspecified.

      (parameterize ((current-output-port (open-output-string)))
        (display "piece")
        (display " by piece ")
        (display "by piece.")
        (newline)
        (get-output-string (current-output-port)))

          ⇒ "piece by piece by piece.\n"

procedure: call-with-output-string procedure ¶

  The procedure is called with one argument, a textual output port. The values yielded by procedure are ignored. When procedure returns, `call-with-output-string` returns the port’s accumulated output as a string. If the result string is modified, the effect is unspecified.

  This procedure could have been defined as follows:

      (define (call-with-output-string procedure)
        (let ((port (open-output-string)))
          (procedure port)
          (get-output-string port)))

procedure: call-with-truncated-output-string limit procedure ¶

  Similar to `call-with-output-string`, except that the output is limited to at most limit characters. The returned value is a pair; the car of the pair is `#t` if procedure attempted to write more than limit characters, and `#f` otherwise. The cdr of the pair is a newly allocated string containing the accumulated output.

  This procedure could have been defined as follows:

      (define (call-with-truncated-output-string limit procedure)
        (let ((port (open-output-string)))
          (let ((truncated?
                 (call-with-truncated-output-port limit port
                                                  procedure)))
            (cons truncated? (get-output-string port)))))

  This procedure is helpful for displaying circular lists, as shown in this example:

      (define inf (list 'inf))
      (call-with-truncated-output-string 40
        (lambda (port)
          (write inf port)))                  ⇒  (#f . "(inf)")
      (set-cdr! inf inf)
      (call-with-truncated-output-string 40
        (lambda (port)
          (write inf port)))
              ⇒  (#t . "(inf inf inf inf inf inf inf inf inf inf")

procedure: write-to-string object [limit] ¶

  Writes object to a string output port, and returns the resulting string.

  If limit is supplied and not `#f`, then this procedure is equivalent to the following and returns a pair instead of just a string:

      (call-with-truncated-output-string limit
        (lambda (port)
          (write object port)))

obsolete procedure: with-input-from-string string thunk ¶
obsolete procedure: with-output-to-string thunk ¶
obsolete procedure: with-output-to-truncated-string limit thunk ¶

  These procedures are deprecated; instead use `open-input-string`, `call-with-output-string`, or `call-with-truncated-output-string` along with `parameterize`.

### 14.4 Bytevector Ports

This section describes binary input ports that read their input from given bytevectors, and binary output ports that accumulate their output and return it as a bytevector.

standard procedure: open-input-bytevector bytevector [start [end]] ¶

  Takes a bytevector and returns a binary input port that delivers bytes from the bytevector. If the bytevector is modified, the effect is unspecified.

  The optional arguments start and end may be used to specify that the bytevector port delivers bytes from a portion of bytevector; if not given, start defaults to `0` and end defaults to `(bytevector-length bytevector)`.

standard procedure: open-output-bytevector ¶

  Returns a binary output port that will accumulate bytes for retrieval by `get-output-bytevector`.

standard procedure: get-output-bytevector port ¶

  It is an error if port was not created with open-output-bytevector.

  Returns a bytevector consisting of the bytes that have been output to the port so far in the order they were output. If the result bytevector is modified, the effect is unspecified.

procedure: call-with-output-bytevector procedure ¶

  The procedure is called with one argument, a binary output port. The values yielded by procedure are ignored. When procedure returns, `call-with-output-bytevector` returns the port’s accumulated output as a newly allocated bytevector.

  This procedure could have been defined as follows:

      (define (call-with-output-bytevector procedure)
        (let ((port (open-output-bytevector)))
          (procedure port)
          (get-output-bytevector port)))

### 14.5 Input Procedures

This section describes the procedures that read input. Input procedures can read either from the current input port or from a given port. Remember that to read from a file, you must first open a port to the file.

Input ports can be divided into two types, called interactive and non-interactive. Interactive input ports are ports that read input from a source that is time-dependent; for example, a port that reads input from a terminal or from another program. Non-interactive input ports read input from a time-independent source, such as an ordinary file or a character string.

In this section, all optional arguments called port default to the current input port.

standard procedure: read [port] ¶

  The `read` procedure converts external representations of Scheme objects into the objects themselves. It returns the next object parsable from the given textual input port, updating port to point to the first character past the end of the external representation of the object.

  Implementations may support extended syntax to represent record types or other types that do not have datum representations.

  If an end of file is encountered in the input before any characters are found that can begin an object, then an end-of-file object is returned. The port remains open, and further attempts to read will also return an end-of-file object. If an end of file is encountered after the beginning of an object’s external representation, but the external representation is incomplete and therefore not parsable, an error that satisfies `read-error?` is signaled.

  The port remains open, and further attempts to read will also return an end-of-file object. If an end of file is encountered after the beginning of an object’s written representation, but the written representation is incomplete and therefore not parsable, an error is signalled.

standard procedure: read-char [port] ¶

  Returns the next character available from the textual input port, updating port to point to the following character. If no more characters are available, an end-of-file object is returned.

  In MIT/GNU Scheme, if port is an interactive input port and no characters are immediately available, `read-char` will hang waiting for input, even if the port is in non-blocking mode.

procedure: read-char-no-hang [port] ¶

  This procedure behaves exactly like `read-char` except when port is an interactive port in non-blocking mode, and there are no characters immediately available. In that case this procedure returns `#f` without blocking.

procedure: unread-char char [port] ¶

  The given char must be the most-recently read character from the textual input port. This procedure “unreads” the character, updating port as if the character had never been read.

  Note that this only works with characters returned by `read-char` or `read-char-no-hang`.

standard procedure: peek-char [port] ¶

  Returns the next character available from the textual input port, without updating port to point to the following character. If no more characters are available, an end-of-file object is returned.

  Note: The value returned by a call to `peek-char` is the same as the value that would have been returned by a call to `read-char` on the same port. The only difference is that the very next call to `read-char` or `peek-char` on that port will return the value returned by the preceding call to `peek-char`. In particular, a call to `peek-char` on an interactive port will hang waiting for input whenever a call to `read-char` would have hung.

standard procedure: read-line [port] ¶

  Returns the next line of text available from the textual input port, updating the port to point to the following character. If an end of line is read, a string containing all of the text up to (but not including) the end of line is returned, and the port is updated to point just past the end of line. If an end of file is encountered before any end of line is read, but some characters have been read, a string containing those characters is returned. If an end of file is encountered before any characters are read, an end-of-file object is returned. For the purpose of this procedure, an end of line consists of either a linefeed character, a carriage return character, or a sequence of a carriage return character followed by a linefeed character. Implementations may also recognize other end of line characters or sequences.

  In MIT/GNU Scheme, if port is an interactive input port and no characters are immediately available, `read-line` will hang waiting for input, even if the port is in non-blocking mode.

standard procedure: eof-object? object ¶

  Returns `#t` if object is an end-of-file object, otherwise returns `#f`. The precise set of end-of-file objects will vary among implementations, but in any case no end-of-file object will ever be an object that can be read in using `read`.

standard procedure: eof-object ¶

  Returns an end-of-file object, not necessarily unique.

standard procedure: char-ready? [port] ¶

  Returns `#t` if a character is ready on the textual input port and returns `#f` otherwise. If `char-ready?` returns `#t` then the next `read-char` operation on the given port is guaranteed not to hang. If the port is at end of file then `char-ready?` returns `#t`.

  Rationale: The `char-ready?` procedure exists to make it possible for a program to accept characters from interactive ports without getting stuck waiting for input. Any input editors associated with such ports must ensure that characters whose existence has been asserted by `char-ready?` cannot be removed from the input. If `char-ready?` were to return `#f` at end of file, a port at end of file would be indistinguishable from an interactive port that has no ready characters.

standard procedure: read-string k [port] ¶

  Reads the next k characters, or as many as are available before the end of file, from the textual input port into a newly allocated string in left-to-right order and returns the string. If no characters are available before the end of file, an end-of-file object is returned.

  Note: MIT/GNU Scheme previously defined this procedure differently, and this alternate usage is deprecated; please use `read-delimited-string` instead. For now, `read-string` will redirect to `read-delimited-string` as needed, but this redirection will be eliminated in a future release.

procedure: read-string! string [port [start [end]]] ¶

  Reads the next end-start characters, or as many as are available before the end of file, from the textual input port into string in left-to-right order beginning at the start position. If end is not supplied, reads until the end of string has been reached. If start is not supplied, reads beginning at position `0`. Returns the number of characters read. If no characters are available, an end-of-file object is returned.

  In MIT/GNU Scheme, if port is an interactive port in non-blocking mode and no characters are immediately available, `#f` is returned without any modification of string.

  However, if one or more characters are immediately available, the region is filled using the available characters. The procedure then returns the number of characters filled in, without waiting for further characters, even if the number of filled characters is less than the size of the region.

obsolete procedure: read-substring! string start end [port] ¶

  This procedure is deprecated; use `read-string!` instead.

standard procedure: read-u8 [port] ¶

  Returns the next byte available from the binary input port, updating the port to point to the following byte. If no more bytes are available, an end-of-file object is returned.

  In MIT/GNU Scheme, if port is an interactive input port in non-blocking mode and no characters are immediately available, `read-u8` will return `#f`.

standard procedure: peek-u8 [port] ¶

  Returns the next byte available from the binary input port, but without updating the port to point to the following byte. If no more bytes are available, an end-of-file object is returned.

  In MIT/GNU Scheme, if port is an interactive input port in non-blocking mode and no characters are immediately available, `peek-u8` will return `#f`.

standard procedure: u8-ready? [port] ¶

  Returns `#t` if a byte is ready on the binary input `port` and returns `#f` otherwise. If `u8-ready?` returns `#t` then the next `read-u8` operation on the given port is guaranteed not to hang. If the port is at end of file then `u8-ready?` returns `#t`.

standard procedure: read-bytevector k [port] ¶

  Reads the next k bytes, or as many as are available before the end of file, from the binary input port into a newly allocated bytevector in left-to-right order and returns the bytevector. If no bytes are available before the end of file, an end-of-file object is returned.

  In MIT/GNU Scheme, if port is an interactive input port in non-blocking mode and no characters are immediately available, `read-bytevector` will return `#f`.

  However, if one or more bytes are immediately available, they are read and returned as a bytevector, without waiting for further bytes, even if the number of bytes is less than k.

standard procedure: read-bytevector! bytevector [port [start [end]]] ¶

  Reads the next end-start bytes, or as many as are available before the end of file, from the binary input port into bytevector in left-to-right order beginning at the start position. If end is not supplied, reads until the end of bytevector has been reached. If start is not supplied, reads beginning at position `0`. Returns the number of bytes read. If no bytes are available, an end-of-file object is returned.

  In MIT/GNU Scheme, if port is an interactive input port in non-blocking mode and no characters are immediately available, `read-bytevector!` will return `#f`.

  However, if one or more bytes are immediately available, the region is filled using the available bytes. The procedure then returns the number of bytes filled in, without waiting for further bytes, even if the number of filled bytes is less than the size of the region.

procedure: read-delimited-string char-set [port] ¶

  Reads characters from port until it finds a terminating character that is a member of char-set (see Character Sets) or encounters end of file. The port is updated to point to the terminating character, or to end of file if no terminating character was found. `read-delimited-string` returns the characters, up to but excluding the terminating character, as a newly allocated string.

  This procedure ignores the blocking mode of the port, blocking unconditionally until it sees either a delimiter or end of file. If end of file is encountered before any characters are read, an end-of-file object is returned.

  On many input ports, this operation is significantly faster than the following equivalent code using `peek-char` and `read-char`:

      (define (read-delimited-string char-set port)
        (let ((char (peek-char port)))
          (if (eof-object? char)
              char
              (list->string
               (let loop ((char char))
                 (if (or (eof-object? char)
                         (char-in-set? char char-set))
                     '()
                     (begin
                       (read-char port)
                       (cons char
                             (loop (peek-char port))))))))))

#### 14.5.1 Reader Controls

The following parameters control the behavior of the `read` procedure.

parameter: param:reader-radix ¶

  This parameter defines the radix used by the reader when it parses numbers. This is similar to passing a radix argument to `string->number`. The value of the parameter must be one of `2`, `8`, `10`, or `16`; an error is signaled if the parameter is bound to any other value.

  Note that much of the number syntax is invalid for radixes other than `10`. The reader detects cases where such invalid syntax is used and signals an error. However, problems can still occur when `param:reader-radix` is bound to `16`, because syntax that normally denotes symbols can now denote numbers (e.g. `abc`). Because of this, it is usually undesirable to bind this parameter to anything other than the default.

  The default value of this parameter is `10`.

parameter: param:reader-fold-case? ¶

  This parameter controls whether the parser folds the case of symbols, character names, and certain other syntax. If it is bound to its default value of `#t`, symbols read by the parser are case-folded prior to being interned. Otherwise, symbols are interned without folding.

  At present, it is a bad idea to use this feature, as it doesn’t really make Scheme case-sensitive, and therefore can break features of the Scheme runtime that depend on case-folded symbols. Instead, use the `#!fold-case` or `#!no-fold-case` markers in your code.

obsolete variable: *parser-radix* ¶
obsolete variable: *parser-canonicalize-symbols?* ¶

  These variables are deprecated; instead use the corresponding parameter objects.

### 14.6 Output Procedures

Output ports may or may not support buffering of output, in which output characters are collected together in a buffer and then sent to the output device all at once. (Most of the output ports implemented by the runtime system support buffering.) Sending all of the characters in the buffer to the output device is called flushing the buffer. In general, output procedures do not flush the buffer of an output port unless the buffer is full.

However, the standard output procedures described in this section perform what is called discretionary flushing of the buffer. Discretionary output flushing works as follows. After a procedure performs its output (writing characters to the output buffer), it checks to see if the port implements an operation called `discretionary-flush-output`. If so, then that operation is invoked to flush the buffer. At present, only the console port defines `discretionary-flush-output`; this is used to guarantee that output to the console appears immediately after it is written, without requiring calls to `flush-output-port`.

In this section, all optional arguments called port default to the current output port.

Note: MIT/GNU Scheme doesn’t support datum labels, so any behavior in `write`, `write-shared`, or `write-simple` that depends on datum labels is not implemented. At present all three of these procedures are equivalent. This will be remedied in a future release.

standard procedure: write object [port] ¶

  Writes a representation of object to the given textual output port. Strings that appear in the written representation are enclosed in quotation marks, and within those strings backslash and quotation mark characters are escaped by backslashes. Symbols that contain non-ASCII characters are escaped with vertical lines. Character objects are written using the `#\` notation.

  If object contains cycles which would cause an infinite loop using the normal written representation, then at least the objects that form part of the cycle must be represented using datum labels. Datum labels must not be used if there are no cycles.

  Implementations may support extended syntax to represent record types or other types that do not have datum representations.

  The `write` procedure returns an unspecified value.

  On MIT/GNU Scheme `write` performs discretionary output flushing.

standard procedure: write-shared object [port] ¶

  The `write-shared` procedure is the same as `write`, except that shared structure must be represented using datum labels for all pairs and vectors that appear more than once in the output.

standard procedure: write-simple object [port] ¶

  The `write-simple` procedure is the same as `write`, except that shared structure is never represented using datum labels. This can cause `write-simple` not to terminate if object contains circular structure.

standard procedure: display object [port] ¶

  Writes a representation of object to the given textual output port. Strings that appear in the written representation are output as if by write-string instead of by write. Symbols are not escaped. Character objects appear in the representation as if written by `write-char` instead of by `write`. The display representation of other objects is unspecified. However, `display` must not loop forever on self-referencing pairs, vectors, or records. Thus if the normal `write` representation is used, datum labels are needed to represent cycles as in `write`.

  Implementations may support extended syntax to represent record types or other types that do not have datum representations.

  The `display` procedure returns an unspecified value.

  Rationale: The `write` procedure is intended for producing machine-readable output and `display` for producing human-readable output.

standard procedure: newline [port] ¶

  Writes an end of line to textual output port. Exactly how this is done differs from one operating system to another. Returns an unspecified value.

standard procedure: write-char char [port] ¶

  Writes the character char (not an external representation of the character) to the given textual output port and returns an unspecified value.

standard procedure: write-string string [port [start [end]]] ¶

  Writes the characters of string from start to end in left-to-right order to the textual output port.

obsolete procedure: write-substring string start end [port] ¶

  This procedure is deprecated; use `write-string` instead.

standard procedure: write-u8 byte [port] ¶

  Writes the byte to the given binary output port and returns an unspecified value.

  In MIT/GNU Scheme, if port is an interactive output port in non-blocking mode and writing a byte would block, `write-u8` immediately returns `#f` without writing anything. Otherwise byte is written and `1` is returned.

standard procedure: write-bytevector bytevector [port [start [end]]] ¶

  Writes the bytes of bytevector from start to end in left-to-right order to the binary output port.

  In MIT/GNU Scheme, if port is an interactive output port in non-blocking mode `write-bytevector` will write as many bytes as it can without blocking, then returns the number of bytes written; if no bytes can be written without blocking, returns `#f` without writing anything. Otherwise `write-bytevector` returns the number of bytes actually written, which may be less than the number requested if unable to write all the bytes. (For example, if writing to a file and the file system is full.)

standard procedure: flush-output-port [port] ¶

  Flushes any buffered output from the buffer of port to the underlying file or device and returns an unspecified value.

obsolete procedure: flush-output [port] ¶

  This procedure is deprecated; use `flush-output-port` instead.

procedure: fresh-line [port] ¶

  Most output ports are able to tell whether or not they are at the beginning of a line of output. If port is such a port, this procedure writes an end-of-line to the port only if the port is not already at the beginning of a line. If port is not such a port, this procedure is identical to `newline`. In either case, `fresh-line` performs discretionary output flushing and returns an unspecified value.

procedure: write-line object [port] ¶

  Like `write`, except that it writes an end-of-line to port after writing object’s representation. This procedure performs discretionary output flushing and returns an unspecified value.

procedure: beep [port] ¶

  Performs a “beep” operation on port, performs discretionary output flushing, and returns an unspecified value. On the console port, this usually causes the console bell to beep, but more sophisticated interactive ports may take other actions, such as flashing the screen. On most output ports, e.g. file and string output ports, this does nothing.

procedure: clear [port] ¶

  “Clears the screen” of port, performs discretionary output flushing, and returns an unspecified value. On a terminal or window, this has a well-defined effect. On other output ports, e.g. file and string output ports, this does nothing.

procedure: pp object [port [as-code?]] ¶

  `pp` prints object in a visually appealing and structurally revealing manner on port. If object is a procedure, `pp` attempts to print the source text. If the optional argument as-code? is true, `pp` prints lists as Scheme code, providing appropriate indentation; by default this argument is false. `pp` performs discretionary output flushing and returns an unspecified value.

The following parameters may be used with `parameterize` to change the behavior of the `write` and `display` procedures.

parameter: param:printer-radix ¶

  This parameter specifies the default radix used to print numbers. Its value must be one of the exact integers `2`, `8`, `10`, or `16`; the default is `10`. For values other than `10`, numbers are prefixed to indicate their radix.

parameter: param:printer-list-breadth-limit ¶

  This parameter specifies a limit on the length of the printed representation of a list or vector; for example, if the limit is `4`, only the first four elements of any list are printed, followed by ellipses to indicate any additional elements. The value of this parameter must be an exact non-negative integer, or `#f` meaning no limit; the default is `#f`.

      (parameterize ((param:printer-list-breadth-limit 4))
        (write-to-string '(a b c d)))
                                      ⇒ "(a b c d)"
      (parameterize ((param:printer-list-breadth-limit 4))
        (write-to-string '(a b c d e)))
                                      ⇒ "(a b c d ...)"

parameter: param:printer-list-depth-limit ¶

  This parameter specifies a limit on the nesting of lists and vectors in the printed representation. If lists (or vectors) are more deeply nested than the limit, the part of the representation that exceeds the limit is replaced by ellipses. The value of this parameter must be an exact non-negative integer, or `#f` meaning no limit; the default is `#f`.

      (parameterize ((param:printer-list-depth-limit 4))
        (write-to-string '((((a))) b c d)))
                                      ⇒ "((((a))) b c d)"
      (parameterize ((param:printer-list-depth-limit 4))
        (write-to-string '(((((a)))) b c d)))
                                      ⇒ "((((...))) b c d)"

parameter: param:printer-string-length-limit ¶

  This parameter specifies a limit on the length of the printed representation of strings. If a string’s length exceeds this limit, the part of the printed representation for the characters exceeding the limit is replaced by ellipses. The value of this parameter must be an exact non-negative integer, or `#f` meaning no limit; the default is `#f`.

      (parameterize ((param:printer-string-length-limit 4))
        (write-to-string "abcd"))
                                      ⇒ "\"abcd\""
      (parameterize ((param:printer-string-length-limit 4))
        (write-to-string "abcde"))
                                      ⇒ "\"abcd...\""

parameter: param:print-with-maximum-readability? ¶

  This parameter, which takes a boolean value, tells the printer to use a special printed representation for objects that normally print in a form that cannot be recognized by `read`. These objects are printed using the representation `#@n`, where n is the result of calling `hash` on the object to be printed. The reader recognizes this syntax, calling `unhash` on n to get back the original object. Note that this printed representation can only be recognized by the Scheme program in which it was generated, because these hash numbers are different for each invocation of Scheme.

obsolete variable: *unparser-radix* ¶
obsolete variable: *unparser-list-breadth-limit* ¶
obsolete variable: *unparser-list-depth-limit* ¶
obsolete variable: *unparser-string-length-limit* ¶
obsolete variable: *unparse-with-maximum-readability?* ¶

  These variables are deprecated; instead use the corresponding parameter objects.

### 14.7 Blocking Mode

An interactive port is always in one of two modes: blocking or non-blocking. This mode is independent of the terminal mode: each can be changed independently of the other. Furthermore, if it is an interactive I/O port, there are separate blocking modes for input and for output.

If an input port is in blocking mode, attempting to read from it when no input is available will cause Scheme to “block”, i.e. suspend itself, until input is available. If an input port is in non-blocking mode, attempting to read from it when no input is available will cause the reading procedure to return immediately, indicating the lack of input in some way (exactly how this situation is indicated is separately specified for each procedure or operation).

An output port in blocking mode will block if the output device is not ready to accept output. In non-blocking mode it will return immediately after performing as much output as the device will allow (again, each procedure or operation reports this situation in its own way).

Interactive ports are initially in blocking mode; this can be changed at any time with the procedures defined in this section.

These procedures represent blocking mode by the symbol `blocking`, and non-blocking mode by the symbol `nonblocking`. An argument called mode must be one of these symbols. A port argument to any of these procedures may be any port, even if that port does not support blocking mode; in that case, the port is not modified in any way.

procedure: input-port-blocking-mode input-port ¶
procedure: output-port-blocking-mode output-port ¶

  Returns the blocking mode of input-port or output-port. Returns `#f` if the given port doesn’t support blocking mode.

procedure: set-input-port-blocking-mode! input-port mode ¶
procedure: set-output-port-blocking-mode output-port mode ¶

  Changes the blocking mode of input-port or output-port to be mode and returns an unspecified value.

procedure: with-input-port-blocking-mode input-port mode thunk ¶
procedure: with-output-port-blocking-mode output-port mode thunk ¶

  Thunk must be a procedure of no arguments.

  Binds the blocking mode of input-port or output-port to be mode, and calls thunk. When thunk returns, the original blocking mode is restored and the values yielded by thunk are returned.

obsolete procedure: port/input-blocking-mode input-port ¶
obsolete procedure: port/set-input-blocking-mode input-port mode ¶
obsolete procedure: port/with-input-blocking-mode input-port mode thunk ¶
obsolete procedure: port/output-blocking-mode output-port ¶
obsolete procedure: port/set-output-blocking-mode output-port mode ¶
obsolete procedure: port/with-output-blocking-mode output-port mode thunk ¶

  These procedures are deprecated; instead use the corresponding procedures above.

### 14.8 Terminal Mode

A port that reads from or writes to a terminal has a terminal mode; this is either cooked or raw. This mode is independent of the blocking mode: each can be changed independently of the other. Furthermore, a terminal I/O port has independent terminal modes both for input and for output.

A terminal port in cooked mode provides some standard processing to make the terminal easy to communicate with. For example, under unix, cooked mode on input reads from the terminal a line at a time and provides editing within the line, while cooked mode on output might translate linefeeds to carriage-return/linefeed pairs. In general, the precise meaning of cooked mode is operating-system dependent, and furthermore might be customizable by means of operating-system utilities. The basic idea is that cooked mode does whatever is necessary to make the terminal handle all of the usual user-interface conventions for the operating system, while keeping the program’s interaction with the port as normal as possible.

A terminal port in raw mode disables all of that processing. In raw mode, characters are directly read from and written to the device without any translation or interpretation by the operating system. On input, characters are available as soon as they are typed, and are not echoed on the terminal by the operating system. In general, programs that put ports in raw mode have to know the details of interacting with the terminal. In particular, raw mode is used for writing programs such as text editors.

Terminal ports are initially in cooked mode; this can be changed at any time with the procedures defined in this section.

These procedures represent cooked mode by the symbol `cooked`, and raw mode by the symbol `raw`. An argument called mode must be one of these symbols. A port argument to any of these procedures may be any port, even if that port does not support terminal mode; in that case, the port is not modified in any way.

procedure: input-port-terminal-mode input-port ¶
procedure: output-port-terminal-mode output-port ¶

  Returns the terminal mode of input-port or output-port. Returns `#f` if the given port is not a terminal port.

procedure: set-input-port-terminal-mode! input-port mode ¶
procedure: set-output-port-terminal-mode! output-port mode ¶

  Changes the terminal mode of input-port or output-port to be mode and returns an unspecified value.

procedure: with-input-port-terminal-mode input-port mode thunk ¶
procedure: with-output-port-terminal-mode output-port mode thunk ¶

  Thunk must be a procedure of no arguments.

  Binds the terminal mode of input-port or output-port to be mode, and calls thunk. When thunk returns, the original terminal mode is restored and the values yielded by thunk are returned.

obsolete procedure: port/input-terminal-mode input-port ¶
obsolete procedure: port/set-input-terminal-mode input-port mode ¶
obsolete procedure: port/with-input-terminal-mode input-port mode thunk ¶
obsolete procedure: port/output-terminal-mode output-port ¶
obsolete procedure: port/set-output-terminal-mode output-port mode ¶
obsolete procedure: port/with-output-terminal-mode output-port mode thunk ¶

  These procedures are deprecated; instead use the corresponding procedures above.

### 14.9 Format

The procedure `format` is very useful for producing nicely formatted text, producing good-looking messages, and so on. MIT/GNU Scheme’s implementation of `format` is similar to that of Common Lisp, except that Common Lisp defines many more directives.14

`format` is a run-time-loadable option. To use it, execute

    (load-option 'format)

once before calling it.

procedure: format destination control-string argument … ¶

  Writes the characters of control-string to destination, except that a tilde (`~`) introduces a format directive. The character after the tilde, possibly preceded by prefix parameters and modifiers, specifies what kind of formatting is desired. Most directives use one or more arguments to create their output; the typical directive puts the next argument into the output, formatted in some special way. It is an error if no argument remains for a directive requiring an argument, but it is not an error if one or more arguments remain unprocessed by a directive.

  The output is sent to destination. If destination is `#f`, a string is created that contains the output; this string is returned as the value of the call to `format`. In all other cases `format` returns an unspecified value. If destination is `#t`, the output is sent to the current output port. Otherwise, destination must be an output port, and the output is sent there.

  This procedure performs discretionary output flushing (see Output Procedures).

  A `format` directive consists of a tilde (`~`), optional prefix parameters separated by commas, optional colon (`:`) and at-sign (`@`) modifiers, and a single character indicating what kind of directive this is. The alphabetic case of the directive character is ignored. The prefix parameters are generally integers, notated as optionally signed decimal numbers. If both the colon and at-sign modifiers are given, they may appear in either order.

  In place of a prefix parameter to a directive, you can put the letter ‘`V`’ (or ‘`v`’), which takes an argument for use as a parameter to the directive. Normally this should be an exact integer. This feature allows variable-width fields and the like. You can also use the character ‘`#`’ in place of a parameter; it represents the number of arguments remaining to be processed.

  It is an error to give a format directive more parameters than it is described here as accepting. It is also an error to give colon or at-sign modifiers to a directive in a combination not specifically described here as being meaningful.

  `~A`

    The next argument, which may be any object, is printed as if by `display`. `~mincolA` inserts spaces on the right, if necessary, to make the width at least mincol columns. The `@` modifier causes the spaces to be inserted on the left rather than the right.

  `~S`

    The next argument, which may be any object, is printed as if by `write`. `~mincolS` inserts spaces on the right, if necessary, to make the width at least mincol columns. The `@` modifier causes the spaces to be inserted on the left rather than the right.

  `~%`

    This outputs a `#\newline` character. `~n%` outputs n newlines. No argument is used. Simply putting a newline in control-string would work, but `~%` is often used because it makes the control string look nicer in the middle of a program.

  `~~`

    This outputs a tilde. `~n~` outputs n tildes.

  `~newline`

    Tilde immediately followed by a newline ignores the newline and any following non-newline whitespace characters. With an `@`, the newline is left in place, but any following whitespace is ignored. This directive is typically used when control-string is too long to fit nicely into one line of the program:

        (define (type-clash-error procedure arg spec actual)
          (format
           #t
           "~%Procedure ~S~%requires its %A argument ~
            to be of type ~S,~%but it was called with ~
            an argument of type ~S.~%"
           procedure arg spec actual))

        (type-clash-error 'vector-ref
                          "first"
                          'integer
                          'vector)

        prints

        Procedure vector-ref
        requires its first argument to be of type integer,
        but it was called with an argument of type vector.

    Note that in this example newlines appear in the output only as specified by the `~%` directives; the actual newline characters in the control string are suppressed because each is preceded by a tilde.

### 14.10 Custom Output

MIT/GNU Scheme provides hooks for specifying that specified objects have special written representations. There are no restrictions on the written representations.

procedure: define-print-method predicate print-method ¶

  Defines the print method for objects satisfying predicate to be print-method. The predicate argument must be a unary procedure that returns true for the objects to print specially, and print-method must be a binary procedure that accepts one of those objects and a textual output port.

  Although print-method can print the object in any way, we strongly recomment using one of the following special printers.

procedure: standard-print-method name [get-parts] ¶

  The name argument may be a unary procedure, a string, or a symbol; if it is a procedure it is called with the object to be printed as its argument and should return a string or a symbol. The get-parts argument, if provided, must be a unary procedure that is called with the object to be printed and must return a list of objects. If get-parts is not provided, it defaults to a procedure that returns an empty list.

  The output generated by this method is in a standard format:

      #[<name> <hash> <part>…]

  where `<name>` is the string or symbol from name as printed by `display`, `<hash>` is a unique nonnegative integer generated by calling `hash-object` on the object, and the `<part>`s are the result of calling get-parts as printed by `write` and separated by spaces.

  One significant advantage of print methods generated by `standard-print-method` is that the parts returned by get-parts are examined when searching for circular structure (as by `write`) or shared structure (as by `write-shared`). In effect the printer sees one of these objects as a compound object containing those parts.

procedure: bracketed-print-method name printer ¶

  The name argument may be a unary procedure, a string, or a symbol; if it is a procedure it is called with the object to be printed as its argument and should return a string or a symbol. The printer argument must be a binary procedure, which is called with the object to print and a textual output port as its arguments.

  Similar to `standard-print-method`, this procedure prints an object

      #[<name> <hash><output>]

  where `<name>` is the string or symbol from name as printed by `display`, `<hash>` is a unique nonnegative integer generated by calling `hash-object` on the object, and `<output>` is the text written by printer.

  This procedure has the benefit of printing objects using the standard bracketed form, but because its output is unstructured can not be examined for sharing or circularity. Generally speaking it’s preferable to use `standard-print-method` instead.

The following are deprecated procedures that have been replaced by the above.

obsolete procedure: set-record-type-unparser-method! record-type unparser-method ¶

  This procedure is deprecated; instead use

      (define-print-method (record-predicate record-type)
        unparser-method)

  provided that unparser-method is really a print method.

obsolete procedure: unparser/set-tagged-vector-method! tag unparser-method ¶
obsolete procedure: unparser/set-tagged-pair-method! tag unparser-method ¶

  These procedures arg deprecated. There is no direct replacement for them.

  These were primarily used by `define-structure`, which now generates new-style print methods. If you have other uses of these, it should be possible to translate them to use `define-print-method` with hand-written predicates.

obsolete procedure: standard-unparser-method name procedure ¶

  This procedure is deprecated; it is currently an alias for `bracketed-print-method`.

obsolete procedure: with-current-unparser-state unparser-state procedure ¶

  This procedure is deprecated, with no direct replacement. In general just use procedure without wrapping it.

### 14.11 Prompting

This section describes procedures that prompt the user for input. Why should the programmer use these procedures when it is possible to do prompting using ordinary input and output procedures? One reason is that the prompting procedures are more succinct. However, a second and better reason is that the prompting procedures can be separately customized for each user interface, providing more natural interaction. The interfaces for Edwin and for GNU Emacs have already been customized in this fashion; because Edwin and Emacs are very similar editors, their customizations provide very similar behavior.

Each of these procedure accepts an optional argument called port, which if given must be an I/O port. If not given, this port defaults to the value of `(interaction-i/o-port)`; this is initially the console I/O port.

procedure: prompt-for-command-expression prompt [port] ¶

  Prompts the user for an expression that is to be executed as a command. This is the procedure called by the REP loop to read the user’s expressions.

  If prompt is a string, it is used verbatim as the prompt string. Otherwise, it must be a pair whose car is the symbol ‘`standard`’ and whose cdr is a string; in this case the prompt string is formed by prepending to the string the current REP loop “level number” and a space. Also, a space is appended to the string, unless it already ends in a space or is an empty string.

  The default behavior of this procedure is to print a fresh line, a newline, and the prompt string; flush the output buffer; then read an object and return it. v Under Edwin and Emacs, before the object is read, the interaction buffer is put into a mode that allows expressions to be edited and submitted for input using specific editor commands. The first expression that is submitted is returned as the value of this procedure.

procedure: prompt-for-command-char prompt [port] ¶

  Prompts the user for a single character that is to be executed as a command; the returned character is guaranteed to satisfy `char-graphic?`. If at all possible, the character is read from the user interface using a mode that reads the character as a single keystroke; in other words, it should not be necessary for the user to follow the character with a carriage return or something similar.

  This is the procedure called by `debug` and `where` to read the user’s commands.

  If prompt is a string, it is used verbatim as the prompt string. Otherwise, it must be a pair whose car is `standard` and whose cdr is a string; in this case the prompt string is formed by prepending to the string the current REP loop “level number” and a space. Also, a space is appended to the string, unless it already ends in a space or is an empty string.

  The default behavior of this procedure is to print a fresh line, a newline, and the prompt string; flush the output buffer; read a character in raw mode, echo that character, and return it.

  Under Edwin and Emacs, instead of reading a character, the interaction buffer is put into a mode in which graphic characters submit themselves as input. After this mode change, the first such character submitted is returned as the value of this procedure.

procedure: prompt-for-expression prompt [port] ¶

  Prompts the user for an expression.

  The prompt string is formed by appending a colon and a space to prompt, unless prompt already ends in a space or is the null string.

  The default behavior of this procedure is to print a fresh line, a newline, and the prompt string; flush the output buffer; then read an object and return it.

  Under Edwin and Emacs, the expression is read in the minibuffer.

procedure: prompt-for-evaluated-expression prompt [environment [port]] ¶

  Prompts the user for an evaluated expression. Calls `prompt-for-expression` to read an expression, then evaluates the expression using environment; if environment is not given, the REP loop environment is used.

procedure: prompt-for-confirmation prompt [port] ¶

  Prompts the user for confirmation. The result yielded by this procedure is a boolean.

  The prompt string is formed by appending the string `" (y or n)? "` to prompt, unless prompt already ends in a space or is the null string.

  The default behavior of this procedure is to print a fresh line, a newline, and the prompt string; flush the output buffer; then read a character in raw mode. If the character is `#\y`, `#\Y`, or `#\space`, the procedure returns `#t`; If the character is `#\n`, `#\N`, or `#\rubout`, the procedure returns `#f`. Otherwise the prompt is repeated.

  Under Edwin or Emacs, the confirmation is read in the minibuffer.

### 14.12 Textual Port Primitives

This section describes the low-level operations that can be used to build and manipulate textual I/O ports. The purpose of these operations is to allow programmers to construct new kinds of textual I/O ports.

The mechanisms described in this section are exclusively for textual ports; binary ports can’t be customized. In this section, any reference to a “port” that isn’t modified by “textual” or “binary” is assumed to be a textual port.

The abstract model of a textual I/O port, as implemented here, is a combination of a set of named operations and a state. The state is an arbitrary object, the meaning of which is determined by the operations. The operations are defined by a mapping from names to procedures.

The set of named operations is represented by an object called a textual port type. A port type is constructed from a set of named operations, and is subsequently used to construct a port. The port type completely specifies the behavior of the port. Port types also support a simple form of inheritance, allowing you to create new ports that are similar to existing ports.

The port operations are divided into two classes:

Standard operations

  There is a specific set of standard operations for input ports, and a different set for output ports. Applications can assume that the standard input operations are implemented for all input ports, and likewise the standard output operations are implemented for all output ports.

Custom operations

  Some ports support additional operations. For example, ports that implement output to terminals (or windows) may define an operation named `y-size` that returns the height of the terminal in characters. Because only some ports will implement these operations, programs that use custom operations must test each port for their existence, and be prepared to deal with ports that do not implement them.

#### 14.12.1 Textual Port Types

The procedures in this section provide means for constructing port types with standard and custom operations, and accessing their operations.

procedure: make-textual-port-type operations port-type ¶

  Creates and returns a new port type. Operations must be a list; each element is a list of two elements, the name of the operation (a symbol) and the procedure that implements it. Port-type is either `#f` or a port type; if it is a port type, any operations implemented by port-type but not specified in operations will be implemented by the resulting port type.

  Operations need not contain definitions for all of the standard operations; the procedure will provide defaults for any standard operations that are not defined. At a minimum, the following operations must be defined: for input ports, `read-char` and `peek-char`; for output ports, either `write-char` or `write-substring`. I/O ports must supply the minimum operations for both input and output.

  If an operation in operations is defined to be `#f`, then the corresponding operation in port-type is not inherited.

  If `read-char` is defined in operations, then any standard input operations defined in port-type are ignored. Likewise, if `write-char` or `write-substring` is defined in operations, then any standard output operations defined in port-type are ignored. This feature allows overriding the standard operations without having to enumerate them.

procedure: textual-port-type? object ¶
procedure: textual-input-port-type? object ¶
procedure: textual-output-port-type? object ¶
procedure: textual-i/o-port-type? object ¶

  These predicates return `#t` if object is a port type, input-port type, output-port type, or I/O-port type, respectively. Otherwise, they return `#f`.

obsolete procedure: make-port-type operations port-type ¶
obsolete procedure: port-type? object ¶
obsolete procedure: input-port-type? object ¶
obsolete procedure: output-port-type? object ¶
obsolete procedure: i/o-port-type? object ¶

  These procedures are deprecated; use the procedures defined above.

obsolete procedure: port-type/operations port-type ¶
obsolete procedure: port-type/operation-names port-type ¶
obsolete procedure: port-type/operation port-type symbol ¶

  These procedures are deprecated and will be removed in the near future. There are no replacements planned.

#### 14.12.2 Constructors and Accessors for Textual Ports

The procedures in this section provide means for constructing ports, accessing the type of a port, and manipulating the state of a port.

procedure: make-textual-port port-type state ¶

  Returns a new port with type port-type and the given state. The port will be an input, output, or I/O port according to port-type.

procedure: textual-port-type textual-port ¶

  Returns the port type of textual-port.

procedure: textual-port-state textual-port ¶

  Returns the state component of textual-port.

procedure: set-textual-port-state! textual-port object ¶

  Changes the state component of textual-port to be object. Returns an unspecified value.

procedure: textual-port-operation textual-port symbol ¶

  Returns the operation named symbol for textual-port. If textual-port has no such operation, returns `#f`.

procedure: textual-port-operation-names textual-port ¶

  Returns a newly allocated list whose elements are the names of the operations implemented by textual-port.

obsolete procedure: make-port port-type state ¶
obsolete procedure: port/type textual-port ¶
obsolete procedure: port/state textual-port ¶
obsolete procedure: set-port/state! textual-port object ¶
obsolete procedure: port/operation textual-port symbol ¶
obsolete procedure: port/operation-names port ¶

  These procedures are deprecated; use the procedures defined above.

#### 14.12.3 Textual Input Port Operations

This section describes the standard operations on textual input ports. Following that, some useful custom operations are described.

operation on textual input port: read-char port ¶

  Removes the next character available from port and returns it. If port has no more characters and will never have any (e.g. at the end of an input file), this operation returns an end-of-file object. If port has no more characters but will eventually have some more (e.g. a terminal where nothing has been typed recently), and it is in non-blocking mode, `#f` is returned; otherwise the operation hangs until input is available.

operation on textual input port: peek-char port ¶

  Reads the next character available from port and returns it. The character is not removed from port, and a subsequent attempt to read from the port will get that character again. In other respects this operation behaves like `read-char`.

operation on textual input port: char-ready? port k ¶

  `char-ready?` returns `#t` if at least one character is available to be read from port. If no characters are available, the operation waits up to k milliseconds before returning `#f`, returning immediately if any characters become available while it is waiting.

operation on textual input port: read-string port char-set ¶
operation on textual input port: discard-chars port char-set ¶

  These operations are like `read-char`, except that they read or discard multiple characters at once. All characters up to, but excluding, the first character in char-set (or end of file) are read from port. `read-string` returns these characters as a newly allocated string, while `discard-chars` discards them and returns an unspecified value. These operations hang until sufficient input is available, even if port is in non-blocking mode. If end of file is encountered before any input characters, `read-string` returns an end-of-file object.

operation on textual input port: read-substring port string start end ¶

  Reads characters from port into the substring defined by string, start, and end until either the substring has been filled or there are no more characters available. Returns the number of characters written to the substring.

  If port is an interactive port, and at least one character is immediately available, the available characters are written to the substring and this operation returns immediately. If no characters are available, and port is in blocking mode, the operation blocks until at least one character is available. Otherwise, the operation returns `#f` immediately.

  This is an extremely fast way to read characters from a port.

procedure: input-port/read-char textual-input-port ¶
procedure: input-port/peek-char textual-input-port ¶
procedure: input-port/char-ready? textual-input-port k ¶
procedure: input-port/read-string textual-input-port char-set ¶
procedure: input-port/discard-chars textual-input-port char-set ¶
procedure: input-port/read-substring textual-input-port string start end ¶

  Each of these procedures invokes the respective operation on textual-input-port. For example, the following are equivalent:

      (input-port/read-char textual-input-port)
      ((textual-port-operation textual-input-port 'read-char)
       textual-input-port)

The following custom operations are implemented for input ports to files, and will also work with some other kinds of input ports:

operation on textual input port: eof? port ¶

  Returns `#t` if port is known to be at end of file, otherwise it returns `#f`.

operation on textual input port: chars-remaining port ¶

  Returns an estimate of the number of characters remaining to be read from port. This is useful only when port is a file port in binary mode; in other cases, it returns `#f`.

operation on textual input port: buffered-input-chars port ¶

  Returns the number of unread characters that are stored in port’s buffer. This will always be less than or equal to the buffer’s size.

operation on textual input port: input-buffer-size port ¶

  Returns the maximum number of characters that port’s buffer can hold.

operation on textual input port: set-input-buffer-size port size ¶

  Resizes port’s buffer so that it can hold at most size characters. Characters in the buffer are discarded. Size must be an exact non-negative integer.

#### 14.12.4 Textual Output Port Operations

This section describes the standard operations on output ports. Following that, some useful custom operations are described.

operation on textual output port: write-char port char ¶

  Writes char to port and returns an unspecified value.

operation on textual output port: write-substring port string start end ¶

  Writes the substring specified by string, start, and end to port and returns an unspecified value. Equivalent to writing the characters of the substring, one by one, to port, but is implemented very efficiently.

operation on textual output port: fresh-line port ¶

  Most output ports are able to tell whether or not they are at the beginning of a line of output. If port is such a port, end-of-line is written to the port only if the port is not already at the beginning of a line. If port is not such a port, an end-of-line is unconditionally written to the port. Returns an unspecified value.

operation on textual output port: flush-output port ¶

  If port is buffered, this causes its buffer to be written out. Otherwise it has no effect. Returns an unspecified value.

operation on textual output port: discretionary-flush-output port ¶

  Normally, this operation does nothing. However, ports that support discretionary output flushing implement this operation identically to `flush-output`.

procedure: output-port/write-char textual-output-port char ¶
procedure: output-port/write-substring textual-output-port string start end ¶
procedure: output-port/fresh-line textual-output-port ¶
procedure: output-port/flush-output textual-output-port ¶
procedure: output-port/discretionary-flush-output textual-output-port ¶

  Each of these procedures invokes the respective operation on textual-output-port. For example, the following are equivalent:

      (output-port/write-char textual-output-port char)
      ((textual-port-operation textual-output-port 'write-char)
       textual-output-port char)

procedure: output-port/write-string textual-output-port string ¶

  Writes string to textual-output-port. Equivalent to

      (output-port/write-substring textual-output-port
                                   string
                                   0
                                   (string-length string))

The following custom operations are generally useful.

operation on textual output port: buffered-output-chars port ¶

  Returns the number of unwritten characters that are stored in port’s buffer. This will always be less than or equal to the buffer’s size.

operation on textual output port: output-buffer-size port ¶

  Returns the maximum number of characters that port’s buffer can hold.

operation on textual output port: set-output-buffer-size port size ¶

  Resizes port’s buffer so that it can hold at most size characters. Characters in the buffer are discarded. Size must be an exact non-negative integer.

operation on textual output port: x-size port ¶

  Returns an exact positive integer that is the width of port in characters. If port has no natural width, e.g. if it is a file port, `#f` is returned.

operation on textual output port: y-size port ¶

  Returns an exact positive integer that is the height of port in characters. If port has no natural height, e.g. if it is a file port, `#f` is returned.

procedure: output-port/x-size textual-output-port ¶

  This procedure invokes the custom operation whose name is the symbol `x-size`, if it exists. If the `x-size` operation is both defined and returns a value other than `#f`, that value is returned as the result of this procedure. Otherwise, `output-port/x-size` returns a default value (currently `80`).

  `output-port/x-size` is useful for programs that tailor their output to the width of the display (a fairly common practice). If the output device is not a display, such programs normally want some reasonable default width to work with, and this procedure provides exactly that.

procedure: output-port/y-size textual-output-port ¶

  This procedure invokes the custom operation whose name is the symbol `y-size`, if it exists. If the `y-size` operation is defined, the value it returns is returned as the result of this procedure; otherwise, `#f` is returned.

### 14.13 Parser Buffers

The parser buffer mechanism facilitates construction of parsers for complex grammars. It does this by providing an input stream with unbounded buffering and backtracking. The amount of buffering is under program control. The stream can backtrack to any position in the buffer.

The mechanism defines two data types: the parser buffer and the parser-buffer pointer. A parser buffer is like an input port with buffering and backtracking. A parser-buffer pointer is a pointer into the stream of characters provided by a parser buffer.

Note that all of the procedures defined here consider a parser buffer to contain a stream of Unicode characters.

There are several constructors for parser buffers:

procedure: textual-input-port->parser-buffer textual-input-port ¶
obsolete procedure: input-port->parser-buffer textual-input-port ¶

  Returns a parser buffer that buffers characters read from textual-input-port.

procedure: substring->parser-buffer string start end ¶

  Returns a parser buffer that buffers the characters in the argument substring. This is equivalent to creating a string input port and calling `textual-input-port->parser-buffer`, but it runs faster and uses less memory.

procedure: string->parser-buffer string ¶

  Like `substring->parser-buffer` but buffers the entire string.

procedure: source->parser-buffer source ¶

  Returns a parser buffer that buffers the characters returned by calling source. Source is a procedure of three arguments: a string, a start index, and an end index (in other words, a substring specifier). Each time source is called, it writes some characters in the substring, and returns the number of characters written. When there are no more characters available, it returns zero. It must not return zero in any other circumstance.

Parser buffers and parser-buffer pointers may be distinguished from other objects:

procedure: parser-buffer? object ¶

  Returns `#t` if object is a parser buffer, otherwise returns `#f`.

procedure: parser-buffer-pointer? object ¶

  Returns `#t` if object is a parser-buffer pointer, otherwise returns `#f`.

Characters can be read from a parser buffer much as they can be read from an input port. The parser buffer maintains an internal pointer indicating its current position in the input stream. Additionally, the buffer remembers all characters that were previously read, and can look at characters arbitrarily far ahead in the stream. It is this buffering capability that facilitates complex matching and backtracking.

procedure: read-parser-buffer-char buffer ¶

  Returns the next character in buffer, advancing the internal pointer past that character. If there are no more characters available, returns `#f` and leaves the internal pointer unchanged.

procedure: peek-parser-buffer-char buffer ¶

  Returns the next character in buffer, or `#f` if no characters are available. Leaves the internal pointer unchanged.

procedure: parser-buffer-ref buffer index ¶

  Returns a character in buffer. Index is a non-negative integer specifying the character to be returned. If index is zero, returns the next available character; if it is one, returns the character after that, and so on. If index specifies a position after the last character in buffer, returns `#f`. Leaves the internal pointer unchanged.

The internal pointer of a parser buffer can be read or written:

procedure: get-parser-buffer-pointer buffer ¶

  Returns a parser-buffer pointer object corresponding to the internal pointer of buffer.

procedure: set-parser-buffer-pointer! buffer pointer ¶

  Sets the internal pointer of buffer to the position specified by pointer. Pointer must have been returned from a previous call of `get-parser-buffer-pointer` on buffer. Additionally, if some of buffer’s characters have been discarded by `discard-parser-buffer-head!`, pointer must be outside the range that was discarded.

procedure: get-parser-buffer-tail buffer pointer ¶

  Returns a newly-allocated string consisting of all of the characters in buffer that fall between pointer and buffer’s internal pointer. Pointer must have been returned from a previous call of `get-parser-buffer-pointer` on buffer. Additionally, if some of buffer’s characters have been discarded by `discard-parser-buffer-head!`, pointer must be outside the range that was discarded.

procedure: discard-parser-buffer-head! buffer ¶

  Discards all characters in buffer that have already been read; in other words, all characters prior to the internal pointer. After this operation has completed, it is no longer possible to move the internal pointer backwards past the current position by calling `set-parser-buffer-pointer!`.

The next rather large set of procedures does conditional matching against the contents of a parser buffer. All matching is performed relative to the buffer’s internal pointer, so the first character to be matched against is the next character that would be returned by `peek-parser-buffer-char`. The returned value is always `#t` for a successful match, and `#f` otherwise. For procedures whose names do not end in ‘`-no-advance`’, a successful match also moves the internal pointer of the buffer forward to the end of the matched text; otherwise the internal pointer is unchanged.

procedure: match-parser-buffer-char buffer char ¶
procedure: match-parser-buffer-char-ci buffer char ¶
procedure: match-parser-buffer-not-char buffer char ¶
procedure: match-parser-buffer-not-char-ci buffer char ¶
procedure: match-parser-buffer-char-no-advance buffer char ¶
procedure: match-parser-buffer-char-ci-no-advance buffer char ¶
procedure: match-parser-buffer-not-char-no-advance buffer char ¶
procedure: match-parser-buffer-not-char-ci-no-advance buffer char ¶

  Each of these procedures compares a single character in buffer to char. The basic comparison `match-parser-buffer-char` compares the character to char using `char=?`. The procedures whose names contain the ‘`-ci`’ modifier do case-insensitive comparison (i.e. they use `char-ci=?`). The procedures whose names contain the ‘`not-`’ modifier are successful if the character doesn’t match char.

procedure: match-parser-buffer-char-in-set buffer char-set ¶
procedure: match-parser-buffer-char-in-set-no-advance buffer char-set ¶

  These procedures compare the next character in buffer against char-set using `char-in-set?`.

procedure: match-parser-buffer-string buffer string ¶
procedure: match-parser-buffer-string-ci buffer string ¶
procedure: match-parser-buffer-string-no-advance buffer string ¶
procedure: match-parser-buffer-string-ci-no-advance buffer string ¶

  These procedures match string against buffer’s contents. The ‘`-ci`’ procedures do case-insensitive matching.

procedure: match-parser-buffer-substring buffer string start end ¶
procedure: match-parser-buffer-substring-ci buffer string start end ¶
procedure: match-parser-buffer-substring-no-advance buffer string start end ¶
procedure: match-parser-buffer-substring-ci-no-advance buffer string start end ¶

  These procedures match the specified substring against buffer’s contents. The ‘`-ci`’ procedures do case-insensitive matching.

The remaining procedures provide information that can be used to identify locations in a parser buffer’s stream.

procedure: parser-buffer-position-string pointer ¶

  Returns a string describing the location of pointer in terms of its character and line indexes. This resulting string is meant to be presented to an end user in order to direct their attention to a feature in the input stream. In this string, the indexes are presented as one-based numbers.

  Pointer may alternatively be a parser buffer, in which case it is equivalent to having specified the buffer’s internal pointer.

procedure: parser-buffer-pointer-index pointer ¶
procedure: parser-buffer-pointer-line pointer ¶

  Returns the character or line index, respectively, of pointer. Both indexes are zero-based.

### 14.14 Parser Language

Although it is possible to write parsers using the parser-buffer abstraction (see Parser Buffers), it is tedious. The problem is that the abstraction isn’t closely matched to the way that people think about syntactic structures. In this section, we introduce a higher-level mechanism that greatly simplifies the implementation of a parser.

The parser language described here allows the programmer to write BNF-like specifications that are translated into efficient Scheme code at compile time. The language is declarative, but it can be freely mixed with Scheme code; this allows the parsing of grammars that aren’t conveniently described in the language.

The language also provides backtracking. For example, this expression matches any sequence of alphanumeric characters followed by a single alphabetic character:

    (*matcher
     (seq (* (char-set char-set:alphanumeric))
          (char-set char-set:alphabetic)))

The way that this works is that the matcher matches alphanumeric characters in the input stream until it finds a non-alphanumeric character. It then tries to match an alphabetic character, which of course fails. At this point, if it matched at least one alphanumeric character, it backtracks: the last matched alphanumeric is “unmatched”, and it again attempts to match an alphabetic character. The backtracking can be arbitrarily deep; the matcher will continue to back up until it finds a way to match the remainder of the expression.

So far, this sounds a lot like regular-expression matching (see Regular Expressions). However, there are some important differences.

- The parser language uses a Scheme-like syntax that is easier to read and write than regular-expression notation.
- The language provides macros so that common syntactic constructs can be abstracted.
- The language mixes easily with Scheme code, allowing the full power of Scheme to be applied to program around limitations in the parser language.
- The language provides expressive facilities for converting syntax into parsed structure. It also makes it easy to convert parsed strings into meaningful objects (e.g. numbers).
- The language is compiled into machine language; regular expressions are usually interpreted.

Here is an example that shows off several of the features of the parser language. The example is a parser for XML start tags:

    (*parser
     (with-pointer p
       (seq "<"
            parse-name
            parse-attribute-list
            (alt (match ">")
                 (match "/>")
                 (sexp
                  (lambda (b)
                    (error
                     (string-append
                      "Unterminated start tag at "
                      (parser-buffer-position-string p)))))))))

This shows that the basic description of a start tag is very similar to its BNF. Non-terminal symbols `parse-name` and `parse-attribute-list` do most of the work, and the noise strings `"<"` and `">"` are the syntactic markers delimiting the form. There are two alternate endings for start tags, and if the parser doesn’t find either of the endings, the Scheme code (wrapped in `sexp`) is run to signal an error. The error procedure `perror` takes a pointer `p`, which it uses to indicate the position in the input stream at which the error occurred. In this case, that is the beginning of the start tag, i.e. the position of the leading `"<"` marker.

This example still looks pretty complicated, mostly due to the error-signalling code. In practice, this is abstracted into a macro, after which the expression is quite succinct:

    (*parser
     (bracket "start tag"
         (seq (noise (string "<")) parse-name)
         (match (alt (string ">") (string "/>")))
       parse-attribute-list))

The `bracket` macro captures the pattern of a bracketed item, and hides much of the detail.

The parser language actually consists of two languages: one for defining matchers, and one for defining parsers. The languages are intentionally very similar, and are meant to be used together. Each sub-language is described below in its own section.

The parser language is a run-time-loadable option; to use it, execute

    (load-option '*parser)

once before compiling any code that uses the language.

#### 14.14.1 *Matcher

The matcher language is a declarative language for specifying a matcher procedure. A matcher procedure is a procedure that accepts a single parser-buffer argument and returns a boolean value indicating whether the match it performs was successful. If the match succeeds, the internal pointer of the parser buffer is moved forward over the matched text. If the match fails, the internal pointer is unchanged.

For example, here is a matcher procedure that matches the character ‘`a`’:

    (lambda (b) (match-parser-buffer-char b #\a))

Here is another example that matches two given characters, c1 and c2, in sequence:

    (lambda (b)
      (let ((p (get-parser-buffer-pointer b)))
        (if (match-parser-buffer-char b c1)
            (if (match-parser-buffer-char b c2)
                #t
                (begin
                  (set-parser-buffer-pointer! b p)
                  #f))
            #f)))

This is code is clear, but has lots of details that get in the way of understanding what it is doing. Here is the same example in the matcher language:

    (*matcher (seq (char c1) (char c2)))

This is much simpler and more intuitive. And it generates virtually the same code:

    (pp (*matcher (seq (char c1) (char c2))))
    -| (lambda (#[b1])
    -|   (let ((#[p1] (get-parser-buffer-pointer #[b1])))
    -|     (and (match-parser-buffer-char #[b1] c1)
    -|          (if (match-parser-buffer-char #[b1] c2)
    -|              #t
    -|              (begin
    -|                (set-parser-buffer-pointer! #[b1] #[p1])
    -|                #f)))))

Now that we have seen an example of the language, it’s time to look at the detail. The `*matcher` special form is the interface between the matcher language and Scheme.

special form: *matcher mexp ¶

  The operand mexp is an expression in the matcher language. The `*matcher` expression expands into Scheme code that implements a matcher procedure.

Here are the predefined matcher expressions. New matcher expressions can be defined using the macro facility (see Parser-language Macros). We will start with the primitive expressions.

matcher expression: char expression ¶
matcher expression: char-ci expression ¶
matcher expression: not-char expression ¶
matcher expression: not-char-ci expression ¶

  These expressions match a given character. In each case, the expression operand is a Scheme expression that must evaluate to a character at run time. The ‘`-ci`’ expressions do case-insensitive matching. The ‘`not-`’ expressions match any character other than the given one.

matcher expression: string expression ¶
matcher expression: string-ci expression ¶

  These expressions match a given string. The expression operand is a Scheme expression that must evaluate to a string at run time. The `string-ci` expression does case-insensitive matching.

matcher expression: char-set expression ¶

  These expressions match a single character that is a member of a given character set. The expression operand is a Scheme expression that must evaluate to a character set at run time.

matcher expression: end-of-input ¶

  The `end-of-input` expression is successful only when there are no more characters available to be matched.

matcher expression: discard-matched ¶

  The `discard-matched` expression always successfully matches the null string. However, it isn’t meant to be used as a matching expression; it is used for its effect. `discard-matched` causes all of the buffered text prior to this point to be discarded (i.e. it calls `discard-parser-buffer-head!` on the parser buffer).

  Note that `discard-matched` may not be used in certain places in a matcher expression. The reason for this is that it deliberately discards information needed for backtracking, so it may not be used in a place where subsequent backtracking will need to back over it. As a rule of thumb, use `discard-matched` only in the last operand of a `seq` or `alt` expression (including any `seq` or `alt` expressions in which it is indirectly contained).

In addition to the above primitive expressions, there are two convenient abbreviations. A character literal (e.g. ‘`#\A`’) is a legal primitive expression, and is equivalent to a `char` expression with that literal as its operand (e.g. ‘`(char #\A)`’). Likewise, a string literal is equivalent to a `string` expression (e.g. ‘`(string "abc")`’).

Next there are several combinator expressions. These closely correspond to similar combinators in regular expressions. Parameters named mexp are arbitrary expressions in the matcher language.

matcher expression: seq mexp … ¶

  This matches each mexp operand in sequence. For example,

      (seq (char-set char-set:alphabetic)
           (char-set char-set:numeric))

  matches an alphabetic character followed by a numeric character, such as ‘`H4`’.

  Note that if there are no mexp operands, the `seq` expression successfully matches the null string.

matcher expression: alt mexp … ¶

  This attempts to match each mexp operand in order from left to right. The first one that successfully matches becomes the match for the entire `alt` expression.

  The `alt` expression participates in backtracking. If one of the mexp operands matches, but the overall match in which this expression is embedded fails, the backtracking mechanism will cause the `alt` expression to try the remaining mexp operands. For example, if the expression

      (seq (alt "ab" "a") "b")

  is matched against the text ‘`abc`’, the `alt` expression will initially match its first operand. But it will then fail to match the second operand of the `seq` expression. This will cause the `alt` to be restarted, at which time it will match ‘`a`’, and the overall match will succeed.

  Note that if there are no mexp operands, the `alt` match will always fail.

matcher expression: * mexp ¶

  This matches zero or more occurrences of the mexp operand. (Consequently this match always succeeds.)

  The `*` expression participates in backtracking; if it matches N occurrences of mexp, but the overall match fails, it will backtrack to N-1 occurrences and continue. If the overall match continues to fail, the `*` expression will continue to backtrack until there are no occurrences left.

matcher expression: + mexp ¶

  This matches one or more occurrences of the mexp operand. It is equivalent to

      (seq mexp (* mexp))

matcher expression: ? mexp ¶

  This matches zero or one occurrences of the mexp operand. It is equivalent to

      (alt mexp (seq))

matcher expression: sexp expression ¶

  The `sexp` expression allows arbitrary Scheme code to be embedded inside a matcher. The expression operand must evaluate to a matcher procedure at run time; the procedure is called to match the parser buffer. For example,

      (*matcher
       (seq "a"
            (sexp parse-foo)
            "b"))

  expands to

      (lambda (#[b1])
        (let ((#[p1] (get-parser-buffer-pointer #[b1])))
          (and (match-parser-buffer-char #[b1] #\a)
               (if (parse-foo #[b1])
                   (if (match-parser-buffer-char #[b1] #\b)
                       #t
                       (begin
                         (set-parser-buffer-pointer! #[b1] #[p1])
                         #f))
                   (begin
                     (set-parser-buffer-pointer! #[b1] #[p1])
                     #f)))))

  The case in which expression is a symbol is so common that it has an abbreviation: ‘`(sexp symbol)`’ may be abbreviated as just symbol.

matcher expression: with-pointer identifier mexp ¶

  The `with-pointer` expression fetches the parser buffer’s internal pointer (using `get-parser-buffer-pointer`), binds it to identifier, and then matches the pattern specified by mexp. Identifier must be a symbol.

  This is meant to be used on conjunction with `sexp`, as a way to capture a pointer to a part of the input stream that is outside the `sexp` expression. An example of the use of `with-pointer` appears above (see with-pointer example).

#### 14.14.2 *Parser

The parser language is a declarative language for specifying a parser procedure. A parser procedure is a procedure that accepts a single parser-buffer argument and parses some of the input from the buffer. If the parse is successful, the procedure returns a vector of objects that are the result of the parse, and the internal pointer of the parser buffer is advanced past the input that was parsed. If the parse fails, the procedure returns `#f` and the internal pointer is unchanged. This interface is much like that of a matcher procedure, except that on success the parser procedure returns a vector of values rather than `#t`.

The `*parser` special form is the interface between the parser language and Scheme.

special form: *parser pexp ¶

  The operand pexp is an expression in the parser language. The `*parser` expression expands into Scheme code that implements a parser procedure.

There are several primitive expressions in the parser language. The first two provide a bridge to the matcher language (see *Matcher):

parser expression: match mexp ¶

  The `match` expression performs a match on the parser buffer. The match to be performed is specified by mexp, which is an expression in the matcher language. If the match is successful, the result of the `match` expression is a vector of one element: a string containing that text.

parser expression: noise mexp ¶

  The `noise` expression performs a match on the parser buffer. The match to be performed is specified by mexp, which is an expression in the matcher language. If the match is successful, the result of the `noise` expression is a vector of zero elements. (In other words, the text is matched and then thrown away.)

  The mexp operand is often a known character or string, so in the case that mexp is a character or string literal, the `noise` expression can be abbreviated as the literal. In other words, ‘`(noise "foo")`’ can be abbreviated just ‘`"foo"`’.

parser expression: values expression … ¶

  Sometimes it is useful to be able to insert arbitrary values into the parser result. The `values` expression supports this. The expression arguments are arbitrary Scheme expressions that are evaluated at run time and returned in a vector. The `values` expression always succeeds and never modifies the internal pointer of the parser buffer.

parser expression: discard-matched ¶

  The `discard-matched` expression always succeeds, returning a vector of zero elements. In all other respects it is identical to the `discard-matched` expression in the matcher language.

Next there are several combinator expressions. Parameters named pexp are arbitrary expressions in the parser language. The first few combinators are direct equivalents of those in the matcher language.

parser expression: seq pexp … ¶

  The `seq` expression parses each of the pexp operands in order. If all of the pexp operands successfully match, the result is the concatenation of their values (by `vector-append`).

parser expression: alt pexp … ¶

  The `alt` expression attempts to parse each pexp operand in order from left to right. The first one that successfully parses produces the result for the entire `alt` expression.

  Like the `alt` expression in the matcher language, this expression participates in backtracking.

parser expression: * pexp ¶

  The `*` expression parses zero or more occurrences of pexp. The results of the parsed occurrences are concatenated together (by `vector-append`) to produce the expression’s result.

  Like the `*` expression in the matcher language, this expression participates in backtracking.

parser expression: + pexp ¶

  The `*` expression parses one or more occurrences of pexp. It is equivalent to

      (seq pexp (* pexp))

parser expression: ? pexp ¶

  The `*` expression parses zero or one occurrences of pexp. It is equivalent to

      (alt pexp (seq))

The next three expressions do not have equivalents in the matcher language. Each accepts a single pexp argument, which is parsed in the usual way. These expressions perform transformations on the returned values of a successful match.

parser expression: transform expression pexp ¶

  The `transform` expression performs an arbitrary transformation of the values returned by parsing pexp. Expression is a Scheme expression that must evaluate to a procedure at run time. If pexp is successfully parsed, the procedure is called with the vector of values as its argument, and must return a vector or `#f`. If it returns a vector, the parse is successful, and those are the resulting values. If it returns `#f`, the parse fails and the internal pointer of the parser buffer is returned to what it was before pexp was parsed.

  For example:

      (transform (lambda (v) (if (= 0 (vector-length v)) #f v)) …)

parser expression: encapsulate expression pexp ¶

  The `encapsulate` expression transforms the values returned by parsing pexp into a single value. Expression is a Scheme expression that must evaluate to a procedure at run time. If pexp is successfully parsed, the procedure is called with the vector of values as its argument, and may return any Scheme object. The result of the `encapsulate` expression is a vector of length one containing that object. (And consequently `encapsulate` doesn’t change the success or failure of pexp, only its value.)

  For example:

      (encapsulate vector->list …)

parser expression: map expression pexp ¶

  The `map` expression performs a per-element transform on the values returned by parsing pexp. Expression is a Scheme expression that must evaluate to a procedure at run time. If pexp is successfully parsed, the procedure is mapped (by `vector-map`) over the values returned from the parse. The mapped values are returned as the result of the `map` expression. (And consequently `map` doesn’t change the success or failure of pexp, nor the number of values returned.)

  For example:

      (map string->symbol …)

Finally, as in the matcher language, we have `sexp` and `with-pointer` to support embedding Scheme code in the parser.

parser expression: sexp expression ¶

  The `sexp` expression allows arbitrary Scheme code to be embedded inside a parser. The expression operand must evaluate to a parser procedure at run time; the procedure is called to parse the parser buffer. This is the parser-language equivalent of the `sexp` expression in the matcher language.

  The case in which expression is a symbol is so common that it has an abbreviation: ‘`(sexp symbol)`’ may be abbreviated as just symbol.

parser expression: with-pointer identifier pexp ¶

  The `with-pointer` expression fetches the parser buffer’s internal pointer (using `get-parser-buffer-pointer`), binds it to identifier, and then parses the pattern specified by pexp. Identifier must be a symbol. This is the parser-language equivalent of the `with-pointer` expression in the matcher language.

#### 14.14.3 Parser-language Macros

The parser and matcher languages provide a macro facility so that common patterns can be abstracted. The macro facility allows new expression types to be independently defined in the two languages. The macros are defined in hierarchically organized tables, so that different applications can have private macro bindings.

special form: define-*matcher-macro formals expression ¶
special form: define-*parser-macro formals expression ¶

  These special forms are used to define macros in the matcher and parser language, respectively. Formals is like the formals list of a `define` special form, and expression is a Scheme expression.

  If formals is a list (or improper list) of symbols, the first symbol in the list is the name of the macro, and the remaining symbols are interpreted as the formals of a lambda expression. A lambda expression is formed by combining the latter formals with the expression, and this lambda expression, when evaluated, becomes the expander. The defined macro accepts the same number of operands as the expander. A macro instance is expanded by applying the expander to the list of operands; the result of the application is interpreted as a replacement expression for the macro instance.

  If formals is a symbol, it is the name of the macro. In this case, the expander is a procedure of no arguments whose body is expression. When the formals symbol appears by itself as an expression in the language, the expander is called with no arguments, and the result is interpreted as a replacement expression for the symbol.

procedure: define-*matcher-expander identifier expander ¶
procedure: define-*parser-expander identifier expander ¶

  These procedures provide a procedural interface to the macro-definition mechanism. Identifier must be a symbol, and expander must be an expander procedure, as defined above. Instances of the `define-*matcher-macro` and `define-*parser-macro` special forms expand into calls to these procedures.

The remaining procedures define the interface to the parser-macros table abstraction. Each parser-macro table has a separate binding space for macros in the matcher and parser languages. However, the table inherits bindings from one specified table; it’s not possible to inherit matcher-language bindings from one table and parser-language bindings from another.

procedure: make-parser-macros parent-table ¶

  Create and return a new parser-macro table that inherits from parent-table. Parent-table must be either a parser-macro table, or `#f`; usually it is specified as the value of `global-parser-macros`.

procedure: parser-macros? object ¶

  This is a predicate for parser-macro tables.

procedure: global-parser-macros ¶

  Return the global parser-macro table. This table is predefined and contains all of the bindings documented here.

There is a “current” table at all times, and macro definitions are always placed in this table. By default, the current table is the global macro table, but the following procedures allow this to be changed.

procedure: current-parser-macros ¶

  Return the current parser-macro table.

procedure: set-current-parser-macros! table ¶

  Change the current parser-macro table to table, which must satisfy `parser-macros?`.

procedure: with-current-parser-macros table thunk ¶

  Bind the current parser-macro table to table, call thunk with no arguments, then restore the original table binding. The value returned by thunk is the returned as the value of this procedure. Table must satisfy `parser-macros?`, and thunk must be a procedure of no arguments.

### 14.15 XML Support

MIT/GNU Scheme provides a simple non-validating XML parser. This parser is believed to be conformant with XML 1.0. It passes all of the tests in the "xmltest" directory of the XML conformance tests (dated 2001-03-15). The parser supports XML namespaces; it doesn’t support external document type declarations (DTDs), and it doesn’t yet support XML 1.1. The output of the parser is a record tree that closely reflects the structure of the XML document.

MIT/GNU Scheme also provides support for writing an XML record tree to an output port. There is no guarantee that parsing an XML document and writing it back out will make a verbatim copy of the document. The output will be semantically identical but may have small syntactic differences. For example, entities are substituted during the parsing process.

The purpose of the XML support is to provide a mechanism for reading and writing simple XML documents. In the future this support may be further developed to support a standard interface such as DOM or SAX.

The XML support is a run-time-loadable option; to use it, execute

    (load-option 'xml)

once before running any code that uses it.

#### 14.15.1 XML Input

The primary entry point for the XML parser is `read-xml`, which reads characters from a port and returns an XML document record. The character coding of the input is determined by reading some of the input stream and looking for a byte order mark and/or an encoding in the XML declaration. We support all ISO 8859 codings, as well as UTF-8, UTF-16, and UTF-32.

When an XHTML document is read, the parser provides entity definitions for all of the named XHTML characters; for example, it defines ‘`&nbsp;`’ and ‘`&copy;`’. In order for a document to be recognized as XHTML, it must contain an XHTML DTD, such as this:

    <!DOCTYPE html
              PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
              "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

At present the parser recognizes XHTML Strict 1.0 and XHTML 1.1 documents.

procedure: read-xml port [pi-handlers] ¶

  Read an XML document from port and return the corresponding XML document record.

  Pi-handlers, if specified, must be an association list. Each element of pi-handlers must be a list of two elements: a symbol and a procedure. When the parser encounters processing instructions with a name that appears in pi-handlers, the procedure is called with one argument, which is the text of the processing instructions. The procedure must return a list of XML structure records that are legal for the context of the processing instructions.

procedure: read-xml-file pathname [pi-handlers] ¶

  This convenience procedure simplifies reading XML from a file. It is roughly equivalent to

      (define (read-xml-file pathname #!optional pi-handlers)
        (call-with-input-file pathname
          (lambda (port)
            (read-xml port pi-handlers))))

procedure: string->xml string [start [end [pi-handlers]]] ¶

  This convenience procedure simplifies reading XML from a string. The string argument may be a string or a wide string. It is roughly equivalent to

      (define (string->xml string #!optional start end pi-handlers)
        (read-xml (open-input-string string start end)
                  pi-handlers))

#### 14.15.2 XML Output

The following procedures serialize XML document records into character sequences. All are virtually identical except for the way that the character sequence is represented.

Each procedure will accept either an `xml-document` record or any of the other XML record types. This makes it possible to write fragments of XML documents, although you should keep in mind that such fragments aren’t documents and won’t generally be accepted by any XML parser.

If the xml being written is an `xml-document` record, the procedures `write-xml` and `write-xml-file` will look for a contained `xml-declaration` record and its `encoding` attribute. If the `encoding` is a supported value, the output will be encoded as specified; otherwise it will be encoded as UTF-8.

When an XHTML document record is written, named XHTML characters are translated into their corresponding entities. For example, the character ‘`#\U+00A0`’ is written as ‘`&nbsp;`’. In order for an XML document record to be recognized as XHTML, it must have a DTD record that satisfies the predicate `html-dtd?`.

procedure: write-xml xml port ¶

  Write xml to port. Note that character encoding will only be done if port supports it.

procedure: write-xml-file xml pathname ¶

  Write xml to the file specified by pathname. Roughly equivalent to

      (define (write-xml-file xml pathname)
        (call-with-output-file pathname
          (lambda (port)
            (write-xml xml port))))

procedure: xml->wide-string xml ¶

  Convert xml to a wide string. No character encoding is used, since wide strings can represent all characters without encoding. Roughly equivalent to

      (define (xml->wide-string xml)
        (call-with-wide-output-string
         (lambda (port)
           (write-xml xml port))))

procedure: xml->string xml ¶

  Convert xml to a character string encoded as UTF-8. Roughly equivalent to

      (define (xml->string xml)
        (wide-string->utf8-string (xml->wide-string xml)))

#### 14.15.3 XML Names

MIT/GNU Scheme implements XML names in a slightly complex way. Unfortunately, this complexity is a direct consequence of the definition of XML names rather than a mis-feature of this implementation.

The reason that XML names are complex is that XML namespace support, which was added after XML was standardized, is not very well integrated with the core XML definition. The most obvious problem is that names can’t have associated namespaces when they appear in the DTD of a document, even if the body of the document uses them. Consequently, it must be possible to compare non-associated names with associated names.

An XML name consists of two parts: the qname, which is a symbol, possibly including a namespace prefix; and the Uniform Resource Identifier (URI), which identifies an optional namespace.

procedure: make-xml-name qname uri ¶

  Creates and returns an XML name. Qname must be a symbol whose name satisfies `string-is-xml-name?`. Uri must satisfy either `absolute-uri?` or `null-xml-namespace-uri?`. The returned value is an XML name that satisfies `xml-name?`.

  If uri is the null namespace (satisfies `null-xml-namespace-uri?`), the returned value is a symbol equivalent to qname. This means that an ordinary symbol can be used as an XML name when there is no namespace associated with the name.

  For convenience, qname may be a string, in which case it is converted to a symbol using `make-xml-qname`.

  For convenience, uri may be any object that `->uri` is able to convert to a URI record, provided the resulting URI meets the above restrictions.

procedure: xml-name? object ¶

  Returns `#t` if object is an XML name, and `#f` otherwise.

procedure: xml-name->symbol xml-name ¶

  Returns the symbol part of xml-name.

procedure: xml-name-uri xml-name ¶

  Returns the URI of xml-name. The result always satisfies `absolute-uri?` or `null-xml-namespace-uri?`.

procedure: xml-name-string xml-name ¶

  Returns the qname of xml-name as a string. Equivalent to

      (symbol->string (xml-name->symbol xml-name))

The next two procedures get the prefix and local part of an XML name, respectively. The prefix of an XML name is the part of the qname to the left of the colon, while the local part is the part of the qname to the right of the colon. If there is no colon in the qname, the local part is the entire qname, and the prefix is the null symbol (i.e. ‘`||`’).

procedure: xml-name-prefix xml-name ¶

  Returns the prefix of xml-name as a symbol.

procedure: xml-name-local xml-name ¶

  Returns the local part of xml-name as a symbol.

The next procedure compares two XML names for equality. The rules for equality are slightly complex, in order to permit comparing names in the DTD with names in the document body. So, if both of the names have non-null namespace URIs, then the names are equal if and only if their local parts are equal and their URIs are equal. (The prefixes of the names are not considered in this case.) Otherwise, the names are equal if and only if their qnames are equal.

procedure: xml-name=? xml-name-1 xml-name-2 ¶

  Returns `#t` if xml-name-1 and xml-name-2 are the same name, and `#f` otherwise.

These next procedures define the data abstraction for qnames. While qnames are represented as symbols, only symbols whose names satisfy `string-is-xml-name?` are qnames.

procedure: make-xml-qname string ¶

  String must satisfy `string-is-xml-name?`. Returns the qname corresponding to string (the symbol whose name is string).

procedure: xml-qname? object ¶

  Returns `#t` if object is a qname, otherwise returns `#f`.

procedure: xml-qname-prefix qname ¶

  Returns the prefix of qname as a symbol.

procedure: xml-qname-local qname ¶

  Returns the local part of qname as a symbol.

The prefix of a qname or XML name may be absent if there is no colon in the name. The absent, or null, prefix is abstracted by the next two procedures. Note that the null prefix is a symbol, just like non-null prefixes.

procedure: null-xml-name-prefix ¶

  Returns the null prefix.

procedure: null-xml-name-prefix? object ¶

  Returns `#t` if object is the null prefix, otherwise returns `#f`.

The namespace URI of an XML name may be null, meaning that there is no namespace associated with the name. This namespace is represented by a relative URI record whose string representation is the null string.

procedure: null-xml-namespace-uri ¶

  Returns the null namespace URI record.

procedure: null-xml-namespace-uri? object ¶

  Returns `#t` if object is the null namespace URI record, otherwise returns `#f`.

The following values are two distinguished URI records.

variable: xml-uri ¶

  `xml-uri` is the URI reserved for use by the XML recommendation. This URI must be used with the ‘`xml`’ prefix.

variable: xmlns-uri ¶

  `xmlns-uri` is the URI reserved for use by the XML namespace recommendation. This URI must be used with the ‘`xmlns`’ prefix.

procedure: make-xml-nmtoken string ¶

procedure: xml-nmtoken? object ¶

procedure: string-is-xml-name? string ¶

procedure: string-is-xml-nmtoken? string ¶

#### 14.15.4 XML Structure

The output from the XML parser and the input to the XML output procedure is a complex data structure composed of a hierarchy of typed components. Each component is a record whose fields correspond to parts of the XML structure that the record represents. There are no special operations on these records; each is a tuple with named subparts. The root record type is `xml-document`, which represents a complete XML document.

Each record type type has the following associated bindings:

`<type>`

  is a variable bound to the record-type descriptor for type. The record-type descriptor may be used as a specializer in SOS method definitions, which greatly simplifies code to dispatch on these types.

`type?`

  is a predicate for records of type type. It accepts one argument, which can be any object, and returns `#t` if the object is a record of this type, or `#f` otherwise.

`make-type`

  is a constructor for records of type type. It accepts one argument for each field of type, in the same order that they are written in the type description, and returns a newly-allocated record of that type.

`type-field`

  is an accessor procedure for the field field in records of type type. It accepts one argument, which must be a record of that type, and returns the contents of the corresponding field in the record.

`set-type-field!`

  is a modifier procedure for the field field in records of type type. It accepts two arguments: the first must be a record of that type, and the second is a new value for the corresponding field. The record’s field is modified to have the new value.

record type: xml-document declaration misc-1 dtd misc-2 root misc-3 ¶

  The `xml-document` record is the top-level record representing a complete XML document. Declaration is either an `xml-declaration` object or `#f`. Dtd is either an `xml-dtd` object or `#f`. Root is an `xml-element` object. Misc-1, misc-2, and misc-3 are lists of miscellaneous items; a miscellaneous item is either an `xml-comment` object, an `xml-processing-instructions` object, or a string of whitespace.

record type: xml-declaration version encoding standalone ¶

  The `xml-declaration` record represents the ‘`<?xml … ?>`’ declaration that optionally appears at the beginning of an XML document. Version is a version string, typically `"1.0"`. Encoding is either an encoding string or `#f`. Standalone is either `"yes"`, `"no"`, or `#f`.

record type: xml-element name attributes contents ¶

  The `xml-element` record represents general XML elements; the bulk of a typical XML document consists of these elements. Name is the element name (an XML name). Attributes is a list of XML attribute objects. Contents is a list of the contents of the element. Each element of this list is either a string, an `xml-element` record or an `xml-processing-instructions` record.

record type: xml-processing-instructions name text ¶

  The `xml-processing-instructions` record represents processing instructions, which have the form ‘`<?name … ?>`’. These instructions are intended to contain non-XML data that will be processed by another interpreter; for example they might contain PHP programs. The name field is the processor name (a symbol), and the text field is the body of the instructions (a string).

record type: xml-dtd root external internal ¶

  The `xml-dtd` record represents a document type declaration. The root field is an XML name for the root element of the document. External is either an `xml-external-id` record or `#f`. Internal is a list of DTD element records (e.g. `xml-!element`, `xml-!attlist`, etc.).

The remaining record types are valid only within a DTD.

record type: xml-!element name content-type ¶

  The `xml-!element` record represents an element-type declaration. Name is the XML name of the type being declared (a symbol). Content-type describes the type and can have several different values, as follows:

  - The XML names ‘`EMPTY`’ and ‘`ANY`’ correspond to the XML keywords of the same name.
  - A list ‘`(MIX type …)`’ corresponds to the ‘`(#PCDATA | type | …)`’ syntax.

record type: xml-!attlist name definitions ¶

  The `xml-!attlist` record represents an attribute-list declaration. Name is the XML name of the type for which attributes are being declared (a symbol). Definitions is a list of attribute definitions, each of which is a list of three elements `(name type default)`. Name is an XML name for the name of the attribute (a symbol). Type describes the attribute type, and can have one of the following values:

  - The XML names ‘`CDATA`’, ‘`IDREFS`’, ‘`IDREF`’, ‘`ID`’, ‘`ENTITY`’, ‘`ENTITIES`’, ‘`NMTOKENS`’, and ‘`NMTOKEN`’ correspond to the XML keywords of the same names.
  - A list ‘`(NOTATION name1 name2 …)`’ corresponds to the ‘`NOTATION (name1 | name2 …)`’ syntax.
  - A list ‘`(ENUMERATED name1 name2 …)`’ corresponds to the ‘`(name1 | name2 …)`’ syntax.

  Default describes the default value for the attribute, and can have one of the following values:

  - The XML names ‘`#REQUIRED`’ and ‘`#IMPLIED`’ correspond to the XML keywords of the same names.
  - A list ‘`(#FIXED value)`’ corresponds to the ‘`#FIXED "value"`’ syntax. Value is represented as a string.
  - A list ‘`(DEFAULT value)`’ corresponds to the ‘`"value"`’ syntax. Value is represented as a string.

record type: xml-!entity name value ¶

  The `xml-!entity` record represents a general entity declaration. Name is an XML name for the entity. Value is the entity’s value, either a string or an `xml-external-id` record.

record type: xml-parameter-!entity name value ¶

  The `xml-parameter-!entity` record represents a parameter entity declaration. Name is an XML name for the entity. Value is the entity’s value, either a string or an `xml-external-id` record.

record type: xml-unparsed-!entity name id notation ¶

  The `xml-unparsed-!entity` record represents an unparsed entity declaration. `Name` is an XML name for the entity. Id is an `xml-external-id` record. Notation is an XML name for the notation.

record type: xml-!notation name id ¶

  The `xml-!notation` record represents a notation declaration. `Name` is an XML name for the notation. Id is an `xml-external-id` record.

record type: xml-external-id id uri ¶

  The `xml-external-id` record is a reference to an external DTD. This reference consists of two parts: id is a public ID literal, corresponding to the ‘`PUBLIC`’ keyword, while uri is a system literal, corresponding to the ‘`SYSTEM`’ keyword. Either or both may be present, depending on the context. Id is represented as a string, while uri is represented as a URI record.

## 15 Operating-System Interface

The Scheme standard provides a simple mechanism for reading and writing files: file ports. MIT/GNU Scheme provides additional tools for dealing with other aspects of the operating system:

- Pathnames are a reasonably operating-system independent tool for manipulating the component parts of file names. This can be useful for implementing defaulting of file name components.
- Control over the current working directory: the place in the file system from which relative file names are interpreted.
- Procedures that rename, copy, delete, and test for the existence of files. Also, procedures that return detailed information about a particular file, such as its type (directory, link, etc.) or length.
- Procedures for reading the contents of a directory.
- Procedures for obtaining times in various formats, converting between the formats, and generating human-readable time strings.
- Procedures to run other programs as subprocesses of Scheme, to read their output, and write input to them.
- A means to determine the operating system Scheme is running under.

### 15.1 Pathnames

MIT/GNU Scheme programs need to use names to designate files. The main difficulty in dealing with names of files is that different file systems have different naming formats for files. For example, here is a table of several file systems (actually, operating systems that provide file systems) and what equivalent file names might look like for each one:

    System          File Name
    ------          ---------
    TOPS-20         <LISPIO>FORMAT.FASL.13
    TOPS-10         FORMAT.FAS[1,4]
    ITS             LISPIO;FORMAT FASL
    MULTICS         >udd>LispIO>format.fasl
    TENEX           <LISPIO>FORMAT.FASL;13
    VAX/VMS         [LISPIO]FORMAT.FAS;13
    UNIX            /usr/lispio/format.fasl
    DOS             C:\USR\LISPIO\FORMAT.FAS

It would be impossible for each program that deals with file names to know about each different file name format that exists; a new operating system to which Scheme was ported might use a format different from any of its predecessors. Therefore, MIT/GNU Scheme provides two ways to represent file names: filenames (also called namestrings), which are strings in the implementation-dependent form customary for the file system, and pathnames, which are special abstract data objects that represent file names in an implementation-independent way. Procedures are provided to convert between these two representations, and all manipulations of files can be expressed in machine-independent terms by using pathnames.

In order to allow MIT/GNU Scheme programs to operate in a network environment that may have more than one kind of file system, the pathname facility allows a file name to specify which file system is to be used. In this context, each file system is called a host, in keeping with the usual networking terminology.15

Note that the examples given in this section are specific to unix pathnames. Pathnames for other operating systems have different external representations.

#### 15.1.1 Filenames and Pathnames

Pathname objects are usually created by parsing filenames (character strings) into component parts. MIT/GNU Scheme provides operations that convert filenames into pathnames and vice versa.

procedure: ->pathname object ¶

  Returns a pathname that is the equivalent of object. Object must be a pathname or a string. If object is a pathname, it is returned. If object is a string, this procedure returns the pathname that corresponds to the string; in this case it is equivalent to `(parse-namestring object #f #f)`.

      (->pathname "foo")          ⇒  #[pathname 65 "foo"]
      (->pathname "/usr/morris")  ⇒  #[pathname 66 "/usr/morris"]

procedure: parse-namestring thing [host [defaults]] ¶

  This turns thing into a pathname. Thing must be a pathname or a string. If thing is a pathname, it is returned. If thing is a string, this procedure returns the pathname that corresponds to the string, parsed according to the syntax of the file system specified by host.

  This procedure does not do defaulting of pathname components.

  The optional arguments are used to determine what syntax should be used for parsing the string. In general this is only really useful if your implementation of MIT/GNU Scheme supports more than one file system, otherwise you would use `->pathname`. If given, host must be a host object or `#f`, and defaults must be a pathname. Host specifies the syntax used to parse the string. If host is not given or `#f`, the host component from defaults is used instead; if defaults is not given, the host component from `param:default-pathname-defaults` is used.

procedure: ->namestring pathname ¶

  `->namestring` returns a newly allocated string that is the filename corresponding to pathname.

      (->namestring (->pathname "/usr/morris/minor.van"))
           ⇒  "/usr/morris/minor.van"

procedure: pathname-simplify pathname ¶

  Returns a pathname that locates the same file or directory as pathname, but is in some sense simpler. Note that `pathname-simplify` might not always be able to simplify the pathname, e.g. on unix with symbolic links the directory `/usr/morris/../` need not be the same as `/usr/`. In cases of uncertainty the behavior is conservative, returning the original or a partly simplified pathname.

      (pathname-simplify "/usr/morris/../morris/dance")
           ⇒  #[pathname "/usr/morris/dance"]

#### 15.1.2 Components of Pathnames

A pathname object always has six components, described below. These components are the common interface that allows programs to work the same way with different file systems; the mapping of the pathname components into the concepts peculiar to each file system is taken care of by the Scheme implementation.

host

  The name of the file system on which the file resides. In the current implementation, this component is always a host object that is filled in automatically by the runtime system. When specifying the host component, use either `#f` or the value of the variable `local-host`.

device

  Corresponds to the “device” or “file structure” concept in many host file systems: the name of a (logical or physical) device containing files. This component is the drive letter for PC file systems, and is unused for unix file systems.

directory

  Corresponds to the “directory” concept in many host file systems: the name of a group of related files (typically those belonging to a single user or project). This component is always used for all file systems.

name

  The name of a group of files that can be thought of as conceptually the “same” file. This component is always used for all file systems.

type

  Corresponds to the “filetype” or “extension” concept in many host file systems. This says what kind of file this is. Files with the same name but different type are usually related in some specific way, such as one being a source file, another the compiled form of that source, and a third the listing of error messages from the compiler. This component is currently used for all file systems, and is formed by taking the characters that follow the last dot in the namestring.

version

  Corresponds to the “version number” concept in many host file systems. Typically this is a number that is incremented every time the file is modified. This component is currently unused for all file systems.

Note that a pathname is not necessarily the name of a specific file. Rather, it is a specification (possibly only a partial specification) of how to access a file. A pathname need not correspond to any file that actually exists, and more than one pathname can refer to the same file. For example, the pathname with a version of `newest` may refer to the same file as a pathname with the same components except a certain number as the version. Indeed, a pathname with version `newest` may refer to different files as time passes, because the meaning of such a pathname depends on the state of the file system. In file systems with such facilities as “links”, multiple file names, logical devices, and so on, two pathnames that look quite different may turn out to address the same file. To access a file given a pathname, one must do a file-system operation such as `open-input-file`.

Two important operations involving pathnames are parsing and merging. Parsing is the conversion of a filename (which might be something supplied interactively by the users when asked to supply the name of a file) into a pathname object. This operation is implementation-dependent, because the format of filenames is implementation-dependent. Merging takes a pathname with missing components and supplies values for those components from a source of default values.

Not all of the components of a pathname need to be specified. If a component of a pathname is missing, its value is `#f`. Before the file system interface can do anything interesting with a file, such as opening the file, all the missing components of a pathname must be filled in. Pathnames with missing components are used internally for various purposes; in particular, parsing a namestring that does not specify certain components will result in a pathname with missing components.

Any component of a pathname may be the symbol `unspecific`, meaning that the component simply does not exist, for file systems in which such a value makes no sense. For example, unix and Windows file systems usually do not support version numbers, so the version component for such a host might be `unspecific`.16

In addition to `#f` and `unspecific`, the components of a pathname may take on the following meaningful values:

host

  An implementation-defined type which may be tested for using the `host?` predicate.

device

  On systems that support this component (Windows), it may be specified as a string containing a single alphabetic character, for which the alphabetic case is ignored.

directory

  A non-empty list, which represents a directory path: a sequence of directories, each of which has a name in the previous directory, the last of which is the directory specified by the entire path. Each element in such a path specifies the name of the directory relative to the directory specified by the elements to its left. The first element of the list is either the symbol `absolute` or the symbol `relative`. If the first element in the list is the symbol `absolute`, then the directory component (and subsequently the pathname) is absolute; the first component in the sequence is to be found at the “root” of the file system. If the directory is relative then the first component is to be found in some as yet unspecified directory; typically this is later specified to be the current working directory.

  Aside from `absolute` and `relative`, which may only appear as the first element of the list, each subsequent element in the list is either: a string, which is a literal component; the symbol `wild`, meaningful only when used in conjunction with the directory reader; or the symbol `up`, meaning the next directory is the “parent” of the previous one. `up` corresponds to the file `..` in unix and PC file systems.

  (The following note does not refer to any file system currently supported by MIT/GNU Scheme, but is included for completeness.) In file systems that do not have “hierarchical” structure, a specified directory component will always be a list whose first element is `absolute`. If the system does not support directories other than a single global directory, the list will have no other elements. If the system supports “flat” directories, i.e. a global set of directories with no subdirectories, then the list will contain a second element, which is either a string or `wild`. In other words, a non-hierarchical file system is treated as if it were hierarchical, but the hierarchical features are unused. This representation is somewhat inconvenient for such file systems, but it discourages programmers from making code depend on the lack of a file hierarchy.

name

  A string, which is a literal component; or the symbol `wild`, meaningful only when used in conjunction with the directory reader.

type

  A string, which is a literal component; or the symbol `wild`, meaningful only when used in conjunction with the directory reader.

version

  An exact positive integer, which is a literal component; the symbol `newest`, which means to choose the largest available version number for that file; the symbol `oldest`, which means to choose the smallest version number; or the symbol `wild`, meaningful only when used in conjunction with the directory reader. In the future some other possible values may be added, e.g. `installed`. Note that currently no file systems support version numbers; thus this component is not used and should be specified as `#f`.

procedure: make-pathname host device directory name type version ¶

  Returns a pathname object whose components are the respective arguments. Each argument must satisfy the restrictions for the corresponding component, which were outlined above.

      (make-pathname #f
                     #f
                     '(absolute "usr" "morris")
                     "foo"
                     "scm"
                     #f)
           ⇒  #[pathname 67 "/usr/morris/foo.scm"]

procedure: pathname-host pathname ¶
procedure: pathname-device pathname ¶
procedure: pathname-directory pathname ¶
procedure: pathname-name pathname ¶
procedure: pathname-type pathname ¶
procedure: pathname-version pathname ¶

  Returns a particular component of pathname.

      (define x (->pathname "/usr/morris/foo.scm"))
      (pathname-host x)       ⇒  #[host 1]
      (pathname-device x)     ⇒  unspecific
      (pathname-directory x)  ⇒  (absolute "usr" "morris")
      (pathname-name x)       ⇒  "foo"
      (pathname-type x)       ⇒  "scm"
      (pathname-version x)    ⇒  unspecific

procedure: pathname-new-device pathname device ¶
procedure: pathname-new-directory pathname directory ¶
procedure: pathname-new-name pathname name ¶
procedure: pathname-new-type pathname type ¶
procedure: pathname-new-version pathname version ¶

  Returns a new copy of pathname with the respective component replaced by the second argument. Pathname is unchanged. Portable programs should not explicitly replace a component with `unspecific` because this might not be permitted in some situations.

      (define p (->pathname "/usr/blisp/rel15"))
      p
           ⇒  #[pathname 71 "/usr/blisp/rel15"]
      (pathname-new-name p "rel100")
           ⇒  #[pathname 72 "/usr/blisp/rel100"]
      (pathname-new-directory p '(relative "test" "morris"))
           ⇒  #[pathname 73 "test/morris/rel15"]
      p
           ⇒  #[pathname 71 "/usr/blisp/rel15"]

procedure: pathname-default-device pathname device ¶
procedure: pathname-default-directory pathname directory ¶
procedure: pathname-default-name pathname name ¶
procedure: pathname-default-type pathname type ¶
procedure: pathname-default-version pathname version ¶

  These operations are similar to the `pathname-new-component` operations, except that they only change the specified component if it has the value `#f` in pathname.

#### 15.1.3 Operations on Pathnames

procedure: pathname? object ¶

  Returns `#t` if object is a pathname; otherwise returns `#f`.

procedure: pathname=? pathname1 pathname2 ¶

  Returns `#t` if pathname1 is equivalent to pathname2; otherwise returns `#f`. Pathnames are equivalent if all of their components are equivalent, hence two pathnames that are equivalent must identify the same file or equivalent partial pathnames. However, the converse is not true: non-equivalent pathnames may specify the same file (e.g. via absolute and relative directory components), and pathnames that specify no file at all (e.g. name and directory components unspecified) may be equivalent.

procedure: pathname-absolute? pathname ¶

  Returns `#t` if pathname is an absolute rather than relative pathname object; otherwise returns `#f`. Specifically, this procedure returns `#t` when the directory component of pathname is a list starting with the symbol `absolute`, and returns `#f` in all other cases. All pathnames are either absolute or relative, so if this procedure returns `#f`, the argument is a relative pathname.

procedure: directory-pathname? pathname ¶

  Returns `#t` if pathname has only directory components and no file components. This is roughly equivalent to

      (define (directory-pathname? pathname)
        (string-null? (file-namestring pathname)))

  except that it is faster.

procedure: pathname-wild? pathname ¶

  Returns `#t` if pathname contains any wildcard components; otherwise returns `#f`.

procedure: merge-pathnames pathname [defaults [default-version]] ¶

  Returns a pathname whose components are obtained by combining those of pathname and defaults. Defaults defaults to the value of `param:default-pathname-defaults` and default-version defaults to `newest`.

  The pathnames are combined by components: if pathname has a non-missing component, that is the resulting component, otherwise the component from defaults is used. The default version can be `#f` to preserve the information that the component was missing from pathname. The directory component is handled specially: if both pathnames have directory components that are lists, and the directory component from pathname is relative (i.e. starts with `relative`), then the resulting directory component is formed by appending pathname’s component to defaults’s component. For example:

      (define path1 (->pathname "scheme/foo.scm"))
      (define path2 (->pathname "/usr/morris"))
      path1
           ⇒  #[pathname 74 "scheme/foo.scm"]
      path2
           ⇒  #[pathname 75 "/usr/morris"]
      (merge-pathnames path1 path2)
           ⇒  #[pathname 76 "/usr/scheme/foo.scm"]
      (merge-pathnames path2 path1)
           ⇒  #[pathname 77 "/usr/morris.scm"]

  The merging rules for the version are more complex and depend on whether pathname specifies a name. If pathname does not specify a name, then the version, if not provided, will come from defaults. However, if pathname does specify a name then the version is not affected by defaults. The reason is that the version “belongs to” some other file name and is unlikely to have anything to do with the new one. Finally, if this process leaves the version missing, then default-version is used.

  The net effect is that if the user supplies just a name, then the host, device, directory and type will come from defaults, but the version will come from default-version. If the user supplies nothing, or just a directory, the name, type and version will come over from defaults together.

parameter: param:default-pathname-defaults ¶

  The value of this parameter (see Parameters) is the default pathname-defaults pathname; if any pathname primitive that needs a set of defaults is not given one, it uses this one. Modifying the `working-directory-pathname` parameter also changes this parameter to the same value, computed by merging the new working directory with the parameter’s old value.

variable: *default-pathname-defaults* ¶

  This variable is deprecated; use `param:default-pathname-defaults` instead.

procedure: pathname-default pathname device directory name type version ¶

  This procedure defaults all of the components of pathname simultaneously. It could have been defined by:

      (define (pathname-default pathname
                                device directory name type version)
        (make-pathname (pathname-host pathname)
                       (or (pathname-device pathname) device)
                       (or (pathname-directory pathname) directory)
                       (or (pathname-name pathname) name)
                       (or (pathname-type pathname) type)
                       (or (pathname-version pathname) version)))

procedure: file-namestring pathname ¶
procedure: directory-namestring pathname ¶
procedure: host-namestring pathname ¶
procedure: enough-namestring pathname [defaults] ¶

  These procedures return a string corresponding to a subset of the pathname information. `file-namestring` returns a string representing just the name, type and version components of pathname; the result of `directory-namestring` represents just the host, device, and directory components; and `host-namestring` returns a string for just the host portion.

  `enough-namestring` takes another argument, defaults. It returns an abbreviated namestring that is just sufficient to identify the file named by pathname when considered relative to the defaults (which defaults to the value of `param:default-pathname-defaults`).

      (file-namestring "/usr/morris/minor.van")
           ⇒  "minor.van"
      (directory-namestring "/usr/morris/minor.van")
           ⇒  "/usr/morris/"
      (enough-namestring "/usr/morris/men")
           ⇒  "men"      ;perhaps

procedure: file-pathname pathname ¶
procedure: directory-pathname pathname ¶
procedure: enough-pathname pathname [defaults] ¶

  These procedures return a pathname corresponding to a subset of the pathname information. `file-pathname` returns a pathname with just the name, type and version components of pathname. The result of `directory-pathname` is a pathname containing the host, device and directory components of pathname.

  `enough-pathname` takes another argument, defaults. It returns an abbreviated pathname that is just sufficient to identify the file named by pathname when considered relative to the defaults (which defaults to the value of `param:default-pathname-defaults`).

  These procedures are similar to `file-namestring`, `directory-namestring` and `enough-namestring`, but they return pathnames instead of strings.

procedure: directory-pathname-as-file pathname ¶

  Returns a pathname that is equivalent to pathname, but in which the directory component is represented as a file. The last directory is removed from the directory component and converted into name and type components. This is the inverse operation to `pathname-as-directory`.

      (directory-pathname-as-file (->pathname "/usr/blisp/"))
           ⇒  #[pathname "/usr/blisp"]

procedure: pathname-as-directory pathname ¶

  Returns a pathname that is equivalent to pathname, but in which any file components have been converted to a directory component. If pathname does not have name, type, or version components, it is returned without modification. Otherwise, these file components are converted into a string, and the string is added to the end of the list of directory components. This is the inverse operation to `directory-pathname-as-file`.

      (pathname-as-directory (->pathname "/usr/blisp/rel5"))
           ⇒  #[pathname "/usr/blisp/rel5/"]

#### 15.1.4 Miscellaneous Pathname Procedures

This section gives some standard operations on host objects, and some procedures that return some useful pathnames.

variable: local-host ¶

  This variable has as its value the host object that describes the local host’s file system.

procedure: host? object ¶

  Returns `#t` if object is a pathname host; otherwise returns `#f`.

procedure: host=? host1 host2 ¶

  Returns `#t` if host1 and host2 denote the same pathname host; otherwise returns `#f`.

procedure: init-file-pathname [host] ¶

  Returns a pathname for the user’s initialization file on host. The host argument defaults to the value of `local-host`. If the initialization file does not exist this procedure returns `#f`.

  Under unix, the init file is called `.scheme.init`; under Windows, the init file is called `scheme.ini`. In either case, it is located in the user’s home directory, which is computed by `user-homedir-pathname`.

procedure: user-homedir-pathname [host] ¶

  Returns a pathname for the user’s “home directory” on host. The host argument defaults to the value of `local-host`. The concept of a “home directory” is itself somewhat implementation-dependent, but it should be the place where the user keeps personal files, such as initialization files and mail.

  Under unix, the user’s home directory is specified by the `HOME` environment variable. If this variable is undefined, the user name is computed using the `getlogin` system call, or if that fails, the `getuid` system call. The resulting user name is passed to the `getpwnam` system call to obtain the home directory.

  Under Windows, several heuristics are tried to find the user’s home directory. The user’s home directory is computed by examining several environment variables, in the following order:

  - `HOMEDRIVE` and `HOMEPATH` are both defined and `%HOMEDRIVE%%HOMEPATH%` is an existing directory. (These variables are automatically defined by Windows NT.)
  - `HOME` is defined and `%HOME%` is an existing directory.
  - `USERDIR` and `USERNAME` are defined and `%USERDIR%\%USERNAME%` is an existing directory.
  - `USERDIR` and `USER` are defined and `%USERDIR%\%USER%` is an existing directory.
  - `USERNAME` is defined and `%USERNAME%` is an existing directory on the Windows system drive.
  - `USER` is defined and `%USER%` is an existing directory on the Windows system drive.
  - Finally, if all else fails, the Windows system drive is used as the home directory.

procedure: system-library-pathname pathname ¶

  Locates pathname in MIT/GNU Scheme’s system library directory. An error of type `condition-type:file-operation-error` is signalled if pathname cannot be located on the library search path.

      (system-library-pathname "compiler.com")
        ⇒ #[pathname 45 "/usr/local/lib/mit-scheme/compiler.com"]

procedure: system-library-directory-pathname pathname ¶

  Locates the pathname of an MIT/GNU Scheme system library directory. An error of type `condition-type:file-operation-error` is signalled if pathname cannot be located on the library search path.

      (system-library-directory-pathname "options")
           ⇒ #[pathname 44 "/usr/local/lib/mit-scheme/options/"]

### 15.2 Working Directory

When MIT/GNU Scheme is started, the current working directory (or simply, working directory) is initialized in an operating-system dependent manner; usually, it is the directory in which Scheme was invoked. The working directory can be determined from within Scheme by calling the `pwd` procedure, and changed by calling the `cd` procedure. Each REP loop has its own working directory, and inferior REP loops initialize their working directory from the value in effect in their superior at the time they are created.

procedure: working-directory-pathname ¶
procedure: pwd ¶

  Returns the current working directory as a pathname that has no name, type, or version components, just host, device, and directory components. `pwd` is an alias for `working-directory-pathname`; the long name is intended for programs and the short name for interactive use.

procedure: set-working-directory-pathname! filename ¶
procedure: cd filename ¶

  Makes filename the current working directory and returns the new current working directory as a pathname. Filename is coerced to a pathname using `pathname-as-directory`. `cd` is an alias for `set-working-directory-pathname!`; the long name is intended for programs and the short name for interactive use.

  When this procedure is executed in the top-level REP loop, it changes the working directory of the running Scheme executable.

      (set-working-directory-pathname! "/usr/morris/blisp")
           ⇒  #[pathname "/usr/morris/blisp/"]
      (set-working-directory-pathname! "~")
           ⇒  #[pathname "/usr/morris/"]

  This procedure signals an error if filename does not refer to an existing directory.

  If filename describes a relative rather than absolute pathname, this procedure interprets it as relative to the current working directory, before changing the working directory.

      (working-directory-pathname)
           ⇒  #[pathname "/usr/morris/"]
      (set-working-directory-pathname! "foo")
           ⇒  #[pathname "/usr/morris/foo/"]

procedure: with-working-directory-pathname filename thunk ¶

  This procedure dynamically binds the current working directory to filename and returns the value of thunk (a procedure of no arguments). Filename is coerced to a pathname using `pathname-as-directory`.

### 15.3 File Manipulation

This section describes procedures that manipulate files and directories. Any of these procedures can signal a number of errors for many reasons. The specifics of these errors are much too operating-system dependent to document here. However, if such an error is signalled by one of these procedures, it will be of type `condition-type:file-operation-error`.

procedure: file-exists? filename ¶
procedure: file-exists-direct? filename ¶
procedure: file-exists-indirect? filename ¶

  These procedures return `#t` if filename is an existing file or directory; otherwise they return `#f`. In operating systems that support symbolic links, if the file is a symbolic link, `file-exists-direct?` tests for the existence of the link, while `file-exists-indirect?` and `file-exists?` test for the existence of the file pointed to by the link.

procedure: copy-file source-filename target-filename ¶

  Makes a copy of the file named by source-filename. The copy is performed by creating a new file called target-filename, and filling it with the same data as source-filename.

procedure: rename-file source-filename target-filename ¶

  Changes the name of source-filename to be target-filename. In the unix implementation, this will not rename across file systems.

procedure: delete-file filename ¶

  Deletes the file named filename.

procedure: delete-file-no-errors filename ¶

  Like `delete-file`, but returns a boolean value indicating whether an error occurred during the deletion. If no errors occurred, `#t` is returned. If an error of type `condition-type:file-error` or `condition-type:port-error` is signalled, `#f` is returned.

procedure: hard-link-file source-filename target-filename ¶

  Makes a hard link from source-filename to target-filename. This operation gives the file specified by source-filename a new name, in addition to the old name.

  This currently works only on unix systems. It is further restricted to work only when source-filename and target-filename refer to names in the same file system.

procedure: soft-link-file source-filename target-filename ¶

  Creates a new soft link called target-filename that points at the file source-filename. (Soft links are also sometimes called symbolic links.) Note that source-filename will be interpreted as a string (although you may specify it as a pathname object, if you wish). The contents of this string will be stored in the file system as the soft link. When a file operation attempts to open the link, the contents of the link are interpreted relative to the link’s location at that time.

  This currently works only on unix systems.

procedure: make-directory filename ¶

  Creates a new directory named filename. Signals an error if filename already exists, or if the directory cannot be created.

procedure: delete-directory filename ¶

  Deletes the directory named filename. Signals an error if the directory does not exist, is not a directory, or contains any files or subdirectories.

procedure: ->truename filename ¶

  This procedure attempts to discover and return the “true name” of the file associated with filename within the file system. An error of type `condition-type:file-operation-error` is signalled if the appropriate file cannot be located within the file system.

procedure: call-with-temporary-file-pathname procedure ¶

  Calls `temporary-file-pathname` to create a temporary file, then calls procedure with one argument, the pathname referring to that file. When procedure returns, if the temporary file still exists, it is deleted; then, the value yielded by procedure is returned. If procedure escapes from its continuation, and the file still exists, it is deleted.

procedure: temporary-file-pathname [directory] ¶

  Creates a new empty temporary file and returns a pathname referring to it. The temporary file is created with Scheme’s default permissions, so barring unusual circumstances it can be opened for input and/or output without error. The temporary file will remain in existence until explicitly deleted. If the file still exists when the Scheme process terminates, it will be deleted.

  If directory is specified, the temporary file will be stored there. If it is not specified, or if it is `#f`, the temporary file will be stored in the directory returned by `temporary-directory-pathname`.

procedure: temporary-directory-pathname ¶

  Returns the pathname of an existing directory that can be used to store temporary files. These directory names are tried, in order, until a writeable directory is found:

  - The directories specified by the environment variables `TMPDIR`, `TEMP`, or `TMP`.
  - Under unix, the directories `/var/tmp`, `/usr/tmp`, or `/tmp`.
  - Under Windows, the following directories on the system drive: `\temp`, `\tmp`, or `\`.
  - Under Windows, the current directory, as specified by `param:default-pathname-defaults`.

procedure: file-directory? filename ¶

  Returns `#t` if the file named filename exists and is a directory. Otherwise returns `#f`. In operating systems that support symbolic links, if filename names a symbolic link, this examines the file linked to, not the link itself.

  This is equivalent to

      (eq? 'directory (file-type-indirect filename))

procedure: file-regular? filename ¶

  Returns `#t` if the file named filename exists and is a regular file (i.e. not a directory, symbolic link, device file, etc.). Otherwise returns `#f`. In operating systems that support symbolic links, if filename names a symbolic link, this examines the file linked to, not the link itself.

  This is equivalent to

      (eq? 'regular (file-type-indirect filename))

procedure: file-symbolic-link? filename ¶

  In operating systems that support symbolic links, if the file named filename exists and is a symbolic link, this procedure returns the contents of the symbolic link as a newly allocated string. The returned value is the name of the file that the symbolic link points to and must be interpreted relative to the directory of filename. If filename either does not exist or is not a symbolic link, or if the operating system does not support symbolic links, this procedure returns `#f`.

procedure: file-type-direct filename ¶
procedure: file-type-indirect filename ¶

  If the file named filename exists, `file-type-direct` returns a symbol specifying what type of file it is. For example, if filename refers to a directory, the symbol `directory` is returned. If filename doesn’t refer to an existing file, `#f` is returned.

  If filename refers to a symbolic link, `file-type-direct` returns the type of the link itself, while `file-type-indirect` returns the type of the file linked to.

  At this time, the symbols that can be returned are the following. The names are intended to be self-explanatory. Most of these names can only be returned on particular operating systems, and so the operating-system name is prefixed to the name.

      regular
      directory
      unix-symbolic-link
      unix-character-device
      unix-block-device
      unix-named-pipe
      unix-socket
      win32-named-pipe

procedure: file-readable? filename ¶

  Returns `#t` if filename names a file that can be opened for input; i.e. a readable file. Otherwise returns `#f`.

procedure: file-writeable? filename ¶

  Returns `#t` if filename names a file that can be opened for output; i.e. a writeable file. Otherwise returns `#f`.

procedure: file-executable? filename ¶

  Returns `#t` if filename names a file that can be executed. Otherwise returns `#f`. Under unix, an executable file is identified by its mode bits. Under Windows, an executable file has one of the file extensions `.exe`, `.com`, or `.bat`.

procedure: file-access filename mode ¶

  Mode must be an exact integer between `0` and `7` inclusive; it is a bitwise-encoded predicate selector with `1` meaning “executable”, `2` meaning “writeable”, and `4` meaning “readable”. `file-access` returns `#t` if filename exists and satisfies the predicates selected by mode. For example, if mode is `5`, then filename must be both readable and executable. If filename doesn’t exist, or if it does not satisfy the selected predicates, `#f` is returned.

procedure: file-eq? filename1 filename2 ¶

  Determines whether filename1 and filename2 refer to the same file. Under unix, this is done by comparing the inodes and devices of the two files. Under Windows, this is done by comparing the filename strings.

procedure: file-modes filename ¶

  If filename names an existing file, `file-modes` returns an exact non-negative integer encoding the file’s permissions. The encoding of this integer is operating-system dependent. Under unix, it is the least-significant 12 bits of the `st_mode` element of the `struct stat` structure. Under Windows, it is the file attribute bits, which are described below. If filename does not name an existing file, `#f` is returned.

procedure: set-file-modes! filename modes ¶

  Filename must name an existing file. Modes must be an exact non-negative integer that could have been returned by a call to `file-modes`. `set-file-modes!` modifies the file’s permissions to be those encoded by modes.

variable: nt-file-mode/read-only ¶
variable: nt-file-mode/hidden ¶
variable: nt-file-mode/system ¶
variable: nt-file-mode/directory ¶
variable: nt-file-mode/archive ¶
variable: nt-file-mode/normal ¶
variable: nt-file-mode/temporary ¶
variable: nt-file-mode/compressed ¶

  The values of these variables are the “mode bits” that comprise the value returned by `file-modes` under Windows. These bits are small integers that are combined by adding to form a complete set of modes. The integer zero represents a set of modes in which none of these bits are set.

procedure: file-modification-time filename ¶

  Returns the modification time of filename as an exact non-negative integer. The result may be compared to other file times using ordinary integer arithmetic. If filename names a file that does not exist, `file-modification-time` returns `#f`.

  In operating systems that support symbolic links, if filename names a symbolic link, `file-modification-time` returns the modification time of the file linked to. An alternate procedure, `file-modification-time-direct`, returns the modification time of the link itself; in all other respects it is identical to `file-modification-time`. For symmetry, `file-modification-time-indirect` is a synonym of `file-modification-time`.

procedure: file-access-time filename ¶

  Returns the access time of filename as an exact non-negative integer. The result may be compared to other file times using ordinary integer arithmetic. If filename names a file that does not exist, `file-access-time` returns `#f`.

  In operating systems that support symbolic links, if filename names a symbolic link, `file-access-time` returns the access time of the file linked to. An alternate procedure, `file-access-time-direct`, returns the access time of the link itself; in all other respects it is identical to `file-access-time`. For symmetry, `file-access-time-indirect` is a synonym of `file-access-time`.

procedure: set-file-times! filename access-time modification-time ¶

  Filename must name an existing file, while access-time and modification-time must be valid file times that might have been returned by `file-access-time` and `file-modification-time`, respectively. `set-file-times!` alters the access and modification times of the file specified by filename to the values given by access-time and modification-time, respectively. For convenience, either of the time arguments may be specified as `#f`; in this case the corresponding time is not changed. `set-file-times!` returns an unspecified value.

procedure: current-file-time ¶

  Returns the current time as an exact non-negative integer, in the same format used by the above file-time procedures. This number can be compared to other file times using ordinary arithmetic operations.

procedure: file-touch filename ¶

  Touches the file named filename. If the file already exists, its modification time is set to the current file time and `#f` is returned. Otherwise, the file is created and `#t` is returned. This is an atomic test-and-set operation, so it is useful as a synchronization mechanism.

procedure: file-length filename ¶

  Returns the length, in bytes, of the file named filename as an exact non-negative integer.

procedure: file-attributes filename ¶

  This procedure determines if the file named filename exists, and returns information about it if so; if the file does not exist, it returns `#f`.

  In operating systems that support symbolic links, if filename names a symbolic link, `file-attributes` returns the attributes of the link itself. An alternate procedure, `file-attributes-indirect`, returns the attributes of the file linked to; in all other respects it is identical to `file-attributes`. For symmetry, `file-attributes-direct` is a synonym of `file-attributes`.

The information returned by `file-attributes` is decoded by accessor procedures. The following accessors are defined in all operating systems:

procedure: file-attributes/type attributes ¶

  The file type: `#t` if the file is a directory, a character string (the name linked to) if a symbolic link, or `#f` for all other types of file.

procedure: file-attributes/access-time attributes ¶

  The last access time of the file, an exact non-negative integer.

procedure: file-attributes/modification-time attributes ¶

  The last modification time of the file, an exact non-negative integer.

procedure: file-attributes/change-time attributes ¶

  The last change time of the file, an exact non-negative integer.

procedure: file-attributes/length attributes ¶

  The length of the file in bytes.

procedure: file-attributes/mode-string attributes ¶

  The mode string of the file, a newly allocated string showing the file’s mode bits. Under unix, this string is in unix format. Under Windows, this string shows the standard “DOS” attributes in their usual format.

procedure: file-attributes/n-links attributes ¶

  The number of links to the file, an exact positive integer. Under Windows, this is always `1`.

The following additional accessors are defined under unix:

procedure: file-attributes/uid attributes ¶

  The user id of the file’s owner, an exact non-negative integer.

procedure: file-attributes/gid attributes ¶

  The group id of the file’s group, an exact non-negative integer.

procedure: file-attributes/inode-number attributes ¶

  The inode number of the file, an exact non-negative integer.

The following additional accessor is defined under Windows:

procedure: file-attributes/modes attributes ¶

  The attribute bits of the file. This is an exact non-negative integer containing the file’s attribute bits, exactly as specified by the operating system’s API.

### 15.4 Directory Reader

procedure: directory-read directory [sort?] ¶

  Directory must be an object that can be converted into a pathname by
  `->pathname`. The directory specified by directory is read, and the contents of the directory is returned as a newly allocated list of absolute pathnames. The result is sorted according to the usual sorting conventions for directories, unless sort? is specified as `#f`. If directory has name, type, or version components, the returned list contains only those pathnames whose name, type, and version components match those of directory; `wild` or `#f` as one of these components means “match anything”.

  The Windows implementation supports “globbing”, in which the characters `*` and `?` are interpreted to mean “match anything” and “match any character”, respectively. This “globbing” is supported only in the file part of directory.

### 15.5 Date and Time

MIT/GNU Scheme provides a simple set of procedures for manipulating date and time information. There are four time representations, each of which serves a different purpose. Each representation may be converted to any of the others.

The primary time representation, universal time, is an exact non-negative integer counting the number of seconds that have elapsed since midnight January 1, 1900 UTC. (UTC stands for Coordinated Universal Time, and is the modern name for Greenwich Mean Time.) This format is produced by `get-universal-time` and `decoded-time->universal-time`.

The second representation, decoded time, is a record structure in which the time is broken down into components, such as month, minute, etc. Decoded time is always relative to a particular time zone, which is a component of the structure. This format is produced by `global-decoded-time` and `local-decoded-time`.

The third representation, file time, is an exact non-negative integer that is larger for increasing time. Unlike universal time, this representation is operating-system dependent. This format is produced by all of the file-attribute procedures, for example `file-modification-time` and `file-attributes`.

The fourth representation, the time string, is an external representation for time. This format is defined by RFC-822, Standard for the format of ARPA Internet text messages, with the modification that years are represented as four-digit numbers rather than two-digit numbers. This format is the standard format for Internet email and numerous other network protocols.

Within this section, argument variables named universal-time, decoded-time, file-time, and time-string are respectively required to be of the corresponding format.

#### 15.5.1 Universal Time

procedure: get-universal-time ¶

  Return the current time in universal format.

      (get-universal-time) ⇒ 3131453078

variable: epoch ¶

  `epoch` is the representation of midnight January 1, 1970 UTC in universal-time format.

      epoch ⇒ 2208988800

#### 15.5.2 Decoded Time

Objects representing standard time components, such as seconds and minutes, are required to be exact non-negative integers. Seconds and minutes must be inclusively between `0` and `59`; hours between `0` and `23`; days between `1` and `31`; months between `1` and `12`; years are represented in “four-digit” form, in which 1999 is represented as `1999` — not `99`.

procedure: local-decoded-time ¶

  Return the current time in decoded format. The decoded time is represented in the local time zone.

      (pp (local-decoded-time))
      -| #[decoded-time 76]
      -| (second 2)
      -| (minute 12)
      -| (hour 11)
      -| (day 27)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 1)
      -| (daylight-savings-time 1)
      -| (zone 5)

procedure: global-decoded-time ¶

  Return the current time in decoded format. The decoded time is represented in UTC.

      (pp (global-decoded-time))
      -| #[decoded-time 77]
      -| (second 8)
      -| (minute 12)
      -| (hour 15)
      -| (day 27)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 1)
      -| (daylight-savings-time 0)
      -| (zone 0)

procedure: make-decoded-time second minute hour day month year [zone] ¶

  Return a new decoded-time object representing the given time. The arguments must be valid components according to the above rules, and must form a valid date.

  If zone is not supplied or is `#f`, the resulting decoded time will be represented in the local time zone. Otherwise, zone must be a valid time zone, and the result will be represented in that zone.

  Warning: because this procedure depends on the operating system’s runtime library, it is not capable of representing all dates. In particular, on most unix systems, it is not possible to encode dates that occur prior to midnight, January 1, 1970 UTC. Attempting to do this will signal an error.

      (pp (make-decoded-time 0 9 11 26 3 1999))
      -| #[decoded-time 19]
      -| (second 0)
      -| (minute 9)
      -| (hour 11)
      -| (day 26)
      -| (month 3)
      -| (year 1999)
      -| (day-of-week 4)
      -| (daylight-savings-time 0)
      -| (zone 5)

      (pp (make-decoded-time 0 9 11 26 3 1999 3))
      -| #[decoded-time 80]
      -| (second 0)
      -| (minute 9)
      -| (hour 11)
      -| (day 26)
      -| (month 3)
      -| (year 1999)
      -| (day-of-week 4)
      -| (daylight-savings-time 0)
      -| (zone 3)

procedure: decoded-time/second decoded-time ¶
procedure: decoded-time/minute decoded-time ¶
procedure: decoded-time/hour decoded-time ¶
procedure: decoded-time/day decoded-time ¶
procedure: decoded-time/month decoded-time ¶
procedure: decoded-time/year decoded-time ¶

  Return the corresponding component of decoded-time.

      (decoded-time/second (local-decoded-time)) ⇒ 17
      (decoded-time/year (local-decoded-time)) ⇒ 1999
      (decoded-time/day (local-decoded-time)) ⇒ 26

procedure: decoded-time/day-of-week decoded-time ¶

  Return the day of the week on which decoded-time falls, encoded as an exact integer between `0` (Monday) and `6` (Sunday), inclusive.

      (decoded-time/day-of-week (local-decoded-time)) ⇒ 4

procedure: decoded-time/daylight-savings-time? decoded-time ¶

  Return `#t` if decoded-time is represented using daylight savings time. Otherwise return `#f`.

      (decoded-time/daylight-savings-time? (local-decoded-time))
                        ⇒ #f

procedure: decoded-time/zone decoded-time ¶

  Return the time zone in which decoded-time is represented. This is an exact rational number between `-24` and `+24` inclusive, that when multiplied by `3600` is an integer. The value is the number of hours west of UTC.

      (decoded-time/zone (local-decoded-time)) ⇒ 5

procedure: time-zone? object ¶

  Returns `#t` if object is an exact number between `-24` and `+24` inclusive, that when multiplied by `3600` is an integer.

      (time-zone? -5)   ⇒ #t
      (time-zone? 11/2) ⇒ #t
      (time-zone? 11/7) ⇒ #f

procedure: month/max-days month ¶

  Returns the maximum number of days possible in month. Month must be an exact integer between `1` and `12` inclusive.

      (month/max-days 2) ⇒ 29
      (month/max-days 3) ⇒ 31
      (month/max-days 4) ⇒ 30

#### 15.5.3 File Time

As stated above, file time is operating-system dependent. As of this writing, two formats are used. For unix and Windows systems, file time is the number of seconds since midnight January 1, 1970 UTC (the standard unix time convention).

The following procedures generate their results in file-time format:

    file-access-time
    file-access-time-direct
    file-access-time-indirect
    file-modification-time
    file-modification-time-direct
    file-modification-time-indirect
    file-attributes/access-time
    file-attributes/modification-time
    file-attributes/change-time

Additionally, `set-file-times!` accepts its time arguments in file-time format.

#### 15.5.4 Time-Format Conversion

The procedures described in this section convert times from one format to another.

procedure: universal-time->local-decoded-time universal-time ¶
procedure: universal-time->global-decoded-time universal-time ¶

  Converts an argument in universal-time format to decoded-time format. The result is in the local time zone or UTC, respectively.

      (pp (universal-time->local-decoded-time (get-universal-time)))
      -| #[decoded-time 21]
      -| (second 23)
      -| (minute 57)
      -| (hour 17)
      -| (day 29)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 3)
      -| (daylight-savings-time 1)
      -| (zone 5)

      (pp (universal-time->global-decoded-time
           (get-universal-time)))
      -| #[decoded-time 22]
      -| (second 27)
      -| (minute 57)
      -| (hour 21)
      -| (day 29)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 3)
      -| (daylight-savings-time 0)
      -| (zone 0)

procedure: universal-time->file-time universal-time ¶

  Converts an argument in universal-time format to file-time format.

      (universal-time->file-time (get-universal-time))
          ⇒ 925422988

procedure: universal-time->local-time-string universal-time ¶
procedure: universal-time->global-time-string universal-time ¶

  Converts an argument in universal-time format to a time string. The result is in the local time zone or UTC, respectively.

      (universal-time->local-time-string (get-universal-time))
          ⇒ "Thu, 29 Apr 1999 17:55:31 -0400"
      (universal-time->global-time-string (get-universal-time))
          ⇒ "Thu, 29 Apr 1999 21:55:51 +0000"

procedure: decoded-time->universal-time decoded-time ¶

  Converts an argument in decoded-time format to universal-time format.

      (decoded-time->universal-time (local-decoded-time))
          ⇒ 3134411942
      (decoded-time->universal-time (global-decoded-time))
          ⇒ 3134411947

procedure: decoded-time->file-time decoded-time ¶

  Converts an argument in decoded-time format to file-time format.

      (decoded-time->file-time (local-decoded-time))
          ⇒ 925423191
      (decoded-time->file-time (global-decoded-time))
          ⇒ 925423195

procedure: decoded-time->string decoded-time ¶

  Convert an argument in decoded-time format to a time string.

      (decoded-time->string (local-decoded-time))
          ⇒ "Thu, 29 Apr 1999 18:00:43 -0400"
      (decoded-time->string (global-decoded-time))
          ⇒ "Thu, 29 Apr 1999 22:00:46 +0000"

procedure: file-time->universal-time file-time ¶

  Converts an argument in universal-time format to file-time format.

      (file-time->universal-time (file-modification-time "/"))
          ⇒ 3133891907

procedure: file-time->local-decoded-time file-time ¶
procedure: file-time->global-decoded-time file-time ¶

  Converts an argument in file-time format to decoded-time format. The result is in the local time zone or UTC, respectively.

      (pp (file-time->local-decoded-time
           (file-modification-time "/")))
      -| #[decoded-time 26]
      -| (second 47)
      -| (minute 31)
      -| (hour 17)
      -| (day 23)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 4)
      -| (daylight-savings-time 1)
      -| (zone 5)

      (pp (file-time->global-decoded-time
           (file-modification-time "/")))
      -| #[decoded-time 27]
      -| (second 47)
      -| (minute 31)
      -| (hour 21)
      -| (day 23)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 4)
      -| (daylight-savings-time 0)
      -| (zone 0)

procedure: file-time->local-time-string file-time ¶
procedure: file-time->global-time-string file-time ¶

  Converts an argument in file-time format to a time string. The result is in the local time zone or UTC, respectively.

      (file-time->local-time-string (file-modification-time "/"))
          ⇒ "Fri, 23 Apr 1999 17:31:47 -0400"
      (file-time->global-time-string (file-modification-time "/"))
          ⇒ "Fri, 23 Apr 1999 21:31:47 +0000"

procedure: string->universal-time time-string ¶

  Converts a time-string argument to universal-time format.

      (string->universal-time "Fri, 23 Apr 1999 21:31:47 +0000")
          ⇒ 3133888307
      (string->universal-time "Fri, 23 Apr 1999 17:31:47 -0400")
          ⇒ 3133888307

procedure: string->decoded-time time-string ¶

  Converts a time-string argument to decoded-time format.

      (pp (string->decoded-time "Fri, 23 Apr 1999 17:31:47 -0400"))
      -| #[decoded-time 30]
      -| (second 47)
      -| (minute 31)
      -| (hour 17)
      -| (day 23)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 4)
      -| (daylight-savings-time 0)
      -| (zone 4)

procedure: string->file-time time-string ¶

  Converts a time-string argument to file-time format.

      (string->file-time "Fri, 23 Apr 1999 17:31:47 -0400")
          ⇒ 924899507

#### 15.5.5 External Representation of Time

The normal external representation for time is the time string, as described above. The procedures in this section generate alternate external representations of time which are more verbose and may be more suitable for presentation to human readers.

procedure: decoded-time/date-string decoded-time ¶
procedure: decoded-time/time-string decoded-time ¶

  These procedures return strings containing external representations of the date and time, respectively, represented by decoded-time. The results are implicitly in local time.

      (decoded-time/date-string (local-decoded-time))
          ⇒ "Tuesday March 30, 1999"
      (decoded-time/time-string (local-decoded-time))
          ⇒ "11:22:38 AM"

procedure: day-of-week/long-string day-of-week ¶
procedure: day-of-week/short-string day-of-week ¶

  Returns a string representing the given day-of-week. The argument must be an exact non-negative integer between `0` and `6` inclusive. `day-of-week/long-string` returns a long string that fully spells out the name of the day. `day-of-week/short-string` returns a shortened string that abbreviates the day to three letters.

      (day-of-week/long-string 0)  ⇒ "Monday"
      (day-of-week/short-string 0) ⇒ "Mon"
      (day-of-week/short-string 3) ⇒ "Thu"

procedure: month/long-string month ¶
procedure: month/short-string month ¶

  Returns a string representing the given month. The argument must be an exact non-negative integer between `1` and `12` inclusive. `month/long-string` returns a long string that fully spells out the name of the month. `month/short-string` returns a shortened string that abbreviates the month to three letters.

      (month/long-string 1)   ⇒ "January"
      (month/short-string 1)  ⇒ "Jan"
      (month/short-string 10) ⇒ "Oct"

procedure: time-zone->string ¶

  Returns a string corresponding to the given time zone. This string is the same string that is used to generate RFC-822 time strings.

      (time-zone->string 5)    ⇒ "-0500"
      (time-zone->string -4)   ⇒ "+0400"
      (time-zone->string 11/2) ⇒ "-0530"

### 15.6 Machine Time

The previous section dealt with procedures that manipulate clock time. This section describes procedures that deal with computer time: elapsed CPU time, elapsed real time, and so forth. These procedures are useful for measuring the amount of time it takes to execute code.

Some of the procedures in this section manipulate a time representation called ticks. A tick is a unit of time that is unspecified here but can be converted to and from seconds by supplied procedures. A count in ticks is represented as an exact integer. At present each tick is one millisecond, but this may change in the future.

procedure: process-time-clock ¶

  Returns the amount of process time, in ticks, that has elapsed since Scheme was started. Process time is measured by the operating system and is time during which the Scheme process is computing. It does not include time in system calls, but depending on the operating system it may include time used by subprocesses.

      (process-time-clock) ⇒ 21290

procedure: real-time-clock ¶

  Returns the amount of real time, in ticks, that has elapsed since Scheme was started. Real time is the time measured by an ordinary clock.

      (real-time-clock) ⇒ 33474836

procedure: internal-time/ticks->seconds ticks ¶

  Returns the number of seconds corresponding to ticks. The result is always a real number.

      (internal-time/ticks->seconds 21290) ⇒ 21.29
      (internal-time/ticks->seconds 33474836) ⇒ 33474.836

procedure: internal-time/seconds->ticks seconds ¶

  Returns the number of ticks corresponding to seconds. Seconds must be a real number.

      (internal-time/seconds->ticks 20.88) ⇒ 20880
      (internal-time/seconds->ticks 20.83) ⇒ 20830

procedure: system-clock ¶

  Returns the amount of process time, in seconds, that has elapsed since Scheme was started. Roughly equivalent to:

      (internal-time/ticks->seconds (process-time-clock))

  Example:

      (system-clock) ⇒ 20.88

procedure: runtime ¶

  Returns the amount of process time, in seconds, that has elapsed since Scheme was started. However, it does not include time spent in garbage collection.

      (runtime) ⇒ 20.83

procedure: with-timings thunk receiver ¶

  Calls thunk with no arguments. After thunk returns, receiver is called with three arguments describing the time spent while computing thunk: the elapsed run time, the amount of time spent in the garbage collector, and the elapsed real time. All three times are in ticks.

  This procedure is most useful for doing performance measurements, and is designed to have relatively low overhead.

      (with-timings
       (lambda () … hairy computation …)
       (lambda (run-time gc-time real-time)
         (write (internal-time/ticks->seconds run-time))
         (write-char #\space)
         (write (internal-time/ticks->seconds gc-time))
         (write-char #\space)
         (write (internal-time/ticks->seconds real-time))
         (newline)))

procedure: measure-interval runtime? procedure ¶

  Calls procedure, passing it the current process time, in seconds, as an argument. The result of this call must be another procedure. When procedure returns, the resulting procedure is tail-recursively called with the ending time, in seconds, as an argument.

  If runtime? is `#f`, the elapsed time is deducted from the elapsed system time returned by `runtime`.

  While this procedure can be used for time measurement, its interface is somewhat clumsy for that purpose. We recommend that you use `with-timings` instead, because it is more convenient and has lower overhead.

      (measure-interval #t
                        (lambda (start-time)
                          (let ((v … hairy computation …))
                            (lambda (end-time)
                              (write (- end-time start-time))
                              (newline)
                              v))))

### 15.7 Subprocesses

MIT/GNU Scheme provides the ability to run and control subprocesses. This support is divided into two parts: a low-level set of primitives that maps onto the underlying operating system’s process-control primitives, and a high-level set of procedures for starting a subprocess and running it to completion in a single call. Subprocesses that are run in the latter fashion are referred to as synchronous, because they are started and stopped in synchrony with a Scheme procedure call.

This chapter documents Scheme’s high-level synchronous-subprocess support. The low-level support is not documented but is available for those who are willing to read the source code.

Synchronous-subprocess support is a run-time-loadable option. To use it, execute

    (load-option 'synchronous-subprocess)

once before calling it.

#### 15.7.1 Subprocess Procedures

There are two commands for running synchronous subprocesses under Scheme. `run-shell-command` is very simple to use, provides access to all shell features, and is to be preferred in most situations. `run-synchronous-subprocess` allows direct execution of a program and precise control of the command-line arguments passed to the program, but does not provide file globbing, I/O redirection, or other shell features.

procedure: run-shell-command command option … ¶

  Runs command, which must be a string. Command is passed to a command shell for interpretation; how the shell is chosen is detailed below.

  The options are a sequence of keyword/value pairs that specify optional behavior. See Subprocess Options.

  `run-shell-command` waits until the subprocess completes its execution and returns the exit code from the subprocess. If the subprocess is killed or stopped, an error is signalled and the procedure does not return.

procedure: run-synchronous-subprocess program arguments option … ¶

  Runs program, passing it the given command-line arguments. Program must be either the name of a program on the path, or else a pathname to a specific program. Arguments must be a list of strings; each string is a single command-line argument to the program.

  The options are a sequence of keyword/value pairs that specify optional behavior. See Subprocess Options.

  `run-synchronous-subprocess` waits until the subprocess completes its execution and returns the exit code from the subprocess. If the subprocess is killed or stopped, an error is signalled and the procedure does not return.

#### 15.7.2 Subprocess Conditions

If a subprocess spawned by one of the above procedures is killed or suspended, then one of the following errors will be signalled.

condition type: condition-type:subprocess-signalled subprocess reason ¶

  This condition type is a subtype of `condition-type:subprocess-abnormal-termination`. It is signalled when the subprocess is killed.

  Subprocess is an object that represents the subprocess involved. The internals of this object can be accessed but the interface is not documented at this time; see the source code for details.

  Reason is interesting only on unix systems, where it is the signal that killed the process. On other systems it has a fixed value that conveys no useful information.

condition type: condition-type:subprocess-stopped subprocess reason ¶

  This condition type is a subtype of `condition-type:subprocess-abnormal-termination`. It is signalled when the subprocess is stopped or suspended.

  Subprocess is an object that represents the subprocess involved. The internals of this object can be accessed but the interface is not documented at this time; see the source code for details.

  Reason is interesting only on unix systems, where it is the signal that stopped the process. On other systems it has a fixed value that conveys no useful information.

condition type: condition-type:subprocess-abnormal-termination subprocess reason ¶

  This condition type is a subtype of `condition-type:error`. This is an abstract type that is never signalled. It is provided so that condition handlers can be bound to it.

#### 15.7.3 Subprocess Options

The following subprocess options may be passed to `run-shell-command` or `run-synchronous-subprocess`. These options are passed as alternating keyword/value pairs, for example:

    (run-shell-command "ls /"
                       'output my-output-port
                       'output-buffer-size 8192)

The example shows a shell command being run with two options specified: `output` and `output-buffer-size`.

subprocess option: input port ¶

  Specifies the standard input of the subprocess. Port may be an input port, in which case characters are read from port and fed to the subprocess until port reaches end-of-file. Alternatively, port may be `#f`, indicating that the subprocess has no standard input.

  The default value of this option is `#f`.

      (call-with-input-file "foo.in"
        (lambda (port)
          (run-shell-command "cat > /dev/null" 'input port)))

subprocess option: input-line-translation line-ending ¶

  Specifies how line-endings should be translated when writing characters to the subprocess. Ignored if the `input` option is `#f`. Line-ending must be either a string specifying the line ending, or the symbol `default`, meaning to use the operating system’s standard line ending. In either case, newline characters to be written to the `input` port are translated to the specified line ending before being written.

  The default value of this option is `default`.

      (call-with-input-file "foo.in"
        (lambda (port)
          (run-shell-command "cat > /dev/null"
                             'input port
                             'input-line-translation "\r\n")))

subprocess option: input-buffer-size n ¶

  Specifies the size of the input buffer for the standard input of the subprocess. (This is the buffer on the Scheme side, and has nothing to do with any buffering done on the subprocess side.) Ignored if the `input` option is `#f`. N must be an exact positive integer specifying the number of characters the buffer can hold.

  The default value of this option is `512`.

      (call-with-input-file "foo.in"
        (lambda (port)
          (run-shell-command "cat > /dev/null"
                             'input port
                             'input-buffer-size 4096)))

subprocess option: output port ¶

  Specifies the standard output and standard error of the subprocess. Port may be an output port, in which case characters are read from the subprocess and fed to port until the subprocess finishes. Alternatively, port may be `#f`, indicating that the subprocess has no standard output or standard error.

  The default value of this option is the value of `(current-output-port)`.

      (call-with-output-file "foo.out"
        (lambda (port)
          (run-shell-command "ls -la /etc" 'output port)))

subprocess option: output-line-translation line-ending ¶

  Specifies how line-endings should be translated when reading characters from the standard output of the subprocess. Ignored if the `output` option is `#f`. Line-ending must be either a string specifying the line ending, or the symbol `default`, meaning to use the operating system’s standard line ending. In either case, newline characters read from the subprocess port are translated to the specified line ending.

  The default value of this option is `default`.

      (call-with-output-file "foo.out"
        (lambda (port)
          (run-shell-command "ls -la /etc"
                             'output port
                             'output-line-translation "\r\n")))

subprocess option: output-buffer-size n ¶

  Specifies the size of the output buffer for the standard output of the subprocess. (This is the buffer on the Scheme side, and has nothing to do with any buffering done on the subprocess side.) Ignored if the `output` option is `#f`. N must be an exact positive integer specifying the number of characters the buffer can hold.

  The default value of this option is `512`.

      (call-with-output-file "foo.out"
        (lambda (port)
          (run-shell-command "ls -la /etc"
                             'output port
                             'output-buffer-size 4096)))

subprocess option: redisplay-hook thunk ¶

  Specifies that thunk is to be run periodically when output from the subprocess is available. Thunk must be a procedure of no arguments, or `#f` indicating that no hook is supplied. This option is mostly useful for interactive systems. For example, the Edwin text editor uses this to update output buffers when running some subprocesses.

  The default value of this option is `#f`.

      (run-shell-command "ls -la /etc"
                         'redisplay-hook
                         (lambda ()
                           (update-buffer-contents buffer)))

subprocess option: environment environment ¶

  Specifies the environment variables that are to be used for the subprocess. Environment must be either a vector of strings or `#f` indicating the default environment. If it is a vector of strings, each string must be a name/value pair where the name and value are separated by an equal sign, for example, `"foo=bar"`. To define a variable with no value, just omit the value, as in `"foo="`.

  Note that the variable `scheme-subprocess-environment` is bound to the default subprocess environment.

  The default value of this option is `#f`.

      (run-shell-command "ls -la /etc"
                         'environment
                         (let* ((v scheme-subprocess-environment)
                                (n (vector-length v))
                                (v (vector-grow v (+ n 1))))
                           (vector-set! v n "TERM=none")
                           v))

subprocess option: working-directory pathname ¶

  Specifies the working directory in which the subprocess will run.

  The default value of this option is `(working-directory-pathname)`.

      (run-shell-command "ls -la" 'working-directory "/etc/")

subprocess option: use-pty? boolean ¶

  This option is meaningful only on unix systems; on other systems it is ignored. Specifies whether to communicate with the subprocess using PTY devices; if true, PTYs will be used, otherwise pipes will be used.

  The default value of this option is `#f`.

      (run-shell-command "ls -la /etc" 'use-pty? #t)

subprocess option: shell-file-name pathname ¶

  Specifies the shell program to use for `run-shell-command`.

  The default value of this option is `(os/shell-file-name)`. This is the value of the environment variable `SHELL`, or if `SHELL` is not set, the value is operating-system dependent as follows:

  - On unix systems, `/bin/sh` is used.
  - On Windows systems, the value of the environment variable `COMSPEC` is used. If that is not set, `cmd.exe` is used for Windows NT, or `command.com` is used for Windows 9x; in each case the shell is found by searching the path.

      (run-shell-command "ls -la /etc"
                         'shell-file-name "/usr/local/bin/bash")

### 15.8 TCP Sockets

MIT/GNU Scheme provides access to sockets, which are a mechanism for inter-process communication. TCP stream sockets are supported, which communicate between computers over a TCP/IP network. TCP sockets are supported on all operating systems.

TCP sockets have two distinct interfaces: one interface to implement a client and another to implement a server. The basic protocol is that servers set up a listening port and wait for connections from clients. Implementation of clients is simpler and will be treated first.

The socket procedures accept two special arguments, called host-name and service. Host-name is a string which must be the name of an internet host. It is looked up using the ordinary lookup rules for your computer. For example, if your host is `foo.mit.edu` and host-name is `"bar"`, then it specifies `bar.mit.edu`.

Service specifies the service to which you will connect. A networked computer normally provides several different services, such as telnet or FTP. Each service is associated with a unique port number; for example, the `"www"` service is associated with port `80`. The service argument specifies the port number, either as a string, or directly as an exact non-negative integer. Port strings are decoded by the operating system using a table; for example, on unix the table is in `/etc/services`. Usually you will use a port string rather than a number.

procedure: open-tcp-stream-socket host-name service ¶

  `open-tcp-stream-socket` opens a connection to the host specified by host-name. Host-name is looked up using the ordinary lookup rules for your computer. The connection is established to the service specified by service. The returned value is an I/O port, to which you can read and write characters using ordinary Scheme I/O procedures such as `read-char` and `write-char`.

  When you wish to close the connection, just use `close-port`.

  As an example, here is how you can open a connection to a web server:

      (open-tcp-stream-socket "web.mit.edu" "www")

Next we will treat setting up a TCP server, which is slightly more complicated. Creating a server is a two-part process. First, you must open a server socket, which causes the operating system to listen to the network on a port that you specify. Once the server socket is opened, the operating system will allow clients to connect to your computer on that port.

In the second step of the process, you accept the connection, which completes the connection initiated by the client, and allows you to communicate with the client. Accepting a connection does not affect the server socket; it continues to listen for additional client connections. You can have multiple client connections to the same server socket open simultaneously.

procedure: open-tcp-server-socket service [address] ¶

  This procedure opens a server socket that listens for connections to service; the socket will continue to listen until you close it. The returned value is a server socket object.

  An error is signalled if another process is already listening on the service. Additionally, ports whose number is less than `1024` are privileged on many operating systems, and cannot be used by non-privileged processes; if service specifies such a port and you do not have administrative privileges, an error may be signalled.

  The optional argument address specifies the IP address on which the socket will listen. If this argument is not supplied or is given as `#f`, then the socket listens on all IP addresses for this machine. (This is equivalent to passing the result of calling `host-address-any`.)

procedure: tcp-server-connection-accept server-socket block? peer-address [line-translation] ¶

  Checks to see if a client has connected to server-socket. If so, an I/O port is returned. The returned port can be read and written using ordinary Scheme I/O procedures such as `read-char` and `write-char`.

  The argument block? says what to do if no client has connected at the time of the call. If `#f`, it says to return immediately with two values of `#f`. Otherwise, the call waits until a client connects.

  The argument peer-address is either `#f` or an IP address as allocated by `allocate-host-address`. If it is an IP address, the address is modified to be the address of the client making the connection.

  The optional argument line-translation specifies how end-of-line characters will be translated when reading or writing to the returned socket. If this is unspecified or `#f`, then lines will be terminated by CR-LF, which is the standard for most internet protocols. Otherwise, it must be a string, which specifies the line-ending character sequence to use.

  Note that closing the port returned by this procedure does not affect server-socket; it just closes the particular client connection that was opened by the call. To close server-socket, use `close-tcp-server-socket`.

procedure: close-tcp-server-socket server-socket ¶

  Closes the server socket server-socket. The operating system will cease listening for network connections to that service. Client connections to server-socket that have already been accepted will not be affected.

### 15.9 Miscellaneous OS Facilities

This section contains assorted operating-system facilities that don’t fit into other categories.

variable: microcode-id/operating-system ¶
obsolete variable: microcode-id/operating-system-name ¶

  `microcode-id/operating-system` is bound to a symbol that specifies the type of operating system that Scheme is running under. There are two possible values: `unix` or `nt`.

  The deprecated variable `microcode-id/operating-system-name` is a string that’s equivalent to `microcode-id/operating-system`.

variable: microcode-id/operating-system-variant ¶

  This variable is a string that identifies the particular variant of the operating system that Scheme is running under. Here are some of the possible values:

      "GNU/Linux"
      "MacOSX"
      "Microsoft Windows NT 4.0 (Build 1381; Service Pack 3)"

  For Windows systems, it is recommended that you match on the prefix of this string and ignore the `"Build"` suffix. This is because the suffix may contain information about service packs or fixes, while the prefix will be constant for a particular version of Windows.

The next few procedures provide access to the domain name service (DNS), which maintains associations between internet host names such as `"www.swiss.ai.mit.edu"` and IP addresses, such as `18.23.0.16`. In MIT/GNU Scheme, we represent an internet host name as a string, and an IP address as a byte vector of length 4 (byte vectors are just character strings that are accessed using `vector-8b-ref` rather than `string-ref`). The bytes in an IP address read in the same order as they do when written out:

    (get-host-by-name "www.swiss") ⇒ #("\022\027\000\020")

procedure: get-host-by-name host-name ¶

  Looks up the internet host name host-name using the DNS, returning a vector of IP addresses for the corresponding host, or `#f` if there is no such host. Usually the returned vector has only one element, but if a host has more than one network interface, the vector might have more than one element.

      (get-host-by-name "www.swiss") ⇒ #("\022\027\000\020")

procedure: get-host-by-address ip-address ¶

  Does a reverse DNS lookup on ip-address, returning the internet host name corresponding to that address, or `#f` if there is no such host.

      (get-host-by-address "\022\027\000\020") ⇒ "swissnet.ai.mit.edu"

procedure: canonical-host-name host-name ¶

  Finds the “canonical” internet host name for host-name. For example:

      (canonical-host-name "zurich")    ⇒ "zurich.ai.mit.edu"
      (canonical-host-name "www.swiss") ⇒ "swissnet.ai.mit.edu"

  In both examples, the default internet domain ‘`ai.mit.edu`’ is added to host-name. In the second example, `"www.swiss"` is an alias for another computer named `"swissnet"`.

procedure: get-host-name ¶

  Returns the string that identifies the computer that MIT/GNU Scheme is running on. Usually this is an unqualified internet host name, i.e. the host name without the domain suffix:

      (get-host-name) ⇒ "aarau"

procedure: os/hostname ¶

  Returns the canonical internet host name of the computer that MIT/GNU Scheme is running on. So, in contrast to the example for `get-host-name`:

      (os/hostname) ⇒ "aarau.ai.mit.edu"

procedure: allocate-host-address ¶

  Allocates and returns an IP address object. This is just a string of a fixed length (current 4 bytes) into which an IP address may be stored. This procedure is used to generate an appropriate argument to be passed to `tcp-server-connection-accept`.

      (allocate-host-address) ⇒ "Xe\034\241"

procedure: host-address-any ¶

  Return an IP address object that specifies “any host”. This object is useful only when passed as the address argument to `open-tcp-server-socket`.

      (host-address-any) ⇒ "\000\000\000\000"

procedure: host-address-loopback ¶

  Return an IP address object that specifies the local loopback network interface. The loopback interface is a software network interface that can be used only for communicating between processes on the same computer. This address object is useful only when passed as the address argument to `open-tcp-server-socket`.

      (host-address-loopback) ⇒ "\177\000\000\001"

## 16 Error System

The MIT/GNU Scheme error system provides a uniform mechanism for the signalling of errors and other exceptional conditions. The simplest and most generally useful procedures in the error system are:

`error`

  is used to signal simple errors, specifying a message and some irritant objects (see Condition Signalling). Errors are usually handled by stopping the computation and putting the user in an error REPL.

`warn`

  is used to signal warnings (see Condition Signalling). Warnings are usually handled by printing a message on the console and continuing the computation normally.

`ignore-errors`

  is used to suppress the normal handling of errors within a given dynamic extent (see Condition Handling). Any error that occurs within the extent is trapped, returning immediately to the caller of `ignore-errors`.

More demanding applications require more powerful facilities. To give a concrete example, suppose you want floating-point division to return a very large number whenever the denominator is zero. This behavior can be implemented using the error system.

The Scheme arithmetic system can signal many different kinds of errors, including floating-point divide by zero. In our example, we would like to handle this particular condition specially, allowing the system to handle other arithmetic errors in its usual way.

The error system supports this kind of application by providing mechanisms for distinguishing different types of error conditions and for specifying where control should be transferred should a given condition arise. In this example, there is a specific object that represents the “floating-point divide by zero” condition type, and it is possible to dynamically specify an arbitrary Scheme procedure to be executed when a condition of that type is signalled. This procedure then finds the stack frame containing the call to the division operator, and returns the appropriate value from that frame.

Another useful kind of behavior is the ability to specify uniform handling for related classes of conditions. For example, it might be desirable, when opening a file for input, to gracefully handle a variety of different conditions associated with the file system. One such condition might be that the file does not exist, in which case the program will try some other action, perhaps opening a different file instead. Another related condition is that the file exists, but is read protected, so it cannot be opened for input. If these or any other related conditions occur, the program would like to skip this operation and move on to something else.

At the same time, errors unrelated to the file system should be treated in their usual way. For example, calling `car` on the argument `3` should signal an error. Or perhaps the name given for the file is syntactically incorrect, a condition that probably wants to be handled differently from the case of the file not existing.

To facilitate the handling of classes of conditions, the error system taxonomically organizes all condition types. The types are related to one another by taxonomical links, which specify that one type is a “kind of” another type. If two types are linked this way, one is considered to be a specialization of the other; or vice-versa, the second is a generalization of the first. In our example, all of the errors associated with opening an input file would be specializations of the condition type “cannot open input file”.

The taxonomy of condition types permits any condition type to have no more than one immediate generalization. Thus, the condition types form a forest (set of trees). While users can create new trees, the standard taxonomy (see Condition-Type Taxonomy) is rooted at `condition-type:serious-condition`, `condition-type:warning`, `condition-type:simple-condition`, and `condition-type:breakpoint`; users are encouraged to add new subtypes to these condition types rather than create new trees in the forest.

To summarize, the error system provides facilities for the following tasks. The sections that follow will describe these facilities in more detail.

Signalling a condition ¶

  A condition may be signalled in a number of different ways. Simple errors may be signalled, without explicitly defining a condition type, using `error`. The `signal-condition` procedure provides the most general signalling mechanism.

Handling a condition ¶

  The programmer can dynamically specify handlers for particular condition types or for classes of condition types, by means of the `bind-condition-handler` procedure. Individual handlers have complete control over the handling of a condition, and additionally may decide not to handle a particular condition, passing it on to previously bound handlers.

Restarting from a handler ¶

  The `with-restart` procedure provides a means for condition-signalling code to communicate to condition-handling code what must be done to proceed past the condition. Handlers can examine the restarts in effect when a condition was signalled, allowing a structured way to continue an interrupted computation.

Packaging condition state

  Each condition is represented by an explicit object. Condition objects contain information about the nature of the condition, information that describes the state of the computation from which the condition arose, and information about the ways the computation can be restarted.

Classification of conditions ¶

  Each condition has a type, represented by a condition type object. Each condition type may be a specialization of some other condition types. A group of types that share a common generalization can be handled uniformly by specifying a handler for the generalization.

### 16.1 Condition Signalling

Once a condition instance has been created using `make-condition` (or any condition constructor), it can be signalled. The act of signalling a condition is separated from the act of creating the condition to allow more flexibility in how conditions are handled. For example, a condition instance could be returned as the value of a procedure, indicating that something unusual has happened, to allow the caller to clean up some state. The caller could then signal the condition once it is ready.

A more important reason for having a separate condition-signalling mechanism is that it allows resignalling. When a signalled condition has been caught by a particular handler, and the handler decides that it doesn’t want to process that particular condition, it can signal the condition again. This is one way to allow other handlers to get a chance to see the condition.

procedure: error reason argument … ¶

  This is the simplest and most common way to signal a condition that requires intervention before a computation can proceed (when intervention is not required, `warn` is more appropriate). `error` signals a condition (using `signal-condition`), and if no handler for that condition alters the flow of control (by invoking a restart, for example) it calls the procedure `standard-error-handler`, which normally prints an error message and stops the computation, entering an error REPL. Under normal circumstances `error` will not return a value (although an interactive debugger can be used to force this to occur).

  Precisely what condition is signalled depends on the first argument to `error`. If reason is a condition, then that condition is signalled and the arguments are ignored. If reason is a condition type, then a new instance of this type is generated and signalled; the arguments are used to generate the values of the fields for this condition type (they are passed as the field-plist argument to `make-condition`). In the most common case, however, reason is neither a condition nor a condition type, but rather a string or symbol. In this case a condition of type `condition-type:simple-error` is created with the message field containing the reason and the irritants field containing the arguments.

procedure: warn reason argument … ¶

  When a condition is not severe enough to warrant intervention, it is appropriate to signal the condition with `warn` rather than `error`. As with `error`, `warn` first calls `signal-condition`; the condition that is signalled is chosen exactly as in `error` except that a condition of type `condition-type:simple-warning` is signalled if reason is neither a condition nor a condition type. If the condition is not handled, `warn` calls the procedure `standard-warning-handler`, which normally prints a warning message and continues the computation by returning from `warn`.

  `warn` establishes a restart named `muffle-warning` before calling `signal-condition`. This allows a signal handler to prevent the generation of the warning message by calling `muffle-warning`. The value of a call to `warn` is unspecified.

procedure: signal-condition condition ¶

  This is the fundamental operation for signalling a condition. The precise operation of `signal-condition` depends on the condition type of which condition is an instance, the condition types set by `break-on-signals`, and the handlers established by `bind-condition-handler` and `bind-default-condition-handler`.

  If the condition is an instance of a type that is a specialization of any of the types specified by `break-on-signals`, then a breakpoint REPL is initiated. Otherwise (or when that REPL returns), the handlers established by `bind-condition-handler` are checked, most recent first. Each applicable handler is invoked, and the search for a handler continues if the handler returns normally. If all applicable handlers return, then the applicable handlers established by `bind-default-condition-handler` are checked, again most recent first. Finally, if no handlers apply (or all return in a normal manner), `signal-condition` returns an unspecified value.

  Note: unlike many other systems, the MIT/GNU Scheme runtime library does not establish handlers of any kind. (However, the Edwin text editor uses condition handlers extensively.) Thus, calls to `signal-condition` will return to the caller unless there are user supplied condition handlers, as the following example shows:

      (signal-condition
       (make-condition
        condition-type:error
        (call-with-current-continuation (lambda (x) x))
        '()    ; no restarts
        '()))  ; no fields
      ⇒  unspecified

### 16.2 Error Messages

By convention, error messages (and in general, the reports generated by `write-condition-report`) should consist of one or more complete sentences. The usual rules for sentences should be followed: the first word of the sentence should be capitalized, and the sentence should be terminated by a period. The message should not contain extraneous whitespace such as line breaks or indentation.

The error system provides a simple formatting language that allows the programmer to have some control over the printing of error messages. This formatting language will probably be redesigned in a future release.

Error messages typically consist of a string describing the error, followed by some irritant objects. The string is printed using `display`, and the irritants are printed using `write`, typically with a space between each irritant. To allow simple formatting, we introduce a noise object, printed using `display`. The irritant list may contain ordinary objects interspersed with noise objects. Each noise object is printed using `display`, with no extra whitespace, while each normal object is printed using `write`, prefixed by a single space character.

Here is an example:

    (define (error-within-procedure message irritant procedure)
      (error message
             irritant
             (error-irritant/noise "within procedure")
             procedure
             (error-irritant/noise ".")))

This would format as follows:

    (error-within-procedure "Bad widget" 'widget-32 'invert-widget) error→

    Bad widget widget-32 within procedure invert-widget.

Here are the operations supporting error messages:

procedure: format-error-message message irritants port ¶

  Message is typically a string (although this is not required), irritants a list of irritant objects, and port an output port. Formats message and irritants to port in the standard way. Note that, during the formatting process, the depth and breadth to which lists are printed are each limited to small numbers, to guarantee that the output from each irritant is not arbitrarily large.

procedure: error-irritant/noise value ¶

  Creates and returns a noise object whose value is value.

### 16.3 Condition Handling

The occurrence of a condition is signalled using `signal-condition`. `signal-condition` attempts to locate and invoke a condition handler that is prepared to deal with the type of condition that has occurred. A condition handler is a procedure of one parameter, the condition that is being signalled. A procedure is installed as a condition handler by calling `bind-condition-handler` (to establish a handler that is in effect only while a particular thunk is executing) or `bind-default-condition-handler` (to establish a handler that is in effect permanently). As implied by the name, handlers created by `bind-default-condition-handler` are invoked only after all other applicable handlers have been invoked.

A handler may process a signal in any way it deems appropriate, but the common patterns are:

Ignore the condition.

  By returning from the handler in the usual manner.

Handle the condition.

  By doing some processing and then invoking a restart (or, less preferably, a continuation) that was established at some point prior to the call to `signal-condition`.

Resignal a condition.

  By doing some processing and calling `signal-condition` with either the same condition or a newly created one. In order to support this, `signal-condition` runs handler in such a way that a subsequent call to `signal-condition` sees only the handlers that were established prior to this one.

As an aid to debugging condition handlers, Scheme maintains a set of condition types that will cause an interactive breakpoint to occur prior to normal condition signalling. That is, `signal-condition` creates a new REPL prior to its normal operation when its argument is a condition that is a specialization of any of these types. The procedure `break-on-signals` establishes this set of condition types.

procedure: ignore-errors thunk ¶

  Executes thunk with a condition handler that intercepts the signalling of any specialization of `condition-type:error` (including those produced by calls to `error`) and immediately terminates the execution of thunk and returns from the call to `ignore-errors` with the signalled condition as its value. If thunk returns normally, its value is returned from `ignore-errors`.

  Notice that `ignore-errors` does not “turn off signalling” or condition handling. Condition handling takes place in the normal manner but conditions specialized from `condition-type:error` are trapped rather than propogated as they would be by default.

procedure: bind-condition-handler condition-types handler thunk ¶

  Invokes thunk after adding handler as a condition handler for the conditions specified by condition-types. Condition-types must be a list of condition types; signalling a condition whose type is a specialization of any of these types will cause the handler to be invoked. See `signal-condition` for a description of the mechanism used to invoke handlers.

  By special extension, if condition-types is the empty list then the handler is called for all conditions.

procedure: bind-default-condition-handler condition-types handler ¶

  Installs handler as a (permanent) condition handler for the conditions specified by condition-types. Condition-types must be a list of condition types; signalling a condition whose type is a specialization of any of these types will cause the handler to be invoked. See `signal-condition` for a description of the mechanism used to invoke handlers.

  By special extension, if condition-types is the empty list then the handler is called for all conditions.

procedure: break-on-signals condition-types ¶

  Arranges for `signal-condition` to create an interactive REPL before it signals a condition that is a specialization of any of the types in the list of condition-types. This can be extremely helpful when trying to debug code that uses custom condition handlers. In order to create a REPL when any condition type is signalled it is best to actually put a breakpoint on entry to `signal-condition`.

procedure: standard-error-handler condition ¶

  Called internally by `error` after it calls `signal-condition`. Normally creates a new REPL with the prompt `"error>"` (but see `standard-error-hook`). In order to simulate the effect of calling `error`, code may call `signal-condition` directly and then call `standard-error-handler` if `signal-condition` returns.

parameter: standard-error-hook ¶

  This parameter controls the behavior of the procedure `standard-error-handler`, and hence `error`. It is intended to be bound with `parameterize` and is normally `#f`. It may be changed to a procedure of one argument and will then be invoked (with `standard-error-hook` rebound to `#f`) by `standard-error-handler` just prior to starting the error REPL. It is passed one argument, the condition being signalled.

procedure: standard-warning-handler condition ¶

  This is the procedure called internally by `warn` after it calls `signal-condition`. The normal behavior of `standard-warning-handler` is to print a message (but see `standard-warning-hook`). More precisely, the message is printed to the port returned by `notification-output-port`. The message is formed by first printing the string `"Warning: "` to this port, and then calling `write-condition-report` on condition and the port.

  In order to simulate the effect of calling `warn`, code may call `signal-condition` directly and then call `standard-warning-handler` if `signal-condition` returns. (This is not sufficient to implement the `muffle-warning` protocol, however. For that purpose an explicit restart must be provided.)

parameter: standard-warning-hook ¶

  This parameter controls the behavior of the procedure `standard-warning-handler`, and hence `warn`. It is intended to be bound with `parameterize` and is normally `#f`. It may be changed to a procedure of one argument and will then be invoked (with `standard-warning-hook` rebound to `#f`) by `standard-warning-handler` in lieu of writing the warning message. It is passed one argument, the condition being signalled.

### 16.4 Restarts

The Scheme error system provides a mechanism, known as restarts, that helps coordinate condition-signalling code with condition-handling code. A module of code that detects and signals conditions can provide procedures (using `with-simple-restart` or `with-restart`) to be invoked by handlers that wish to continue, abort, or restart the computation. These procedures, called restart effectors, are encapsulated in restart objects.

When a condition object is created, it contains a set of restart objects, each of which contains a restart effector. Condition handlers can inspect the condition they are handling (using `find-restart` to find restarts by name, or `condition/restarts` to see the entire set), and they can invoke the associated effectors (using `invoke-restart` or `invoke-restart-interactively`). Effectors can take arguments, and these may be computed directly by the condition-handling code or by gathering them interactively from the user.

The names of restarts can be chosen arbitrarily, but the choice of name is significant. These names are used to coordinate between the signalling code (which supplies names for restarts) and the handling code (which typically chooses a restart effector by the name of its restart). Thus, the names specify the restart protocol implemented by the signalling code and invoked by the handling code. The protocol indicates the number of arguments required by the effector code as well as the semantics of the arguments.

Scheme provides a conventional set of names (hence, protocols) for common use. By choosing the names of restarts from this set, signalling code can indicate that it is able to perform a small set of fairly common actions (`abort`, `continue`, `muffle-warning`, `retry`, `store-value`, `use-value`). In turn, simple condition-handling code can look for the kind of action it wishes to perform and simply invoke it by name. All of Scheme’s conventional names are symbols, although in general restart names are not restricted to any particular data type. In addition, the object `#f` is reserved to indicate the “not for automated use” protocol: these restarts should be activated only under human control.

Restarts themselves are first-class objects. They encapsulate their name, a procedure (known as the effector) to be executed if they are invoked, and a thunk (known as the reporter) that can be invoked to display a description of the restart (used, for example, by the interactive debugger). Invoking a restart is an indication that a handler has chosen to accept control for a condition; as a consequence, the effector of the restart should not return, since this would indicate that the handler declined to handle the condition. Thus, the effector should call a continuation captured before the condition-signalling process began. The most common pattern of usage by signalling code is encapsulated in `with-simple-restart`.

Within this chapter, a parameter named restarts will accept any of the following values:

- A list of restart objects.
- A condition. The procedure `condition/restarts` is called on the condition, and the resulting list of restarts is used in place of the condition.
- The symbol `bound-restarts`. The procedure `bound-restarts` is called (with no arguments), and the resulting list of restarts is used in place of the symbol.
- If the restarts parameter is optional and is not supplied, it is equivalent to having specified the symbol `bound-restarts`.

#### 16.4.1 Establishing Restart Code

procedure: with-simple-restart name reporter thunk ¶

  Invokes thunk in a dynamic environment created by adding a restart named name to the existing named restarts. Reporter may be used during the execution of thunk to produce a description of the newly created restart; it must either be a procedure of one argument (a port) or a string. By convention, the description generated by reporter should be a short complete sentence, with first word capitalized and terminated by a period. The sentence should fit on one line with a little room to spare (see the examples below); usually this means that the sentence should be 70 characters or less in length.

  If the restart created by `with-simple-restart` is invoked it simply aborts the computation in progress by returning an unspecified value from the call to `with-simple-restart`. Otherwise `with-simple-restart` returns the value computed by thunk.

    (with-simple-restart 'george "This restart is named george."
      (lambda () 3)) ⇒ 3

    (with-simple-restart 'george "This restart is named george."
      (lambda ()
        (invoke-restart (find-restart 'george)))) ⇒ unspecific

    (with-simple-restart 'george "This restart is named george."
      (lambda () (car 3)))
    ;The object 3, passed as the first argument to car,
    ; is not the correct type.
    ;To continue, call RESTART with an option number:
    ; (RESTART 3) => Specify an argument to use in its place.
    ; (RESTART 2) => This restart is named george.
    ; (RESTART 1) => Return to read-eval-print level 1.

procedure: with-restart name reporter effector interactor thunk ¶

  Invokes thunk in a dynamic environment created by adding a restart named name to the existing named restarts. Reporter may be used during the execution of thunk to produce a description of the newly created restart; it must either be a procedure of one argument (a port) or a string. Effector is a procedure which will be called when the restart is invoked by `invoke-restart`. Interactor specifies the arguments that are to be passed to effector when it is invoked interactively; it may be either a procedure of no arguments, or `#f`. If interactor is `#f`, this restart is not meant to be invoked interactively.

  The value returned by `with-restart` is the value returned by thunk. Should the restart be invoked by a condition handler, however, the effector will not return back to the handler that invoked it. Instead, the effector should call a continuation created before the condition-signalling process began, and `with-restart` will therefore not return in the normal manner.

    (define (by-george! thunk)
      ; This code handles conditions that arise while executing thunk
      ; by invoking the GEORGE restart, passing 1 and 2 to the restart’s
      ; effector code.
      (bind-condition-handler '() ; All conditions
       (lambda (condition)
         (invoke-restart (find-restart 'george) 1 2))
       thunk))

    (define (can-george! thunk)
      ; This code provides a way of handling errors: the GEORGE restart.
      ; In order to GEORGE you must supply two values.
      (lambda ()
        (call-with-current-continuation
         (lambda (kappa)
           (with-restart
            'george                         ; Name
            "This restart is named george." ; Reporter
            (lambda (a b)                   ; Effector
              (kappa (list 'george a b)))
            values                          ; Interactor
            thunk)))))                      ; Thunk

    (by-george! (can-george! (lambda () -3))        ⇒ -3
    (by-george! (can-george! (lambda () (car 'x)))) ⇒ (george 1 2)

#### 16.4.2 Invoking Standard Restart Code

Scheme supports six standard protocols for restarting from a condition, each encapsulated using a named restart (for use by condition-signalling code) and a simple procedure (for use by condition-handling code). Unless otherwise specified, if one of these procedures is unable to find its corresponding restart, it returns immediately with an unspecified value.

Each of these procedures accepts an optional argument restarts, which is described above in Restarts.

procedure: abort [restarts] ¶

  Abort the computation, using the restart named `abort`. The corresponding effector takes no arguments and abandons the current line of computation. This is the restart provided by Scheme’s REPL.

  If there is no restart named `abort`, this procedure signals an error of type `condition-type:no-such-restart`.

procedure: continue [restarts] ¶

  Continue the current computation, using the restart named `continue`. The corresponding effector takes no arguments and continues the computation beyond the point at which the condition was signalled.

procedure: muffle-warning [restarts] ¶

  Continue the current computation, using the restart named `muffle-warning`. The corresponding effector takes no arguments and continues the computation beyond the point at which any warning message resulting from the condition would be presented to the user. The procedure `warn` establishes a `muffle-warning` restart for this purpose.

  If there is no restart named `muffle-warning`, this procedure signals an error of type `condition-type:no-such-restart`.

procedure: retry [restarts] ¶

  Retry the current computation, using the restart named `retry`. The corresponding effector takes no arguments and simply retries the same computation that triggered the condition. The condition may reoccur, of course, if the root cause has not been eliminated. The code that signals a “file does not exist” error can be expected to supply a `retry` restart. The restart would be invoked after first creating the missing file, since the computation is then likely to succeed if it is simply retried.

procedure: store-value new-value [restarts] ¶

  Retry the current computation, using the restart named `store-value`, after first storing new-value. The corresponding effector takes one argument, new-value, and stores it away in a restart-dependent location, then retries the same computation that triggered the condition. The condition may reoccur, of course, if the root cause has not been eliminated. The code that signals an “unassigned variable” error can be expected to supply a `store-value` restart; this would store the value in the variable and continue the computation.

procedure: use-value new-value [restarts] ¶

  Retry the current computation, using the restart named `use-value`, but substituting new-value for a value that previously caused a failure. The corresponding effector takes one argument, new-value, and retries the same computation that triggered the condition with the new value substituted for the failing value. The condition may reoccur, of course, if the new value also induces the condition.

  The code that signals an “unassigned variable” error can be expected to supply a `use-value` restart; this would simply continue the computation with new-value instead of the value of the variable. Contrast this with the `retry` and `store-value` restarts. If the `retry` restart is used it will fail because the variable still has no value. The `store-value` restart could be used, but it would alter the value of the variable, so that future references to the variable would not be detected.

#### 16.4.3 Finding and Invoking General Restart Code

Restarts are a general mechanism for establishing a protocol between condition-signalling and condition-handling code. The Scheme error system provides “packaging” for a number of common protocols. It also provides lower-level hooks that are intended for implementing customized protocols. The mechanism used by signalling code (`with-restart` and `with-simple-restart`) is used for both purposes.

Four additional operations are provided for the use of condition-handling code. Two operations (`bound-restarts` and `find-restart`) allow condition-handling code to locate active restarts. The other two operations (`invoke-restart` and `invoke-restart-interactively`) allow restart effectors to be invoked once the restart object has been located.

In addition, there is a data abstraction that provides access to the information encapsulated in restart objects.

procedure: bound-restarts ¶

  Returns a list of all currently active restart objects, most recently installed first. `bound-restarts` should be used with caution by condition-handling code, since it reveals all restarts that are active at the time it is called, rather than at the time the condition was signalled. It is useful, however, for collecting the list of restarts for inclusion in newly generated condition objects or for inspecting the current state of the system.

procedure: find-restart name [restarts] ¶

  Returns the first restart object named name in the list of restarts (permissible values for restarts are described above in Restarts). When used in a condition handler, `find-restart` is usually passed the name of a particular restart and the condition object that has been signalled. In this way the handler finds only restarts that were available when the condition was created (usually the same as when it was signalled). If restarts is omitted, the currently active restarts would be used, and these often include restarts added after the condition ocurred.

procedure: invoke-restart restart argument … ¶

  Calls the restart effector encapsulated in restart, passing the specified arguments to it. `invoke-restart` is intended for use by condition-handling code that understands the protocol implemented by restart, and can therefore calculate and pass an appropriate set of arguments.

  If a condition handler needs to interact with a user to gather the arguments for an effector (e.g. if it does not understand the protocol implemented by restart) `invoke-restart-interactively` should be used instead of `invoke-restart`.

procedure: invoke-restart-interactively restart ¶

  First calls the interactor encapsulated in restart to interactively gather the arguments needed for restart’s effector. It then calls the effector, passing these arguments to it.

  `invoke-restart-interactively` is intended for calling interactive restarts (those for which `restart/interactor` is not `#f`). For convenience, `invoke-restart-interactively` will call the restart’s effector with no arguments if the restart has no interactor; this behavior may change in the future.

#### 16.4.4 The Named Restart Abstraction

A restart object is very simple, since it encapsulates only a name, effector, interactor, and description.

procedure: restart? object ¶

  Returns `#f` if and only if object is not a restart.

procedure: restart/name restart ¶

  Returns the name of restart. While the Scheme error system uses only symbols and the object `#f` for its predefined names, programs may use arbitrary objects (name equivalence is tested using `eq?`).

procedure: restart/effector restart ¶

  Returns the effector encapsulated in restart. Normally this procedure is not used since `invoke-restart` and `invoke-restart-interactively` capture the most common invocation patterns.

procedure: restart/interactor restart ¶

  Returns the interactor encapsulated in restart. This is either a procedure of no arguments or the object `#f`. Normally this procedure is not used since `invoke-restart-interactively` captures the most common usage. Thus `restart/interactor` is most useful as a predicate to determine if restart is intended to be invoked interactively.

procedure: write-restart-report restart port ¶

  Writes a description of restart to port. This works by either displaying (if it is a string) or calling (if it is a procedure) the reporter that was supplied when the restart was created.

### 16.5 Condition Instances

A condition, in addition to the information associated with its type, usually contains other information that is not shared with other conditions of the same type. For example, the condition type associated with “unbound variable” errors does not specify the name of the variable that was unbound. The additional information is captured in a condition object, also called a condition instance.

In addition to information that is specific to a given type of condition (such as the variable name for “unbound variable” conditions), every condition instance also contains a continuation that encapsulates the state of the computation in which the condition occurred. This continuation is used for analyzing the computation to learn more about the context in which the condition occurred. It is not intended to provide a mechanism for continuing the computation; that mechanism is provided by restarts.

#### 16.5.1 Generating Operations on Conditions

Scheme provides four procedures that take a condition type as input and produce operations on the corresponding condition object. These are reminiscent of the operations on record types that produce record operators (see Records). Given a condition type it is possible to generate: a constructor for instances of the type (using `condition-constructor`); an accessor to extract the contents of a field in instances of the type (using `condition-accessor`); a predicate to test for instances of the type (using `condition-predicate`); and a procedure to create and signal an instance of the type (using `condition-signaller`).

Notice that the creation of a condition object is distinct from signalling an occurrence of the condition. Condition objects are first-class; they may be created and never signalled, or they may be signalled more than once. Further notice that there are no procedures for modifying conditions; once created, a condition cannot be altered.

procedure: condition-constructor condition-type field-names ¶

  Returns a constructor procedure that takes as arguments values for the fields specified in field-names and creates a condition of type condition-type. Field-names must be a list of symbols that is a subset of the field-names in condition-type. The constructor procedure returned by `condition-constructor` has signature

      (lambda (continuation restarts . field-values) …)

  where the field-names correspond to the field-values. The constructor argument restarts is described in Restarts. Conditions created by the constructor procedure have `#f` for the values of all fields other than those specified by field-names.

  For example, the following procedure `make-simple-warning` constructs a condition of type `condition-type:simple-warning` given a continuation (where the condition occurred), a description of the restarts to be made available, a warning message, and a list of irritants that caused the warning:

      (define make-simple-warning
        (condition-constructor condition-type:simple-warning
                               '(message irritants)))

procedure: condition-accessor condition-type field-name ¶

  Returns a procedure that takes as input a condition object of type condition-type and extracts the contents of the specified field-name. `condition-accessor` signals `error:bad-range-argument` if the field-name isn’t one of the named fields of condition-type; the returned procedure will signal `error:wrong-type-argument` if passed an object other than a condition of type condition-type or one of its specializations.

  If it is known in advance that a particular field of a condition will be accessed repeatedly it is worth constructing an accessor for the field using `condition-accessor` rather than using the (possibly more convenient, but slower) `access-condition` procedure.

procedure: condition-predicate condition-type ¶

  Returns a predicate procedure for testing whether an object is a condition of type condition-type or one of its specializations (there is no predefined way to test for a condition of a given type but not a specialization of that type).

procedure: condition-signaller condition-type field-names default-handler ¶

  Returns a signalling procedure with parameters field-names. When the signalling procedure is called it creates and signals a condition of type condition-type. If the condition isn’t handled (i.e. if no handler is invoked that causes an escape from the current continuation) the signalling procedure reduces to a call to default-handler with the condition as its argument.

  There are several standard procedures that are conventionally used for default-handler. If condition-type is a specialization of `condition-type:error`, default-handler should be the procedure
  `standard-error-handler`. If condition-type is a specialization of `condition-type:warning`, default-handler should be the procedure `standard-warning-handler`. If condition-type is a specialization of `condition-type:breakpoint`, default-handler should be the procedure `standard-breakpoint-handler`.

#### 16.5.2 Condition Abstraction

The condition data type is abstracted through a predicate `condition?` and a set of accessor procedures.

procedure: condition? object ¶

  Returns `#f` if and only if object is not a condition.

procedure: condition/type condition ¶

  Returns the condition type of which condition is an instance.

procedure: condition/error? condition ¶

  Returns `#t` if the condition is an instance of condition type `condition-type:error` or a specialization of it, `#f` otherwise.

procedure: condition/restarts condition ¶

  Returns the list of restarts specified when condition was created.

procedure: condition/continuation condition ¶

  Returns the continuation specified when condition was created. This is provided for inspecting the state of the system when the condition occurred, not for continuing or restarting the computation.

procedure: write-condition-report condition port ¶

  Writes a description of condition to port, using the reporter function from the condition type associated with condition. See also `condition/report-string`.

#### 16.5.3 Simple Operations on Condition Instances

The simple procedures described in this section are built on top of the more detailed abstraction of condition objects described above. While these procedures are sometimes easier to use, they are often less efficient.

procedure: make-condition condition-type continuation restarts field-plist ¶

  Create a new condition object as an instance of condition-type, associated with continuation. The continuation is provided for inspection purposes only, not for restarting the computation. The restarts argument is described in Restarts. The field-plist is an alternating list of field names and values for those fields, where the field names are those that would be returned by `(condition-type/field-names condition-type)`. It is used to provide values for fields in the condition object; fields with no value specified are set to `#f`. Once a condition object has been created there is no way to alter the values of these fields.

procedure: access-condition condition field-name ¶

  Returns the value stored in the field field-name within condition. Field-name must be one of the names returned by `(condition-type/field-names (condition/type condition))`. `access-condition` looks up the field-name at runtime, so it is more efficient to use `condition-accessor` to create an access function if the same field is to be extracted from several instances of the same condition type.

procedure: condition/report-string condition ¶

  Returns a string containing a report of the condition. This is generated by calling `write-condition-report` on condition and a string output port, and returning the output collected by the port as a string.

### 16.6 Condition Types

Each condition has a condition type object associated with it. These objects are used as a means of focusing on related classes of conditions, first by concentrating all of the information about a specific class of condition in a single place, and second by specifying an inheritance relationship between types. This inheritance relationship forms the taxonomic structure of the condition hierarchy (see Condition-Type Taxonomy).

The following procedures consititute the abstraction for condition types.

procedure: make-condition-type name generalization field-names reporter ¶

  Creates and returns a (new) condition type that is a specialization of generalization (if it is a condition type) or is the root of a new tree of condition types (if generalization is `#f`). For debugging purposes, the condition type has a name, and instances of this type contain storage for the fields specified by field-names (a list of symbols) in addition to the fields common to all conditions (type, continuation and restarts).

  Reporter is used to produce a description of a particular condition of this type. It may be a string describing the condition, a procedure of arity two (the first argument will be a condition of this type and the second a port) that will `write` the message to the given port, or `#f` to specify that the reporter should be taken from the condition type generalization (or produce an “undocumented condition of type …” message if generalization is `#f`). The conventions used to form descriptions are spelled out in Error Messages.

procedure: condition-type/error? condition-type ¶

  Returns `#t` if the condition-type is `condition-type:error` or a specialization of it, `#f` otherwise.

procedure: condition-type/field-names condition-type ¶

  Returns a list of all of the field names for a condition of type condition-type. This is the set union of the fields specified when this condition-type was created with the `condition-type/field-names` of the generalization of this condition-type.

procedure: condition-type/generalizations condition-type ¶

  Returns a list of all of the generalizations of condition-type. Notice that every condition type is considered a generalization of itself.

procedure: condition-type? object ¶

  Returns `#f` if and only if object is not a condition type.

### 16.7 Condition-Type Taxonomy

The MIT/GNU Scheme error system provides a rich set of predefined condition types. These are organized into a forest through taxonomic links providing the relationships for “specializes” and “generalizes”. The chart appearing below shows these relationships by indenting all the specializations of a given type relative to the type. Note that the variables that are bound to these condition types are prefixed by ‘`condition-type:`’; for example, the type appearing in the following table as ‘`simple-error`’ is stored in the variable `condition-type:simple-error`. Users are encouraged to add new condition types by creating specializations of existing ones.

Following the chart are detailed descriptions of the predefined condition types. Some of these types are marked as abstract types. Abstract types are not intended to be used directly as the type of a condition; they are to be used as generalizations of other types, and for binding condition handlers. Types that are not marked as abstract are concrete; they are intended to be explicitly used as a condition’s type.

    serious-condition
        error
            simple-error
            illegal-datum
                wrong-type-datum
                    wrong-type-argument
                    wrong-number-of-arguments
                datum-out-of-range
                    bad-range-argument
                inapplicable-object
            file-error
                file-operation-error
                derived-file-error
            port-error
                derived-port-error
            variable-error
                unbound-variable
                unassigned-variable
            arithmetic-error
                divide-by-zero
                floating-point-overflow
                floating-point-underflow
            control-error
                no-such-restart
            not-loading
            primitive-procedure-error
                system-call-error
    warning
        simple-warning
    simple-condition
    breakpoint

condition type: condition-type:serious-condition ¶

  This is an abstract type. All serious conditions that require some form of intervention should inherit from this type. In particular, all errors inherit from this type.

condition type: condition-type:error ¶

  This is an abstract type. All errors should inherit from this type.

condition type: condition-type:simple-error message irritants ¶

  This is the condition generated by the `error` procedure when its first argument is not a condition or condition type. The fields message and irritants are taken directly from the arguments to `error`; message contains an object (usually a string) and irritants contains a list of objects. The reporter for this type uses `format-error-message` to generate its output from message and irritants.

condition type: condition-type:illegal-datum datum ¶

  This is an abstract type. This type indicates the class of errors in which a program discovers an object that lacks specific required properties. Most commonly, the object is of the wrong type or is outside a specific range. The datum field contains the offending object.

condition type: condition-type:wrong-type-datum datum type ¶

  This type indicates the class of errors in which a program discovers an object that is of the wrong type. The type field contains a string describing the type that was expected, and the datum field contains the object that is of the wrong type.

    (error:wrong-type-datum 3.4 "integer")  error→
    ;The object 3.4 is not an integer.
    ;To continue, call RESTART with an option number:
    ; (RESTART 1) => Return to read-eval-print level 1.

procedure: error:wrong-type-datum datum type ¶

  This procedure signals a condition of type `condition-type:wrong-type-datum`. The datum and type fields of the condition are filled in from the corresponding arguments to the procedure.

condition type: condition-type:wrong-type-argument datum type operator operand ¶

  This type indicates that a procedure was passed an argument of the wrong type. The operator field contains the procedure (or a symbol naming the procedure), the operand field indicates the argument position that was involved (this field contains either a symbol, a non-negative integer, or `#f`), the type field contains a string describing the type that was expected, and the datum field contains the offending argument.

    (+ 'a 3)                                error→
    ;The object a, passed as the first argument to integer-add,
    ; is not the correct type.
    ;To continue, call RESTART with an option number:
    ; (RESTART 2) => Specify an argument to use in its place.
    ; (RESTART 1) => Return to read-eval-print level 1.

    (list-copy 3)
    ;The object 3, passed as an argument to list-copy, is not a list.
    ;To continue, call RESTART with an option number:
    ; (RESTART 1) => Return to read-eval-print level 1.

procedure: error:wrong-type-argument datum type operator ¶

  This procedure signals a condition of type `condition-type:wrong-type-argument`. The datum, type and operator fields of the condition are filled in from the corresponding arguments to the procedure; the operand field of the condition is set to `#f`.

condition type: condition-type:wrong-number-of-arguments datum type operands ¶

  This type indicates that a procedure was called with the wrong number of arguments. The datum field contains the procedure being called, the type field contains the number of arguments that the procedure accepts, and the operands field contains a list of the arguments that were passed to the procedure.

    (car 3 4)                               error→
    ;The procedure car has been called with 2 arguments;
    ; it requires exactly 1 argument.
    ;To continue, call RESTART with an option number:
    ; (RESTART 1) => Return to read-eval-print level 1.

procedure: error:wrong-number-of-arguments datum type operands ¶

  This procedure signals a condition of type `condition-type:wrong-number-of-arguments`. The datum, type and operands fields of the condition are filled in from the corresponding arguments to the procedure.

condition type: condition-type:datum-out-of-range datum ¶

  This type indicates the class of errors in which a program discovers an object that is of the correct type but is otherwise out of range. Most often, this type indicates that an index to some data structure is outside of the range of indices for that structure. The datum field contains the offending object.

    (error:datum-out-of-range 3)            error→
    ;The object 3 is not in the correct range.
    ;To continue, call RESTART with an option number:
    ; (RESTART 1) => Return to read-eval-print level 1.

procedure: error:datum-out-of-range datum ¶

  This procedure signals a condition of type `condition-type:datum-out-of-range`. The datum field of the condition is filled in from the corresponding argument to the procedure.

condition type: condition-type:bad-range-argument datum operator operand ¶

  This type indicates that a procedure was passed an argument that is of the correct type but is otherwise out of range. Most often, this type indicates that an index to some data structure is outside of the range of indices for that structure. The operator field contains the procedure (or a symbol naming the procedure), the operand field indicates the argument position that was involved (this field contains either a symbol, a non-negative integer, or `#f`), and the datum field is the offending argument.

    (string-ref "abc" 3)                    error→
    ;The object 3, passed as the second argument to string-ref,
    ; is not in the correct range.
    ;To continue, call RESTART with an option number:
    ; (RESTART 2) => Specify an argument to use in its place.
    ; (RESTART 1) => Return to read-eval-print level 1.

procedure: error:bad-range-argument datum operator ¶

  This procedure signals a condition of type `condition-type:bad-range-argument`. The datum and operator fields of the condition are filled in from the corresponding arguments to the procedure; the operand field of the condition is set to `#f`.

condition type: condition-type:inapplicable-object datum operands ¶

  This type indicates an error in which a program attempted to apply an object that is not a procedure. The object being applied is saved in the datum field, and the arguments being passed to the object are saved as a list in the operands field.

    (3 4)                                   error→
    ;The object 3 is not applicable.
    ;To continue, call RESTART with an option number:
    ; (RESTART 2) => Specify a procedure to use in its place.
    ; (RESTART 1) => Return to read-eval-print level 1.

condition type: condition-type:file-error filename ¶

  This is an abstract type. It indicates that an error associated with a file has occurred. For example, attempting to delete a nonexistent file will signal an error. The filename field contains a filename or pathname associated with the operation that failed.

condition type: condition-type:file-operation-error filename verb noun reason operator operands ¶

  This is the most common condition type for file system errors. The filename field contains the filename or pathname that was being operated on. The verb field contains a string which is the verb or verb phrase describing the operation being performed, and the noun field contains a string which is a noun or noun phrase describing the object being operated on. The reason field contains a string describing the error that occurred. The operator field contains the procedure performing the operation (or a symbol naming that procedure), and the operands field contains a list of the arguments that were passed to that procedure. For example, an attempt to delete a nonexistent file would have the following field values:

      filename        "/zu/cph/tmp/no-such-file"
      verb            "delete"
      noun            "file"
      reason          "no such file or directory"
      operator        file-remove
      operands        ("/zu/cph/tmp/no-such-file")

  and would generate a message like this:

      (delete-file "/zu/cph/tmp/no-such-file") error→
      ;Unable to delete file "/zu/cph/tmp/no-such-file" because:
      ; No such file or directory.
      ;To continue, call RESTART with an option number:
      ; (RESTART 3) => Try to delete the same file again.
      ; (RESTART 2) => Try to delete a different file.
      ; (RESTART 1) => Return to read-eval-print level 1.

procedure: error:file-operation index verb noun reason operator operands ¶

  This procedure signals a condition of type `condition-type:file-operation-error`. The fields of the condition are filled in from the corresponding arguments to the procedure, except that the filename is taken as the indexth element of operands.

condition type: condition-type:derived-file-error filename condition ¶

  This is another kind of file error, which is generated by obscure file-system errors that do not fit into the standard categories. The filename field contains the filename or pathname that was being operated on, and the condition field contains a condition describing the error in more detail. Usually the condition field contains a condition of type `condition-type:system-call-error`.

procedure: error:derived-file filename condition ¶

  This procedure signals a condition of type `condition-type:derived-file-error`. The filename and condition fields of the condition are filled in from the corresponding arguments to the procedure.

condition type: condition-type:port-error port ¶

  This is an abstract type. It indicates that an error associated with a I/O port has occurred. For example, writing output to a file port can signal an error if the disk containing the file is full; that error would be signalled as a port error. The port field contains the associated port.

condition type: condition-type:derived-port-error port condition ¶

  This is a concrete type that is signalled when port errors occur. The port field contains the port associated with the error, and the condition field contains a condition object that describes the error in more detail. Usually the condition field contains a condition of type `condition-type:system-call-error`.

procedure: error:derived-port port condition ¶

  This procedure signals a condition of type `condition-type:derived-port-error`. The port and condition fields of the condition are filled in from the corresponding arguments to the procedure.

condition type: condition-type:variable-error location environment ¶

  This is an abstract type. It indicates that an error associated with a variable has occurred. The location field contains the name of the variable, and the environment field contains the environment in which the variable was referenced.

condition type: condition-type:unbound-variable location environment ¶

  This type is generated when a program attempts to access or modify a variable that is not bound. The location field contains the name of the variable, and the environment field contains the environment in which the reference occurred.

    foo                                     error→
    ;Unbound variable: foo
    ;To continue, call RESTART with an option number:
    ; (RESTART 3) => Specify a value to use instead of foo.
    ; (RESTART 2) => Define foo to a given value.
    ; (RESTART 1) => Return to read-eval-print level 1.

condition type: condition-type:unassigned-variable location environment ¶

  This type is generated when a program attempts to access a variable that is not assigned. The location field contains the name of the variable, and the environment field contains the environment in which the reference occurred.

    foo                                     error→
    ;Unassigned variable: foo
    ;To continue, call RESTART with an option number:
    ; (RESTART 3) => Specify a value to use instead of foo.
    ; (RESTART 2) => Set foo to a given value.
    ; (RESTART 1) => Return to read-eval-print level 1.

condition type: condition-type:arithmetic-error operator operands ¶

  This is an abstract type. It indicates that a numerical operation was unable to complete because of an arithmetic error. (For example, division by zero.) The operator field contains the procedure that implements the operation (or a symbol naming the procedure), and the operands field contains a list of the arguments that were passed to the procedure.

condition type: condition-type:divide-by-zero operator operands ¶

  This type is generated when a program attempts to divide by zero. The operator field contains the procedure that implements the failing operation (or a symbol naming the procedure), and the operands field contains a list of the arguments that were passed to the procedure.

    (/ 1 0)
    ;Division by zero signalled by /.
    ;To continue, call RESTART with an option number:
    ; (RESTART 1) => Return to read-eval-print level 1.

procedure: error:divide-by-zero operator operands ¶

  This procedure signals a condition of type `condition-type:divide-by-zero`. The operator and operands fields of the condition are filled in from the corresponding arguments to the procedure.

condition type: condition-type:floating-point-overflow operator operands ¶

  This type is generated when a program performs an arithmetic operation that results in a floating-point overflow. The operator field contains the procedure that implements the operation (or a symbol naming the procedure), and the operands field contains a list of the arguments that were passed to the procedure.

condition type: condition-type:floating-point-underflow operator operands ¶

  This type is generated when a program performs an arithmetic operation that results in a floating-point underflow. The operator field contains the procedure that implements the operation (or a symbol naming the procedure), and the operands field contains a list of the arguments that were passed to the procedure.

condition type: condition-type:primitive-procedure-error operator operands ¶

  This is an abstract type. It indicates that an error was generated by a primitive procedure call. Primitive procedures are distinguished from ordinary procedures in that they are not written in Scheme but instead in the underlying language of the Scheme implementation. The operator field contains the procedure that implements the operation (or a symbol naming the procedure), and the operands field contains a list of the arguments that were passed to the procedure.

condition type: condition-type:system-call-error operator operands system-call error-type ¶

  This is the most common condition type generated by primitive procedures. A condition of this type indicates that the primitive made a system call to the operating system, and that the system call signalled an error. The system-call error is reflected back to Scheme as a condition of this type, except that many common system-call errors are automatically translated by the Scheme implementation into more useful forms; for example, a system-call error that occurs while trying to delete a file will be translated into a condition of type `condition-type:file-operation-error`. The operator field contains the procedure that implements the operation (or a symbol naming the procedure), and the operands field contains a list of the arguments that were passed to the procedure. The system-call and error-type fields contain symbols that describe the specific system call that was being made and the error that occurred, respectively; these symbols are completely operating-system dependent.

condition type: condition-type:control-error ¶

  This is an abstract type. It describes a class of errors relating to program control flow.

condition type: condition-type:no-such-restart name ¶

  This type indicates that a named restart was not active when it was expected to be. Conditions of this type are signalled by several procedures that look for particular named restarts, for example `muffle-warning`. The name field contains the name that was being searched for.

    (muffle-warning)                        error→
    ;The restart named muffle-warning is not bound.
    ;To continue, call RESTART with an option number:
    ; (RESTART 1) => Return to read-eval-print level 1.

procedure: error:no-such-restart name ¶

  This procedure signals a condition of type `condition-type:no-such-restart`. The name field of the condition is filled in from the corresponding argument to the procedure.

condition type: condition-type:not-loading ¶

  A condition of this type is generated when the procedure `current-load-pathname` is called from somewhere other than inside a file being loaded.

    (current-load-pathname)                 error→
    ;No file being loaded.
    ;To continue, call RESTART with an option number:
    ; (RESTART 1) => Return to read-eval-print level 1.

condition type: condition-type:warning ¶

  This is an abstract type. All warnings should inherit from this type. Warnings are a class of conditions that are usually handled by informing the user of the condition and proceeding the computation normally.

condition type: condition-type:simple-warning message irritants ¶

  This is the condition generated by the `warn` procedure. The fields message and irritants are taken directly from the arguments to `warn`; message contains an object (usually a string) and irritants contains a list of objects. The reporter for this type uses `format-error-message` to generate its output from message and irritants.

condition type: condition-type:simple-condition message irritants ¶

  This is an unspecialized condition that does not fall into any of the standard condition classes. The message field contains an object (usually a string) and irritants contains a list of objects. The reporter for this type uses `format-error-message` to generate its output from message and irritants.

condition type: condition-type:breakpoint environment message prompt ¶

  A condition of this type is generated by the breakpoint mechanism. The contents of its fields are beyond the scope of this document.

## 17 Graphics

MIT/GNU Scheme has a simple two-dimensional line-graphics interface that is suitable for many graphics applications. In particular it is often used for plotting data points from experiments. The interface is generic in that it can support different types of graphics devices in a uniform manner. At the present time only one type of graphics device is implemented on each operating system.

Procedures are available for drawing points, lines, and text; defining the coordinate system; clipping graphics output; controlling some of the drawing characteristics; and controlling the output buffer (for devices that perform buffering). Additionally, devices may support custom operations, such as control of colors.

There are some constraints on the arguments to the procedures described in this chapter. Any argument named graphics-device must be a graphics device object that was returned from a call to `make-graphics-device`. Any argument that is a coordinate must be either an exact integer or an inexact real.

### 17.1 Opening and Closing of Graphics Devices

procedure: graphics-type-available? graphics-device-type ¶

  This predicate returns `#t` if the graphics system named by the symbol graphics-device-type is implemented by the Scheme system. Otherwise it returns `#f`, in which case it is an error to attempt to make a graphics device using graphics-device-type.

procedure: enumerate-graphics-types ¶

  This procedure returns a list of symbols which are the names of all the graphics device types that are supported by the Scheme system. The result is useful in deciding what additional arguments to supply to `make-graphics-device`, as each device type typically has a unique way of specifying the initial size, shape and other attributes.

procedure: make-graphics-device graphics-device-type object … ¶

  This operation creates and returns a graphics device object. Graphics-device-type is a symbol naming a graphics device type, and both the number and the meaning of the remaining arguments is determined by that type (see the description of each device type for details); graphics-device-type must satisfy `graphics-type-available?`. Graphics-device-type may also be `#f`, in which case the graphics device type is chosen by the system from what is available. This allows completely portable graphics programs to be written provided no custom graphics operations are used. When graphics-device-type is `#f` no further arguments may be given; each graphics device type will use some “sensible” defaults. If more control is required then the program should use one of the two procedures above to dispatch on the available types.

  This procedure opens and initializes the device, which remains valid until explicitly closed by the procedure `graphics-close`. Depending on the implementation of the graphics device, if this object is reclaimed by the garbage collector, the graphics device may remain open or it may be automatically closed. While a graphics device remains open the resources associated with it are not released.

procedure: graphics-close graphics-device ¶

  Closes graphics-device, releasing its resources. Subsequently it is an error to use graphics-device.

### 17.2 Coordinates for Graphics

Each graphics device has two different coordinate systems associated with it: device coordinates and virtual coordinates. Device coordinates are generally defined by low-level characteristics of the device itself, and often cannot be changed. Most device coordinate systems are defined in terms of pixels, and usually the upper-left-hand corner is the origin of the coordinate system, with x coordinates increasing to the right and y coordinates increasing downwards.

In contrast, virtual coordinates are more flexible in the units employed, the position of the origin, and even the direction in which the coordinates increase. A virtual coordinate system is defined by assigning coordinates to the edges of a device. Because these edge coordinates are arbitrary real numbers, any Cartesian coordinate system can be defined.

All graphics procedures that use coordinates are defined on virtual coordinates. For example, to draw a line at a particular place on a device, the virtual coordinates for the endpoints of that line are given.

When a graphics device is initialized, its virtual coordinate system is reset so that the left edge corresponds to an x-coordinate of `-1`, the right edge to x-coordinate `1`, the bottom edge to y-coordinate `-1`, and the top edge to y-coordinate `1`.

procedure: graphics-device-coordinate-limits graphics-device ¶

  Returns (as multiple values) the device coordinate limits for graphics-device. The values, which are exact non-negative integers, are: x-left, y-bottom, x-right, and y-top.

procedure: graphics-coordinate-limits graphics-device ¶

  Returns (as multiple values) the virtual coordinate limits for graphics-device. The values, which are real numbers, are: x-left, y-bottom, x-right, and y-top.

procedure: graphics-set-coordinate-limits graphics-device x-left y-bottom x-right y-top ¶

  Changes the virtual coordinate limits of graphics-device to the given arguments. X-left, y-bottom, x-right, and y-top must be real numbers. Subsequent calls to `graphics-coordinate-limits` will return the new limits. This operation has no effect on the device’s displayed contents.

  Note: This operation usually resets the clip rectangle, although it is not guaranteed to do so. If a clip rectangle is in effect when this procedure is called, it is necessary to redefine the clip rectangle afterwards.

### 17.3 Drawing Graphics

The procedures in this section provide the basic drawing capabilities of Scheme’s graphics system.

procedure: graphics-clear graphics-device ¶

  Clears the display of graphics-device. Unaffected by the current drawing mode.

procedure: graphics-draw-point graphics-device x y ¶

  Draws a single point on graphics-device at the virtual coordinates given by x and y, using the current drawing mode.

procedure: graphics-erase-point graphics-device x y ¶

  Erases a single point on graphics-device at the virtual coordinates given by x and y. This procedure is unaffected by the current drawing mode.

This is equivalent to

    (lambda (device x y)
      (graphics-bind-drawing-mode device 0
        (lambda ()
          (graphics-draw-point device x y))))

procedure: graphics-draw-line graphics-device x-start y-start x-end y-end ¶

  X-start, y-start, x-end, and y-end must be real numbers. Draws a line on graphics-device that connects the points (x-start, y-start) and (x-end, y-end). The line is drawn using the current drawing mode and line style.

procedure: graphics-draw-text graphics-device x y string ¶

  Draws the characters of string at the point (x, y) on graphics-device, using the current drawing mode. The characteristics of the characters drawn are device-dependent, but all devices are initialized so that the characters are drawn upright, from left to right, with the leftmost edge of the leftmost character at x, and the baseline of the characters at y.

The following two procedures provide an alternate mechanism for drawing lines, which is more akin to using a plotter. They maintain a cursor, which can be positioned to a particular point and then dragged to another point, producing a line. Sequences of connected line segments can be drawn by dragging the cursor from point to point.

Many graphics operations have an unspecified effect on the cursor. The following exceptions are guaranteed to leave the cursor unaffected:

    graphics-device-coordinate-limits
    graphics-coordinate-limits
    graphics-enable-buffering
    graphics-disable-buffering
    graphics-flush
    graphics-bind-drawing-mode
    graphics-set-drawing-mode
    graphics-bind-line-style
    graphics-set-line-style

The initial state of the cursor is unspecified.

procedure: graphics-move-cursor graphics-device x y ¶

  Moves the cursor for graphics-device to the point (x, y). The contents of the device’s display are unchanged.

procedure: graphics-drag-cursor graphics-device x y ¶

  Draws a line from graphics-device’s cursor to the point (x, y), simultaneously moving the cursor to that point. The line is drawn using the current drawing mode and line style.

### 17.4 Characteristics of Graphics Output

Two characteristics of graphics output are so useful that they are supported uniformly by all graphics devices: drawing mode and line style. A third characteristic, color, is equally useful (if not more so), but implementation restrictions prohibit a uniform interface.

The drawing mode, an exact integer in the range `0` to `15` inclusive, determines how the figure being drawn is combined with the background over which it is drawn to generate the final result. Initially the drawing mode is set to “source”, so that the new output overwrites whatever appears in that place. Useful alternative drawing modes can, for example, erase what was already there, or invert it.

Altogether 16 boolean operations are available for combining the source (what is being drawn) and the destination (what is being drawn over). The source and destination are combined by the device on a pixel-by-pixel basis as follows:

    Mode    Meaning
    ----    -------
    0       ZERO [erase; use background color]
    1       source AND destination
    2       source AND (NOT destination)
    3       source
    4       (NOT source) AND destination
    5       destination
    6       source XOR destination
    7       source OR destination
    8       NOT (source OR destination)
    9       NOT (source XOR destination)
    10      NOT destination
    11      source OR (NOT destination)
    12      NOT source
    13      (NOT source) OR destination
    14      (NOT source) OR (NOT destination)
    15      ONE [use foreground color]

The line style, an exact integer in the range `0` to `7` inclusive, determines which parts of a line are drawn in the foreground color, and which in the background color. The default line style, “solid”, draws the entire line in the foreground color. Alternatively, the “dash” style alternates between foreground and background colors to generate a dashed line. This capability is useful for plotting several things on the same graph.

Here is a table showing the name and approximate pattern of the different styles. A ‘`1`’ in the pattern represents a foreground pixel, while a ‘`-`’ represents a background pixel. Note that the precise output for each style will vary from device to device. The only style that is guaranteed to be the same for every device is “solid”.

    Style   Name                    Pattern
    -----   -------                 -------
    0       solid                   1111111111111111
    1       dash                    11111111--------
    2       dot                     1-1-1-1-1-1-1-1-
    3       dash dot                1111111111111-1-
    4       dash dot dot            11111111111-1-1-
    5       long dash               11111111111-----
    6       center dash             111111111111-11-
    7       center dash dash        111111111-11-11-

procedure: graphics-bind-drawing-mode graphics-device drawing-mode thunk ¶
procedure: graphics-bind-line-style graphics-device line-style thunk ¶

  These procedures bind the drawing mode or line style, respectively, of graphics-device, invoke the procedure thunk with no arguments, then undo the binding when thunk returns. The value of each procedure is the value returned by thunk. Graphics operations performed during thunk’s dynamic extent will see the newly bound mode or style as current.

procedure: graphics-set-drawing-mode graphics-device drawing-mode ¶
procedure: graphics-set-line-style graphics-device line-style ¶

  These procedures change the drawing mode or line style, respectively, of graphics-device. The mode or style will remain in effect until subsequent changes or bindings.

### 17.5 Buffering of Graphics Output

To improve performance of graphics output, most graphics devices provide some form of buffering. By default, Scheme’s graphics procedures flush this buffer after every drawing operation. The procedures in this section allow the user to control the flushing of the output buffer.

procedure: graphics-enable-buffering graphics-device ¶

  Enables buffering for graphics-device. In other words, after this procedure is called, graphics operations are permitted to buffer their drawing requests. This usually means that the drawing is delayed until the buffer is flushed explicitly by the user, or until it fills up and is flushed by the system.

procedure: graphics-disable-buffering graphics-device ¶

  Disables buffering for graphics-device. By default, all graphics devices are initialized with buffering disabled. After this procedure is called, all drawing operations perform their output immediately, before returning.

  Note: `graphics-disable-buffering` flushes the output buffer if necessary.

procedure: graphics-flush graphics-device ¶

  Flushes the graphics output buffer for graphics-device. This operation has no effect for devices that do not support buffering, or if buffering is disabled for the device.

### 17.6 Clipping of Graphics Output

Scheme provides a rudimentary mechanism for restricting graphics output to a given rectangular subsection of a graphics device. By default, graphics output that is drawn anywhere within the device’s virtual coordinate limits will appear on the device. When a clip rectangle is specified, however, output that would have appeared outside the clip rectangle is not drawn.

Note that changing the virtual coordinate limits for a device will usually reset the clip rectangle for that device, as will any operation that affects the size of the device (such as a window resizing operation). However, programs should not depend on this.

procedure: graphics-set-clip-rectangle graphics-device x-left y-bottom x-right y-top ¶

  Specifies the clip rectangle for graphics-device in virtual coordinates. X-left, y-bottom, x-right, and y-top must be real numbers. Subsequent graphics output is clipped to the intersection of this rectangle and the device’s virtual coordinate limits.

procedure: graphics-reset-clip-rectangle graphics-device ¶

  Eliminates the clip rectangle for graphics-device. Subsequent graphics output is clipped to the virtual coordinate limits of the device.

### 17.7 Custom Graphics Operations

In addition to the standard operations, a graphics device may support custom operations. For example, most devices have custom operations to control color. `graphics-operation` is used to invoke custom operations.

procedure: graphics-operation graphics-device name object … ¶

  Invokes the graphics operation on graphics-device whose name is the symbol name, passing it the remaining arguments. This procedure can be used to invoke the standard operations, as well as custom operations that are specific to a particular graphics device type. The names of the standard graphics operations are formed by removing the `graphics-` prefix from the corresponding procedure. For example, the following are equivalent:

      (graphics-draw-point device x y)
      (graphics-operation device 'draw-point x y)

  For information on the custom operations for a particular device, see the documentation for its type.

### 17.8 Images

Some graphics device types support images, which are rectangular pieces of picture that may be drawn into a graphics device. Images are often called something else in the host graphics system, such as bitmaps or pixmaps. The operations supported vary between devices, so look under the different device types to see what operations are available. All devices that support images support the following operations.

operation on graphics-device: create-image width height ¶

  Images are created using the `create-image` graphics operation, specifying the width and height of the image in device coordinates (pixels).

      (graphics-operation device 'create-image 200 100)

  The initial contents of an image are unspecified.

  `create-image` is a graphics operation rather than a procedure because the kind of image returned depends on the kind of graphics device used and the options specified in its creation. The image may be used freely with other graphics devices created with the same attributes, but the effects of using an image with a graphics device with different attributes (for example, different colors) is undefined. Under X, the image is display dependent.

operation on graphics-device: draw-image x y image ¶

  The image is copied into the graphics device at the specified position.

operation on graphics-device: draw-subimage x y image im-x im-y w h ¶

  Part of the image is copied into the graphics device at the specified (x, y) position. The part of the image that is copied is the rectangular region at im-x and im-y and of width w and height h. These four numbers are given in device coordinates (pixels).

procedure: image? object ¶

  Returns `#t` if object is an image, otherwise returns `#f`.

procedure: image/destroy image ¶

  This procedure destroys image, returning storage to the system. Programs should destroy images after they have been used because even modest images may use large amounts of memory. Images are reclaimed by the garbage collector, but they may be implemented using memory outside of Scheme’s heap. If an image is reclaimed before being destroyed, the implementation might not deallocate that non-heap memory, which can cause a subsequent call to `create-image` to fail because it is unable to allocate enough memory.

procedure: image/height image ¶

  Returns the height of the image in device coordinates.

procedure: image/width image ¶

  Returns the width of the image in device coordinates.

procedure: image/fill-from-byte-vector image bytes ¶

  The contents of image are set in a device-dependent way, using one byte per pixel from bytes (a string). Pixels are filled row by row from the top of the image to the bottom, with each row being filled from left to right. There must be at least `(* (image/height image) (image/width image))` bytes in bytes.

### 17.9 X Graphics

MIT/GNU Scheme supports graphics in the X window system (version 11). Arbitrary numbers of displays may be opened, and arbitrary numbers of graphics windows may be created for each display. A variety of operations is available to manipulate various aspects of the windows, to control their size, position, colors, and mapping. The X graphics device type supports images, which are implemented as Xlib `XImage` objects. X display, window, and image objects are automatically closed if they are reclaimed by the garbage collector.

#### 17.9.1 X Graphics Type

A graphics device for X windows is created by passing the symbol `x` as the graphics device type name to `make-graphics-device`:

    (make-graphics-device 'x #!optional display geometry suppress-map?)

where display is either a display object, `#f`, or a string; geometry is either `#f` or a string; and suppress-map? is a boolean or a vector (see below). A new window is created on the appropriate display, and a graphics device representing that window is returned.

Display specifies which X display the window is to be opened on; if it is `#f` or a string, it is passed as an argument to `x-open-display`, and the value returned by that procedure is used in place of the original argument. Geometry is an X geometry string, or `#f` which means to use the default geometry (which is specified as a resource).

Suppress-map?, if given, may take two forms. First, it may be a boolean: if `#f` (the default), the window is automatically mapped after it is created; otherwise, `#t` means to suppress this automatic mapping. The second form is a vector of three elements. The first element is a boolean with the same meaning as the boolean form of suppress-map?. The second element is a string, which specifies an alternative resource name to be used for looking up the window’s resources. The third element is also a string, which specifies a class name for looking up the window’s resources. The default value for suppress-map? is `#f`.

The default resource and class names are `"schemeGraphics"` and `"SchemeGraphics"` respectively.

The window is initialized using the resource and class names specified by suppress-map?, and is sensitive to the following resource properties:

    Property        Class           Default
    --------        -----           -------
    geometry        Geometry        512x384+0+0
    font            Font            fixed
    borderWidth     BorderWidth     2
    internalBorder  BorderWidth     [border width]
    background      Background      white
    foreground      Foreground      black
    borderColor     BorderColor     [foreground color]
    cursorColor     Foreground      [foreground color]
    pointerColor    Foreground      [foreground color]

The window is created with a `backing_store` attribute of `Always`. The window’s name and icon name are initialized to `"scheme-graphics"`.

#### 17.9.2 Utilities for X Graphics

procedure: x-graphics/open-display display-name ¶

  Opens a connection to the display whose name is display-name, returning a display object. If unable to open a connection, `#f` is returned. Display-name is normally a string, which is an X display name in the usual form; however, `#f` is also allowed, meaning to use the value of the unix environment variable `DISPLAY`.

procedure: x-graphics/close-display display ¶

  Closes display; after calling this procedure, it is an error to use display for any purpose. Any windows that were previously opened on display are destroyed and their resources returned to the operating system.

procedure: x-close-all-displays ¶

  Closes all open connections to X displays. Equivalent to calling `x-close-display` on all open displays.

procedure: x-geometry-string x y width height ¶

  This procedure creates and returns a standard X geometry string from the given arguments. X and y must be either exact integers or `#f`, while width and height must be either exact non-negative integers or `#f`. Usually either x and y are both specified or both `#f`; similarly for width and height. If only one of the elements of such a pair is specified, it is ignored.

  Examples:

      (x-geometry-string #f #f 100 200) ⇒ "100x200"
      (x-geometry-string 2 -3 100 200) ⇒ "100x200+2-3"
      (x-geometry-string 2 -3 #f #f) ⇒ "+2-3"

  Note that the x and y arguments cannot distinguish between `+0` and `-0`, even though these have different meanings in X. If either of those arguments is `0`, it means `+0` in X terminology. If you need to distinguish these two cases you must create your own geometry string using Scheme’s string and number primitives.

#### 17.9.3 Custom Operations on X Graphics Devices

Custom operations are invoked using the procedure `graphics-operation`. For example,

    (graphics-operation device 'set-foreground-color "blue")

operation on x-graphics-device: set-background-color color-name ¶
operation on x-graphics-device: set-foreground-color color-name ¶
operation on x-graphics-device: set-border-color color-name ¶
operation on x-graphics-device: set-mouse-color color-name ¶

  These operations change the colors associated with a window. Color-name must be a string, which is the X server’s name for the desired color. `set-border-color` and `set-mouse-color` immediately change the border and mouse-cursor colors. `set-background-color` and `set-foreground-color` change the colors to be used when drawing, but have no effect on anything drawn prior to their invocation. Because changing the background color affects the entire window, we recommend calling `graphics-clear` on the window’s device afterwards. Color names include both mnemonic names, like `"red"`, and intensity names specified in the `"#rrggbb"` notation.

operation on x-graphics-device: draw-arc x y radius-x radius-y angle-start angle-sweep fill? ¶

  Operation `draw-arc` draws or fills an arc. An arc is a segment of a circle, which may have been stretched along the x- or y- axis to form an ellipse.

  The parameters x, y, radius-x and radius-y describe the circle and angle-start and angle-sweep choose which part of the circle is drawn. The arc is drawn on the graphics device with the center of the circle at the virtual coordinates given by x and y. radius-x and radius-y determine the size of the circle in virtual coordinate units.

  The parameter angle-start determines where the arc starts. It is measured in degrees in an anti-clockwise direction, starting at 3 o’clock. angle-sweep determines how much of the circle is drawn. It too is measured anti-clockwise in degrees. A negative value means the measurement is in a clockwise direction.

  Note that the angles are determined on a unit circle before it is stretched into an ellipse, so the actual angles that you will see on the computer screen depends on all of: radius-x and radius-y, the window size, and the virtual coordinates.

  If fill? is `#f` then just the segment of the circle is drawn, otherwise the arc is filled in a pie-slice fashion.

  This draws a quarter circle pie slice, standing on its point, with point at virtual coordinates (3,5):

      (graphics-operation g 'draw-arc 3 5 .5 .5 45 90 #t)

operation on x-graphics-device: draw-circle x y radius ¶
operation on x-graphics-device: fill-circle x y radius ¶

  These operations draw a circle (outline) or a filled circle (solid) at on the graphics device at the virtual coordinates given by x and y. These operations could be implemented trivially interms of the `draw-arc` operation.

operation on x-graphics-device: set-border-width width ¶
operation on x-graphics-device: set-internal-border-width width ¶

  These operations change the external and internal border widths of a window. Width must be an exact non-negative integer, specified in pixels. The change takes place immediately. Note that changing the internal border width can cause displayed graphics to be garbled; we recommend calling `graphics-clear` on the window’s device after doing so.

operation on x-graphics-device: set-font font-name ¶

  Changes the font used when drawing text in a window. Font-name must be a string that is a font name known to the X server. This operation does not affect text drawn prior to its invocation.

operation on x-graphics-device: set-mouse-shape shape-number ¶

  Changes the shape of the mouse cursor. Shape-number is an exact non-negative integer that is used as an index into the mouse-shape font; when multiplied by 2 this number corresponds to an index in the file
  `/usr/include/X11/cursorfont.h`.

operation on x-graphics-device: map-window ¶
operation on x-graphics-device: withdraw-window ¶

  These operations control the mapping of windows. They correspond directly to Xlib’s `XMapWindow` and `XWithdrawWindow`.

operation on x-graphics-device: resize-window width height ¶

  Changes the size of a window. Width and height must be exact non-negative integers. The operation corresponds directly to Xlib’s `XResizeWindow`.

  This operation resets the virtual coordinate system and the clip rectangle.

operation on x-graphics-device: move-window x y ¶

  Changes the position of a window on the display. X and y must be exact integers. The operation corresponds directly to Xlib’s `XMoveWindow`. Note that the coordinates x and y do not take the external border into account, and therefore will not position the window as you might like. The only reliable way to position a window is to ask a window manager to do it for you.

operation on x-graphics-device: get-default resource property ¶

  This operation corresponds directly to Xlib’s `XGetDefault`. Resource and property must be strings. The operation returns the character string corresponding to the association of resource and property; if no such association exists, `#f` is returned.

operation on x-graphics-device: copy-area source-x-left source-y-top width height destination-x-left destination-y-top ¶

  This operation copies the contents of the rectangle specified by source-x-left, source-y-top, width, and height to the rectangle of the same dimensions at destination-x-left and destination-y-top.

operation on x-graphics-device: font-structure font-name ¶

  Returns a Scheme equivalent of the X font structure for the font named font-name. If the string font-name does not name a font known to the X server, or names a 16-bit font, `#f` is returned.

procedure: x-font-structure/name font-structure ¶
procedure: x-font-structure/direction font-structure ¶
procedure: x-font-structure/all-chars-exist font-structure ¶
procedure: x-font-structure/default-char font-structure ¶
procedure: x-font-structure/min-bounds font-structure ¶
procedure: x-font-structure/max-bounds font-structure ¶
procedure: x-font-structure/start-index font-structure ¶
procedure: x-font-structure/character-bounds font-structure ¶
procedure: x-font-structure/max-ascent font-structure ¶
procedure: x-font-structure/max-descent font-structure ¶

  These procedures extract the components of the font description structure returned by the X graphics operation `font-structure`. A more complete description of these components appears in documentation of the `XLoadQueryFont` Xlib call. `start-index` is the index of the first character available in the font. The `min-bounds` and `max-bounds` components are structures of type `x-character-bounds`, and the `character-bounds` component is a vector of the same type.

procedure: x-character-bounds/lbearing character-bounds ¶
procedure: x-character-bounds/rbearing character-bounds ¶
procedure: x-character-bounds/width character-bounds ¶
procedure: x-character-bounds/ascent character-bounds ¶
procedure: x-character-bounds/descent character-bounds ¶

  These procedures extract components of objects of type `x-character-bounds`. A more complete description of them appears in documentation of the
  `XLoadQueryFont` Xlib call.

### 17.10 Win32 Graphics

MIT/GNU Scheme supports graphics on Microsoft Windows 95, Windows 98, and Windows NT. In addition to the usual operations, there are operations to control the size, position and colors of a graphics window. Win32 devices support images, which are implemented as device independent bitmaps (DIBs).

The Win32 graphics device type is implemented as a top level window. `graphics-enable-buffering` is implemented and gives a 2x to 4x speedup on many graphics operations. As a convenience, when buffering is enabled clicking on the graphics window’s title bar effects a `graphics-flush` operation. The user has the benefit of the increased performance and the ability to view the progress in drawing at the click of a mouse button.

#### 17.10.1 Win32 Graphics Type

Win32 graphics devices are created by specifying the symbol `win32` as the graphics-device-type argument to `make-graphics-device`. The Win32 graphics device type is implemented as a top-level window and supports color drawing in addition to the standard Scheme graphics operations.

Graphics devices are opened as follows:

    (make-graphics-device 'win32 #!optional width height palette)

where width and height specify the size, in pixels, of the drawing area in the graphics window (i.e. excluding the frame). Palette determines the colors available for drawing in the window.

When a color is specified for drawing, the nearest color available in the palette is used. Permitted values for palette are

`'grayscale`

  The window allocates colors from a grayscale palette of approximately 236 shades of gray.

`'grayscale-128`

  The window allocates colors from a grayscale palette of 128 shades of gray.

`'standard`

  The standard palette has good selection of colors and grays.

`#f` or `'system`

  The colors available are those in the system palette. There are usually 16 to 20 colors in the system palette and these are usually sufficent for simple applications like line drawings and x-vs-y graphs of mathematical functions. Drawing with the system palette can be more efficient.

If palette is not specified then the `standard` palette is used.

#### 17.10.2 Custom Operations for Win32 Graphics

Custom operations are invoked using the procedure `graphics-operation`. For example,

    (graphics-operation device 'set-foreground-color "blue")

operation on win32-graphics-device: set-background-color color-name ¶
operation on win32-graphics-device: set-foreground-color color-name ¶

  These operations change the colors associated with a window. Color-name must be of one of the valid color specification forms listed below. `set-background-color` and `set-foreground-color` change the colors to be used when drawing, but have no effect on anything drawn prior to their invocation. Because changing the background color affects the entire window, we recommend calling `graphics-clear` on the window’s device afterwards.

  The foreground color affects the drawing of text, points, lines, ellipses and filled polygons.

  Colors are specified in one of three ways:

  An integer

    This is the Win32 internal RGB value.

  By name

    A limited number of names are understood by the system. Names are strings, e.g. `"red"`, `"blue"`, `"black"`. More names can be registered with the `define-color` operation.

  RGB (Red-Green-Blue) triples

    A triple is either a vector or list of three integers in the range 0–255 inclusive which specify the intensity of the red, green and blue components of the color. Thus `#(0 0 0)` is black, `(0 0 128)` is dark blue and `#(255 255 255)` is white.

  If the color is not available in the graphics device then the nearest available color is used instead.

operation on win32-graphics-device: define-color name spec ¶

  Define the string name to be the color specified by spec. Spec may be any acceptable color specification. Note that the color names defined this way are available to any Win32 graphics device, and the names do not have to be defined for each device.

  Color names defined by this interface may also be used when setting the colors of the Scheme console window, or the colors of Edwin editor windows.

operation on win32-graphics-device: find-color name ¶

  Looks up a color previously defined by `define-color`. This returns the color in its most efficient form for operations `set-foreground-color` or `set-background-color`.

operation on win32-graphics-device: draw-ellipse left top right bottom ¶

  Draw an ellipse. Left, top, right and bottom indicate the coordinates of the bounding rectangle of the ellipse. Circles are merely ellipses with equal width and height. Note that the bounding rectangle has horizontal and vertical sides. Ellipses with rotated axes cannot be drawn. The rectangle applies to the center of the line used to draw the ellipse; if the line width has been set to greater than 1 then the ellipse will spill outside the bounding rectange by half of the line width.

operation on win32-graphics-device: fill-polygon points ¶

  Draws a filled polygon using the current foreground color. Points is a vector of real numbers. The numbers are in the order x1 y1 x2 y2 … xn yn. For example,

      (graphics-operation device 'fill-polygon #(0 0 0 1 1 0))

  draws a solid triangular region between the points (0, 0), (0, 1) and (1, 0).

operation on win32-graphics-device: load-bitmap pathname ¶

  The graphics device contents and size are initialized from the windows bitmap file specified by pathname. If no file type is supplied then a `".BMP"` extension is added. If a clip rectangle is in effect when this procedure is called, it is necessary to redefine the clip rectangle afterwards.

operation on win32-graphics-device: save-bitmap pathname ¶

  The graphics device contents are saved as a bitmap to the file specified by pathname. If no file type is supplied then a `".BMP"` extension is added. The saved bitmap may be incorporated into documents or printed.

operation on win32-graphics-device: move-window x y ¶

  The graphics device window is moved to the screen position specified by x and y.

operation on win32-graphics-device: resize-window width height ¶

  The graphics device window is resized to the specified width and height in device coordinates (pixels). If a clip rectangle is in effect when this procedure is called, it is necessary to redefine the clip rectangle afterwards.

operation on win32-graphics-device: set-line-width width ¶

  This operation sets the line width for future drawing of lines, points and ellipses. It does not affect existing lines and has no effect on filled polygons. The line width is specified in device units. The default and initial value of this parameter is 1 pixel.

operation on win32-graphics-device: set-window-name name ¶

  This sets the window title to the string name. The window is given the name `"Scheme Graphics"` at creation.

operation on win32-graphics-device: set-font handle ¶

  Sets the font for drawing text. Currently not well supported. If you can get a Win32 font handle it can be used here.

operation on win32-graphics-device: copy-area source-x-left source-y-top width height destination-x-left destination-y-top ¶

  This operation copies the contents of the rectangle specified by source-x-left, source-y-top, width, and height to the rectangle of the same dimensions at destination-x-left and destination-y-top.

## 18 Win32 Package Reference

### 18.1 Overview

The Win32 implementation is still in a state of development. It is expected that changes will be necessary when MIT/GNU Scheme is ported to Windows NT on the DEC Alpha architecture. In particular, the current system is not arranged in a way that adequately distinguishes between issues that are a consequence of the NT operating system and those which are a consequence of the Intel x86 architecture.

Thus this documentation is not definitive, it merely outlines how the current system works. Parts of the system will change and any project implemented using the win32 system must plan for a re-implementation stage.

The Win32 implementation has several components:

- Special microcode primitives.
- A foreign function interface (FFI) for calling procedures in dynamically linked libraries (DLLs).
- An interface for Edwin.
- The Win32 package provides support for using the features of the Windows 3.1 and Windows NT 3.1 environments.
- Device Independent Bitmap utilities. These are used by the win32 Scheme Graphics implementation. (The Scheme Graphics implementation is described in the Reference Manual).

Note that all the names in the Win32 support are part of the `win32` package. The names are bound in the `(win32)` environment, and do not appear as bindings in the user or root environments. An effect of this is that it is far easier to develop Win32 software in the `(win32)` package environment or a child environment.

### 18.2 Foreign Function Interface

The Win32 foreign function interface (FFI) is a primitive and fairly simple system for calling procedures written in C in a dynamically linked library (DLL). Both user’s procedures from a custom DLL and system procedures (e.g. MessageBox) are called using the same mechanism.

Warning: The FFI as it stands has several flaws which make it difficult to use reliably. It is expected that both the interface to and the mechanisms used by the FFI will be changed in the future. We provide it, and this documentation, only to give people an early start in accessing some of the features of Win32 from Scheme. Should you use it in an experiment we welcome any feedback.

The FFI is designed for calling C procedures that use C data types rather than Scheme data objects. Thus it is not possible to write and call a C procedure that returns, for example, a Scheme list. The object returned will always be an integer (which may represent the address of a C data structure).

Warning: It is extremely dangerous to try to pass Scheme callback procedures to C procedures. It is only possible by passing integer ‘handles’ rather than the actual procedures, and even so, if a garbage collection occurs during the execution of the callback procedure objects in Scheme’s heap will have moved. Thus in a foreign procedure that has a callback and a string, after calling the callback the string value may no longer be valid. Playing this game requires a profound knowledge of the implementation.

The interface to the FFI has two main components: a language for declaring the types of values passed to and returned from the foreign procedures and a form for declaring foreign procedures.

#### 18.2.1 Windows Types

Foreign types are designed to represent a correspondence between a Scheme data type that is used to represent an object within the Scheme world and a C data type that represents the data object in the C world. Thus we cannot manipulate true C objects in Scheme, nor can we manipulate Scheme objects in C.

Each foreign type has four aspects that together ensure that the correspondence between the Scheme and C objects is maintained. These aspects are all encoded as procedures that either check for validity or convert between representations. Thus a foreign type is not a declarative type so much as a procedural description of how to pass the type. The underlying foreign procedure call mechanism can pass integers and vector-like Scheme objects, and returns integer values. All other objects must be translated into integers or some other basic type, and must be recovered from integers.

The aspects are:

check

  A predicate that returns `#t` if the argument is of an acceptable Scheme type, otherwise returns `#f`. The check procedure is used for type-checking.

convert

  A procedure of one argument which returns a Scheme object of one of the basic types. It is used to convert an object into a ‘simpler’ object that will eventually be converted into a C object. The legal simpler objects are integers and strings.

return-convert

  A procedure of one argument that, given an integer, returns a Scheme object of a type satisfying check. Its purpose is to convert the result returned by the foreign procedure into a Scheme value.

revert

  Some C procedures modify one or more of their arguments. These arguments are passed by reference, i.e. as a pointer to their address. Since a Scheme object might have a different memory layout and storage conventions, it must be passed by copy-in and copy-out rather than by reference. Revert is a procedure of two parameters, the original object passed and the result of convert on that object. Revert may then inspect the converted object and copy back the changes to the original.

special form: define-windows-type name check convert return revert ¶
special form: define-similar-windows-type name model [check [convert [return [revert]]]] ¶

  Both forms define a windows type. The first form defines a type in terms of its aspects as described above. The second defines the type as being like another type, except for certain aspects, which are redefined. Name is the name of the type. Model is the name of a type. Check, convert, return and revert are procedures or the value `#f`. A `#f` means use the default value, which in the second form means use the definition provided for model. The defaults are

  check

    `(lambda (x) #t)`, i.e. unchecked.

  convert

    `(lambda (x) x)`, i.e. no translation performed.

  return

    `(lambda (x) x)`, i.e. no translation performed.

  revert

    `(lambda (x y) unspecific)`, i.e. no update performed

  The `unchecked` windows type (see below) is defined as:

      (define-windows-type unchecked #f #f #f #f)

  Windows types are not first class values, so they cannot be stored in variables or defined using `define`:

      (define my-type unchecked)            error→  Unbound variable
      (define-similar-windows-type my-type unchecked)
                                            ;; the correct way

  Scheme characters must be converted to integers. This is accomplished as follows:

      (define-windows-type char
         char?          ; check
         char->integer  ; convert
         integer->char  ; convert return value
         #f             ; cannot be passed by reference
      )

windows type: unchecked ¶

  The type which is not checked and undergoes only the basic conversion from a Scheme integer to a C integer or from a Scheme string to a C pointer to the first byte of the string. Returned `unchecked` values are returned as integers.

windows type: bool ¶

  Scheme booleans are analogous to C integers `0` and `1`. Windows type `bool` have been defined as:

      (define-windows-type bool
         boolean?
         (lambda (x) (if x 1 0))
         (lambda (x) (if (eq? x 0) #f #t))
         #f)

windows type: char ¶

  Scheme characters are converted into C objects of type `char`, which are indistinguishable from small integers.

windows type: int ¶
windows type: uint ¶
windows type: long ¶
windows type: ulong ¶
windows type: short ¶
windows type: ushort ¶
windows type: word ¶
windows type: byte ¶

  Various integer types that are passed without conversion.

windows type: string ¶

  A string that is passed as a C pointer of type `char*` to the first character in the string.

windows type: char* ¶

  A string or `#f`. The string is passed as a pointer to characters. The string is correctly null-terminated. `#f` is passed as the null pointer. This is an example where there is a more complex mapping between C objects and Scheme objects. C’s `char*` type is represented as one of two Scheme types depending on its value. This allows us us to distinguish between the C string (pointer) that points to the empty sequence of characters and the null pointer (which doesnt point anywhere).

windows type: handle ¶
windows type: hbitmap ¶
windows type: hbrush ¶
windows type: hcursor ¶
windows type: hdc ¶
windows type: hicon ¶
windows type: hinstance ¶
windows type: hmenu ¶
windows type: hpalette ¶
windows type: hpen ¶
windows type: hrgn ¶
windows type: hwnd ¶

  Various kinds of Win32 handle. These names correspond to the same, but all uppercase, names in the Windows C language header files. Win32 API calls are the source of values of this type and the values are meaningless except as arguments to other Win32 API calls. Currently these values are represented as integers but we expect that Win32 handles will in future be represented by allocated Scheme objects (e.g. records) that will allow predicates (e.g. `hmenu?`) and sensible interlocking with the garbage collector to free the programmer of the current tedious allocation and deallocation of handles.

windows type: resource-id ¶

  A Windows resource identifier is either a small integer or a string. In C, this distinction is possible because pointers look like larger integers, so a machine word representing a small integer can be distinguished from a machine word that is a pointer to the text of the name of the resource.

#### 18.2.2 Windows Foreign Procedures

Foreign procedures are declared as callable entry-points in a module, usually a dynamically linked library (DLL).

procedure: find-module name ¶

  Returns a module suitable for use in creating procedures with `windows-procedure`. Name is a string which is the name of a DLL file. Internally, `find-module` uses the `LoadLibrary` Win32 API, so name should conform to the specifications for this call. Name should be either a full path name of a DLL, or the name of a DLL that resides in the same directory as the Scheme binary `SCHEME.EXE` or in the system directory.

  The module returned is a description for the DLL, and the DLL need not necessarily be linked at or immediately after this call. DLL modules are linked on need and unlinked before Scheme exits and when there are no remaining references to entry points after a garbage-collection. This behavior ensures that the Scheme system can run when a DLL is absent, provided the DLL is not actually used (i.e. no attempt is made to call a procedure in the DLL).

variable: gdi32.dll ¶

  This variable is bound to the module describing the `GDI32.DLL` library, which contains the Win32 API graphics calls, e.g. `LineTo`.

variable: kernel32.dll ¶

  This variable is bound to the module describing the `KERNEL32.DLL` library.

variable: user32.dll ¶

  This variable is bound to the module describing the `USER32.DLL` library. This module contains many useful Win32 API procedures, like `MessageBox` and `SetWindowText`.

special form: windows-procedure (name (parameter type) …) return-type module entry-name [options] ¶

  This form creates a procedure, and could be thought of as “foreign-named-lambda”. The form creates a Scheme procedure that calls the C procedure identified by the exported entry point entry-name in the module identified by the value of module. Both entry-name and module are evaluated at procedure creation time, so either may be expression. Entry-name must evaluate to a string and module must evaluate to a module as returned by `find-module`. These are the only parts of the form that are evaluated at procedure creation time.

  Name is the name of the procedure and is for documentation purposes only. This form does not define a procedure called name. It is more like `lambda`. The name might be used for debugging and pretty-printing.

  A windows procedure has a fixed number of parameters (i.e. no ‘rest’ parameters or ‘varargs’), each of which is named and associated with a windows type type. Both the name parameter and the windows type type must be symbols and are not evaluated. The procedure returns a value of the windows type return-type.

  The following example creates a procedure that takes a window handle (`hwnd`) and a string and returns a boolean (`bool`) result. The procedure does this by calling the `SetWindowText` entry in the module that is the value of the variable `user32.dll`. The variable `set-window-title` is defined to have this procedure as it’s value.

      (define set-window-title
        (windows-procedure
         (set-window-text (window hwnd) (text string))
         bool user32.dll "SetWindowText"))

      (set-window-title my-win "Hi")
                               ⇒  #t
                               ;; Changes window’s title/text

      set-window-title         ⇒  #[compiled-procedure  …]
      set-window-text          error→  Unbound variable

  When there are no options the created procedure will (a) check its arguments against the types, (b) convert the arguments, (c) call the C procedure and (d) convert the returned value. No reversion is performed, even if one of the types has a reversion defined. (Reverted types are rare [I have never used one], so paying a cost for this unless it is used seems silly).

  The following options are allowed:

  `with-reversions`

    The reversions are included in the type conversions.

  `expand`

    A synonym for `with-reversions`.

  Scheme code

    The Scheme code is placed between steps (a) and (b) in the default process. The Scheme code can enforce constraints on the arguments, including constraints between arguments such as checking that an index refers to a valid position in a string.

  If both options (i.e. `with-reversions` and Scheme code) are used, `with-reversions` must appear first. There can be arbitrarily many Scheme expression.

#### 18.2.3 Win32 API names and procedures

This section is a moving target.

The `#define` values from `wingdi.h` and `winuser.h` are available as bindings in the `(win32)` package environment. The `#define` symbols are all uppercase; these have been translated to all lowercase Scheme identifiers, thus `WM_LBUTTONUP` is the scheme variable `wm_lbuttonup`. As Scheme is case insensitive, the upper-case version may be used and probably should to make the code look more like conventional Windows code. The Scheme bindings have been produced automagically. Most of the `#define`-symbols contain an underscore so there are not many name clashes. There is one very notable name clash, however: `ERROR` is `#define`d to 0, which shadows the scheme procedure `error` in the root package environment. To signal an error, use `access` to get `error` from the system global environment:

    (declare (usual-integrations))
    …
    ((access error system-global-environment) "Complain" …)

The set of procedures is incomplete because procedures have been added on a by-need basis for the implementation of other parts of the system, e.g. Scheme Graphics. Look in the implementation for further details.

Win32 API procedure names have been uniformly converted into Scheme identifiers as follows:

- A leading uppercase letter is translated into a lowercase letter.
- Subsequent sequences of uppercase letters are translated into lowercase letters preceeded by a hyphen (minus symbol), i.e. hyphens are inserted at a lowercase to uppercase transition.
- Predicates beginning with `Is` finally have a question-mark appended.

Example: applying these rules to `IsWindow` yields `is-window?`, and `GetDC` is translated into `get-dc`.

### 18.3 Device Independent Bitmap Utilities

The Device Independent Bitmap (DIB) utilities library `DIBUTILS.DLL` and the associated procedures in `dib.scm` in the Win32 system source is an example of how to use the foreign function interface to access and manipulate non-Scheme objects.

windows type: dib ¶

  In the C world a DIB is a handle to a piece of memory containing the bits that represent information about the image and the pixels of the image. The handle is a machine-word sized piece of data which may be thought of as a 32 bit integer. The handle may be null (i.e. zero), indicating that there is no block of memory describing the DIB. The null value is usually returned by C functions that are supposed to create a DIB but failed, for some reason like the memory could not be allocated or a file could not be opened.

  In the Scheme world a DIB is a structure containing information about the bitmap (specifically the integer that represents the handle). We also include `#f` in the `dib` windows type to mirror the null handle error value.

      (define dib-result
        (lambda (handle)
          (if (= handle 0)
              #f
              (make-dib handle))))

      (define dib-arg
        (lambda (dib)
          (if dib
              (cell-contents (dib-handle dib))
              0)))

      (define-windows-type dib
        (lambda (thing) (or (dib? thing) (eq? thing #f)))
        dib-arg
        dib-result)

#### 18.3.1 DIB procedures

The following procedures have typed parameters, using the same convention as `windows-procedure`.

procedure: open-dib (filename string) ¶

  Return type: dib. Calls the `OpenDIB` entry of `DIBUTILS.DLL`. If the return value is not `#f` then the file filename was found, successfully opened, and the contents were suitable for loading into memory as a device independent bitmap.

procedure: write-dib (filename string) (dib dib) ¶

  Return type: bool. Calls the `WriteDIB` entry of `DIBUTILS.DLL`. Returns `#t` if the file filename could be opened and written to. After this operation the file contains the bitmap data in a standard format that is understood by `open-dib` and various system utilities like the bitmap editor. Any problems resulting in failure are signalled by a `#f` return value.

procedure: bitmap-from-dib (dib dib) (palette hpalette) ¶

  Return type: hbitmap. Calls the `BitmapFromDib` entry of `DIBUTILS.DLL`. The returned value is a device dependent bitmap. The colours from the DIB are matched against colors in palette.

procedure: dib-from-bitmap (bitmap hbitmap) (style dword) (bits word) (palette hpalette) ¶

  Return type: dib. Returns a DIB containing the same image as the device dependent bitmap bitmap. Style determines the kind of DIB, e.g. compression style. Calls the `DibFromBitmap` entry of `DIBUTILS.DLL`.

procedure: dib-blt (dest hdc) (x int) (y int) (w int) (h int) (src dib) (src-x int) (src-y int) (raster-op long) ¶

  Return type: bool. Calls the `DibBlt` entry of `DIBUTILS.DLL`. Similar to the Win32 API `BitBlt` call, but draws a DIB rather than a piece of another device context. Draws the dib on device context hdc at position (x,y). A rectangle of width w and height h is copied from position (src-x,src-y) of dib. Raster-op is supposed to allow the source and destination to be combined but I don’t think I got this right so stick to `SRCCOPY`.

procedure: delete-dib (dib dib) ¶

  Return type: bool. This procedure reclaims the storage occupied by a DIB. After being deleted, the DIB should not be used. This procedure allows the programmer to reclaim external heap storage rather than risking it running out before the next garbage collection.

procedure: dib-height (dib dib) ¶

  Return type: int. Calls the `DibHeight` expand entry of `DIBUTILS.DLL`, which returns the height of the bitmap in pixels.

procedure: dib-width (dib dib) ¶

  Return type: int. Calls the `DibWidth` entry of `DIBUTILS.DLL`, which returns the width of the bitmap in pixels.

procedure: copy-bitmap (bm hbitmap) ¶

  Return type: hbitmap. Calls the `CopyBitmap` of `DIBUTILS.DLL`, which creates a new bitmap with the same size and contents as the original.

procedure: create-dib (width int) (height int) (style int) (depth int) (palette hpalette) ¶

  Return type: dib. Calls the `CreateDIB` entry of `DIBUTILS.DLL`. Creates a DIB of width by height pixels and depth bits of colour information. The style parameter determines how the bitmap is stored. I have only ever used `BI_RGB`. If depth<=8 then the palette determines the DIB’s colour table.

procedure: crop-bitmap (bm hbitmap) (left int) (top int) (right int) (bottom int) ¶

  Return type: hbitmap. Calls the `CropBitmap` entry of `DIBUTILS.DLL`. Returns a new bitmap containing the image from a region of the original.

procedure: dib-set-pixels-unaligned dib (pixels string) ¶

  Return type: bool. Calls the `DIBSetPixelsUnaligned` entry of
  `DIBUTILS.DLL`. Stuffs bytes from pixels into the bitmap. There are no alignment constraints on pixels (the usual way of doing this is to use the `SetDIBits` function which requires that every scan line of the bitmap is 32-bit word aligned, even if the scan lines are not a multiple of 4 bytes long). doing this

#### 18.3.2 Other parts of the DIB Utilities implementation

The `DIBUTILS.DLL` library is an ordinary DLL. See the standard Microsoft Windows documentation on how to create DLLs. Look at the code in the `WIN32/DIBUTILS` directory of the Scheme source.

Please note:

- For the foreign function interface to find the procedures they must be declared as exports in the `.DEF` definition file.
- To load the `.DLL` file use the `find-module` Scheme function. Look at `WIN32/DIB.SCM` to see how this is done.
- The current system works with C procedures with the `__stdcall` and `__cdecl` calling conventions but not the `__fastcall` calling convention.

## 19 Standards Support

### 19.1 Revised7 Report on the Algorithmic Language Scheme

Description

  The core standard for the Scheme language.

URL

  https://small.r7rs.org/attachment/r7rs.pdf

Support

  Fully supported, with exceptions:

  - `syntax-rules` doesn’t handle additional patterns after a segment pattern. This means that `(syntax-rules () ((a b ... c) …))` doesn’t work, even though `(syntax-rules () ((a b ...) …))` does work.
  - `syntax-rules` doesn’t handle vector patterns or templates.
  - Multiple values aren’t supported correctly. Instead `values` returns a special record containing the values, and `call-with-values` takes that record apart and passes the values to the receiver. For many uses of multiple values this is adequate.
Libraries

  `(scheme base)` `(scheme case-lambda)` `(scheme char)` `(scheme complex)`
  `(scheme cxr)` `(scheme eval)` `(scheme file)` `(scheme inexact)`
  `(scheme lazy)` `(scheme load)` `(scheme process-context)` `(scheme r5rs)`
  `(scheme read)` `(scheme repl)` `(scheme time)` `(scheme write)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`*` | `+` | `-`
`...` | `/` | `<`
`<=` | `=` | `=>`
`>` | `>=` | `_`
`abs` | `acos` | `and`
`angle` | `append` | `apply`
`asin` | `assoc` | `assq`
`assv` | `atan` | `begin`
`binary-port?` | `boolean=?` | `boolean?`
`bytevector` | `bytevector-append` | `bytevector-copy`
`bytevector-copy!` | `bytevector-length` | `bytevector-u8-ref`
`bytevector-u8-set!` | `bytevector?` | `caaaar`
`caaadr` | `caaar` | `caadar`
`caaddr` | `caadr` | `caar`
`cadaar` | `cadadr` | `cadar`
`caddar` | `cadddr` | `caddr`
`cadr` | `call-with-current-continuation` | `call-with-input-file`
`call-with-output-file` | `call-with-port` | `call-with-values`
`call/cc` | `car` | `case`
`case-lambda` | `cdaaar` | `cdaadr`
`cdaar` | `cdadar` | `cdaddr`
`cdadr` | `cdar` | `cddaar`
`cddadr` | `cddar` | `cdddar`
`cddddr` | `cdddr` | `cddr`
`cdr` | `ceiling` | `char->integer`
`char-alphabetic?` | `char-ci<=?` | `char-ci<?`
`char-ci=?` | `char-ci>=?` | `char-ci>?`
`char-downcase` | `char-foldcase` | `char-lower-case?`
`char-numeric?` | `char-ready?` | `char-upcase`
`char-upper-case?` | `char-whitespace?` | `char<=?`
`char<?` | `char=?` | `char>=?`
`char>?` | `char?` | `close-input-port`
`close-output-port` | `close-port` | `command-line`
`complex?` | `cond` | `cond-expand`
`cons` | `cos` | `current-error-port`
`current-input-port` | `current-jiffy` | `current-output-port`
`current-second` | `define` | `define-record-type`
`define-syntax` | `define-values` | `delay`
`delay-force` | `delete-file` | `denominator`
`digit-value` | `display` | `do`
`dynamic-wind` | `else` | `emergency-exit`
`environment` | `eof-object` | `eof-object?`
`eq?` | `equal?` | `eqv?`
`error` | `error-object-irritants` | `error-object-message`
`error-object?` | `eval` | `even?`
`exact` | `exact-integer-sqrt` | `exact-integer?`
`exact?` | `exit` | `exp`
`expt` | `features` | `file-error?`
`file-exists?` | `finite?` | `floor`
`floor-quotient` | `floor-remainder` | `floor/`
`flush-output-port` | `for-each` | `force`
`gcd` | `get-environment-variable` | `get-environment-variables`
`get-output-bytevector` | `get-output-string` | `guard`
`if` | `imag-part` | `include`
`include-ci` | `inexact` | `inexact?`
`infinite?` | `input-port-open?` | `input-port?`
`integer->char` | `integer?` | `interaction-environment`
`jiffies-per-second` | `lambda` | `lcm`
`length` | `let` | `let*`
`let*-values` | `let-syntax` | `let-values`
`letrec` | `letrec*` | `letrec-syntax`
`list` | `list->string` | `list->vector`
`list-copy` | `list-ref` | `list-set!`
`list-tail` | `list?` | `load`
`log` | `magnitude` | `make-bytevector`
`make-list` | `make-parameter` | `make-polar`
`make-promise` | `make-rectangular` | `make-string`
`make-vector` | `map` | `max`
`member` | `memq` | `memv`
`min` | `modulo` | `nan?`
`negative?` | `newline` | `not`
`null?` | `number->string` | `number?`
`numerator` | `odd?` | `open-binary-input-file`
`open-binary-output-file` | `open-input-bytevector` | `open-input-file`
`open-input-string` | `open-output-bytevector` | `open-output-file`
`open-output-string` | `or` | `output-port-open?`
`output-port?` | `pair?` | `parameterize`
`peek-char` | `peek-u8` | `port?`
`positive?` | `procedure?` | `promise?`
`quasiquote` | `quote` | `quotient`
`raise` | `raise-continuable` | `rational?`
`rationalize` | `read` | `read-bytevector`
`read-bytevector!` | `read-char` | `read-error?`
`read-line` | `read-string` | `read-u8`
`real-part` | `real?` | `remainder`
`reverse` | `round` | `set!`
`set-car!` | `set-cdr!` | `sin`
`sqrt` | `square` | `string`
`string->list` | `string->number` | `string->symbol`
`string->utf8` | `string->vector` | `string-append`
`string-ci<=?` | `string-ci<?` | `string-ci=?`
`string-ci>=?` | `string-ci>?` | `string-copy`
`string-copy!` | `string-downcase` | `string-fill!`
`string-foldcase` | `string-for-each` | `string-length`
`string-map` | `string-ref` | `string-set!`
`string-upcase` | `string<=?` | `string<?`
`string=?` | `string>=?` | `string>?`
`string?` | `substring` | `symbol->string`
`symbol=?` | `symbol?` | `syntax-error`
`syntax-rules` | `tan` | `textual-port?`
`truncate` | `truncate-quotient` | `truncate-remainder`
`truncate/` | `u8-ready?` | `unless`
`unquote` | `unquote-splicing` | `utf8->string`
`values` | `vector` | `vector->list`
`vector->string` | `vector-append` | `vector-copy`
`vector-copy!` | `vector-fill!` | `vector-for-each`
`vector-length` | `vector-map` | `vector-ref`
`vector-set!` | `vector?` | `when`
`with-exception-handler` | `with-input-from-file` | `with-output-to-file`
`write` | `write-bytevector` | `write-char`
`write-shared` | `write-simple` | `write-string`

### 19.2 SRFI 1: List Library

Description

  An extensive set of procedures for working with lists that is a superset of the list procedures defined by R7RS.

URL

  https://srfi.schemers.org/srfi-1/srfi-1.html

Support

  Fully supported.

Libraries

  `(srfi 1)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`alist-cons` | `alist-copy` | `alist-delete`
`alist-delete!` | `any` | `append`
`append!` | `append-map` | `append-map!`
`append-reverse` | `append-reverse!` | `assoc`
`assq` | `assv` | `break`
`break!` | `caaaar` | `caaadr`
`caaar` | `caadar` | `caaddr`
`caadr` | `caar` | `cadaar`
`cadadr` | `cadar` | `caddar`
`cadddr` | `caddr` | `cadr`
`car` | `car+cdr` | `cdaaar`
`cdaadr` | `cdaar` | `cdadar`
`cdaddr` | `cdadr` | `cdar`
`cddaar` | `cddadr` | `cddar`
`cdddar` | `cddddr` | `cdddr`
`cddr` | `cdr` | `circular-list`
`circular-list?` | `concatenate` | `concatenate!`
`cons` | `cons*` | `count`
`delete` | `delete!` | `delete-duplicates`
`delete-duplicates!` | `dotted-list?` | `drop`
`drop-right` | `drop-right!` | `drop-while`
`eighth` | `every` | `fifth`
`filter` | `filter!` | `filter-map`
`find` | `find-tail` | `first`
`fold` | `fold-right` | `for-each`
`fourth` | `iota` | `last`
`last-pair` | `length` | `length+`
`list` | `list-copy` | `list-index`
`list-ref` | `list-tabulate` | `list=`
`lset-adjoin` | `lset-diff+intersection` | `lset-diff+intersection!`
`lset-difference` | `lset-difference!` | `lset-intersection`
`lset-intersection!` | `lset-union` | `lset-union!`
`lset-xor` | `lset-xor!` | `lset<=`
`lset=` | `make-list` | `map`
`map!` | `map-in-order` | `member`
`memq` | `memv` | `ninth`
`not-pair?` | `null-list?` | `null?`
`pair-fold` | `pair-fold-right` | `pair-for-each`
`pair?` | `partition` | `partition!`
`proper-list?` | `reduce` | `reduce-right`
`remove` | `remove!` | `reverse`
`reverse!` | `second` | `set-car!`
`set-cdr!` | `seventh` | `sixth`
`span` | `span!` | `split-at`
`split-at!` | `take` | `take!`
`take-right` | `take-while` | `take-while!`
`tenth` | `third` | `unfold`
`unfold-right` | `unzip1` | `unzip2`
`unzip3` | `unzip4` | `unzip5`
`xcons` | `zip`

### 19.3 SRFI 2: `and-let*`

Description

  A macro combining features of `let*` and `and`.

URL

  https://srfi.schemers.org/srfi-2/srfi-2.html

Support

  Fully supported.

Libraries

  `(srfi 2)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`and-let*`

### 19.4 SRFI 8: `receive`

Description

  Early syntax for binding multiple values. R7RS provides the more flexible `let-values`, which should be preferred.

URL

  https://srfi.schemers.org/srfi-8/srfi-8.html

Support

  Fully supported.

Libraries

  `(srfi 8)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`receive`

### 19.5 SRFI 9: Record Types

Description

  A basic implementation of record types. Superseded by R7RS.

URL

  https://srfi.schemers.org/srfi-9/srfi-9.html

Support

  The MIT/GNU Scheme implementation is fully compatible.

Libraries

  `(srfi 9)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`define-record-type`

### 19.6 SRFI 14: Character-set Library

Description

  An extensive set of definitions for working with sets of characters.

URL

  https://srfi.schemers.org/srfi-14/srfi-14.html

Support

  Fully supported.

Libraries

  `(srfi 14)`

Global

  Most names are bound in the MIT/GNU Scheme global environment. Exceptions are the linear-update procedures whose names end in `!`.

`->char-set` | `char-set`
`char-set->list` | `char-set->string`
`char-set-adjoin` | `char-set-adjoin!`
`char-set-any` | `char-set-complement`
`char-set-complement!` | `char-set-contains?`
`char-set-copy` | `char-set-count`
`char-set-cursor` | `char-set-cursor-next`
`char-set-delete` | `char-set-delete!`
`char-set-diff+intersection` | `char-set-diff+intersection!`
`char-set-difference` | `char-set-difference!`
`char-set-every` | `char-set-filter`
`char-set-filter!` | `char-set-fold`
`char-set-for-each` | `char-set-hash`
`char-set-intersection` | `char-set-intersection!`
`char-set-map` | `char-set-ref`
`char-set-size` | `char-set-unfold`
`char-set-unfold!` | `char-set-union`
`char-set-union!` | `char-set-xor`
`char-set-xor!` | `char-set:ascii`
`char-set:blank` | `char-set:digit`
`char-set:empty` | `char-set:full`
`char-set:graphic` | `char-set:hex-digit`
`char-set:iso-control` | `char-set:letter`
`char-set:letter+digit` | `char-set:lower-case`
`char-set:printing` | `char-set:punctuation`
`char-set:symbol` | `char-set:title-case`
`char-set:upper-case` | `char-set:whitespace`
`char-set<=` | `char-set=`
`char-set?` | `end-of-char-set?`
`list->char-set` | `list->char-set!`
`string->char-set` | `string->char-set!`
`ucs-range->char-set` | `ucs-range->char-set!`

### 19.7 SRFI 23: Error Reporting Mechanism

Description

  A simple implementation of `error`. Superseded by R7RS.

URL

  https://srfi.schemers.org/srfi-23/srfi-23.html

Support

  Fully supported.

Libraries

  `(srfi 23)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`error`

### 19.8 SRFI 27: Sources of Random Bits

Description

  Procedures to obtain pseudo-random numbers in various formats.

URL

  https://srfi.schemers.org/srfi-27/srfi-27.html

Support

  Fully supported.

Libraries

  `(srfi 27)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`default-random-source` | `make-random-source`
`random-integer` | `random-real`
`random-source-make-integers` | `random-source-make-reals`
`random-source-make-reals` | `random-source-pseudo-randomize!`
`random-source-randomize!` | `random-source-state-ref`
`random-source-state-set!` | `random-source?`

### 19.9 SRFI 39: Parameter Objects

Description

  A mechanism for dynamic binding. Superseded by R7RS.

URL

  https://srfi.schemers.org/srfi-39/srfi-39.html

Support

  Fully supported.

Libraries

  `(srfi 39)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`make-parameter` | `parameterize`

### 19.10 SRFI 69: Basic Hash Tables

Description

  A standard interface for hash tables. Superseded by SRFI 125.

URL

  https://srfi.schemers.org/srfi-69/srfi-69.html

Support

  Fully supported.

Libraries

  `(srfi 69)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`alist->hash-table` | `hash`
`hash-by-identity` | `hash-table->alist`
`hash-table-copy` | `hash-table-delete!`
`hash-table-equivalence-function` | `hash-table-exists?`
`hash-table-fold` | `hash-table-hash-function`
`hash-table-keys` | `hash-table-merge!`
`hash-table-ref` | `hash-table-ref/default`
`hash-table-set!` | `hash-table-size`
`hash-table-update!` | `hash-table-update!/default`
`hash-table-values` | `hash-table-walk`
`hash-table?` | `make-hash-table`
`string-ci-hash` | `string-hash`

### 19.11 SRFI 115: Scheme Regular Expressions

Description

  An implementation of regular expressions using Scheme syntax.

URL

  https://srfi.schemers.org/srfi-115/srfi-115.html

Support

  Fully supported.

Libraries

  `(srfi 115)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`char-set->sre` | `regexp`
`regexp-extract` | `regexp-fold`
`regexp-match->list` | `regexp-match-count`
`regexp-match-submatch` | `regexp-match-submatch-end`
`regexp-match-submatch-start` | `regexp-match?`
`regexp-matches` | `regexp-matches?`
`regexp-partition` | `regexp-replace`
`regexp-replace-all` | `regexp-search`
`regexp-split` | `regexp?`
`rx` | `valid-sre?`

### 19.12 SRFI 124: Ephemerons

Description

  Support for ephemerons, a weak-pointer mechanism.

URL

  https://srfi.schemers.org/srfi-124/srfi-124.html

Support

  Fully supported.

Libraries

  `(srfi 124)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`ephemeron-broken?` | `ephemeron-datum` | `ephemeron-key`
`ephemeron?` | `make-ephemeron` | `reference-barrier`

### 19.13 SRFI 125: Intermediate Hash Tables

Description

  A comprehensive set of procedures for hash tables. Supersedes SRFI 69.

URL

  https://srfi.schemers.org/srfi-125/srfi-125.html

Support

  Fully supported.

Libraries

  `(srfi 125)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`alist->hash-table` | `hash`
`hash-by-identity` | `hash-table`
`hash-table->alist` | `hash-table-clear!`
`hash-table-contains?` | `hash-table-copy`
`hash-table-count` | `hash-table-delete!`
`hash-table-difference!` | `hash-table-empty-copy`
`hash-table-empty?` | `hash-table-entries`
`hash-table-equivalence-function` | `hash-table-exists?`
`hash-table-find` | `hash-table-fold`
`hash-table-for-each` | `hash-table-hash-function`
`hash-table-intern!` | `hash-table-intersection!`
`hash-table-keys` | `hash-table-map`
`hash-table-map!` | `hash-table-map->list`
`hash-table-merge!` | `hash-table-mutable?`
`hash-table-pop!` | `hash-table-prune!`
`hash-table-ref` | `hash-table-ref/default`
`hash-table-set!` | `hash-table-size`
`hash-table-unfold` | `hash-table-union!`
`hash-table-update!` | `hash-table-update!/default`
`hash-table-values` | `hash-table-walk`
`hash-table-xor!` | `hash-table=?`
`hash-table?` | `make-hash-table`
`string-ci-hash` | `string-hash`

### 19.14 SRFI 128: Comparators (reduced)

Description

  An implementation of comparators, which encapsulate type, equality, ordering, and hashing.

URL

  https://srfi.schemers.org/srfi-128/srfi-128.html

Support

  Fully supported.

Libraries

  `(srfi 128)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`<=?` | `<?`
`=?` | `>=?`
`>?` | `boolean-hash`
`char-ci-hash` | `char-hash`
`comparator-check-type` | `comparator-equality-predicate`
`comparator-hash` | `comparator-hash-function`
`comparator-hashable?` | `comparator-if<=>`
`comparator-ordered?` | `comparator-ordering-predicate`
`comparator-register-default!` | `comparator-test-type`
`comparator-type-test-predicate` | `comparator?`
`default-hash` | `hash-bound`
`hash-salt` | `make-comparator`
`make-default-comparator` | `make-eq-comparator`
`make-equal-comparator` | `make-eqv-comparator`
`make-list-comparator` | `make-pair-comparator`
`make-vector-comparator` | `number-hash`
`string-ci-hash` | `string-hash`
`symbol-hash`

### 19.15 SRFI 131: ERRR5RS Record Syntax (reduced)

Description

  An enhanced record syntax supporting abbreviations and single inheritance.

URL

  https://srfi.schemers.org/srfi-131/srfi-131.html

Support

  The MIT/GNU Scheme implementation is fully compatible.

Libraries

  `(srfi 131)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`define-record-type`

### 19.16 SRFI 133: Vector Library (R7RS-compatible)

Description

  A set of procedures for working with vectors.

URL

  https://srfi.schemers.org/srfi-133/srfi-133.html

Support

  Fully supported.

Libraries

  `(srfi 133)`

Global

  No bindings in the MIT/GNU Scheme global environment. Some of the same names are there, but they aren’t the same implementations.

`reverse-list->vector` | `reverse-vector->list`
`vector-any` | `vector-append-subvectors`
`vector-cumulate` | `vector-empty?`
`vector-every` | `vector-fold`
`vector-fold-right` | `vector-index`
`vector-index-right` | `vector-map!`
`vector-partition` | `vector-reverse!`
`vector-reverse-copy` | `vector-skip`
`vector-skip-right` | `vector-swap!`
`vector-unfold` | `vector-unfold!`
`vector-unfold-right` | `vector-unfold-right!`
`vector=` | `vector-binary-search`
`vector-concatenate` | `vector-count`
`vector-reverse-copy!`

### 19.17 SRFI 140: Immutable Strings

Description

  A comprehensive library for using immutable strings.

URL

  https://srfi.schemers.org/srfi-140/srfi-140.html

Support

  Partially supported: only immutable strings are implemented.

Libraries

  `(srfi 140)`

Global

  Some bindings in the MIT/GNU Scheme global environment; see below.

Names that are bound in the MIT/GNU Scheme global environment:

`istring?` | `list->string`
`string` | `string->list`
`string->utf16` | `string->utf16be`
`string->utf16le` | `string->utf8`
`string->vector` | `string-append`
`string-ci<=?` | `string-ci<?`
`string-ci=?` | `string-ci>=?`
`string-ci>?` | `string-concatenate`
`string-downcase` | `string-fold`
`string-fold-right` | `string-foldcase`
`string-for-each` | `string-length`
`string-map` | `string-null?`
`string-ref` | `string-titlecase`
`string-upcase` | `string<=?`
`string<?` | `string=?`
`string>=?` | `string>?`
`string?` | `substring`
`utf16->string` | `utf16be->string`
`utf16le->string` | `utf8->string`
`vector->string`

Names that are not bound in the MIT/GNU Scheme global environment:

`reverse-list->string` | `string-any`
`string-concatenate-reverse` | `string-contains`
`string-contains-right` | `string-count`
`string-drop` | `string-drop-right`
`string-every` | `string-filter`
`string-for-each-index` | `string-index`
`string-index-right` | `string-join`
`string-map-index` | `string-pad`
`string-pad-right` | `string-prefix-length`
`string-prefix?` | `string-remove`
`string-repeat` | `string-replace`
`string-skip` | `string-skip-right`
`string-split` | `string-suffix-length`
`string-suffix?` | `string-tabulate`
`string-take` | `string-take-right`
`string-trim` | `string-trim-both`
`string-trim-right` | `string-unfold`
`string-unfold-right` | `xsubstring`

### 19.18 SRFI 143: Fixnums

Description

  Definitions of fixnum-specific procedures.

URL

  https://srfi.schemers.org/srfi-143/srfi-143.html

Support

  Fully supported with efficient compilation.

Libraries

  `(srfi 143)`

Global

  All names are bound in the MIT/GNU Scheme global environment.

`fixnum?` | `fx*`
`fx*/carry` | `fx+`
`fx+/carry` | `fx-`
`fx-/carry` | `fx-greatest`
`fx-least` | `fx-width`
`fx<=?` | `fx<?`
`fx=?` | `fx>=?`
`fx>?` | `fxabs`
`fxand` | `fxarithmetic-shift`
`fxarithmetic-shift-left` | `fxarithmetic-shift-right`
`fxbit-count` | `fxbit-field`
`fxbit-field-reverse` | `fxbit-field-rotate`
`fxbit-set?` | `fxcopy-bit`
`fxeven?` | `fxfirst-set-bit`
`fxif` | `fxior`
`fxlength` | `fxmax`
`fxmin` | `fxneg`
`fxnegative?` | `fxnot`
`fxodd?` | `fxpositive?`
`fxquotient` | `fxremainder`
`fxsqrt` | `fxsquare`
`fxxor` | `fxzero?`

### 19.19 SRFI 158: Generators and Accumulators

Description

  Efficient sources and sinks of objects.

URL

  https://srfi.schemers.org/srfi-158/srfi-158.html

Support

  Partially supported.

Libraries

  No libraries.

Global

  All names are bound in the MIT/GNU Scheme global environment.

`bytevector->generator` | `circular-generator`
`gappend` | `gcons*`
`gdelete` | `gdrop`
`gdrop-while` | `generator`
`generator->list` | `generator->reverse-list`
`generator->string` | `generator->vector`
`generator-any` | `generator-count`
`generator-every` | `generator-find`
`generator-fold` | `generator-fold-right`
`generator-for-each` | `generator-map->list`
`generator-unfold` | `gfilter`
`gflatten` | `gmap`
`gremove` | `gtake`
`gtake-while` | `list->generator`
`make-iota-generator` | `make-range-generator`
`reverse-vector->generator` | `string->generator`
`vector->generator`

## Appendix A GNU Free Documentation License

Version 1.2, November 2002

    Copyright © 2000,2001,2002 Free Software Foundation, Inc.
    51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA

    Everyone is permitted to copy and distribute verbatim copies
    of this license document, but changing it is not allowed.

0. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

   This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

   A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

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4. MODIFICATIONS

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## Appendix B Binding Index

Jump to: | " # ' ( ) * + , - . / 1 2 8 < = > ? \ _ ` A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

| Index Entry |  | Section

" |  |
| `"`: |  | Strings

# |  |
| `#`: |  | Syntax of numerical constants
| `#!optional`: |  | Additional Notations
| `#!optional`: |  | Lambda Expressions
| `#!rest`: |  | Additional Notations
| `#!rest`: |  | Lambda Expressions
| `#(`: |  | Vectors
| `#b`: |  | Syntax of numerical constants
| `#d`: |  | Syntax of numerical constants
| `#e`: |  | Syntax of numerical constants
| `#f`: |  | True and False
| `#f`: |  | Conditionals
| `#f`: |  | Booleans
| `#i`: |  | Syntax of numerical constants
| `#o`: |  | Syntax of numerical constants
| `#t`: |  | True and False
| `#t`: |  | Conditionals
| `#t`: |  | Booleans
| `#x`: |  | Syntax of numerical constants
| `#\`: |  | Characters
| `#\alarm`: |  | Characters
| `#\backspace`: |  | Characters
| `#\delete`: |  | Characters
| `#\escape`: |  | Characters
| `#\newline`: |  | Characters
| `#\null`: |  | Characters
| `#\return`: |  | Characters
| `#\space`: |  | Characters
| `#\space`: |  | Strings
| `#\tab`: |  | Characters

' |  |
| `'`: |  | Quoting
| `'`: |  | Lists

( |  |
| `(`: |  | Lists
| `()`: |  | Lists

) |  |
| `)`: |  | Lists

* |  |
| `*`: |  | Procedure Call Syntax
| `*`: |  | Numerical operations
| `*`: |  | Regular S-Expressions
| `*`: |  | *Matcher
| `*`: |  | *Parser
| `*`: |  | R7RS
| `**`: |  | Regular S-Expressions
| `**`: |  | Regular S-Expressions
| `**?`: |  | Regular S-Expressions
| `*?`: |  | Regular S-Expressions
| `*default-pathname-defaults*`: |  | Operations on Pathnames
| `*matcher`: |  | *Matcher
| `*parser`: |  | *Parser
| `*parser-canonicalize-symbols?*`: |  | Input Procedures
| `*parser-radix*`: |  | Input Procedures
| `*random-state*`: |  | Random Number Generation
| `*unparse-with-maximum-readability?*`: |  | Output Procedures
| `*unparser-list-breadth-limit*`: |  | Output Procedures
| `*unparser-list-depth-limit*`: |  | Output Procedures
| `*unparser-radix*`: |  | Output Procedures
| `*unparser-string-length-limit*`: |  | Output Procedures

+ |  |
| `+`: |  | Procedure Call Syntax
| `+`: |  | Exactness
| `+`: |  | Numerical operations
| `+`: |  | Regular S-Expressions
| `+`: |  | *Matcher
| `+`: |  | *Parser
| `+`: |  | R7RS
| `+?`: |  | Regular S-Expressions
| `+inf.0`: |  | Flonum Operations
| `+nan.0`: |  | Flonum Operations
| `+snan.1`: |  | Flonum Operations

, |  |
| `,`: |  | Quoting
| `,`: |  | Lists
| `,@`: |  | Quoting
| `,@`: |  | Lists

- |  |
| `-`: |  | Numerical operations
| `-`: |  | R7RS
| `-1+`: |  | Numerical operations
| `->char-set`: |  | SRFI 14
| `->namestring`: |  | Filenames and Pathnames
| `->pathname`: |  | Filenames and Pathnames
| `->pathname`: |  | Working Directory
| `->truename`: |  | File Manipulation
| `-inf.0`: |  | Flonum Operations
| `-nan.0`: |  | Flonum Operations
| `-snan.1`: |  | Flonum Operations

. |  |
| `.`: |  | Lists
| `...`: |  | Pattern Language
| `...`: |  | R7RS

/ |  |
| `/`: |  | Implementation restrictions
| `/`: |  | Numerical operations
| `/`: |  | R7RS

1 |  |
| `1+`: |  | Numerical operations
| `1d-table/alist`: |  | 1D Tables
| `1d-table/get`: |  | 1D Tables
| `1d-table/lookup`: |  | 1D Tables
| `1d-table/put!`: |  | 1D Tables
| `1d-table/remove!`: |  | 1D Tables
| `1d-table?`: |  | 1D Tables

2 |  |
| `2d-get`: |  | The Association Table
| `2d-get-alist-x`: |  | The Association Table
| `2d-get-alist-y`: |  | The Association Table
| `2d-put!`: |  | The Association Table
| `2d-remove!`: |  | The Association Table

8 |  |
| `8-bit-char-set?`: |  | Character Sets

< |  |
| `<`: |  | Numerical operations
| `<`: |  | R7RS
| `<=`: |  | Numerical operations
| `<=`: |  | R7RS
| `<=?`: |  | SRFI 128
| `<?`: |  | SRFI 128
| `<xml-!attlist>`: |  | XML Structure
| `<xml-!element>`: |  | XML Structure
| `<xml-!entity>`: |  | XML Structure
| `<xml-!notation>`: |  | XML Structure
| `<xml-declaration>`: |  | XML Structure
| `<xml-document>`: |  | XML Structure
| `<xml-dtd>`: |  | XML Structure
| `<xml-element>`: |  | XML Structure
| `<xml-external-id>`: |  | XML Structure
| `<xml-parameter-!entity>`: |  | XML Structure
| `<xml-processing-instructions>`: |  | XML Structure
| `<xml-unparsed-!entity>`: |  | XML Structure

= |  |
| `=`: |  | Equivalence Predicates
| `=`: |  | Equivalence Predicates
| `=`: |  | Numerical operations
| `=`: |  | Numerical operations
| `=`: |  | Strings
| `=`: |  | R7RS
| `=>`: |  | Conditionals
| `=>`: |  | R7RS
| `=?`: |  | SRFI 128

> |  |
| `>`: |  | Numerical operations
| `>`: |  | R7RS
| `>=`: |  | Numerical operations
| `>=`: |  | R7RS
| `>=?`: |  | SRFI 128
| `>?`: |  | SRFI 128

? |  |
| `?`: |  | Regular S-Expressions
| `?`: |  | *Matcher
| `?`: |  | *Parser
| `??`: |  | Regular S-Expressions

\ |  |
| `\"`: |  | Strings
| `\a`: |  | Strings
| `\b`: |  | Strings
| `\n`: |  | Strings
| `\r`: |  | Strings
| `\t`: |  | Strings
| `\x`: |  | Strings
| `\\`: |  | Strings
| `\|`: |  | Strings

_ |  |
| `_`: |  | R7RS

` |  |
| ```: |  | Quoting
| ```: |  | Lists

A |  |
| `abort`: |  | Restarts
| `abort`: |  | Invoking Standard Restart Code
| `abs`: |  | Numerical operations
| `abs`: |  | R7RS
| `access`: |  | Assignments
| `access-condition`: |  | Generating Operations on Conditions
| `access-condition`: |  | Simple Condition Instance Operations
| `acos`: |  | Numerical operations
| `acos`: |  | R7RS
| `alist->amap`: |  | Amap constructors
| `alist->hash-table`: |  | Construction of Hash Tables
| `alist->hash-table`: |  | Construction of Hash Tables
| `alist->hash-table`: |  | SRFI 69
| `alist->hash-table`: |  | SRFI 125
| `alist->rb-tree`: |  | Red-Black Trees
| `alist->wt-tree`: |  | Construction of Weight-Balanced Trees
| `alist-cons`: |  | SRFI 1
| `alist-copy`: |  | Association Lists
| `alist-copy`: |  | SRFI 1
| `alist-delete`: |  | SRFI 1
| `alist-delete!`: |  | SRFI 1
| `alist?`: |  | Association Lists
| `allocate-host-address`: |  | Miscellaneous OS Facilities
| `alt`: |  | Regular S-Expressions
| `alt`: |  | *Matcher
| `alt`: |  | *Parser
| `amap->alist`: |  | Amap copying and conversion
| `amap-args`: |  | Amap accessors
| `amap-clean!`: |  | Amap mutators
| `amap-clear!`: |  | Amap mutators
| `amap-comparator`: |  | Amap accessors
| `amap-contains?`: |  | Amap predicates
| `amap-copy`: |  | Amap copying and conversion
| `amap-count`: |  | Amap contents
| `amap-delete!`: |  | Amap mutators
| `amap-difference!`: |  | Amaps as sets
| `amap-empty-copy`: |  | Amap copying and conversion
| `amap-empty?`: |  | Amap predicates
| `amap-entries`: |  | Amap contents
| `amap-find`: |  | Amap contents
| `amap-fold`: |  | Amap mapping and folding
| `amap-for-each`: |  | Amap mapping and folding
| `amap-implementation-name`: |  | Amap accessors
| `amap-implementation-names`: |  | Amap constructors
| `amap-implementation-supported-args`: |  | Amap constructors
| `amap-implementation-supports-args?`: |  | Amap constructors
| `amap-implementation-supports-comparator?`: |  | Amap constructors
| `amap-intern!`: |  | Amap mutators
| `amap-intersection!`: |  | Amaps as sets
| `amap-keys`: |  | Amap contents
| `amap-map`: |  | Amap mapping and folding
| `amap-map!`: |  | Amap mapping and folding
| `amap-map->list`: |  | Amap mapping and folding
| `amap-mutable?`: |  | Amap predicates
| `amap-pop!`: |  | Amap mutators
| `amap-prune!`: |  | Amap mapping and folding
| `amap-ref`: |  | Amap accessors
| `amap-ref/default`: |  | Amap accessors
| `amap-set!`: |  | Amap mutators
| `amap-size`: |  | Amap contents
| `amap-unfold`: |  | Amap constructors
| `amap-union!`: |  | Amaps as sets
| `amap-update!`: |  | Amap mutators
| `amap-update!/default`: |  | Amap mutators
| `amap-values`: |  | Amap contents
| `amap-xor!`: |  | Amaps as sets
| `amap=?`: |  | Amap predicates
| `amap?`: |  | Amap predicates
| `and`: |  | Conditionals
| `and`: |  | Booleans
| `and`: |  | R7RS
| `and-let*`: |  | and-let* (SRFI 2)
| `and-let*`: |  | SRFI 2
| `angle`: |  | Numerical operations
| `angle`: |  | Numerical operations
| `angle`: |  | R7RS
| `any`: |  | Mapping of Lists
| `any`: |  | SRFI 1
| `any-char`: |  | Regular S-Expressions
| `append`: |  | Cutting and Pasting Lists
| `append`: |  | R7RS
| `append`: |  | SRFI 1
| `append!`: |  | Cutting and Pasting Lists
| `append!`: |  | SRFI 1
| `append-map`: |  | Mapping of Lists
| `append-map`: |  | SRFI 1
| `append-map!`: |  | Mapping of Lists
| `append-map!`: |  | SRFI 1
| `append-map*`: |  | Mapping of Lists
| `append-map*!`: |  | Mapping of Lists
| `append-reverse`: |  | SRFI 1
| `append-reverse!`: |  | SRFI 1
| `apply`: |  | Procedure Operations
| `apply`: |  | R7RS
| `apply-hook-extra`: |  | Application Hooks
| `apply-hook-procedure`: |  | Application Hooks
| `apply-hook?`: |  | Application Hooks
| `asin`: |  | Numerical operations
| `asin`: |  | R7RS
| `assoc`: |  | Association Lists
| `assoc`: |  | R7RS
| `assoc`: |  | SRFI 1
| `association-procedure`: |  | Association Lists
| `assq`: |  | Association Lists
| `assq`: |  | R7RS
| `assq`: |  | SRFI 1
| `assv`: |  | Association Lists
| `assv`: |  | R7RS
| `assv`: |  | SRFI 1
| `atan`: |  | Numerical operations
| `atan`: |  | Numerical operations
| `atan`: |  | Flonum Operations
| `atan`: |  | R7RS

B |  |
| `beep`: |  | Output Procedures
| `begin`: |  | Sequencing
| `begin`: |  | R7RS
| `binary-port?`: |  | Ports
| `binary-port?`: |  | R7RS
| `bind-cell-contents!`: |  | Parameters
| `bind-condition-handler`: |  | Error System
| `bind-condition-handler`: |  | Condition Signalling
| `bind-condition-handler`: |  | Condition Handling
| `bind-condition-handler`: |  | Condition Handling
| `bind-default-condition-handler`: |  | Condition Signalling
| `bind-default-condition-handler`: |  | Condition Handling
| `bind-default-condition-handler`: |  | Condition Handling
| `bit-string->signed-integer`: |  | Integer Conversions of Bit Strings
| `bit-string->unsigned-integer`: |  | Integer Conversions of Bit Strings
| `bit-string-allocate`: |  | Construction of Bit Strings
| `bit-string-and`: |  | Bitwise Operations on Bit Strings
| `bit-string-and!`: |  | Bitwise Operations on Bit Strings
| `bit-string-andc`: |  | Bitwise Operations on Bit Strings
| `bit-string-andc!`: |  | Bitwise Operations on Bit Strings
| `bit-string-append`: |  | Cutting and Pasting Bit Strings
| `bit-string-clear!`: |  | Selecting Bit String Components
| `bit-string-copy`: |  | Construction of Bit Strings
| `bit-string-fill!`: |  | Modification of Bit Strings
| `bit-string-length`: |  | Selecting Bit String Components
| `bit-string-move!`: |  | Modification of Bit Strings
| `bit-string-movec!`: |  | Bitwise Operations on Bit Strings
| `bit-string-not`: |  | Bitwise Operations on Bit Strings
| `bit-string-or`: |  | Bitwise Operations on Bit Strings
| `bit-string-or!`: |  | Bitwise Operations on Bit Strings
| `bit-string-ref`: |  | Selecting Bit String Components
| `bit-string-set!`: |  | Selecting Bit String Components
| `bit-string-xor`: |  | Bitwise Operations on Bit Strings
| `bit-string-xor!`: |  | Bitwise Operations on Bit Strings
| `bit-string-zero?`: |  | Bitwise Operations on Bit Strings
| `bit-string=?`: |  | Bitwise Operations on Bit Strings
| `bit-string?`: |  | Selecting Bit String Components
| `bit-substring`: |  | Cutting and Pasting Bit Strings
| `bit-substring-find-next-set-bit`: |  | Selecting Bit String Components
| `bit-substring-move-right!`: |  | Modification of Bit Strings
| `bitless-char?`: |  | Character implementation
| `bitmap-from-dib`: |  | DIB procedures
| `bool`: |  | Windows Types
| `boolean-hash`: |  | SRFI 128
| `boolean/and`: |  | Booleans
| `boolean/or`: |  | Booleans
| `boolean=?`: |  | R7RS
| `boolean?`: |  | Booleans
| `boolean?`: |  | R7RS
| `both`: |  | Strings
| `bound-restart`: |  | Finding and Invoking General Restart Code
| `bound-restarts`: |  | Finding and Invoking General Restart Code
| `bracketed-print-method`: |  | Custom Output
| `break`: |  | SRFI 1
| `break!`: |  | SRFI 1
| `break-on-signals`: |  | Condition Signalling
| `break-on-signals`: |  | Condition Handling
| `break-on-signals`: |  | Condition Handling
| `buffered-input-chars on textual input port`: |  | Textual Input Port Operations
| `buffered-output-chars on textual output port`: |  | Textual Output Port Operations
| `byte`: |  | Windows Types
| `bytevector`: |  | R7RS
| `bytevector->generator`: |  | SRFI 158
| `bytevector-append`: |  | R7RS
| `bytevector-copy`: |  | R7RS
| `bytevector-copy!`: |  | R7RS
| `bytevector-length`: |  | R7RS
| `bytevector-u8-ref`: |  | R7RS
| `bytevector-u8-set!`: |  | R7RS
| `bytevector?`: |  | R7RS

C |  |
| `caaaar`: |  | Pairs
| `caaaar`: |  | R7RS
| `caaaar`: |  | SRFI 1
| `caaadr`: |  | Pairs
| `caaadr`: |  | R7RS
| `caaadr`: |  | SRFI 1
| `caaar`: |  | Pairs
| `caaar`: |  | R7RS
| `caaar`: |  | SRFI 1
| `caadar`: |  | Pairs
| `caadar`: |  | R7RS
| `caadar`: |  | SRFI 1
| `caaddr`: |  | Pairs
| `caaddr`: |  | R7RS
| `caaddr`: |  | SRFI 1
| `caadr`: |  | Pairs
| `caadr`: |  | R7RS
| `caadr`: |  | SRFI 1
| `caar`: |  | Pairs
| `caar`: |  | R7RS
| `caar`: |  | SRFI 1
| `cadaar`: |  | Pairs
| `cadaar`: |  | R7RS
| `cadaar`: |  | SRFI 1
| `cadadr`: |  | Pairs
| `cadadr`: |  | R7RS
| `cadadr`: |  | SRFI 1
| `cadar`: |  | Pairs
| `cadar`: |  | R7RS
| `cadar`: |  | SRFI 1
| `caddar`: |  | Pairs
| `caddar`: |  | R7RS
| `caddar`: |  | SRFI 1
| `cadddr`: |  | Pairs
| `cadddr`: |  | R7RS
| `cadddr`: |  | SRFI 1
| `caddr`: |  | Pairs
| `caddr`: |  | R7RS
| `caddr`: |  | SRFI 1
| `cadr`: |  | Pairs
| `cadr`: |  | R7RS
| `cadr`: |  | SRFI 1
| `call-with-binary-input-file`: |  | File Ports
| `call-with-binary-input-file`: |  | File Ports
| `call-with-binary-output-file`: |  | File Ports
| `call-with-binary-output-file`: |  | File Ports
| `call-with-current-continuation`: |  | Continuations
| `call-with-current-continuation`: |  | R7RS
| `call-with-input-file`: |  | File Ports
| `call-with-input-file`: |  | R7RS
| `call-with-output-bytevector`: |  | Bytevector Ports
| `call-with-output-file`: |  | File Ports
| `call-with-output-file`: |  | R7RS
| `call-with-output-string`: |  | String Ports
| `call-with-port`: |  | Ports
| `call-with-port`: |  | File Ports
| `call-with-port`: |  | File Ports
| `call-with-port`: |  | R7RS
| `call-with-temporary-file-pathname`: |  | File Manipulation
| `call-with-truncated-output-port`: |  | Ports
| `call-with-truncated-output-port`: |  | String Ports
| `call-with-truncated-output-string`: |  | String Ports
| `call-with-truncated-output-string`: |  | String Ports
| `call-with-truncated-output-string`: |  | String Ports
| `call-with-values`: |  | Continuations
| `call-with-values`: |  | R7RS
| `call/cc`: |  | R7RS
| `canonical-host-name`: |  | Miscellaneous OS Facilities
| `capture-syntactic-environment`: |  | SC Transformer Definition
| `car`: |  | Storage Model
| `car`: |  | Pairs
| `car`: |  | Streams
| `car`: |  | R7RS
| `car`: |  | SRFI 1
| `car+cdr`: |  | Pairs
| `car+cdr`: |  | SRFI 1
| `case`: |  | Conditionals
| `case`: |  | Sequencing
| `case`: |  | R7RS
| `case-lambda`: |  | R7RS
| `cd`: |  | Working Directory
| `cdaaar`: |  | Pairs
| `cdaaar`: |  | R7RS
| `cdaaar`: |  | SRFI 1
| `cdaadr`: |  | Pairs
| `cdaadr`: |  | R7RS
| `cdaadr`: |  | SRFI 1
| `cdaar`: |  | Pairs
| `cdaar`: |  | R7RS
| `cdaar`: |  | SRFI 1
| `cdadar`: |  | Pairs
| `cdadar`: |  | R7RS
| `cdadar`: |  | SRFI 1
| `cdaddr`: |  | Pairs
| `cdaddr`: |  | R7RS
| `cdaddr`: |  | SRFI 1
| `cdadr`: |  | Pairs
| `cdadr`: |  | R7RS
| `cdadr`: |  | SRFI 1
| `cdar`: |  | Pairs
| `cdar`: |  | R7RS
| `cdar`: |  | SRFI 1
| `cddaar`: |  | Pairs
| `cddaar`: |  | R7RS
| `cddaar`: |  | SRFI 1
| `cddadr`: |  | Pairs
| `cddadr`: |  | R7RS
| `cddadr`: |  | SRFI 1
| `cddar`: |  | Pairs
| `cddar`: |  | R7RS
| `cddar`: |  | SRFI 1
| `cdddar`: |  | Pairs
| `cdddar`: |  | R7RS
| `cdddar`: |  | SRFI 1
| `cddddr`: |  | Pairs
| `cddddr`: |  | R7RS
| `cddddr`: |  | SRFI 1
| `cdddr`: |  | Pairs
| `cdddr`: |  | R7RS
| `cdddr`: |  | SRFI 1
| `cddr`: |  | Pairs
| `cddr`: |  | R7RS
| `cddr`: |  | SRFI 1
| `cdr`: |  | Entry Format
| `cdr`: |  | Pairs
| `cdr`: |  | Streams
| `cdr`: |  | R7RS
| `cdr`: |  | SRFI 1
| `ceiling`: |  | Numerical operations
| `ceiling`: |  | R7RS
| `ceiling->exact`: |  | Numerical operations
| `cell-contents`: |  | Parameters
| `cell?`: |  | Parameters
| `char`: |  | *Matcher
| `char`: |  | Windows Types
| `char*`: |  | Windows Types
| `char->bitless-char`: |  | Character implementation
| `char->digit`: |  | Characters
| `char->integer`: |  | Characters
| `char->integer`: |  | R7RS
| `char->name`: |  | Characters
| `char-alphabetic?`: |  | Characters
| `char-alphabetic?`: |  | R7RS
| `char-alphanumeric?`: |  | Characters
| `char-bits`: |  | Character implementation
| `char-bits-limit`: |  | Character implementation
| `char-ci`: |  | Regular S-Expressions
| `char-ci`: |  | *Matcher
| `char-ci-hash`: |  | SRFI 128
| `char-ci-predicate`: |  | Character implementation
| `char-ci<=?`: |  | Characters
| `char-ci<=?`: |  | R7RS
| `char-ci<?`: |  | Characters
| `char-ci<?`: |  | R7RS
| `char-ci=?`: |  | Characters
| `char-ci=?`: |  | R7RS
| `char-ci>=?`: |  | Characters
| `char-ci>=?`: |  | R7RS
| `char-ci>?`: |  | Characters
| `char-ci>?`: |  | R7RS
| `char-code`: |  | Character implementation
| `char-code-limit`: |  | Character implementation
| `char-downcase`: |  | Characters
| `char-downcase`: |  | R7RS
| `char-foldcase`: |  | Characters
| `char-foldcase`: |  | R7RS
| `char-general-category`: |  | Unicode
| `char-graphic?`: |  | Prompting
| `char-hash`: |  | SRFI 128
| `char-in`: |  | Regular S-Expressions
| `char-in-set?`: |  | Character Sets
| `char-lower-case?`: |  | Characters
| `char-lower-case?`: |  | R7RS
| `char-not-in`: |  | Regular S-Expressions
| `char-numeric?`: |  | Characters
| `char-numeric?`: |  | R7RS
| `char-predicate`: |  | Character implementation
| `char-ready?`: |  | Input Procedures
| `char-ready?`: |  | R7RS
| `char-ready? on textual input port`: |  | Textual Input Port Operations
| `char-set`: |  | Character Sets
| `char-set`: |  | *Matcher
| `char-set`: |  | SRFI 14
| `char-set*`: |  | Character Sets
| `char-set->code-points`: |  | Character Sets
| `char-set->list`: |  | SRFI 14
| `char-set->sre`: |  | SRFI 115
| `char-set->string`: |  | SRFI 14
| `char-set-adjoin`: |  | SRFI 14
| `char-set-adjoin!`: |  | SRFI 14
| `char-set-any`: |  | SRFI 14
| `char-set-complement`: |  | SRFI 14
| `char-set-complement!`: |  | SRFI 14
| `char-set-contains?`: |  | SRFI 14
| `char-set-copy`: |  | SRFI 14
| `char-set-count`: |  | SRFI 14
| `char-set-cursor`: |  | SRFI 14
| `char-set-cursor-next`: |  | SRFI 14
| `char-set-delete`: |  | SRFI 14
| `char-set-delete!`: |  | SRFI 14
| `char-set-diff+intersection`: |  | SRFI 14
| `char-set-diff+intersection!`: |  | SRFI 14
| `char-set-difference`: |  | Character Sets
| `char-set-difference`: |  | SRFI 14
| `char-set-difference!`: |  | SRFI 14
| `char-set-every`: |  | SRFI 14
| `char-set-filter`: |  | SRFI 14
| `char-set-filter!`: |  | SRFI 14
| `char-set-fold`: |  | SRFI 14
| `char-set-for-each`: |  | SRFI 14
| `char-set-hash`: |  | SRFI 14
| `char-set-intersection`: |  | Character Sets
| `char-set-intersection`: |  | SRFI 14
| `char-set-intersection!`: |  | SRFI 14
| `char-set-intersection*`: |  | Character Sets
| `char-set-invert`: |  | Character Sets
| `char-set-map`: |  | SRFI 14
| `char-set-predicate`: |  | Character Sets
| `char-set-ref`: |  | SRFI 14
| `char-set-size`: |  | SRFI 14
| `char-set-unfold`: |  | SRFI 14
| `char-set-unfold!`: |  | SRFI 14
| `char-set-union`: |  | Character Sets
| `char-set-union`: |  | SRFI 14
| `char-set-union!`: |  | SRFI 14
| `char-set-union*`: |  | Character Sets
| `char-set-xor`: |  | SRFI 14
| `char-set-xor!`: |  | SRFI 14
| `char-set:alphabetic`: |  | Character Sets
| `char-set:alphanumeric`: |  | Character Sets
| `char-set:ascii`: |  | SRFI 14
| `char-set:blank`: |  | SRFI 14
| `char-set:digit`: |  | SRFI 14
| `char-set:empty`: |  | SRFI 14
| `char-set:full`: |  | SRFI 14
| `char-set:graphic`: |  | SRFI 14
| `char-set:hex-digit`: |  | SRFI 14
| `char-set:iso-control`: |  | SRFI 14
| `char-set:letter`: |  | SRFI 14
| `char-set:letter+digit`: |  | SRFI 14
| `char-set:lower-case`: |  | Character Sets
| `char-set:lower-case`: |  | SRFI 14
| `char-set:numeric`: |  | Character Sets
| `char-set:printing`: |  | SRFI 14
| `char-set:punctuation`: |  | SRFI 14
| `char-set:symbol`: |  | SRFI 14
| `char-set:title-case`: |  | SRFI 14
| `char-set:upper-case`: |  | Character Sets
| `char-set:upper-case`: |  | SRFI 14
| `char-set:whitespace`: |  | Character Sets
| `char-set:whitespace`: |  | Strings
| `char-set:whitespace`: |  | SRFI 14
| `char-set<=`: |  | SRFI 14
| `char-set=`: |  | SRFI 14
| `char-set=?`: |  | Character Sets
| `char-set?`: |  | Character Sets
| `char-set?`: |  | SRFI 14
| `char-upcase`: |  | Characters
| `char-upcase`: |  | R7RS
| `char-upper-case?`: |  | Characters
| `char-upper-case?`: |  | R7RS
| `char-whitespace?`: |  | Characters
| `char-whitespace?`: |  | Strings
| `char-whitespace?`: |  | R7RS
| `char<=?`: |  | Characters
| `char<=?`: |  | R7RS
| `char<?`: |  | Characters
| `char<?`: |  | R7RS
| `char=?`: |  | Equivalence Predicates
| `char=?`: |  | Equivalence Predicates
| `char=?`: |  | Characters
| `char=?`: |  | R7RS
| `char>=?`: |  | Characters
| `char>=?`: |  | R7RS
| `char>?`: |  | Characters
| `char>?`: |  | R7RS
| `char?`: |  | Characters
| `char?`: |  | R7RS
| `chars-remaining on textual input port`: |  | Textual Input Port Operations
| `circular-generator`: |  | SRFI 158
| `circular-list`: |  | Miscellaneous List Operations
| `circular-list`: |  | SRFI 1
| `circular-list?`: |  | Selecting List Components
| `circular-list?`: |  | SRFI 1
| `clear`: |  | Output Procedures
| `close-all-open-files`: |  | File Ports
| `close-input-port`: |  | Ports
| `close-input-port`: |  | R7RS
| `close-output-port`: |  | Ports
| `close-output-port`: |  | R7RS
| `close-port`: |  | Ports
| `close-port`: |  | R7RS
| `close-syntax`: |  | SC Transformer Definition
| `close-tcp-server-socket`: |  | TCP Sockets
| `code-point-general-category`: |  | Unicode
| `code-point-in-set?`: |  | Character Sets
| `command-line`: |  | R7RS
| `comparator-check-type`: |  | SRFI 128
| `comparator-equality-predicate`: |  | SRFI 128
| `comparator-hash`: |  | SRFI 128
| `comparator-hash-function`: |  | SRFI 128
| `comparator-hashable?`: |  | SRFI 128
| `comparator-if<=>`: |  | SRFI 128
| `comparator-ordered?`: |  | SRFI 128
| `comparator-ordering-predicate`: |  | SRFI 128
| `comparator-register-default!`: |  | SRFI 128
| `comparator-test-type`: |  | SRFI 128
| `comparator-type-test-predicate`: |  | SRFI 128
| `comparator?`: |  | SRFI 128
| `compile-regsexp`: |  | Regsexp Procedures
| `compiled-procedure?`: |  | Procedure Operations
| `complex?`: |  | Numerical operations
| `complex?`: |  | R7RS
| `compound-procedure?`: |  | Procedure Operations
| `compute-char-set`: |  | Character Sets
| `conc-name`: |  | Structure Definitions
| `concatenate`: |  | SRFI 1
| `concatenate!`: |  | SRFI 1
| `cond`: |  | Entry Format
| `cond`: |  | Conditionals
| `cond`: |  | Sequencing
| `cond`: |  | Booleans
| `cond`: |  | R7RS
| `cond-expand`: |  | cond-expand (SRFI 0)
| `cond-expand`: |  | R7RS
| `condition-accessor`: |  | Generating Operations on Conditions
| `condition-accessor`: |  | Generating Operations on Conditions
| `condition-accessor`: |  | Simple Condition Instance Operations
| `condition-constructor`: |  | Generating Operations on Conditions
| `condition-constructor`: |  | Generating Operations on Conditions
| `condition-predicate`: |  | Generating Operations on Conditions
| `condition-predicate`: |  | Generating Operations on Conditions
| `condition-signaller`: |  | Generating Operations on Conditions
| `condition-signaller`: |  | Generating Operations on Conditions
| `condition-type/error?`: |  | Condition Types
| `condition-type/field-names`: |  | Condition Types
| `condition-type/generalizations`: |  | Condition Types
| `condition-type:arithmetic-error`: |  | Taxonomy
| `condition-type:bad-range-argument`: |  | Numerical operations
| `condition-type:bad-range-argument`: |  | Integer Conversions of Bit Strings
| `condition-type:bad-range-argument`: |  | Integer Conversions of Bit Strings
| `condition-type:bad-range-argument`: |  | Object Hashing
| `condition-type:bad-range-argument`: |  | Taxonomy
| `condition-type:breakpoint`: |  | Error System
| `condition-type:breakpoint`: |  | Taxonomy
| `condition-type:control-error`: |  | Taxonomy
| `condition-type:datum-out-of-range`: |  | Taxonomy
| `condition-type:derived-file-error`: |  | Taxonomy
| `condition-type:derived-port-error`: |  | Taxonomy
| `condition-type:divide-by-zero`: |  | Taxonomy
| `condition-type:error`: |  | Condition Handling
| `condition-type:error`: |  | Condition State
| `condition-type:error`: |  | Condition Types
| `condition-type:error`: |  | Taxonomy
| `condition-type:file-error`: |  | Taxonomy
| `condition-type:file-operation-error`: |  | Miscellaneous Pathnames
| `condition-type:file-operation-error`: |  | File Manipulation
| `condition-type:file-operation-error`: |  | File Manipulation
| `condition-type:file-operation-error`: |  | Taxonomy
| `condition-type:floating-point-overflow`: |  | Taxonomy
| `condition-type:floating-point-underflow`: |  | Taxonomy
| `condition-type:illegal-datum`: |  | Taxonomy
| `condition-type:inapplicable-object`: |  | Taxonomy
| `condition-type:macro-binding`: |  | Environment Operations
| `condition-type:no-such-restart`: |  | Invoking Standard Restart Code
| `condition-type:no-such-restart`: |  | Invoking Standard Restart Code
| `condition-type:no-such-restart`: |  | Taxonomy
| `condition-type:not-loading`: |  | Taxonomy
| `condition-type:port-error`: |  | Taxonomy
| `condition-type:primitive-procedure-error`: |  | Taxonomy
| `condition-type:serious-condition`: |  | Error System
| `condition-type:serious-condition`: |  | Taxonomy
| `condition-type:simple-condition`: |  | Error System
| `condition-type:simple-condition`: |  | Taxonomy
| `condition-type:simple-error`: |  | Condition Signalling
| `condition-type:simple-error`: |  | Taxonomy
| `condition-type:simple-warning`: |  | Condition Signalling
| `condition-type:simple-warning`: |  | Taxonomy
| `condition-type:subprocess-abnormal-termination`: |  | Subprocess Conditions
| `condition-type:subprocess-signalled`: |  | Subprocess Conditions
| `condition-type:subprocess-stopped`: |  | Subprocess Conditions
| `condition-type:system-call-error`: |  | Taxonomy
| `condition-type:unassigned-variable`: |  | Variable Bindings
| `condition-type:unassigned-variable`: |  | Environment Operations
| `condition-type:unassigned-variable`: |  | Taxonomy
| `condition-type:unbound-variable`: |  | Environment Concepts
| `condition-type:unbound-variable`: |  | Dynamic Binding
| `condition-type:unbound-variable`: |  | Taxonomy
| `condition-type:variable-error`: |  | Taxonomy
| `condition-type:warning`: |  | Error System
| `condition-type:warning`: |  | Taxonomy
| `condition-type:wrong-number-of-arguments`: |  | Lambda Expressions
| `condition-type:wrong-number-of-arguments`: |  | Lambda Expressions
| `condition-type:wrong-number-of-arguments`: |  | Arity
| `condition-type:wrong-number-of-arguments`: |  | Taxonomy
| `condition-type:wrong-type-argument`: |  | Entry Format
| `condition-type:wrong-type-argument`: |  | Taxonomy
| `condition-type:wrong-type-datum`: |  | Taxonomy
| `condition-type?`: |  | Condition Types
| `condition/continuation`: |  | Condition State
| `condition/error?`: |  | Condition State
| `condition/report-string`: |  | Simple Condition Instance Operations
| `condition/restarts`: |  | Generating Operations on Conditions
| `condition/restarts`: |  | Condition State
| `condition/restarts`: |  | Simple Condition Instance Operations
| `condition/type`: |  | Condition State
| `condition?`: |  | Condition State
| `conjugate`: |  | Numerical operations
| `cons`: |  | Pairs
| `cons`: |  | R7RS
| `cons`: |  | SRFI 1
| `cons*`: |  | Construction of Lists
| `cons*`: |  | SRFI 1
| `cons-stream`: |  | Streams
| `console-i/o-port`: |  | Ports
| `constructor`: |  | Structure Definitions
| `continuation?`: |  | Continuations
| `continue`: |  | Restarts
| `continue`: |  | Invoking Standard Restart Code
| `copier`: |  | Structure Definitions
| `copy-area on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `copy-area on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `copy-bitmap`: |  | DIB procedures
| `copy-file`: |  | File Manipulation
| `copysign`: |  | Numerical operations
| `cos`: |  | Numerical operations
| `cos`: |  | R7RS
| `count`: |  | SRFI 1
| `create-dib`: |  | DIB procedures
| `create-image on graphics-device`: |  | Images
| `crop-bitmap`: |  | DIB procedures
| `current-error-port`: |  | Ports
| `current-error-port`: |  | R7RS
| `current-file-time`: |  | File Manipulation
| `current-input-port`: |  | Ports
| `current-input-port`: |  | File Ports
| `current-input-port`: |  | R7RS
| `current-jiffy`: |  | R7RS
| `current-output-port`: |  | Ports
| `current-output-port`: |  | File Ports
| `current-output-port`: |  | R7RS
| `current-parser-macros`: |  | Parser-language Macros
| `current-second`: |  | R7RS

D |  |
| `day-of-week/long-string`: |  | External Representation of Time
| `day-of-week/short-string`: |  | External Representation of Time
| `debug`: |  | Prompting
| `decoded-time->file-time`: |  | Time-Format Conversion
| `decoded-time->string`: |  | Time-Format Conversion
| `decoded-time->universal-time`: |  | Time-Format Conversion
| `decoded-time/date-string`: |  | External Representation of Time
| `decoded-time/day`: |  | Decoded Time
| `decoded-time/day-of-week`: |  | Decoded Time
| `decoded-time/daylight-savings-time?`: |  | Decoded Time
| `decoded-time/hour`: |  | Decoded Time
| `decoded-time/minute`: |  | Decoded Time
| `decoded-time/month`: |  | Decoded Time
| `decoded-time/second`: |  | Decoded Time
| `decoded-time/time-string`: |  | External Representation of Time
| `decoded-time/year`: |  | Decoded Time
| `decoded-time/zone`: |  | Decoded Time
| `default-hash`: |  | SRFI 128
| `default-object?`: |  | Lambda Expressions
| `default-random-source`: |  | Random Number Generation
| `default-random-source`: |  | SRFI 27
| `define`: |  | Environment Concepts
| `define`: |  | Definitions
| `define`: |  | Definitions
| `define`: |  | Internal Definitions
| `define`: |  | Sequencing
| `define`: |  | Environment Variables
| `define`: |  | R7RS
| `define-*matcher-expander`: |  | Parser-language Macros
| `define-*matcher-macro`: |  | Parser-language Macros
| `define-*parser-expander`: |  | Parser-language Macros
| `define-*parser-macro`: |  | Parser-language Macros
| `define-color on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `define-print-method`: |  | Custom Output
| `define-record-type`: |  | define-record-type (SRFI 9)
| `define-record-type`: |  | R7RS
| `define-record-type`: |  | SRFI 9
| `define-record-type`: |  | SRFI 131
| `define-similar-windows-type`: |  | Windows Types
| `define-structure`: |  | Structure Definitions
| `define-structure`: |  | Records
| `define-syntax`: |  | Syntactic Binding Constructs
| `define-syntax`: |  | R7RS
| `define-values`: |  | R7RS
| `define-windows-type`: |  | Windows Types
| `del-assoc`: |  | Association Lists
| `del-assoc!`: |  | Association Lists
| `del-assq`: |  | Association Lists
| `del-assq!`: |  | Association Lists
| `del-assv`: |  | Association Lists
| `del-assv!`: |  | Association Lists
| `delay`: |  | Lexical Binding
| `delay`: |  | Promises
| `delay`: |  | R7RS
| `delay-force`: |  | R7RS
| `delete`: |  | Filtering Lists
| `delete`: |  | SRFI 1
| `delete!`: |  | Filtering Lists
| `delete!`: |  | Filtering Lists
| `delete!`: |  | SRFI 1
| `delete-association-procedure`: |  | Association Lists
| `delete-dib`: |  | DIB procedures
| `delete-directory`: |  | File Manipulation
| `delete-duplicates`: |  | SRFI 1
| `delete-duplicates!`: |  | SRFI 1
| `delete-file`: |  | File Manipulation
| `delete-file`: |  | R7RS
| `delete-file-no-errors`: |  | File Manipulation
| `delete-member-procedure`: |  | Filtering Lists
| `delq`: |  | Filtering Lists
| `delq!`: |  | Filtering Lists
| `delv`: |  | Filtering Lists
| `delv`: |  | Filtering Lists
| `delv!`: |  | Filtering Lists
| `denominator`: |  | Numerical operations
| `denominator`: |  | R7RS
| `dib`: |  | Device Independent Bitmap Utilities
| `dib-blt`: |  | DIB procedures
| `dib-from-bitmap`: |  | DIB procedures
| `dib-height`: |  | DIB procedures
| `dib-set-pixels-unaligned`: |  | DIB procedures
| `dib-width`: |  | DIB procedures
| `digit->char`: |  | Characters
| `digit-value`: |  | Characters
| `digit-value`: |  | R7RS
| `directory-namestring`: |  | Operations on Pathnames
| `directory-pathname`: |  | Operations on Pathnames
| `directory-pathname-as-file`: |  | Operations on Pathnames
| `directory-pathname?`: |  | Operations on Pathnames
| `directory-read`: |  | Directory Reader
| `discard-chars on textual input port`: |  | Textual Input Port Operations
| `discard-matched`: |  | *Matcher
| `discard-matched`: |  | *Parser
| `discard-parser-buffer-head!`: |  | Parser Buffers
| `discretionary-flush-output`: |  | Output Procedures
| `discretionary-flush-output on textual output port`: |  | Textual Output Port Operations
| `display`: |  | Output Procedures
| `display`: |  | Error Messages
| `display`: |  | R7RS
| `do`: |  | Environment Concepts
| `do`: |  | Sequencing
| `do`: |  | Iteration
| `do`: |  | R7RS
| `dotted-list?`: |  | Selecting List Components
| `dotted-list?`: |  | SRFI 1
| `draw-arc`: |  | Custom Operations on X Graphics Devices
| `draw-arc on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `draw-circle`: |  | Custom Operations on X Graphics Devices
| `draw-circle on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `draw-ellipse on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `draw-image on graphics-device`: |  | Images
| `draw-subimage on graphics-device`: |  | Images
| `drop`: |  | Selecting List Components
| `drop`: |  | Cutting and Pasting Lists
| `drop`: |  | SRFI 1
| `drop-right`: |  | SRFI 1
| `drop-right!`: |  | SRFI 1
| `drop-while`: |  | SRFI 1
| `dynamic-wind`: |  | Continuations
| `dynamic-wind`: |  | R7RS

E |  |
| `eighth`: |  | Selecting List Components
| `eighth`: |  | SRFI 1
| `else`: |  | Entry Format
| `else`: |  | Conditionals
| `else`: |  | Conditionals
| `else`: |  | R7RS
| `emergency-exit`: |  | R7RS
| `encapsulate`: |  | *Parser
| `end-of-char-set?`: |  | SRFI 14
| `end-of-input`: |  | *Matcher
| `enough-namestring`: |  | Operations on Pathnames
| `enough-pathname`: |  | Operations on Pathnames
| `entity-extra`: |  | Application Hooks
| `entity-procedure`: |  | Application Hooks
| `entity?`: |  | Application Hooks
| `enumerate-graphics-types`: |  | Opening and Closing of Graphics Devices
| `environment`: |  | Subprocess Options
| `environment`: |  | R7RS
| `environment-assign!`: |  | Environment Operations
| `environment-assignable?`: |  | Environment Operations
| `environment-assigned?`: |  | Environment Operations
| `environment-bindings`: |  | Environment Operations
| `environment-bound-names`: |  | Environment Operations
| `environment-bound?`: |  | Environment Operations
| `environment-definable?`: |  | Environment Operations
| `environment-define`: |  | Environment Operations
| `environment-define-macro`: |  | Environment Operations
| `environment-has-parent?`: |  | Environment Operations
| `environment-lookup`: |  | Environment Operations
| `environment-lookup-macro`: |  | Environment Operations
| `environment-macro-names`: |  | Environment Operations
| `environment-parent`: |  | Environment Operations
| `environment-reference-type`: |  | Environment Operations
| `environment?`: |  | Environment Operations
| `eof-object`: |  | Input Procedures
| `eof-object`: |  | R7RS
| `eof-object?`: |  | Input Procedures
| `eof-object?`: |  | R7RS
| `eof? on textual input port`: |  | Textual Input Port Operations
| `ephemeron-broken?`: |  | Ephemerons
| `ephemeron-broken?`: |  | SRFI 124
| `ephemeron-datum`: |  | Ephemerons
| `ephemeron-datum`: |  | SRFI 124
| `ephemeron-key`: |  | Ephemerons
| `ephemeron-key`: |  | SRFI 124
| `ephemeron?`: |  | Ephemerons
| `ephemeron?`: |  | SRFI 124
| `epoch`: |  | Universal Time
| `eq-hash`: |  | Construction of Hash Tables
| `eq-hash`: |  | Address Hashing
| `eq-hash-mod`: |  | Address Hashing
| `eq?`: |  | Equivalence Predicates
| `eq?`: |  | Equivalence Predicates
| `eq?`: |  | Symbols
| `eq?`: |  | Symbols
| `eq?`: |  | Association Lists
| `eq?`: |  | Association Lists
| `eq?`: |  | Association Lists
| `eq?`: |  | The Association Table
| `eq?`: |  | Construction of Hash Tables
| `eq?`: |  | Construction of Hash Tables
| `eq?`: |  | Construction of Hash Tables
| `eq?`: |  | Construction of Hash Tables
| `eq?`: |  | Construction of Hash Tables
| `eq?`: |  | Object Hashing
| `eq?`: |  | The Named Restart Abstraction
| `eq?`: |  | R7RS
| `equal-hash`: |  | Construction of Hash Tables
| `equal-hash`: |  | Address Hashing
| `equal-hash-mod`: |  | Address Hashing
| `equal?`: |  | Quoting
| `equal?`: |  | Equivalence Predicates
| `equal?`: |  | Equivalence Predicates
| `equal?`: |  | Association Lists
| `equal?`: |  | Association Lists
| `equal?`: |  | Association Lists
| `equal?`: |  | Construction of Hash Tables
| `equal?`: |  | Construction of Hash Tables
| `equal?`: |  | R7RS
| `eqv-hash`: |  | Address Hashing
| `eqv-hash-mod`: |  | Address Hashing
| `eqv?`: |  | Storage Model
| `eqv?`: |  | Conditionals
| `eqv?`: |  | Equivalence Predicates
| `eqv?`: |  | Equivalence Predicates
| `eqv?`: |  | Pairs
| `eqv?`: |  | Association Lists
| `eqv?`: |  | Association Lists
| `eqv?`: |  | Association Lists
| `eqv?`: |  | Construction of Hash Tables
| `eqv?`: |  | Construction of Hash Tables
| `eqv?`: |  | Construction of Hash Tables
| `eqv?`: |  | Construction of Hash Tables
| `eqv?`: |  | R7RS
| `er-macro-transformer`: |  | Explicit Renaming
| `error`: |  | Errors
| `error`: |  | Error System
| `error`: |  | Condition Signalling
| `error`: |  | Condition Signalling
| `error`: |  | Condition Handling
| `error`: |  | Condition Handling
| `error`: |  | R7RS
| `error`: |  | SRFI 23
| `error-irritant/noise`: |  | Error Messages
| `error-object-irritants`: |  | R7RS
| `error-object-message`: |  | R7RS
| `error-object?`: |  | R7RS
| `error:bad-range-argument`: |  | Taxonomy
| `error:datum-out-of-range`: |  | Taxonomy
| `error:derived-file`: |  | Taxonomy
| `error:derived-port`: |  | Taxonomy
| `error:divide-by-zero`: |  | Taxonomy
| `error:file-operation`: |  | Taxonomy
| `error:no-such-restart`: |  | Taxonomy
| `error:wrong-number-of-arguments`: |  | Taxonomy
| `error:wrong-type-argument`: |  | Taxonomy
| `error:wrong-type-datum`: |  | Taxonomy
| `eval`: |  | Environment Operations
| `eval`: |  | R7RS
| `even?`: |  | Numerical operations
| `even?`: |  | R7RS
| `every`: |  | Mapping of Lists
| `every`: |  | SRFI 1
| `exact`: |  | Exactness
| `exact`: |  | Numerical operations
| `exact`: |  | R7RS
| `exact->inexact`: |  | Numerical operations
| `exact-integer-sqrt`: |  | R7RS
| `exact-integer?`: |  | Numerical operations
| `exact-integer?`: |  | R7RS
| `exact-nonnegative-integer?`: |  | Numerical operations
| `exact-rational?`: |  | Numerical operations
| `exact?`: |  | Numerical operations
| `exact?`: |  | R7RS
| `except-last-pair`: |  | Cutting and Pasting Lists
| `except-last-pair!`: |  | Cutting and Pasting Lists
| `exit`: |  | R7RS
| `exp`: |  | Numerical operations
| `exp`: |  | R7RS
| `expm1`: |  | Numerical operations
| `export-random-state`: |  | Random Number Generation
| `expt`: |  | Numerical operations
| `expt`: |  | R7RS
| `extend-top-level-environment`: |  | Top-level Environments

F |  |
| `false`: |  | Booleans
| `false?`: |  | Booleans
| `features`: |  | R7RS
| `fifth`: |  | Selecting List Components
| `fifth`: |  | SRFI 1
| `file-access`: |  | File Manipulation
| `file-access-time`: |  | File Manipulation
| `file-access-time-direct`: |  | File Manipulation
| `file-access-time-indirect`: |  | File Manipulation
| `file-attributes`: |  | File Manipulation
| `file-attributes-direct`: |  | File Manipulation
| `file-attributes-indirect`: |  | File Manipulation
| `file-attributes/access-time`: |  | File Manipulation
| `file-attributes/change-time`: |  | File Manipulation
| `file-attributes/gid`: |  | File Manipulation
| `file-attributes/inode-number`: |  | File Manipulation
| `file-attributes/length`: |  | File Manipulation
| `file-attributes/mode-string`: |  | File Manipulation
| `file-attributes/modes`: |  | File Manipulation
| `file-attributes/modification-time`: |  | File Manipulation
| `file-attributes/n-links`: |  | File Manipulation
| `file-attributes/type`: |  | File Manipulation
| `file-attributes/uid`: |  | File Manipulation
| `file-directory?`: |  | File Manipulation
| `file-eq?`: |  | File Manipulation
| `file-error?`: |  | File Ports
| `file-error?`: |  | File Ports
| `file-error?`: |  | File Ports
| `file-error?`: |  | R7RS
| `file-executable?`: |  | File Manipulation
| `file-exists-direct?`: |  | File Manipulation
| `file-exists-indirect?`: |  | File Manipulation
| `file-exists?`: |  | File Manipulation
| `file-exists?`: |  | R7RS
| `file-length`: |  | File Manipulation
| `file-modes`: |  | File Manipulation
| `file-modification-time`: |  | File Manipulation
| `file-modification-time-direct`: |  | File Manipulation
| `file-modification-time-indirect`: |  | File Manipulation
| `file-namestring`: |  | Operations on Pathnames
| `file-pathname`: |  | Operations on Pathnames
| `file-readable?`: |  | File Manipulation
| `file-regular?`: |  | File Manipulation
| `file-symbolic-link?`: |  | File Manipulation
| `file-time->global-decoded-time`: |  | Time-Format Conversion
| `file-time->global-time-string`: |  | Time-Format Conversion
| `file-time->local-decoded-time`: |  | Time-Format Conversion
| `file-time->local-time-string`: |  | Time-Format Conversion
| `file-time->universal-time`: |  | Time-Format Conversion
| `file-touch`: |  | File Manipulation
| `file-type-direct`: |  | File Manipulation
| `file-type-indirect`: |  | File Manipulation
| `file-writeable?`: |  | File Manipulation
| `fill-circle`: |  | Custom Operations on X Graphics Devices
| `fill-circle on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `fill-polygon`: |  | Custom Operations for Win32 Graphics
| `fill-polygon on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `fill-with`: |  | Strings
| `filter`: |  | Filtering Lists
| `filter`: |  | SRFI 1
| `filter!`: |  | Filtering Lists
| `filter!`: |  | SRFI 1
| `filter-map`: |  | SRFI 1
| `find`: |  | Searching Lists
| `find`: |  | SRFI 1
| `find-color on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `find-module`: |  | Windows Foreign Procedures
| `find-restart`: |  | Restarts
| `find-restart`: |  | Finding and Invoking General Restart Code
| `find-restart`: |  | Finding and Invoking General Restart Code
| `find-tail`: |  | Searching Lists
| `find-tail`: |  | SRFI 1
| `finite?`: |  | R7RS
| `first`: |  | Selecting List Components
| `first`: |  | SRFI 1
| `fix:*`: |  | Fixnum Operations
| `fix:+`: |  | Fixnum Operations
| `fix:-`: |  | Fixnum Operations
| `fix:-1+`: |  | Fixnum Operations
| `fix:1+`: |  | Fixnum Operations
| `fix:<`: |  | Fixnum Operations
| `fix:<=`: |  | Fixnum Operations
| `fix:=`: |  | Fixnum Operations
| `fix:>`: |  | Fixnum Operations
| `fix:>=`: |  | Fixnum Operations
| `fix:and`: |  | Fixnum Operations
| `fix:andc`: |  | Fixnum Operations
| `fix:divide`: |  | Fixnum Operations
| `fix:fixnum?`: |  | Fixnum Operations
| `fix:gcd`: |  | Fixnum Operations
| `fix:lsh`: |  | Fixnum Operations
| `fix:negative?`: |  | Fixnum Operations
| `fix:not`: |  | Fixnum Operations
| `fix:or`: |  | Fixnum Operations
| `fix:positive?`: |  | Fixnum Operations
| `fix:quotient`: |  | Fixnum Operations
| `fix:remainder`: |  | Fixnum Operations
| `fix:xor`: |  | Fixnum Operations
| `fix:zero?`: |  | Fixnum Operations
| `fixnum?`: |  | SRFI 143
| `flo:*`: |  | Flonum Operations
| `flo:*+`: |  | Flonum Operations
| `flo:+`: |  | Flonum Operations
| `flo:-`: |  | Flonum Operations
| `flo:/`: |  | Flonum Operations
| `flo:<`: |  | Flonum Operations
| `flo:<=`: |  | Flonum Operations
| `flo:<>`: |  | Flonum Operations
| `flo:=`: |  | Flonum Operations
| `flo:>`: |  | Flonum Operations
| `flo:>=`: |  | Flonum Operations
| `flo:abs`: |  | Flonum Operations
| `flo:acos`: |  | Flonum Operations
| `flo:acosh`: |  | Flonum Operations
| `flo:asin`: |  | Flonum Operations
| `flo:asinh`: |  | Flonum Operations
| `flo:atan`: |  | Flonum Operations
| `flo:atan2`: |  | Flonum Operations
| `flo:atanh`: |  | Flonum Operations
| `flo:cbrt`: |  | Flonum Operations
| `flo:ceiling`: |  | Flonum Operations
| `flo:ceiling->exact`: |  | Flonum Operations
| `flo:classify`: |  | Flonum Operations
| `flo:clear-exceptions!`: |  | Floating-Point Exceptions
| `flo:copysign`: |  | Flonum Operations
| `flo:cos`: |  | Flonum Operations
| `flo:cosh`: |  | Flonum Operations
| `flo:default-environment`: |  | Floating-Point Environment
| `flo:default-rounding-mode`: |  | Floating-Point Rounding Mode
| `flo:default-trapped-exceptions`: |  | Floating-Point Exceptions
| `flo:defer-exception-traps!`: |  | Floating-Point Exceptions
| `flo:deferring-exception-traps`: |  | Floating-Point Exceptions
| `flo:environment`: |  | Floating-Point Environment
| `flo:erf`: |  | Flonum Operations
| `flo:erfc`: |  | Flonum Operations
| `flo:error-bound`: |  | Flonum Operations
| `flo:exception:divide-by-zero`: |  | Floating-Point Exceptions
| `flo:exception:inexact-result`: |  | Floating-Point Exceptions
| `flo:exception:invalid-operation`: |  | Floating-Point Exceptions
| `flo:exception:overflow`: |  | Floating-Point Exceptions
| `flo:exception:subnormal-operand`: |  | Floating-Point Exceptions
| `flo:exception:underflow`: |  | Floating-Point Exceptions
| `flo:exceptions->names`: |  | Floating-Point Exceptions
| `flo:exp`: |  | Flonum Operations
| `flo:expm1`: |  | Flonum Operations
| `flo:expt`: |  | Flonum Operations
| `flo:fast-fma?`: |  | Flonum Operations
| `flo:finite?`: |  | Flonum Operations
| `flo:flonum?`: |  | Flonum Operations
| `flo:floor`: |  | Flonum Operations
| `flo:floor->exact`: |  | Flonum Operations
| `flo:fma`: |  | Flonum Operations
| `flo:gamma`: |  | Flonum Operations
| `flo:greatest-normal-exponent-base-10`: |  | Flonum Operations
| `flo:greatest-normal-exponent-base-2`: |  | Flonum Operations
| `flo:greatest-normal-exponent-base-e`: |  | Flonum Operations
| `flo:have-trap-enable/disable?`: |  | Floating-Point Exceptions
| `flo:hypot`: |  | Flonum Operations
| `flo:ignoring-exception-traps`: |  | Floating-Point Exceptions
| `flo:infinite?`: |  | Flonum Operations
| `flo:j0`: |  | Flonum Operations
| `flo:j1`: |  | Flonum Operations
| `flo:jn`: |  | Flonum Operations
| `flo:largest-positive-normal`: |  | Flonum Operations
| `flo:ldexp`: |  | Flonum Operations
| `flo:least-normal-exponent-base-10`: |  | Flonum Operations
| `flo:least-normal-exponent-base-2`: |  | Flonum Operations
| `flo:least-normal-exponent-base-e`: |  | Flonum Operations
| `flo:least-subnormal-exponent-base-10`: |  | Flonum Operations
| `flo:least-subnormal-exponent-base-2`: |  | Flonum Operations
| `flo:least-subnormal-exponent-base-e`: |  | Flonum Operations
| `flo:lgamma`: |  | Flonum Operations
| `flo:log`: |  | Flonum Operations
| `flo:log-error-bound`: |  | Flonum Operations
| `flo:log-ulp-of-one`: |  | Flonum Operations
| `flo:log1p`: |  | Flonum Operations
| `flo:logb`: |  | Flonum Operations
| `flo:make-nan`: |  | Flonum Operations
| `flo:max`: |  | Flonum Operations
| `flo:max-mag`: |  | Flonum Operations
| `flo:min`: |  | Flonum Operations
| `flo:min-mag`: |  | Flonum Operations
| `flo:names->exceptions`: |  | Floating-Point Exceptions
| `flo:nan-payload`: |  | Flonum Operations
| `flo:nan-quiet?`: |  | Flonum Operations
| `flo:nan?`: |  | Flonum Operations
| `flo:negate`: |  | Flonum Operations
| `flo:negative?`: |  | Flonum Operations
| `flo:nextafter`: |  | Flonum Operations
| `flo:normal-exponent-max`: |  | Flonum Operations
| `flo:normal-exponent-min`: |  | Flonum Operations
| `flo:normal?`: |  | Flonum Operations
| `flo:positive?`: |  | Flonum Operations
| `flo:precision`: |  | Flonum Operations
| `flo:preserving-environment`: |  | Floating-Point Environment
| `flo:radix`: |  | Flonum Operations
| `flo:radix.`: |  | Flonum Operations
| `flo:raise-exceptions!`: |  | Floating-Point Exceptions
| `flo:random-unit`: |  | Random Number Generation
| `flo:random-unit-closed`: |  | Random Number Generation
| `flo:random-unit-open`: |  | Random Number Generation
| `flo:restore-exception-flags!`: |  | Floating-Point Exceptions
| `flo:round`: |  | Flonum Operations
| `flo:round->exact`: |  | Flonum Operations
| `flo:rounding-mode`: |  | Floating-Point Rounding Mode
| `flo:rounding-modes`: |  | Floating-Point Rounding Mode
| `flo:safe-zero?`: |  | Flonum Operations
| `flo:safe<`: |  | Flonum Operations
| `flo:safe<=`: |  | Flonum Operations
| `flo:safe<>`: |  | Flonum Operations
| `flo:safe=`: |  | Flonum Operations
| `flo:safe>`: |  | Flonum Operations
| `flo:safe>=`: |  | Flonum Operations
| `flo:save-exception-flags`: |  | Floating-Point Exceptions
| `flo:scalbn`: |  | Flonum Operations
| `flo:set-environment!`: |  | Floating-Point Environment
| `flo:set-rounding-mode!`: |  | Floating-Point Rounding Mode
| `flo:set-trapped-exceptions!`: |  | Floating-Point Exceptions
| `flo:sign-negative?`: |  | Flonum Operations
| `flo:signed-lgamma`: |  | Flonum Operations
| `flo:sin`: |  | Flonum Operations
| `flo:sinh`: |  | Flonum Operations
| `flo:smallest-positive-normal`: |  | Flonum Operations
| `flo:smallest-positive-subnormal`: |  | Flonum Operations
| `flo:sqrt`: |  | Flonum Operations
| `flo:subnormal-exponent-min`: |  | Flonum Operations
| `flo:subnormal?`: |  | Flonum Operations
| `flo:supported-exceptions`: |  | Floating-Point Exceptions
| `flo:tan`: |  | Flonum Operations
| `flo:tanh`: |  | Flonum Operations
| `flo:test-exception-flags`: |  | Floating-Point Exceptions
| `flo:test-exceptions`: |  | Floating-Point Exceptions
| `flo:total-mag<`: |  | Flonum Operations
| `flo:total-order`: |  | Flonum Operations
| `flo:total-order-mag`: |  | Flonum Operations
| `flo:total<`: |  | Flonum Operations
| `flo:trap-exceptions!`: |  | Floating-Point Exceptions
| `flo:trapped-exceptions`: |  | Floating-Point Exceptions
| `flo:truncate`: |  | Flonum Operations
| `flo:truncate->exact`: |  | Flonum Operations
| `flo:ulp`: |  | Flonum Operations
| `flo:ulp-of-one`: |  | Flonum Operations
| `flo:unordered?`: |  | Flonum Operations
| `flo:untrap-exceptions!`: |  | Floating-Point Exceptions
| `flo:update-environment!`: |  | Floating-Point Environment
| `flo:with-exceptions-trapped`: |  | Floating-Point Exceptions
| `flo:with-exceptions-untrapped`: |  | Floating-Point Exceptions
| `flo:with-rounding-mode`: |  | Floating-Point Rounding Mode
| `flo:with-trapped-exceptions`: |  | Floating-Point Exceptions
| `flo:y0`: |  | Flonum Operations
| `flo:y1`: |  | Flonum Operations
| `flo:yn`: |  | Flonum Operations
| `flo:zero?`: |  | Flonum Operations
| `flonum-parser-fast?`: |  | Numerical input and output
| `flonum-unparser-cutoff`: |  | Numerical input and output
| `floor`: |  | Numerical operations
| `floor`: |  | R7RS
| `floor->exact`: |  | Numerical operations
| `floor-quotient`: |  | R7RS
| `floor-remainder`: |  | R7RS
| `floor/`: |  | R7RS
| `fluid-let`: |  | Dynamic Binding
| `flush-output`: |  | Output Procedures
| `flush-output on textual output port`: |  | Textual Output Port Operations
| `flush-output-port`: |  | Output Procedures
| `flush-output-port`: |  | Output Procedures
| `flush-output-port`: |  | R7RS
| `fold`: |  | Folding of Lists
| `fold`: |  | SRFI 1
| `fold-left`: |  | Folding of Lists
| `fold-right`: |  | Folding of Lists
| `fold-right`: |  | SRFI 1
| `font-structure on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `for-each`: |  | Mapping of Lists
| `for-each`: |  | R7RS
| `for-each`: |  | SRFI 1
| `force`: |  | Promises
| `force`: |  | Streams
| `force`: |  | R7RS
| `format`: |  | Format
| `format-error-message`: |  | Error Messages
| `fourth`: |  | Selecting List Components
| `fourth`: |  | SRFI 1
| `fresh-line`: |  | Output Procedures
| `fresh-line on textual output port`: |  | Textual Output Port Operations
| `fx*`: |  | SRFI 143
| `fx*/carry`: |  | SRFI 143
| `fx+`: |  | SRFI 143
| `fx+/carry`: |  | SRFI 143
| `fx-`: |  | SRFI 143
| `fx-/carry`: |  | SRFI 143
| `fx-greatest`: |  | SRFI 143
| `fx-least`: |  | SRFI 143
| `fx-width`: |  | SRFI 143
| `fx<=?`: |  | SRFI 143
| `fx<?`: |  | SRFI 143
| `fx=?`: |  | SRFI 143
| `fx>=?`: |  | SRFI 143
| `fx>?`: |  | SRFI 143
| `fxabs`: |  | SRFI 143
| `fxand`: |  | SRFI 143
| `fxarithmetic-shift`: |  | SRFI 143
| `fxarithmetic-shift-left`: |  | SRFI 143
| `fxarithmetic-shift-right`: |  | SRFI 143
| `fxbit-count`: |  | SRFI 143
| `fxbit-field`: |  | SRFI 143
| `fxbit-field-reverse`: |  | SRFI 143
| `fxbit-field-rotate`: |  | SRFI 143
| `fxbit-set?`: |  | SRFI 143
| `fxcopy-bit`: |  | SRFI 143
| `fxeven?`: |  | SRFI 143
| `fxfirst-set-bit`: |  | SRFI 143
| `fxif`: |  | SRFI 143
| `fxior`: |  | SRFI 143
| `fxlength`: |  | SRFI 143
| `fxmax`: |  | SRFI 143
| `fxmin`: |  | SRFI 143
| `fxneg`: |  | SRFI 143
| `fxnegative?`: |  | SRFI 143
| `fxnot`: |  | SRFI 143
| `fxodd?`: |  | SRFI 143
| `fxpositive?`: |  | SRFI 143
| `fxquotient`: |  | SRFI 143
| `fxremainder`: |  | SRFI 143
| `fxsqrt`: |  | SRFI 143
| `fxsquare`: |  | SRFI 143
| `fxxor`: |  | SRFI 143
| `fxzero?`: |  | SRFI 143

G |  |
| `gappend`: |  | SRFI 158
| `gc-reclaimed-object`: |  | Weak Pairs
| `gc-reclaimed-object?`: |  | Weak Pairs
| `gcd`: |  | Numerical operations
| `gcd`: |  | R7RS
| `gcons*`: |  | SRFI 158
| `gdelete`: |  | SRFI 158
| `gdi32.dll`: |  | Windows Foreign Procedures
| `gdrop`: |  | SRFI 158
| `gdrop-while`: |  | SRFI 158
| `ge`: |  | Initial and Current Environments
| `ge`: |  | REPL Environment
| `general-car-cdr`: |  | Pairs
| `generate-uninterned-symbol`: |  | Symbols
| `generator`: |  | SRFI 158
| `generator->list`: |  | SRFI 158
| `generator->reverse-list`: |  | SRFI 158
| `generator->string`: |  | SRFI 158
| `generator->vector`: |  | SRFI 158
| `generator-any`: |  | SRFI 158
| `generator-count`: |  | SRFI 158
| `generator-every`: |  | SRFI 158
| `generator-find`: |  | SRFI 158
| `generator-fold`: |  | SRFI 158
| `generator-fold-right`: |  | SRFI 158
| `generator-for-each`: |  | SRFI 158
| `generator-map->list`: |  | SRFI 158
| `generator-unfold`: |  | SRFI 158
| `get-default on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `get-environment-variable`: |  | R7RS
| `get-environment-variables`: |  | R7RS
| `get-host-by-address`: |  | Miscellaneous OS Facilities
| `get-host-by-name`: |  | Miscellaneous OS Facilities
| `get-host-name`: |  | Miscellaneous OS Facilities
| `get-output-bytevector`: |  | Bytevector Ports
| `get-output-bytevector`: |  | Bytevector Ports
| `get-output-bytevector`: |  | R7RS
| `get-output-string`: |  | String Ports
| `get-output-string`: |  | String Ports
| `get-output-string`: |  | R7RS
| `get-parser-buffer-pointer`: |  | Parser Buffers
| `get-parser-buffer-tail`: |  | Parser Buffers
| `get-universal-time`: |  | Universal Time
| `gfilter`: |  | SRFI 158
| `gflatten`: |  | SRFI 158
| `global-decoded-time`: |  | Decoded Time
| `global-parser-macros`: |  | Parser-language Macros
| `gmap`: |  | SRFI 158
| `grapheme-cluster-length`: |  | Strings
| `grapheme-cluster-slice`: |  | Strings
| `graphics-bind-drawing-mode`: |  | Characteristics of Graphics Output
| `graphics-bind-line-style`: |  | Characteristics of Graphics Output
| `graphics-clear`: |  | Drawing Graphics
| `graphics-clear`: |  | Custom Operations on X Graphics Devices
| `graphics-clear`: |  | Custom Operations on X Graphics Devices
| `graphics-close`: |  | Opening and Closing of Graphics Devices
| `graphics-coordinate-limits`: |  | Coordinates for Graphics
| `graphics-device-coordinate-limits`: |  | Coordinates for Graphics
| `graphics-disable-buffering`: |  | Buffering of Graphics Output
| `graphics-drag-cursor`: |  | Drawing Graphics
| `graphics-draw-line`: |  | Drawing Graphics
| `graphics-draw-point`: |  | Drawing Graphics
| `graphics-draw-text`: |  | Drawing Graphics
| `graphics-enable-buffering`: |  | Buffering of Graphics Output
| `graphics-erase-point`: |  | Drawing Graphics
| `graphics-flush`: |  | Buffering of Graphics Output
| `graphics-move-cursor`: |  | Drawing Graphics
| `graphics-operation`: |  | Custom Graphics Operations
| `graphics-reset-clip-rectangle`: |  | Clipping of Graphics Output
| `graphics-set-clip-rectangle`: |  | Clipping of Graphics Output
| `graphics-set-coordinate-limits`: |  | Coordinates for Graphics
| `graphics-set-drawing-mode`: |  | Characteristics of Graphics Output
| `graphics-set-line-style`: |  | Characteristics of Graphics Output
| `graphics-type-available?`: |  | Opening and Closing of Graphics Devices
| `gremove`: |  | SRFI 158
| `group`: |  | Regular S-Expressions
| `group-ref`: |  | Regular S-Expressions
| `gtake`: |  | SRFI 158
| `gtake-while`: |  | SRFI 158
| `guarantee-procedure-of-arity`: |  | Arity
| `guard`: |  | R7RS

H |  |
| `handle`: |  | Windows Types
| `hard-link-file`: |  | File Manipulation
| `hash`: |  | Address Hashing
| `hash`: |  | Object Hashing
| `hash`: |  | SRFI 69
| `hash`: |  | SRFI 125
| `hash-bound`: |  | SRFI 128
| `hash-by-eqv`: |  | Address Hashing
| `hash-by-identity`: |  | Address Hashing
| `hash-by-identity`: |  | SRFI 69
| `hash-by-identity`: |  | SRFI 125
| `hash-object`: |  | Object Hashing
| `hash-salt`: |  | SRFI 128
| `hash-table`: |  | SRFI 125
| `hash-table->alist`: |  | Basic Hash Table Operations
| `hash-table->alist`: |  | SRFI 69
| `hash-table->alist`: |  | SRFI 125
| `hash-table-clean!`: |  | Basic Hash Table Operations
| `hash-table-clear!`: |  | Basic Hash Table Operations
| `hash-table-clear!`: |  | SRFI 125
| `hash-table-constructor`: |  | Construction of Hash Tables
| `hash-table-constructor`: |  | Construction of Hash Tables
| `hash-table-contains?`: |  | SRFI 125
| `hash-table-copy`: |  | SRFI 69
| `hash-table-copy`: |  | SRFI 125
| `hash-table-count`: |  | SRFI 125
| `hash-table-delete!`: |  | Basic Hash Table Operations
| `hash-table-delete!`: |  | SRFI 69
| `hash-table-delete!`: |  | SRFI 125
| `hash-table-difference!`: |  | SRFI 125
| `hash-table-empty-copy`: |  | SRFI 125
| `hash-table-empty?`: |  | SRFI 125
| `hash-table-entries`: |  | SRFI 125
| `hash-table-entry-type:datum-ephemeral`: |  | Construction of Hash Tables
| `hash-table-entry-type:datum-weak`: |  | Construction of Hash Tables
| `hash-table-entry-type:key&datum-ephemeral`: |  | Construction of Hash Tables
| `hash-table-entry-type:key&datum-weak`: |  | Construction of Hash Tables
| `hash-table-entry-type:key-ephemeral`: |  | Construction of Hash Tables
| `hash-table-entry-type:key-weak`: |  | Construction of Hash Tables
| `hash-table-entry-type:key/datum-weak`: |  | Construction of Hash Tables
| `hash-table-entry-type:strong`: |  | Construction of Hash Tables
| `hash-table-equivalence-function`: |  | SRFI 69
| `hash-table-equivalence-function`: |  | SRFI 125
| `hash-table-exists?`: |  | SRFI 69
| `hash-table-exists?`: |  | SRFI 125
| `hash-table-find`: |  | SRFI 125
| `hash-table-fold`: |  | SRFI 69
| `hash-table-fold`: |  | SRFI 125
| `hash-table-for-each`: |  | SRFI 125
| `hash-table-grow-size`: |  | Resizing of Hash Tables
| `hash-table-hash-function`: |  | SRFI 69
| `hash-table-hash-function`: |  | SRFI 125
| `hash-table-intern!`: |  | Basic Hash Table Operations
| `hash-table-intern!`: |  | SRFI 125
| `hash-table-intersection!`: |  | SRFI 125
| `hash-table-keys`: |  | Basic Hash Table Operations
| `hash-table-keys`: |  | SRFI 69
| `hash-table-keys`: |  | SRFI 125
| `hash-table-map`: |  | SRFI 125
| `hash-table-map!`: |  | SRFI 125
| `hash-table-map->list`: |  | SRFI 125
| `hash-table-merge!`: |  | SRFI 69
| `hash-table-merge!`: |  | SRFI 125
| `hash-table-mutable?`: |  | SRFI 125
| `hash-table-pop!`: |  | SRFI 125
| `hash-table-prune!`: |  | SRFI 125
| `hash-table-ref`: |  | Basic Hash Table Operations
| `hash-table-ref`: |  | SRFI 69
| `hash-table-ref`: |  | SRFI 125
| `hash-table-ref/default`: |  | Basic Hash Table Operations
| `hash-table-ref/default`: |  | SRFI 69
| `hash-table-ref/default`: |  | SRFI 125
| `hash-table-rehash-size`: |  | Resizing of Hash Tables
| `hash-table-rehash-threshold`: |  | Resizing of Hash Tables
| `hash-table-set!`: |  | Basic Hash Table Operations
| `hash-table-set!`: |  | SRFI 69
| `hash-table-set!`: |  | SRFI 125
| `hash-table-shrink-size`: |  | Resizing of Hash Tables
| `hash-table-size`: |  | Basic Hash Table Operations
| `hash-table-size`: |  | SRFI 69
| `hash-table-size`: |  | SRFI 125
| `hash-table-unfold`: |  | SRFI 125
| `hash-table-union!`: |  | SRFI 125
| `hash-table-update!`: |  | Basic Hash Table Operations
| `hash-table-update!`: |  | SRFI 69
| `hash-table-update!`: |  | SRFI 125
| `hash-table-update!/default`: |  | Basic Hash Table Operations
| `hash-table-update!/default`: |  | SRFI 69
| `hash-table-update!/default`: |  | SRFI 125
| `hash-table-values`: |  | Basic Hash Table Operations
| `hash-table-values`: |  | SRFI 69
| `hash-table-values`: |  | SRFI 125
| `hash-table-walk`: |  | Basic Hash Table Operations
| `hash-table-walk`: |  | SRFI 69
| `hash-table-walk`: |  | SRFI 125
| `hash-table-xor!`: |  | SRFI 125
| `hash-table/clean!`: |  | Basic Hash Table Operations
| `hash-table/clear!`: |  | Basic Hash Table Operations
| `hash-table/constructor`: |  | Construction of Hash Tables
| `hash-table/count`: |  | Basic Hash Table Operations
| `hash-table/datum-list`: |  | Basic Hash Table Operations
| `hash-table/for-each`: |  | Basic Hash Table Operations
| `hash-table/get`: |  | Basic Hash Table Operations
| `hash-table/intern!`: |  | Basic Hash Table Operations
| `hash-table/key-list`: |  | Basic Hash Table Operations
| `hash-table/lookup`: |  | Basic Hash Table Operations
| `hash-table/make`: |  | Object Hashing
| `hash-table/modify!`: |  | Basic Hash Table Operations
| `hash-table/put!`: |  | Basic Hash Table Operations
| `hash-table/rehash-size`: |  | Resizing of Hash Tables
| `hash-table/rehash-threshold`: |  | Resizing of Hash Tables
| `hash-table/remove!`: |  | Basic Hash Table Operations
| `hash-table/size`: |  | Resizing of Hash Tables
| `hash-table=?`: |  | SRFI 125
| `hash-table?`: |  | Basic Hash Table Operations
| `hash-table?`: |  | SRFI 69
| `hash-table?`: |  | SRFI 125
| `hbitmap`: |  | Windows Types
| `hbrush`: |  | Windows Types
| `hcursor`: |  | Windows Types
| `hdc`: |  | Windows Types
| `hicon`: |  | Windows Types
| `hinstance`: |  | Windows Types
| `hmenu`: |  | Windows Types
| `host-address-any`: |  | Miscellaneous OS Facilities
| `host-address-loopback`: |  | Miscellaneous OS Facilities
| `host-namestring`: |  | Operations on Pathnames
| `host=?`: |  | Miscellaneous Pathnames
| `host?`: |  | Miscellaneous Pathnames
| `hpalette`: |  | Windows Types
| `hpen`: |  | Windows Types
| `hrgn`: |  | Windows Types
| `hwnd`: |  | Windows Types

I |  |
| `i/o-port-type?`: |  | Textual Port Types
| `i/o-port?`: |  | Ports
| `identifier=?`: |  | SC Identifiers
| `identifier?`: |  | SC Identifiers
| `if`: |  | Conditionals
| `if`: |  | Booleans
| `if`: |  | R7RS
| `ignore-error`: |  | Condition Handling
| `ignore-errors`: |  | Condition Handling
| `imag-part`: |  | Numerical operations
| `imag-part`: |  | R7RS
| `image/destroy`: |  | Images
| `image/fill-from-byte-vector`: |  | Images
| `image/height`: |  | Images
| `image/width`: |  | Images
| `image?`: |  | Images
| `implemented-primitive-procedure?`: |  | Primitive Procedures
| `import-random-state`: |  | Random Number Generation
| `include`: |  | R7RS
| `include-ci`: |  | R7RS
| `inexact`: |  | Numerical operations
| `inexact`: |  | R7RS
| `inexact->exact`: |  | Numerical operations
| `inexact?`: |  | Numerical operations
| `inexact?`: |  | R7RS
| `infinite?`: |  | R7RS
| `init-file-pathname`: |  | Miscellaneous Pathnames
| `initial-offset`: |  | Structure Definitions
| `input`: |  | Subprocess Options
| `input-buffer-size`: |  | Subprocess Options
| `input-buffer-size on textual input port`: |  | Textual Input Port Operations
| `input-line-translation`: |  | Subprocess Options
| `input-port->parser-buffer`: |  | Parser Buffers
| `input-port-blocking-mode`: |  | Blocking Mode
| `input-port-open?`: |  | Ports
| `input-port-open?`: |  | R7RS
| `input-port-terminal-mode`: |  | Terminal Mode
| `input-port-type?`: |  | Textual Port Types
| `input-port/char-ready?`: |  | Textual Input Port Operations
| `input-port/discard-chars`: |  | Textual Input Port Operations
| `input-port/peek-char`: |  | Textual Input Port Operations
| `input-port/read-char`: |  | Textual Input Port Operations
| `input-port/read-string`: |  | Textual Input Port Operations
| `input-port/read-substring`: |  | Textual Input Port Operations
| `input-port?`: |  | Ports
| `input-port?`: |  | R7RS
| `int`: |  | Windows Types
| `integer->char`: |  | Characters
| `integer->char`: |  | R7RS
| `integer-ceiling`: |  | Numerical operations
| `integer-divide`: |  | Numerical operations
| `integer-divide`: |  | Fixnum Operations
| `integer-divide-quotient`: |  | Numerical operations
| `integer-divide-quotient`: |  | Fixnum Operations
| `integer-divide-remainder`: |  | Numerical operations
| `integer-divide-remainder`: |  | Fixnum Operations
| `integer-floor`: |  | Numerical operations
| `integer-round`: |  | Numerical operations
| `integer-truncate`: |  | Numerical operations
| `integer-truncate`: |  | Numerical operations
| `integer?`: |  | Numerical operations
| `integer?`: |  | R7RS
| `interaction-environment`: |  | R7RS
| `interaction-i/o-port`: |  | Ports
| `interaction-i/o-port`: |  | Prompting
| `intern`: |  | Symbols
| `intern-soft`: |  | Symbols
| `internal-time/seconds->ticks`: |  | Machine Time
| `internal-time/ticks->seconds`: |  | Machine Time
| `interpreter-environment?`: |  | Top-level Environments
| `invoke-restart`: |  | Restarts
| `invoke-restart`: |  | Establishing Restart Code
| `invoke-restart`: |  | Finding and Invoking General Restart Code
| `invoke-restart`: |  | Finding and Invoking General Restart Code
| `invoke-restart`: |  | The Named Restart Abstraction
| `invoke-restart-interactively`: |  | Restarts
| `invoke-restart-interactively`: |  | Finding and Invoking General Restart Code
| `invoke-restart-interactively`: |  | Finding and Invoking General Restart Code
| `invoke-restart-interactively`: |  | Finding and Invoking General Restart Code
| `invoke-restart-interactively`: |  | The Named Restart Abstraction
| `invoke-restart-interactively`: |  | The Named Restart Abstraction
| `iota`: |  | Construction of Lists
| `iota`: |  | SRFI 1
| `istring?`: |  | SRFI 140

J |  |
| `jiffies-per-second`: |  | R7RS

K |  |
| `kernel32.dll`: |  | Windows Foreign Procedures
| `keyword-constructor`: |  | Structure Definitions

L |  |
| `lambda`: |  | Entry Format
| `lambda`: |  | Static Scoping
| `lambda`: |  | Additional Notations
| `lambda`: |  | Procedure Call Syntax
| `lambda`: |  | Lambda Expressions
| `lambda`: |  | Lexical Binding
| `lambda`: |  | Definitions
| `lambda`: |  | Internal Definitions
| `lambda`: |  | Sequencing
| `lambda`: |  | Procedures
| `lambda`: |  | R7RS
| `last`: |  | Cutting and Pasting Lists
| `last`: |  | SRFI 1
| `last-pair`: |  | Cutting and Pasting Lists
| `last-pair`: |  | SRFI 1
| `lcm`: |  | Numerical operations
| `lcm`: |  | R7RS
| `leading`: |  | Strings
| `leading`: |  | Strings
| `length`: |  | Implementation restrictions
| `length`: |  | Selecting List Components
| `length`: |  | R7RS
| `length`: |  | SRFI 1
| `length+`: |  | Selecting List Components
| `length+`: |  | SRFI 1
| `let`: |  | Environment Concepts
| `let`: |  | Lexical Binding
| `let`: |  | Dynamic Binding
| `let`: |  | Definitions
| `let`: |  | Internal Definitions
| `let`: |  | Sequencing
| `let`: |  | Iteration
| `let`: |  | R7RS
| `let*`: |  | Environment Concepts
| `let*`: |  | Lexical Binding
| `let*`: |  | Definitions
| `let*`: |  | Internal Definitions
| `let*`: |  | Sequencing
| `let*`: |  | R7RS
| `let*-syntax`: |  | Syntactic Binding Constructs
| `let*-values`: |  | Lexical Binding
| `let*-values`: |  | Definitions
| `let*-values`: |  | Internal Definitions
| `let*-values`: |  | Sequencing
| `let*-values`: |  | R7RS
| `let-syntax`: |  | Syntactic Binding Constructs
| `let-syntax`: |  | R7RS
| `let-values`: |  | Lexical Binding
| `let-values`: |  | Definitions
| `let-values`: |  | Internal Definitions
| `let-values`: |  | Sequencing
| `let-values`: |  | R7RS
| `letrec`: |  | Environment Concepts
| `letrec`: |  | Lexical Binding
| `letrec`: |  | Definitions
| `letrec`: |  | Internal Definitions
| `letrec`: |  | Internal Definitions
| `letrec`: |  | Sequencing
| `letrec`: |  | R7RS
| `letrec*`: |  | Lexical Binding
| `letrec*`: |  | Definitions
| `letrec*`: |  | Internal Definitions
| `letrec*`: |  | Sequencing
| `letrec*`: |  | R7RS
| `letrec-syntax`: |  | Syntactic Binding Constructs
| `letrec-syntax`: |  | R7RS
| `line-end`: |  | Regular S-Expressions
| `line-start`: |  | Regular S-Expressions
| `link-variables`: |  | Top-level Environments
| `list`: |  | Entry Format
| `list`: |  | Construction of Lists
| `list`: |  | Construction of Lists
| `list`: |  | Miscellaneous List Operations
| `list`: |  | Construction of Vectors
| `list`: |  | String Ports
| `list`: |  | R7RS
| `list`: |  | SRFI 1
| `list->char-set`: |  | SRFI 14
| `list->char-set!`: |  | SRFI 14
| `list->generator`: |  | SRFI 158
| `list->stream`: |  | Streams
| `list->string`: |  | Strings
| `list->string`: |  | R7RS
| `list->string`: |  | SRFI 140
| `list->vector`: |  | Construction of Lists
| `list->vector`: |  | Construction of Vectors
| `list->vector`: |  | R7RS
| `list-copy`: |  | Construction of Lists
| `list-copy`: |  | Association Lists
| `list-copy`: |  | R7RS
| `list-copy`: |  | SRFI 1
| `list-deletor`: |  | Filtering Lists
| `list-deletor`: |  | Filtering Lists
| `list-deletor`: |  | Association Lists
| `list-deletor!`: |  | Filtering Lists
| `list-deletor!`: |  | Filtering Lists
| `list-deletor!`: |  | Association Lists
| `list-head`: |  | Cutting and Pasting Lists
| `list-index`: |  | SRFI 1
| `list-ref`: |  | Selecting List Components
| `list-ref`: |  | R7RS
| `list-ref`: |  | SRFI 1
| `list-set!`: |  | R7RS
| `list-tabulate`: |  | Construction of Lists
| `list-tabulate`: |  | SRFI 1
| `list-tail`: |  | Cutting and Pasting Lists
| `list-tail`: |  | R7RS
| `list=`: |  | SRFI 1
| `list?`: |  | Selecting List Components
| `list?`: |  | Association Lists
| `list?`: |  | 1D Tables
| `list?`: |  | R7RS
| `load`: |  | R7RS
| `load-bitmap on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `load-option`: |  | Format
| `load-option`: |  | Parser Language
| `load-option`: |  | XML Support
| `local-decoded-time`: |  | Decoded Time
| `local-host`: |  | Miscellaneous Pathnames
| `log`: |  | Numerical operations
| `log`: |  | R7RS
| `log-logistic`: |  | Numerical operations
| `log1mexp`: |  | Numerical operations
| `log1p`: |  | Numerical operations
| `log1pexp`: |  | Numerical operations
| `logistic`: |  | Numerical operations
| `logistic-1/2`: |  | Numerical operations
| `logit`: |  | Numerical operations
| `logit-exp`: |  | Numerical operations
| `logit1/2+`: |  | Numerical operations
| `logsumexp`: |  | Numerical operations
| `long`: |  | Windows Types
| `lset-adjoin`: |  | SRFI 1
| `lset-diff+intersection`: |  | SRFI 1
| `lset-diff+intersection!`: |  | SRFI 1
| `lset-difference`: |  | SRFI 1
| `lset-difference!`: |  | SRFI 1
| `lset-intersection`: |  | SRFI 1
| `lset-intersection!`: |  | SRFI 1
| `lset-union`: |  | SRFI 1
| `lset-union!`: |  | SRFI 1
| `lset-xor`: |  | SRFI 1
| `lset-xor!`: |  | SRFI 1
| `lset<=`: |  | SRFI 1
| `lset=`: |  | SRFI 1

M |  |
| `magnitude`: |  | Numerical operations
| `magnitude`: |  | R7RS
| `make-1d-table`: |  | 1D Tables
| `make-amap`: |  | Amap constructors
| `make-apply-hook`: |  | Application Hooks
| `make-bit-string`: |  | Construction of Bit Strings
| `make-bytevector`: |  | R7RS
| `make-cell`: |  | Parameters
| `make-char`: |  | Character implementation
| `make-circular-list`: |  | Miscellaneous List Operations
| `make-comparator`: |  | SRFI 128
| `make-condition`: |  | Condition Signalling
| `make-condition`: |  | Condition Signalling
| `make-condition`: |  | Simple Condition Instance Operations
| `make-condition-type`: |  | Condition Types
| `make-datum-weak-eq-hash-table`: |  | Construction of Hash Tables
| `make-datum-weak-eqv-hash-table`: |  | Construction of Hash Tables
| `make-decoded-time`: |  | Decoded Time
| `make-default-comparator`: |  | SRFI 128
| `make-directory`: |  | File Manipulation
| `make-entity`: |  | Application Hooks
| `make-ephemeron`: |  | Ephemerons
| `make-ephemeron`: |  | SRFI 124
| `make-eq-comparator`: |  | SRFI 128
| `make-eq-hash-table`: |  | Construction of Hash Tables
| `make-equal-comparator`: |  | SRFI 128
| `make-equal-hash-table`: |  | Construction of Hash Tables
| `make-eqv-comparator`: |  | SRFI 128
| `make-eqv-hash-table`: |  | Construction of Hash Tables
| `make-graphics-device`: |  | Opening and Closing of Graphics Devices
| `make-hash-table`: |  | Construction of Hash Tables
| `make-hash-table`: |  | Construction of Hash Tables
| `make-hash-table`: |  | SRFI 69
| `make-hash-table`: |  | SRFI 125
| `make-hash-table*`: |  | Construction of Hash Tables
| `make-hash-table-type`: |  | Construction of Hash Tables
| `make-hash-table-type*`: |  | Construction of Hash Tables
| `make-initialized-list`: |  | Construction of Lists
| `make-initialized-vector`: |  | Construction of Vectors
| `make-iota-generator`: |  | SRFI 158
| `make-key-ephemeral-eq-hash-table`: |  | Construction of Hash Tables
| `make-key-ephemeral-eqv-hash-table`: |  | Construction of Hash Tables
| `make-key-weak-eq-hash-table`: |  | Construction of Hash Tables
| `make-key-weak-eqv-hash-table`: |  | Construction of Hash Tables
| `make-list`: |  | Construction of Lists
| `make-list`: |  | R7RS
| `make-list`: |  | SRFI 1
| `make-list-comparator`: |  | SRFI 128
| `make-object-hash-table`: |  | Construction of Hash Tables
| `make-object-hasher`: |  | Object Hashing
| `make-pair-comparator`: |  | SRFI 128
| `make-parameter`: |  | Parameters
| `make-parameter`: |  | R7RS
| `make-parameter`: |  | SRFI 39
| `make-parser-macros`: |  | Parser-language Macros
| `make-pathname`: |  | Components of Pathnames
| `make-polar`: |  | Numerical operations
| `make-polar`: |  | R7RS
| `make-port`: |  | Constructors and Accessors for Textual Ports
| `make-port-type`: |  | Textual Port Types
| `make-primitive-procedure`: |  | Primitive Procedures
| `make-procedure-arity`: |  | Arity
| `make-promise`: |  | R7RS
| `make-random-source`: |  | Random Number Generation
| `make-random-source`: |  | SRFI 27
| `make-random-state`: |  | Random Number Generation
| `make-range-generator`: |  | SRFI 158
| `make-rb-tree`: |  | Red-Black Trees
| `make-record-type`: |  | Records
| `make-rectangular`: |  | Numerical operations
| `make-rectangular`: |  | Numerical operations
| `make-rectangular`: |  | R7RS
| `make-root-top-level-environment`: |  | Top-level Environments
| `make-settable-parameter`: |  | Parameters
| `make-string`: |  | Strings
| `make-string`: |  | R7RS
| `make-string-hash-table`: |  | Construction of Hash Tables
| `make-strong-eq-hash-table`: |  | Construction of Hash Tables
| `make-strong-eqv-hash-table`: |  | Construction of Hash Tables
| `make-symbol-hash-table`: |  | Construction of Hash Tables
| `make-syntactic-closure`: |  | SC Transformer Definition
| `make-synthetic-identifier`: |  | SC Identifiers
| `make-textual-port`: |  | Constructors and Accessors for Textual Ports
| `make-textual-port-type`: |  | Textual Port Types
| `make-top-level-environment`: |  | Top-level Environments
| `make-unsettable-parameter`: |  | Parameters
| `make-vector`: |  | Construction of Vectors
| `make-vector`: |  | R7RS
| `make-vector-comparator`: |  | SRFI 128
| `make-weak-eq-hash-table`: |  | Construction of Hash Tables
| `make-weak-eqv-hash-table`: |  | Construction of Hash Tables
| `make-wt-tree`: |  | Construction of Weight-Balanced Trees
| `make-wt-tree-type`: |  | Construction of Weight-Balanced Trees
| `make-xml-!attlist`: |  | XML Structure
| `make-xml-!element`: |  | XML Structure
| `make-xml-!entity`: |  | XML Structure
| `make-xml-!notation`: |  | XML Structure
| `make-xml-declaration`: |  | XML Structure
| `make-xml-document`: |  | XML Structure
| `make-xml-dtd`: |  | XML Structure
| `make-xml-element`: |  | XML Structure
| `make-xml-external-id`: |  | XML Structure
| `make-xml-name`: |  | XML Names
| `make-xml-nmtoken`: |  | XML Names
| `make-xml-parameter-!entity`: |  | XML Structure
| `make-xml-processing-instructions`: |  | XML Structure
| `make-xml-qname`: |  | XML Names
| `make-xml-unparsed-!entity`: |  | XML Structure
| `map`: |  | Mapping of Lists
| `map`: |  | *Parser
| `map`: |  | R7RS
| `map`: |  | SRFI 1
| `map!`: |  | SRFI 1
| `map*`: |  | Mapping of Lists
| `map-in-order`: |  | SRFI 1
| `map-window on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `match`: |  | *Parser
| `match-parser-buffer-char`: |  | Parser Buffers
| `match-parser-buffer-char-ci`: |  | Parser Buffers
| `match-parser-buffer-char-ci-no-advance`: |  | Parser Buffers
| `match-parser-buffer-char-in-set`: |  | Parser Buffers
| `match-parser-buffer-char-in-set-no-advance`: |  | Parser Buffers
| `match-parser-buffer-char-no-advance`: |  | Parser Buffers
| `match-parser-buffer-not-char`: |  | Parser Buffers
| `match-parser-buffer-not-char-ci`: |  | Parser Buffers
| `match-parser-buffer-not-char-ci-no-advance`: |  | Parser Buffers
| `match-parser-buffer-not-char-no-advance`: |  | Parser Buffers
| `match-parser-buffer-string`: |  | Parser Buffers
| `match-parser-buffer-string-ci`: |  | Parser Buffers
| `match-parser-buffer-string-ci-no-advance`: |  | Parser Buffers
| `match-parser-buffer-string-no-advance`: |  | Parser Buffers
| `match-parser-buffer-substring`: |  | Parser Buffers
| `match-parser-buffer-substring-ci`: |  | Parser Buffers
| `match-parser-buffer-substring-ci-no-advance`: |  | Parser Buffers
| `match-parser-buffer-substring-no-advance`: |  | Parser Buffers
| `max`: |  | Numerical operations
| `max`: |  | R7RS
| `measure-interval`: |  | Machine Time
| `member`: |  | Searching Lists
| `member`: |  | R7RS
| `member`: |  | SRFI 1
| `member-procedure`: |  | Searching Lists
| `memq`: |  | Searching Lists
| `memq`: |  | R7RS
| `memq`: |  | SRFI 1
| `memv`: |  | Searching Lists
| `memv`: |  | R7RS
| `memv`: |  | SRFI 1
| `merge-pathnames`: |  | File Ports
| `merge-pathnames`: |  | Operations on Pathnames
| `merge-sort`: |  | Miscellaneous List Operations
| `merge-sort!`: |  | Modifying Vectors
| `microcode-id/operating-system`: |  | Miscellaneous OS Facilities
| `microcode-id/operating-system-name`: |  | Miscellaneous OS Facilities
| `microcode-id/operating-system-variant`: |  | Miscellaneous OS Facilities
| `min`: |  | Numerical operations
| `min`: |  | R7RS
| `modexp`: |  | Numerical operations
| `modulo`: |  | Numerical operations
| `modulo`: |  | R7RS
| `month/long-string`: |  | External Representation of Time
| `month/max-days`: |  | Decoded Time
| `month/short-string`: |  | External Representation of Time
| `move-window on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `move-window on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `muffle-warning`: |  | Condition Signalling
| `muffle-warning`: |  | Condition Handling
| `muffle-warning`: |  | Restarts
| `muffle-warning`: |  | Invoking Standard Restart Code

N |  |
| `name->char`: |  | Characters
| `named`: |  | Structure Definitions
| `named-lambda`: |  | Lambda Expressions
| `named-lambda`: |  | Definitions
| `named-lambda`: |  | Sequencing
| `NaN (not a number)`: |  | Flonum Operations
| `nan?`: |  | R7RS
| `nearest-repl/environment`: |  | REPL Environment
| `negative?`: |  | Numerical operations
| `negative?`: |  | R7RS
| `newline`: |  | Output Procedures
| `newline`: |  | R7RS
| `nil`: |  | Booleans
| `ninth`: |  | Selecting List Components
| `ninth`: |  | SRFI 1
| `noise`: |  | *Parser
| `not`: |  | Booleans
| `not`: |  | R7RS
| `not-char`: |  | *Matcher
| `not-char-ci`: |  | *Matcher
| `not-pair?`: |  | SRFI 1
| `notification-output-port`: |  | Ports
| `notification-output-port`: |  | Condition Handling
| `nt-file-mode/archive`: |  | File Manipulation
| `nt-file-mode/compressed`: |  | File Manipulation
| `nt-file-mode/directory`: |  | File Manipulation
| `nt-file-mode/hidden`: |  | File Manipulation
| `nt-file-mode/normal`: |  | File Manipulation
| `nt-file-mode/read-only`: |  | File Manipulation
| `nt-file-mode/system`: |  | File Manipulation
| `nt-file-mode/temporary`: |  | File Manipulation
| `null-list?`: |  | Selecting List Components
| `null-list?`: |  | SRFI 1
| `null-xml-name-prefix`: |  | XML Names
| `null-xml-name-prefix?`: |  | XML Names
| `null-xml-namespace-uri`: |  | XML Names
| `null-xml-namespace-uri?`: |  | XML Names
| `null?`: |  | Selecting List Components
| `null?`: |  | Selecting List Components
| `null?`: |  | Streams
| `null?`: |  | R7RS
| `null?`: |  | SRFI 1
| `number->string`: |  | Numerical input and output
| `number->string`: |  | R7RS
| `number-hash`: |  | SRFI 128
| `number-wt-type`: |  | Construction of Weight-Balanced Trees
| `number?`: |  | Numerical operations
| `number?`: |  | R7RS
| `numerator`: |  | Numerical operations
| `numerator`: |  | R7RS

O |  |
| `object-hash`: |  | Object Hashing
| `object-hashed?`: |  | Object Hashing
| `object-unhash`: |  | Object Hashing
| `odd?`: |  | Numerical operations
| `odd?`: |  | R7RS
| `open-binary-i/o-file`: |  | File Ports
| `open-binary-input-file`: |  | File Ports
| `open-binary-input-file`: |  | File Ports
| `open-binary-input-file`: |  | R7RS
| `open-binary-output-file`: |  | File Ports
| `open-binary-output-file`: |  | File Ports
| `open-binary-output-file`: |  | R7RS
| `open-dib`: |  | DIB procedures
| `open-i/o-file`: |  | File Ports
| `open-input-bytevector`: |  | Bytevector Ports
| `open-input-bytevector`: |  | R7RS
| `open-input-file`: |  | File Ports
| `open-input-file`: |  | File Ports
| `open-input-file`: |  | R7RS
| `open-input-string`: |  | String Ports
| `open-input-string`: |  | R7RS
| `open-output-bytevector`: |  | Bytevector Ports
| `open-output-bytevector`: |  | Bytevector Ports
| `open-output-bytevector`: |  | R7RS
| `open-output-file`: |  | File Ports
| `open-output-file`: |  | File Ports
| `open-output-file`: |  | R7RS
| `open-output-string`: |  | String Ports
| `open-output-string`: |  | String Ports
| `open-output-string`: |  | String Ports
| `open-output-string`: |  | R7RS
| `open-tcp-server-socket`: |  | TCP Sockets
| `open-tcp-stream-socket`: |  | TCP Sockets
| `or`: |  | Conditionals
| `or`: |  | Booleans
| `or`: |  | R7RS
| `os/hostname`: |  | Miscellaneous OS Facilities
| `output`: |  | Subprocess Options
| `output-buffer-size`: |  | Subprocess Options
| `output-buffer-size on textual output port`: |  | Textual Output Port Operations
| `output-line-translation`: |  | Subprocess Options
| `output-port-blocking-mode`: |  | Blocking Mode
| `output-port-open?`: |  | Ports
| `output-port-open?`: |  | R7RS
| `output-port-terminal-mode`: |  | Terminal Mode
| `output-port-type?`: |  | Textual Port Types
| `output-port/discretionary-flush-output`: |  | Textual Output Port Operations
| `output-port/flush-output`: |  | Textual Output Port Operations
| `output-port/fresh-line`: |  | Textual Output Port Operations
| `output-port/write-char`: |  | Textual Output Port Operations
| `output-port/write-string`: |  | Textual Output Port Operations
| `output-port/write-substring`: |  | Textual Output Port Operations
| `output-port/x-size`: |  | Textual Output Port Operations
| `output-port/y-size`: |  | Textual Output Port Operations
| `output-port?`: |  | Ports
| `output-port?`: |  | R7RS

P |  |
| `pair-fold`: |  | SRFI 1
| `pair-fold-right`: |  | SRFI 1
| `pair-for-each`: |  | SRFI 1
| `pair?`: |  | Pairs
| `pair?`: |  | Selecting List Components
| `pair?`: |  | Weak Pairs
| `pair?`: |  | R7RS
| `pair?`: |  | SRFI 1
| `param:default-pathname-defaults`: |  | Operations on Pathnames
| `param:flonum-printer-cutoff`: |  | Numerical input and output
| `param:print-with-maximum-readability?`: |  | Output Procedures
| `param:printer-list-breadth-limit`: |  | Output Procedures
| `param:printer-list-depth-limit`: |  | Output Procedures
| `param:printer-radix`: |  | Output Procedures
| `param:printer-string-length-limit`: |  | Output Procedures
| `param:reader-fold-case?`: |  | Input Procedures
| `param:reader-radix`: |  | Input Procedures
| `parameterize`: |  | Dynamic Binding
| `parameterize`: |  | Definitions
| `parameterize`: |  | Internal Definitions
| `parameterize`: |  | Sequencing
| `parameterize`: |  | Ports
| `parameterize`: |  | File Ports
| `parameterize`: |  | File Ports
| `parameterize`: |  | Output Procedures
| `parameterize`: |  | R7RS
| `parameterize`: |  | SRFI 39
| `parameterize*`: |  | Parameters
| `parse-namestring`: |  | Filenames and Pathnames
| `parser-buffer-pointer-index`: |  | Parser Buffers
| `parser-buffer-pointer-line`: |  | Parser Buffers
| `parser-buffer-pointer?`: |  | Parser Buffers
| `parser-buffer-position-string`: |  | Parser Buffers
| `parser-buffer-ref`: |  | Parser Buffers
| `parser-buffer?`: |  | Parser Buffers
| `parser-macros?`: |  | Parser-language Macros
| `partition`: |  | Filtering Lists
| `partition`: |  | SRFI 1
| `partition!`: |  | Filtering Lists
| `partition!`: |  | SRFI 1
| `pathname-absolute?`: |  | Operations on Pathnames
| `pathname-as-directory`: |  | Operations on Pathnames
| `pathname-as-directory`: |  | Working Directory
| `pathname-default`: |  | Operations on Pathnames
| `pathname-default-device`: |  | Components of Pathnames
| `pathname-default-directory`: |  | Components of Pathnames
| `pathname-default-name`: |  | Components of Pathnames
| `pathname-default-type`: |  | Components of Pathnames
| `pathname-default-version`: |  | Components of Pathnames
| `pathname-device`: |  | Components of Pathnames
| `pathname-directory`: |  | Components of Pathnames
| `pathname-host`: |  | Components of Pathnames
| `pathname-name`: |  | Components of Pathnames
| `pathname-new-device`: |  | Components of Pathnames
| `pathname-new-directory`: |  | Components of Pathnames
| `pathname-new-name`: |  | Components of Pathnames
| `pathname-new-type`: |  | Components of Pathnames
| `pathname-new-version`: |  | Components of Pathnames
| `pathname-simplify`: |  | Filenames and Pathnames
| `pathname-type`: |  | Components of Pathnames
| `pathname-version`: |  | Components of Pathnames
| `pathname-wild?`: |  | Operations on Pathnames
| `pathname=?`: |  | Operations on Pathnames
| `pathname?`: |  | Operations on Pathnames
| `peek-char`: |  | Input Procedures
| `peek-char`: |  | R7RS
| `peek-char on textual input port`: |  | Textual Input Port Operations
| `peek-parser-buffer-char`: |  | Parser Buffers
| `peek-u8`: |  | Input Procedures
| `peek-u8`: |  | R7RS
| `port-type/operation`: |  | Textual Port Types
| `port-type/operation-names`: |  | Textual Port Types
| `port-type/operations`: |  | Textual Port Types
| `port-type?`: |  | Textual Port Types
| `port/input-blocking-mode`: |  | Blocking Mode
| `port/input-terminal-mode`: |  | Terminal Mode
| `port/operation`: |  | Constructors and Accessors for Textual Ports
| `port/operation-names`: |  | Constructors and Accessors for Textual Ports
| `port/output-blocking-mode`: |  | Blocking Mode
| `port/output-terminal-mode`: |  | Terminal Mode
| `port/set-input-blocking-mode`: |  | Blocking Mode
| `port/set-input-terminal-mode`: |  | Terminal Mode
| `port/set-output-blocking-mode`: |  | Blocking Mode
| `port/set-output-terminal-mode`: |  | Terminal Mode
| `port/state`: |  | Constructors and Accessors for Textual Ports
| `port/type`: |  | Constructors and Accessors for Textual Ports
| `port/with-input-blocking-mode`: |  | Blocking Mode
| `port/with-input-terminal-mode`: |  | Terminal Mode
| `port/with-output-blocking-mode`: |  | Blocking Mode
| `port/with-output-terminal-mode`: |  | Terminal Mode
| `port?`: |  | Ports
| `port?`: |  | R7RS
| `positive?`: |  | Numerical operations
| `positive?`: |  | R7RS
| `pp`: |  | Output Procedures
| `predicate`: |  | Structure Definitions
| `primitive-procedure-name`: |  | Primitive Procedures
| `primitive-procedure?`: |  | Procedure Operations
| `print-procedure`: |  | Structure Definitions
| `procedure`: |  | Booleans
| `procedure-arity`: |  | Arity
| `procedure-arity-max`: |  | Arity
| `procedure-arity-min`: |  | Arity
| `procedure-arity-valid?`: |  | Arity
| `procedure-arity?`: |  | Arity
| `procedure-environment`: |  | Procedure Operations
| `procedure-of-arity?`: |  | Arity
| `procedure?`: |  | Procedure Operations
| `procedure?`: |  | R7RS
| `process-time-clock`: |  | Machine Time
| `promise-forced?`: |  | Promises
| `promise-value`: |  | Promises
| `promise?`: |  | Promises
| `promise?`: |  | R7RS
| `prompt-for-command-char`: |  | Prompting
| `prompt-for-command-expression`: |  | Prompting
| `prompt-for-confirmation`: |  | Prompting
| `prompt-for-evaluated-expression`: |  | Prompting
| `prompt-for-expression`: |  | Prompting
| `proper-list?`: |  | Selecting List Components
| `proper-list?`: |  | SRFI 1
| `pwd`: |  | Working Directory

Q |  |
| `quasiquote`: |  | Quoting
| `quasiquote`: |  | Lists
| `quasiquote`: |  | R7RS
| `quick-sort`: |  | Miscellaneous List Operations
| `quick-sort!`: |  | Modifying Vectors
| `quote`: |  | Quoting
| `quote`: |  | Lists
| `quote`: |  | R7RS
| `quotient`: |  | Numerical operations
| `quotient`: |  | Numerical operations
| `quotient`: |  | Numerical operations
| `quotient`: |  | R7RS

R |  |
| `raise`: |  | R7RS
| `raise-continuable`: |  | R7RS
| `random`: |  | Random Number Generation
| `random-bytevector`: |  | Random Number Generation
| `random-bytevector!`: |  | Random Number Generation
| `random-integer`: |  | Random Number Generation
| `random-integer`: |  | SRFI 27
| `random-real`: |  | Random Number Generation
| `random-real`: |  | SRFI 27
| `random-source-make-integers`: |  | Random Number Generation
| `random-source-make-integers`: |  | SRFI 27
| `random-source-make-reals`: |  | Random Number Generation
| `random-source-make-reals`: |  | SRFI 27
| `random-source-make-reals`: |  | SRFI 27
| `random-source-pseudo-randomize!`: |  | Random Number Generation
| `random-source-pseudo-randomize!`: |  | SRFI 27
| `random-source-randomize!`: |  | Random Number Generation
| `random-source-randomize!`: |  | SRFI 27
| `random-source-state-ref`: |  | Random Number Generation
| `random-source-state-ref`: |  | SRFI 27
| `random-source-state-set!`: |  | Random Number Generation
| `random-source-state-set!`: |  | SRFI 27
| `random-source?`: |  | Random Number Generation
| `random-source?`: |  | SRFI 27
| `random-state?`: |  | Random Number Generation
| `rational?`: |  | Numerical operations
| `rational?`: |  | R7RS
| `rationalize`: |  | Numerical operations
| `rationalize`: |  | R7RS
| `rationalize->exact`: |  | Numerical operations
| `rb-tree->alist`: |  | Red-Black Trees
| `rb-tree/copy`: |  | Red-Black Trees
| `rb-tree/datum-list`: |  | Red-Black Trees
| `rb-tree/delete!`: |  | Red-Black Trees
| `rb-tree/delete-max!`: |  | Red-Black Trees
| `rb-tree/delete-max-datum!`: |  | Red-Black Trees
| `rb-tree/delete-max-pair!`: |  | Red-Black Trees
| `rb-tree/delete-min!`: |  | Red-Black Trees
| `rb-tree/delete-min-datum!`: |  | Red-Black Trees
| `rb-tree/delete-min-pair!`: |  | Red-Black Trees
| `rb-tree/empty?`: |  | Red-Black Trees
| `rb-tree/equal?`: |  | Red-Black Trees
| `rb-tree/height`: |  | Red-Black Trees
| `rb-tree/insert!`: |  | Red-Black Trees
| `rb-tree/key-list`: |  | Red-Black Trees
| `rb-tree/lookup`: |  | Red-Black Trees
| `rb-tree/max`: |  | Red-Black Trees
| `rb-tree/max-datum`: |  | Red-Black Trees
| `rb-tree/max-pair`: |  | Red-Black Trees
| `rb-tree/min`: |  | Red-Black Trees
| `rb-tree/min-datum`: |  | Red-Black Trees
| `rb-tree/min-pair`: |  | Red-Black Trees
| `rb-tree/size`: |  | Red-Black Trees
| `rb-tree?`: |  | Red-Black Trees
| `read`: |  | Overview
| `read`: |  | External Representations
| `read`: |  | Lists
| `read`: |  | Symbols
| `read`: |  | Symbols
| `read`: |  | Symbols
| `read`: |  | Ports
| `read`: |  | Input Procedures
| `read`: |  | R7RS
| `read-bytevector`: |  | Input Procedures
| `read-bytevector`: |  | R7RS
| `read-bytevector!`: |  | Input Procedures
| `read-bytevector!`: |  | R7RS
| `read-char`: |  | Ports
| `read-char`: |  | Input Procedures
| `read-char`: |  | Input Procedures
| `read-char`: |  | Input Procedures
| `read-char`: |  | R7RS
| `read-char on textual input port`: |  | Textual Input Port Operations
| `read-char-no-hang`: |  | Input Procedures
| `read-delimited-string`: |  | Input Procedures
| `read-error?`: |  | R7RS
| `read-line`: |  | Input Procedures
| `read-line`: |  | R7RS
| `read-only`: |  | Structure Definitions
| `read-parser-buffer-char`: |  | Parser Buffers
| `read-string`: |  | Input Procedures
| `read-string`: |  | R7RS
| `read-string on textual input port`: |  | Textual Input Port Operations
| `read-string!`: |  | Input Procedures
| `read-substring on textual input port`: |  | Textual Input Port Operations
| `read-substring!`: |  | Input Procedures
| `read-u8`: |  | Ports
| `read-u8`: |  | Input Procedures
| `read-u8`: |  | R7RS
| `read-xml`: |  | XML Input
| `read-xml-file`: |  | XML Input
| `real-part`: |  | Numerical operations
| `real-part`: |  | R7RS
| `real-time-clock`: |  | Machine Time
| `real?`: |  | Numerical operations
| `real?`: |  | R7RS
| `receive`: |  | receive (SRFI 8)
| `receive`: |  | SRFI 8
| `record-accessor`: |  | Records
| `record-constructor`: |  | Records
| `record-keyword-constructor`: |  | Records
| `record-modifier`: |  | Records
| `record-predicate`: |  | Records
| `record-type-descriptor`: |  | Records
| `record-type-field-names`: |  | Records
| `record-type-name`: |  | Records
| `record-type?`: |  | Records
| `record?`: |  | Records
| `redisplay-hook`: |  | Subprocess Options
| `reduce`: |  | Folding of Lists
| `reduce`: |  | SRFI 1
| `reduce-left`: |  | Folding of Lists
| `reduce-right`: |  | Folding of Lists
| `reduce-right`: |  | SRFI 1
| `reference-barrier`: |  | Reference barriers
| `reference-barrier`: |  | SRFI 124
| `regexp`: |  | SRFI 115
| `regexp-extract`: |  | SRFI 115
| `regexp-fold`: |  | SRFI 115
| `regexp-match->list`: |  | SRFI 115
| `regexp-match-count`: |  | SRFI 115
| `regexp-match-submatch`: |  | SRFI 115
| `regexp-match-submatch-end`: |  | SRFI 115
| `regexp-match-submatch-start`: |  | SRFI 115
| `regexp-match?`: |  | SRFI 115
| `regexp-matches`: |  | SRFI 115
| `regexp-matches?`: |  | SRFI 115
| `regexp-partition`: |  | SRFI 115
| `regexp-replace`: |  | SRFI 115
| `regexp-replace-all`: |  | SRFI 115
| `regexp-search`: |  | SRFI 115
| `regexp-split`: |  | SRFI 115
| `regexp?`: |  | SRFI 115
| `regsexp-match-string`: |  | Regsexp Procedures
| `regsexp-search-string-forward`: |  | Regsexp Procedures
| `remainder`: |  | Numerical operations
| `remainder`: |  | Numerical operations
| `remainder`: |  | R7RS
| `remove`: |  | Filtering Lists
| `remove`: |  | SRFI 1
| `remove!`: |  | Filtering Lists
| `remove!`: |  | SRFI 1
| `rename-file`: |  | File Manipulation
| `resize-window on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `resize-window on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `resource-id`: |  | Windows Types
| `restart/effector`: |  | The Named Restart Abstraction
| `restart/interactor`: |  | Finding and Invoking General Restart Code
| `restart/interactor`: |  | The Named Restart Abstraction
| `restart/name`: |  | The Named Restart Abstraction
| `restart?`: |  | The Named Restart Abstraction
| `retry`: |  | Restarts
| `retry`: |  | Invoking Standard Restart Code
| `retry`: |  | Invoking Standard Restart Code
| `reverse`: |  | Miscellaneous List Operations
| `reverse`: |  | R7RS
| `reverse`: |  | SRFI 1
| `reverse!`: |  | Miscellaneous List Operations
| `reverse!`: |  | SRFI 1
| `reverse-list->string`: |  | SRFI 140
| `reverse-list->vector`: |  | SRFI 133
| `reverse-vector->generator`: |  | SRFI 158
| `reverse-vector->list`: |  | SRFI 133
| `round`: |  | Numerical operations
| `round`: |  | R7RS
| `round->exact`: |  | Numerical operations
| `rsc-macro-transformer`: |  | SC Transformer Definition
| `run-shell-command`: |  | Subprocess Procedures
| `run-synchronous-subprocess`: |  | Subprocess Procedures
| `runtime`: |  | Machine Time
| `rx`: |  | SRFI 115

S |  |
| `safe-accessors`: |  | Structure Definitions
| `save-bitmap on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `sc-macro-transformer`: |  | SC Transformer Definition
| `scheme-subprocess-environment`: |  | Subprocess Options
| `second`: |  | Selecting List Components
| `second`: |  | SRFI 1
| `seq`: |  | Regular S-Expressions
| `seq`: |  | *Matcher
| `seq`: |  | *Parser
| `set!`: |  | Top-Level Definitions
| `set!`: |  | Assignments
| `set!`: |  | R7RS
| `set-apply-hook-extra!`: |  | Application Hooks
| `set-apply-hook-procedure!`: |  | Application Hooks
| `set-background-color`: |  | Custom Operations for Win32 Graphics
| `set-background-color on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `set-background-color on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `set-border-color on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `set-border-width on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `set-car!`: |  | Pairs
| `set-car!`: |  | R7RS
| `set-car!`: |  | SRFI 1
| `set-cdr!`: |  | Lists
| `set-cdr!`: |  | Pairs
| `set-cdr!`: |  | String Ports
| `set-cdr!`: |  | R7RS
| `set-cdr!`: |  | SRFI 1
| `set-cell-contents!`: |  | Parameters
| `set-current-input-port!`: |  | Ports
| `set-current-output-port!`: |  | Ports
| `set-current-parser-macros!`: |  | Parser-language Macros
| `set-entity-extra!`: |  | Application Hooks
| `set-entity-procedure!`: |  | Application Hooks
| `set-ephemeron-datum!`: |  | Ephemerons
| `set-ephemeron-key!`: |  | Ephemerons
| `set-file-modes!`: |  | File Manipulation
| `set-file-times!`: |  | File Manipulation
| `set-font on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `set-font on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `set-foreground-color`: |  | Custom Operations for Win32 Graphics
| `set-foreground-color on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `set-foreground-color on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `set-hash-table-rehash-size!`: |  | Resizing of Hash Tables
| `set-hash-table-rehash-threshold!`: |  | Resizing of Hash Tables
| `set-hash-table/rehash-size!`: |  | Resizing of Hash Tables
| `set-hash-table/rehash-threshold!`: |  | Resizing of Hash Tables
| `set-input-buffer-size on textual input port`: |  | Textual Input Port Operations
| `set-input-port-blocking-mode!`: |  | Blocking Mode
| `set-input-port-terminal-mode!`: |  | Terminal Mode
| `set-interaction-i/o-port!`: |  | Ports
| `set-internal-border-width on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `set-line-width on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `set-mouse-color on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `set-mouse-shape on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `set-notification-output-port!`: |  | Ports
| `set-output-buffer-size on textual output port`: |  | Textual Output Port Operations
| `set-output-port-blocking-mode`: |  | Blocking Mode
| `set-output-port-terminal-mode!`: |  | Terminal Mode
| `set-parser-buffer-pointer!`: |  | Parser Buffers
| `set-port/state!`: |  | Constructors and Accessors for Textual Ports
| `set-record-type-unparser-method!`: |  | Custom Output
| `set-textual-port-state!`: |  | Constructors and Accessors for Textual Ports
| `set-trace-output-port!`: |  | Ports
| `set-window-name on win32-graphics-device`: |  | Custom Operations for Win32 Graphics
| `set-working-directory-pathname!`: |  | Working Directory
| `set-xml-!attlist-definitions!`: |  | XML Structure
| `set-xml-!attlist-name!`: |  | XML Structure
| `set-xml-!element-content-type!`: |  | XML Structure
| `set-xml-!element-name!`: |  | XML Structure
| `set-xml-!entity-name!`: |  | XML Structure
| `set-xml-!entity-value!`: |  | XML Structure
| `set-xml-!notation-id!`: |  | XML Structure
| `set-xml-!notation-name!`: |  | XML Structure
| `set-xml-declaration-encoding!`: |  | XML Structure
| `set-xml-declaration-standalone!`: |  | XML Structure
| `set-xml-declaration-version!`: |  | XML Structure
| `set-xml-document-declaration!`: |  | XML Structure
| `set-xml-document-dtd!`: |  | XML Structure
| `set-xml-document-misc-1!`: |  | XML Structure
| `set-xml-document-misc-2!`: |  | XML Structure
| `set-xml-document-misc-3!`: |  | XML Structure
| `set-xml-document-root!`: |  | XML Structure
| `set-xml-dtd-external!`: |  | XML Structure
| `set-xml-dtd-internal!`: |  | XML Structure
| `set-xml-dtd-root!`: |  | XML Structure
| `set-xml-element-attributes!`: |  | XML Structure
| `set-xml-element-content!`: |  | XML Structure
| `set-xml-element-name!`: |  | XML Structure
| `set-xml-external-id-id!`: |  | XML Structure
| `set-xml-external-id-uri!`: |  | XML Structure
| `set-xml-parameter-!entity-name!`: |  | XML Structure
| `set-xml-parameter-!entity-value!`: |  | XML Structure
| `set-xml-processing-instructions-name!`: |  | XML Structure
| `set-xml-processing-instructions-text!`: |  | XML Structure
| `set-xml-unparsed-!entity-id!`: |  | XML Structure
| `set-xml-unparsed-!entity-name!`: |  | XML Structure
| `set-xml-unparsed-!entity-notation!`: |  | XML Structure
| `seventh`: |  | Selecting List Components
| `seventh`: |  | SRFI 1
| `sexp`: |  | *Matcher
| `sexp`: |  | *Parser
| `shell-file-name`: |  | Subprocess Options
| `short`: |  | Windows Types
| `signal-condition`: |  | Error System
| `signal-condition`: |  | Condition Signalling
| `signal-condition`: |  | Condition Signalling
| `signal-condition`: |  | Condition Signalling
| `signal-condition`: |  | Condition Handling
| `signal-condition`: |  | Condition Handling
| `signal-condition`: |  | Condition Handling
| `signal-condition`: |  | Condition Handling
| `signed-integer->bit-string`: |  | Integer Conversions of Bit Strings
| `simplest-exact-rational`: |  | Numerical operations
| `simplest-rational`: |  | Numerical operations
| `sin`: |  | Numerical operations
| `sin`: |  | R7RS
| `singleton-wt-tree`: |  | Construction of Weight-Balanced Trees
| `sixth`: |  | Selecting List Components
| `sixth`: |  | SRFI 1
| `soft-link-file`: |  | File Manipulation
| `sort`: |  | Miscellaneous List Operations
| `sort!`: |  | Modifying Vectors
| `source->parser-buffer`: |  | Parser Buffers
| `span`: |  | SRFI 1
| `span!`: |  | SRFI 1
| `split-at`: |  | SRFI 1
| `split-at!`: |  | SRFI 1
| `sqrt`: |  | Implementation restrictions
| `sqrt`: |  | Numerical operations
| `sqrt`: |  | R7RS
| `square`: |  | R7RS
| `standard-error-handler`: |  | Condition Handling
| `standard-error-handler`: |  | Condition Handling
| `standard-error-hook`: |  | Condition Handling
| `standard-print-method`: |  | Custom Output
| `standard-unparser-method`: |  | Custom Output
| `standard-warning-handler`: |  | Condition Handling
| `standard-warning-handler`: |  | Condition Handling
| `standard-warning-hook`: |  | Condition Handling
| `store-value`: |  | Restarts
| `store-value`: |  | Invoking Standard Restart Code
| `store-value`: |  | Invoking Standard Restart Code
| `stream`: |  | Streams
| `stream->list`: |  | Streams
| `stream-car`: |  | Streams
| `stream-cdr`: |  | Streams
| `stream-first`: |  | Streams
| `stream-head`: |  | Streams
| `stream-length`: |  | Streams
| `stream-map`: |  | Streams
| `stream-null?`: |  | Streams
| `stream-pair?`: |  | Streams
| `stream-ref`: |  | Streams
| `stream-rest`: |  | Streams
| `stream-tail`: |  | Streams
| `string`: |  | Strings
| `string`: |  | *Matcher
| `string`: |  | Windows Types
| `string`: |  | R7RS
| `string`: |  | SRFI 140
| `string*`: |  | Strings
| `string->char-set`: |  | SRFI 14
| `string->char-set!`: |  | SRFI 14
| `string->decoded-time`: |  | Time-Format Conversion
| `string->file-time`: |  | Time-Format Conversion
| `string->generator`: |  | SRFI 158
| `string->list`: |  | Strings
| `string->list`: |  | R7RS
| `string->list`: |  | SRFI 140
| `string->nfc`: |  | Strings
| `string->nfd`: |  | Strings
| `string->number`: |  | Numerical input and output
| `string->number`: |  | R7RS
| `string->parser-buffer`: |  | Parser Buffers
| `string->symbol`: |  | Symbols
| `string->symbol`: |  | R7RS
| `string->uninterned-symbol`: |  | Symbols
| `string->universal-time`: |  | Time-Format Conversion
| `string->utf16`: |  | SRFI 140
| `string->utf16be`: |  | SRFI 140
| `string->utf16le`: |  | SRFI 140
| `string->utf8`: |  | R7RS
| `string->utf8`: |  | SRFI 140
| `string->vector`: |  | Construction of Vectors
| `string->vector`: |  | R7RS
| `string->vector`: |  | SRFI 140
| `string->xml`: |  | XML Input
| `string-any`: |  | Strings
| `string-any`: |  | SRFI 140
| `string-append`: |  | Strings
| `string-append`: |  | R7RS
| `string-append`: |  | SRFI 140
| `string-append*`: |  | Strings
| `string-builder`: |  | Strings
| `string-ci`: |  | Regular S-Expressions
| `string-ci`: |  | *Matcher
| `string-ci-hash`: |  | SRFI 69
| `string-ci-hash`: |  | SRFI 125
| `string-ci-hash`: |  | SRFI 128
| `string-ci<=?`: |  | Strings
| `string-ci<=?`: |  | R7RS
| `string-ci<=?`: |  | SRFI 140
| `string-ci<?`: |  | Strings
| `string-ci<?`: |  | R7RS
| `string-ci<?`: |  | SRFI 140
| `string-ci=?`: |  | Strings
| `string-ci=?`: |  | Strings
| `string-ci=?`: |  | R7RS
| `string-ci=?`: |  | SRFI 140
| `string-ci>=?`: |  | Strings
| `string-ci>=?`: |  | R7RS
| `string-ci>=?`: |  | SRFI 140
| `string-ci>?`: |  | Strings
| `string-ci>?`: |  | R7RS
| `string-ci>?`: |  | SRFI 140
| `string-compare`: |  | Strings
| `string-compare-ci`: |  | Strings
| `string-concatenate`: |  | SRFI 140
| `string-concatenate-reverse`: |  | SRFI 140
| `string-contains`: |  | SRFI 140
| `string-contains-right`: |  | SRFI 140
| `string-copy`: |  | Strings
| `string-copy`: |  | R7RS
| `string-copy!`: |  | Strings
| `string-copy!`: |  | R7RS
| `string-count`: |  | Strings
| `string-count`: |  | SRFI 140
| `string-downcase`: |  | Strings
| `string-downcase`: |  | R7RS
| `string-downcase`: |  | SRFI 140
| `string-drop`: |  | SRFI 140
| `string-drop-right`: |  | SRFI 140
| `string-end`: |  | Regular S-Expressions
| `string-every`: |  | Strings
| `string-every`: |  | SRFI 140
| `string-fill!`: |  | Strings
| `string-fill!`: |  | R7RS
| `string-filter`: |  | SRFI 140
| `string-find-first-index`: |  | Searching and Matching Strings
| `string-find-last-index`: |  | Searching and Matching Strings
| `string-find-next-char`: |  | Searching and Matching Strings
| `string-find-next-char-ci`: |  | Searching and Matching Strings
| `string-find-next-char-in-set`: |  | Searching and Matching Strings
| `string-find-previous-char`: |  | Searching and Matching Strings
| `string-find-previous-char-ci`: |  | Searching and Matching Strings
| `string-find-previous-char-in-set`: |  | Searching and Matching Strings
| `string-fold`: |  | SRFI 140
| `string-fold-right`: |  | SRFI 140
| `string-foldcase`: |  | Strings
| `string-foldcase`: |  | R7RS
| `string-foldcase`: |  | SRFI 140
| `string-for-each`: |  | Strings
| `string-for-each`: |  | R7RS
| `string-for-each`: |  | SRFI 140
| `string-for-each-index`: |  | SRFI 140
| `string-hash`: |  | Strings
| `string-hash`: |  | Symbols
| `string-hash`: |  | Construction of Hash Tables
| `string-hash`: |  | SRFI 69
| `string-hash`: |  | SRFI 125
| `string-hash`: |  | SRFI 128
| `string-hash-ci`: |  | Strings
| `string-head`: |  | Strings
| `string-in-nfc?`: |  | Strings
| `string-in-nfd?`: |  | Strings
| `string-index`: |  | SRFI 140
| `string-index-right`: |  | SRFI 140
| `string-is-xml-name?`: |  | XML Names
| `string-is-xml-nmtoken?`: |  | XML Names
| `string-join`: |  | SRFI 140
| `string-joiner`: |  | Strings
| `string-joiner*`: |  | Strings
| `string-length`: |  | Implementation restrictions
| `string-length`: |  | Strings
| `string-length`: |  | R7RS
| `string-length`: |  | SRFI 140
| `string-lower-case?`: |  | Strings
| `string-map`: |  | Strings
| `string-map`: |  | R7RS
| `string-map`: |  | SRFI 140
| `string-map-index`: |  | SRFI 140
| `string-match-backward`: |  | Searching and Matching Strings
| `string-match-forward`: |  | Searching and Matching Strings
| `string-null?`: |  | Strings
| `string-null?`: |  | SRFI 140
| `string-pad`: |  | SRFI 140
| `string-pad-left`: |  | Strings
| `string-pad-right`: |  | Strings
| `string-pad-right`: |  | SRFI 140
| `string-padder`: |  | Strings
| `string-prefix-ci?`: |  | Searching and Matching Strings
| `string-prefix-length`: |  | SRFI 140
| `string-prefix?`: |  | Searching and Matching Strings
| `string-prefix?`: |  | SRFI 140
| `string-ref`: |  | Storage Model
| `string-ref`: |  | Strings
| `string-ref`: |  | R7RS
| `string-ref`: |  | SRFI 140
| `string-remove`: |  | SRFI 140
| `string-repeat`: |  | SRFI 140
| `string-replace`: |  | Strings
| `string-replace`: |  | SRFI 140
| `string-search-all`: |  | Searching and Matching Strings
| `string-search-backward`: |  | Searching and Matching Strings
| `string-search-forward`: |  | Searching and Matching Strings
| `string-set!`: |  | Storage Model
| `string-set!`: |  | Strings
| `string-set!`: |  | Symbols
| `string-set!`: |  | R7RS
| `string-skip`: |  | SRFI 140
| `string-skip-right`: |  | SRFI 140
| `string-slice`: |  | Strings
| `string-split`: |  | SRFI 140
| `string-splitter`: |  | Strings
| `string-start`: |  | Regular S-Expressions
| `string-suffix-ci?`: |  | Searching and Matching Strings
| `string-suffix-length`: |  | SRFI 140
| `string-suffix?`: |  | Searching and Matching Strings
| `string-suffix?`: |  | SRFI 140
| `string-tabulate`: |  | SRFI 140
| `string-tail`: |  | Strings
| `string-take`: |  | SRFI 140
| `string-take-right`: |  | SRFI 140
| `string-titlecase`: |  | Strings
| `string-titlecase`: |  | SRFI 140
| `string-trim`: |  | Strings
| `string-trim`: |  | SRFI 140
| `string-trim-both`: |  | SRFI 140
| `string-trim-left`: |  | Strings
| `string-trim-right`: |  | Strings
| `string-trim-right`: |  | SRFI 140
| `string-trimmer`: |  | Strings
| `string-unfold`: |  | SRFI 140
| `string-unfold-right`: |  | SRFI 140
| `string-upcase`: |  | Strings
| `string-upcase`: |  | R7RS
| `string-upcase`: |  | SRFI 140
| `string-upper-case?`: |  | Strings
| `string-word-breaks`: |  | Strings
| `string-wt-type`: |  | Construction of Weight-Balanced Trees
| `string<=?`: |  | Strings
| `string<=?`: |  | R7RS
| `string<=?`: |  | SRFI 140
| `string<?`: |  | Strings
| `string<?`: |  | R7RS
| `string<?`: |  | SRFI 140
| `string=?`: |  | Equivalence Predicates
| `string=?`: |  | Equivalence Predicates
| `string=?`: |  | Strings
| `string=?`: |  | Strings
| `string=?`: |  | Symbols
| `string=?`: |  | Symbols
| `string=?`: |  | Construction of Hash Tables
| `string=?`: |  | Construction of Hash Tables
| `string=?`: |  | R7RS
| `string=?`: |  | SRFI 140
| `string>=?`: |  | Strings
| `string>=?`: |  | R7RS
| `string>=?`: |  | SRFI 140
| `string>?`: |  | Strings
| `string>?`: |  | R7RS
| `string>?`: |  | SRFI 140
| `string?`: |  | Strings
| `string?`: |  | R7RS
| `string?`: |  | SRFI 140
| `strong-hash-table/constructor`: |  | Construction of Hash Tables
| `sublist`: |  | Cutting and Pasting Lists
| `substring`: |  | Strings
| `substring`: |  | R7RS
| `substring`: |  | SRFI 140
| `substring->parser-buffer`: |  | Parser Buffers
| `substring?`: |  | Searching and Matching Strings
| `subvector`: |  | Cutting Vectors
| `subvector->list`: |  | Construction of Lists
| `subvector-fill!`: |  | Modifying Vectors
| `subvector-move-left!`: |  | Modifying Vectors
| `subvector-move-right!`: |  | Modifying Vectors
| `symbol->string`: |  | Storage Model
| `symbol->string`: |  | Equivalence Predicates
| `symbol->string`: |  | Equivalence Predicates
| `symbol->string`: |  | Symbols
| `symbol->string`: |  | R7RS
| `symbol-append`: |  | Symbols
| `symbol-hash`: |  | Symbols
| `symbol-hash`: |  | SRFI 128
| `symbol-hash-mod`: |  | Symbols
| `symbol<?`: |  | Symbols
| `symbol=?`: |  | R7RS
| `symbol?`: |  | Symbols
| `symbol?`: |  | R7RS
| `syntax-error`: |  | R7RS
| `syntax-rules`: |  | Pattern Language
| `syntax-rules`: |  | R7RS
| `system-clock`: |  | Machine Time
| `system-global-environment`: |  | Environment Variables
| `system-library-directory-pathname`: |  | Miscellaneous Pathnames
| `system-library-pathname`: |  | Miscellaneous Pathnames

T |  |
| `t`: |  | Booleans
| `take`: |  | Cutting and Pasting Lists
| `take`: |  | SRFI 1
| `take!`: |  | SRFI 1
| `take-right`: |  | SRFI 1
| `take-while`: |  | SRFI 1
| `take-while!`: |  | SRFI 1
| `tan`: |  | Numerical operations
| `tan`: |  | R7RS
| `tcp-server-connection-accept`: |  | TCP Sockets
| `template`: |  | Entry Format
| `temporary-directory-pathname`: |  | File Manipulation
| `temporary-file-pathname`: |  | File Manipulation
| `tenth`: |  | Selecting List Components
| `tenth`: |  | SRFI 1
| `textual-i/o-port-type?`: |  | Textual Port Types
| `textual-input-port->parser-buffer`: |  | Parser Buffers
| `textual-input-port-type?`: |  | Textual Port Types
| `textual-output-port-type?`: |  | Textual Port Types
| `textual-port-operation`: |  | Constructors and Accessors for Textual Ports
| `textual-port-operation-names`: |  | Constructors and Accessors for Textual Ports
| `textual-port-state`: |  | Constructors and Accessors for Textual Ports
| `textual-port-type`: |  | Constructors and Accessors for Textual Ports
| `textual-port-type?`: |  | Textual Port Types
| `textual-port?`: |  | Ports
| `textual-port?`: |  | R7RS
| `the-environment`: |  | Top-level Environments
| `third`: |  | Selecting List Components
| `third`: |  | SRFI 1
| `thunk?`: |  | Arity
| `time-zone->string`: |  | External Representation of Time
| `time-zone?`: |  | Decoded Time
| `top-level-environment?`: |  | Top-level Environments
| `trace-output-port`: |  | Ports
| `trailing`: |  | Strings
| `trailing`: |  | Strings
| `transform`: |  | *Parser
| `tree-copy`: |  | Pairs
| `true`: |  | Booleans
| `truncate`: |  | Numerical operations
| `truncate`: |  | R7RS
| `truncate->exact`: |  | Numerical operations
| `truncate-quotient`: |  | R7RS
| `truncate-remainder`: |  | R7RS
| `truncate/`: |  | R7RS
| `type`: |  | Structure Definitions
| `type`: |  | Structure Definitions
| `type-descriptor`: |  | Structure Definitions

U |  |
| `u8-ready?`: |  | Input Procedures
| `u8-ready?`: |  | R7RS
| `ucs-range->char-set`: |  | SRFI 14
| `ucs-range->char-set!`: |  | SRFI 14
| `uint`: |  | Windows Types
| `ulong`: |  | Windows Types
| `unbind-variable`: |  | Top-level Environments
| `unchecked`: |  | Windows Types
| `unfold`: |  | SRFI 1
| `unfold-right`: |  | SRFI 1
| `unhash`: |  | Object Hashing
| `unhash-object`: |  | Object Hashing
| `unicode-char?`: |  | Unicode
| `unicode-code-point?`: |  | Unicode
| `unicode-scalar-value?`: |  | Unicode
| `universal-time->file-time`: |  | Time-Format Conversion
| `universal-time->global-decoded-time`: |  | Time-Format Conversion
| `universal-time->global-time-string`: |  | Time-Format Conversion
| `universal-time->local-decoded-time`: |  | Time-Format Conversion
| `universal-time->local-time-string`: |  | Time-Format Conversion
| `unless`: |  | R7RS
| `unparser/set-tagged-pair-method!`: |  | Custom Output
| `unparser/set-tagged-vector-method!`: |  | Custom Output
| `unquote`: |  | Quoting
| `unquote`: |  | Lists
| `unquote`: |  | R7RS
| `unquote-splicing`: |  | Quoting
| `unquote-splicing`: |  | Lists
| `unquote-splicing`: |  | R7RS
| `unread-char`: |  | Input Procedures
| `unsigned-integer->bit-string`: |  | Integer Conversions of Bit Strings
| `unzip1`: |  | SRFI 1
| `unzip2`: |  | SRFI 1
| `unzip3`: |  | SRFI 1
| `unzip4`: |  | SRFI 1
| `unzip5`: |  | SRFI 1
| `use-pty?`: |  | Subprocess Options
| `use-value`: |  | Restarts
| `use-value`: |  | Invoking Standard Restart Code
| `user-homedir-pathname`: |  | Miscellaneous Pathnames
| `user-initial-environment`: |  | Initial and Current Environments
| `user-initial-environment`: |  | Environment Variables
| `user-initial-environment`: |  | REPL Environment
| `user32.dll`: |  | Windows Foreign Procedures
| `ushort`: |  | Windows Types
| `utf16->string`: |  | SRFI 140
| `utf16be->string`: |  | SRFI 140
| `utf16le->string`: |  | SRFI 140
| `utf8->string`: |  | R7RS
| `utf8->string`: |  | SRFI 140

V |  |
| `valid-hash-number?`: |  | Object Hashing
| `valid-object-hash?`: |  | Object Hashing
| `valid-sre?`: |  | SRFI 115
| `values`: |  | Continuations
| `values`: |  | *Parser
| `values`: |  | R7RS
| `vector`: |  | Construction of Vectors
| `vector`: |  | R7RS
| `vector->generator`: |  | SRFI 158
| `vector->list`: |  | Construction of Lists
| `vector->list`: |  | Construction of Vectors
| `vector->list`: |  | R7RS
| `vector->string`: |  | Construction of Vectors
| `vector->string`: |  | R7RS
| `vector->string`: |  | SRFI 140
| `vector-any`: |  | SRFI 133
| `vector-append`: |  | R7RS
| `vector-append-subvectors`: |  | SRFI 133
| `vector-binary-search`: |  | Selecting Vector Components
| `vector-binary-search`: |  | SRFI 133
| `vector-concatenate`: |  | SRFI 133
| `vector-copy`: |  | Construction of Vectors
| `vector-copy`: |  | R7RS
| `vector-copy!`: |  | R7RS
| `vector-count`: |  | SRFI 133
| `vector-cumulate`: |  | SRFI 133
| `vector-eighth`: |  | Selecting Vector Components
| `vector-empty?`: |  | SRFI 133
| `vector-every`: |  | SRFI 133
| `vector-fifth`: |  | Selecting Vector Components
| `vector-fill!`: |  | Modifying Vectors
| `vector-fill!`: |  | R7RS
| `vector-first`: |  | Selecting Vector Components
| `vector-fold`: |  | SRFI 133
| `vector-fold-right`: |  | SRFI 133
| `vector-for-each`: |  | R7RS
| `vector-fourth`: |  | Selecting Vector Components
| `vector-grow`: |  | Construction of Vectors
| `vector-head`: |  | Cutting Vectors
| `vector-index`: |  | SRFI 133
| `vector-index-right`: |  | SRFI 133
| `vector-length`: |  | Implementation restrictions
| `vector-length`: |  | Selecting Vector Components
| `vector-length`: |  | R7RS
| `vector-map`: |  | Construction of Vectors
| `vector-map`: |  | R7RS
| `vector-map!`: |  | SRFI 133
| `vector-partition`: |  | SRFI 133
| `vector-ref`: |  | Storage Model
| `vector-ref`: |  | Selecting Vector Components
| `vector-ref`: |  | R7RS
| `vector-reverse!`: |  | SRFI 133
| `vector-reverse-copy`: |  | SRFI 133
| `vector-reverse-copy!`: |  | SRFI 133
| `vector-second`: |  | Selecting Vector Components
| `vector-set!`: |  | Selecting Vector Components
| `vector-set!`: |  | R7RS
| `vector-seventh`: |  | Selecting Vector Components
| `vector-sixth`: |  | Selecting Vector Components
| `vector-skip`: |  | SRFI 133
| `vector-skip-right`: |  | SRFI 133
| `vector-swap!`: |  | SRFI 133
| `vector-tail`: |  | Cutting Vectors
| `vector-third`: |  | Selecting Vector Components
| `vector-unfold`: |  | SRFI 133
| `vector-unfold!`: |  | SRFI 133
| `vector-unfold-right`: |  | SRFI 133
| `vector-unfold-right!`: |  | SRFI 133
| `vector=`: |  | SRFI 133
| `vector?`: |  | Selecting Vector Components
| `vector?`: |  | R7RS

W |  |
| `warn`: |  | Condition Signalling
| `warn`: |  | Condition Signalling
| `warn`: |  | Invoking Standard Restart Code
| `weak-car`: |  | Weak Pairs
| `weak-cdr`: |  | Weak Pairs
| `weak-cons`: |  | Weak Pairs
| `weak-hash-table/constructor`: |  | Construction of Hash Tables
| `weak-pair/car?`: |  | Weak Pairs
| `weak-pair?`: |  | Weak Pairs
| `weak-set-car!`: |  | Weak Pairs
| `weak-set-cdr!`: |  | Weak Pairs
| `when`: |  | R7RS
| `where`: |  | Prompting
| `windows-procedure`: |  | Windows Foreign Procedures
| `with-current-parser-macros`: |  | Parser-language Macros
| `with-current-unparser-state`: |  | Custom Output
| `with-exception-handler`: |  | R7RS
| `with-input-from-binary-file`: |  | File Ports
| `with-input-from-file`: |  | File Ports
| `with-input-from-file`: |  | R7RS
| `with-input-from-port`: |  | Ports
| `with-input-from-string`: |  | String Ports
| `with-input-port-blocking-mode`: |  | Blocking Mode
| `with-input-port-terminal-mode`: |  | Terminal Mode
| `with-interaction-i/o-port`: |  | Ports
| `with-notification-output-port`: |  | Ports
| `with-output-port-blocking-mode`: |  | Blocking Mode
| `with-output-port-terminal-mode`: |  | Terminal Mode
| `with-output-to-binary-file`: |  | File Ports
| `with-output-to-file`: |  | File Ports
| `with-output-to-file`: |  | R7RS
| `with-output-to-port`: |  | Ports
| `with-output-to-string`: |  | String Ports
| `with-output-to-truncated-string`: |  | String Ports
| `with-pointer`: |  | *Matcher
| `with-pointer`: |  | *Parser
| `with-restart`: |  | Error System
| `with-restart`: |  | Restarts
| `with-restart`: |  | Establishing Restart Code
| `with-restart`: |  | Finding and Invoking General Restart Code
| `with-simple-restart`: |  | Restarts
| `with-simple-restart`: |  | Restarts
| `with-simple-restart`: |  | Establishing Restart Code
| `with-simple-restart`: |  | Finding and Invoking General Restart Code
| `with-timings`: |  | Machine Time
| `with-trace-output-port`: |  | Ports
| `with-working-directory-pathname`: |  | Working Directory
| `withdraw-window on x-graphics-device`: |  | Custom Operations on X Graphics Devices
| `within-continuation`: |  | Continuations
| `word`: |  | Windows Types
| `working-directory`: |  | Subprocess Options
| `working-directory-pathname`: |  | Working Directory
| `write`: |  | External Representations
| `write`: |  | Symbols
| `write`: |  | Symbols
| `write`: |  | Ports
| `write`: |  | String Ports
| `write`: |  | String Ports
| `write`: |  | Output Procedures
| `write`: |  | Error Messages
| `write`: |  | R7RS
| `write-bytevector`: |  | Output Procedures
| `write-bytevector`: |  | R7RS
| `write-char`: |  | Entry Format
| `write-char`: |  | Ports
| `write-char`: |  | Output Procedures
| `write-char`: |  | R7RS
| `write-char on textual output port`: |  | Textual Output Port Operations
| `write-condition-report`: |  | Condition Handling
| `write-condition-report`: |  | Condition State
| `write-condition-report`: |  | Simple Condition Instance Operations
| `write-dib`: |  | DIB procedures
| `write-line`: |  | Output Procedures
| `write-restart-report`: |  | The Named Restart Abstraction
| `write-shared`: |  | Output Procedures
| `write-shared`: |  | R7RS
| `write-simple`: |  | Output Procedures
| `write-simple`: |  | R7RS
| `write-string`: |  | Output Procedures
| `write-string`: |  | Format
| `write-string`: |  | R7RS
| `write-substring`: |  | Output Procedures
| `write-substring on textual output port`: |  | Textual Output Port Operations
| `write-to-string`: |  | String Ports
| `write-u8`: |  | Ports
| `write-u8`: |  | Output Procedures
| `write-u8`: |  | R7RS
| `write-xml`: |  | XML Output
| `write-xml-file`: |  | XML Output
| `wt-tree/add`: |  | Basic Operations on Weight-Balanced Trees
| `wt-tree/add!`: |  | Basic Operations on Weight-Balanced Trees
| `wt-tree/delete`: |  | Basic Operations on Weight-Balanced Trees
| `wt-tree/delete!`: |  | Basic Operations on Weight-Balanced Trees
| `wt-tree/delete-min`: |  | Indexing Operations on Weight-Balanced Trees
| `wt-tree/delete-min!`: |  | Indexing Operations on Weight-Balanced Trees
| `wt-tree/difference`: |  | Advanced Operations on Weight-Balanced Trees
| `wt-tree/empty?`: |  | Basic Operations on Weight-Balanced Trees
| `wt-tree/fold`: |  | Advanced Operations on Weight-Balanced Trees
| `wt-tree/for-each`: |  | Advanced Operations on Weight-Balanced Trees
| `wt-tree/index`: |  | Indexing Operations on Weight-Balanced Trees
| `wt-tree/index-datum`: |  | Indexing Operations on Weight-Balanced Trees
| `wt-tree/index-pair`: |  | Indexing Operations on Weight-Balanced Trees
| `wt-tree/intersection`: |  | Advanced Operations on Weight-Balanced Trees
| `wt-tree/lookup`: |  | Basic Operations on Weight-Balanced Trees
| `wt-tree/member?`: |  | Basic Operations on Weight-Balanced Trees
| `wt-tree/min`: |  | Indexing Operations on Weight-Balanced Trees
| `wt-tree/min-datum`: |  | Indexing Operations on Weight-Balanced Trees
| `wt-tree/min-pair`: |  | Indexing Operations on Weight-Balanced Trees
| `wt-tree/rank`: |  | Indexing Operations on Weight-Balanced Trees
| `wt-tree/set-equal?`: |  | Advanced Operations on Weight-Balanced Trees
| `wt-tree/size`: |  | Basic Operations on Weight-Balanced Trees
| `wt-tree/split<`: |  | Advanced Operations on Weight-Balanced Trees
| `wt-tree/split>`: |  | Advanced Operations on Weight-Balanced Trees
| `wt-tree/subset?`: |  | Advanced Operations on Weight-Balanced Trees
| `wt-tree/union`: |  | Advanced Operations on Weight-Balanced Trees
| `wt-tree/union-merge`: |  | Advanced Operations on Weight-Balanced Trees
| `wt-tree?`: |  | Basic Operations on Weight-Balanced Trees

X |  |
| `x-character-bounds/ascent`: |  | Custom Operations on X Graphics Devices
| `x-character-bounds/descent`: |  | Custom Operations on X Graphics Devices
| `x-character-bounds/lbearing`: |  | Custom Operations on X Graphics Devices
| `x-character-bounds/rbearing`: |  | Custom Operations on X Graphics Devices
| `x-character-bounds/width`: |  | Custom Operations on X Graphics Devices
| `x-close-all-displays`: |  | Utilities for X Graphics
| `x-font-structure/all-chars-exist`: |  | Custom Operations on X Graphics Devices
| `x-font-structure/character-bounds`: |  | Custom Operations on X Graphics Devices
| `x-font-structure/default-char`: |  | Custom Operations on X Graphics Devices
| `x-font-structure/direction`: |  | Custom Operations on X Graphics Devices
| `x-font-structure/max-ascent`: |  | Custom Operations on X Graphics Devices
| `x-font-structure/max-bounds`: |  | Custom Operations on X Graphics Devices
| `x-font-structure/max-descent`: |  | Custom Operations on X Graphics Devices
| `x-font-structure/min-bounds`: |  | Custom Operations on X Graphics Devices
| `x-font-structure/name`: |  | Custom Operations on X Graphics Devices
| `x-font-structure/start-index`: |  | Custom Operations on X Graphics Devices
| `x-geometry-string`: |  | Utilities for X Graphics
| `x-graphics/close-display`: |  | Utilities for X Graphics
| `x-graphics/open-display`: |  | Utilities for X Graphics
| `x-open-display`: |  | X Graphics Type
| `x-size on textual output port`: |  | Textual Output Port Operations
| `xcons`: |  | Pairs
| `xcons`: |  | SRFI 1
| `xml-!attlist`: |  | XML Structure
| `xml-!attlist-definitions`: |  | XML Structure
| `xml-!attlist-name`: |  | XML Structure
| `xml-!attlist?`: |  | XML Structure
| `xml-!element`: |  | XML Structure
| `xml-!element-content-type`: |  | XML Structure
| `xml-!element-name`: |  | XML Structure
| `xml-!element?`: |  | XML Structure
| `xml-!entity`: |  | XML Structure
| `xml-!entity-name`: |  | XML Structure
| `xml-!entity-value`: |  | XML Structure
| `xml-!entity?`: |  | XML Structure
| `xml-!notation`: |  | XML Structure
| `xml-!notation-id`: |  | XML Structure
| `xml-!notation-name`: |  | XML Structure
| `xml-!notation?`: |  | XML Structure
| `xml->string`: |  | XML Output
| `xml->wide-string`: |  | XML Output
| `xml-declaration`: |  | XML Structure
| `xml-declaration-encoding`: |  | XML Structure
| `xml-declaration-standalone`: |  | XML Structure
| `xml-declaration-version`: |  | XML Structure
| `xml-declaration?`: |  | XML Structure
| `xml-document`: |  | XML Structure
| `xml-document-declaration`: |  | XML Structure
| `xml-document-dtd`: |  | XML Structure
| `xml-document-misc-1`: |  | XML Structure
| `xml-document-misc-2`: |  | XML Structure
| `xml-document-misc-3`: |  | XML Structure
| `xml-document-root`: |  | XML Structure
| `xml-document?`: |  | XML Structure
| `xml-dtd`: |  | XML Structure
| `xml-dtd-external`: |  | XML Structure
| `xml-dtd-internal`: |  | XML Structure
| `xml-dtd-root`: |  | XML Structure
| `xml-dtd?`: |  | XML Structure
| `xml-element`: |  | XML Structure
| `xml-element-attributes`: |  | XML Structure
| `xml-element-content`: |  | XML Structure
| `xml-element-name`: |  | XML Structure
| `xml-element?`: |  | XML Structure
| `xml-external-id`: |  | XML Structure
| `xml-external-id-id`: |  | XML Structure
| `xml-external-id-uri`: |  | XML Structure
| `xml-external-id?`: |  | XML Structure
| `xml-name->symbol`: |  | XML Names
| `xml-name-local`: |  | XML Names
| `xml-name-prefix`: |  | XML Names
| `xml-name-string`: |  | XML Names
| `xml-name-uri`: |  | XML Names
| `xml-name=?`: |  | XML Names
| `xml-name?`: |  | XML Names
| `xml-nmtoken?`: |  | XML Names
| `xml-parameter-!entity`: |  | XML Structure
| `xml-parameter-!entity-name`: |  | XML Structure
| `xml-parameter-!entity-value`: |  | XML Structure
| `xml-parameter-!entity?`: |  | XML Structure
| `xml-processing-instructions`: |  | XML Structure
| `xml-processing-instructions-name`: |  | XML Structure
| `xml-processing-instructions-text`: |  | XML Structure
| `xml-processing-instructions?`: |  | XML Structure
| `xml-qname-local`: |  | XML Names
| `xml-qname-prefix`: |  | XML Names
| `xml-qname?`: |  | XML Names
| `xml-unparsed-!entity`: |  | XML Structure
| `xml-unparsed-!entity-id`: |  | XML Structure
| `xml-unparsed-!entity-name`: |  | XML Structure
| `xml-unparsed-!entity-notation`: |  | XML Structure
| `xml-unparsed-!entity?`: |  | XML Structure
| `xml-uri`: |  | XML Names
| `xmlns-uri`: |  | XML Names
| `xsubstring`: |  | SRFI 140

Y |  |
| `y-size`: |  | Textual Port Primitives
| `y-size on textual output port`: |  | Textual Output Port Operations

Z |  |
| `zero?`: |  | Numerical operations
| `zero?`: |  | Numerical operations
| `zero?`: |  | R7RS
| `zip`: |  | SRFI 1

Jump to: | " # ' ( ) * + , - . / 1 2 8 < = > ? \ _ ` A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

## Appendix C Concept Index

Jump to: | ! " # ' ( ) , - . 1 ; = ? [ \ ] ` A B C D E F G H I J K L M N O P Q R S T U V W X Z

| Index Entry |  | Section

! |  |
| ! in mutation procedure names: |  | Naming Conventions

" |  |
| " as external representation: |  | Strings

# |  |
| # as format parameter: |  | Format
| # in external representation of number: |  | Syntax of numerical constants
| #( as external representation: |  | Vectors
| #* as external representation: |  | Bit Strings
| #b as external representation: |  | Syntax of numerical constants
| #d as external representation: |  | Syntax of numerical constants
| #e as external representation: |  | Syntax of numerical constants
| #f as external representation: |  | Booleans
| #i as external representation: |  | Syntax of numerical constants
| #o as external representation: |  | Syntax of numerical constants
| #t as external representation: |  | Booleans
| #x as external representation: |  | Syntax of numerical constants
| #[ as external representation: |  | Custom Output
| #\ as external representation: |  | Characters
| #| as external representation: |  | Comments

' |  |
| ’ as external representation: |  | Quoting

( |  |
| ( as external representation: |  | Lists

) |  |
| ) as external representation: |  | Lists

, |  |
| , as external representation: |  | Quoting
| ,@ as external representation: |  | Quoting

- |  |
| -ci, in string procedure name: |  | Strings
| -| notational convention: |  | Examples

. |  |
| . as external representation: |  | Lists
| ... in entries: |  | Entry Format

1 |  |
| 1D table (defn): |  | 1D Tables

; |  |
| ; as external representation: |  | Comments

= |  |
| => in cond clause: |  | Conditionals
| => notational convention: |  | Examples

? |  |
| ? in predicate names: |  | Naming Conventions

[ |  |
| [ in entries: |  | Entry Format

\ |  |
| \ as escape character in string: |  | Strings

] |  |
| ] in entries: |  | Entry Format

` |  |
| ‘ as external representation: |  | Quoting

A |  |
| absolute pathname (defn): |  | Working Directory
| absolute value, of number: |  | Numerical operations
| access time, of file: |  | File Manipulation
| access, used with set!: |  | Assignments
| addition, of numbers: |  | Numerical operations
| address hashing: |  | Address Hashing
| alias: |  | Syntax Terminology
| alias: |  | SC Identifiers
| alist (defn): |  | Association Lists
| alphabetic case, of interned symbol: |  | Symbols
| alphabetic case-insensitivity of programs (defn): |  | Uppercase and Lowercase
| anonymous syntactic keyword: |  | Macros
| apostrophe, as external representation: |  | Quoting
| appending, of bit strings: |  | Cutting and Pasting Bit Strings
| appending, of lists: |  | Cutting and Pasting Lists
| appending, of symbols: |  | Symbols
| appending, to output file: |  | File Ports
| application hook (defn): |  | Procedures
| application hook (defn): |  | Application Hooks
| application, of procedure: |  | Procedure Operations
| apply hook (defn): |  | Application Hooks
| argument evaluation order: |  | Procedure Call Syntax
| arity: |  | Arity
| ASCII character: |  | Character implementation
| assignment: |  | Assignments
| association list (defn): |  | Association Lists
| association table (defn): |  | The Association Table
| asterisk, as external representation: |  | Bit Strings
| attribute, of file: |  | File Manipulation

B |  |
| backquote, as external representation: |  | Quoting
| backslash, as escape character in string: |  | Strings
| Backtracking, in parser language: |  | Parser Language
| balanced binary trees: |  | Red-Black Trees
| balanced binary trees: |  | Weight-Balanced Trees
| barrier, reference: |  | Reference barriers
| bell, ringing on console: |  | Output Procedures
| binary port (defn): |  | Ports
| binary trees: |  | Red-Black Trees
| binary trees: |  | Weight-Balanced Trees
| binary trees, as discrete maps: |  | Weight-Balanced Trees
| binary trees, as sets: |  | Weight-Balanced Trees
| binding expression (defn): |  | Static Scoping
| binding expression, dynamic: |  | Dynamic Binding
| binding expression, lexical: |  | Lexical Binding
| binding, of variable: |  | Variable Bindings
| binding, syntactic keyword: |  | Environment Operations
| binding, unassigned: |  | Environment Operations
| binding, variable: |  | Environment Operations
| bit string (defn): |  | Bit Strings
| bit string index (defn): |  | Bit Strings
| bit string length (defn): |  | Bit Strings
| bitless character: |  | Character implementation
| bitmaps: |  | Custom Operations for Win32 Graphics
| bitmaps, graphics: |  | Images
| bitwise-logical operations, on fixnums: |  | Fixnum Operations
| block structure: |  | Lexical Binding
| blocking mode, of port: |  | Blocking Mode
| BOA constructor: |  | Structure Definitions
| BOA constructor (defn): |  | Structure Definitions
| body, of special form (defn): |  | Entry Format
| boolean object: |  | True and False
| boolean object (defn): |  | Booleans
| boolean object, equivalence predicate: |  | Booleans
| bound variable (defn): |  | Variable Bindings
| bound-restarts: |  | Generating Operations on Conditions
| bound-restarts: |  | Simple Condition Instance Operations
| bracket, in entries: |  | Entry Format
| broken ephemeron: |  | Ephemerons
| bucky bit, of character (defn): |  | Character implementation
| bucky bit, prefix (defn): |  | Characters
| buffering, of graphics output: |  | Buffering of Graphics Output
| buffering, of output: |  | Output Procedures
| built-in procedure: |  | Procedures
| bytevector, input and output ports: |  | Bytevector Ports

C |  |
| call by need evaluation (defn): |  | Promises
| car field, of pair (defn): |  | Lists
| case clause: |  | Conditionals
| case conversion, of character: |  | Characters
| case folding, of character: |  | Characters
| case sensitivity, of string operations: |  | Strings
| case, of interned symbol: |  | Symbols
| case-insensitivity of programs (defn): |  | Uppercase and Lowercase
| cdr field, of pair (defn): |  | Lists
| cell (defn): |  | Parameters
| character (defn): |  | Characters
| character bits (defn): |  | Character implementation
| character code (defn): |  | Character implementation
| character set: |  | Character Sets
| character, bitless: |  | Character implementation
| character, input from port: |  | Input Procedures
| character, input from textual port: |  | Textual Input Port Operations
| character, output to textual port: |  | Textual Output Port Operations
| character, searching string for: |  | Searching and Matching Strings
| characters, special, in programs: |  | Additional Notations
| child, of environment (defn): |  | Environment Concepts
| circle, graphics: |  | Custom Operations for Win32 Graphics
| circles, drawing: |  | Custom Operations on X Graphics Devices
| circles, drawing: |  | Custom Operations on X Graphics Devices
| circular list: |  | Selecting List Components
| circular list: |  | Miscellaneous List Operations
| circular structure: |  | Equivalence Predicates
| clause, of case expression: |  | Conditionals
| clause, of cond expression: |  | Conditionals
| clearing the console screen: |  | Output Procedures
| client socket: |  | TCP Sockets
| clip rectangle, graphics (defn): |  | Clipping of Graphics Output
| clipping, of graphics: |  | Clipping of Graphics Output
| closing environment, of procedure (defn): |  | Lambda Expressions
| closing, of file port: |  | File Ports
| closing, of port: |  | Ports
| code point: |  | Unicode
| code, of character (defn): |  | Character implementation
| code-point list: |  | Character Sets
| code-point range: |  | Character Sets
| code-point range: |  | Character Sets
| color: |  | Custom Operations for Win32 Graphics
| combination (defn): |  | Procedure Call Syntax
| comma, as external representation: |  | Quoting
| comment, extended, in programs (defn): |  | Comments
| comment, in programs (defn): |  | Comments
| comparison predicate: |  | Explicit Renaming
| comparison, for equivalence: |  | Equivalence Predicates
| comparison, of bit strings: |  | Bitwise Operations on Bit Strings
| comparison, of boolean objects: |  | Booleans
| comparison, of characters: |  | Characters
| comparison, of numbers: |  | Numerical operations
| comparison, of XML names: |  | XML Names
| compiled, procedure type: |  | Procedures
| component selection, of bit string: |  | Selecting Bit String Components
| component selection, of cell: |  | Parameters
| component selection, of character: |  | Character implementation
| component selection, of ephemeron: |  | Ephemerons
| component selection, of list: |  | Selecting List Components
| component selection, of pair: |  | Pairs
| component selection, of stream: |  | Streams
| component selection, of vector: |  | Selecting Vector Components
| component selection, of weak pair: |  | Weak Pairs
| components, of pathname: |  | Components of Pathnames
| compound procedure: |  | Procedures
| cond clause: |  | Conditionals
| condition (defn): |  | Condition Instances
| condition handler (defn): |  | Condition Handling
| condition instance (defn): |  | Condition Instances
| condition signalling (defn): |  | Condition Signalling
| condition type: |  | Error System
| condition type: |  | Condition Types
| conditional expression (defn): |  | Conditionals
| console, clearing: |  | Output Procedures
| console, port: |  | Ports
| console, ringing the bell: |  | Output Procedures
| constant: |  | Storage Model
| constant expression (defn): |  | Literal Expressions
| constant, and quasiquote: |  | Quoting
| constant, and quote: |  | Quoting
| construction, of bit string: |  | Construction of Bit Strings
| construction, of cell: |  | Parameters
| construction, of character: |  | Character implementation
| construction, of circular list: |  | Miscellaneous List Operations
| construction, of continuation: |  | Continuations
| construction, of EOF object: |  | Input Procedures
| construction, of ephemeron: |  | Ephemerons
| construction, of hash table: |  | Construction of Hash Tables
| construction, of list: |  | Construction of Lists
| construction, of pair: |  | Pairs
| construction, of pathname: |  | Filenames and Pathnames
| construction, of pathname: |  | Filenames and Pathnames
| construction, of pathname: |  | Components of Pathnames
| construction, of procedure: |  | Lambda Expressions
| construction, of promise: |  | Promises
| construction, of stream: |  | Streams
| construction, of symbols: |  | Symbols
| construction, of textual port type: |  | Textual Port Types
| construction, of vector: |  | Construction of Vectors
| construction, of weak pair: |  | Weak Pairs
| continuation: |  | Continuations
| continuation, alternate invocation: |  | Continuations
| continuation, and dynamic binding: |  | Dynamic Binding
| control, bucky bit prefix (defn): |  | Characters
| conventions for error messages: |  | Error Messages
| conventions, lexical: |  | Lexical Conventions
| conventions, naming: |  | Naming Conventions
| conventions, notational: |  | Notational Conventions
| conversion, pathname to string: |  | Filenames and Pathnames
| conversion, pathname to string: |  | Operations on Pathnames
| cooked mode, of terminal port: |  | Terminal Mode
| coordinates, graphics: |  | Coordinates for Graphics
| copying, of alist: |  | Association Lists
| copying, of bit string: |  | Construction of Bit Strings
| copying, of file: |  | File Manipulation
| copying, of tree: |  | Pairs
| copying, of vector: |  | Construction of Vectors
| current environment: |  | Top-level Environments
| current environment (defn): |  | Initial and Current Environments
| current error port (defn): |  | Ports
| current input port (defn): |  | Ports
| current input port, rebinding: |  | File Ports
| current interaction port (defn): |  | Ports
| current notification port (defn): |  | Ports
| current output port (defn): |  | Ports
| current output port, rebinding: |  | File Ports
| current tracing output port (defn): |  | Ports
| current working directory: |  | Operating-System Interface
| current working directory (defn): |  | Working Directory
| cursor, graphics (defn): |  | Drawing Graphics
| custom operations, on graphics device: |  | Custom Graphics Operations
| custom operations, on textual port: |  | Textual Port Primitives
| cutting, of bit string: |  | Cutting and Pasting Bit Strings
| cutting, of list: |  | Cutting and Pasting Lists
| cutting, of vector: |  | Cutting Vectors

D |  |
| d, as exponent marker in number: |  | Syntax of numerical constants
| decoded time: |  | Date and Time
| default environment, floating-point: |  | Floating-Point Environment
| default object (defn): |  | Lambda Expressions
| defaulting, of pathname: |  | Operations on Pathnames
| defaulting, of pathname: |  | Operations on Pathnames
| define, procedure (defn): |  | Definitions
| defining foreign procedures: |  | Windows Foreign Procedures
| defining foreign types: |  | Windows Types
| defining foreign types: |  | Windows Types
| definition: |  | Definitions
| definition, internal: |  | Internal Definitions
| definition, internal (defn): |  | Definitions
| definition, top-level: |  | Top-Level Definitions
| definition, top-level (defn): |  | Definitions
| deletion, of alist element: |  | Association Lists
| deletion, of file: |  | File Manipulation
| deletion, of list element: |  | Filtering Lists
| delimiter, in programs (defn): |  | Delimiters
| denormal: |  | Flonum Operations
| device coordinates, graphics (defn): |  | Coordinates for Graphics
| device, pathname component: |  | Components of Pathnames
| difference, of numbers: |  | Numerical operations
| directive, format (defn): |  | Format
| directory path (defn): |  | Components of Pathnames
| directory, converting pathname to: |  | Operations on Pathnames
| directory, current working (defn): |  | Working Directory
| directory, pathname component: |  | Components of Pathnames
| directory, predicate for: |  | File Manipulation
| directory, reading: |  | Miscellaneous Pathnames
| directory, reading: |  | Directory Reader
| discrete maps, using binary trees: |  | Weight-Balanced Trees
| discretionary flushing, of buffered output: |  | Output Procedures
| disembodied property list: |  | Symbols
| display, clearing: |  | Output Procedures
| display, X graphics: |  | Utilities for X Graphics
| divide-by-zero exception: |  | Floating-Point Exceptions
| division, of integers: |  | Numerical operations
| division, of numbers: |  | Numerical operations
| DLL, DIBUTILS.DLL: |  | Device Independent Bitmap Utilities
| DLL, exports: |  | Other parts of the DIB Utilities implementation
| DLL, GDI32.DLL: |  | Windows Foreign Procedures
| DLL, KERNEL32.DLL: |  | Windows Foreign Procedures
| DLL, loading: |  | Windows Foreign Procedures
| DLL, USER32.DLL: |  | Windows Foreign Procedures
| dot, as external representation: |  | Lists
| dotted notation, for pair (defn): |  | Lists
| dotted pair (see pair): |  | Lists
| double precision, of inexact number: |  | Syntax of numerical constants
| double quote, as external representation: |  | Strings
| drawing arcs and circles, graphics: |  | Custom Operations on X Graphics Devices
| drawing arcs and circles, graphics: |  | Custom Operations on X Graphics Devices
| drawing mode, graphics (defn): |  | Characteristics of Graphics Output
| dynamic binding: |  | Dynamic Binding
| dynamic binding: |  | Condition Handling
| dynamic binding: |  | Condition Handling
| dynamic binding, and continuations: |  | Dynamic Binding
| dynamic binding, versus static scoping: |  | Static Scoping
| dynamic environment: |  | Dynamic Binding
| dynamic extent: |  | Dynamic Binding
| dynamic parameter (defn): |  | Parameters
| dynamic types (defn): |  | Overview

E |  |
| e, as exponent marker in number: |  | Syntax of numerical constants
| effector, restart (defn): |  | Restarts
| element, of list (defn): |  | Lists
| ellipse, graphics: |  | Custom Operations for Win32 Graphics
| ellipsis, in entries: |  | Entry Format
| else clause, of case expression (defn): |  | Conditionals
| else clause, of cond expression (defn): |  | Conditionals
| empty list (defn): |  | Lists
| empty list, external representation: |  | Lists
| empty list, predicate for: |  | Selecting List Components
| empty stream, predicate for: |  | Streams
| empty string, predicate for: |  | Strings
| end of file object (see EOF object): |  | Input Procedures
| end, of subvector (defn): |  | Vectors
| entity (defn): |  | Application Hooks
| entry format: |  | Entry Format
| environment (defn): |  | Environment Concepts
| environment, current: |  | Top-level Environments
| environment, current (defn): |  | Initial and Current Environments
| environment, extension (defn): |  | Environment Concepts
| environment, initial (defn): |  | Initial and Current Environments
| environment, interpreter: |  | Top-level Environments
| environment, of procedure: |  | Lambda Expressions
| environment, procedure closing (defn): |  | Lambda Expressions
| environment, procedure invocation (defn): |  | Lambda Expressions
| environment, top-level: |  | Top-level Environments
| EOF object, construction: |  | Input Procedures
| EOF object, predicate for: |  | Input Procedures
| ephemerally held data, of hash table: |  | Construction of Hash Tables
| ephemerally held keys, of hash table: |  | Construction of Hash Tables
| ephemeron (defn): |  | Ephemerons
| ephemeron, broken: |  | Ephemerons
| equality, of XML names: |  | XML Names
| equivalence predicate (defn): |  | Equivalence Predicates
| equivalence predicate, for bit strings: |  | Bitwise Operations on Bit Strings
| equivalence predicate, for boolean objects: |  | Booleans
| equivalence predicate, for characters: |  | Characters
| equivalence predicate, for fixnums: |  | Fixnum Operations
| equivalence predicate, for flonums: |  | Flonum Operations
| equivalence predicate, for flonums: |  | Flonum Operations
| equivalence predicate, for numbers: |  | Numerical operations
| equivalence predicate, for pathname host: |  | Miscellaneous Pathnames
| equivalence predicate, for pathnames: |  | Operations on Pathnames
| equivalence predicate, of hash table: |  | Construction of Hash Tables
| error messages, conventions: |  | Error Messages
| error port, current (defn): |  | Ports
| error, in examples: |  | Examples
| error, unassigned variable: |  | Variable Bindings
| error, unbound variable (defn): |  | Environment Concepts
| error–> notational convention: |  | Examples
| errors, notational conventions: |  | Errors
| escape character, for string: |  | Strings
| escape procedure (defn): |  | Continuations
| escape procedure, alternate invocation: |  | Continuations
| evaluation order, of arguments: |  | Procedure Call Syntax
| evaluation, call by need (defn): |  | Promises
| evaluation, in examples: |  | Examples
| evaluation, lazy (defn): |  | Promises
| evaluation, of s-expression: |  | Environment Operations
| even number: |  | Numerical operations
| exactness: |  | Exactness
| examples: |  | Examples
| existence, testing of file: |  | File Manipulation
| exit, non-local: |  | Continuations
| explicit renaming: |  | Explicit Renaming
| exponent marker (defn): |  | Syntax of numerical constants
| expression (defn): |  | Expressions
| expression, binding (defn): |  | Static Scoping
| expression, conditional (defn): |  | Conditionals
| expression, constant (defn): |  | Literal Expressions
| expression, input from port: |  | Input Procedures
| expression, iteration (defn): |  | Iteration
| expression, literal (defn): |  | Literal Expressions
| expression, procedure call (defn): |  | Procedure Call Syntax
| expression, special form (defn): |  | Special Form Syntax
| extended comment, in programs (defn): |  | Comments
| extended real line: |  | Flonum Operations
| extension, of environment (defn): |  | Environment Concepts
| extent, of dynamic binding (defn): |  | Dynamic Binding
| extent, of objects: |  | Overview
| external representation (defn): |  | External Representations
| external representation, and quasiquote: |  | Quoting
| external representation, and quote: |  | Quoting
| external representation, for bit string: |  | Bit Strings
| external representation, for empty list: |  | Lists
| external representation, for list: |  | Lists
| external representation, for number: |  | Syntax of numerical constants
| external representation, for pair: |  | Lists
| external representation, for procedure: |  | Procedures
| external representation, for string: |  | Strings
| external representation, for symbol: |  | Symbols
| external representation, for vector: |  | Vectors
| external representation, parsing: |  | Input Procedures
| extra object, of application hook: |  | Application Hooks

F |  |
| f, as exponent marker in number: |  | Syntax of numerical constants
| false, boolean object: |  | True and False
| false, boolean object (defn): |  | Booleans
| false, in conditional expression (defn): |  | Conditionals
| false, predicate for: |  | Booleans
| FDL, GNU Free Documentation License: |  | GNU Free Documentation License
| file (regular), predicate for: |  | File Manipulation
| file name: |  | Pathnames
| file time: |  | Date and Time
| file type, procedure for: |  | File Manipulation
| file, converting pathname directory to: |  | Operations on Pathnames
| file, end-of-file marker (see EOF object): |  | Input Procedures
| file, input and output ports: |  | File Ports
| filename (defn): |  | Pathnames
| filling, of bit string: |  | Modification of Bit Strings
| filling, of vector: |  | Modifying Vectors
| filtering, of list: |  | Filtering Lists
| fixnum (defn): |  | Fixnum Operations
| floating-point comparison, ordered: |  | Flonum Operations
| floating-point comparison, unordered: |  | Flonum Operations
| floating-point environment: |  | Floating-Point Environment
| floating-point environment, default: |  | Floating-Point Environment
| floating-point number, infinite: |  | Flonum Operations
| floating-point number, normal: |  | Flonum Operations
| floating-point number, not a number: |  | Flonum Operations
| floating-point number, subnormal: |  | Flonum Operations
| floating-point number, zero: |  | Flonum Operations
| flonum (defn): |  | Flonum Operations
| flushing, of buffered output: |  | Output Procedures
| folding, of list: |  | Folding of Lists
| forcing, of promise: |  | Promises
| foreign type declarations: |  | Windows Types
| form: |  | Syntax Terminology
| form, special (defn): |  | Special Form Syntax
| formal parameter list, of lambda (defn): |  | Lambda Expressions
| format directive (defn): |  | Format
| format, entry: |  | Entry Format

G |  |
| generalization, of condition types: |  | Error System
| generalization, of condition types: |  | Condition Signalling
| generalization, of condition types: |  | Condition Types
| generalization, of condition types: |  | Condition Types
| generalization, of condition types: |  | Condition Types
| generalization, of condition types (defn): |  | Error System
| gensym (see uninterned symbol): |  | Symbols
| geometry string, X graphics: |  | Utilities for X Graphics
| grapheme cluster: |  | Strings
| graphics: |  | Graphics
| graphics, bitmaps: |  | Images
| graphics, buffering of output: |  | Buffering of Graphics Output
| graphics, circle: |  | Custom Operations for Win32 Graphics
| graphics, clipping: |  | Clipping of Graphics Output
| graphics, coordinate systems: |  | Coordinates for Graphics
| graphics, cursor (defn): |  | Drawing Graphics
| graphics, custom operations: |  | Custom Graphics Operations
| graphics, device coordinates (defn): |  | Coordinates for Graphics
| graphics, drawing: |  | Drawing Graphics
| graphics, drawing arcs and circles: |  | Custom Operations on X Graphics Devices
| graphics, drawing arcs and circles: |  | Custom Operations on X Graphics Devices
| graphics, drawing mode (defn): |  | Characteristics of Graphics Output
| graphics, ellipse: |  | Custom Operations for Win32 Graphics
| graphics, images: |  | Images
| graphics, line style (defn): |  | Characteristics of Graphics Output
| graphics, opening and closing devices: |  | Opening and Closing of Graphics Devices
| graphics, output characteristics: |  | Characteristics of Graphics Output
| graphics, virtual coordinates (defn): |  | Coordinates for Graphics
| greatest common divisor, of numbers: |  | Numerical operations
| growing, of vector: |  | Construction of Vectors

H |  |
| handler, condition (defn): |  | Condition Handling
| hard linking, of file: |  | File Manipulation
| hash table: |  | Hash Tables
| hashing, of key in hash table: |  | Construction of Hash Tables
| hashing, of object: |  | Object Hashing
| hashing, of string: |  | Strings
| hashing, of symbol: |  | Symbols
| home directory, as pathname: |  | Miscellaneous Pathnames
| home directory, as pathname: |  | Miscellaneous Pathnames
| hook, application (defn): |  | Procedures
| host, in filename: |  | Pathnames
| host, pathname component: |  | Components of Pathnames
| hostname, TCP: |  | TCP Sockets
| hygienic: |  | Macros
| hyper, bucky bit prefix (defn): |  | Characters

I |  |
| I/O, to bytevectors: |  | Bytevector Ports
| I/O, to files: |  | File Ports
| I/O, to strings: |  | String Ports
| identifier: |  | Syntax Terminology
| identifier (defn): |  | Identifiers
| identity, additive: |  | Numerical operations
| identity, multiplicative: |  | Numerical operations
| images, graphics: |  | Images
| immutable: |  | Storage Model
| immutable string: |  | Strings
| implementation restriction: |  | Implementation restrictions
| implicit begin: |  | Sequencing
| improper list (defn): |  | Lists
| index, of bit string (defn): |  | Bit Strings
| index, of list (defn): |  | Selecting List Components
| index, of string (defn): |  | Strings
| index, of subvector (defn): |  | Vectors
| index, of vector (defn): |  | Vectors
| inexact-result exception: |  | Floating-Point Exceptions
| infinity (`+inf.0`, `-inf.0`): |  | Flonum Operations
| inheritance, of environment bindings (defn): |  | Environment Concepts
| initial environment (defn): |  | Initial and Current Environments
| initial size, of hash table: |  | Resizing of Hash Tables
| input: |  | Input/Output
| input form: |  | SC Transformer Definition
| input form, to macro: |  | Explicit Renaming
| input operations: |  | Input Procedures
| input port (defn): |  | Ports
| input port, bytevector: |  | Bytevector Ports
| input port, console: |  | Ports
| input port, current (defn): |  | Ports
| input port, file: |  | File Ports
| input port, string: |  | String Ports
| input, XML: |  | XML Input
| insensitivity, to case in programs (defn): |  | Uppercase and Lowercase
| installed, as pathname component: |  | Components of Pathnames
| instance, of condition (defn): |  | Condition Instances
| integer division: |  | Numerical operations
| integer, converting to bit string: |  | Integer Conversions of Bit Strings
| interaction port, current (defn): |  | Ports
| interactive input ports (defn): |  | Input Procedures
| internal definition: |  | Internal Definitions
| internal definition (defn): |  | Definitions
| internal representation, for character: |  | Character implementation
| internal representation, for inexact number: |  | Syntax of numerical constants
| interned symbol (defn): |  | Symbols
| interning, of symbols: |  | Symbols
| interpreted, procedure type: |  | Procedures
| interpreter environment: |  | Top-level Environments
| invalid-operation exception: |  | Flonum Operations
| invalid-operation exception: |  | Floating-Point Exceptions
| inverse, additive, of number: |  | Numerical operations
| inverse, multiplicative, of number: |  | Numerical operations
| inverse, of bit string: |  | Bitwise Operations on Bit Strings
| inverse, of boolean object: |  | Booleans
| invocation environment, of procedure (defn): |  | Lambda Expressions
| iteration expression (defn): |  | Iteration

J |  |
| joiner procedure, of strings: |  | Strings
| joining, of strings: |  | Strings

K |  |
| key, of association list element (defn): |  | Association Lists
| keyword: |  | Macros
| keyword binding: |  | Environment Operations
| keyword constructor: |  | Structure Definitions
| keyword constructor (defn): |  | Structure Definitions
| keyword, of special form (defn): |  | Special Form Syntax

L |  |
| l, as exponent marker in number: |  | Syntax of numerical constants
| lambda expression (defn): |  | Lambda Expressions
| lambda list (defn): |  | Lambda Expressions
| lambda, implicit in define: |  | Definitions
| lambda, implicit in let: |  | Lexical Binding
| latent types (defn): |  | Overview
| lazy evaluation (defn): |  | Promises
| least common multiple, of numbers: |  | Numerical operations
| length, of bit string: |  | Selecting Bit String Components
| length, of bit string (defn): |  | Bit Strings
| length, of list (defn): |  | Lists
| length, of stream: |  | Streams
| length, of string (defn): |  | Strings
| length, of vector (defn): |  | Vectors
| letrec, implicit in define: |  | Internal Definitions
| lexical binding expression: |  | Lexical Binding
| lexical conventions: |  | Lexical Conventions
| lexical scoping (defn): |  | Static Scoping
| library, system pathname: |  | Miscellaneous Pathnames
| library, system pathname: |  | Miscellaneous Pathnames
| limitations: |  | Win32 Package Overview
| limitations: |  | Foreign function interface
| line style, graphics (defn): |  | Characteristics of Graphics Output
| linking (hard), of file: |  | File Manipulation
| linking (soft), of file: |  | File Manipulation
| list (defn): |  | Lists
| list index (defn): |  | Selecting List Components
| list, association (defn): |  | Association Lists
| list, converting to stream: |  | Streams
| list, converting to vector: |  | Construction of Vectors
| list, external representation: |  | Lists
| list, improper (defn): |  | Lists
| literal expression (defn): |  | Literal Expressions
| literal, and quasiquote: |  | Quoting
| literal, and quote: |  | Quoting
| literal, identifier as: |  | Identifiers
| loading DLLs: |  | Windows Foreign Procedures
| local part, of XML name: |  | XML Names
| location: |  | Storage Model
| location, of variable: |  | Variable Bindings
| log-odds: |  | Numerical operations
| log-odds: |  | Numerical operations
| log-probability: |  | Numerical operations
| log-probability: |  | Numerical operations
| logical operations, on fixnums: |  | Fixnum Operations
| long precision, of inexact number: |  | Syntax of numerical constants
| loopback interface: |  | Miscellaneous OS Facilities
| looping (see iteration expressions): |  | Iteration
| lowercase: |  | Uppercase and Lowercase
| lowercase, character conversion: |  | Characters

M |  |
| macro: |  | Macros
| macro keyword: |  | Macros
| macro transformer: |  | Macros
| macro transformer: |  | SC Transformer Definition
| macro transformer: |  | Explicit Renaming
| macro use: |  | Macros
| magnitude, of real number: |  | Numerical operations
| manifest types (defn): |  | Overview
| mapping, of list: |  | Mapping of Lists
| mapping, of stream: |  | Streams
| mapping, of vector: |  | Construction of Vectors
| Matcher language: |  | *Matcher
| Matcher procedure: |  | *Matcher
| matching, of strings: |  | Searching and Matching Strings
| maximum, of numbers: |  | Numerical operations
| memoization, of promise: |  | Promises
| merging, of pathnames: |  | Operations on Pathnames
| meta, bucky bit prefix (defn): |  | Characters
| minimum, of numbers: |  | Numerical operations
| modification time, of file: |  | File Manipulation
| modification, of bit string: |  | Modification of Bit Strings
| modification, of vector: |  | Modifying Vectors
| modulus, of hashing procedure: |  | Construction of Hash Tables
| modulus, of integers: |  | Numerical operations
| moving, of bit string elements: |  | Modification of Bit Strings
| moving, of vector elements: |  | Modifying Vectors
| multiple values, from procedure: |  | Continuations
| multiplication, of numbers: |  | Numerical operations
| must be, notational convention: |  | Errors
| mutable: |  | Storage Model
| mutable string: |  | Strings
| mutation procedure (defn): |  | Naming Conventions

N |  |
| name, of file: |  | File Manipulation
| name, of symbol: |  | Symbols
| name, of value (defn): |  | Variable Bindings
| name, pathname component: |  | Components of Pathnames
| named lambda (defn): |  | Lambda Expressions
| named let (defn): |  | Iteration
| names, XML: |  | XML Names
| naming conventions: |  | Naming Conventions
| naming conventions: |  | Win32 API names and procedures
| NaN: |  | Flonum Operations
| negative infinity (`-inf.0`): |  | Flonum Operations
| negative number: |  | Numerical operations
| nesting, of quasiquote expressions: |  | Quoting
| newest, as pathname component: |  | Components of Pathnames
| NFC: |  | Strings
| NFD: |  | Strings
| non-local exit: |  | Continuations
| normal floating-point number: |  | Flonum Operations
| Normalization Form C (NFC): |  | Strings
| Normalization Form D (NFD): |  | Strings
| not a number (NaN, `+nan.0`): |  | Flonum Operations
| notation, dotted (defn): |  | Lists
| notational conventions: |  | Notational Conventions
| notification port, current (defn): |  | Ports
| null string, predicate for: |  | Strings
| number: |  | Numbers
| number, external representation: |  | Syntax of numerical constants
| number, pseudorandom generation: |  | Random Number Generation
| numeric precision, inexact: |  | Syntax of numerical constants
| numerical input and output: |  | Numerical input and output
| numerical operations: |  | Numerical operations
| numerical types: |  | Numerical types

O |  |
| object hashing: |  | Object Hashing
| odd number: |  | Numerical operations
| oldest, as pathname component: |  | Components of Pathnames
| one-dimensional table (defn): |  | 1D Tables
| operand, of procedure call (defn): |  | Procedure Call Syntax
| Operating-System Interface: |  | Operating-System Interface
| operator, of procedure call (defn): |  | Procedure Call Syntax
| option, run-time-loadable: |  | Format
| option, run-time-loadable: |  | Parser Language
| option, run-time-loadable: |  | XML Support
| optional component, in entries: |  | Entry Format
| optional parameter (defn): |  | Lambda Expressions
| order, of argument evaluation: |  | Procedure Call Syntax
| ordered comparison: |  | Flonum Operations
| ordering, of characters: |  | Characters
| ordering, of numbers: |  | Numerical operations
| output: |  | Input/Output
| output form: |  | SC Transformer Definition
| output port (defn): |  | Ports
| output port, bytevector: |  | Bytevector Ports
| output port, console: |  | Ports
| output port, current (defn): |  | Ports
| output port, file: |  | File Ports
| output port, string: |  | String Ports
| output procedures: |  | Output Procedures
| output, XML: |  | XML Output
| overflow exception: |  | Floating-Point Exceptions

P |  |
| padder procedure: |  | Strings
| padding, of string: |  | Strings
| pair (defn): |  | Lists
| pair, external representation: |  | Lists
| pair, weak (defn): |  | Weak Pairs
| parameter list, of lambda (defn): |  | Lambda Expressions
| parameter, dynamic (defn): |  | Parameters
| parameter, entry category: |  | Entry Format
| parameter, optional (defn): |  | Lambda Expressions
| parameter, required (defn): |  | Lambda Expressions
| parameter, rest (defn): |  | Lambda Expressions
| parent, of directory: |  | Components of Pathnames
| parent, of environment (defn): |  | Environment Concepts
| parenthesis, as external representation: |  | Lists
| parenthesis, as external representation: |  | Vectors
| Parser buffer: |  | Parser Buffers
| Parser language: |  | Parser Language
| Parser language: |  | *Parser
| Parser procedure: |  | *Parser
| parser, XML: |  | XML Input
| Parser-buffer pointer: |  | Parser Buffers
| parsing, of external representation: |  | Input Procedures
| pasting, of bit strings: |  | Cutting and Pasting Bit Strings
| pasting, of lists: |  | Cutting and Pasting Lists
| pasting, of symbols: |  | Symbols
| path, directory (defn): |  | Components of Pathnames
| pathname: |  | Operating-System Interface
| pathname (defn): |  | Pathnames
| pathname components: |  | Components of Pathnames
| pathname, absolute (defn): |  | Working Directory
| pathname, relative (defn): |  | Working Directory
| period, as external representation: |  | Lists
| physical size, of hash table (defn): |  | Resizing of Hash Tables
| port: |  | Input/Output
| port (defn): |  | Ports
| port number, TCP: |  | TCP Sockets
| port, bytevector: |  | Bytevector Ports
| port, console: |  | Ports
| port, current: |  | Ports
| port, file: |  | File Ports
| port, string: |  | String Ports
| positive infinity (`+inf.0`): |  | Flonum Operations
| positive number: |  | Numerical operations
| precision, of inexact number: |  | Syntax of numerical constants
| predicate (defn): |  | Naming Conventions
| predicate (defn): |  | Equivalence Predicates
| predicate, equivalence (defn): |  | Equivalence Predicates
| prefix, of string: |  | Searching and Matching Strings
| prefix, of XML name: |  | XML Names
| pretty printer: |  | Output Procedures
| primitive procedure (defn): |  | Procedures
| primitive, procedure type: |  | Procedures
| print name, of symbol: |  | Symbols
| printed output, in examples: |  | Examples
| printing graphics output: |  | Custom Operations for Win32 Graphics
| procedure: |  | Procedures
| procedure call (defn): |  | Procedure Call Syntax
| procedure define (defn): |  | Definitions
| procedure, closing environment (defn): |  | Lambda Expressions
| procedure, compiled: |  | Procedures
| procedure, compound: |  | Procedures
| procedure, construction: |  | Lambda Expressions
| procedure, entry format: |  | Entry Format
| procedure, escape (defn): |  | Continuations
| procedure, interpreted: |  | Procedures
| procedure, invocation environment (defn): |  | Lambda Expressions
| procedure, of application hook: |  | Application Hooks
| procedure, primitive: |  | Procedures
| procedure, type: |  | Procedures
| product, of numbers: |  | Numerical operations
| promise (defn): |  | Promises
| promise, construction: |  | Promises
| promise, forcing: |  | Promises
| prompting: |  | Prompting
| proper tail recursion (defn): |  | Overview
| property list: |  | Associations
| property list: |  | 1D Tables
| property list: |  | The Association Table
| property list, of symbol: |  | Symbols
| protocol, restart (defn): |  | Restarts
| pseudorandom number generation: |  | Random Number Generation

Q |  |
| qname, of XML name: |  | XML Names
| quiet NaN: |  | Flonum Operations
| quiet NaN: |  | Floating-Point Exceptions
| quote, as external representation: |  | Quoting
| quotient, of integers: |  | Numerical operations
| quotient, of numbers: |  | Numerical operations
| quoting: |  | Quoting

R |  |
| R4RS: |  | Overview
| random number generation: |  | Random Number Generation
| rational, simplest (defn): |  | Numerical operations
| raw mode, of terminal port: |  | Terminal Mode
| record-type descriptor (defn): |  | Records
| recursion (see tail recursion): |  | Overview
| red-black binary trees: |  | Red-Black Trees
| reference barrier: |  | Reference barriers
| reference, strong (defn): |  | Weak References
| reference, variable (defn): |  | Variable References
| reference, weak (defn): |  | Weak References
| referentially transparent: |  | Macros
| region of variable binding, do: |  | Iteration
| region of variable binding, internal definition: |  | Internal Definitions
| region of variable binding, lambda: |  | Lambda Expressions
| region of variable binding, let: |  | Lexical Binding
| region of variable binding, let*: |  | Lexical Binding
| region of variable binding, letrec: |  | Lexical Binding
| region, of variable binding (defn): |  | Static Scoping
| regular file, predicate for: |  | File Manipulation
| rehash size, of hash table (defn): |  | Resizing of Hash Tables
| rehash threshold, of hash table (defn): |  | Resizing of Hash Tables
| relative pathname (defn): |  | Working Directory
| remainder, of integers: |  | Numerical operations
| renaming procedure: |  | Explicit Renaming
| renaming, of file: |  | File Manipulation
| REP loop: |  | Condition Signalling
| REP loop: |  | Condition Signalling
| REP loop: |  | Condition Handling
| REP loop: |  | Condition Handling
| REP loop: |  | Condition Handling
| REP loop: |  | Condition Handling
| REP loop: |  | Invoking Standard Restart Code
| REP loop (defn): |  | Initial and Current Environments
| REP loop, environment of: |  | Initial and Current Environments
| representation, external (defn): |  | External Representations
| required parameter (defn): |  | Lambda Expressions
| resizing, of hash table: |  | Resizing of Hash Tables
| resources, X graphics: |  | X Graphics Type
| rest parameter (defn): |  | Lambda Expressions
| restart (defn): |  | Restarts
| restart effector (defn): |  | Restarts
| restart protocol: |  | Restarts
| restarts, bound: |  | Generating Operations on Conditions
| restarts, bound: |  | Simple Condition Instance Operations
| result of evaluation, in examples: |  | Examples
| result, unspecified (defn): |  | Examples
| reversal, of list: |  | Miscellaneous List Operations
| ringing the console bell: |  | Output Procedures
| root, as pathname component: |  | Components of Pathnames
| run-time-loadable option: |  | Format
| run-time-loadable option: |  | Parser Language
| run-time-loadable option: |  | XML Support
| runtime system: |  | Overview

S |  |
| s, as exponent marker in number: |  | Syntax of numerical constants
| s-expression: |  | Environment Operations
| scalar value: |  | Unicode
| scheme concepts: |  | Scheme Concepts
| Scheme standard: |  | Overview
| scope (see region): |  | Overview
| scoping, lexical (defn): |  | Static Scoping
| scoping, static: |  | Static Scoping
| screen, clearing: |  | Output Procedures
| searching, of alist: |  | Association Lists
| searching, of bit string: |  | Selecting Bit String Components
| searching, of list: |  | Searching Lists
| searching, of string: |  | Searching and Matching Strings
| searching, of vector: |  | Selecting Vector Components
| selecting, of stream component: |  | Streams
| selection, components of pathname: |  | Operations on Pathnames
| selection, of bit string component: |  | Selecting Bit String Components
| selection, of cell component: |  | Parameters
| selection, of character component: |  | Character implementation
| selection, of ephemeron component: |  | Ephemerons
| selection, of list component: |  | Selecting List Components
| selection, of pair component: |  | Pairs
| selection, of vector component: |  | Selecting Vector Components
| selection, of weak pair component: |  | Weak Pairs
| semicolon, as external representation: |  | Comments
| sensitivity, to case in programs (defn): |  | Uppercase and Lowercase
| sequencing expressions: |  | Sequencing
| server socket: |  | TCP Sockets
| server socket: |  | TCP Sockets
| service, TCP: |  | TCP Sockets
| set, of characters: |  | Character Sets
| sets, using binary trees: |  | Weight-Balanced Trees
| shadowing, of variable binding (defn): |  | Environment Concepts
| short precision, of inexact number: |  | Syntax of numerical constants
| signal an error (defn): |  | Errors
| signalling NaN: |  | Flonum Operations
| signalling NaN: |  | Floating-Point Exceptions
| signalling, of condition (defn): |  | Condition Signalling
| signed zero: |  | Flonum Operations
| simplest rational (defn): |  | Numerical operations
| simplification, of pathname: |  | Filenames and Pathnames
| single precision, of inexact number: |  | Syntax of numerical constants
| size, of hash table (defn): |  | Resizing of Hash Tables
| slice, of string: |  | Strings
| socket: |  | TCP Sockets
| soft linking, of file: |  | File Manipulation
| special characters, in programs: |  | Additional Notations
| special form: |  | Special Forms
| special form (defn): |  | Special Form Syntax
| special form, entry category: |  | Entry Format
| specialization, of condition types: |  | Error System
| specialization, of condition types: |  | Condition Signalling
| specialization, of condition types: |  | Generating Operations on Conditions
| specialization, of condition types: |  | Generating Operations on Conditions
| specialization, of condition types: |  | Condition State
| specialization, of condition types: |  | Condition Types
| specialization, of condition types (defn): |  | Error System
| specified result, in examples: |  | Examples
| splitter procedure: |  | Strings
| splitting, of string: |  | Strings
| SRFI 0: |  | cond-expand (SRFI 0)
| SRFI 2: |  | and-let* (SRFI 2)
| SRFI 8: |  | receive (SRFI 8)
| SRFI 9: |  | define-record-type (SRFI 9)
| SRFI syntax: |  | SRFI syntax
| standard operations, on textual port: |  | Textual Port Primitives
| standard Scheme (defn): |  | Overview
| start, of subvector (defn): |  | Vectors
| static scoping: |  | Static Scoping
| static scoping (defn): |  | Overview
| static types (defn): |  | Overview
| stream (defn): |  | Streams
| stream, converting to list: |  | Streams
| string builder procedure: |  | Strings
| string index (defn): |  | Strings
| string length (defn): |  | Strings
| string slice: |  | Strings
| string, character (defn): |  | Strings
| string, input and output ports: |  | String Ports
| string, input from port: |  | Input Procedures
| string, input from textual port: |  | Textual Input Port Operations
| string, interning as symbol: |  | Symbols
| string, of bits (defn): |  | Bit Strings
| string, searching string for: |  | Searching and Matching Strings
| strong reference (defn): |  | Weak References
| strong types (defn): |  | Overview
| strongly held data, of hash table: |  | Construction of Hash Tables
| strongly held keys, of hash table: |  | Construction of Hash Tables
| subnormal floating-point number: |  | Flonum Operations
| subnormal-operand exception: |  | Floating-Point Exceptions
| subprocess: |  | Subprocesses
| substring, of bit string: |  | Cutting and Pasting Bit Strings
| substring, output to textual port: |  | Textual Output Port Operations
| subtraction, of numbers: |  | Numerical operations
| subvector (defn): |  | Vectors
| suffix, of string: |  | Searching and Matching Strings
| sum, of numbers: |  | Numerical operations
| super, bucky bit prefix (defn): |  | Characters
| symbol (defn): |  | Symbols
| symbolic link, predicate for: |  | File Manipulation
| symbolic linking, of file: |  | File Manipulation
| synchronous subprocess: |  | Subprocesses
| syntactic closure: |  | Syntax Terminology
| syntactic closures: |  | Syntactic Closures
| syntactic environment: |  | Syntax Terminology
| syntactic keyword: |  | Procedure Call Syntax
| syntactic keyword: |  | Macros
| syntactic keyword (defn): |  | Special Form Syntax
| syntactic keyword binding: |  | Environment Operations
| syntactic keyword, identifier as: |  | Identifiers
| synthetic identifier: |  | Syntax Terminology

T |  |
| table, association (defn): |  | The Association Table
| table, one-dimensional (defn): |  | 1D Tables
| tail recursion (defn): |  | Overview
| tail recursion, vs. iteration expression: |  | Iteration
| taxonomical link, of condition type (defn): |  | Error System
| terminal mode, of port: |  | Terminal Mode
| terminal screen, clearing: |  | Output Procedures
| tetrachotomy: |  | Flonum Operations
| textual input port operations: |  | Textual Input Port Operations
| textual output port operations: |  | Textual Output Port Operations
| textual port (defn): |  | Ports
| textual port primitives: |  | Textual Port Primitives
| textual port type: |  | Textual Port Primitives
| tick: |  | Machine Time
| time, decoded: |  | Date and Time
| time, file: |  | Date and Time
| time, string: |  | Date and Time
| time, universal: |  | Date and Time
| token, in programs (defn): |  | Whitespace
| top-level definition: |  | Top-Level Definitions
| top-level definition (defn): |  | Definitions
| top-level environment: |  | Top-level Environments
| total ordering (defn): |  | Miscellaneous List Operations
| tracing output port, current (defn): |  | Ports
| transformer environment: |  | SC Transformer Definition
| tree, copying: |  | Pairs
| trees, balanced binary: |  | Red-Black Trees
| trees, balanced binary: |  | Weight-Balanced Trees
| trichotomy: |  | Flonum Operations
| trimmer procedure: |  | Strings
| trimming, of string: |  | Strings
| true, boolean object: |  | True and False
| true, boolean object (defn): |  | Booleans
| true, in conditional expression (defn): |  | Conditionals
| truename, of input file: |  | File Manipulation
| type predicate, for 1D table: |  | 1D Tables
| type predicate, for alist: |  | Association Lists
| type predicate, for apply hook: |  | Application Hooks
| type predicate, for bit string: |  | Selecting Bit String Components
| type predicate, for boolean: |  | Booleans
| type predicate, for cell: |  | Parameters
| type predicate, for character: |  | Characters
| type predicate, for character set: |  | Character Sets
| type predicate, for compiled procedure: |  | Procedure Operations
| type predicate, for compound procedure: |  | Procedure Operations
| type predicate, for continuation: |  | Continuations
| type predicate, for empty list: |  | Selecting List Components
| type predicate, for entity: |  | Application Hooks
| type predicate, for environment: |  | Environment Operations
| type predicate, for EOF object: |  | Input Procedures
| type predicate, for ephemeron: |  | Ephemerons
| type predicate, for fixnum: |  | Fixnum Operations
| type predicate, for flonum: |  | Flonum Operations
| type predicate, for hash table: |  | Basic Hash Table Operations
| type predicate, for list: |  | Selecting List Components
| type predicate, for number: |  | Numerical operations
| type predicate, for pair: |  | Pairs
| type predicate, for pathname: |  | Operations on Pathnames
| type predicate, for pathname host: |  | Miscellaneous Pathnames
| type predicate, for port: |  | Ports
| type predicate, for primitive procedure: |  | Procedure Operations
| type predicate, for procedure: |  | Procedure Operations
| type predicate, for promise: |  | Promises
| type predicate, for record: |  | Records
| type predicate, for record type: |  | Records
| type predicate, for stream pair: |  | Streams
| type predicate, for symbol: |  | Symbols
| type predicate, for top-level environment: |  | Top-level Environments
| type predicate, for vector: |  | Selecting Vector Components
| type predicate, for weak pair: |  | Weak Pairs
| type, condition: |  | Error System
| type, of condition: |  | Condition Types
| type, of procedure: |  | Procedures
| type, pathname component: |  | Components of Pathnames
| types, latent (defn): |  | Overview
| types, manifest (defn): |  | Overview
| types, Windows: |  | Windows Types

U |  |
| unassigned binding: |  | Environment Operations
| unassigned variable: |  | Variable References
| unassigned variable (defn): |  | Variable Bindings
| unassigned variable, and assignment: |  | Assignments
| unassigned variable, and definition: |  | Top-Level Definitions
| unassigned variable, and dynamic bindings: |  | Dynamic Binding
| unassigned variable, and named let: |  | Iteration
| unbound variable: |  | Variable References
| unbound variable (defn): |  | Environment Concepts
| underflow exception: |  | Floating-Point Exceptions
| Unicode: |  | Unicode
| Unicode code point: |  | Unicode
| Unicode normalization forms: |  | Strings
| Unicode scalar value: |  | Unicode
| Uniform Resource Identifier: |  | XML Names
| uninterned symbol (defn): |  | Symbols
| universal time: |  | Date and Time
| unordered comparison: |  | Flonum Operations
| unspecified result (defn): |  | Examples
| up, as pathname component: |  | Components of Pathnames
| uppercase: |  | Uppercase and Lowercase
| uppercase, character conversion: |  | Characters
| URI, of XML name: |  | XML Names
| usable size, of hash table (defn): |  | Resizing of Hash Tables
| usage environment: |  | SC Transformer Definition

V |  |
| V as format parameter: |  | Format
| valid index, of bit string (defn): |  | Bit Strings
| valid index, of list (defn): |  | Selecting List Components
| valid index, of string (defn): |  | Strings
| valid index, of subvector (defn): |  | Vectors
| valid index, of vector (defn): |  | Vectors
| value, of variable (defn): |  | Variable Bindings
| values, multiple: |  | Continuations
| variable binding: |  | Variable Bindings
| variable binding: |  | Environment Operations
| variable binding, do: |  | Iteration
| variable binding, fluid-let: |  | Dynamic Binding
| variable binding, internal definition: |  | Internal Definitions
| variable binding, lambda: |  | Lambda Expressions
| variable binding, let: |  | Lexical Binding
| variable binding, let*: |  | Lexical Binding
| variable binding, letrec: |  | Lexical Binding
| variable binding, top-level definition: |  | Top-Level Definitions
| variable reference (defn): |  | Variable References
| variable, adding to environment: |  | Definitions
| variable, assigning values to: |  | Assignments
| variable, binding region (defn): |  | Static Scoping
| variable, entry category: |  | Entry Format
| variable, identifier as: |  | Identifiers
| vector (defn): |  | Vectors
| vector index (defn): |  | Vectors
| vector length (defn): |  | Vectors
| vector, converting to list: |  | Construction of Lists
| version, pathname component: |  | Components of Pathnames
| virtual coordinates, graphics (defn): |  | Coordinates for Graphics

W |  |
| warning: |  | Foreign function interface
| weak pair (defn): |  | Weak Pairs
| weak pair, and 1D table: |  | 1D Tables
| weak reference (defn): |  | Weak References
| weak types (defn): |  | Overview
| weakly held data, of hash table: |  | Construction of Hash Tables
| weakly held keys, of hash table: |  | Construction of Hash Tables
| weight-balanced binary trees: |  | Weight-Balanced Trees
| whitespace, in programs (defn): |  | Whitespace
| Win32 API names: |  | Win32 API names and procedures
| Win32 graphics: |  | Win32 Graphics
| Windows types: |  | Windows Types
| working directory (see current working directory): |  | Working Directory

X |  |
| X display, graphics: |  | Utilities for X Graphics
| X geometry string, graphics: |  | Utilities for X Graphics
| X graphics: |  | X Graphics
| X resources, graphics: |  | X Graphics Type
| X window system: |  | X Graphics
| XML input: |  | XML Input
| XML names: |  | XML Names
| XML output: |  | XML Output
| XML parser: |  | XML Input

Z |  |
| zero: |  | Numerical operations
| zero: |  | Flonum Operations

Jump to: | ! " # ' ( ) , - . 1 ; = ? [ \ ] ` A B C D E F G H I J K L M N O P Q R S T U V W X Z

#### Footnotes

##### (1)

MIT/GNU Scheme implements the whole tower of numerical types. It has unlimited-precision exact integers and exact rationals. Flonums are used to implement all inexact reals; on machines that support IEEE floating-point arithmetic these are double-precision floating-point numbers.

##### (2)

MIT/GNU Scheme implements all of the written notations for numbers.

##### (3)

In MIT/GNU Scheme the `rational?` procedure is the same as `real?`, and the `complex?` procedure is the same as `number?`.

##### (4)

MIT/GNU Scheme signals an error of type `condition-type:bad-range-argument` in this case.

##### (5)

The above definitions imply that all lists have finite length and are terminated by the empty list.

##### (6)

Note that path is restricted to a machine-dependent range, usually the size of a machine word. On many machines, this means that the maximum length of path will be 30 operations (32 bits, less the sign bit and the “end-of-sequence” bit).

##### (7)

Although they are often used as predicates, `memq`, `memv`, and `member` do not have question marks in their names because they return useful values rather than just `#t` or `#f`.

##### (8)

In older dialects of Lisp, uninterned symbols were fairly important. This was true because symbols were complicated data structures: in addition to having value cells (and sometimes, function cells), these structures contained property lists. Because of this, uninterned symbols were often used merely for their property lists — sometimes an uninterned symbol used this way was referred to as a disembodied property list. In MIT/GNU Scheme, symbols do not have property lists, or any other components besides their names. There is a different data structure similar to disembodied property lists: one-dimensional tables (see 1D Tables). For these reasons, uninterned symbols are not very useful in MIT/GNU Scheme. In fact, their primary purpose is to simplify the generation of unique variable names in programs that generate Scheme code.

##### (9)

MIT/GNU Scheme reserves a specific set of interned symbols for its own use. If you use these reserved symbols it is possible that you could break specific pieces of software that depend on them. The reserved symbols all have names beginning with the characters ‘`#[`’ and ending with the character ‘`]`’; thus none of these symbols can be read by the procedure `read` and hence are not likely to be used by accident. For example, `(intern "#[unnamed-procedure]")` produces a reserved symbol.

##### (10)

In MIT/GNU Scheme, the returned list is always newly allocated.

##### (11)

This introduction is taken from Common Lisp, The Language, second edition, p. 431.

##### (12)

Although they are often used as predicates, `assq`, `assv`, and `assoc` do not have question marks in their names because they return useful values rather than just `#t` or `#f`.

##### (13)

SRFI 69 does not give hash functions the flexibility to return new hash values after a garbage collection, which prevents a system whose garbage collector may relocate objects from hashing based on the addresses of objects in memory (see Address Hashing). SRFI 69 also does not specify circumstances when procedures passed as arguments to hash table operations may not themselves modify the hash table, which requires defensive copying and defensive repetitions of lookups.

##### (14)

This description of `format` is adapted from Common Lisp, The Language, second edition, section 22.3.3.

##### (15)

This introduction is adapted from Common Lisp, The Language, second edition, section 23.1.

##### (16)

This description is adapted from Common Lisp, The Language, second edition, section 23.1.1.