► |  | The Racket Reference

1 | Language Model
2 | Syntactic Forms
3 | Datatypes
4 | Structures
5 | Classes and Objects
6 | Units
7 | Contracts
8 | Pattern Matching
9 | Control Flow
10 | Concurrency and Parallelism
11 | Macros
12 | Input and Output
13 | Reflection and Security
14 | Operating System
15 | Memory Management
16 | Unsafe Operations
17 | Running Racket
| Bibliography
| Index

▼ | 12 | Input and Output

12.1 | Ports
12.2 | Byte and String Input
12.3 | Byte and String Output
12.4 | Reading
12.5 | Writing
12.6 | Pretty Printing
12.7 | Reader Extension
12.8 | Printer Extension
12.9 | Serialization
12.10 | Fast-Load Serialization

► | 12.6 | Pretty Printing

12.6.1 | Basic Pretty-Print Options
12.6.2 | Per-Symbol Special Printing
12.6.3 | Line-Output Hook
12.6.4 | Value Output Hook
12.6.5 | Additional Custom-Output Support

On this page:

pretty-print
pretty-write
pretty-display
pretty-format
pretty-print-handler
12.6.1`` Basic Pretty-Print Options
pretty-print-columns
pretty-print-depth
pretty-print-exact-as-decimal
pretty-print-.-symbol-without-bars
pretty-print-show-inexactness
12.6.2`` Per-Symbol Special Printing
pretty-print-abbreviate-read-macros
pretty-print-style-table?
pretty-print-current-style-table
pretty-print-extend-style-table
pretty-print-remap-stylable
12.6.3`` Line-Output Hook
pretty-print-newline
pretty-print-print-line
12.6.4`` Value Output Hook
pretty-print-size-hook
pretty-print-print-hook
pretty-print-pre-print-hook
pretty-print-post-print-hook
12.6.5`` Additional Custom-Output Support
pretty-printing
make-tentative-pretty-print-output-port
tentative-pretty-print-port-transfer
tentative-pretty-print-port-cancel

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#### 12.6`` Pretty Printing

(require racket/pretty)

The bindings documented in this section are provided by the racket/pretty and racket libraries, but not racket/base.

> > procedure

> > (pretty-print v [port quote-depth]) → void?

> v : any/c
> port : output-port? = (current-output-port)
> quote-depth : (or/c 0 1) = 0

Pretty-prints the value v using the same printed form as the default print mode, but with newlines and whitespace inserted to avoid lines longer than (pretty-print-columns), as controlled by (pretty-print-current-style-table). The printed form ends in a newline, unless the pretty-print-columns parameter is set to 'infinity. When port has line counting enabled (see Counting Positions, Lines, and Columns), then printing is sensitive to the column when printing starts—both for determining an initial line break and indenting subsequent lines.

In addition to the parameters defined in this section, pretty-print conforms to the print-graph, print-struct, print-hash-table, print-vector-length, print-box, and print-as-expression parameters.

The pretty printer detects structures that have the prop:custom-write property and calls the corresponding custom-write procedure. The custom-write procedure can check the parameter pretty-printing to cooperate with the pretty-printer. Recursive printing to the port automatically uses pretty printing, but if the structure has multiple recursively printed sub-expressions, a custom-write procedure may need to cooperate more to insert explicit newlines. Use port-next-location to determine the current output column, use pretty-print-columns to determine the target printing width, and use pretty-print-newline to insert a newline (so that the function in the pretty-print-print-line parameter can be called appropriately). Use make-tentative-pretty-print-output-port to obtain a port for tentative recursive prints (e.g., to check the length of the output).

> > procedure

> > (pretty-write v [port]) → void?

> v : any/c
> port : output-port? = (current-output-port)

Same as pretty-print, but v is printed like write instead of like print.

> > procedure

> > (pretty-display v [port]) → void?

> v : any/c
> port : output-port? = (current-output-port)

Same as pretty-print, but v is printed like display instead of like print.

> > procedure

> > (pretty-format v [columns]) → string?

> v : any/c
> columns : exact-nonnegative-integer? = (pretty-print-columns)

Like pretty-print, except that it returns a string containing the pretty-printed value, rather than sending the output to a port.

The optional argument columns argument is used to parameterize pretty-print-columns.

> > procedure

> > (pretty-print-handler v) → void?

> v : any/c

Pretty-prints v if v is not #<void>, or prints nothing if v is #<void>. Pass this procedure to current-print to install the pretty printer into the REPL run by read-eval-print-loop.

##### 12.6.1`` Basic Pretty-Print Options

> > parameter

> > (pretty-print-columns)

> → (or/c exact-positive-integer? 'infinity)
> (pretty-print-columns width) → void?
> width : (or/c exact-positive-integer? 'infinity)

A parameter that determines the default width for pretty printing.

If the display width is 'infinity, then pretty-printed output is never broken into lines, and a newline is not added to the end of the output.

> > parameter

> > (pretty-print-depth) → (or/c exact-nonnegative-integer? #f)

> (pretty-print-depth depth) → void?
> depth : (or/c exact-nonnegative-integer? #f)

Parameter that controls the default depth for recursive pretty printing. Printing to depth means that elements nested more deeply than depth are replaced with “...”; in particular, a depth of 0 indicates that only simple values are printed. A depth of #f (the default) allows printing to arbitrary depths.

> > parameter

> > (pretty-print-exact-as-decimal) → boolean?

> (pretty-print-exact-as-decimal as-decimal?) → void?
> as-decimal? : any/c

A parameter that determines how exact non-integers are printed. If the parameter’s value is #t, then an exact non-integer with a decimal representation is printed as a decimal number instead of a fraction. The initial value is #f.

> > parameter

> > (pretty-print-.-symbol-without-bars) → boolean?

> (pretty-print-.-symbol-without-bars on?) → void?
> on? : any/c

A parameter that controls the printing of the symbol whose print name is just a period. If set to a true value, then such a symbol is printed as only the period. If set to a false value, it is printed as a period with vertical bars surrounding it.

> > parameter

> > (pretty-print-show-inexactness) → boolean?

> (pretty-print-show-inexactness show?) → void?
> show? : any/c

A parameter that determines how inexact numbers are printed. If the parameter’s value is #t, then inexact numbers are always printed with a leading #i. The initial value is #f.

##### 12.6.2`` Per-Symbol Special Printing

> > parameter

> > (pretty-print-abbreviate-read-macros) → boolean?

> (pretty-print-abbreviate-read-macros abbrev?) → void?
> abbrev? : any/c

A parameter that controls whether or not quote, unquote, unquote-splicing, etc., are abbreviated with ', ,, ,@, etc. By default, the abbreviations are enabled.

See also pretty-print-remap-stylable.

> > procedure

> > (pretty-print-style-table? v) → boolean?

> v : any/c

Returns #t if v is a style table for use with pretty-print-current-style-table, #f otherwise.

> > parameter

> > (pretty-print-current-style-table) → pretty-print-style-table?

> (pretty-print-current-style-table style-table) → void?
> style-table : pretty-print-style-table?

A parameter that holds a table of style mappings. See pretty-print-extend-style-table.

> > procedure

> > (pretty-print-extend-style-table |  | style-table |
> > |  | symbol-list |
> > |  | like-symbol-list) |

> → pretty-print-style-table?
> style-table : pretty-print-style-table?
> symbol-list : (listof symbol?)
> like-symbol-list : (listof symbol?)

Creates a new style table by extending an existing style-table, so that the style mapping for each symbol of like-symbol-list in the original table is used for the corresponding symbol of symbol-list in the new table. The symbol-list and like-symbol-list lists must have the same length. The style-table argument can be #f, in which case the default mappings are used from the original table (see below).

The style mapping for a symbol controls the way that whitespace is inserted when printing a list that starts with the symbol. In the absence of any mapping, when a list is broken across multiple lines, each element of the list is printed on its own line, each with the same indentation.

The default style mapping includes mappings for the following symbols, so that the output follows popular code-formatting rules:

> 'lambda 'case-lambda
> 'define 'define-macro 'define-syntax
> 'let 'letrec 'let*
> 'let-syntax 'letrec-syntax
> 'let-values 'letrec-values 'let*-values
> 'let-syntaxes 'letrec-syntaxes
> 'begin 'begin0 'do
> 'if 'set! 'set!-values
> 'unless 'when
> 'cond 'case 'and 'or
> 'module
> 'syntax-rules 'syntax-case 'letrec-syntaxes+values
> 'import 'export 'link
> 'require 'require-for-syntax 'require-for-template 'provide
> 'public 'private 'override 'rename 'inherit 'field 'init
> 'shared 'send 'class 'instantiate 'make-object

> > parameter

> > (pretty-print-remap-stylable)

> → (any/c . -> . (or/c symbol? #f))
> (pretty-print-remap-stylable proc) → void?
> proc : (any/c . -> . (or/c symbol? #f))

A parameter that controls remapping for styles and for the determination of the reader shorthands.

This procedure is called with each sub-expression that appears as the first element in a sequence. If it returns a symbol, the style table is used, as if that symbol were at the head of the sequence. If it returns #f, the style table is treated normally. Similarly, when determining whether to abbreviate reader macros, this parameter is consulted.

##### 12.6.3`` Line-Output Hook

> > procedure

> > (pretty-print-newline port width) → void?

> port : out-port?
> width : exact-nonnegative-integer?

Calls the procedure associated with the pretty-print-print-line parameter to print a newline to port, if port is the output port that is redirected to the original output port for printing, otherwise a plain newline is printed to port. The width argument should be the target column width, typically obtained from pretty-print-columns.

> > parameter

> > (pretty-print-print-line)

> →

> ((or/c exact-nonnegative-integer? #f)
> output-port?
> exact-nonnegative-integer?
> (or/c exact-nonnegative-integer? 'infinity)
> . -> .
> exact-nonnegative-integer?)

> (pretty-print-print-line proc) → void?

> proc

> :

> ((or/c exact-nonnegative-integer? #f)
> output-port?
> exact-nonnegative-integer?
> (or/c exact-nonnegative-integer? 'infinity)
> . -> .
> exact-nonnegative-integer?)

A parameter that determines a procedure for printing the newline separator between lines of a pretty-printed value. The procedure is called with four arguments: a new line number, an output port, the old line’s length, and the number of destination columns. The return value from proc is the number of extra characters it printed at the beginning of the new line.

The proc procedure is called before any characters are printed with 0 as the line number and 0 as the old line length; proc is called after the last character of a value has been printed with #f as the line number and with the length of the last line. Whenever the pretty-printer starts a new line, proc is called with the new line’s number (where the first new line is numbered 1) and the just-finished line’s length. The destination-columns argument to proc is always the total width of the destination printing area, or 'infinity if pretty-printed values are not broken into lines.

The default proc procedure prints a newline whenever the line number is not 0 and the column count is not 'infinity, always returning 0. A custom proc procedure can be used to print extra text before each line of pretty-printed output; the number of characters printed before each line should be returned by proc so that the next line break can be chosen correctly.

The destination port supplied to proc is generally not the port supplied to pretty-print or pretty-display (or the current output port), but output to this port is ultimately redirected to the port supplied to pretty-print or pretty-display.

##### 12.6.4`` Value Output Hook

> > parameter

> > (pretty-print-size-hook)

> →

> (any/c boolean? output-port?
> . -> .
> (or/c #f exact-nonnegative-integer?))

> (pretty-print-size-hook proc) → void?

> proc

> :

> (any/c boolean? output-port?
> . -> .
> (or/c #f exact-nonnegative-integer?))

A parameter that determines a sizing hook for pretty-printing.

The sizing hook is applied to each value to be printed. If the hook returns #f, then printing is handled internally by the pretty-printer. Otherwise, the value should be an integer specifying the length of the printed value in characters; the print hook will be called to actually print the value (see pretty-print-print-hook).

The sizing hook receives three arguments. The first argument is the value to print. The second argument is a boolean: #t for printing like display and #f for printing like write. The third argument is the destination port; the port is the one supplied to pretty-print or pretty-display (or the current output port). The sizing hook may be applied to a single value multiple times during pretty-printing.

> > parameter

> > (pretty-print-print-hook)

> → (any/c boolean? output-port? . -> . void?)
> (pretty-print-print-hook proc) → void?
> proc : (any/c boolean? output-port? . -> . void?)

A parameter that determines a print hook for pretty-printing. The print-hook procedure is applied to a value for printing when the sizing hook (see pretty-print-size-hook) returns an integer size for the value.

The print hook receives three arguments. The first argument is the value to print. The second argument is a boolean: #t for printing like display and #f for printing like write. The third argument is the destination port; this port is generally not the port supplied to pretty-print or pretty-display (or the current output port), but output to this port is ultimately redirected to the port supplied to pretty-print or pretty-display.

> > parameter

> > (pretty-print-pre-print-hook)

> → (any/c output-port? . -> . void)
> (pretty-print-pre-print-hook proc) → void?
> proc : (any/c output-port? . -> . void)

A parameter that determines a hook procedure to be called just before an object is printed. The hook receives two arguments: the object and the output port. The port is the one supplied to pretty-print or pretty-display (or the current output port).

> > parameter

> > (pretty-print-post-print-hook)

> → (any/c output-port? . -> . void)
> (pretty-print-post-print-hook proc) → void?
> proc : (any/c output-port? . -> . void)

A parameter that determines a hook procedure to be called just after an object is printed. The hook receives two arguments: the object and the output port. The port is the one supplied to pretty-print or pretty-display (or the current output port).

##### 12.6.5`` Additional Custom-Output Support

> > parameter

> > (pretty-printing) → boolean?

> (pretty-printing on?) → void?
> on? : any/c

A parameter that is set to #t when the pretty printer calls a custom-write procedure (see prop:custom-write) for output in a mode that supports line breaks. When pretty printer calls a custom-write procedure merely to detect cycles or to try to print on a single line, it sets this parameter to #f.

> > procedure

> > (make-tentative-pretty-print-output-port |  | out |
> > |  | width |
> > |  | overflow-thunk) |

> → output-port?
> out : output-port?
> width : exact-nonnegative-integer?
> overflow-thunk : (-> any)

Produces an output port that is suitable for recursive pretty printing without actually producing output. Use such a port to tentatively print when proper output depends on the size of recursive prints. After printing, determine the size of the tentative output using file-position.

The out argument should be a pretty-printing port, such as the one supplied to a custom-write procedure when pretty-printing is set to true, or another tentative output port. The width argument should be a target column width, usually obtained from pretty-print-columns, possibly decremented to leave room for a terminator. The overflow-thunk procedure is called if more than width items are printed to the port or if a newline is printed to the port via pretty-print-newline; it can escape from the recursive print through a continuation as a shortcut, but overflow-thunk can also return, in which case it is called every time afterward that additional output is written to the port.

After tentative printing, either accept the result with tentative-pretty-print-port-transfer or reject it with tentative-pretty-print-port-cancel. Failure to accept or cancel properly interferes with graph-structure printing, calls to hook procedures, etc. Explicitly cancel the tentative print even when overflow-thunk escapes from a recursive print.

> > procedure

> > (tentative-pretty-print-port-transfer |  | tentative-out |  |  |  |
> > |  | orig-out) |  | → |  | void?

> tentative-out : output-port?
> orig-out : output-port?

Causes the data written to tentative-out to be transferred as if written to orig-out. The tentative-out argument should be a port produced by make-tentative-pretty-print-output-port, and orig-out should be either a pretty-printing port (provided to a custom-write procedure) or another tentative output port.

> > procedure

> > (tentative-pretty-print-port-cancel tentative-out) → void?

> tentative-out : output-port?

Cancels the content of tentative-out, which was produced by make-tentative-pretty-print-output-port. The main effect of canceling is that graph-reference definitions are undone, so that a future print of a graph-referenced object includes the defining #‹n›=.
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