### 4.2 Format (version 3.1) ¶

`(require 'format)` or `(require 'srfi-28)`

#### 4.2.1 Format Interface ¶

Function: format destination format-string . arguments ¶

  An almost complete implementation of Common LISP format description according to the CL reference book Common LISP from Guy L. Steele, Digital Press. Backward compatible to most of the available Scheme format implementations.

  Returns `#t`, `#f` or a string; has side effect of printing according to format-string. If destination is `#t`, the output is to the current output port and `#t` is returned. If destination is `#f`, a formatted string is returned as the result of the call. NEW: If destination is a string, destination is regarded as the format string; format-string is then the first argument and the output is returned as a string. If destination is a number, the output is to the current error port if available by the implementation. Otherwise destination must be an output port and `#t` is returned.

  format-string must be a string. In case of a formatting error format returns `#f` and prints a message on the current output or error port. Characters are output as if the string were output by the `display` function with the exception of those prefixed by a tilde (~). For a detailed description of the format-string syntax please consult a Common LISP format reference manual. For a test suite to verify this format implementation load `formatst.scm`.

#### 4.2.2 Format Specification (Format version 3.1) ¶

Please consult a Common LISP format reference manual for a detailed description of the format string syntax. For a demonstration of the implemented directives see `formatst.scm`.

This implementation supports directive parameters and modifiers (`:` and `@` characters). Multiple parameters must be separated by a comma (`,`). Parameters can be numerical parameters (positive or negative), character parameters (prefixed by a quote character (`'`), variable parameters (`v`), number of rest arguments parameter (`#`), empty and default parameters. Directive characters are case independent. The general form of a directive is:

directive ::= ~{directive-parameter,}[:][@]directive-character

directive-parameter ::= [ [-|+]{0-9}+ | ’character | v | # ]

#### 4.2.2.1 Implemented CL Format Control Directives ¶

Documentation syntax: Uppercase characters represent the corresponding control directive characters. Lowercase characters represent control directive parameter descriptions.

`~A`

  Any (print as `display` does).

  `~@A`

    left pad.

  `~mincol,colinc,minpad,padcharA`

    full padding.

`~S`

  S-expression (print as `write` does).

  `~@S`

    left pad.

  `~mincol,colinc,minpad,padcharS`

    full padding.

`~D`

  Decimal.

  `~@D`

    print number sign always.

  `~:D`

    print comma separated.

  `~mincol,padchar,commacharD`

    padding.

`~X`

  Hexadecimal.

  `~@X`

    print number sign always.

  `~:X`

    print comma separated.

  `~mincol,padchar,commacharX`

    padding.

`~O`

  Octal.

  `~@O`

    print number sign always.

  `~:O`

    print comma separated.

  `~mincol,padchar,commacharO`

    padding.

`~B`

  Binary.

  `~@B`

    print number sign always.

  `~:B`

    print comma separated.

  `~mincol,padchar,commacharB`

    padding.

`~nR`

  Radix n.

  `~n,mincol,padchar,commacharR`

    padding.

`~@R`

  print a number as a Roman numeral.

`~:@R`

  print a number as an “old fashioned” Roman numeral.

`~:R`

  print a number as an ordinal English number.

`~R`

  print a number as a cardinal English number.

`~P`

  Plural.

  `~@P`

    prints `y` and `ies`.

  `~:P`

    as `~P but jumps 1 argument backward.`

  `~:@P`

    as `~@P but jumps 1 argument backward.`

`~C`

  Character.

  `~@C`

    prints a character as the reader can understand it (i.e. `#\` prefixing).

  `~:C`

    prints a character as emacs does (eg. `^C` for ASCII 03).

`~F`

  Fixed-format floating-point (prints a flonum like mmm.nnn).

  `~width,digits,scale,overflowchar,padcharF`
  `~@F`

    If the number is positive a plus sign is printed.

`~E`

  Exponential floating-point (prints a flonum like mmm.nnn `E` ee).

  `~width,digits,exponentdigits,scale,overflowchar,padchar,exponentcharE`
  `~@E`

    If the number is positive a plus sign is printed.

`~G`

  General floating-point (prints a flonum either fixed or exponential).

  `~width,digits,exponentdigits,scale,overflowchar,padchar,exponentcharG`
  `~@G`

    If the number is positive a plus sign is printed.

`~$`

  Dollars floating-point (prints a flonum in fixed with signs separated).

  `~digits,scale,width,padchar$`
  `~@$`

    If the number is positive a plus sign is printed.

  `~:@$`

    A sign is always printed and appears before the padding.

  `~:$`

    The sign appears before the padding.

`~%`

  Newline.

  `~n%`

    print n newlines.

`~&`

  print newline if not at the beginning of the output line.

  `~n&`

    prints `~&` and then n-1 newlines.

`~|`

  Page Separator.

  `~n|`

    print n page separators.

`~~`

  Tilde.

  `~n~`

    print n tildes.

`~`<newline>

  Continuation Line.

  `~:`<newline>

    newline is ignored, white space left.

  `~@`<newline>

    newline is left, white space ignored.

`~T`

  Tabulation.

  `~@T`

    relative tabulation.

  `~colnum,colincT`

    full tabulation.

`~?`

  Indirection (expects indirect arguments as a list).

  `~@?`

    extracts indirect arguments from format arguments.

`~(str~)`

  Case conversion (converts by `string-downcase`).

  `~:(str~)`

    converts by `string-capitalize`.

  `~@(str~)`

    converts by `string-capitalize-first`.

  `~:@(str~)`

    converts by `string-upcase`.

`~*`

  Argument Jumping (jumps 1 argument forward).

  `~n*`

    jumps n arguments forward.

  `~:*`

    jumps 1 argument backward.

  `~n:*`

    jumps n arguments backward.

  `~@*`

    jumps to the 0th argument.

  `~n@*`

    jumps to the nth argument (beginning from 0)

`~[str0~;str1~;...~;strn~]`

  Conditional Expression (numerical clause conditional).

  `~n[`

    take argument from n.

  `~@[`

    true test conditional.

  `~:[`

    if-else-then conditional.

  `~;`

    clause separator.

  `~:;`

    default clause follows.

`~{str~}`

  Iteration (args come from the next argument (a list)). Iteration bounding is controlled by configuration variables format:iteration-bounded and format:max-iterations. With both variables default, a maximum of 100 iterations will be performed.

  `~n{`

    at most n iterations.

  `~:{`

    args from next arg (a list of lists).

  `~@{`

    args from the rest of arguments.

  `~:@{`

    args from the rest args (lists).

`~^`

  Up and out.

  `~n^`

    aborts if n = 0

  `~n,m^`

    aborts if n = m

  `~n,m,k^`

    aborts if n <= m <= k

#### 4.2.2.2 Not Implemented CL Format Control Directives ¶

`~:A`

  print `#f` as an empty list (see below).

`~:S`

  print `#f` as an empty list (see below).

`~<~>`

  Justification.

`~:^`

  (sorry I don’t understand its semantics completely)

#### 4.2.2.3 Extended, Replaced and Additional Control Directives ¶

`~mincol,padchar,commachar,commawidthD`
`~mincol,padchar,commachar,commawidthX`
`~mincol,padchar,commachar,commawidthO`
`~mincol,padchar,commachar,commawidthB`
`~n,mincol,padchar,commachar,commawidthR`

  commawidth is the number of characters between two comma characters.

`~I`

  print a R4RS complex number as `~F~@Fi` with passed parameters for `~F`.

`~Y`

  Pretty print formatting of an argument for scheme code lists.

`~K`

  Same as `~?.`

`~!`

  Flushes the output if format destination is a port.

`~_`

  Print a `#\space` character

  `~n_`

    print n `#\space` characters.

`~/`

  Print a `#\tab` character

  `~n/`

    print n `#\tab` characters.

`~nC`

  Takes n as an integer representation for a character. No arguments are consumed. n is converted to a character by `integer->char`. n must be a positive decimal number.

`~:S`

  Print out readproof. Prints out internal objects represented as `#<...>` as strings `"#<...>"` so that the format output can always be processed by `read`.

`~:A`

  Print out readproof. Prints out internal objects represented as `#<...>` as strings `"#<...>"` so that the format output can always be processed by `read`.

`~Q`

  Prints information and a copyright notice on the format implementation.

  `~:Q`

    prints format version.

`~F, ~E, ~G, ~$`

  may also print number strings, i.e. passing a number as a string and format it accordingly.

#### 4.2.2.4 Configuration Variables ¶

Format has some configuration variables at the beginning of `format.scm` to suit the systems and users needs. There should be no modification necessary for the configuration that comes with SLIB. If modification is desired the variable should be set after the format code is loaded. Format detects automatically if the running scheme system implements floating point numbers and complex numbers.

format:symbol-case-conv

  Symbols are converted by `symbol->string` so the case type of the printed symbols is implementation dependent. `format:symbol-case-conv` is a one arg closure which is either `#f` (no conversion), `string-upcase`, `string-downcase` or `string-capitalize`. (default `#f`)

format:iobj-case-conv

  As format:symbol-case-conv but applies for the representation of implementation internal objects. (default `#f`)

format:expch

  The character prefixing the exponent value in `~E` printing. (default `#\E`)

format:iteration-bounded

  When `#t`, a `~{...~}` control will iterate no more than the number of times specified by format:max-iterations regardless of the number of iterations implied by modifiers and arguments. When `#f`, a `~{...~}` control will iterate the number of times implied by modifiers and arguments, unless termination is forced by language or system limitations. (default `#t`)

format:max-iterations

  The maximum number of iterations performed by a `~{...~}` control. Has effect only when format:iteration-bounded is `#t`. (default 100)

#### 4.2.2.5 Compatibility With Other Format Implementations ¶

SLIB format 2.x:

  See `format.doc`.

SLIB format 1.4:

  Downward compatible except for padding support and `~A`, `~S`, `~P`, `~X` uppercase printing. SLIB format 1.4 uses C-style `printf` padding support which is completely replaced by the CL `format` padding style.

MIT C-Scheme 7.1:

  Downward compatible except for `~`, which is not documented (ignores all characters inside the format string up to a newline character). (7.1 implements `~a`, `~s`, ~newline, `~~`, `~%`, numerical and variable parameters and `:/@` modifiers in the CL sense).

Elk 1.5/2.0:

  Downward compatible except for `~A` and `~S` which print in uppercase. (Elk implements `~a`, `~s`, `~~`, and `~%` (no directive parameters or modifiers)).

Scheme->C 01nov91:

  Downward compatible except for an optional destination parameter: S2C accepts a format call without a destination which returns a formatted string. This is equivalent to a #f destination in S2C. (S2C implements `~a`, `~s`, `~c`, `~%`, and `~~` (no directive parameters or modifiers)).

This implementation of format is solely useful in the SLIB context because it requires other components provided by SLIB.