# SRFI 28: Basic Format Strings

by Scott G. Miller

> > > This copy of the SRFI 28 specification document is distributed as part of the Racket package srfi-doc.

> > > The canonical source of this document is https://srfi.schemers.org/srfi-28/srfi-28.html.

## Status

This SRFI is currently in final status. Here is an explanation of each status that a SRFI can hold. To provide input on this SRFI, please send email to `srfi-28@nospamsrfi.schemers.org`. To subscribe to the list, follow these instructions. You can access previous messages via the mailing list archive.

- Draft: 2002-03-23--2002-05-23
- Final: 2002-06-25

## Abstract

This document specifies Format Strings, a method of interpreting a Scheme string which contains a number of escape sequences that are replaced with other string data according to the semantics of each sequence.

## Rationale

Many Scheme systems provide access to a function called `format.` This function takes as arguments a format string, an ordinary Scheme string containing zero or more escape sequences, followed zero or more Scheme values. The procedure processes the format string and performs string replacement on the escape sequences according to the rules for each code. This SRFI defines a basic version of `format` which should allow portable code to be written using the function without much (if any) effort on the part of Scheme implementors.

## Specification

`format` `format-string [obj ...] -> string`

> Accepts a message template (a Scheme String), and processes it, replacing any escape sequences in order with one or more characters, the characters themselves dependent on the semantics of the escape sequence encountered.

> An escape sequence is a two character sequence in the string where the first character is a tilde '~'. Each escape code's meaning is as follows:

> - `~a` The corresponding value is inserted into the string as if printed with `display`.
> - `~s` The corresponding value is inserted into the string as if printed with `write`.
> - `~%` A newline is inserted.
> - `~~` A tilde '~' is inserted.

> `~a` and `~s`, when encountered, require a corresponding Scheme value to be present after the format string. The values provided as operands are used by the escape sequences in order. It is an error if fewer values are provided than escape sequences that require them.

> `~%` and `~~` require no corresponding value.

### Examples

    (format "Hello, ~a" "World!")
    ; => "Hello, World!"

    (format "Error, list is too short: ~s~%" '(one "two" 3))
    ; => "Error, list is too short: (one \"two\" 3))"

## Implementation

The implementation below requires SRFI-6 (Basic string ports) and SRFI-23 (Error reporting mechanism).

    (define format
      (lambda (format-string . objects)
        (let ((buffer (open-output-string)))
          (let loop ((format-list (string->list format-string))
                     (objects objects))
            (cond ((null? format-list) (get-output-string buffer))
                  ((char=? (car format-list) #\~)
                   (if (null? (cdr format-list))
                       (error 'format "Incomplete escape sequence")
                       (case (cadr format-list)
                         ((#\a)
                          (if (null? objects)
                              (error 'format "No value for escape sequence")
                              (begin
                                (display (car objects) buffer)
                                (loop (cddr format-list) (cdr objects)))))
    	             ((#\s)
                          (if (null? objects)
                              (error 'format "No value for escape sequence")
                              (begin
                                (write (car objects) buffer)
                                (loop (cddr format-list) (cdr objects)))))
                         ((#\%)
                          (newline buffer)
                          (loop (cddr format-list) objects))
                         ((#\~)
                          (write-char #\~ buffer)
                          (loop (cddr format-list) objects))
                         (else
                          (error 'format "Unrecognized escape sequence")))))
                  (else (write-char (car format-list) buffer)
                        (loop (cdr format-list) objects)))))))

## Copyright

Copyright (C) Scott G. Miller (2002). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Last modified: Mon Apr 8 06:54:37 CDT 2002