# Title

SRFI-6: Basic String Ports

# Author

William D Clinger

# Status

This SRFI is currently in ``final'' status. To see an explanation of each status that a SRFI can hold, see here. You can access the discussion via the archive of the mailing list.

- Received: 1999/04/07
- Draft: 1999/04/26-1999/06/24
- Final: 1999/07/01

# Abstract

Scheme's i/o primitives are extended by adding three new procedures that

- create an input port from a string,
- create an output port whose contents are accumulated in Scheme's working memory instead of an external file, and
- extract the accumulated contents of an in-memory output port and return them in the form of a string.

# Issues

None.

# Rationale

Scheme's procedures for performing input and output from and to ports become more useful when extended by string ports. The interface described here has been in use since 1986 or before, and is currently supported by several of the major implementations.

# Specification

This specification is taken from the MacScheme Reference Manual.

    (OPEN-INPUT-STRING string)                            ;procedure

  Takes a string and returns an input port that delivers characters from the string. The port can be closed by `CLOSE-INPUT-PORT`, though its storage will be reclaimed by the garbage collector if it becomes inaccessible.

              (define p
                (open-input-string "(a . (b . c . ())) 34"))

              (input-port? p)                 -->  #t
              (read p)                        -->  (a b c)
              (read p)                        -->  34
              (eof-object? (peek-char p))     -->  #t

    (OPEN-OUTPUT-STRING)                                  ;procedure

  Returns an output port that will accumulate characters for retrieval by `GET-OUTPUT-STRING`. The port can be closed by the procedure `CLOSE-OUTPUT-PORT`, though its storage will be reclaimed by the garbage collector if it becomes inaccessible.

              (let ((q (open-output-string))
                    (x '(a b c)))
                (write (car x) q)
                (write (cdr x) q)
                (get-output-string q))        -->  "a(b c)"

    (GET-OUTPUT-STRING output-port)                       ;procedure

  Given an output port created by `OPEN-OUTPUT-STRING`, returns a string consisting of the characters that have been output to the port so far.

# Implementation

What follows is just an outline of how these procedures might be implemented, because a real implementation would also have to redefine `READ`, `WRITE`, and so forth to use `PEEK-CHAR`, `READ-CHAR`, and `WRITE-CHAR` as redefined below.

Since the code for `READ` and `WRITE` would be identical to code that already exists in any implementation, however, it should not be necessary for this SRFI to include that code within this SRFI. Including it would only detract from the readability of this implementation.

    ; This implementation is not IEEE- or R5RS-compliant,
    ; for the following reasons:
    ;
    ; This implementation does not redefine procedures
    ; like READ, WRITE, DISPLAY, and NEWLINE to ensure
    ; that they use the redefined PEEK-CHAR, READ-CHAR,
    ; WRITE-CHAR, and so forth.  That should be easy
    ; for an implementor to do, however.
    ;
    ; This implementation obtains an end-of-file object
    ; by reading a Unix-specific file, /dev/null.

    (define open-input-string 0)  ; assigned below
    (define open-output-string 0) ; assigned below
    (define get-output-string 0)  ; assigned below

    ; We have to remember the original procedures before
    ; we can define new ones.

    (define ur-vector? vector?)
    (define ur-vector-length vector-length)
    (define ur-vector-ref vector-ref)
    (define ur-vector-set! vector-set!)
    (define ur-input-port? input-port?)
    (define ur-output-port? output-port?)
    (define ur-close-input-port close-input-port)
    (define ur-close-output-port close-output-port)
    (define ur-peek-char peek-char)
    (define ur-read-char read-char)
    (define ur-write-char write-char)

    ; IEEE/ANSI Scheme insists that we define any global
    ; variables that we are going to assign.  R5RS Scheme
    ; apparently does not require this.

    (define vector? vector?)
    (define vector-length vector-length)
    (define vector-ref vector-ref)
    (define vector-set! vector-set!)
    (define input-port? input-port?)
    (define output-port? output-port?)
    (define close-input-port close-input-port)
    (define close-output-port close-output-port)
    (define peek-char peek-char)
    (define read-char read-char)
    (define write-char write-char)

    (let ((ur-vector? ur-vector?)
          (ur-vector-length ur-vector-length)
          (ur-vector-ref ur-vector-ref)
          (ur-vector-set! ur-vector-set!)
          (ur-input-port? ur-input-port?)
          (ur-output-port? ur-output-port?)
          (ur-close-input-port ur-close-input-port)
          (ur-close-output-port ur-close-output-port)
          (ur-peek-char ur-peek-char)
          (ur-read-char ur-read-char)
          (ur-write-char ur-write-char)
          (eof (call-with-input-file "/dev/null" read-char))
          (input-string-tag (list 'input-string-tag))
          (output-string-tag (list 'output-string-tag)))

      (define (error)
        (display "You're not supposed to do that!")
        (newline)
        (if #f #f))

      (define (restrict f pred?)
        (lambda (x . rest)
          (if (pred? x)
              (apply f x rest)
              (error))))

      (define (my-vector? x)
        (and (ur-vector? x)
             (not (input-string? x))
             (not (output-string? x))))

      (define (input-string? x)
        (and (ur-vector? x)
             (positive? (ur-vector-length x))
             (eq? input-string-tag (ur-vector-ref x 0))))

      (define (output-string? x)
        (and (ur-vector? x)
             (positive? (ur-vector-length x))
             (eq? output-string-tag (ur-vector-ref x 0))))

      (define (selector pred? i)
        (lambda (x)
          (if (pred? x)
              (ur-vector-ref x i)
              (error))))

      (define (setter pred? i)
        (lambda (x y)
          (if (pred? x)
              (begin (ur-vector-set! x i y)
                     (if #f #f))
              (error))))

      (set! vector?       my-vector?)
      (set! vector-length (restrict ur-vector-length my-vector?))
      (set! vector-ref    (restrict ur-vector-ref  my-vector?))
      (set! vector-set!   (restrict ur-vector-set! my-vector?))

      (let ()

        ; The guts of the implementation begin here.

        (define (make-input-string s)
          (vector input-string-tag #t s (string-length s) 0))

        (define input-string:open?  (selector input-string? 1))
        (define input-string:open?! (setter   input-string? 1))
        (define input-string:string (selector input-string? 2))
        (define input-string:size   (selector input-string? 3))
        (define input-string:next   (selector input-string? 4))
        (define input-string:next!  (setter   input-string? 4))

        (define (make-output-string)
          (vector output-string-tag #t '()))

        (define output-string:open?     (selector output-string? 1))
        (define output-string:open?!    (setter   output-string? 1))
        (define output-string:contents  (selector output-string? 2))
        (define output-string:contents! (setter   output-string? 2))

        (set! open-input-string make-input-string)
        (set! open-output-string make-output-string)
        (set! get-output-string
              (lambda (x)
                (list->string (reverse (output-string:contents x)))))

        (set! input-port?
              (lambda (x)
                (or (ur-input-port? x)
                    (input-string? x))))

        (set! output-port?
              (lambda (x)
                (or (ur-output-port? x)
                    (output-string? x))))

        (set! close-input-port
              (lambda (x)
                (if (input-string? x)
                    (input-string:open?! x #f)
                    (ur-close-input-port x))))

        (set! close-output-port
              (lambda (x)
                (if (output-string? x)
                    (output-string:open?! x #f)
                    (ur-close-output-port x))))

        (set! peek-char
              (lambda args
                (if (null? args)
                    (ur-peek-char)
                    (let ((x (car args)))
                      (if (input-string? x)
                          (let ((s (input-string:string x))
                                (i (input-string:next x))
                                (n (input-string:size x)))
                            (if (input-string:open? x)
                                (if (< i n)
                                    (string-ref s i)
                                    eof)
                                (error)))
                          (ur-peek-char x))))))

        (set! read-char
              (lambda args
                (if (null? args)
                    (ur-read-char)
                    (let ((x (car args)))
                      (if (input-string? x)
                          (let ((s (input-string:string x))
                                (i (input-string:next x))
                                (n (input-string:size x)))
                            (if (input-string:open? x)
                                (if (< i n)
                                    (let ((c (string-ref s i)))
                                      (input-string:next! x (+ i 1))
                                      c)
                                    eof)
                                (error)))
                          (ur-read-char x))))))

        (set! write-char
              (lambda (c . rest)
                (if (null? rest)
                    (ur-write-char c)
                    (let ((x (car rest)))
                      (if (output-string? x)
                          (if (output-string:open? x)
                              (output-string:contents!
                               x
                               (cons c (output-string:contents x)))
                              (error))
                          (ur-write-char c x))))))

        (if #f #f)))

# Copyright

Copyright (C) William D Clinger (1999). All Rights Reserved.

This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Scheme Request For Implementation process or editors, except as needed for the purpose of developing SRFIs in which case the procedures for copyrights defined in the SRFI process must be followed, or as required to translate it into languages other than English.

The limited permissions granted above are perpetual and will not be revoked by the authors or their successors or assigns.

This document and the information contained herein is provided on an "AS IS" basis and THE AUTHOR AND THE SRFI EDITORS DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Last modified: Wed Aug 13 11:49:23 CDT 2003