by William D Clinger
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Scheme's i/o primitives are extended by adding three new procedures that:
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.
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
Returns an output port that will accumulate characters for
GET-OUTPUT-STRING. The port can be
closed by the procedure
its storage will be reclaimed by the garbage collector if it
(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
returns a string consisting of the characters that have been
output to the port so far.
What follows is just an outline of how these procedures might be
implemented, because a real implementation would also have to
WRITE, and so forth
WRITE-CHAR as redefined below.
Since the code for
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 (C) William D Clinger (1999). All Rights Reserved.
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