srfi/srfi-10.scm

   1 ;;; srfi-10.scm --- Hash-Comma Reader Extension
   2
   3 ;;      Copyright (C) 2001, 2002 Free Software Foundation, Inc.
   4 ;;
   5 ;; This program is free software; you can redistribute it and/or
   6 ;; modify it under the terms of the GNU General Public License as
   7 ;; published by the Free Software Foundation; either version 2, or
   8 ;; (at your option) any later version.
   9 ;;
  10 ;; This program is distributed in the hope that it will be useful,
  11 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13 ;; General Public License for more details.
  14 ;;
  15 ;; You should have received a copy of the GNU General Public License
  16 ;; along with this software; see the file COPYING.  If not, write to
  17 ;; the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
  18 ;; Boston, MA 02111-1307 USA
  19 ;;
  20 ;; As a special exception, the Free Software Foundation gives permission
  21 ;; for additional uses of the text contained in its release of GUILE.
  22 ;;
  23 ;; The exception is that, if you link the GUILE library with other files
  24 ;; to produce an executable, this does not by itself cause the
  25 ;; resulting executable to be covered by the GNU General Public License.
  26 ;; Your use of that executable is in no way restricted on account of
  27 ;; linking the GUILE library code into it.
  28 ;;
  29 ;; This exception does not however invalidate any other reasons why
  30 ;; the executable file might be covered by the GNU General Public License.
  31 ;;
  32 ;; This exception applies only to the code released by the
  33 ;; Free Software Foundation under the name GUILE.  If you copy
  34 ;; code from other Free Software Foundation releases into a copy of
  35 ;; GUILE, as the General Public License permits, the exception does
  36 ;; not apply to the code that you add in this way.  To avoid misleading
  37 ;; anyone as to the status of such modified files, you must delete
  38 ;; this exception notice from them.
  39 ;;
  40 ;; If you write modifications of your own for GUILE, it is your choice
  41 ;; whether to permit this exception to apply to your modifications.
  42 ;; If you do not wish that, delete this exception notice.
  43
  44 ;;; Commentary:
  45
  46 ;; This module implements the syntax extension #,(), also called
  47 ;; hash-comma, which is defined in SRFI-10.
  48 ;;
  49 ;; The support for SRFI-10 consists of the procedure
  50 ;; `define-reader-ctor' for defining new reader constructors and the
  51 ;; read syntax form
  52 ;;
  53 ;; #,(<ctor> <datum> ...)
  54 ;;
  55 ;; where <ctor> must be a symbol for which a read constructor was
  56 ;; defined previously.
  57 ;;
  58 ;; Example:
  59 ;;
  60 ;; (define-reader-ctor 'file open-input-file)
  61 ;; (define f '#,(file "/etc/passwd"))
  62 ;; (read-line f)
  63 ;; =>
  64 ;; "root:x:0:0:root:/root:/bin/bash"
  65 ;;
  66 ;; Please note the quote before the #,(file ...) expression.  This is
  67 ;; necessary because ports are not self-evaluating in Guile.
  68 ;;
  69 ;; This module is fully documented in the Guile Reference Manual.
  70
  71 ;;; Code:
  72
  73 (define-module (srfi srfi-10)
  74   :use-module (ice-9 rdelim)
  75   :export (define-reader-ctor))
  76
  77 (cond-expand-provide (current-module) '(srfi-10))
  78
  79 ;; This hash table stores the association between comma-hash tags and
  80 ;; the corresponding constructor procedures.
  81 ;;
  82 (define reader-ctors (make-hash-table 31))
  83
  84 ;; This procedure installs the procedure @var{proc} as the constructor
  85 ;; for the comma-hash tag @var{symbol}.
  86 ;;
  87 (define (define-reader-ctor symbol proc)
  88   (hashq-set! reader-ctors symbol proc)
  89   (if #f #f))                           ; Return unspecified value.
  90
  91 ;; Retrieve the constructor procedure for the tag @var{symbol} or
  92 ;; throw an error if no such tag is defined.
  93 ;;
  94 (define (lookup symbol)
  95   (let ((p (hashq-ref reader-ctors symbol #f)))
  96     (if (procedure? p)
  97         p
  98         (error "unknown hash-comma tag " symbol))))
  99
 100 ;; This is the actual reader extension.
 101 ;;
 102 (define (hash-comma char port)
 103   (let* ((obj (read port)))
 104     (if (and (list? obj) (positive? (length obj)) (symbol? (car obj)))
 105         (let ((p (lookup (car obj))))
 106           (let ((res (apply p (cdr obj))))
 107             res))
 108         (error "syntax error in hash-comma expression"))))
 109
 110 ;; Install the hash extension.
 111 ;;
 112 (read-hash-extend #\, hash-comma)
 113
 114 ;;; srfi-10.scm ends here