srfi/srfi-10.scm

   1 ;;; srfi-10.scm --- Hash-Comma Reader Extension
   2
   3 ;;      Copyright (C) 2001, 2002, 2006 Free Software Foundation, Inc.
   4 ;;
   5 ;; This library is free software; you can redistribute it and/or
   6 ;; modify it under the terms of the GNU Lesser General Public
   7 ;; License as published by the Free Software Foundation; either
   8 ;; version 2.1 of the License, or (at your option) any later version.
   9 ;;
  10 ;; This library 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 ;; Lesser General Public License for more details.
  14 ;;
  15 ;; You should have received a copy of the GNU Lesser General Public
  16 ;; License along with this library; if not, write to the Free Software
  17 ;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  18
  19 ;;; Commentary:
  20
  21 ;; This module implements the syntax extension #,(), also called
  22 ;; hash-comma, which is defined in SRFI-10.
  23 ;;
  24 ;; The support for SRFI-10 consists of the procedure
  25 ;; `define-reader-ctor' for defining new reader constructors and the
  26 ;; read syntax form
  27 ;;
  28 ;; #,(<ctor> <datum> ...)
  29 ;;
  30 ;; where <ctor> must be a symbol for which a read constructor was
  31 ;; defined previously.
  32 ;;
  33 ;; Example:
  34 ;;
  35 ;; (define-reader-ctor 'file open-input-file)
  36 ;; (define f '#,(file "/etc/passwd"))
  37 ;; (read-line f)
  38 ;; =>
  39 ;; "root:x:0:0:root:/root:/bin/bash"
  40 ;;
  41 ;; Please note the quote before the #,(file ...) expression.  This is
  42 ;; necessary because ports are not self-evaluating in Guile.
  43 ;;
  44 ;; This module is fully documented in the Guile Reference Manual.
  45
  46 ;;; Code:
  47
  48 (define-module (srfi srfi-10)
  49   :use-module (ice-9 rdelim)
  50   :export (define-reader-ctor))
  51
  52 (cond-expand-provide (current-module) '(srfi-10))
  53
  54 ;; This hash table stores the association between comma-hash tags and
  55 ;; the corresponding constructor procedures.
  56 ;;
  57 (define reader-ctors (make-hash-table 31))
  58
  59 ;; This procedure installs the procedure @var{proc} as the constructor
  60 ;; for the comma-hash tag @var{symbol}.
  61 ;;
  62 (define (define-reader-ctor symbol proc)
  63   (hashq-set! reader-ctors symbol proc)
  64   (if #f #f))                           ; Return unspecified value.
  65
  66 ;; Retrieve the constructor procedure for the tag @var{symbol} or
  67 ;; throw an error if no such tag is defined.
  68 ;;
  69 (define (lookup symbol)
  70   (let ((p (hashq-ref reader-ctors symbol #f)))
  71     (if (procedure? p)
  72         p
  73         (error "unknown hash-comma tag " symbol))))
  74
  75 ;; This is the actual reader extension.
  76 ;;
  77 (define (hash-comma char port)
  78   (let* ((obj (read port)))
  79     (if (and (list? obj) (positive? (length obj)) (symbol? (car obj)))
  80         (let ((p (lookup (car obj))))
  81           (let ((res (apply p (cdr obj))))
  82             res))
  83         (error "syntax error in hash-comma expression"))))
  84
  85 ;; Install the hash extension.
  86 ;;
  87 (read-hash-extend #\, hash-comma)
  88
  89 ;;; srfi-10.scm ends here