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
90