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[bpt/guile.git] / srfi / srfi-10.scm

;;;; srfi-10.scm --- SRFI-10 read hash extension for Guile
;;;;
;;;; 	Copyright (C) 2001 Free Software Foundation, Inc.
;;;; 
;;;; This program is free software; you can redistribute it and/or
;;;; modify it under the terms of the GNU General Public License as
;;;; published by the Free Software Foundation; either version 2, or
;;;; (at your option) any later version.
;;;; 
;;;; This program is distributed in the hope that it will be useful,
;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;;;; General Public License for more details.
;;;; 
;;;; You should have received a copy of the GNU General Public License
;;;; along with this software; see the file COPYING.  If not, write to
;;;; the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
;;;; Boston, MA 02111-1307 USA
;;;;
;;;; As a special exception, the Free Software Foundation gives permission
;;;; for additional uses of the text contained in its release of GUILE.
;;;;
;;;; The exception is that, if you link the GUILE library with other files
;;;; to produce an executable, this does not by itself cause the
;;;; resulting executable to be covered by the GNU General Public License.
;;;; Your use of that executable is in no way restricted on account of
;;;; linking the GUILE library code into it.
;;;;
;;;; This exception does not however invalidate any other reasons why
;;;; the executable file might be covered by the GNU General Public License.
;;;;
;;;; This exception applies only to the code released by the
;;;; Free Software Foundation under the name GUILE.  If you copy
;;;; code from other Free Software Foundation releases into a copy of
;;;; GUILE, as the General Public License permits, the exception does
;;;; not apply to the code that you add in this way.  To avoid misleading
;;;; anyone as to the status of such modified files, you must delete
;;;; this exception notice from them.
;;;;
;;;; If you write modifications of your own for GUILE, it is your choice
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;;;; If you do not wish that, delete this exception notice.

;;; Commentary:

;;; This module implements the syntax extension #,(), also called
;;; hash-comma, which is defined in SRFI-10.
;;;
;;; The support for SRFI-10 consists of the procedure
;;; `define-reader-ctor' for defining new reader constructors and the
;;; read syntax form 
;;;
;;; #,(<ctor> <datum> ...)
;;;
;;; where <ctor> must be a symbol for which a read constructor was
;;; defined previously.
;;;
;;; Example:
;;;
;;; (define-reader-ctor 'file open-input-file)
;;; (define f '#,(file "/etc/passwd"))
;;; (read-line f)
;;; =>
;;; "root:x:0:0:root:/root:/bin/bash"
;;;
;;; Please note the quote before the #,(file ...) expression.  This is
;;; necessary because ports are not self-evaluating in Guile.

;;; Code:

(define-module (srfi srfi-10)
  :use-module (ice-9 rdelim)
  :export (define-reader-ctor))

(cond-expand-provide (current-module) '(srfi-10))

;; This hash table stores the association between comma-hash tags and
;; the corresponding constructor procedures.
;;
(define reader-ctors (make-hash-table 31))

;; This procedure installs the procedure @var{proc} as the constructor
;; for the comma-hash tag @var{symbol}.
;;
(define (define-reader-ctor symbol proc)
  (hashq-set! reader-ctors symbol proc)
  (if #f #f))				; Return unspecified value.

;; Retrieve the constructor procedure for the tag @var{symbol} or
;; throw an error if no such tag is defined.
;;
(define (lookup symbol)
  (let ((p (hashq-ref reader-ctors symbol #f)))
    (if (procedure? p)
	p
	(error "unknown hash-comma tag " symbol))))

;; This is the actual reader extension.
;;
(define (hash-comma char port)
  (let* ((obj (read port)))
    (if (and (list? obj) (positive? (length obj)) (symbol? (car obj)))
	(let ((p (lookup (car obj))))
	  (let ((res (apply p (cdr obj))))
	    res))
	(error "syntax error in hash-comma expression"))))

;; Install the hash extension.
;;
(read-hash-extend #\, hash-comma)