srfi/srfi-10.scm

   1 ;;;; srfi-10.scm --- SRFI-10 read hash extension for Guile
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  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 ;;; Code:
  70
  71 (define-module (srfi srfi-10)
  72   #:use-module (ice-9 rdelim))
  73
  74 (export define-reader-ctor)
  75
  76 (cond-expand-provide (current-module) '(srfi-10))
  77
  78 ;; This hash table stores the association between comma-hash tags and
  79 ;; the corresponding constructor procedures.
  80 ;;
  81 (define reader-ctors (make-hash-table 31))
  82
  83 ;; This procedure installs the procedure @var{proc} as the constructor
  84 ;; for the comma-hash tag @var{symbol}.
  85 ;;
  86 (define (define-reader-ctor symbol proc)
  87   (hashq-set! reader-ctors symbol proc)
  88   (if #f #f))                           ; Return unspecified value.
  89
  90 ;; Retrieve the constructor procedure for the tag @var{symbol} or
  91 ;; throw an error if no such tag is defined.
  92 ;;
  93 (define (lookup symbol)
  94   (let ((p (hashq-ref reader-ctors symbol #f)))
  95     (if (procedure? p)
  96         p
  97         (error "unknown hash-comma tag " symbol))))
  98
  99 ;; This is the actual reader extension.
 100 ;;
 101 (define (hash-comma char port)
 102   (let* ((obj (read port)))
 103     (if (and (list? obj) (positive? (length obj)) (symbol? (car obj)))
 104         (let ((p (lookup (car obj))))
 105           (let ((res (apply p (cdr obj))))
 106             res))
 107         (error "syntax error in hash-comma expression"))))
 108
 109 ;; Install the hash extension.
 110 ;;
 111 (read-hash-extend #\, hash-comma)