module/srfi/srfi-31.scm

   1 ;;; srfi-31.scm --- special form for recursive evaluation
   2
   3 ;;      Copyright (C) 2004, 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 ;;; Original author: Rob Browning <rlb@defaultvalue.org>
  20
  21 (define-module (srfi srfi-31)
  22   :export-syntax (rec))
  23
  24 (define-macro (rec arg-form . body)
  25   (cond
  26    ((and (symbol? arg-form) (= 1 (length body)))
  27     ;; (rec S (cons 1 (delay S)))
  28     `(letrec ((,arg-form ,(car body)))
  29        ,arg-form))
  30    ;; (rec (f x) (+ x 1))
  31    ((list? arg-form)
  32     `(letrec ((,(car arg-form) (lambda ,(cdr arg-form) ,@body)))
  33        ,(car arg-form)))
  34    (else
  35     (error "syntax error in rec form" `(rec ,arg-form ,@body)))))