1 ;;; srfi-16.scm --- case-lambda
3 ;; Copyright (C) 2001, 2002, 2006 Free Software Foundation, Inc.
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.
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.
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
19 ;;; Author: Martin Grabmueller
23 ;; Implementation of SRFI-16. `case-lambda' is a syntactic form
24 ;; which permits writing functions acting different according to the
25 ;; number of arguments passed.
27 ;; The syntax of the `case-lambda' form is defined in the following
31 ;; --> (case-lambda <case-lambda-clause>)
32 ;; <case-lambda-clause>
33 ;; --> (<signature> <definition-or-command>*)
35 ;; --> (<identifier>*)
36 ;; | (<identifier>* . <identifier>)
39 ;; The value returned by a `case-lambda' form is a procedure which
40 ;; matches the number of actual arguments against the signatures in
41 ;; the various clauses, in order. The first matching clause is
42 ;; selected, the corresponding values from the actual parameter list
43 ;; are bound to the variable names in the clauses and the body of the
44 ;; clause is evaluated.
48 (define-module (srfi srfi-16)
49 :export-syntax (case-lambda))
51 (cond-expand-provide (current-module) '(srfi-16))
53 (define-macro (case-lambda . clauses)
55 ;; Return the length of the list @var{l}, but allow dotted list.
59 ((pair? l) (+ 1 (alength (cdr l))))
62 ;; Return @code{#t} if @var{l} is a dotted list, @code{#f} if it is
67 ((pair? l) (dotted? (cdr l)))
70 ;; Return the expression for accessing the @var{index}th element of
71 ;; the list called @var{args-name}. If @var{tail?} is true, code
72 ;; for accessing the list-tail is generated, otherwise for accessing
73 ;; the list element itself.
75 (define (accessor args-name index tail?)
79 ((1) `(cdr ,args-name))
80 ((2) `(cddr ,args-name))
81 ((3) `(cdddr ,args-name))
82 ((4) `(cddddr ,args-name))
83 (else `(list-tail ,args-name ,index)))
85 ((0) `(car ,args-name))
86 ((1) `(cadr ,args-name))
87 ((2) `(caddr ,args-name))
88 ((3) `(cadddr ,args-name))
89 (else `(list-ref ,args-name ,index)))))
91 ;; Generate the binding lists of the variables of one case-lambda
92 ;; clause. @var{vars} is the (possibly dotted) list of variables
93 ;; and @var{args-name} is the generated name used for the argument
96 (define (gen-temps vars args-name)
97 (let lp ((v vars) (i 0))
100 (cons `(,(car v) ,(accessor args-name i #f))
101 (lp (cdr v) (+ i 1))))
102 (else `((,v ,(accessor args-name i #t)))))))
104 ;; Generate the cond clauses for each of the clauses of case-lambda,
105 ;; including the parameter count check, binding of the parameters
106 ;; and the code of the corresponding body.
108 (define (gen-clauses l length-name args-name)
109 (cond ((null? l) (list '(else (error "too few arguments"))))
112 `((,(if (dotted? (caar l)) '>= '=)
113 ,length-name ,(alength (caar l)))
114 (let ,(gen-temps (caar l) args-name)
116 (gen-clauses (cdr l) length-name args-name)))))
118 (let ((args-name (gensym))
119 (length-name (gensym)))
122 (let ((,length-name (length ,args-name)))
123 (cond ,@(gen-clauses clauses length-name args-name))))))
126 ;;; srfi-16.scm ends here