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;;; srfi-16.scm --- case-lambda
;; Copyright (C) 2001, 2002, 2006 Free Software Foundation, Inc.
;;
;; This library is free software; you can redistribute it and/or
;; modify it under the terms of the GNU Lesser General Public
;; License as published by the Free Software Foundation; either
;; version 3 of the License, or (at your option) any later version.
;;
;; This library 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
;; Lesser General Public License for more details.
;;
;; You should have received a copy of the GNU Lesser General Public
;; License along with this library; if not, write to the Free Software
;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
;;; Author: Martin Grabmueller
;;; Commentary:
;; Implementation of SRFI-16. `case-lambda' is a syntactic form
;; which permits writing functions acting different according to the
;; number of arguments passed.
;;
;; The syntax of the `case-lambda' form is defined in the following
;; EBNF grammar.
;;
;; <case-lambda>
;; --> (case-lambda <case-lambda-clause>)
;; <case-lambda-clause>
;; --> (<signature> <definition-or-command>*)
;; <signature>
;; --> (<identifier>*)
;; | (<identifier>* . <identifier>)
;; | <identifier>
;;
;; The value returned by a `case-lambda' form is a procedure which
;; matches the number of actual arguments against the signatures in
;; the various clauses, in order. The first matching clause is
;; selected, the corresponding values from the actual parameter list
;; are bound to the variable names in the clauses and the body of the
;; clause is evaluated.
;;; Code:
(define-module (srfi srfi-16)
:export-syntax (case-lambda))
(cond-expand-provide (current-module) '(srfi-16))
(define-macro (case-lambda . clauses)
;; Return the length of the list @var{l}, but allow dotted list.
;;
(define (alength l)
(cond ((null? l) 0)
((pair? l) (+ 1 (alength (cdr l))))
(else 0)))
;; Return @code{#t} if @var{l} is a dotted list, @code{#f} if it is
;; a normal list.
;;
(define (dotted? l)
(cond ((null? l) #f)
((pair? l) (dotted? (cdr l)))
(else #t)))
;; Return the expression for accessing the @var{index}th element of
;; the list called @var{args-name}. If @var{tail?} is true, code
;; for accessing the list-tail is generated, otherwise for accessing
;; the list element itself.
;;
(define (accessor args-name index tail?)
(if tail?
(case index
((0) `,args-name)
((1) `(cdr ,args-name))
((2) `(cddr ,args-name))
((3) `(cdddr ,args-name))
((4) `(cddddr ,args-name))
(else `(list-tail ,args-name ,index)))
(case index
((0) `(car ,args-name))
((1) `(cadr ,args-name))
((2) `(caddr ,args-name))
((3) `(cadddr ,args-name))
(else `(list-ref ,args-name ,index)))))
;; Generate the binding lists of the variables of one case-lambda
;; clause. @var{vars} is the (possibly dotted) list of variables
;; and @var{args-name} is the generated name used for the argument
;; list.
;;
(define (gen-temps vars args-name)
(let lp ((v vars) (i 0))
(cond ((null? v) '())
((pair? v)
(cons `(,(car v) ,(accessor args-name i #f))
(lp (cdr v) (+ i 1))))
(else `((,v ,(accessor args-name i #t)))))))
;; Generate the cond clauses for each of the clauses of case-lambda,
;; including the parameter count check, binding of the parameters
;; and the code of the corresponding body.
;;
(define (gen-clauses l length-name args-name)
(cond ((null? l) (list '(else (error "too few arguments"))))
(else
(cons
`((,(if (dotted? (caar l)) '>= '=)
,length-name ,(alength (caar l)))
(let ,(gen-temps (caar l) args-name)
,@(cdar l)))
(gen-clauses (cdr l) length-name args-name)))))
(let ((args-name (gensym))
(length-name (gensym)))
(let ((proc
`(lambda ,args-name
(let ((,length-name (length ,args-name)))
(cond ,@(gen-clauses clauses length-name args-name))))))
proc)))
;;; srfi-16.scm ends here
|
