073f886203
* module/srfi/srfi-16.scm: New file. * AUTHORS: Mention it. * module/srfi/srfi-16.mes: New file.
127 lines
4.1 KiB
Scheme
127 lines
4.1 KiB
Scheme
;;; srfi-16.scm --- case-lambda
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;; Copyright (C) 2001, 2002, 2006 Free Software Foundation, Inc.
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;;
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;; This library is free software; you can redistribute it and/or
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;; modify it under the terms of the GNU Lesser General Public
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;; License as published by the Free Software Foundation; either
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;; version 2.1 of the License, or (at your option) any later version.
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;;
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;; This library is distributed in the hope that it will be useful,
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;; but WITHOUT ANY WARRANTY; without even the implied warranty of
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;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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;; Lesser General Public License for more details.
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;;
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;; You should have received a copy of the GNU Lesser General Public
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;; License along with this library; if not, write to the Free Software
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;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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;;; Author: Martin Grabmueller
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;;; Commentary:
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;; Implementation of SRFI-16. `case-lambda' is a syntactic form
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;; which permits writing functions acting different according to the
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;; number of arguments passed.
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;;
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;; The syntax of the `case-lambda' form is defined in the following
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;; EBNF grammar.
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;;
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;; <case-lambda>
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;; --> (case-lambda <case-lambda-clause>)
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;; <case-lambda-clause>
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;; --> (<signature> <definition-or-command>*)
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;; <signature>
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;; --> (<identifier>*)
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;; | (<identifier>* . <identifier>)
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;; | <identifier>
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;;
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;; The value returned by a `case-lambda' form is a procedure which
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;; matches the number of actual arguments against the signatures in
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;; the various clauses, in order. The first matching clause is
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;; selected, the corresponding values from the actual parameter list
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;; are bound to the variable names in the clauses and the body of the
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;; clause is evaluated.
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;;; Code:
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(define-module (srfi srfi-16)
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:export-syntax (case-lambda))
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(cond-expand-provide (current-module) '(srfi-16))
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(define-macro (case-lambda . clauses)
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;; Return the length of the list @var{l}, but allow dotted list.
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;;
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(define (alength l)
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(cond ((null? l) 0)
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((pair? l) (+ 1 (alength (cdr l))))
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(else 0)))
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;; Return @code{#t} if @var{l} is a dotted list, @code{#f} if it is
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;; a normal list.
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;;
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(define (dotted? l)
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(cond ((null? l) #f)
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((pair? l) (dotted? (cdr l)))
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(else #t)))
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;; Return the expression for accessing the @var{index}th element of
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;; the list called @var{args-name}. If @var{tail?} is true, code
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;; for accessing the list-tail is generated, otherwise for accessing
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;; the list element itself.
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;;
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(define (accessor args-name index tail?)
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(if tail?
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(case index
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((0) `,args-name)
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((1) `(cdr ,args-name))
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((2) `(cddr ,args-name))
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((3) `(cdddr ,args-name))
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((4) `(cddddr ,args-name))
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(else `(list-tail ,args-name ,index)))
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(case index
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((0) `(car ,args-name))
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((1) `(cadr ,args-name))
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((2) `(caddr ,args-name))
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((3) `(cadddr ,args-name))
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(else `(list-ref ,args-name ,index)))))
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;; Generate the binding lists of the variables of one case-lambda
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;; clause. @var{vars} is the (possibly dotted) list of variables
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;; and @var{args-name} is the generated name used for the argument
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;; list.
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;;
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(define (gen-temps vars args-name)
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(let lp ((v vars) (i 0))
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(cond ((null? v) '())
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((pair? v)
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(cons `(,(car v) ,(accessor args-name i #f))
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(lp (cdr v) (+ i 1))))
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(else `((,v ,(accessor args-name i #t)))))))
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;; Generate the cond clauses for each of the clauses of case-lambda,
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;; including the parameter count check, binding of the parameters
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;; and the code of the corresponding body.
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;;
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(define (gen-clauses l length-name args-name)
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(cond ((null? l) (list '(else (error "too few arguments"))))
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(else
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(cons
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`((,(if (dotted? (caar l)) '>= '=)
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,length-name ,(alength (caar l)))
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(let ,(gen-temps (caar l) args-name)
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,@(cdar l)))
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(gen-clauses (cdr l) length-name args-name)))))
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(let ((args-name (gensym))
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(length-name (gensym)))
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(let ((proc
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`(lambda ,args-name
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(let ((,length-name (length ,args-name)))
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(cond ,@(gen-clauses clauses length-name args-name))))))
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proc)))
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;;; srfi-16.scm ends here
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