16: Syntax for procedures of variable arity.16: Syntax for procedures of variable arity.by Lars T Hansen
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CASE-LAMBDA, a syntax for procedures with a variable number of arguments, is introduced.
CASE-LAMBDA reduces the clutter of procedures that execute different code depending on the number of arguments they were passed; it is a pattern-matching mechanism that matches on the number of arguments. CASE-LAMBDA is available in some Scheme systems.
While CASE-LAMBDA can be implemented as a macro using only facilities available in R5RS Scheme, it admits considerable implementation-specific optimization.
Each <clause> should have the form (<formals> <body>), where <formals> is a formal arguments list as for LAMBDA, cf section 4.1.4 of the R5RS. Each <body> is a <tail-body>, cf section 3.5 of the R5RS.
A CASE-LAMBDA expression evaluates to a procedure that accepts a variable number of arguments and is lexically scoped in the same manner as procedures resulting from LAMBDA expressions. When the procedure is called with some arguments V1 .. Vk, then the first <clause> for which the arguments agree with <formals> is selected, where agreement is specified as for the <formals> of a LAMBDA expression. The variables of <formals> are bound to fresh locations, the values V1 .. Vk are stored in those locations, the <body> is evaluated in the extended environment, and the results of <body> are returned as the results of the procedure call.
It is an error for the arguments not to agree with the <formals> of any <clause>.
(define plus
(case-lambda
(() 0)
((x) x)
((x y) (+ x y))
((x y z) (+ (+ x y) z))
(args (apply + args))))
(plus) --> 0
(plus 1) --> 1
(plus 1 2 3) --> 6
((case-lambda
((a) a)
((a b) (* a b)))
1 2 3) --> error
The following implementation is written in R5RS Scheme. It is not compatible with the IEEE Scheme standard because the IEEE standard does not contain the high-level macro system.
The implementation assumes that some top-level names defined by the R5RS are bound to their original values.
;; This code is in the public domain.
(define-syntax case-lambda
(syntax-rules ()
((case-lambda)
(lambda args
(error "CASE-LAMBDA without any clauses.")))
((case-lambda
(?a1 ?e1 ...)
?clause1 ...)
(lambda args
(let ((l (length args)))
(case-lambda "CLAUSE" args l
(?a1 ?e1 ...)
?clause1 ...))))
((case-lambda "CLAUSE" ?args ?l
((?a1 ...) ?e1 ...)
?clause1 ...)
(if (= ?l (length '(?a1 ...)))
(apply (lambda (?a1 ...) ?e1 ...) ?args)
(case-lambda "CLAUSE" ?args ?l
?clause1 ...)))
((case-lambda "CLAUSE" ?args ?l
((?a1 . ?ar) ?e1 ...)
?clause1 ...)
(case-lambda "IMPROPER" ?args ?l 1 (?a1 . ?ar) (?ar ?e1 ...)
?clause1 ...))
((case-lambda "CLAUSE" ?args ?l
(?a1 ?e1 ...)
?clause1 ...)
(let ((?a1 ?args))
?e1 ...))
((case-lambda "CLAUSE" ?args ?l)
(error "Wrong number of arguments to CASE-LAMBDA."))
((case-lambda "IMPROPER" ?args ?l ?k ?al ((?a1 . ?ar) ?e1 ...)
?clause1 ...)
(case-lambda "IMPROPER" ?args ?l (+ ?k 1) ?al (?ar ?e1 ...)
?clause1 ...))
((case-lambda "IMPROPER" ?args ?l ?k ?al (?ar ?e1 ...)
?clause1 ...)
(if (>= ?l ?k)
(apply (lambda ?al ?e1 ...) ?args)
(case-lambda "CLAUSE" ?args ?l
?clause1 ...)))))
Copyright (C) Lars T Hansen (1999). All Rights Reserved.
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