by Chongkai Zhu
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This SRFI proposes an extension to the
to allow the
=> clauses as in
case is introduced as a syntax sugar based on
cond, which helps to save a explicit calling to
But without the
=> clause, if the
needs the value of
let can't be saved. For an
easy example, suppose we want the following:
(case (get-symbol) ((true) #t) ((false) #f) (else => (lambda (x) x)))
=> clause in
case, we have to
(let ((key (get-symbol))) (cond ((eq? key 'true) #t) ((eq? key 'false) #f) (else key)))
(Based on R5RS section 4.2.1 Conditionals)
((<datum1> ...) <expression1> <expression2> ...),
where each <datum> is an external representation of some object. All the <datum>s must be distinct. The last <clause> may be an "else clause," which has the form
(else <expression1> <expression2> ...).
Alternatively, a <clause> may be of the form
((<datum1> ...) => <expression>)
and the last <clause> may be of the form
(else => <expression>)
Semantics: A `case' expression is evaluated as
follows. <Key> is evaluated and its result is compared against each
<datum>. If the result of evaluating <key> is equivalent (in the
sense of `eqv?'; see section see section 6.1 Equivalence
predicates) to a <datum>, then the expressions in the corresponding
<clause> are evaluated from left to right and the result(s) of the last
expression in the <clause> is(are) returned as the result(s) of the
`case' expression. If the result of evaluating <key> is
different from every <datum>, then if there is an else clause its
expressions are evaluated and the result(s) of the last is(are) the result(s)
of the `case' expression; otherwise the result of the
`case' expression is unspecified. If the
selected <clause> uses the
=> alternate form, then the
<expression> is evaluated. Its value must be a procedure that accepts
one argument; this procedure is then called on the value of <Key> and
the value(s) returned by this procedure is(are) returned by the
(Based on R5RS section 3.5 Proper tail recursion)
If a cond or case expression is in a tail context, and has a clause of the form (<expression1> => <expression2>) then the (implied) call to the procedure that results from the evaluation of <expression2> is in a tail context. <expression2> itself is not in a tail context.
(define-syntax case (syntax-rules (else =>) ((case (key ...) clauses ...) (let ((atom-key (key ...))) (case atom-key clauses ...))) ((case key (else => result)) (result key)) ((case key ((atoms ...) => result)) (if (memv key '(atoms ...)) (result key))) ((case key ((atoms ...) => result) clause clauses ...) (if (memv key '(atoms ...)) (result key) (case key clause clauses ...))) ((case key (else result1 result2 ...)) (begin result1 result2 ...)) ((case key ((atoms ...) result1 result2 ...)) (if (memv key '(atoms ...)) (begin result1 result2 ...))) ((case key ((atoms ...) result1 result2 ...) clause clauses ...) (if (memv key '(atoms ...)) (begin result1 result2 ...) (case key clause clauses ...)))))
Copyright (C) 2006 Chongkai Zhu. All Rights Reserved.
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