SRFI-87: => in case clauses
This SRFI proposes an extension to the case syntax to allow the => clauses as in cond.
For more information see: SRFI-87: => in case clauses
case is introduced as a syntax sugar based on cond, which helps to save a explicit calling to let. But without the => clause, if the result expression needs the value of key, the 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)))
Without the => clause in case, we have to write:
(let ((key (get-symbol))) (cond ((eq? key 'true) #t) ((eq? key 'false) #f) (else key)))
(Based on R5RS section 4.2.1 Conditionals)
<Key> may be any expression. Each <clause> should have the form
- ((<datum1> ...) <expression1> <expression2> ...)syntax
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> ...)syntax
Alternatively, a <clause> may be of the form
- ((<datum1> ...) => <expression>)syntax
and the last <clause> may be of the form
- (else => <expression>)syntax
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 case expression.
(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 (<expression> => <expression>) then the (implied) call to the procedure that results from the evaluation of <expression> is in a tail context. <expression> itself is not in a tail context.
- Chongkai Zhu
- Packaged for Chicken Scheme 5 by Sergey Goldgaber
Copyright (C) 2006 Chongkai Zhu. All Rights Reserved.
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- 0.1 - Ported to Chicken Scheme 5