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casesyntax to allow the
=>clauses as in
caseis introduced as a syntax sugar based on
cond, which helps to save a explicit calling to
let. But without the
=>clause, if the
resultexpression needs the value of
letcan'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 write:
(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
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
=> 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.
(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 ...)))))
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