2.13 Dispatch: case
syntax
(case val-expr case-clause ...)
case-clause = [(datum ...) then-body ...+] | [else then-body ...+]
Evaluates val-expr and uses the result to select a
case-clause. The selected clause is the first one with a
datum whose quoted form is equal? to the
result of val-expr. If no such datum is present, the
else case-clause is selected; if no else
case-clause is present, either, then the result of the
case form is #<void>.
The case form of racket differs from that of R6RS: Scheme or R5RS: Legacy Scheme by being based equal? instead of eqv? (in addition to allowing internal definitions).
For the selected case-clause, the results of the last then-body, which is in tail position with respect to the case form, are the results for the whole case form.
A case-clause that starts with else must be the last case-clause.
The case form can dispatch to a matching case-clause in O(log N) time for N datums.
Examples: | ||||||||||||||||||||||||||||||
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(define (classify c) (case (char-general-category c) [(ll lu lt ln lo) "letter"] [(nd nl no) "number"] [else "other"]))
> (classify #\A) "letter"
> (classify #\1) "number"
> (classify #\!) "other"