3.13 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:
> (case (+ 7 5) [(1 2 3) 'small] [(10 11 12) 'big]) 'big
> (case (- 7 5) [(1 2 3) 'small] [(10 11 12) 'big]) 'small
> (case (string-append "do" "g") [("cat" "dog" "mouse") "animal"] [else "mineral or vegetable"]) "animal"
> (case (list 'y 'x) [((a b) (x y)) 'forwards] [((b a) (y x)) 'backwards]) 'backwards
> (case 'x [(x) "ex"] [('x) "quoted ex"]) "ex"
> (case (list 'quote 'x) [(x) "ex"] [('x) "quoted ex"]) "quoted ex"
(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"