2.13 Dispatch: case

(case val-expr case-clause ...)

case-clause   =   [(datum ...) then-expr ...+]     |   [else then-expr ...+]

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 eqv? 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>.

For the selected case-clause, the results of the last then-expr, 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.

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

(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"