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3.13 Dispatch: case

(case val-expr case-clause ...)

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

Evaluates

and uses the result to select a

. The selected clause is the first one with a

whose

d form is

to the result of

. If no such

is present, the

is selected; if no

is present, either, then the result of the

form is

.

The case form of racket differs from that of R6RS or R5RS by being based on 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

"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"

> (classify #\A)

"letter"

> (classify #\1)

"number"

> (classify #\!)

"other"

Added in version 8.11.1.8 of package base.

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