chickadee » scheme » case

Syntax: <Key> may be any expression. Each <clause> should have the form

((<datum[1]> ...) <expression[1]> <expression[2]> ...),

where each <datum> is an external representation of some object. Alternatively, as per R7RS, a <clause> may be of the form

((<datum[1]> ...) => <expression>).

All the <datum>s must be distinct. The last <clause> may be an "else clause," which has one of the following two forms:

(else <expression[1]> <expression[2]> ...)
(else => <expression>).      ; R7RS extension

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 below) 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 selected <clause> uses the => alternate form (an R7RS extension), then the <expression> is evaluated. Its value must be a procedure that accepts one argument; this procedure is then called on the value of the <key> and the value(s) returned by this procedure is(are) returned by 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.

(case (* 2 3)
  ((2 3 5 7) 'prime)
  ((1 4 6 8 9) 'composite))             ===>  composite
(case (car '(c d))
  ((a) 'a)
  ((b) 'b))                             ===>  unspecified
(case (car '(c d))
  ((a e i o u) 'vowel)
  ((w y) 'semivowel)
  (else 'consonant))                    ===>  consonant