case -- r7rs Definition

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Kind

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syntax;

Implemented by

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• case (from vonuvoli);

Syntax signature

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Syntax keywords:

• else: literal;
• value: expression;
• variant: value of type any;
• then-expression: expression;
• clause: pattern with variants:
  • ((variant |...|));
  • ((variant |...|) then-expression |...|);
  • (else);
  • (else then-expression |...|);

Syntax variants:

• (_ value)
• (_ value clause |...|)

Exports

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• scheme:base -- (scheme base);

Exports recursive

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• scheme -- (scheme);

Description

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(case <key> <clause_1> <clause_2> ...)
else    ; auxiliary syntax
=>      ; auxiliary syntax

Syntax: <Key> can be any expression. Each <clause> has the form

((<datum_1> ...) <expression_1> <expression_2> ...)

where each <datum> is an external representation of some object. It is an error if any of the <datum>s are the same anywhere in the expression. Alternatively, a <clause> can be of the form

((<datum_1> ...) => <expression>)

The last <clause> can be an "else clause", which has one of the forms

(else <expression_1> <expression_2> ...)

or

(else => <expression>)

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 the same (in the sense of eqv?; see section on eqv?) to a <datum>, then the expressions in the corresponding <clause> are evaluated in order and the results of the last expression in the <clause> are returned as the results of 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 results of the last are the results of the case expression; otherwise the result of the case expression is unspecified.

If the selected <clause> or else clause uses the => alternate form, then the <expression> is evaluated. It is an error if its value is not a procedure accepting one argument. This procedure is then called on the value of the <key> and the values returned by this procedure are returned by the case expression.

(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 => (lambda (x) x)))     ===>  c

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The text herein was sourced and adapted as described in the "R7RS attribution of various text snippets" appendix.

Referenced-types

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• any;