cond -- r7rs Definition

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Kind

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syntax;

Implemented by

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• cond (from vonuvoli);

Syntax signature

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Syntax keywords:

• else: literal;
• condition: expression;
• then-expression: expression;
• clause: pattern with variants:
  • (condition);
  • (condition then-expression |...|);
  • (else);
  • (else then-expression |...|);

Syntax variants:

• (_)
• (_ clause |...|)

Exports

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• scheme:base -- (scheme base);

Exports recursive

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• scheme -- (scheme);

Description

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

Syntax: <Clauses> take one of two forms, either

(<test> <expression_1> ...)

where <test> is any expression, or

(<test> => <expression>)

The last <clause> can be an "else clause", which has the form

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

Semantics: A cond expression is evaluated by evaluating the <test> expressions of successive <clause>s in order until one of them evaluates to a true value (see section on booleans). When a <test> evaluates to a true value, the remaining <expression>s in its <clause> are evaluated in order, and the results of the last <expression> in the <clause> are returned as the results of the entire cond expression.

If the selected <clause> contains only the <test> and no <expression>s, then the value of the <test> is returned as the result. If the selected <clause> uses the => alternate form, then the <expression> is evaluated. It is an error if its value is not a procedure that accepts one argument. This procedure is then called on the value of the <test> and the values returned by this procedure are returned by the cond expression.

If all <test>s evaluate to #f, and there is no else clause, then the result of the conditional expression is unspecified; if there is an else clause, then its <expression>s are evaluated in order, and the values of the last one are returned.

(cond ((> 3 2) 'greater)
      ((< 3 2) 'less))         ===>  greater

(cond ((> 3 3) 'greater)
      ((< 3 3) 'less)
      (else 'equal))            ===>  equal

(cond ((assv 'b '((a 1) (b 2))) => cadr)
      (else #f))         ===>  2

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