cond-expand -- r7rs Definition

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Kind

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syntax;

Implemented by

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• cond-expand (from vonuvoli);

Exports

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• scheme:base -- (scheme base);

Exports recursive

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• scheme -- (scheme);

Description

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(cond-expand <ce-clause_1> <ce-clause_2> ...)

Syntax: The cond-expand expression type provides a way to statically expand different expressions depending on the implementation. A <ce-clause> takes the following form:

(<feature-requirement> <expression> ...)

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

(else <expression> ...)

A <feature-requirement> takes one of the following forms:

• <feature-identifier>
• (library <library-name>)
• (and <feature-requirement> ...)
• (or <feature-requirement> ...)
• (not <feature-requirement>)

Semantics: Each implementation maintains a list of feature identifiers which are present, as well as a list of libraries which can be imported. The value of a <feature-requirement> is determined by replacing each <feature-identifier> and (library <library-name>) on the implementation's lists with #t, and all other feature identifiers and library names with #f, then evaluating the resulting expression as a Scheme boolean expression under the normal interpretation of and, or, and not.

A cond-expand is then expanded by evaluating the <feature-requirement>s of successive <ce-clause>s in order until one of them returns #t. When a true clause is found, the corresponding <expression>s are expanded to a begin, and the remaining clauses are ignored. If none of the <feature-requirement>s evaluate to #t, then if there is an else clause, its <expression>s are included. Otherwise, the behavior of the cond-expand is unspecified. Unlike cond, cond-expand does not depend on the value of any variables.

The exact features provided are implementation-defined, but for portability a core set of features is given in appendix on standard feature identifiers.

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