10.1 srfi-0 - Feature conditional

Module: srfi-0

Provides SRFI-0 cond-expand macro form. Now this module is autoloaded when cond-expand is used, so you don't need to say (use srfi-0) explicitly. Portable programs can't use 'use' form anyway.

Macro: cond-expand (feature-requirement command-or-definition …) …

[SRFI-0] This macro expands to command-or-definition … if Gauche supports feature-requirement. In a way, it can be used to absorb the different feature sets supported by different Scheme implementations.

feature-requirement must be in the following syntax:

feature-requirement : feature-identifier | (and feature-requirement …) | (or feature-requirement …) | (not feature-requirement)

feature-identifier is a symbol that indicates a feature. If such a feature is supported in Gauche, it satisfies the feature-requirement. You can do boolean combination of feature-requirements to compose more complex conditions.

The macro tests each feature-requirement in order, and if one is satisfied, the macro itself expands to the corresponding command-or-definition ….

The last clause may begin with a symbol else instead of feature-requirement. If no condition is fulfilled before it, then the macro expands to the command-or-definitions of the else clause. If there's no else clause and no condition is fulfilled, an error is signalled.

Currently, Gauche recognizes srfi names (e.g. srfi-1) that it supports (See section Standard conformance, for the list of supported SRFIs), and a symbol gauche as feature-identifier.

Suppose you are writing a program that requires srfi-13 (string library). In Gauche, the standard way to use srfi-13 is like this:

(use srfi-13) ;; ... your program ...

However, the 'use' macro is Gauche specific and won't work on other Scheme implementations. You can write instead:

(cond-expand (srfi-13 ;; ... your program ... ))

If the implementation supports srfi-13, the form expands to your program. Note that Gauche issues (use srfi-13) for you in this case.

The above form signals an error if the implementation doesn't support srfi-13. However, you might want to run your program on such implementations as well, by providing alternative library definitions.

(cond-expand (srfi-13 ;; ... your program ... ) (else (load "my-srfi-13-implementation") ;; ... your program ... ))

This document was generated by Shiro Kawai on October, 7 2008 using texi2html 1.78.