Feature Expressions

ToC

Allegro CL

ANSI Common Lisp

24 System Construction

24.1 System Construction Concepts

24.1.2 Features

24.1.2.1 Feature Expressions

Boolean combinations of features, called feature expressions, are used by the #+ and #- reader macros in order to direct conditional reading of expressions by the Lisp reader.

The rules for interpreting a feature expression are as follows:

feature
If a symbol naming a feature is used as a feature expression, the feature expression succeeds if that feature is present; otherwise it fails.
(not feature-conditional)
A not feature expression succeeds if its argument feature-conditional fails; otherwise, it succeeds.
(and {feature-conditional}*)
An and feature expression succeeds if all of its argument feature-conditionals succeed; otherwise, it fails.
(or {feature-conditional}*)
An or feature expression succeeds if any of its argument feature-conditionals succeed; otherwise, it fails.

24.1.2.1.1 Examples of Feature Expressions