Feature Expressions (ANSI and GNU Common Lisp Document)

Next: Examples of Feature Expressions, Previous: Features, Up: 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.