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  ANSI Common Lisp   24 System Construction   24.2 Dictionary of System Construction

24.2.5 *features* Variable

Value Type:
a proper list.

Initial Value:
implementation-dependent.

Description:
The value of *features* is called the features list. It is a list of symbols, called features, that correspond to some aspect of the implementation or environment.

Most features have implementation-dependent meanings; The following meanings have been assigned to feature names:

  • :cltl1

    If present, indicates that the lisp package purports to conform to the 1984 specification Common Lisp: The Language. It is possible, but not required, for a conforming implementation to have this feature because this specification specifies that its symbols are to be in the common-lisp package, not the lisp package.

  • :cltl2

    If present, indicates that the implementation purports to conform to Common Lisp: The Language, Second Edition. This feature must not be present in any conforming implementation, since conformance to that document is not compatible with conformance to this specification. The name, however, is reserved by this specification in order to help programs distinguish implementations which conform to that document from implementations which conform to this specification.

  • :ieee-floating-point

    If present, indicates that the implementation purports to conform to the requirements of IEEE Standard for Binary Floating-Point Arithmetic.

  • :x3j13

    If present, indicates that the implementation conforms to some particular working draft of this specification, or to some subset of features that approximates a belief about what this specification might turn out to contain. A conforming implementation might or might not contain such a feature. (This feature is intended primarily as a stopgap in order to provide implementors something to use prior to the availability of a draft standard, in order to discourage them from introducing the :draft-ansi-cl and :ansi-cl features prematurely.)

  • :draft-ansi-cl

    If present, indicates that the implementation purports to conform to the first full draft of this specification, which went to public review in 1992. A conforming implementation which has the :draft-ansi-cl-2 or :ansi-cl feature is not permitted to retain the :draft-ansi-cl feature since incompatible changes were made subsequent to the first draft.

  • :draft-ansi-cl-2

    If present, indicates that a second full draft of this specification has gone to public review, and that the implementation purports to conform to that specification. (If additional public review drafts are produced, this keyword will continue to refer to the second draft, and additional keywords will be added to identify conformance with such later drafts. As such, the meaning of this keyword can be relied upon not to change over time.) A conforming implementation which has the :ansi-cl feature is only permitted to retain the :draft-ansi-cl feature if the finally approved standard is not incompatible with the draft standard.

  • :ansi-cl

    If present, indicates that this specification has been adopted by ANSI as an official standard, and that the implementation purports to conform.

  • :common-lisp

    This feature must appear in *features* for any implementation that has one or more of the features :x3j13, :draft-ansi-cl, or :ansi-cl. It is intended that it should also appear in implementations which have the features :cltl1 or :cltl2, but this specification cannot force such behavior. The intent is that this feature should identify the language family named "Common Lisp," rather than some specific dialect within that family.

See Also:
Section 1.5.2.1.1 Use of Read-Time Conditionals, Section 2.4 Standard Macro Characters

Notes:
The value of *features* is used by the #+ and #- reader syntax.

Symbols in the features list may be in any package, but in practice they are generally in the keyword package. This is because keyword is the package used by default when reading2 feature expressions in the #+ and #- reader macros. Code that needs to name a feature2 in a package P (other than keyword) can do so by making explicit use of a package prefix for P, but note that such code must also assure that the package P exists in order for the feature expression to be read2 - even in cases where the feature expression is expected to fail.

It is generally considered wise for an implementation to include one or more features identifying the specific implementation, so that conditional expressions can be written which distinguish idiosyncrasies of one implementation from those of another. Since features are normally symbols in the keyword package where name collisions might easily result, and since no uniquely defined mechanism is designated for deciding who has the right to use which symbol for what reason, a conservative strategy is to prefer names derived from one's own company or product name, since those names are often trademarked and are hence less likely to be used unwittingly by another implementation.

Allegro CL Implementation Details:
None.

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