Keyword (computer programming): Difference between revisions

Revision as of 10:45, 23 February 2009

Revision as of 17:42, 15 March 2009

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In [[computer programming]], a '''keyword''' is a [[word]] or [[identifier]] that has a particular meaning to the [[programming language]]. The meaning of keywords — and, indeed, the meaning of the notion of ''keyword'' — differs widely from language to language.

In many languages, such as [[C (programming language)|C]] and similar environments like [[C++]], a ''keyword'' is a [[reserved word]] which identifies a syntactic form. Words used in [[control flow]] constructs, such as <tt>if</tt>, <ttcode>then</ttcode>, and <ttcode>else</ttcode> are keywords. In these languages, keywords cannot also be used as the names of variables or functions.

Some languages, such as [[PostScript]], are extremely liberal in this approach, allowing core keywords to be redefined for specific purposes.

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In [[Common Lisp]], the term "keyword" (or "keyword symbol") is used for a special sort of symbol, or identifier. Unlike other symbols, which usually stand for variables or functions, keywords are self-[[Lisp (programming language)#Self-evaluating forms and quoting|quoting]] and evaluate to themselves. Keywords are usually used to label named arguments to functions, and to represent symbolic values.

Languages vary as to what is provided as a keyword and what is a [[library routine]]. Some languages, for instance, provide keywords for input/output operations whereas in others these are library routines. In [[Python (programming language)|Python]] (versions earlier than 3.0) and many [[BASIC]] dialects, <code>print</code> is a keyword. In contrast, the C and Lisp equivalents <code>printf</code> and <code>format</code> are functions in the standard library.

Typically, when a programmer attempts to use a keyword for a variable or function name, a compilation error will be triggered. In most modern editors, the keywords are automatically set to have a particular text colour to remind or inform the programmers that they are keywords.

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In languages with [[macro (computer science)|macros]] or [[lazy evaluation]], control flow constructs such as <code>if</code> can be implemented as macros or functions. In languages without these expressive features, they are generally keywords.

== See also ==

* [[Java keywords]].

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