Keyword (computer programming): Difference between revisions
m moved Keyword (computer) to Keyword (computer programming): need to distiguish between two computer-related uses: internet searches and computer programming |
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In many languages, such as [[C programming language|C]] and its kin, a ''keyword'' is a [[reserved word]] which identifies a syntactic form. Words used in [[control flow]] constructs, such as <tt>if</tt>, <tt>then</tt>, and <tt>else</tt> are keywords. In these languages, keywords cannot also be used as the names of variables or functions. |
In many languages, such as [[C programming language|C]] and its kin, a ''keyword'' is a [[reserved word]] which identifies a syntactic form. Words used in [[control flow]] constructs, such as <tt>if</tt>, <tt>then</tt>, and <tt>else</tt> are keywords. In these languages, keywords cannot also be used as the names of variables or functions. |
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Some languages, such as [[PostScript]], are extremely liberal in this approach |
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]], in contrast, ''keywords'' (or ''keyword symbols'') are a special sort of symbol, or identifier. Unlike other symbols, which usually stand for variables or functions, keywords are self-[[quoting]] and evaluate to themselves. Keywords are usually used to label named arguments to functions, and to represent symbolic values. |
In [[Common Lisp]], in contrast, ''keywords'' (or ''keyword symbols'') are a special sort of symbol, or identifier. Unlike other symbols, which usually stand for variables or functions, keywords are self-[[quoting]] and evaluate to themselves. Keywords are usually used to label named arguments to functions, and to represent symbolic values. |
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Revision as of 12:03, 19 September 2006
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 and its kin, a keyword is a reserved word which identifies a syntactic form. Words used in control flow constructs, such as if, then, and else 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.
In Common Lisp, in contrast, keywords (or keyword symbols) are a special sort of symbol, or identifier. Unlike other symbols, which usually stand for variables or functions, keywords are self-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 and many BASIC dialects, print is a keyword. In contrast, the C and Lisp equivalents printf and format are functions in the standard library.
In languages with macros or lazy evaluation, control flow constructs such as if can be implemented as macros or functions. In languages without these expressive features, they are generally keywords.