Keyword (computing)

In a computer language, a reserved word (also known as a reserved identifier) is a word that cannot be used as an identifier, such as the name of a variable, function, or label – it is "reserved from use". This is a syntactic definition, and a reserved word may have no meaning.

A closely related and often conflated notion is a keyword, which is a word with special meaning in a particular context. This is a semantic definition. By contrast, names in a standard library but not built into the language are not considered reserved words or keywords. The terms "reserved word" and "keyword" are often used interchangeably – one may say that a reserved word is "reserved for use as a keyword" – and formal use varies from language to language; for this article we distinguish as above.

In general reserved words and keywords need not coincide, but in most modern languages keywords are a subset of reserved words, as this makes parsing easier, since keywords cannot be confused with identifiers. In some languages, like C or Python, reserved words and keywords coincide, while in other languages, like Java, all keywords are reserved words, but some reserved words are not keywords – these are "reserved for future use". In yet other languages, such as the older languages ALGOL, FORTRAN and PL/I, there are keywords but no reserved words, with keywords being distinguished from identifiers by other means. This makes parsing more difficult with look-ahead parsers necessary.

The sets of reserved words and keywords in a language often coincide or are almost equal, and the distinction is subtle, so the terms are often used interchangeably. However, in careful usage they are distinguished.

Making keywords be reserved words makes lexing easier, as a string of characters will unambiguously be either a keyword or an identifier, without depending on context; thus keywords are usually a subset of reserved words. However, reserved words need not be keywords – for example, in Java, goto is a reserved word, but has no meaning and does not appear in any production rules in the grammar. This is usually done for forward compatibility, so a reserved word may become a keyword in a future version without breaking existing programs.

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