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A metasyntactic variable is a placeholder name used in computer science, a word without meaning intended to be substituted by some objects pertaining to the context where it is used. The word foo as used in IETF Requests for Comments is a good example.〔(RFC 3092 (rfc3092) - Etymology of "Foo" )〕 By mathematical analogy, a metasyntactic variable is a word that is a variable for other words, just as in algebra letters are used as variables for numbers.〔 Any symbol or word which does not violate the syntactic rules of the language can be used as a metasyntactic variable. For specifications written in natural language, nonsense words are commonly used as metasyntactic variables. Metasyntactic variables have a secondary, implied meaning to the reader (often students), which makes them different from normal metavariables. It is understood by those who have studied computer science that certain words are placeholders or examples only and should or must be replaced in a production-level computer program. In hacker culture, "metasyntactic variable" has come to denote some typical (otherwise meaningless) words used as metavariables in computing; see reification. For example, ''The Hacker's Dictionary'' (1st ed.) defined FOO as "the first metasyntactic variable" and BAR as "the second metasyntactic variable", explaining that "When you have to invent an arbitrary temporary name for something for the sake of exposition, FOO is usually used. If you need a second one, BAR or BAZ is usually used; there is a slight preference at MIT for bar and at Stanford for baz. Clearly, bar was the original, for the concatenation FOOBAR is widely used also, and this in turn can be traced to the obscene acronym 'FUBAR' that arose in the armed forces during World War II. () A hacker avoids using 'foo' as the real name of anything. Indeed, a standard convention is that any file with 'foo' in its name is temporary and can be deleted on sight."〔As reproduced in 〕 The names of these consecrated "metasyntactic variables" are also commonly used as actual identifiers (for variables, functions, etc.) in tutorial programming examples when their purpose is to emphasize syntax; in this usage, "metasyntactic variable" is synonymous with "meaningless word". ==Construction== * ''meta-'' means providing information about, or transcending, * ''syntax'' denotes the grammatical arrangement of words or the grammatical rules of a programming language, and * a ''variable'' is something that can assume a value, or something likely to vary. So ''metasyntactic variable'' denotes a word that "transcends grammar and can assume a value" or one that is "more comprehensive than suggested by its grammatical arrangement and is likely to vary". It may also denote a word that provides information about the grammatical arrangement of words by being able to assume a value that is expected to vary. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Metasyntactic variable」の詳細全文を読む スポンサード リンク
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