Metasyntactic variable

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A metasyntactic variable (pronounced /ˌmɛtəsɪnˈtæktᵻk ˈvɛriəbl/) is a placeholder name or an alias term commonly used to denote the subject matter under discussion or an arbitrary member of a class of things under discussion. The term originates from computer programming and other technical contexts, and is commonly used in examples by hackers and programmers. The use of a metasyntactic variable is helpful in freeing a programmer from creating a logically named variable, although the invented term may also become sufficiently popular and enter the language as a neologism. The word foo is the canonical example.^[1]

Any word can be used as a metasyntactic variable, but "nonsense words" are commonly used. The same concept is employed in other fields where it is expressed by terms such as schematic variable (see logical form).

1 Explanation of the concept
- 1.1 Dissecting the word
- 1.2 Mathematical analogy
2 Words commonly used as metasyntactic variables
3 See also
4 References
5 External links

[edit] Explanation of the concept

[edit] Dissecting the word

'Meta' means information about, more comprehensive, or transcending.
'Syntax' means the grammatical arrangement of words or The grammatical rules of a programming language.
'Variable' means something that can assume a value, or something likely to vary.

So we have a word that

transcends grammar and can assume a value

or one that

is more comprehensive than grammatical arrangement and is likely to vary.

[edit] Mathematical analogy

A metasyntactic variable is as a word that is a variable for other words, just as in algebra letters are used as variables for numbers.^[1]

[edit] Words commonly used as metasyntactic variables

A "standard list of metasyntactic variables used in syntax examples" is: foo, bar, baz, qux, quux, corge, grault, garply, waldo, fred, plugh, xyzzy, thud.^[1] The word foo occurs in over 330 RFCs and bar occurs in over 290.^{[citation needed]}

Foo is used in one section of a tutorial on the C programming language by Brian W. Kernighan. Here, he illustrates the use of the extern declaration:^[2]

Second, at the beginning of any file
that contains functions needing a variable whose definition is in some other file,
put in an extern declaration, outside of any function:

       extern  int     foo;

       f1( ) { ... }
          etc.

Spam, ham, and eggs are the canonical metasyntactic variables used in the Python programming language.^[3] This is a reference to the famous comedy sketch, Spam, by Monty Python, the namesake of the language.^[4] In this extract from the Python tutorial showing the use of comments,^[5] STRING is a metasyntactic variable.

Some examples:

# this is the first comment
SPAM = 1                 # and this is the second comment
                         # ... and now a third!
STRING = "# This is not a comment."