Metasyntactic variable

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A metasyntactic variable is a meaningless word used as a placeholder in computer science, 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.[1]

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.[1]

"A standard convention is that any file with 'foo' in its name is temporary and can be deleted on sight."[2] 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.[3]

The term[edit]

The term metasyntactic variable is an amalgamation of three components:

  • meta, meaning providing information about, or transcending
  • syntactic, denoting the grammatical arrangement of words or the grammatical rules of a programming language
  • variable, something that can assume a value, or a thing likely to vary


Both the IETF RFCs and computer programming languages are rendered in plain text, making it necessary to distinguish metasyntactic variables by a naming convention, more or less obvious from context. If rich text formatting is available, e.g. as in the HTML produced from texinfo sources, then a typographical convention may be used, as done for the example in the GNU Fortran manual:[4]

Plain text example:

RFC 772 (cited in RFC 3092) contains for instance:

 All is well; now the recipients can be specified.

     S: MRCP TO:<Foo@Y> <CRLF>
     R: 200 OK

     S: MRCP TO:<Raboof@Y> <CRLF>
     R: 553  No such user here

     S: MRCP TO:<bar@Y> <CRLF>
     R: 200 OK

     S: MRCP TO:<@Y,@X,fubar@Z> <CRLF>
     R: 200 OK

  Note that the failure of "Raboof" has no effect on the storage of
  mail for "Foo", "bar" or the mail to be forwarded to "fubar@Z"
  through host "X".

(The documentation for texinfo emphasizes the distinction between metavariables and mere variables used in a programming language being documented in some texinfo file as: "Use the @var command to indicate metasyntactic variables. A metasyntactic variable is something that stands for another piece of text. For example, you should use a metasyntactic variable in the documentation of a function to describe the arguments that are passed to that function. Do not use @var for the names of particular variables in programming languages. These are specific names from a program, so @code is correct for them."[5])

Another point reflected in the above example is the convention that a metavariable is to be uniformly substituted with the same instance in all its appearances in a given schema. This is in contrast with nonterminal symbols in formal grammars where the nonterminals on the right of a production can be substituted by different instances.[6]

A third example of the use of the "metasyntactic variables" foo and bar, this time as actual identifiers in a programming (interview) example is contrasting the following C++ function prototypes for their different argument passing mechanisms:[7]

void foo(Fruit bar);
void foo(Fruit* bar);
void foo(Fruit& bar);

Words commonly used as metasyntactic variables[edit]


Metasyntactic variables used in the United States include foobar, foo, bar, baz, qux, quux, quuz, corge, grault, garply, waldo, fred, plugh, xyzzy, and thud.[1][8] Wibble, wobble, wubble, and flob are used in the UK.[9] And there are a reported blep, blah, and boop from Australia.[10][11]

Due to English being the foundation-language, or lingua franca, of most computer programming languages these variables are also commonly seen even in programs and examples of programs written for other spoken-language audiences.

The typical names may depend however on the subculture that has developed around a given programming language. For example, spam, ham, and eggs are the principal metasyntactic variables used in the Python programming language.[12] This is a reference to the comedy sketch Spam by Monty Python, the eponym of the language.[13]

The R programming language often adds norf to the list.[14]


In French, the words are toto and its derivatives, replacing o by other vowels: tata, titi, tutu... It may come from Toto who is the main character in many French jokes.[15][better source needed]


In Japanese, the words hoge and piyo are commonly used, with other common words and variants being fuga, hogera, and hogehoge.[16] Note that -ra is a pluralizing ending in Japanese, and reduplication is also used for pluralizing. The origin of hoge as a metasyntactic variable is not known, but it is believed to date to the early 1980s.[16]


In Portuguese, the words fulano, sicrano and beltrano are commonly used to refer to people.[17]


In Spanish, the words fulano,[18] mengano[19] perengano,[20] and zutano[21] are commonly used, often followed by de tal ("of such"), mocking a lastname in Spanish form (e.g. Fulano de Tal).


In Turkish, the words falan, filan, hede, hödö are commonly used.

See also[edit]


  1. ^ a b c RFC 3092 (rfc3092) - Etymology of "Foo"
  2. ^ As reproduced in Hank Bromley; Richard Lamson (1987). Lisp lore: a guide to programming the Lisp machine. Kluwer Academic. p. 291. 
  3. ^ Mark Slagell (2002). Sams Teach Yourself Ruby in 21 Days. Sams Publishing. p. 108. ISBN 978-0-672-32252-5. 
  4. ^
  5. ^
  6. ^ R. D. Tennent (2002). Specifying Software: A Hands-On Introduction. Cambridge University Press. pp. 36–37 and 210. ISBN 978-0-521-00401-5. 
  7. ^ John Mongan; Noah Kindler; Eric Giguere (2012). Programming Interviews Exposed: Secrets to Landing Your Next Job. John Wiley & Sons. p. 242. ISBN 978-1-118-28720-0. 
  8. ^ quuz. Retrieved November 18, 2016, from programming@ProgClub
  9. ^ wibble. (n.d.). Jargon File 4.4.7. Retrieved February 23, 2010, from [1]
  10. ^ blep. Retrieved November 18, 2016, from programming@ProgClub
  11. ^ blah, boop. Retrieved November 19, 2016, from programming@ProgClub
  12. ^ Python Tutorial
  13. ^ General Python FAQ
  14. ^
  15. ^ fr:Variable métasyntaxique#Toto
  16. ^ a b メタ構文変数 (in Japanese)
  17. ^
  18. ^
  19. ^
  20. ^
  21. ^

External links[edit]