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Metasyntactic variable

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This article is about metasyntactic variables in computer science and programming. For metasyntactic variables as used in formal logic, see Metavariable (logic). For usage in spoken languages, see Placeholder name.

A metasyntactic variable is a specific word or set of words identified as a placeholder in computer science and specifically computer programming. These words are commonly found in source code and are intended to be modified or substituted before real-world usage. For example, foo and bar are used in over 330 Internet Engineering Task Force Requests for Comments, the documents which define foundational internet technologies like HTTP (web), TCP/IP, and email protocols.[1][2]

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]

Metasyntactic variables are used to name entities such as variables, functions, and commands whose exact identity is unimportant and serve only to demonstrate a concept, which is useful for teaching programming.

Common metasyntactic variables

Due to English being the foundation-language, or lingua franca, of most computer programming languages, these variables are 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.

General usage

Metasyntactic variables used commonly across all programming languages include foobar, foo, bar, baz, qux, quux, corge, grault, garply, waldo, fred, plugh, xyzzy, and thud; several of these words are references to the game Colossal Cave Adventure.[1][3]

A complete reference can be found in a MIT Press book titled The Hacker's Dictionary.

Japanese

In Japanese, the words hoge (ほげ)[4] and fuga (ふが) are commonly used, with other common words and variants being piyo (ぴよ), hogera (ほげら), and hogehoge (ほげほげ).[5][circular reference] The origin of hoge as a metasyntactic variable is not known, but it is believed to date to the early 1980s.[5]

French

In France, the word toto is widely used, with variants tata, titi, tutu as related placeholders. One commonly-raised source for the use of toto is a reference to the stock character used to tell jokes with Tête à Toto.[citation needed]

Usage examples

A screenshot of a metasyntactic variable FOO assigned and echoed in an interactive shell session.

C

See also: C programming language

In the following example the function name foo and the variable name bar are both metasyntactic variables. Lines beginning with // are comments. 

// The function named foo
int foo(void)
{
   // Declare the variable bar and set the value to 1
   int bar = 1;

   return bar;
}

C++

See also: C++

Function prototypes with examples of different argument passing mechanisms:[6] 

void Foo(Fruit bar);
void Foo(Fruit* bar);
void Foo(const Fruit& bar);

Example showing the function overloading capabilities of the C++ language 

void Foo(int bar);
void Foo(int bar, int baz);
void Foo(int bar, int baz, int qux);

Python

Spam, ham, and eggs are the principal metasyntactic variables used in the Python programming language.[7] This is a reference to the famous comedy sketch, "Spam", by Monty Python, the eponym of the language.[8] In the following example spam, ham, and eggs are metasyntactic variables and lines beginning with # are comments. 

# Define a function named spam
def spam():

    # Define the variable ham
    ham = "Hello World!"

    # Define the variable eggs
    eggs = 1

    return

IETF Requests for Comments

Both the IETF RFCs and computer programming languages are rendered in plain text, making it necessary to distinguish metasyntactic variables by a naming convention, since it would not be obvious from context.

Here is an example from the official IETF document explaining the e-mail protocols (from RFC 772 - cited in RFC 3092): 

 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."[9])

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.[10]

Example data

SQL

It is common to use the name ACME in example SQL Databases and as placeholder company-name for the purpose of teaching. The term 'ACME Database' is commonly used to mean a training or example-only set of database data used solely for training or testing. ACME is also commonly used in documentation which shows SQL usage examples, a common practice with in many educational texts as well as technical documentation from companies such as Microsoft and Oracle.[11][12][13]

See also

References

  1. ^ a b c Eastlake 3rd, Donald E.; Manros, Carl-Uno; Raymond, Eric S. Etymology of "Foo". doi:10.17487/RFC3092. RFC 3092.{{citation}}: CS1 maint: numeric names: authors list (link)
  2. ^ "Document Retrieval". RFC Editor.
  3. ^ Laughlin, Stuart (November 18, 2016). "Metasyntactic variable". programming@ProgClub (Mailing list). Archived from the original on December 2, 2022. Retrieved November 18, 2016.
  4. ^ "hogeの意味・使い方 - 英和辞典 Weblio 辞書". ejje.weblio.jp.
  5. ^ a b メタ構文変数 (in Japanese)
  6. ^ Mongan, John; Kindler, Noah; Giguere, Eric (2012). Programming Interviews Exposed: Secrets to Landing Your Next Job. John Wiley & Sons. p. 242. ISBN 978-1-118-28720-0.
  7. ^ "The Python Tutorial — Python 3.8.1 documentation". docs.python.org.
  8. ^ "General Python FAQ — Python 3.9.7 documentation". docs.python.org. Python Software Foundation. 2021-09-04. Why is it called Python?. Archived from the original on 2021-08-26. Retrieved 2021-09-05.
  9. ^ "Marking Words and Phrases". Texinfo 4.0. The GNU Documentation Format. Archived from the original on 2009-11-06. Retrieved 2012-11-25.
  10. ^ R. D. Tennent (2002). Specifying Software: A Hands-On Introduction. Cambridge University Press. pp. 36–37 and 210. ISBN 978-0-521-00401-5.
  11. ^ Kriegel, Alex (2008). SQL bible. Indianapolis, Ind: Wiley. ISBN 978-0-470-22906-4. OCLC 402446308.
  12. ^ Ruel, Chris (2014). Oracle 12c for dummies (in Danish). Hoboken, NJ: John Wiley & Sons. ISBN 978-1-118-74531-1. OCLC 870334544.
  13. ^ "Work with data in ASP.NET Core Apps". docs.microsoft.com.

External links

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