Editing Metasyntactic variable (section)

== Usage examples ==
[[File:Metasyntactic Variable FOO in Shell.png|thumb|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 <code>foo</code> and the variable name <code>bar</code> are both metasyntactic variables. Lines beginning with <code>//</code> are comments.
<syntaxhighlight lang="cpp">
// The function named foo
int foo(void)
{
   // Declare the variable bar and set the value to 1
   int bar = 1;

   return bar;
}
</syntaxhighlight>

===C++===
{{See also|C++}}
Function prototypes with examples of different [[Parameter (computer programming)#Argument passing|argument passing]] mechanisms:<ref name="MonganKindler2012">{{cite book|first1=John|last1=Mongan|first2=Noah|last2=Kindler|first3=Eric|last3=Giguere|title=Programming Interviews Exposed: Secrets to Landing Your Next Job|url=https://books.google.com/books?id=GQE4r2e5fAsC&pg=PA242|year=2012|publisher=John Wiley & Sons|isbn=978-1-118-28720-0|page=242}}</ref>
<syntaxhighlight lang="cpp">
void Foo(Fruit bar);
void Foo(Fruit* bar);
void Foo(const Fruit& bar);
</syntaxhighlight>

Example showing the [[function overloading]] capabilities of the C++ language
<syntaxhighlight lang="cpp">
void Foo(int bar);
void Foo(int bar, int baz);
void Foo(int bar, int baz, int qux);
</syntaxhighlight>

===Python===
[[Spam (food)|Spam]], [[ham]], and [[Egg (food)|eggs]] are the principal metasyntactic variables used in the [[Python programming language]].<ref>{{Cite web|url=https://docs.python.org/3/tutorial/|title=The Python Tutorial — Python 3.8.1 documentation|website=docs.python.org}}</ref> This is a reference to the famous comedy sketch, "[[Spam (Monty Python)|Spam]]", by [[Monty Python]], the eponym of the language.<ref>{{Cite web |date=2021-09-04 |title=General Python FAQ — Python 3.9.7 documentation |url=https://docs.python.org/3/faq/general.html |url-status=live |archive-url=https://web.archive.org/web/20210826153036/https://docs.python.org/3/faq/general.html#why-is-it-called-python |archive-date=2021-08-26 |access-date=2021-09-05 |website=docs.python.org |publisher=Python Software Foundation |at=Why is it called Python?}}</ref>
In the following example <code>spam</code>, <code>ham</code>, and <code>eggs</code> are metasyntactic variables and lines beginning with <code>#</code> are comments.
<syntaxhighlight lang="python">
# Define a function named spam
def spam():

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

    # Define the variable eggs
    eggs = 1

    return
</syntaxhighlight>

===IETF Requests for Comments===
Both the [[IETF]] [[Request for Comments|RFC]]s and [[computer programming language]]s 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 [[Variable (programming)|variables]] in programming languages. These are specific names from a program, so @code is correct for them."<ref>{{cite web |url=http://sunsite.ualberta.ca/Documentation/Gnu/texinfo-4.0/html_chapter/texinfo_10.html |title=Marking Words and Phrases |work=Texinfo 4.0. The GNU Documentation Format|access-date=2012-11-25 |url-status=dead |archive-url=https://web.archive.org/web/20091106004856/http://sunsite.ualberta.ca/Documentation/Gnu/texinfo-4.0/html_chapter/texinfo_10.html |archive-date=2009-11-06 }}</ref>)

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.<ref name="Tennent2002">{{cite book|author=R. D. Tennent|title=Specifying Software: A Hands-On Introduction|year=2002|publisher=Cambridge University Press|isbn=978-0-521-00401-5|pages=36–37 and 210}}</ref>