Metasyntactic variable

In logic, mathematics and computer science, a metasyntactic variable (also metalinguistic variable) (pronounced /ˌmɛtəsɪnˈtæktɪk ˈvɛəriəbl/) is a symbol or set of symbols in a metalanguage which stands for a symbol or set of symbols in some object language. For instance, in the sentence:

Let A and B be two arbitrary formulas of a formal language .

The symbols A and B are not symbols of the object language , they are metasyntactic variables in the metalanguage (in this case, English) which is talking about the object language .

In computer science, programmers use metasyntactic variables to describe 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 use of a metasyntactic variable is helpful in freeing a programmer from creating a logically named variable. The word foo is the principal example.^[1]

In Terry Pratchett's book Men at Arms, the dog Gaspode says, "Clothing has never been what you might call a thingy of dog wossname.", and then adds, "Two metasyntactic variables there. Sorry.".

Any symbol or 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).

Etymology

• 'Meta' means providing information about, 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.

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]

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.^[2]

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:^[3]

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.

In Python

Spam, ham, and eggs are the principal^{[citation needed]} metasyntactic variables used in the Python programming language.^[4] This is a reference to the famous comedy sketch, Spam, by Monty Python, the namesake of the language.^[5] In this extract from the Python tutorial showing the use of comments,^[6] SPAM and STRING are metasyntactic variables.

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."

Wibble, wobble, wubble and flob are often used in the UK^[7].

References

See also

• Placeholder name
• Free variables and bound variables
• gadget
• widget
• Hello World
• Lorem Ipsum
• Fnord
• Alice and Bob
• John Doe

External links

• Definition of metasyntactic variable, with examples.
• Examples of metasyntactic variables used in Commonwealth Hackish, such as wombat.
• Variable "foo" and Other Programming Oddities