165 lines
6.4 KiB
Markdown
165 lines
6.4 KiB
Markdown
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# Metasyntactic variable
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## Tools
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From Wikipedia, the free encyclopedia
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This article is about metasyntactic variables in computer science and programming. For metasyntactic
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variables as used in formal logic, see Metavariable (logic). For usage in spoken languages, see
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Placeholder name.
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A metasyntactic variable is a specific word or set of words identified as a placeholder in computer
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science and specifically computer programming. These words are commonly found in source code and are
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intended to be modified or substituted before real-world usage. For example, foo and bar are used in
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over 330 Internet Engineering Task Force Requests for Comments, the documents which define
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foundational internet technologies like HTTP (web), TCP/IP, and email protocols.[1][2]
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By mathematical analogy, a metasyntactic variable is a word that is a variable for other words,
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just as in algebra letters are used as variables for numbers.[1]
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Metasyntactic variables are used to name entities such as variables, functions, and commands whose
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exact identity is unimportant and serve only to demonstrate a concept, which is useful for teaching
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programming.
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Common metasyntactic variables
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Due to English being the foundation-language, or lingua franca, of most computer programming
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languages, these variables are commonly seen even in programs and examples of programs written for other spoken-language audiences.
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The typical names may depend however on the subculture that has developed around a given programming
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language.
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## General usage
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Metasyntactic variables used commonly across all programming languages include foobar, foo, bar,
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baz, qux, quux, corge, grault, garply, waldo, fred, plugh, xyzzy, and thud; several of these words
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are references to the game Colossal Cave Adventure.[1][3]
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A complete reference can be found in a MIT Press book titled The Hacker's Dictionary.
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## Japanese
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In Japanese, the words hoge (ほげ)[4] and fuga (ふが) are commonly used, with other common words
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and variants being piyo (ぴよ), hogera (ほげら), and hogehoge (ほげほげ).[5][circular reference]
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The origin of hoge as a metasyntactic variable is not known, but it is believed to date to the early
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1980s.[5]
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## French
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In France, the word toto is widely used, with variants tata, titi, tutu as related placeholders.
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One commonly-raised source for the use of toto is a reference to the stock character used to tell
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jokes with Tête à Toto.[citation needed]
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## Turkish
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In Turkey, the words hede and hödö (usually spelt hodo due to ASCII-only naming constraints of
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programming languages) are well-known metasyntactic variables stemmed from popular humorous cartoon
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magazines of the 90's like LeMan. The words don't mean anything, and specifically used in place of
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things that don't mean anything. The terms have been popularized to the masses by the actor and
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stand-up comedian Cem Yılmaz in the late 90's and early 2000's.[6]
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## Usage examples
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A screenshot of a metasyntactic variable FOO assigned and echoed in an interactive shell session.
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## C
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See also: C programming language
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In the following example the function name foo and the variable name bar are both metasyntactic
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variables. Lines beginning with // are comments.
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```c
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// The function named foo
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int foo(void)
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{
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// Declare the variable bar and set the value to 1
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int bar = 1;
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return bar;
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}
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```
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## C++
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See also: C++
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Function prototypes with examples of different argument passing mechanisms:[7]
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```cpp
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void Foo(Fruit bar);
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void Foo(Fruit* bar);
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void Foo(const Fruit& bar);
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Example showing the function overloading capabilities of the C++ language
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void Foo(int bar);
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void Foo(int bar, int baz);
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void Foo(int bar, int baz, int qux);
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```
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## Python
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Spam, ham, and eggs are the principal metasyntactic variables used in the Python programming
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language.[8] This is a reference to the famous comedy sketch, "Spam", by Monty Python, the eponym
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of the language.[9] In the following example spam, ham, and eggs are metasyntactic variables and
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lines beginning with # are comments.
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```python
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# Define a function named spam
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def spam():
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# Define the variable ham
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ham = "Hello World!"
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# Define the variable eggs
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eggs = 1
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return
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```
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## IETF Requests for Comments
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Both the IETF RFCs and computer programming languages are rendered in plain text, making it
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necessary to distinguish metasyntactic variables by a naming convention, since it would not be
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obvious from context.
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Here is an example from the official IETF document explaining the e-mail protocols (from RFC 772 -
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cited in RFC 3092):
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```text
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All is well; now the recipients can be specified.
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S: MRCP TO:<Foo@Y> <CRLF>
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R: 200 OK
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S: MRCP TO:<Raboof@Y> <CRLF>
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R: 553 No such user here
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S: MRCP TO:<bar@Y> <CRLF>
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R: 200 OK
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S: MRCP TO:<@Y,@X,fubar@Z> <CRLF>
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R: 200 OK
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Note that the failure of "Raboof" has no effect on the storage of
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mail for "Foo", "bar" or the mail to be forwarded to "fubar@Z"
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through host "X".
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```
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(The documentation for texinfo emphasizes the distinction between metavariables and mere variables
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used in a programming language being documented in some texinfo file as: "Use the @var command to
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indicate metasyntactic variables. A metasyntactic variable is something that stands for another
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piece of text. For example, you should use a metasyntactic variable in the documentation of a
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function to describe the arguments that are passed to that function. Do not use @var for the names
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of particular variables in programming languages. These are specific names from a program, so
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@code is correct for them."[10])
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Another point reflected in the above example is the convention that a metavariable is to be
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uniformly substituted with the same instance in all its appearances in a given schema. This is in
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contrast with nonterminal symbols in formal grammars where the nonterminals on the right of a
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production can be substituted by different instances.[11]
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Example data
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SQL
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It is common to use the name ACME in example SQL Databases and as placeholder company-name for the
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purpose of teaching. The term 'ACME Database' is commonly used to mean a training or example-only
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set of database data used solely for training or testing. ACME is also commonly used in
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documentation which shows SQL usage examples, a common practice with in many educational texts as
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well as technical documentation from companies such as Microsoft and Oracle.[12][13][14]
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