CODEWITHSUNDEEP
language-agnosticsymbols
Charles Holbrow
Charles Holbrow

Reputation: 4231

What does a double colon followed by an equals sign (::=) mean in programming documentation?

What does ::= mean in programming documentation?
For example in the Lua documentation: or in Python documentation.

Upvotes: 71

Views: 39053

Answers (5)

Syed Shaharyaar Hussain
Syed Shaharyaar Hussain

Reputation: 679

This is BNF notation that is used to describe the syntax or semantics of different language elements in programming language. To give you an example, in Transact-SQL this BNF notation is used to define what expression is or how expression is defined:

<expression> ::=
{
    constant
    | scalar_function
    | column
    | variable
    | ( expression )
    | { unary_operator } expression
    | expression { binary_operator } expression
}

"|" symbol is used to indicate that we have to make single choice from available options. You can think of BNF system as meta-language that is used to describe some other language e.g. SQL, Python or C etc.

Reference: https://learn.microsoft.com/en-us/sql/t-sql/language-elements/expressions-transact-sql?view=sql-server-ver16

Upvotes: 0

martineg
martineg

Reputation: 1359

This is Backus-Naur Form (BNF) notation describing the language. ::= in this context means is defined as.

For example, in the Python language documentation you refer to, an identifier is defined as a letter or an underscore, followed by a letter, a digit or an underscore. The notation then goes on to describe what a letter and a digit is defined as, and so on.

Upvotes: 22

soulcheck
soulcheck

Reputation: 36767

It symbolizes 'symbol derivation rule' in Backus–Naur Form

Meaning that in:

<symbol> ::= __expression__

nonterminal <symbol> consists of (is defined as, is constructed from, derives from) __expression__

It's used to describe language grammars.

Notice that both examples are in Extended Backus–Naur Form, but using a traditional BNF symbol-expression separator (::=).

Upvotes: 66

NPE
NPE

Reputation: 500257

As others have already said, it's part of the BNF notation. Wikipedia has an explanation and some examples, which I won't repeat here.

The history and evolution of the ::= symbol itself is explained in The History of the ALGOL Effort (p29 onwards).

Upvotes: 7

m0skit0
m0skit0

Reputation: 25873

The given element syntax. For example:

identifier ::=  (letter|"_") (letter | digit | "_")*

Means all identifiers must conform to the given syntax rule.

Upvotes: 5

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