问题
What does ::= mean in programming documentation?
For example in the Lua documentation: or in Python documentation.
回答1:
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 (::=).
回答2:
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.
回答3:
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).
回答4:
The given element syntax. For example:
identifier ::= (letter|"_") (letter | digit | "_")*
Means all identifiers must conform to the given syntax rule.
来源：https://stackoverflow.com/questions/9196066/what-does-a-double-colon-followed-by-an-equals-sign-mean-in-programming-do