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

language-agnostic symbols
Charles Holbrow picture Charles Holbrow · Feb 8, 2012 · Viewed 20.7k times · Source

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

Answer

soulcheck picture soulcheck · Feb 8, 2012

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 (::=).

Related questions

What and where are the stack and heap?

Programming language books explain that value types are created on the stack, and reference types are created on the heap, without explaining what these two things are. I haven't read a clear explanation of this. I understand what a stack …

Read More
What is dependency injection?

There have been several questions already posted with specific questions about dependency injection, such as when to use it and what frameworks are there for it. However, What is dependency injection and when/why should or shouldn't it be used?

Read More
Regex to match only letters

How can I write a regex that matches only letters?

Read More