# Meaning of double colon $::$

I've came across this expression regarding function composition in some notes by people from computer science, and I don't know what the notation "$::$" means.

$$g \circ f : A \rightarrow C :: a \mapsto g(f(a))$$

I get what this expression is trying to tell me, but I am pretty disturbed by the fact that I don't know what "$::$" means exactly and how to read it. Can anyone tell me what is "$::$"?

• Any way you could post the notes? CS uses a lot of overloaded notation, so there's no way we can tell you what it means without some context. – jmite Mar 18 '17 at 21:57
• It is just an example of a category, specifically the category of sets and functions. I've found this specific example in a PhD thesis, but similar notation is used the following notes as well:cs.ox.ac.uk/teaching/materials16-17/catsproofsprocs/… – Jerry Mar 18 '17 at 22:01

It stands for "given by" or "defined by". The function $g \circ f$ is a function from $A$ to $C$ which maps $a$ to $g(f(a))$.