Learning Functional Programming in Go

Official definition of a function

A function is a subset of Cartesian product of A and B, which is relation of AxB (A cross B):

f is equal to or subset of AxB 

Here, f is a subset of pairs.

For all of A, there is a unique B (b:B) such that the subset <a,b> is a relation of that relation f:

<a,b> ∈  f