Learning Functional Programming in Go

Functor definition

The Functor (https://hackage.haskell.org/package/base-4.8.1.0/docs/Data-Functor.html#t:Functor) class is used for types that can be mapped over.

We'll use Haskell syntax because it so clearly defines FP algebraic data types, including their structures, rules, and logic. fmap is the map function. The period . notation is the compose operator.

Instances of Functor should satisfy the following identity and associativity laws:

fmap id  ==  idfmap (f . g)  ==  fmap f . fmap g

We should recognize these two rules from Chapter 11, Category Theory That Applies.