Swift Functional Programming - Second Edition

Functors

In Chapter 3, Types and Type Casting, we talked a little about category theory. Here we go a little further in category theory and talk about Functors. A Functor contains morphisms such as a map function, which transforms the Functor. We can think about a Functor as a functional design pattern.

Knowing category theory is great, but we do not have to. So simply put, a Functor is a structure or container that we can map over. In other words, a Functor is any type that implements the map function. Examples of Functors are Dictionary, Array, Set, Optional, and Closure types.

A Functor applies morphisms or functions to the values it contains, instead of itself. Suppose if we call a function such as doSomething(param: Double), then it ...