Composing Software

Functors & Categories

A functor data type is something you can map over. It’s a container which has a map operation which can be used to apply a function to the values inside it. When you see a functor datatype, you should think “mappable”. In JavaScript, functor types are typically represented as an object with a .map() method that maps from inputs to outputs, e.g., Array.prototype.map(). A common analogy is to think of a functor data structure as a box, and map as a way to apply a function to each item contained inside the box, which creates a new box containing the outputs.

In category theory, a functor is a structure preserving map from category ...