Something is Mappable if there is a way to map a function over it.

mapping means to apply a transformation to the contents, without changing the structure.

This module both contains the Mappable behaviour, which might be added to your modules/structures by using use Mappable from within them, as well as the Mappable.map(mappable, fun) function, which will dispatch to whatever structure is passed in as first argument.

In Category Theory, something that is Mappable is called a Functor.

Fruit Salad Example

Say we have an apple. There are many operations we could do with an apple, such as peel it, slice it, juice it, etc.

However, right now we don't have an apple, but a bowl filled with apples. How can we make sure we can still use all the operations we could on single apples?

The answer: We need to take the apples one-by-one from the bowl, perform the desired operation, and then put them back in the bowl.

This 'take -> perform -> put back' is the implementation of map for a bowl. It works similar for other data structures:

Exactly how to take something and put a result back, and when to perform the desired operation (if we have an empty bowl, for instance, there's nothing to do) is what you need to define in your implementation.