A functor is a thing that can be mapped over. For example:

    map abs [-1, 2, 3] => [1, 2, 3]

Arrays are the simplest example, but it looks like an iterable. However functors retain shape whereas iterables don't. For example if we had a tree (represented visually):

    oak = -1
          / \
         2   3

If we used oak as an iterable, we would lose the structure of the tree:

    map abs iter(oak) => [1, 2, 3]

However if tree belongs to the functor typeclass (i.e. implements functor interface), then:

    map abs oak => 1
                  / \
                 2   3

The alternatives to functors are:

1. mutate the existing data structure

2. copy a new one and then mutate (two passes)

3. create a new one while mutating (one pass, increased complexity / bugs)