After learning about Functor, the next piece with no dependency is Foldable.
Similarly to Functor, this concept is taken from Category Theory and used in Functional Programing.
The operation fold allows is an aggregation. It takes a starting element and combine it with the Foldable type following the recipe provided by the method f.
Fold can the used to implement reduce. You should try it ! The difference with reduce is that the start element is either the identity of the operation provided in f, meaning an element that does not change the value, for instance empty string "" for the string concatenation operation or the 0 for the + operation in Int type. It is possible to implement a version of reduce where the start element is simply the first element to be combined in the fold if you have access to a function head for instance. You will have to handle cases when the Foldable provided is empty too.
As you can see fold implementation is relying on recursion. We talked about recursion in a previous episode.
Now you have seen Functor and Foldable !