Question

From what I understand Foldable basically represents structures that have a number of elements of the same type that can be iterated over, i.e. lists, maps, sets, etc.

Is there a class like Appendable or Insertable which basically represents structures one can add elements to? Of course there would be no guarantee of the order of which elements are retrieved.

I’d rather not create a class myself if there already is one.

Answer 1

You should look at the Data.Collections package. It contains the Unfoldable typeclass with the following methods:

class Unfoldable c i | c -> i where
  insert :: i -> c -> c
  empty :: c
  singleton :: i -> c

It also provides the insertMany and insertManySorted methods, to insert all the elements from a Foldable into an Unfoldable.

If you make your type an instance of both Foldable and Unfoldable then you can both insert and retrieve elements from it.