Is there a function to flatten a nested list of elements?

Question

How can I flatten a nested list like this:

[1, 2, 3, 4] == flatten [[[1,2],[3]],[[4]]]

Answer 1

An arbitrarily nested list can be approximated by a Data.Tree, which can be flattened by the appropriately named function flatten.

I say approximated because Data.Tree allows a data item to be attached to every node, not just the leaves. However, you could create a Data.Tree (Maybe a), and attach Nothing to the body nodes, and flatten with catMaybes . flatten.