data Tree1 a = Leaf1 a | Node1 [Tree1 a]

flatten :: Tree1 -> [a]

But this type needs it:

data Tree2 a = Leaf2 a | Node2 (Tree2 [a])

flatten2 :: Tree2 -> [a]

Tree1 can vary in depth between nodes, while Tree2 is either a, [a], [[a]], [[[a]]] etc.