Kata

The concat function on lists has the following type:

concat :: [[a]] -> [a]

Simply put, it flattens a list:

concat [[1, 2], [3, 4], [], [5]] == [1, 2, 3, 4, 5]

Farmer Thomas wants to write concat in terms of foldr. Specifically, he wants you to find p, q and r such that:

concat = foldr (foldr p q) r

Now get working!

After doing this one, you may want to check out Lambda Calculus: Lists as folds II (this might act as a hint for this kata :P).