William Murphy wrote:
I've spent a lot of time trying to write a version of concat, which concatenates lists of any "depth":
It is a little bit more involved, but quite possible. The code is not much longer than the one you wrote (essentially, three lines: one class and two instance declarations). Here is the complete code: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} {-# LANGUAGE TypeFamilies, FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} module DeepFlat where class DeepFlat a b | a -> b where dflat :: [a] -> [b] -- If we flatten a list of lists instance DeepFlat a b => DeepFlat [a] b where dflat = concatMap dflat -- If we are given a list of non-lists instance a ~ b => DeepFlat a b where dflat = id test1 = dflat "abracadabra" -- "abracadabra" test2 = dflat ["abra","cadabra"] test3 = dflat [["ab","ra"],["cad","abra"]] test4 = dflat [[["a","b"],["ra"]],[["cad","abra"]]]