Discussion: Allow custom constraint for elem in Foldable

The Foldable class offers the method elem :: Eq a => a -> t a -> Bool Unfortunately, this is really awful for sets, hash maps, etc. See https://stackoverflow.com/questions/45361801/implement-an-olog-n-foldable-el... for an example. We could fix it, kinda: class Foldable t where type ElemConstr t :: * -> Constraint type ElemConstr t = Eq elem :: ElemConstr t a => a -> t a -> Bool default elem :: (ElemConstr t ~ Eq, Eq a) => a -> t a -> Bool One might legitimately complain that such a wild type is ad hoc, but one might counter that complaint by saying that most of Foldable is already ad hoc. David