Dear Cafe, we have Data.Set, Data.IntSet, Data.HashSet, and they all have similar API, where the only difference is the constraint on the elements. (Same thing for maps.) Can we unify this as follows: {-# language ConstraintKinds, TypeFamilies #-} class SetC s where type Con s :: * -> Constraint singleton :: (Con s a) => a -> s a foldMap :: (Con s a, Monoid m) => (a -> m) -> s a -> m ... Then for Data.Set, we write instance SetC S.Set where type Con S.Set = Ord ; ... It seems to work, and it allows me to write polymorphic code, and switch implementations from the top. Full source: https://gitlab.imn.htwk-leipzig.de/waldmann/pure-matchbox/tree/master/src/Da... Example use case (switch implementation): https://gitlab.imn.htwk-leipzig.de/waldmann/pure-matchbox/blob/master/src/Ma... Still, there are some clumsy corners in this code, perhaps you can help: * for instance SetC HashSet, there are two constraints. I want to write type Con HashSet = \ e -> (Hashable e, Eq, e) but this does not work (there is no "type lambda"?) * for maps, I want to write class (forall k . Foldable m k) => MapC m but this seems impossible now (This is would work with -XQuantifiedConstraints ?) * in some other code using the same idea (the class exports the constraint), I had an instance where the constraint was empty. Again, I cannot write type Con Foo = \ s -> () - J.W.