I posted the following on stackoverflow http://stackoverflow.com/questions/43827374/what-is-a-can-be-derived-from-co..., but it hasn't got too much attention so I thought I'd ask here: I can write the following: {-# LANGUAGE RankNTypes #-}{-# LANGUAGE FlexibleInstances #-}{-# LANGUAGE UndecidableInstances #-}{-# LANGUAGE ConstraintKinds #-} f :: Integral a => (forall b. Num b => b) -> a f = id And all is good. Presumably GHC can derive Integral from Num so all is well. I can be a bit tricker, yet I'm still fine: class Integral x => MyIntegral xinstance Integral x => MyIntegral x class Num x => MyNum xinstance Num x => MyNum x f' :: MyIntegral a => (forall b. MyNum b => b) -> a f' = id So lets say I want to generalise this, like so: g :: c2 a => (forall b. c1 b => b) -> a g = id Now obviously this will spit the dummy, because GHC can not derive c2 from c1, as c2 is not constrained. What do I need to add to the type signature of g to say that "you can derive c2 from c1"?