I was trying to create a typeclass for an abstract Stack class, and ran into some problems. The following 'works' fine: {-# OPTIONS_GHC -XEmptyDataDecls -XFlexibleContexts -fno-monomorphism-restriction #-} module Stack where data Void class Stack s where push_ :: s a r -> b -> s b (s a r) empty :: s () Void top :: s a (s b r) -> (a, s b r) first :: s a r -> a instance Stack (,) where push_ s a = (a,s) empty = ((),undefined::Void) top = id first = fst p = flip push_ test0 = top . p 2 . p 3 $ empty -- But the following doesn't - I get an "Ambiguous type variable `s' in the contraint `Stack s' arising from the use of `first': test1 = first . p 2 . p 3 $ empty -- sure, that makes sense, it somehow needs to know what flavour of Stack to use even though (or perhaps because) the answer is independent of it. -- So I listen to the "probable fix" and add a type signature: test1 :: Stack (,) => Integer -- This does not however help at all! The only way I have found of 'fixing' this requires annotating the code itself, which I most definitely do not want to do because I specifically want the code to be polymorphic in that way. But GHC 6.8.2 does not want to let me do this. What are my options? Jacques