Hi cafe, in my code I use Data.Data and OverlappingInstances. My problem now is, that I can't use functions using these instances as arguments for fromConstrM. This is what my problem looks like: {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE UndecidableInstances #-} import Data.Data class Test a where foo :: Monad m => m a instance Num a => Test a where foo = return 1 instance Test Int where foo = return 2 test constr = fromConstrM foo constr but when I compile I get: test.hs:15:26: Overlapping instances for Test d arising from a use of `foo' at test.hs:15:26-28 Matching instances: instance [overlap ok] (Num a) => Test a -- Defined at test.hs:9:9-23 instance [overlap ok] Test Int -- Defined at test.hs:12:9-16 (The choice depends on the instantiation of `d' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) In the first argument of `fromConstrM', namely `foo' In the expression: fromConstrM foo constr In the definition of `test': test constr = fromConstrM foo constr Failed, modules loaded: none. Is there a way out? Right now I use a "case (typeOf x) of" kind of construct, but it gets pretty messy and I even have to unsafeCoerce at one point. - Timo