The following code {-# LANGUAGE FlexibleInstances, TypeFamilies #-} import Control.Applicative class Z t where type W t z :: t -> W t instance Z (a -> b) where type W (a -> b) = a -> b z = id instance Z (IO (a -> b)) where type W (IO (a -> b)) = IO a -> IO b z = (<*>) works fine, but if I try and generalize to from IO to the Applicative classes instance (Applicative m) => Z (m (a -> b)) where type W (m (a -> b)) = m a -> m b z = (<*>) I get the following error Temp.hs:10:9: Conflicting family instance declarations: type instance W (a -> b) -- Defined at Temp.hs:10:9 type instance W (m (a -> b)) -- Defined at Temp.hs:14:9 Failed, modules loaded: none. unless I remove one of the instances, and then it is happy. Is this correct? I don't claim to really understand the rules regarding type classes, but I can't see why these are overlapping. Thanks! -Tyson