Here's a small program defining a collection class: ------------------ class Sat a where dict :: a data EqD a = EqD {eq :: a->a->Bool} instance Sat (EqD a) => Eq a where (==) = eq dict class Collection c cxt | c -> cxt where empty :: Sat (cxt a) => c a single :: Sat (cxt a) => a -> c a union :: Sat (cxt a) => c a -> c a -> c a member :: Sat (cxt a) => a -> c a -> Bool instance Collection [] EqD where empty = [] single x = [x] union = (++) member = elem ------------------ Loading this program with overlapping instances turned on produces the error message: Type checking ERROR "C:\windows\Skrivbord\HugsBug.hs":24 - Cannot justify constraints in instance member binding *** Expression : member *** Type : (Collection [] EqD, Sat (EqD a)) => a -> [a] -> Bool *** Given context : (Collection [] EqD, Sat (EqD a)) *** Constraints : Eq a Very strange, given the instance declaration instance Sat (EqD a) => Eq a The behaviour is the same in the February and December 2001 versions. John Hughes