Nicolas Frisby described the problem and the solution of the type equality comparison that takes into account the top-most type constructor only. Thus `Maybe Bool' should be considered equal to `Maybe Char' because both types have the same top constructor Maybe. The following complete code shows another solution. The code includes several tests, test1 through test6, along with the expected answers. {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} -- type equality for the top-level type constructor -- That is, decide two types to be equal if the top-most type constructor -- matches. Thus `Maybe Bool' should be considered equal to `Maybe Char' module TEQ where -- Our approach is simple. Given a type, we normalize it: -- replace all arguments of the type constructor with unit. -- After that, we use the regular HList's TypeEq. class Normalize t r | t -> r instance TypeCast (c ()) r => Normalize (c x) r instance TypeCast (c () ()) r => Normalize (c x y) r instance TypeCast (c () () ()) r => Normalize (c x y z) r -- add instances for type constructors of higher arities instance TypeCast y x => Normalize x y top_eq :: (Normalize a a', Normalize b b', TypeEq a' b' r) => a -> b -> r top_eq = undefined test1 = top_eq (undefined::Int) (undefined::Char) -- :t test1 -- test1 :: HFalse test2 = top_eq (undefined::Bool) (undefined::Bool) -- test2 :: HTrue test3 = top_eq (undefined::Maybe Bool) (undefined::Maybe Char) -- test3 :: HTrue test4 = top_eq (undefined::Maybe Bool) (undefined::[Char]) -- test4 :: HFalse test5 = top_eq (undefined::Either Int Char) (undefined::Either Char Int) -- test5 :: HTrue test6 = top_eq (undefined::Either Int Char) (undefined::[Char]) -- test6 :: HFalse -- The following is borrowed from HList and included here for completeness data HTrue data HFalse class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x class TypeEq x y b | x y -> b instance TypeEq x x HTrue instance TypeCast HFalse b => TypeEq x y b