Hi, I have the following code, using equality constraints and (I believe) RankNTypes: {-# LANGUAGE MultiParamTypeClasses, TypeFamilies, RankNTypes, ExistentialQuantification #-} {-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-} -- import Math.Algebra.Group.PermutationGroup -- Vector space over field k with basis b data Vect k b = V [(b,k)] deriving (Eq,Show) data TensorBasis a b = T a b deriving (Eq, Ord, Show) -- Tensor product of two vector spaces type Tensor u v = (u ~ Vect k a, v ~ Vect k b) => Vect k (TensorBasis a b) -- ** class Algebra k v where -- "v is a k-algebra" unit :: k -> v mult :: Tensor v v -> v type GroupAlgebra k = Vect k Int -- (Permutation Int) instance Num k => Algebra k (GroupAlgebra k) where unit 0 = V [] unit x = V [(1,x)] mult (V ts) = V [(g*h,x) | (T g h, x) <- ts] Everything is fine except for the last line, which causes the following error message: Couldn't match expected type `Tensor (GroupAlgebra k) (GroupAlgebra k)' against inferred type `Vect k1 b' In the pattern: V ts In the definition of `mult': mult (V ts) = V [(g * h, x) | (T g h, x) <- ts] In the instance declaration for `Algebra k (GroupAlgebra k)' But according to me, I've told it that these two types are the same at the line marked -- ** How do I help it out with type inference? (It, in this case, is GHCi 6.12.1) Any ideas?