newtype Vector2 a = Vector2 (a,a)
     deriving (Show,Eq)

class VectorSpace v where
     vZero :: (Num a) => v a
     vSum :: (Num a) => v a -> v a -> v a
     vScalarProd :: (Num a) => a -> v a -> v a
     vMagnitude :: (Floating a) => v a -> a

Necesita definir que Vector2 sea instancias de la clase de tipo VectorSpace.