Hi, I study opengl and decided to improve the Vertex3 type. data Vertex3 a => Vertex3 a! a! a! a vertex and a vector are same. It is often need to multiple a vector at a scalar or to calculate a scalar production of two vectors. I have realized the Num class for the Vertex3 type: instance (Num a) => Num (Vertex3 a) where (Vertex3 x y z) * (Vertex3 x' y' z') = (Vertex3 (x*x') (y*y') (z*z')) (Vertex3 x y z) + (Vertex3 x' y' z') = (Vertex3 (x+x') (y+y') (z+z')) (Vertex3 x y z) - (Vertex3 x' y' z') = (Vertex3 (x-x') (y-y') (z-z')) abs (Vertex3 x y z) = (Vertex3 (abs x) (abs y) (abs z)) signum (Vertex3 x y z) = (Vertex3 (signum x) (signum y) (signum z)) negate (Vertex3 x y z) = (Vertex3 (-x) (-y) (-z)) instance (Fractional a) => Fractional (Vertex3 a) where (Vertex3 x y z) / (Vertex3 x' y' z') = (Vertex3 (x/x') (y/y') (z/z')) -------------- But I can't instance my own class: class (Num a) => SVect a where (***) :: Num b => a -> b -> a instance (Num t) => SVect (Vertex3 t) where (Vertex3 x y z) *** c = Vertex3 (c*x) (c*y) (c*z) GHC posts about the error: surface.hs:107:36: Couldn't match expected type `b' against inferred type `t' `b' is a rigid type variable bound by the type signature for `***' at surface.hs:103:14 `t' is a rigid type variable bound by the instance declaration at surface.hs:105:14 In the second argument of `(*)', namely `x' In the first argument of `Vertex3', namely `(c * x)' In the expression: Vertex3 (c * x) (c * y) (c * z) ------------------------ I understand the "c" argument must have type as x,y and z, but I don't know what need to do. Daneel Yaitskov.