# how to instance a class

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## how to instance a class

 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.