Help need "Cannot use type synonym in instance head"

14 Apr 2002

      type Poly = [Float]

addPoly::Poly->Poly->Poly
addPoly x y = zipWith (+) x y

instance (Float a) => Num (Poly a) where
               (+) = addPoly

I got the error message "Cannot use type synonym in instance head" when I was trying to compile the codes above. Can anyone tell me why and how to solve it?

Cheers

 Benny