Benny writes:

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?

You can either substitute in the type synonym:

instance Num [Float] where (+) = addPoly

Or change Poly to newtype:

newtype Poly = Poly [Float]

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

HTH

Cheers

Benny

Jon Cast