Help need "Cannot use type synonym in instance head"

[Jon Cast]

Please keep this on the list, so everyone can benefit.

Benny <neon_2001@hotmail.com> wrote:

> Hello Jon

> Thanks for the help. But it still didn't work after I changed the
> codes from:

> > type Poly = [Float]
> > addPoly::Poly->Poly->Poly
> > addPoly x y = zipWith (+) x y
> > instance (Float a) => Num (Poly a) where
> >               (+) = addPoly

> to your suggestion:

> type Poly = [Float]

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

> instance Num [Float] where
>    (+) = addPoly

> I received this erreor message "Syntax error in instance head
> (variable expected)".

I sent this suggestion without actually trying it; unfortunately, as
you discovered, it violates the rules of the Haskell 98 language.
Please disregard this suggestion.

> Then I tried your second suggestion, so I changed to codes to:

> type Poly = [Float]

> newtype Poly = Poly [Float]

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

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

> Then I received this error message " Repeated definition of type
> constructor "Poly"".

What I meant was the following:

> newtype Poly = Poly [Float]

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

> instance Num Poly where
>                (+) = addPoly

I.e., delete the type declaration as well as the extraneous
information from the instance.

<snip>

Btw., you'll also want instances of Eq and Show to get Num to go through.

> Thank you very much

You're welcome

> Cheers

> Benny

Jon Cast