Re: Help need "Cannot use type synonym in instance head"

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