[Haskell-cafe] Re: Type question in instance of a class

[J. Garrett Morris]

On Mon, Nov 17, 2008 at 10:38 AM, Maurí­cio <briqueabraque at yahoo.com> wrote:
> newtype ComplexWithDouble = ComplexWithDouble (ComplexNumber Double)
>  deriving ...

Perhaps you want something like:

class Complex r c | c -> r
    where makeComplex :: r -> r -> c
          realPart :: c -> r
          imagPart :: c -> r

data ComplexNumber t = CN t t
instance Complex t (ComplexNumber t)
    where makeComplex = CN
          realPart (CN r _) = r
          imagPart (CN _ i) = i

newtype ComplexWithDouble = CWD (ComplexNumber Double)
    deriving (Complex Double)

Having the parameters "backwards" is somewhat annoying, I suppose, but
it's unavoidable if you're hoping to use generalized newtype deriving
I believe.

 /g

-- 
I am in here