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

[Maurí­cio]

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.

Great! Actually, I would never imagine that (ComplexNumber Double)
would yield a class with a single type variable... What is the logic
behind that? I know about kinds (of types). Do classes have "kinds"
of their own?

Thanks,
Maurício

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