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