The intended semantics are: Two objects are equal if they have the same name. The type of the name depends on the type of the object
3) How would I write an instance of Named that implements the name function correctly.
I think (but someone with better knowledge) the term "dependent type" means something else: http://en.wikipedia.org/wiki/Dependent_types I'm sure you'll be interested in type families, which offer a better and richer syntax to what you want: http://www.haskell.org/ghc/docs/latest/html/users_guide/type-families.html I tried rewriting your example using ghc-6.8.2 but I wasn't able to build it, and since ghc-6.8.* isn't actually supposed to properly support type families I though I could give you a bad example even if I get it to build :) So I'm leaving it here just in case someone wants to use it as a base: ------ module Main (main) where data (Eq a) => Object a = Object a class (Eq (NameType o)) => Named o where type NameType o :: * name :: o -> NameType o equals :: o -> o -> Bool equals o1 o2 = (name o1) == (name o2) instance (Named a) => Eq a where (==) = equals instance Named (Object Integer) where type NameType (Object Integer) = Integer name (Object i) = i f,g :: Object Integer f = Object 1 g = Object 2 main = putStrLn $ show $ f == g ------ Best, Maurício