Maurício, Thanks for your rapid feedback, I will study your code.
I think (but someone with better knowledge) the term "dependent type" means something else: http://en.wikipedia.org/wiki/Dependent_types
From your first link I think the term *dependent type* is defined as a type that depends on *values* of other types. The semantic that I am trying to express is that one type depends on another type. Specifically, the type of a name depends on the type of the object. For example a Person with a Name would give a Persons-Name and a Dog with a Name would give a Dogs-Name. The paper [1] describe several styles of dependency Types depending on values called dependent types Types depending on types called parametric and type-indexed types
[1]Comparing Approaches to Generic Programming Haskell by Hinze, Jeuring, and LÄoh Best regards, Pat Maurício wrote:
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
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