On Wed, Jan 19, 2011 at 11:56 PM, Patrick Browne
I am trying to see what how this requirement can be represented using just the normal instance-implements-class relation for a comparison with a specification language approach.
If there is no simple way to do this using type classes then I am obviously mis-using the technique.
Going back to your original message: -- My intension is that the PERSON class should *specify* -- that a person has a constant id called p1 -- and that a person has a name that can be found from the id. You can do this with type level numerals. data Z data S a type Zero = Z type One = S Zero type Two = S One -- etc class PersonId a where name :: a -> String instance PersonId Z where name _ = "John" instance PersonId (S Z) where name _ = "Julie" main = putStrLn (name (undefined :: One)) -- prints "Julie" -- ryan