Yes. There is a significant difference in both the internal representation and the high level representation. ExistsEq is a structure with two internal fields the dictionary and the function, while the RankNEq has only one field. As Andreas Rossberg noted ExistsEq is useless without extra fields. Here is an example which might be more useful: data Exists = Eq a => Exists (a -> a) a a next :: Exists -> Maybe Exists next (Exists f x y) | x == y = Nothing | otherwise = Just (Exists f (f x) y) Note that the only functions which I can apply over type 'a' are (==) and f. Cheers, Krasimir On 4/27/05, Andre Pang <ozone@algorithm.com.au> wrote:
On 27/04/2005, at 9:26 PM, Krasimir Angelov wrote:
RankN and Exists are completelly different. The types of RankNEq and ExistsEq constructors are:
RankNEq :: (forall a. Eq a => a -> a -> Bool) -> RankN ExistsEq :: forall a. Eq a => (a -> a -> Bool) -> Exists
i.e. RankNEq requires one argument, which is a polymorfic function that have to be applied to Eq dictonary. ExistsEq have two arguments: an Eq dictonary and function of type (a -> a -> Bool).
Thanks Krasimir and Tomasz for that clarification. Unfortunately, I'm still a bit confused :). Krasimir, from what you say, this sounds like a big difference in implementation, but from a Haskell (non-type-wizard) user's point of view, is there a practical difference between the two? I can't think of a situation where rank-N/existentials couldn't be substituted for the other ...
-- % Andre Pang : trust.in.love.to.save <http://www.algorithm.com.au/>