Okay thanks I got the difference between both.
The 'exists' syntax seems very useful. Is it planned to be added to GHC in a
near future?
2011/2/28 Heinrich Apfelmus
Yves Parès wrote:
takeC :: Int -> Compoz a b -> (exists c. Compoz a c)
dropC :: Int -> Compoz a b -> (exists c. Compoz c b)
What does 'exists' means? To create a rank-2 type can't you use:
takeC :: Int -> Compoz a b -> (forall c. Compoz a c)
??
Ah, (exists c. Compoz a c) means "There exists a type c such that the whole thing has type Compoz a c ".
What you describe would be the type "For any type c the whole thing can be treated as having type Compoz a c " which is something entirely different.
The point is that in the former version, the function takeC determines what the type c should be and the caller has no clue what it is. In the latter version, the function that calls takeC can choose which type it should be.
What I wrote is *not* legal Haskell. (At least not in GHC. If I remember correctly, the EHC from Utrecht supports first-class existential quantification ). You have to encode it in some way, for instance with a data type
data Exists f = forall c . Exists (f c) takeC :: Int -> Compoz a b -> Exists (Compoz a)
Regards, Heinrich Apfelmus
-- http://apfelmus.nfshost.com
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