data RankN = RankNEq (forall a. Eq a => a -> a -> Bool) | RankNOrd (forall a. Ord a => a -> a -> Bool)
data Exists = forall a. Eq a => ExistsEq (a -> a -> Bool) | forall a. Ord a => ExistsOrd (a -> a -> Bool)
So, the RankN type uses rank-2 polymorphism to "hide" the expression inside the type, whereas the Exists type uses existentially quantified types instead. The two seem pretty equivalent to me, since the data constructors have the same type. However, I can't help but feel that I'm missing something fundamental about a difference between them. Are the two completely isomorphic? Is there some advantage or disadvantage to using one over the other?
They don't have the same type. The types are
RankNEq :: (forall a.Eq a => a->a->Bool) -> RankN ExistsEq :: forall a.Eq a => (a->a->Bool -> Exists)
These are quite different beasts.
The difference really shows up when you *use* (deconstruct) them:
g (RankNEq f) = (f 4 5, f True False)
This allows the embedded function to be used polymorphically. But:
h (ExistsEq f) = ???
Here, you cannot use f at all (well, except with undefined). The type is not polymorphic in "a" on the RHS, it is abstract! You'd need to encapsulate a value of the same type (or a constructing function) as well to this type useful.
-- Andreas Rossberg, [EMAIL PROTECTED]
Let's get rid of those possible thingies! -- TB
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