On Sat, Apr 21, 2012 at 6:05 PM, Bas van Dijk
Hi,
I just found out that with the new ConstraintKinds extension we can parameterize the constraint of an existentially quantified type:
{-# LANGUAGE KindSignatures, ConstraintKinds, ExistentialQuantification #-} import GHC.Exts data Some (c :: * -> Constraint) = forall a. c a => Some a
I have a package containing this type, under a different name: http://hackage.haskell.org/packages/archive/exists/0.1/doc/html/Data-Exists....
This could be used to define SomeException for example:
import Control.Exception (Exception) type SomeException = Some Exception
Unfortunately it's difficult to make this an instance of Exception, because Exception requires Typeable, and there's no support at present for deriving Typeable on anything with Constraints in it. Though I suppose you could write it manually. (Or maybe StandaloneDeriving?). Alternately we can just wait for the new Typeable using PolyKinds. The other thing is that while it's cool that you can factor out the common existential-type-with-a-class-constraint pattern into this one datatype, I'm not sure I can see any practical benefit to actually doing it.
Are there any other use cases?
Surprisingly (for me) the only other class I found looking around in base and a few other popular libraries which looked useful in conjunction with this type was Show. If anyone knows of others I'll be glad to add them. However, with the * -> * kinded version (which I'm calling Exists1) you can use Functor/Foldable/Traversable. Can anyone think of a practical application for that? (I also have instances for various Comonad classes, but I'm pretty sure those are useless because there's no way to apply a 'run' function to them (and I might remove them if anyone's bothered by the dependencies).)
Bas
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
-- Work is punishment for failing to procrastinate effectively.