Re: [Haskell-cafe] Class Instance with ExistentialQuantification

7 Jan 2014

      * Andrew Gibiansky  [2014-01-06 22:17:21-0500]
...
Why is the following not allowed?
{-# LANGUAGE ExistentialQuantification, ExplicitForAll, RankNTypes,
FlexibleInstances #-}
class Class a where
  test :: a -> Bool
instance Class (forall m. m -> m) where
  test _ = True
main = do
  putStrLn $ test id
Is there a reason that this is forbidden? Just curious.
I believe the rule is that all constraints (including class constraints)
range over monotypes.

What are you trying to achieve?

You can do this, for example:

  newtype Poly = Poly (forall a . a -> a)
  instance Class Poly where test = const True

  main = print $ test $ Poly id

BTW, this has nothing to do with ExistentialQuantification.

Roman

Re: [Haskell-cafe] Class Instance with ExistentialQuantification

Roman Cheplyaka