Re: [Haskell-cafe] Class Instance with ExistentialQuantification

Ah, I see. I wasn't aware that constraints had to be over monotypes. I figured that since you could write a function f :: (forall a. a -> a) -> Bool Then you could also do similar things with a class. (The reason I was doing this was that I wanted a typeclass to match something like "return 'a'" without using IncoherentInstances or other sketchiness, and found that trying to have a typeclass with an instance for 'forall m. Monad m => m Char` gave me this error.) Thanks! Andrew On Tue, Jan 7, 2014 at 5:18 AM, Roman Cheplyaka wrote:

* 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