Re: [Haskell-cafe] Class Instance with ExistentialQuantification

Hi, is it not allowed simply because none has needed it yet, or is there a deeper theoretical problem with it? I’m asking because the implementation of Coercible behaves as if there is an instance instance forall a. (Coercible (t1 a) (t2 a)) => Coercible (forall a. t1 a) (forall a. t2 a) and if were theoretically dubious, I’d like to know about it :-) Greetings, Joachim Am Dienstag, den 07.01.2014, 10:11 -0500 schrieb Andrew Gibiansky:

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

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