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 <roma@ro-che.info> wrote:
* Andrew Gibiansky <andrew.gibiansky@gmail.com> [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