[Haskell-cafe] Class Instance with ExistentialQuantification

[Roman Cheplyaka]

* Joachim Breitner <mail at joachim-breitner.de> [2014-01-07 15:16:15+0000]
> Hi,
> 
> is it not allowed simply because none has needed it yet, or is there a
> deeper theoretical problem with it?

FWIW, here's Simon's answer on a similar topic:
http://osdir.com/ml/glasgow-haskell-users@haskell.org/2013-03/msg00048.html

> 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 <roma at ro-che.info>
> > wrote:
> >         * Andrew Gibiansky <andrew.gibiansky at 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
> > 
> > 
> > _______________________________________________
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> 
> -- 
> Joachim “nomeata” Breitner
>   mail at joachim-breitner.de • http://www.joachim-breitner.de/
>   Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0x4743206C
>   Debian Developer: nomeata at debian.org



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