[Haskell-cafe] Class Instance with ExistentialQuantification
Roman Cheplyaka
roma at ro-che.info
Tue Jan 7 22:33:07 UTC 2014
* 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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> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>
> --
> 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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