[ghc-steering-committee] Mind the gap

[Richard Eisenberg]

I find that surprising, and disappointing. I think of PolyKinds as largely innocuous, except in strange scenarios, which is why PolyKinds is included in GHC2021. If that is wrong, perhaps we should consider not including PolyKinds in GHC2023...

It would be interesting to learn more about what's going wrong.

Richard

> On Jul 5, 2022, at 10:26 AM, Chris Dornan <chris at chrisdornan.com> wrote:
> 
> yup, i should have started there—all my recent troubles have come from PolyKinds!
> 
> On Tue, 5 Jul 2022 at 15:00, Richard Eisenberg <lists at richarde.dev <mailto:lists at richarde.dev>> wrote:
> This is PolyKinds, which generalizes the kind of D, meaning that satisfying the Typeable a superclass constraint of C also requires a Typeable k constraint, where (a :: k).
> 
> Richard
> 
> > On Jul 2, 2022, at 12:49 AM, Chris Dornan <chris at chrisdornan.com <mailto:chris at chrisdornan.com>> wrote:
> > 
> > Quick quiz: the below Haskell2010 ‘Phantoms' module (also in this Gist: https://gist.github.com/cdornan/f75cd8024434d998c87610cbb7fb6ab3 <https://gist.github.com/cdornan/f75cd8024434d998c87610cbb7fb6ab3>) appears (for me on GHC 9.2.2 and 9.2.3) to not be a GHC2021 module, reporting this error under the latter configuration:
> > 
> > Phantoms.hs:22:10: error:
> >    • Could not deduce (Typeable k)
> >        arising from the superclasses of an instance declaration
> >      from the context: (C a, Typeable a)
> >        bound by the instance declaration
> >        at Phantoms.hs:22:10-36
> >    • In the instance declaration for ‘S (D a)’
> >   |
> > 22 | instance (C a,Typeable a) => S (D a) where smethod = undefined
> >   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > 
> > Phantoms.hs:22:10: error:
> >    • Could not deduce (Typeable k)
> >        arising from the superclasses of an instance declaration
> >      from the context: (C a, Typeable a)
> >        bound by the instance declaration
> >        at Phantoms.hs:22:10-36
> >    • In the instance declaration for ‘S (D a)’
> >   |
> > 22 | instance (C a,Typeable a) => S (D a) where smethod = undefined
> > 
> > The question is which language extension(s) are giving rise to these errors?
> > 
> > The ‘Phantoms' module is this:
> > 
> > {-# LANGUAGE DerivingVia #-}
> > 
> > module Phantoms where
> > 
> > import Data.Typeable
> > 
> > 
> > class C a where
> >  cmethod :: Proxy a -> ()
> > 
> > class (Show a, Typeable a) => S a where
> >  smethod :: a -> Int
> > 
> > 
> > newtype UsingD a = UsingD { getUsingD :: a }
> > 
> > 
> > newtype D a = D { getD :: Int }
> >    deriving (Show) via UsingD (D a)
> > 
> > 
> > instance (C a,Typeable a) => S (D a) where smethod = undefined
> > 
> > instance (C a,Typeable a) => Show (UsingD a) where showsPrec = undefined
> > 
> > 
> > instance (C a) => C (D a) where cmethod _ = undefined
> > 
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> 

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