[Haskell-cafe] Promoting associated data types

[Alp Mestanogullari]

Hello,

Sorry to chime in, but Richard: could you expand a little bit on what your
ongoing work there consists in and what will be possible that wasn't
before? You got me quite curious here.

Thanks!

On Thu, Jun 25, 2015 at 2:31 PM, Richard Eisenberg <eir at cis.upenn.edu>
wrote:

> Hi Косырев,
>
> Promoting type/data families isn't a matter of safety, exactly, but a
> matter of drastically altering the internal type system of GHC to support
> such things. Happily, my research is all about doing so. You can expect
> your first example `class C1 a (b :: D I)` to work in GHC 7.12. If you're
> overly interested, my work is happening at github.com/goldfirere/ghc on
> the `nokinds` branch. But there won't be much (any?) action there for a
> little while as I'm working on anther project for the next few weeks.
>
> Your second example, though, is ill-formed: you can't have a type variable
> of kind `D`, as `D`, by itself, isn't a kind. You could say this, if you
> wanted, though:
>
> > class C1 a (b :: D a)
>
> Do you have an use case for this? I'm always curious to see how Haskellers
> want to use these advanced features!
>
> Thanks,
> Richard
>
> On Jun 25, 2015, at 2:44 AM, Kosyrev Serge <_deepfire at feelingofgreen.ru>
> wrote:
>
> > Good day!
> >
> > DataKinds doesn't allow promotion of type/data families.
> >
> > However, in the specific case of associated data families -- aren't
> > they constrained more, and sufficiently so that the following, for
> example,
> > could be made to work without stepping into dangerous territories:
> >
> > --
> > {-# LANGUAGE DataKinds #-}
> > {-# LANGUAGE KindSignatures #-}
> > {-# LANGUAGE TypeFamilies #-}
> > {-# LANGUAGE UnicodeSyntax #-}
> >
> > module M where
> >
> > class C a where
> >    data D a ∷ *
> >
> > data I
> >
> > instance C I where
> >    data D I
> >
> > class C1 a (b ∷ D I) where
> >
> > -- ..or, maybe, perhaps even the following:
> >
> > class C1 a (b ∷ D) where
> >
> > --
> > respectfully,
> > Косырев Серёга
> > _______________________________________________
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> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> >
>
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-- 
Alp Mestanogullari
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