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
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@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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
-- Alp Mestanogullari