[Haskell-cafe] Strange error with GADT DataKinds -- needs TypeInType!?

[Tom Ellis]

Yes indeed, that's very helpful, thanks!  The following *does* work so I
assume "existential types" do not fall under the purview of GADTs in this
context?



{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}

data T a b where
  E :: T a b -> T b c -> T a c

type family F (a :: T ka kb)

type instance F ('E a b) = ()


On Thu, May 03, 2018 at 06:16:13PM -0400, Richard Eisenberg wrote:
> Data constructor I is a GADT constructor -- it constrains the values of its return type. Specifically, it says that T's two parameters must be the same. K is not a GADT constructor (it's just a GADT-syntax constructor), as it doesn't constraint the return type at all. In order to use a GADT constructor in a type, you need -XTypeInType; this ability was not available before GHC 8.0.
> 
> There are concrete plans to smooth out this wrinkle: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0020-no-type-in-type.rst <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0020-no-type-in-type.rst>
> 
> I hope this helps!
> Richard
> 
> > On May 3, 2018, at 5:24 PM, Tom Ellis <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
> > 
> > Can anyone explain why the type instance for K is fine but the one for I
> > needs TypeInType?  (It does indeed work when I turn on TypeInType.)
> > 
> > 
> > 
> > {-# LANGUAGE GADTs #-}
> > {-# LANGUAGE TypeFamilies #-}
> > {-# LANGUAGE DataKinds #-}
> > {-# LANGUAGE PolyKinds #-}
> > 
> > data T a b where
> >  K :: b -> T a b
> >  I :: T a a
> > 
> > type family F (a :: T ka kb)
> > 
> > -- Data constructor ‘I’ cannot be used here
> > --        (Perhaps you intended to use TypeInType)
> > -- type instance F 'I = ()
> > 
> > type instance F ('K b) = ()
> > _______________________________________________
> > Haskell-Cafe mailing list
> > To (un)subscribe, modify options or view archives go to:
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> > Only members subscribed via the mailman list are allowed to post.
> 

> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.