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

4 May 2018

      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... https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0020-no...
I hope this helps!
Richard
...
On May 3, 2018, at 5:24 PM, Tom Ellis  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.