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

[Richard Eisenberg]

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) = ()
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