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

I hope this helps!
Richard

On May 3, 2018, at 5:24 PM, Tom Ellis <tom-lists-haskell-cafe-2013@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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