Question about TypeInType

[Richard Eisenberg]

I think this is #12088. The problem is that open type family instances aren't used in kind checking in the same region of a module. The workaround is to put a top-level Template Haskell splice

> $(return [])

between the two type instances. This is far from optimal, but fixing it is a Major Project. See https://ghc.haskell.org/trac/ghc/ticket/12088#comment:15 <https://ghc.haskell.org/trac/ghc/ticket/12088#comment:15>

Richard

> On Apr 12, 2018, at 7:47 PM, Iavor Diatchki <iavor.diatchki at gmail.com> wrote:
> 
> Hello,
> 
> I was experimenting with TypeInType and run into a problem, that can be reduced to the following example.  Does anyone have any insight on what causes the error, in particular why is `IxKind` not being reduced?
> 
> -Iavor
> 
> 
> {-# Language TypeInType, TypeFamilies #-}
> 
> module Help where
> 
> import Data.Kind
> 
> type family IxKind (m :: Type) :: Type
> type family Value (m :: Type) :: IxKind m -> Type
> 
> data T (k :: Type) (f :: k -> Type)
> 
> type instance IxKind (T k f) = k
> type instance Value (T k f) = f
> 
> {-
> [1 of 1] Compiling Help             ( Desktop/Help.hs, interpreted )
> Desktop/Help.hs:13:31: error:
>     • Expected kind ‘IxKind (T k f) -> *’, but ‘f’ has kind ‘k -> *’
>     • In the type ‘f’
>       In the type instance declaration for ‘Value’
>    |
> 13 | type instance Value (T k f) = f
>    |                               ^
> Failed, no modules loaded.
> -}
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