Question about TypeInType

[Richard Eisenberg]

Done.

> On Apr 16, 2018, at 11:08 AM, Simon Peyton Jones <simonpj at microsoft.com> wrote:
> 
> if it is, please can someone add a new regression example to #12088.  You can never have too many.
>  
> Thanks
>  
> Simon
>  
> From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Richard Eisenberg
> Sent: 13 April 2018 02:39
> To: Iavor Diatchki <iavor.diatchki at gmail.com>
> Cc: ghc-devs at haskell.org
> Subject: Re: Question about TypeInType
>  
> 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 <mailto: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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