On Apr 16, 2018, at 11:08 AM, Simon Peyton Jones <simonpj@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@haskell.org> On Behalf Of Richard Eisenberg
Sent: 13 April 2018 02:39
To: Iavor Diatchki <iavor.diatchki@gmail.com>
Cc: ghc-devs@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

 

Richard

On Apr 12, 2018, at 7:47 PM, Iavor Diatchki <iavor.diatchki@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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