Question about TypeInType

Simon Peyton Jones simonpj at
Mon Apr 16 15:08:57 UTC 2018

if it is, please can someone add a new regression example to #12088.  You can never have too many.



From: ghc-devs <ghc-devs-bounces at> On Behalf Of Richard Eisenberg
Sent: 13 April 2018 02:39
To: Iavor Diatchki <iavor.diatchki at>
Cc: ghc-devs at
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


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


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?


{-# 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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