if it is, please can someone add a new regression example to #12088. You can never have too many.
Thanks
Simon
From: ghc-devs On Behalf Of Richard Eisenberg
Sent: 13 April 2018 02:39
To: Iavor Diatchki
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 mailto: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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