Re: Question about TypeInType

[Richard Eisenberg]

Done.

> On Apr 16, 2018, at 11:08 AM, Simon Peyton Jones  
> 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  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 
> <https://ghc.haskell.org/trac/ghc/ticket/12088#comment:15>
>  
> Richard
> 
> 
> On Apr 12, 2018, at 7:47 PM, Iavor Diatchki  <mailto:iavor.diatc...@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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RE: Question about TypeInType

[Simon Peyton Jones via ghc-devs]

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.diatc...@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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Re: 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 


Richard

> On Apr 12, 2018, at 7:47 PM, Iavor Diatchki  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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