[GHC] #13992: Error message, room for improvement (polykinds)

[GHC]

#13992: Error message, room for improvement (polykinds)
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  PolyKinds
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by Iceland_jack:

Old description:

> Something like
>
> {{{#!hs
> --    • No instance for (Show (Compose Proxy Proxy a))
> --        arising from the 'deriving' clause of a data type declaration
> --       Possible fix:
> --         use a standalone 'deriving instance' declaration,
> --           so you can specify the instance context yourself
> --     • When deriving the instance for (Show (FlipProxy a))
>
> {-# Language DerivingStrategies, GeneralizedNewtypeDeriving, PolyKinds,
> KindSignatures #-}
>
> import Data.Functor.Compose
> import Data.Proxy
> import Data.Kind
>
> newtype FlipProxy a = FlipProxy_ (Compose Proxy Proxy a)
>   deriving newtype
>     Show
> }}}
>
> where the solution is to constraint the kind of `a :: Type`, it would be
> nice if GHC could reference kind variables.

New description:

 Something like

 {{{#!hs
 --    • No instance for (Show (Compose Proxy Proxy a))
 --        arising from the 'deriving' clause of a data type declaration
 --       Possible fix:
 --         use a standalone 'deriving instance' declaration,
 --           so you can specify the instance context yourself
 --     • When deriving the instance for (Show (FlipProxy a))

 {-# Language DerivingStrategies, GeneralizedNewtypeDeriving, PolyKinds,
 KindSignatures #-}

 import Data.Functor.Compose
 import Data.Proxy
 import Data.Kind

 newtype FlipProxy a = FlipProxy_ (Compose Proxy Proxy a)
   deriving newtype
     Show
 }}}

 where the solution is to constrain the kind of `a` to `Type`, it would be
 nice if GHC could reference kind variables.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13992#comment:1>
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