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

18 Jul 2017

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

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13992
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler