Re: [GHC] #16194: deriving, wrong code: newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving stock Functor

18 Jan 2019

      #16194: deriving, wrong code: newtype T cat a = MkT ((forall xx. cat xx xx) -> a)
deriving stock Functor
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.7
      Resolution:  wontfix           |             Keywords:  DeriveFunctor
                                     |  deriving RankNTypes
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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Comment (by Iceland_jack):

 Thought: The second representation (`g & \g' -> f (g' x)`) can maybe be
 captured as a newtype with an instance (like `DList` changes the
 representation of  change the representation of `[a]` to `Endo [a]`). GHC
 would generate the newtype form, the instance takes care of the operation
 and coerce back.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16194#comment:5
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