[GHC] #12224: Replace placeHolderPunRhs with a PlaceHolder value

[GHC]

#12224: Replace placeHolderPunRhs with a PlaceHolder value
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        Reporter:  alanz             |                Owner:  alanz
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
  (Parser)                           |
      Resolution:                    |             Keywords:
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 RyanGlScott):

 alanz, presumably you're talking this datatype in `HsPat`?

 {{{#!hs
 -- For details on above see note [Api annotations] in ApiAnnotation
 data HsRecField' id arg = HsRecField {
         hsRecFieldLbl :: Located id,
         hsRecFieldArg :: arg,           -- ^ Filled in by renamer when
 punning
         hsRecPun      :: Bool           -- ^ Note [Punning]
   } deriving (Data, Functor, Foldable, Traversable)
 }}}

 And you want to change `hsRecFieldArg` to be of type `PostRn id arg`? If
 so, I don't think you'll be able to derive `Functor`, `Foldable`, or
 `Traversable` anymore, since GHC won't know what code to fill in for the
 irreducible type family `PostRn id arg`. One possible workaround is to
 define your `Functor` instances manually:

 {{{#!hs
 instance Functor (HsRecField' Id) where
   fmap f (HsRecField a b c) = HsRecField a (f b) c

 instance Functor (HsRecField' Name) where
   fmap _ (HsRecField a b c) = HsRecField a b c

 instance Functor (HsRecField' RdrName) where
   fmap _ (HsRecField a b c) = HsRecField a b c
 }}}

 and similarly for `Foldable` and `Traversable`. It's not the prettiest
 solution, but I think it might work for GHC's purposes.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12224#comment:2>
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