[GHC] #7808: data families and TH names do not mix well (e.g. cannot use TH deriving)

[GHC]

#7808: data families and TH names do not mix well (e.g. cannot use TH deriving)
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        Reporter:  duncan            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  7.6.2
      Resolution:                    |             Keywords:  TypeFamilies
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):

 Apologies for commenting on an extremely old Trac ticket.

 The way I tackle this problem in libraries like `bifunctors` (see
 [http://hackage.haskell.org/package/bifunctors-5.4.1/docs/Data-Bifunctor-
 TH.html#v:deriveBifunctor Data.Bifunctor.TH]) is by allowing functions of
 the form `:: Name -> Q a` to take either a type constructor `Name` (for a
 plain datatype) or a data constructor `Name` (for a data family instance).
 The TH code is able to look up the original data family instance from any
 of its data constructor names, so it works, although it's a bit awkward.
 There's also the drawback that you can't do this trick for data family
 instances with no constructors... but how useful are those, really? :)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7808#comment:7>
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