[GHC] #10598: DeriveAnyClass and GND don't work well together

[GHC]

#10598: DeriveAnyClass and GND don't work well together
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        Reporter:  osa1              |                Owner:  RyanGlScott
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:  Generics
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):

 > That magic somewhat disturbs me, especially since it means a type
 synonym is not equivalent to its expansion.

 I'm not proposing making these type synonyms expand to something different
 than what they're defined as above. The only magic comes when it's used in
 a `deriving` statement—that would cause its underlying (expanded) type to
 have a different derived //instance//, but the types are honest.

 > But if you think it's better than (backwards-compatibly) extending the
 parser to reuse keywords... (I guess `default` is a bit of a stretch
 anyway.)

 It's not quite backwards-compatible—we'd also have to make a breaking
 change to `template-haskell` in order to accommodate the presence of these
 new pragmas. The type synonym approach, however, is only backportable to
 GHC 7.6.

 (It should be noted that many uses of this feature would not be usable on
 older GHCs in the first place, so perhaps this isn't a huge concern.)

 > In any case, you should also decide how to annotate a `class`
 definition, which is where I actually suggested a pragma.

 I'm not sure what you mean. Are talking about annotating when you should
 use `DeriveAnyClass` (`DAC`)? Or something else?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10598#comment:25>
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