[GHC] #9858: Typeable instances should be kind-aware

[GHC]

#9858: Typeable instances should be kind-aware
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        Reporter:  dreixel           |                   Owner:  dreixel
            Type:  bug               |                  Status:  new
        Priority:  highest           |               Milestone:  7.10.1
       Component:  Compiler          |                 Version:  7.9
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by oerjan):

 Replying to [comment:30 simonpj]:
 >  * And the kinds would have to be included in the fingerprint of the
 `TyCon` generated by the standalone derived instance.

 Once you have the mechanism for getting a kind representation to put in
 the fingerprint, how much harder is it to just make it work
 polymorphically while you're at it?

 I'm not a GHC developer by any means, but I still had the hunch that this
 could be a main stumbling block to getting the proper solution working.

 > The "right" solution is, I think, at the bottom of comment:20.  But NB
 that solution will mean that all these standalone derived instances would
 become illegal, so there is a backward compatibility cost.  The drastic
 solution does not have this problem.

 If *all* types later become `Typeable` automatically, then wouldn't it be
 safe and backward compatible to just ignore the explicit instances with a
 redundancy warning?

 (I've occasionally been thinking that multiple derived instances for the
 same type and class should often be safely compatible with each other.  If
 made by the same mechanism, they're guaranteed to have the same code,
 after all.  Although I guess standalone deriving with explicit restricted
 type signature could complicate that.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9858#comment:32>
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