[GHC] #10011: The Data instance for Ratio violates internal invariants.

[GHC]

#10011: The Data instance for Ratio violates internal invariants.
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        Reporter:  ekmett            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.10.1
       Component:  Core Libraries    |                 Version:  7.10.1-rc1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D625
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Comment (by ekmett):

 This work scoped in this ticket is done, but you are right, we should
 definitely add a regression test to ensure this doesn't backslide in the
 future.

 Along the way we found a much less critical, but related issue (ok,
 really, feature request), which I'll file a separate ticket for -- with an
 eye towards fixing it on a longer time horizon:

 I'd eventually like to fix up the "virtual" data constructor we talk about
 in the Data instance to use `(%)`, since we're using a virtual constructor
 here, and that would let `gshow` and the like do the right thing, but when
 we tried that as part of this patch,  the fact that `dataToExpQ` currently
 filters out things that are illegal constructors caused a test failure.

 Fixing _that_ is a big enough issue to punt to 7.12. Fixing that right
 might also let us fix things like dataToExpQ for things like Maps, Sets,
 and other containers that also have to use the virtual constructor model
 to preserve internal invariants, which would be a pretty big boon to heavy
 users of template haskell.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10011#comment:11>
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