[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:  ekmett
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Core        |           Version:  7.10.1-rc1
  Libraries                          |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 I found this when Simon was cleaning up unused dependencies in

 {{{
 https://phabricator.haskell.org/rGHCc409b6f30373535b6eed92e55d4695688d32be9e#10730
 }}}

 The Data instance for Ratio just uses the raw `(:%)` constructor and
 doesn't check that the result is reduced to normal form.

 It strikes me that the fix is to add back the Integral constraint on the
 Data instance and to use `(%)` rather than `(:%)` in the `gfoldl` and
 `gunfold` code.

 This restores the invariant and matches the behavior of "virtual
 constructors" we've used to patch up such problems  elsewhere.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10011>
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