[GHC] #11796: Warn about unwanted instances in a modular way

[GHC]

#11796: Warn about unwanted instances in a modular way
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        Reporter:  Lemming           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
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 Lemming):

 It could resolve the long discussion on libraries mailing list about
 whether we want instances like Foldable (,) or not. With this warning we
 could both have the instances for those who prefer to use pairs for
 everything, but do not harm those who consider `length (2,3)` an error.

 See https://mail.haskell.org/pipermail/libraries/2016-February/026678.html
 .

 Conflicting "warn-instance" strings might be resolved with package
 qualification as I have mentioned in the last paragraph of the proposal.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11796#comment:2>
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