[GHC] #9757: Warn about derivable instances

[GHC]

#9757: Warn about derivable instances
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              Reporter:  dfeuer      |            Owner:
                  Type:  feature     |           Status:  closed
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.9
             Component:  Compiler    |         Keywords:
            Resolution:  wontfix     |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by NeilMitchell):

 Indeed, I'm certainly happy to discuss this on the HLint issue tracker. I
 think GHC warnings are generally general-purpose and clear issues (e.g.
 patterns are inexhaustive) while HLint ones can be more style-based and
 instance-specific (e.g. use notElem instead of not and elem).

 My guess is that it's probably impossible in any remotely general way to
 detect instances that would be better off done with generalised newtype
 deriving (and obviously actually impossible in the general case), but if
 anyone can come up with ideas/code I'd certainly consider it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9757#comment:7>
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