[GHC] #3399: Generalize the type of Data.List.{deleteBy, deleteFirstsBy}

[GHC]

#3399: Generalize the type of Data.List.{deleteBy, deleteFirstsBy}
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        Reporter:  iago              |                Owner:
            Type:  proposal          |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  libraries/base    |              Version:  6.10.4
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2526
       Wiki Page:                    |
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Comment (by bgamari):

 There was recently a
 [[https://mail.haskell.org/pipermail/libraries/2016-September/027319.html|discussion]]
 on the libraries list but it's not clear to me what the conclusion was.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3399#comment:9>
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