[GHC] #10457: Revise/remove custom mapM implementation for lists

[GHC]

#10457: Revise/remove custom mapM implementation for lists
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        Reporter:  dolio             |                   Owner:
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:  7.10.2
       Component:  libraries/base    |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D924
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Comment (by simonmar):

 Hmmm, so this is only a transparent change for Monads that have `<*> =
 ap`.  Arguably since it is possible to write legal (though unconventional)
 Haskell that behaves differently, and it's not a bug fix, it should not go
 into 7.10.2.

 (arguing against my own best interests, because it being in 7.10.2 would
 be helpful for me, but I've been working around it for a while so I can
 continue to do so)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10457#comment:6>
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