[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:  closed
        Priority:  normal            |            Milestone:  8.0.1
       Component:  libraries/base    |              Version:  7.10.1
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1124
       Wiki Page:                    |
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Comment (by nomeata):

 JFTR: This instance of the regression has been fixed, because the `State`
 implementation got a sensible `Applicative` instance. The change in
 changeset:60297486/ghc still has the ability to regress any user code that
 has `(<*>) = ap`. OTOH, there is little we can do about it besides telling
 people to implement their Applicatives properly.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10457#comment:20>
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