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

[GHC]

#10457: Revise/remove custom mapM implementation for lists
-------------------------------------+-------------------------------------
        Reporter:  dolio             |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  libraries/base    |              Version:  7.10.1
      Resolution:                    |             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
-------------------------------------+-------------------------------------
Changes (by thomie):

 * failure:  None/Unknown => Runtime performance bug


Comment:

 nofib/allocs/cryptarithm2       13094088        + 64.55%        21545696
 bytes

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10457#comment:18>
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