[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:
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Comment (by dolio):

 `mapM_` is in a similar situation, despite not being in the `Foldable`
 class. It is currently implemented with `(>>)`, but it could be
 implemented with `(<*)` or just as `mapM_ = traverse_`. This is desired as
 well (see the most recent message in the libraries thread at the time of
 this comment).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10457#comment:1>
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