[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:  patch
        Priority:  normal            |               Milestone:  7.12.1
       Component:  libraries/base    |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D1124
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Changes (by bgamari):

 * differential:  Phab:D924 => Phab:D1124


Old description:

> Recently, Simon Marlow asked why the list instance for Traversable had a
> custom mapM implementation that used the Monad operations. Having looked
> a bit, I don't think there's any good reason. The only fusion that the
> custom mapM can participate in is due to it being written as foldr, but
> traverse is, as well. So as long as 'mapM = traverse' is able to inline
> appropriately, there should be no difference.
>
> Further, this can be changed, in principle, for 7.10.2. It doesn't change
> any types, only the implementation.
>
> mapM = traverse is the class default definition, so this could possibly
> be completed by just removing the custom definition.
>
> Link to the libraries thread:
> https://mail.haskell.org/pipermail/libraries/2015-May/025708.html

New description:

 Recently, Simon Marlow asked why the list instance for `Traversable` had a
 custom `mapM` implementation that used the `Monad` operations. Having
 looked a bit, I don't think there's any good reason. The only fusion that
 the custom `mapM` can participate in is due to it being written as
 `foldr`, but `traverse` is, as well. So as long as `mapM = traverse` is
 able to inline appropriately, there should be no difference.

 Further, this can be changed, in principle, for 7.10.2. It doesn't change
 any types, only the implementation.

 `mapM = traverse` is the class default definition, so this could possibly
 be completed by just removing the custom definition.

 Link to the libraries thread:
 https://mail.haskell.org/pipermail/libraries/2015-May/025708.html

--

Comment:

 Phab:D924 proposed a redefinition of `mapM_` in addition to the removal of
 list's override of `mapM`. This changed the performance characteristics of
 user code, as seen in #10711.

 I've extracted the removal of the `mapM` override into Phab:D1124 and
 forwarded the `mapM_` issue to the Core Libraries Committee.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10457#comment:13>
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