[GHC] #14349: Semigroup/Monoid instances for System.Exit.ExitCode

[GHC]

#14349: Semigroup/Monoid instances for System.Exit.ExitCode
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        Reporter:  neil.mayhew       |                Owner:  (none)
            Type:  feature request   |               Status:  upstream
        Priority:  low               |            Milestone:
       Component:  libraries/base    |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by neil.mayhew):

 * cc: hvr, ekmett, core-libraries-committee (added)


Comment:

 As far as I can see, the only other reasonable `Monoid` instance would be
 to have success if any succeeded, but this is almost never what you want.
 It would be possible to use a similar approach to the `Any`/`All` newtypes
 for `Bool`, but this seems like overthinking it.

 It would be possible to favour the rightmost failure, but I don't see much
 point in this. That's why I mentioned `set -e`, which returns the exit
 code of the first, ie leftmost, failure. The fact that bash short-circuits
 the rest of the evaluation is mostly an issue of lazy evaluation. The
 semantics of the summary exit code are the same, although of course the
 side effects are different.

 Adding a named function would be a valid approach, but it's a pity to add
 a single-use function when we already have a nice general way to view the
 problem with `Monoid`.

 If `mempty` is an issue then it could be a `Semigroup` instance. However,
 it seems natural to me that running no child processes is considered
 successful.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14349#comment:4>
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