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

[GHC]

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

 {{{#!hs
 instance Monoid ExitCode where
     mempty = ExitSuccess
     mappend ExitSuccess b = b
     mappend a           _ = a
 }}}

 This allows the summary result of multiple child processes to be computed
 naturally. For example:

 {{{#!hs
 mconcat <$> mapM system commands
 }}}

 The result is `ExitSuccess` if they all succeeded, and the leftmost
 failure otherwise

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