[GHC] #13131: add effectful version of atomicModifyMutVar#

[GHC]

#13131: add effectful version of atomicModifyMutVar#
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           Reporter:  carter         |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 something like
 {{{
 atomicEffectfulModifyMutVar# :: MutVar# s a -> ((# a, State# s #) -> (#
 State# s , a  , b#)) -> State# s -> (#State# s, b #)

 }}}

 this isn't quite safely expressible currently  without some unsafeCoercion
 highjinks that I'm not sure are robust

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13131>
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