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

[GHC]

#13131: add effectful version of atomicModifyMutVar#
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        Reporter:  carter            |                Owner:
            Type:  feature request   |               Status:  infoneeded
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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 rwbarton):

 * status:  new => infoneeded


Comment:

 I don't understand what the semantics of this would be. How can you
 atomically update a `MutVar#` while allowing arbitrary side effects when
 computing the new value?

 The pure version works because we can speculatively create a thunk
 containing the old value in a compare-and-swap loop.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13131#comment:1>
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