[GHC] #7787: modifyMVar does not restore value if callback returns error value

[GHC]

#7787: modifyMVar does not restore value if callback returns error value
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    Reporter:  joeyadams         |       Owner:                             
        Type:  bug               |      Status:  new                        
    Priority:  high              |   Milestone:  7.8.1                      
   Component:  libraries/base    |     Version:  7.7                        
    Keywords:                    |          Os:  Unknown/Multiple           
Architecture:  Unknown/Multiple  |     Failure:  Incorrect result at runtime
  Difficulty:  Unknown           |    Testcase:                             
   Blockedby:                    |    Blocking:                             
     Related:                    |  
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Comment(by parcs):

 Replying to [comment:1 ezyang]:
 > Another possibility is to replace the pattern match with an irrefutable
 pattern. I'm not sure which is better, though an irrefutable pattern could
 make laziness bugs worse.

 Btw this is what `atomicModifyIORef` essentially does.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7787#comment:4>
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