[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:  normal                       |      Component:  libraries/base  
 Version:  7.7                          |       Keywords:                  
      Os:  Unknown/Multiple             |   Architecture:  Unknown/Multiple
 Failure:  Incorrect result at runtime  |      Blockedby:                  
Blocking:                               |        Related:                  
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Comment(by 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.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7787#comment:1>
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