[GHC] #8006: Asynchronous exception rethrown synchronously inside runStmt

[GHC]

#8006: Asynchronous exception rethrown synchronously inside runStmt
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    Reporter:  edsko             |       Owner:                  
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.6.3           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by edsko):

 "I don't think an asynch exn should ever become the value of a thunk." --
 yes, that would be ideal. Unfortunately, that's very hard to do, because
 there is no way to install an exception handler for synchronous exceptions
 only -- in particular, the almost-primitive `GHC.IO.catchException`
 catches all exceptions, does a runtime type check, and rethrows the
 exception using `raiseIO#` if the types don't match. This means that
 asynchronous exceptions become synchronous, and hence can become the value
 of a thunk. Manually re-throwing the exception using `throwTo` is not
 really a solution either (at least, not a generic solution, though
 sometimes a possible workaround), as then we need to decide in user code
 how to resume the thread. The ghc bugs I cite at the start of the issue
 are all related to this problem.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/8006#comment:4>
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