[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 simonpj):

 Ok, well, before thinking about workarounds, let's just check that
 something closer to ideal is not possible.  Here is a tiny example
 {{{
 let x = unsafePerformIO $
         do { let v = expensive 100
            ; printToFile "logfile" ("Computed v" ++ show v)
            ; return v }
 }}}
 Here I am writing to a log-file as an example of the kind of benign side
 effect that should be ok in `unsafePerformIO`.

 So what should happen if an asynch exn is caught inside `printToFile` some
 I/O stuff is restored, and the exception is re-thrown?  I would have that
 this principle should hold:
  * exactly the same thing should happen as would have happened if the
 exception was delivered when we ''were not'' inside any catch-blocks in
 `printToFile`.
 Namely, freeze and resume.

 I'd be inclined to resume just before the `throwTo`. So that might (just)
 cause the log file to be written to twice, but it's supposed be a benign
 side effect.

 It's a good point though.  I wonder what the semantics says?

 Adding a new kind of catch does not seem like the right solution.
 `printToFile` might really really want to restore some invariants before
 aborting; if it can't catch async exns, it couldn't do so.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/8006#comment:7>
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