[GHC] #799: runtime error when using par/seq in a monad

#799: runtime error when using par/seq in a monad
--------------------------------------+-------------------------------------
    Reporter:  mafuchs@xxxxxxxxxxxxx  |        Owner:         
        Type:  bug                    |       Status:  new    
    Priority:  normal                 |    Milestone:         
   Component:  Runtime System         |      Version:  6.5    
    Severity:  normal                 |     Keywords:         
          Os:  Unknown                |   Difficulty:  Unknown
Architecture:  Unknown                |  
--------------------------------------+-------------------------------------
The following version of parfib dies with the following message, but the
 version after the error message works:

 parfib 0 = return 1
 parfib 1 = return 1
 parfib n = do
               n1 <- parfib (n - 1)
               n2 <- parfib (n - 2)
               n3 <- (n1 `par` (n2 `seq` (return (n1 + n2 + 1))))
               return n3

 sh-2.04$ ./Main +RTS -N4 -sstderr
 c:\msys\1.0\ghc6.5\bin\Main.exe +RTS -N4 -sstderr
 Main.exe: internal error: evacuate: strange closure type 8208
     (GHC version 6.5.20060619 for i386_unknown_mingw32)
     Please report this as a GHC bug:
 http://www.haskell.org/ghc/reportabug

 This application has requested the Runtime to terminate it in an unusual
 way.
 Please contact the application's support team for more information.

 This version of parfib works

 parfib 0 = return 1
 parfib 1 = return 1
 parfib n = do
               n1 <- parfib (n - 1)
               n2 <- parfib (n - 2)
               return (n1 `par` (n2 `seq` (n1 + n2 + 1)))

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/799>
GHC <http://www.haskell.org/ghc/>
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