[GHC] #8502: Document a surprising {{{unsafeDupablePerformIO}}} limitation.

[GHC]

#8502: Document a surprising {{{unsafeDupablePerformIO}}} limitation.
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       Reporter:  int-e              |             Owner:
           Type:  task               |            Status:  new
       Priority:  normal             |         Milestone:  7.8.1
      Component:  libraries/base     |           Version:  7.6.3
       Keywords:                     |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple   |   Type of failure:  Documentation
     Difficulty:  Easy (less than 1  |  bug
  hour)                              |         Test Case:
     Blocked By:                     |          Blocking:
Related Tickets:                     |
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 We just discussed the following code on #ghc.
 {{{
 import System.IO.Unsafe
 import Control.Concurrent.MVar
 import Control.Concurrent
 import GHC.Conc (par, pseq)

 x :: MVar Int -> Int -> Int
 x m a = unsafeDupablePerformIO $
     withMVar m $ \i -> do
         sum [1..1000000 + a] `seq` return i

 main1 = do
     m <- newMVar 42
     let w = x m 0
     w `par` print w
 {-
 spark: thread blocked indefinitely in an MVar operation
 -}

 main2 = do
     m <- newMVar 42
     let w = x m 0
     forkIO $ print w
     forkIO $ print w
     print w
 {-
 spark: <<loop>>
 spark: <<loop>>
 -}

 main = main2
 }}}

 This is not a bug; what happens is that the RTS suspends one of the
 duplicated computations of {{{w}}}, effectively killing the IO action that
 was passed to {{{unsafeDupablePerformIO}}} in the middle. This limitation
 of {{{unsafeDupablePerformIO}}} should be documented.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8502>
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