On 18/03/13 19:07, s9gf4ult wrote:
nor STM, becuase IO is not acceptable inside STM transaction.
I just need some thread-safe blocking variable like MVar
modifyMVar :: MVar
<http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Control-Concurrent-MVar.html#t:MVar>
a -> (a -> IO
<http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/System-IO.html#t:IO>
(a, b)) -> IO
<http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/System-IO.html#t:IO>
b
Whilst it's true that IO cannot be performed within an STM action, a
common pattern is to return the necessary IO action from the STM action,
and then run it once the STM transaction has completed successfully.
Tim
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