If you are doing IO operations, then the operation is hardly atomic, is it? Just take from the MVar, compute, and when you're done, put a value back on the MVar. So long as you can guarantee all users of the MVar take before putting, you will have the desired semantics. Something worth considering: what are the desired semantics if an asynchronous exception is thrown on the thread servicing the MVar? If the answer is to just quit, what if it has already performed externally visible IO actions? If the answer is to ignore it, what if the thread gets wedged? Edward Excerpts from s9gf4ult's message of Mon Mar 18 01:07:42 -0700 2013:
18.03.2013 13:26, Alexander V Vershilov ?????:
I can not use atomicModifyIORef because it works with pure computation
atomicModifyIORef :: IORef http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-IORef.... a -> (a -> (a, b)) -> IO http://hackage.haskell.org/packages/archive/base/latest/doc/html/System-IO.h... b
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-Co... a -> (a -> IO http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/System-IO.... (a, b)) -> IO http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/System-IO.... b