Re: [Haskell-cafe] MVar which can not be null ?

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