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

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