Control.Concurrent.AdvSTM

When a TVar was modified in a transaction and this transaction commits, this update remains invisible to other threads until the corresponding onCommit action was run.
If the onCommit action throws an exception, the original value of the TVars will be restored.
Accessing a modified TVar within the onCommit action will cause a Deadlock exception to be thrown.

As a general rule, onCommit should only be used for "real" (i.e. without atomic blocks) IO actions and is certainly not the right place to fiddle with TVars. For example, if you wanted to write a TVar value to a file on commit, you could write:

 tvar <- newTVarIO "bla"
 atomically $ do 
    x <- readTVar tvar 
    onCommit (writeFile "myfile" x)

unsafeRetryWith :: IO () -> m b

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Retries the transaction and uses unsafeIOToSTM to fork off a thread that runs the given IO action. Since a transaction might be rerun several times by the runtime system, it is your responsibility to ensure that the IO-action is idempotent and releases all acquired locks.

orElse :: m a -> m a -> m a

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See orElse

retry :: m a

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See retry

check :: Bool -> m ()

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See check

alwaysSucceeds :: m a -> m ()

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See alwaysSucceeds

always :: m Bool -> m ()

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See always

catchSTM :: Exception e => m a -> (e -> m a) -> m a

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See catchSTM

liftAdv :: STM a -> m a

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Lifts STM actions to MonadAdvSTM.

readTVar :: TVar a -> m a

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Reads a value from a TVar. Blocks until the IO onCommit action(s) of the corresponding transaction are complete. See onCommit for a more detailed description of this behaviour.

writeTVar :: TVar a -> a -> m ()

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Writes a value to a TVar. Blocks until the onCommit IO-action(s) are complete. See onCommit for details.

newTVar :: a -> m (TVar a)

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See newTVar