stm-io-hooks-0.6.0: An STM monad with IO hooks

Portabilitynon-portable (requires STM)
MaintainerPeter Robinson <>



Provides the type class MonadAdvSTM and the AdvSTM monad. Parts of this implementation were taken from the HaskellWiki Page of MonadAdvSTM (see package description).



class Monad m => MonadAdvSTM m whereSource

A type class for extended-STM monads. For a concrete instantiation see AdvSTM


onCommit :: IO () -> m ()Source

Takes an IO action that will be executed iff the transaction commits.

  • When a TVar was modified in a transaction and the transaction tries to commit, this update remains invisible to other threads until the corresponding onCommit action is dispatched.
  • 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)

Note: If you really need to access the TVar within an onCommit action (e.g. to recover from an IO exception), you can use writeTVarAsync.

unsafeRetryWith :: IO () -> m bSource

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 aSource

See orElse

retry :: m aSource

See retry

check :: Bool -> m ()Source

See check

alwaysSucceeds :: m a -> m ()Source

always :: m Bool -> m ()Source

See always

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

liftAdv :: STM a -> m aSource

Lifts STM actions to MonadAdvSTM.

readTVar :: TVar a -> m aSource

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 ()Source

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

readTVarAsync :: TVar a -> m aSource

Reads a value directly from the TVar. Does not block when the onCommit actions aren't done yet. NOTE: Only use this function when you know what you're doing.

writeTVarAsync :: TVar a -> a -> m ()Source

Writes a value directly to the TVar. Does not block when onCommit actions aren't done yet. This function comes in handy for error recovery of exceptions that occur in onCommit.

newTVar :: a -> m (TVar a)Source

See newTVar

data AdvSTM a Source

Drop-in replacement for the STM monad