Control.Monad.AdvSTM

stm-io-hooks-0.5.4: An STM monad with IO hooks

Portability	non-portable (requires STM)
Stability	experimental
Maintainer	Peter Robinson <robinson@ecs.tuwien.ac.at>

Description

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

Synopsis

class Monad m => MonadAdvSTM m where

onCommit :: IO () -> m ()

unsafeRetryWith :: IO () -> m b

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

retry :: m a

check :: Bool -> m ()

alwaysSucceeds :: m a -> m ()

always :: m Bool -> m ()

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

liftAdv :: STM a -> m a

readTVar :: TVar a -> m a

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

newTVar :: a -> m (TVar a)

data AdvSTM a

Documentation

class Monad m => MonadAdvSTM m where

Source

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

Methods

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 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

Source

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

Source

See orElse

retry :: m a

Source

See retry

check :: Bool -> m ()

Source

See check

alwaysSucceeds :: m a -> m ()

Source

See alwaysSucceeds

always :: m Bool -> m ()

Source

See always

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

Source

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 ()

Source

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

newTVar :: a -> m (TVar a)

Source

See newTVar