Portability | non-portable (requires STM) |
---|---|
Stability | experimental |
Maintainer | Peter Robinson <robinson@ecs.tuwien.ac.at> |
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 where
- onCommitWith :: ([IO ()] -> IO ()) -> m ()
- 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 ()
- readTVarAsync :: TVar a -> m a
- writeTVarAsync :: TVar a -> a -> m ()
- newTVar :: a -> m (TVar a)
- unsafeIOToSTM :: IO a -> m a
- data AdvSTM a
Documentation
class Monad m => MonadAdvSTM m whereSource
A type class for extended-STM monads. For a concrete instantiation see
AdvSTM
Takes a closure IO action and a commit IO action. The commit IO action will be executed iff the transaction commits. Commit actions are sequenced (within the same transaction), i.e.,
onCommitWith id (putStr "hello") onCommitWith id (putStr " world")
will print "hello world".
The closure action is useful for encapsulating the commit actions, e.g., within a database transaction. The last call of onCommitWith in the transaction is applied to the sequence of commit actions, i.e.:
onCommitWith id (putStr "hello") onCommitWith (\s -> do { putStrLn "start"; s; putStrLn "\nend"}) (putStr " world")
- 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
.
onCommit :: IO () -> m ()Source
Works like onCommitWith
without closure action:
'onCommit = onCommitWith id'
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
See retry
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 aSource
See catchSTM
Lifts STM actions to MonadAdvSTM
.
readTVar :: TVar a -> m aSource
Reads a value from a TVar. Blocks until the IO onCommit aidction(s) of
the corresponding transaction are complete.is not the last function
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
unsafeIOToSTM :: IO a -> m aSource
MonadAdvSTM AdvSTM | |
MonadAdvSTM m => MonadAdvSTM (ReaderT r m) | |
MonadAdvSTM m => MonadAdvSTM (StateT s m) | |
(MonadAdvSTM m, Monoid w) => MonadAdvSTM (WriterT w m) |