stm-io-hooks-1.1.0: STM with IO hooks

Copyright(c) HaskellWiki 2006-2007, Peter Robinson 2008
MaintainerPeter Robinson <>
Portabilitynon-portable (requires STM)
Safe HaskellNone



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 Source

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


onCommitWith Source


:: ([IO ()] -> IO ())

closure action

-> m () 

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 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

always :: m Bool -> m () Source

See always

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

liftAdv :: STM a -> m a Source

Lifts STM actions to MonadAdvSTM.

readTVar :: TVar a -> m a Source

Reads a value from a TVar. Blocks until the IO onCommit aidction(s) of the corresponding transaction are 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 a Source

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

unsafeIOToSTM :: IO a -> m a Source