Control.Concurrent.STM.OrElseIO

Choose between the return value of an IO action and an STM operation, depending on which is available first.

Documentation

runOrElse :: IO a -> STM b -> IO (Either a b)

runOrElse io stm runs the IO action io. If its result is available when runOrElse itself returns, then that value is used as the function's return value. If not, the STM operation stm is attempted. Then, whichever of io's and stm's return value is then available first is returned from runOrElse, with a preference to that of io if both are available. runOrElse' reverses this priority.

It can happen that stm is never attempted. If it is, however, its result is used as return value only if it is available before that of io. Note that in that case, a long-running io will keep running until completed, even if runOrElse has already returned with the result of stm. A future version will probably kill off the io thread if its value is not needed (i.e. if that of stm value is used), but that is not currently the case.

runOrElse' :: STM a -> IO b -> IO (Either a b)

A version of runOrElse that prefers the STM operation to the IO action. In this case, the IO action is always run, but its value is only used if the return value of the STM operation is not available when the function returns.

The same caveat regarding long-running IO operations as for runOrElse also applies here.

runOrTakeTMVar :: IO a -> TMVar b -> IO (Either a b)

runOrTakeTMVar io tm = runOrElse io (takeTMVar tm).

runOrTakeTMVar' :: TMVar a -> IO b -> IO (Either a b)

runOrTakeTMVar' tm io = runOrElse' (takeTMVar tm) io.

runOrRun :: IO a -> IO b -> IO (Either a b)

A version of runOrElse where the STM operation is to run another IO action and takeTMVar the associated TMVar holding its return value. The first is preferred to the second.