Control.Concurrent.Event

An Event is a simple mechanism for communication between threads: one thread signals an event and other threads wait for it.

Each event has an internal state which is either "Set" or "Cleared". This state can be changed with the corresponding functions set and clear. The wait function blocks until the state is "Set". An important property of setting an event is that all threads waiting for it are woken.

It was inspired by the Python Event object. See:

http://docs.python.org/3.1/library/threading.html#event-objects

This module is designed to be imported qualified. We suggest importing it like:

 import           Control.Concurrent.Event          ( Event )
 import qualified Control.Concurrent.Event as Event ( ... )

Documentation

data Event

An event is in one of two possible states: "Set" or "Cleared".

Instances

Eq Event

Typeable Event

new :: IO Event

Create an event in the "Cleared" state.

newSet :: IO Event

Create an event in the "Set" state.

wait :: Event -> IO ()

Block until the event is set.

If the state of the event is already "Set" this function will return immediately. Otherwise it will block until another thread calls set.

You can also stop a thread that is waiting for an event by throwing an asynchronous exception.

waitTimeout :: Event -> Integer -> IO Bool

Block until the event is set or until a timer expires.

Like wait, but with a timeout. A return value of False indicates a timeout occurred.

The timeout is specified in microseconds. A timeout of 0 μs will cause the function to return False without blocking in case the event state is "Cleared". Negative timeouts are treated the same as a timeout of 0 μs.

set :: Event -> IO ()

Changes the state of the event to "Set". All threads that where waiting for this event are woken. Threads that wait after the state is changed to "Set" will not block at all.

clear :: Event -> IO ()

Changes the state of the event to "Cleared". Threads that wait after the state is changed to "Cleared" will block until the state is changed to "Set".

isSet :: Event -> IO Bool

Returns True if the state of the event is "Set" and False if the state is "Cleared".

Notice that this is only a snapshot of the state. By the time a program reacts on its result it may already be out of date.