The `concurrent-extra` package offers among other things the following
selection of synchronisation primitives:
* `Broadcast`: Wake multiple threads by broadcasting a value.
* `Event`: Wake multiple threads by signalling an event.
* `Lock`: Enforce exclusive access to a resource. Also known as a
binary semaphore or mutex. The package additionally provides an
alternative that works in the `STM` monad.
* `RLock`: A lock which can be acquired multiple times by the same
thread. Also known as a reentrant mutex.
* `ReadWriteLock`: Multiple-reader, single-writer locks. Used to
protect shared resources which may be concurrently read, but only
sequentially written.
* `ReadWriteVar`: Concurrent read, sequential write variables.
Please consult the API documentation of the individual modules for
more detailed information.
This package was inspired by the concurrency libraries of
[Java](http://download.oracle.com/javase/6/docs/technotes/guides/concurrency/index.html)
and [Python](http://docs.python.org/py3k/library/threading.html).