Parallel and Concurrent Programming in Haskell

Chapter 9. Cancellation and Timeouts

In an interactive application, it is often important for one thread to interrupt the execution of another thread after the occurrence of some particular condition. Some examples of this kind of behavior include the following:

• When the user clicks the “stop” button in a web browser, the browser may need to interrupt several activities, such as a thread downloading the page, a thread rendering the page, and a thread running scripts.
• A server application typically wants to give a client a set amount of time to issue a request before closing its connection, so as to avoid letting dormant connections use up resources.
• An application that has a thread running a user interface and a separate thread performing some compute-intensive task (say, generating a visualization of some data) needs to interrupt the computation when the user changes the parameters via the user interface.

The crucial design decision in supporting cancellation is whether the intended victim should have to poll for the cancellation condition or whether the thread is immediately cancelled in some way. This is a tradeoff:

1. If the thread has to poll, then there is a danger that the programmer may forget to poll regularly enough, and the thread will become unresponsive, perhaps permanently so. Unresponsive threads lead to hangs and deadlocks, which are particularly unpleasant from a user’s perspective.
2. If cancellation happens asynchronously, critical sections that modify state need ...