Mutable, doubly linked lists for STM

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BSD3 licensed by Joey Adams

Doubly linked lists are often used in imperative languages to maintain a collection of objects such that an object can remove itself easily. The same idea applies here. Thanks to STM, these lists can be shared among threads with little effort.

Suppose you have a thread listening for events. You want to dispatch events to any thread that opts in. You could write a function that installs an event handler, performs an action, and removes the handler:

withEventHandler list handler action =
    bracket (atomically $ LinkedList.append handler list)
            (atomically . LinkedList.delete)
            (\_ -> action)

Then, you could have your listener thread dispatch events using a function like this:

dispatchEvent list event = do
    handlers <- atomically $ LinkedList.toList list
    forM_ handlers $ \handler -> handler event
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