For example, consider John Meacham's simple example. As G.Russel pointed out, (almost) all the event-based implementation suffer from a problem: if the user clicks the button twice, fastly, the program does not exit.
This is because the programs are written like
onClick button callback1
callback1 = doSomething >> unregister callback1 >> onClick button callback2 = doSomethingElse
The first ">>" in callback1 can be the point on wich the user clicks the button for the second time!
Just for the record: this is *not* true for most (all?) event based implementations as they are *not* concurrent (at least not pre-emptively concurrent). Somewhere down, there is an event loop -- a loop that waits for an event, executes a callback, and loops again. Even though more events can happen while executing the callback, it will only propegate into Haskell once the callback returns and the event loop pops it off the OS event queue. [note: You can achieve more concurrency in Haskell by using Haskell threads but even than one has to wait till the callbacks are done or otherwise the runtime system sits idle while waiting for the next event (as this is a C call).] -- Daan