Daan Leijen
One can implement all other models on top of this: - you just keep this model and use callbacks. This is what GIO, HsGTK and TkGofer do. - you make the callbacks behave concurrently (this would lead to the bug in the example though). How could you do that? well, when in Haskell, you fork a thread to handle an event and return directly. Would it work? No! because the eventloop will immediately wait for the next event. As this is a C call, the haskell runtime system will not run any haskell threads.
Note that HGL (which works on both Hugs and GHC) does something a lot like what you describe. The reason it works on HGL is that the event loop thread calls 'yield' before executing the blocking call to get the next event (on X11) or returning from the event callback (on Win32). Yield pushes the current thread down to the lowest priority so all the other threads get to run until they terminate, block or yield before the event loop thread gets to continue. [Caveat: The semantics of yield is the Hugs semantics, the GHC semantics are a little different and should, perhaps be strengthened.] In short, it's perfectly possible to have the callbacks run concurrently. Whether it's a good way to structure systems I leave to the usual contributors to this list. -- Alastair Reid alastair@reid-consulting-uk.ltd.uk Reid Consulting (UK) Limited http://www.reid-consulting-uk.ltd.uk/alastair/