[GUI] GUIs and events/callbacks.

[Wolfgang Thaller]

Am I overlooking some subtle semantic differences? To me, it looks like 
(non-composable) events can easily be implemented on top of callbacks 
using Control.Concurrent.Chan.
And it shouldn't be too difficult to implement callbacks on top of 
events...

If the above is true, then I'd say we implement callbacks first, we can 
easily add events later. Events look like a higher-level concept to me, 
as they seem to require concurrency to be meaningful. They could be 
added as an add-on-library, but IMHO we probably don't need them in the 
spec right now.

Which leads us to another big question: Concurrency.
The CGA should probably be safe to use from many threads at once 
(although many backends aren't, so the library would have to do a lot 
of synchronizing). Should callbacks for multiple clicks on one button 
really run concurrently, or should it behave like most toolkits for 
other languages? I know of no "conventional" toolkits where event 
handlers for one event source are run concurrently. It sounds 
dangerous, but it might have advantages.

Cheers,

Wolfgang