OK, I finally see what you mean, I assumed that when I called (send dialog show #t), the program would enter a loop that plucks messages out of the dialog event queue and dispatches them *synchronously* to the appropriate callbacks. A small test I did indicates that no event dispatch occurs while a callback is beinghandled, so at least the dispatch does appear synchronous. So, in theory at least, for my particular example, I think an escape continuation could still take me out of a callback and back into my own code. As you point out however in the non-modal case, there must be a message loop, or watch dog,  running on another process and in that case I suppose invoking the escape continuation would replace the watch dog stack with the one on my original process, and, since I suspect there is only one watch dog for all the windows in the application, kill all UI updates. I can see why there would be need for a continuation barrier. Does this make any sense, or I am totally of base here? Thank you for your patience, -pp <div>On 10/7/06, Matthias Felleisen < <a href="mailto:matthias@ccs.neu.edu">matthias@ccs.neu.edu</a>> wrote:<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> No you didn't. A callback doesn't return and can't return. It is an asynchronous call from somewhere deep inside when an event occurs. In a sense, it is run in concurrently to the rest of the program. In your special case, it was waiting (due to the modal dialog) and one could argue that there is a sequential flow of control. But that's just a special case. So, in general, some concurrent process watches the "world" and waits for events to happen. When they do happen, it checks whether you want to know about it. This watch dog finds out that you have installed a callback and calls. If you raise an exception, the exception erases the control stack of this call and everything around it. In other words, it stops the execution of the watchdog process. Now your own process is spared. After all, there is no connection between the watchdog and your application. This has little to do with the fearsome first-class continuation objects in Scheme but everything with "GUI concurrency" and "exceptional flow of control." Since callbacks can't return, they have void as a return type. You are therefore forced to communicate all results and values across this "void wall" via side effects. ;; --- If you really want to dig deep: all this is the fundamental flaw of call-back based programming. It's the dominant and efficient model. I sure wish, however, we could come up with something better. -- Matthias </blockquote></div>