var news;
waitfor {
news = http.get("http://news.bbc.co.uk");
}
or {
hold(1000);
news = http.get("http://news.cnn.com");
}
or {
hold(1000*60);
throw "sorry, no news. timeout";
}
show(news);
Of course in this case this only means a wasted request. But what in case of side effects?
Further, "hold" is almost certainly a magical statement that actually compiles into a pattern of calls to .setTimeout and various handlers, and has no literal existence, so there probably isn't any point at which the hold is "executed".
You can't actually turn a single-thread runtime into a threaded runtime at the user level. You can apply a series of increasingly sophisticated hacks that may make program like it's multithreaded, but you can't escape the fact you have only one program counter. (This isn't a criticism of the library. It looks quite useful. It just doesn't magically make browser Javascript truly multithreaded.)
This approach is actually quite useful, and is part of what makes the event-based programming style practical. For all the stuff that's going on, you do always have the guarantee that any given event handler will fully execute, and no other hunk of code will be able to observe the half-executed state of any given handler. Without that property you'd basically just be doing conventional multithreading with a really inconvenient code structure.
In the case you describe the CNN request will automatically be cancelled by honoring any try/finally or try/retract clauses in the http.get function. (try/retract is a new SJS thing). To see how cancellation/retraction works check out the slides starting at http://onilabs.com/presentations/ugent/cancellation.html .
I've whipped up a tiny project with the priorities inverted: readable output at the expense of completeness: https://github.com/lalcmellkmal/nestless
It only does the minimum desugaring to prevent nesting in the common case, no more.