StratifiedJS: Javascript + structured concurrency

Re:StratifiedJS: Javascript + structured concurrency by skrebbel on Wednesday February 01, 2012 @10:24AM (#3538284)

I'm probably missing something, but isn't this race condition galore? As per the front page example:

    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);

This starts the first 'or' clause the moment the http.get(BBC) suspends, which it does quickly because http.get is async. Now, if the BBC get returns just after my hold(1000) has completed but before http.get(CNN) had the time to really launch the http request and suspend, i'll have done the CNN get for nothing.

Of course in this case this only means a wasted request. But what in case of side effects?

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Re:StratifiedJS: Javascript + structured concurrency by jerf on Wednesday February 01, 2012 @12:12PM (#3538866)

Ignoring WebWorkers, the underlying Javascript engine is single-threaded, so the problem you describe can't exist. Once you start down a code path, it will continue until it finishes and yields execution.

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.

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Re:StratifiedJS: Javascript + structured concurrency by afri on Wednesday February 01, 2012 @11:03AM (#3538472)

Ah, this is one of the clever bits of StratifiedJS which differentiates it from a lot of the other async extensions out there.

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 .

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