On Oct 19, 2011, at 12:55 PM, Eric Jacobs wrote:
> Kris Zyp wrote:
>> I believe that the overwhelming need that is continually and
>> constantly expressed and felt in the JS community in terms of handling
>> asynchronous activity is fundamentally a cry for top-down controlled
>> single-frame continuations (obviously not always stated in such terms,
>> but that is the effective need/solution). In terms of an actual code
>> example, essentially what is desired is to be able to write functions
>> like:
>>
>> element.onclick = function(){
>> // suspend execution after doSomethingAsync() to wait for result
>> var result = <some operator> doSomethingAsync();
>> // resume and do something else
>> alert(result);
>> };
>>
> I really like the direction that this is going, but I'm curious: Why not
> look into having full coroutines support?
Asked and answered many times, e.g.
https://mail.mozilla.org/pipermail/es-discuss/2006-June/003497.html
on the implementation problems with requiring suspending across native frames.
The other objection is that (ignoring some evil native APIs such as sync XHR)
JS has run-to-completion execution model now. You can model
assert_invariants();
f();
assert_invariants_not_affected_by_f_etc();
where "etc" means functions called from f. No data races, no preemption points
even if "voluntary" -- the immediately preempted function may have volunteered,
but in programming in the large, the sum of its ancestors in all call graphs
may well *not* have volunteered to lose their invariants.
This second objection is not an implementor issue, rather a
security/integrity/pedagogy concern. It's a big one too.
/be
> Coroutines support the above
> pattern well, plus they can be used to implement generators using the
> same mechanisms (although slightly differently than JS1.7/Python
> generators.) To allow code to suspend execution as you have shown above
> while avoiding reentrancy, you'll need some kind of "fork" or "spawn"
> primitive as well, and coroutines provide a nice paradigm for that.
>
> -Eric
>
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