I pretty much abandoned that line of investigation with the conclusion that
generators:
http://wiki.ecmascript.org/doku.php?id=strawman:generators
are a good (and well-tested, in Python and SpiderMonkey) design for
single-frame continuations. They hang together well; in particular, they don't
have the issues with `finally' that some of the alternatives I talked about do.
Moreover, the continuation-capture mechanisms based on call/cc or shift/reset
require additional power in the VM to essentially tail-call their argument
expression. When I tried prototyping this in SpiderMonkey, I found this to be
one of the biggest challenges -- and that was just in the straight-up
interpreter, not in the tracing JIT or method JIT.
Generators work well for lightweight concurrency. As a proof of concept, I put
together a little library of "tasks" based on generators:
http://github.com/dherman/jstask
Somebody reminded me that Neil Mix had written a very similar library several
years ago, called Thread.js:
http://www.neilmix.com/2007/02/07/threading-in-javascript-17/
and there's another library called Er.js that built off of that to create some
Erlang-like abstractions:
http://www.beatniksoftware.com/erjs/
Dave
On Dec 8, 2010, at 11:36 PM, Tom Van Cutsem wrote:
> The spirit of the proposal is that this special type of statement be a linear
> sequence of function executions (as opposed to nested function-reference
> callbacks delegating execution to other code).
>
> The special behavior is that in between each part/expression of the
> statement, evaluation of the statement itself (NOT the rest of the program)
> may be "suspended" until the previous part/expression is fulfilled. This
> would conceptually be like a yield/continuation localized to ONLY the
> statement in question, not affecting the linear execution of the rest of the
> program.
>
> This reminds me of a proposal by Kris Zyp a couple of months ago ("single
> frame continuations")
> https://mail.mozilla.org/pipermail/es-discuss/2010-March/010865.html
>
> I don't think that discussion lead to a clear outcome, but it's definitely
> related, both in terms of goals as well as in mechanism.
> I also recall it prompted Dave Herman to sketch the design space of
> (single-frame) continuations for JS:
> https://mail.mozilla.org/pipermail/es-discuss/2010-April/010894.html
>
> Cheers,
> Tom
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