By example: Continuation-passing style in JavaScript | Hacker News Reader

By example: Continuation-passing style in JavaScript

matt.might.net

62 pointsbudu19 comments

What the author is missing: tail recursion in constant space is an implicit requirement for practical use of CPS transformations. In fact this is probably the single biggest reason for wanting efficient tail calls in a language.

ECMA doesn't require efficient tail recursion. Caveat programmer.

Indeed!

You can work around it with a trampoline (http://en.wikipedia.org/wiki/Tail_call#Through_trampolining), which is what I did with cfa-js. (http://dl.dropbox.com/u/6600185/cfa/cfademo.html)

Good point. True.

If you're going to do real CPS programming in JavaScript, you'll want to use a trampoline or a timer mechanism to reset the stack.

But, in most cases, you do a little computation and then block waiting for an event.

When the continuation gets invoked by the event-handling loop, the stack is reset.

When the continuation gets invoked by the event-handling loop, the stack is reset.

You're not compiling to C here: "the stack" may be a chain of heap-allocated activation records that are still live as far as GC is concerned. What you're describing might work but you'd want to test it to be sure.

EDIT: On reflection I agree that the setTimeout() approach should free the stack, however it's implemented.

> ECMA doesn't require efficient tail recursion. Caveat programmer.

You can always use setTimeout to call your tail, which has the neat effect that your CPS "threads" become cooperatively multitasked and can progress alongside each other. Just replace "foo()" tail calls with "setTimeout(foo, 0)".

And the downside that you're nuking your stack every time, making debugging quite a bit harder.

CPS is about much more than just callbacks, though.

Twisted (a python asynchronous network programming framework) does something similar, but inverted. Twisted functions return a Deferred object without blocking. A Deferred is a promise of a value, so rather than passing the callback into the function, you register a callback with the deferred, and the function promises to call it later.

This type of thing has a few nice properties -- you can easily register more than one callback, as well as using them as a pipeline (kind-of like monads in haskell), which are awkward to implement with callbacks.

As an aside, in python you can trick generators into turning synchronous-looking code into asynchronous, which can actually make things look a lot neater if done properly -- i'll provide code if anyone's interested.

> python you can trick generators into turning synchronous-looking code into asynchronous

Example: http://dieselweb.org/lib/ or https://github.com/saucelabs/monocle

Although Diesel2 is much more interesting, with greenlets support. No more messing about with `yield`, since the blocking functions themselves force a context switch. https://github.com/jamwt/diesel

That looks really interesting, though it seems a bit dangerous -- greenlet feels like a bit of a hack.

I wish I'd found diesel before twisted; it seems a lot more elegant if you don't need any of the built-in protocols. The twisted model is good, there's just too much of it.

Could someone explain how the 'call/cc in javascript' from the article is supposed to be used?

  function callcc (f,cc) { 
    f(function(x,k) { cc(x) },cc);
  }

Suppose you wrote a compiler that translates to JavaScript in continuation-passing style. (Or suppose you wrote all your JavaScript in continuation-passing style.)

That's the implementation of call/cc.

For how to use call/cc, I recommend my other article on continuations:

http://matt.might.net/articles/programming-with-continuation...

I found this link before posting the question but I still can't imagine how to use javascript call/cc implementation from your article (on the other hand I think I know how to use call/cc in scheme). Maybe I am too tired today.

If you posted a simple example of JavaScript written in continuation-passing style and using this call/cc implementation probably everything would be clear.

People interesting in this article might also like Eric Lippert's articles on the subject.

http://blogs.msdn.com/b/ericlippert/archive/2010/10/21/conti...

They're more from a language design perspective rather than the more practical article here.

Basically, this article is just about what callbacks are and how they are all nice and convenient?

Unfortunately, yes.

Yes... including how they're "just" nice and convenient as a compilation target for a language with exceptions and continuations.

No. I think you're missing the point.

Continuations become a highly constrained kind of callback in continuation-passing style.

Callbacks by themselves don't make for a nice intermediate format for compilation, or a way to model exceptions or distributed computation.

They only gain these abilities when constrained as continuations in CPS.

For good descriptions of using CPS as an intermediate representation in a compiler, check out Appel's _Compiling with Continuations_, Guy L. Steele's "Rabbit" thesis, Krantz's "Orbit" thesis, and chapter 12 of the first edition of _Essentials of Programming Languages_ (they cut it from later editions, unfortunately). Appel uses ML, the rest use Scheme.

doesn't jquery already do this with $.post?

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