Asynchronous Error Handling | Hacker News Reader

Asynchronous Error Handling

swannodette.github.io

56 pointsllambda18 comments

This criticizes promises on two grounds: they need to propagate and handle errors, and the Javascript syntax looks different from the corresponding sequential code. OTOH with channels, if I understand it, the system needs to propagate and handle errors ("If an asynchronous process writes an error onto its channel we will convert it into an exception") -- can you explain the advantage? And the better syntax we're offered is ClojureScript or yield.

For a cleaner comparison we should avoid unnecessary differences in the code. Let's fix that:

    getTweetsFor("swannodette")
      .then(function (tweets) {
        return expandUrl(parseUrls(tweets)[0]);
      })
      .then(httpGet)
      .then(
        function (responseBody) {
          console.log("Most recent link text:", responseBody);
        },
        function (error) {
          console.error("Error with the twitterverse:", error);
        }
      );

Besides yield, ES6 has shorter function expressions (you can try them right now in Firefox):

    getTweetsFor("swannodette")
      .then(tweets => expandUrl(parseUrls(tweets)[0]))
      .then(httpGet)
      .then(responseBody => console.log("Most recent link text:", responseBody),
            error => console.error("Error with the twitterverse:", error));

I'd agree that this could be better still; I'd like either built-in syntax dealing with promises, like E had 15 or so years ago, or something more general like Haskell's do-notation. But yield seems nice too.

Thanks for the cleanup though I think it's easy to see it's not much of an improvement. It's also not really equivalent to the core.async / ES6 Generator version as the error must propagate through the promise pipeline instead of short circuiting as it would thrown as a real exception.

UPDATE: also see the comment below about stack traces. Since we don't have to wrap promises at every step of the pipeline you can recover much cleaner stack traces devoid of useless promise machinery.

Which leads us to the points from my other post about the problem with promises :) While this is fine for coarse grained asynchrony like this example - the allocation overheads for each step of the pipeline is unacceptable for say mouse events - forcing you to leverage another abstraction like Rx. So does JS need sugar for both promises & async streams? I should hope not, and core.async / ES6 Generators demonstrate that you don't need two different abstractions and you can get a lot more efficiency and less error indirection than promises can possibly offer. This is more or less what Erik Meijer's been saying about C# async/await for a long time now.

not much of an improvement

I just wanted our thumbs off the scale.

the error must propagate through the promise pipeline instead of short circuiting

Along a queue vs. up a stack, I'm not sure there's a fundamental efficiency difference, if that's what you're saying. Maybe. When it comes to distributed apps, OTOH, just one round trip saved by automatic promise pipelining http://erights.org/elib/distrib/pipeline.html can outweigh a lot of that sort of thing.

stack traces

That post says Q's stack traces are fine if you set a flag, and the same for generator-based libraries (optional because it costs extra). I expect ClojureScript does better because it can compile its own code, and the same would go for a promise-based language (or JS preprocessor).

So this is what you were getting at with "needs to propagate and handle errors" vs. ClojureScript -- you're saying the latter's more efficient?

does JS need sugar for both promises & async streams?

I'm not sure what you mean about async streams, but I wasn't seriously after more changes to JS syntax, just wishing (1) E had taken off and (2) new languages learn from it.

That post says Q's stack traces are fine if you set a flag, and that generator-based libraries also pay extra for full stack traces. I expect ClojureScript does better because it can compile its own code, and the same would go for a promise-based language (or JS preprocessor).

Q explicitly says that you don't want to enable stack traces for production, which seems like a fairly high cost to pay. Neither core.async nor ES6 generators need to do anything special to have clean traces - it's fundamental to how the system works.

I'm not sure what you mean about async streams, but I wasn't seriously after more changes to JS syntax, just wishing (1) E had taken off and (2) new languages learn from it.

Sorry for not being clear, my posts are all part of continuum. My point is hardly anyone uses promises for fine grained asynchrony because of the costs and debugging indirection. My point is that you can do both coarse/fine grained asynchrony at little cost and without obfuscating your errors during development or in production if you move beyond promises.

"the debugging story for generators is somewhat bad, especially because of the missing stack traces for thrown errors. Fortunately, there are solutions and workarounds, like those implemented by genny (obtrusive, reduces performance) and galaxy (unobtrusive, but requires native modules)."

From the post you brought up, http://spion.github.io/posts/analysis-generators-and-other-a... -- by the author of Genny. As I said, that's at least qualitatively the same as Q; quantitatively I haven't looked. At https://github.com/spion/genny there's the example line

    require('genny').longStackSupport = true

and "This results with CPU overhead of approximately 500% and memory overhead of approximately 40%. In the future, the overhead will probably be eliminated in node but not in browsers." The benchmark post says Catcher is worse.

The README for Q shows the same switch and says "This feature does come with somewhat-serious performance and memory overhead, however. If you're working with lots of promises, or trying to scale a server to many users, you should probably keep it off. But in development, go for it!"

I'm not a current user of any of these systems, but this is why I said the first-order cause of any performance difference appears to be that ClojureScript is not a library.

I can't really speak to other generators libraries nor their how good their debugging stories are based on how good or bad their designs are. core.async is just a library and not directly part of ClojureScript. I do know that we need nothing to support good traces and we do not take any performance hit of any kind for this "feature".

Not a Javascript library. The post advocates generators for Javascript programmers. For core.async to tell us the efficiency of JS code of this style, it'd need to expose a generator-based interface and compile to generator-based code like a human writes. If I understand right, it doesn't, and the JS libraries that do work this way are much less efficient when full stack traces are turned on. (If there are others, I hope someone will chime in.)

If you think those two libraries are just bad, I'll update somewhat in that direction.

No the post advocates generators plus a competent channel abstraction. It's going to be hard to compare generator libraries to core.async, far as I can tell none employ the comprehensive set of optimizations that we do for time and memory. Perhaps when someone ports our ideas to JavaScript a real comparison can be made.

This would be the equivalent in Kal: http://goo.gl/TwsL2h

It's a different approach, but it doesn't require ES6 or any libraries (once compiled). Forgive the slight funkiness with the indentation.

  try
    wait for tweets from getTweetsFor "swannodette"
    url = parseUrls(tweets)[0]
    wait for responseBody from httpGet url
    print "Most recent link text: " + responseBody
  catch error
    console.error "Error with the twitterverse: " + error

This is nice but I am of course biased about the CSP model. While `wait` is good for one off tasks, I now prefer CSP channels as they just as easily capture Rx style interactive event programming.

I wrote a detailed comparison of JS async coding patterns here (emphasis on ES6 generators):

http://spion.github.io/posts/analysis-generators-and-other-a...

Unfortunately most patterns that consolidate error handling also result with useless stack traces, or have to resort to performance-reducing trickery (Q). This also includes generators.

This is another problem with promises is the the mangling of the stack trace. In my version the stack traces are more or less left clean since you don't have to wrap at every step of your async pipeline.

This is a very interesting idea. Although I still prefer the "jquery" way of passing in success/ failure as options. In fact, depending on what framework I'm using, I even trigger a success or failure event so it looks more like..

model.fetch();

model.on('error', function() { throw new Error(); });

But this article really got me thinking though, thank you!

The problem with passing success/failure as options is they don't compose well. Try to write the twitter example in the blog post this way. Then try to abstract each step as a reusable piece. And lastly have a mechanism for handling errors at any step. This looks ugly and is hard to follow. Promises (or generators) provide an API for thinking about async code as composable parts, which leads to easier to understand functions.

There is a problem with relying on setImmediate() in your channel implementation:

1. httpGet...

2. setImmediate() in a tight loop using 100% CPU until httpGet() returns

...

I'd rather not burn ~100% CPU for what could be a couple of seconds waiting for a slow web server (even if we're nicely burning it).

swannodette, would you consider upping the font size 1-3px? The font is very thin and I found it hard to read at even 15px.

I agree. I find Computer Modern to be more suited for mathematics and making it justified as well is not good for readability.

But TeX justifies (and hyphenates and microtypes...) by default? I'm curious why it would hurt legibility. It's the default text font for non-math stuff there too

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