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This criticizes promises on two grounds: they need to propagate and handle errors, and the Javascript syntax looks different from the corresponding sequential c

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.

swannodette 13y ago
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.
- abecedarius 13y ago
  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 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.
  - swannodette 13y ago
    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.
    - abecedarius 13y ago
Eleanor_Erwin 13y ago
my AuntySamantha just got an awesome cream Mercedes E-Class Diesel just by some parttime working online with a macbook air. his explanation.... http://7.ly/6gn http://7.ly/6gn

swannodette 13y ago

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.

abecedarius 13y ago
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 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.
- swannodette 13y ago
  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.
  - abecedarius 13y ago

Eleanor_Erwin 13y ago

my AuntySamantha just got an awesome cream Mercedes E-Class Diesel just by some parttime working online with a macbook air. his explanation.... http://7.ly/6gn http://7.ly/6gn