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