ClojureScript Core.async Todos
rigsomelight.com
rigsomelight.com
First of all, it is by nature a very "imperative" solution... it goes (to some degree at least) counter to the whole idea that state changes need to be limited as much as possible and isolated from the bulk of the "functional" code.
Also, it seems people are using this idea to create little localized event handlers, whereas before you'd solve this in a functional approach by having a single global event handler.
Sure, having a global event handler can be derided by saying "this global handler is spaghetti code because it doesn't isolate concerns."
But in the same way, this type of channel-driven programming can be called spaghetti code because the many different goroutines can step on each other as they manipulate the DOM.
For instance, if OP creates another channel-driven system for "editing todos" he would have to do some hard thinking to make sure the "add todos" system and the "edit todos" system don't interfere with each other and cause undefined behavior.
I should point out I haven't built a system yet that is channel-based (I plan to do this soon) so my knowledge of this approach is as of yet limited and it's possible I will like channels better once I've used them more heavily.
It is interesting. In normal Js applications there is tremendous gravity towards having mutable application state because the callbacks need a handle of some sort (normally some instance of something). So I am wondering what you mean by saying that the callback pattern is more functional?
In addition to mutable application state we have another global mutable state, one that's even harder to opt out of: The Dom. Regardless wether we use channels or callbacks this is going to be a problem whenever we have more than one actor changing it, whether that is done at the same time or sequentially.
To me the place for core.async or the newish yield operator is that blocking will allow us to wire together complex event behavior in some fairly sophisticated ways. How long does it take to get a slideshow to really work well in callback land? Or responsive autocomplete with correct lower and upper bound timeout thresholds?
So we can handle the wiring of events and when we get to the right state we do our functional application state transition.
Then as long as we are driving the Dom off of our application state then we should be good right? * waiving my hands a bunch
Those are not hard problems, really. Doesn't core.async solve bigger issues?
I would say that is the bigger issue 'expression'. I have to say though I am not advocating the creation of huge systems this way but I am very interested in this as a different approach and I am going to see how far it takes me.
I agree that the callback approach is not functional, and that the channel approach is certainly much better than this status quo approach.
I'm comparing instead against an approach where you break down all relevant events into JSON, and then push those into a global fully-functional "event handler" function that translates events (along the "world state") into a set of DOM manipulation primitives.
With that type of approach, no channels are necessary. I agree this approach has difficulties with time-sensitive operations (such as responsive autocomplete, etc) and that channels offer advantages for this.
I'm not yet sure which approach leads to the least tradeoffs or whether there is a third approach that combines the best of both worlds that I don't quite comprehend yet.
> another global mutable state, one that's even harder to opt out of: The Dom
Well, I think the success of angular.js shows that it's possible (and probably desirable) to completely isolate DOM state changes from other program code... though the OP program doesn't do this, this is clearly just a simplified example, so I want to see what is possible once someone builds a client-side framework that is built on channels...
The decoupled queue approach seems to be rare in practice and obviously superior. Wish I had thought about it long ago. I just followed the examples available to me.
I really believe the channel approach lifts this notion (values on queues) to the forefront with the addition of blocking sequential semantics.
Yeah my examples were a bit simplified, but I did try to make the DOM state a function of the application state. There will be more examples coming in the near future :)
How common is this approach? I'd be very interested to hear people's experiences with it. It's what we do, except we run the global event processor on a JS timer so that it works like a game loop picking items off a queue. Our DOM event handlers (the things you call addEventListener with) are all trivial stubs that simply create JSON descriptions of events as they happen and append them to that queue. We tried many different approaches before we hit on this, and it works surprisingly well in the two hardest areas: managing complexity and keeping the UI snappy.
You can also create perceived concurrency this way: you do a little work on an item in the queue and then, if it's not done, put it back at the back of the line. That's how we do the "time-sensitive operations" you mention.
I'm not sure what to call this design. I think of it as a state machine, but that's a metaphor rather than a precise description.
I've only done simple things in JS and don't understand what advantage you're getting by queueing events yourself, since JS already runs an event loop. Is it that you can inspect the queue and drop/modify events superseded by subsequent ones? I tried to do that once and it didn't help, but I've gotta admit I'm the opposite of an expert on JS UIs.
http://www.cs.ucf.edu/courses/cop4020/sum2009/CSP-hoare.pdf
On my list to read too.
I wrote a reply that is perhaps a bit too long to put here, so I put it at http://pastebin.com/4nN04Hj3 instead.
The short answers are: (1) this event loop is different from the browser's event loop because it's app-specific. Having a single interception point where you translate the browser event stream (mousedown, keypress etc.) into higher-level app-specific terms, and then write the rest of the UI in the latter, is a strategy for reducing complexity—an application of bottom-up programming to web UIs, really; and (2) emphatically yes, making the "official" event loop trivial and running all the complex stuff off your own app-specific loop allows you to control how much processing to do and when, which can be a way out of some thorny performance problems (such as sluggish scrolling) as well as an easy path to simulated concurrency.
It's interesting that you should have said "drop/modify events superseded by subsequent ones" because that's precisely what led us to this design originally—we were desperate, in fact, for a way to do that. The bottom-up programming aspect and the concurrency aspect only became clear later.
Sounds like I gave up too easily on queuing and pruning events, or it was the particular platform (touch tracking on Nexus 7 and first-gen iPad).
Thanks for the experience report. :-) It might help me remember this when I get back to http://wry.me/hacking/lissajous.html to finish & polish -- I let it get all cut-and-pastey in doing drag-controls for the first time.
http://web.archive.org/web/20070626045558/http://www.drjava....
This model deserves to better known, especially since all this stuff is in play again nowadays. Are there other implementations of it? (Node.js might be a natural platform, given its event loop and the popularity of promises there.) I also wonder what the biggest differences are between it and Erlang.
In your pipelining example (t3 := (x <- a()) <- c(y <- b())), what happens if the messages arrive out of order? (Or does the system prevent that and if so how?) I suppose if an expression can't be evaluated because a promise hasn't resolved yet, it could just go back to the end of the queue and eventually everything will bubble up?
Also: that Xanadu? Wow, no idea.
Edit: I forgot another thing I wanted to include in this omnibus comment. The paper "On the Development of Reactive Systems" that's mentioned at your first link and which can be found at [1], starts off with a very interesting and exciting distinction between "transformational" systems (which take inputs and transform them into outputs) and "reactive" systems that "are repeatedly prompted by the outside world and [whose] role is to continuously respond to external inputs. A reactive system, in general, does not compute or perform a function, but is supposed to maintain a certain ongoing relationship, so to speak, with its environment". I think this is extraordinarily lucid and that the authors are right to talk about concurrency—and probably complexity in general—in the latter sort of system as being a different challenge than in the former. Unfortunately, the paper quickly sinks into a pit of software process goo and never returns. A little Googling gives me the impression that important technical work did come out of it, though, and if anyone knows what the high points are I would like to see them.
[1] http://www.wisdom.weizmann.ac.il/~dharel/SCANNED.PAPERS/Reac...
The VatTP protocol is meant to guarantee 'just enough' ordering as at http://www.erights.org/elib/concurrency/partial-order.html -- there was a talk about VatTP, with slides, but I haven't seen a proper write-up and never learned it.
I think http://ai.eecs.umich.edu/~tpkelly/Ken/ is the most active descendant (and V8Ken linked from there which they intend to hook up with node.js). Daira Hopwood's new language Noether sounds promising: https://thestrangeloop.com/sessions/noether-a-concurrent-sec...
About Erlang, besides capabilities and mutable data I think the biggest difference comes from blocking receive. This gives it more of a CSP flavor. My own experience is limited -- my exposure to Erlang was mostly helping other hackers. (I also made an Erlang-influenced Scheme dialect I was unsatisfied with for boring reasons, but it was kind of in between: immutable data, channels as capabilities, synchronous messages.)
I think it is what could be called "Haskell syndrome" when people tend to write abstract things in "very fancy way" without asking that simple question.
There are tons of blog posts like "look, I could write this abstraction in Haskell this way using this kind of monads!". Well, you can, very clever, the question is - what are the chances to encounter this abstraction in real-world problems? Isn't there a straightforward but less fancy solution?
Should I really have "all this fancy stuff" to process onClick events which can be done in 3 lines of plain ugly JS?)
"All this fancy stuff" is just an exploration of what's possible. The verbosity can (and will) be abstracted away via Clojure's excellent tools of abstraction (higher-order functions and macros). Your "3 lines of plain ugly JS" aren't going to scale because they rely on mutation. Mutation is deceptively quick and easy when you're just starting out. However, once your program grows large it turns into a sprawling mess of complexity.
That's not a good reason for advocating ClojureScript. Why not Dart? TypeScript? CoffeeScript? Tell me something good about core.async...
Edit: Really, downvotes?
> Clojurescript is the browser language I always
> wanted. JS seems like such a hack.
Start with Rob Pike's "Go Concurrency Patterns" (https://www.youtube.com/watch?v=f6kdp27TYZs)
- whoa, this is cool!
- this is harder to read than the callback approach right now
There's no doubt channel support in clojure is awesome, just a personal matter of whether I'll end up preferring this paradigm for UI development.
Another analogy - Alan Kay compares software engineering to architecture and how so much engineering is brute force (think pyramids). core.async (really CSP) is like an arch (think gothic cathedral). Common UI patterns no longer seem arduous - and UI patterns which once seemed far out reach can be accomplished with a reasonable amount of engineering effort.
Also - thanks for all the awesome core.async gists - I feel like I want to take a geek sabbatical just so I can study them :)
Why do you think this is (I'm asking honestly, not trying to be snarky). Is it because you are not familiar with reading Lisp? The notation used to access channels is confusing? Unfamiliarity with the go-block concept?
It took me a while to fully internalize the combination of syntax and power of go blocks, but now that I have reading core.async code (I have not yet had a need to actually use them, and no time to port an existing codebase) is much easier.
I question whether the statement that callbacks are easier to read and understand is merely due to being accustomed to them. Core.async represents a radical departure from existing designs, and that can be jarring. Does this mean better syntactic sugar won't be created as developers gain experience? Or course not.
That said, this feels like an imperative solution to something that a state machine could handle equally well.
Webkit, blah.
Maybe my country of origin makes me have a bit of D. Quixote, who knows.