Signals, the push-pull based algorithm | Hacker News Reader

Signals, the push-pull based algorithm

willybrauner.com

170 pointsmpweiher36 comments

I know it is out of scope for this article, but there are variants where the operations are monadic rather than applicative and the shape of the graph can change depending on values. And also variations with state - where history can be taken into account.

Jane street briefly summarizes some options here: https://blog.janestreet.com/breaking-down-frp/

And they have an interesting talk on the trade-offs and how their own system, incremental, evolved: https://blog.janestreet.com/seven-implementations-of-increme...

Overall, very nice article. A few notes:

* I think the first implementation in JS land was Flapjax, which was around 2008: https://www.flapjax-lang.org/publications/

* The article didn't discuss glitch-freedom, which I think is fairly important.

I didn't know about Flapjax, thanks I'll check it out. Glitch-freedom is indeed a gap in this article. I focused on the signal algorithm exclusively without some implementation optimisation like batching updates; there is so much more to cover! Maybe in a next one, Thanks!

And before there was "reactive", there were (oneway) dataflow constraints.

And a lot of literature on the algorithms.

I wrote a bit about the connection here:

https://blog.metaobject.com/2014/03/the-siren-call-of-kvo-an...

(It starts in a slightly different place, but gets there)

Also about constraints as an architectural connector.

https://dl.acm.org/doi/10.1145/2889443.2889456?cid=813164912...

Batching isn't just an optimisation, not having transactional semantics can cause bugs.

I wrote a whole screed here about how glitches are evil and Rx is evil for teaching people they’re normal, but then I thought about it a bit more—

The system as described isn’t actually glitchy, is it? It doesn’t eagerly run any user computations, just dirtying, and that is idempotent so the order is irrelevant. It’s also a bit useless because it only allows you to pull out values of your own initiative, not subscribe to them, but that’s fixable by notifying all subscribers after the dirtying is done, which can’t cause glitches (unless the subscribers violate the rules of the game by triggering more signals).

So now I’m confused whether all the fiddly priority-queue needlepoint is actually needed for anything but the ability to avoid recomputation when an intermediate node decides it doesn’t want to change its output despite a change in one of its inputs. I remember the priority queue being one of the biggest performance killers in Sodium, so that can’t be it, right?..

I’m also confused about whether push-pull as TFA understands it has much to do with Conal Elliott’s definition. I don’t think it does? I feel like I need to reread the paper again.

Also also, some mention of weak references would probably be warranted.

> whether push-pull as TFA understands it has much to do with Conal Elliott’s definition.

Virtually nothing that is getting sold/branded as "FRP" has anything to do with Conal Eliott's definition.

I once gave a long talk about this here in Berlin, but I don't remember if there was a video.

I've also explained it on twitter a bunch of times, including this memorable sequence:

https://x.com/mpweiher/status/1353716926325915648

Kinda like the Marshall McLuhan scene in Annie Hall ("if only real life were like this")

https://www.youtube.com/watch?t=136&v=sXJ8tKRlW3E

>> whether push-pull as TFA understands it has much to do with Conal Elliott’s definition.

> Virtually nothing that is getting sold/branded as "FRP" has anything to do with Conal Eliott's definition.

True but not what I meant. The article implicitly (and, in the links at the end, explicitly) refers to his 2009 paper “Push-pull functional reactive programming”, which describes a semantic model together with an specific implementation strategy.

So I was wondering if TFA’s “push-pull” has anything to do with Elliott 2009’s “push-pull”. I don’t think so, because I remember the latter doing wholly push-based recomputation of discrete reactive entities (Events and Reactives) and pull-based only for continuous entities that require eventual sampling (Behaviors).

With that said, I find it difficult to squeeze an actual algorithm out of Elliott’s high-level, semantics-oriented discussion, and usually realize that I misunderstood or misremembered something whenever I reread that paper (every few years). So if the author went all the way to reference this specific work out of all the FRP literature, I’m willing to believe that they are implying some sort of link that I’m not seeing. I would just like to know where it is.

After wondering what the heck glitch-freedom is and learning about it, I agree with you. It seems like it deserves at least a brief explanation in an article about how signals work.

I've gone with the universal `alien-signals` package for my project (which doesn't use a frontend framework that includes signals). They show benchmarks of being by far the fastest and have strict limits on code complexity. Those limits are also supposed to avoid glitches by design, and now at least some of that is tested[1].

[1]: https://github.com/stackblitz/alien-signals/pull/39

I do not want to distract from the content of the article, which is highly relevant for folks who built UIs with frameworks that are conceptually based on signals, but the way that the reading experience is designed really great, in particular the guided reading flow through the instructive code path is something that I rarely have seen done at all, and this even works pretty well on mobile. It's a delightful reminder on how a dynamic medium can be more than the simulation of print on screens.

I searched for the "right way" to explain this algorithm, and ultimately parsing the code was the clearest for me. This isn't always true; sometimes a graph or an interactive module has more impact; it really depends on the topic. Thanks for the feedback.

Kudos! Minor detail: the code could use a border or bg color different than the text, so that they’re more distinct.

The visual examples are rather misleading:

The "2 * x" is rather - why would the reaction from a change in X display many gradual increments of 1 instead of showing the final value once? And then why does Z =Y+1 instead of +1 to Y repeats all the steps again from X? That's not how real signal frameworks work, and also not how you'd imagine they should work

Then the next cascading example: ok, if Signal is a button, not the underlying mechanism behind it, then "computed 1" is also a signal, why isn't it called that? (though intuitively you'd think the moving dots are signals, not buttons)

Beautiful presentation... @willybrauner, I would like to read your spin on a follow-up piece on `glitch-freedom`. But in all honesty, this journal entry/post is a work of art; a testament to your journey as a technologist!.

Cheers

Writing technical articles without adding a graphic and playful touch would indeed interest me less. I find that it's the intersection of these two worlds that makes reading enjoyable. Thanks for your kind words!

What an amazing article. I really like the presentation of text scrolling together with the code. Wonder how this is done under the hood.

Thanks a lot! The left column containing the text is sticky, The right column is absolute and translate on Y and depend on an intersection observer triggered by each scroll section from the left.

Super cool, thanks for explaining! Is the code available somewhere?

No, unfortunately, my website is not open source. I may consider externalizing the blog in the future, while keeping all the internal article modules.

Relevant project for reactive stream programming that gets overlooked https://github.com/Point72/csp

The writeup is written very well. Kudos!

However, I really wonder if this scales to real applications…

I’ve seen too many people get initially mesmerized by “event driven” programming only to find the system eventually becomes a steaming mess that nobody can comprehend or debug. Or it manages to work but has serious performance issues (in an exclusively “push” design)…

Maybe in a single process, single threaded environment, it is more difficult to screw up?

Why create an array each time it iterates a Set rather than just iterating the Set?

I create a new copy of the Set because the functions called during the loop might modify it by removing elements via cleanup functions. Without a copy, iterating directly on the Set could cause unpredictable behavior.

But as the article points out, this is a naive implementation. It could be discussed and greatly improved.

Nice presentation, looks like the same thing I implemented in Tcl here: https://wiki.tcl-lang.org/page/ReacTcl :-)

Sodium (and the book that goes with it) is a great resource too https://github.com/SodiumFRP/sodium

How is error handling expected to happen here?

Result<T> should work

I love this pattern, but I always felt like squeezing idiom B into idiom A when doing this in e.g. TS. I never tried effect-ts, so maybe I'm missing out. Anyone have experience in this?

try-catch

Now that Angular is adopting signals, building in it has become a much better experience.

Mind you, the framework still has a hostile learning curve, but for those who already made that investment, it's a boon.

Also in https://svelte.dev/blog/runes

Reply on news.ycombinator.com