What Are Signals? | Hacker News Reader

What Are Signals?

signia.tldraw.dev

I don't really understand why people like signals again. I used a signals-like reactive programming model in VueJS a while ago and hated that you could never be sure exactly where things were being changed. Thankfully it seems that the React creators and maintainers are not hopping on the signals train and are instead adamant about unidirectional dataflow with explicit mapping of state to UI, which, as has been the reason for why React had been invented in the first place, makes reasoning about the application much easier [0][1].

This is a good thread about their drawbacks [2]. Apparently, both the below examples actually do different things:

    function One(props) {
      const doubleCount = props.count * 2;
      return <div>Count: {doubleCount}</div>
    }


    function Two(props) {
      return <div>Count: {props.count * 2}</div>
    }

Like the tweeter says, signals are mutable state. I'll stick with unidirectional data flow in React.

[0] https://twitter.com/jordwalke/status/1629663133039214593

[1] https://twitter.com/dan_abramov/status/1629539600489119744

[2] https://twitter.com/devongovett/status/1629540226589663233

That difference in the snippet you mentioned has 100% to do with how Solid is compiling its JSX, and 0% to do with how signals must work.

If you don't like what Solid is doing you may consider it a self-inflicted wound, nothing is forcing Solid to work that way.

My impression is that Javascript is just an horrible language to create reactive systems, and that default purity with explicit effects is extremely underrated. (But then, I was already biased.)

It is hard to even make sense of the discussion without imagining the details of how those frameworks are implemented. And the discussion on the level of normal usage, not something advanced or development oriented.

What language(s) would you recommend for building reactive systems?

Well, the OP links to 3 large threads from JS framework designers that do nothing but talk about how they deal with the lack of referential transparency.

So, I'd guess, something with referential transparency.

Because it's nicer. The mental model of reading the functions using signals is; it's all about initialize|constructor|new function. And then those off-jsx stuff starts to make sense since you won't re-initialize those things. They need to exist somewhere in particular execution model. In Solidjs, it's effects (compiled from jsx where signals are read). I think those who are familiar with static type languages, naturally understand how signals needs to be unwrap/wrap .. like a monoid, not a big deal.

When you have a big app, it becomes a spaghetti mess, and I speak from personal experience having to untangle that spaghetti.

What approach do you prefer for big apps?

The state has to go somewhere, curious how you choose to handle it.

Personally I just use Redux Toolkit with RTK Query for any synchronization with backend state. What I like about Redux is that it takes the immutable functional approach, you cannot change previous state but must instead derive new state from the old state very explicitly. In this way, it's the antithesis of signal-based state.

> What I like about Redux is that it takes the immutable functional approach

Strange that you don't like spaghetti, but then praise Redux that is spaghetti exemplified: dozens of functions, and wrappers, and hooks eerywhere. Trying to trace how a value gets changed through the three-four layers of wrappers is like pulling teeth through the anus.

And looking at the current reincarnation of this abomination, it is marginally less spaghetti, and converging on the same signals code that everyone is converging on.

Redux Toolkit is very impressive, especially with TypeScript. I'd agree that there was a lot of boilerplate in the before-times, but today, it's not "marginally less," it's significantly less boilerplate. The debugging tools are top notch as well, I can trace values and their changes pretty easily these days.

The main point of Redux is that it follows functional programming paradigms of immutable data, which makes reasoning about changes much easier. Compare that to the global state everywhere of signals/reactive/observables-based approaches like MobX or RxJS.

> Compare that to the global state everywhere of signals/reactive/observables-based approaches like MobX or RxJS.

What are you on about? Redux is literally about global state everywhere. But with three to four layers of abstractions between.

You define your state globally. Then use hooks to fetch data from that global state, and then "dispatch" aka call actions on that global state. If something else somewhere else updates data in that global state your component will be affected if it uses that data.

But it also requires you to write an insane amount of useless stuff like "define your store, then reducers, then slices, then god knows what" to arrive at almost the same code:

   // redux

   import { useSelector, useDispatch } from 'react-redux'
   import { decrement, increment } from './counterSlice'

   export function Counter() {
     // note how we reach into magical global state that useSelector knows about
     const count = useSelector((state: RootState) => state.counter.value)
     const dispatch = useDispatch()

     return <>
        <button onClick={() => dispatch(increment())}>Increment</button>
        <span>{count}</span>
        <button onClick={() => dispatch(decrement())}>Decrement</button>
     </>;
   }


   // solid

   // state is immutable. It's also not magical, you explicitly refrence and import it
   //
   // increment and decrement would use a `set` function to update the store to required value
   // see https://www.solidjs.com/docs/latest/api#createstore
   // and https://www.solidjs.com/docs/latest/api#updating-stores
   //
   import { state, increment, decrement } from './counter-store';

   export function Counter() {
     return <>
        <button onClick={() => increment()}>Increment</button>
        <span>{state.count}</span>
        <button onClick={() => decrement()}>Decrement</button>
     </>;
   }

Solid is basically using the same Flux architecture that Redux and Vuex is using then, with stores and actions. It's just more hidden away than in Redux which is more explicit. Yes, I do want to explicitly see what my data store looks like and in Redux I'm able to. That I have to create slices (which are automatically mapped to TypeScript by the way) is fine with me, it's only a few lines more than in your Solid example.

To add, this is also why I explicitly mentioned Rx and Mob, which are not as clean as the Flux architecture in terms of creating spaghetti code. And I believe we are also defining spaghetti differently. Spaghetti != boilerplate.

Counterpoint: I've worked on a big application that used MobX and had no such issues.

I'm not a JS/Vue/React programmer but I would definitely expect function One to return <div>Count: 4</div> if props.count == 2 when One is called. For function Two I would be more suspicious and would not be overly surprised if it returned <div>Count: {props.count * 2}</div> to be evaluated at a later stage.

Yes, it's more of a design problem of some of these libraries though.

Ideally, signalling shouldn't be retrofitted on normal variables. It assumes then that you overload variable access (assignment and reads). Then it's hard to know which variables are reactive and which ones are not. The problem stems from the overloading of semantics.

A UI is mutable so it's not about mutable state being wrong but the unit of variability are not program variables but rendered props which are basically DOM Element properties. In react, it just happens that it uses variables whose values are injected in the DOM elements via jsx but that's an issue (especially when dealing with exports, prop drilling etc)

So until a plain variable and a reactive variable are made to look different everywhere in code, it will remain confusing. And even then, it's not optimal.

Disclaimer: I'm currently implementing a cross-platform UI framework in Go and that's the reason why I had to think about it since traditional languages do not have reactive variables.

Elm managed to move away from signals back in 2016 (controversial at the time, but IMO a great move).

https://elm-lang.org/news/farewell-to-frp

Controversial only because they were hard to learn and those that invested in learning lost that investment. The new architecture is so much better and not hard to explain.

This one sentence probably explains 90%+ of all programming advocacy.

So sad Elm is essentially dead.

That sounds like a limitation of solid. In Vue, you can use return type to differentiate if you are in ts atleast

That example makes a lot of sense once you realize how JSX works in SolidJS (and its very different from React):

1. The function is called only once at component construction, and never again. Unlike React, it is _not_ called for every render.

2. Every expression in `{}` is compiled to a thunk function - not to a direct JS expression.

For example, the expression

   <>{props.count * 2}</>

compiles to

    memo(() => props.count * 2)

Another interesting bit - as the article above rightfully points out, `useState` and `useMemo` are signals. The main difference is that you have to track and specify your dependencies manually, whereas Solid auto-tracks dependencies based on their usage while the computation runs.

here’s a debunking of the third tweet with reference: https://twitter.com/antonycourtney/status/162970259437243597...

Your 2 is not a signals drawback, but a SolidJS one.

I love signals in VueJS, and it doesn’t even preclude a redux-like/lite state- management library like Pinia.

I would rather use the proper Vue term, reactive references/refs. Signals is such an awkward term.

https://vuejs.org/guide/extras/reactivity-in-depth.html#conn...

That’s why I italicized it.

If those two pieces of code do different things, it’s a bad library plain and simple lol.

I’ve implemented this pattern with mobx and those two pieces of code are equivalent.

In my opinion:

- Unidirectional data flow patterns prevent a class of problems junior developers tend to make but require much deeper discipline to keep the app understandable

- Conventional bi-directional data flow patterns are susceptible to junior developer mistakes but generally are hard to make unreadable

I haven't had much success using signals as a software composition mechanism at scale ... Despite being familiar with Conal Elliott's "functional reactive animations" concepts and Racket's FrTime.

"patch languages" like PureData, Max/MSP/Jitter, QuartzComposer are much better interfaces to work with signals ... Or rather with signal processors.

A series of articles from the author of Solid discussing all those tweets, and more, including the history of signals and their vurrent state.

- The Evolution of Signals in JavaScript, https://dev.to/this-is-learning/the-evolution-of-signals-in-...

- React vs Signals: 10 Years Later, https://dev.to/this-is-learning/react-vs-signals-10-years-la...

- Making the Case for Signals in JavaScript , https://dev.to/this-is-learning/making-the-case-for-signals-...

Those two code examples are different because that's the tradeoff that Solid made. It allows Solid to track changes in a uniform way even if the data isn't inside a component. Because in Solid components are just a way to organize code, not a unit of rendering (like in React).

And on top of that if you think that React still somehow has unidirectional data flow, it doesn't. Hooks make it anything but unidirectional. And not that different from signals: https://res.cloudinary.com/practicaldev/image/fetch/s--upMC6... But often less predictable.

See also The Cost of Consistency in UI Frameworks, https://dev.to/this-is-learning/the-cost-of-consistency-in-u... which is slightly related to this.

Dan Abramov replied to those as well. Personally, the purity of the React model with bringing in only local rather than global state in every function is what appeals to me. Even hooks are still local state, if you change the value in useState, you're not going to suddenly have something change elsewhere, where it wasn't explicit where you changed it.

useEffect, useContext, and usSyncExternalStore are no longer about "local only".

And you never use only local state. You advocate for Redux in a different comment which is literally a global state where you are going to have something suddenly change somewhere from the point of view of a component.

You are misunderstanding me. I never said to use only local state with no global state at all. My point is that signals automatically make all state global because you can reference what should be a local value in a component, in another component entirely which is not at all connected to the initial component either in a parent or a child relationship. In other words, you are making a graph, not a tree.

For Redux, at least that's explicit, for signals, it's really not.

At first I thought this was talking about Unix signals

Then I thought for a moment it was talking about a "signal" in the information theoretical sense

All I came away with was that they've invented a new name for event driven architectures and/or data flow programming.

I know naming is hard but we need to stop overloading terms.

Signals are a common term for this. Someone else invented the term not the people who wrote this.

“Signal” was used for a time-varying value in a reactive system for a very long time (I thought it was as far back as Fran in 1998, but I was remembering wrong—I was only able to find usage in Grapefruit in 2009, older references welcome). One way or another, in a functional reactive programming system, you need distinct names for:

(a) A thing that happens (or might happen but ultimately doesn’t) and is associated with a value of some sort, such as the mouse coordinates of first left-button click the program sees;

(b) A list of the above with monotonically increasing times, such as all the left-button click coordinates in order;

(d) A (conceptually) piecewise continuous function of time, such as the position of an animated object.

[To avoid “time leaks”, i.e. accidentally retaining all change history from the beginning of time, you also need

(e) A computation that reads the current time, such as the mouse coordinates of the next left-button click the program will see;

but that’s not supposed to be obvious—it took more than a decade to figure out.]

Possible names include: event occurrence [for (a)], event stream [for (b)], event [either (a) or (b)]; behaviour [(c) or (d)—the library may not distinguish them in the API or only provide (d)]; reactive [Conal Elliot’s name for (c) as opposed to (d)]; signal [(b), (c), or both]. All of these make sense in isolation. I don’t think you can win here.

[Perhaps the worst word to use is “stream“, because people start trying to fit streams with backpressure in there. FRP only makes sense as far as you can assume any recalculations happen instantaneously, meaning at the very least before the next input change happens. If you forget that, you get Rx instead of FRP, and that’s not amenable to human comprehension—or indeed sanity.]

I've always referred to and heard these referred to as "events" and "sequences" with d/e being subtypes of either or containing additional contexts.

I'm not sure what you mean by "Rx" in this context.

My background is much more in the systems (and indeed, "signals" in the theoretical sense).

I still think the name "signal" here is quite bad, since it's the abstract concept of something-that-carries-information

> I’m not sure what you mean by "Rx" in this context.

From “reactive extensions”, a name for a family of libraries[1] (RxJava, Rx.NET, RxJS), AFAIK one of the first attempted implementations of mature FRP ideas in the imperative world and one messy enough that it took React’s success for anything similar to reënter the mainstream.

Compare the enthusiastic HN reception of “Deprecating the observer pattern” in 2011[2], mostly by people who heard of FRP in the functional-language setting, and the vitriol it received in 2018[3], apparently from people for whom FRP came to mean Rx (and similarly confused things like Bacon.js). It is this vitriol that I meant to preemptively redirect by the mention of FRP ≠ Rx, so if you’re not aware of this history it’s no big loss to ignore it.

(Yes, I am very much No-True-Scotsmanning this. That should not be taken to mean that reconciling FRP with side effects is a trivial problem, though,—I’m not sure it’s even a solved one. Only that Rx and its ilk suck as a solution.)

> My background is much more in the systems (and indeed, "signals" in the theoretical sense).

> I still think the name "signal" here is quite bad, since it's the abstract concept of something-that-carries-information

I think the intention was to allude to physical wires and conceptual analog or digital things they transmit, as in “reset signal”, “differential signalling”, etc. It does seem to me to be an apt term for a (continuous-)time-dependent doodad in a program, given “variable” is taken.

Your description sounds more like the information-theoretic “channel” or “transmission medium”.

[1] https://reactivex.io/

[2] https://news.ycombinator.com/item?id=2972581

[3] https://news.ycombinator.com/item?id=17845341

I felt similarly disappointed, as I was expecting a cool intro to DSP or something similar. But nope, another BS buzzword from the magical world of webdev.

The term signal was used, because RxJS had already taken the term "Observable" to refer to their primitives, which are actually more like streams.

Because of this, Ryan Carniato of SolidJS referred to his own observable primitives as "signals" and this has become the default terminology to talk about this type of pattern within the context of web development.

Really, signals should be thought of as a type of observable.

Signal isn't overloaded. It's use here is the same as it's use elsewhere. It just requires contextual information to know what it means. A Qt programmer might think of signals and slots, a UNIX programmer might think of UNIX signals, etc. Jargon sure. Overloaded? I don't think so. Only thing here that bothers me is how vague the title is.

The terminology of signals (and slots) has been used in Qt [0] since the 1990s I believe.

Still, it is unfortunate that the article begins with “Let's start with an extremely broad definition” (of signals), then continues with what sounds like signals in general, mentioning React only as an example, but then turns out wanting to only talk about signals in the particular reactive UI sense common in web development, without properly introducing that narrowing of focus.

I guess that focus can be expected from a blog on a site about a TypeScript signals library, but from the way the first few paragraphs are written, it’s certainly confusing when reading the article without further context, coming from HN.

[0] https://en.wikipedia.org/wiki/Signals_and_slots

> The terminology of signals (and slots) has been used in Qt [0] since the 1990s I believe.

Was confusing then and is still confusing now.

I was equally confused, it just needs a paragraph at the beginning explaining the context

signals represent continuous time varying quantities, like an electrical voltage, an audio signal or the current mouse coordinates. streams (event streams) represent a sequence of discrete events, like key presses or network packets or financial transactions.

the key difference is in backpressure strategy: signals are canonically lazy, they don’t compute or do work until sampled, and only the latest value is relevant (nobody cares where the mouse was a moment ago when nobody was looking). streams are eager, you can’t skip a keyboard event or a financial transaction, even if the pipes are backed up – instead you have to tell upstream to slow down so you can catch up. The benefit of event streams is the guarantee that you'll see every event, which means streams are suitable for driving sequences of side effects (keyboard event -> network request -> database transaction).

signals are a good fit for rendering because you only want to render at up to say 60fps (even if the mouse updates faster, which it does). and you only want to render what’s onscreen, and only when the tab is focused. rendering (say dom effects) is indeed effectful but not in the discrete way; the dom is a resource, it has a mount/unmount object lifecycle, and due to this symmetry it is a good fit for rendering whereas isolated effects (without a corresponding undo operation) are a terrible fit for signals because backpressure will drop events and corrupt the system state.

You can use streams for rendering too, but it's dis-optimal, and potentially by a lot. If your app chokes on a burst of events, you want to skip ahead and render the final state without bothering to render all the intermediate historical states. Signal laziness is what enables this "work skipping"; a stream would have to process each individual event in sequence.

I have no idea if JS projects get the backpressure right, can anyone confirm?

It’s kinda funny to see “signals” coming round into fashion or popular discussion in front-end tech Twitter. Mobx is a signals state management library has been around for 7 years. Notion used signals internally for state management and often when engineers joined they’d ask why we use signals over Redux, can we switch to Redux because it’s more modern, etc. Now it’s 4 years later and for some reason this pattern is all the rage. I’m glad I didn’t waste time on Redux just in time for trends to move on.

To discuss more specifically TLDraw’s Signia library, I think the ability to do manual diff tracking & incremental updates for computed stores is an interesting “escape hatch”. Docs here: https://signia.tldraw.dev/docs/incremental

Most signal libraries I’ve seen try to lean hard into magic auto-tracking of dependencies which means they make it really easy for the developer to correctly observe a lot of dependencies, cool on correctness, but then have a very limited set of tools to deal with the performance implication of some computation needing to re-run a whole bunch. The differential tracking here means that if you see such a hotspot, you can get really manual optimization of recomputation without needing to squeeze into the libraries pre-packaged observable collection API.

Downside of this API is it seems quite easy to get it wrong.

Another thing I like about Signia is the use of logical time. I saw this first in Jotai internals, then in Starbeam. I haven’t dug into the source of the library yet but I think logical time is a good approach and makes inspecting internals make a bit more sense than inspecting systems (like Notion’s) that rely purely on update notification listeners.

Same here, I have been spreading mobx everywhere I have gone. There's usually some resistance but once people got used to automatic dependency tracking, they never go back.

I've seen so many projects get bogged down in props hell, then gobs of context api and performance problems when too many things react at once.

Mobx has been solving these problems for a long time now!

I recently started ripping Mobx out of my app. Mobx solved exactly 1 problem for me, which was passing data horizontally (to siblings/cousins) and even to non-React components.

But now `useSyncExternalStore` solves that. No context needed. Can read and write state anywhere, even outside components. No "observe" or "track" shenanigans. Efficient updates (can listen to a subsection of the tree).

That appears to be in beta still? What store are you using with it?

I'll keep an eye on it! But mobx does more than that, like a lot of component optimizations and utilities. It's been so good for so long, I'm not in a rush to get rid of it.

`useSyncExternalStore` was shipped live in React 18.0 and is fully ready for production use.

Source: I'm the primary Redux maintainer, and worked with Andrew Clark of the React team to nail down the semantics and behavior needed by `useSyncExternalStore` in practice. They had the idea, but discussed a lot of the necessary use cases with us and other lib maintainers, and a lot of its internal implementation is directly related to how React-Redux's `useSelector` hook was implemented already.

I built the first working code that used it by prototyping React-Redux v8's switch from our own internal subscription handling to `useSyncExternalStore` instead and gave Andrew feedback:

- https://github.com/reduxjs/react-redux/pull/1808

valtio is a modern mobx-like implementation that uses `useSyncExternalStore` (via useSnapshot - https://valtio.pmnd.rs/docs/api/basic/useSnapshot).

Its still not quite as nice as MobX, as passing larger objects down the component tree means you need to mix snapshot and non-snapshot state. But it makes me hopeful about `useSyncExternalStore`

There is a schism between "JS influencer Twitter devs" and "Building actual apps devs"

I have stopped taking seriously people that rely on novelty to sell you their next training.

Similarly Angular 1 allowed component oriented development far before react. But people seem to think React invented it.

The problem with angular 1 components (isolated scope directives) was simply that not many understood it as a best practice until far too late, and the framework allowed you to do all kinds of terrible designs like shared scope/multiple controllers per file, which was the more common way to develop. Also performance was poor.

Feel the same way about signals. Vue has been doing it for years (just not tied into render optimizations). I used a similar model via angular 1 back in 2014 even, via watch chains. Much less performant though. Can also do it via redux by defining a special third arg which defines state depenencies (which we do here).

Nothing really novel about a tree-like series of derived states. Interesting that social media is ablaze like its something new

Building 'signal graphs' because views subscribe to err signals is an old hat for re-frame users who are benefiting from a clean and simple mechanism to react to data changes since forever.

Subscriptions grow from the leaf up to the root (which is a single app "db") and so computation is minimal. If something writes to the "db", only relevant subscriptions are recomputed and only views subscribing to those are "repainted".

Works like magic.

Hmmm... attempt to define signals using ReactJS' useState()/useMemo() vocabulary appears as really bad idea.

PReact's signal() as a concept is better - more pure. https://preactjs.com/guide/v10/signals/

I did some intense Linux/POSIX signals work in C over the last year, and expected this article to be about those creatures, since so foundational.

Nope.

Why does Signia exist when there's mobx?

This does not differ from the logic that comes with React hooks. "We've added new magic for you. Everything is done in the background. You just code the way we want.". This logic generally results in new complexities eventually. Because I don't think there's any benefit in trying to hide the obvious complexity in this way.

Signal is a comonad in the category of endofunctors, what's the problem ?

Remember when signals - aka the observer pattern - were a stupid overengineered enterprise design pattern object oriented programmers had to use because they were too dumb to understand functional programming?

Now they're back, baby. Happy to see them used again, even if we've had to razzle it up a bit with a new name.

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