
When deciding what to talk about in 2024, I wanted to showcase how powerful
state machines and XState can be by building something complicated, but not
_too_ complicated. That was the balance I was trying to strike. The year before,
I had built a little autocomplete dropdown component for my
[Model Based E2E Testing with State Machines](https://erikras.com/blog/model-based-e2e-testing)
talk, which desipite not being the state machine used in the talk, _was_
implemented with a relatively simple state machine. It seemed like the perfect
amount of complexity and asynchronous loading state. I turned it into a nice 30
minute talk, that I was lucky enough to be able to give at
[CityJS Athens](https://greece.cityjsconf.org/),
[What The Stack](https://wts.sh/), in Skopye, North Macedonia,
[React Alicante](https://reactalicante.es/), and
[React Africa](https://react-africa.com/), in Casablanca, Morocco.

Below I will share some photos of my vists to those places.

## Talk

<video>6yM839PycHU</video>

The code can be found
[on Github](https://github.com/erikras/search-state-machine).

<p>
<img
  src="/images/ui-with-state-machine-chart.png"
  alt="The state chart"
  width={3118}
  height={1580}
/>

<caption>
  The state chart.
</caption>
</p>

## CityJS – Athens, Greece

<p>
<img
  src="/images/2024-athens01.jpg"
  alt="The Acropolis at night"
  width={4032}
  height={3024}
/>

<caption>
  The Acropolis at night.
</caption>
</p>

<p>
<img
  src="/images/2024-athens02.jpg"
  alt="The Temple of Poseidon with Kadi"
  width={4032}
  height={3024}
/>

<caption>
  The Temple of Poseidon with <a href="https://bsky.app/profile/kadi.bsky.social">Kadi</a>.
</caption>
</p>

<p>
<img
  src="/images/2024-athens03.jpg"
  alt="All the speakers at the Temple of Poseidon"
  width={2048}
  height={1536}
/>

<caption>
  All the speakers at the Temple of Poseidon
</caption>
</p>

<p>
<img
  src="/images/2024-athens04.jpg"
  alt="The Parthenon"
  width={2048}
  height={1536}
/>

<caption>
  The <a href="https://en.wikipedia.org/wiki/Parthenon">Parthenon</a>
</caption>
</p>

## React Alicante – Alicante, Spain

<p>
<img
  src="/images/2024-alicante01.jpg"
  alt="Speakers Selfie"
  width={3895}
  height={2720}
/>

<caption>
  Speakers Selfie
</caption>
</p>

<p>
<img
  src="/images/2024-alicante02.jpg"
  alt="Plisplay with Stately"
  width={4032}
  height={3024}
/>

<caption>
  <em>Plisplay</em> with <a href="https://stately.ai/">Stately</a>
</caption>
</p>

<p>
<img
  src="/images/2024-alicante03.jpg"
  alt="Speakers' Night Out"
  width={3528}
  height={1608}
/>

<caption>
  Speakers' Night Out
</caption>
</p>

## What The Stack – Skopje, North Macedonia

<p>
<img
  src="/images/2024-skopje01.jpg"
  alt="Alexander the Great"
  width={4032}
  height={3024}
/>

<caption>
  Statue of Alexander the Great, the most famous Macedonian.
</caption>
</p>

<p>
<img
  src="/images/2024-skopje02.jpg"
  alt="The organizers, Blagica and Darko"
  width={4032}
  height={3024}
/>

<caption>
  The organizers, Blagica and <a href="https://bsky.app/profile/darko.io">Darko</a>
</caption>
</p>

## React Africa – Casablanca, Morocco

<p>
<img
  src="/images/2024-casablanca01.jpg"
  alt="Hassan II Mosque"
  width={4032}
  height={3024}
/>

<caption>
  <a href="https://en.wikipedia.org/wiki/Hassan_II_Mosque">Hassan II Mosque</a>, the must-see building in Casablanca
</caption>
</p>

<p>
<img
  src="/images/2024-casablanca02.jpg"
  alt="Interior of the Hassan II Mosque"
  width={4032}
  height={3024}
/>

<caption>
  Interior of the Hassan II Mosque. As a non-muslim, I was not allowed inside.
</caption>
</p>

<p>
<img
  src="/images/2024-casablanca03.jpg"
  alt="Colorful grains at the market"
  width={3024}
  height={4032}
/>

<caption>
  Colorful grains at the market
</caption>
</p>

<p>
<img
  src="/images/2024-casablanca04.jpg"
  alt="Speakers on a bus"
  width={4032}
  height={3024}
/>

<caption>
  We took a speaker trip to Rabat.
</caption>
</p>

I am so grateful to the organizers that invited me to their conferences,
[Aris](https://x.com/arismarko), [Darko](https://bsky.app/profile/darko.io),
[Victoria](https://bsky.app/profile/vicqr9.bsky.social), and
[Aymen](https://bsky.app/profile/aymenbenamor.bsky.social); I love you all. 🫶

What a great year! I can't wait to do it all again next year. I've already got
my talk planned. Stay tuned...


Building Complex UI with State Machines


The more I play with [Solid](https://www.solidjs.com/), the more I find its
abstraction incredibly fascinating. I think it might be as revolutionary as
React's "just render everything every time and diff the DOMs" approach was back
in 2013. In my own words as a total n00b:

> When you write your JSX with conditionals and props that are
> [signals](https://www.solidjs.com/docs/latest/api#createsignal), Solid is able
> to notice exactly which pieces of your UI depend on which signal value, and
> can rerender _only that dependent UI_ when the signal value changes.

When I wrote my [Proof of Concept](/blog/solid-final-form-poc) using an external
state management system with Solid, I realized that I had to communicate via
signals, i.e. I had to keep the state I wanted to use in the UI in a signal.
This is _vaguely_ analogous to how the way to force a React component to
rerender is to change some part of its state with `setState()`.

> One important difference in Solid, when coming from React, is realizing that
> your "render function" (the component body itself) is **ONLY CALLED ONCE** on
> first render. All UI updates are dependent on signal changes _from inside the
> JSX_.

So let's try and plug my favorite framework agnostic state management library
into Solid!

## XState

The way you use XState is that you create a machine definition of your finite
state machine, and then you "boot up an instance" of that machine using
`interpret()`, which gives you a "service". Once you `.start()` your service,
you can `.send()` events to it, and generally observe it via listeners or
grabbing a snapshot of the state of the service at any given time.

### `select`

The primary way that you can subscribe to one particular slice of the machine's
"context" (where it stores data) is via a "selector", very much
[like Redux](https://redux.js.org/usage/deriving-data-selectors).

### `matches`

The way you determine what state the machine is in is with a `.matches()`
function. From
[the XState docs](https://xstate.js.org/docs/guides/states.html#state-matches-parentstatevalue):

> The `state.matches(parentStateValue)` method determines whether the current
> state.value is a subset of the given `parentStateValue`. The method determines
> if the parent state value “matches” the state value. For example, assuming the
> current `state.value` is `{ red: 'stop' }`:
>
> ```ts
> state.matches("red"); // => true
> state.matches("red.stop"); // => true
> state.matches({ red: "stop" }); // => true
> state.matches("green"); // => false
> ```

### `can`

A predicate that tells you if you "can" send a given event – i.e. that the
machine is currently listening for that event.

Quoth
[the docs](https://xstate.js.org/docs/guides/states.html#state-methods-and-properties):

> The `state.can(event)` method determines whether an `event` will cause a state
> change if sent to the interpreted machine. The method will return `true` if
> the state will change due to the event being sent; otherwise the method will
> return `false`.

This is especially useful for visually disabling buttons that don't do anything
right now.

### `send`

A state machine isn't much good if you can't send events to it, so we're going
to need to expose the `.send()` function as well.

## `useMachine()`

I've written a ~~hook~~ function called `useMachine()` that takes a state
machine definition and then exposes the above API to be consumed in Solid.
`can`, and `matches` use signals internally, and `select` returns the actual
signal to the data selected. Remember that we must use signals if we want Solid
to rerender parts of our UI.

Oh, and it's all strongly typed AF.

This is what it looks like to consume:

```ts
import { Component, Match, Switch } from "solid-js";
import { useMachine } from "./useMachine";
import { trafficMachine } from "./trafficMachine";
import "./App.css";

export const App: Component = () => {
  const { can, matches, select, send } = useMachine(trafficMachine);
  const cycles = select((s) => s.context.cycles); // defines a signal on first render
  return (
    <div class="app">
      <div>
        <div>Cycles: {cycles()}</div>
        <button onClick={() => send("Reset")}>Reset</button>
      </div>
      <div>
        <Switch>
          <Match when={matches("Stopped")}>
            <div>Stopped</div>
            <button disabled={!can("Start")} onClick={() => send("Start")}>
              Start
            </button>
          </Match>
          <Match when={matches("Started")}>
            <div>Started</div>
            <button disabled={!can("Stop")} onClick={() => send("Stop")}>
              Stop
            </button>
          </Match>
        </Switch>
      </div>
      <div>
        <div class="traffic-light">
          <div class={`red ${matches("Started.Red") ? "on" : "off"}`} />
          <div class={`yellow ${matches("Started.Yellow") ? "on" : "off"}`} />
          <div class={`green ${matches("Started.Green") ? "on" : "off"}`} />
        </div>
      </div>
    </div>
  );
};
```

## State Chart

Just a humble traffic light that counts how many times it's gone through `Red`.

<img
  src="/images/xstate-in-solid-state-chart.png"
  alt="Traffic Light State Chart"
  width={2320}
  height={1592}
/>

## Demo

Click "Open Sandbox" to dig into the actual `useMachine()` code.

<iframe
  src="https://codesandbox.io/embed/xstate-solid-ztsy8t?fontsize=14&hidenavigation=1&theme=dark"
  style={{
    width: "100%",
    height: 500,
    border: 0,
    borderRadius: 4,
    overflow: "hidden",
  }}
  title="XState Solid"
  allow="accelerometer; ambient-light-sensor; camera; encrypted-media; geolocation; gyroscope; hid; microphone; midi; payment; usb; vr; xr-spatial-tracking"
  sandbox="allow-forms allow-modals allow-popups allow-presentation allow-same-origin allow-scripts"
></iframe>

If you inspect the DOM, you'll see that _only the nodes that need to change are
changing_.

<img
  src="/images/xstate-in-solid-dom.gif"
  alt="Traffic Light DOM GIF"
  width={642}
  height={228}
/>

## Conclusion

This was mostly a learning exercise for me. I'd be happy to contribute this code
to initiate an official `@xstate/solid` library. The Stately people, that
maintain XState, know how to contact me.

Thanks for reading, and I hope you learned something.


XState in Solid


When Remix was first announced, by the gentlemen behind React Router, I knew it
was gonna rock...eventually. I figured they would need some time to work out the
exact APIs. That it was released initially as a paid offering was all I needed
to ignore it for a bit to let it mature. When it became open source several
months later, my interest was piqued.

I've been using [Next.js](https://nextjs.org) since it was in version 1.0. I
fully understood all that it was doing for me, because I had extensive
experience cobbling together such a site using Webpack, React Router, Express,
Server Side Rendering, Hot Reloading, and all the hot new technologies in 2015.
Even after authoring open source libraries used by millions of companies around
the globe,
[React Redux Universal Hot Example](https://github.com/erikras/react-redux-universal-hot-example)
remains my repository with more Github stars than any other. People are still
starring it, despite all the cobwebs, as I have not touched it in six years.
Every once in a while, someone will come up to me a conference and thank me for
it, telling me that they built and launched and grew a startup using that
starter kit. So I've been starting every project with Next.js and recommending
that everyone do the same for years and years.

All that is to say that I was highly skeptical that any framework could come
along and unseat my beloved Next.js from its pedestal. Nevertheless, I went
through the [Remix Tutorial](https://remix.run/docs/en/v1/tutorials/blog) –
which is different (better) now than it was when I did it – to see what these
Router boys were up to.

Within an hour, I was hooked. I will never start a project with Next.js again.

I've been building websites since _all_ pages were static files. When it was
discovered that you could execute code on the server – I ❤️ U `/cgi-bin`! – to
_dynamically_ change the HTML that was downloaded we were off to the races!

So I also come to Remix with a fundamental understanding of the ...errr...
fundamentals of HTTP and how the web works. Seeing Remix embrace The Platform™
warms my heart. That said, I remember why we all moved away from The
Platform™... because browsers sucked! But now that Internet Explorer is rotting
in its grave, I think it might be just the right time to return to the
fundamentals.

When [Remix Conf 2022](https://remix.run/conf/2022) was announced, I immediately
began brainstorming ideas interesting enough to attract the attention of the
Remix team. In my dayjob at [Centered](https://centered.app/erik), I have been
introducing [XState](https://xstate.js.org) state machines at every opportunity,
and it invariably improves the codebase. There was even one particularly complex
bit of backend logic, accepting a webhook from Linear, where I chose to
implement it as a state machine, and, wouldn't you know it?, it improved the
readability and robustness of the code. Remix is so elegant about how it does
stuff on the backend, I began to wonder if I couldn't come up with some sort of
an app to use a state machine on the backend using Remix.
[My previous talk](./infobip-shift-2021) was about using XState to implement an
e-commerce checkout flow, so what if I could do that entirely on the backend?

Narrator: "He pulled it off."

I wrote the prototype entirely on a transatlantic flight in February, and shared
the code with both the Remix and XState teams. My talk was _sort of_ accepted at
RemixConf. I was chosen as a backup speaker. This was a new concept to me, but
it makes total sense. I was to have all the perks of a speaker, travel,
expenses, hotel, etc., but rather than give my talk in front of the conference
attendees, it would be to an empty auditorium the night before, and the talk
would be released on YouTube with all the others, so future people wouldn't even
know it was to an empty crowd. Great plan! I was just delighted to attend the
conference!

As luck would have it, one of the speakers had some visa issues, and I was
bumped up to give a talk in front of an audience. And it made all the
difference! I've never given a talk with such an enthusiastic audience!

So, without further ado...

## The Talk

<video>r4xI6LPf6iQ</video>

<caption>
  The slides can be found{" "}
  <a
    href="https://speakerdeck.com/erikras/state-machines-on-the-edge-remixconf-2022"
    target="_blank"
  >
    here
  </a>
  , and the code <a
    href="https://github.com/erikras/remix-conf-2022"
    target="_blank"
  >
    here
  </a>, if you so desire.
</caption>

## Photos

<p>
<img
  src="/images/remixconf-2022-2.jpg"
  alt="The audience during my talk"
  width={2500}
  height={1669}
/>

<caption>
  The audience during my talk. I had the perfect slot, right before the morning coffee break.
</caption>
</p>

<p>
<img
  src="/images/remixconf-2022-3.jpg"
  alt="View from the back"
  width={2500}
  height={1668}
/>

<caption>
  The view from the back. That's Kent C. Dodds' pink sleeve on the far left, furiously live tweeting.
</caption>
</p>

<p>
<img
  src="/images/remixconf-2022-4.jpg"
  alt="Fisheye from the stage"
  width={2500}
  height={1669}
/>

<caption>
  Fisheye from the stage.
</caption>
</p>

<p>
<img
  src="/images/remixconf-2022-5.jpg"
  alt="Heroes and friends"
  width={4032}
  height={3024}
/>

<caption>
  A few of my heroes that I now call friends, <a href="https://twitter.com/tannerlinsley">Tanner Linsley</a>, <a href="https://twitter.com/jen_ayy_">Jenny Truong</a>, <a href="https://twitter.com/mattpocockuk">Matt Pocock</a>, <a href="https://twitter.com/DavidKPiano">David Khourshid</a>, and <a href="https://twitter.com/chancethedev">Chance Strickland</a>.
</caption>
</p>

<p>
<img
  src="/images/remixconf-2022-6.jpg"
  alt="Michael Chan and Chance Strickland"
  width={4032}
  height={3024}
/>

<caption>
  Photogenic lads, <a href="https://twitter.com/chantastic">Michael Chan</a> and <a href="https://twitter.com/chancethedev">Chance Strickland</a>.
</caption>
</p>

<p>
<img
  src="/images/remixconf-2022-7.jpg"
  alt="Van Selfie"
  width={4032}
  height={3024}
/>

<caption>
  We morbidly joked about what a loss to the JavaScript community it would be if this van crashed.
</caption>
</p>

<p>
<img
  src="/images/remixconf-2022-8.jpg"
  alt="Scooter Crew"
  width={4032}
  height={3024}
/>

<caption>
  <a href="https://twitter.com/johnlindquist">John Lindquist</a> lent some scooters to <a href="https://twitter.com/jen_ayy_">Jenny Truong</a>, <a href="https://twitter.com/mattpocockuk">Matt Pocock</a>, <a href="https://twitter.com/DavidKPiano">David Khourshid</a>, and me to ride around his neighborhood.
</caption>
</p>

<p>
<img
  src="/images/remixconf-2022-9.jpg"
  alt="Smash Burgers"
  width={4032}
  height={3024}
/>

<caption>
  <a href="https://twitter.com/jlengstorf">Jason Lengstorf</a> taught me the meaning of "smash burgers", and my life will never be the same.
</caption>
</p>

## Salt Lake City – Aerial Shots

<p>
<img
  src="/images/slc-2022-1.jpg"
  alt="Flying into Salt Lake City"
  width={4032}
  height={3024}
/>

<caption>
  As an East Coast boy whose never been to the rocky mountains, the terrain around Salt Lake City was <em>gorgeous</em>!
</caption>
</p>

<p>
<img
  src="/images/slc-2022-2.jpg"
  alt="Salt Lake"
  width={4032}
  height={3024}
/>

<caption>
  Apparently they have a big – dare I say "great"? – lake of salt there.
</caption>
</p>


State Machines on the Edge


It was _so_ nice to go back to an in-person conference! The
[Infobip Shift](https://shift.infobip.com) conference organizers did an amazing
job of making it as safe as possible. Everyone entering the conference venue was
required to show a European COVID vaccination certificate (which I have), or a
negative test from that morning. Then, once inside... _WE COULD REMOVE OUR
MASKS_!!! I've never been so happy to see so many noses and mouths!

It was also great to finally meet in person some folks I've known online for
years, like [Shawn Wang](https://twitter.com/swyx),
[Nader Dabit](https://twitter.com/dabit3),
[Ives van Hoorne](https://twitter.com/CompuIves), and
[Kent C. Dodds](https://twitter.com/kentcdodds). And it was also great to hang
out with some other folks whom I already consider friends, like
[David 🎹](https://twitter.com/DavidKPiano),
[Tomasz Łakomy](https://twitter.com/tlakomy), and
[Kitze](https://twitter.com/thekitze), as well meeting many new friends that I
hope to see again. I'd like to say a special _obrigada_ to
[Atila Fassina](https://twitter.com/atilafassina); we were complete strangers
when we met up at the hotel lobby, but we decided to spend a day together
walking around the historical district of Zadar. We had a great time, talking
not about tech, but about being expat fathers.

The venue for this particular talk was pretty wild. It was in a
[UFO-shaped basketball stadium](https://www.youtube.com/watch?v=yWqQoovu93M),
where I was in the center of the circle, and the audience was all around me at
360º. Because I'm such a big fan of live coding talks, both seeing and giving, I
was a little restricted in the time where I could be facing those behind me, but
I did my best.

## The Talk

<video>0bY9_DasGEg</video>

<caption>
  The slides can be found{" "}
  <a
    href="https://speakerdeck.com/erikras/youre-a-wizard-form-harry"
    target="_blank"
  >
    here
  </a>
  , and the code <a
    href="https://github.com/erikras/potionocean"
    target="_blank"
  >
    here
  </a>, if you so desire.
</caption>

## Photos

<p>
<img
  src="/images/infobip-shift-2021-1.jpg"
  alt="Pre-talk Selfie"
  width={4032}
  height={3024}
/>

<caption>Obligatory Pre-talk Selfie</caption>
</p>

<p>
<img
  src="/images/infobip-shift-2021-2.jpg"
  alt="Front Row Groupies"
  width={4032}
  height={3024}
/>

<caption>
  My front-row groupies: <a href="https://twitter.com/tlakomy">Tomasz Łakomy</a>, <a href="https://twitter.com/jen_ayy_">Jenny Truong</a>, <a href="https://twitter.com/jacobmparis">Jacob Paris</a>, <a href="https://twitter.com/DavidKPiano">David 🎹</a>, and <a href="https://twitter.com/kentcdodds">Kent C. Dodds</a>.
</caption>
</p>

<p>
<img
  src="/images/infobip-shift-2021-3.jpg"
  alt="360º Crowd"
  width={4032}
  height={3024}
/>

<caption>
  360º Crowd. Photo by <a href="https://twitter.com/swyx">Shawn Wang</a>.
</caption>
</p>

<p>
<img
  src="/images/infobip-shift-2021-4.jpg"
  alt="Panorama"
  width={13056}
  height={3918}
/>

<caption>
  It was a <em>huge</em> venue! Photo by <a href="https://twitter.com/swyx">Shawn Wang</a>.
</caption>
</p>

<p>
<img
  src="/images/infobip-shift-2021-5.jpg"
  alt="Flow"
  width={3024}
  height={4032}
/>

<caption>
  I ended with a pitch for my new employer, <a href="https://www.centered.app/">Centered</a>.
  Photo by <a href="https://twitter.com/swyx">Shawn Wang</a>.
</caption>
</p>

<p>
<img
  src="/images/infobip-shift-2021-6.jpg"
  alt="Speaker's Dinner"
  width={4032}
  height={3024}
/>

<caption>
  We had a speakers dinner in an abandoned church, where I enjoyed the company of <a href="https://twitter.com/atilafassina">Atila Fassina</a>, <a href="https://twitter.com/mhevery">Misko Hevery</a>, <a href="https://twitter.com/kentcdodds">Kent C. Dodds</a>, and <a href="https://twitter.com/CompuIves">Ives van Hoorne</a>.
</caption>
</p>

<p>
<img
  src="/images/infobip-shift-2021-7.jpg"
  alt="Ives and Kitze"
  width={4032}
  height={3024}
/>

<caption>
  Me with the legendary <a href="https://twitter.com/CompuIves">Ives van Hoorne</a> and <a href="https://twitter.com/thekitze">Kitze</a>.
</caption>
</p>


You're a Wizard Form, Harry.


It's hard to state just how difficult it is to talk about state management; it
all depends on the context of the conversation. The more I have been thinking
about state machines, and learning about XState, the more I like the way XState
talks about "application state".

In the context (🙄) of state machines:

**state**: which of a finite number of states the machine is currently in. e.g.
`fetching`, `loaded`, `error`.

**context**: the environment in which events are taking place. This includes
which user is logged in, any data that has been loaded, and errors that have
been returned.

Notice how these are the same words that React uses for different things. The
state machine "context" is what you would put into a `useState()` hook. In
Redux, the state machine "context" would be in your store.

Let's build the simplest possible state machine, in my new favorite language,
[ReScript](https://rescript-lang.org):

## The Ubiquitous Counterexample

In ReScript, our states, represented by
[variants](https://rescript-lang.org/docs/manual/latest/variant), can have a
[payload](https://rescript-lang.org/docs/manual/latest/variant#constructor-arguments),
which is where we will place our context. In this case, our context is a simple
integer.

```reason
type state = Idle(int)
type event = Increment | Decrement | Reset

export let initial = Idle(0)

export let transition = (state:state, event:event) =>
  switch (state, event) {
    | (Idle(value), Increment) => Idle(value + 1)
    | (Idle(value), Decrement) => Idle(value - 1)
    | (Idle(_), Reset) => initial
  }
```

Notice that we are always returning to our only state, `Idle`, and our events
are only modifying our context.

<img
  src="/images/single-state-machine.png"
  width={888}
  height={730}
  alt="A single-state state machine"
/>
<caption>American Idle</caption>

What have we created here? A simple **_reducer_**. This is the equivalent, in
TypeScript, of:

```ts
export const initial = 0;
export function transition(
  state: number,
  event: "Increment" | "Decrement" | "Reset"
) {
  switch (event) {
    case "Increment":
      return state + 1;
    case "Decrement":
      return state - 1;
    case "Reset":
      return initial;
  }
}
```

It's the kind of thing that powers Redux or what you might give to a
`useReducer()` hook in React.

I can hear you rolling your eyes already.

### Yeah, so what?

Well, what if we wanted to temporarily disable our counter, so that the value
couldn't be changed? You could add another `disabled: boolean` to your reducer's
`state` value... But if you were implementing it as a state machine, you could
have your `Pause` event shift the machine into the `Paused` state, where it
would ignore the `Increment` and `Decrement` events.

```reason
type state = Idle(int) | Paused(int)
type event = Increment | Decrement | Reset | Pause | Unpause

export let initial = Idle(0)

export let transition = (. state:state, event:event) =>
  switch (state, event) {
    | (Idle(value), Increment) => Idle(value + 1)
    | (Idle(value), Decrement) => Idle(value - 1)
    | (Idle(_), Reset) => initial
    | (Idle(value), Pause) => Paused(value)
    | (Paused(value), Unpause) => Idle(value)
    | (Idle(_), Unpause)
    | (Paused(_), _) => state
  }
```

These are very, very trivially simple examples, but I think there may be a
significant difference "at scale" with a real application.


A reducer is a single-state state machine


After hearing my friend, [David 🎹](https://twitter.com/davidkpiano), go on and
on about [finite] state machines and the value they can provide to us as
frontend programmers, a few months ago, I was able to actually use his library,
[XState](https://xstate.js.org), in anger at work, and the result was nothing
short of glorious!

> It eliminates a whole class of bugs!

👆 This is a phrase that has come out of my mouth very often in the last couple
of years since being baptized into the Holy Church of TypeScript. I have spread
the Good Word far and wide, and even created a meme which I have used multiple
times on PRs with my team.

<s>1308736441753767937</s>

## ReScript

[ReScript](https://rescript-lang.org), the artist formerly known as BuckleScript
and Reason, takes "It eliminates a whole class of bugs" to the next level. Quoth
the docs:

> ReScript has **no** pitfalls, aka the type system is "sound" (the types will
> always be correct). e.g. If a type isn't marked as nullable, its value will
> never lie and let through some undefined value silently. **ReScript code has
> no null/undefined errors.**

Okay, you have my attention! I read through the docs
[one day](https://twitter.com/erikras/status/1370007991202758657) like it was a
_Jack Reacher_ novel, very excited to see the sort of tradeoffs ReScript was
making.

In my n00b experience so far, the interop edges where ReScript must consume –
moreso than provide – third party JavaScript libraries feels a bit awkward...at
least more awkward than modern 2021 TypeScript which has huge community support
(so the comparison is not really fair).

Since acquiring this itch, I've been looking for a simple example project to try
out my ReScript skills.

## Twitter's "Follow" Button

Last week, I noticed an interesting thing about the "Follow" button on Twitter's
webapp. Rather than having four states, `not-following`, `following`,
`not-following-hover`, and `following-hover`, it actually has _five_! There's a
special state when you've just clicked "Follow", but are still hovering over the
button.

<s>1372838015656136708</s>

**Perfect!** This is a job perfect for a state machine, and also tiny enough
that maybe I could implement it in ReScript!

At the time of this writing, there are virtually zero search results that come
back for "XState ReScript". There is one article about ReScript and state
machines called
[Modelling domain with state machines in ReasonML](https://dev.to/margaretkrutikova/modelling-domain-with-state-machines-in-reasonml-n29),
from almost a year ago by
[Margarita Krutikova](https://twitter.com/rita_krutikova), in which she mentions
XState, but codes in ~~ReasonML~~ ReScript. That is what I shall attempt to do,
as well. One could imagine a ReScript interop layer for XState existing
(ReXState? 🦖), but it doesn't seem to at the moment.

## "Follow" Button State Machine

I admit to usually rolling my eyes at the point of David 🎹's talks where he
recommends:

> Start with a sheet of paper or a whiteboard, and draw out the states you need.

"I can see the Matrix and visualize all the states just looking at the code", I
think smugly to myself. However, this time, darned if he wasn't right! I
struggled with considering all the interactions writing just code (plus, this
coding language is foreign to me), so I found great benefit in drawing up my
state machine by hand first. Of course I used
[Excalidraw](https://excalidraw.com/), not paper – I'm not a caveman!

<img
  src="/images/follow-button-state-chart.png"
  width={1480}
  height={801}
  alt="Follow Button Finite State Machine"
/>
<caption>
  The clicks without hovering are there for completeness (and maybe
  accessibility?).
</caption>

Notice that from `not-following-hover`, it's pretty hard to get to
`following-hover` (with red background and "Unfollow" text), when you would
think would be the next state; you have to:

1. Click
2. Mouse out
3. Mouse over again

## ReScript Implementation

### Types

Let's start with our types:

```reason
type state =
  | Following
  | FollowingHover
  | FollowingNeverMouseOut
  | NotFollowing
  | NotFollowingHover
type event = Click | MouseOver | MouseOut
type style = Highlight | Danger | Hover | None
```

The actual Twitter "Follow" button doesn't have a style for `NotFollowingHover`,
but I decided to improve on that and make a `Hover` style.

### `getText()`

What should the text of the button be in each state? Notice that you can "fall
through" just like you can with a `switch-case` statement in JavaScript if you
leave out the `break`.

```reason
@genType
let getText = (state: state) => {
  switch state {
  | NotFollowing
  | NotFollowingHover => "Follow"
  | FollowingHover => "Unfollow"
  | Following
  | FollowingNeverMouseOut => "Following"
  }
}
```

Notice the `@genType`. One fun fact that made me want to attempt adopting
ReScript is that: **_IT GENERATES TYPESCRIPT TYPES FOR YOU_!!!** 🤯

It's not the most beautiful TS ever, but it gives you the typings and then
defers the calls to the generated JavaScript. For example, here's an excerpt of
the TypeScript the code above generated:

```ts
// tslint:disable-next-line:no-var-requires
const followButtonBS = require("./followButton.bs");

// tslint:disable-next-line:interface-over-type-literal
export type state =
  | "Following"
  | "FollowingHover"
  | "NotFollowing"
  | "NotFollowingHover"
  | "FollowingNeverMouseOut";

// tslint:disable-next-line:interface-over-type-literal
export type event = "Click" | "MouseOver" | "MouseOut";

// tslint:disable-next-line:interface-over-type-literal
export type style = "Highlight" | "Danger" | "Hover" | "None";

export const getText: (state: state) => string = function (Arg1: any) {
  const result = followButtonBS.getText($$toRE161065015[Arg1]);
  return result;
};
```

However, more interesting, is the JavaScript that it generates. Check this out:

```js
function getText(state) {
  switch (state) {
    case /* FollowingHover */ 1:
      return "Unfollow";
    case /* NotFollowing */ 2:
    case /* NotFollowingHover */ 3:
      return "Follow";
    case /* Following */ 0:
    case /* FollowingNeverMouseOut */ 4:
      return "Following";
  }
}
```

All nicely documented so you can follow along.

### `getStyle()`

Now we declaratively define how the state relates to the style (think, "CSS
class") of the button.

```reason
@genType
let getStyle = (state: state) => {
  switch state {
  | FollowingHover => Danger
  | Following => Highlight
  | NotFollowing => None
  | NotFollowingHover
  | FollowingNeverMouseOut => Hover
  }
}
```

Pretty similar to `getText()`. Notice that the "fifth state", `FollowingHover`,
is special in both functions.

### The Machine

Here's where it gets fun! The machine is a simple mapping from state to state
via events. This is just the code that represents the arrows in the
drawing...nothing more.

```reason
@genType
let machine = (. state:state, event: event) =>
  switch state {
    | NotFollowing => switch (event) {
      | Click => Following
      | MouseOver => NotFollowingHover
      | MouseOut => state
    }
    | NotFollowingHover => switch event {
      | Click => FollowingNeverMouseOut
      | MouseOut => NotFollowing
      | MouseOver => state
    }
    | FollowingNeverMouseOut => switch event {
      | Click => NotFollowingHover
      | MouseOut => Following
      | MouseOver => state
    }
    | Following => switch event {
      | MouseOver => FollowingHover
      | Click => NotFollowing
      | MouseOut => state
    }
    | FollowingHover => switch event {
      | MouseOut => Following
      | Click => NotFollowingHover
      | MouseOver => state
    }
  }
```

...which compiles nicely – in milliseconds! – to this efficient JS code:

```js
function machine(state, $$event) {
  switch (state) {
    case /* Following */ 0:
      switch ($$event) {
        case /* Click */ 0:
          return /* NotFollowing */ 2;
        case /* MouseOver */ 1:
          return /* FollowingHover */ 1;
        case /* MouseOut */ 2:
          return state;
      }
    case /* FollowingHover */ 1:
      switch ($$event) {
        case /* Click */ 0:
          return /* NotFollowingHover */ 3;
        case /* MouseOver */ 1:
          return state;
        case /* MouseOut */ 2:
          return /* Following */ 0;
      }
    case /* NotFollowing */ 2:
      switch ($$event) {
        case /* Click */ 0:
          return /* Following */ 0;
        case /* MouseOver */ 1:
          return /* NotFollowingHover */ 3;
        case /* MouseOut */ 2:
          return state;
      }
    case /* NotFollowingHover */ 3:
      switch ($$event) {
        case /* Click */ 0:
          return /* FollowingNeverMouseOut */ 4;
        case /* MouseOver */ 1:
          return state;
        case /* MouseOut */ 2:
          return /* NotFollowing */ 2;
      }
    case /* FollowingNeverMouseOut */ 4:
      switch ($$event) {
        case /* Click */ 0:
          return /* NotFollowingHover */ 3;
        case /* MouseOver */ 1:
          return state;
        case /* MouseOut */ 2:
          return /* Following */ 0;
      }
  }
}
```

## Demo Time

The following button (pun intended) is written using the logic described above,
with the actual JavaScript compiled from ReScript.

<div style={{ textAlign: "center", marginTop: "1em" }}>
  <FsmFollowButton />
</div>

Notice how there are two states where it reads "Following", the one right after
you've clicked it, and the button is still gray, and the one after you've moused
away and the button is purple.

So concludes my first of what will hopefully be
[many posts about ReScript](/blog/categories/rescript). Feel free to follow me
on Twitter as I ask questions from people smarter than myself and get help
learning this promising language.


Finite State Machines

Posted in Finite State Machines