ReasonML and Parcel (or zero-config type-safe React)

I would like to start this by saying that I’m obviously no expert on either ReasonML or Javascript bundlers: for the last 2 years I’ve been using the (great) create-react-app boilerplate and never thought on moving away… until now. Javascript and Typescript also always met my needs, until JS’s constant unexpected behavior and TS’s really bad type inference started bothering me for real.

I’d also like to tell you that I am, by no means, a expert on front-end development. I’ve started on 2017 and, for almost an year, worked exclusively with ASP.NET Webforms and Django templates.

With that said, let’s dive into the

Wonderful World of ReasonML

Let’s start by creating a folder, then initializing a npm project. Name it whatever you want, I’ll name it “nice-memes”!

mkdir nice-memes
cd nice-memes
npm init -y

Alright, now that we have package.json, let’s install Parcel as a dev dependency:

npm i -D parcel-bundler

And put this in your scrips on package.json:

  "scripts": {
    "start": "NODE_ENV=development parcel src/index.html --open --port 3000"

If you try to run right now… It will break. Why? Because there’s nothing on index.html yet, obviously 🤷‍♂️ So… let’s create our files! Start creating a src folder, an index.html and an Index.re files:

mkdir src
cd src
touch index.html
touch Index.re

Explaining how to write a basic HTML5 file is out of the scope of this post, but your body has to look like this:

  <div id="root"></div>
  <script src="./Index.re"></script>

Wait… WHAT??? Are you telling me that I can just import my Reason file into HTML????

That was exactly what I thought at the time I read the Parcel documentation on ReasonML, and… it works! I mean, try to run npm start right now and go to http://localhost:3000. What do you see? Only a white screen, right? Let’s take care of that.

Let’s start by installing our dependencies, so run npm i -S react react-dom reason-react. If you try to run it right now, you’ll get an error related to bsb. Of course! How could we run ReasonML on the browser without bucklescript? Just run npm i -D bs-platform, create a bsconfig.json in your project’s root directory and put the following content in it:

  "name": "professor-escroto",
  "reason": { "react-jsx": 3 },
  "bsc-flags": ["-bs-super-errors"],
  "sources": [
      "dir": "src",
      "subdirs": true
  "package-specs": [
      "module": "commonjs",
      "in-source": true
  "suffix": ".bs.js",
  "namespace": true,
  "bs-dependencies": ["reason-react"],
  "refmt": 3

One more time, try to npm start your way into glory! You’ll notice that it runs, your Reason file is built, and… you still have a blank page. Because we didn’t write anything on the Index.re, so open it and be ready for some exciting ReasonReact action, putting the following code in it:

module App = {
  let make = () => {
        "I like high quality memes!" -> React.string

  <App />,

Check your browser and… Ta-da, you like high quality memes!

That’s all, folks. Thanks for your attention!

Celso Bonutti Filho
Hey, I'm Celso! I'm a developer from Salvador, Bahia. I created this blog so I can talk about random stuff I like, study or like and study.
I love memes btw.