ReasonReact is built to work with Melange and Reason. This installation guide will help you get started.

If you want to build a new app or a new website with ReasonReact and Melange, we recommend to use a package manager such as opam to download and install the required dependencies.

There are other alternatives available, such as esy or nix. See Melange documentation for details.

• melange v2.0.0 or higher
• dune v3.8.0 or higher

Melange is the toolchain that compiles Reason and OCaml to JavaScript. It integrates with dune to provide a seamless experience for building and running your project.

Follow the getting started guide on Melange's documentation to continue.

Since Reason is an alternative syntax for OCaml, we integrate seamlessly into the official OCaml editor toolchain. Following the recommendation from Melange's documentation.

• For VSCode, we recommend using the vscode-ocaml-platform plugin
• For other editors (Emacs and Vim), we recommend using a language server client plugin of your choice, and pairing it with ocaml-lsp.

Melange opam template comes with the minimum required to build a ReasonReact application with all dependencies up-to-date and ready.

The .re/.ml files compile to straightforward .js files inside your _build folder. You can open index.html directly from the file system. No server needed! Change any file to see that page automatically refreshed.

# clone the repo
git clone https://github.com/melange-re/melange-opam-template my-reason-react-app

# This will initialise the opam switch and install all the dependencies (from both opam and npm)
cd my-reason-react-app && npm run init
# We install native dependencies (melange, dune, reason and reason-react) from opam
# while JavaScript dependencies (react, react-dom, webpack) from npm

# In separate terminals:
npm run watch # It will watch for changes in your Reason/OCaml files and compile them to JavaScript
npm run serve # Serves the application with a local HTTP server

Read more about the template in the opam-template README.

Set up manually is straight forward. Install native dependencies, add some scripts and create a dune file.

1. Install the following dependencies in your opam switch with

opam install melange dune reason reason-react reason-react-ppx -y

2. Enable melange inside dune under a dune-project file in the root of your project with the following:

(lang dune 3.8)
(using melange 0.1)

Check dune docs for all fields available.

3. Create a dune file in the source of your Reason code with the following content:

(melange.emit
  (target melange) ; or any target name you want
  (libraries reason-react)
  (preprocess (pps reason-react-ppx)))

The target name is the name of the folder where the compiled code will live and also an "alias" to run the build commands (see below).

4. Run those commands to build your code:

# "@melange" is an alias created automatically from the target field in the dune file
opam exec -- dune build @melange
# or if you want to watch for changes
opam exec -- dune build @melange --watch

All compiled code will live under _build/default/{{target_name}} folder with the same structure as your source code.

Head to the Intro example guide for a tour of the most important Reason React concepts you will encounter every day.