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This page is a detailed explanation on how to install Reason with opam, both manually and using a template. There are other options available, such as esy, but we recommend using opam for the best experience. Check the esy installation page if you want to use esy instead.

System requirements

• macOS and Linux are supported natively
• Windows is supported via WSL (https://ocaml.org/docs/ocaml-on-windows)

Install the package manager

opam (short for OCaml Package Manager) is a tool for managing packages and environments. If you haven't installed opam yet, please follow the instructions provided on the official website. Be aware that the initial setup might be notably slow due to compiler retrieval and build processes.

Using a template

This template serves as a solid foundation, integrating essential configurations and structure to expedite your development process. Simply follow the link to access the template and initiate your ReasonML project effortlessly.

We recommend this method for beginners.

Repository melange-re/melange-opam-template

The template comes from the Melange team, since it's a common duo to use Reason with Melange. Melange is a compiler that emits JavaScript, from Reason code. You can find all the information about Melange in their docs: https://melange.re

Setup a new environment manually

opam needs to be initialized. This initialization will create a folder in ~/.opam, where the packages will be stored. This step is only required once.

opam init -y

During opam init, you will be asked if you want to add a hook to your shell to put the tools available in the current opam switch on your PATH.

eval $(opam env)

Create switch

opam is now installed and configured. Before installing Reason, we need to create a switch which is set of packages that are installed in a given compiler version. This is similar to how nvm manages different versions of packages on a Node version.

opam switch create . 5.1.0 --deps-only

For a comprehensive guide on how to create an opam switch, refer to the following link: ocaml.org/docs/installing-ocaml#create-an-opam-switch.

Install reason

opam install reason

Once the installation of the reason package is done you will have available the following tools refmt and rtop and there's no configuration change to enable the build system (dune) to build your code.

Make sure you have installed the latest version of refmt and rtop by running the following command:

refmt --version

Install the build system

We introduced the build-system called dune that Reason projects can use to specify libraries, executables and applications. It’s optimized for monorepos and makes project maintenance easier. Check their docs in case you want to learn more about it: dune.build.

opam install dune

Hello world

To wrap up the installation process, let's create a simple hello world project. With the basic setup done:

Create a file hello.re with the following content:

print_endline("Hello world!");

Create a file dune-project with the following content (check dune.build for latest version):

(lang dune 3.6)

Create an empty file hello.opam

Create a file dune with the following content:

(executable
 (name hello)
 (public_name hello))

Note: dune uniformly uses the .exe extension to build native executables, even on Unix where programs don’t usually have a .exe extension.

The executable stanza is used to define executables and the name field is used to specify the name of the executable (Can run with dune exec src/hello.exe). The public_name field is used to specify the name of the executable when it is installed and allows you to run the executable with hello directly: dune exec hello.

Run the project (this will compile the project and run the executable):

dune exec hello

If you want to build only:

dune build

What's Next?

After you have successfully installed Reason, it's time to set up your editor to get access to all the nice features such as auto-completion. Later on you can check out the language basics to get a basic understanding of all the Reason language constructs.