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Melange

Melange compiles OCaml / Reason to JavaScript

Powered by the versatile OCaml type system, with best-in-class type inference, Melange produces robust JavaScript code.

Sponsored by:

Installation

OPAM

Melange is released to OPAM. Install it with:

$ opam install melange
Template

melange-re/melange-opam-template provides a GitHub template repository that can be used as a project starter.

Esy

Get Esy on NPM:

  • npm install -g esy installs Esy globally
  • if npm is installed, npx esy can be used to run Esy locally

An Esy project starter also exists.

After cloning the template, run esy in the project root.

Nix

Melange has good support for Nix:

  • github:melange-re/melange can be added as a flake input
  • the melange flake provides a melange.overlays.default overlay that adds melange to pkgs.ocamlPackages.melange
  • the melc binary can be run with nix run github:melange-re/melange, e.g. nix run github:melange-re/melange -- --help

OCaml version compatibility

The current Melange distribution targets OCaml 4.14. There's an old version of Melange based on OCaml 4.12 that requires version 4.12.0+mel of melange-compiler-libs.

Editor integration

Melange has first class support in Dune. ocaml-lsp or Merlin works as usual. In VSCode, the VSCode OCaml Platform extension is recommended.

Community

FAQ

How does this project relate to other tools?

This project is forked from the ReScript compiler, focused on a deeper integration with the OCaml ecosystem. This allows sharing code between backend and frontend using Dune's virtual libraries.

Melange also introduces a ReScript compatibility layer to maintain compatibility with ReScript syntax - preserving access to ReScript's package ecosystem.

A small write-up with more details on the motivation behind this project can be found in the announcement blog post.

Below is a quick comparison between Melange and other tools:

Name

Purpose

Dependencies

Notes

Esy

Package manager

Installed with NPM

Obtaining dependencies such as dune or reason

OPAM

Package manager

None

Obtaining dependencies such as dune or reason

Dune

Build tool

Installed with e.g. esy or opam

Composable build tool for OCaml; supports composing custom rules to build any project

Reason

Syntax

Installed with e.g. esy or opam

Alternative syntax to OCaml

Melange

Compiler that emits Script

Esy / OPAM (to install), Dune (to build)

Supports OCaml, Reason and ReScript syntax; derived from ReScript, focused on deeper integration with OCaml

ReScript

The brand around a syntax and a compiler that emits JavaScript

None

Distributed via NPM as prebuilt binaries; previously called BuckleScript

Can I use ReScript syntax?

Yes! ReScript syntax is supported, but ReScript won't have as many features as the OCaml or Reason syntaxes due to ReScript being built on top of an old OCaml version (4.06 - released in 2018). (e.g. letop binding operators, generalized module open expressions, or local substitutions in signatures).

Contributing

See CONTRIBUTING.md.

Acknowledgments

  • Thanks to the ReScript project, its author and maintainer @bobzhang, and its many contributors. Melange is a fork of ReScript, and continues to incorporate patches to ReScript on a regular basis.
  • Thanks to the OCaml team, obviously, without such a beautiful yet practical language, this project would not exist.
  • Thanks to Bloomberg and Facebook. The ReScript project began at Bloomberg and was published in 2016; without the support of Bloomberg, it would not have happened. ReScript was funded by Facebook since July 2017.

See also Credits.md concerning some individual components of Melange.

Licensing

See COPYING and COPYING.LESSER

See Credits for more details.