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
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.
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.
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.
FAQ
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.