A mixture of tooling combined to produce JavaScript from OCaml & Reason
991
stars
12,794
commits
OCaml
primary language
Sep 10, 2026
updated
README
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
Check melange.re to get started.
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.
Nix
Melange has good support for Nix:
github:melange-re/melangecan be added as a flake input- the melange flake provides a
melange.overlays.defaultoverlay that adds melange topkgs.ocamlPackages.melange - the
melcbinary can be run withnix run github:melange-re/melange, e.g.nix run github:melange-re/melange/2.0.0 -- --help
OCaml version compatibility
- Melange v3.0 works on OCaml 4.14 (v3-414) and 5.1 (v3-51).
- Melange v2.0 works on OCaml 5.1 only.
- Melange v1.0 can build projects with OCaml >= 4.13 (including OCaml 5.x).
- Editor integration only works on OCaml 4.14, because Melange emits
.cmtartifacts targeting the OCaml 4.14 binary format.
- Editor integration only works on OCaml 4.14, because Melange emits
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
- There's a
#melangechannel in the ReasonML Discord
FAQ
How does this project relate to other tools?
This project is forked from an earlier version of the ReScript compiler, focused on a deeper integration with the OCaml ecosystem. Such integration makes it easy to share code between backend and frontend using e.g. Dune's virtual libraries.
Melange 1.0 includes a ReScript compatibility layer to maintain compatibility with ReScript syntax - preserving access to ReScript's package ecosystem. Both ReScript and Melange have diverged significantly since then, and this compatibility layer was removed in Melange 2.0.
A small write-up with more details on the motivation behind this project can be found in the announcement blog post. Additional write-ups exist at anmonteiro.substack.com
Below is a quick comparison between Melange and other tools:
| Name | Purpose | Dependencies | Notes |
|---|---|---|---|
| OPAM | Package manager | None | Obtaining dependencies such as dune or reason |
| Dune | Build tool | Installed with e.g. opam | Composable build tool for OCaml; supports composing custom rules to build any project |
| Reason | Syntax | Installed with e.g. opam | Alternative syntax to OCaml |
| Melange | Compiler that emits JavaScript | OPAM (to install), Dune (to build) | Supports OCaml and Reason; 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 |
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.
Contributors
(top 30 of 317)
Similar repos
Languages
OCaml
96.5%
Reason
1.5%
Perl
1.0%
A mixture of tooling combined to produce JavaScript from OCaml & Reason
991
stars
12,794
commits
OCaml
primary language
Sep 10, 2026
updated
README
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
Check melange.re to get started.
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.
Nix
Melange has good support for Nix:
github:melange-re/melangecan be added as a flake input- the melange flake provides a
melange.overlays.defaultoverlay that adds melange topkgs.ocamlPackages.melange - the
melcbinary can be run withnix run github:melange-re/melange, e.g.nix run github:melange-re/melange/2.0.0 -- --help
OCaml version compatibility
- Melange v3.0 works on OCaml 4.14 (v3-414) and 5.1 (v3-51).
- Melange v2.0 works on OCaml 5.1 only.
- Melange v1.0 can build projects with OCaml >= 4.13 (including OCaml 5.x).
- Editor integration only works on OCaml 4.14, because Melange emits
.cmtartifacts targeting the OCaml 4.14 binary format.
- Editor integration only works on OCaml 4.14, because Melange emits
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
- There's a
#melangechannel in the ReasonML Discord
FAQ
How does this project relate to other tools?
This project is forked from an earlier version of the ReScript compiler, focused on a deeper integration with the OCaml ecosystem. Such integration makes it easy to share code between backend and frontend using e.g. Dune's virtual libraries.
Melange 1.0 includes a ReScript compatibility layer to maintain compatibility with ReScript syntax - preserving access to ReScript's package ecosystem. Both ReScript and Melange have diverged significantly since then, and this compatibility layer was removed in Melange 2.0.
A small write-up with more details on the motivation behind this project can be found in the announcement blog post. Additional write-ups exist at anmonteiro.substack.com
Below is a quick comparison between Melange and other tools:
| Name | Purpose | Dependencies | Notes |
|---|---|---|---|
| OPAM | Package manager | None | Obtaining dependencies such as dune or reason |
| Dune | Build tool | Installed with e.g. opam | Composable build tool for OCaml; supports composing custom rules to build any project |
| Reason | Syntax | Installed with e.g. opam | Alternative syntax to OCaml |
| Melange | Compiler that emits JavaScript | OPAM (to install), Dune (to build) | Supports OCaml and Reason; 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 |
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.
Similar repos
Contributors
(top 30 of 317)
Languages
OCaml
96.5%
Reason
1.5%
Perl
1.0%