What's possible with Melange


Since dune got the integration with Melange (and Ahrefs migrated to Melange Ahrefs is now built with Melange) I wanted to write about the benefits of using it.

I wrote a blog post comparing Melange with the previous compiler (BuckleScript/ReScript) and what’s possible. I tried to be objetive and focus only on the technical parts of it.

The link to the post is: What's possible with Melange | sancho.dev

Hope you like it, and feel free to ask anything!


Hi! Thanks for the write and for sharing :slight_smile: I posted a question a few days back that seems on topic, but which, afaict, is not answered in your post (which, iiuc, is target at reason/rescript users more than OCaml users):

As a Jsoo user, my naive assumption was that anything that didn’t involve system interactions would compile, but this seems to be the case. I couldn’t find any readily available documentation explaining these limitations tho, nor what is required to make dependencies available for code that comiles via melange. Any pointers here would be helpful and gratefully received :slight_smile:


We should make it probably clearer on the documentation, I will open an issue. For now let me try to draft some requisites from the top of my head.

Making a library “melange ready” there are the following rules:

  • To have all dependencies modes melange
  • To have dune 3.1 and OCaml 5.0/5.1 (depending on Melange’s version)
  • Don’t rely on system dependencies (similar on jsoo)

Having all dependencies modes melange is the biggest difference from jsoo, where you can add modes js in your “entry” and all subsequent libraries will be compiled to js (ofc unless some system dependency is used and no shim is available).

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Having recently moved a code base from rescript to melange: I think it’s just great!
(the server/back end code is in OCaml)

*I tried JSOO and didn’t find sufficient documentation to get started with a typical web FE application, like using it with a bundler, CSS modules, translations etc.
*JS as JavaScript not JaneStreet

Some of the tradeoffs with rescript, that you mentioned

  • Variants representation as strings : I really miss this, it makes interop with css, DOM, JS libraries much simpler
  • uncurry by default : I like uncurry by default (in JS land), the generated code is much more idiomatic and I wish there was a middle ground that both allows interop with OCaml, and doesn’t generate JS with ugly curry calls.

but I would trade all of these to be able to share code between the BE and FE.

Thanks for sharing the lessons learnt for having a bigger code base in Rescript and the limitations that come with that. I’m not a company, so when making choices about technologies to use for personal projects I find such reads very reassuring - I don’t want to be the first person to encounter every typical workflow problem.
(a general issue I find with OCaml is there’s not enough noise from companies using it - is it used successfully for web applications, am I going to be the first person to need to solve translations etc.)

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Thanks for reading and feedback.

I found that the web is very extensive, heavily dependent on specific use cases, and uniquely different from one another.

There’s definitively not a bunch of companies sharing their solutions to the world when they work with ReScript/Melange, and I only can talk about mine.

Most frontend problems can be solved with the same fashion as the JavaScript ecosystem. In ahrefs we have worked on some of them ourselves, as an example: translations are solved by GitHub - ahrefs/melange-react-intl: ocaml ppx to reduce boilerplate from react-intl messages. JSON encoding decoding by GitHub - ahrefs/atd: Static types for JSON APIs and we maintain some bindings.

With our implication on the “ecosystem” we hope to get more efforts into making it bigger/better/stable sooner.

Hope it helps.