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Melange 4.0 is here


Today, we're introducing Melange 4.0, the latest version of our backend for the OCaml compiler that emits JavaScript.

Melange is now compatible with OCaml 5.2! In this release, Melange starts emitting more ES6 constructs, supports more JavaScript builtins, improves error handling, and makes using Dune virtual libraries fully supported. Read on for more details.

Embracing OCaml 5.2

OCaml 5.2 was released just last week. With Melange 4, you can now leverage all the power and enhancements in this latest OCaml version. We've upgraded the OCaml type checker and the standard library to match OCaml 5.2's, ensuring you can select a 5.2-compatible Melange in your projects.

This does not mean Melange is only compatible with OCaml 5.2. As previously mentioned in the Melange 3 announcement, each Melange version ships one release for every compiler version that it supports; for OCaml 5.1, this would correspond to Melange 4.0.0-51.

Full Support for Dune Virtual Libraries

Melange now fully supports Dune virtual libraries, which requires Dune 3.15.2. There were a few bumps in road when we thought it wouldn't be possible to support this use case in Melange. We were thankfully proven wrong (#1067), and we think that virtual libraries can become an interesting way to write libraries that work across Melange and native OCaml, sharing a common interface.

Emitting ES6 and Enhanced JavaScript Interop

In this release, Melange starts emitting ES6, particularly:

  • Melange 4.0 emits let instead of var, and const where possible (#1019, #1059).
  • Emitting let allowed us to remove a bit of legacy code related to compiling for loops, taking advantage of let's lexical scoping. (#1020)

In our effort to expand the API surface area for JavaScript interoperability, we've added new bindings to a few more JavaScript features in the melange.js library (whose main entrypoint is the Js module):

  • Js.Bigint: #1044 added bindings to work with BigInt;
  • The Js.Set and Js.Map modules now bind to even more methods available in these JavaScript data structures (#1047, #1101).
  • JS Iterators: We introduced minimal bindings for JavaScript iterators, making it easier to work with iterable objects, which some other modules' methods can now return (#1060).
  • WeakMap and WeakSet: Bindings for these weakly referenced collections have also been added (#1058).

Improved Error Handling and Code Generation

In this release, we've also made significant improvements in how Melange handles errors and generates JavaScript code:

  • Instead of throwing JavaScript objects with Melange exception payloads, we now emit a Melange-specific error (throw new MelangeError(..)) for more consistent error handling (#1036).
    • An interesting corollary to this change is that catching Melange errors from external JavaScript only needs to check instanceof MelangeError.

Additional Fixes & Enhancements

  • Slimmer Executable: We've removed unnecessary internal code from melange-compiler-libs, making the Melange compiled executable smaller in size and faster to build (#1075).
  • Float Operations: Fixed runtime primitives for Float.{min,max} and related functions, ensuring more accurate mathematical operations (#1050).
  • Warning 51 (wrong-tailcall-expectation): Melange 4 ships with support for enabling warning 51 and triggering the warning when [@tailcall] is used (#1075).


The Melange 4.0 changelog lists all the changes that made it to this release.

Thanks for reading and stay tuned for more updates. Happy hacking!