Linking an OCaml library into a Rust program

The section will cover how to create a program in Rust that calls functions from an OCaml library.

Example project layout:

Cargo.toml
build.rs
src: contains Rust code
lib: contains OCaml code and dune file
dune-project
example.opam

Add the following to your Cargo.toml:

[dependencies]
ocaml = "*"

[build-dependencies]
ocaml-build = {version = "*", features=["dune"]}

And add a build.rs, this example assumes your OCaml library and the following dune file are in lib at the root of your Rust project:

extern crate ocaml_build;
pub fn main() {
  ocaml_build::Dune::new("lib").build()
}

If the dune root is not the root of your project you can use Dune::with_root to set the correct path.

Next you will need to add setup a dune project to handle compilation of your OCaml code. Here is an example dune file that will generate object files that can be linked into your Rust program, in this case the OCaml library is named example:

(executable
 (name example)
 (public_name example)
 (modes exe object))

NOTE: The OCaml code needs to be built with dune in object mode

In lib/example.ml

let hello_world () = "Hello, world"
let () = Callback.register "hello_world" hello_world

In lib/main.rs:

extern crate ocaml;

ocaml::import! {
  fn hello_world() -> String;
}

pub fn main() {
  let gc = ocaml::init(); // Initialize OCaml runtime

  let s = unsafe { hello_world(&gc).unwrap() };
  println!("{s}");
}

NOTE: ocaml::init needs to be called before attempting to access any OCaml functions.

To simplify the full setup process, take a look at rust-ocaml-starter.

ocaml-rs

Linking an OCaml library into a Rust program