ocaml-rs - OCaml extensions in Rust

ocaml-rs allows for OCaml extensions to be written directly in Rust with no C stubs. It was originally forked from raml, but has been almost entirely re-written thanks to support from the OCaml Software Foundation.

Works with OCaml versions 4.06.0 and up

Please report any issues on github

NOTE: While ocaml-rs can be used safely, it does not prevent a wide range of potential errors or mistakes. It should be thought of as a Rust implementation of the existing C API. ocaml-interop can be used to perform safe OCaml/Rust interop. The latest version of ocaml-rs actually uses ocaml-interop behind the scenes to interact with the garbage collector. ocaml-rs also exports an interop module, which is an alias for ocaml_interop and the two interfaces can be combined if desired.

Getting started

Take a look at the ocaml-rust-starter project for a basic example to help get started with ocaml-rs.

On the Rust side, you will need to add the following to your Cargo.toml:

toml ocaml = "*"

or

toml ocaml = {git = "https://github.com/zshipko/ocaml-rs"}

For macOS you will need also to add the following to your project's .cargo/config file:

toml [build] rustflags = ["-C", "link-args=-Wl,-undefined,dynamic_lookup"]

This is because macOS doesn't allow undefined symbols in dynamic libraries by default.

Additionally, if you plan on releasing to opam, you will need to vendor your Rust dependencies to avoid making network requests during the build phase, since reaching out to crates.io/github will be blocked by the opam sandbox. To do this you should run:

shell cargo vendor

then follow the instructions for editing .cargo/config

Build options

By default, building ocaml-sys will invoke the ocamlopt command to figure out the version and location of the OCaml compiler. There are a few environment variables to control this.

If both OCAML_VERSION and OCAML_WHERE_PATH are present, their values are used without invoking ocamlopt. If any of those two env variables is undefined, then ocamlopt will be invoked to obtain both values.

Defining the OCAML_VERSION and OCAML_WHERE_PATH variables is useful for saving time in CI environments where an OCaml install is not really required (to run clippy for example).

Features

Documentation

https://docs.rs/ocaml

Examples

``rust // Automatically deriveIntoValueandFromValue`

[derive(ocaml::IntoValue, ocaml::FromValue)]

struct Example<'a> { name: &'a str, i: ocaml::Int, }

[ocaml::func]

pub fn incr_example(mut e: Example) -> Example { e.i += 1; e }

[ocaml::func]

pub fn build_tuple(i: ocaml::Int) -> (ocaml::Int, ocaml::Int, ocaml::Int) { (i + 1, i + 2, i + 3) }

[ocaml::func]

pub fn average(arr: ocaml::Array) -> Result { let mut sum = 0f64;

for i in 0..arr.len() {
    sum += arr.get_double(i)?;
}

Ok(sum / arr.len() as f64)

}

// A native_func must take ocaml::Value for every argument and return an ocaml::Value // these functions have minimal overhead compared to wrapping with func

[ocaml::native_func]

pub fn incr(value: ocaml::Value) -> ocaml::Value { let i = value.int_val(); ocaml::Value::int(i + 1) }

// This is equivalent to:

[no_mangle]

pub extern "C" fn incr2(value: ocaml::Value) -> ocaml::Value { ocaml::body!(gc: (value) { let i = value.int_val(); ocaml::Value::int( i + 1) }) }

// ocaml::native_func is responsible for: // - Ensures that #[no_mangle] and extern "C" are added, in addition to wrapping // - Wraps the function body using ocaml::body!

// Finally, if your function is marked [@@unboxed] and [@@noalloc] in OCaml then you can avoid // boxing altogether for f64 arguments using a plain C function and a bytecode function // definition:

[no_mangle]

pub extern "C" fn incrf(input: f64) -> f64 { input + 1.0 }

[cfg(feature = "derive")]

[ocaml::bytecode_func]

pub fn incrf_bytecode(input: f64) -> f64 { incrf(input) } ```

Note: By default the func macro will create a bytecode wrapper (using bytecode_func) for functions with more than 5 arguments.

The OCaml stubs would look like this:

```ocaml type example = { name: string; i: int; }

external increxample: example -> example = "increxample" external buildtuple: int -> int * int * int = "buildtuple" external average: float array -> float = "average" external incr: int -> int = "incr" external incr2: int -> int = "incr2" external incrf: float -> float = "incrf_bytecode" "incrf" [@@unboxed] [@@noalloc] ```

For more examples see test/src or ocaml-vec.

Type conversion

This chart contains the mapping between Rust and OCaml types used by ocaml::func

| Rust type | OCaml type | | ---------------- | -------------------- | | () | unit | | isize | int | | usize | int | | i8 | int | | u8 | int | | i16 | int | | u16 | int | | i32 | int32 | | u32 | int32 | | i64 | int64 | | u64 | int64 | | f32 | float | | f64 | float | | str | string | | [u8] | bytes | | String | string | | Option<A> | 'a option | | Result<A, B> | exception | | (A, B, C) | 'a * 'b * 'c | | &[Value] | 'a array (no copy) | | Vec<A>, &[A] | 'a array | | BTreeMap<A, B> | ('a, 'b) list | | LinkedList<A> | 'a list |

NOTE: Even though &[Value] is specifically marked as no copy, any type like Option<Value> would also qualify since the inner value is not converted to a Rust type. However, Option<String> will do full unmarshaling into Rust types. Another thing to note: FromValue for str and &[u8] is zero-copy, however IntoValue for str and &[u8] creates a new value - this is necessary to ensure the string is registered with the OCaml runtime.

If you're concerned with minimizing allocations/conversions you should use Value type directly.

Pointers to Rust values on the OCaml heap

Pointer<T> can be used to create and access Rust types on the OCaml heap.

For example, for a type that implements Custom:

```rust use ocaml::FromValue;

struct MyType;

unsafe extern "C" fn mytypefinalizer(v: ocaml::Value) { let ptr: ocaml::Pointer = ocaml::Pointer::fromvalue(v); ptr.dropinplace() }

ocaml::customfinalize!(MyType, mytypefinalizer);

[ocaml::func]

pub fn newmytype() -> ocaml::Pointer { ocaml::Pointer::alloccustom(gc, MyType) // ocaml::Pointer::allocfinal(gc, MyType, finalizer, None) can also be used // if you don't intend to implement Custom }

[ocaml::func]

pub fn mytypeexample(t: ocaml::Pointer) { let mytype = t.asmut(); // MyType has no fields, but normally you // would do something with MyType here } ```

Custom exception type

When a Rust panic or Err is encountered it will be raised as a Failure on the OCaml side, to configure a custom exception type you can register it with the OCaml runtime using the name Rust_exception:

```ocaml exception Rust

let () = Callback.registerexception "Rusterror" (Rust "") ```

It must take a single string argument.

Upgrading

Since 0.10 and later have a much different API compared to earlier version, here is are some major differences that should be considered when upgrading: