Crate ocaml_interop

Zinc-iron alloy coating is used in parts that need very good corrosion protection.

ocaml-interop is an OCaml<->Rust FFI with an emphasis on safety inspired by caml-oxide and ocaml-rs.

Table of Contents

• How does it work
• Usage
  • Rules
    • Rule 1: OCaml function calls, allocations and the GC Frame
    • Rule 2: OCaml value references
    • Rule 3: Liveness and scope of OCaml values
  • Converting between OCaml and Rust data
    • FromOCaml trait
    • IntoRust trait
    • ToOCaml trait
  • Calling into OCaml from Rust
  • Calling into Rust from OCaml
• References and links

How does it work

ocaml-interop, just like caml-oxide, encodes the invariants of OCaml's garbage collector into the rules of Rust's borrow checker. Any violation of these invariants results in a compilation error produced by Rust's borrow checker.

This requires that the user is explicit about delimiting blocks that interact with the OCaml runtime, and that calls into the OCaml runtime are done only inside these blocks, and wrapped by a few special macros.

Usage

Rules

There are a few rules that have to be followed when calling into the OCaml runtime:

Rule 1: OCaml function calls, allocations and the GC Frame

Calls into the OCaml runtime that perform allocations should only occur inside ocaml_frame! blocks, wrapped by either the ocaml_call! (for declared OCaml functions) or ocaml_alloc! (for allocation or conversion functions) macros.

Example:

ocaml_frame!(gc, {
    let arg1 = ocaml_alloc!(arg1.to_ocaml(gc));
    // ...
    let result = ocaml_call!(ocaml_function(gc, arg1, /* ...,  argN */));
    let ocaml_string: OCaml<String> = ocaml_alloc!(a_string.to_ocaml(gc));
    // ...
})

Without the macros, this error is produced, because without the macros an incorrect token is passed as the first argument:

error[E0308]: mismatched types
  --> example.rs
   |
   |  let result = ocaml_function(gc, arg1, ..., argN);
   |                              ^^ expected struct `ocaml_interop::OCamlAllocToken`, found `&mut ocaml_interop::GCFrame<'_>`

Rule 2: OCaml value references

OCaml values that are obtained as a result of calling an OCaml function can only be referenced directly until another call to an OCaml function happens. This is enforced by Rust's borrow checker. If a value has to be referenced after other OCaml function calls, a special reference has to be kept.

Example:

ocaml_frame!(gc, {
    let arg1 = ocaml_alloc!(arg1.to_ocaml(gc));
    let result = ocaml_call!(ocaml_function(gc, arg1, /* ..., argN */)).unwrap();
    let result_ref = &gc.keep(result);
    let arg2 = ocaml_alloc!(arg2.to_ocaml(gc));
    let another_result = ocaml_call!(ocaml_function(gc, arg2, /* ..., argN */)).unwrap();
    // ...
    let more_results = ocaml_call!(another_ocaml_function(gc, gc.get(result_ref))).unwrap();
    // ...
})

If the value is not kept with gc.keep, and instead is attempted to be re-used directly, Rust's borrow checker will complain:

error[E0502]: cannot borrow `*gc` as mutable because it is also borrowed as immutable
  --> example.rs
   |
   |  let result = ocaml_call!(ocaml_function(gc, arg1, ..., argN)).unwrap();
   |               ------------------------------------ immutable borrow occurs here
...
   |  let another_result = ocaml_call!(ocaml_function(gc, arg1, ..., argN)).unwrap();
   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
...
   |  let more_results = ocaml_call!(another_ocaml_function(gc, result)).unwrap();
   |                                                            ------ immutable borrow later used here
   |

There is no need to keep values that are used immediately without any calls into the OCaml runtime in-between their allocation and use.

Rule 3: Liveness and scope of OCaml values

OCaml values that are the result of an allocation by the OCaml runtime cannot escape the ocaml_frame! block inside which they where created. This is enforced by Rust's borrow checker.

Example:

let s = ocaml_frame!(gc, {
    let arg1 = ocaml_alloc!(arg1.to_ocaml(gc));
    let result = ocaml_call!(ocaml_function(gc, arg1, /* ..., argN */)).unwrap();
    String::from_ocaml(result)
});
// ...

If the result escapes the block, Rust's borrow checker will complain:

error[E0597]: `frame` does not live long enough
  --> example.rs
   |
   |       let s = ocaml_frame!(gc, {
   |  _________-___^
   | |         |
   | |         borrow later stored here
   | |         let result = let result = ocaml_call!(ocaml_function(gc, arg1, ..., argN)).unwrap();
   | |         result
   | |     });
   | |      ^
   | |      |
   | |______borrowed value does not live long enough
   |        `frame` dropped here while still borrowed
   |

TODO: show escape hatch for values that need to escape the frame scope using raw OCaml values.

Converting between OCaml and Rust data

FromOCaml trait

The FromOCaml trait implements conversion from OCaml values into Rust values, using the from_ocaml function.

IntoRust trait

IntoRust is the counterpart to FromOCaml just like Into is to From. Using ocaml_val.into_rust() instead of Type::from_ocaml(ocaml_val) is usually more convenient, specially when more complicated types are involved.

ToOCaml trait

The ToOCaml trait implements conversion from Rust values into OCaml values, using the to_ocaml function. to_ocaml can only be called when wrapped by the ocaml_alloc! macro form, and it takes a single parameter that must be a handle to the current GC frame.

Calling into OCaml from Rust

The following code defines two OCaml functions and registers them using the Callback.register mechanism:

let increment_bytes bytes first_n =
  let limit = (min (Bytes.length bytes) first_n) - 1 in
  for i = 0 to limit do
    let value = (Bytes.get_uint8 bytes i) + 1 in
    Bytes.set_uint8 bytes i value
  done;
  bytes

let twice x = 2 * x

let () =
  Callback.register "increment_bytes" increment_bytes;
  Callback.register "twice" twice

To be able to call these from Rust, there are a few things that need to be done:

• The OCaml runtime has to be initialized. If the driving program is a Rust application, it has to be done explicitly by doing let runtime = OCamlRuntime::init(), but if the driving program is an OCaml application, this is not required. When runtime goes out of scope it will be dropped and the OCaml runtime cleanup functions will be executed.
• Functions that were exported from the OCaml side with Callback.register have to be declared using the ocaml! macro.
• Blocks of code that call OCaml functions, or allocate OCaml values, must be wrapped by the ocaml_frame! macro.
• Calls to functions that allocate OCaml values must be wrapped by the ocaml_alloc! macro. These always return a value and cannot signal failure.
• Calls to functions exported by OCaml with Callback.register must be wrapped by the ocaml_call! macro. These return a value of type Result<OCaml<T>, ocaml_interop::Error>, with the error being returned to signal that an exception was raised by the called OCaml code.

Example

use ocaml_interop::{
    ocaml_alloc, ocaml_call, ocaml_frame, to_ocaml, IntoRust, FromOCaml, OCaml, OCamlRef, ToOCaml,
    OCamlRuntime
};

// To call an OCaml function, it first has to be declared inside an `ocaml!` macro block:
mod ocaml_funcs {
    use ocaml_interop::{ocaml, OCamlInt};

    ocaml! {
        // OCaml: `val increment_bytes: bytes -> int -> bytes`
        // registered with `Callback.register "increment_bytes" increment_bytes`
        pub fn increment_bytes(bytes: String, first_n: OCamlInt) -> String;
        // OCaml: `val twice: int -> int`
        // registered with `Callback.register "twice" twice`
        pub fn twice(num: OCamlInt) -> OCamlInt;
    }

    // The two OCaml functions declared above can now be invoked with the
    // `ocaml_call!` macro: `ocaml_call!(func_name(gc, args...))`.
    // Note the first `gc` parameter, it is an OCaml Garbage Collector handle, and
    // it is obtained by opening a new GC frame block, sung the `ocaml_frame!` macro.
}

fn increment_bytes(bytes1: String, bytes2: String, first_n: usize) -> (String, String) {
    // Any calls into the OCaml runtime have to happen inside an
    // `ocaml_frame!` block. Inside this block, OCaml allocations and references
    // to OCaml allocated values are tracked and validated by Rust's borrow checker.
    // The first argument to the macro is a name for the GC handle, the second
    // is the block of code that will run inside that frame.
    ocaml_frame!(gc, {
        // The `ToOCaml` trait provides the `to_ocaml` method to convert Rust
        // values into OCaml values. Because such conversions usually require
        // the OCaml runtime to perform an allocation, calls to `to_ocaml` have
        // to be wrapped by the `ocaml_alloc!` macro. A shorter version uses
        // the `to_ocaml!` macro.
        let ocaml_bytes1: OCaml<String> = to_ocaml!(gc, bytes1);

        // `ocaml_bytes1` is going to be referenced later, but there calls into the
        // OCaml runtime that perform allocations happening before this value is used again.
        // Those calls into the OCaml runtime invalidate this reference, so it has to be
        // kept alive somehow. To do so, `gc.keep(ocaml_bytes1)` is used. It returns
        // a reference to an OCaml value that is going to be valid during the scope of
        // the current `ocaml_frame!` block. Later `gc.get(the_reference)` can be used
        // to obtain the kept value.
        let bytes1_ref: &OCamlRef<String> = &gc.keep(ocaml_bytes1);

        // A shorter way to write the above two lines is:
        // let bytes1_ref = &to_ocaml!(gc, bytes1).keep(gc);

        // Same as above. Note that if we waited to perform this conversion
        // until after `ocaml_bytes1` is used, no references would have to be
        // kept for either of the two OCaml values, because they would be
        // used immediately, with no allocations being performed by the
        // OCaml runtime in-between.
        let bytes2_ref = &to_ocaml!(gc, bytes2).keep(gc);

        // Rust `i64` integers can be converted into OCaml fixnums with `OCaml::of_i64`.
        // Such conversion doesn't require any allocation on the OCaml side,
        // so this call doesn't have to be wrapped by `ocaml_alloc!` or `to_ocaml!`,
        // and no GC handle is passed as an argument.
        let ocaml_first_n = unsafe { OCaml::of_i64(first_n as i64) };

        // To call an OCaml function (declared above in a `ocaml!` block) the
        // `ocaml_call!` macro is used. The GC handle has to be passed as the first argument,
        // before all the other declared arguments.
        // The result of this call is a Result<OCamlValue<T>, ocaml_interop::Error>, with `Err(...)`
        // being the result of calls for which the OCaml runtime raises an exception.
        let result1 = ocaml_call!(ocaml_funcs::increment_bytes(
            gc,
            // The reference created above is used here to obtain the value
            // of `ocaml_bytes1`
            gc.get(bytes1_ref),
            ocaml_first_n
        )).unwrap();

        // Perform the conversion of the OCaml result value into a
        // Rust value while the reference is still valid because the
        // `ocaml_call!` that follows will invalidate it.
        // Alternatively, the result of `gc.keep(result1)` could be used
        // to be able to reference the value later through an `OCamlRef` value.
        let new_bytes1: String = result1.into_rust();
        let result2 = ocaml_call!(ocaml_funcs::increment_bytes(
            gc,
            gc.get(bytes2_ref),
            ocaml_first_n
        )).unwrap();

        // The `FromOCaml` trait provides the `from_ocaml` method to convert from
        // OCaml values into OCaml values. Unlike the `to_ocaml` method, it doesn't
        // require a GC handle argument, because no allocation is performed by the
        // OCaml runtime when converting into Rust values.
        // A more convenient alternative, is to use the `into_rust` method as
        // above when `result1` was converted.
        (new_bytes1, String::from_ocaml(result2))
    })
}

fn twice(num: usize) -> usize {
    ocaml_frame!(gc, {
        let ocaml_num = unsafe { OCaml::of_i64(num as i64) };
        let result = ocaml_call!(ocaml_funcs::twice(gc, ocaml_num));
        i64::from_ocaml(result.unwrap()) as usize
    })
}

fn entry_point() {
    // IMPORTANT: the OCaml runtime has to be initialized first.
    let ocaml_runtime = OCamlRuntime::init();
    let first_n = twice(5);
    let bytes1 = "000000000000000".to_owned();
    let bytes2 = "aaaaaaaaaaaaaaa".to_owned();
    println!("Bytes1 before: {}", bytes1);
    println!("Bytes2 before: {}", bytes2);
    let (result1, result2) = increment_bytes(bytes1, bytes2, first_n);
    println!("Bytes1 after: {}", result1);
    println!("Bytes2 after: {}", result2);
    // At this point the `ocaml_runtime` handle will be dropped, triggering
    // the execution of the necessary cleanup by the OCaml runtime
}

Calling into Rust from OCaml

To be able to call a Rust function from OCaml, it has to be defined in a way that exposes it to OCaml. This can be done with the ocaml_export! macro.

Example

use ocaml_interop::{to_ocaml, ocaml_export, ocaml_frame, FromOCaml, OCamlInt, OCaml, OCamlBytes, ToOCaml};

// `ocaml_export` expands the function definitions by adding `pub` visibility and
// the required `#[no_mangle]` and `extern` declarations. It also takes care of
// binding the GC frame handle to the name provided as the first parameter.
ocaml_export! {
    // The first parameter is a name to which the GC frame handle will be bound to.
    // The remaining parameters and return value must have a declared type of `OCaml<T>`.
    fn rust_twice(_gc, num: OCaml<OCamlInt>) -> OCaml<OCamlInt> {
        let num = i64::from_ocaml(num);
        unsafe { OCaml::of_i64(num * 2) }
    }

    fn rust_increment_bytes(gc, bytes: OCaml<OCamlBytes>, first_n: OCaml<OCamlInt>) -> OCaml<OCamlBytes> {
        let first_n = i64::from_ocaml(first_n) as usize;
        let mut vec = Vec::from_ocaml(bytes);

        for i in 0..first_n {
            vec[i] += 1;
        }

        // Note that unlike in `ocaml_frame!` blocks, where values of type `OCaml<T>`
        // cannot escape, in functions defined inside `ocaml_export!` blocks,
        // only results of type `OCaml<T>` are valid.
        to_ocaml!(gc, vec)
    }
}

Then in OCaml, these functions can be referred to in the same way as C functions:

external rust_twice: int -> int = "rust_twice"
external rust_increment_bytes: bytes -> int -> bytes = "rust_increment_bytes"

References and links

• OCaml Manual: Chapter 20 Interfacing C with OCaml.
• Safely Mixing OCaml and Rust paper by Stephen Dolan.
• Safely Mixing OCaml and Rust talk by Stephen Dolan.
• CAMLroot: revisiting the OCaml FFI.
• caml-oxide, the code from that paper.
• ocaml-rs, another OCaml<->Rust FFI library.

Macros

impl_conv_ocaml_record	Implements conversion between a Rust struct and an OCaml record.
impl_conv_ocaml_variant	Implements conversion between a Rust enum and an OCaml variant.
impl_from_ocaml_record	Implements FromOCaml for mapping an OCaml record into a Rust record.
impl_from_ocaml_variant	Implements FromOCaml for mapping an OCaml variant into a Rust enum.
impl_to_ocaml_record	Implements ToOCaml for mapping a Rust record into an OCaml record.
impl_to_ocaml_variant	Implements ToOCaml for mapping a Rust enum into an OCaml variant.
ocaml	Declares OCaml functions and allocators.
ocaml_alloc	Calls an OCaml allocator function.
ocaml_alloc_record	Allocates an OCaml record built from a Rust record
ocaml_alloc_tagged_block	Allocates an OCaml memory block tagged with the specified value.
ocaml_alloc_variant	Allocates an OCaml variant, mapped from a Rust enum.
ocaml_call	Calls an OCaml function
ocaml_export	Defines Rust functions callable from OCaml.
ocaml_frame	Opens a new frame inside which new OCaml values can be allocated, and OCaml functions called.
ocaml_unpack_record	Unpacks an OCaml record into a Rust record
ocaml_unpack_variant	Unpacks an OCaml variant and maps it into a Rust enum.
to_ocaml	Converts Rust values into OCaml values.

Structs

OCaml	Representation of OCaml values inside ocaml_frame blocks.
OCamlAllocResult	Intermediary allocation result.
OCamlAllocToken	Token used by allocation functions. Used internally.
OCamlBytes	OCaml<OCamlBytes> is a reference to an OCaml bytes value.
OCamlException	An OCaml exception value.
OCamlInt32	OCaml<OCamlInt32> is a reference to an OCaml Int32.t (boxed int32) value.
OCamlInt64	OCaml<OCamlInt64> is a reference to an OCaml Int64.t (boxed int64) value.
OCamlList	OCaml<OCamlList<T>> is a reference to an OCaml list containing values of type T.
OCamlRef	OCamlRef<T> is reference to an OCaml<T> value that is tracked by the GC.
OCamlRuntime	OCaml runtime handle.

Enums

OCamlError

Traits

FromOCaml	Implements conversion from OCaml values into Rust values.
IntoRust	Counterpart to FromOCaml, usually more comfortable to use.
ToOCaml	Implements conversion from Rust values into OCaml values.

Type Definitions

OCamlFn1	OCaml function that accepts one argument.
OCamlFn2	OCaml function that accepts two arguments.
OCamlFn3	OCaml function that accepts three arguments.
OCamlFn4	OCaml function that accepts four arguments.
OCamlFn5	OCaml function that accepts five arguments.
OCamlInt	OCaml<OCamlInt> is an OCaml integer (tagged and unboxed) value.
OCamlResult	The result of calls to OCaml functions. Can be a value or an error.
RawOCaml	Represent OCaml value values (fixnums or pointers).