lwt.rs

ocaml::import! {
    fn lwt_poll(x: ocaml::Value) -> Option<ocaml::Value>;
}

pub struct Lwt<'a, T> {
    value: ocaml::Value,
    rt: &'a ocaml::Runtime,
    _t: std::marker::PhantomData<T>,
}

impl<'a, T: ocaml::FromValue + Unpin> std::future::Future for Lwt<'a, T> {
    type Output = Result<T, ocaml::Error>;

    fn poll(
        self: std::pin::Pin<&mut Self>,
        _ctx: &mut std::task::Context,
    ) -> std::task::Poll<Self::Output> {
        let p = std::pin::Pin::get_mut(self);

        let x = unsafe { lwt_poll(&p.rt, p.value.clone()) };
        match x {
            Ok(Some(x)) => std::task::Poll::Ready(Ok(ocaml::FromValue::from_value(x))),
            Ok(None) => std::task::Poll::Pending,
            Err(e) => std::task::Poll::Ready(Err(e)),
        }
    }
}

rust_lwt.ml

let () = 
  Callback.register "lwt_poll" (Lwt.poll