Handling of float arrays about ocaml-rs

[LaurentMazare]

Ah thanks, not sure how I managed to miss this one but that looks like exactly what I would want :)

[zshipko]

Just opened a PR: #89

[zshipko]

I'm not opposed to adding a C file to ocaml-sys, I was trying to avoid it for a while but it's probably a better idea than rewriting the macros as Rust macros. This instance is a also little more complicated than the others since it depends on FLAT_FLOAT_ARRAY.

I made a branch with your proposed additions: https://github.com/zshipko/ocaml-rs/tree/float-array

This branch defines ocaml_sys::FLAT_FLOAT_ARRAY and adds sys/src/ocaml-sys.c which contains wrappers for Double_val, Store_double_val, Double_field and Store_double_field (see https://github.com/zshipko/ocaml-rs/blob/float-array/sys/src/mlvalues.rs#L143)

Does that work for you? Or did you have something else in mind?

[LaurentMazare]

That looks great and I think would fit my use case.
One thing I find a bit surprising is that there doesn't seem to be any function in mlvalues.h that would allocate a float array from the C side using the appropriate representation (i.e. with a Double_array_tag if FLAT_FLOAT_ARRAY is set and a 0 tag otherwise) but it feels that it's easy to do this on the Rust side using the newly added ocaml_sys::FLAT_FLOAT_ARRAY constant. Maybe it would be worth having some helper function for this?

[zshipko]

It looks like there is CAMLextern value caml_alloc_float_array (mlsize_t len); in alloc.h, I will add that as well.

[LaurentMazare]

Thanks, I only plan on using the ocaml-sys bit that your PR is introducing, but maybe at some point you also want to use this in the ocaml-rs part when converting a Vec<f64> to an OCaml float array. It seems that currently this would result in an array of boxed floats and somehow the OCaml runtime manages to use it but I ran into some issues while doing the same thing with the new flambda backend.

[zshipko]

Thanks, I will take a look at that before I merge this!

[zshipko]

FYI, I just released ocamlsys` 0.23 with these changes

[LaurentMazare]

Great, thanks!