OCaml has full multicore support with algebraic effects now. The effect system makes things like async very nice as there's no function "coloring" problem: https://discuss.ocaml.org/t/ocaml-5-0-0-is-out/10974

But I don't believe the effects are tracked in the type system yet, but that's on it way.

The type system for effects is an ongoing research effort. For now you get unhandled effect exceptions at runtime.

With Multicore OCaml we gained thread sanitizer support and a reasonable memory model. Combined they give you tools for reasoning about data races and finding them. https://ocaml.org/manual/5.3/tsan.html