Upcoming OCaml multicore won't expose effects at all. I believe the intention is to finish work on typed effects[1] and only then add it to the language.
They have an effects branch but admitted it won't be ready in time for OCaml 5.0 (supposedly arriving around end of 2021 / early 2022) so they have a separate compiler variant with it where the progress can be tracked.
Last I read, they really want to nail down the entire multi-threaded environment to the last detail first (which mostly means make sure no parallel bugs are introduced + single-threaded code won't become slower; both pretty huge goals).
Last I read, they really want to nail down the entire multi-threaded environment to the last detail first (which mostly means make sure no parallel bugs are introduced + single-threaded code won't become slower; both pretty huge goals).