Multicore upstreaming wasn't blocked on having fully-fledged effects. If you look at the diff between multicore 5.00 and trunk OCaml, the changes required for f

Multicore upstreaming wasn't blocked on having fully-fledged effects. If you look at the diff between multicore 5.00 and trunk OCaml, the changes required for fibers is pretty small relative to the multicore GC and making the rest of the runtime thread-safe. The original plan was to upstream only the multicore GC. This was sped up on the suggestion of the core developers and now 5.0 will bring parallelism and effect handlers (though without syntactic support for the latter). https://discuss.ocaml.org/t/multicore-ocaml-september-2021-effect-handlers-will-be-in-ocaml-5-0/8554 https://discuss.ocaml.org/t/multicore-ocaml-september-2021-e... has a good explanation of effect handlers, syntax and what will be available in 5.0.