The effect system is here, and it can already be used. However, the integration within the (surface) language and its type system is still a work in progress. Thus to explore the use of effects, you need to use the experimental Effect (https://github.com/ocaml/ocaml/blob/trunk/stdlib/effect.mli) module which gives you access to low-level primitives for defining effects, installing an effect handler and resuming a continuation. The module will be considered as experimental in OCaml 5.0 and subsequent version of OCaml 5 will refine the integration within the language. The objective here was to decouple the multicore runtime work from the higher-level language design questions.