With algebraic effect handlers (the concurrency story of multicore OCaml), you can write your I/O code in direct-style, but retain the advantage of non-blocking I/O. As such, you have the advantage of goroutines, but this is also an opt-in. The asynchronous I/O is implemented as a library and is not baked into the language, and hence you can write your own I/O library. For more details, see our recent draft paper: http://kcsrk.info/papers/system_effects_may_17.pdf.