It's optional to the point that you can write single-threaded version without any io.async/io.concurrent, but you will need to pass the io parameter around, if you want to do I/O. You are mistaking what is called "async" here for what other languages call async/await. It's a very different concept. Async in this context means just "spawn this function in a background, but if you can't, just run it right now".