ZioRateLimiter.scala

private def runQueue(data: RateLimiterQueue[IO[Nothing, Unit]], 
                     queue: IOQueue[RateLimiterMsg]): IO[Nothing, Unit] = {
  queue
    // (1) take a message from the queue (or wait until one is available)
    .take
    // (2) modify the data structure accordingly
    .map {
      case ScheduledRunQueue => data.notScheduled
      case Schedule(t)       => data.enqueue(t)
    }
    // (3) run the rate limiter queue: obtain the rate-limiter-tasks to be run
    .map(_.run(System.currentTimeMillis()))
    .flatMap {
      case (tasks, d) =>
        tasks
          // (4) convert each rate-limiter-task to an IO
          .map {
            case Run(run)         => run
            case RunAfter(millis) => 
              IO.sleep[Nothing](millis.millis)
                .flatMap(_ => queue.offer(ScheduledRunQueue))
          }
          // (5) fork each converted IO so that it runs in the background
          .map(_.fork[Nothing])
          // (6) sequence a list of IOs which spawn background fibers
          // into one big IO which, when run, will spawn all of them
          .sequence_
          .map(_ => d)
    }
    // (7) recursive call to handle the next message,
    // using the updated data structure
    .flatMap(d => runQueue(d, queue))
}