scheduleAtFixedRate.scala

import scala.concurrent.duration._

def scheduleTaskAtFixedRate[A](initialDelay: FiniteDuration, period: FiniteDuration, task: Task[A]): Task[Unit] =
  Task.create { (s, callback) =>
    def loop(delay: Long): Task[Unit] =
      Task.defer {
        val startedAt = s.currentTimeMillis()
        val t = task.flatMap { _ =>
          val duration = s.currentTimeMillis() - startedAt
          val nextDelay = {
            val v = period.toMillis - duration
            if (v < 0) 0 else v
          }

          loop(nextDelay)
        }

        if (delay > 0) t.delayExecution(delay.millis) else t
      }

    loop(initialDelay.toMillis)
      .executeWithOptions(_.enableAutoCancelableRunLoops)
      .runAsync(callback)(s)
  }