monix-run-loop.scala

import monix.execution._
import monix.execution.RunLoop.FrameId

def fibbonacciWithMacros(iterations: Int, callback: BigInt => Unit)
  (implicit s: Scheduler): Unit = {

  def loop(idx: Int, a: BigInt, b: BigInt, frameId: FrameId): Unit =
    if (idx >= iterations) callback(b) else 
      RunLoop.step(frameId) { frameId => loop(idx+1, b, a+b, frameId) }
      
  RunLoop.start(frameId => loop(0, BigInt(0), BigInt(1), frameId))
}

def fibbonacciSimple(iterations: Int, callback: BigInt => Unit)
  (implicit s: Scheduler): Unit = {

  def loop(idx: Int, a: BigInt, b: BigInt, frameId: Int): Unit = 
    if (idx >= iterations) callback(b) else {
      val nextFrameId = (frameId + 1) & s.batchedExecutionModulus
      if (nextFrameId > 0)
        loop(idx+1, b, a+b, nextFrameId)
      else // call-stack full, inserting asynchronous boundary
        s.execute(new Runnable { 
          def run() = loop(idx+1, b, a+b, nextFrameId)
        })
    }
    
  // Starting the loop  
  if (s.batchedExecutionModulus == 0) // execution always async?
    s.execute(new Runnable { def run() = loop(0, BigInt(0), BigInt(0), 0) })
  else
    loop(0, BigInt(0), BigInt(1), 0)
}

// To play with these ...
import monix.execution.Scheduler.Implicits.global