Scala.js Bouncing Taco

Main.scala

import Math._

object ScalaJSExample extends js.JSApp{
  class HTMLImageElement extends dom.raw.HTMLImageElement {
    var onload: js.Function1[dom.Event, _] = ???
  }
  
  def newUnit() = 2 * Math.random
  
  def newMutPlus() = {
    val unit = newUnit()
    (i:Double) => i + unit
  }
  
  def newMutMinus() = {
    val unit = newUnit()
    (i:Double) => i - unit
  }
  
  def main(): Unit = {
    val (h, w) = (Page.canvas.height, Page.canvas.width)
    var x = 0.0
    var y = 0.0
    
    val drawing = dom.document.createElement("img").asInstanceOf[HTMLImageElement]
    drawing.src = "http://3.bp.blogspot.com/_HmGRbHGeQ0o/TT43TfKPwwI/AAAAAAAAGoI/NJBsM3g5OtY/s1600/taco-supreme.jpg"

    var latMut:(Double => Double) = newMutPlus()
    var vertMut:(Double => Double) = newMutPlus()

    drawing.onload = { evt:dom.Event =>
      val shrunkH = drawing.height / 2
      val shrunkW = drawing.width / 2
        dom.setInterval(() => {
          if(x + drawing.width >= w) latMut = newMutMinus()
          else if(x <= 0) latMut = newMutPlus()

          if(y + drawing.height >= h) vertMut = newMutMinus()
          else if(y <= 0) vertMut = newMutPlus()

          Page.renderer.clearRect(0, 0, w, h)
          Page.renderer.drawImage(drawing, x, y)
          x = latMut(x)
          y = vertMut(y)
        }, 10)
    }
  }
}