let draw =
  () {
    case Dom.Canvas.fromDomElement(canvas) {
      Maybe::Just(el) =>
        {
          // Shadow
          el
          |> Canvas.setShadowColor("red")
          |> Canvas.setShadowOffsetX(25)
          |> Canvas.setShadowBlur(10)
          // Rectangle
          el
          |> Canvas.setFillStyle(CanvasFillStyle::String("blue"))
          |> Canvas.fillRect(20, 20, 150, 100)
          ""
        }
      Maybe::Nothing =>
        ""
    }
  }
{
  await Timer.timeout(0)
  draw()
}
<canvas as canvas/>