let draw =
  () {
    case Dom.Canvas.fromDomElement(canvas) {
      Maybe::Just(el) =>
        {
          el
          |> Canvas.setLineWidth(15)
          |> Canvas.beginPath
          |> Canvas.moveTo(20, 20)
          |> Canvas.lineTo(130, 130)
          |> Canvas.rect(40, 40, 70, 70)
          |> Canvas.stroke
          ""
        }
      Maybe::Nothing =>
        ""
    }
  }
{
  await Timer.timeout(0)
  draw()
}
<canvas as canvas/>