let draw =
  () {
    case Dom.Canvas.fromDomElement(canvas) {
      Maybe::Just(el) =>
        {
          let dims =
            el
            |> Dom.Canvas.toDomElement
            |> Dom.getDimensions
          // Draw yellow background
          el
          |> Canvas.beginPath
          |> Canvas.setFillStyle(CanvasFillStyle::String("#ff6"))
          |> Canvas.fillRect(0, 0, dims.width, dims.height)
          // Draw blue triangle
          el
          |> Canvas.beginPath
          |> Canvas.setFillStyle(CanvasFillStyle::String("blue"))
          |> Canvas.moveTo(20, 20)
          |> Canvas.lineTo(180, 20)
          |> Canvas.lineTo(130, 130)
          |> Canvas.closePath
          |> Canvas.fill
          // Clear part of the canvas
          el
          |> Canvas.clearRect(10, 10, 120, 100)
          ""
        }
      Maybe::Nothing =>
        ""
    }
  }
{
  await Timer.timeout(0)
  draw()
}
<canvas as canvas/>