let draw =
  () {
    case Dom.Canvas.fromDomElement(canvas) {
      Maybe::Just(el) =>
        {
          let dims =
            el
            |> Dom.Canvas.toDomElement
            |> Dom.getDimensions
          // Create clipping path
          let region =
            Path2D.create()
            |> Path2D.rect(80, 10, 20, 130)
            |> Path2D.rect(40, 50, 100, 50)
          // Apply the clipping path
          el
          |> Canvas.clipPath(region, CanvasFillRule::EvenOdd)
          // Draw stuff that gets clipped
          el
          |> Canvas.setFillStyle(CanvasFillStyle::String("blue"))
          |> Canvas.fillRect(0, 0, dims.width, dims.height)
          ""
        }
      Maybe::Nothing =>
        ""
    }
  }
{
  await Timer.timeout(0)
  draw()
}
<canvas as canvas/>