let draw =
  () {
    case Dom.Canvas.fromDomElement(canvas) {
      Maybe::Just(el) =>
        {
          let width =
            el
            |> Canvas.measureText("Hello world")
            |> TextMetrics.width
          el
          |> Canvas.setFont("24px serif")
          |> Canvas.fillText("width: #{width}", 0, 50)
          ""
        }
      Maybe::Nothing =>
        ""
    }
  }
{
  await Timer.timeout(0)
  draw()
}
<canvas as canvas/>