let draw =
  () {
    case Dom.Canvas.fromDomElement(canvas) {
      Maybe::Just(el) =>
        {
          let imageData =
            Canvas.createImageData(el, 100, 100)
          Array.range(0, 100 * 100 - 1)
          |> Array.mapWithIndex(
            (val : Number, index : Number) {
              ImageData.set(imageData, index * 4, 190)
              ImageData.set(imageData, index * 4 + 1, 0)
              ImageData.set(imageData, index * 4 + 2, 210)
              ImageData.set(imageData, index * 4 + 3, 255)
              val
            })
          el
          |> Canvas.putImageData(imageData, 20, 20)
          ""
        }
      Maybe::Nothing =>
        ""
    }
  }
{
  await Timer.timeout(0)
  draw()
}
<canvas as canvas/>