let circle =
  Path2D.create()
  |> Path2D.arc(150, 75, 50, 0, 2 * `Math.PI`)
let draw =
  () {
    case Dom.Canvas.fromDomElement(canvas) {
      Maybe::Just(el) =>
        {
          el
          |> Canvas.setFillStyle(CanvasFillStyle::String("red"))
          |> Canvas.fillPath(circle)
          ""
        }
      Maybe::Nothing =>
        ""
    }
  }
// Listen for mouse moves
let onMouseMove =
  (e : Html.Event) {
    case Dom.Canvas.fromDomElement(canvas) {
      Maybe::Just(el) =>
        {
          let dims =
            el
            |> Dom.Canvas.toDomElement
            |> Dom.getDimensions
          let x =
            e.clientX - dims.left
          let y =
            e.clientY - dims.top
          // Check whether point is inside circle
          let isPointInPath =
            el
            |> Canvas.isPointInPath(x, y, CanvasFillRule::Nonzero, Maybe::Just(circle))
          let color =
            CanvasFillStyle::String(
              if isPointInPath {
                "green"
              } else {
                "red"
              })
          el
          |> Canvas.setFillStyle(color)
          |> Canvas.clearRect(0, 0, dims.width, dims.height)
          |> Canvas.fillPath(circle)
          ""
        }
      Maybe::Nothing =>
        ""
    }
  }
{
  await Timer.timeout(0)
  draw()
}
<canvas as canvas onMouseMove={onMouseMove}/>