Draw ellipses on WPF on button click

#r @"PresentationCore"
#r @"PresentationFramework"
#r @"WindowsBase"
#r @"System.Xaml"
#r @"UIAutomationTypes"

open System
open System.Windows
open System.Windows.Media
open System.Windows.Shapes
open System.Windows.Controls

let window = Window(Height = 400.0, Width = 400.0)
window.Title <- "Draw test"
let stackPanel = StackPanel()
window.Content <- stackPanel
stackPanel.Orientation <- Orientation.Vertical

let button1 = Button()
button1.Content <- "Click me to draw a blue ellipse"
stackPanel.Children.Add button1

let button2 = Button()
button2.Content <- "Click me to draw a red ellipse"
stackPanel.Children.Add button2

let clearButton = Button()
clearButton.Content <- "Click me to clear the canvas"
stackPanel.Children.Add clearButton

let canvas = Canvas()
canvas.Width <- window.Width
canvas.Height <- window.Height
stackPanel.Children.Add canvas

let buildEllipse height width fill stroke =
    let ellipse = Ellipse()
    ellipse.Height  <- height
    ellipse.Width   <- width
    ellipse.Fill    <- fill
    ellipse.Stroke  <- stroke
    ellipse

let ellipse1 = buildEllipse 100.0 200.0 Brushes.Aqua Brushes.Black
Canvas.SetLeft(ellipse1, canvas.Width / 10.0)
Canvas.SetTop(ellipse1, canvas.Height / 10.0)
let ellipse2 = buildEllipse 200.0 100.0 Brushes.Red Brushes.DarkViolet
Canvas.SetLeft(ellipse2, canvas.Width / 4.0)
Canvas.SetTop(ellipse2, canvas.Height / 5.0)

let addEllipseToCanvas (canvas:Canvas) (ellipse:Ellipse) =
    match canvas.Children with
    | c when c.Contains ellipse ->
        canvas.Children.Remove ellipse
        canvas.Children.Add(ellipse) |> ignore //needs to be removed and readded or the canvas complains
    | _ ->
        canvas.Children.Add(ellipse) |> ignore

button1.Click.Add(fun _ -> addEllipseToCanvas canvas ellipse1)
button2.Click.Add(fun _ -> addEllipseToCanvas canvas ellipse2)
clearButton.Click.Add(fun _ -> canvas.Children.Clear())

#if INTERACTIVE
window.Show()
#else
[<EntryPoint; STAThread>]
let main argv =
    let app = new Application()
    app.Run(window)
#endif