// Ulteriori informazioni su F# all'indirizzo http://fsharp.net

open System
open System.Drawing
open System.Windows.Forms

let form =
    let temp = new Form(Text = "Ciao!", Height = 600, Width = 800)

    let pointsMasterList = ref []
    let pointsTempList = ref[]
    let mouseDown = ref false
    let pen = ref(new Pen(Color.Black))

    temp.MouseDown.Add(fun _ -> mouseDown := true)

    let leftMouse, rightMouse =
        temp.MouseDown
        |> Event.partition (fun e -> e.Button = MouseButtons.Left)

    leftMouse.Add(fun _ -> pen := new Pen(Color.Black))
    rightMouse.Add(fun _ -> pen := new Pen(Color.Red))

    let mouseUp _ =
        mouseDown := false
        if List.length !pointsTempList > 1 then
            let points = List.toArray !pointsTempList
            pointsMasterList :=
                (!pen, points) :: !pointsMasterList
        pointsTempList := []
        temp.Invalidate()

    let mouseMove (e: MouseEventArgs) =
        pointsTempList := e.Location :: !pointsTempList
        temp.Invalidate()

    let paint (e: PaintEventArgs) =
        if List.length !pointsTempList > 1 then
            e.Graphics.DrawLines
                (!pen, List.toArray !pointsTempList)
        !pointsMasterList
        |> List.iter
            (fun (pen, points) ->
                e.Graphics.DrawLines(pen, points))

    temp.MouseUp |> Event.add mouseUp
    temp.MouseMove
        |> Event.filter(fun _ -> !mouseDown)
        |> Event.add mouseMove

    temp.Paint |> Event.add paint

    temp

Application.Run(form)