Question

I’m currently working my way through “Real World Functional Programming”. I am trying to get example 1.12 working, a “hello world” program using windows forms. This is the code:-

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

    type HelloWindow() =
         let frm = new Form(Width = 400, Height = 140)
         let fnt = new Font("Times New Roman", 28.0f)
         let lbl = new Label(Dock = DockStyle.Fill, Font = fnt,
                               TextAlign = ContentAlignment.MiddleCenter)
         do frm.Controls.Add(lbl)


         member x.SayHello(name) =
              let msg = "Hello" + name + "!"
              lbl.Text <- msg

         member x.Run() =
              Application.Run(frm);;

    let hello = new HelloWindow();;
    hello.SayHello("you");;
    hello.Run();;

Unfortunately, this throws an error – “Starting a second message loop on a single thread is not a valid operation.” So obviously there is a window opening and not terminating and that is confusing the program. I can’t see how to fix the error, can anyone help me out?

I have also tried inputting the final code block as:-

    let hello = new HelloWindow()
    hello.SayHello("you")
    hello.Run();;

But that does not help. The code runs fine but produces no result with the last line commented out.

Answer 1

The example was meant to compile and run as a Windows Form application. If you would like to run it in F# Interactive, you have to use frm.Show() instead of Application.Run(frm).

You could make the example work both in F# Interactive and in compiled projects using compiler directives:

type HelloWindow() =
    let frm = new Form(Width = 400, Height = 140)
    // ...
    // The same as before

    member x.Run() =
        #if INTERACTIVE
        frm.Show()
        #else
        Application.Run(frm)
        #endif