Type Elmish.WPF main() function reports binding () -> #Window as an error

Why does my version of the Elmish.WPF Sample NewWindow (XAML code and F# Core) emit the error ...

The type 'unit -> 'a' is not compatible with the type 'Window'.

The same line emits the warning...

This construct causes code to be less generic than indicated by its type annotations. The type variable implied by the use of a '#', '_' or other type annotation at or near <line reference to createWindow_Window2 in the let bindings = statement">

Why am I getting this error and warning?

What I am doing is merging the Elmish.WPF Samples SingleCounter (XAML code and F# Core) and NewWindow (XAML code and F# Core) to have the Model, bindings(), and Msg parts in Program.fs instead of App.fs (as it was in the NewWindow sample).

My goal is to make a SimpleCounter able to open a NewWindow.

The XAML code passes Func<Window2> into the F# code here...

let main mainWindow (createWindow2: Func<#Window>)

...I define bindings as...

let bindings = Platform.bindings createWindow_Window2

I am down to the one compiler error I mentioned above (and that also appears in line beginning "let bindings = ..." below)...

let main mainWindow (createWindow_Window2: Func<#Window>) =

  let logger =
      LoggerConfiguration()
            .MinimumLevel.Override("Elmish.WPF.Update", Events.LogEventLevel.Verbose)
            .MinimumLevel.Override("Elmish.WPF.Bindings", Events.LogEventLevel.Verbose)
            .MinimumLevel.Override("Elmish.WPF.Performance", Events.LogEventLevel.Verbose)
            .WriteTo.Console()
            .CreateLogger()
  let createWindow_Window2 = 
      let window = createWindow_Window2.Invoke()
      window.Owner <- mainWindow
      window

  let bindings = Platform.bindings createWindow_Window2
  WpfProgram.mkProgramWithCmdMsg (fun _ -> m_init, []) update bindings toCmd
    |> WpfProgram.withLogger (new SerilogLoggerFactory(logger))
    |> WpfProgram.startElmishLoop mainWindow

The top of Program.bindings is defined as ...

 let bindings (createWindow_Window2: unit -> #Window) () : Binding<Model, Msg> list = [
        "Window_Window2_Show|> Binding.cmd Window_AboutProduct_Show
        "Window_Window2" |> Binding.subModelWin(
                                                Window_Window2.get >> WindowState.ofOption,
                                                snd,
                                                Window_Window2ct.mapInOutMsg,
                                                Window_Window2_Module.Window_Window2.bindings,
                                                createWindow_Window2,
                                                isModal = true)
...bindings continue but are not relevant to this question...

... and reports no errors!

The compiler seems to think Platform.bindings is expecting unit -> `a when createWindow_Window2 is correctly typed as unit -> #Window.

You can see the call into Program.fs:main is correctly called from App.xaml.cs with a lambda function returning a Window2 as follows...

        private void StartElmish(object sender, EventArgs e)
        {
            this.Activated -= StartElmish;
            Program.main(MainWindow, () => new Window2());
        }

So my question is why am I getting this mismatch type error when it seems I am passing the correct types?

Thank you!

Solution

A big thank you to: Brian Berns, Bent Tranberg, and Tomas Petricek for helping me!

The answer to my problem was astonishingly simple to an expert but to a relative novice only yielded after meticulous comparison with the working Elmish.WPF code for NewWindow.

For easy comparison I have pushed up to rfreytag/Elmish.WPF a version of the NewWindow (XAML code and F# Core) sample that compiles and runs (REMEMBER to build the NewWindow sample).

This version of NewWindow (XAML code and F# Core) when compiled shows the error and warning I reported above …

The type 'unit -> 'a' is not compatible with the type 'Window'. 
See also C:\Workspace\Elmish.WPF\src\Samples\AnotherNewWindow.Core\Program.fs(78,72)-(78,79).

...and the warning...

This construct causes code to be less generic than indicated by its 
type annotations. The type variable implied by the use of a '#', '_' 
or other type annotation at or near 
'C:\Workspace\Elmish.WPF\src\Samples\AnotherNewWindow.Core\Program.fs(78,72)-(78,79)' 
has been constrained to be type 'unit -> 'a'.

To see the precise fix you can compare the working and breaking branches on my copy of Elmish.WPF. Which is that I had forgotten the () following the definition of let createWindow_Window2 = ...

let createWindow_Window2 = 
      let window = createWindow_Window2.Invoke()
      window.Owner <- mainWindow
      window

... returns the function () -> Window while the correct ...

let createWindow_Window2 () = 
      let window = createWindow_Window2.Invoke()
      window.Owner <- mainWindow
      window

... returns the needed Window.

A newbie looks at unit → ‘a and doesn’t immediately recognize it as a function. And of course, functions are not C# Window objects.

Did not help me that the error message doesn’t follow the F# form of...

expecting a <type sought by context> 
but was given a <passed in type>

...which has become familiar from working with the F# compiler.

Not sure why this message diverged from that pattern. Maybe someone can explain that?

The warning is alerting to the less-specific #Window (see # ‘flexible type’ definition) possibly clashing at run-time with the unit → ‘a function.

As usual, learning a new framework means learning the compiler and linker messages.