This is what i would like to do;
I know the alternatives using extra variable(s) but i would like it this way.
Just a simple flag 0 or 1 to skip DefWindowProc() but also set a new return value.
This way i can write code more easily and in 1 single line.

Function CallMySetCursorFunction( ByVal hWnd as Long, RetVal As Long PTR ) As Long
    FUNCTION = 1
    @RetVal = %Whatever
End Function
Function wnd_Proc( ByVal hWnd AS LONG, ByVal wMsg AS LONG, ByVal wParam AS LONG, ByVal lParam AS LONG )  As LONG
    Select Case wMsg
        If CallMySetCursorFunction( hWnd, VarPtr( FUNCTION ) ) Then Exit Function
    End Select
    FUNCTION = DefWindowProc( hWnd, wMsg, wParam, lParam )
End Function