The Window structure contains functions and structures to create and operate on windows. Many other operations are available by sending messages to windows.

structure Window :
    type HWND and HINSTANCE and HMENU
    type POINT = { x: int, y: int }
    type RECT =  { left: int, top: int, right: int, bottom: int }

    structure Style:
        include BIT_FLAGS
        val WS_OVERLAPPED: flags and WS_POPUP: flags and WS_CHILD: flags and WS_MINIMIZE: flags
        and WS_VISIBLE: flags and WS_DISABLED:flags and WS_CLIPSIBLINGS:flags
        and WS_CLIPCHILDREN:flags and WS_MAXIMIZE:flags and WS_CAPTION:flags
        and WS_BORDER:flags and WS_DLGFRAME:flags and WS_VSCROLL:flags and WS_HSCROLL:flags
        and WS_SYSMENU:flags and WS_THICKFRAME:flags and WS_GROUP:flags and WS_TABSTOP:flags
        and WS_MINIMIZEBOX:flags and WS_MAXIMIZEBOX:flags and WS_TILED:flags and WS_ICONIC:flags
        and WS_SIZEBOX:flags and WS_OVERLAPPEDWINDOW:flags and WS_TILEDWINDOW:flags
        and WS_POPUPWINDOW:flags and WS_CHILDWINDOW:flags

    datatype WindowPositionStyle =
        |    SWP_DEFERERASE
        |    SWP_HIDEWINDOW
        |    SWP_NOACTIVATE
        |    SWP_NOCOPYBITS
        |    SWP_NOMOVE
        |    SWP_NOREDRAW
        |    SWP_NOSIZE
        |    SWP_NOZORDER
        |    SWP_SHOWWINDOW
        |    SWP_OTHER of int

    datatype ShowWindowOptions =
    |    SW_MAXIMIZE
    |    SW_MINIMIZE
    |    SW_RESTORE
    |    SW_SHOW
    |    SW_SHOWNA

    val SW_NORMAL: ShowWindowOptions
    val SW_MAX: ShowWindowOptions

    val ShowWindow: HWND * ShowWindowOptions -> bool

    datatype GetWindowFlags =
    |    GW_HWNDLAST
    |    GW_HWNDNEXT
    |    GW_HWNDPREV
    |    GW_OWNER

    datatype ParentType =
          ChildWindow of {id: int, parent: HWND}
        | PopupWindow of HMENU
        | PopupWithClassMenu

    val GWL_EXSTYLE : int
    val GWL_HINSTANCE : int
    val GWL_HWNDPARENT : int
    val GWL_ID : int
    val GWL_STYLE : int
    val GWL_USERDATA : int

    val AdjustWindowRect : RECT * Style.flags * bool -> RECT
    val AdjustWindowRectEx :  RECT * Style.flags * bool * int -> RECT
    val ArrangeIconicWindows : HWND -> int
    val BringWindowToTop : HWND -> unit
    val CW_USEDEFAULT : int
    val ChildWindowFromPoint : HWND * POINT -> HWND option
    val CloseWindow : HWND -> unit
    val CreateWindow :
       {x: int, y: int, init: 'a, name: string, class: 'a Class.ATOM,
         style: Style.flags, width: int, height: int,
         instance: HINSTANCE, relation: ParentType} -> HWND
    val DestroyWindow: HWND -> unit
    val FindWindow: string * string option -> HWND
    val FindWindowEx: HWND option * HWND option * string * string option -> HWND
    val GetClassName : HWND -> string
    val GetClientRect : HWND -> RECT
    val GetDesktopWindow : unit -> HWND
    val GetForegroundWindow : unit -> HWND
    val GetLastActivePopup : HWND -> HWND
    val GetNextWindow : HWND * GetWindowFlags -> HWND
    val GetParent : HWND -> HWND option
    val GetTopWindow : HWND option-> HWND option
    val GetWindow : HWND * GetWindowFlags -> HWND option
    val GetWindowContextHelpId : HWND -> int
    val GetWindowLong : HWND * int -> int
    val GetWindowRect : HWND -> RECT
    val GetWindowText : HWND -> string
    val GetWindowTextLength : HWND -> int
    val IsChild : HWND * HWND -> bool
    val IsIconic : HWND -> bool
    val IsWindow : HWND -> bool
    val IsWindowVisible : HWND -> bool
    val IsZoomed : HWND -> bool
    val MoveWindow : {x: int, y: int, hWnd: HWND, width: int, height: int, repaint: bool} -> unit
    val OpenIcon : HWND -> unit
    val SetForegroundWindow : HWND -> bool
    val SetParent : HWND * HWND option -> HWND
    val SetWindowContextHelpId : HWND * int -> unit
    val SetWindowLong : HWND * int * int -> int
    val SetWindowPos : HWND * HWND * int * int * int * int * WindowPositionStyle list -> unit
    val SetWindowText : HWND * string -> unit
    val SubclassWindow :
       HWND *
       (HWND * Message.Message * 'a -> Message.LRESULT option * 'a) * 'a -> unit
    val WindowFromPoint : POINT -> HWND option

datatype ParentType = ChildWindow of {id: int, parent: HWND} | PopupWindow of HMENU   |  PopupWithClassMenu
The ParentType datatype is used as one of the arguments to CreateWindow.  PopupWithClassMenu and PopupWindow both create top-level windows.  PopupWindow provides a menu to be used whereas PopupWithClassMenu uses the menu, if any, given when the class was created.  ChildWindow creates a child of the given parent window and provides an id for the child.  All children must have different identifiers.   The identifier is used in WM_NOTIFY messages to notify the parent about changes in the child and can be used to find a child window from the parent using GetDlgItem.



It is possible to replace a window procedure for a window by subclassing the window.  This is useful where a window is required which is almost the same as, say an Edit window, but it needs to process a few messages in a special way.  A new window procedure is installed which process the particular messages but calls the original window procedure for any of the others.

(The terminology is, in my opinion confusing: subclassing a window does not create a new class of window but changes only a single window).  In C this is done using the SetWindowLong function: in ML there is a SubclassWindow function which does this and automatically calls the window procedure of the base class for any messages which are not otherwise processed.