Dear all, A lot has been said about the register/unregister of callbacks. In my opinion a programmer should not have to register/unregister callbacks explicitly. The reason for this is that a decent GUI program already is supposed to do proper subsetting of the available callbacks by a number of means. For instance by opening/closing menu (elements), or enabling/disabling mouse, keyboard, window (element)s, menu(elements)... . It is up to the implementation of the platform what the appropriate action is in order to achieve an efficient implementation. On the X Windows platform this seems to be registering/unregistering of callbacks. The MS Windows platform also does not like to keep track of unnecessary mouse movements. A program must explicitly set a flag in order to receive these events. For Object I/O this flag is always set. In addition, a 'filter' function is associated with the keyboard and mouse callback. Suppose mouse events have type MouseState, then the corresponding filter function is of type (MouseState -> Bool). Before the OS sends a mouse event to the callback, it first checks if it passes the filter, and only if so, sends the event. This call is cheap and does not invoke IO actions (unless you use unsafePerformIO). Regards, Peter