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Interface programming is based on callback functions which provide a
response to interrupts or events in the underlying system environment. These
are processed in qtHaskell via the normal event handling loop mechanisms
provided by Qt. In most cases a call to qApplicationExec ()
or exec someQDialogObject () is all that is
needed to start event processing in a qtHaskell program.
The term "callback function" will be used in the following text in the general sense to refer to any Haskell functions which may be passed to the underlying C++ wrapper classes via the Haskell Foreign Function Interface. There are three categories of callback functions that can be specified in qtHaskell programs:
Slots. These are activated by Signals in the Qt MetaObject system;
Handlers. These correspond to virtual functions in the C++ definition of Qt Classes.
User Defined Functions. These are similar to handlers except that they allow the user to create a new virtual function for a class rather than overriding an existing virtual function.
Note that all virtual methods of a Qt class can be overridden by specifying a handler function in qtHaskell even though the method does not correspond to a system event, i.e. the method is only called from the application's own code. This is the case for all User Defined Functions.