Callbacks can optionally specify a list of field values that are to be unified with the contents of the X11 event record, and/or used as arguments to the callback goal. ProXL handles all the conversion between the C objects and data structures that the X server delivers, and the Prolog objects and data structures that your callback is given. Thus, your application is given symbolic atoms instead of numbers whenever possible, and furthermore, it is safe for your application to assert these objects in the data base.
This section describes the field specifications that can be used for each event and the possible values that these values can take. See the X11 documentation for more details on what individual values mean.