Adding messages

To add new messages, define clauses for generate_message_hook/3. For instance, to create a new message term indicating that the world is round, one could choose the message term round_world and provide the definition:

     generate_message_hook(round_world) --> ['The world is round.'-[],nl]
     

Please note: The atom nl is used for breaking the message into lines. Using the format specification ~n (new-line) is strongly discouraged, since applications typically require explicit control over new-lines.

Though this was not obvious from the round_world example, message terms parse to a list of Control-Args pairs, where Control is a format specification string, and Args is the list of arguments for the given Control string. To illustrate, if we wanted to say that the world was flat and wet, we could write

     
     user:generate_message_hook(world(X,Y))-->
                            ['The world is ~w and ~w.'-[X,Y],nl].
     
     ?- print_message(help,world(flat,wet)).
     

Here is an example of how one might implement a new exception, bad_font, when the File associated with that font doesn't exist.

     check_font(Font,File):-
         (   file_exists(File)->true
         ;   raise_exception(bad_font(Font, File))
         ).
     
     generate_message_hook(bad_font(Font,File)) -->
         ['Can''t find the file ~w corresponding to font ~w'-
             [File, Font],nl]).
     

Instead of using the message facility, you could use an existing error message in this way:

     check_font1(Font,File):-
         (   file_exists(File)->true
         ;   raise_exception(existence_error(check_font1(Font,File),2,
                                                         file, File,0))
         ).