After a message is parsed, but before the message is printed out,
print_message/2 calls
user:message_hook(+Message,+Severity,+Lines)
where Lines is of the form [Line1, Line2, ...], where each Linei is of the form [Control_1-Args_1,Control_2-Args_2, ...].
In our example,
message_hook(world(flat,wet),error,
[['The world is ~w and ~w.'-[flat,wet]]])
would be called.
If the call to user:message_hook/3 succeeds, print_message/2
succeeds without further processing. Otherwise, the built-in message
display is used.
An example of using a hook to redirect output can be seen
in the reference page for print_message_lines/3.
It is often useful for a message hook to execute some code for its effects, and then fail. This allows other message hooks a chance to run, and allows the printing of the message to proceed normally.
For example, we might want to ring a bell when printing an error message, and print a count of error messages seen so far. This could be done as follows:
:- dynamic error_count/1.
user:message_hook(_, error, _) :-
put(7), % ASCII 7 rings bell
( error_count(N) -> true
; N = 1
),
format('! Error ~w:~n', [N]),
N1 is N+1,
retractall(error_count(_)),
assert(error_count(N1)),
fail.