After a message is parsed, but before the message is printed out,
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:
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
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.