'QU_messages':generate_message/3 extendable


:- multifile 'QU_messages':generate_message/3.

generate_message(+MessageTerm, -S0, -S)

For a given MessageTerm, generates a list composed of Control-Arg pairs and the atom nl. This can be translated into a nested list of Control-Arg pairs, which can be used as input to print_message_lines/3.


MessageTerm term
May be any term.
S0 list of pair
the resulting list of Control-Arg pairs.
S list of pair
the remaining list.


Clauses for generate_message/3 underly all messages from Prolog. They may be examined and altered. They are found in messages(language('QU_messages')), which by default is qplib('embed/english/QU_messages.pl').

The purpose of this predicate is to allow you to totally redefine the content of Prolog's messages. In particular, it is possible to translate all the messages from English into some other language.

This predicate should not be modified if all you want to do is modify or add a few messages: user:generate_message_hook/3 is provided for that purpose.

The Prolog system uses the built-in predicate print_message/2 to print all its messages. When print_message/2 is called, it calls


to generate the message. If that fails,


is called instead.

If generate_message/3 succeeds, L is assumed to have been bound to a list whose elements are either Control-Args pairs or the atom nl. Each Control-Arg pair should be such that the call

     format(user_error, Control, Args)

is valid. The atom nl is used for breaking the message into lines. Using the format specification ~n (new-line) is discouraged, since the routine that actually prints the message (see user:message_hook/3 and print_message_lines/3) may need to have control over newlines.

'QU_messages':generate_message/3 is not included by default in runtime systems, since end-users of application programs should probably not be seeing any messages from the Prolog system. If a runtime system does require the messages facility its source code should include a goal such as:

     :- ensure_loaded(library(

If there is a call to print_message/2 when 'QU_messages':generate_message/3 is undefined, or if generate_message/3 fails for some reason, the message term itself is printed. Here is an example of what happens when generate_message/3 fails.

     | ?- print_message(error,unexpected_error(37)).
     ! unexpected_error(37)

generate_message/3 failed because the message term was not recognized. In the following example print_message/2 is being called by the default exception handler.

     | ?- write(A,B).
     ! Instantiation error in argument 1 of write/2
     ! goal:  write(_2107,_2108)
     | ?- abolish('QU_messages':generate_message/3).
     | ?- write(A,B).
     ! instantiation_error(write(_2187,_2188),1)

Note that a call to 'QU_messages':generate_message/3 simply fails if the predicate is undefined; an existence_error is never signalled.


The following example shows how the output of generate_message/3 is translated and passed to print_message_lines/3.

     gen_message_and_print_lines(Msg, Stream, Prefix) :-
             generate_message(Msg, L, []),
             lines(L, Lines, []),
             print_message_lines(Stream, Prefix, Lines)
     lines([]) --> [].
     lines([H|T]) --> line(H), [nl], lines(T).
     line([]) --> [].
     line([Control-Args|T]) --> [Control-Args], line(T).


When print_message/2 calls 'QU_messages':generate_message/3 it handles any exceptions that arise by printing out an error message. It then writes out the original message.

See Also

print_message/2, message_hook/3, format/[2,3], print_message_lines/3, user:generate_message_hook/3, QU_messages':query_abbreviation/2 ref-msg