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This file is part of Logtalk
Copyright 1998-2021 Paulo Moura
SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


By default, debug and debug(Group) messages are only printed when the debug flag is turned on. These messages are also suppressed when compiling code with the optimize flag turned on. This tool supports selective enabling of debug and debug(Group) messages in normal and debug modes.

API documentation

This tool API documentation is available at:


For general information on debugging, open in a web browser the following link and consult the debugging section of the User Manual:



This tool can be loaded using the query:

| ?- logtalk_load(debug_messages(loader)).


To test this tool, load the tester.lgt file:

| ?- logtalk_load(debug_messages(tester)).


The tool provides two sets of predicates. The first set allows enabling and disabling of all debug and debug(Group) messages for a given component. The second set allows enabling and disabling of debug(Group) messages for a given group and component for fine-grained control.

Upon loading the tool, all debug messages are skipped. The user is then expected to use the tool API to selectively enable the messages that will be printed. As an example, consider the following object, part of a xyz component:

:- object(foo).

	:- public([bar/0, baz/0]).
	:- uses(logtalk, [print_message/3]).

	bar :-
		print_message(debug(bar), xyz, @'bar/0 called').

	baz :-
		print_message(debug(baz), xyz, @'baz/0 called').

:- end_object.

Assuming the object foo is compiled and loaded in normal or debug mode, after also loading this tool, bar/0 and baz/0 messages will not print any debug messages:

| ?- {debug_messages(loader), foo}.

| ?- foo::(bar, baz).

We can then enable all debug messages for the xyz component:

| ?- debug_messages::enable(xyx).

| ?- foo::(bar, baz).
bar/0 called
baz/0 called

Or we can selectively enable only debug messages for a specific group:

| ?- debug_messages::disable(xyx).

| ?- debug_messages::enable(xyx, bar).

| ?- foo::(bar, baz).
bar/0 called