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This file is part of Logtalk
Copyright 1998-2018 Paulo Moura

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.


This tool provides the default Logtalk command-line debugger.

API documentation

To consult this tool API documentation, open in a web browser the link:


For more information on using the debugger, open in a web browser the following file and consult the debugging section of the User Manual:



This tool can be loaded using the query:

| ?- logtalk_load(debugger(loader)).

Note that this tool cannot be loaded at the same time as other tools (e.g. the ports profiler) that also provide a debug handler, which must be unique in a running session.

When the code to be debugged runs computationally expensive initializations, loading this tool after the code may have a noticeable impact in loading times.


Debugging Logtalk source code (with this debugger) requires compiling source files using the debug(on) compiler flag. For example:

| ?- logtalk_load(my_buggy_code, [debug(on)]).

In alternative, you may also turn on the debug flag globally by typing:

| ?- set_logtalk_flag(debug, on).

But note that loader files may override this flag setting (e.g. by using debug(off) or optimize(on) options for loaded files). If that's the case, you will need to either edit the loader files or write customized loader files enabling debugging.

Logtalk also provides basic support for the SWI-Prolog graphical tracer. The required settings are described in the settings-sample.lgt file.

The debugger_messages.lgt source file defines the default debugger message translations.

The dump_trace.lgt contains a simple code example on how to redirect a goal trace to a file.

Known issues

Line number spy points require a Prolog backend compiler that supports accessing read term starting line but only some systems (B-Prolog, JIProlog, Lean Prolog, SWI-Prolog, and YAP) provide accurate line numbers.

As a workaround, you can check the start line number for an entity predicate definition using a query such as:

| ?- object_property(Entity, defines(Functor/Arity, Properties)).

and checking the returned line_count/1 property to find if there's any offset to the source file number of the predicate clause that you want to trace. This issue, if present, usually only affects the first predicate clause.

Line number spy points are currently not available when using XSB as the Prolog backend compiler.

Other notes

All source files are formatted using tabs (the recommended setting is a tab width equivalent to 4 spaces).