This tool provides the default Logtalk command-line debugger. Unlike Prolog systems, the Logtalk debugger is a regular application, using a public API. As a consequence, it must be explicitly loaded by the programmer, either manually at the top-level interpreter or automatically from a settings file.

API documentation

This tool API documentation is available at:



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.


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

| ?- logtalk_load(debugger(tester)).


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. For detailed information on using the debugger, consult the debugging section of the User Manual:


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

The dump_trace.lgt provides a simple solution for dumping a goal trace to a file. For example:

| ?- dump_trace::start_redirect_to_file('trace.txt', some_goal),

A full trace can also be obtained at the top-level by using the S (Skip) command at the call port for the top-levle goal when tracing it.

Alternative debugger tools

Logtalk provides basic support for the SWI-Prolog graphical tracer. The required settings are described in the settings-sample.lgt file. Logtalk queries can be traced using this tool by using the gtrace/0-1 predicates. For example:

| ?- gtrace(foo::bar).


| ?- gtrace, foo::bar.

You can also use the gspy/1 predicate to spy a Logtalk predicate specified as Entity::Functor/Arity when using the graphical tracer. When using this tool, internal Logtalk compiler/runtime predicates and compiled predicates that resulted from the term-expansion mechanism may be exposed in some cases. This issue is shared with Prolog code and results from the non-availability of source code for the predicates being traced.

Known issues

Line number spy points (aka breakpoints) require a Prolog backend compiler that supports accessing read term starting line but only some backends (B-Prolog, GNU Prolog, JIProlog, LVM, SICStus Prolog, SWI-Prolog, Trealla 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.