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.
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.
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
| ?- set_logtalk_flag(debug, on).
But note that loader files may override this flag setting (e.g. by using
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:
debugger_messages.lgt source file defines the default debugger
dump_trace.lgt contains a simple code example on how to redirect
a goal trace to a file.
Alternative debugger tools¶
Logtalk provides basic support for the SWI-Prolog graphical tracer. The
required settings are described in the
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
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
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, Lean Prolog, LVM, SICStus 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
Line number spy points are currently not available when using XSB as the Prolog backend compiler.