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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.
To test this tool, load the
| ?- 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
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
settings-sample.lgt file. Logtalk
queries can be traced using this tool by using the
| ?- gtrace(foo::bar).
| ?- gtrace, foo::bar.
You can also use the
gspy/1 predicate to spy a Logtalk predicate specified
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.
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 predicate
Line number spy points are currently not available when using XSB as the Prolog backend compiler.