2.10.2 Trace Mode Example

The trace/0 predicate turns on "trace mode", which, by default, produces a trace and pauses at every port of every predicate to allow inspection of the state of the program. This is normally done from the Prolog console window, but for embedded Prolog systems or when Prolog runs as a daemon it can also be done by getting a prompt via the libssh package.

Note: If the native graphics plugin (XPCE) is available, the commands gtrace/0 and gspy/1 activate the graphical debugger while tdebug/0 and tspy/1 allow debugging of arbitrary threads.

Each goal is printed using the Prolog predicate write_term/2. The style is defined by the Prolog flag debugger_write_options and can be modified using this flag or using the w, p and d commands of the tracer (section 2.10.4.3).

Here's an example debugging session that shows the basic flow. The unify port is off by default since it doesn't add a lot of information in most cases for the command line debugger.

is_a(rock1, rock).
is_a(rock2, rock).
color(rock1, red).

noun(X, Type) :- is_a(X, Type).
adjective(X, color, Value) :- color(X, Value).

?- trace.
true.

[trace]  ?- noun(X, rock), adjective(X, color, red).
   Call: (11) noun(_9774, rock) ? creep

The trace/0 predicate turned on trace mode, which is now indicated at every prompt by [trace] ?-. The initial query provided by the user was noun(X, rock), adjective(X, color, red) which is asking to find a "red rock". Finally: the first port triggered was a Call to the first predicate in the initial query, indicating the engine is about to look for the first rule that matches noun(_9774, rock).

Pressing spacebar, c, or enter caused the tracer to print creep followed by the next trace. There are many additional commands available that are described later in the overview.

is_a(rock1, rock).
is_a(rock2, rock).
color(rock1, red).

noun(X, Type) :- is_a(X, Type).
adjective(X, color, Value) :- color(X, Value).

[trace]  ?- noun(X, rock), adjective(X, color, red).
...
   Call: (12) is_a(_9774, rock) ? creep
   Exit: (12) is_a(rock1, rock) ? creep
   Exit: (11) noun(rock1, rock) ? creep
...

Next, the first clause of noun/2 gets a call trace since the engine is trying to find the next rule that matches is_a(_9774, rock). Since there is a fact that can unify: is_a(rock1, rock), the trace shows exit (i.e. succeeded) along with that value. Since that was the final predicate in the body of noun/2 , noun/2 also gets an exit trace that shows the unified value of its head: noun(rock1, rock).

is_a(rock1, rock).
is_a(rock2, rock).
color(rock1, red).

noun(X, Type) :- is_a(X, Type).
adjective(X, color, Value) :- color(X, Value).

[trace]  ?- noun(X, rock), adjective(X, color, red).
...
   Call: (11) adjective(rock1, color, red) ? creep
   Call: (12) color(rock1, red) ? creep
   Exit: (12) color(rock1, red) ? creep
   Exit: (11) adjective(rock1, color, red) ? creep
   X = rock1 ;
...

Prolog then moved to the next predicate in the initial query: adjective/3 and solved it in a similar way. Since that was the last predicate in the query, an answer was returned. Pressing ; requested the next answer and began Prolog backtracking.

is_a(rock1, rock).
is_a(rock2, rock).
color(rock1, red).

noun(X, Type) :- is_a(X, Type).
adjective(X, color, Value) :- color(X, Value).

[trace]  ?- noun(X, rock), adjective(X, color, red).
...
   Redo: (12) is_a(_9774, rock) ? creep
   Exit: (12) is_a(rock2, rock) ? creep
   Exit: (11) noun(rock2, rock) ? creep
   Call: (11) adjective(rock2, color, red) ? creep
   Call: (12) color(rock2, red) ? creep
   Fail: (12) color(rock2, red) ? creep
   Fail: (11) adjective(rock2, color, red) ? creep
false.

The only choice point to redo (i.e. backtrack over) was the is_a/2 clause of noun/2 since there was one potential match left to attempt to unify: is_a(rock2, rock). This succeeds with an exit trace since it does unify with the redo predicate and causes noun(rock2, rock) to also succeed with exit just as above.

As the traces continue, you can see the fail port get activated for color(rock2, red) since there is no way to prove that predicate and thus the whole query returns false.

Tracing will continue for every query you pose until you enter notrace. to turn off trace mode.