A slight detour is useful to describe some related predicates that
can be confusing: To only trace a single or select set of predicates,
the
trace/1 or trace/2
predicates can be used to set a trace point. Even though they use
the same base predicate name trace, these predicates ignore
the leash/1 and visible/1
global settings and don't pause when they trace a port. They really are
a different feature that also happens to do tracing.
A trace point is set on a particular predicate and traces the
ports of that predicate whether or not you are in trace/0
trace mode. Each trace point can trace different ports if the trace/2
variant is used.
?- trace(is_a/2).
% is_a/2: [all]
true.
?- noun(X, rock), adjective(X, color, red).
T Call: is_a(_25702, rock)
T Exit: is_a(rock1, rock)
X = rock1 ;
T Redo: is_a(rock1, rock)
T Exit: is_a(rock2, rock)
false.
Notice that trace mode did not have to be turned on using trace/0
and that this only traced out the ports hit while executing
is_a/2
and that the program was not ever paused.
In fact, if trace mode is turned on while using a trace point, things
get very confusing because the trace point infrastructure itself will be
traced!
?- trace(is_a/2).
% is_a/2: [all]
true.
?- trace.
true.
[trace] ?- noun(X, rock), adjective(X, color, red).
Call: (11) noun(_29318, rock) ? creep
Call: (12) is_a(_29318, rock) ? creep
Call: (13) print_message(debug, frame(user:is_a(_29318, rock), trace(call))) ? creep
Call: (18) push_msg(frame(user:is_a(_29318, rock), trace(call))) ? creep
Call: (21) exception(undefined_global_variable, '$inprint_message', _30046) ? creep
Fail: (21) exception(undefined_global_variable, '$inprint_message', _30090) ? creep
Exit: (18) push_msg(frame(user:is_a(_29318, rock), trace(call))) ? creep
Call: (19) prolog:message(frame(user:is_a(_29318, rock), trace(call)), _30140, _30142) ? creep
Fail: (19) prolog:message(frame(user:is_a(_29318, rock), trace(call)), _30140, _30142) ? creep
Call: (19) message_property(debug, stream(_30192)) ? creep
Fail: (19) message_property(debug, stream(_30192)) ? creep
Call: (20) message_property(debug, prefix(_30200)) ? creep
Fail: (20) message_property(debug, prefix(_30200)) ? creep
T Call: is_a(_29318, rock)
Call: (17) pop_msg ? creep
Exit: (17) pop_msg ? creep
...Lots more after this...
So, trace points are a confusingly named and separate feature
from trace mode.
