2.10.5 Trace Mode vs. Trace Point

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.