2.10.3 Trace Mode Options: leash/1 and visible/1
When you enable trace mode with trace/0, the tracer will, by default, pause and wait for a command at every port it hits on every predicate. The leash/1 predicate can be used to modify the ports to pause at. This is a global setting, so changes will remain until they are changed again or SWI-Prolog is restarted. Disabling the tracer via notrace/0 doesn't affect which ports are leashed.
The leash/1 argument
must start with +
to add, or -
to remove,
followed by the name of a port such as call
, exit
,
etc. There are special terms like all
which can be used
instead of manually adding or removing every port.
To stop only at the fail port, use leash/1 like this:
?- leash(-all). true. ?- leash(+fail). true. ?- trace. true. [trace] ?- noun(X, rock), adjective(X, color, red). Call: (11) noun(_3794, rock) Call: (12) is_a(_3794, rock) Exit: (12) is_a(rock1, rock) Exit: (11) noun(rock1, rock) Call: (11) adjective(rock1, color, red) Call: (12) color(rock1, red) Exit: (12) color(rock1, red) Exit: (11) adjective(rock1, color, red) X = rock1 ; Redo: (12) is_a(_3794, rock) Exit: (12) is_a(rock2, rock) Exit: (11) noun(rock2, rock) Call: (11) adjective(rock2, color, red) Call: (12) color(rock2, red) Fail: (12) color(rock2, red) ? creep Fail: (11) adjective(rock2, color, red) ? creep false.
Now, only the lines that start with "Fail:" have "creep" after them
because that was the only time the tracer paused for a command. To never
pause and just see all the traces, use leash(-all)
and
don't turn any ports back on.
The default ports are still printed out because a different setting,
visible/1, controls
which ports are printed. visible/1
takes the same form of argument as leash/1.
To only stop and show the fail
port, use
leash/1 and visible/1
like this:
?- leash(-all). true. ?- leash(+fail). true. ?- visible(-all). true. ?- visible(+fail). true. ?- trace. true. [trace] ?- noun(X, rock), adjective(X, color, red). X = rock1 ; Fail: (12) color(rock2, red) ? creep Fail: (11) adjective(rock2, color, red) ? creep false.