Back to trace mode features: Because the tracing output of a Prolog
program can often be quite large, sometimes it is useful to start trace
mode at a particular point deep in the program. This is what a spy
point is for. It specifies a predicate that should turn on trace
mode.
A spy point is enabled like this: spy(mypredicate/2).
After that command, the first time mypredicate/2 is encountered, trace
mode will turn on and work just like it does normally. This includes
paying attention to the global leash/1
and visible/1
settings. The spy point can be removed using nospy/1
or nospyall/0.
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).
?- spy(is_a/2).
% Spy point on is_a/2
true.
[debug] ?- noun(X, rock), adjective(X, color, red).
* Call: (12) is_a(_1858, rock) ? creep
* Exit: (12) is_a(rock1, rock) ? creep
Exit: (11) noun(rock1, rock) ? creep
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 ;
* Redo: (12) is_a(_1858, 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.
After the spy point is hit, the output above is identical to the
traces generated by running trace/0
with the initial query, but is obviously missing all of the traces
before the spy point.
Note that after spy/1
is called, there is a new tag in front of ?-, the [debug]
tag:
?- spy(is_a/2).
% Spy point on is_a/2
true.
[debug] ?-
This means the system is in "debug mode". Debug mode does two things:
it tells the system to watch for spy points and it turns off some
optimizations that would make the traces confusing. The ideal 4-port
model (Byrd, 1980)
as described in many Prolog books (Clocksin &
Melish, 1987) is not visible in many Prolog implementations
because code optimisation removes part of the choice and exit points.
Backtrack points are not shown if either the goal succeeded
deterministically or its alternatives were removed using the cut. When
running in debug mode, choice points are only destroyed when removed by
the cut and last call optimisation is switched off. [Note: This implies
the system can run out of stack in debug mode, while no problems arise
when running in non-debug mode.]
Debug mode can be turned off again using nodebug/0,
but then the spy point will be ignored (but remembered). Turning debug
mode back on via
debug/0 will hit the
spy point again.
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).
?- spy(is_a/2).
% Spy point on is_a/2
true.
[debug] ?- nodebug.
true.
?- noun(X, rock).
X = rock1 ;
X = rock2.
?- debug.
true.
[debug] ?- noun(X, rock).
* Call: (11) is_a(_47826, rock) ? creep
* Exit: (11) is_a(rock1, rock) ? creep
Exit: (10) noun(rock1, rock) ? creep
X = rock1 ;
* Redo: (11) is_a(_47826, rock) ? creep
* Exit: (11) is_a(rock2, rock) ? creep
Exit: (10) noun(rock2, rock) ? creep
X = rock2.
So, debug mode allows Prolog to watch for spy points and enable trace
mode when it hits one. The tracing/0
and debugging/0
predicates will report if the system is in either of those modes.
