SWI-Prolog has a 6-port tracer, extending the standard 4-port Byrd box model tracer Byrd, 1980, Clocksin & Melish, 1987 with two additional ports. The optional unify port allows the user to inspect the result after unification of the head. The exception port shows exceptions raised by throw/1 or one of the built-in predicates. See section 4.9.
The standard ports are called
unify. The tracer is started by the trace/0
command, when a spy point is reached and the system is in debugging mode
or when an exception is raised.
The interactive top-level goal trace/0
means ``trace the next query''. The tracer shows the port, displaying
the port name, the current depth of the recursion and the goal. The goal
is printed using the Prolog predicate write_term/2.
The style is defined by the Prolog flag
and can be modified using this flag or using the
d commands of the tracer.
1 ?- visible(+all), leash(-exit). Yes 2 ?- trace, min([3, 2], X). Call: ( 3) min([3, 2], G235) ? creep Unify: ( 3) min([3, 2], G235) Call: ( 4) min(, G244) ? creep Unify: ( 4) min(, 2) Exit: ( 4) min(, 2) Call: ( 4) min(3, 2, G235) ? creep Unify: ( 4) min(3, 2, G235) Call: ( 5) 3 < 2 ? creep Fail: ( 5) 3 < 2 ? creep Redo: ( 4) min(3, 2, G235) ? creep Exit: ( 4) min(3, 2, 2) Exit: ( 3) min([3, 2], 2) Yes [trace] 3 ?-
|Figure 2 : Example trace|
On leashed ports (set with the predicate leash/1,
the user is prompted for an action. All actions are single character
commands which are executed without waiting for a return, unless
the command-line option -tty is active. Tracer options:
|/f||Search for any fail port|
|/fe solve||Search for a fail or exit
port of any goal with name |
|/c solve(a, _)||Search for a call to solve/2
whose first argument is a variable or the atom |
|/a member(_, _)||Search for any port on member/2. This is equivalent to setting a spy point on member/2.|
on, the context module of the goal is displayed between square brackets (see section 5). Default is
max_depth(Depth)option of debugger_print_options, limiting the depth to which terms are printed. See also the
[quoted(true), attributes(write), priority(699)], bypassing portray/1, etc.
The ideal 4 port Byrd box 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 (debug/0) choice points are only destroyed when removed by the cut. In debug mode, last call optimisation is switched off.5This implies the system can run out of local stack in debug mode, while no problems arise when running in non-debug mode.
Reference information to all predicates available for manipulating the debugger is in section 4.38.