Port denotes the reason to activate the tracer (`port’in the 4/5-port, but with some additions):

call

Normal entry through the call port of the 4-port debugger.

redo(PC)

Normal entry through the redo port of the 4-port debugger. The redo port signals resuming a predicate to generate alternative solutions. If PC is 0 (zero), clause indexing has found another clause that will be tried next. Otherwise, PC is the program counter in the current clause where execution continues. This implies we are dealing with an in-clause choice point left by, e.g., ;/2. Note that non-determinism in foreign predicates are also handled using an in-clause choice point.

unify

The unify port represents the neck instruction, signalling the end of the head-matching process. This port is normally invisible. See leash/1 and visible/1.

exit

The exit port signals the goal is proved. It is possible for the goal to have alternatives. See prolog_frame_attribute/3 to examine the goal stack.

fail

The fail port signals final failure of the goal.

exception(Except)

An exception is raised and still pending. This port is activated on each parent frame of the frame generating the exception until the exception is caught or the user restarts normal computation using retry. Except is the pending exception term.

cut_call(PC)

A cut is encountered at PC. This port is used by the graphical debugger to visualise the effect of the cut.

cut_exit(PC)

A cut has been executed. See cut_call(PC) for more information.

Frame is a reference to the current local stack frame, which can be examined using prolog_frame_attribute/3. Choice is a reference to the last choice point and can be examined using prolog_choice_attribute/3. Action must be unified with a term that specifies how execution must continue. The following actions are defined:

abort

Abort execution. See abort/0.

continue

Continue (i.e., creep in the command line debugger).

fail

Make the current goal fail.

ignore

Step over the current goal without executing it.

nodebug

Continue execution in normal nodebugging mode. See nodebug/0.

retry

Retry the current frame.

retry(Frame)

Retry the given frame. This must be a parent of the current frame.

skip

Skip over the current goal (i.e., skip in the command line debugger).

up

Skip to the parent goal (i.e., up in the command line debugger).

Together with the predicates described in section 4.39 and the other predicates of this chapter, this predicate enables the Prolog user to define a complete new debugger in Prolog. Besides this, it enables the Prolog programmer to monitor the execution of a program. The example below records all goals trapped by the tracer in the database.

prolog_trace_interception(Port, Frame, _PC, continue) :-
        prolog_frame_attribute(Frame, goal, Goal),
        prolog_frame_attribute(Frame, level, Level),
        recordz(trace, trace(Port, Level, Goal)).

To trace the execution of‘go’this way the following query should be given:

?- trace, go, notrace.

As of version 9.1.12, unification against variables in the passed data as well as changes to backtrackable global variables persist. The hook should not unify variables in its arguments. One solution to this is to backtrace over the body of the interceptor. Note that the Action needs to be preserved.

user:prolog_trace_interception(Port, Frame, Choice, Action) :-
    State = state(0),
    (   my_trace_interception(Port, Frame, Choice, Action),
        nb_setarg(1, State, Action),
        fail
    ;   arg(1, State, Action)
    ).