SWI-Prolog 8.4.0 Reference Manual: Section B.3

## B.3 Intercepting the Tracer

prolog_trace_interception(+Port, +Frame, +Choice, -Action)
  Dynamic predicate, normally not defined. This predicate is called from the SWI-Prolog debugger just before it would show a port. If this predicate succeeds, the debugger assumes that the trace action has been taken care of and continues execution as described by Action. Otherwise the normal Prolog debugger actions are performed.

  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.
  break(PC)
    A `break` instruction is executed. PC is program counter. This port is used by the graphical debugger.
  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.

prolog_skip_frame(-Frame)
  Indicate Frame as a skipped frame and set the‘skip level' (see prolog_skip_level/2 to the recursion depth of Frame. The effect of the skipped flag is that a redo on a child of this frame is handled differently. First, a `redo` trace is called for the child, where the skip level is set to `redo_in_skip`. Next, the skip level is set to skip level of the skipped frame.
prolog_skip_level(-Old, +New)
  Unify Old with the old value of‘skip level' and then set this level according to New. New is an integer, the atom `very_deep` (meaning don't skip) or the atom `skip_in_redo` (see prolog_skip_frame/1). The‘skip level' is a setting of each Prolog thread that disables the debugger on all recursion levels deeper than the level of the variable. See also prolog_skip_frame/1.