/usr/lib/swi-prolog/doc/Manual/tracehook.html

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<h2 id="sec:tracehook"><a id="sec:B.4"><span class="sec-nr">B.4</span> <span class="sec-title">Intercepting 
the Tracer</span></a></h2>

<a id="sec:tracehook"></a>

<dl class="latex">
<dt class="pubdef"><a id="prolog_trace_interception/4"><strong>prolog_trace_interception</strong>(<var>+Port, 
+Frame, +Choice, -Action</var>)</a></dt>
<dd class="defbody">
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 <var>Action</var>. 
Otherwise the normal Prolog debugger actions are performed.

<p><var>Port</var> denotes the reason to activate the tracer (`port' in 
the 4/5-port, but with some additions):

<dl class="latex">
<dt><strong>call</strong></dt>
<dd class="defbody">
Normal entry through the call port of the 4-port debugger.</dd>
<dt><strong>redo</strong>(<var>PC</var>)</dt>
<dd class="defbody">
Normal entry through the redo port of the 4-port debugger. The
<code>redo</code> port signals resuming a predicate to generate 
alternative solutions. If <var>PC</var> is 0 (zero), clause indexing has 
found another clause that will be tried next. Otherwise, <var>PC</var> 
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., <a class="pred" href="control.html#;/2">;/2</a>. Note that 
non-determinism in foreign predicates are also handled using an 
in-clause choice point.</dd>
<dt><strong>unify</strong></dt>
<dd class="defbody">
The unify port represents the <em>neck</em> instruction, signalling the 
end of the head-matching process. This port is normally invisible. See
<a id="idx:leash1:1849"></a><a class="pred" href="debugger.html#leash/1">leash/1</a> 
and <a id="idx:visible1:1850"></a><a class="pred" href="debugger.html#visible/1">visible/1</a>.</dd>
<dt><strong>exit</strong></dt>
<dd class="defbody">
The exit port signals the goal is proved. It is possible for the goal to 
have alternatives. See <a id="idx:prologframeattribute3:1851"></a><a class="pred" href="manipstack.html#prolog_frame_attribute/3">prolog_frame_attribute/3</a> 
to examine the goal stack.</dd>
<dt><strong>fail</strong></dt>
<dd class="defbody">
The fail port signals final failure of the goal.</dd>
<dt><strong>exception</strong>(<var>Except</var>)</dt>
<dd class="defbody">
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
<code>retry</code>. <var>Except</var> is the pending exception term.</dd>
<dt><strong>break</strong>(<var>PC</var>)</dt>
<dd class="defbody">
A <code>break</code> instruction is executed. <var>PC</var> is program 
counter. This port is used by the graphical debugger.</dd>
<dt><strong>cut_call</strong>(<var>PC</var>)</dt>
<dd class="defbody">
A cut is encountered at <var>PC</var>. This port is used by the 
graphical debugger to visualise the effect of the cut.</dd>
<dt><strong>cut_exit</strong>(<var>PC</var>)</dt>
<dd class="defbody">
A cut has been executed. See <code>cut_call(PC)</code> for more 
information.
</dd>
</dl>

<p><var>Frame</var> is a reference to the current local stack frame, 
which can be examined using <a id="idx:prologframeattribute3:1852"></a><a class="pred" href="manipstack.html#prolog_frame_attribute/3">prolog_frame_attribute/3</a>. <var>Choice</var> 
is a reference to the last choice point and can be examined using
<a id="idx:prologchoiceattribute3:1853"></a><a class="pred" href="manipstack.html#prolog_choice_attribute/3">prolog_choice_attribute/3</a>. <var>Action</var> 
must be unified with a term that specifies how execution must continue. 
The following actions are defined:

<dl class="latex">
<dt><strong>abort</strong></dt>
<dd class="defbody">
Abort execution. See <a id="idx:abort0:1854"></a><a class="pred" href="toplevel.html#abort/0">abort/0</a>.
</dd>
<dt><strong>continue</strong></dt>
<dd class="defbody">
Continue (i.e., <em>creep</em> in the command line debugger).
</dd>
<dt><strong>fail</strong></dt>
<dd class="defbody">
Make the current goal fail.
</dd>
<dt><strong>ignore</strong></dt>
<dd class="defbody">
Step over the current goal without executing it.
</dd>
<dt><strong>nodebug</strong></dt>
<dd class="defbody">
Continue execution in normal nodebugging mode. See <a id="idx:nodebug0:1855"></a><a class="pred" href="debugger.html#nodebug/0">nodebug/0</a>.
</dd>
<dt><strong>retry</strong></dt>
<dd class="defbody">
Retry the current frame.
</dd>
<dt><strong>retry</strong>(<var>Frame</var>)</dt>
<dd class="defbody">
Retry the given frame. This must be a parent of the current frame.
</dd>
<dt><strong>skip</strong></dt>
<dd class="defbody">
Skip over the current goal (i.e., <em>skip</em> in the command line 
debugger).
</dd>
</dl>

<p>Together with the predicates described in <a class="sec" href="debugger.html">section 
4.38</a> 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.

<pre class="code">
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)).
</pre>

<p>To trace the execution of `go' this way the following query should be 
given:

<pre class="code">
?- trace, go, notrace.
</pre>

</dd>
<dt class="pubdef"><a id="prolog_skip_frame/1"><strong>prolog_skip_frame</strong>(<var>-Frame</var>)</a></dt>
<dd class="defbody">
Indicate <var>Frame</var> as a skipped frame and set the `skip level' 
(see
<a id="idx:prologskiplevel2:1856"></a><a class="pred" href="tracehook.html#prolog_skip_level/2">prolog_skip_level/2</a> 
to the recursion depth of <var>Frame</var>. The effect of the skipped 
flag is that a redo on a child of this frame is handled differently. 
First, a <code>redo</code> trace is called for the child, where the skip 
level is set to <code>redo_in_skip</code>. Next, the skip level is set 
to skip level of the skipped frame.</dd>
<dt class="pubdef"><a id="prolog_skip_level/2"><strong>prolog_skip_level</strong>(<var>-Old, 
+New</var>)</a></dt>
<dd class="defbody">
Unify <var>Old</var> with the old value of `skip level' and then set 
this level according to <var>New</var>. <var>New</var> is an integer, 
the atom
<code>very_deep</code> (meaning don't skip) or the atom <code>skip_in_redo</code> 
(see <a id="idx:prologskipframe1:1857"></a><a class="pred" href="tracehook.html#prolog_skip_frame/1">prolog_skip_frame/1</a>). 
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 <a id="idx:prologskipframe1:1858"></a><a class="pred" href="tracehook.html#prolog_skip_frame/1">prolog_skip_frame/1</a>.
</dd>
</dl>

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