/usr/share/doc/swi-prolog-doc/Manual/debugging.html

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<H2><A NAME="sec:7.4"><SPAN class="sec-nr">7.4</SPAN> <SPAN class="sec-title">Debugging</SPAN></A></H2>

<A NAME="sec:debugging"></A> The CHR debugging facilities are currently 
rather limited. Only tracing is currently available. To use the CHR 
debugging facilities for a CHR file it must be compiled for debugging. 
Generating debug info is controlled by the CHR option <A class="flag" href="flags.html#flag:debug">debug</A>, 
whose default is derived from the SWI-Prolog flag <A class="flag" href="flags.html#flag:generate_debug_info">generate_debug_info</A>. 
Therefore debug info is provided unless the <STRONG>-nodebug</STRONG> is 
used.

<H3><A NAME="sec:7.4.1"><SPAN class="sec-nr">7.4.1</SPAN> <SPAN class="sec-title">Ports</SPAN></A></H3>

<A NAME="sec:chrports"></A> For CHR constraints the four standard ports 
are defined:

<DL>
<DT><STRONG>call</STRONG></DT>
<DD class="defbody">
A new constraint is called and becomes active.
</DD>
<DT><STRONG>exit</STRONG></DT>
<DD class="defbody">
An active constraint exits: it has either been inserted in the store 
after trying all rules or has been removed from the constraint store.
</DD>
<DT><STRONG>fail</STRONG></DT>
<DD class="defbody">
An active constraint fails.
</DD>
<DT><STRONG>redo</STRONG></DT>
<DD class="defbody">
An active constraint starts looking for an alternative solution.
</DD>
</DL>

<P>In addition to the above ports, CHR constraints have five additional 
ports:

<DL>
<DT><STRONG>wake</STRONG></DT>
<DD class="defbody">
A suspended constraint is woken and becomes active.
</DD>
<DT><STRONG>insert</STRONG></DT>
<DD class="defbody">
An active constraint has tried all rules and is suspended in the 
constraint store.
</DD>
<DT><STRONG>remove</STRONG></DT>
<DD class="defbody">
An active or passive constraint is removed from the constraint store.
</DD>
<DT><STRONG>try</STRONG></DT>
<DD class="defbody">
An active constraints tries a rule with possibly some passive 
constraints. The try port is entered just before committing to the rule.
</DD>
<DT><STRONG>apply</STRONG></DT>
<DD class="defbody">
An active constraints commits to a rule with possibly some passive 
constraints. The apply port is entered just after committing to the 
rule.
</DD>
</DL>

<H3><A NAME="sec:7.4.2"><SPAN class="sec-nr">7.4.2</SPAN> <SPAN class="sec-title">Tracing</SPAN></A></H3>

Tracing is enabled with the <A NAME="idx:chrtrace0:1250"></A><A class="pred" href="debugging.html#chr_trace/0">chr_trace/0</A> 
predicate and disabled with the <A NAME="idx:chrnotrace0:1251"></A><A class="pred" href="debugging.html#chr_notrace/0">chr_notrace/0</A> 
predicate.

<P>When enabled the tracer will step through the <CODE>call</CODE>,
<CODE>exit</CODE>, <CODE>fail</CODE>, <CODE>wake</CODE> and <CODE>apply</CODE> 
ports, accepting debug commands, and simply write out the other ports.

<P>The following debug commands are currently supported:

<PRE class="code">
        CHR debug options:

                &lt;cr&gt;    creep           c       creep
                s       skip
                g       ancestors
                n       nodebug
                b       break
                a       abort
                f       fail
                ?       help            h       help
</PRE>

<P>Their meaning is:

<DL>
<DT><STRONG>creep</STRONG></DT>
<DD class="defbody">
Step to the next port.
</DD>
<DT><STRONG>skip</STRONG></DT>
<DD class="defbody">
Skip to exit port of this call or wake port.
</DD>
<DT><STRONG>ancestors</STRONG></DT>
<DD class="defbody">
Print list of ancestor call and wake ports.
</DD>
<DT><STRONG>nodebug</STRONG></DT>
<DD class="defbody">
Disable the tracer.
</DD>
<DT><STRONG>break</STRONG></DT>
<DD class="defbody">
Enter a recursive Prolog top-level. See <A NAME="idx:break0:1252"></A><A class="pred" href="toplevel.html#break/0">break/0</A>.
</DD>
<DT><STRONG>abort</STRONG></DT>
<DD class="defbody">
Exit to the top-level. See <A NAME="idx:abort0:1253"></A><A class="pred" href="toplevel.html#abort/0">abort/0</A>.
</DD>
<DT><STRONG>fail</STRONG></DT>
<DD class="defbody">
Insert failure in execution.
</DD>
<DT><STRONG>help</STRONG></DT>
<DD class="defbody">
Print the above available debug options.
</DD>
</DL>

<H3><A NAME="sec:7.4.3"><SPAN class="sec-nr">7.4.3</SPAN> <SPAN class="sec-title">CHR 
Debugging Predicates</SPAN></A></H3>

<A NAME="sec:predicates"></A> The <CODE>library(chr)</CODE> module 
contains several predicates that allow inspecting and printing the 
content of the constraint store.

<DL>
<DT class="pubdef"><A NAME="chr_trace/0"><STRONG>chr_trace</STRONG></A></DT>
<DD class="defbody">
Activate the CHR tracer. By default the CHR tracer is activated and 
deactivated automatically by the Prolog predicates <A NAME="idx:trace0:1254"></A><A class="pred" href="debugger.html#trace/0">trace/0</A> 
and
<A NAME="idx:notrace0:1255"></A><A class="pred" href="debugger.html#notrace/0">notrace/0</A>.</DD>
<DT class="pubdef"><A NAME="chr_notrace/0"><STRONG>chr_notrace</STRONG></A></DT>
<DD class="defbody">
De-activate the CHR tracer. By default the CHR tracer is activated and 
deactivated automatically by the Prolog predicates <A NAME="idx:trace0:1256"></A><A class="pred" href="debugger.html#trace/0">trace/0</A> 
and
<A NAME="idx:notrace0:1257"></A><A class="pred" href="debugger.html#notrace/0">notrace/0</A>.</DD>
<DT class="pubdef"><A NAME="chr_leash/1"><STRONG>chr_leash</STRONG>(<VAR>+Spec</VAR>)</A></DT>
<DD class="defbody">
Define the set of CHR ports on which the CHR tracer asks for user 
intervention (i.e. stops). <VAR>Spec</VAR> is either a list of ports as 
defined in <A class="sec" href="debugging.html">section 7.4.1</A> or a 
predefined `alias'. Defined aliases are: <CODE>full</CODE> to stop at 
all ports, <CODE>none</CODE> or <CODE>off</CODE> to never stop, and <CODE>default</CODE> 
to stop at the <CODE>call</CODE>,
<CODE>exit</CODE>, <CODE>fail</CODE>, <CODE>wake</CODE> and <CODE>apply</CODE> 
ports. See also <A NAME="idx:leash1:1258"></A><A class="pred" href="debugger.html#leash/1">leash/1</A>.</DD>
<DT class="pubdef"><A NAME="chr_show_store/1"><STRONG>chr_show_store</STRONG>(<VAR>+Mod</VAR>)</A></DT>
<DD class="defbody">
Prints all suspended constraints of module <VAR>Mod</VAR> to the 
standard output. This predicate is automatically called by the 
SWI-Prolog top-level at the end of each query for every CHR module 
currently loaded. The Prolog flag
<CODE>chr_toplevel_show_store</CODE> controls whether the top-level 
shows the constraint stores. The value <CODE>true</CODE> enables it. Any 
other value disables it.</DD>
<DT class="pubdef"><A NAME="find_chr_constraint/1"><STRONG>find_chr_constraint</STRONG>(<VAR>-Constraint</VAR>)</A></DT>
<DD class="defbody">
Returns a constraint in the constraint store. Via backtracking, all 
constraints in the store can be enumerated.
</DD>
</DL>

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