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

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<h2 id="sec:debug"><a id="sec:A.10"><span class="sec-nr">A.10</span> <span class="sec-title">library(debug): 
Print debug messages and test assertions</span></a></h2>

<p><a id="sec:debug"></a>

<dl class="tags">
<dt class="tag">author</dt>
<dd>
Jan Wielemaker
</dd>
</dl>

<p>This library is a replacement for <a class="pred" href="format.html#format/3">format/3</a> 
for printing debug messages. Messages are assigned a <i>topic</i>. By 
dynamically enabling or disabling topics the user can select desired 
messages. Debug statements are removed when the code is compiled for 
optimization.

<p>See manual for details. With XPCE, you can use the call below to 
start a graphical monitoring tool.

<pre class="code">
?- prolog_ide(debug_monitor).
</pre>

<p>Using the predicate <a class="pred" href="debug.html#assertion/1">assertion/1</a> 
you can make assumptions about your program explicit, trapping the 
debugger if the condition does not hold.

<dl class="latex">
<dt class="pubdef"><span class="pred-tag">[semidet]</span><a id="debugging/1"><strong>debugging</strong>(<var>+Topic</var>)</a></dt>
<dt class="pubdef"><span class="pred-tag">[nondet]</span><a id="debugging/1"><strong>debugging</strong>(<var>-Topic</var>)</a></dt>
<dt class="pubdef"><span class="pred-tag">[nondet]</span><a id="debugging/2"><strong>debugging</strong>(<var>?Topic, 
?Bool</var>)</a></dt>
<dd class="defbody">
Examine debug topics. The form <code>debugging(+Topic)</code> may be 
used to perform more complex debugging tasks. A typical usage skeleton 
is:

<pre class="code">
      (   debugging(mytopic)
      -&gt;  &lt;perform debugging actions&gt;
      ;   true
      ),
      ...
</pre>

<p>The other two calls are intended to examine existing and enabled 
debugging tokens and are typically not used in user programs.</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="debug/1"><strong>debug</strong>(<var>+Topic</var>)</a></dt>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="nodebug/1"><strong>nodebug</strong>(<var>+Topic</var>)</a></dt>
<dd class="defbody">
Add/remove a topic from being printed. <code>nodebug(_)</code> removes 
all topics. Gives a warning if the topic is not defined unless it is 
used from a directive. The latter allows placing debug topics at the 
start of a (load-)file without warnings.

<p>For <a class="pred" href="debug.html#debug/1">debug/1</a>, <var>Topic</var> 
can be a term <var>Topic</var> <var>&gt;</var> Out, where Out is either 
a stream or stream-alias or a filename (atom). This redirects debug 
information on this topic to the given output.</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="list_debug_topics/0"><strong>list_debug_topics</strong></a></dt>
<dd class="defbody">
List currently known debug topics and their setting.</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="debug_message_context/1"><strong>debug_message_context</strong>(<var>+What</var>)</a></dt>
<dd class="defbody">
Specify additional context for debug messages. <var>What</var> is one of 
+Context or -Context, and Context is one of <code>thread</code>, <code>time</code> 
or
<code>time(Format)</code>, where Format is a format specification for
<a class="pred" href="system.html#format_time/3">format_time/3</a> 
(default is <code>%T.%3f</code>). Initially, <a class="pred" href="debug.html#debug/3">debug/3</a> 
shows only thread information.</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="debug/3"><strong>debug</strong>(<var>+Topic, 
+Format, :Args</var>)</a></dt>
<dd class="defbody">
<var>Format</var> a message if debug topic is enabled. Similar to <a class="pred" href="format.html#format/3">format/3</a> 
to <code>user_error</code>, but only prints if <var>Topic</var> is 
activated through
<a class="pred" href="debug.html#debug/1">debug/1</a>. <var>Args</var> 
is a meta-argument to deal with goal for the @-command. Output is first 
handed to the hook
<a class="pred" href="debug.html#prolog:debug_print_hook/3">prolog:debug_print_hook/3</a>. 
If this fails, <var>Format</var>+<var>Args</var> is translated to text 
using the message-translation (see
<a class="pred" href="exception.html#print_message/2">print_message/2</a>) 
for the term <code>debug(Format, Args)</code> and then printed to every 
matching destination (controlled by <a class="pred" href="debug.html#debug/1">debug/1</a>) 
using <a class="pred" href="exception.html#print_message_lines/3">print_message_lines/3</a>.

<p>The message is preceded by '% ' and terminated with a newline.

<dl class="tags">
<dt class="tag">See also</dt>
<dd>
<a class="pred" href="format.html#format/3">format/3</a>.
</dd>
</dl>

</dd>
<dt class="multidef"><span class="pred-tag">[semidet,multifile]</span><a id="prolog:debug_print_hook/3"><span class="module">prolog</span>:<strong>debug_print_hook</strong>(<var>+Topic, 
+Format, +Args</var>)</a></dt>
<dd class="defbody">
Hook called by <a class="pred" href="debug.html#debug/3">debug/3</a>. 
This hook is used by the graphical frontend that can be activated using <a class="pred" href="idepreds.html#prolog_ide/1">prolog_ide/1</a>:

<pre class="code">
?- prolog_ide(debug_monitor).
</pre>

</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="assertion/1"><strong>assertion</strong>(<var>:Goal</var>)</a></dt>
<dd class="defbody">
Acts similar to C <code>assert()</code> macro. It has no effect if <var>Goal</var> 
succeeds. If <var>Goal</var> fails or throws an exception, the following 
steps are taken:

<p>
<ul class="latex">
<li>call <a class="pred" href="debug.html#prolog:assertion_failed/2">prolog:assertion_failed/2</a>. 
If <a class="pred" href="debug.html#prolog:assertion_failed/2">prolog:assertion_failed/2</a> 
fails, then:

<p>
<ul class="latex">
<li>If this is an interactive toplevel thread, print a message, the 
stack-trace, and finally trap the debugger.
<li>Otherwise, throw <code>error(assertion_error(Reason, G),_)</code> 
where Reason is one of <code>fail</code> or the exception raised.
</ul>
</ul>
</dd>
<dt class="multidef"><span class="pred-tag">[semidet,multifile]</span><a id="prolog:assertion_failed/2"><span class="module">prolog</span>:<strong>assertion_failed</strong>(<var>+Reason, 
+Goal</var>)</a></dt>
<dd class="defbody">
This hook is called if the <var>Goal</var> of <a class="pred" href="debug.html#assertion/1">assertion/1</a> 
fails. <var>Reason</var> is unified with either <code>fail</code> if <var>Goal</var> 
simply failed or an exception call otherwise. If this hook fails, the 
default behaviour is activated. If the hooks throws an exception it will 
be propagated into the caller of <a class="pred" href="debug.html#assertion/1">assertion/1</a>.
</dd>
</dl>

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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / debug.html
