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<span id="trace"></span><div class="header">
<p>
Next: <a href="step.html" accesskey="n" rel="next">step</a>, Previous: <a href="describe_002dobject.html" accesskey="p" rel="prev">describe-object</a>, Up: <a href="Environment-Dictionary.html" accesskey="u" rel="up">Environment Dictionary</a> </p>
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<hr>
<span id="trace_002c-untrace-_005bMacro_005d"></span><h4 class="subsection">25.2.8 trace, untrace [Macro]</h4>
<p><code>trace</code> <i>{<i>function-name</i>}*</i> ⇒ <i>trace-result</i>
</p>
<p><code>untrace</code> <i>{<i>function-name</i>}*</i> ⇒ <i>untrace-result</i>
</p>
<span id="Arguments-and-Values_003a_003a-494"></span><h4 class="subsubheading">Arguments and Values::</h4>
<p><i>function-name</i>—a <i>function name</i>.
</p>
<p><i>trace-result</i>—<i>implementation-dependent</i>,
unless no <i>function-names</i> are supplied,
in which case <i>trace-result</i> is a <i>list</i> of <i>function names</i>.
</p>
<p><i>untrace-result</i>—<i>implementation-dependent</i>.
</p>
<span id="Description_003a_003a-656"></span><h4 class="subsubheading">Description::</h4>
<p><b>trace</b> and <b>untrace</b> control the invocation of the trace facility.
</p>
<p>Invoking <b>trace</b> with one or more <i>function-names</i> causes
the denoted <i>functions</i> to be “traced.”
Whenever a traced <i>function</i> is invoked, information
about the call,
about the arguments passed,
and about any eventually returned values
is printed to <i>trace output</i>.
If <b>trace</b> is used with no <i>function-names</i>,
no tracing action is performed;
instead, a list of the <i>functions</i> currently being traced is returned.
</p>
<p>Invoking <b>untrace</b> with one or more function names causes those
functions to be “untraced” (<i>i.e.</i>, no longer traced).
If <b>untrace</b> is used with no <i>function-names</i>,
all <i>functions</i> currently being traced are untraced.
</p>
<p>If a <i>function</i> to be traced has been open-coded
(<i>e.g.</i>, because it was declared <b>inline</b>),
a call to that <i>function</i> might not produce trace output.
</p>
<span id="Examples_003a_003a-466"></span><h4 class="subsubheading">Examples::</h4>
<div class="example">
<pre class="example"> (defun fact (n) (if (zerop n) 1 (* n (fact (- n 1)))))
⇒ FACT
(trace fact)
⇒ (FACT)
;; Of course, the format of traced output is implementation-dependent.
(fact 3)
<tt> |> </tt> 1 Enter FACT 3
<tt> |> </tt> | 2 Enter FACT 2
<tt> |> </tt> | 3 Enter FACT 1
<tt> |> </tt> | | 4 Enter FACT 0
<tt> |> </tt> | | 4 Exit FACT 1
<tt> |> </tt> | 3 Exit FACT 1
<tt> |> </tt> | 2 Exit FACT 2
<tt> |> </tt> 1 Exit FACT 6
⇒ 6
</pre></div>
<span id="Side-Effects_003a_003a-84"></span><h4 class="subsubheading">Side Effects::</h4>
<p>Might change the definitions of the <i>functions</i> named by <i>function-names</i>.
</p>
<span id="Affected-By_003a_003a-124"></span><h4 class="subsubheading">Affected By::</h4>
<p>Whether the functions named are defined or already being traced.
</p>
<span id="Exceptional-Situations_003a_003a-239"></span><h4 class="subsubheading">Exceptional Situations::</h4>
<p>Tracing an already traced function,
or untracing a function not currently being traced,
should produce no harmful effects, but might signal a warning.
</p>
<span id="See-Also_003a_003a-534"></span><h4 class="subsubheading">See Also::</h4>
<p><b>*trace-output*</b>,
<a href="step.html">step</a>
</p>
<span id="Notes_003a_003a-327"></span><h4 class="subsubheading">Notes::</h4>
<p><b>trace</b> and <b>untrace</b> may also accept additional
<i>implementation-dependent</i> argument formats. The format of the trace
output is <i>implementation-dependent</i>.
</p>
<p>Although <b>trace</b> can be extended to permit non-standard options,
<i>implementations</i> are nevertheless encouraged (but not required)
to warn about the use of syntax or options
that are neither specified by this standard
nor added as an extension by the <i>implementation</i>,
since they could be symptomatic of typographical errors
or of reliance on features supported in <i>implementations</i>
other than the current <i>implementation</i>.
</p>
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