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<html><head><title>XLISP listp</title>
<link rel="stylesheet" type="text/css" href="reference.css">
</head>
<body>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="reference-index.htm">Reference</a>
<hr>
<h1>listp</h1>
<hr>
<p><table cellpadding="0" cellspacing="0" style="margin-left:10px"><tbody>
<tr valign="top">
<td><nobr>Type:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>predicate function (subr)</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xlbfun.c</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<dl>
<dt>(listp <i>expr</i>)</dt>
<dd><i>expr</i> - the expression to check<br>
returns - <a href="t.htm"> T </a> if the value is a
list or <nobr><a href="nil.htm">NIL</a> ,</nobr>
<a href="nil.htm">NIL</a> otherwise</dd>
</dl>
<h2>Description</h2>
<p>The 'listp' predicate function checks if the 'expr' is a list.
<a href="t.htm"> T </a> is returned if 'expr' is a
list or an empty list [the <a href="nil.htm">NIL</a> value],
<a href="nil.htm">NIL</a> is returned otherwise.</p>
<h2>Examples</h2>
<pre class="example">
(listp '(a b)) <font color="#008844">; returns T - list</font>
(listp nil) <font color="#008844">; returns T - NIL</font>
(listp '(a . b)) <font color="#008844">; returns T - dotted pair list</font>
(listp (lambda (x) (print x))) <font color="#008844">; returns NIL - closure - lambda</font>
(listp #(1 2 3)) <font color="#008844">; returns NIL - array</font>
(listp *standard-output*) <font color="#008844">; returns NIL - stream</font>
(listp 1.2) <font color="#008844">; returns NIL - float</font>
(listp #'quote) <font color="#008844">; returns NIL - fsubr</font>
(listp 1) <font color="#008844">; returns NIL - integer</font>
(listp object) <font color="#008844">; returns NIL - object</font>
(listp "str") <font color="#008844">; returns NIL - string</font>
(listp #'car) <font color="#008844">; returns NIL - subr</font>
(listp 'a) <font color="#008844">; returns NIL - symbol</font>
</pre>
<p><b>Note:</b> <a href="nil.htm">NIL</a> or '() is used in many
places as a list-class or atom-class expression. Both
<a href="atom.htm">atom</a> and 'listp', when applied to
<nobr><a href="nil.htm">NIL</a> ,</nobr> return
<a href="t.htm"> T </a>. If you wish to check
for a non-empty list, use the <a href="consp.htm">consp</a>
predicate function.</p>
<p>See the
<a href="../manual/xlisp-man-018.htm#listp">listp</a>
function in the <nobr>XLISP 2.0</nobr> manual.</p>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
<hr>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="reference-index.htm">Reference</a>
</body></html>
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