docsrc/xlisp/xlisp-doc/reference/when.htm


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<html><head><title>XLISP when</title>

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</head>

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<a href="../start.htm">Nyquist / XLISP 2.0</a>&nbsp; -&nbsp;
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<hr>

<h1>when</h1>

<hr>

<p><table cellpadding="0" cellspacing="0" style="margin-left:10px"><tbody>
<tr valign="top">
  <td><nobr>Type:</nobr></td>
  <td><nobr>&nbsp;&nbsp;-&nbsp;&nbsp;</nobr></td>
  <td width="100%"><nobr>special form (fsubr)</nobr></td>
</tr>
<tr valign="top">
  <td><nobr>Source:</nobr></td>
  <td><nobr>&nbsp;&nbsp;-&nbsp;&nbsp;</nobr></td>
  <td width="100%"><nobr>xlcont.c</nobr></td>
</tr>
</tbody></table></p>

<h2>Syntax</h2>

<dl>
<dt>(when <i>test</i> [<i>expr</i> ... ])</dt>
<dd><i>test</i> - an expression, <a href="nil.htm">NIL</a>
or non-<a href="nil.htm">NIL</a><br>
<i>expr</i> - expressions comprising a body of code<br>
returns - the value of the last expression or
<a href="nil.htm">NIL</a></dd>
</dl>

<h2>Description</h2>

<p>The 'when' macro executes the 'expr' forms if 'test' evaluates to a
non-<a href="nil.htm">NIL</a> value. If 'test' is
<nobr>non-<a href="nil.htm">NIL</a> ,</nobr> the value of the
last 'expr' is returned as the result. If 'test' is
<nobr><a href="nil.htm">NIL</a> ,</nobr>
<a href="nil.htm">NIL</a> is returned with none of 'expr'
evaluated.</p>

<h2>Examples</h2>

<pre class="example">
(when NIL)                  <font color="#008844">; returns NIL</font>
(when T)                    <font color="#008844">; returns T</font>

(when T (print "hi") 'foo)  <font color="#008844">; prints "hi"  returns FOO</font>

(when (listp '(a))
      (print "a list"))     <font color="#008844">; prints  "a list"</font>
                            <font color="#008844">; returns "a list"</font>
</pre>

<p>See the
<a href="../manual/xlisp-man-019.htm#when">when</a>
special form in the <nobr>XLISP 2.0</nobr> manual.</p>

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