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<html><head><title>XLISP &optional</title>
<link rel="stylesheet" type="text/css" href="reference.css">
</head>
<body>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
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<hr>
<h1>&optional</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>keyword</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xleval.c</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<dl>
<dt>&optional [<i>opt-arg</i> | (<i>opt-arg</i> [<i>opt-value</i> [<i>supplied-p</i>]])] ...</dt>
<dd><i>opt-arg</i> - optional argument<br>
<i>opt-value</i> - optional argument initialization value<br>
<i>supplied-p</i> - optional argument existence variable</dd>
</dl>
<h2>Description</h2>
<p>In XLISP, there are several times that you define a formal argument list
for a body of code like
<nobr><a href="defun.htm">defun</a> ,</nobr>
<nobr><a href="defmacro.htm">defmacro</a> ,</nobr>
<a href="keyword-answer.htm">:answer</a> and
<a href="lambda.htm">lambda</a>. All of the formal arguments
that are defined are required to appear in the invocation of the defined
function or operation. If there are any '&optional' arguments defined,
they will be filled in order. If there are insufficient parameters for the
'&optional' arguments, they will contain
<nobr><a href="nil.htm">NIL</a> ,</nobr> unless the arguments
have an 'opt-value' initial value specified. The 'supplied-p' variable, if
specified, will contain <a href="t.htm"> T </a> if
the 'opt-arg' was supplied by the function call and
<a href="nil.htm">NIL</a> if it was not supplied by the function
call. This 'supplied-p' variable allows the programmer to test for an
arguments existence. At the end of the function or operation execution,
these local symbols and their values are are removed.</p>
<h2>Examples</h2>
<pre class="example">
(defun foo <font color="#008844">; define function FOO</font>
(a &optional b (c 1) ) <font color="#008844">; with some optional args</font>
(print a) (print b) (print c))
(foo) <font color="#008844">; error: too few arguments</font>
(foo 1) <font color="#008844">; prints 1 NIL 1</font>
(foo 1 2) <font color="#008844">; prints 1 2 1</font>
(foo 1 2 3) <font color="#008844">; prints 1 2 3</font>
(defun fee <font color="#008844">; define function FEE</font>
(a &optional (b 9 b-passed) ) <font color="#008844">; with some optional args</font>
(print a) (print b)
(if b-passed (print "b was passed")
(print "b not passed")))
(fee 1) <font color="#008844">; prints 1 9 "b not passed"</font>
(fee 1 2) <font color="#008844">; prints 1 2 "b was passed"</font>
</pre>
<p>See the
<a href="../manual/xlisp-man-009.htm#9-1-2">&optional</a>
keyword in the <nobr>XLISP 2.0</nobr> manual.</p>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
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