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<html><head><title>XLISP &key</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>&key</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>&key <i>key-arg</i> ...<br>
&key (<i>key-arg</i> [<i>key-value</i> [<i>supplied-p-var</i>]]) ...<br>
&key ((<i>key-symbol key-arg</i>) [<i>key-value</i> [<i>supplied-p-var</i>]]) ...</dt>
<dd><i>key-arg</i> - keyword argument<br>
<i>key-symbol</i> - keyword argument symbol<br>
<i>key-value</i> - keyword argument initialization<br>
<i>supplied-p-var</i> - 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
<a href="lambda-keyword-optional.htm">&optional</a> arguments defined, they will
be filled in order.</p>
<p>There are other optional arguments called 'keyword' arguments. These
arguments are not position dependent but can be specified in any order by a
preceding keyword [a symbol with a leading colon ':']. If there is no
'key-symbol' specified in the argument list, the keyword will be constructed
from the 'key-arg' name by adding a leading colon ':'. For example a
'key-arg' of 'furter' will generate a keyword symbol of ':furter'.</p>
<p>Like the <a href="lambda-keyword-optional.htm">&optional</a> arguments, there
can be initialization values provided via the 'key-value' argument. If there
is no 'key-value' argument and no value is provided by the function call,
the 'key-arg' value will be <a href="nil.htm">NIL</a>.</p>
<p>The 'supplied-p-var', if it is specified, will contain a
<a href="t.htm"> T </a> if the 'key-arg' value was
supplied by the function call and a <a href="nil.htm">NIL</a> if
it was not supplied by the function call. This 'supplied-p-var' allows the
programmer to test for an argument's 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 (a &key b c)
(print a) (print b) (print c))
(foo) <font color="#008844">; error: too few arguments</font>
(foo 1) <font color="#008844">; prints 1 NIL NIL</font>
(foo 1 2) <font color="#008844">; prints 1 NIL NIL</font>
(foo 1 :b 2 :c 3) <font color="#008844">; prints 1 2 3</font>
(foo 1 :c 3 :b 2) <font color="#008844">; prints 1 2 3</font>
(foo 1 :b 3 :b 2) <font color="#008844">; prints 1 3 NIL</font>
(defun fee (a &key (b 9 b-passed))
(print a) (print b)
(if b-passed (print "b was passed")
(print "b not passed")))
(fee) <font color="#008844">; error: too few arguments</font>
(fee 1) <font color="#008844">; prints 1 9 "b not passed"</font>
(fee 1 2) <font color="#008844">; prints 1 9 "b not passed"</font>
(fee 1 :b 2) <font color="#008844">; prints 1 2 "b was passed"</font>
(defun fi (a &key ((:mykey b) 9 b-passed))
(print a) (print b)
(if b-passed (print "b was passed")
(print "b not passed")))
(fi) <font color="#008844">; error: too few arguments</font>
(fi 1) <font color="#008844">; prints 1 9 "b not passed"</font>
(fi 1 2) <font color="#008844">; prints 1 9 "b not passed"</font>
(fi 1 :b 2) <font color="#008844">; prints 1 9 "b not passed"</font>
(fi 1 :mykey 2) <font color="#008844">; prints 1 2 "b was passed"</font>
</pre>
<p><b>Note:</b> There is a '&allow-other-keys' keyword in XLISP and
Common Lisp. In the case of XLISP, this keyword is extraneous since the
default for keyword arguments is to allow other keys (without errors).</p>
<p>See the
<a href="../manual/xlisp-man-009.htm#9-1-4">&key</a>
keyword in the <nobr>XLISP 2.0</nobr> manual.</p>
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