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<html><head><title>XLISP labels</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>labels</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>special form (fsubr)</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xlcont.c</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<dl>
<dt>(labels ([<i>function</i> ... ]) <i>expr</i> ... )</dt>
<dd><i>function</i> - a function definition binding which is of the form:<br>
<dl><dd>(<i>symbol arg-list body</i>)</dd>
<dl><dd><i>symbol</i> - the symbol specifying the function name<br>
<i>arg-list</i> - the argument list for the function<br>
<i>body</i> - the body of the function</dd></dl></dl>
<i>expr</i> - an expression<br>
returns - the value of the last expression</dd>
</dl>
<h2>Description</h2>
<p>The 'labels' special form is basically a local block construct that
allows local 'function' definitions followed by a block of code to evaluate.
The first form after the labels is the 'binding' form. It contains a series
of 'functions'. 'labels' allows the'functions' to be defined in a mutually
recursive manner. [The similar <a href="flet.htm">flet</a> form
does not allow this.] The 'labels' form will go through and define the
'symbols' of the 'functions' and then sequentially execute the 'exprs'. The
value of the last 'expr' evaluated is returned. When the 'labels' is
finished execution, the 'symbols' that were defined will no longer
exist.</p>
<h2>Examples</h2>
<pre class="example">
(labels ((fuzz (x) (+ x x))) <font color="#008844">; a LABELS with a local function FUZZ</font>
(fuzz 2)) <font color="#008844">; returns 4</font>
<font color="#008844">; FUZZ no longer exists</font>
(fuzz 2) <font color="#008844">; error: unbound function - FUZZ</font>
<font color="#008844">; an empty LABELS</font>
(labels () (print 'a)) <font color="#008844">; prints A</font>
<font color="#008844">; LABELS form including</font>
(labels ((inc (arg) (est arg)) <font color="#008844">; INC definition using EST</font>
(est (var) (* .1 var))) <font color="#008844">; EST definition</font>
(inc 99) ) <font color="#008844">; returns 9.9</font>
<font color="#008844">; FLET form including</font>
(flet ((inc (arg) (est arg)) <font color="#008844">; INC definition using EST</font>
(est (var) (* .1 var))) <font color="#008844">; EST definition</font>
(inc 99) <font color="#008844">; error: unbound function - EST</font>
</pre>
<p><b>Note:</b> <a href="flet.htm">flet</a> does not allow
recursive definitions of functions. The 'label' special form does allow
this.</p>
<p>See the
<a href="../manual/xlisp-man-019.htm#labels">labels</a>
special form in the <nobr>XLISP 2.0</nobr> manual.</p>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
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