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<html><head><title>XLISP unwind-protect</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>unwind-protect</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>(unwind-protect <i>protect-form clean-up-form</i> ... )</dt>
<dd><i>protect-form</i> - a form that is to be protected<br>
<i>clean-up-form</i> - a sequence forms to execute after <i>protect-form</i><br>
returns - the value of <i>protect-form</i></dd>
</dl>
<h2>Description</h2>
<p> The 'unwind-protect' special form allows the protecting [trapping] of
all forms of exit from the 'protect-form'. The exits that are trapped
include errors, <nobr><a href="throw.htm">throw</a> ,</nobr> <a
href="return.htm">return</a> and <a
href="go.htm">go</a>. The 'clean-up-form' will be executed in
all cases, when there is an exit from 'protect-form' and when the form does
not have exit. 'unwind-protect' will return the result from the
'protect-form', not from the 'clean-up-forms'. Errors or exits that occur in
the 'clean-up-form' are not protected. It is possible to trap these with
another 'unwind-protect'.</p>
<p><div class="box">
<p><b>Note:</b> 'unwind-protext' will not protect against errors signalled
by <nobr>built-in</nobr> functions if
<a href="global-breakenable.htm">*breakenable*</a> is <nobr>not
<a href="nil.htm">NIL</a></nobr></nobr>.</p>
</div></p>
<h2>Examples</h2>
<pre class="example">
(unwind-protect
(+ 2 2) <font color="#008844">; protected form</font>
(print "an exit")) <font color="#008844">; clean up form</font>
<font color="#008844">; prints "an exit"</font>
<font color="#008844">; returns 4</font>
</pre>
<pre class="example">
(setq *breakenable* nil) <font color="#008844">; to turn off break loop traps</font>
(unwind-protect
(+ 1 "2") <font color="#008844">; protected form</font>
(print "something happened")) <font color="#008844">; clean up form</font>
<font color="#008844">; error: bad argument type - "2"</font>
<font color="#008844">; prints "something happened"</font>
</pre>
<pre class="example">
(catch 'mytag
(unwind-protect
(throw 'mytag) <font color="#008844">; protected form</font>
(print "an exit"))) <font color="#008844">; clean up form</font>
<font color="#008844">; prints "an exit"</font>
</pre>
<pre class="example">
(setq *breakenable* nil) <font color="#008844">; to turn off break loop traps</font>
(unwind-protect
(throw 'notag) <font color="#008844">; protected form</font>
(print "an exit")) <font color="#008844">; clean up form</font>
<font color="#008844">; error: no target for THROW</font>
<font color="#008844">; prints "an exit"</font>
</pre>
<pre class="example">
(prog () (print "start")
(unwind-protect
(go end) <font color="#008844">; protected form</font>
(print "an exit")) <font color="#008844">; clean-up form</font>
end (print "end")) <font color="#008844">; prints "start"</font>
<font color="#008844">; prints "an exit"</font>
<font color="#008844">; prints "end"</font>
</pre>
<pre class="example">
(prog () (print "start")
(unwind-protect
(return "I'm done") <font color="#008844">; protected form</font>
(print "but first")) <font color="#008844">; clean-up form</font>
(print "won't get here")) <font color="#008844">; prints "start"</font>
<font color="#008844">; prints "but first"</font>
<font color="#008844">; returns "I'm done"</font>
</pre>
<p>See the
<a href="../manual/xlisp-man-019.htm#unwind-protect">unwind-protect</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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