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<html><head><title>XLISP *evalhook*</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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<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="reference-index.htm">Reference</a>
<hr>
<h1>*evalhook*</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>system variable</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> *evalhook*</dt>
</dl>
<h2>Description</h2>
<p>*evalhook* is a system variable whose value is user code that will
intercept evaluations either through normal system evaluation or through
calls to <a href="evalhook.htm">evalhook</a>. The default value for
*evalhook* is <nobr><a href="nil.htm">NIL</a> ,</nobr> which
specifies to use the built in system evaluator. If *evalhook* is
<nobr>non-<a href="nil.htm">NIL</a> ,</nobr> the routine is
called with expression and environment parameters. If the environment
argument is <nobr><a href="nil.htm">NIL</a> ,</nobr> then the the
current global environment is used. The environment, if
<nobr>non-<a href="nil.htm">NIL</a> ,</nobr> is a structure
composed of dotted pairs constructed of the symbol and its value which have
the form:</p>
<pre class="example">
(((<font color="#008844"><i>sym1</i></font> . <font color="#008844"><i>val1</i></font>) (<font color="#008844"><i>sym2</i></font> . <font color="#008844"><i>val2</i></font>) ... )))
</pre>
<h2>Examples</h2>
<pre class="example">
(defun myeval (exp env) <font color="#008844">; define MYEVAL routine</font>
(princ "exp: ") (print exp)
(princ "env: ") (print env)
(evalhook exp #'myeval NIL env))
(defun foo (a) (+ a a)) <font color="#008844">; create simple function</font>
(setq *evalhook* #'myeval) <font color="#008844">; and install MYEVAL as hook</font>
(foo 1) <font color="#008844">; prints exp: (FOO 1) env:NIL</font>
<font color="#008844">; exp: 1 env:NIL</font>
<font color="#008844">; exp: (+ A A) env:((((A . 1))))</font>
<font color="#008844">; exp: A env:((((A . 1))))</font>
<font color="#008844">; exp: A env:((((A . 1))))</font>
<font color="#008844">; returns 2</font>
(top-level) <font color="#008844">; to clean up *evalhook*</font>
</pre>
<p><b>Note:</b> The <a href="evalhook.htm">evalhook</a> function and
*evalhook* system variable are very useful in the construction of debugging
facilities within XLISP. The <a href="trace.htm">trace</a> and
<a href="untrace.htm">untrace</a> functions use
<a href="evalhook.htm">evalhook</a> and *evalhook* to implement
their functionality. The other useful aspect of
<a href="evalhook.htm">evalhook</a> and *evalhook* is to help in
understanding how XLISP works to see the expressions, their environment
and how they are evaluated.</p>
<p><b>Caution:</b> Be careful when using *evalhook* and
<a href="evalhook.htm">evalhook</a>. If you put in a bad definition
into *evalhook*, you might not be able to do anything and will need to
exit XLISP.</p>
<p><b>Unusual behaviour:</b> The <a href="evalhook.htm">evalhook</a>
function and *evalhook* system variable, by their nature, cause some unusual
things to happen. After you have set *evalhook* to some
non-<a href="nil.htm">NIL</a> value, your function will be
called. However, when you are all done and set *evalhook* to
<a href="nil.htm">NIL</a> or some other new routine, it will
never be set. This is because the
<a href="evalhook.htm">evalhook</a> function [in the 'xlbfun.c'
source file] saves the old value of *evalhook* before calling your routine,
and then restores it after the evaluation. The mechanism to reset
*evalhook* is to execute the
<a href="top-level.htm">top-level</a> function, which sets
*evalhook* to <a href="nil.htm">NIL</a>.</p>
<p>See the
<a href="../manual/xlisp-man-011.htm#evalhook">*evalhook*</a>
system variable in the <nobr>XLISP 2.0</nobr> manual.</p>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
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