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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> -
<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>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>function (subr)</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xlbfun.c, xleval.c</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<dl>
<dt>(evalhook<i> expr eval-expr apply-expr</i> [<i>env</i>])</dt>
<dd><i>expr</i> - an expression to evaluate<br>
<i>eval-expr</i> - an expression for the evaluation routine<br>
<i>apply-expr</i> - an expression for
<a href="apply.htm">apply</a> [not used]<br>
<i>env</i> - an environment expression, default is
<a href="nil.htm">NIL</a><br>
returns - the result of evaluating the expression</dd>
</dl>
<h2>Description</h2>
<p>The 'evalhook' function is a function that performs evaluation.
The routine specified by 'eval-expr' is called with the 'expr' and 'env'
parameters. If 'eval-expr' is
<nobr><a href="nil.htm">NIL</a> ,</nobr> then the normal system
evaluator is called. The 'apply-hook' is a dummy parameter that is not used
in the current XLISP system. The 'expr' contains the expression to be
evaluated. If the 'env' argument to 'evalhook' is not specified,
<a href="nil.htm">NIL</a> is used, which specifies to use the
current global environment. The 'env', if specified, 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">
(setq a 100) <font color="#008844">; set up global values</font>
(setq b 200)
(evalhook '(+ a b) NIL NIL) <font color="#008844">; returns 300 - no 'env' was given</font>
(evalhook '(+ a b) NIL NIL <font color="#008844">; eval with a=1 and b=2</font>
'((((a . 1)(b . 2))))) <font color="#008844">; returns 3</font>
(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 'evalhook' function and
<a href="global-evalhook.htm">*evalhook*</a> 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 'evalhook' and
<a href="global-evalhook.htm">*evalhook*</a> to implement their
functionality. The other useful aspect of 'evalhook' and
<a href="global-evalhook.htm">*evalhook*</a> 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
<a href="global-evalhook.htm">*evalhook*</a> and 'evalhook'.
If you put in a bad definition into
<nobr><a href="global-evalhook.htm">*evalhook*</a> ,</nobr> you might not
be able to do anything and will need to exit XLISP.</p>
<p><b>Unusual behaviour:</b> The 'evalhook' function and
<a href="global-evalhook.htm">*evalhook*</a> system variable, by their
nature, cause some unusual things to happen. After you have set
<a href="global-evalhook.htm">*evalhook*</a> to some
non-<a href="nil.htm">NIL</a> value, your function will be
called. However, when you are all done and set
<a href="global-evalhook.htm">*evalhook*</a> to
<a href="nil.htm">NIL</a> or some other new routine, it will
never be set. This is because the 'xevalhook' function [in the 'xlbfun.c'
source file] saves the old value of
<a href="global-evalhook.htm">*evalhook*</a> before calling your routine,
and then restores it after the evaluation. The mechanism to reset
<a href="global-evalhook.htm">*evalhook*</a> is to execute the
<a href="top-level.htm">top-level</a> function, which sets
<a href="global-evalhook.htm">*evalhook*</a> to
<a href="nil.htm">NIL</a>.</p>
<p>See the
<a href="../manual/xlisp-man-022.htm#evalhook">evalhook</a>
function in the <nobr>XLISP 2.0</nobr> manual.</p>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
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