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<html><head><title>XLISP: An Object-oriented Lisp</title></head>
<link rel="stylesheet" type="text/css" href="manual.css">
<body>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="contents.htm">Contents</a> |
<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="../reference/reference-index.htm">Reference</a>
<hr>
<h1>XLISP Object System</h1>
<hr>
<ol>
<li><nobr><a href="#1">Definitions</a></nobr></li>
<ul>
<li><nobr><a href="#define-selector">selector</a> - a symbol used to select an appropriate method</nobr></li>
<li><nobr><a href="#define-message">message</a> - a selector and a list of actual arguments</nobr></li>
<li><nobr><a href="#define-method">method</a> - the code that implements a message</nobr></li>
<li><nobr><a href="#define-object">object</a> - the top of the class hierarchy</nobr></li>
<li><nobr><a href="#define-class">class</a> - the class of all object classes</nobr></li>
</ul>
<li><nobr><a href="#2">The 'send' Function</a></nobr></li>
<li><nobr><a href="#3">The 'self' Symbol</a></nobr></li>
<li><nobr><a href="#4">The 'send-super' Function</a></nobr></li>
<li><nobr><a href="#5">The 'object' Class</a></nobr></li>
<ul>
<li><nobr><a href="#object">object</a> - the top of the class hierarchy</nobr></li>
<ul>
<li><nobr><a href="#object-show">:show</a> - show an object's instance variables</nobr></li>
<li><nobr><a href="#object-class">:class</a> - return the class of an object</nobr></li>
<li><nobr><a href="#object-isnew">:isnew</a> - the default object initialization routine</nobr></li>
<li><nobr><a href="#send-super">send-super</a> - send superclass a message</nobr></li>
</ul>
</ul>
<li><nobr><a href="#6">The 'class' Class</a></nobr></li>
<ul>
<li><nobr><a href="#class">class</a> - class of all object classes</nobr></li>
<ul>
<li><nobr><a href="#class-new">:new</a> - create a new instance of a class</nobr></li>
<li><nobr><a href="#class-isnew">:isnew</a> - initialize a new class</nobr></li>
<li><nobr><a href="#class-answer">:answer</a> - add a message to a class</nobr></li>
</ul>
</ul>
</ol>
<a name="1"></a>
<a name="define-selector"></a>
<a name="define-message"></a>
<a name="define-method"></a>
<a name="define-object"></a>
<a name="define-class"></a>
<hr>
<h2>1 Definitions</h2>
<hr>
<p><div class="box">
<p><table cellpadding="0" cellspacing="0"><tbody>
<tr>
<td align="right"><nobr>selector</nobr></td>
<td valign="top"><nobr> — </nobr></td>
<td width="100%"><nobr>a symbol used to select an appropriate method, usually a keyword</nobr></td>
</tr>
<tr>
<td align="right"><nobr>message</nobr></td>
<td valign="top"><nobr> — </nobr></td>
<td width="100%"><nobr>a selector symbol and a list of actual arguments</nobr></td>
</tr>
<tr>
<td align="right"><nobr>method</nobr></td>
<td valign="top"><nobr> — </nobr></td>
<td width="100%"><nobr>the Lisp code that implements a message</nobr></td>
</tr>
<tr>
<td align="right"><nobr><a href="../reference/object.htm">object</a></nobr></td>
<td valign="top"><nobr> — </nobr></td>
<td width="100%"><nobr>the top of the class hierarchy</nobr></td>
</tr>
<tr>
<td align="right"><nobr><a href="../reference/class.htm">class</a></nobr></td>
<td valign="top"><nobr> — </nobr></td>
<td width="100%"><nobr>the class of all object classes [including itself]</nobr></td>
</tr>
</tbody></table></p>
</div></p>
<p>Since XLISP was created to provide a simple basis for experimenting with
<nobr>object-oriented</nobr> programming, one of the primitive data types
included is <a href="../reference/object.htm">object</a>. <nobr>In
XLISP</nobr>, an object consists of a data structure containing a pointer to
the object's <a href="../reference/class.htm">class</a> as well as an array
containing the values of the object's instance variables.</p>
<p>Officially, there is no way to see inside an object [look at the values
of its instance variables]. The only way to communicate with an object is by
sending it a message.</p>
<p><nobr> <a href="#top">Back to Top</a></nobr></p>
<a name="2"></a>
<hr>
<h2>2 The 'send' Function</h2>
<hr>
<p>You can send a message to an object using the
<a href="../reference/send.htm">send</a> function:</p>
<p><div class="box">
<dl>
<dt>(<b>send</b> <i>object selector</i> [<i>args</i>])</dt>
<dd><i>object</i> - an <a href="object.htm">object</a><br>
<i>selector</i> - message selector for <i>object</i><br>
<i>arg</i> - parameter sent to <i>object</i> method<br>
returns - the <i>object</i></dd>
</dl>
</div></p>
<p>The <a href="../reference/send.htm">send</a> function takes the object as
its first argument, the message selector as its second argument [which must
be a symbol] and the message arguments as its remaining arguments. <nobr>It
determines</nobr> the class of the receiving object and attempts to find a
method corresponding to the message selector in the set of messages defined
for that class. <nobr>If the</nobr> message is not found in the object's
class and the class has a <nobr>super-class</nobr>, the search continues by
looking at the messages defined for the <nobr>super-class</nobr>. This
process continues from one <nobr>super-class</nobr> to the next until a
method for the message is found. <nobr>If no</nobr> method is found, an
error occurs.</p>
<p><nobr> <a href="#top">Back to Top</a></nobr></p>
<a name="3"></a>
<hr>
<h2>3 The 'self' Symbol</h2>
<hr>
<p>When a method is found, the evaluator binds the receiving object to the
symbol <a href="../reference/self.htm">self</a> and evaluates the method
using the remaining elements of the original list as arguments to the
method. These arguments are always evaluated prior to being bound to their
corresponding formal arguments. The result of evaluating the method becomes
the result of the expression.</p>
<p>Within the body of a method, a message can be sent to the current object
by calling:</p>
<pre class="example">
(send self ... )
</pre>
<p>The method lookup starts with the object's class regardless of the class
containing the current method.</p>
<p><nobr> <a href="#top">Back to Top</a></nobr></p>
<a name="4"></a>
<hr>
<h2>4 The 'send-super' Function</h2>
<hr>
<p>Sometimes it is desirable to invoke a general method in a superclass even
when it is overridden by a more specific method in a subclass. This can be
accomplished by calling <a
href="../reference/send-super.htm">send-super</a>, which begins the
method lookup in the superclass of the class defining the current method
rather than in the class of the current object:</p>
<p><div class="box">
<dl>
<dt>(<b>send-super</b> <i>selector</i> [<i>args</i>])</dt>
<dd><i>selector</i> - the message selector<br>
<i>args</i> - the optional message arguments<br>
returns - the result of sending the message</dd>
</dl>
</div></p>
<p>The <a href="../reference/send-super.htm">send-super</a> function
takes a selector as its first argument [which must be a symbol] and the
message arguments as its remaining arguments. Notice that <a
href="../reference/send-super.htm">send-super</a> can only be sent from
within a method, and the target of the message is always the current object
<a href="../reference/self.htm">self</a>.</p>
<pre class="example">
(send-super ... )
</pre>
<p>is similar to:</p>
<pre class="example">
(send self ... )
</pre>
<p>except that method lookup begins in the superclass of the class
containing the current method rather than the class of the current
object.</p>
<p><nobr> <a href="#top">Back to Top</a></nobr></p>
<a name="5"></a>
<a name="object"></a>
<a name="object-show"></a>
<a name="object-class"></a>
<a name="object-isnew"></a>
<a name="send-super"></a>
<hr>
<h2>5 The 'object' Class</h2>
<hr>
<p><div class="box">
<p><a href="../reference/object.htm">object</a> - the top of the class hierarchy.</p>
</div></p>
<p>Messages:</p>
<p><div class="box">
<dl>
<dt>(<a href="../reference/send.htm">send</a> <i>object</i> <a href="../reference/keyword-show.htm">:show</a>)
- show an object's instance variables.</dt>
<dd>returns - the object</dd>
<br>
<dt>(<a href="../reference/send.htm">send</a> <i>object</i> <a href="../reference/keyword-class.htm">:class</a>)
- return the class of an object</dt>
<dd>returns - the class of the object</dd>
<br>
<dt>(<a href="../reference/send.htm">send</a> <i>object</i> <a href="../reference/keyword-isnew.htm">:isnew</a> <i>args</i>)
- run the default object initialization routine</dt>
<dd>returns - the object</dd>
<br>
<dt>(<a href="../reference/send.htm">send</a> <i>object</i> <a href="../reference/keyword-isa.htm">:isa</a> <i>class</i>)
- test if <i>object</i> inherits from <i>class</i></dt>
<dd>returns - <a href="../reference/t.htm"> T </a> if object
is an instance of class or a subclass of class, otherwise
<a href="../reference/nil.htm">NIL</a></dd>
</dl>
</div></p>
<p><nobr> <a href="#top">Back to Top</a></nobr></p>
<a name="6"></a>
<a name="class"></a>
<a name="class-new"></a>
<a name="class-isnew"></a>
<a name="class-answer"></a>
<hr>
<h2>6 The 'class' Class</h2>
<hr>
<p><div class="box">
<p><a href="../reference/class.htm">class</a>
- class of all object classes (including itself)</p>
</div></p>
<p>Messages:</p>
<p><div class="box">
<dl>
<dt>(<a href="../reference/send.htm">send</a> <i>class</i> <a href="../reference/keyword-new.htm">:new</a> <i>ivars</i> [<i>cvars</i> [<i>super</i>]])
- create a new instance of a class</dt>
<dd>returns - the new class object</dd>
<br>
<dt>(<a href="../reference/send.htm">send</a> <i>class</i> <a href="../reference/keyword-isnew.htm">:isnew</a> <i>ivars</i> [<i>cvars</i> [<i>super</i>]])
- initialize a new class</dt>
<dd><i>ivars</i> - list of instance variable symbols<br>
<i>cvars</i> - list of class variable symbols<br>
<i>super</i> - the superclass, default is <a href="../reference/object.htm">object</a><br>
returns - the new class object</dd>
<br>
<dt>(<a href="../reference/send.htm">send</a> <i>class</i> <a href="../reference/keyword-answer.htm">:answer</a> <i>selector</i> <i>fargs</i> <i>body</i>)
- add a message to a class</dt>
<dd><i>selector</i> - a message selector symbol<br>
<i>fargs</i> - the formal argument list, a lambda list<br>
<i>body</i> - a list of executable expressions<br>
returns - the object</dd>
</dl>
</div></p>
<p>When a new instance of a
<a href="../reference/class.htm">class</a> is created by sending
the message <a href="../reference/keyword-new.htm">:new</a>
to an existing <a href="../reference/class.htm">class</a>, the
message <a href="../reference/keyword-isnew.htm">:isnew</a> followed by
whatever parameters were passed to the
<a href="../reference/keyword-new.htm">:new</a> message is sent to the
newly created <a href="../reference/object.htm">object</a>.</p>
<p>When a new <a href="../reference/class.htm">class</a> is created
by sending the <a href="../reference/keyword-new.htm">:new</a> message to
the object <a href="../reference/class.htm">class</a>, an optional
parameter may be specified indicating the superclass of the new
<a href="../reference/class.htm">class</a>. If this parameter is
omitted, the new <a href="../reference/class.htm">class</a> will be
a subclass of <a href="../reference/object.htm">object</a>. A
<a href="../reference/class.htm">class</a> inherits all instance
variables, class variables, and methods from its superclass.</p>
<p><nobr> <a href="#top">Back to Top</a></nobr></p>
<hr>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="contents.htm">Contents</a> |
<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="../reference/reference-index.htm">Reference</a>
</body></html>
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