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<title>Slot Description Example (Guile Reference Manual)</title>
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<p>
Next: <a href="Methods-and-Generic-Functions.html" accesskey="n" rel="next">Methods and Generic Functions</a>, Previous: <a href="Slot-Options.html" accesskey="p" rel="prev">Slot Options</a>, Up: <a href="GOOPS.html" accesskey="u" rel="up">GOOPS</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
<h3 class="section" id="Illustrating-Slot-Description"><span>8.5 Illustrating Slot Description<a class="copiable-link" href="#Illustrating-Slot-Description"> &para;</a></span></h3>
<p>To illustrate slot description, we can redefine the <code class="code">&lt;my-complex&gt;</code>
class seen before. A definition could be:
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define-class &lt;my-complex&gt; (&lt;number&gt;)
(r #:init-value 0 #:getter get-r #:setter set-r! #:init-keyword #:r)
(i #:init-value 0 #:getter get-i #:setter set-i! #:init-keyword #:i))
</pre></div>
<p>With this definition, the <code class="code">r</code> and <code class="code">i</code> slots are set to 0 by
default, and can be initialised to other values by calling <code class="code">make</code>
with the <code class="code">#:r</code> and <code class="code">#:i</code> keywords. Also the generic functions
<code class="code">get-r</code>, <code class="code">set-r!</code>, <code class="code">get-i</code> and <code class="code">set-i!</code> are
automatically defined to read and write the slots.
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define c1 (make &lt;my-complex&gt; #:r 1 #:i 2))
(get-r c1) &rArr; 1
(set-r! c1 12)
(get-r c1) &rArr; 12
(define c2 (make &lt;my-complex&gt; #:r 2))
(get-r c2) &rArr; 2
(get-i c2) &rArr; 0
</pre></div>
<p>Accessors can both read and write a slot. So, another definition of the
<code class="code">&lt;my-complex&gt;</code> class, using the <code class="code">#:accessor</code> option, could be:
</p>
<a class="index-entry-id" id="index-set_0021-1"></a>
<div class="example lisp">
<pre class="lisp-preformatted">(define-class &lt;my-complex&gt; (&lt;number&gt;)
(r #:init-value 0 #:accessor real-part #:init-keyword #:r)
(i #:init-value 0 #:accessor imag-part #:init-keyword #:i))
</pre></div>
<p>With this definition, the <code class="code">r</code> slot can be read with:
</p><div class="example lisp">
<pre class="lisp-preformatted">(real-part c)
</pre></div>
<p>and set with:
</p><div class="example lisp">
<pre class="lisp-preformatted">(set! (real-part c) new-value)
</pre></div>
<p>Suppose now that we want to manipulate complex numbers with both
rectangular and polar coordinates. One solution could be to have a
definition of complex numbers which uses one particular representation
and some conversion functions to pass from one representation to the
other. A better solution is to use virtual slots, like this:
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define-class &lt;my-complex&gt; (&lt;number&gt;)
;; True slots use rectangular coordinates
(r #:init-value 0 #:accessor real-part #:init-keyword #:r)
(i #:init-value 0 #:accessor imag-part #:init-keyword #:i)
;; Virtual slots access do the conversion
(m #:accessor magnitude #:init-keyword #:magn
#:allocation #:virtual
#:slot-ref (lambda (o)
(let ((r (slot-ref o 'r)) (i (slot-ref o 'i)))
(sqrt (+ (* r r) (* i i)))))
#:slot-set! (lambda (o m)
(let ((a (slot-ref o 'a)))
(slot-set! o 'r (* m (cos a)))
(slot-set! o 'i (* m (sin a))))))
(a #:accessor angle #:init-keyword #:angle
#:allocation #:virtual
#:slot-ref (lambda (o)
(atan (slot-ref o 'i) (slot-ref o 'r)))
#:slot-set! (lambda(o a)
(let ((m (slot-ref o 'm)))
(slot-set! o 'r (* m (cos a)))
(slot-set! o 'i (* m (sin a)))))))
</pre></div>
<p>In this class definition, the magnitude <code class="code">m</code> and angle <code class="code">a</code>
slots are virtual, and are calculated, when referenced, from the normal
(i.e. <code class="code">#:allocation #:instance</code>) slots <code class="code">r</code> and <code class="code">i</code>, by
calling the function defined in the relevant <code class="code">#:slot-ref</code> option.
Correspondingly, writing <code class="code">m</code> or <code class="code">a</code> leads to calling the
function defined in the <code class="code">#:slot-set!</code> option. Thus the
following expression
</p>
<a class="index-entry-id" id="index-_0023_003aslot_002dset_0021-2"></a>
<a class="index-entry-id" id="index-_0023_003aslot_002dref-2"></a>
<div class="example lisp">
<pre class="lisp-preformatted">(slot-set! c 'a 3)
</pre></div>
<p>permits to set the angle of the <code class="code">c</code> complex number.
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define c (make &lt;my-complex&gt; #:r 12 #:i 20))
(real-part c) &rArr; 12
(angle c) &rArr; 1.03037682652431
(slot-set! c 'i 10)
(set! (real-part c) 1)
(describe c)
-|
#&lt;&lt;my-complex&gt; 401e9b58&gt; is an instance of class &lt;my-complex&gt;
Slots are:
r = 1
i = 10
m = 10.0498756211209
a = 1.47112767430373
</pre></div>
<p>Since initialization keywords have been defined for the four slots, we
can now define the standard Scheme primitives <code class="code">make-rectangular</code>
and <code class="code">make-polar</code>.
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define make-rectangular
(lambda (x y) (make &lt;my-complex&gt; #:r x #:i y)))
(define make-polar
(lambda (x y) (make &lt;my-complex&gt; #:magn x #:angle y)))
</pre></div>
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Next: <a href="Methods-and-Generic-Functions.html">Methods and Generic Functions</a>, Previous: <a href="Slot-Options.html">Slot Options</a>, Up: <a href="GOOPS.html">GOOPS</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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