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<h4 class="subsection">3.1.6 Deconstruction Functions</h4>

<p>A fast native implementation of the deconstruction operation is provided
<a name="index-deconstruction-485"></a>by the functions declared in <samp><span class="file">decons.h</span></samp>.

<div class="defun">
&mdash; Function: void <b>avm_initialize_decons</b> ()<var><a name="index-avm_005finitialize_005fdecons-486"></a></var><br>
<blockquote><p>This should be called prior to the first call to
<code>avm_deconstruction</code>, so as to initialize some necessary internal
data structures. Results will be undefined if it is not. 
</p></blockquote></div>

<div class="defun">
&mdash; Function: void <b>avm_count_decons</b> ()<var><a name="index-avm_005fcount_005fdecons-487"></a></var><br>
<blockquote><p>For ecologically sound memory management, this function should be called
at the end of a run to verify that there have been no leaks due to the
deconstruction functions, which there won't be unless the code in
<samp><span class="file">decons.c</span></samp> has been ineptly modified. An error message to the
effect of unreclaimed &ldquo;points&rdquo; could be the result otherwise. 
</p></blockquote></div>

<div class="defun">
&mdash; Function: list <b>avm_deconstruction</b> (<var>list pointer, list operand, int *fault</var>)<var><a name="index-avm_005fdeconstruction-488"></a></var><br>
<blockquote><p>Deconstructions are performed by this function, as described in
<a href="Field.html#Field">Field</a>. In the <code>silly</code> program notation (<a href="A-Simple-Lisp-Like-Language.html#A-Simple-Lisp-Like-Language">A Simple Lisp Like Language</a>), this function computes the value
of ([[<code>field</code>]] <var>pointer</var>) <var>operand</var>.

        <p>For example, using the fixed list <code>avm_join(NULL,NULL)</code> as the
<var>pointer</var> parameter will cause a copy of the operand itself
to be returned as the result. A <var>pointer</var> equal to
<code>avm_join(NULL,avm_join(NULL,NULL))</code> will cause a copy of
<code>operand-&gt;tail</code> to be returned, and so on. A <code>NULL</code>
<var>pointer</var> causes an internal error.

        <p>If the deconstruction is invalid, as in the case of the tail of an empty
list, the invalid deconstruction error message is returned as the
result, and the <code>*</code><var>fault</var> parameter is set to a non-zero value. 
The <code>*</code><var>fault</var> parameter is also set to a non-zero value in the event
of a memory overflow, and the memory overflow message is returned. 
</p></blockquote></div>

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