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<title>avram - a virtual machine code interpreter: 2.7.13.2 Filter</title>

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<a name="Filter"></a>
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<a name="Filter-1"></a>
<h4 class="subsubsection">2.7.13.2 Filter</h4>

<a name="index-filter"></a>
<p>Another well known list operation is that which applies a predicate to
every item of a list, and deletes those for which the predicate is
false. For a predicate with virtual code <code><var>p</var></code>, such an
application can be coded conveniently in this form,
</p>
<dl compact="compact">
<dt> <em>T23</em></dt>
<dd><p>[[<code>filter</code>]] <code><var>p</var></code> = <code>((nil,nil),(nil,(<var>p</var>,nil)))</code>
</p></dd>
</dl>

<p>which is to say that writing <code>((nil,nil),(nil,(<var>p</var>,nil)))</code> in
<code>silly</code> is the same as writing <code>filter <var>p</var></code>.
</p>
<p>The virtual machine detects code of this form provided that
<code><var>p</var></code> is other than <code>nil</code>, and evaluates it consistently
with the following properties, causing it to have the meaning that it
does.
</p>
<dl compact="compact">
<dt> <em>P26</em></dt>
<dd><p>([[<code>filter</code>]] <code><var>p</var></code>) <code>nil</code> = <code>nil</code>
</p></dd>
<dt> <em>P27</em></dt>
<dd><p>([[<code>filter</code>]] <code><var>p</var></code>) <code>(<var>x</var>,<var>y</var>)</code> =
([[<code>filter</code>]] <code><var>p</var></code>) <code><var>y</var></code> if <code><var>p</var> <code><var>x</var></code></code> = <code>nil</code>
</p></dd>
<dt> <em>P28</em></dt>
<dd><p>([[<code>filter</code>]] <code><var>p</var></code>) <code>(<var>x</var>,<var>y</var>)</code> =
<code>(<var>x</var>,</code>([[<code>filter</code>]] <code><var>p</var></code>) <code><var>y</var>)</code> if <code><var>p</var> <var>x</var></code> is a non-<code>nil</code> tree
</p></dd>
</dl>

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