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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>
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