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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<title>Pathological Answer</title>
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<link rel="stylesheet" href="../../stylesheets/pyke.css" type="text/css" />
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<tr id="header1"><th id="header" colspan="3">
&nbsp;
</th></tr>
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<th id="crumb-left"></th>
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<div id="nav">
<ul>
<li><a href="../../index.html">Home</a></li>
<li>&gt;</li>
<li><a href="../index.html">Logic Programming</a></li>
<li>&gt;</li>
<li><a href="index.html">Pattern Matching</a></li>
<li>&gt;</li>
<li>Pathological Answer</li>
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<div class="document" id="pathological-answer">
<h1 class="title">Pathological Answer</h1>
<p>This is the answer to the following question:</p>
<div class="section" id="pathological-question">
<h2>Pathological Question</h2>
<p>What is the bound value of pattern variable <tt class="docutils literal">$y</tt> after matching the
following two tuple patterns:</p>
<blockquote>
<dl class="docutils">
<dt>Tuple pattern A:</dt>
<dd><tt class="docutils literal">((ho, $_, ($a, <span class="pre">$a)),</span> ($a, $a, $b), ($a, *$b))</tt></dd>
<dt>Tuple pattern B:</dt>
<dd><tt class="docutils literal">($x, $x, $y)</tt></dd>
</dl>
</blockquote>
</div>
<div class="section" id="answer">
<h2>Answer</h2>
<p>Let's take this step by step, matching each element of the two tuple patterns
in turn.</p>
<ol class="arabic">
<li><p class="first">Match <tt class="docutils literal">(ho, $_, ($a, $a))</tt> to <tt class="docutils literal">$x</tt>.</p>
<p>This succeeds with the following binding:</p>
<dl class="docutils">
<dt><tt class="docutils literal">$x</tt>:</dt>
<dd><p class="first last"><tt class="docutils literal">(ho, $_, ($a, $a))</tt></p>
</dd>
</dl>
</li>
<li><p class="first">Match <tt class="docutils literal">($a, $a, $b)</tt> to <tt class="docutils literal">$x</tt>.</p>
<p>Because <tt class="docutils literal">$x</tt> is bound to a value, this becomes the same as matching:</p>
<ul class="simple">
<li><tt class="docutils literal">($a, $a, $b)</tt> to</li>
<li><tt class="docutils literal">(ho, $_, ($a, $a))</tt></li>
</ul>
<p>Which succeeds, binding:</p>
<dl class="docutils">
<dt><tt class="docutils literal">$a</tt>:</dt>
<dd><p class="first last"><tt class="docutils literal">ho</tt></p>
</dd>
<dt><tt class="docutils literal">$b</tt>:</dt>
<dd><p class="first last"><tt class="docutils literal">($a, $a)</tt></p>
</dd>
</dl>
<p><tt class="docutils literal">$_</tt> is an anonymous variable, so it is not bound (or bound to).</p>
</li>
<li><p class="first">Match <tt class="docutils literal">($a, *$b)</tt> to <tt class="docutils literal">$y</tt>.</p>
<p>Because both <tt class="docutils literal">$a</tt> and <tt class="docutils literal">$b</tt> have bound values, this becomes the same as
matching:</p>
<ul class="simple">
<li><tt class="docutils literal">(ho, ho, ho)</tt> to</li>
<li><tt class="docutils literal">$y</tt></li>
</ul>
<p>Which succeeds, binding:</p>
<dl class="docutils">
<dt><tt class="docutils literal">$y</tt>:</dt>
<dd><p class="first last"><tt class="docutils literal">(ho, ho, ho)</tt></p>
</dd>
</dl>
</li>
</ol>
<p>So the overall match succeeds with the following bindings:</p>
<blockquote>
<dl class="docutils">
<dt><tt class="docutils literal">$x</tt>:</dt>
<dd><tt class="docutils literal">(ho, $_, ($a, $a))</tt></dd>
<dt><tt class="docutils literal">$a</tt>:</dt>
<dd><tt class="docutils literal">ho</tt></dd>
<dt><tt class="docutils literal">$b</tt>:</dt>
<dd><tt class="docutils literal">($a, $a)</tt></dd>
<dt><tt class="docutils literal">$y</tt>:</dt>
<dd><tt class="docutils literal">(ho, ho, ho)</tt></dd>
</dl>
</blockquote>
<p>And so <tt class="docutils literal">$y</tt> is <tt class="docutils literal">(ho, ho, ho)</tt>!</p>
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">If you got this right, you should really be using Pyke!</p>
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<h3>More:</h3>
<div class="right-item"><a href="literal_patterns.html">Literal Patterns</a><p>Explanation of <em>literal patterns</em>.</p>
</div>
<div class="right-item"><a href="pattern_variables.html">Pattern Variables</a><p>Explanation of <em>pattern variables</em>.</p>
</div>
<div class="right-item"><a href="tuple_patterns.html">Tuple Patterns</a><p>Explanation of <em>tuple patterns</em>.</p>
</div>
<div class="right-item"><a href="matching_patterns.html">Matching Two Patterns</a><p>Explanation of matching two patterns together, vs matching a pattern
to data.</p>
</div>
</div>
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Copyright &copy; 2007-2009 Bruce Frederiksen
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<div id="last-modified">
Page last modified
Mon, Oct 27 2008.
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