1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
|
<!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>Matching Two Patterns</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<link rel="stylesheet" href="../../stylesheets/pyke.css" type="text/css" />
</head>
<body>
<table id="page-table">
<thead class="head">
<tr id="header1"><th id="header" colspan="3">
</th></tr>
<tr id="header2">
<th id="crumb-left"></th>
<th id="crumb-line">
<div id="nav">
<ul>
<li><a href="../../index.html">Home</a></li>
<li>></li>
<li><a href="../index.html">Logic Programming</a></li>
<li>></li>
<li><a href="index.html">Pattern Matching</a></li>
<li>></li>
<li>Matching 2 Patterns</li>
</ul>
</div>
</th>
<th id="crumb-right"></th>
</tr>
</thead>
<tbody id="body">
<tr id="body-tr">
<td id="left-nav">
<div id="left-nav-div">
<div class="title-nav"><a href="../../index.html">Home</a></div><div class="nav-branch">
<div class="normal-nav"><a href="../../about_pyke/index.html">About Pyke</a></div>
<div class="title-nav"><a href="../index.html">Logic Programming</a></div><div class="nav-branch">
<div class="normal-nav"><a href="../statements.html">Statements</a></div>
<div class="title-nav"><a href="index.html">Pattern Matching</a></div><div class="nav-branch">
<div class="normal-nav"><a href="literal_patterns.html">Literal Patterns</a></div>
<div class="normal-nav"><a href="pattern_variables.html">Pattern Variables</a></div>
<div class="normal-nav"><a href="tuple_patterns.html">Tuple Patterns</a></div>
<div class="normal-nav"><a href="matching_patterns.html">Matching 2 Patterns</a></div>
</div>
<div class="normal-nav"><a href="../rules/index.html">Rules</a></div>
<div class="normal-nav"><a href="../plans.html">Plans</a></div>
</div>
<div class="normal-nav"><a href="../../knowledge_bases/index.html">Knowledge Bases</a></div>
<div class="normal-nav"><a href="../../pyke_syntax/index.html">Pyke Syntax</a></div>
<div class="normal-nav"><a href="../../using_pyke/index.html">Using Pyke</a></div>
<div class="normal-nav"><a href="../../examples.html">Examples</a></div>
<div class="normal-nav"><a href="../../PyCon2008-paper.html">PyCon 2008 Paper</a></div>
</div>
</div>
<div id="icons">
<div id="project-page">
<a href="http://sourceforge.net/projects/pyke/">Pyke Project Page</a>
</div>
Please Make a Donation:<br />
<a href="http://sourceforge.net/donate/index.php?group_id=207724">
<img src="http://images.sourceforge.net/images/project-support.jpg"
width="88" height="32" border="0"
alt="Support This Project" /> </a> <br /><br />
Hosted by: <br />
<a href="http://sourceforge.net/projects/pyke">
<img src="http://sflogo.sourceforge.net/sflogo.php?group_id=207724&type=14"
width="150" height="40"
alt="Get Python Knowledge Engine (PyKE) at SourceForge.net. Fast, secure and Free Open Source software downloads" /></a>
</div>
</td>
<td id="main-td">
<div id="main">
<a name="startcontent" id="startcontent"></a>
<div class="document" id="matching-two-patterns">
<h1 class="title">Matching Two Patterns</h1>
<p>You've now seen all of the different kinds of patterns and how they are
matched to data. Not too bad so far...</p>
<p>Patterns are used to generalize statements. One situation where you need to
generalize a statement is when you want to ask a question. That's been
covered above.</p>
<p>The other situation where you need to generalize a statement is when you write
<a class="reference external" href="../rules/index.html">rules</a>, which are explained later.</p>
<p>But rules also need to be able to match one pattern to another <em>pattern</em>,
and not just match patterns to <em>data</em> as we have discussed so far.</p>
<p>So before we can move on to rules, we need to examine how one pattern is
matched to another pattern.</p>
<p>The short answer is that it all comes down to pattern variables and that
pattern variables may not only be bound to <em>data</em>, but may also be bound to
other <em>patterns</em>.</p>
<div class="section" id="binding-to-a-literal-pattern">
<h2>Binding to a Literal Pattern</h2>
<p>Binding a pattern variable to a literal pattern is just like binding it to the
data within that literal pattern. Nothing fancy here!</p>
</div>
<div class="section" id="binding-to-another-pattern-variable">
<h2>Binding to Another Pattern Variable</h2>
<p>When pattern variable A is bound to pattern variable B, they essentially
become the <em>same pattern variable</em>. Basically, pattern variable A <em>becomes</em>
pattern variable B (or, you might say, <em>defers</em> to pattern variable B).</p>
<p>Let's say that pattern variable A has been bound to pattern variable B and
that pattern variable B is still unbound.</p>
<ol class="arabic simple">
<li>Prior to binding pattern variable B to a value, it doesn't matter whether
you ask if pattern variable A is bound or pattern variable B is bound, the
answer is False for both.</li>
<li>It doesn't matter whether you match pattern variable A to a value or
pattern variable B to a value. In both cases, it is pattern variable B
that gets bound to this value.</li>
<li>And once pattern variable B is bound to a value, it doesn't matter whether
you ask for the bound value of pattern variable A or pattern variable B,
you will get the same value.</li>
</ol>
<p>So for all intents and purposes pattern variable A and pattern variable B
become the same pattern variable.</p>
</div>
<div class="section" id="binding-to-a-tuple-pattern">
<h2>Binding to a Tuple Pattern</h2>
<p>Because pattern variables may be bound to tuple patterns, the term <em>fully
bound</em> is introduced. Asking whether the pattern variable is <em>fully bound</em>
means that not only is it bound to a value (the tuple pattern), but that all
of the subordinate patterns (recursively) within the tuple pattern are also
bound to values.</p>
<p>Being <em>fully bound</em> means that the bound value of the pattern variable can be
converted into standard Python data without any pattern variables in it. This
is important when Pyke wants to talk to Python because Python has no concept
of storing <em>variables</em> within its data structures.</p>
</div>
<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>
<p>The answer is <a class="reference external" href="pathological_answer.html">here</a> (but no peeking!).</p>
<!-- ADD_LINKS MARKER -->
</div>
</div>
<!-- <div id="return-to-top">
<a href="#">Return to Top</a>
</div>
-->
</div>
</td>
<td id="right-nav">
<div id="right-nav-div">
<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>
</td>
</tr>
</tbody>
<tfoot id="foot">
<tr id="foot2">
<td id="copyright" colspan="3">
Copyright © 2007-2009 Bruce Frederiksen
</td>
</tr>
</tfoot>
</table>
<div id="last-modified">
Page last modified
Mon, Oct 27 2008.
</div>
<script type="text/javascript">
var gaJsHost = (("https:" == document.location.protocol) ?
"https://ssl." : "http://www.");
document.write(unescape("%3Cscript src='" + gaJsHost +
"google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E"));
</script>
<script type="text/javascript">
try {
var pageTracker = _gat._getTracker("UA-6310805-1");
pageTracker._trackPageview();
} catch(err) {}
</script>
</body>
</html>
|
