/tutorial/backtracking/expand_expression/expand_expression.html

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  3. <title>The tutorial expand_expression</title>
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  73. 

  74. <h1>The tutorial expand_expression</h1>
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  75. <div class="i">
    76. 

  76. <p>This tutorial shows basic use of the cluster backtracking.</p>
    77. 

  77. <p>The same example: expanding syntactic expression, is
    78. 

  78. written using two differents ways: <span class="class">ABSTRACT</span> and <span class="class">TREE</span>.
    79. 

  79. It is recommended to start with <span class="class">TREE</span>.</p></div>
    80. 

  80. <a name="sommaire"/><h2>Sommaire</h2>
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  81. <div class="s"><ul class="s">
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  82.  <li><a href="#chap-1">How to compile?</a></li>
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  83.  <li><a href="#chap-2">What does it do?</a></li>
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  84.  <li><a href="#chap-3">How does 'tree' works?</a></li>
    85. 

  85.  <li><a href="#chap-4">How does 'abstract' works?</a></li>
    86. 

  86. </ul></div>
    87. 

  87. 

  88. <a name="chap-1"/><h2>How to compile?</h2>
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  89. <div class="c2">
    90. 

  90. <p>In both case, just type:</p>
    91. 

  91. <p>  se c -clean -o a_nick_name expand_expression</p>
    92. 

  92. </div>
    93. 

  93. <a name="chap-2"/><h2>What does it do?</h2>
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  94. <div class="c2">
    95. 

  95. <p>Each of the 2 programs performs syntaxic expansion.
    96. 

  96. For example, putting the line</p>
    97. 

  97. <p><pre>
    98. 

  98. 	(Hello + Hi) the ( + great) world!
    99. 

  99. </pre></p>
    100. 

  100. <p>at input will produce the output:</p>
    101. 

  101. <p><pre>
    102. 

  102.         (1)     Hello the world!
    103. 

  103.         (2)     Hello the great world!
    104. 

  104.         (3)     Hi the world!
    105. 

  105.         (4)     Hi the great world!
    106. 

  106. </pre></p>
    107. 

  107. <p>The grammar of the inputs are:</p>
    108. 

  108. <p><pre>
    109. 

  109.   input       ::= alternative;
    110. 

  110.   alternative ::= sequence ['+' sequence]...;
    111. 

  111.   sequence    ::= [term]...;
    112. 

  112.   term        ::= '(' alternative ')' | "a term";
    113. 

  113. </pre></p>
    114. 

  114. <p>Exercice: write a program that does the same syntaxic expansion.
    115. 

  115. (trick: reuse the parser of the tutorial)</p>
    116. 

  116. </div>
    117. 

  117. <a name="chap-3"/><h2>How does 'tree' works?</h2>
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  118. <div class="c2">
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  119. <p>The tree example uses the most usable class of the
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  120. backtracking cluster: the class <span class="class">BACKTRACKING</span>.</p>
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  121. <p>The class <span class="class">BACKTRACKING</span> make an exploration of the
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  122. and/or graphes made of <span class="class">BACKTRACKING_NODE</span> instances. Many
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  123. usefull <span class="class">BACKTRACKING_NODE</span> inheriters are defined and 
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  124. useables.</p>
    125. 

  125. <p>The syntaxic expression is transformed to a such
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  126. and/or structure. Alternatives does use <span class="class">BACKTRACKING_NODE_OR_PAIR</span>
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  127. that make a or between two nodes. Sequences does use
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  128. <span class="class">BACKTRACKING_NODE_AND_PAIR</span> that make a and between to nodes.
    129. 

  129. In both cases, the order of the nodes is meaningful.
    130. 

  130. Two other predefined nodes are used: the_true_node
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  131. (of class <span class="class">BACKTRACKING_NODE_TRUE</span>) for empty terms, and,
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  132. the_false_node (of class <span class="class">BACKTRACKING_NODE_FALSE</span>) for 
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  133. errors of syntax.</p>
    134. 

  134. <p>To get practical results, the terms are using a local
    135. 

  135. class <span class="class">STRING_NODE</span> that records what to print in a local
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  136. field. When explored these nodes push the string on the
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  137. and call the feature continue. Not that covariance is
    138. 

  138. used.</p>
    139. 

  139. <p>The use of that class is easy: define the features
    140. 

  140. 'get_context' and 'restore_context'. These features
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  141. will be called during the exploration to save the
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  142. context before to explore an alternative and to restore
    143. 

  143. it during the backtracking. These feature are defined
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  144. as returning <span class="class">ANY</span> but please make a covariant redefinition
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  145. as in exemples, where the context, an <span class="class">INTEGER</span>, records the 
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  146. count of items pushed.</p>
    147. 

  147. </div>
    148. 

  148. <a name="chap-4"/><h2>How does 'abstract' works?</h2>
    149. 

  149. <div class="c2">
    150. 

  150. <p>The abstract example does exactly the same that tree except
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  151. that it uses a more abstract class <span class="class">ABSTRACT_BACKTRACKING</span>.</p>
    152. 

  152. <p>That class is intended to be used when structures to be
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  153. explored can not be derived from <span class="class">BACKTRACKING_NODE</span>. </p>
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  154. </div>
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