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  35   <div class="section" id="treating-trees-ii-treestep">
  36 <h1>Treating Trees II: <code class="docutils literal notranslate"><span class="pre">treestep</span></code><a class="headerlink" href="#treating-trees-ii-treestep" title="Permalink to this headline">¶</a></h1>
  37 <p>Let’s consider a tree structure, similar to one described <a class="reference external" href="https://www.cs.kent.ac.uk/people/staff/dat/miranda/whyfp90.pdf">“Why
  38 functional programming matters” by John
  39 Hughes</a>,
  40 that consists of a node value followed by zero or more child trees. (The
  41 asterisk is meant to indicate the <a class="reference external" href="https://en.wikipedia.org/wiki/Kleene_star">Kleene
  42 star</a>.)</p>
  43 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">tree</span> <span class="o">=</span> <span class="p">[]</span> <span class="o">|</span> <span class="p">[</span><span class="n">node</span> <span class="n">tree</span><span class="o">*</span><span class="p">]</span>
  44 </pre></div>
  45 </div>
  46 <p>In the spirit of <code class="docutils literal notranslate"><span class="pre">step</span></code> we are going to define a combinator
  47 <code class="docutils literal notranslate"><span class="pre">treestep</span></code> which expects a tree and three additional items: a
  48 base-case function <code class="docutils literal notranslate"><span class="pre">[B]</span></code>, and two quoted programs <code class="docutils literal notranslate"><span class="pre">[N]</span></code> and <code class="docutils literal notranslate"><span class="pre">[C]</span></code>.</p>
  49 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">tree</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="n">treestep</span>
  50 </pre></div>
  51 </div>
  52 <p>If the current tree node is empty then just execute <code class="docutils literal notranslate"><span class="pre">B</span></code>:</p>
  53 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>   <span class="p">[]</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="n">treestep</span>
  54 <span class="o">---------------------------</span>
  55    <span class="p">[]</span>  <span class="n">B</span>
  56 </pre></div>
  57 </div>
  58 <p>Otherwise, evaluate <code class="docutils literal notranslate"><span class="pre">N</span></code> on the node value, <code class="docutils literal notranslate"><span class="pre">map</span></code> the whole function
  59 (abbreviated here as <code class="docutils literal notranslate"><span class="pre">K</span></code>) over the child trees recursively, and then
  60 combine the result with <code class="docutils literal notranslate"><span class="pre">C</span></code>.</p>
  61 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>   <span class="p">[</span><span class="n">node</span> <span class="n">tree</span><span class="o">*</span><span class="p">]</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="n">treestep</span>
  62 <span class="o">---------------------------------------</span> <span class="n">w</span><span class="o">/</span> <span class="n">K</span> <span class="o">==</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="n">treestep</span>
  63        <span class="n">node</span> <span class="n">N</span> <span class="p">[</span><span class="n">tree</span><span class="o">*</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">C</span>
  64 </pre></div>
  65 </div>
  66 <p>(Later on we’ll experiment with making <code class="docutils literal notranslate"><span class="pre">map</span></code> part of <code class="docutils literal notranslate"><span class="pre">C</span></code> so you can
  67 use other combinators.)</p>
  68 <div class="section" id="derive-the-recursive-function">
  69 <h2>Derive the recursive function.<a class="headerlink" href="#derive-the-recursive-function" title="Permalink to this headline">¶</a></h2>
  70 <p>We can begin to derive it by finding the <code class="docutils literal notranslate"><span class="pre">ifte</span></code> stage that <code class="docutils literal notranslate"><span class="pre">genrec</span></code>
  71 will produce.</p>
  72 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">K</span> <span class="o">==</span> <span class="p">[</span><span class="ow">not</span><span class="p">]</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">R0</span><span class="p">]</span>   <span class="p">[</span><span class="n">R1</span><span class="p">]</span> <span class="n">genrec</span>
  73   <span class="o">==</span> <span class="p">[</span><span class="ow">not</span><span class="p">]</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">R0</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">R1</span><span class="p">]</span> <span class="n">ifte</span>
  74 </pre></div>
  75 </div>
  76 <p>So we just have to derive <code class="docutils literal notranslate"><span class="pre">J</span></code>:</p>
  77 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span> <span class="o">==</span> <span class="n">R0</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">R1</span>
  78 </pre></div>
  79 </div>
  80 <p>The behavior of <code class="docutils literal notranslate"><span class="pre">J</span></code> is to accept a (non-empty) tree node and arrive at
  81 the desired outcome.</p>
  82 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>       <span class="p">[</span><span class="n">node</span> <span class="n">tree</span><span class="o">*</span><span class="p">]</span> <span class="n">J</span>
  83 <span class="o">------------------------------</span>
  84    <span class="n">node</span> <span class="n">N</span> <span class="p">[</span><span class="n">tree</span><span class="o">*</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">C</span>
  85 </pre></div>
  86 </div>
  87 <p>So <code class="docutils literal notranslate"><span class="pre">J</span></code> will have some form like:</p>
  88 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span> <span class="o">==</span> <span class="o">...</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="o">...</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="o">...</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">...</span>
  89 </pre></div>
  90 </div>
  91 <p>Let’s dive in. First, unquote the node and <code class="docutils literal notranslate"><span class="pre">dip</span></code> <code class="docutils literal notranslate"><span class="pre">N</span></code>.</p>
  92 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">node</span> <span class="n">tree</span><span class="o">*</span><span class="p">]</span> <span class="n">uncons</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="n">dip</span>
  93 <span class="n">node</span> <span class="p">[</span><span class="n">tree</span><span class="o">*</span><span class="p">]</span>        <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="n">dip</span>
  94 <span class="n">node</span> <span class="n">N</span> <span class="p">[</span><span class="n">tree</span><span class="o">*</span><span class="p">]</span>
  95 </pre></div>
  96 </div>
  97 <p>Next, <code class="docutils literal notranslate"><span class="pre">map</span></code> <code class="docutils literal notranslate"><span class="pre">K</span></code> over the child trees and combine with <code class="docutils literal notranslate"><span class="pre">C</span></code>.</p>
  98 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">node</span> <span class="n">N</span> <span class="p">[</span><span class="n">tree</span><span class="o">*</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">C</span>
  99 <span class="n">node</span> <span class="n">N</span> <span class="p">[</span><span class="n">tree</span><span class="o">*</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">C</span>
 100 <span class="n">node</span> <span class="n">N</span> <span class="p">[</span><span class="n">K</span><span class="o">.</span><span class="n">tree</span><span class="o">*</span><span class="p">]</span>       <span class="n">C</span>
 101 </pre></div>
 102 </div>
 103 <p>So:</p>
 104 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span> <span class="o">==</span> <span class="n">uncons</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="n">dip</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">C</span>
 105 </pre></div>
 106 </div>
 107 <p>Plug it in and convert to <code class="docutils literal notranslate"><span class="pre">genrec</span></code>:</p>
 108 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">K</span> <span class="o">==</span> <span class="p">[</span><span class="ow">not</span><span class="p">]</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">J</span>                       <span class="p">]</span> <span class="n">ifte</span>
 109   <span class="o">==</span> <span class="p">[</span><span class="ow">not</span><span class="p">]</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">uncons</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="n">dip</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">C</span><span class="p">]</span> <span class="n">ifte</span>
 110   <span class="o">==</span> <span class="p">[</span><span class="ow">not</span><span class="p">]</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">uncons</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span>   <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span> <span class="n">genrec</span>
 111 </pre></div>
 112 </div>
 113 </div>
 114 <div class="section" id="extract-the-givens-to-parameterize-the-program">
 115 <h2>Extract the givens to parameterize the program.<a class="headerlink" href="#extract-the-givens-to-parameterize-the-program" title="Permalink to this headline">¶</a></h2>
 116 <p>Working backwards:</p>
 117 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="ow">not</span><span class="p">]</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span>          <span class="p">[</span><span class="n">uncons</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span>                  <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span> <span class="n">genrec</span>
 118 <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="ow">not</span><span class="p">]</span> <span class="n">swap</span>     <span class="p">[</span><span class="n">uncons</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span>                  <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span> <span class="n">genrec</span>
 119 <span class="p">[</span><span class="n">B</span><span class="p">]</span>                <span class="p">[</span><span class="n">uncons</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="p">[[</span><span class="ow">not</span><span class="p">]</span> <span class="n">swap</span><span class="p">]</span> <span class="n">dip</span> <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span> <span class="n">genrec</span>
 120                                     <span class="o">^^^^^^^^^^^^^^^^</span>
 121 <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[[</span><span class="n">N</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span>      <span class="p">[</span><span class="n">uncons</span><span class="p">]</span> <span class="n">swoncat</span> <span class="p">[[</span><span class="ow">not</span><span class="p">]</span> <span class="n">swap</span><span class="p">]</span> <span class="n">dip</span> <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span> <span class="n">genrec</span>
 122 <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="p">[</span><span class="n">dip</span><span class="p">]</span> <span class="n">cons</span> <span class="p">[</span><span class="n">uncons</span><span class="p">]</span> <span class="n">swoncat</span> <span class="p">[[</span><span class="ow">not</span><span class="p">]</span> <span class="n">swap</span><span class="p">]</span> <span class="n">dip</span> <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span> <span class="n">genrec</span>
 123         <span class="o">^^^^^^^^^^^^^^^^^^^^^^^^^^^</span>
 124 </pre></div>
 125 </div>
 126 <p>Extract a couple of auxiliary definitions:</p>
 127 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">TS</span><span class="o">.</span><span class="mi">0</span> <span class="o">==</span> <span class="p">[[</span><span class="ow">not</span><span class="p">]</span> <span class="n">swap</span><span class="p">]</span> <span class="n">dip</span>
 128 <span class="n">TS</span><span class="o">.</span><span class="mi">1</span> <span class="o">==</span> <span class="p">[</span><span class="n">dip</span><span class="p">]</span> <span class="n">cons</span> <span class="p">[</span><span class="n">uncons</span><span class="p">]</span> <span class="n">swoncat</span>
 129 </pre></div>
 130 </div>
 131 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="n">TS</span><span class="o">.</span><span class="mi">1</span> <span class="n">TS</span><span class="o">.</span><span class="mi">0</span> <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span>                         <span class="n">genrec</span>
 132 <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span>           <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span>         <span class="p">[</span><span class="n">TS</span><span class="o">.</span><span class="mi">1</span> <span class="n">TS</span><span class="o">.</span><span class="mi">0</span><span class="p">]</span> <span class="n">dip</span> <span class="n">genrec</span>
 133 <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span>         <span class="p">[</span><span class="nb">map</span><span class="p">]</span> <span class="n">swoncat</span> <span class="p">[</span><span class="n">TS</span><span class="o">.</span><span class="mi">1</span> <span class="n">TS</span><span class="o">.</span><span class="mi">0</span><span class="p">]</span> <span class="n">dip</span> <span class="n">genrec</span>
 134 </pre></div>
 135 </div>
 136 <p>The givens are all to the left so we have our definition.</p>
 137 <div class="section" id="alternate-extract-the-givens-to-parameterize-the-program">
 138 <h3>(alternate) Extract the givens to parameterize the program.<a class="headerlink" href="#alternate-extract-the-givens-to-parameterize-the-program" title="Permalink to this headline">¶</a></h3>
 139 <p>Working backwards:</p>
 140 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="ow">not</span><span class="p">]</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span>           <span class="p">[</span><span class="n">uncons</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span>            <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span> <span class="n">genrec</span>
 141 <span class="p">[</span><span class="ow">not</span><span class="p">]</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span>       <span class="p">[</span><span class="n">dip</span><span class="p">]</span> <span class="n">cons</span> <span class="p">[</span><span class="n">uncons</span><span class="p">]</span> <span class="n">swoncat</span> <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span> <span class="n">genrec</span>
 142 <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="p">[</span><span class="ow">not</span><span class="p">]</span> <span class="n">roll</span><span class="o">&gt;</span> <span class="p">[</span><span class="n">dip</span><span class="p">]</span> <span class="n">cons</span> <span class="p">[</span><span class="n">uncons</span><span class="p">]</span> <span class="n">swoncat</span> <span class="p">[</span><span class="nb">map</span> <span class="n">C</span><span class="p">]</span> <span class="n">genrec</span>
 143         <span class="o">^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^</span>
 144 </pre></div>
 145 </div>
 146 </div>
 147 </div>
 148 <div class="section" id="define-treestep">
 149 <h2>Define <code class="docutils literal notranslate"><span class="pre">treestep</span></code><a class="headerlink" href="#define-treestep" title="Permalink to this headline">¶</a></h2>
 150 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">notebook_preamble</span> <span class="k">import</span> <span class="n">D</span><span class="p">,</span> <span class="n">J</span><span class="p">,</span> <span class="n">V</span><span class="p">,</span> <span class="n">define</span><span class="p">,</span> <span class="n">DefinitionWrapper</span>
 151 </pre></div>
 152 </div>
 153 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">DefinitionWrapper</span><span class="o">.</span><span class="n">add_definitions</span><span class="p">(</span><span class="s1">&#39;&#39;&#39;</span>
 154
 155 <span class="s1">    _treestep_0 == [[not] swap] dip</span>
 156 <span class="s1">    _treestep_1 == [dip] cons [uncons] swoncat</span>
 157 <span class="s1">    treegrind == [_treestep_1 _treestep_0] dip genrec</span>
 158 <span class="s1">    treestep == [map] swoncat treegrind</span>
 159
 160 <span class="s1">&#39;&#39;&#39;</span><span class="p">,</span> <span class="n">D</span><span class="p">)</span>
 161 </pre></div>
 162 </div>
 163 </div>
 164 <div class="section" id="examples">
 165 <h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
 166 <p>Consider trees, the nodes of which are integers. We can find the sum of
 167 all nodes in a tree with this function:</p>
 168 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sumtree</span> <span class="o">==</span> <span class="p">[</span><span class="n">pop</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[]</span> <span class="p">[</span><span class="nb">sum</span> <span class="o">+</span><span class="p">]</span> <span class="n">treestep</span>
 169 </pre></div>
 170 </div>
 171 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">define</span><span class="p">(</span><span class="s1">&#39;sumtree == [pop 0] [] [sum +] treestep&#39;</span><span class="p">)</span>
 172 </pre></div>
 173 </div>
 174 <p>Running this function on an empty tree value gives zero:</p>
 175 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>   <span class="p">[]</span> <span class="p">[</span><span class="n">pop</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[]</span> <span class="p">[</span><span class="nb">sum</span> <span class="o">+</span><span class="p">]</span> <span class="n">treestep</span>
 176 <span class="o">------------------------------------</span>
 177            <span class="mi">0</span>
 178 </pre></div>
 179 </div>
 180 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[] sumtree&#39;</span><span class="p">)</span>  <span class="c1"># Empty tree.</span>
 181 </pre></div>
 182 </div>
 183 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">0</span>
 184 </pre></div>
 185 </div>
 186 <p>Running it on a non-empty node:</p>
 187 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">n</span> <span class="n">tree</span><span class="o">*</span><span class="p">]</span>  <span class="p">[</span><span class="n">pop</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[]</span> <span class="p">[</span><span class="nb">sum</span> <span class="o">+</span><span class="p">]</span> <span class="n">treestep</span>
 188 <span class="n">n</span> <span class="p">[</span><span class="n">tree</span><span class="o">*</span><span class="p">]</span> <span class="p">[[</span><span class="n">pop</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[]</span> <span class="p">[</span><span class="nb">sum</span> <span class="o">+</span><span class="p">]</span> <span class="n">treestep</span><span class="p">]</span> <span class="nb">map</span> <span class="nb">sum</span> <span class="o">+</span>
 189 <span class="n">n</span> <span class="p">[</span> <span class="o">...</span> <span class="p">]</span>                                   <span class="nb">sum</span> <span class="o">+</span>
 190 <span class="n">n</span> <span class="n">m</span>                                             <span class="o">+</span>
 191 <span class="n">n</span><span class="o">+</span><span class="n">m</span>
 192 </pre></div>
 193 </div>
 194 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[23] sumtree&#39;</span><span class="p">)</span>  <span class="c1"># No child trees.</span>
 195 </pre></div>
 196 </div>
 197 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">23</span>
 198 </pre></div>
 199 </div>
 200 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[23 []] sumtree&#39;</span><span class="p">)</span>  <span class="c1"># Child tree, empty.</span>
 201 </pre></div>
 202 </div>
 203 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">23</span>
 204 </pre></div>
 205 </div>
 206 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[23 [2 [4]] [3]] sumtree&#39;</span><span class="p">)</span>  <span class="c1"># Non-empty child trees.</span>
 207 </pre></div>
 208 </div>
 209 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">32</span>
 210 </pre></div>
 211 </div>
 212 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[23 [2 [8] [9]] [3] [4 []]] sumtree&#39;</span><span class="p">)</span>  <span class="c1"># Etc...</span>
 213 </pre></div>
 214 </div>
 215 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">49</span>
 216 </pre></div>
 217 </div>
 218 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[23 [2 [8] [9]] [3] [4 []]] [pop 0] [] [cons sum] treestep&#39;</span><span class="p">)</span>  <span class="c1"># Alternate &quot;spelling&quot;.</span>
 219 </pre></div>
 220 </div>
 221 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">49</span>
 222 </pre></div>
 223 </div>
 224 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[23 [2 [8] [9]] [3] [4 []]] [] [pop 23] [cons] treestep&#39;</span><span class="p">)</span>  <span class="c1"># Replace each node.</span>
 225 </pre></div>
 226 </div>
 227 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">23</span> <span class="p">[</span><span class="mi">23</span> <span class="p">[</span><span class="mi">23</span><span class="p">]</span> <span class="p">[</span><span class="mi">23</span><span class="p">]]</span> <span class="p">[</span><span class="mi">23</span><span class="p">]</span> <span class="p">[</span><span class="mi">23</span> <span class="p">[]]]</span>
 228 </pre></div>
 229 </div>
 230 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[23 [2 [8] [9]] [3] [4 []]] [] [pop 1] [cons] treestep&#39;</span><span class="p">)</span>
 231 </pre></div>
 232 </div>
 233 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">1</span><span class="p">]]</span> <span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">1</span> <span class="p">[]]]</span>
 234 </pre></div>
 235 </div>
 236 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[23 [2 [8] [9]] [3] [4 []]] [] [pop 1] [cons] treestep sumtree&#39;</span><span class="p">)</span>
 237 </pre></div>
 238 </div>
 239 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">6</span>
 240 </pre></div>
 241 </div>
 242 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[23 [2 [8] [9]] [3] [4 []]] [pop 0] [pop 1] [sum +] treestep&#39;</span><span class="p">)</span>  <span class="c1"># Combine replace and sum into one function.</span>
 243 </pre></div>
 244 </div>
 245 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">6</span>
 246 </pre></div>
 247 </div>
 248 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[4 [3 [] [7]]] [pop 0] [pop 1] [sum +] treestep&#39;</span><span class="p">)</span>  <span class="c1"># Combine replace and sum into one function.</span>
 249 </pre></div>
 250 </div>
 251 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">3</span>
 252 </pre></div>
 253 </div>
 254 </div>
 255 <div class="section" id="redefining-the-ordered-binary-tree-in-terms-of-treestep">
 256 <h2>Redefining the Ordered Binary Tree in terms of <code class="docutils literal notranslate"><span class="pre">treestep</span></code>.<a class="headerlink" href="#redefining-the-ordered-binary-tree-in-terms-of-treestep" title="Permalink to this headline">¶</a></h2>
 257 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">Tree</span> <span class="o">=</span> <span class="p">[]</span> <span class="o">|</span> <span class="p">[[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="n">left</span> <span class="n">right</span><span class="p">]</span>
 258 </pre></div>
 259 </div>
 260 <p>What kind of functions can we write for this with our <code class="docutils literal notranslate"><span class="pre">treestep</span></code>?</p>
 261 <p>The pattern for processing a non-empty node is:</p>
 262 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">node</span> <span class="n">N</span> <span class="p">[</span><span class="n">tree</span><span class="o">*</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">C</span>
 263 </pre></div>
 264 </div>
 265 <p>Plugging in our BTree structure:</p>
 266 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="n">N</span> <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">C</span>
 267 </pre></div>
 268 </div>
 269 <div class="section" id="traversal">
 270 <h3>Traversal<a class="headerlink" href="#traversal" title="Permalink to this headline">¶</a></h3>
 271 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="n">first</span> <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">i</span>
 272 <span class="n">key</span> <span class="p">[</span><span class="n">value</span><span class="p">]</span>       <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">i</span>
 273 <span class="n">key</span>               <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">i</span>
 274 <span class="n">key</span>               <span class="p">[</span><span class="n">lkey</span> <span class="n">rkey</span> <span class="p">]</span>         <span class="n">i</span>
 275 <span class="n">key</span>                <span class="n">lkey</span> <span class="n">rkey</span>
 276 </pre></div>
 277 </div>
 278 <p>This doesn’t quite work:</p>
 279 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[[3 0] [[2 0] [][]] [[9 0] [[5 0] [[4 0] [][]] [[8 0] [[6 0] [] [[7 0] [][]]][]]][]]] [&quot;B&quot;] [first] [i] treestep&#39;</span><span class="p">)</span>
 280 </pre></div>
 281 </div>
 282 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">3</span> <span class="s1">&#39;B&#39;</span> <span class="s1">&#39;B&#39;</span>
 283 </pre></div>
 284 </div>
 285 <p>Doesn’t work because <code class="docutils literal notranslate"><span class="pre">map</span></code> extracts the <code class="docutils literal notranslate"><span class="pre">first</span></code> item of whatever its
 286 mapped function produces. We have to return a list, rather than
 287 depositing our results directly on the stack.</p>
 288 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="n">N</span>     <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">C</span>
 289
 290 <span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="n">first</span> <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">flatten</span> <span class="n">cons</span>
 291 <span class="n">key</span>               <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="nb">map</span> <span class="n">flatten</span> <span class="n">cons</span>
 292 <span class="n">key</span>               <span class="p">[[</span><span class="n">lk</span><span class="p">]</span> <span class="p">[</span><span class="n">rk</span><span class="p">]</span> <span class="p">]</span>         <span class="n">flatten</span> <span class="n">cons</span>
 293 <span class="n">key</span>               <span class="p">[</span> <span class="n">lk</span>   <span class="n">rk</span>  <span class="p">]</span>                 <span class="n">cons</span>
 294                   <span class="p">[</span><span class="n">key</span>  <span class="n">lk</span>   <span class="n">rk</span>  <span class="p">]</span>
 295 </pre></div>
 296 </div>
 297 <p>So:</p>
 298 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[]</span> <span class="p">[</span><span class="n">first</span><span class="p">]</span> <span class="p">[</span><span class="n">flatten</span> <span class="n">cons</span><span class="p">]</span> <span class="n">treestep</span>
 299 </pre></div>
 300 </div>
 301 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[[3 0] [[2 0] [] []] [[9 0] [[5 0] [[4 0] [] []] [[8 0] [[6 0] [] [[7 0] [] []]] []]] []]]   [] [first] [flatten cons] treestep&#39;</span><span class="p">)</span>
 302 </pre></div>
 303 </div>
 304 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">3</span> <span class="mi">2</span> <span class="mi">9</span> <span class="mi">5</span> <span class="mi">4</span> <span class="mi">8</span> <span class="mi">6</span> <span class="mi">7</span><span class="p">]</span>
 305 </pre></div>
 306 </div>
 307 <p>There we go.</p>
 308 </div>
 309 <div class="section" id="in-order-traversal">
 310 <h3>In-order traversal<a class="headerlink" href="#in-order-traversal" title="Permalink to this headline">¶</a></h3>
 311 <p>From here:</p>
 312 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">key</span> <span class="p">[[</span><span class="n">lk</span><span class="p">]</span> <span class="p">[</span><span class="n">rk</span><span class="p">]]</span> <span class="n">C</span>
 313 <span class="n">key</span> <span class="p">[[</span><span class="n">lk</span><span class="p">]</span> <span class="p">[</span><span class="n">rk</span><span class="p">]]</span> <span class="n">i</span>
 314 <span class="n">key</span>  <span class="p">[</span><span class="n">lk</span><span class="p">]</span> <span class="p">[</span><span class="n">rk</span><span class="p">]</span> <span class="n">roll</span><span class="o">&lt;</span>
 315 <span class="p">[</span><span class="n">lk</span><span class="p">]</span> <span class="p">[</span><span class="n">rk</span><span class="p">]</span> <span class="n">key</span> <span class="n">swons</span> <span class="n">concat</span>
 316 <span class="p">[</span><span class="n">lk</span><span class="p">]</span> <span class="p">[</span><span class="n">key</span> <span class="n">rk</span><span class="p">]</span>       <span class="n">concat</span>
 317 <span class="p">[</span><span class="n">lk</span>   <span class="n">key</span> <span class="n">rk</span><span class="p">]</span>
 318 </pre></div>
 319 </div>
 320 <p>So:</p>
 321 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[]</span> <span class="p">[</span><span class="n">i</span> <span class="n">roll</span><span class="o">&lt;</span> <span class="n">swons</span> <span class="n">concat</span><span class="p">]</span> <span class="p">[</span><span class="n">first</span><span class="p">]</span> <span class="n">treestep</span>
 322 </pre></div>
 323 </div>
 324 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[[3 0] [[2 0] [] []] [[9 0] [[5 0] [[4 0] [] []] [[8 0] [[6 0] [] [[7 0] [] []]] []]] []]]   [] [uncons pop] [i roll&lt; swons concat] treestep&#39;</span><span class="p">)</span>
 325 </pre></div>
 326 </div>
 327 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">2</span> <span class="mi">3</span> <span class="mi">4</span> <span class="mi">5</span> <span class="mi">6</span> <span class="mi">7</span> <span class="mi">8</span> <span class="mi">9</span><span class="p">]</span>
 328 </pre></div>
 329 </div>
 330 </div>
 331 </div>
 332 <div class="section" id="with-treegrind">
 333 <h2>With <code class="docutils literal notranslate"><span class="pre">treegrind</span></code>?<a class="headerlink" href="#with-treegrind" title="Permalink to this headline">¶</a></h2>
 334 <p>The <code class="docutils literal notranslate"><span class="pre">treegrind</span></code> function doesn’t include the <code class="docutils literal notranslate"><span class="pre">map</span></code> combinator, so
 335 the <code class="docutils literal notranslate"><span class="pre">[C]</span></code> function must arrange to use some combinator on the quoted
 336 recursive copy <code class="docutils literal notranslate"><span class="pre">[K]</span></code>. With this function, the pattern for processing a
 337 non-empty node is:</p>
 338 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">node</span> <span class="n">N</span> <span class="p">[</span><span class="n">tree</span><span class="o">*</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">C</span>
 339 </pre></div>
 340 </div>
 341 <p>Plugging in our BTree structure:</p>
 342 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="n">N</span> <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">C</span>
 343 </pre></div>
 344 </div>
 345 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[[&quot;key&quot; &quot;value&quot;] [&quot;left&quot;] [&quot;right&quot;] ] [&quot;B&quot;] [&quot;N&quot;] [&quot;C&quot;] treegrind&#39;</span><span class="p">)</span>
 346 </pre></div>
 347 </div>
 348 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="s1">&#39;key&#39;</span> <span class="s1">&#39;value&#39;</span><span class="p">]</span> <span class="s1">&#39;N&#39;</span> <span class="p">[[</span><span class="s1">&#39;left&#39;</span><span class="p">]</span> <span class="p">[</span><span class="s1">&#39;right&#39;</span><span class="p">]]</span> <span class="p">[[</span><span class="ow">not</span><span class="p">]</span> <span class="p">[</span><span class="s1">&#39;B&#39;</span><span class="p">]</span> <span class="p">[</span><span class="n">uncons</span> <span class="p">[</span><span class="s1">&#39;N&#39;</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="p">[</span><span class="s1">&#39;C&#39;</span><span class="p">]</span> <span class="n">genrec</span><span class="p">]</span> <span class="s1">&#39;C&#39;</span>
 349 </pre></div>
 350 </div>
 351 </div>
 352 <div class="section" id="treegrind-with-step">
 353 <h2><code class="docutils literal notranslate"><span class="pre">treegrind</span></code> with <code class="docutils literal notranslate"><span class="pre">step</span></code><a class="headerlink" href="#treegrind-with-step" title="Permalink to this headline">¶</a></h2>
 354 <p>Iteration through the nodes</p>
 355 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[[3 0] [[2 0] [] []] [[9 0] [[5 0] [[4 0] [] []] [[8 0] [[6 0] [] [[7 0] [] []]] []]] []]]   [pop] [&quot;N&quot;] [step] treegrind&#39;</span><span class="p">)</span>
 356 </pre></div>
 357 </div>
 358 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="mi">3</span> <span class="mi">0</span><span class="p">]</span> <span class="s1">&#39;N&#39;</span> <span class="p">[</span><span class="mi">2</span> <span class="mi">0</span><span class="p">]</span> <span class="s1">&#39;N&#39;</span> <span class="p">[</span><span class="mi">9</span> <span class="mi">0</span><span class="p">]</span> <span class="s1">&#39;N&#39;</span> <span class="p">[</span><span class="mi">5</span> <span class="mi">0</span><span class="p">]</span> <span class="s1">&#39;N&#39;</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">0</span><span class="p">]</span> <span class="s1">&#39;N&#39;</span> <span class="p">[</span><span class="mi">8</span> <span class="mi">0</span><span class="p">]</span> <span class="s1">&#39;N&#39;</span> <span class="p">[</span><span class="mi">6</span> <span class="mi">0</span><span class="p">]</span> <span class="s1">&#39;N&#39;</span> <span class="p">[</span><span class="mi">7</span> <span class="mi">0</span><span class="p">]</span> <span class="s1">&#39;N&#39;</span>
 359 </pre></div>
 360 </div>
 361 <p>Sum the nodes’ keys.</p>
 362 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;0 [[3 0] [[2 0] [] []] [[9 0] [[5 0] [[4 0] [] []] [[8 0] [[6 0] [] [[7 0] [] []]] []]] []]]   [pop] [first +] [step] treegrind&#39;</span><span class="p">)</span>
 363 </pre></div>
 364 </div>
 365 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">44</span>
 366 </pre></div>
 367 </div>
 368 <p>Rebuild the tree using <code class="docutils literal notranslate"><span class="pre">map</span></code> (imitating <code class="docutils literal notranslate"><span class="pre">treestep</span></code>.)</p>
 369 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;[[3 0] [[2 0] [] []] [[9 0] [[5 0] [[4 0] [] []] [[8 0] [[6 0] [] [[7 0] [] []]] []]] []]]   [] [[100 +] infra] [map cons] treegrind&#39;</span><span class="p">)</span>
 370 </pre></div>
 371 </div>
 372 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[[</span><span class="mi">103</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[[</span><span class="mi">102</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[]</span> <span class="p">[]]</span> <span class="p">[[</span><span class="mi">109</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[[</span><span class="mi">105</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[[</span><span class="mi">104</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[]</span> <span class="p">[]]</span> <span class="p">[[</span><span class="mi">108</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[[</span><span class="mi">106</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[]</span> <span class="p">[[</span><span class="mi">107</span> <span class="mi">0</span><span class="p">]</span> <span class="p">[]</span> <span class="p">[]]]</span> <span class="p">[]]]</span> <span class="p">[]]]</span>
 373 </pre></div>
 374 </div>
 375 </div>
 376 <div class="section" id="do-we-have-the-flexibility-to-reimplement-tree-get">
 377 <h2>Do we have the flexibility to reimplement <code class="docutils literal notranslate"><span class="pre">Tree-get</span></code>?<a class="headerlink" href="#do-we-have-the-flexibility-to-reimplement-tree-get" title="Permalink to this headline">¶</a></h2>
 378 <p>I think we do:</p>
 379 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">N</span><span class="p">]</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="n">treegrind</span>
 380 </pre></div>
 381 </div>
 382 <p>We’ll start by saying that the base-case (the key is not in the tree) is
 383 user defined, and the per-node function is just the query key literal:</p>
 384 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="p">[</span><span class="n">query_key</span><span class="p">]</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="n">treegrind</span>
 385 </pre></div>
 386 </div>
 387 <p>This means we just have to define <code class="docutils literal notranslate"><span class="pre">C</span></code> from:</p>
 388 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="n">query_key</span> <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">C</span>
 389 </pre></div>
 390 </div>
 391 <p>Let’s try <code class="docutils literal notranslate"><span class="pre">cmp</span></code>:</p>
 392 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">C</span> <span class="o">==</span> <span class="n">P</span> <span class="p">[</span><span class="n">T</span><span class="o">&gt;</span><span class="p">]</span> <span class="p">[</span><span class="n">E</span><span class="p">]</span> <span class="p">[</span><span class="n">T</span><span class="o">&lt;</span><span class="p">]</span> <span class="nb">cmp</span>
 393
 394 <span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="n">query_key</span> <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">P</span> <span class="p">[</span><span class="n">T</span><span class="o">&gt;</span><span class="p">]</span> <span class="p">[</span><span class="n">E</span><span class="p">]</span> <span class="p">[</span><span class="n">T</span><span class="o">&lt;</span><span class="p">]</span> <span class="nb">cmp</span>
 395 </pre></div>
 396 </div>
 397 <div class="section" id="the-predicate-p">
 398 <h3>The predicate <code class="docutils literal notranslate"><span class="pre">P</span></code><a class="headerlink" href="#the-predicate-p" title="Permalink to this headline">¶</a></h3>
 399 <p>Seems pretty easy (we must preserve the value in case the keys are
 400 equal):</p>
 401 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="n">query_key</span> <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">P</span>
 402 <span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="n">query_key</span> <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">roll</span><span class="o">&lt;</span>
 403 <span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">query_key</span>       <span class="p">[</span><span class="n">roll</span><span class="o">&lt;</span> <span class="n">uncons</span> <span class="n">swap</span><span class="p">]</span> <span class="n">dip</span>
 404
 405 <span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span> <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">roll</span><span class="o">&lt;</span> <span class="n">uncons</span> <span class="n">swap</span> <span class="n">query_key</span>
 406 <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="p">[</span><span class="n">key</span> <span class="n">value</span><span class="p">]</span>       <span class="n">uncons</span> <span class="n">swap</span> <span class="n">query_key</span>
 407 <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="n">key</span> <span class="p">[</span><span class="n">value</span><span class="p">]</span>              <span class="n">swap</span> <span class="n">query_key</span>
 408 <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="p">[</span><span class="n">value</span><span class="p">]</span> <span class="n">key</span>                   <span class="n">query_key</span>
 409
 410 <span class="n">P</span> <span class="o">==</span> <span class="n">roll</span><span class="o">&lt;</span> <span class="p">[</span><span class="n">roll</span><span class="o">&lt;</span> <span class="n">uncons</span> <span class="n">swap</span><span class="p">]</span> <span class="n">dip</span>
 411 </pre></div>
 412 </div>
 413 <p>(Possibly with a swap at the end? Or just swap <code class="docutils literal notranslate"><span class="pre">T&lt;</span></code> and <code class="docutils literal notranslate"><span class="pre">T&gt;</span></code>.)</p>
 414 <p>So now:</p>
 415 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="p">[</span><span class="n">value</span><span class="p">]</span> <span class="n">key</span> <span class="n">query_key</span> <span class="p">[</span><span class="n">T</span><span class="o">&gt;</span><span class="p">]</span> <span class="p">[</span><span class="n">E</span><span class="p">]</span> <span class="p">[</span><span class="n">T</span><span class="o">&lt;</span><span class="p">]</span> <span class="nb">cmp</span>
 416 </pre></div>
 417 </div>
 418 <p>Becomes one of these three:</p>
 419 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="p">[</span><span class="n">value</span><span class="p">]</span> <span class="n">T</span><span class="o">&gt;</span>
 420 <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="p">[</span><span class="n">value</span><span class="p">]</span> <span class="n">E</span>
 421 <span class="p">[</span><span class="n">left</span> <span class="n">right</span><span class="p">]</span> <span class="p">[</span><span class="n">K</span><span class="p">]</span> <span class="p">[</span><span class="n">value</span><span class="p">]</span> <span class="n">T</span><span class="o">&lt;</span>
 422 </pre></div>
 423 </div>
 424 </div>
 425 <div class="section" id="e">
 426 <h3><code class="docutils literal notranslate"><span class="pre">E</span></code><a class="headerlink" href="#e" title="Permalink to this headline">¶</a></h3>
 427 <p>Easy.</p>
 428 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">E</span> <span class="o">==</span> <span class="n">roll</span><span class="o">&gt;</span> <span class="n">popop</span> <span class="n">first</span>
 429 </pre></div>
 430 </div>
 431 </div>
 432 <div class="section" id="t-and-t">
 433 <h3><code class="docutils literal notranslate"><span class="pre">T&lt;</span></code> and <code class="docutils literal notranslate"><span class="pre">T&gt;</span></code><a class="headerlink" href="#t-and-t" title="Permalink to this headline">¶</a></h3>
 434 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">T</span><span class="o">&lt;</span> <span class="o">==</span> <span class="n">pop</span> <span class="p">[</span><span class="n">first</span><span class="p">]</span> <span class="n">dip</span> <span class="n">i</span>
 435 <span class="n">T</span><span class="o">&gt;</span> <span class="o">==</span> <span class="n">pop</span> <span class="p">[</span><span class="n">second</span><span class="p">]</span> <span class="n">dip</span> <span class="n">i</span>
 436 </pre></div>
 437 </div>
 438 </div>
 439 </div>
 440 <div class="section" id="putting-it-together">
 441 <h2>Putting it together<a class="headerlink" href="#putting-it-together" title="Permalink to this headline">¶</a></h2>
 442 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">T</span><span class="o">&gt;</span> <span class="o">==</span> <span class="n">pop</span> <span class="p">[</span><span class="n">first</span><span class="p">]</span> <span class="n">dip</span> <span class="n">i</span>
 443 <span class="n">T</span><span class="o">&lt;</span> <span class="o">==</span> <span class="n">pop</span> <span class="p">[</span><span class="n">second</span><span class="p">]</span> <span class="n">dip</span> <span class="n">i</span>
 444 <span class="n">E</span> <span class="o">==</span> <span class="n">roll</span><span class="o">&gt;</span> <span class="n">popop</span> <span class="n">first</span>
 445 <span class="n">P</span> <span class="o">==</span> <span class="n">roll</span><span class="o">&lt;</span> <span class="p">[</span><span class="n">roll</span><span class="o">&lt;</span> <span class="n">uncons</span> <span class="n">swap</span><span class="p">]</span> <span class="n">dip</span>
 446
 447 <span class="n">Tree</span><span class="o">-</span><span class="n">get</span> <span class="o">==</span> <span class="p">[</span><span class="n">P</span> <span class="p">[</span><span class="n">T</span><span class="o">&gt;</span><span class="p">]</span> <span class="p">[</span><span class="n">E</span><span class="p">]</span> <span class="p">[</span><span class="n">T</span><span class="o">&lt;</span><span class="p">]</span> <span class="nb">cmp</span><span class="p">]</span> <span class="n">treegrind</span>
 448 </pre></div>
 449 </div>
 450 <p>To me, that seems simpler than the <code class="docutils literal notranslate"><span class="pre">genrec</span></code> version.</p>
 451 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">DefinitionWrapper</span><span class="o">.</span><span class="n">add_definitions</span><span class="p">(</span><span class="s1">&#39;&#39;&#39;</span>
 452
 453 <span class="s1">    T&gt; == pop [first] dip i</span>
 454 <span class="s1">    T&lt; == pop [second] dip i</span>
 455 <span class="s1">    E == roll&gt; popop first</span>
 456 <span class="s1">    P == roll&lt; [roll&lt; uncons swap] dip</span>
 457
 458 <span class="s1">    Tree-get == [P [T&gt;] [E] [T&lt;] cmp] treegrind</span>
 459
 460 <span class="s1">&#39;&#39;&#39;</span><span class="p">,</span> <span class="n">D</span><span class="p">)</span>
 461 </pre></div>
 462 </div>
 463 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;&#39;&#39;</span><span class="se">\</span>
 464
 465 <span class="s1">[[3 13] [[2 12] [] []] [[9 19] [[5 15] [[4 14] [] []] [[8 18] [[6 16] [] [[7 17] [] []]] []]] []]]</span>
 466
 467 <span class="s1">[] [5] Tree-get</span>
 468
 469 <span class="s1">&#39;&#39;&#39;</span><span class="p">)</span>
 470 </pre></div>
 471 </div>
 472 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">15</span>
 473 </pre></div>
 474 </div>
 475 <div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">J</span><span class="p">(</span><span class="s1">&#39;&#39;&#39;</span><span class="se">\</span>
 476
 477 <span class="s1">[[3 13] [[2 12] [] []] [[9 19] [[5 15] [[4 14] [] []] [[8 18] [[6 16] [] [[7 17] [] []]] []]] []]]</span>
 478
 479 <span class="s1">[pop &quot;nope&quot;] [25] Tree-get</span>
 480
 481 <span class="s1">&#39;&#39;&#39;</span><span class="p">)</span>
 482 </pre></div>
 483 </div>
 484 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="s1">&#39;nope&#39;</span>
 485 </pre></div>
 486 </div>
 487 </div>
 488 </div>
 489
 490
 491           </div>
 492         </div>
 493       </div>
 494       <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 495         <div class="sphinxsidebarwrapper">
 496   <h3><a href="../index.html">Table Of Contents</a></h3>
 497   <ul>
 498 <li><a class="reference internal" href="#">Treating Trees II: <code class="docutils literal notranslate"><span class="pre">treestep</span></code></a><ul>
 499 <li><a class="reference internal" href="#derive-the-recursive-function">Derive the recursive function.</a></li>
 500 <li><a class="reference internal" href="#extract-the-givens-to-parameterize-the-program">Extract the givens to parameterize the program.</a><ul>
 501 <li><a class="reference internal" href="#alternate-extract-the-givens-to-parameterize-the-program">(alternate) Extract the givens to parameterize the program.</a></li>
 502 </ul>
 503 </li>
 504 <li><a class="reference internal" href="#define-treestep">Define <code class="docutils literal notranslate"><span class="pre">treestep</span></code></a></li>
 505 <li><a class="reference internal" href="#examples">Examples</a></li>
 506 <li><a class="reference internal" href="#redefining-the-ordered-binary-tree-in-terms-of-treestep">Redefining the Ordered Binary Tree in terms of <code class="docutils literal notranslate"><span class="pre">treestep</span></code>.</a><ul>
 507 <li><a class="reference internal" href="#traversal">Traversal</a></li>
 508 <li><a class="reference internal" href="#in-order-traversal">In-order traversal</a></li>
 509 </ul>
 510 </li>
 511 <li><a class="reference internal" href="#with-treegrind">With <code class="docutils literal notranslate"><span class="pre">treegrind</span></code>?</a></li>
 512 <li><a class="reference internal" href="#treegrind-with-step"><code class="docutils literal notranslate"><span class="pre">treegrind</span></code> with <code class="docutils literal notranslate"><span class="pre">step</span></code></a></li>
 513 <li><a class="reference internal" href="#do-we-have-the-flexibility-to-reimplement-tree-get">Do we have the flexibility to reimplement <code class="docutils literal notranslate"><span class="pre">Tree-get</span></code>?</a><ul>
 514 <li><a class="reference internal" href="#the-predicate-p">The predicate <code class="docutils literal notranslate"><span class="pre">P</span></code></a></li>
 515 <li><a class="reference internal" href="#e"><code class="docutils literal notranslate"><span class="pre">E</span></code></a></li>
 516 <li><a class="reference internal" href="#t-and-t"><code class="docutils literal notranslate"><span class="pre">T&lt;</span></code> and <code class="docutils literal notranslate"><span class="pre">T&gt;</span></code></a></li>
 517 </ul>
 518 </li>
 519 <li><a class="reference internal" href="#putting-it-together">Putting it together</a></li>
 520 </ul>
 521 </li>
 522 </ul>
 523 <div class="relations">
 524 <h3>Related Topics</h3>
 525 <ul>
 526   <li><a href="../index.html">Documentation overview</a><ul>
 527   <li><a href="index.html">Essays about Programming in Joy</a><ul>
 528       <li>Previous: <a href="Ordered_Binary_Trees.html" title="previous chapter">Treating Trees I: Ordered Binary Trees</a></li>
 529       <li>Next: <a href="Generator_Programs.html" title="next chapter">Using <code class="docutils literal notranslate"><span class="pre">x</span></code> to Generate Values</a></li>
 530   </ul></li>
 531   </ul></li>
 532 </ul>
 533 </div>
 534   <div role="note" aria-label="source link">
 535     <h3>This Page</h3>
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 562 <span xmlns:dct="http://purl.org/dc/terms/" property="dct:title">Thun Documentation</span> by <a xmlns:cc="http://creativecommons.org/ns#" href="https://joypy.osdn.io/" property="cc:attributionName" rel="cc:attributionURL">Simon Forman</a> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/">Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License</a>.<br />Based on a work at <a xmlns:dct="http://purl.org/dc/terms/" href="https://osdn.net/projects/joypy/" rel="dct:source">https://osdn.net/projects/joypy/</a>.
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