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<div class="section" id="traversing-datastructures-with-zippers">
<h1>Traversing Datastructures with Zippers<a class="headerlink" href="#traversing-datastructures-with-zippers" title="Permalink to this headline"></a></h1>
<p>This notebook is about using the “zipper” with joy datastructures. See
the <a class="reference external" href="https://en.wikipedia.org/wiki/Zipper_%28data_structure%29">Zipper wikipedia
entry</a> or
the original paper: <a class="reference external" href="https://www.st.cs.uni-saarland.de/edu/seminare/2005/advanced-fp/docs/huet-zipper.pdf">“FUNCTIONAL PEARL The Zipper” by Gérard
Huet</a></p>
<p>Given a datastructure on the stack we can navigate through it, modify
it, and rebuild it using the “zipper” technique.</p>
<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">J</span><span class="p">,</span> <span class="n">V</span><span class="p">,</span> <span class="n">define</span>
</pre></div>
</div>
<div class="section" id="trees">
<h2>Trees<a class="headerlink" href="#trees" title="Permalink to this headline"></a></h2>
<p>In Joypy there aren’t any complex datastructures, just ints, floats,
strings, Symbols (strings that are names of functions) and sequences
(aka lists, aka quoted literals, aka aggregates, etc…), but we can build
<a class="reference external" href="https://en.wikipedia.org/wiki/Tree_%28data_structure%29">trees</a> out
of sequences.</p>
<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;[1 [2 [3 4 25 6] 7] 8]&#39;</span><span class="p">)</span>
</pre></div>
</div>
<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">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span>
</pre></div>
</div>
</div>
<div class="section" id="zipper-in-joy">
<h2>Zipper in Joy<a class="headerlink" href="#zipper-in-joy" title="Permalink to this headline"></a></h2>
<p>Zippers work by keeping track of the current item, the already-seen
items, and the yet-to-be seen items as you traverse a datastructure (the
datastructure used to keep track of these items is the zipper.)</p>
<p>In Joy we can do this with the following words:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">z</span><span class="o">-</span><span class="n">down</span> <span class="o">==</span> <span class="p">[]</span> <span class="n">swap</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="n">z</span><span class="o">-</span><span class="n">up</span> <span class="o">==</span> <span class="n">swons</span> <span class="n">swap</span> <span class="n">shunt</span>
<span class="n">z</span><span class="o">-</span><span class="n">right</span> <span class="o">==</span> <span class="p">[</span><span class="n">swons</span><span class="p">]</span> <span class="n">cons</span> <span class="n">dip</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="n">z</span><span class="o">-</span><span class="n">left</span> <span class="o">==</span> <span class="n">swons</span> <span class="p">[</span><span class="n">uncons</span> <span class="n">swap</span><span class="p">]</span> <span class="n">dip</span> <span class="n">swap</span>
</pre></div>
</div>
<p>Let’s use them to change 25 into 625. The first time a word is used I
show the trace so you can see how it works. If we were going to use
these a lot it would make sense to write Python versions for efficiency,
but see below.</p>
<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;z-down == [] swap uncons swap&#39;</span><span class="p">)</span>
<span class="n">define</span><span class="p">(</span><span class="s1">&#39;z-up == swons swap shunt&#39;</span><span class="p">)</span>
<span class="n">define</span><span class="p">(</span><span class="s1">&#39;z-right == [swons] cons dip uncons swap&#39;</span><span class="p">)</span>
<span class="n">define</span><span class="p">(</span><span class="s1">&#39;z-left == swons [uncons swap] dip swap&#39;</span><span class="p">)</span>
</pre></div>
</div>
<div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">V</span><span class="p">(</span><span class="s1">&#39;[1 [2 [3 4 25 6] 7] 8] z-down&#39;</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="o">.</span> <span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="n">z</span><span class="o">-</span><span class="n">down</span>
<span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="o">.</span> <span class="n">z</span><span class="o">-</span><span class="n">down</span>
<span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="o">.</span> <span class="p">[]</span> <span class="n">swap</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="p">[]</span> <span class="o">.</span> <span class="n">swap</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[]</span> <span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="o">.</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[]</span> <span class="mi">1</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="o">.</span> <span class="n">swap</span>
<span class="p">[]</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="mi">1</span> <span class="o">.</span>
</pre></div>
</div>
<div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">V</span><span class="p">(</span><span class="s1">&#39;[] [[2 [3 4 25 6] 7] 8] 1 z-right&#39;</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="o">.</span> <span class="p">[]</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="mi">1</span> <span class="n">z</span><span class="o">-</span><span class="n">right</span>
<span class="p">[]</span> <span class="o">.</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="mi">1</span> <span class="n">z</span><span class="o">-</span><span class="n">right</span>
<span class="p">[]</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="o">.</span> <span class="mi">1</span> <span class="n">z</span><span class="o">-</span><span class="n">right</span>
<span class="p">[]</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="mi">1</span> <span class="o">.</span> <span class="n">z</span><span class="o">-</span><span class="n">right</span>
<span class="p">[]</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="mi">1</span> <span class="o">.</span> <span class="p">[</span><span class="n">swons</span><span class="p">]</span> <span class="n">cons</span> <span class="n">dip</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[]</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="mi">1</span> <span class="p">[</span><span class="n">swons</span><span class="p">]</span> <span class="o">.</span> <span class="n">cons</span> <span class="n">dip</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[]</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">1</span> <span class="n">swons</span><span class="p">]</span> <span class="o">.</span> <span class="n">dip</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[]</span> <span class="o">.</span> <span class="mi">1</span> <span class="n">swons</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[]</span> <span class="mi">1</span> <span class="o">.</span> <span class="n">swons</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[]</span> <span class="mi">1</span> <span class="o">.</span> <span class="n">swap</span> <span class="n">cons</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="mi">1</span> <span class="p">[]</span> <span class="o">.</span> <span class="n">cons</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">.</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="o">.</span> <span class="n">uncons</span> <span class="n">swap</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="o">.</span> <span class="n">swap</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="o">.</span>
</pre></div>
</div>
<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;[1] [8] [2 [3 4 25 6] 7] z-down&#39;</span><span class="p">)</span>
</pre></div>
</div>
<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="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[]</span> <span class="p">[[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">2</span>
</pre></div>
</div>
<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;[1] [8] [] [[3 4 25 6] 7] 2 z-right&#39;</span><span class="p">)</span>
</pre></div>
</div>
<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="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span>
</pre></div>
</div>
<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;[1] [8] [2] [7] [3 4 25 6] z-down&#39;</span><span class="p">)</span>
</pre></div>
</div>
<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="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">3</span>
</pre></div>
</div>
<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;[1] [8] [2] [7] [] [4 25 6] 3 z-right&#39;</span><span class="p">)</span>
</pre></div>
</div>
<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="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">4</span>
</pre></div>
</div>
<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;[1] [8] [2] [7] [3] [25 6] 4 z-right&#39;</span><span class="p">)</span>
</pre></div>
</div>
<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="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">25</span>
</pre></div>
</div>
<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;[1] [8] [2] [7] [4 3] [6] 25 sqr&#39;</span><span class="p">)</span>
</pre></div>
</div>
<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="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">625</span>
</pre></div>
</div>
<div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">V</span><span class="p">(</span><span class="s1">&#39;[1] [8] [2] [7] [4 3] [6] 625 z-up&#39;</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="o">.</span> <span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">625</span> <span class="n">z</span><span class="o">-</span><span class="n">up</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">.</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">625</span> <span class="n">z</span><span class="o">-</span><span class="n">up</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="o">.</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">625</span> <span class="n">z</span><span class="o">-</span><span class="n">up</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="o">.</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">625</span> <span class="n">z</span><span class="o">-</span><span class="n">up</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="o">.</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">625</span> <span class="n">z</span><span class="o">-</span><span class="n">up</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="o">.</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">625</span> <span class="n">z</span><span class="o">-</span><span class="n">up</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="o">.</span> <span class="mi">625</span> <span class="n">z</span><span class="o">-</span><span class="n">up</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">625</span> <span class="o">.</span> <span class="n">z</span><span class="o">-</span><span class="n">up</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">625</span> <span class="o">.</span> <span class="n">swons</span> <span class="n">swap</span> <span class="n">shunt</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="mi">625</span> <span class="o">.</span> <span class="n">swap</span> <span class="n">cons</span> <span class="n">swap</span> <span class="n">shunt</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="mi">625</span> <span class="p">[</span><span class="mi">6</span><span class="p">]</span> <span class="o">.</span> <span class="n">cons</span> <span class="n">swap</span> <span class="n">shunt</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="o">.</span> <span class="n">swap</span> <span class="n">shunt</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span> <span class="mi">3</span><span class="p">]</span> <span class="o">.</span> <span class="n">shunt</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="o">.</span>
</pre></div>
</div>
<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;[1] [8] [2] [7] [3 4 625 6] z-up&#39;</span><span class="p">)</span>
</pre></div>
</div>
<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="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span>
</pre></div>
</div>
<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;[1] [8] [2 [3 4 625 6] 7] z-up&#39;</span><span class="p">)</span>
</pre></div>
</div>
<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">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span>
</pre></div>
</div>
</div>
<div class="section" id="dip-and-infra">
<h2><code class="docutils literal notranslate"><span class="pre">dip</span></code> and <code class="docutils literal notranslate"><span class="pre">infra</span></code><a class="headerlink" href="#dip-and-infra" title="Permalink to this headline"></a></h2>
<p>In Joy we have the <code class="docutils literal notranslate"><span class="pre">dip</span></code> and <code class="docutils literal notranslate"><span class="pre">infra</span></code> combinators which can “target”
or “address” any particular item in a Joy tree structure.</p>
<div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">V</span><span class="p">(</span><span class="s1">&#39;[1 [2 [3 4 25 6] 7] 8] [[[[[[sqr] dipd] infra] dip] infra] dip] infra&#39;</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="o">.</span> <span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="p">[[[[[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span>
<span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="o">.</span> <span class="p">[[[[[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span>
<span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="p">[[[[[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="o">.</span> <span class="n">infra</span>
<span class="mi">8</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">1</span> <span class="o">.</span> <span class="p">[[[[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">8</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">1</span> <span class="p">[[[[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="o">.</span> <span class="n">dip</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">8</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="o">.</span> <span class="p">[[[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">8</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="p">[[[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="o">.</span> <span class="n">infra</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">7</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">2</span> <span class="o">.</span> <span class="p">[[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">7</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">2</span> <span class="p">[[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="o">.</span> <span class="n">dip</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">7</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="o">.</span> <span class="p">[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="n">infra</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">7</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">25</span> <span class="mi">6</span><span class="p">]</span> <span class="p">[[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span><span class="p">]</span> <span class="o">.</span> <span class="n">infra</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">6</span> <span class="mi">25</span> <span class="mi">4</span> <span class="mi">3</span> <span class="o">.</span> <span class="p">[</span><span class="n">sqr</span><span class="p">]</span> <span class="n">dipd</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">6</span> <span class="mi">25</span> <span class="mi">4</span> <span class="mi">3</span> <span class="p">[</span><span class="n">sqr</span><span class="p">]</span> <span class="o">.</span> <span class="n">dipd</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">6</span> <span class="mi">25</span> <span class="o">.</span> <span class="n">sqr</span> <span class="mi">4</span> <span class="mi">3</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">6</span> <span class="mi">25</span> <span class="o">.</span> <span class="n">dup</span> <span class="n">mul</span> <span class="mi">4</span> <span class="mi">3</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">6</span> <span class="mi">25</span> <span class="mi">25</span> <span class="o">.</span> <span class="n">mul</span> <span class="mi">4</span> <span class="mi">3</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">6</span> <span class="mi">625</span> <span class="o">.</span> <span class="mi">4</span> <span class="mi">3</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">6</span> <span class="mi">625</span> <span class="mi">4</span> <span class="o">.</span> <span class="mi">3</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">6</span> <span class="mi">625</span> <span class="mi">4</span> <span class="mi">3</span> <span class="o">.</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">6</span> <span class="mi">625</span> <span class="mi">4</span> <span class="mi">3</span> <span class="p">[</span><span class="mi">7</span><span class="p">]</span> <span class="o">.</span> <span class="n">swaack</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">7</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="o">.</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">7</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">2</span> <span class="o">.</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">7</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">2</span> <span class="p">[</span><span class="mi">8</span><span class="p">]</span> <span class="o">.</span> <span class="n">swaack</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">8</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="o">.</span> <span class="mi">1</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">8</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">1</span> <span class="o">.</span> <span class="p">[]</span> <span class="n">swaack</span>
<span class="mi">8</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">1</span> <span class="p">[]</span> <span class="o">.</span> <span class="n">swaack</span>
<span class="p">[</span><span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span> <span class="o">.</span>
</pre></div>
</div>
<p>If you read the trace carefully you’ll see that about half of it is the
<code class="docutils literal notranslate"><span class="pre">dip</span></code> and <code class="docutils literal notranslate"><span class="pre">infra</span></code> combinators de-quoting programs and “digging” into
the subject datastructure. Instead of maintaining temporary results on
the stack they are pushed into the pending expression (continuation).
When <code class="docutils literal notranslate"><span class="pre">sqr</span></code> has run the rest of the pending expression rebuilds the
datastructure.</p>
</div>
<div class="section" id="z">
<h2><code class="docutils literal notranslate"><span class="pre">Z</span></code><a class="headerlink" href="#z" title="Permalink to this headline"></a></h2>
<p>Imagine a function <code class="docutils literal notranslate"><span class="pre">Z</span></code> that accepts a sequence of <code class="docutils literal notranslate"><span class="pre">dip</span></code> and
<code class="docutils literal notranslate"><span class="pre">infra</span></code> combinators, a quoted program <code class="docutils literal notranslate"><span class="pre">[Q]</span></code>, and a datastructure to
work on. It would effectively execute the quoted program as if it had
been embedded in a nested series of quoted programs, e.g.:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="p">[</span><span class="o">...</span><span class="p">]</span> <span class="p">[</span><span class="n">Q</span><span class="p">]</span> <span class="p">[</span><span class="n">dip</span> <span class="n">dip</span> <span class="n">infra</span> <span class="n">dip</span> <span class="n">infra</span> <span class="n">dip</span> <span class="n">infra</span><span class="p">]</span> <span class="n">Z</span>
<span class="o">-------------------------------------------------------------</span>
<span class="p">[</span><span class="o">...</span><span class="p">]</span> <span class="p">[[[[[[[</span><span class="n">Q</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">Z</span></code> function isn’t hard to make.</p>
<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;Z == [[] cons cons] step i&#39;</span><span class="p">)</span>
</pre></div>
</div>
<p>Here it is in action in a simplified scenario.</p>
<div class="code ipython2 highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">V</span><span class="p">(</span><span class="s1">&#39;1 [2 3 4] Z&#39;</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="o">.</span> <span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="n">Z</span>
<span class="mi">1</span> <span class="o">.</span> <span class="p">[</span><span class="mi">2</span> <span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="n">Z</span>
<span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="o">.</span> <span class="n">Z</span>
<span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="o">.</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="mi">1</span> <span class="p">[</span><span class="mi">2</span> <span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="o">.</span> <span class="n">step</span> <span class="n">i</span>
<span class="mi">1</span> <span class="mi">2</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="o">.</span> <span class="n">i</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="mi">1</span> <span class="mi">2</span> <span class="o">.</span> <span class="p">[]</span> <span class="n">cons</span> <span class="n">cons</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="mi">1</span> <span class="mi">2</span> <span class="p">[]</span> <span class="o">.</span> <span class="n">cons</span> <span class="n">cons</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="mi">1</span> <span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="o">.</span> <span class="n">cons</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="o">.</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="o">.</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="o">.</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="o">.</span> <span class="n">i</span> <span class="p">[</span><span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span> <span class="o">.</span> <span class="p">[]</span> <span class="n">cons</span> <span class="n">cons</span> <span class="p">[</span><span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span> <span class="p">[]</span> <span class="o">.</span> <span class="n">cons</span> <span class="n">cons</span> <span class="p">[</span><span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="o">.</span> <span class="n">cons</span> <span class="p">[</span><span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="o">.</span> <span class="p">[</span><span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span><span class="p">]</span> <span class="o">.</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span><span class="p">]</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="o">.</span> <span class="n">step</span> <span class="n">i</span>
<span class="p">[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="mi">4</span> <span class="p">[[]</span> <span class="n">cons</span> <span class="n">cons</span><span class="p">]</span> <span class="o">.</span> <span class="n">i</span> <span class="n">i</span>
<span class="p">[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="mi">4</span> <span class="o">.</span> <span class="p">[]</span> <span class="n">cons</span> <span class="n">cons</span> <span class="n">i</span>
<span class="p">[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="mi">4</span> <span class="p">[]</span> <span class="o">.</span> <span class="n">cons</span> <span class="n">cons</span> <span class="n">i</span>
<span class="p">[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="p">[</span><span class="mi">4</span><span class="p">]</span> <span class="o">.</span> <span class="n">cons</span> <span class="n">i</span>
<span class="p">[[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="mi">4</span><span class="p">]</span> <span class="o">.</span> <span class="n">i</span>
<span class="o">.</span> <span class="p">[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="mi">4</span>
<span class="p">[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="o">.</span> <span class="mi">4</span>
<span class="p">[[</span><span class="mi">1</span> <span class="mi">2</span><span class="p">]</span> <span class="mi">3</span><span class="p">]</span> <span class="mi">4</span> <span class="o">.</span>
</pre></div>
</div>
<p>And here it is doing the main thing.</p>
<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;[1 [2 [3 4 25 6] 7] 8] [sqr] [dip dip infra dip infra dip infra] Z&#39;</span><span class="p">)</span>
</pre></div>
</div>
<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">2</span> <span class="p">[</span><span class="mi">3</span> <span class="mi">4</span> <span class="mi">625</span> <span class="mi">6</span><span class="p">]</span> <span class="mi">7</span><span class="p">]</span> <span class="mi">8</span><span class="p">]</span>
</pre></div>
</div>
</div>
<div class="section" id="addressing">
<h2>Addressing<a class="headerlink" href="#addressing" title="Permalink to this headline"></a></h2>
<p>Because we are only using two combinators we could replace the list with
a string made from only two characters.</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="p">[</span><span class="o">...</span><span class="p">]</span> <span class="p">[</span><span class="n">Q</span><span class="p">]</span> <span class="s1">&#39;ddididi&#39;</span> <span class="n">Zstr</span>
<span class="o">-------------------------------------------------------------</span>
<span class="p">[</span><span class="o">...</span><span class="p">]</span> <span class="p">[[[[[[[</span><span class="n">Q</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span><span class="p">]</span> <span class="n">dip</span><span class="p">]</span> <span class="n">infra</span>
</pre></div>
</div>
<p>The string can be considered a name or address for an item in the
subject datastructure.</p>
</div>
<div class="section" id="determining-the-right-path-for-an-item-in-a-tree">
<h2>Determining the right “path” for an item in a tree.<a class="headerlink" href="#determining-the-right-path-for-an-item-in-a-tree" title="Permalink to this headline"></a></h2>
<p>It’s easy to read off (in reverse) the right sequence of “d” and “i”
from the subject datastructure:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>[ n [ n [ n n x ...
i d i d i d d Bingo!
</pre></div>
</div>
</div>
</div>
</div>
</div>
</div>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Traversing Datastructures with Zippers</a><ul>
<li><a class="reference internal" href="#trees">Trees</a></li>
<li><a class="reference internal" href="#zipper-in-joy">Zipper in Joy</a></li>
<li><a class="reference internal" href="#dip-and-infra"><code class="docutils literal notranslate"><span class="pre">dip</span></code> and <code class="docutils literal notranslate"><span class="pre">infra</span></code></a></li>
<li><a class="reference internal" href="#z"><code class="docutils literal notranslate"><span class="pre">Z</span></code></a></li>
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