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  <div class="section" id="design-doc-switch">
<span id="design-doc-switch"></span><h1>Design Doc: Switch<a class="headerlink" href="#design-doc-switch" title="永久链接至标题">¶</a></h1>
</div>
<div class="section" id="background">
<span id="background"></span><h1>Background<a class="headerlink" href="#background" title="永久链接至标题">¶</a></h1>
<p>Many programming languages provide <code class="docutils literal"><span class="pre">switch</span></code> as a generalization of <code class="docutils literal"><span class="pre">if-elif-else</span></code>.  We want to add it to Fluid.</p>
<p>The following example shows the usage of <code class="docutils literal"><span class="pre">fluid.switch</span></code>.</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="n">a</span> <span class="o">=</span> <span class="n">fluid</span><span class="o">.</span><span class="n">Var</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
<span class="n">b</span> <span class="o">=</span> <span class="n">fluid</span><span class="o">.</span><span class="n">Var</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>

<span class="k">with</span> <span class="n">switch</span><span class="p">()</span> <span class="k">as</span> <span class="n">switch</span><span class="p">:</span>
    <span class="k">with</span> <span class="n">switch</span><span class="o">.</span><span class="n">case</span><span class="p">(</span><span class="n">fluid</span><span class="o">.</span><span class="n">less_equal</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="mi">10</span><span class="p">)):</span>
        <span class="n">fluid</span><span class="o">.</span><span class="k">print</span><span class="p">(</span><span class="s2">&quot;Case 1&quot;</span><span class="p">)</span>
    <span class="k">with</span> <span class="n">switch</span><span class="o">.</span><span class="n">case</span><span class="p">(</span><span class="n">fluid</span><span class="o">.</span><span class="n">larger</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="mi">0</span><span class="p">)):</span>
        <span class="n">fluid</span><span class="o">.</span><span class="k">print</span><span class="p">(</span><span class="s2">&quot;Case 2&quot;</span><span class="p">)</span>
    <span class="k">with</span> <span class="n">switch</span><span class="o">.</span><span class="n">default</span><span class="p">():</span>
        <span class="n">fluid</span><span class="o">.</span><span class="k">print</span><span class="p">(</span><span class="s2">&quot;Case 3&quot;</span><span class="p">)</span>
</pre></div>
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</div>
<div class="section" id="the-semantics">
<span id="the-semantics"></span><h1>The Semantics<a class="headerlink" href="#the-semantics" title="永久链接至标题">¶</a></h1>
<ol class="simple">
<li>A <code class="docutils literal"><span class="pre">switch</span></code> control-flow checks cases one-by-one.</li>
<li>The condition of each case is a boolean value, which is a scalar, and differs from the <code class="docutils literal"><span class="pre">fluid.if_else</span></code> control-flow, which condition could be a vector of boolean values.</li>
<li>It runs the first matched case, or the default case if there is one.</li>
<li>Once it matches a case, it runs the corresponding branch and only that branch.  It&#8217;s like there is a C&#8217;s <code class="docutils literal"><span class="pre">break</span></code> keyword at the end of each case.</li>
</ol>
<p>The above program should print and print only &#8220;Case 1&#8221;.</p>
<p>The implementation of the backward pass of the <code class="docutils literal"><span class="pre">switch</span></code> control-flow is easier than the backward of the <code class="docutils literal"><span class="pre">if_else</span></code>, because <code class="docutils literal"><span class="pre">switch</span></code> runs at most one branch, whereas <code class="docutils literal"><span class="pre">if-else</span></code> could run more than one branches.</p>
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