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+            
+                OOP Visitors在Polymorphic情况下的类型错误
+            
+            
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+        <span class="post-date">
+            Jun 28, 2023
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+    <p>像Java里面:</p>
+<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">interface</span> <span class="title class_">Tree</span> &#123;</span><br><span class="line">  <span class="keyword">public</span> &lt;B&gt; B <span class="title function_">accept</span><span class="params">(TreeVisitor&lt;B&gt; v)</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">interface</span> <span class="title class_">TreeVisitor</span>&lt;B&gt; &#123;</span><br><span class="line">  <span class="keyword">public</span> B <span class="title function_">visitLeaf</span><span class="params">(Leaf t)</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Leaf</span> <span class="keyword">implements</span> <span class="title class_">Tree</span> &#123;</span><br><span class="line">  <span class="keyword">public</span> <span class="keyword">final</span> <span class="type">int</span> value;</span><br><span class="line"></span><br><span class="line">  <span class="keyword">public</span> <span class="title function_">Leaf</span><span class="params">(<span class="type">int</span> value)</span> &#123;</span><br><span class="line">    <span class="built_in">this</span>.value = value;</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  <span class="keyword">public</span> &lt;B&gt; B <span class="title function_">accept</span><span class="params">(TreeVisitor&lt;B&gt; v)</span> &#123;</span><br><span class="line">    <span class="keyword">return</span> v.visitLeaf(<span class="built_in">this</span>);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
+
+<p>要是从字面上直接翻译成OCaml:</p>
+<figure class="highlight ocaml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="keyword">virtual</span> tree =</span><br><span class="line">  <span class="keyword">object</span></span><br><span class="line">    <span class="keyword">method</span> <span class="keyword">virtual</span> accept : <span class="symbol">&#x27;a</span> . <span class="symbol">&#x27;a</span> tree_visitor -&gt; <span class="symbol">&#x27;a</span></span><br><span class="line">  <span class="keyword">end</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">and</span> <span class="keyword">virtual</span> [<span class="symbol">&#x27;a</span>] tree_visitor =</span><br><span class="line">  <span class="keyword">object</span></span><br><span class="line">    <span class="keyword">method</span> <span class="keyword">virtual</span> visit_leaf : (leaf -&gt; <span class="symbol">&#x27;a</span>)</span><br><span class="line">  <span class="keyword">end</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">and</span> leaf (i : <span class="built_in">int</span>) =</span><br><span class="line">  <span class="keyword">object</span> (self)</span><br><span class="line">    <span class="keyword">method</span> get = i</span><br><span class="line"></span><br><span class="line">    <span class="keyword">method</span> accept (v : <span class="symbol">&#x27;a</span> tree_visitor) : <span class="symbol">&#x27;a</span> =</span><br><span class="line">      v#visit_leaf (self :&gt; leaf)</span><br><span class="line">  <span class="keyword">end</span></span><br></pre></td></tr></table></figure>
+
+<p>是行不通滴:</p>
+<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">File &quot;visitor.ml&quot;, line 3, characters 28-54:</span><br><span class="line">3 |     method virtual accept : &#x27;a . &#x27;a tree_visitor -&gt; &#x27;a</span><br><span class="line">                                ^^^^^^^^^^^^^^^^^^^^^^^^^^</span><br><span class="line">Error: The universal type variable &#x27;a cannot be generalized:</span><br><span class="line">       it escapes its scope.</span><br></pre></td></tr></table></figure>
+
+<p>OCaml的类在定义时是单态的，比如:</p>
+<figure class="highlight ocaml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> [<span class="symbol">&#x27;a</span>] c (x:<span class="symbol">&#x27;a</span>) = </span><br><span class="line">  <span class="keyword">object</span></span><br><span class="line">    <span class="keyword">method</span> x = x</span><br><span class="line">  <span class="keyword">end</span></span><br><span class="line"><span class="keyword">and</span> d = </span><br><span class="line">  <span class="keyword">object</span></span><br><span class="line">    <span class="keyword">method</span> strange = (<span class="keyword">new</span> c <span class="number">0</span>)#x</span><br><span class="line">  <span class="keyword">end</span></span><br></pre></td></tr></table></figure>
+
+<p>这里c是:</p>
+<figure class="highlight ocaml"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> [<span class="symbol">&#x27;a</span>] c : <span class="symbol">&#x27;a</span> -&gt; <span class="keyword">object</span> <span class="keyword">constraint</span> <span class="symbol">&#x27;a</span> = <span class="built_in">int</span> <span class="keyword">method</span> x : <span class="symbol">&#x27;a</span> <span class="keyword">end</span></span><br></pre></td></tr></table></figure>
+
+<p>因为在d里面<code>new c 0</code>了一个 <code>int c</code> 的对象。</p>
+<p>所以正确的写法应该是:</p>
+<figure class="highlight ocaml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="keyword">virtual</span> [<span class="symbol">&#x27;a</span>, <span class="symbol">&#x27;leaf</span>, <span class="symbol">&#x27;node</span>, <span class="symbol">&#x27;empty</span>] open_tree_visitor = </span><br><span class="line">  <span class="keyword">object</span></span><br><span class="line">    <span class="keyword">method</span> <span class="keyword">virtual</span> visit_leaf :  <span class="symbol">&#x27;leaf</span> -&gt; <span class="symbol">&#x27;a</span></span><br><span class="line">    <span class="keyword">method</span> <span class="keyword">virtual</span> visit_node :  <span class="symbol">&#x27;node</span> -&gt; <span class="symbol">&#x27;a</span></span><br><span class="line">    <span class="keyword">method</span> <span class="keyword">virtual</span> visit_empty :  <span class="symbol">&#x27;empty</span> -&gt; <span class="symbol">&#x27;a</span></span><br><span class="line">  <span class="keyword">end</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="keyword">virtual</span> tree = </span><br><span class="line">  <span class="keyword">object</span></span><br><span class="line">    <span class="keyword">method</span> <span class="keyword">virtual</span> accept : <span class="symbol">&#x27;a</span> .</span><br><span class="line">      (<span class="symbol">&#x27;a</span>, <span class="symbol">&#x27;leaf</span>, <span class="symbol">&#x27;node</span>, <span class="symbol">&#x27;empty</span>) open_tree_visitor -&gt; <span class="symbol">&#x27;a</span></span><br><span class="line">  <span class="keyword">end</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">and</span> leaf (i : <span class="built_in">int</span>) = </span><br><span class="line">  <span class="keyword">object</span> (self:<span class="symbol">&#x27;self</span>)</span><br><span class="line">    <span class="keyword">inherit</span> tree</span><br><span class="line">    <span class="keyword">method</span> get = i</span><br><span class="line">    <span class="keyword">method</span> accept v = v#visit_leaf (self :&gt; leaf)</span><br><span class="line">  <span class="keyword">end</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">and</span> node (x : tree) (y: tree) = </span><br><span class="line">  <span class="keyword">object</span> (self:<span class="symbol">&#x27;self</span>)</span><br><span class="line">    <span class="keyword">inherit</span> tree</span><br><span class="line">    <span class="keyword">method</span> left = x</span><br><span class="line">    <span class="keyword">method</span> right = y</span><br><span class="line">    <span class="keyword">method</span> accept v = v#visit_node (self :&gt; node)</span><br><span class="line">  <span class="keyword">end</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">and</span> empty = </span><br><span class="line">  <span class="keyword">object</span> (self:<span class="symbol">&#x27;self</span>)</span><br><span class="line">    <span class="keyword">inherit</span> tree</span><br><span class="line">    <span class="keyword">method</span> accept v = v#visit_empty (self :&gt; empty)</span><br><span class="line">  <span class="keyword">end</span></span><br></pre></td></tr></table></figure>
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