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-rw-r--r--2023/06/28/OOP-Visitors在Polymorphic情况下的类型错误/index.html243
-rw-r--r--2023/06/28/caml-release-runtime-system导致的死锁问题/index.html247
-rw-r--r--2023/06/28/隐藏一些OCaml-Effect的机制,让其语法在精神上更接近delimcc/index.html237
3 files changed, 727 insertions, 0 deletions
diff --git a/2023/06/28/OOP-Visitors在Polymorphic情况下的类型错误/index.html b/2023/06/28/OOP-Visitors在Polymorphic情况下的类型错误/index.html
new file mode 100644
index 00000000..2f715eea
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+++ b/2023/06/28/OOP-Visitors在Polymorphic情况下的类型错误/index.html
@@ -0,0 +1,243 @@
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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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+
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+ <div class="post-content">
+ <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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diff --git a/2023/06/28/caml-release-runtime-system导致的死锁问题/index.html b/2023/06/28/caml-release-runtime-system导致的死锁问题/index.html
new file mode 100644
index 00000000..77754735
--- /dev/null
+++ b/2023/06/28/caml-release-runtime-system导致的死锁问题/index.html
@@ -0,0 +1,247 @@
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+ caml_release_runtime_system导致的死锁问题
+
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+ Jun 28, 2023
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+ <div class="post-img">
+
+ <div class="h-line-primary"></div>
+
+ </div>
+</div>
+ <div class="post-content">
+ <p>给出一个<code>test.ml</code>:</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="type">Printf</span>.eprintf <span class="string">&quot;hello from OCaml\n%!&quot;</span></span><br></pre></td></tr></table></figure>
+
+<p>和<code>test-ocaml-5.c</code>:</p>
+<figure class="highlight c"><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="meta">#<span class="keyword">include</span> <span class="string">&lt;stdio.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string">&lt;stdlib.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string">&lt;caml/misc.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string">&lt;caml/callback.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string">&lt;caml/threads.h&gt;</span></span></span><br><span class="line"></span><br><span class="line"><span class="type">int</span></span><br><span class="line"><span class="title function_">main</span> <span class="params">(<span class="type">int</span> argc, <span class="type">char</span> *argv[])</span></span><br><span class="line"> &#123;</span><br><span class="line"> <span class="built_in">fprintf</span> (<span class="built_in">stderr</span>, <span class="string">&quot;starting up ...\n&quot;</span>);</span><br><span class="line"> caml_startup (argv);</span><br><span class="line"> <span class="built_in">fprintf</span> (<span class="built_in">stderr</span>, <span class="string">&quot;acquiring ...\n&quot;</span>);</span><br><span class="line"> caml_acquire_runtime_system ();</span><br><span class="line"> <span class="built_in">fprintf</span> (<span class="built_in">stderr</span>, <span class="string">&quot;acquired\n&quot;</span>);</span><br><span class="line"> <span class="comment">// here is where I would be calling an OCaml callback</span></span><br><span class="line"> <span class="comment">// omitted for simplicity</span></span><br><span class="line"> caml_release_runtime_system ();</span><br><span class="line"> <span class="built_in">exit</span> (<span class="number">0</span>);</span><br><span class="line"> &#125;</span><br></pre></td></tr></table></figure>
+
+<p>然后用OCaml5的ocaml native compiler编译一下(这里我用的是<code>ocaml-variants.5.0.0+options</code>):<br><code>ocamlopt -g test-ocaml-5.c test.ml -o test-ocaml-5</code></p>
+<p>运行<code>test-ocaml-5</code> 会出现:</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><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">starting up ...</span><br><span class="line">hello from OCaml</span><br><span class="line">acquiring ...</span><br><span class="line">Fatal error: Fatal error during lock: Resource deadlock avoided</span><br><span class="line"></span><br><span class="line">[1] 8998 IOT instruction (core dumped) ./test-ocaml-5</span><br></pre></td></tr></table></figure>
+
+<p>这里我尝试了一下 4.14.0 和 4.14.1 , 都没有出现这个情况,而如果用<code>threads</code>编译的话:<br><code>ocamlopt -g -I +unix unix.cmxa -I +threads threads.cmxa test-ocaml-5.c test.ml -o test-ocaml-5</code></p>
+<p>无论在5.0还是4.14.x,都会挂起。</p>
+<p>出现这个问题的一个可能原因是,在<code>test-ocaml-5.c</code>中,我在开头调用了<code>caml_startup()</code>,这会让当前线程获取锁,而<code>caml_acquire_runtime_system()</code>会再次获取它,<code>caml_acquire_runtime_system()</code> 应该在<code>caml_release_runtime_system()</code>之后调用:</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></pre></td><td class="code"><pre><span class="line"><span class="built_in">int</span></span><br><span class="line">main (<span class="built_in">int</span> argc, <span class="built_in">char</span> *argv<span class="literal">[]</span>)</span><br><span class="line"> &#123;</span><br><span class="line"> fprintf (stderr, <span class="string">&quot;starting up ...\n&quot;</span>);</span><br><span class="line"> caml_startup (argv);</span><br><span class="line"> fprintf (stderr, <span class="string">&quot;acquiring ...\n&quot;</span>);</span><br><span class="line"> caml_release_runtime_system <span class="literal">()</span>;</span><br><span class="line"> caml_acquire_runtime_system <span class="literal">()</span>;</span><br><span class="line"> fprintf (stderr, <span class="string">&quot;acquired\n&quot;</span>);</span><br><span class="line"> exit (<span class="number">0</span>);</span><br><span class="line"> &#125;</span><br></pre></td></tr></table></figure>
+
+<p>运行结果为:</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></pre></td><td class="code"><pre><span class="line">starting up ...</span><br><span class="line">hello from OCaml</span><br><span class="line">acquiring ...</span><br><span class="line">acquired</span><br></pre></td></tr></table></figure>
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diff --git a/2023/06/28/隐藏一些OCaml-Effect的机制,让其语法在精神上更接近delimcc/index.html b/2023/06/28/隐藏一些OCaml-Effect的机制,让其语法在精神上更接近delimcc/index.html
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+ 隐藏一些OCaml Effect的机制,让其语法在精神上更接近delimcc
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+ <p><a target="_blank" rel="noopener" href="https://github.com/kayceesrk/delimcc_of_fxhandler">delimcc_of_fxhandler这个库</a>在OCaml5的effect handlers上实现了一些delimcc原语(shift&#x2F;reset, control&#x2F;prompt这些):</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> p = new_prompt <span class="literal">()</span> <span class="keyword">in</span></span><br><span class="line"><span class="keyword">assert</span> (<span class="literal">[]</span> = push_prompt p (<span class="keyword">fun</span> <span class="literal">()</span> -&gt;</span><br><span class="line"> <span class="number">1</span>::<span class="number">2</span>::take_subcont p (<span class="keyword">fun</span> _k -&gt; <span class="literal">[]</span>)));</span><br><span class="line"><span class="keyword">assert</span> ([<span class="number">1</span>;<span class="number">2</span>] = push_prompt p (<span class="keyword">fun</span> <span class="literal">()</span> -&gt;</span><br><span class="line"> <span class="number">1</span>::<span class="number">2</span>::take_subcont p (<span class="keyword">fun</span> k -&gt; push_subcont k <span class="literal">[]</span>)));</span><br><span class="line"><span class="keyword">assert</span> (<span class="number">135</span> =</span><br><span class="line"> <span class="keyword">let</span> p1 = new_prompt <span class="literal">()</span> <span class="keyword">in</span></span><br><span class="line"> <span class="keyword">let</span> p2 = new_prompt <span class="literal">()</span> <span class="keyword">in</span></span><br><span class="line"> <span class="keyword">let</span> p3 = new_prompt <span class="literal">()</span> <span class="keyword">in</span></span><br><span class="line"> <span class="keyword">let</span> pushtwice sk =</span><br><span class="line"> sk (<span class="keyword">fun</span> <span class="literal">()</span> -&gt;</span><br><span class="line"> sk (<span class="keyword">fun</span> <span class="literal">()</span> -&gt;</span><br><span class="line"> shift0 p2 (<span class="keyword">fun</span> sk2 -&gt; sk2 (<span class="keyword">fun</span> <span class="literal">()</span> -&gt;</span><br><span class="line"> sk2 (<span class="keyword">fun</span> <span class="literal">()</span> -&gt; <span class="number">3</span>))) <span class="literal">()</span>))</span><br><span class="line"> <span class="keyword">in</span></span><br><span class="line"> push_prompt p1 (<span class="keyword">fun</span> <span class="literal">()</span> -&gt;</span><br><span class="line"> push_prompt p2 (<span class="keyword">fun</span> <span class="literal">()</span> -&gt;</span><br><span class="line"> push_prompt p3 (<span class="keyword">fun</span> <span class="literal">()</span> -&gt; shift0 p1 pushtwice <span class="literal">()</span>) + <span class="number">10</span>) + <span class="number">1</span>) + <span class="number">100</span>);</span><br><span class="line"></span><br><span class="line">print_endline <span class="string">&quot;Success!&quot;</span></span><br></pre></td></tr></table></figure>
+
+<p>另外, <a target="_blank" rel="noopener" href="https://github.com/avsm/ocaml/commits/effect-syntax">avsm这里</a>可以看到一些OCaml的Effect Syntax进展。</p>
+<p>还有 <a target="_blank" rel="noopener" href="https://github.com/dhil/ocaml-multicont">multi-shot continuations in OCaml</a>,在这个仓库里面还讨论了一些有趣的问题,例如,OCaml 编译器和runtime会做出一些假设从而进行一些优化,这些优化在使用multi-shot continutation时是不可取的(或完全错误的)。编译器优化导致错误的一个例子是堆到栈的转换,例如:</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><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><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">(* An illustration of how the heap to stack optimisation is broken.</span></span><br><span class="line"><span class="comment"> * This example is adapted from de Vilhena and Pottier (2021).</span></span><br><span class="line"><span class="comment"> * file: heap2stack.ml</span></span><br><span class="line"><span class="comment"> * compile: ocamlopt -I $(opam var lib)/multicont multicont.cmxa heap2stack.ml</span></span><br><span class="line"><span class="comment"> * run: ./a.out *)</span></span><br><span class="line"></span><br><span class="line"><span class="comment">(* We first require a little bit of setup. The following declares an</span></span><br><span class="line"><span class="comment"> operation `Twice&#x27; which we use to implement multiple returns. *)</span></span><br><span class="line"><span class="keyword">type</span> _ <span class="type">Effect</span>.t += <span class="type">Twice</span> : <span class="built_in">unit</span> <span class="type">Effect</span>.t</span><br><span class="line"></span><br><span class="line"><span class="comment">(* The handler `htwice&#x27; interprets `Twice&#x27; by simply invoking its</span></span><br><span class="line"><span class="comment"> continuation twice. *)</span></span><br><span class="line"><span class="keyword">let</span> htwice : (<span class="built_in">unit</span>, <span class="built_in">unit</span>) <span class="type">Effect</span>.<span class="type">Deep</span>.handler</span><br><span class="line"> = &#123; retc = (<span class="keyword">fun</span> x -&gt; x)</span><br><span class="line"> ; exnc = (<span class="keyword">fun</span> e -&gt; raise e)</span><br><span class="line"> ; effc = (<span class="keyword">fun</span> (<span class="keyword">type</span> a) (eff : a <span class="type">Effect</span>.t) -&gt;</span><br><span class="line"> <span class="keyword">let</span> <span class="keyword">open</span> <span class="type">Effect</span>.<span class="type">Deep</span> <span class="keyword">in</span></span><br><span class="line"> <span class="keyword">match</span> eff <span class="keyword">with</span></span><br><span class="line"> | <span class="type">Twice</span> -&gt; <span class="type">Some</span> (<span class="keyword">fun</span> (k : (a, _) continuation) -&gt;</span><br><span class="line"> continue (<span class="type">Multicont</span>.<span class="type">Deep</span>.clone_continuation k) <span class="literal">()</span>;</span><br><span class="line"> continue k <span class="literal">()</span>)</span><br><span class="line"> | _ -&gt; <span class="type">None</span>) &#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">(* Now for the interesting stuff. In the code below, the compiler will</span></span><br><span class="line"><span class="comment"> perform an escape analysis on the reference `i&#x27; and deduce that it</span></span><br><span class="line"><span class="comment"> does not escape the local scope, because it is unaware of the</span></span><br><span class="line"><span class="comment"> semantics of `perform Twice&#x27;, hence the optimiser will transform</span></span><br><span class="line"><span class="comment"> `i&#x27; into an immediate on the stack to save a heap allocation. As a</span></span><br><span class="line"><span class="comment"> consequence, the assertion `(!i = 1)&#x27; will succeed twice, whereas</span></span><br><span class="line"><span class="comment"> it should fail after the second return of `perform Twice&#x27;. *)</span></span><br><span class="line"><span class="keyword">let</span> heap2stack <span class="literal">()</span> =</span><br><span class="line"> <span class="type">Effect</span>.<span class="type">Deep</span>.match_with</span><br><span class="line"> (<span class="keyword">fun</span> <span class="literal">()</span> -&gt;</span><br><span class="line"> <span class="keyword">let</span> i = <span class="built_in">ref</span> <span class="number">0</span> <span class="keyword">in</span></span><br><span class="line"> <span class="type">Effect</span>.perform <span class="type">Twice</span>;</span><br><span class="line"> i := !i + <span class="number">1</span>;</span><br><span class="line"> <span class="type">Printf</span>.printf <span class="string">&quot;i = %d\n%!&quot;</span> !i;</span><br><span class="line"> <span class="keyword">assert</span> (!i = <span class="number">1</span>))</span><br><span class="line"> <span class="literal">()</span> htwice</span><br><span class="line"></span><br><span class="line"><span class="comment">(* The following does not trigger an assertion failure. *)</span></span><br><span class="line"><span class="keyword">let</span> _ = heap2stack <span class="literal">()</span></span><br><span class="line"></span><br><span class="line"><span class="comment">(* To fix this issue, we can wrap reference allocations in an instance</span></span><br><span class="line"><span class="comment"> of `Sys.opaque_identity&#x27;. However, this is not really a viable fix</span></span><br><span class="line"><span class="comment"> in general, as we may not have access to the client code that</span></span><br><span class="line"><span class="comment"> allocates the reference! *)</span></span><br><span class="line"><span class="keyword">let</span> heap2stack&#x27; <span class="literal">()</span> =</span><br><span class="line"> <span class="type">Effect</span>.<span class="type">Deep</span>.match_with</span><br><span class="line"> (<span class="keyword">fun</span> <span class="literal">()</span> -&gt;</span><br><span class="line"> <span class="keyword">let</span> i = <span class="type">Sys</span>.opaque_identity (<span class="built_in">ref</span> <span class="number">0</span>) <span class="keyword">in</span></span><br><span class="line"> <span class="type">Effect</span>.perform <span class="type">Twice</span>;</span><br><span class="line"> i := !i + <span class="number">1</span>;</span><br><span class="line"> <span class="type">Printf</span>.printf <span class="string">&quot;i = %d\n%!&quot;</span> !i;</span><br><span class="line"> <span class="keyword">assert</span> (!i = <span class="number">1</span>))</span><br><span class="line"> <span class="literal">()</span> htwice</span><br><span class="line"></span><br><span class="line"><span class="comment">(* The following triggers an assertion failure. *)</span></span><br><span class="line"><span class="keyword">let</span> _ = heap2stack&#x27; <span class="literal">()</span></span><br></pre></td></tr></table></figure>
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