doc/guide/localhost/Closure

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  26 <h1>Closure</h1>
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  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p>A closure is a data structure that is the run-time representation of a
  32 function.</p></div>
  33 </div>
  34 </div>
  35 <div class="sect1">
  36 <h2 id="_typical_implementation">Typical Implementation</h2>
  37 <div class="sectionbody">
  38 <div class="paragraph"><p>In a typical implementation, a closure consists of a <em>code pointer</em>
  39 (indicating what the function does) and an <em>environment</em> containing
  40 the values of the free variables of the function.  For example, in the
  41 expression</p></div>
  42 <div class="listingblock">
  43 <div class="content"><div class="highlight"><pre><span class="k">let</span><span class="w"></span>
  44 <span class="w">   </span><span class="k">val</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">5</span><span class="w"></span>
  45 <span class="k">in</span><span class="w"></span>
  46 <span class="w">   </span><span class="k">fn</span><span class="w"> </span><span class="n">y</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="n">+</span><span class="w"> </span><span class="n">y</span><span class="w"></span>
  47 <span class="k">end</span><span class="w"></span>
  48 </pre></div></div></div>
  49 <div class="paragraph"><p>the closure for <span class="monospaced">fn y =&gt; x + y</span> contains a pointer to a piece of code
  50 that knows to take its argument and add the value of <span class="monospaced">x</span> to it, plus
  51 the environment recording the value of <span class="monospaced">x</span> as <span class="monospaced">5</span>.</p></div>
  52 <div class="paragraph"><p>To call a function, the code pointer is extracted and jumped to,
  53 passing in some agreed upon location the environment and the argument.</p></div>
  54 </div>
  55 </div>
  56 <div class="sect1">
  57 <h2 id="_mlton_8217_s_implementation">MLton&#8217;s Implementation</h2>
  58 <div class="sectionbody">
  59 <div class="paragraph"><p>MLton does not implement closures traditionally.  Instead, based on
  60 whole-program higher-order control-flow analysis, MLton represents a
  61 function as an element of a sum type, where the variant indicates
  62 which function it is and carries the free variables as arguments.  See
  63 <a href="ClosureConvert">ClosureConvert</a> and <a href="References#CejtinEtAl00">CejtinEtAl00</a> for details.</p></div>
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