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      Closure
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A closure is a data structure that is the run-time representation of a function. <h2 id="head-c89ca68e44beb2aeaa7fa392ca452792f302e18c">Typical Implementation</h2>
<p>
In a typical implementation, a closure consists of a <em>code pointer</em> (indicating what the function does) and an <em>environment</em> containing the values of the free variables of the function.  For example, in the expression 
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<pre class=code>
<B><FONT COLOR="#A020F0">let</FONT></B>
   <B><FONT COLOR="#A020F0">val</FONT></B> x = <B><FONT COLOR="#5F9EA0">5</FONT></B>
<B><FONT COLOR="#A020F0">in</FONT></B>
   <B><FONT COLOR="#A020F0">fn</FONT></B> y =&gt; x + y
<B><FONT COLOR="#A020F0">end</FONT></B>
</PRE>
<p>
 
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<p>
the closure for <tt>fn&nbsp;y&nbsp;=&gt;&nbsp;x&nbsp;+&nbsp;y</tt> contains a pointer to a piece of code that knows to take its argument and add the value of <tt>x</tt> to it, plus the environment recording the value of <tt>x</tt> as <tt>5</tt>. 
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<p>
To call a function, the code pointer is extracted and jumped to, passing in some agreed upon location the environment and the argument. 
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<h2 id="head-ca4b0e2ce966921179ffc843f53997c622b12c20">MLton's Implementation</h2>
<p>
MLton does not implement closures traditionally.  Instead, based on whole-program higher-order control-flow analysis, MLton represents a function as an element of a sum type, where the variant indicates which function it is and carries the free variables as arguments.  See <a href="ClosureConvert">ClosureConvert</a> and <a href = "References#CejtinEtAl00">CejtinEtAl00</a> for details. 
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Last edited on 2005-11-30 23:25:36 by <span title="ppp-71-139-183-221.dsl.snfc21.pacbell.net"><a href="StephenWeeks">StephenWeeks</a></span>.
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