<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta name="robots" content="index,nofollow">
<title>Closure - MLton Standard ML Compiler (SML Compiler)</title>
<link rel="stylesheet" type="text/css" charset="iso-8859-1" media="all" href="common.css">
<link rel="stylesheet" type="text/css" charset="iso-8859-1" media="screen" href="screen.css">
<link rel="stylesheet" type="text/css" charset="iso-8859-1" media="print" href="print.css">
<link rel="Start" href="Home">
</head>
<body lang="en" dir="ltr">
<script src="http://www.google-analytics.com/urchin.js" type="text/javascript">
</script>
<script type="text/javascript">
_uacct = "UA-833377-1";
urchinTracker();
</script>
<table bgcolor = lightblue cellspacing = 0 style = "border: 0px;" width = 100%>
<tr>
<td style = "
border: 0px;
color: darkblue;
font-size: 150%;
text-align: left;">
<a class = mltona href="Home">MLton MLTONWIKIVERSION</a>
<td style = "
border: 0px;
font-size: 150%;
text-align: center;
width: 50%;">
Closure
<td style = "
border: 0px;
text-align: right;">
<table cellspacing = 0 style = "border: 0px">
<tr style = "vertical-align: middle;">
</table>
<tr style = "background-color: white;">
<td colspan = 3
style = "
border: 0px;
font-size:70%;
text-align: right;">
<a href = "Home">Home</a>
<a href = "TitleIndex">Index</a>
</table>
<div id="content" lang="en" dir="ltr">
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
</p>
<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 => x + y
<B><FONT COLOR="#A020F0">end</FONT></B>
</PRE>
<p>
</p>
<p>
the closure for <tt>fn y => x + 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>.
</p>
<p>
To call a function, the code pointer is extracted and jumped to, passing in some agreed upon location the environment and the argument.
</p>
<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.
</p>
</div>
<p>
<hr>
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>.
</body></html>
