<!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>EtaExpansion - 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%;"> EtaExpansion <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"> Eta expansion is a simple syntactic change used to work around the <a href="ValueRestriction">ValueRestriction</a> in <a href="StandardML">Standard ML</a>. <p> The eta expansion of an expression <tt>e</tt> is the expression <tt>fn z => e z</tt>, where <tt>z</tt> does not occur in <tt>e</tt>. This only makes sense if <tt>e</tt> denotes a function, i.e. is of arrow type. Eta expansion delays the evaluation of <tt>e</tt> until the function is applied, and will re-evaluate <tt>e</tt> each time the function is applied. </p> <p> The name "eta expansion" comes from the eta-conversion rule of the <a href="LambdaCalculus">lambda calculus</a>. Expansion refers to the directionality of the equivalence being used, namely taking <tt>e</tt> to <tt>fn z => e z</tt> rather than <tt>fn z => e z</tt> to <tt>e</tt> (eta contraction). </p> </div> <p> <hr> Last edited on 2006-03-28 00:57:50 by <span title="adsl-71-141-33-187.dsl.snfc21.sbcglobal.net"><a href="StephenWeeks">StephenWeeks</a></span>. </body></html>