<!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>Lazy - 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%;"> Lazy <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"> In a lazy (or non-strict) language, the arguments to a function are not evaluated before calling the function. Instead, the arguments are suspended and only evaluated by the function if needed. <p> <a href="StandardML">Standard ML</a> is an eager (or strict) language, not a lazy language. However, it is easy to delay evaluation of an expression in SML by creating a <em>thunk</em>, which is a nullary function. In SML, a thunk is written <tt>fn () => e</tt>. Another essential feature of laziness is <em>memoization</em>, meaning that once a suspended argument is evaluated, subsequent references look up the value. We can express this in SML with a function that maps a thunk to a memoized thunk. </p> <pre class=code> <B><FONT COLOR="#0000FF">signature</FONT></B> LAZY = <B><FONT COLOR="#0000FF">sig</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> lazy: (unit -> 'a) -> unit -> 'a <B><FONT COLOR="#0000FF">end</FONT></B> </PRE> <p> </p> <p> This is easy to implement in SML. </p> <pre class=code> <B><FONT COLOR="#0000FF">structure</FONT></B> Lazy: LAZY = <B><FONT COLOR="#0000FF">struct</FONT></B> <B><FONT COLOR="#A020F0">fun</FONT></B> lazy (th: unit -> 'a): unit -> 'a = <B><FONT COLOR="#A020F0">let</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> r: 'a option ref = ref NONE <B><FONT COLOR="#A020F0">in</FONT></B> <B><FONT COLOR="#A020F0">fn</FONT></B> () => <B><FONT COLOR="#A020F0">case</FONT></B> !r <B><FONT COLOR="#A020F0">of</FONT></B> NONE => <B><FONT COLOR="#A020F0">let</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> a = th () <B><FONT COLOR="#A020F0">val</FONT></B> () = r := SOME a <B><FONT COLOR="#A020F0">in</FONT></B> a <B><FONT COLOR="#A020F0">end</FONT></B> | SOME a => a <B><FONT COLOR="#A020F0">end</FONT></B> <B><FONT COLOR="#0000FF">end</FONT></B> </PRE> <p> </p> </div> <p> <hr> Last edited on 2005-01-26 20:33:55 by <span title="cfs36.cs.cornell.edu"><a href="MatthewFluet">MatthewFluet</a></span>. </body></html>