<!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>GenerativeDatatype - 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%;"> GenerativeDatatype <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 href="StandardML">Standard ML</a>, datatype declarations are said to be <em>generative</em>, because each time a datatype declaration is evaluated, it yields a new type. Thus, any attempt to mix the types will lead to a type error at compile-time. The following program, which does not type check, demonstrates this. <pre class=code> <B><FONT COLOR="#0000FF">functor</FONT></B> F () = <B><FONT COLOR="#0000FF">struct</FONT></B> <B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">T</FONT> </FONT></B><B><FONT COLOR="#0000FF">end</FONT></B> <B><FONT COLOR="#0000FF">structure</FONT></B> S1 = F () <B><FONT COLOR="#0000FF">structure</FONT></B> S2 = F () <B><FONT COLOR="#A020F0">val</FONT></B> _: S1.t -> S2.t = <B><FONT COLOR="#A020F0">fn</FONT></B> x => x </PRE> <p> </p> <p> Generativity also means that two different datatype declarations define different types, even if they define identical constructors. The following program does not type check due to this. </p> <pre class=code> <B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">A</FONT> </FONT></B>|<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">B</FONT> </FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> a1 = A <B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">A</FONT> </FONT></B>|<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">B</FONT> </FONT></B><B><FONT COLOR="#A020F0">val</FONT></B> a2 = A <B><FONT COLOR="#A020F0">val</FONT></B> _ = <B><FONT COLOR="#A020F0">if</FONT></B> true <B><FONT COLOR="#A020F0">then</FONT></B> a1 <B><FONT COLOR="#A020F0">else</FONT></B> a2 </PRE> <p> </p> <h2 id="head-a4bc8bf5caf54b18cea9f58e83dd4acb488deb17">Also see</h2> <ul> <li> <p> <a href="GenerativeException">GenerativeException</a> </p> </li> </ul> </div> <p> <hr> Last edited on 2010-03-02 15:11:35 by <span title="fenrir.cs.rit.edu"><a href="MatthewFluet">MatthewFluet</a></span>. </body></html>