<!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>TypeConstructor - 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%;"> TypeConstructor <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>, a type constructor is a function from types to types. Type constructors can be <em>nullary</em>, meaning that they take no arguments, as in <tt>char</tt>, <tt>int</tt>, and <tt>real</tt>. Type constructors can be <em>unary</em>, meaning that they take one argument, as in <tt>array</tt>, <tt>list</tt>, and <tt>vector</tt>. A program can define a new type constructor in two ways: a <tt>type</tt> definition or a <tt>datatype</tt> declaration. User-defined type constructors can can take any number of arguments. <pre class=code> <B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">T</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> int * real <I><FONT COLOR="#B22222">(* 0 arguments *)</FONT></I> </FONT></B><B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> 'a t </FONT></B>=<B><FONT COLOR="#228B22"> 'a * int <I><FONT COLOR="#B22222">(* 1 argument *)</FONT></I> </FONT></B><B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> ('a, 'b) t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">A</FONT> </FONT></B>|<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">B</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> 'a * 'b <I><FONT COLOR="#B22222">(* 2 arguments *)</FONT></I> </FONT></B><B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> ('a, 'b, 'c) t </FONT></B>=<B><FONT COLOR="#228B22"> 'a * ('b -> 'c) <I><FONT COLOR="#B22222">(* 3 arguments *)</FONT></I> </FONT></B></PRE> <p> </p> <p> Here are the syntax rules for type constructor application. </p> <ul> <li> <p> Type constructor application is written in postfix. So, one writes <tt>int list</tt>, not <tt>list int</tt>. </p> </li> <li class="gap"> <p> Unary type constructors drop the parens, so one writes <tt>int list</tt>, not <tt>(int) list</tt>. </p> </li> <li class="gap"> <p> Nullary type constructors drop the argument entirely, so one writes <tt>int</tt>, not <tt>() int</tt>. </p> </li> <li class="gap"> <p> N-ary type constructors use tuple notation; for example, <tt>(int, real) t</tt>. </p> </li> <li class="gap"> <p> Type constructor application associates to the left. So, <tt>int ref list</tt> is the same as <tt>(int ref) list</tt>. </p> </li> </ul> </div> <p> <hr> Last edited on 2005-12-02 04:26:23 by <span title="ppp-71-139-183-221.dsl.snfc21.pacbell.net"><a href="StephenWeeks">StephenWeeks</a></span>. </body></html>