<!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>ProductType - 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%;"> ProductType <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 href="StandardML">Standard ML</a> has special syntax for products (tuples). A product type is written as <pre>t1 * t2 * ... * tN </pre> and a product pattern is written as <pre>(p1, p2, ..., pN) </pre><p> In most situations the syntax is quite convenient. However, there are situations where the syntax is cumbersome. There are also situations in which it is useful to construct and destruct n-ary products inductively, especially when using <a href="Fold">Fold</a>. </p> <p> In such situations, it is useful to have a binary product datatype with an infix constructor defined as follows. <pre class=code> <B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> ('a, 'b) product </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">&</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> 'a * 'b </FONT></B><B><FONT COLOR="#A020F0">infix</FONT></B> & </PRE> </p> <p> With these definitions, one can write an n-ary product as a nested binary product quite conveniently. <pre>x1 & x2 & ... & xn </pre> </p> <p> Because of left associativity, this is the same as <pre>(((x1 & x2) & ...) & xn) </pre> </p> <p> Because <tt>&</tt> is a constructor, the syntax can also be used for patterns. </p> <p> The symbol <tt>&</tt> is inspired by the Curry-Howard isomorphism: the proof of a conjunction <tt>(A & B)</tt> is a pair of proofs <tt>(a, b)</tt>. </p> <h2 id="head-058073812447f89d85b57e47ba6958fee7a17440">Example: parser combinators</h2> <p> A typical parser combinator library provides a combinator that has a type of the form. </p> <pre>'a parser * 'b parser -> ('a * 'b) parser </pre><p> and produces a parser for the concatenation of two parsers. When more than two parsers are concatenated, the result of the resulting parser is a nested structure of pairs </p> <pre>(...((p1, p2), p3)..., pN) </pre><p> which is somewhat cumbersome. </p> <p> By using a product type, the type of the concatenation combinator then becomes </p> <pre>'a parser * 'b parser -> ('a, 'b) product parser </pre><p> While this doesn't stop the nesting, it makes the pattern significantly easier to write. Instead of </p> <pre>(...((p1, p2), p3)..., pN) </pre>the pattern is written as <pre>p1 & p2 & p3 & ... & pN </pre>which is considerably more concise. <h2 id="head-a4bc8bf5caf54b18cea9f58e83dd4acb488deb17">Also see</h2> <ul> <li> <p> <a href="VariableArityPolymorphism">VariableArityPolymorphism</a> </p> </li> <li> <p> <a href="Utilities">Utilities</a> </p> </li> </ul> </div> <p> <hr> Last edited on 2007-08-26 19:59:19 by <span title="c-71-57-91-146.hsd1.il.comcast.net"><a href="MatthewFluet">MatthewFluet</a></span>. </body></html>