<!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>EqualityTypeVariable - 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%;"> EqualityTypeVariable <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"> An equality type variable is a type variable that starts with two or more primes, as in <tt>''a</tt> or <tt>''b</tt>. The canonical use of equality type variables is in specifying the type of the <a href="PolymorphicEquality">PolymorphicEquality</a> function, which is <tt>''a * ''a -> bool</tt>. Equality type variables ensure that polymorphic equality is only used on <a href="EqualityType">equality types</a>, by requiring that at every use of a polymorphic value, equality type variables are instantiated by equality types. <p> For example, the following program is type correct because polymorphic equality is applied to variables of type <tt>''a</tt>. </p> <pre class=code> <B><FONT COLOR="#A020F0">fun</FONT></B> f (x: ''a, y: ''a): bool = x = y </PRE> <p> </p> <p> On the other hand, the following program is not type correct, because polymorphic equality is applied to variables of type <tt>'a</tt>, which is not an equality type. </p> <pre class=code> <B><FONT COLOR="#A020F0">fun</FONT></B> f (x: 'a, y: 'a): bool = x = y </PRE> <p> </p> <p> MLton reports the following error, indicating that polymorphic equality expects equality types, but didn't get them. </p> <pre>Error: z.sml 1.32. Function applied to incorrect argument. expects: [<equality>] * [<equality>] but got: [<non-equality>] * [<non-equality>] in: = (x, y) </pre><p> As an example of using such a function that requires equality types, suppose that <tt>f</tt> has polymorphic type <tt>''a -> unit</tt>. Then, <tt>f 13</tt> is type correct because <tt>int</tt> is an equality type. On the other hand, <tt>f 13.0</tt> and <tt>f (fn x => x)</tt> are not type correct, because <tt>real</tt> and arrow types are not equality types. We can test these facts with the following short programs. First, we verify that such an <tt>f</tt> can be applied to integers. </p> <pre class=code> <B><FONT COLOR="#0000FF">functor</FONT></B> Ok (<B><FONT COLOR="#A020F0">val</FONT></B> f: ''a -> unit): <B><FONT COLOR="#0000FF">sig</FONT></B> <B><FONT COLOR="#0000FF">end</FONT></B> = <B><FONT COLOR="#0000FF">struct</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> () = f <B><FONT COLOR="#5F9EA0">13</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> () = f <B><FONT COLOR="#5F9EA0">14</FONT></B> <B><FONT COLOR="#0000FF">end</FONT></B> </PRE> <p> </p> <p> We can do better, and verify that such an <tt>f</tt> can be applied to any integer. </p> <pre class=code> <B><FONT COLOR="#0000FF">functor</FONT></B> Ok (<B><FONT COLOR="#A020F0">val</FONT></B> f: ''a -> unit): <B><FONT COLOR="#0000FF">sig</FONT></B> <B><FONT COLOR="#0000FF">end</FONT></B> = <B><FONT COLOR="#0000FF">struct</FONT></B> <B><FONT COLOR="#A020F0">fun</FONT></B> g (x: int) = f x <B><FONT COLOR="#0000FF">end</FONT></B> </PRE> <p> </p> <p> Even better, we don't need to introduce a dummy function name; we can use a type constraint. </p> <pre class=code> <B><FONT COLOR="#0000FF">functor</FONT></B> Ok (<B><FONT COLOR="#A020F0">val</FONT></B> f: ''a -> unit): <B><FONT COLOR="#0000FF">sig</FONT></B> <B><FONT COLOR="#0000FF">end</FONT></B> = <B><FONT COLOR="#0000FF">struct</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> _ = f: int -> unit <B><FONT COLOR="#0000FF">end</FONT></B> </PRE> <p> </p> <p> Even better, we can use a signature constraint. </p> <pre class=code> <B><FONT COLOR="#0000FF">functor</FONT></B> Ok (S: <B><FONT COLOR="#0000FF">sig</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> f: ''a -> unit <B><FONT COLOR="#0000FF">end</FONT></B>): <B><FONT COLOR="#0000FF">sig</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> f: int -> unit <B><FONT COLOR="#0000FF">end</FONT></B> = S </PRE> <p> </p> <p> This functor concisely verifies that a function of polymorphic type <tt>''a -> unit</tt> can be safely used as a function of type <tt>int -> unit</tt>. </p> <p> As above, we can verify that such an <tt>f</tt> can not be used at non equality types. </p> <pre class=code> <B><FONT COLOR="#0000FF">functor</FONT></B> Bad (S: <B><FONT COLOR="#0000FF">sig</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> f: ''a -> unit <B><FONT COLOR="#0000FF">end</FONT></B>): <B><FONT COLOR="#0000FF">sig</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> f: real -> unit <B><FONT COLOR="#0000FF">end</FONT></B> = S </PRE> <p> </p> <pre class=code> <B><FONT COLOR="#0000FF">functor</FONT></B> Bad (S: <B><FONT COLOR="#0000FF">sig</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> f: ''a -> unit <B><FONT COLOR="#0000FF">end</FONT></B>): <B><FONT COLOR="#0000FF">sig</FONT></B> <B><FONT COLOR="#A020F0">val</FONT></B> f: ('a -> 'a) -> unit <B><FONT COLOR="#0000FF">end</FONT></B> = S </PRE> <p> </p> <p> For each of these programs, MLton reports the following error. </p> <pre>Error: z.sml 2.4. Variable type in structure disagrees with signature. variable: f structure: [<equality>] -> _ signature: [<non-equality>] -> _ </pre><h2 id="head-298e9694028673faa9fcbabf774bc23e4d2fbbe0">Equality type variables in type and datatype declarations</h2> <p> Equality type variables can be used in type and datatype declarations; however they play no special role. For example, </p> <pre class=code> <B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> 'a t </FONT></B>=<B><FONT COLOR="#228B22"> 'a * int </FONT></B></PRE> <p> </p> <p> is completely identical to </p> <pre class=code> <B><FONT COLOR="#A020F0">type</FONT></B><B><FONT COLOR="#228B22"> ''a t </FONT></B>=<B><FONT COLOR="#228B22"> ''a * int </FONT></B></PRE> <p> </p> <p> In particular, such a definition does <em>not</em> require that <tt>t</tt> only be applied to equality types. </p> <p> Similarly, </p> <pre class=code> <B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> 'a 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 </FONT></B></PRE> <p> </p> <p> is completely identical to </p> <pre class=code> <B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> ''a 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 </FONT></B></PRE> <p> </p> </div> <p> <hr> Last edited on 2005-12-01 04:00:38 by <span title="ppp-71-139-183-221.dsl.snfc21.pacbell.net"><a href="StephenWeeks">StephenWeeks</a></span>. </body></html>