doc/guide/localhost/Overloading

   1 <!DOCTYPE html>
   2 <html lang="en">
   3 <head>
   4 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
   5 <meta name="generator" content="AsciiDoc 8.6.9">
   6 <title>Overloading</title>
   7 <link rel="stylesheet" href="./asciidoc.css" type="text/css">
   8 <link rel="stylesheet" href="./pygments.css" type="text/css">
   9
  10
  11 <script type="text/javascript" src="./asciidoc.js"></script>
  12 <script type="text/javascript">
  13 /*<![CDATA[*/
  14 asciidoc.install();
  15 /*]]>*/
  16 </script>
  17 <link rel="stylesheet" href="./mlton.css" type="text/css">
  18 </head>
  19 <body class="article">
  20 <div id="banner">
  21 <div id="banner-home">
  22 <a href="./Home">MLton 20180207</a>
  23 </div>
  24 </div>
  25 <div id="header">
  26 <h1>Overloading</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p>In <a href="StandardML">Standard ML</a>, constants (like <span class="monospaced">13</span>, <span class="monospaced">0w13</span>, <span class="monospaced">13.0</span>)
  32 are overloaded, meaning that they can denote a constant of the
  33 appropriate type as determined by context.  SML defines the
  34 overloading classes <em>Int</em>, <em>Real</em>, and <em>Word</em>, which denote the sets
  35 of types that integer, real, and word constants may take on.  In
  36 MLton, these are defined as follows.</p></div>
  37 <table class="tableblock frame-all grid-all"
  38 style="
  39 width:100%;
  40 ">
  41 <col style="width:25%;">
  42 <col style="width:75%;">
  43 <tbody>
  44 <tr>
  45 <td class="tableblock halign-center valign-top" ><p class="tableblock"><em>Int</em></p></td>
  46 <td class="tableblock halign-left valign-top" ><p class="tableblock"><span class="monospaced">Int2.int</span>, <span class="monospaced">Int3.int</span>, &#8230; <span class="monospaced">Int32.int</span>, <span class="monospaced">Int64.int</span>, <span class="monospaced">Int.int</span>, <span class="monospaced">IntInf.int</span>, <span class="monospaced">LargeInt.int</span>, <span class="monospaced">FixedInt.int</span>, <span class="monospaced">Position.int</span></p></td>
  47 </tr>
  48 <tr>
  49 <td class="tableblock halign-center valign-top" ><p class="tableblock"><em>Real</em></p></td>
  50 <td class="tableblock halign-left valign-top" ><p class="tableblock"><span class="monospaced">Real32.real</span>, <span class="monospaced">Real64.real</span>, <span class="monospaced">Real.real</span>, <span class="monospaced">LargeReal.real</span></p></td>
  51 </tr>
  52 <tr>
  53 <td class="tableblock halign-center valign-top" ><p class="tableblock"><em>Word</em></p></td>
  54 <td class="tableblock halign-left valign-top" ><p class="tableblock"><span class="monospaced">Word2.word</span>, <span class="monospaced">Word3.word</span>, &#8230; <span class="monospaced">Word32.word</span>, <span class="monospaced">Word64.word</span>, <span class="monospaced">Word.word</span>, <span class="monospaced">LargeWord.word</span>, <span class="monospaced">SysWord.word</span></p></td>
  55 </tr>
  56 </tbody>
  57 </table>
  58 <div class="paragraph"><p>The <a href="DefinitionOfStandardML">Definition</a> allows flexibility in how
  59 much context is used to resolve overloading.  It says that the context
  60 is <em>no larger than the smallest enclosing structure-level
  61 declaration</em>, but that <em>an implementation may require that a smaller
  62 context determines the type</em>.  MLton uses the largest possible context
  63 allowed by SML in resolving overloading.  If the type of a constant is
  64 not determined by context, then it takes on a default type.  In MLton,
  65 these are defined as follows.</p></div>
  66 <table class="tableblock frame-all grid-all"
  67 style="
  68 width:100%;
  69 ">
  70 <col style="width:25%;">
  71 <col style="width:75%;">
  72 <tbody>
  73 <tr>
  74 <td class="tableblock halign-center valign-top" ><p class="tableblock"><em>Int</em></p></td>
  75 <td class="tableblock halign-left valign-top" ><p class="tableblock"><span class="monospaced">Int.int</span></p></td>
  76 </tr>
  77 <tr>
  78 <td class="tableblock halign-center valign-top" ><p class="tableblock"><em>Real</em></p></td>
  79 <td class="tableblock halign-left valign-top" ><p class="tableblock"><span class="monospaced">Real.real</span></p></td>
  80 </tr>
  81 <tr>
  82 <td class="tableblock halign-center valign-top" ><p class="tableblock"><em>Word</em></p></td>
  83 <td class="tableblock halign-left valign-top" ><p class="tableblock"><span class="monospaced">Word.word</span></p></td>
  84 </tr>
  85 </tbody>
  86 </table>
  87 <div class="paragraph"><p>Other implementations may use a smaller context or different default
  88 types.</p></div>
  89 </div>
  90 </div>
  91 <div class="sect1">
  92 <h2 id="_also_see">Also see</h2>
  93 <div class="sectionbody">
  94 <div class="ulist"><ul>
  95 <li>
  96 <p>
  97 <a href="http://www.standardml.org/Basis/top-level-chapter.html">discussion of overloading in the Basis Library</a>
  98 </p>
  99 </li>
 100 </ul></div>
 101 </div>
 102 </div>
 103 <div class="sect1">
 104 <h2 id="_examples">Examples</h2>
 105 <div class="sectionbody">
 106 <div class="ulist"><ul>
 107 <li>
 108 <p>
 109 The following program is rejected.
 110 </p>
 111 <div class="listingblock">
 112 <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">S</span><span class="p">:</span><span class="w"></span>
 113 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
 114 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">x</span><span class="p">:</span><span class="w"> </span><span class="n">Word8</span><span class="p">.</span><span class="n">word</span><span class="w"></span>
 115 <span class="w">   </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 116 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
 117 <span class="w">      </span><span class="k">val</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">0w0</span><span class="w"></span>
 118 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 119 </pre></div></div></div>
 120 <div class="paragraph"><p>The smallest enclosing structure declaration for <span class="monospaced">0w0</span> is
 121 <span class="monospaced">val x = 0w0</span>.  Hence, <span class="monospaced">0w0</span> receives the default type for words,
 122 which is <span class="monospaced">Word.word</span>.</p></div>
 123 </li>
 124 </ul></div>
 125 </div>
 126 </div>
 127 </div>
 128 <div id="footnotes"><hr></div>
 129 <div id="footer">
 130 <div id="footer-text">
 131 </div>
 132 <div id="footer-badges">
 133 </div>
 134 </div>
 135 </body>
 136 </html>