doc/guide/localhost/GenerativeDatatype

   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>GenerativeDatatype</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>GenerativeDatatype</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>, datatype declarations are said to be
  32 <em>generative</em>, because each time a datatype declaration is evaluated,
  33 it yields a new type.  Thus, any attempt to mix the types will lead to
  34 a type error at compile-time.  The following program, which does not
  35 type check, demonstrates this.</p></div>
  36 <div class="listingblock">
  37 <div class="content"><div class="highlight"><pre><span class="k">functor</span><span class="w"> </span><span class="n">F</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
  38 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
  39 <span class="w">      </span><span class="k">datatype</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">T</span><span class="w"></span>
  40 <span class="w">   </span><span class="k">end</span><span class="w"></span>
  41 <span class="k">structure</span><span class="w"> </span><span class="n">S1</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">F</span><span class="w"> </span><span class="p">()</span><span class="w"></span>
  42 <span class="k">structure</span><span class="w"> </span><span class="n">S2</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">F</span><span class="w"> </span><span class="p">()</span><span class="w"></span>
  43 <span class="k">val</span><span class="w"> </span><span class="p">_:</span><span class="w"> </span><span class="n">S1</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">S2</span><span class="p">.</span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">x</span><span class="w"></span>
  44 </pre></div></div></div>
  45 <div class="paragraph"><p>Generativity also means that two different datatype declarations
  46 define different types, even if they define identical constructors.
  47 The following program does not type check due to this.</p></div>
  48 <div class="listingblock">
  49 <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">B</span><span class="w"></span>
  50 <span class="k">val</span><span class="w"> </span><span class="n">a1</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">A</span><span class="w"></span>
  51 <span class="k">datatype</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">B</span><span class="w"></span>
  52 <span class="k">val</span><span class="w"> </span><span class="n">a2</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">A</span><span class="w"></span>
  53 <span class="k">val</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="n">true</span><span class="w"> </span><span class="k">then</span><span class="w"> </span><span class="n">a1</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="n">a2</span><span class="w"></span>
  54 </pre></div></div></div>
  55 </div>
  56 </div>
  57 <div class="sect1">
  58 <h2 id="_also_see">Also see</h2>
  59 <div class="sectionbody">
  60 <div class="ulist"><ul>
  61 <li>
  62 <p>
  63 <a href="GenerativeException">GenerativeException</a>
  64 </p>
  65 </li>
  66 </ul></div>
  67 </div>
  68 </div>
  69 </div>
  70 <div id="footnotes"><hr></div>
  71 <div id="footer">
  72 <div id="footer-text">
  73 </div>
  74 <div id="footer-badges">
  75 </div>
  76 </div>
  77 </body>
  78 </html>