HCoop / hcoop / debian / mlton.git / blame
| Commit | Line | Data |
|---|---|---|
| 7f918cf1 CE |
1 | <!DOCTYPE html>\r |
| 2 | <html lang="en">\r | |
| 3 | <head>\r | |
| 4 | <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">\r | |
| 5 | <meta name="generator" content="AsciiDoc 8.6.9">\r | |
| 6 | <title>GenerativeDatatype</title>\r | |
| 7 | <link rel="stylesheet" href="./asciidoc.css" type="text/css">\r | |
| 8 | <link rel="stylesheet" href="./pygments.css" type="text/css">\r | |
| 9 | \r | |
| 10 | \r | |
| 11 | <script type="text/javascript" src="./asciidoc.js"></script>\r | |
| 12 | <script type="text/javascript">\r | |
| 13 | /*<![CDATA[*/\r | |
| 14 | asciidoc.install();\r | |
| 15 | /*]]>*/\r | |
| 16 | </script>\r | |
| 17 | <link rel="stylesheet" href="./mlton.css" type="text/css">\r | |
| 18 | </head>\r | |
| 19 | <body class="article">\r | |
| 20 | <div id="banner">\r | |
| 21 | <div id="banner-home">\r | |
| 22 | <a href="./Home">MLton 20180207</a>\r | |
| 23 | </div>\r | |
| 24 | </div>\r | |
| 25 | <div id="header">\r | |
| 26 | <h1>GenerativeDatatype</h1>\r | |
| 27 | </div>\r | |
| 28 | <div id="content">\r | |
| 29 | <div id="preamble">\r | |
| 30 | <div class="sectionbody">\r | |
| 31 | <div class="paragraph"><p>In <a href="StandardML">Standard ML</a>, datatype declarations are said to be\r | |
| 32 | <em>generative</em>, because each time a datatype declaration is evaluated,\r | |
| 33 | it yields a new type. Thus, any attempt to mix the types will lead to\r | |
| 34 | a type error at compile-time. The following program, which does not\r | |
| 35 | type check, demonstrates this.</p></div>\r | |
| 36 | <div class="listingblock">\r | |
| 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>\r | |
| 38 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
| 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>\r | |
| 40 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
| 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>\r | |
| 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>\r | |
| 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">-></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">=></span><span class="w"> </span><span class="n">x</span><span class="w"></span>\r | |
| 44 | </pre></div></div></div>\r | |
| 45 | <div class="paragraph"><p>Generativity also means that two different datatype declarations\r | |
| 46 | define different types, even if they define identical constructors.\r | |
| 47 | The following program does not type check due to this.</p></div>\r | |
| 48 | <div class="listingblock">\r | |
| 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>\r | |
| 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>\r | |
| 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>\r | |
| 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>\r | |
| 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>\r | |
| 54 | </pre></div></div></div>\r | |
| 55 | </div>\r | |
| 56 | </div>\r | |
| 57 | <div class="sect1">\r | |
| 58 | <h2 id="_also_see">Also see</h2>\r | |
| 59 | <div class="sectionbody">\r | |
| 60 | <div class="ulist"><ul>\r | |
| 61 | <li>\r | |
| 62 | <p>\r | |
| 63 | <a href="GenerativeException">GenerativeException</a>\r | |
| 64 | </p>\r | |
| 65 | </li>\r | |
| 66 | </ul></div>\r | |
| 67 | </div>\r | |
| 68 | </div>\r | |
| 69 | </div>\r | |
| 70 | <div id="footnotes"><hr></div>\r | |
| 71 | <div id="footer">\r | |
| 72 | <div id="footer-text">\r | |
| 73 | </div>\r | |
| 74 | <div id="footer-badges">\r | |
| 75 | </div>\r | |
| 76 | </div>\r | |
| 77 | </body>\r | |
| 78 | </html>\r |