HCoop / hcoop / debian / mlton.git / blame
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| 7f918cf1 CE |
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| 6 | <title>TypeConstructor</title>\r | |
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| 22 | <a href="./Home">MLton 20180207</a>\r | |
| 23 | </div>\r | |
| 24 | </div>\r | |
| 25 | <div id="header">\r | |
| 26 | <h1>TypeConstructor</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>, a type constructor is a function from\r | |
| 32 | types to types. Type constructors can be <em>nullary</em>, meaning that\r | |
| 33 | they take no arguments, as in <span class="monospaced">char</span>, <span class="monospaced">int</span>, and <span class="monospaced">real</span>.\r | |
| 34 | Type constructors can be <em>unary</em>, meaning that they take one\r | |
| 35 | argument, as in <span class="monospaced">array</span>, <span class="monospaced">list</span>, and <span class="monospaced">vector</span>. A program\r | |
| 36 | can define a new type constructor in two ways: a <span class="monospaced">type</span> definition\r | |
| 37 | or a <span class="monospaced">datatype</span> declaration. User-defined type constructors can\r | |
| 38 | can take any number of arguments.</p></div>\r | |
| 39 | <div class="listingblock">\r | |
| 40 | <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">T</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">real</span><span class="w"> </span><span class="cm">(* 0 arguments *)</span><span class="w"></span>\r | |
| 41 | <span class="k">type</span><span class="w"> </span><span class="n">'a</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="n">*</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="cm">(* 1 argument *)</span><span class="w"></span>\r | |
| 42 | <span class="k">datatype</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="p">,</span><span class="w"> </span><span class="n">'b</span><span class="p">)</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><span class="k">of</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'b</span><span class="w"> </span><span class="cm">(* 2 arguments *)</span><span class="w"></span>\r | |
| 43 | <span class="k">type</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="p">,</span><span class="w"> </span><span class="n">'b</span><span class="p">,</span><span class="w"> </span><span class="n">'c</span><span class="p">)</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="n">*</span><span class="w"> </span><span class="p">(</span><span class="n">'b</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'c</span><span class="p">)</span><span class="w"> </span><span class="cm">(* 3 arguments *)</span><span class="w"></span>\r | |
| 44 | </pre></div></div></div>\r | |
| 45 | <div class="paragraph"><p>Here are the syntax rules for type constructor application.</p></div>\r | |
| 46 | <div class="ulist"><ul>\r | |
| 47 | <li>\r | |
| 48 | <p>\r | |
| 49 | Type constructor application is written in postfix. So, one writes\r | |
| 50 | <span class="monospaced">int list</span>, not <span class="monospaced">list int</span>.\r | |
| 51 | </p>\r | |
| 52 | </li>\r | |
| 53 | <li>\r | |
| 54 | <p>\r | |
| 55 | Unary type constructors drop the parens, so one writes\r | |
| 56 | <span class="monospaced">int list</span>, not <span class="monospaced">(int) list</span>.\r | |
| 57 | </p>\r | |
| 58 | </li>\r | |
| 59 | <li>\r | |
| 60 | <p>\r | |
| 61 | Nullary type constructors drop the argument entirely, so one writes\r | |
| 62 | <span class="monospaced">int</span>, not <span class="monospaced">() int</span>.\r | |
| 63 | </p>\r | |
| 64 | </li>\r | |
| 65 | <li>\r | |
| 66 | <p>\r | |
| 67 | N-ary type constructors use tuple notation; for example,\r | |
| 68 | <span class="monospaced">(int, real) t</span>.\r | |
| 69 | </p>\r | |
| 70 | </li>\r | |
| 71 | <li>\r | |
| 72 | <p>\r | |
| 73 | Type constructor application associates to the left. So,\r | |
| 74 | <span class="monospaced">int ref list</span> is the same as <span class="monospaced">(int ref) list</span>.\r | |
| 75 | </p>\r | |
| 76 | </li>\r | |
| 77 | </ul></div>\r | |
| 78 | </div>\r | |
| 79 | </div>\r | |
| 80 | </div>\r | |
| 81 | <div id="footnotes"><hr></div>\r | |
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| 84 | </div>\r | |
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| 86 | </div>\r | |
| 87 | </div>\r | |
| 88 | </body>\r | |
| 89 | </html>\r |