HCoop / hcoop / debian / mlton.git / blame
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| 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>Closure</title>\r | |
| 7 | <link rel="stylesheet" href="./asciidoc.css" type="text/css">\r | |
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| 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>Closure</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>A closure is a data structure that is the run-time representation of a\r | |
| 32 | function.</p></div>\r | |
| 33 | </div>\r | |
| 34 | </div>\r | |
| 35 | <div class="sect1">\r | |
| 36 | <h2 id="_typical_implementation">Typical Implementation</h2>\r | |
| 37 | <div class="sectionbody">\r | |
| 38 | <div class="paragraph"><p>In a typical implementation, a closure consists of a <em>code pointer</em>\r | |
| 39 | (indicating what the function does) and an <em>environment</em> containing\r | |
| 40 | the values of the free variables of the function. For example, in the\r | |
| 41 | expression</p></div>\r | |
| 42 | <div class="listingblock">\r | |
| 43 | <div class="content"><div class="highlight"><pre><span class="k">let</span><span class="w"></span>\r | |
| 44 | <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">5</span><span class="w"></span>\r | |
| 45 | <span class="k">in</span><span class="w"></span>\r | |
| 46 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">y</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="n">+</span><span class="w"> </span><span class="n">y</span><span class="w"></span>\r | |
| 47 | <span class="k">end</span><span class="w"></span>\r | |
| 48 | </pre></div></div></div>\r | |
| 49 | <div class="paragraph"><p>the closure for <span class="monospaced">fn y => x + y</span> contains a pointer to a piece of code\r | |
| 50 | that knows to take its argument and add the value of <span class="monospaced">x</span> to it, plus\r | |
| 51 | the environment recording the value of <span class="monospaced">x</span> as <span class="monospaced">5</span>.</p></div>\r | |
| 52 | <div class="paragraph"><p>To call a function, the code pointer is extracted and jumped to,\r | |
| 53 | passing in some agreed upon location the environment and the argument.</p></div>\r | |
| 54 | </div>\r | |
| 55 | </div>\r | |
| 56 | <div class="sect1">\r | |
| 57 | <h2 id="_mlton_8217_s_implementation">MLton’s Implementation</h2>\r | |
| 58 | <div class="sectionbody">\r | |
| 59 | <div class="paragraph"><p>MLton does not implement closures traditionally. Instead, based on\r | |
| 60 | whole-program higher-order control-flow analysis, MLton represents a\r | |
| 61 | function as an element of a sum type, where the variant indicates\r | |
| 62 | which function it is and carries the free variables as arguments. See\r | |
| 63 | <a href="ClosureConvert">ClosureConvert</a> and <a href="References#CejtinEtAl00">CejtinEtAl00</a> for details.</p></div>\r | |
| 64 | </div>\r | |
| 65 | </div>\r | |
| 66 | </div>\r | |
| 67 | <div id="footnotes"><hr></div>\r | |
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| 70 | </div>\r | |
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| 72 | </div>\r | |
| 73 | </div>\r | |
| 74 | </body>\r | |
| 75 | </html>\r |