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>Lazy</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>Lazy</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 lazy (or non-strict) language, the arguments to a function are\r | |
| 32 | not evaluated before calling the function. Instead, the arguments are\r | |
| 33 | suspended and only evaluated by the function if needed.</p></div>\r | |
| 34 | <div class="paragraph"><p><a href="StandardML">Standard ML</a> is an eager (or strict) language, not a lazy\r | |
| 35 | language. However, it is easy to delay evaluation of an expression in\r | |
| 36 | SML by creating a <em>thunk</em>, which is a nullary function. In SML, a\r | |
| 37 | thunk is written <span class="monospaced">fn () => e</span>. Another essential feature of laziness\r | |
| 38 | is <em>memoization</em>, meaning that once a suspended argument is evaluated,\r | |
| 39 | subsequent references look up the value. We can express this in SML\r | |
| 40 | with a function that maps a thunk to a memoized thunk.</p></div>\r | |
| 41 | <div class="listingblock">\r | |
| 42 | <div class="content"><div class="highlight"><pre><span class="k">signature</span><span class="w"> </span><span class="n">LAZY</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
| 43 | <span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
| 44 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">lazy</span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"></span>\r | |
| 45 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
| 46 | </pre></div></div></div>\r | |
| 47 | <div class="paragraph"><p>This is easy to implement in SML.</p></div>\r | |
| 48 | <div class="listingblock">\r | |
| 49 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">Lazy</span><span class="p">:</span><span class="w"> </span><span class="n">LAZY</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
| 50 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
| 51 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">lazy</span><span class="w"> </span><span class="p">(</span><span class="n">th</span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="p">):</span><span class="w"> </span><span class="n">unit</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>\r | |
| 52 | <span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
| 53 | <span class="w"> </span><span class="k">datatype</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">lazy_result</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">Unevaluated</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">(</span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="p">)</span><span class="w"></span>\r | |
| 54 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">Evaluated</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">'a</span><span class="w"></span>\r | |
| 55 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">Failed</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">exn</span><span class="w"></span>\r | |
| 56 | \r | |
| 57 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">ref</span><span class="w"> </span><span class="p">(</span><span class="n">Unevaluated</span><span class="w"> </span><span class="n">th</span><span class="p">)</span><span class="w"></span>\r | |
| 58 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
| 59 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
| 60 | <span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="n">!r</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
| 61 | <span class="w"> </span><span class="n">Unevaluated</span><span class="w"> </span><span class="n">th</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
| 62 | <span class="w"> </span><span class="k">val</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">th</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
| 63 | <span class="w"> </span><span class="k">handle</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">(</span><span class="n">r</span><span class="w"> </span><span class="n">:=</span><span class="w"> </span><span class="n">Failed</span><span class="w"> </span><span class="n">x</span><span class="p">;</span><span class="w"> </span><span class="k">raise</span><span class="w"> </span><span class="n">x</span><span class="p">)</span><span class="w"></span>\r | |
| 64 | <span class="w"> </span><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="n">r</span><span class="w"> </span><span class="n">:=</span><span class="w"> </span><span class="n">Evaluated</span><span class="w"> </span><span class="n">a</span><span class="w"></span>\r | |
| 65 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
| 66 | <span class="w"> </span><span class="n">a</span><span class="w"></span>\r | |
| 67 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
| 68 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">Evaluated</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">a</span><span class="w"></span>\r | |
| 69 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">Failed</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">raise</span><span class="w"> </span><span class="n">x</span><span class="w"></span>\r | |
| 70 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
| 71 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
| 72 | </pre></div></div></div>\r | |
| 73 | </div>\r | |
| 74 | </div>\r | |
| 75 | </div>\r | |
| 76 | <div id="footnotes"><hr></div>\r | |
| 77 | <div id="footer">\r | |
| 78 | <div id="footer-text">\r | |
| 79 | </div>\r | |
| 80 | <div id="footer-badges">\r | |
| 81 | </div>\r | |
| 82 | </div>\r | |
| 83 | </body>\r | |
| 84 | </html>\r |