doc/guide/localhost/Lazy

   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>Lazy</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>Lazy</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p>In a lazy (or non-strict) language, the arguments to a function are
  32 not evaluated before calling the function.  Instead, the arguments are
  33 suspended and only evaluated by the function if needed.</p></div>
  34 <div class="paragraph"><p><a href="StandardML">Standard ML</a> is an eager (or strict) language, not a lazy
  35 language.  However, it is easy to delay evaluation of an expression in
  36 SML by creating a <em>thunk</em>, which is a nullary function.  In SML, a
  37 thunk is written <span class="monospaced">fn () =&gt; e</span>.  Another essential feature of laziness
  38 is <em>memoization</em>, meaning that once a suspended argument is evaluated,
  39 subsequent references look up the value.  We can express this in SML
  40 with a function that maps a thunk to a memoized thunk.</p></div>
  41 <div class="listingblock">
  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>
  43 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
  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">-&gt;</span><span class="w"> </span><span class="n">&#39;a</span><span class="p">)</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">&#39;a</span><span class="w"></span>
  45 <span class="w">   </span><span class="k">end</span><span class="w"></span>
  46 </pre></div></div></div>
  47 <div class="paragraph"><p>This is easy to implement in SML.</p></div>
  48 <div class="listingblock">
  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>
  50 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
  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">-&gt;</span><span class="w"> </span><span class="n">&#39;a</span><span class="p">):</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">&#39;a</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
  52 <span class="w">         </span><span class="k">let</span><span class="w"></span>
  53 <span class="w">            </span><span class="k">datatype</span><span class="w"> </span><span class="n">&#39;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">-&gt;</span><span class="w"> </span><span class="n">&#39;a</span><span class="p">)</span><span class="w"></span>
  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">&#39;a</span><span class="w"></span>
  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>
  56
  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>
  58 <span class="w">         </span><span class="k">in</span><span class="w"></span>
  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">=&gt;</span><span class="w"></span>
  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>
  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">=&gt;</span><span class="w"> </span><span class="k">let</span><span class="w"></span>
  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>
  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">=&gt;</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>
  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>
  65 <span class="w">                                     </span><span class="k">in</span><span class="w"></span>
  66 <span class="w">                                       </span><span class="n">a</span><span class="w"></span>
  67 <span class="w">                                     </span><span class="k">end</span><span class="w"></span>
  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">=&gt;</span><span class="w"> </span><span class="n">a</span><span class="w"></span>
  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">=&gt;</span><span class="w"> </span><span class="k">raise</span><span class="w"> </span><span class="n">x</span><span class="w"></span>
  70 <span class="w">         </span><span class="k">end</span><span class="w"></span>
  71 <span class="w">   </span><span class="k">end</span><span class="w"></span>
  72 </pre></div></div></div>
  73 </div>
  74 </div>
  75 </div>
  76 <div id="footnotes"><hr></div>
  77 <div id="footer">
  78 <div id="footer-text">
  79 </div>
  80 <div id="footer-badges">
  81 </div>
  82 </div>
  83 </body>
  84 </html>