doc/guide/localhost/DefineTypeBeforeUse

   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>DefineTypeBeforeUse</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>DefineTypeBeforeUse</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p><a href="StandardML">Standard ML</a> requires types to be defined before they are
  32 used.  Because of type inference, the use of a type can be implicit;
  33 hence, this requirement is more subtle than it might appear.  For
  34 example, the following program is not type correct, because the type
  35 of <span class="monospaced">r</span> is <span class="monospaced">t option ref</span>, but <span class="monospaced">t</span> is defined after <span class="monospaced">r</span>.</p></div>
  36 <div class="listingblock">
  37 <div class="content"><div class="highlight"><pre><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="n">NONE</span><span class="w"></span>
  38 <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>
  39 <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">SOME</span><span class="w"> </span><span class="n">A</span><span class="w"></span>
  40 </pre></div></div></div>
  41 <div class="paragraph"><p>MLton reports the following error, indicating that the type defined on
  42 line 2 is used on line 1.</p></div>
  43 <div class="listingblock">
  44 <div class="content monospaced">
  45 <pre>Error: z.sml 3.10-3.20.
  46   Function applied to incorrect argument.
  47     expects: _ * [???] option
  48     but got: _ * [t] option
  49     in: := (r, SOME A)
  50     note: type would escape its scope: t
  51     escape from: z.sml 2.10-2.10
  52     escape to: z.sml 1.1-1.16
  53 Warning: z.sml 1.5-1.5.
  54   Type of variable was not inferred and could not be generalized: r.
  55     type: ??? option ref
  56     in: val r = ref NONE</pre>
  57 </div></div>
  58 <div class="paragraph"><p>While the above example is benign, the following example shows how to
  59 cast an integer to a function by (implicitly) using a type before it
  60 is defined.  In the example, the ref cell <span class="monospaced">r</span> is of type
  61 <span class="monospaced">t option ref</span>, where <span class="monospaced">t</span> is defined <em>after</em> <span class="monospaced">r</span>, as a parameter to
  62 functor <span class="monospaced">F</span>.</p></div>
  63 <div class="listingblock">
  64 <div class="content"><div class="highlight"><pre><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="n">NONE</span><span class="w"></span>
  65 <span class="k">functor</span><span class="w"> </span><span class="n">F</span><span class="w"> </span><span class="p">(</span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span>
  66 <span class="w">           </span><span class="k">val</span><span class="w"> </span><span class="n">x</span><span class="p">:</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
  67 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
  68 <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">SOME</span><span class="w"> </span><span class="n">x</span><span class="w"></span>
  69 <span class="w">      </span><span class="k">fun</span><span class="w"> </span><span class="n">get</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">valOf</span><span class="w"> </span><span class="p">(</span><span class="n">!r</span><span class="p">)</span><span class="w"></span>
  70 <span class="w">   </span><span class="k">end</span><span class="w"></span>
  71 <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="k">type</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">unit</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">unit</span><span class="w"></span>
  72 <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="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="p">())</span><span class="w"></span>
  73 <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="k">type</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">int</span><span class="w"></span>
  74 <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">13</span><span class="p">)</span><span class="w"></span>
  75 <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">S1</span><span class="p">.</span><span class="n">get</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">()</span><span class="w"></span>
  76 </pre></div></div></div>
  77 <div class="paragraph"><p>MLton reports the following error.</p></div>
  78 <div class="listingblock">
  79 <div class="content monospaced">
  80 <pre>Warning: z.sml 1.5-1.5.
  81   Type of variable was not inferred and could not be generalized: r.
  82     type: ??? option ref
  83     in: val r = ref NONE
  84 Error: z.sml 5.16-5.26.
  85   Function applied to incorrect argument.
  86     expects: _ * [???] option
  87     but got: _ * [t] option
  88     in: := (r, SOME x)
  89     note: type would escape its scope: t
  90     escape from: z.sml 2.17-2.17
  91     escape to: z.sml 1.1-1.16
  92 Warning: z.sml 6.11-6.13.
  93   Type of variable was not inferred and could not be generalized: get.
  94     type: unit -&gt; ???
  95     in: fun get () = (valOf (! r))
  96 Error: z.sml 12.10-12.18.
  97   Function not of arrow type.
  98     function: [unit]
  99     in: (S1.get ()) ()</pre>
 100 </div></div>
 101 </div>
 102 </div>
 103 </div>
 104 <div id="footnotes"><hr></div>
 105 <div id="footer">
 106 <div id="footer-text">
 107 </div>
 108 <div id="footer-badges">
 109 </div>
 110 </div>
 111 </body>
 112 </html>