HCoop / hcoop / debian / mlton.git / blame
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1 | <!DOCTYPE html>\r |
| 2 | <html lang="en">\r | |
| 3 | <head>\r | |
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| 5 | <meta name="generator" content="AsciiDoc 8.6.9">\r | |
| 6 | <title>DefineTypeBeforeUse</title>\r | |
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| 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>DefineTypeBeforeUse</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 href="StandardML">Standard ML</a> requires types to be defined before they are\r | |
| 32 | used. Because of type inference, the use of a type can be implicit;\r | |
| 33 | hence, this requirement is more subtle than it might appear. For\r | |
| 34 | example, the following program is not type correct, because the type\r | |
| 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>\r | |
| 36 | <div class="listingblock">\r | |
| 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>\r | |
| 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>\r | |
| 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>\r | |
| 40 | </pre></div></div></div>\r | |
| 41 | <div class="paragraph"><p>MLton reports the following error, indicating that the type defined on\r | |
| 42 | line 2 is used on line 1.</p></div>\r | |
| 43 | <div class="listingblock">\r | |
| 44 | <div class="content monospaced">\r | |
| 45 | <pre>Error: z.sml 3.10-3.20.\r | |
| 46 | Function applied to incorrect argument.\r | |
| 47 | expects: _ * [???] option\r | |
| 48 | but got: _ * [t] option\r | |
| 49 | in: := (r, SOME A)\r | |
| 50 | note: type would escape its scope: t\r | |
| 51 | escape from: z.sml 2.10-2.10\r | |
| 52 | escape to: z.sml 1.1-1.16\r | |
| 53 | Warning: z.sml 1.5-1.5.\r | |
| 54 | Type of variable was not inferred and could not be generalized: r.\r | |
| 55 | type: ??? option ref\r | |
| 56 | in: val r = ref NONE</pre>\r | |
| 57 | </div></div>\r | |
| 58 | <div class="paragraph"><p>While the above example is benign, the following example shows how to\r | |
| 59 | cast an integer to a function by (implicitly) using a type before it\r | |
| 60 | is defined. In the example, the ref cell <span class="monospaced">r</span> is of type\r | |
| 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\r | |
| 62 | functor <span class="monospaced">F</span>.</p></div>\r | |
| 63 | <div class="listingblock">\r | |
| 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>\r | |
| 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>\r | |
| 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>\r | |
| 67 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
| 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>\r | |
| 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>\r | |
| 70 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
| 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">-></span><span class="w"> </span><span class="n">unit</span><span class="w"></span>\r | |
| 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">=></span><span class="w"> </span><span class="p">())</span><span class="w"></span>\r | |
| 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>\r | |
| 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>\r | |
| 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>\r | |
| 76 | </pre></div></div></div>\r | |
| 77 | <div class="paragraph"><p>MLton reports the following error.</p></div>\r | |
| 78 | <div class="listingblock">\r | |
| 79 | <div class="content monospaced">\r | |
| 80 | <pre>Warning: z.sml 1.5-1.5.\r | |
| 81 | Type of variable was not inferred and could not be generalized: r.\r | |
| 82 | type: ??? option ref\r | |
| 83 | in: val r = ref NONE\r | |
| 84 | Error: z.sml 5.16-5.26.\r | |
| 85 | Function applied to incorrect argument.\r | |
| 86 | expects: _ * [???] option\r | |
| 87 | but got: _ * [t] option\r | |
| 88 | in: := (r, SOME x)\r | |
| 89 | note: type would escape its scope: t\r | |
| 90 | escape from: z.sml 2.17-2.17\r | |
| 91 | escape to: z.sml 1.1-1.16\r | |
| 92 | Warning: z.sml 6.11-6.13.\r | |
| 93 | Type of variable was not inferred and could not be generalized: get.\r | |
| 94 | type: unit -> ???\r | |
| 95 | in: fun get () = (valOf (! r))\r | |
| 96 | Error: z.sml 12.10-12.18.\r | |
| 97 | Function not of arrow type.\r | |
| 98 | function: [unit]\r | |
| 99 | in: (S1.get ()) ()</pre>\r | |
| 100 | </div></div>\r | |
| 101 | </div>\r | |
| 102 | </div>\r | |
| 103 | </div>\r | |
| 104 | <div id="footnotes"><hr></div>\r | |
| 105 | <div id="footer">\r | |
| 106 | <div id="footer-text">\r | |
| 107 | </div>\r | |
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| 109 | </div>\r | |
| 110 | </div>\r | |
| 111 | </body>\r | |
| 112 | </html>\r |