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22 | <a href="./Home">MLton 20180207</a>\r | |
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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 | |
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