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22 | <a href="./Home">MLton 20180207</a>\r | |
23 | </div>\r | |
24 | </div>\r | |
25 | <div id="header">\r | |
26 | <h1>TypeChecking</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>MLton’s type checker follows the <a href="DefinitionOfStandardML">Definition</a>\r | |
32 | closely, so you may find differences between MLton and other SML\r | |
33 | compilers that do not follow the Definition so closely. In\r | |
34 | particular, SML/NJ has many deviations from the Definition — please\r | |
35 | see <a href="SMLNJDeviations">SMLNJDeviations</a> for those that we are aware of.</p></div>\r | |
36 | <div class="paragraph"><p>In some respects MLton’s type checker is more powerful than other SML\r | |
37 | compilers, so there are programs that MLton accepts that are rejected\r | |
38 | by some other SML compilers. These kinds of programs fall into a few\r | |
39 | simple categories.</p></div>\r | |
40 | <div class="ulist"><ul>\r | |
41 | <li>\r | |
42 | <p>\r | |
43 | MLton resolves flexible record patterns using a larger context than\r | |
44 | many other SML compilers. For example, MLton accepts the\r | |
45 | following.\r | |
46 | </p>\r | |
47 | <div class="listingblock">\r | |
48 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">{</span><span class="n">x</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><span class="n">x</span><span class="w"></span>\r | |
49 | <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">f</span><span class="w"> </span><span class="p">{</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><span class="n">y</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="s">"foo"</span><span class="p">}</span><span class="w"></span>\r | |
50 | </pre></div></div></div>\r | |
51 | </li>\r | |
52 | <li>\r | |
53 | <p>\r | |
54 | MLton uses as large a context as possible to resolve the type of\r | |
55 | variables constrained by the value restriction to be monotypes. For\r | |
56 | example, MLton accepts the following.\r | |
57 | </p>\r | |
58 | <div class="listingblock">\r | |
59 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">S</span><span class="p">:</span><span class="w"></span>\r | |
60 | <span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
61 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">f</span><span class="p">:</span><span class="w"> </span><span class="n">int</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">int</span><span class="w"></span>\r | |
62 | <span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
63 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
64 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">x</span><span class="p">)</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">y</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">y</span><span class="p">)</span><span class="w"></span>\r | |
65 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
66 | </pre></div></div></div>\r | |
67 | </li>\r | |
68 | </ul></div>\r | |
69 | </div>\r | |
70 | </div>\r | |
71 | <div class="sect1">\r | |
72 | <h2 id="_type_error_messages">Type error messages</h2>\r | |
73 | <div class="sectionbody">\r | |
74 | <div class="paragraph"><p>To aid in the understanding of type errors, MLton’s type checker\r | |
75 | displays type errors differently than other SML compilers. In\r | |
76 | particular, when two types are different, it is important for the\r | |
77 | programmer to easily understand why they are different. So, MLton\r | |
78 | displays only the differences between two types that don’t match,\r | |
79 | using underscores for the parts that match. For example, if a\r | |
80 | function expects <span class="monospaced">real * int</span> but gets <span class="monospaced">real * real</span>, the type error\r | |
81 | message would look like</p></div>\r | |
82 | <div class="listingblock">\r | |
83 | <div class="content monospaced">\r | |
84 | <pre>expects: _ * [int]\r | |
85 | but got: _ * [real]</pre>\r | |
86 | </div></div>\r | |
87 | <div class="paragraph"><p>As another aid to spotting differences, MLton places brackets <span class="monospaced">[]</span>\r | |
88 | around the parts of the types that don’t match. A common situation is\r | |
89 | when a function receives a different number of arguments than it\r | |
90 | expects, in which case you might see an error like</p></div>\r | |
91 | <div class="listingblock">\r | |
92 | <div class="content monospaced">\r | |
93 | <pre>expects: [int * real]\r | |
94 | but got: [int * real * string]</pre>\r | |
95 | </div></div>\r | |
96 | <div class="paragraph"><p>The brackets make it easy to see that the problem is that the tuples\r | |
97 | have different numbers of components — not that the components don’t\r | |
98 | match. Contrast that with a case where a function receives the right\r | |
99 | number of arguments, but in the wrong order, in which case you might\r | |
100 | see an error like</p></div>\r | |
101 | <div class="listingblock">\r | |
102 | <div class="content monospaced">\r | |
103 | <pre>expects: [int] * [real]\r | |
104 | but got: [real] * [int]</pre>\r | |
105 | </div></div>\r | |
106 | <div class="paragraph"><p>Here the brackets make it easy to see that the components do not match.</p></div>\r | |
107 | <div class="paragraph"><p>We appreciate feedback on any type error messages that you find\r | |
108 | confusing, or suggestions you may have for improvements to error\r | |
109 | messages.</p></div>\r | |
110 | </div>\r | |
111 | </div>\r | |
112 | <div class="sect1">\r | |
113 | <h2 id="_the_shortest_most_recent_rule_for_type_names">The shortest/most-recent rule for type names</h2>\r | |
114 | <div class="sectionbody">\r | |
115 | <div class="paragraph"><p>In a type error message, MLton often has a number of choices in\r | |
116 | deciding what name to use for a type. For example, in the following\r | |
117 | type-incorrect program</p></div>\r | |
118 | <div class="listingblock">\r | |
119 | <div class="content"><div class="highlight"><pre><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 | |
120 | <span class="k">fun</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">(</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><span class="n">x</span><span class="w"></span>\r | |
121 | <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">f</span><span class="w"> </span><span class="s">"foo"</span><span class="w"></span>\r | |
122 | </pre></div></div></div>\r | |
123 | <div class="paragraph"><p>MLton reports the error message</p></div>\r | |
124 | <div class="listingblock">\r | |
125 | <div class="content monospaced">\r | |
126 | <pre>Error: z.sml 3.9-3.15.\r | |
127 | Function applied to incorrect argument.\r | |
128 | expects: [t]\r | |
129 | but got: [string]\r | |
130 | in: f "foo"</pre>\r | |
131 | </div></div>\r | |
132 | <div class="paragraph"><p>MLton could have reported <span class="monospaced">expects: [int]</span> instead of <span class="monospaced">expects: [t]</span>.\r | |
133 | However, MLton uses the shortest/most-recent rule in order to decide\r | |
134 | what type name to display. This rule means that, at the point of the\r | |
135 | error, MLton first looks for the shortest name for a type in terms of\r | |
136 | number of structure identifiers (e.g. <span class="monospaced">foobar</span> is shorter than <span class="monospaced">A.t</span>).\r | |
137 | Next, if there are multiple names of the same length, then MLton uses\r | |
138 | the most recently defined name. It is this tiebreaker that causes\r | |
139 | MLton to prefer <span class="monospaced">t</span> to <span class="monospaced">int</span> in the above example.</p></div>\r | |
140 | <div class="paragraph"><p>In signature matching, most recently defined is not taken to include\r | |
141 | all of the definitions introduced by the structure (since the matching\r | |
142 | takes place outside the structure and before it is defined). For\r | |
143 | example, in the following type-incorrect program</p></div>\r | |
144 | <div class="listingblock">\r | |
145 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">S</span><span class="p">:</span><span class="w"></span>\r | |
146 | <span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
147 | <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">int</span><span class="w"></span>\r | |
148 | <span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
149 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
150 | <span class="w"> </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 | |
151 | <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="s">"foo"</span><span class="w"></span>\r | |
152 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
153 | </pre></div></div></div>\r | |
154 | <div class="paragraph"><p>MLton reports the error message</p></div>\r | |
155 | <div class="listingblock">\r | |
156 | <div class="content monospaced">\r | |
157 | <pre>Error: z.sml 2.4-4.6.\r | |
158 | Variable in structure disagrees with signature (type): x.\r | |
159 | structure: val x: [string]\r | |
160 | defn at: z.sml 7.11-7.11\r | |
161 | signature: val x: [int]\r | |
162 | spec at: z.sml 3.11-3.11</pre>\r | |
163 | </div></div>\r | |
164 | <div class="paragraph"><p>If there is a type that only exists inside the structure being\r | |
165 | matched, then the prefix <span class="monospaced">_str.</span> is used. For example, in the\r | |
166 | following type-incorrect program</p></div>\r | |
167 | <div class="listingblock">\r | |
168 | <div class="content"><div class="highlight"><pre><span class="k">structure</span><span class="w"> </span><span class="n">S</span><span class="p">:</span><span class="w"></span>\r | |
169 | <span class="w"> </span><span class="k">sig</span><span class="w"></span>\r | |
170 | <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">int</span><span class="w"></span>\r | |
171 | <span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
172 | <span class="w"> </span><span class="k">struct</span><span class="w"></span>\r | |
173 | <span class="w"> </span><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">T</span><span class="w"></span>\r | |
174 | <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="n">T</span><span class="w"></span>\r | |
175 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
176 | </pre></div></div></div>\r | |
177 | <div class="paragraph"><p>MLton reports the error message</p></div>\r | |
178 | <div class="listingblock">\r | |
179 | <div class="content monospaced">\r | |
180 | <pre>Error: z.sml 2.4-4.6.\r | |
181 | Variable in structure disagrees with signature (type): x.\r | |
182 | structure: val x: [_str.t]\r | |
183 | defn at: z.sml 7.11-7.11\r | |
184 | signature: val x: [int]\r | |
185 | spec at: z.sml 3.11-3.11</pre>\r | |
186 | </div></div>\r | |
187 | <div class="paragraph"><p>in which the <span class="monospaced">[_str.t]</span> refers to the type defined in the structure.</p></div>\r | |
188 | </div>\r | |
189 | </div>\r | |
190 | </div>\r | |
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