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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 |
| | 191 | <div id="footnotes"><hr></div>\r |
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HCoop / hcoop / debian / mlton.git / blame_incremental