doc/guide/localhost/TypeChecking

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  22 <a href="./Home">MLton 20180207</a>
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  26 <h1>TypeChecking</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p>MLton&#8217;s type checker follows the <a href="DefinitionOfStandardML">Definition</a>
  32 closely, so you may find differences between MLton and other SML
  33 compilers that do not follow the Definition so closely.  In
  34 particular, SML/NJ has many deviations from the Definition&#8201;&#8212;&#8201;please
  35 see <a href="SMLNJDeviations">SMLNJDeviations</a> for those that we are aware of.</p></div>
  36 <div class="paragraph"><p>In some respects MLton&#8217;s type checker is more powerful than other SML
  37 compilers, so there are programs that MLton accepts that are rejected
  38 by some other SML compilers.  These kinds of programs fall into a few
  39 simple categories.</p></div>
  40 <div class="ulist"><ul>
  41 <li>
  42 <p>
  43 MLton resolves flexible record patterns using a larger context than
  44 many other SML compilers.  For example, MLton accepts the
  45 following.
  46 </p>
  47 <div class="listingblock">
  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>
  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">&quot;foo&quot;</span><span class="p">}</span><span class="w"></span>
  50 </pre></div></div></div>
  51 </li>
  52 <li>
  53 <p>
  54 MLton uses as large a context as possible to resolve the type of
  55 variables constrained by the value restriction to be monotypes.  For
  56 example, MLton accepts the following.
  57 </p>
  58 <div class="listingblock">
  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>
  60 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
  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">-&gt;</span><span class="w"> </span><span class="n">int</span><span class="w"></span>
  62 <span class="w">   </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
  63 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
  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">=&gt;</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">=&gt;</span><span class="w"> </span><span class="n">y</span><span class="p">)</span><span class="w"></span>
  65 <span class="w">   </span><span class="k">end</span><span class="w"></span>
  66 </pre></div></div></div>
  67 </li>
  68 </ul></div>
  69 </div>
  70 </div>
  71 <div class="sect1">
  72 <h2 id="_type_error_messages">Type error messages</h2>
  73 <div class="sectionbody">
  74 <div class="paragraph"><p>To aid in the understanding of type errors, MLton&#8217;s type checker
  75 displays type errors differently than other SML compilers.  In
  76 particular, when two types are different, it is important for the
  77 programmer to easily understand why they are different.  So, MLton
  78 displays only the differences between two types that don&#8217;t match,
  79 using underscores for the parts that match.  For example, if a
  80 function expects <span class="monospaced">real * int</span> but gets <span class="monospaced">real * real</span>, the type error
  81 message would look like</p></div>
  82 <div class="listingblock">
  83 <div class="content monospaced">
  84 <pre>expects: _ * [int]
  85 but got: _ * [real]</pre>
  86 </div></div>
  87 <div class="paragraph"><p>As another aid to spotting differences, MLton places brackets <span class="monospaced">[]</span>
  88 around the parts of the types that don&#8217;t match.  A common situation is
  89 when a function receives a different number of arguments than it
  90 expects, in which case you might see an error like</p></div>
  91 <div class="listingblock">
  92 <div class="content monospaced">
  93 <pre>expects: [int * real]
  94 but got: [int * real * string]</pre>
  95 </div></div>
  96 <div class="paragraph"><p>The brackets make it easy to see that the problem is that the tuples
  97 have different numbers of components&#8201;&#8212;&#8201;not that the components don&#8217;t
  98 match.  Contrast that with a case where a function receives the right
  99 number of arguments, but in the wrong order, in which case you might
 100 see an error like</p></div>
 101 <div class="listingblock">
 102 <div class="content monospaced">
 103 <pre>expects: [int] * [real]
 104 but got: [real] * [int]</pre>
 105 </div></div>
 106 <div class="paragraph"><p>Here the brackets make it easy to see that the components do not match.</p></div>
 107 <div class="paragraph"><p>We appreciate feedback on any type error messages that you find
 108 confusing, or suggestions you may have for improvements to error
 109 messages.</p></div>
 110 </div>
 111 </div>
 112 <div class="sect1">
 113 <h2 id="_the_shortest_most_recent_rule_for_type_names">The shortest/most-recent rule for type names</h2>
 114 <div class="sectionbody">
 115 <div class="paragraph"><p>In a type error message, MLton often has a number of choices in
 116 deciding what name to use for a type.  For example, in the following
 117 type-incorrect program</p></div>
 118 <div class="listingblock">
 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>
 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>
 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">&quot;foo&quot;</span><span class="w"></span>
 122 </pre></div></div></div>
 123 <div class="paragraph"><p>MLton reports the error message</p></div>
 124 <div class="listingblock">
 125 <div class="content monospaced">
 126 <pre>Error: z.sml 3.9-3.15.
 127   Function applied to incorrect argument.
 128     expects: [t]
 129     but got: [string]
 130     in: f "foo"</pre>
 131 </div></div>
 132 <div class="paragraph"><p>MLton could have reported <span class="monospaced">expects: [int]</span> instead of <span class="monospaced">expects: [t]</span>.
 133 However, MLton uses the shortest/most-recent rule in order to decide
 134 what type name to display.  This rule means that, at the point of the
 135 error, MLton first looks for the shortest name for a type in terms of
 136 number of structure identifiers (e.g. <span class="monospaced">foobar</span> is shorter than <span class="monospaced">A.t</span>).
 137 Next, if there are multiple names of the same length, then MLton uses
 138 the most recently defined name.  It is this tiebreaker that causes
 139 MLton to prefer <span class="monospaced">t</span> to <span class="monospaced">int</span> in the above example.</p></div>
 140 <div class="paragraph"><p>In signature matching, most recently defined is not taken to include
 141 all of the definitions introduced by the structure (since the matching
 142 takes place outside the structure and before it is defined).  For
 143 example, in the following type-incorrect program</p></div>
 144 <div class="listingblock">
 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>
 146 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
 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>
 148 <span class="w">   </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 149 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
 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>
 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">&quot;foo&quot;</span><span class="w"></span>
 152 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 153 </pre></div></div></div>
 154 <div class="paragraph"><p>MLton reports the error message</p></div>
 155 <div class="listingblock">
 156 <div class="content monospaced">
 157 <pre>Error: z.sml 2.4-4.6.
 158   Variable in structure disagrees with signature (type): x.
 159     structure: val x: [string]
 160     defn at: z.sml 7.11-7.11
 161     signature: val x: [int]
 162     spec at: z.sml 3.11-3.11</pre>
 163 </div></div>
 164 <div class="paragraph"><p>If there is a type that only exists inside the structure being
 165 matched, then the prefix <span class="monospaced">_str.</span> is used.  For example, in the
 166 following type-incorrect program</p></div>
 167 <div class="listingblock">
 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>
 169 <span class="w">   </span><span class="k">sig</span><span class="w"></span>
 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>
 171 <span class="w">   </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
 172 <span class="w">   </span><span class="k">struct</span><span class="w"></span>
 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>
 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>
 175 <span class="w">   </span><span class="k">end</span><span class="w"></span>
 176 </pre></div></div></div>
 177 <div class="paragraph"><p>MLton reports the error message</p></div>
 178 <div class="listingblock">
 179 <div class="content monospaced">
 180 <pre>Error: z.sml 2.4-4.6.
 181   Variable in structure disagrees with signature (type): x.
 182     structure: val x: [_str.t]
 183     defn at: z.sml 7.11-7.11
 184     signature: val x: [int]
 185     spec at: z.sml 3.11-3.11</pre>
 186 </div></div>
 187 <div class="paragraph"><p>in which the <span class="monospaced">[_str.t]</span> refers to the type defined in the structure.</p></div>
 188 </div>
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