<!DOCTYPE html> <html lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <meta name="generator" content="AsciiDoc 8.6.8"> <title>TypeChecking</title> <link rel="stylesheet" href="./asciidoc.css" type="text/css"> <link rel="stylesheet" href="./pygments.css" type="text/css"> <script type="text/javascript" src="./asciidoc.js"></script> <script type="text/javascript"> /*<![CDATA[*/ asciidoc.install(); /*]]>*/ </script> <link rel="stylesheet" href="./mlton.css" type="text/css"/> </head> <body class="article"> <div id="banner"> <div id="banner-home"> <a href="./Home">MLton 20130715</a> </div> </div> <div id="header"> <h1>TypeChecking</h1> </div> <div id="content"> <div id="preamble"> <div class="sectionbody"> <div class="paragraph"><p>MLton’s type checker follows the <a href="DefinitionOfStandardML">Definition</a> closely, so you may find differences between MLton and other SML compilers that do not follow the Definition so closely. In particular, SML/NJ has many deviations from the Definition — please see <a href="SMLNJDeviations">SMLNJDeviations</a> for those that we are aware of.</p></div> <div class="paragraph"><p>In some respects MLton’s type checker is more powerful than other SML compilers, so there are programs that MLton accepts that are rejected by some other SML compilers. These kinds of programs fall into a few simple categories.</p></div> <div class="ulist"><ul> <li> <p> MLton resolves flexible record patterns using a larger context than many other SML compilers. For example, MLton accepts the following. </p> <div class="listingblock"> <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> <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> </pre></div></div></div> </li> <li> <p> MLton uses as large a context as possible to resolve the type of variables constrained by the value restriction to be monotypes. For example, MLton accepts the following. </p> <div class="listingblock"> <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> <span class="w"> </span><span class="k">sig</span><span class="w"></span> <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> <span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span> <span class="w"> </span><span class="k">struct</span><span class="w"></span> <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> <span class="w"> </span><span class="k">end</span><span class="w"></span> </pre></div></div></div> </li> </ul></div> </div> </div> <div class="sect1"> <h2 id="_type_error_messages">Type error messages</h2> <div class="sectionbody"> <div class="paragraph"><p>To aid in the understanding of type errors, MLton’s type checker displays type errors differently than other SML compilers. In particular, when two types are different, it is important for the programmer to easily understand why they are different. So, MLton displays only the differences between two types that don’t match, using underscores for the parts that match. For example, if a function expects <span class="monospaced">real * int</span> but gets <span class="monospaced">real * real</span>, the type error message would look like</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>expects: _ * [int] but got: _ * [real]</pre> </div></div> <div class="paragraph"><p>As another aid to spotting differences, MLton places brackets <span class="monospaced">[]</span> around the parts of the types that don’t match. A common situation is when a function receives a different number of arguments than it expects, in which case you might see an error like</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>expects: [int * real] but got: [int * real * string]</pre> </div></div> <div class="paragraph"><p>The brackets make it easy to see that the problem is that the tuples have different numbers of components — not that the components don’t match. Contrast that with a case where a function receives the right number of arguments, but in the wrong order, in which case you might see an error like</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>expects: [int] * [real] but got: [real] * [int]</pre> </div></div> <div class="paragraph"><p>Here the brackets make it easy to see that the components do not match.</p></div> <div class="paragraph"><p>We appreciate feedback on any type error messages that you find confusing, or suggestions you may have for improvements to error messages.</p></div> </div> </div> <div class="sect1"> <h2 id="_the_shortest_most_recent_rule_for_type_names">The shortest/most-recent rule for type names</h2> <div class="sectionbody"> <div class="paragraph"><p>In a type error message, MLton often has a number of choices in deciding what name to use for a type. For example, in the following type-incorrect program</p></div> <div class="listingblock"> <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> <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> <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> </pre></div></div></div> <div class="paragraph"><p>MLton reports the error message</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>Error: z.sml 3.9. Function applied to incorrect argument. expects: [t] but got: [string] in: f "foo"</pre> </div></div> <div class="paragraph"><p>MLton could have reported <span class="monospaced">expects: [int]</span> instead of <span class="monospaced">expects: [t]</span>. However, MLton uses the shortest/most-recent rule in order to decide what type name to display. This rule means that, at the point of the error, MLton first looks for the shortest name for a type in terms of number of structure identifiers (e.g. <span class="monospaced">foobar</span> is shorter than <span class="monospaced">A.t</span>). Next, if there are multiple names of the same length, then MLton uses the most recently defined name. It is this tiebreaker that causes MLton to prefer <span class="monospaced">t</span> to <span class="monospaced">int</span> in the above example.</p></div> <div class="paragraph"><p>In signature matching, most recently defined is taken to include all of the definitions introduced by the structure. For example</p></div> <div class="listingblock"> <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> <span class="w"> </span><span class="k">sig</span><span class="w"></span> <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> <span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span> <span class="w"> </span><span class="k">struct</span><span class="w"></span> <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> <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> <span class="w"> </span><span class="k">end</span><span class="w"></span> </pre></div></div></div> <div class="paragraph"><p>MLton reports the error message</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>Error: z.sml 2.4. Variable type in structure disagrees with signature. variable: x structure: [string] signature: [t]</pre> </div></div> <div class="paragraph"><p>in which the <span class="monospaced">[t]</span> refers to the type defined in the structure, since that is more recent than the definition of <span class="monospaced">int</span>.</p></div> <div class="paragraph"><p>In signatures with type equations, this can be somewhat confusing. For example.</p></div> <div class="listingblock"> <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> <span class="w"> </span><span class="k">sig</span><span class="w"></span> <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">t</span><span class="w"></span> <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">t</span><span class="w"></span> <span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="p">=</span><span class="w"></span> <span class="w"> </span><span class="k">struct</span><span class="w"></span> <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> <span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">char</span><span class="w"></span> <span class="w"> </span><span class="k">end</span><span class="w"></span> </pre></div></div></div> <div class="paragraph"><p>MLton reports the error message</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>Error: z.sml 2.4. Type definition in structure disagrees with signature. type: u structure: [u] signature: [t]</pre> </div></div> <div class="paragraph"><p>This error reflects the fact that the signature requires type <span class="monospaced">u</span> to equal <span class="monospaced">t</span>, but that in the structure, <span class="monospaced">u</span> is defined to be <span class="monospaced">char</span>, whose most-recent name is <span class="monospaced">u</span>, while the signature requires <span class="monospaced">u</span> to be <span class="monospaced">int</span>, whose most-recent name is <span class="monospaced">t</span>.</p></div> </div> </div> </div> <div id="footnotes"><hr></div> <div id="footer"> <div id="footer-text"> </div> <div id="footer-badges"> </div> </div> </body> </html>