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<a href="./Home">MLton 20130715</a>
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<h1>UnresolvedBugs</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>Here are the places where MLton deviates from
<a href="DefinitionOfStandardML">The Definition of Standard ML (Revised)</a>. In
general, MLton complies with the <a href="DefinitionOfStandardML">Definition</a>
quite closely, typically much more closely than other SML compilers
(see, e.g., our list of <a href="SMLNJDeviations">SML/NJ’s deviations</a>). In
fact, the four deviations listed here are the only known deviations,
and we have no plans to fix them. If you find a deviation not listed
here, please report a <a href="Bug">Bug</a>.</p></div>
<div class="paragraph"><p>We don’t plan to fix these bugs because the first (parsing nested
cases) has historically never been accepted by any SML compiler, the
other three clearly indicate problems in the
<a href="DefinitionOfStandardML">Definition</a>.</p></div>
<div class="ulist"><ul>
<li>
<p>
MLton does not correctly parse case expressions nested within other
matches. For example, the following fails.
</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="mi">0</span><span class="w"> </span><span class="n">y</span><span class="w"> </span><span class="p">=</span><span class="w"></span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="k">of</span><span class="w"></span>
<span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="mi">2</span><span class="w"></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><span class="mi">3</span><span class="w"></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="w"> </span><span class="n">y</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">4</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><p>To do this in a program, simply parenthesize the case expression.</p></div>
<div class="paragraph"><p>Allowing such expressions, although compliant with the Definition,
would be a mistake, since using parentheses is clearer and no SML
compiler has ever allowed them. Furthermore, implementing this would
require serious yacc grammar rewriting followed by postprocessing.</p></div>
</li>
<li>
<p>
MLton rejects rebinding of constructors with <span class="monospaced">val rec</span>
declarations, as in
</p>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="k">rec</span><span class="w"> </span><span class="n">NONE</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>
</pre></div></div></div>
<div class="paragraph"><p>The Definition (bizarrely) requires this program to type check, but to
raise <span class="monospaced">Bind</span>.</p></div>
<div class="paragraph"><p>We have no plans to change this behavior, as the Definition’s behavior
is clearly an error, a mismatch between the static semantics and the
dynamic semantics.</p></div>
</li>
<li>
<p>
MLton does not hide the equality aspect of types declared in
<span class="monospaced">abstype</span> declarations. So, MLton accepts programs like the following,
while the Definition rejects them.
</p>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span class="k">abstype</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><span class="k">with</span><span class="w"> </span><span class="k">end</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="k">fn</span><span class="w"> </span><span class="p">(</span><span class="n">t1</span><span class="p">,</span><span class="w"> </span><span class="n">t2</span><span class="w"> </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">t1</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">t2</span><span class="w"></span>
<span class="k">abstype</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><span class="k">with</span><span class="w"> </span><span class="k">val</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">T</span><span class="w"> </span><span class="k">end</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">a</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">a</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><p>One consequence of this choice is that MLton accepts the following
program, in accordance with the Definition.</p></div>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span class="k">abstype</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><span class="k">with</span><span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">eq</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">op</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">end</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="k">fn</span><span class="w"> </span><span class="p">(</span><span class="n">t1</span><span class="p">,</span><span class="w"> </span><span class="n">t2</span><span class="w"> </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">eq</span><span class="w"> </span><span class="p">(</span><span class="n">t1</span><span class="p">,</span><span class="w"> </span><span class="n">t2</span><span class="p">)</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><p>Other implementations will typically reject this program, because they
make an early choice for the type of <span class="monospaced">eq</span> to be <span class="monospaced">''a * ''a -> bool</span>
instead of <span class="monospaced">t * t -> bool</span>. The choice is understandable, since the
Definition accepts the following program.</p></div>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span class="k">abstype</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><span class="k">with</span><span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">eq</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">op</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">end</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">eq</span><span class="w"> </span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">)</span><span class="w"></span>
</pre></div></div></div>
</li>
<li>
<p>
MLton (re-)type checks each functor definition at every
corresponding functor application (the compilation technique of
defunctorization). One consequence of this implementation is that
MLton accepts the following program, while the Definition rejects
it.
</p>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span class="k">functor</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="k">sig</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="k">end</span><span class="p">)</span><span class="w"> </span><span class="p">=</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="n">id</span><span class="w"> </span><span class="n">id</span><span class="w"></span>
<span class="k">end</span><span class="w"></span>
<span class="k">structure</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">F</span><span class="w"> </span><span class="p">(</span><span class="k">struct</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="k">end</span><span class="p">)</span><span class="w"></span>
<span class="k">structure</span><span class="w"> </span><span class="n">B</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">struct</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">bool</span><span class="w"> </span><span class="k">end</span><span class="p">)</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">A</span><span class="p">.</span><span class="n">f</span><span class="w"> </span><span class="mi">10</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">B</span><span class="p">.</span><span class="n">f</span><span class="w"> </span><span class="s">"dude"</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><p>On the other hand, other implementations will typically reject the
following program, while MLton and the Definition accept it.</p></div>
<div class="listingblock">
<div class="content"><div class="highlight"><pre><span class="k">functor</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="k">sig</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="k">end</span><span class="p">)</span><span class="w"> </span><span class="p">=</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="n">id</span><span class="w"> </span><span class="n">id</span><span class="w"></span>
<span class="k">end</span><span class="w"></span>
<span class="k">structure</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">F</span><span class="w"> </span><span class="p">(</span><span class="k">struct</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="k">end</span><span class="p">)</span><span class="w"></span>
<span class="k">structure</span><span class="w"> </span><span class="n">B</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">struct</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">bool</span><span class="w"> </span><span class="k">end</span><span class="p">)</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">A</span><span class="p">.</span><span class="n">f</span><span class="w"> </span><span class="mi">10</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">B</span><span class="p">.</span><span class="n">f</span><span class="w"> </span><span class="n">false</span><span class="w"></span>
</pre></div></div></div>
<div class="paragraph"><p>See <a href="References#DreyerBlume07">DreyerBlume07</a> for more details.</p></div>
</li>
</ul></div>
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