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<main>
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<h1 id="specialization"><a class="header" href="#specialization">Specialization</a></h1>
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<p><strong>TODO</strong>: where does Chalk fit in? Should we mention/discuss it here?</p>
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<p>Defined in the <code>specialize</code> module.</p>
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<p>The basic strategy is to build up a <em>specialization graph</em> during
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coherence checking (coherence checking looks for <a href="../coherence.html">overlapping impls</a>).
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Insertion into the graph locates the right place
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to put an impl in the specialization hierarchy; if there is no right
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place (due to partial overlap but no containment), you get an overlap
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error. Specialization is consulted when selecting an impl (of course),
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and the graph is consulted when propagating defaults down the
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specialization hierarchy.</p>
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<p>You might expect that the specialization graph would be used during
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selection – i.e. when actually performing specialization. This is
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not done for two reasons:</p>
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<ul>
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<li>
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<p>It's merely an optimization: given a set of candidates that apply,
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we can determine the most specialized one by comparing them directly
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for specialization, rather than consulting the graph. Given that we
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also cache the results of selection, the benefit of this
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optimization is questionable.</p>
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</li>
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<li>
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<p>To build the specialization graph in the first place, we need to use
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selection (because we need to determine whether one impl specializes
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another). Dealing with this reentrancy would require some additional
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mode switch for selection. Given that there seems to be no strong
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reason to use the graph anyway, we stick with a simpler approach in
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selection, and use the graph only for propagating default
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implementations.</p>
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</li>
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</ul>
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<p>Trait impl selection can succeed even when multiple impls can apply,
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as long as they are part of the same specialization family. In that
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case, it returns a <em>single</em> impl on success – this is the most
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specialized impl <em>known</em> to apply. However, if there are any inference
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variables in play, the returned impl may not be the actual impl we
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will use at codegen time. Thus, we take special care to avoid projecting
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associated types unless either (1) the associated type does not use
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<code>default</code> and thus cannot be overridden or (2) all input types are
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known concretely.</p>
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<h2 id="additional-resources"><a class="header" href="#additional-resources">Additional Resources</a></h2>
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<p><a href="https://www.youtube.com/watch?v=rZqS4bLPL24">This talk</a> by @sunjay may be useful. Keep in mind that the talk only
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gives a broad overview of the problem and the solution (it was presented about
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halfway through @sunjay's work). Also, it was given in June 2018, and some
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things may have changed by the time you watch it.</p>
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