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