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<h3>
Richard Eisenberg pushed to branch wip/cfuneqcan-refactor
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
</h4>
<ul>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/f9ab3c4528939a179cf98c91212c8d03647be264">f9ab3c45</a></strong>
<div>
<span>by Richard Eisenberg</span>
<i>at 2020-11-25T23:53:55-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Remove flattening variables
This patch redesigns the flattener to simplify type family applications
directly instead of using flattening meta-variables and skolems. The key new
innovation is the CanEqLHS type and the new CEqCan constraint (Ct). A CanEqLHS
is either a type variable or exactly-saturated type family application; either
can now be rewritten using a CEqCan constraint in the inert set.
Because the flattener no longer reduces all type family applications to
variables, there was some performance degradation if a lengthy type family
application is now flattened over and over (not making progress). To
compensate, this patch contains some extra optimizations in the flattener,
leading to a number of performance improvements.
Close #18875.
Close #18910.
There are many extra parts of the compiler that had to be affected in writing
this patch:
* The family-application cache (formerly the flat-cache) sometimes stores
coercions built from Given inerts. When these inerts get kicked out, we must
kick out from the cache as well. (This was, I believe, true previously, but
somehow never caused trouble.) Kicking out from the cache requires adding a
filterTM function to TrieMap.
* This patch obviates the need to distinguish "blocking" coercion holes from
non-blocking ones (which, previously, arose from CFunEqCans). There is thus
some simplification around coercion holes.
* Extra commentary throughout parts of the code I read through, to preserve
the knowledge I gained while working.
* A change in the pure unifier around unifying skolems with other types.
Unifying a skolem now leads to SurelyApart, not MaybeApart, as documented
in Note [Binding when looking up instances] in GHC.Core.InstEnv.
* Some more use of MCoercion where appropriate.
* Previously, class-instance lookup automatically noticed that e.g. C Int was
a "unifier" to a target [W] C (F Bool), because the F Bool was flattened to
a variable. Now, a little more care must be taken around checking for
unifying instances.
* Previously, tcSplitTyConApp_maybe would split (Eq a => a). This is silly,
because (=>) is not a tycon in Haskell. Fixed now, but there are some
knock-on changes in e.g. TrieMap code and in the canonicaliser.
* New function anyFreeVarsOf{Type,Co} to check whether a free variable
satisfies a certain predicate.
* Type synonyms now remember whether or not they are "forgetful"; a forgetful
synonym drops at least one argument. This is useful when flattening; see
flattenView.
* The pattern-match completeness checker invokes the solver. This invocation
might need to look through newtypes when checking representational equality.
Thus, the desugarer needs to keep track of the in-scope variables to know
what newtype constructors are in scope. I bet this bug was around before but
never noticed.
* Extra-constraints wildcards are no longer simplified before printing.
See Note [Do not simplify ConstraintHoles] in GHC.Tc.Solver.
* Whether or not there are Given equalities has become slightly subtler.
See the new HasGivenEqs datatype.
* Note [Type variable cycles in Givens] in GHC.Tc.Solver.Canonical
explains a significant new wrinkle in the new approach.
* See Note [What might match later?] in GHC.Tc.Solver.Interact, which
explains the fix to #18910.
Though I (Richard) did the implementation, Simon PJ was very involved
in design and review.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/faad5f02b2728c022c69d930679cee16707956ad">faad5f02</a></strong>
<div>
<span>by Richard Eisenberg</span>
<i>at 2020-11-25T23:53:55-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Rename the flattener to become the rewriter.
Now that flattening doesn't produce flattening variables,
it's not really flattening anything: it's rewriting. This
change also means that the rewriter can no longer be confused
the core flattener (in GHC.Core.Unify), which is sometimes used
during type-checking.
</pre>
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<h4>30 changed files:</h4>
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<a href="#4fbb1c5f251c38fc01fcde693420ee6d02df7e45">
compiler/GHC/Cmm/Dataflow/Label.hs
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<a href="#783e5dae6e86931f06700fc088fb7d48c8a07386">
compiler/GHC/Core/Coercion.hs
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<a href="#b0ab2032bd0c006bb9cda1e5433173de35930f53">
compiler/GHC/Core/Coercion.hs-boot
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<a href="#f10ed7a2470454dfdd8691a08beba67d8b78ee70">
compiler/GHC/Core/Coercion/Axiom.hs
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<a href="#975dc08a8e7942b32d621f617d5a9c1b668601dd">
compiler/GHC/Core/Coercion/Opt.hs
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compiler/GHC/Core/FamInstEnv.hs
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compiler/GHC/Core/InstEnv.hs
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compiler/GHC/Core/Map/Expr.hs
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compiler/GHC/Core/Map/Type.hs
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compiler/GHC/Core/TyCo/FVs.hs
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<a href="#578c7c3857d66d963736ff6742f3433a0e8d01b7">
compiler/GHC/Core/TyCo/Rep.hs
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compiler/GHC/Core/TyCon.hs
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compiler/GHC/Core/TyCon/Env.hs
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compiler/GHC/Core/Type.hs
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compiler/GHC/Core/Unify.hs
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compiler/GHC/Data/Bag.hs
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compiler/GHC/Data/Maybe.hs
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compiler/GHC/Data/TrieMap.hs
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compiler/GHC/Driver/Flags.hs
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compiler/GHC/Driver/Session.hs
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compiler/GHC/HsToCore/Monad.hs
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compiler/GHC/HsToCore/Types.hs
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compiler/GHC/Stg/CSE.hs
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compiler/GHC/Tc/Errors.hs
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compiler/GHC/Tc/Gen/Bind.hs
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compiler/GHC/Tc/Gen/HsType.hs
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compiler/GHC/Tc/Gen/Pat.hs
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<a href="#07389a8f0516c26409d86f6bb651ed00759b5d4c">
compiler/GHC/Tc/Instance/Family.hs
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<a href="#f5efba4115c6981cb7f758f67b351e37479ae329">
compiler/GHC/Tc/Plugin.hs
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compiler/GHC/Tc/Solver.hs
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<h5>The diff was not included because it is too large.</h5>
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