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<h3 style="margin-top: 20px; margin-bottom: 10px;">
Simon Peyton Jones pushed to branch wip/T22194-flags at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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Commits:
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<ul>
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<strong style="font-weight: bold;"><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/77b5eb804b66a28e53a50df43fdc61abed037376">77b5eb80</a></strong>
<div>
<span> by Simon Peyton Jones </span> <i> at 2023-03-28T23:52:43+01:00 </i>
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<pre class="commit-message" style='white-space: pre-wrap; display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; border-radius: 2px; margin: 0; padding: 8px 12px; border: 1px solid #dcdcde;'>Major refactor in the handling of equality constraints
This MR substantially refactors the way in which the constraint
solver deals with equality constraints. The big thing is:
* Intead of a pipeline in which we /first/ canonicalise and /then/
interact (the latter including performing unification) the two steps
are more closely integreated into one. That avoids the current
rather indirect communication between the two steps.
The proximate cause for this refactoring is fixing #22194, which involve
solving [W] alpha[2] ~ Maybe (F beta[4])
by doing this:
alpha[2] := Maybe delta[2]
[W] delta[2] ~ F beta[4]
That is, we don't promote beta[4]! This is very like introducing a cycle
breaker, and was very awkward to do before, but now it is all nice.
See GHC.Tc.Utils.Unify Note [Promotion and level-checking] and
Note [Family applications in canonical constraints].
The big change is this:
* Several canonicalisation checks (occurs-check, cycle-breaking,
checking for concreteness) are combined into one new function:
GHC.Tc.Utils.Unify.checkTyEqRhs
This function is controlled by `TyEqFlags`, which says what to do
for foralls, type families etc.
* `canEqCanLHSFinish` now sees if unification is possible, and if so,
actually does it: see `canEqCanLHSFinish_try_unification`.
There are loads of smaller changes:
* The on-the-fly unifier `GHC.Tc.Utils.Unify.unifyType` has a
cheap-and-cheerful version of `checkTyEqRhs`, called
`simpleUnifyCheck`. If `simpleUnifyCheck` succeeds, it can unify,
otherwise it defers by emitting a constraint. This is simpler than
before.
* I simplified the swapping code in `GHC.Tc.Solver.Equality.canEqCanLHS`.
Especially the nasty stuff involving `swap_for_occurs` and
`canEqTyVarFunEq`. Much nicer now. See
Note [Orienting TyVarLHS/TyFamLHS]
Note [Orienting TyFamLHS/TyFamLHS]
* Added `cteSkolemOccurs`, `cteConcrete`, and `cteCoercionHole` to the
problems that can be discovered by `checkTyEqRhs`.
Yet smaller:
* Added a `synIsConcrete` flag to `SynonymTyCon` (alongside `synIsFamFree`)
to reduce the need for synonym expansion when checking concreteness.
Use it in `isConcreteType`.
* Renamed `isConcrete` to `isConcreteType`
* Defined `GHC.Core.TyCo.FVs.isInjectiveInType` as a more efficient
way to find if a particular type variable is used injectively than
finding all the injective variables. It is called in
`GHC.Tc.Utils.Unify.definitely_poly`, which in turn is used quite a
lot.
* Moved `rewriterView` to `GHC.Core.Type`, so we can use it from the
constraint solver.
Fixes #22194
Compile times decrease by an average of 0.1%; but there is a 7.4%
drop in compiler allocation on T15703.
Metric Decrease:
T15703
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18 changed files:
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<a href="#ae6d91a5d028418bbf1431347d659e744e0a3128">
compiler/GHC/Core/Opt/Simplify/Iteration.hs
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<a href="#039e8f5676356ef05c90af828aae48aac7296e47">
compiler/GHC/Core/TyCo/FVs.hs
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<a href="#578c7c3857d66d963736ff6742f3433a0e8d01b7">
compiler/GHC/Core/TyCo/Rep.hs
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<a href="#4aad0050db1a8a20db8bbca149111de99cb299c9">
compiler/GHC/Core/TyCon.hs
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<a href="#84c9d3ba34173297e7d7747e02caa2e004b164fc">
compiler/GHC/Core/Type.hs
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<a href="#a11863ba405cb13e5c25518fb7c6048b20e277f7">
compiler/GHC/Core/Type.hs-boot
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<a href="#2fefe3125c649bc5377b06a7cca18e785855b79c">
compiler/GHC/Data/Bag.hs
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<a href="#f4ae6618b39d5ee4af8302b821aa7e38097a84b7">
compiler/GHC/Data/Maybe.hs
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<a href="#e4b9d5e9d2de37561bfd06a2c97a5c924a1bab76">
compiler/GHC/Tc/Errors.hs
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<a href="#1dd9c7a6bbc222c976a6ec1c4b772232ae60f7e5">
compiler/GHC/Tc/Errors/Ppr.hs
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<a href="#54436e21ea530a66a8a71bcc01c836f4740b8f02">
compiler/GHC/Tc/Solver.hs
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<a href="#03ec4f8741105271af29a61fdb47c8feba9bbcfa">
compiler/GHC/Tc/Solver/Equality.hs
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<a href="#b97557795e2f14fc492ff8c685b149c247b5e480">
compiler/GHC/Tc/Solver/InertSet.hs
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<a href="#0c6b585a8a21f8340a85ca2cef89c4b1bf5a8953">
compiler/GHC/Tc/Solver/Monad.hs
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<a href="#3b6d78a50c8462304f843b952a88e531718dbbd2">
compiler/GHC/Tc/Types/Constraint.hs
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<a href="#286002f016636e4fbd86ac6bafaacea29ac4633d">
compiler/GHC/Tc/Utils/Concrete.hs
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<a href="#94c96afe1bb3b771ffbcfeeca34ce43ebbe76ccb">
compiler/GHC/Tc/Utils/TcMType.hs
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<li class="file-stats">
<a href="#5461e7beeb7ae73d7c01f720cbaa3fa2c5333590">
compiler/GHC/Tc/Utils/TcType.hs
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The diff was not included because it is too large.
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