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97adaf89
by Simon Peyton Jones at 2026-10-14T00:07:27+01:00
Line up simpleUnifyCheck and check_ty_eq_rhs
Even after #26543 was allegedly fixed, the original repo case in the
Description continued to fail. The reason was that the QuickLook
unifier, `qlUnify`, used `simpleUnifyCheck` for checking unification
invariants; and `simpleUnifyCheck` conservatively rejected any RHS
with a coercion hole in it. The upshot was that QuickLook was not
as clever as it should be, wrongly failing to do an impredicative
instantiation.
Interestingly `check_ty_eq_rhs`, which does the same job during
constraint solving, was more liberal: it just looked at the free vars
of the coercion, and allowed coercion holes.
This MR lines them up, adding some careful notes. See
Note [simpleUnifyCheck]
Note [Occurs check and coercions]
Note [Unification preconditions] esp (COERCIONS)
Things are better than before, but I am still uncomfortable about the
possibilty that a hole might be filled with a coercion that mentions
the LHS type variable; for now I have left this discomfort documented
in Note [Occurs check and coercions].
Main changes
* `qlUnify` now only unifies /instantiation variables/ not regular unification
variables. This makes QuickLook more predictable. See (UQL6) in
Note [QuickLook unification].
* `qlUnify` no longer needs to call `simpleUnifyCheck`, so we can get rid of
`UC_QuickLook` and its code in `simpleUnifyCheck`.
* I found I could get rid of `mkOccFolders` in favour of `anyFreeVarsOfType`.
Hooray.
* Improve documentation of CycleBreakerTv, and rename it to GivenCycleBreakerTv
because they are only uesd for Givens!
* I accepted a 9% worsening in compile time allocation on T5030
because the newly-consistent story triggered #27934.
Other changes
* I refactored `atvFolder` so that it pairs the "interesting-var" function
with the in-scope set, to avoid building a closure with a free variable.
Now we get nice tight loops for `anyFreeVarsOfType` and friends.
See Note [Any-free-var folder].
Metric Increase:
T5030
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5f7b3047
by Simon Peyton Jones at 2026-10-14T00:07:27+01:00
Fix order-dependence in QuickLook
Ticket #27917 pointed out that QuickLook was order-dependent
-- very bad!
The solution is in `qlUnify`: spot when we are unifying a type with a
definite monotype, and then monomorphise any free instantiation
variables of that type.
See (UQL8) and (UQL9) in Note [QuickLook unification].
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c391c70c
by Simon Peyton Jones at 2026-10-14T00:07:28+01:00
Add some comments about RuntimeUnk
This commit just adds comments explaining why we don't need to worry about
RuntimeUnkTv in the constraint solver.
It also tightens up docmentation of earlier (overlapping) commits in
this MR.
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7df3ba2f
by Andrea Vezzosi at 2026-10-14T06:15:48-04:00
add runGhcWithSignalHandlers and remove handling from runGhc[T]
it only ever makes sense to call `withSignalHandlers` from `main`, otherwise
signals can be too easily swallowed without notification to the rest of your
process.
This commit takes withSignalHandlers out of `runGhc/runGhcT` and provides
`runGhcWithSignalHandlers` for the old behaviour.
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b19b9a47
by Florian Ragwitz at 2026-10-14T03:37:48-07:00
hpc: Traverse patterns to tick contained HsExprs
So we can have coverage for view patterns.
Fixes #18590.