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Diffstat (limited to 'compiler/GHC/Tc/Utils/Unify.hs')
| -rw-r--r-- | compiler/GHC/Tc/Utils/Unify.hs | 32 |
1 files changed, 0 insertions, 32 deletions
diff --git a/compiler/GHC/Tc/Utils/Unify.hs b/compiler/GHC/Tc/Utils/Unify.hs index 7b8920f555..dc1fd382d2 100644 --- a/compiler/GHC/Tc/Utils/Unify.hs +++ b/compiler/GHC/Tc/Utils/Unify.hs @@ -1764,38 +1764,6 @@ better in practice. Revisited in Nov '20, along with removing flattening variables. Problem is still present, and the solution is still the same. -Note [Refactoring hazard: metaTyVarUpdateOK] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -I (Richard E.) have a sad story about refactoring this code, retained here -to prevent others (or a future me!) from falling into the same traps. - -It all started with #11407, which was caused by the fact that the TyVarTy -case of defer_me didn't look in the kind. But it seemed reasonable to -simply remove the defer_me check instead. - -It referred to two Notes (since removed) that were out of date, and the -fast_check code in occurCheckExpand seemed to do just about the same thing as -defer_me. The one piece that defer_me did that wasn't repeated by -occurCheckExpand was the type-family check. (See Note [Prevent unification -with type families].) So I checked the result of occurCheckExpand for any -type family occurrences and deferred if there were any. This was done -in commit e9bf7bb5cc9fb3f87dd05111aa23da76b86a8967 . - -This approach turned out not to be performant, because the expanded -type was bigger than the original type, and tyConsOfType (needed to -see if there are any type family occurrences) looks through type -synonyms. So it then struck me that we could dispense with the -defer_me check entirely. This simplified the code nicely, and it cut -the allocations in T5030 by half. But, as documented in Note [Prevent -unification with type families], this destroyed performance in -T3064. Regardless, I missed this regression and the change was -committed as 3f5d1a13f112f34d992f6b74656d64d95a3f506d . - -Bottom lines: - * defer_me is back, but now fixed w.r.t. #11407. - * Tread carefully before you start to refactor here. There can be - lots of hard-to-predict consequences. - Note [Type synonyms and the occur check] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Generally speaking we try to update a variable with type synonyms not |