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author | Richard Eisenberg <rae@richarde.dev> | 2020-11-25 15:22:16 -0500 |
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committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2020-12-01 19:57:41 -0500 |
commit | 8bb52d9186655134e3e06b4dc003e060379f5417 (patch) | |
tree | cf62438a5f5b3587fe666d72d77561201253306a /compiler/GHC/HsToCore/Types.hs | |
parent | 0dd45d0adbade7eaae973b09b4d0ff1acb1479b8 (diff) | |
download | haskell-8bb52d9186655134e3e06b4dc003e060379f5417.tar.gz |
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.
* The inert_count field of InertCans wasn't actually used, so I removed
it.
Though I (Richard) did the implementation, Simon PJ was very involved
in design and review.
This updates the Haddock submodule to avoid #18932 by adding
a type signature.
-------------------------
Metric Decrease:
T12227
T5030
T9872a
T9872b
T9872c
Metric Increase:
T9872d
-------------------------
Diffstat (limited to 'compiler/GHC/HsToCore/Types.hs')
-rw-r--r-- | compiler/GHC/HsToCore/Types.hs | 4 |
1 files changed, 4 insertions, 0 deletions
diff --git a/compiler/GHC/HsToCore/Types.hs b/compiler/GHC/HsToCore/Types.hs index 68da67d21e..aeeeb0c530 100644 --- a/compiler/GHC/HsToCore/Types.hs +++ b/compiler/GHC/HsToCore/Types.hs @@ -12,6 +12,7 @@ import GHC.Types.CostCentre.State import GHC.Types.Name.Env import GHC.Types.SrcLoc import GHC.Types.Var +import GHC.Types.Name.Reader (GlobalRdrEnv) import GHC.Hs (LForeignDecl, HsExpr, GhcTc) import GHC.Tc.Types (TcRnIf, IfGblEnv, IfLclEnv, CompleteMatches) import GHC.HsToCore.Pmc.Types (Nablas) @@ -42,6 +43,9 @@ data DsGblEnv = DsGblEnv { ds_mod :: Module -- For SCC profiling , ds_fam_inst_env :: FamInstEnv -- Like tcg_fam_inst_env + , ds_gbl_rdr_env :: GlobalRdrEnv -- needed *only* to know what newtype + -- constructors are in scope during + -- pattern-match satisfiability checking , ds_unqual :: PrintUnqualified , ds_msgs :: IORef Messages -- Warning messages , ds_if_env :: (IfGblEnv, IfLclEnv) -- Used for looking up global, |