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@@ -65,7 +65,6 @@ import GHC.Unit.Module.ModGuts |
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import GHC.Core.Unfold
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import Data.List( partition )
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-import Data.List.NonEmpty ( NonEmpty (..) )
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import GHC.Core.Subst (substTickish)
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import GHC.Core.TyCon (tyConClass_maybe)
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import GHC.Core.DataCon (dataConTyCon)
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@@ -1280,6 +1279,36 @@ specCase :: SpecEnv |
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, OutId
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, [OutAlt]
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, UsageDetails)
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+{-
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+------------------
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+SLPJ: I am commenting out this entire special case.
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+Reading Note [Floating dictionaries out of cases] carefully, I just don't get it.
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+* We never explicitly pattern-match on a dictionary; rather the class-op rules
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+ select superclasses from it.
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+* Calling `interestingDict` on every scrutinee is hardly sensible;
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+ generally `interestingDict` is called only on Constraint-kinded things.
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+* It was giving a Lint scope error in !14272
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+
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+So I think this code does no good; it's a waste of time and complexity.
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+
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+The commit that introduced it is back in 2010:
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+
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+ commit c107a00ccf1e641a2d008939cf477c71caa028d5
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+ Author: Simon Peyton Jones <simonpj@microsoft.com>
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+ Date: Thu Aug 12 13:11:33 2010 +0000
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+
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+ Improve the Specialiser, fixing Trac #4203
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+
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+ Simply fixing #4203 is a tiny fix: in case alterantives we should
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+ do dumpUDs *including* the case binder.
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+
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+ But I realised that we can do better and wasted far too much time
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+ implementing the idea. It's described in
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+ Note [Floating dictionaries out of cases]
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+
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+There is no compelling motivation and no test case
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+----------------------
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+
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specCase env scrut' case_bndr [Alt con args rhs]
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| -- See Note [Floating dictionaries out of cases]
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-- interestingDict scrut' (idType case_bndr)
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@@ -1340,7 +1369,7 @@ specCase env scrut' case_bndr [Alt con args rhs] |
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&& tyCoVarsOfType var_ty `disjointVarSet` arg_set
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where
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var_ty = idType var
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-
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+-}
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specCase env scrut case_bndr alts
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= do { (alts', uds_alts) <- mapAndCombineSM spec_alt alts
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