Simon Peyton Jones pushed to branch wip/andreask/spec_tyfams at Glasgow Haskell Compiler / GHC Commits: a76cbf2d by Simon Peyton Jones at 2025-04-27T12:55:52+01:00 Typos - - - - - 752ecf79 by Simon Peyton Jones at 2025-04-27T12:56:00+01:00 Remove a special case that appears to do nothing See the commented-out block of `specCase` - - - - - 2 changed files: - compiler/GHC/Core/Opt/Specialise.hs - compiler/GHC/Tc/Solver/Monad.hs Changes: ===================================== compiler/GHC/Core/Opt/Specialise.hs ===================================== @@ -65,7 +65,6 @@ import GHC.Unit.Module.ModGuts import GHC.Core.Unfold import Data.List( partition ) -import Data.List.NonEmpty ( NonEmpty (..) ) import GHC.Core.Subst (substTickish) import GHC.Core.TyCon (tyConClass_maybe) import GHC.Core.DataCon (dataConTyCon) @@ -1280,6 +1279,36 @@ specCase :: SpecEnv , OutId , [OutAlt] , UsageDetails) +{- +------------------ +SLPJ: I am commenting out this entire special case. +Reading Note [Floating dictionaries out of cases] carefully, I just don't get it. +* We never explicitly pattern-match on a dictionary; rather the class-op rules + select superclasses from it. +* Calling `interestingDict` on every scrutinee is hardly sensible; + generally `interestingDict` is called only on Constraint-kinded things. +* It was giving a Lint scope error in !14272 + +So I think this code does no good; it's a waste of time and complexity. + +The commit that introduced it is back in 2010: + + commit c107a00ccf1e641a2d008939cf477c71caa028d5 + Author: Simon Peyton Jones <simonpj@microsoft.com> + Date: Thu Aug 12 13:11:33 2010 +0000 + + Improve the Specialiser, fixing Trac #4203 + + Simply fixing #4203 is a tiny fix: in case alterantives we should + do dumpUDs *including* the case binder. + + But I realised that we can do better and wasted far too much time + implementing the idea. It's described in + Note [Floating dictionaries out of cases] + +There is no compelling motivation and no test case +---------------------- + specCase env scrut' case_bndr [Alt con args rhs] | -- See Note [Floating dictionaries out of cases] -- interestingDict scrut' (idType case_bndr) @@ -1340,7 +1369,7 @@ specCase env scrut' case_bndr [Alt con args rhs] && tyCoVarsOfType var_ty `disjointVarSet` arg_set where var_ty = idType var - +-} specCase env scrut case_bndr alts = do { (alts', uds_alts) <- mapAndCombineSM spec_alt alts ===================================== compiler/GHC/Tc/Solver/Monad.hs ===================================== @@ -397,7 +397,7 @@ updInertDicts dict_ct@(DictCt { di_cls = cls, di_ev = ev, di_tys = tys }) does_not_mention_ip_for :: Type -> DictCt -> Bool does_not_mention_ip_for str_ty (DictCt { di_cls = cls, di_tys = tys }) = not $ mightMentionIP (not . typesAreApart str_ty) (const True) cls tys - -- See Note [Using typesAreApart when calling mightMmentionIP] + -- See Note [Using typesAreApart when calling mightMentionIP] -- in GHC.Core.Predicate updInertIrreds :: IrredCt -> TcS () @@ -586,7 +586,7 @@ Note [Using isCallStackTy in mightMentionIP] To implement Note [Don't add HasCallStack constraints to the solved set], we need to check whether a constraint contains a HasCallStack or HasExceptionContext constraint. We do this using the 'mentionsIP' function, but as per -Note [Using typesAreApart when calling mightMentions] we don't want to simply do: +Note [Using typesAreApart when calling mightMentionIP] we don't want to simply do: mightMentionIP (const True) -- (ignore the implicit parameter string) View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/279442edcad54e8b5b23240c0354c87... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/279442edcad54e8b5b23240c0354c87... You're receiving this email because of your account on gitlab.haskell.org.