From 2060788524c41ae87ba87276cb59e7cab574fe68 Mon Sep 17 00:00:00 2001 From: "simonpj@microsoft.com" Date: Thu, 7 Oct 2010 08:41:04 +0000 Subject: [PATCH] Fix a looping bug in the new occur-check code --- compiler/typecheck/TcInteract.lhs | 238 ++++++++++++++++++------------------- 1 file changed, 118 insertions(+), 120 deletions(-) diff --git a/compiler/typecheck/TcInteract.lhs b/compiler/typecheck/TcInteract.lhs index 807dd61..2a53503 100644 --- a/compiler/typecheck/TcInteract.lhs +++ b/compiler/typecheck/TcInteract.lhs @@ -35,7 +35,7 @@ import Outputable import TcRnTypes import TcErrors import TcSMonad -import qualified Bag as Bag +import Bag import qualified Data.Map as Map import Maybes @@ -481,6 +481,8 @@ spontaneousSolveStage workItem inerts -- Note, just passing the inerts through for the skolem equivalence classes trySpontaneousSolve :: WorkItem -> InertSet -> TcS (Maybe SWorkList) trySpontaneousSolve (CTyEqCan { cc_id = cv, cc_flavor = gw, cc_tyvar = tv1, cc_rhs = xi }) inerts + | isGiven gw + = return Nothing | Just tv2 <- tcGetTyVar_maybe xi = do { tch1 <- isTouchableMetaTyVar tv1 ; tch2 <- isTouchableMetaTyVar tv2 @@ -583,23 +585,17 @@ The spontaneous solver has to return a given which mentions the unified unificat variable *on the left* of the equality. Here is what happens if not: Original wanted: (a ~ alpha), (alpha ~ Int) We spontaneously solve the first wanted, without changing the order! - given : a ~ alpha [having unifice alpha := a] + given : a ~ alpha [having unified alpha := a] Now the second wanted comes along, but he cannot rewrite the given, so we simply continue. At the end we spontaneously solve that guy, *reunifying* [alpha := Int] -We avoid this problem by orienting the given so that the unification variable is on the left. -[Note that alternatively we could attempt to enforce this at canonicalization] +We avoid this problem by orienting the given so that the unification +variable is on the left. [Note that alternatively we could attempt to +enforce this at canonicalization] -Avoiding double unifications is yet another reason to disallow touchable unification variables -as RHS of type family equations: F xis ~ alpha. Consider having already spontaneously solved -a wanted (alpha ~ [b]) by setting alpha := [b]. So the inert set looks like: - given : alpha ~ [b] -And now a new wanted (F tau ~ alpha) comes along. Since it does not react with anything -we will be left with a constraint (F tau ~ alpha) that must cause a unification of -(alpha := F tau) at some point (either in spontaneous solving, or at the end). But alpha -is *already* unified so we must not do anything to it. By disallowing naked touchables in -the RHS of constraints (in favor of introduced flatten skolems) we do not have to worry at -all about unifying or spontaneously solving (F xis ~ alpha) by unification. +See also Note [No touchables as FunEq RHS] in TcSMonad; avoiding +double unifications is the main reason we disallow touchable +unification variables as RHS of type family equations: F xis ~ alpha. \begin{code} ---------------- @@ -608,117 +604,121 @@ solveWithIdentity :: InertSet -> TcS (Maybe SWorkList) -- Solve with the identity coercion -- Precondition: kind(xi) is a sub-kind of kind(tv) +-- Precondition: CtFlavor is not Given -- See [New Wanted Superclass Work] to see why we do this for *given* as well solveWithIdentity inerts cv gw tv xi - | not (isGiven gw) = do { tybnds <- getTcSTyBindsBag - ; case occ_check_ok tybnds xi of - Nothing -> return Nothing - Just (xi_unflat,coi) -- coi : xi_unflat ~ xi - -> do { traceTcS "Sneaky unification:" $ + ; case occurCheck tybnds inerts tv xi of + Nothing -> return Nothing + Just (xi_unflat,coi) -> solve_with xi_unflat coi } + where + solve_with xi_unflat coi -- coi : xi_unflat ~ xi + = do { traceTcS "Sneaky unification:" $ vcat [text "Coercion variable: " <+> ppr gw, text "Coercion: " <+> pprEq (mkTyVarTy tv) xi, text "Left Kind is : " <+> ppr (typeKind (mkTyVarTy tv)), text "Right Kind is : " <+> ppr (typeKind xi) - ] - ; setWantedTyBind tv xi_unflat -- Set tv := xi_unflat - ; cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi_unflat xi_unflat - ; let flav = mkGivenFlavor gw UnkSkol - ; (cts, co) <- case coi of - ACo co -> do { can_eqs <- canEq flav cv_given (mkTyVarTy tv) xi_unflat - ; return (can_eqs, co) } - IdCo co -> return $ - (singleCCan (CTyEqCan { cc_id = cv_given - , cc_flavor = mkGivenFlavor gw UnkSkol - , cc_tyvar = tv, cc_rhs = xi } - -- xi, *not* xi_unflat because - -- xi_unflat may require flattening! - ), co) - ; case gw of - Wanted {} -> setWantedCoBind cv co - Derived {} -> setDerivedCoBind cv co - _ -> pprPanic "Can't spontaneously solve *given*" empty - - -- See Note [Avoid double unifications] - - ; return (Just cts) } - } - | otherwise - = return Nothing - - where occ_check_ok :: Bag.Bag (TcTyVar, TcType) -> TcType -> Maybe (TcType,CoercionI) - occ_check_ok ty_binds_bag ty = ok ty - where - -- @ok ty@ - -- Traverse @ty@ to make sure that @tv@ does not appear under some flatten skolem. - -- If it appears under some flatten skolem look in that flatten skolem equivalence class - -- (see Note [InertSet FlattenSkolemEqClass], [Loopy Spontaneous Solving]) to see if you - -- can find a different flatten skolem to use, that is, one that does not mention @tv@. - -- - -- Postcondition: Just (ty',coi) <- ok ty - -- coi :: ty' ~ ty - -- - -- NB: The returned type may not be flat! - -- - -- NB: There is no need to do the tcView thing here to expand synonyms, because - -- expanded synonyms have the same or fewer variables than their expanded definitions, - -- but never more. - -- See Note [Type synonyms and the occur check] in TcUnify for the handling of type synonyms. - ok this_ty@(TyConApp tc tys) - | Just tys_cois <- allMaybes (map ok tys) - = let (tys',cois') = unzip tys_cois - in Just (TyConApp tc tys', mkTyConAppCoI tc cois') - | isSynTyCon tc, Just ty_expanded <- tcView this_ty - = ok ty_expanded - ok (PredTy sty) - | Just (sty',coi) <- ok_pred sty - = Just (PredTy sty', coi) - where ok_pred (ClassP cn tys) - | Just tys_cois <- allMaybes $ map ok tys - = let (tys', cois') = unzip tys_cois - in Just (ClassP cn tys', mkClassPPredCoI cn cois') - ok_pred (IParam nm ty) - | Just (ty',co') <- ok ty - = Just (IParam nm ty', mkIParamPredCoI nm co') - ok_pred (EqPred ty1 ty2) - | Just (ty1',coi1) <- ok ty1, Just (ty2',coi2) <- ok ty2 - = Just (EqPred ty1' ty2', mkEqPredCoI coi1 coi2) - ok_pred _ = Nothing - ok (FunTy arg res) - | Just (arg', coiarg) <- ok arg, Just (res', coires) <- ok res - = Just (FunTy arg' res', mkFunTyCoI coiarg coires) - ok (AppTy fun arg) - | Just (fun', coifun) <- ok fun, Just (arg', coiarg) <- ok arg - = Just (AppTy fun' arg', mkAppTyCoI coifun coiarg) - ok (ForAllTy tv1 ty1) - -- WARNING: What if it is a (t1 ~ t2) => t3? It's not handled properly at the moment. - | Just (ty1', coi) <- ok ty1 - = Just (ForAllTy tv1 ty1', mkForAllTyCoI tv1 coi) - - -- Variable cases - ok this_ty@(TyVarTy tv') - | not $ isTcTyVar tv' = Just (this_ty, IdCo this_ty) -- Bound variable - | tv == tv' = Nothing -- Occurs check error - -- Flatten skolem - ok (TyVarTy fsk) | FlatSkol zty <- tcTyVarDetails fsk - = case ok zty of - Nothing -> go_down_eq_class $ getFskEqClass inerts fsk - Just (zty',ico) -> Just (zty',ico) - where go_down_eq_class [] = Nothing - go_down_eq_class ((fsk1,co1):rest) - = case ok (TyVarTy fsk1) of - Nothing -> go_down_eq_class rest - Just (ty1,co1i') -> Just (ty1, mkTransCoI co1i' (ACo co1)) - -- Finally, check if bound already - ok this_ty@(TyVarTy tv0) - = case Bag.foldlBag find_bind Nothing ty_binds_bag of - Nothing -> Just (this_ty, IdCo this_ty) - Just ty0 -> ok ty0 - where find_bind Nothing (tvx,tyx) | tv0 == tvx = Just tyx - find_bind m _ = m - -- Fall through - ok _ty = Nothing - + ] + ; setWantedTyBind tv xi_unflat -- Set tv := xi_unflat + ; cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi_unflat xi_unflat + ; let flav = mkGivenFlavor gw UnkSkol + ; (cts, co) <- case coi of + ACo co -> do { can_eqs <- canEq flav cv_given (mkTyVarTy tv) xi_unflat + ; return (can_eqs, co) } + IdCo co -> return $ + (singleCCan (CTyEqCan { cc_id = cv_given + , cc_flavor = mkGivenFlavor gw UnkSkol + , cc_tyvar = tv, cc_rhs = xi } + -- xi, *not* xi_unflat because + -- xi_unflat may require flattening! + ), co) + ; case gw of + Wanted {} -> setWantedCoBind cv co + Derived {} -> setDerivedCoBind cv co + _ -> pprPanic "Can't spontaneously solve *given*" empty + -- See Note [Avoid double unifications] + ; return (Just cts) } + +occurCheck :: Bag (TcTyVar, TcType) -> InertSet + -> TcTyVar -> TcType -> Maybe (TcType,CoercionI) +-- Traverse @ty@ to make sure that @tv@ does not appear under some flatten skolem. +-- If it appears under some flatten skolem look in that flatten skolem equivalence class +-- (see Note [InertSet FlattenSkolemEqClass], [Loopy Spontaneous Solving]) to see if you +-- can find a different flatten skolem to use, that is, one that does not mention @tv@. +-- +-- Postcondition: Just (ty', coi) = occurCheck binds inerts tv ty +-- coi :: ty' ~ ty +-- NB: The returned type ty' may not be flat! + +occurCheck ty_binds_bag inerts tv ty + = ok emptyVarSet ty + where + ok bad this_ty@(TyConApp tc tys) + | Just tys_cois <- allMaybes (map (ok bad) tys) + , (tys',cois') <- unzip tys_cois + = Just (TyConApp tc tys', mkTyConAppCoI tc cois') + | isSynTyCon tc, Just ty_expanded <- tcView this_ty + = ok bad ty_expanded -- See Note [Type synonyms and the occur check] in TcUnify + ok bad (PredTy sty) + | Just (sty',coi) <- ok_pred bad sty + = Just (PredTy sty', coi) + ok bad (FunTy arg res) + | Just (arg', coiarg) <- ok bad arg, Just (res', coires) <- ok bad res + = Just (FunTy arg' res', mkFunTyCoI coiarg coires) + ok bad (AppTy fun arg) + | Just (fun', coifun) <- ok bad fun, Just (arg', coiarg) <- ok bad arg + = Just (AppTy fun' arg', mkAppTyCoI coifun coiarg) + ok bad (ForAllTy tv1 ty1) + -- WARNING: What if it is a (t1 ~ t2) => t3? It's not handled properly at the moment. + | Just (ty1', coi) <- ok bad ty1 + = Just (ForAllTy tv1 ty1', mkForAllTyCoI tv1 coi) + + -- Variable cases + ok _bad this_ty@(TyVarTy tv') + | not $ isTcTyVar tv' = Just (this_ty, IdCo this_ty) -- Bound variable + | tv == tv' = Nothing -- Occurs check error + + ok bad (TyVarTy fsk) + | FlatSkol zty <- tcTyVarDetails fsk + = if fsk `elemVarSet` bad then + -- its type has been checked + go_down_eq_class bad $ getFskEqClass inerts fsk + else + -- its type is not yet checked + case ok bad zty of + Nothing -> go_down_eq_class (bad `extendVarSet` fsk) $ + getFskEqClass inerts fsk + Just (zty',ico) -> Just (zty',ico) + + -- Check if there exists a ty bind already, as a result of sneaky unification. + ok bad this_ty@(TyVarTy tv0) + = case Bag.foldlBag find_bind Nothing ty_binds_bag of + Nothing -> Just (this_ty, IdCo this_ty) + Just ty0 -> ok bad ty0 + where find_bind Nothing (tvx,tyx) | tv0 == tvx = Just tyx + find_bind m _ = m + -- Fall through + ok _bad _ty = Nothing + + ok_pred bad (ClassP cn tys) + | Just tys_cois <- allMaybes $ map (ok bad) tys + = let (tys', cois') = unzip tys_cois + in Just (ClassP cn tys', mkClassPPredCoI cn cois') + ok_pred bad (IParam nm ty) + | Just (ty',co') <- ok bad ty + = Just (IParam nm ty', mkIParamPredCoI nm co') + ok_pred bad (EqPred ty1 ty2) + | Just (ty1',coi1) <- ok bad ty1, Just (ty2',coi2) <- ok bad ty2 + = Just (EqPred ty1' ty2', mkEqPredCoI coi1 coi2) + ok_pred _ _ = Nothing + + go_down_eq_class _bad_tvs [] = Nothing + go_down_eq_class bad_tvs ((fsk1,co1):rest) + | fsk1 `elemVarSet` bad_tvs = go_down_eq_class bad_tvs rest + | otherwise + = case ok bad_tvs (TyVarTy fsk1) of + Nothing -> go_down_eq_class (bad_tvs `extendVarSet` fsk1) rest + Just (ty1,co1i') -> Just (ty1, mkTransCoI co1i' (ACo co1)) \end{code} @@ -1884,5 +1884,3 @@ matchClassInst clas tys loc } } \end{code} - - -- 1.7.10.4