{-# LANGUAGE CPP #-} {-# LANGUAGE DeriveFunctor #-} {-# LANGUAGE DerivingStrategies #-} {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE MultiWayIf #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE ViewPatterns #-} {-# OPTIONS_GHC -Wno-incomplete-record-updates -Wno-orphans #-} -- | Monadic definitions for the constraint solver module GHC.Tc.Solver.Monad ( -- The TcS monad TcS, runTcS, runTcSEarlyAbort, runTcSWithEvBinds, runTcSInerts, failTcS, warnTcS, addErrTcS, wrapTcS, runTcSEqualities, nestTcS, nestImplicTcS, setEvBindsTcS, emitImplicationTcS, emitTvImplicationTcS, selectNextWorkItem, getWorkList, updWorkListTcS, pushLevelNoWorkList, runTcPluginTcS, addUsedGRE, addUsedGREs, keepAlive, matchGlobalInst, TcM.ClsInstResult(..), QCInst(..), -- Tracing etc panicTcS, traceTcS, traceFireTcS, bumpStepCountTcS, csTraceTcS, wrapErrTcS, wrapWarnTcS, resetUnificationFlag, setUnificationFlag, -- Evidence creation and transformation MaybeNew(..), freshGoals, isFresh, getEvExpr, newTcEvBinds, newNoTcEvBinds, newWantedEq, emitNewWantedEq, newWanted, newWantedNC, newWantedEvVarNC, newBoundEvVarId, unifyTyVar, reportUnifications, touchabilityTest, TouchabilityTestResult(..), setEvBind, setWantedEq, setWantedEvTerm, setEvBindIfWanted, newEvVar, newGivenEvVar, newGivenEvVars, checkReductionDepth, getSolvedDicts, setSolvedDicts, getInstEnvs, getFamInstEnvs, -- Getting the environments getTopEnv, getGblEnv, getLclEnv, setLclEnv, getTcEvBindsVar, getTcLevel, getTcEvTyCoVars, getTcEvBindsMap, setTcEvBindsMap, tcLookupClass, tcLookupId, -- Inerts updInertTcS, updInertCans, updInertDicts, updInertIrreds, getHasGivenEqs, setInertCans, getInertEqs, getInertCans, getInertGivens, getInertInsols, getInnermostGivenEqLevel, getTcSInerts, setTcSInerts, getUnsolvedInerts, removeInertCts, getPendingGivenScs, addInertCan, insertFunEq, addInertForAll, emitWorkNC, emitWork, lookupInertDict, -- The Model kickOutAfterUnification, -- Inert Safe Haskell safe-overlap failures addInertSafehask, insertSafeOverlapFailureTcS, updInertSafehask, getSafeOverlapFailures, -- Inert solved dictionaries addSolvedDict, lookupSolvedDict, -- Irreds foldIrreds, -- The family application cache lookupFamAppInert, lookupFamAppCache, extendFamAppCache, pprKicked, instDFunType, -- Instantiation -- MetaTyVars newFlexiTcSTy, instFlexi, instFlexiX, cloneMetaTyVar, tcInstSkolTyVarsX, TcLevel, isFilledMetaTyVar_maybe, isFilledMetaTyVar, zonkTyCoVarsAndFV, zonkTcType, zonkTcTypes, zonkTcTyVar, zonkCo, zonkTyCoVarsAndFVList, zonkSimples, zonkWC, zonkTyCoVarKind, -- References newTcRef, readTcRef, writeTcRef, updTcRef, -- Misc getDefaultInfo, getDynFlags, getGlobalRdrEnvTcS, matchFam, matchFamTcM, checkWellStagedDFun, pprEq, -- Smaller utils, re-exported from TcM -- TODO (DV): these are only really used in the -- instance matcher in GHC.Tc.Solver. I am wondering -- if the whole instance matcher simply belongs -- here breakTyEqCycle_maybe, rewriterView ) where import GHC.Prelude import GHC.Driver.Env import qualified GHC.Tc.Utils.Instantiate as TcM import GHC.Core.InstEnv import GHC.Tc.Instance.Family as FamInst import GHC.Core.FamInstEnv import qualified GHC.Tc.Utils.Monad as TcM import qualified GHC.Tc.Utils.TcMType as TcM import qualified GHC.Tc.Instance.Class as TcM( matchGlobalInst, ClsInstResult(..) ) import qualified GHC.Tc.Utils.Env as TcM ( checkWellStaged, tcGetDefaultTys, tcLookupClass, tcLookupId, topIdLvl , tcInitTidyEnv ) import GHC.Tc.Instance.Class( InstanceWhat(..), safeOverlap, instanceReturnsDictCon ) import GHC.Tc.Utils.TcType import GHC.Driver.Session import GHC.Core.Type import qualified GHC.Core.TyCo.Rep as Rep -- this needs to be used only very locally import GHC.Core.Coercion import GHC.Core.Reduction import GHC.Tc.Solver.Types import GHC.Tc.Solver.InertSet import GHC.Tc.Types.Evidence import GHC.Core.Class import GHC.Core.TyCon import GHC.Tc.Errors.Types import GHC.Types.Error ( mkPlainError, noHints ) import GHC.Types.Name import GHC.Types.TyThing import GHC.Unit.Module ( HasModule, getModule, extractModule ) import GHC.Types.Name.Reader ( GlobalRdrEnv, GlobalRdrElt ) import qualified GHC.Rename.Env as TcM import GHC.Types.Var import GHC.Types.Var.Env import GHC.Types.Var.Set import GHC.Utils.Outputable import GHC.Utils.Panic import GHC.Utils.Logger import GHC.Utils.Misc (HasDebugCallStack) import GHC.Data.Bag as Bag import GHC.Types.Unique.Supply import GHC.Tc.Types import GHC.Tc.Types.Origin import GHC.Tc.Types.Constraint import GHC.Tc.Utils.Unify import GHC.Core.Predicate import GHC.Types.Unique.Set (nonDetEltsUniqSet) import Control.Monad import GHC.Utils.Monad import Data.IORef import GHC.Exts (oneShot) import Data.List ( mapAccumL, partition, find ) #if defined(DEBUG) import GHC.Data.Graph.Directed #endif {- ********************************************************************* * * Inert instances: inert_insts * * ********************************************************************* -} addInertForAll :: QCInst -> TcS () -- Add a local Given instance, typically arising from a type signature addInertForAll new_qci = do { ics <- getInertCans ; ics1 <- add_qci ics -- Update given equalities. C.f updateGivenEqs ; tclvl <- getTcLevel ; let pred = qci_pred new_qci not_equality = isClassPred pred && not (isEqPred pred) -- True <=> definitely not an equality -- A qci_pred like (f a) might be an equality ics2 | not_equality = ics1 | otherwise = ics1 { inert_given_eq_lvl = tclvl , inert_given_eqs = True } ; setInertCans ics2 } where add_qci :: InertCans -> TcS InertCans -- See Note [Do not add duplicate quantified instances] add_qci ics@(IC { inert_insts = qcis }) | any same_qci qcis = do { traceTcS "skipping duplicate quantified instance" (ppr new_qci) ; return ics } | otherwise = do { traceTcS "adding new inert quantified instance" (ppr new_qci) ; return (ics { inert_insts = new_qci : qcis }) } same_qci old_qci = tcEqType (ctEvPred (qci_ev old_qci)) (ctEvPred (qci_ev new_qci)) {- Note [Do not add duplicate quantified instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this (#15244): f :: (C g, D g) => .... class S g => C g where ... class S g => D g where ... class (forall a. Eq a => Eq (g a)) => S g where ... Then in f's RHS there are two identical quantified constraints available, one via the superclasses of C and one via the superclasses of D. The two are identical, and it seems wrong to reject the program because of that. But without doing duplicate-elimination we will have two matching QCInsts when we try to solve constraints arising from f's RHS. The simplest thing is simply to eliminate duplicates, which we do here. -} {- ********************************************************************* * * Adding an inert * * ************************************************************************ Note [Adding an equality to the InertCans] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When adding an equality to the inerts: * Kick out any constraints that can be rewritten by the thing we are adding. Done by kickOutRewritable. * Note that unifying a:=ty, is like adding [G] a~ty; just use kickOutRewritable with Nominal, Given. See kickOutAfterUnification. Note [Kick out existing binding for implicit parameter] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have (typecheck/should_compile/ImplicitParamFDs) flub :: (?x :: Int) => (Int, Integer) flub = (?x, let ?x = 5 in ?x) When we are checking the last ?x occurrence, we guess its type to be a fresh unification variable alpha and emit an (IP "x" alpha) constraint. But the given (?x :: Int) has been translated to an IP "x" Int constraint, which has a functional dependency from the name to the type. So fundep interaction tells us that alpha ~ Int, and we get a type error. This is bad. Instead, we wish to excise any old given for an IP when adding a new one. We also must make sure not to float out any IP constraints outside an implication that binds an IP of the same name; see GHC.Tc.Solver.floatConstraints. -} addInertCan :: Ct -> TcS () -- Precondition: item /is/ canonical -- See Note [Adding an equality to the InertCans] addInertCan ct = do { traceTcS "addInertCan {" $ text "Trying to insert new inert item:" <+> ppr ct ; mkTcS (\TcSEnv{tcs_abort_on_insoluble=abort_flag} -> when (abort_flag && insolubleEqCt ct) TcM.failM) ; ics <- getInertCans ; ics <- maybeKickOut ics ct ; tclvl <- getTcLevel ; setInertCans (addInertItem tclvl ics ct) ; traceTcS "addInertCan }" $ empty } maybeKickOut :: InertCans -> Ct -> TcS InertCans -- For a CEqCan, kick out any inert that can be rewritten by the CEqCan maybeKickOut ics ct | CEqCan { cc_lhs = lhs, cc_ev = ev, cc_eq_rel = eq_rel } <- ct = do { (_, ics') <- kickOutRewritable (ctEvFlavour ev, eq_rel) lhs ics ; return ics' } -- See [Kick out existing binding for implicit parameter] | isGivenCt ct , CDictCan { cc_class = cls, cc_tyargs = [ip_name_strty, _ip_ty] } <- ct , isIPClass cls , Just ip_name <- isStrLitTy ip_name_strty -- Would this be more efficient if we used findDictsByClass and then delDict? = let dict_map = inert_dicts ics dict_map' = filterDicts doesn't_match_ip_name dict_map doesn't_match_ip_name :: Ct -> Bool doesn't_match_ip_name ct | Just (inert_ip_name, _inert_ip_ty) <- isIPPred_maybe (ctPred ct) = inert_ip_name /= ip_name | otherwise = True in return (ics { inert_dicts = dict_map' }) | otherwise = return ics ----------------------------------------- kickOutRewritable :: CtFlavourRole -- Flavour/role of the equality that -- is being added to the inert set -> CanEqLHS -- The new equality is lhs ~ ty -> InertCans -> TcS (Int, InertCans) kickOutRewritable new_fr new_lhs ics = do { let (kicked_out, ics') = kickOutRewritableLHS new_fr new_lhs ics n_kicked = workListSize kicked_out ; unless (n_kicked == 0) $ do { updWorkListTcS (appendWorkList kicked_out) -- The famapp-cache contains Given evidence from the inert set. -- If we're kicking out Givens, we need to remove this evidence -- from the cache, too. ; let kicked_given_ev_vars = [ ev_var | ct <- wl_eqs kicked_out , CtGiven { ctev_evar = ev_var } <- [ctEvidence ct] ] ; when (new_fr `eqCanRewriteFR` (Given, NomEq) && -- if this isn't true, no use looking through the constraints not (null kicked_given_ev_vars)) $ do { traceTcS "Given(s) have been kicked out; drop from famapp-cache" (ppr kicked_given_ev_vars) ; dropFromFamAppCache (mkVarSet kicked_given_ev_vars) } ; csTraceTcS $ hang (text "Kick out, lhs =" <+> ppr new_lhs) 2 (vcat [ text "n-kicked =" <+> int n_kicked , text "kicked_out =" <+> ppr kicked_out , text "Residual inerts =" <+> ppr ics' ]) } ; return (n_kicked, ics') } kickOutAfterUnification :: TcTyVar -> TcS Int kickOutAfterUnification new_tv = do { ics <- getInertCans ; (n_kicked, ics2) <- kickOutRewritable (Given,NomEq) (TyVarLHS new_tv) ics -- Given because the tv := xi is given; NomEq because -- only nominal equalities are solved by unification ; setInertCans ics2 ; return n_kicked } -- See Wrinkle (2) in Note [Equalities with incompatible kinds] in GHC.Tc.Solver.Canonical -- It's possible that this could just go ahead and unify, but could there be occurs-check -- problems? Seems simpler just to kick out. kickOutAfterFillingCoercionHole :: CoercionHole -> TcS () kickOutAfterFillingCoercionHole hole = do { ics <- getInertCans ; let (kicked_out, ics') = kick_out ics n_kicked = workListSize kicked_out ; unless (n_kicked == 0) $ do { updWorkListTcS (appendWorkList kicked_out) ; csTraceTcS $ hang (text "Kick out, hole =" <+> ppr hole) 2 (vcat [ text "n-kicked =" <+> int n_kicked , text "kicked_out =" <+> ppr kicked_out , text "Residual inerts =" <+> ppr ics' ]) } ; setInertCans ics' } where kick_out :: InertCans -> (WorkList, InertCans) kick_out ics@(IC { inert_eqs = eqs, inert_funeqs = funeqs }) = (kicked_out, ics { inert_eqs = eqs_to_keep, inert_funeqs = funeqs_to_keep }) where (eqs_to_kick, eqs_to_keep) = partitionInertEqs kick_ct eqs (funeqs_to_kick, funeqs_to_keep) = partitionFunEqs kick_ct funeqs kicked_out = extendWorkListCts (eqs_to_kick ++ funeqs_to_kick) emptyWorkList kick_ct :: Ct -> Bool -- True: kick out; False: keep. kick_ct (CEqCan { cc_rhs = rhs, cc_ev = ctev }) = isWanted ctev && -- optimisation: givens don't have coercion holes anyway rhs `hasThisCoercionHoleTy` hole kick_ct other = pprPanic "kick_ct (coercion hole)" (ppr other) -------------- addInertSafehask :: InertCans -> Ct -> InertCans addInertSafehask ics item@(CDictCan { cc_class = cls, cc_tyargs = tys }) = ics { inert_safehask = addDict (inert_dicts ics) cls tys item } addInertSafehask _ item = pprPanic "addInertSafehask: can't happen! Inserting " $ ppr item insertSafeOverlapFailureTcS :: InstanceWhat -> Ct -> TcS () -- See Note [Safe Haskell Overlapping Instances Implementation] in GHC.Tc.Solver insertSafeOverlapFailureTcS what item | safeOverlap what = return () | otherwise = updInertCans (\ics -> addInertSafehask ics item) getSafeOverlapFailures :: TcS Cts -- See Note [Safe Haskell Overlapping Instances Implementation] in GHC.Tc.Solver getSafeOverlapFailures = do { IC { inert_safehask = safehask } <- getInertCans ; return $ foldDicts consCts safehask emptyCts } -------------- addSolvedDict :: InstanceWhat -> CtEvidence -> Class -> [Type] -> TcS () -- Conditionally add a new item in the solved set of the monad -- See Note [Solved dictionaries] in GHC.Tc.Solver.InertSet addSolvedDict what item cls tys | isWanted item , instanceReturnsDictCon what = do { traceTcS "updSolvedSetTcs:" $ ppr item ; updInertTcS $ \ ics -> ics { inert_solved_dicts = addDict (inert_solved_dicts ics) cls tys item } } | otherwise = return () getSolvedDicts :: TcS (DictMap CtEvidence) getSolvedDicts = do { ics <- getTcSInerts; return (inert_solved_dicts ics) } setSolvedDicts :: DictMap CtEvidence -> TcS () setSolvedDicts solved_dicts = updInertTcS $ \ ics -> ics { inert_solved_dicts = solved_dicts } {- ********************************************************************* * * Other inert-set operations * * ********************************************************************* -} updInertTcS :: (InertSet -> InertSet) -> TcS () -- Modify the inert set with the supplied function updInertTcS upd_fn = do { is_var <- getTcSInertsRef ; wrapTcS (do { curr_inert <- TcM.readTcRef is_var ; TcM.writeTcRef is_var (upd_fn curr_inert) }) } getInertCans :: TcS InertCans getInertCans = do { inerts <- getTcSInerts; return (inert_cans inerts) } setInertCans :: InertCans -> TcS () setInertCans ics = updInertTcS $ \ inerts -> inerts { inert_cans = ics } updRetInertCans :: (InertCans -> (a, InertCans)) -> TcS a -- Modify the inert set with the supplied function updRetInertCans upd_fn = do { is_var <- getTcSInertsRef ; wrapTcS (do { inerts <- TcM.readTcRef is_var ; let (res, cans') = upd_fn (inert_cans inerts) ; TcM.writeTcRef is_var (inerts { inert_cans = cans' }) ; return res }) } updInertCans :: (InertCans -> InertCans) -> TcS () -- Modify the inert set with the supplied function updInertCans upd_fn = updInertTcS $ \ inerts -> inerts { inert_cans = upd_fn (inert_cans inerts) } updInertDicts :: (DictMap Ct -> DictMap Ct) -> TcS () -- Modify the inert set with the supplied function updInertDicts upd_fn = updInertCans $ \ ics -> ics { inert_dicts = upd_fn (inert_dicts ics) } updInertSafehask :: (DictMap Ct -> DictMap Ct) -> TcS () -- Modify the inert set with the supplied function updInertSafehask upd_fn = updInertCans $ \ ics -> ics { inert_safehask = upd_fn (inert_safehask ics) } updInertIrreds :: (Cts -> Cts) -> TcS () -- Modify the inert set with the supplied function updInertIrreds upd_fn = updInertCans $ \ ics -> ics { inert_irreds = upd_fn (inert_irreds ics) } getInertEqs :: TcS InertEqs getInertEqs = do { inert <- getInertCans; return (inert_eqs inert) } getInnermostGivenEqLevel :: TcS TcLevel getInnermostGivenEqLevel = do { inert <- getInertCans ; return (inert_given_eq_lvl inert) } getInertInsols :: TcS Cts -- Returns insoluble equality constraints and TypeError constraints, -- specifically including Givens. -- -- Note that this function only inspects irreducible constraints; -- a DictCan constraint such as 'Eq (TypeError msg)' is not -- considered to be an insoluble constraint by this function. -- -- See Note [Pattern match warnings with insoluble Givens] in GHC.Tc.Solver. getInertInsols = do { inert <- getInertCans ; return $ filterBag insolubleCt (inert_irreds inert) } getInertGivens :: TcS [Ct] -- Returns the Given constraints in the inert set getInertGivens = do { inerts <- getInertCans ; let all_cts = foldIrreds (:) (inert_irreds inerts) $ foldDicts (:) (inert_dicts inerts) $ foldFunEqs (++) (inert_funeqs inerts) $ foldDVarEnv (++) [] (inert_eqs inerts) ; return (filter isGivenCt all_cts) } getPendingGivenScs :: TcS [Ct] -- Find all inert Given dictionaries, or quantified constraints, -- whose cc_pend_sc flag is True -- and that belong to the current level -- Set their cc_pend_sc flag to False in the inert set, and return that Ct getPendingGivenScs = do { lvl <- getTcLevel ; updRetInertCans (get_sc_pending lvl) } get_sc_pending :: TcLevel -> InertCans -> ([Ct], InertCans) get_sc_pending this_lvl ic@(IC { inert_dicts = dicts, inert_insts = insts }) = assertPpr (all isGivenCt sc_pending) (ppr sc_pending) -- When getPendingScDics is called, -- there are never any Wanteds in the inert set (sc_pending, ic { inert_dicts = dicts', inert_insts = insts' }) where sc_pending = sc_pend_insts ++ sc_pend_dicts sc_pend_dicts = foldDicts get_pending dicts [] dicts' = foldr add dicts sc_pend_dicts (sc_pend_insts, insts') = mapAccumL get_pending_inst [] insts get_pending :: Ct -> [Ct] -> [Ct] -- Get dicts with cc_pend_sc = True -- but flipping the flag get_pending dict dicts | Just dict' <- isPendingScDict dict , belongs_to_this_level (ctEvidence dict) = dict' : dicts | otherwise = dicts add :: Ct -> DictMap Ct -> DictMap Ct add ct@(CDictCan { cc_class = cls, cc_tyargs = tys }) dicts = addDict dicts cls tys ct add ct _ = pprPanic "getPendingScDicts" (ppr ct) get_pending_inst :: [Ct] -> QCInst -> ([Ct], QCInst) get_pending_inst cts qci@(QCI { qci_ev = ev }) | Just qci' <- isPendingScInst qci , belongs_to_this_level ev = (CQuantCan qci' : cts, qci') | otherwise = (cts, qci) belongs_to_this_level ev = ctLocLevel (ctEvLoc ev) == this_lvl -- We only want Givens from this level; see (3a) in -- Note [The superclass story] in GHC.Tc.Solver.Canonical getUnsolvedInerts :: TcS ( Bag Implication , Cts ) -- All simple constraints -- Return all the unsolved [Wanted] constraints -- -- Post-condition: the returned simple constraints are all fully zonked -- (because they come from the inert set) -- the unsolved implics may not be getUnsolvedInerts = do { IC { inert_eqs = tv_eqs , inert_funeqs = fun_eqs , inert_irreds = irreds , inert_dicts = idicts } <- getInertCans ; let unsolved_tv_eqs = foldTyEqs add_if_unsolved tv_eqs emptyCts unsolved_fun_eqs = foldFunEqs add_if_unsolveds fun_eqs emptyCts unsolved_irreds = Bag.filterBag isWantedCt irreds unsolved_dicts = foldDicts add_if_unsolved idicts emptyCts unsolved_others = unionManyBags [ unsolved_irreds , unsolved_dicts ] ; implics <- getWorkListImplics ; traceTcS "getUnsolvedInerts" $ vcat [ text " tv eqs =" <+> ppr unsolved_tv_eqs , text "fun eqs =" <+> ppr unsolved_fun_eqs , text "others =" <+> ppr unsolved_others , text "implics =" <+> ppr implics ] ; return ( implics, unsolved_tv_eqs `unionBags` unsolved_fun_eqs `unionBags` unsolved_others) } where add_if_unsolved :: Ct -> Cts -> Cts add_if_unsolved ct cts | isWantedCt ct = ct `consCts` cts | otherwise = cts add_if_unsolveds :: EqualCtList -> Cts -> Cts add_if_unsolveds new_cts old_cts = foldr add_if_unsolved old_cts new_cts getHasGivenEqs :: TcLevel -- TcLevel of this implication -> TcS ( HasGivenEqs -- are there Given equalities? , Cts ) -- Insoluble equalities arising from givens -- See Note [Tracking Given equalities] in GHC.Tc.Solver.InertSet getHasGivenEqs tclvl = do { inerts@(IC { inert_irreds = irreds , inert_given_eqs = given_eqs , inert_given_eq_lvl = ge_lvl }) <- getInertCans ; let given_insols = filterBag insoluble_given_equality irreds -- Specifically includes ones that originated in some -- outer context but were refined to an insoluble by -- a local equality; so no level-check needed -- See Note [HasGivenEqs] in GHC.Tc.Types.Constraint, and -- Note [Tracking Given equalities] in GHC.Tc.Solver.InertSet has_ge | ge_lvl == tclvl = MaybeGivenEqs | given_eqs = LocalGivenEqs | otherwise = NoGivenEqs ; traceTcS "getHasGivenEqs" $ vcat [ text "given_eqs:" <+> ppr given_eqs , text "ge_lvl:" <+> ppr ge_lvl , text "ambient level:" <+> ppr tclvl , text "Inerts:" <+> ppr inerts , text "Insols:" <+> ppr given_insols] ; return (has_ge, given_insols) } where insoluble_given_equality ct = insolubleEqCt ct && isGivenCt ct removeInertCts :: [Ct] -> InertCans -> InertCans -- ^ Remove inert constraints from the 'InertCans', for use when a -- typechecker plugin wishes to discard a given. removeInertCts cts icans = foldl' removeInertCt icans cts removeInertCt :: InertCans -> Ct -> InertCans removeInertCt is ct = case ct of CDictCan { cc_class = cl, cc_tyargs = tys } -> is { inert_dicts = delDict (inert_dicts is) cl tys } CEqCan { cc_lhs = lhs, cc_rhs = rhs } -> delEq is lhs rhs CIrredCan {} -> is { inert_irreds = filterBag (not . eqCt ct) $ inert_irreds is } CQuantCan {} -> panic "removeInertCt: CQuantCan" CNonCanonical {} -> panic "removeInertCt: CNonCanonical" eqCt :: Ct -> Ct -> Bool -- Equality via ctEvId eqCt c c' = ctEvId c == ctEvId c' -- | Looks up a family application in the inerts. lookupFamAppInert :: (CtFlavourRole -> Bool) -- can it rewrite the target? -> TyCon -> [Type] -> TcS (Maybe (Reduction, CtFlavourRole)) lookupFamAppInert rewrite_pred fam_tc tys = do { IS { inert_cans = IC { inert_funeqs = inert_funeqs } } <- getTcSInerts ; return (lookup_inerts inert_funeqs) } where lookup_inerts inert_funeqs | Just ecl <- findFunEq inert_funeqs fam_tc tys , Just (CEqCan { cc_ev = ctev, cc_rhs = rhs }) <- find (rewrite_pred . ctFlavourRole) ecl = Just (mkReduction (ctEvCoercion ctev) rhs, ctEvFlavourRole ctev) | otherwise = Nothing lookupInInerts :: CtLoc -> TcPredType -> TcS (Maybe CtEvidence) -- Is this exact predicate type cached in the solved or canonicals of the InertSet? lookupInInerts loc pty | ClassPred cls tys <- classifyPredType pty = do { inerts <- getTcSInerts ; return (lookupSolvedDict inerts loc cls tys `mplus` fmap ctEvidence (lookupInertDict (inert_cans inerts) loc cls tys)) } | otherwise -- NB: No caching for equalities, IPs, holes, or errors = return Nothing -- | Look up a dictionary inert. lookupInertDict :: InertCans -> CtLoc -> Class -> [Type] -> Maybe Ct lookupInertDict (IC { inert_dicts = dicts }) loc cls tys = case findDict dicts loc cls tys of Just ct -> Just ct _ -> Nothing -- | Look up a solved inert. lookupSolvedDict :: InertSet -> CtLoc -> Class -> [Type] -> Maybe CtEvidence -- Returns just if exactly this predicate type exists in the solved. lookupSolvedDict (IS { inert_solved_dicts = solved }) loc cls tys = case findDict solved loc cls tys of Just ev -> Just ev _ -> Nothing --------------------------- lookupFamAppCache :: TyCon -> [Type] -> TcS (Maybe Reduction) lookupFamAppCache fam_tc tys = do { IS { inert_famapp_cache = famapp_cache } <- getTcSInerts ; case findFunEq famapp_cache fam_tc tys of result@(Just redn) -> do { traceTcS "famapp_cache hit" (vcat [ ppr (mkTyConApp fam_tc tys) , ppr redn ]) ; return result } Nothing -> return Nothing } extendFamAppCache :: TyCon -> [Type] -> Reduction -> TcS () -- NB: co :: rhs ~ F tys, to match expectations of rewriter extendFamAppCache tc xi_args stuff@(Reduction _ ty) = do { dflags <- getDynFlags ; when (gopt Opt_FamAppCache dflags) $ do { traceTcS "extendFamAppCache" (vcat [ ppr tc <+> ppr xi_args , ppr ty ]) ; updInertTcS $ \ is@(IS { inert_famapp_cache = fc }) -> is { inert_famapp_cache = insertFunEq fc tc xi_args stuff } } } -- Remove entries from the cache whose evidence mentions variables in the -- supplied set dropFromFamAppCache :: VarSet -> TcS () dropFromFamAppCache varset = do { inerts@(IS { inert_famapp_cache = famapp_cache }) <- getTcSInerts ; let filtered = filterTcAppMap check famapp_cache ; setTcSInerts $ inerts { inert_famapp_cache = filtered } } where check :: Reduction -> Bool check redn = not (anyFreeVarsOfCo (`elemVarSet` varset) $ reductionCoercion redn) {- ********************************************************************* * * Irreds * * ********************************************************************* -} foldIrreds :: (Ct -> b -> b) -> Cts -> b -> b foldIrreds k irreds z = foldr k z irreds {- ************************************************************************ * * * The TcS solver monad * * * ************************************************************************ Note [The TcS monad] ~~~~~~~~~~~~~~~~~~~~ The TcS monad is a weak form of the main Tc monad All you can do is * fail * allocate new variables * fill in evidence variables Filling in a dictionary evidence variable means to create a binding for it, so TcS carries a mutable location where the binding can be added. This is initialised from the innermost implication constraint. -} data TcSEnv = TcSEnv { tcs_ev_binds :: EvBindsVar, tcs_unified :: IORef Int, -- The number of unification variables we have filled -- The important thing is whether it is non-zero tcs_unif_lvl :: IORef (Maybe TcLevel), -- The Unification Level Flag -- Outermost level at which we have unified a meta tyvar -- Starts at Nothing, then (Just i), then (Just j) where j TcM a } deriving (Functor) -- | Smart constructor for 'TcS', as describe in Note [The one-shot state -- monad trick] in "GHC.Utils.Monad". mkTcS :: (TcSEnv -> TcM a) -> TcS a mkTcS f = TcS (oneShot f) instance Applicative TcS where pure x = mkTcS $ \_ -> return x (<*>) = ap instance Monad TcS where m >>= k = mkTcS $ \ebs -> do unTcS m ebs >>= (\r -> unTcS (k r) ebs) instance MonadIO TcS where liftIO act = TcS $ \_env -> liftIO act instance MonadFail TcS where fail err = mkTcS $ \_ -> fail err instance MonadUnique TcS where getUniqueSupplyM = wrapTcS getUniqueSupplyM instance HasModule TcS where getModule = wrapTcS getModule instance MonadThings TcS where lookupThing n = wrapTcS (lookupThing n) -- Basic functionality -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ wrapTcS :: TcM a -> TcS a -- Do not export wrapTcS, because it promotes an arbitrary TcM to TcS, -- and TcS is supposed to have limited functionality wrapTcS action = mkTcS $ \_env -> action -- a TcM action will not use the TcEvBinds wrap2TcS :: (TcM a -> TcM a) -> TcS a -> TcS a wrap2TcS fn (TcS thing) = mkTcS $ \env -> fn (thing env) wrapErrTcS :: TcM a -> TcS a -- The thing wrapped should just fail -- There's no static check; it's up to the user -- Having a variant for each error message is too painful wrapErrTcS = wrapTcS wrapWarnTcS :: TcM a -> TcS a -- The thing wrapped should just add a warning, or no-op -- There's no static check; it's up to the user wrapWarnTcS = wrapTcS panicTcS :: SDoc -> TcS a failTcS :: TcRnMessage -> TcS a warnTcS, addErrTcS :: TcRnMessage -> TcS () failTcS = wrapTcS . TcM.failWith warnTcS msg = wrapTcS (TcM.addDiagnostic msg) addErrTcS = wrapTcS . TcM.addErr panicTcS doc = pprPanic "GHC.Tc.Solver.Canonical" doc traceTcS :: String -> SDoc -> TcS () traceTcS herald doc = wrapTcS (TcM.traceTc herald doc) {-# INLINE traceTcS #-} -- see Note [INLINE conditional tracing utilities] runTcPluginTcS :: TcPluginM a -> TcS a runTcPluginTcS = wrapTcS . runTcPluginM instance HasDynFlags TcS where getDynFlags = wrapTcS getDynFlags getGlobalRdrEnvTcS :: TcS GlobalRdrEnv getGlobalRdrEnvTcS = wrapTcS TcM.getGlobalRdrEnv bumpStepCountTcS :: TcS () bumpStepCountTcS = mkTcS $ \env -> do { let ref = tcs_count env ; n <- TcM.readTcRef ref ; TcM.writeTcRef ref (n+1) } csTraceTcS :: SDoc -> TcS () csTraceTcS doc = wrapTcS $ csTraceTcM (return doc) {-# INLINE csTraceTcS #-} -- see Note [INLINE conditional tracing utilities] traceFireTcS :: CtEvidence -> SDoc -> TcS () -- Dump a rule-firing trace traceFireTcS ev doc = mkTcS $ \env -> csTraceTcM $ do { n <- TcM.readTcRef (tcs_count env) ; tclvl <- TcM.getTcLevel ; return (hang (text "Step" <+> int n <> brackets (text "l:" <> ppr tclvl <> comma <> text "d:" <> ppr (ctLocDepth (ctEvLoc ev))) <+> doc <> colon) 4 (ppr ev)) } {-# INLINE traceFireTcS #-} -- see Note [INLINE conditional tracing utilities] csTraceTcM :: TcM SDoc -> TcM () -- Constraint-solver tracing, -ddump-cs-trace csTraceTcM mk_doc = do { logger <- getLogger ; when ( logHasDumpFlag logger Opt_D_dump_cs_trace || logHasDumpFlag logger Opt_D_dump_tc_trace) ( do { msg <- mk_doc ; TcM.dumpTcRn False Opt_D_dump_cs_trace "" FormatText msg }) } {-# INLINE csTraceTcM #-} -- see Note [INLINE conditional tracing utilities] runTcS :: TcS a -- What to run -> TcM (a, EvBindMap) runTcS tcs = do { ev_binds_var <- TcM.newTcEvBinds ; res <- runTcSWithEvBinds ev_binds_var tcs ; ev_binds <- TcM.getTcEvBindsMap ev_binds_var ; return (res, ev_binds) } -- | This variant of 'runTcS' will immediatley fail upon encountering an -- insoluble ct. See Note [Speeding up valid hole-fits]. Its one usage -- site does not need the ev_binds, so we do not return them. runTcSEarlyAbort :: TcS a -> TcM a runTcSEarlyAbort tcs = do { ev_binds_var <- TcM.newTcEvBinds ; runTcSWithEvBinds' True True ev_binds_var tcs } -- | This can deal only with equality constraints. runTcSEqualities :: TcS a -> TcM a runTcSEqualities thing_inside = do { ev_binds_var <- TcM.newNoTcEvBinds ; runTcSWithEvBinds ev_binds_var thing_inside } -- | A variant of 'runTcS' that takes and returns an 'InertSet' for -- later resumption of the 'TcS' session. runTcSInerts :: InertSet -> TcS a -> TcM (a, InertSet) runTcSInerts inerts tcs = do ev_binds_var <- TcM.newTcEvBinds runTcSWithEvBinds' False False ev_binds_var $ do setTcSInerts inerts a <- tcs new_inerts <- getTcSInerts return (a, new_inerts) runTcSWithEvBinds :: EvBindsVar -> TcS a -> TcM a runTcSWithEvBinds = runTcSWithEvBinds' True False runTcSWithEvBinds' :: Bool -- ^ Restore type variable cycles afterwards? -- Don't if you want to reuse the InertSet. -- See also Note [Type equality cycles] -- in GHC.Tc.Solver.Canonical -> Bool -> EvBindsVar -> TcS a -> TcM a runTcSWithEvBinds' restore_cycles abort_on_insoluble ev_binds_var tcs = do { unified_var <- TcM.newTcRef 0 ; step_count <- TcM.newTcRef 0 ; inert_var <- TcM.newTcRef emptyInert ; wl_var <- TcM.newTcRef emptyWorkList ; unif_lvl_var <- TcM.newTcRef Nothing ; let env = TcSEnv { tcs_ev_binds = ev_binds_var , tcs_unified = unified_var , tcs_unif_lvl = unif_lvl_var , tcs_count = step_count , tcs_inerts = inert_var , tcs_abort_on_insoluble = abort_on_insoluble , tcs_worklist = wl_var } -- Run the computation ; res <- unTcS tcs env ; count <- TcM.readTcRef step_count ; when (count > 0) $ csTraceTcM $ return (text "Constraint solver steps =" <+> int count) ; when restore_cycles $ do { inert_set <- TcM.readTcRef inert_var ; restoreTyVarCycles inert_set } #if defined(DEBUG) ; ev_binds <- TcM.getTcEvBindsMap ev_binds_var ; checkForCyclicBinds ev_binds #endif ; return res } ---------------------------- #if defined(DEBUG) checkForCyclicBinds :: EvBindMap -> TcM () checkForCyclicBinds ev_binds_map | null cycles = return () | null coercion_cycles = TcM.traceTc "Cycle in evidence binds" $ ppr cycles | otherwise = pprPanic "Cycle in coercion bindings" $ ppr coercion_cycles where ev_binds = evBindMapBinds ev_binds_map cycles :: [[EvBind]] cycles = [c | CyclicSCC c <- stronglyConnCompFromEdgedVerticesUniq edges] coercion_cycles = [c | c <- cycles, any is_co_bind c] is_co_bind (EvBind { eb_lhs = b }) = isEqPrimPred (varType b) edges :: [ Node EvVar EvBind ] edges = [ DigraphNode bind bndr (nonDetEltsUniqSet (evVarsOfTerm rhs)) | bind@(EvBind { eb_lhs = bndr, eb_rhs = rhs}) <- bagToList ev_binds ] -- It's OK to use nonDetEltsUFM here as -- stronglyConnCompFromEdgedVertices is still deterministic even -- if the edges are in nondeterministic order as explained in -- Note [Deterministic SCC] in GHC.Data.Graph.Directed. #endif ---------------------------- setEvBindsTcS :: EvBindsVar -> TcS a -> TcS a setEvBindsTcS ref (TcS thing_inside) = TcS $ \ env -> thing_inside (env { tcs_ev_binds = ref }) nestImplicTcS :: EvBindsVar -> TcLevel -> TcS a -> TcS a nestImplicTcS ref inner_tclvl (TcS thing_inside) = TcS $ \ TcSEnv { tcs_unified = unified_var , tcs_inerts = old_inert_var , tcs_count = count , tcs_unif_lvl = unif_lvl , tcs_abort_on_insoluble = abort_on_insoluble } -> do { inerts <- TcM.readTcRef old_inert_var ; let nest_inert = inerts { inert_cycle_breakers = pushCycleBreakerVarStack (inert_cycle_breakers inerts) , inert_cans = (inert_cans inerts) { inert_given_eqs = False } } -- All other InertSet fields are inherited ; new_inert_var <- TcM.newTcRef nest_inert ; new_wl_var <- TcM.newTcRef emptyWorkList ; let nest_env = TcSEnv { tcs_count = count -- Inherited , tcs_unif_lvl = unif_lvl -- Inherited , tcs_ev_binds = ref , tcs_unified = unified_var , tcs_inerts = new_inert_var , tcs_abort_on_insoluble = abort_on_insoluble , tcs_worklist = new_wl_var } ; res <- TcM.setTcLevel inner_tclvl $ thing_inside nest_env ; out_inert_set <- TcM.readTcRef new_inert_var ; restoreTyVarCycles out_inert_set #if defined(DEBUG) -- Perform a check that the thing_inside did not cause cycles ; ev_binds <- TcM.getTcEvBindsMap ref ; checkForCyclicBinds ev_binds #endif ; return res } nestTcS :: TcS a -> TcS a -- Use the current untouchables, augmenting the current -- evidence bindings, and solved dictionaries -- But have no effect on the InertCans, or on the inert_famapp_cache -- (we want to inherit the latter from processing the Givens) nestTcS (TcS thing_inside) = TcS $ \ env@(TcSEnv { tcs_inerts = inerts_var }) -> do { inerts <- TcM.readTcRef inerts_var ; new_inert_var <- TcM.newTcRef inerts ; new_wl_var <- TcM.newTcRef emptyWorkList ; let nest_env = env { tcs_inerts = new_inert_var , tcs_worklist = new_wl_var } ; res <- thing_inside nest_env ; new_inerts <- TcM.readTcRef new_inert_var -- we want to propagate the safe haskell failures ; let old_ic = inert_cans inerts new_ic = inert_cans new_inerts nxt_ic = old_ic { inert_safehask = inert_safehask new_ic } ; TcM.writeTcRef inerts_var -- See Note [Propagate the solved dictionaries] (inerts { inert_solved_dicts = inert_solved_dicts new_inerts , inert_cans = nxt_ic }) ; return res } emitImplicationTcS :: TcLevel -> SkolemInfoAnon -> [TcTyVar] -- Skolems -> [EvVar] -- Givens -> Cts -- Wanteds -> TcS TcEvBinds -- Add an implication to the TcS monad work-list emitImplicationTcS new_tclvl skol_info skol_tvs givens wanteds = do { let wc = emptyWC { wc_simple = wanteds } ; imp <- wrapTcS $ do { ev_binds_var <- TcM.newTcEvBinds ; imp <- TcM.newImplication ; return (imp { ic_tclvl = new_tclvl , ic_skols = skol_tvs , ic_given = givens , ic_wanted = wc , ic_binds = ev_binds_var , ic_info = skol_info }) } ; emitImplication imp ; return (TcEvBinds (ic_binds imp)) } emitTvImplicationTcS :: TcLevel -> SkolemInfoAnon -> [TcTyVar] -- Skolems -> Cts -- Wanteds -> TcS () -- Just like emitImplicationTcS but no givens and no bindings emitTvImplicationTcS new_tclvl skol_info skol_tvs wanteds = do { let wc = emptyWC { wc_simple = wanteds } ; imp <- wrapTcS $ do { ev_binds_var <- TcM.newNoTcEvBinds ; imp <- TcM.newImplication ; return (imp { ic_tclvl = new_tclvl , ic_skols = skol_tvs , ic_wanted = wc , ic_binds = ev_binds_var , ic_info = skol_info }) } ; emitImplication imp } {- Note [Propagate the solved dictionaries] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It's really quite important that nestTcS does not discard the solved dictionaries from the thing_inside. Consider Eq [a] forall b. empty => Eq [a] We solve the simple (Eq [a]), under nestTcS, and then turn our attention to the implications. It's definitely fine to use the solved dictionaries on the inner implications, and it can make a significant performance difference if you do so. -} -- Getters and setters of GHC.Tc.Utils.Env fields -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- Getter of inerts and worklist getTcSInertsRef :: TcS (IORef InertSet) getTcSInertsRef = TcS (return . tcs_inerts) getTcSWorkListRef :: TcS (IORef WorkList) getTcSWorkListRef = TcS (return . tcs_worklist) getTcSInerts :: TcS InertSet getTcSInerts = getTcSInertsRef >>= readTcRef setTcSInerts :: InertSet -> TcS () setTcSInerts ics = do { r <- getTcSInertsRef; writeTcRef r ics } getWorkListImplics :: TcS (Bag Implication) getWorkListImplics = do { wl_var <- getTcSWorkListRef ; wl_curr <- readTcRef wl_var ; return (wl_implics wl_curr) } pushLevelNoWorkList :: SDoc -> TcS a -> TcS (TcLevel, a) -- Push the level and run thing_inside -- However, thing_inside should not generate any work items #if defined(DEBUG) pushLevelNoWorkList err_doc (TcS thing_inside) = TcS (\env -> TcM.pushTcLevelM $ thing_inside (env { tcs_worklist = wl_panic }) ) where wl_panic = pprPanic "GHC.Tc.Solver.Monad.buildImplication" err_doc -- This panic checks that the thing-inside -- does not emit any work-list constraints #else pushLevelNoWorkList _ (TcS thing_inside) = TcS (\env -> TcM.pushTcLevelM (thing_inside env)) -- Don't check #endif updWorkListTcS :: (WorkList -> WorkList) -> TcS () updWorkListTcS f = do { wl_var <- getTcSWorkListRef ; updTcRef wl_var f } emitWorkNC :: [CtEvidence] -> TcS () emitWorkNC evs | null evs = return () | otherwise = emitWork (map mkNonCanonical evs) emitWork :: [Ct] -> TcS () emitWork [] = return () -- avoid printing, among other work emitWork cts = do { traceTcS "Emitting fresh work" (vcat (map ppr cts)) ; updWorkListTcS (extendWorkListCts cts) } emitImplication :: Implication -> TcS () emitImplication implic = updWorkListTcS (extendWorkListImplic implic) newTcRef :: a -> TcS (TcRef a) newTcRef x = wrapTcS (TcM.newTcRef x) readTcRef :: TcRef a -> TcS a readTcRef ref = wrapTcS (TcM.readTcRef ref) writeTcRef :: TcRef a -> a -> TcS () writeTcRef ref val = wrapTcS (TcM.writeTcRef ref val) updTcRef :: TcRef a -> (a->a) -> TcS () updTcRef ref upd_fn = wrapTcS (TcM.updTcRef ref upd_fn) getTcEvBindsVar :: TcS EvBindsVar getTcEvBindsVar = TcS (return . tcs_ev_binds) getTcLevel :: TcS TcLevel getTcLevel = wrapTcS TcM.getTcLevel getTcEvTyCoVars :: EvBindsVar -> TcS TyCoVarSet getTcEvTyCoVars ev_binds_var = wrapTcS $ TcM.getTcEvTyCoVars ev_binds_var getTcEvBindsMap :: EvBindsVar -> TcS EvBindMap getTcEvBindsMap ev_binds_var = wrapTcS $ TcM.getTcEvBindsMap ev_binds_var setTcEvBindsMap :: EvBindsVar -> EvBindMap -> TcS () setTcEvBindsMap ev_binds_var binds = wrapTcS $ TcM.setTcEvBindsMap ev_binds_var binds unifyTyVar :: TcTyVar -> TcType -> TcS () -- Unify a meta-tyvar with a type -- We keep track of how many unifications have happened in tcs_unified, -- -- We should never unify the same variable twice! unifyTyVar tv ty = assertPpr (isMetaTyVar tv) (ppr tv) $ TcS $ \ env -> do { TcM.traceTc "unifyTyVar" (ppr tv <+> text ":=" <+> ppr ty) ; TcM.writeMetaTyVar tv ty ; TcM.updTcRef (tcs_unified env) (+1) } reportUnifications :: TcS a -> TcS (Int, a) reportUnifications (TcS thing_inside) = TcS $ \ env -> do { inner_unified <- TcM.newTcRef 0 ; res <- thing_inside (env { tcs_unified = inner_unified }) ; n_unifs <- TcM.readTcRef inner_unified ; TcM.updTcRef (tcs_unified env) (+ n_unifs) ; return (n_unifs, res) } data TouchabilityTestResult -- See Note [Solve by unification] in GHC.Tc.Solver.Interact -- which points out that having TouchableSameLevel is just an optimisation; -- we could manage with TouchableOuterLevel alone (suitably renamed) = TouchableSameLevel | TouchableOuterLevel [TcTyVar] -- Promote these TcLevel -- ..to this level | Untouchable instance Outputable TouchabilityTestResult where ppr TouchableSameLevel = text "TouchableSameLevel" ppr (TouchableOuterLevel tvs lvl) = text "TouchableOuterLevel" <> parens (ppr lvl <+> ppr tvs) ppr Untouchable = text "Untouchable" touchabilityTest :: CtFlavour -> TcTyVar -> TcType -> TcS TouchabilityTestResult -- This is the key test for untouchability: -- See Note [Unification preconditions] in GHC.Tc.Utils.Unify -- and Note [Solve by unification] in GHC.Tc.Solver.Interact touchabilityTest flav tv1 rhs | flav /= Given -- See Note [Do not unify Givens] , MetaTv { mtv_tclvl = tv_lvl, mtv_info = info } <- tcTyVarDetails tv1 = do { can_continue_solving <- wrapTcS $ startSolvingByUnification info rhs ; if not can_continue_solving then return Untouchable else do { ambient_lvl <- getTcLevel ; given_eq_lvl <- getInnermostGivenEqLevel ; if | tv_lvl `sameDepthAs` ambient_lvl -> return TouchableSameLevel | tv_lvl `deeperThanOrSame` given_eq_lvl -- No intervening given equalities , all (does_not_escape tv_lvl) free_skols -- No skolem escapes -> return (TouchableOuterLevel free_metas tv_lvl) | otherwise -> return Untouchable } } | otherwise = return Untouchable where (free_metas, free_skols) = partition isPromotableMetaTyVar $ nonDetEltsUniqSet $ tyCoVarsOfType rhs does_not_escape tv_lvl fv | isTyVar fv = tv_lvl `deeperThanOrSame` tcTyVarLevel fv | otherwise = True -- Coercion variables are not an escape risk -- If an implication binds a coercion variable, it'll have equalities, -- so the "intervening given equalities" test above will catch it -- Coercion holes get filled with coercions, so again no problem. {- Note [Do not unify Givens] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this GADT match data T a where T1 :: T Int ... f x = case x of T1 -> True ... So we get f :: T alpha[1] -> beta[1] x :: T alpha[1] and from the T1 branch we get the implication forall[2] (alpha[1] ~ Int) => beta[1] ~ Bool Now, clearly we don't want to unify alpha:=Int! Yet at the moment we process [G] alpha[1] ~ Int, we don't have any given-equalities in the inert set, and hence there are no given equalities to make alpha untouchable. NB: if it were alpha[2] ~ Int, this argument wouldn't hold. But that never happens: invariant (GivenInv) in Note [TcLevel invariants] in GHC.Tc.Utils.TcType. Simple solution: never unify in Givens! -} getDefaultInfo :: TcS ([Type], (Bool, Bool)) getDefaultInfo = wrapTcS TcM.tcGetDefaultTys getWorkList :: TcS WorkList getWorkList = do { wl_var <- getTcSWorkListRef ; wrapTcS (TcM.readTcRef wl_var) } selectNextWorkItem :: TcS (Maybe Ct) -- Pick which work item to do next -- See Note [Prioritise equalities] selectNextWorkItem = do { wl_var <- getTcSWorkListRef ; wl <- readTcRef wl_var ; case selectWorkItem wl of { Nothing -> return Nothing ; Just (ct, new_wl) -> do { -- checkReductionDepth (ctLoc ct) (ctPred ct) -- This is done by GHC.Tc.Solver.Interact.chooseInstance ; writeTcRef wl_var new_wl ; return (Just ct) } } } -- Just get some environments needed for instance looking up and matching -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ getInstEnvs :: TcS InstEnvs getInstEnvs = wrapTcS $ TcM.tcGetInstEnvs getFamInstEnvs :: TcS (FamInstEnv, FamInstEnv) getFamInstEnvs = wrapTcS $ FamInst.tcGetFamInstEnvs getTopEnv :: TcS HscEnv getTopEnv = wrapTcS $ TcM.getTopEnv getGblEnv :: TcS TcGblEnv getGblEnv = wrapTcS $ TcM.getGblEnv getLclEnv :: TcS TcLclEnv getLclEnv = wrapTcS $ TcM.getLclEnv setLclEnv :: TcLclEnv -> TcS a -> TcS a setLclEnv env = wrap2TcS (TcM.setLclEnv env) tcLookupClass :: Name -> TcS Class tcLookupClass c = wrapTcS $ TcM.tcLookupClass c tcLookupId :: Name -> TcS Id tcLookupId n = wrapTcS $ TcM.tcLookupId n -- Setting names as used (used in the deriving of Coercible evidence) -- Too hackish to expose it to TcS? In that case somehow extract the used -- constructors from the result of solveInteract addUsedGREs :: [GlobalRdrElt] -> TcS () addUsedGREs gres = wrapTcS $ TcM.addUsedGREs gres addUsedGRE :: Bool -> GlobalRdrElt -> TcS () addUsedGRE warn_if_deprec gre = wrapTcS $ TcM.addUsedGRE warn_if_deprec gre keepAlive :: Name -> TcS () keepAlive = wrapTcS . TcM.keepAlive -- Various smaller utilities [TODO, maybe will be absorbed in the instance matcher] -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ checkWellStagedDFun :: CtLoc -> InstanceWhat -> PredType -> TcS () -- Check that we do not try to use an instance before it is available. E.g. -- instance Eq T where ... -- f x = $( ... (\(p::T) -> p == p)... ) -- Here we can't use the equality function from the instance in the splice checkWellStagedDFun loc what pred = do mbind_lvl <- checkWellStagedInstanceWhat what case mbind_lvl of Just bind_lvl | bind_lvl > impLevel -> wrapTcS $ TcM.setCtLocM loc $ do { use_stage <- TcM.getStage ; TcM.checkWellStaged pp_thing bind_lvl (thLevel use_stage) } _ -> return () where pp_thing = text "instance for" <+> quotes (ppr pred) -- | Returns the ThLevel of evidence for the solved constraint (if it has evidence) -- See Note [Well-staged instance evidence] checkWellStagedInstanceWhat :: InstanceWhat -> TcS (Maybe ThLevel) checkWellStagedInstanceWhat what | TopLevInstance { iw_dfun_id = dfun_id } <- what = return $ Just (TcM.topIdLvl dfun_id) | BuiltinTypeableInstance tc <- what = do cur_mod <- extractModule <$> getGblEnv return $ Just (if nameIsLocalOrFrom cur_mod (tyConName tc) then outerLevel else impLevel) | otherwise = return Nothing {- Note [Well-staged instance evidence] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Evidence for instances must obey the same level restrictions as normal bindings. In particular, it is forbidden to use an instance in a top-level splice in the module which the instance is defined. This is because the evidence is bound at the top-level and top-level definitions are forbidden from being using in top-level splices in the same module. For example, suppose you have a function.. foo :: Show a => Code Q a -> Code Q () then the following program is disallowed, ``` data T a = T a deriving (Show) main :: IO () main = let x = $$(foo [|| T () ||]) in return () ``` because the `foo` function (used in a top-level splice) requires `Show T` evidence, which is defined at the top-level and therefore fails with an error that we have violated the stage restriction. ``` Main.hs:12:14: error: • GHC stage restriction: instance for ‘Show (T ())’ is used in a top-level splice, quasi-quote, or annotation, and must be imported, not defined locally • In the expression: foo [|| T () ||] In the Template Haskell splice $$(foo [|| T () ||]) In the expression: $$(foo [|| T () ||]) | 12 | let x = $$(foo [|| T () ||]) | ``` Solving a `Typeable (T t1 ...tn)` constraint generates code that relies on `$tcT`, the `TypeRep` for `T`; and we must check that this reference to `$tcT` is well staged. It's easy to know the stage of `$tcT`: for imported TyCons it will be `impLevel`, and for local TyCons it will be `toplevel`. Therefore the `InstanceWhat` type had to be extended with a special case for `Typeable`, which recorded the TyCon the evidence was for and could them be used to check that we were not attempting to evidence in a stage incorrect manner. -} pprEq :: TcType -> TcType -> SDoc pprEq ty1 ty2 = pprParendType ty1 <+> char '~' <+> pprParendType ty2 isFilledMetaTyVar_maybe :: TcTyVar -> TcS (Maybe Type) isFilledMetaTyVar_maybe tv = wrapTcS (TcM.isFilledMetaTyVar_maybe tv) isFilledMetaTyVar :: TcTyVar -> TcS Bool isFilledMetaTyVar tv = wrapTcS (TcM.isFilledMetaTyVar tv) zonkTyCoVarsAndFV :: TcTyCoVarSet -> TcS TcTyCoVarSet zonkTyCoVarsAndFV tvs = wrapTcS (TcM.zonkTyCoVarsAndFV tvs) zonkTyCoVarsAndFVList :: [TcTyCoVar] -> TcS [TcTyCoVar] zonkTyCoVarsAndFVList tvs = wrapTcS (TcM.zonkTyCoVarsAndFVList tvs) zonkCo :: Coercion -> TcS Coercion zonkCo = wrapTcS . TcM.zonkCo zonkTcType :: TcType -> TcS TcType zonkTcType ty = wrapTcS (TcM.zonkTcType ty) zonkTcTypes :: [TcType] -> TcS [TcType] zonkTcTypes tys = wrapTcS (TcM.zonkTcTypes tys) zonkTcTyVar :: TcTyVar -> TcS TcType zonkTcTyVar tv = wrapTcS (TcM.zonkTcTyVar tv) zonkSimples :: Cts -> TcS Cts zonkSimples cts = wrapTcS (TcM.zonkSimples cts) zonkWC :: WantedConstraints -> TcS WantedConstraints zonkWC wc = wrapTcS (TcM.zonkWC wc) zonkTyCoVarKind :: TcTyCoVar -> TcS TcTyCoVar zonkTyCoVarKind tv = wrapTcS (TcM.zonkTyCoVarKind tv) ---------------------------- pprKicked :: Int -> SDoc pprKicked 0 = empty pprKicked n = parens (int n <+> text "kicked out") {- ********************************************************************* * * * The Unification Level Flag * * * ********************************************************************* -} {- Note [The Unification Level Flag] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider a deep tree of implication constraints forall[1] a. -- Outer-implic C alpha[1] -- Simple forall[2] c. ....(C alpha[1]).... -- Implic-1 forall[2] b. ....(alpha[1] ~ Int).... -- Implic-2 The (C alpha) is insoluble until we know alpha. We solve alpha by unifying alpha:=Int somewhere deep inside Implic-2. But then we must try to solve the Outer-implic all over again. This time we can solve (C alpha) both in Outer-implic, and nested inside Implic-1. When should we iterate solving a level-n implication? Answer: if any unification of a tyvar at level n takes place in the ic_implics of that implication. * What if a unification takes place at level n-1? Then don't iterate level n, because we'll iterate level n-1, and that will in turn iterate level n. * What if a unification takes place at level n, in the ic_simples of level n? No need to track this, because the kick-out mechanism deals with it. (We can't drop kick-out in favour of iteration, because kick-out works for skolem-equalities, not just unifications.) So the monad-global Unification Level Flag, kept in tcs_unif_lvl keeps track of - Whether any unifications at all have taken place (Nothing => no unifications) - If so, what is the outermost level that has seen a unification (Just lvl) The iteration done in the simplify_loop/maybe_simplify_again loop in GHC.Tc.Solver. It helpful not to iterate unless there is a chance of progress. #8474 is an example: * There's a deeply-nested chain of implication constraints. ?x:alpha => ?y1:beta1 => ... ?yn:betan => [W] ?x:Int * From the innermost one we get a [W] alpha[1] ~ Int, so we can unify. * It's better not to iterate the inner implications, but go all the way out to level 1 before iterating -- because iterating level 1 will iterate the inner levels anyway. (In the olden days when we "floated" thse Derived constraints, this was much, much more important -- we got exponential behaviour, as each iteration produced the same Derived constraint.) -} resetUnificationFlag :: TcS Bool -- We are at ambient level i -- If the unification flag = Just i, reset it to Nothing and return True -- Otherwise leave it unchanged and return False resetUnificationFlag = TcS $ \env -> do { let ref = tcs_unif_lvl env ; ambient_lvl <- TcM.getTcLevel ; mb_lvl <- TcM.readTcRef ref ; TcM.traceTc "resetUnificationFlag" $ vcat [ text "ambient:" <+> ppr ambient_lvl , text "unif_lvl:" <+> ppr mb_lvl ] ; case mb_lvl of Nothing -> return False Just unif_lvl | ambient_lvl `strictlyDeeperThan` unif_lvl -> return False | otherwise -> do { TcM.writeTcRef ref Nothing ; return True } } setUnificationFlag :: TcLevel -> TcS () -- (setUnificationFlag i) sets the unification level to (Just i) -- unless it already is (Just j) where j <= i setUnificationFlag lvl = TcS $ \env -> do { let ref = tcs_unif_lvl env ; mb_lvl <- TcM.readTcRef ref ; case mb_lvl of Just unif_lvl | lvl `deeperThanOrSame` unif_lvl -> return () _ -> TcM.writeTcRef ref (Just lvl) } {- ********************************************************************* * * * Instantiation etc. * * ********************************************************************* -} -- Instantiations -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ instDFunType :: DFunId -> [DFunInstType] -> TcS ([TcType], TcThetaType) instDFunType dfun_id inst_tys = wrapTcS $ TcM.instDFunType dfun_id inst_tys newFlexiTcSTy :: Kind -> TcS TcType newFlexiTcSTy knd = wrapTcS (TcM.newFlexiTyVarTy knd) cloneMetaTyVar :: TcTyVar -> TcS TcTyVar cloneMetaTyVar tv = wrapTcS (TcM.cloneMetaTyVar tv) instFlexi :: [TKVar] -> TcS TCvSubst instFlexi = instFlexiX emptyTCvSubst instFlexiX :: TCvSubst -> [TKVar] -> TcS TCvSubst instFlexiX subst tvs = wrapTcS (foldlM instFlexiHelper subst tvs) instFlexiHelper :: TCvSubst -> TKVar -> TcM TCvSubst instFlexiHelper subst tv = do { uniq <- TcM.newUnique ; details <- TcM.newMetaDetails TauTv ; let name = setNameUnique (tyVarName tv) uniq kind = substTyUnchecked subst (tyVarKind tv) ty' = mkTyVarTy (mkTcTyVar name kind details) ; TcM.traceTc "instFlexi" (ppr ty') ; return (extendTvSubst subst tv ty') } matchGlobalInst :: DynFlags -> Bool -- True <=> caller is the short-cut solver -- See Note [Shortcut solving: overlap] -> Class -> [Type] -> TcS TcM.ClsInstResult matchGlobalInst dflags short_cut cls tys = wrapTcS (TcM.matchGlobalInst dflags short_cut cls tys) tcInstSkolTyVarsX :: SkolemInfo -> TCvSubst -> [TyVar] -> TcS (TCvSubst, [TcTyVar]) tcInstSkolTyVarsX skol_info subst tvs = wrapTcS $ TcM.tcInstSkolTyVarsX skol_info subst tvs -- Creating and setting evidence variables and CtFlavors -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ data MaybeNew = Fresh CtEvidence | Cached EvExpr isFresh :: MaybeNew -> Bool isFresh (Fresh {}) = True isFresh (Cached {}) = False freshGoals :: [MaybeNew] -> [CtEvidence] freshGoals mns = [ ctev | Fresh ctev <- mns ] getEvExpr :: MaybeNew -> EvExpr getEvExpr (Fresh ctev) = ctEvExpr ctev getEvExpr (Cached evt) = evt setEvBind :: EvBind -> TcS () setEvBind ev_bind = do { evb <- getTcEvBindsVar ; wrapTcS $ TcM.addTcEvBind evb ev_bind } -- | Mark variables as used filling a coercion hole useVars :: CoVarSet -> TcS () useVars co_vars = do { ev_binds_var <- getTcEvBindsVar ; let ref = ebv_tcvs ev_binds_var ; wrapTcS $ do { tcvs <- TcM.readTcRef ref ; let tcvs' = tcvs `unionVarSet` co_vars ; TcM.writeTcRef ref tcvs' } } -- | Equalities only setWantedEq :: HasDebugCallStack => TcEvDest -> Coercion -> TcS () setWantedEq (HoleDest hole) co = do { useVars (coVarsOfCo co) ; fillCoercionHole hole co } setWantedEq (EvVarDest ev) _ = pprPanic "setWantedEq: EvVarDest" (ppr ev) -- | Good for both equalities and non-equalities setWantedEvTerm :: TcEvDest -> EvTerm -> TcS () setWantedEvTerm (HoleDest hole) tm | Just co <- evTermCoercion_maybe tm = do { useVars (coVarsOfCo co) ; fillCoercionHole hole co } | otherwise = -- See Note [Yukky eq_sel for a HoleDest] do { let co_var = coHoleCoVar hole ; setEvBind (mkWantedEvBind co_var tm) ; fillCoercionHole hole (mkTcCoVarCo co_var) } setWantedEvTerm (EvVarDest ev_id) tm = setEvBind (mkWantedEvBind ev_id tm) {- Note [Yukky eq_sel for a HoleDest] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ How can it be that a Wanted with HoleDest gets evidence that isn't just a coercion? i.e. evTermCoercion_maybe returns Nothing. Consider [G] forall a. blah => a ~ T [W] S ~# T Then doTopReactEqPred carefully looks up the (boxed) constraint (S ~ T) in the quantified constraints, and wraps the (boxed) evidence it gets back in an eq_sel to extract the unboxed (S ~# T). We can't put that term into a coercion, so we add a value binding h = eq_sel (...) and the coercion variable h to fill the coercion hole. We even re-use the CoHole's Id for this binding! Yuk! -} fillCoercionHole :: CoercionHole -> Coercion -> TcS () fillCoercionHole hole co = do { wrapTcS $ TcM.fillCoercionHole hole co ; kickOutAfterFillingCoercionHole hole } setEvBindIfWanted :: CtEvidence -> EvTerm -> TcS () setEvBindIfWanted ev tm = case ev of CtWanted { ctev_dest = dest } -> setWantedEvTerm dest tm _ -> return () newTcEvBinds :: TcS EvBindsVar newTcEvBinds = wrapTcS TcM.newTcEvBinds newNoTcEvBinds :: TcS EvBindsVar newNoTcEvBinds = wrapTcS TcM.newNoTcEvBinds newEvVar :: TcPredType -> TcS EvVar newEvVar pred = wrapTcS (TcM.newEvVar pred) newGivenEvVar :: CtLoc -> (TcPredType, EvTerm) -> TcS CtEvidence -- Make a new variable of the given PredType, -- immediately bind it to the given term -- and return its CtEvidence -- See Note [Bind new Givens immediately] in GHC.Tc.Types.Constraint newGivenEvVar loc (pred, rhs) = do { new_ev <- newBoundEvVarId pred rhs ; return (CtGiven { ctev_pred = pred, ctev_evar = new_ev, ctev_loc = loc }) } -- | Make a new 'Id' of the given type, bound (in the monad's EvBinds) to the -- given term newBoundEvVarId :: TcPredType -> EvTerm -> TcS EvVar newBoundEvVarId pred rhs = do { new_ev <- newEvVar pred ; setEvBind (mkGivenEvBind new_ev rhs) ; return new_ev } newGivenEvVars :: CtLoc -> [(TcPredType, EvTerm)] -> TcS [CtEvidence] newGivenEvVars loc pts = mapM (newGivenEvVar loc) pts emitNewWantedEq :: CtLoc -> RewriterSet -> Role -> TcType -> TcType -> TcS Coercion -- | Emit a new Wanted equality into the work-list emitNewWantedEq loc rewriters role ty1 ty2 = do { (ev, co) <- newWantedEq loc rewriters role ty1 ty2 ; updWorkListTcS (extendWorkListEq (mkNonCanonical ev)) ; return co } -- | Create a new Wanted constraint holding a coercion hole -- for an equality between the two types at the given 'Role'. newWantedEq :: CtLoc -> RewriterSet -> Role -> TcType -> TcType -> TcS (CtEvidence, Coercion) newWantedEq loc rewriters role ty1 ty2 = do { hole <- wrapTcS $ TcM.newCoercionHole pty ; traceTcS "Emitting new coercion hole" (ppr hole <+> dcolon <+> ppr pty) ; return ( CtWanted { ctev_pred = pty , ctev_dest = HoleDest hole , ctev_loc = loc , ctev_rewriters = rewriters } , mkHoleCo hole ) } where pty = mkPrimEqPredRole role ty1 ty2 -- | Create a new Wanted constraint holding an evidence variable. -- -- Don't use this for equality constraints: use 'newWantedEq' instead. newWantedEvVarNC :: CtLoc -> RewriterSet -> TcPredType -> TcS CtEvidence -- Don't look up in the solved/inerts; we know it's not there newWantedEvVarNC loc rewriters pty = do { new_ev <- newEvVar pty ; traceTcS "Emitting new wanted" (ppr new_ev <+> dcolon <+> ppr pty $$ pprCtLoc loc) ; return (CtWanted { ctev_pred = pty , ctev_dest = EvVarDest new_ev , ctev_loc = loc , ctev_rewriters = rewriters })} -- | Like 'newWantedEvVarNC', except it might look up in the inert set -- to see if an inert already exists, and uses that instead of creating -- a new Wanted constraint. -- -- Don't use this for equality constraints: this function is only for -- constraints with 'EvVarDest'. newWantedEvVar :: CtLoc -> RewriterSet -> TcPredType -> TcS MaybeNew -- For anything except ClassPred, this is the same as newWantedEvVarNC newWantedEvVar loc rewriters pty = assertPpr (not (isEqPrimPred pty)) (vcat [ text "newWantedEvVar: HoleDestPred" , text "pty:" <+> ppr pty ]) $ do { mb_ct <- lookupInInerts loc pty ; case mb_ct of Just ctev -> do { traceTcS "newWantedEvVar/cache hit" $ ppr ctev ; return $ Cached (ctEvExpr ctev) } _ -> do { ctev <- newWantedEvVarNC loc rewriters pty ; return (Fresh ctev) } } -- | Create a new Wanted constraint, potentially looking up -- non-equality constraints in the cache instead of creating -- a new one from scratch. -- -- Deals with both equality and non-equality constraints. newWanted :: CtLoc -> RewriterSet -> PredType -> TcS MaybeNew newWanted loc rewriters pty | Just (role, ty1, ty2) <- getEqPredTys_maybe pty = Fresh . fst <$> newWantedEq loc rewriters role ty1 ty2 | otherwise = newWantedEvVar loc rewriters pty -- | Create a new Wanted constraint. -- -- Deals with both equality and non-equality constraints. -- -- Does not attempt to re-use non-equality constraints that already -- exist in the inert set. newWantedNC :: CtLoc -> RewriterSet -> PredType -> TcS CtEvidence newWantedNC loc rewriters pty | Just (role, ty1, ty2) <- getEqPredTys_maybe pty = fst <$> newWantedEq loc rewriters role ty1 ty2 | otherwise = newWantedEvVarNC loc rewriters pty -- --------- Check done in GHC.Tc.Solver.Interact.selectNewWorkItem???? --------- -- | Checks if the depth of the given location is too much. Fails if -- it's too big, with an appropriate error message. checkReductionDepth :: CtLoc -> TcType -- ^ type being reduced -> TcS () checkReductionDepth loc ty = do { dflags <- getDynFlags ; when (subGoalDepthExceeded dflags (ctLocDepth loc)) $ wrapErrTcS $ solverDepthError loc ty } matchFam :: TyCon -> [Type] -> TcS (Maybe ReductionN) matchFam tycon args = wrapTcS $ matchFamTcM tycon args matchFamTcM :: TyCon -> [Type] -> TcM (Maybe ReductionN) -- Given (F tys) return (ty, co), where co :: F tys ~N ty matchFamTcM tycon args = do { fam_envs <- FamInst.tcGetFamInstEnvs ; let match_fam_result = reduceTyFamApp_maybe fam_envs Nominal tycon args ; TcM.traceTc "matchFamTcM" $ vcat [ text "Matching:" <+> ppr (mkTyConApp tycon args) , ppr_res match_fam_result ] ; return match_fam_result } where ppr_res Nothing = text "Match failed" ppr_res (Just (Reduction co ty)) = hang (text "Match succeeded:") 2 (vcat [ text "Rewrites to:" <+> ppr ty , text "Coercion:" <+> ppr co ]) solverDepthError :: CtLoc -> TcType -> TcM a solverDepthError loc ty = TcM.setCtLocM loc $ do { ty <- TcM.zonkTcType ty ; env0 <- TcM.tcInitTidyEnv ; let tidy_env = tidyFreeTyCoVars env0 (tyCoVarsOfTypeList ty) tidy_ty = tidyType tidy_env ty msg = TcRnUnknownMessage $ mkPlainError noHints $ vcat [ text "Reduction stack overflow; size =" <+> ppr depth , hang (text "When simplifying the following type:") 2 (ppr tidy_ty) , note ] ; TcM.failWithTcM (tidy_env, msg) } where depth = ctLocDepth loc note = vcat [ text "Use -freduction-depth=0 to disable this check" , text "(any upper bound you could choose might fail unpredictably with" , text " minor updates to GHC, so disabling the check is recommended if" , text " you're sure that type checking should terminate)" ] {- ************************************************************************ * * Breaking type variable cycles * * ************************************************************************ -} -- | Conditionally replace all type family applications in the RHS with fresh -- variables, emitting givens that relate the type family application to the -- variable. See Note [Type equality cycles] in GHC.Tc.Solver.Canonical. -- This only works under conditions as described in the Note; otherwise, returns -- Nothing. breakTyEqCycle_maybe :: CtEvidence -> CheckTyEqResult -- result of checkTypeEq -> CanEqLHS -> TcType -- RHS -> TcS (Maybe ReductionN) -- new RHS that doesn't have any type families breakTyEqCycle_maybe (ctLocOrigin . ctEvLoc -> CycleBreakerOrigin _) _ _ _ -- see Detail (7) of Note = return Nothing breakTyEqCycle_maybe ev cte_result lhs rhs | NomEq <- eq_rel , cte_result `cterHasOnlyProblem` cteSolubleOccurs -- only do this if the only problem is a soluble occurs-check -- See Detail (8) of the Note. = do { should_break <- final_check ; if should_break then do { redn <- go rhs ; return (Just redn) } else return Nothing } where flavour = ctEvFlavour ev eq_rel = ctEvEqRel ev final_check = case flavour of Given -> return True Wanted -- Wanteds work only with a touchable tyvar on the left -- See "Wanted" section of the Note. | TyVarLHS lhs_tv <- lhs -> do { result <- touchabilityTest Wanted lhs_tv rhs ; return $ case result of Untouchable -> False _ -> True } | otherwise -> return False -- This could be considerably more efficient. See Detail (5) of Note. go :: TcType -> TcS ReductionN go ty | Just ty' <- rewriterView ty = go ty' go (Rep.TyConApp tc tys) | isTypeFamilyTyCon tc -- worried about whether this type family is not actually -- causing trouble? See Detail (5) of Note. = do { let (fun_args, extra_args) = splitAt (tyConArity tc) tys fun_app = mkTyConApp tc fun_args fun_app_kind = tcTypeKind fun_app ; fun_redn <- emit_work fun_app_kind fun_app ; arg_redns <- unzipRedns <$> mapM go extra_args ; return $ mkAppRedns fun_redn arg_redns } -- Worried that this substitution will change kinds? -- See Detail (3) of Note | otherwise = do { arg_redns <- unzipRedns <$> mapM go tys ; return $ mkTyConAppRedn Nominal tc arg_redns } go (Rep.AppTy ty1 ty2) = mkAppRedn <$> go ty1 <*> go ty2 go (Rep.FunTy vis w arg res) = mkFunRedn Nominal vis <$> go w <*> go arg <*> go res go (Rep.CastTy ty cast_co) = mkCastRedn1 Nominal ty cast_co <$> go ty go ty@(Rep.TyVarTy {}) = skip ty go ty@(Rep.LitTy {}) = skip ty go ty@(Rep.ForAllTy {}) = skip ty -- See Detail (1) of Note go ty@(Rep.CoercionTy {}) = skip ty -- See Detail (2) of Note skip ty = return $ mkReflRedn Nominal ty emit_work :: TcKind -- of the function application -> TcType -- original function application -> TcS ReductionN -- rewritten type (the fresh tyvar) emit_work fun_app_kind fun_app = case flavour of Given -> do { new_tv <- wrapTcS (TcM.newCycleBreakerTyVar fun_app_kind) ; let new_ty = mkTyVarTy new_tv given_pred = mkHeteroPrimEqPred fun_app_kind fun_app_kind fun_app new_ty given_term = evCoercion $ mkNomReflCo new_ty -- See Detail (4) of Note ; new_given <- newGivenEvVar new_loc (given_pred, given_term) ; traceTcS "breakTyEqCycle replacing type family in Given" (ppr new_given) ; emitWorkNC [new_given] ; updInertTcS $ \is -> is { inert_cycle_breakers = insertCycleBreakerBinding new_tv fun_app (inert_cycle_breakers is) } ; return $ mkReflRedn Nominal new_ty } -- Why reflexive? See Detail (4) of the Note Wanted -> do { new_tv <- wrapTcS (TcM.newFlexiTyVar fun_app_kind) ; let new_ty = mkTyVarTy new_tv ; co <- emitNewWantedEq new_loc (ctEvRewriters ev) Nominal new_ty fun_app ; return $ mkReduction (mkSymCo co) new_ty } -- See Detail (7) of the Note new_loc = updateCtLocOrigin (ctEvLoc ev) CycleBreakerOrigin -- does not fit scenario from Note breakTyEqCycle_maybe _ _ _ _ = return Nothing -- | Fill in CycleBreakerTvs with the variables they stand for. -- See Note [Type equality cycles] in GHC.Tc.Solver.Canonical. restoreTyVarCycles :: InertSet -> TcM () restoreTyVarCycles is = forAllCycleBreakerBindings_ (inert_cycle_breakers is) TcM.writeMetaTyVar {-# SPECIALISE forAllCycleBreakerBindings_ :: CycleBreakerVarStack -> (TcTyVar -> TcType -> TcM ()) -> TcM () #-} -- Unwrap a type synonym only when either: -- The type synonym is forgetful, or -- the type synonym mentions a type family in its expansion -- See Note [Rewriting synonyms] in GHC.Tc.Solver.Rewrite. rewriterView :: TcType -> Maybe TcType rewriterView ty@(Rep.TyConApp tc _) | isForgetfulSynTyCon tc || (isTypeSynonymTyCon tc && not (isFamFreeTyCon tc)) = tcView ty rewriterView _other = Nothing