{-# LANGUAGE CPP #-} module GHC.Tc.Solver( InferMode(..), simplifyInfer, findInferredDiff, growThetaTyVars, simplifyAmbiguityCheck, simplifyDefault, simplifyTop, simplifyTopImplic, simplifyInteractive, solveEqualities, pushLevelAndSolveEqualities, pushLevelAndSolveEqualitiesX, reportUnsolvedEqualities, simplifyWantedsTcM, tcCheckGivens, tcCheckWanteds, tcNormalise, captureTopConstraints, simplifyTopWanteds, promoteTyVarSet, simplifyAndEmitFlatConstraints, -- For Rules we need these solveWanteds, solveWantedsAndDrop, approximateWC, runTcSDeriveds ) where #include "HsVersions.h" import GHC.Prelude import GHC.Data.Bag import GHC.Core.Class ( Class, classKey, classTyCon ) import GHC.Driver.Session import GHC.Tc.Utils.Instantiate import GHC.Data.List.SetOps import GHC.Types.Name import GHC.Types.Id( idType ) import GHC.Utils.Outputable import GHC.Builtin.Utils import GHC.Builtin.Names import GHC.Tc.Errors import GHC.Tc.Types.Evidence import GHC.Tc.Solver.Interact import GHC.Tc.Solver.Canonical ( makeSuperClasses, solveCallStack ) import GHC.Tc.Solver.Rewrite ( rewriteType ) import GHC.Tc.Utils.Unify ( buildTvImplication ) import GHC.Tc.Utils.TcMType as TcM import GHC.Tc.Utils.Monad as TcM import GHC.Tc.Solver.Monad as TcS import GHC.Tc.Types.Constraint import GHC.Core.Predicate import GHC.Tc.Types.Origin import GHC.Tc.Utils.TcType import GHC.Core.Type import GHC.Core.Ppr import GHC.Builtin.Types ( liftedRepTy, manyDataConTy ) import GHC.Core.Unify ( tcMatchTyKi ) import GHC.Utils.Misc import GHC.Utils.Panic import GHC.Types.Var import GHC.Types.Var.Set import GHC.Types.Basic ( IntWithInf, intGtLimit ) import GHC.Types.Error import qualified GHC.LanguageExtensions as LangExt import Control.Monad import Data.Foldable ( toList ) import Data.List ( partition ) import Data.List.NonEmpty ( NonEmpty(..) ) {- ********************************************************************************* * * * External interface * * * ********************************************************************************* -} captureTopConstraints :: TcM a -> TcM (a, WantedConstraints) -- (captureTopConstraints m) runs m, and returns the type constraints it -- generates plus the constraints produced by static forms inside. -- If it fails with an exception, it reports any insolubles -- (out of scope variables) before doing so -- -- captureTopConstraints is used exclusively by GHC.Tc.Module at the top -- level of a module. -- -- Importantly, if captureTopConstraints propagates an exception, it -- reports any insoluble constraints first, lest they be lost -- altogether. This is important, because solveEqualities (maybe -- other things too) throws an exception without adding any error -- messages; it just puts the unsolved constraints back into the -- monad. See GHC.Tc.Utils.Monad Note [Constraints and errors] -- #16376 is an example of what goes wrong if you don't do this. -- -- NB: the caller should bring any environments into scope before -- calling this, so that the reportUnsolved has access to the most -- complete GlobalRdrEnv captureTopConstraints :: forall a. TcM a -> TcM (a, WantedConstraints) captureTopConstraints TcM a thing_inside = do { TcRef WantedConstraints static_wc_var <- forall a gbl lcl. a -> TcRnIf gbl lcl (TcRef a) TcM.newTcRef WantedConstraints emptyWC ; ; (Maybe a mb_res, WantedConstraints lie) <- forall gbl lcl a. (gbl -> gbl) -> TcRnIf gbl lcl a -> TcRnIf gbl lcl a TcM.updGblEnv (\TcGblEnv env -> TcGblEnv env { tcg_static_wc :: TcRef WantedConstraints tcg_static_wc = TcRef WantedConstraints static_wc_var } ) forall a b. (a -> b) -> a -> b $ forall a. TcM a -> TcM (Maybe a, WantedConstraints) TcM.tryCaptureConstraints TcM a thing_inside ; WantedConstraints stWC <- forall a gbl lcl. TcRef a -> TcRnIf gbl lcl a TcM.readTcRef TcRef WantedConstraints static_wc_var -- See GHC.Tc.Utils.Monad Note [Constraints and errors] -- If the thing_inside threw an exception, but generated some insoluble -- constraints, report the latter before propagating the exception -- Otherwise they will be lost altogether ; case Maybe a mb_res of Just a res -> forall (m :: * -> *) a. Monad m => a -> m a return (a res, WantedConstraints lie WantedConstraints -> WantedConstraints -> WantedConstraints `andWC` WantedConstraints stWC) Maybe a Nothing -> do { Bag EvBind _ <- WantedConstraints -> TcM (Bag EvBind) simplifyTop WantedConstraints lie; forall env a. IOEnv env a failM } } -- This call to simplifyTop is the reason -- this function is here instead of GHC.Tc.Utils.Monad -- We call simplifyTop so that it does defaulting -- (esp of runtime-reps) before reporting errors simplifyTopImplic :: Bag Implication -> TcM () simplifyTopImplic :: Bag Implication -> TcM () simplifyTopImplic Bag Implication implics = do { Bag EvBind empty_binds <- WantedConstraints -> TcM (Bag EvBind) simplifyTop (Bag Implication -> WantedConstraints mkImplicWC Bag Implication implics) -- Since all the inputs are implications the returned bindings will be empty ; MASSERT2( isEmptyBag empty_binds, ppr empty_binds ) ; forall (m :: * -> *) a. Monad m => a -> m a return () } simplifyTop :: WantedConstraints -> TcM (Bag EvBind) -- Simplify top-level constraints -- Usually these will be implications, -- but when there is nothing to quantify we don't wrap -- in a degenerate implication, so we do that here instead simplifyTop :: WantedConstraints -> TcM (Bag EvBind) simplifyTop WantedConstraints wanteds = do { String -> SDoc -> TcM () traceTc String "simplifyTop {" forall a b. (a -> b) -> a -> b $ String -> SDoc text String "wanted = " SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr WantedConstraints wanteds ; ((WantedConstraints final_wc, Cts unsafe_ol), EvBindMap binds1) <- forall a. TcS a -> TcM (a, EvBindMap) runTcS forall a b. (a -> b) -> a -> b $ do { WantedConstraints final_wc <- WantedConstraints -> TcS WantedConstraints simplifyTopWanteds WantedConstraints wanteds ; Cts unsafe_ol <- TcS Cts getSafeOverlapFailures ; forall (m :: * -> *) a. Monad m => a -> m a return (WantedConstraints final_wc, Cts unsafe_ol) } ; String -> SDoc -> TcM () traceTc String "End simplifyTop }" SDoc empty ; Bag EvBind binds2 <- WantedConstraints -> TcM (Bag EvBind) reportUnsolved WantedConstraints final_wc ; String -> SDoc -> TcM () traceTc String "reportUnsolved (unsafe overlapping) {" SDoc empty ; forall (f :: * -> *). Applicative f => Bool -> f () -> f () unless (Cts -> Bool isEmptyCts Cts unsafe_ol) forall a b. (a -> b) -> a -> b $ do { -- grab current error messages and clear, warnAllUnsolved will -- update error messages which we'll grab and then restore saved -- messages. ; TcRef (Messages DecoratedSDoc) errs_var <- TcRn (TcRef (Messages DecoratedSDoc)) getErrsVar ; Messages DecoratedSDoc saved_msg <- forall a gbl lcl. TcRef a -> TcRnIf gbl lcl a TcM.readTcRef TcRef (Messages DecoratedSDoc) errs_var ; forall a gbl lcl. TcRef a -> a -> TcRnIf gbl lcl () TcM.writeTcRef TcRef (Messages DecoratedSDoc) errs_var forall e. Messages e emptyMessages ; WantedConstraints -> TcM () warnAllUnsolved forall a b. (a -> b) -> a -> b $ WantedConstraints emptyWC { wc_simple :: Cts wc_simple = Cts unsafe_ol } ; Bag (MsgEnvelope DecoratedSDoc) whyUnsafe <- forall e. Messages e -> Bag (MsgEnvelope e) getWarningMessages forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b <$> forall a gbl lcl. TcRef a -> TcRnIf gbl lcl a TcM.readTcRef TcRef (Messages DecoratedSDoc) errs_var ; forall a gbl lcl. TcRef a -> a -> TcRnIf gbl lcl () TcM.writeTcRef TcRef (Messages DecoratedSDoc) errs_var Messages DecoratedSDoc saved_msg ; Bag (MsgEnvelope DecoratedSDoc) -> TcM () recordUnsafeInfer Bag (MsgEnvelope DecoratedSDoc) whyUnsafe } ; String -> SDoc -> TcM () traceTc String "reportUnsolved (unsafe overlapping) }" SDoc empty ; forall (m :: * -> *) a. Monad m => a -> m a return (EvBindMap -> Bag EvBind evBindMapBinds EvBindMap binds1 forall a. Bag a -> Bag a -> Bag a `unionBags` Bag EvBind binds2) } pushLevelAndSolveEqualities :: SkolemInfo -> [TcTyVar] -> TcM a -> TcM a -- Push level, and solve all resulting equalities -- If there are any unsolved equalities, report them -- and fail (in the monad) -- -- Panics if we solve any non-equality constraints. (In runTCSEqualities -- we use an error thunk for the evidence bindings.) pushLevelAndSolveEqualities :: forall a. SkolemInfo -> [TcTyVar] -> TcM a -> TcM a pushLevelAndSolveEqualities SkolemInfo skol_info [TcTyVar] skol_tvs TcM a thing_inside = do { (TcLevel tclvl, WantedConstraints wanted, a res) <- forall a. String -> TcM a -> TcM (TcLevel, WantedConstraints, a) pushLevelAndSolveEqualitiesX String "pushLevelAndSolveEqualities" TcM a thing_inside ; SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM () reportUnsolvedEqualities SkolemInfo skol_info [TcTyVar] skol_tvs TcLevel tclvl WantedConstraints wanted ; forall (m :: * -> *) a. Monad m => a -> m a return a res } pushLevelAndSolveEqualitiesX :: String -> TcM a -> TcM (TcLevel, WantedConstraints, a) -- Push the level, gather equality constraints, and then solve them. -- Returns any remaining unsolved equalities. -- Does not report errors. -- -- Panics if we solve any non-equality constraints. (In runTCSEqualities -- we use an error thunk for the evidence bindings.) pushLevelAndSolveEqualitiesX :: forall a. String -> TcM a -> TcM (TcLevel, WantedConstraints, a) pushLevelAndSolveEqualitiesX String callsite TcM a thing_inside = do { String -> SDoc -> TcM () traceTc String "pushLevelAndSolveEqualitiesX {" (String -> SDoc text String "Called from" SDoc -> SDoc -> SDoc <+> String -> SDoc text String callsite) ; (TcLevel tclvl, (WantedConstraints wanted, a res)) <- forall a. TcM a -> TcM (TcLevel, a) pushTcLevelM forall a b. (a -> b) -> a -> b $ do { (a res, WantedConstraints wanted) <- forall a. TcM a -> TcM (a, WantedConstraints) captureConstraints TcM a thing_inside ; WantedConstraints wanted <- forall a. TcS a -> TcM a runTcSEqualities (WantedConstraints -> TcS WantedConstraints simplifyTopWanteds WantedConstraints wanted) ; forall (m :: * -> *) a. Monad m => a -> m a return (WantedConstraints wanted,a res) } ; String -> SDoc -> TcM () traceTc String "pushLevelAndSolveEqualities }" ([SDoc] -> SDoc vcat [ String -> SDoc text String "Residual:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr WantedConstraints wanted , String -> SDoc text String "Level:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr TcLevel tclvl ]) ; forall (m :: * -> *) a. Monad m => a -> m a return (TcLevel tclvl, WantedConstraints wanted, a res) } -- | Type-check a thing that emits only equality constraints, solving any -- constraints we can and re-emitting constraints that we can't. -- Use this variant only when we'll get another crack at it later -- See Note [Failure in local type signatures] -- -- Panics if we solve any non-equality constraints. (In runTCSEqualities -- we use an error thunk for the evidence bindings.) solveEqualities :: String -> TcM a -> TcM a solveEqualities :: forall a. String -> TcM a -> TcM a solveEqualities String callsite TcM a thing_inside = do { String -> SDoc -> TcM () traceTc String "solveEqualities {" (String -> SDoc text String "Called from" SDoc -> SDoc -> SDoc <+> String -> SDoc text String callsite) ; (a res, WantedConstraints wanted) <- forall a. TcM a -> TcM (a, WantedConstraints) captureConstraints TcM a thing_inside ; WantedConstraints -> TcM () simplifyAndEmitFlatConstraints WantedConstraints wanted -- simplifyAndEmitFlatConstraints fails outright unless -- the only unsolved constraints are soluble-looking -- equalities that can float out ; String -> SDoc -> TcM () traceTc String "solveEqualities }" SDoc empty ; forall (m :: * -> *) a. Monad m => a -> m a return a res } simplifyAndEmitFlatConstraints :: WantedConstraints -> TcM () -- See Note [Failure in local type signatures] simplifyAndEmitFlatConstraints :: WantedConstraints -> TcM () simplifyAndEmitFlatConstraints WantedConstraints wanted = do { -- Solve and zonk to esablish the -- preconditions for floatKindEqualities WantedConstraints wanted <- forall a. TcS a -> TcM a runTcSEqualities (WantedConstraints -> TcS WantedConstraints solveWanteds WantedConstraints wanted) ; WantedConstraints wanted <- WantedConstraints -> TcM WantedConstraints TcM.zonkWC WantedConstraints wanted ; String -> SDoc -> TcM () traceTc String "emitFlatConstraints {" (forall a. Outputable a => a -> SDoc ppr WantedConstraints wanted) ; case WantedConstraints -> Maybe (Cts, Bag Hole) floatKindEqualities WantedConstraints wanted of Maybe (Cts, Bag Hole) Nothing -> do { String -> SDoc -> TcM () traceTc String "emitFlatConstraints } failing" (forall a. Outputable a => a -> SDoc ppr WantedConstraints wanted) -- Emit the bad constraints, wrapped in an implication -- See Note [Wrapping failing kind equalities] ; TcLevel tclvl <- TcM TcLevel TcM.getTcLevel ; Implication implic <- SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication buildTvImplication SkolemInfo UnkSkol [] (TcLevel -> TcLevel pushTcLevel TcLevel tclvl) WantedConstraints wanted -- ^^^^^^ | ^^^^^^^^^^^^^^^^^ -- it's OK to use UnkSkol | we must increase the TcLevel, -- because we don't bind | as explained in -- any skolem variables here | Note [Wrapping failing kind equalities] ; Implication -> TcM () emitImplication Implication implic ; forall env a. IOEnv env a failM } Just (Cts simples, Bag Hole holes) -> do { Bool _ <- VarSet -> TcM Bool promoteTyVarSet (Cts -> VarSet tyCoVarsOfCts Cts simples) ; String -> SDoc -> TcM () traceTc String "emitFlatConstraints }" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "simples:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr Cts simples , String -> SDoc text String "holes: " SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr Bag Hole holes ] ; Bag Hole -> TcM () emitHoles Bag Hole holes -- Holes don't need promotion ; Cts -> TcM () emitSimples Cts simples } } floatKindEqualities :: WantedConstraints -> Maybe (Bag Ct, Bag Hole) -- Float out all the constraints from the WantedConstraints, -- Return Nothing if any constraints can't be floated (captured -- by skolems), or if there is an insoluble constraint, or -- IC_Telescope telescope error -- Precondition 1: we have tried to solve the 'wanteds', both so that -- the ic_status field is set, and because solving can make constraints -- more floatable. -- Precondition 2: the 'wanteds' are zonked, since floatKindEqualities -- is not monadic -- See Note [floatKindEqualities vs approximateWC] floatKindEqualities :: WantedConstraints -> Maybe (Cts, Bag Hole) floatKindEqualities WantedConstraints wc = VarSet -> WantedConstraints -> Maybe (Cts, Bag Hole) float_wc VarSet emptyVarSet WantedConstraints wc where float_wc :: TcTyCoVarSet -> WantedConstraints -> Maybe (Bag Ct, Bag Hole) float_wc :: VarSet -> WantedConstraints -> Maybe (Cts, Bag Hole) float_wc VarSet trapping_tvs (WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples , wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implics , wc_holes :: WantedConstraints -> Bag Hole wc_holes = Bag Hole holes }) | forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool all Ct -> Bool is_floatable Cts simples = do { (Cts inner_simples, Bag Hole inner_holes) <- forall (m :: * -> *) a b c. Monad m => (a -> m (Bag b, Bag c)) -> Bag a -> m (Bag b, Bag c) flatMapBagPairM (VarSet -> Implication -> Maybe (Cts, Bag Hole) float_implic VarSet trapping_tvs) Bag Implication implics ; forall (m :: * -> *) a. Monad m => a -> m a return ( Cts simples forall a. Bag a -> Bag a -> Bag a `unionBags` Cts inner_simples , Bag Hole holes forall a. Bag a -> Bag a -> Bag a `unionBags` Bag Hole inner_holes) } | Bool otherwise = forall a. Maybe a Nothing where is_floatable :: Ct -> Bool is_floatable Ct ct | Ct -> Bool insolubleEqCt Ct ct = Bool False | Bool otherwise = Ct -> VarSet tyCoVarsOfCt Ct ct VarSet -> VarSet -> Bool `disjointVarSet` VarSet trapping_tvs float_implic :: TcTyCoVarSet -> Implication -> Maybe (Bag Ct, Bag Hole) float_implic :: VarSet -> Implication -> Maybe (Cts, Bag Hole) float_implic VarSet trapping_tvs (Implic { ic_wanted :: Implication -> WantedConstraints ic_wanted = WantedConstraints wanted, ic_given_eqs :: Implication -> HasGivenEqs ic_given_eqs = HasGivenEqs given_eqs , ic_skols :: Implication -> [TcTyVar] ic_skols = [TcTyVar] skols, ic_status :: Implication -> ImplicStatus ic_status = ImplicStatus status }) | ImplicStatus -> Bool isInsolubleStatus ImplicStatus status = forall a. Maybe a Nothing -- A short cut /plus/ we must keep track of IC_BadTelescope | Bool otherwise = do { (Cts simples, Bag Hole holes) <- VarSet -> WantedConstraints -> Maybe (Cts, Bag Hole) float_wc VarSet new_trapping_tvs WantedConstraints wanted ; forall (f :: * -> *). Applicative f => Bool -> f () -> f () when (Bool -> Bool not (forall a. Bag a -> Bool isEmptyBag Cts simples) Bool -> Bool -> Bool && HasGivenEqs given_eqs forall a. Eq a => a -> a -> Bool == HasGivenEqs MaybeGivenEqs) forall a b. (a -> b) -> a -> b $ forall a. Maybe a Nothing -- If there are some constraints to float out, but we can't -- because we don't float out past local equalities -- (c.f GHC.Tc.Solver.approximateWC), then fail ; forall (m :: * -> *) a. Monad m => a -> m a return (Cts simples, Bag Hole holes) } where new_trapping_tvs :: VarSet new_trapping_tvs = VarSet trapping_tvs VarSet -> [TcTyVar] -> VarSet `extendVarSetList` [TcTyVar] skols {- Note [Failure in local type signatures] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When kind checking a type signature, we like to fail fast if we can't solve all the kind equality constraints, for two reasons: * A kind-bogus type signature may cause a cascade of knock-on errors if we let it pass * More seriously, we don't have a convenient term-level place to add deferred bindings for unsolved kind-equality constraints. In earlier GHCs this led to un-filled-in coercion holes, which caused GHC to crash with "fvProv falls into a hole" See #11563, #11520, #11516, #11399 But what about /local/ type signatures, mentioning in-scope type variables for which there might be 'given' equalities? For these we might not be able to solve all the equalities locally. Here's an example (T15076b): class (a ~ b) => C a b data SameKind :: k -> k -> Type where { SK :: SameKind a b } bar :: forall (a :: Type) (b :: Type). C a b => Proxy a -> Proxy b -> () bar _ _ = const () (undefined :: forall (x :: a) (y :: b). SameKind x y) Consider the type signature on 'undefined'. It's ill-kinded unless a~b. But the superclass of (C a b) means that indeed (a~b). So all should be well. BUT it's hard to see that when kind-checking the signature for undefined. We want to emit a residual (a~b) constraint, to solve later. Another possibility is that we might have something like F alpha ~ [Int] where alpha is bound further out, which might become soluble "later" when we learn more about alpha. So we want to emit those residual constraints. BUT it's no good simply wrapping all unsolved constraints from a type signature in an implication constraint to solve later. The problem is that we are going to /use/ that signature, including instantiate it. Say we have f :: forall a. (forall b. blah) -> blah2 f x = <body> To typecheck the definition of f, we have to instantiate those foralls. Moreover, any unsolved kind equalities will be coercion holes in the type. If we naively wrap them in an implication like forall a. (co1:k1~k2, forall b. co2:k3~k4) hoping to solve it later, we might end up filling in the holes co1 and co2 with coercions involving 'a' and 'b' -- but by now we've instantiated the type. Chaos! Moreover, the unsolved constraints might be skolem-escape things, and if we proceed with f bound to a nonsensical type, we get a cascade of follow-up errors. For example polykinds/T12593, T15577, and many others. So here's the plan (see tcHsSigType): * pushLevelAndSolveEqualitiesX: try to solve the constraints * kindGeneraliseSome: do kind generalisation * buildTvImplication: build an implication for the residual, unsolved constraint * simplifyAndEmitFlatConstraints: try to float out every unsolved equality inside that implication, in the hope that it constrains only global type variables, not the locally-quantified ones. * If we fail, or find an insoluble constraint, emit the implication, so that the errors will be reported, and fail. * If we succeed in floating all the equalities, promote them and re-emit them as flat constraint, not wrapped at all (since they don't mention any of the quantified variables. * Note that this float-and-promote step means that anonymous wildcards get floated to top level, as we want; see Note [Checking partial type signatures] in GHC.Tc.Gen.HsType. All this is done: * In GHC.Tc.Gen.HsType.tcHsSigType, as above * solveEqualities. Use this when there no kind-generalisation step to complicate matters; then we don't need to push levels, and can solve the equalities immediately without needing to wrap it in an implication constraint. (You'll generally see a kindGeneraliseNone nearby.) * In GHC.Tc.TyCl and GHC.Tc.TyCl.Instance; see calls to pushLevelAndSolveEqualitiesX, followed by quantification, and then reportUnsolvedEqualities. NB: we call reportUnsolvedEqualities before zonkTcTypeToType because the latter does not expect to see any un-filled-in coercions, which will happen if we have unsolved equalities. By calling reportUnsolvedEqualities first, which fails after reporting errors, we avoid that happening. See also #18062, #11506 Note [Wrapping failing kind equalities] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In simplifyAndEmitFlatConstraints, if we fail to get down to simple flat constraints we will * re-emit the constraints so that they are reported * fail in the monad But there is a Terrible Danger that, if -fdefer-type-errors is on, and we just re-emit an insoluble constraint like (* ~ (*->*)), that we'll report only a warning and proceed with compilation. But if we ever fail in the monad it should be fatal; we should report an error and stop after the type checker. If not, chaos results: #19142. Our solution is this: * Even with -fdefer-type-errors, inside an implication with no place for value bindings (ic_binds = CoEvBindsVar), report failing equalities as errors. We have to do this anyway; see GHC.Tc.Errors Note [Failing equalities with no evidence bindings]. * Right here in simplifyAndEmitFlatConstraints, use buildTvImplication to wrap the failing constraint in a degenerate implication (no skolems, no theta), with ic_binds = CoEvBindsVar. This setting of `ic_binds` means that any failing equalities will lead to an error not a warning, irrespective of -fdefer-type-errors: see Note [Failing equalities with no evidence bindings] in GHC.Tc.Errors, and `maybeSwitchOffDefer` in that module. We still take care to bump the TcLevel of the implication. Partly, that ensures that nested implications have increasing level numbers which seems nice. But more specifically, suppose the outer level has a Given `(C ty)`, which has pending (not-yet-expanded) superclasses. Consider what happens when we process this implication constraint (which we have re-emitted) in that context: - in the inner implication we'll call `getPendingGivenScs`, - we /do not/ want to get the `(C ty)` from the outer level, lest we try to add an evidence term for the superclass, which we can't do because we have specifically set `ic_binds` = `CoEvBindsVar`. - as `getPendingGivenSCcs is careful to only get Givens from the /current/ level, and we bumped the `TcLevel` of the implication, we're OK. TL;DR: bump the `TcLevel` when creating the nested implication. If we don't we get a panic in `GHC.Tc.Utils.Monad.addTcEvBind` (#20043). We re-emit the implication rather than reporting the errors right now, so that the error mesages are improved by other solving and defaulting. e.g. we prefer Cannot match 'Type->Type' with 'Type' to Cannot match 'Type->Type' with 'TYPE r0' Note [floatKindEqualities vs approximateWC] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ floatKindEqualities and approximateWC are strikingly similar to each other, but * floatKindEqualites tries to float /all/ equalities, and fails if it can't, or if any implication is insoluble. * approximateWC just floats out any constraints (not just equalities) that can float; it never fails. -} reportUnsolvedEqualities :: SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM () -- Reports all unsolved wanteds provided; fails in the monad if there are any. -- -- The provided SkolemInfo and [TcTyVar] arguments are used in an implication to -- provide skolem info for any errors. -- reportUnsolvedEqualities :: SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM () reportUnsolvedEqualities SkolemInfo skol_info [TcTyVar] skol_tvs TcLevel tclvl WantedConstraints wanted | WantedConstraints -> Bool isEmptyWC WantedConstraints wanted = forall (m :: * -> *) a. Monad m => a -> m a return () | Bool otherwise = forall r. TcM r -> TcM r checkNoErrs forall a b. (a -> b) -> a -> b $ -- Fail do { Implication implic <- SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication buildTvImplication SkolemInfo skol_info [TcTyVar] skol_tvs TcLevel tclvl WantedConstraints wanted ; WantedConstraints -> TcM () reportAllUnsolved (Bag Implication -> WantedConstraints mkImplicWC (forall a. a -> Bag a unitBag Implication implic)) } -- | Simplify top-level constraints, but without reporting any unsolved -- constraints nor unsafe overlapping. simplifyTopWanteds :: WantedConstraints -> TcS WantedConstraints -- See Note [Top-level Defaulting Plan] simplifyTopWanteds :: WantedConstraints -> TcS WantedConstraints simplifyTopWanteds WantedConstraints wanteds = do { WantedConstraints wc_first_go <- forall a. TcS a -> TcS a nestTcS (WantedConstraints -> TcS WantedConstraints solveWantedsAndDrop WantedConstraints wanteds) -- This is where the main work happens ; DynFlags dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags getDynFlags ; DynFlags -> WantedConstraints -> TcS WantedConstraints try_tyvar_defaulting DynFlags dflags WantedConstraints wc_first_go } where try_tyvar_defaulting :: DynFlags -> WantedConstraints -> TcS WantedConstraints try_tyvar_defaulting :: DynFlags -> WantedConstraints -> TcS WantedConstraints try_tyvar_defaulting DynFlags dflags WantedConstraints wc | WantedConstraints -> Bool isEmptyWC WantedConstraints wc = forall (m :: * -> *) a. Monad m => a -> m a return WantedConstraints wc | WantedConstraints -> Bool insolubleWC WantedConstraints wc , GeneralFlag -> DynFlags -> Bool gopt GeneralFlag Opt_PrintExplicitRuntimeReps DynFlags dflags -- See Note [Defaulting insolubles] = WantedConstraints -> TcS WantedConstraints try_class_defaulting WantedConstraints wc | Bool otherwise = do { [TcTyVar] free_tvs <- [TcTyVar] -> TcS [TcTyVar] TcS.zonkTyCoVarsAndFVList (WantedConstraints -> [TcTyVar] tyCoVarsOfWCList WantedConstraints wc) ; let meta_tvs :: [TcTyVar] meta_tvs = forall a. (a -> Bool) -> [a] -> [a] filter (TcTyVar -> Bool isTyVar forall (f :: * -> *). Applicative f => f Bool -> f Bool -> f Bool <&&> TcTyVar -> Bool isMetaTyVar) [TcTyVar] free_tvs -- zonkTyCoVarsAndFV: the wc_first_go is not yet zonked -- filter isMetaTyVar: we might have runtime-skolems in GHCi, -- and we definitely don't want to try to assign to those! -- The isTyVar is needed to weed out coercion variables ; [Bool] defaulted <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM TcTyVar -> TcS Bool defaultTyVarTcS [TcTyVar] meta_tvs -- Has unification side effects ; if forall (t :: * -> *). Foldable t => t Bool -> Bool or [Bool] defaulted then do { WantedConstraints wc_residual <- forall a. TcS a -> TcS a nestTcS (WantedConstraints -> TcS WantedConstraints solveWanteds WantedConstraints wc) -- See Note [Must simplify after defaulting] ; WantedConstraints -> TcS WantedConstraints try_class_defaulting WantedConstraints wc_residual } else WantedConstraints -> TcS WantedConstraints try_class_defaulting WantedConstraints wc } -- No defaulting took place try_class_defaulting :: WantedConstraints -> TcS WantedConstraints try_class_defaulting :: WantedConstraints -> TcS WantedConstraints try_class_defaulting WantedConstraints wc | WantedConstraints -> Bool isEmptyWC WantedConstraints wc Bool -> Bool -> Bool || WantedConstraints -> Bool insolubleWC WantedConstraints wc -- See Note [Defaulting insolubles] = forall (m :: * -> *) a. Monad m => a -> m a return WantedConstraints wc | Bool otherwise -- See Note [When to do type-class defaulting] = do { Bool something_happened <- WantedConstraints -> TcS Bool applyDefaultingRules WantedConstraints wc -- See Note [Top-level Defaulting Plan] ; if Bool something_happened then do { WantedConstraints wc_residual <- forall a. TcS a -> TcS a nestTcS (WantedConstraints -> TcS WantedConstraints solveWantedsAndDrop WantedConstraints wc) ; WantedConstraints -> TcS WantedConstraints try_class_defaulting WantedConstraints wc_residual } -- See Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence else WantedConstraints -> TcS WantedConstraints try_callstack_defaulting WantedConstraints wc } try_callstack_defaulting :: WantedConstraints -> TcS WantedConstraints try_callstack_defaulting :: WantedConstraints -> TcS WantedConstraints try_callstack_defaulting WantedConstraints wc | WantedConstraints -> Bool isEmptyWC WantedConstraints wc = forall (m :: * -> *) a. Monad m => a -> m a return WantedConstraints wc | Bool otherwise = WantedConstraints -> TcS WantedConstraints defaultCallStacks WantedConstraints wc -- | Default any remaining @CallStack@ constraints to empty @CallStack@s. defaultCallStacks :: WantedConstraints -> TcS WantedConstraints -- See Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence defaultCallStacks :: WantedConstraints -> TcS WantedConstraints defaultCallStacks WantedConstraints wanteds = do Cts simples <- Cts -> TcS Cts handle_simples (WantedConstraints -> Cts wc_simple WantedConstraints wanteds) Bag (Maybe Implication) mb_implics <- forall (m :: * -> *) a b. Monad m => (a -> m b) -> Bag a -> m (Bag b) mapBagM Implication -> TcS (Maybe Implication) handle_implic (WantedConstraints -> Bag Implication wc_impl WantedConstraints wanteds) forall (m :: * -> *) a. Monad m => a -> m a return (WantedConstraints wanteds { wc_simple :: Cts wc_simple = Cts simples , wc_impl :: Bag Implication wc_impl = forall a. Bag (Maybe a) -> Bag a catBagMaybes Bag (Maybe Implication) mb_implics }) where handle_simples :: Cts -> TcS Cts handle_simples Cts simples = forall a. Bag (Maybe a) -> Bag a catBagMaybes forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b <$> forall (m :: * -> *) a b. Monad m => (a -> m b) -> Bag a -> m (Bag b) mapBagM Ct -> TcS (Maybe Ct) defaultCallStack Cts simples handle_implic :: Implication -> TcS (Maybe Implication) -- The Maybe is because solving the CallStack constraint -- may well allow us to discard the implication entirely handle_implic :: Implication -> TcS (Maybe Implication) handle_implic Implication implic | ImplicStatus -> Bool isSolvedStatus (Implication -> ImplicStatus ic_status Implication implic) = forall (m :: * -> *) a. Monad m => a -> m a return (forall a. a -> Maybe a Just Implication implic) | Bool otherwise = do { WantedConstraints wanteds <- forall a. EvBindsVar -> TcS a -> TcS a setEvBindsTcS (Implication -> EvBindsVar ic_binds Implication implic) forall a b. (a -> b) -> a -> b $ -- defaultCallStack sets a binding, so -- we must set the correct binding group WantedConstraints -> TcS WantedConstraints defaultCallStacks (Implication -> WantedConstraints ic_wanted Implication implic) ; Implication -> TcS (Maybe Implication) setImplicationStatus (Implication implic { ic_wanted :: WantedConstraints ic_wanted = WantedConstraints wanteds }) } defaultCallStack :: Ct -> TcS (Maybe Ct) defaultCallStack Ct ct | ClassPred Class cls [Type] tys <- Type -> Pred classifyPredType (Ct -> Type ctPred Ct ct) , Just {} <- Class -> [Type] -> Maybe FastString isCallStackPred Class cls [Type] tys = do { CtEvidence -> EvCallStack -> TcS () solveCallStack (Ct -> CtEvidence ctEvidence Ct ct) EvCallStack EvCsEmpty ; forall (m :: * -> *) a. Monad m => a -> m a return forall a. Maybe a Nothing } defaultCallStack Ct ct = forall (m :: * -> *) a. Monad m => a -> m a return (forall a. a -> Maybe a Just Ct ct) {- Note [When to do type-class defaulting] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In GHC 7.6 and 7.8.2, we did type-class defaulting only if insolubleWC was false, on the grounds that defaulting can't help solve insoluble constraints. But if we *don't* do defaulting we may report a whole lot of errors that would be solved by defaulting; these errors are quite spurious because fixing the single insoluble error means that defaulting happens again, which makes all the other errors go away. This is jolly confusing: #9033. So it seems better to always do type-class defaulting. However, always doing defaulting does mean that we'll do it in situations like this (#5934): run :: (forall s. GenST s) -> Int run = fromInteger 0 We don't unify the return type of fromInteger with the given function type, because the latter involves foralls. So we're left with (Num alpha, alpha ~ (forall s. GenST s) -> Int) Now we do defaulting, get alpha := Integer, and report that we can't match Integer with (forall s. GenST s) -> Int. That's not totally stupid, but perhaps a little strange. Another potential alternative would be to suppress *all* non-insoluble errors if there are *any* insoluble errors, anywhere, but that seems too drastic. Note [Must simplify after defaulting] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We may have a deeply buried constraint (t:*) ~ (a:Open) which we couldn't solve because of the kind incompatibility, and 'a' is free. Then when we default 'a' we can solve the constraint. And we want to do that before starting in on type classes. We MUST do it before reporting errors, because it isn't an error! #7967 was due to this. Note [Top-level Defaulting Plan] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We have considered two design choices for where/when to apply defaulting. (i) Do it in SimplCheck mode only /whenever/ you try to solve some simple constraints, maybe deep inside the context of implications. This used to be the case in GHC 7.4.1. (ii) Do it in a tight loop at simplifyTop, once all other constraints have finished. This is the current story. Option (i) had many disadvantages: a) Firstly, it was deep inside the actual solver. b) Secondly, it was dependent on the context (Infer a type signature, or Check a type signature, or Interactive) since we did not want to always start defaulting when inferring (though there is an exception to this, see Note [Default while Inferring]). c) It plainly did not work. Consider typecheck/should_compile/DfltProb2.hs: f :: Int -> Bool f x = const True (\y -> let w :: a -> a w a = const a (y+1) in w y) We will get an implication constraint (for beta the type of y): [untch=beta] forall a. 0 => Num beta which we really cannot default /while solving/ the implication, since beta is untouchable. Instead our new defaulting story is to pull defaulting out of the solver loop and go with option (ii), implemented at SimplifyTop. Namely: - First, have a go at solving the residual constraint of the whole program - Try to approximate it with a simple constraint - Figure out derived defaulting equations for that simple constraint - Go round the loop again if you did manage to get some equations Now, that has to do with class defaulting. However there exists type variable /kind/ defaulting. Again this is done at the top-level and the plan is: - At the top-level, once you had a go at solving the constraint, do figure out /all/ the touchable unification variables of the wanted constraints. - Apply defaulting to their kinds More details in Note [DefaultTyVar]. Note [Safe Haskell Overlapping Instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In Safe Haskell, we apply an extra restriction to overlapping instances. The motive is to prevent untrusted code provided by a third-party, changing the behavior of trusted code through type-classes. This is due to the global and implicit nature of type-classes that can hide the source of the dictionary. Another way to state this is: if a module M compiles without importing another module N, changing M to import N shouldn't change the behavior of M. Overlapping instances with type-classes can violate this principle. However, overlapping instances aren't always unsafe. They are just unsafe when the most selected dictionary comes from untrusted code (code compiled with -XSafe) and overlaps instances provided by other modules. In particular, in Safe Haskell at a call site with overlapping instances, we apply the following rule to determine if it is a 'unsafe' overlap: 1) Most specific instance, I1, defined in an `-XSafe` compiled module. 2) I1 is an orphan instance or a MPTC. 3) At least one overlapped instance, Ix, is both: A) from a different module than I1 B) Ix is not marked `OVERLAPPABLE` This is a slightly involved heuristic, but captures the situation of an imported module N changing the behavior of existing code. For example, if condition (2) isn't violated, then the module author M must depend either on a type-class or type defined in N. Secondly, when should these heuristics be enforced? We enforced them when the type-class method call site is in a module marked `-XSafe` or `-XTrustworthy`. This allows `-XUnsafe` modules to operate without restriction, and for Safe Haskell inferrence to infer modules with unsafe overlaps as unsafe. One alternative design would be to also consider if an instance was imported as a `safe` import or not and only apply the restriction to instances imported safely. However, since instances are global and can be imported through more than one path, this alternative doesn't work. Note [Safe Haskell Overlapping Instances Implementation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ How is this implemented? It's complicated! So we'll step through it all: 1) `InstEnv.lookupInstEnv` -- Performs instance resolution, so this is where we check if a particular type-class method call is safe or unsafe. We do this through the return type, `ClsInstLookupResult`, where the last parameter is a list of instances that are unsafe to overlap. When the method call is safe, the list is null. 2) `GHC.Tc.Solver.Interact.matchClassInst` -- This module drives the instance resolution / dictionary generation. The return type is `ClsInstResult`, which either says no instance matched, or one found, and if it was a safe or unsafe overlap. 3) `GHC.Tc.Solver.Interact.doTopReactDict` -- Takes a dictionary / class constraint and tries to resolve it by calling (in part) `matchClassInst`. The resolving mechanism has a work list (of constraints) that it process one at a time. If the constraint can't be resolved, it's added to an inert set. When compiling an `-XSafe` or `-XTrustworthy` module, we follow this approach as we know compilation should fail. These are handled as normal constraint resolution failures from here-on (see step 6). Otherwise, we may be inferring safety (or using `-Wunsafe`), and compilation should succeed, but print warnings and/or mark the compiled module as `-XUnsafe`. In this case, we call `insertSafeOverlapFailureTcS` which adds the unsafe (but resolved!) constraint to the `inert_safehask` field of `InertCans`. 4) `GHC.Tc.Solver.simplifyTop`: * Call simplifyTopWanteds, the top-level function for driving the simplifier for constraint resolution. * Once finished, call `getSafeOverlapFailures` to retrieve the list of overlapping instances that were successfully resolved, but unsafe. Remember, this is only applicable for generating warnings (`-Wunsafe`) or inferring a module unsafe. `-XSafe` and `-XTrustworthy` cause compilation failure by not resolving the unsafe constraint at all. * For unresolved constraints (all types), call `GHC.Tc.Errors.reportUnsolved`, while for resolved but unsafe overlapping dictionary constraints, call `GHC.Tc.Errors.warnAllUnsolved`. Both functions convert constraints into a warning message for the user. * In the case of `warnAllUnsolved` for resolved, but unsafe dictionary constraints, we collect the generated warning message (pop it) and call `GHC.Tc.Utils.Monad.recordUnsafeInfer` to mark the module we are compiling as unsafe, passing the warning message along as the reason. 5) `GHC.Tc.Errors.*Unsolved` -- Generates error messages for constraints by actually calling `InstEnv.lookupInstEnv` again! Yes, confusing, but all we know is the constraint that is unresolved or unsafe. For dictionary, all we know is that we need a dictionary of type C, but not what instances are available and how they overlap. So we once again call `lookupInstEnv` to figure that out so we can generate a helpful error message. 6) `GHC.Tc.Utils.Monad.recordUnsafeInfer` -- Save the unsafe result and reason in an IORef called `tcg_safeInfer`. 7) `GHC.Driver.Main.tcRnModule'` -- Reads `tcg_safeInfer` after type-checking, calling `GHC.Driver.Main.markUnsafeInfer` (passing the reason along) when safe-inferrence failed. Note [No defaulting in the ambiguity check] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When simplifying constraints for the ambiguity check, we use solveWantedsAndDrop, not simplifyTopWanteds, so that we do no defaulting. #11947 was an example: f :: Num a => Int -> Int This is ambiguous of course, but we don't want to default the (Num alpha) constraint to (Num Int)! Doing so gives a defaulting warning, but no error. Note [Defaulting insolubles] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If a set of wanteds is insoluble, we have no hope of accepting the program. Yet we do not stop constraint solving, etc., because we may simplify the wanteds to produce better error messages. So, once we have an insoluble constraint, everything we do is just about producing helpful error messages. Should we default in this case or not? Let's look at an example (tcfail004): (f,g) = (1,2,3) With defaulting, we get a conflict between (a0,b0) and (Integer,Integer,Integer). Without defaulting, we get a conflict between (a0,b0) and (a1,b1,c1). I (Richard) find the latter more helpful. Several other test cases (e.g. tcfail005) suggest similarly. So: we should not do class defaulting with insolubles. On the other hand, RuntimeRep-defaulting is different. Witness tcfail078: f :: Integer i => i f = 0 Without RuntimeRep-defaulting, we GHC suggests that Integer should have kind TYPE r0 -> Constraint and then complains that r0 is actually untouchable (presumably, because it can't be sure if `Integer i` entails an equality). If we default, we are told of a clash between (* -> Constraint) and Constraint. The latter seems far better, suggesting we *should* do RuntimeRep-defaulting even on insolubles. But, evidently, not always. Witness UnliftedNewtypesInfinite: newtype Foo = FooC (# Int#, Foo #) This should fail with an occurs-check error on the kind of Foo (with -XUnliftedNewtypes). If we default RuntimeRep-vars, we get Expecting a lifted type, but ‘(# Int#, Foo #)’ is unlifted which is just plain wrong. Conclusion: we should do RuntimeRep-defaulting on insolubles only when the user does not want to hear about RuntimeRep stuff -- that is, when -fprint-explicit-runtime-reps is not set. -} ------------------ simplifyAmbiguityCheck :: Type -> WantedConstraints -> TcM () simplifyAmbiguityCheck :: Type -> WantedConstraints -> TcM () simplifyAmbiguityCheck Type ty WantedConstraints wanteds = do { String -> SDoc -> TcM () traceTc String "simplifyAmbiguityCheck {" (String -> SDoc text String "type = " SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr Type ty SDoc -> SDoc -> SDoc $$ String -> SDoc text String "wanted = " SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr WantedConstraints wanteds) ; (WantedConstraints final_wc, EvBindMap _) <- forall a. TcS a -> TcM (a, EvBindMap) runTcS forall a b. (a -> b) -> a -> b $ WantedConstraints -> TcS WantedConstraints solveWantedsAndDrop WantedConstraints wanteds -- NB: no defaulting! See Note [No defaulting in the ambiguity check] ; String -> SDoc -> TcM () traceTc String "End simplifyAmbiguityCheck }" SDoc empty -- Normally report all errors; but with -XAllowAmbiguousTypes -- report only insoluble ones, since they represent genuinely -- inaccessible code ; Bool allow_ambiguous <- forall gbl lcl. Extension -> TcRnIf gbl lcl Bool xoptM Extension LangExt.AllowAmbiguousTypes ; String -> SDoc -> TcM () traceTc String "reportUnsolved(ambig) {" SDoc empty ; forall (f :: * -> *). Applicative f => Bool -> f () -> f () unless (Bool allow_ambiguous Bool -> Bool -> Bool && Bool -> Bool not (WantedConstraints -> Bool insolubleWC WantedConstraints final_wc)) (forall a. TcM a -> TcM () discardResult (WantedConstraints -> TcM (Bag EvBind) reportUnsolved WantedConstraints final_wc)) ; String -> SDoc -> TcM () traceTc String "reportUnsolved(ambig) }" SDoc empty ; forall (m :: * -> *) a. Monad m => a -> m a return () } ------------------ simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind) simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind) simplifyInteractive WantedConstraints wanteds = String -> SDoc -> TcM () traceTc String "simplifyInteractive" SDoc empty forall (m :: * -> *) a b. Monad m => m a -> m b -> m b >> WantedConstraints -> TcM (Bag EvBind) simplifyTop WantedConstraints wanteds ------------------ simplifyDefault :: ThetaType -- Wanted; has no type variables in it -> TcM Bool -- Return if the constraint is soluble simplifyDefault :: [Type] -> TcM Bool simplifyDefault [Type] theta = do { String -> SDoc -> TcM () traceTc String "simplifyDefault" SDoc empty ; [CtEvidence] wanteds <- CtOrigin -> [Type] -> TcM [CtEvidence] newWanteds CtOrigin DefaultOrigin [Type] theta ; WantedConstraints unsolved <- forall a. TcS a -> TcM a runTcSDeriveds (WantedConstraints -> TcS WantedConstraints solveWantedsAndDrop ([CtEvidence] -> WantedConstraints mkSimpleWC [CtEvidence] wanteds)) ; forall (m :: * -> *) a. Monad m => a -> m a return (WantedConstraints -> Bool isEmptyWC WantedConstraints unsolved) } ------------------ tcCheckGivens :: InertSet -> Bag EvVar -> TcM (Maybe InertSet) -- ^ Return (Just new_inerts) if the Givens are satisfiable, Nothing if definitely -- contradictory tcCheckGivens :: InertSet -> Bag TcTyVar -> TcM (Maybe InertSet) tcCheckGivens InertSet inerts Bag TcTyVar given_ids = do (Bool sat, InertSet new_inerts) <- forall a. InertSet -> TcS a -> TcM (a, InertSet) runTcSInerts InertSet inerts forall a b. (a -> b) -> a -> b $ do String -> SDoc -> TcS () traceTcS String "checkGivens {" (forall a. Outputable a => a -> SDoc ppr InertSet inerts SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr Bag TcTyVar given_ids) TcLclEnv lcl_env <- TcS TcLclEnv TcS.getLclEnv let given_loc :: CtLoc given_loc = TcLevel -> SkolemInfo -> TcLclEnv -> CtLoc mkGivenLoc TcLevel topTcLevel SkolemInfo UnkSkol TcLclEnv lcl_env let given_cts :: [Ct] given_cts = CtLoc -> [TcTyVar] -> [Ct] mkGivens CtLoc given_loc (forall a. Bag a -> [a] bagToList Bag TcTyVar given_ids) -- See Note [Superclasses and satisfiability] [Ct] -> TcS () solveSimpleGivens [Ct] given_cts Cts insols <- TcS Cts getInertInsols Cts insols <- Cts -> TcS Cts try_harder Cts insols String -> SDoc -> TcS () traceTcS String "checkGivens }" (forall a. Outputable a => a -> SDoc ppr Cts insols) forall (m :: * -> *) a. Monad m => a -> m a return (forall a. Bag a -> Bool isEmptyBag Cts insols) forall (m :: * -> *) a. Monad m => a -> m a return forall a b. (a -> b) -> a -> b $ if Bool sat then forall a. a -> Maybe a Just InertSet new_inerts else forall a. Maybe a Nothing where try_harder :: Cts -> TcS Cts -- Maybe we have to search up the superclass chain to find -- an unsatisfiable constraint. Example: pmcheck/T3927b. -- At the moment we try just once try_harder :: Cts -> TcS Cts try_harder Cts insols | Bool -> Bool not (forall a. Bag a -> Bool isEmptyBag Cts insols) -- We've found that it's definitely unsatisfiable = forall (m :: * -> *) a. Monad m => a -> m a return Cts insols -- Hurrah -- stop now. | Bool otherwise = do { [Ct] pending_given <- TcS [Ct] getPendingGivenScs ; [Ct] new_given <- [Ct] -> TcS [Ct] makeSuperClasses [Ct] pending_given ; [Ct] -> TcS () solveSimpleGivens [Ct] new_given ; TcS Cts getInertInsols } tcCheckWanteds :: InertSet -> ThetaType -> TcM Bool -- ^ Return True if the Wanteds are soluble, False if not tcCheckWanteds :: InertSet -> [Type] -> TcM Bool tcCheckWanteds InertSet inerts [Type] wanteds = do [CtEvidence] cts <- CtOrigin -> [Type] -> TcM [CtEvidence] newWanteds CtOrigin PatCheckOrigin [Type] wanteds (Bool sat, InertSet _new_inerts) <- forall a. InertSet -> TcS a -> TcM (a, InertSet) runTcSInerts InertSet inerts forall a b. (a -> b) -> a -> b $ do String -> SDoc -> TcS () traceTcS String "checkWanteds {" (forall a. Outputable a => a -> SDoc ppr InertSet inerts SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] wanteds) -- See Note [Superclasses and satisfiability] WantedConstraints wcs <- WantedConstraints -> TcS WantedConstraints solveWantedsAndDrop ([CtEvidence] -> WantedConstraints mkSimpleWC [CtEvidence] cts) String -> SDoc -> TcS () traceTcS String "checkWanteds }" (forall a. Outputable a => a -> SDoc ppr WantedConstraints wcs) forall (m :: * -> *) a. Monad m => a -> m a return (WantedConstraints -> Bool isSolvedWC WantedConstraints wcs) forall (m :: * -> *) a. Monad m => a -> m a return Bool sat -- | Normalise a type as much as possible using the given constraints. -- See @Note [tcNormalise]@. tcNormalise :: InertSet -> Type -> TcM Type tcNormalise :: InertSet -> Type -> TcM Type tcNormalise InertSet inerts Type ty = do { CtLoc norm_loc <- CtOrigin -> Maybe TypeOrKind -> TcM CtLoc getCtLocM CtOrigin PatCheckOrigin forall a. Maybe a Nothing ; (Type res, InertSet _new_inerts) <- forall a. InertSet -> TcS a -> TcM (a, InertSet) runTcSInerts InertSet inerts forall a b. (a -> b) -> a -> b $ do { String -> SDoc -> TcS () traceTcS String "tcNormalise {" (forall a. Outputable a => a -> SDoc ppr InertSet inerts) ; Type ty' <- CtLoc -> Type -> TcS Type rewriteType CtLoc norm_loc Type ty ; String -> SDoc -> TcS () traceTcS String "tcNormalise }" (forall a. Outputable a => a -> SDoc ppr Type ty') ; forall (f :: * -> *) a. Applicative f => a -> f a pure Type ty' } ; forall (m :: * -> *) a. Monad m => a -> m a return Type res } {- Note [Superclasses and satisfiability] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Expand superclasses before starting, because (Int ~ Bool), has (Int ~~ Bool) as a superclass, which in turn has (Int ~N# Bool) as a superclass, and it's the latter that is insoluble. See Note [The equality types story] in GHC.Builtin.Types.Prim. If we fail to prove unsatisfiability we (arbitrarily) try just once to find superclasses, using try_harder. Reason: we might have a type signature f :: F op (Implements push) => .. where F is a type function. This happened in #3972. We could do more than once but we'd have to have /some/ limit: in the the recursive case, we would go on forever in the common case where the constraints /are/ satisfiable (#10592 comment:12!). For stratightforard situations without type functions the try_harder step does nothing. Note [tcNormalise] ~~~~~~~~~~~~~~~~~~ tcNormalise is a rather atypical entrypoint to the constraint solver. Whereas most invocations of the constraint solver are intended to simplify a set of constraints or to decide if a particular set of constraints is satisfiable, the purpose of tcNormalise is to take a type, plus some locally solved constraints in the form of an InertSet, and normalise the type as much as possible with respect to those constraints. It does *not* reduce type or data family applications or look through newtypes. Why is this useful? As one example, when coverage-checking an EmptyCase expression, it's possible that the type of the scrutinee will only reduce if some local equalities are solved for. See "Wrinkle: Local equalities" in Note [Type normalisation] in "GHC.HsToCore.Pmc". To accomplish its stated goal, tcNormalise first initialises the solver monad with the given InertCans, then uses rewriteType to simplify the desired type with respect to the Givens in the InertCans. *********************************************************************************** * * * Inference * * *********************************************************************************** Note [Inferring the type of a let-bound variable] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider f x = rhs To infer f's type we do the following: * Gather the constraints for the RHS with ambient level *one more than* the current one. This is done by the call pushLevelAndCaptureConstraints (tcMonoBinds...) in GHC.Tc.Gen.Bind.tcPolyInfer * Call simplifyInfer to simplify the constraints and decide what to quantify over. We pass in the level used for the RHS constraints, here called rhs_tclvl. This ensures that the implication constraint we generate, if any, has a strictly-increased level compared to the ambient level outside the let binding. -} -- | How should we choose which constraints to quantify over? data InferMode = ApplyMR -- ^ Apply the monomorphism restriction, -- never quantifying over any constraints | EagerDefaulting -- ^ See Note [TcRnExprMode] in "GHC.Tc.Module", -- the :type +d case; this mode refuses -- to quantify over any defaultable constraint | NoRestrictions -- ^ Quantify over any constraint that -- satisfies 'GHC.Tc.Utils.TcType.pickQuantifiablePreds' instance Outputable InferMode where ppr :: InferMode -> SDoc ppr InferMode ApplyMR = String -> SDoc text String "ApplyMR" ppr InferMode EagerDefaulting = String -> SDoc text String "EagerDefaulting" ppr InferMode NoRestrictions = String -> SDoc text String "NoRestrictions" simplifyInfer :: TcLevel -- Used when generating the constraints -> InferMode -> [TcIdSigInst] -- Any signatures (possibly partial) -> [(Name, TcTauType)] -- Variables to be generalised, -- and their tau-types -> WantedConstraints -> TcM ([TcTyVar], -- Quantify over these type variables [EvVar], -- ... and these constraints (fully zonked) TcEvBinds, -- ... binding these evidence variables Bool) -- True <=> the residual constraints are insoluble simplifyInfer :: TcLevel -> InferMode -> [TcIdSigInst] -> [(Name, Type)] -> WantedConstraints -> TcM ([TcTyVar], [TcTyVar], TcEvBinds, Bool) simplifyInfer TcLevel rhs_tclvl InferMode infer_mode [TcIdSigInst] sigs [(Name, Type)] name_taus WantedConstraints wanteds | WantedConstraints -> Bool isEmptyWC WantedConstraints wanteds = do { -- When quantifying, we want to preserve any order of variables as they -- appear in partial signatures. cf. decideQuantifiedTyVars let psig_tv_tys :: [Type] psig_tv_tys = [ TcTyVar -> Type mkTyVarTy TcTyVar tv | TcIdSigInst sig <- [TcIdSigInst] partial_sigs , (Name _,Bndr TcTyVar tv Specificity _) <- TcIdSigInst -> [(Name, InvisTVBinder)] sig_inst_skols TcIdSigInst sig ] psig_theta :: [Type] psig_theta = [ Type pred | TcIdSigInst sig <- [TcIdSigInst] partial_sigs , Type pred <- TcIdSigInst -> [Type] sig_inst_theta TcIdSigInst sig ] ; CandidatesQTvs dep_vars <- [Type] -> TcM CandidatesQTvs candidateQTyVarsOfTypes ([Type] psig_tv_tys forall a. [a] -> [a] -> [a] ++ [Type] psig_theta forall a. [a] -> [a] -> [a] ++ forall a b. (a -> b) -> [a] -> [b] map forall a b. (a, b) -> b snd [(Name, Type)] name_taus) ; [TcTyVar] qtkvs <- CandidatesQTvs -> TcM [TcTyVar] quantifyTyVars CandidatesQTvs dep_vars ; String -> SDoc -> TcM () traceTc String "simplifyInfer: empty WC" (forall a. Outputable a => a -> SDoc ppr [(Name, Type)] name_taus SDoc -> SDoc -> SDoc $$ forall a. Outputable a => a -> SDoc ppr [TcTyVar] qtkvs) ; forall (m :: * -> *) a. Monad m => a -> m a return ([TcTyVar] qtkvs, [], TcEvBinds emptyTcEvBinds, Bool False) } | Bool otherwise = do { String -> SDoc -> TcM () traceTc String "simplifyInfer {" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "sigs =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [TcIdSigInst] sigs , String -> SDoc text String "binds =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [(Name, Type)] name_taus , String -> SDoc text String "rhs_tclvl =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr TcLevel rhs_tclvl , String -> SDoc text String "infer_mode =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr InferMode infer_mode , String -> SDoc text String "(unzonked) wanted =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr WantedConstraints wanteds ] ; let psig_theta :: [Type] psig_theta = forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b] concatMap TcIdSigInst -> [Type] sig_inst_theta [TcIdSigInst] partial_sigs -- First do full-blown solving -- NB: we must gather up all the bindings from doing -- this solving; hence (runTcSWithEvBinds ev_binds_var). -- And note that since there are nested implications, -- calling solveWanteds will side-effect their evidence -- bindings, so we can't just revert to the input -- constraint. ; EvBindsVar ev_binds_var <- TcM EvBindsVar TcM.newTcEvBinds ; [CtEvidence] psig_evs <- CtOrigin -> [Type] -> TcM [CtEvidence] newWanteds CtOrigin AnnOrigin [Type] psig_theta ; WantedConstraints wanted_transformed_incl_derivs <- forall a. TcLevel -> TcM a -> TcM a setTcLevel TcLevel rhs_tclvl forall a b. (a -> b) -> a -> b $ forall a. EvBindsVar -> TcS a -> TcM a runTcSWithEvBinds EvBindsVar ev_binds_var forall a b. (a -> b) -> a -> b $ WantedConstraints -> TcS WantedConstraints solveWanteds ([CtEvidence] -> WantedConstraints mkSimpleWC [CtEvidence] psig_evs WantedConstraints -> WantedConstraints -> WantedConstraints `andWC` WantedConstraints wanteds) -- psig_evs : see Note [Add signature contexts as wanteds] -- Find quant_pred_candidates, the predicates that -- we'll consider quantifying over -- NB1: wanted_transformed does not include anything provable from -- the psig_theta; it's just the extra bit -- NB2: We do not do any defaulting when inferring a type, this can lead -- to less polymorphic types, see Note [Default while Inferring] ; WantedConstraints wanted_transformed_incl_derivs <- WantedConstraints -> TcM WantedConstraints TcM.zonkWC WantedConstraints wanted_transformed_incl_derivs ; let definite_error :: Bool definite_error = WantedConstraints -> Bool insolubleWC WantedConstraints wanted_transformed_incl_derivs -- See Note [Quantification with errors] -- NB: must include derived errors in this test, -- hence "incl_derivs" wanted_transformed :: WantedConstraints wanted_transformed = WantedConstraints -> WantedConstraints dropDerivedWC WantedConstraints wanted_transformed_incl_derivs quant_pred_candidates :: [Type] quant_pred_candidates | Bool definite_error = [] | Bool otherwise = Cts -> [Type] ctsPreds (Bool -> WantedConstraints -> Cts approximateWC Bool False WantedConstraints wanted_transformed) -- Decide what type variables and constraints to quantify -- NB: quant_pred_candidates is already fully zonked -- NB: bound_theta are constraints we want to quantify over, -- including the psig_theta, which we always quantify over -- NB: bound_theta are fully zonked ; ([TcTyVar] qtvs, [Type] bound_theta, VarSet co_vars) <- InferMode -> TcLevel -> [(Name, Type)] -> [TcIdSigInst] -> [Type] -> TcM ([TcTyVar], [Type], VarSet) decideQuantification InferMode infer_mode TcLevel rhs_tclvl [(Name, Type)] name_taus [TcIdSigInst] partial_sigs [Type] quant_pred_candidates ; [TcTyVar] bound_theta_vars <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM forall gbl lcl. Type -> TcRnIf gbl lcl TcTyVar TcM.newEvVar [Type] bound_theta -- Now emit the residual constraint ; TcLevel -> EvBindsVar -> [(Name, Type)] -> VarSet -> [TcTyVar] -> [TcTyVar] -> WantedConstraints -> TcM () emitResidualConstraints TcLevel rhs_tclvl EvBindsVar ev_binds_var [(Name, Type)] name_taus VarSet co_vars [TcTyVar] qtvs [TcTyVar] bound_theta_vars WantedConstraints wanted_transformed -- All done! ; String -> SDoc -> TcM () traceTc String "} simplifyInfer/produced residual implication for quantification" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "quant_pred_candidates =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] quant_pred_candidates , String -> SDoc text String "psig_theta =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] psig_theta , String -> SDoc text String "bound_theta =" SDoc -> SDoc -> SDoc <+> [TcTyVar] -> SDoc pprCoreBinders [TcTyVar] bound_theta_vars , String -> SDoc text String "qtvs =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [TcTyVar] qtvs , String -> SDoc text String "definite_error =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr Bool definite_error ] ; forall (m :: * -> *) a. Monad m => a -> m a return ( [TcTyVar] qtvs, [TcTyVar] bound_theta_vars, EvBindsVar -> TcEvBinds TcEvBinds EvBindsVar ev_binds_var, Bool definite_error ) } -- NB: bound_theta_vars must be fully zonked where partial_sigs :: [TcIdSigInst] partial_sigs = forall a. (a -> Bool) -> [a] -> [a] filter TcIdSigInst -> Bool isPartialSig [TcIdSigInst] sigs -------------------- emitResidualConstraints :: TcLevel -> EvBindsVar -> [(Name, TcTauType)] -> VarSet -> [TcTyVar] -> [EvVar] -> WantedConstraints -> TcM () -- Emit the remaining constraints from the RHS. emitResidualConstraints :: TcLevel -> EvBindsVar -> [(Name, Type)] -> VarSet -> [TcTyVar] -> [TcTyVar] -> WantedConstraints -> TcM () emitResidualConstraints TcLevel rhs_tclvl EvBindsVar ev_binds_var [(Name, Type)] name_taus VarSet co_vars [TcTyVar] qtvs [TcTyVar] full_theta_vars WantedConstraints wanteds | WantedConstraints -> Bool isEmptyWC WantedConstraints wanteds = forall (m :: * -> *) a. Monad m => a -> m a return () | Bool otherwise = do { Cts wanted_simple <- Cts -> TcM Cts TcM.zonkSimples (WantedConstraints -> Cts wc_simple WantedConstraints wanteds) ; let (Cts outer_simple, Cts inner_simple) = forall a. (a -> Bool) -> Bag a -> (Bag a, Bag a) partitionBag Ct -> Bool is_mono Cts wanted_simple is_mono :: Ct -> Bool is_mono Ct ct = Ct -> Bool isWantedCt Ct ct Bool -> Bool -> Bool && Ct -> TcTyVar ctEvId Ct ct TcTyVar -> VarSet -> Bool `elemVarSet` VarSet co_vars -- Reason for the partition: -- see Note [Emitting the residual implication in simplifyInfer] -- Already done by defaultTyVarsAndSimplify -- ; _ <- TcM.promoteTyVarSet (tyCoVarsOfCts outer_simple) ; let inner_wanted :: WantedConstraints inner_wanted = WantedConstraints wanteds { wc_simple :: Cts wc_simple = Cts inner_simple } ; Bag Implication implics <- if WantedConstraints -> Bool isEmptyWC WantedConstraints inner_wanted then forall (m :: * -> *) a. Monad m => a -> m a return forall a. Bag a emptyBag else do Implication implic1 <- TcM Implication newImplication forall (m :: * -> *) a. Monad m => a -> m a return forall a b. (a -> b) -> a -> b $ forall a. a -> Bag a unitBag forall a b. (a -> b) -> a -> b $ Implication implic1 { ic_tclvl :: TcLevel ic_tclvl = TcLevel rhs_tclvl , ic_skols :: [TcTyVar] ic_skols = [TcTyVar] qtvs , ic_given :: [TcTyVar] ic_given = [TcTyVar] full_theta_vars , ic_wanted :: WantedConstraints ic_wanted = WantedConstraints inner_wanted , ic_binds :: EvBindsVar ic_binds = EvBindsVar ev_binds_var , ic_given_eqs :: HasGivenEqs ic_given_eqs = HasGivenEqs MaybeGivenEqs , ic_info :: SkolemInfo ic_info = SkolemInfo skol_info } ; WantedConstraints -> TcM () emitConstraints (WantedConstraints emptyWC { wc_simple :: Cts wc_simple = Cts outer_simple , wc_impl :: Bag Implication wc_impl = Bag Implication implics }) } where full_theta :: [Type] full_theta = forall a b. (a -> b) -> [a] -> [b] map TcTyVar -> Type idType [TcTyVar] full_theta_vars skol_info :: SkolemInfo skol_info = [(Name, Type)] -> SkolemInfo InferSkol [ (Name name, [TyCoVarBinder] -> [Type] -> Type -> Type mkSigmaTy [] [Type] full_theta Type ty) | (Name name, Type ty) <- [(Name, Type)] name_taus ] -- We don't add the quantified variables here, because they are -- also bound in ic_skols and we want them to be tidied -- uniformly. -------------------- ctsPreds :: Cts -> [PredType] ctsPreds :: Cts -> [Type] ctsPreds Cts cts = [ CtEvidence -> Type ctEvPred CtEvidence ev | Ct ct <- forall a. Bag a -> [a] bagToList Cts cts , let ev :: CtEvidence ev = Ct -> CtEvidence ctEvidence Ct ct ] findInferredDiff :: TcThetaType -> TcThetaType -> TcM TcThetaType -- Given a partial type signature f :: (C a, D a, _) => blah -- and the inferred constraints (X a, D a, Y a, C a) -- compute the difference, which is what will fill in the "_" underscore, -- In this case the diff is (X a, Y a). findInferredDiff :: [Type] -> [Type] -> TcM [Type] findInferredDiff [Type] annotated_theta [Type] inferred_theta | forall (t :: * -> *) a. Foldable t => t a -> Bool null [Type] annotated_theta -- Short cut the common case when the user didn't = forall (m :: * -> *) a. Monad m => a -> m a return [Type] inferred_theta -- write any constraints in the partial signature | Bool otherwise = forall r. TcM r -> TcM r pushTcLevelM_ forall a b. (a -> b) -> a -> b $ do { TcLclEnv lcl_env <- forall gbl lcl. TcRnIf gbl lcl lcl TcM.getLclEnv ; [TcTyVar] given_ids <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM forall gbl lcl. Type -> TcRnIf gbl lcl TcTyVar TcM.newEvVar [Type] annotated_theta ; [CtEvidence] wanteds <- CtOrigin -> [Type] -> TcM [CtEvidence] newWanteds CtOrigin AnnOrigin [Type] inferred_theta ; let given_loc :: CtLoc given_loc = TcLevel -> SkolemInfo -> TcLclEnv -> CtLoc mkGivenLoc TcLevel topTcLevel SkolemInfo UnkSkol TcLclEnv lcl_env given_cts :: [Ct] given_cts = CtLoc -> [TcTyVar] -> [Ct] mkGivens CtLoc given_loc [TcTyVar] given_ids ; WantedConstraints residual <- forall a. TcS a -> TcM a runTcSDeriveds forall a b. (a -> b) -> a -> b $ do { () _ <- [Ct] -> TcS () solveSimpleGivens [Ct] given_cts ; Cts -> TcS WantedConstraints solveSimpleWanteds (forall a. [a] -> Bag a listToBag (forall a b. (a -> b) -> [a] -> [b] map CtEvidence -> Ct mkNonCanonical [CtEvidence] wanteds)) } -- NB: There are no meta tyvars fromn this level annotated_theta -- because we have either promoted them or unified them -- See `Note [Quantification and partial signatures]` Wrinkle 2 ; forall (m :: * -> *) a. Monad m => a -> m a return (forall a b. (a -> b) -> [a] -> [b] map (Type -> Type box_pred forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> Type ctPred) forall a b. (a -> b) -> a -> b $ forall a. Bag a -> [a] bagToList forall a b. (a -> b) -> a -> b $ WantedConstraints -> Cts wc_simple WantedConstraints residual) } where box_pred :: PredType -> PredType box_pred :: Type -> Type box_pred Type pred = case Type -> Pred classifyPredType Type pred of EqPred EqRel rel Type ty1 Type ty2 | Just (Class cls,[Type] tys) <- EqRel -> Type -> Type -> Maybe (Class, [Type]) boxEqPred EqRel rel Type ty1 Type ty2 -> Class -> [Type] -> Type mkClassPred Class cls [Type] tys | Bool otherwise -> forall a. HasCallStack => String -> SDoc -> a pprPanic String "findInferredDiff" (forall a. Outputable a => a -> SDoc ppr Type pred) Pred _other -> Type pred {- Note [Emitting the residual implication in simplifyInfer] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider f = e where f's type is inferred to be something like (a, Proxy k (Int |> co)) and we have an as-yet-unsolved, or perhaps insoluble, constraint [W] co :: Type ~ k We can't form types like (forall co. blah), so we can't generalise over the coercion variable, and hence we can't generalise over things free in its kind, in the case 'k'. But we can still generalise over 'a'. So we'll generalise to f :: forall a. (a, Proxy k (Int |> co)) Now we do NOT want to form the residual implication constraint forall a. [W] co :: Type ~ k because then co's eventual binding (which will be a value binding if we use -fdefer-type-errors) won't scope over the entire binding for 'f' (whose type mentions 'co'). Instead, just as we don't generalise over 'co', we should not bury its constraint inside the implication. Instead, we must put it outside. That is the reason for the partitionBag in emitResidualConstraints, which takes the CoVars free in the inferred type, and pulls their constraints out. (NB: this set of CoVars should be closed-over-kinds.) All rather subtle; see #14584. Note [Add signature contexts as wanteds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this (#11016): f2 :: (?x :: Int) => _ f2 = ?x or this class C a b | a -> b g :: C p q => p -> q f3 :: C Int b => _ f3 = g (3::Int) We'll use plan InferGen because there are holes in the type. But: * For f2 we want to have the (?x :: Int) constraint floating around so that the functional dependencies kick in. Otherwise the occurrence of ?x on the RHS produces constraint (?x :: alpha), and we won't unify alpha:=Int. * For f3 want the (C Int b) constraint from the partial signature to meet the (C Int beta) constraint we get from the call to g; again, fundeps Solution: in simplifyInfer, we add the constraints from the signature as extra Wanteds. Why Wanteds? Wouldn't it be neater to treat them as Givens? Alas that would mess up (GivenInv) in Note [TcLevel invariants]. Consider f :: (Eq a, _) => blah1 f = ....g... g :: (Eq b, _) => blah2 g = ...f... Then we have two psig_theta constraints (Eq a[tv], Eq b[tv]), both with TyVarTvs inside. Ultimately a[tv] := b[tv], but only when we've solved all those constraints. And both have level 1, so we can't put them as Givens when solving at level 1. Best to treat them as Wanteds. But see also #20076, which would be solved if they were Givens. ************************************************************************ * * Quantification * * ************************************************************************ Note [Deciding quantification] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If the monomorphism restriction does not apply, then we quantify as follows: * Step 1. Take the global tyvars, and "grow" them using the equality constraints E.g. if x:alpha is in the environment, and alpha ~ [beta] (which can happen because alpha is untouchable here) then do not quantify over beta, because alpha fixes beta, and beta is effectively free in the environment too We also account for the monomorphism restriction; if it applies, add the free vars of all the constraints. Result is mono_tvs; we will not quantify over these. * Step 2. Default any non-mono tyvars (i.e ones that are definitely not going to become further constrained), and re-simplify the candidate constraints. Motivation for re-simplification (#7857): imagine we have a constraint (C (a->b)), where 'a :: TYPE l1' and 'b :: TYPE l2' are not free in the envt, and instance forall (a::*) (b::*). (C a) => C (a -> b) The instance doesn't match while l1,l2 are polymorphic, but it will match when we default them to LiftedRep. This is all very tiresome. * Step 3: decide which variables to quantify over, as follows: - Take the free vars of the tau-type (zonked_tau_tvs) and "grow" them using all the constraints. These are tau_tvs_plus - Use quantifyTyVars to quantify over (tau_tvs_plus - mono_tvs), being careful to close over kinds, and to skolemise the quantified tyvars. (This actually unifies each quantifies meta-tyvar with a fresh skolem.) Result is qtvs. * Step 4: Filter the constraints using pickQuantifiablePreds and the qtvs. We have to zonk the constraints first, so they "see" the freshly created skolems. -} decideQuantification :: InferMode -> TcLevel -> [(Name, TcTauType)] -- Variables to be generalised -> [TcIdSigInst] -- Partial type signatures (if any) -> [PredType] -- Candidate theta; already zonked -> TcM ( [TcTyVar] -- Quantify over these (skolems) , [PredType] -- and this context (fully zonked) , VarSet) -- See Note [Deciding quantification] decideQuantification :: InferMode -> TcLevel -> [(Name, Type)] -> [TcIdSigInst] -> [Type] -> TcM ([TcTyVar], [Type], VarSet) decideQuantification InferMode infer_mode TcLevel rhs_tclvl [(Name, Type)] name_taus [TcIdSigInst] psigs [Type] candidates = do { -- Step 1: find the mono_tvs ; (VarSet mono_tvs, [Type] candidates, VarSet co_vars) <- InferMode -> [(Name, Type)] -> [TcIdSigInst] -> [Type] -> TcM (VarSet, [Type], VarSet) decideMonoTyVars InferMode infer_mode [(Name, Type)] name_taus [TcIdSigInst] psigs [Type] candidates -- Step 2: default any non-mono tyvars, and re-simplify -- This step may do some unification, but result candidates is zonked ; [Type] candidates <- TcLevel -> VarSet -> [Type] -> TcM [Type] defaultTyVarsAndSimplify TcLevel rhs_tclvl VarSet mono_tvs [Type] candidates -- Step 3: decide which kind/type variables to quantify over ; [TcTyVar] qtvs <- [(Name, Type)] -> [TcIdSigInst] -> [Type] -> TcM [TcTyVar] decideQuantifiedTyVars [(Name, Type)] name_taus [TcIdSigInst] psigs [Type] candidates -- Step 4: choose which of the remaining candidate -- predicates to actually quantify over -- NB: decideQuantifiedTyVars turned some meta tyvars -- into quantified skolems, so we have to zonk again ; [Type] candidates <- [Type] -> TcM [Type] TcM.zonkTcTypes [Type] candidates ; [Type] psig_theta <- [Type] -> TcM [Type] TcM.zonkTcTypes (forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b] concatMap TcIdSigInst -> [Type] sig_inst_theta [TcIdSigInst] psigs) ; let min_theta :: [Type] min_theta = forall a. (a -> Type) -> [a] -> [a] mkMinimalBySCs forall a. a -> a id forall a b. (a -> b) -> a -> b $ -- See Note [Minimize by Superclasses] VarSet -> [Type] -> [Type] pickQuantifiablePreds ([TcTyVar] -> VarSet mkVarSet [TcTyVar] qtvs) [Type] candidates min_psig_theta :: [Type] min_psig_theta = forall a. (a -> Type) -> [a] -> [a] mkMinimalBySCs forall a. a -> a id [Type] psig_theta -- Add psig_theta back in here, even though it's already -- part of candidates, because we always want to quantify over -- psig_theta, and pickQuantifiableCandidates might have -- dropped some e.g. CallStack constraints. c.f #14658 -- equalities (a ~ Bool) -- It's helpful to use the same "find difference" algorithm here as -- we use in GHC.Tc.Gen.Bind.chooseInferredQuantifiers (#20921) -- See Note [Constraints in partial type signatures] ; [Type] theta <- if forall (t :: * -> *) a. Foldable t => t a -> Bool null [Type] psig_theta then forall (m :: * -> *) a. Monad m => a -> m a return [Type] min_theta -- Fast path for the non-partial-sig case else do { [Type] diff <- [Type] -> [Type] -> TcM [Type] findInferredDiff [Type] min_psig_theta [Type] min_theta ; forall (m :: * -> *) a. Monad m => a -> m a return ([Type] min_psig_theta forall a. [a] -> [a] -> [a] ++ [Type] diff) } ; String -> SDoc -> TcM () traceTc String "decideQuantification" ([SDoc] -> SDoc vcat [ String -> SDoc text String "infer_mode:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr InferMode infer_mode , String -> SDoc text String "candidates:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] candidates , String -> SDoc text String "psig_theta:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] psig_theta , String -> SDoc text String "mono_tvs:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr VarSet mono_tvs , String -> SDoc text String "co_vars:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr VarSet co_vars , String -> SDoc text String "qtvs:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [TcTyVar] qtvs , String -> SDoc text String "theta:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] theta ]) ; forall (m :: * -> *) a. Monad m => a -> m a return ([TcTyVar] qtvs, [Type] theta, VarSet co_vars) } {- Note [Constraints in partial type signatures] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have a partial type signature f :: (Eq a, C a, _) => blah We will ultimately quantify f over (Eq a, C a, <diff>), where * <diff> is the result of findInferredDiff (Eq a, C a) <quant-theta> in GHC.Tc.Gen.Bind.chooseInferredQuantifiers * <quant-theta> is the theta returned right here, by decideQuantification At least for single functions we would like to quantify f over precisely the same theta as <quant-theta>, so that we get to take the short-cut path in GHC.Tc.Gen.Bind.mkExport, and avoid calling tcSubTypeSigma for impedence matching. Why avoid? Because it falls over for ambiguous types (#20921). We can get precisely the same theta by using the same algorithm, findInferredDiff. All of this goes wrong if we have (a) mutual recursion, (b) mutiple partial type signatures, (c) with different constraints, and (d) ambiguous types. Something like f :: forall a. Eq a => F a -> _ f x = (undefined :: a) == g x undefined g :: forall b. Show b => F b -> _ -> b g x y = let _ = (f y, show x) in x But that's a battle for another day. -} decideMonoTyVars :: InferMode -> [(Name,TcType)] -> [TcIdSigInst] -> [PredType] -> TcM (TcTyCoVarSet, [PredType], CoVarSet) -- Decide which tyvars and covars cannot be generalised: -- (a) Free in the environment -- (b) Mentioned in a constraint we can't generalise -- (c) Connected by an equality to (a) or (b) -- Also return CoVars that appear free in the final quantified types -- we can't quantify over these, and we must make sure they are in scope decideMonoTyVars :: InferMode -> [(Name, Type)] -> [TcIdSigInst] -> [Type] -> TcM (VarSet, [Type], VarSet) decideMonoTyVars InferMode infer_mode [(Name, Type)] name_taus [TcIdSigInst] psigs [Type] candidates = do { ([Type] no_quant, [Type] maybe_quant) <- InferMode -> [Type] -> TcM ([Type], [Type]) pick InferMode infer_mode [Type] candidates -- If possible, we quantify over partial-sig qtvs, so they are -- not mono. Need to zonk them because they are meta-tyvar TyVarTvs ; [TcTyVar] psig_qtvs <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM HasDebugCallStack => TcTyVar -> TcM TcTyVar zonkTcTyVarToTyVar forall a b. (a -> b) -> a -> b $ forall tv argf. [VarBndr tv argf] -> [tv] binderVars forall a b. (a -> b) -> a -> b $ forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b] concatMap (forall a b. (a -> b) -> [a] -> [b] map forall a b. (a, b) -> b snd forall b c a. (b -> c) -> (a -> b) -> a -> c . TcIdSigInst -> [(Name, InvisTVBinder)] sig_inst_skols) [TcIdSigInst] psigs ; [Type] psig_theta <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM Type -> TcM Type TcM.zonkTcType forall a b. (a -> b) -> a -> b $ forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b] concatMap TcIdSigInst -> [Type] sig_inst_theta [TcIdSigInst] psigs ; [Type] taus <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM (Type -> TcM Type TcM.zonkTcType forall b c a. (b -> c) -> (a -> b) -> a -> c . forall a b. (a, b) -> b snd) [(Name, Type)] name_taus ; TcLevel tc_lvl <- TcM TcLevel TcM.getTcLevel ; let psig_tys :: [Type] psig_tys = [TcTyVar] -> [Type] mkTyVarTys [TcTyVar] psig_qtvs forall a. [a] -> [a] -> [a] ++ [Type] psig_theta co_vars :: VarSet co_vars = [Type] -> VarSet coVarsOfTypes ([Type] psig_tys forall a. [a] -> [a] -> [a] ++ [Type] taus) co_var_tvs :: VarSet co_var_tvs = VarSet -> VarSet closeOverKinds VarSet co_vars -- The co_var_tvs are tvs mentioned in the types of covars or -- coercion holes. We can't quantify over these covars, so we -- must include the variable in their types in the mono_tvs. -- E.g. If we can't quantify over co :: k~Type, then we can't -- quantify over k either! Hence closeOverKinds mono_tvs0 :: VarSet mono_tvs0 = (TcTyVar -> Bool) -> VarSet -> VarSet filterVarSet (Bool -> Bool not forall b c a. (b -> c) -> (a -> b) -> a -> c . TcLevel -> TcTyVar -> Bool isQuantifiableTv TcLevel tc_lvl) forall a b. (a -> b) -> a -> b $ [Type] -> VarSet tyCoVarsOfTypes [Type] candidates -- We need to grab all the non-quantifiable tyvars in the -- candidates so that we can grow this set to find other -- non-quantifiable tyvars. This can happen with something -- like -- f x y = ... -- where z = x 3 -- The body of z tries to unify the type of x (call it alpha[1]) -- with (beta[2] -> gamma[2]). This unification fails because -- alpha is untouchable. But we need to know not to quantify over -- beta or gamma, because they are in the equality constraint with -- alpha. Actual test case: typecheck/should_compile/tc213 mono_tvs1 :: VarSet mono_tvs1 = VarSet mono_tvs0 VarSet -> VarSet -> VarSet `unionVarSet` VarSet co_var_tvs eq_constraints :: [Type] eq_constraints = forall a. (a -> Bool) -> [a] -> [a] filter Type -> Bool isEqPrimPred [Type] candidates mono_tvs2 :: VarSet mono_tvs2 = [Type] -> VarSet -> VarSet growThetaTyVars [Type] eq_constraints VarSet mono_tvs1 constrained_tvs :: VarSet constrained_tvs = (TcTyVar -> Bool) -> VarSet -> VarSet filterVarSet (TcLevel -> TcTyVar -> Bool isQuantifiableTv TcLevel tc_lvl) forall a b. (a -> b) -> a -> b $ ([Type] -> VarSet -> VarSet growThetaTyVars [Type] eq_constraints ([Type] -> VarSet tyCoVarsOfTypes [Type] no_quant) VarSet -> VarSet -> VarSet `minusVarSet` VarSet mono_tvs2) VarSet -> [TcTyVar] -> VarSet `delVarSetList` [TcTyVar] psig_qtvs -- constrained_tvs: the tyvars that we are not going to -- quantify solely because of the monomorphism restriction -- -- (`minusVarSet` mono_tvs2`): a type variable is only -- "constrained" (so that the MR bites) if it is not -- free in the environment (#13785) -- -- (`delVarSetList` psig_qtvs): if the user has explicitly -- asked for quantification, then that request "wins" -- over the MR. Note: do /not/ delete psig_qtvs from -- mono_tvs1, because mono_tvs1 cannot under any circumstances -- be quantified (#14479); see -- Note [Quantification and partial signatures], Wrinkle 3, 4 mono_tvs :: VarSet mono_tvs = VarSet mono_tvs2 VarSet -> VarSet -> VarSet `unionVarSet` VarSet constrained_tvs -- Warn about the monomorphism restriction ; Bool warn_mono <- forall gbl lcl. WarningFlag -> TcRnIf gbl lcl Bool woptM WarningFlag Opt_WarnMonomorphism ; forall (f :: * -> *). Applicative f => Bool -> f () -> f () when (case InferMode infer_mode of { InferMode ApplyMR -> Bool warn_mono; InferMode _ -> Bool False}) forall a b. (a -> b) -> a -> b $ WarnReason -> Bool -> SDoc -> TcM () warnTc (WarningFlag -> WarnReason Reason WarningFlag Opt_WarnMonomorphism) (VarSet constrained_tvs VarSet -> VarSet -> Bool `intersectsVarSet` [Type] -> VarSet tyCoVarsOfTypes [Type] taus) SDoc mr_msg ; String -> SDoc -> TcM () traceTc String "decideMonoTyVars" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "infer_mode =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr InferMode infer_mode , String -> SDoc text String "mono_tvs0 =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr VarSet mono_tvs0 , String -> SDoc text String "no_quant =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] no_quant , String -> SDoc text String "maybe_quant =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] maybe_quant , String -> SDoc text String "eq_constraints =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] eq_constraints , String -> SDoc text String "mono_tvs =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr VarSet mono_tvs , String -> SDoc text String "co_vars =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr VarSet co_vars ] ; forall (m :: * -> *) a. Monad m => a -> m a return (VarSet mono_tvs, [Type] maybe_quant, VarSet co_vars) } where pick :: InferMode -> [PredType] -> TcM ([PredType], [PredType]) -- Split the candidates into ones we definitely -- won't quantify, and ones that we might pick :: InferMode -> [Type] -> TcM ([Type], [Type]) pick InferMode NoRestrictions [Type] cand = forall (m :: * -> *) a. Monad m => a -> m a return ([], [Type] cand) pick InferMode ApplyMR [Type] cand = forall (m :: * -> *) a. Monad m => a -> m a return ([Type] cand, []) pick InferMode EagerDefaulting [Type] cand = do { Bool os <- forall gbl lcl. Extension -> TcRnIf gbl lcl Bool xoptM Extension LangExt.OverloadedStrings ; forall (m :: * -> *) a. Monad m => a -> m a return (forall a. (a -> Bool) -> [a] -> ([a], [a]) partition (Bool -> Type -> Bool is_int_ct Bool os) [Type] cand) } -- For EagerDefaulting, do not quantify over -- over any interactive class constraint is_int_ct :: Bool -> Type -> Bool is_int_ct Bool ovl_strings Type pred | Just (Class cls, [Type] _) <- Type -> Maybe (Class, [Type]) getClassPredTys_maybe Type pred = Bool -> Class -> Bool isInteractiveClass Bool ovl_strings Class cls | Bool otherwise = Bool False pp_bndrs :: SDoc pp_bndrs = forall a. (a -> SDoc) -> [a] -> SDoc pprWithCommas (SDoc -> SDoc quotes forall b c a. (b -> c) -> (a -> b) -> a -> c . forall a. Outputable a => a -> SDoc ppr forall b c a. (b -> c) -> (a -> b) -> a -> c . forall a b. (a, b) -> a fst) [(Name, Type)] name_taus mr_msg :: SDoc mr_msg = SDoc -> Int -> SDoc -> SDoc hang ([SDoc] -> SDoc sep [ String -> SDoc text String "The Monomorphism Restriction applies to the binding" SDoc -> SDoc -> SDoc <> forall a. [a] -> SDoc plural [(Name, Type)] name_taus , String -> SDoc text String "for" SDoc -> SDoc -> SDoc <+> SDoc pp_bndrs ]) Int 2 ([SDoc] -> SDoc hsep [ String -> SDoc text String "Consider giving" , String -> SDoc text (if forall a. [a] -> Bool isSingleton [(Name, Type)] name_taus then String "it" else String "them") , String -> SDoc text String "a type signature"]) ------------------- defaultTyVarsAndSimplify :: TcLevel -> TyCoVarSet -> [PredType] -- Assumed zonked -> TcM [PredType] -- Guaranteed zonked -- Default any tyvar free in the constraints, -- and re-simplify in case the defaulting allows further simplification defaultTyVarsAndSimplify :: TcLevel -> VarSet -> [Type] -> TcM [Type] defaultTyVarsAndSimplify TcLevel rhs_tclvl VarSet mono_tvs [Type] candidates = do { -- Promote any tyvars that we cannot generalise -- See Note [Promote monomorphic tyvars] ; String -> SDoc -> TcM () traceTc String "decideMonoTyVars: promotion:" (forall a. Outputable a => a -> SDoc ppr VarSet mono_tvs) ; Bool any_promoted <- VarSet -> TcM Bool promoteTyVarSet VarSet mono_tvs -- Default any kind/levity vars ; DV {dv_kvs :: CandidatesQTvs -> DTyVarSet dv_kvs = DTyVarSet cand_kvs, dv_tvs :: CandidatesQTvs -> DTyVarSet dv_tvs = DTyVarSet cand_tvs} <- [Type] -> TcM CandidatesQTvs candidateQTyVarsOfTypes [Type] candidates -- any covars should already be handled by -- the logic in decideMonoTyVars, which looks at -- the constraints generated ; Bool poly_kinds <- forall gbl lcl. Extension -> TcRnIf gbl lcl Bool xoptM Extension LangExt.PolyKinds ; [Bool] default_kvs <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM (Bool -> Bool -> TcTyVar -> TcM Bool default_one Bool poly_kinds Bool True) (DTyVarSet -> [TcTyVar] dVarSetElems DTyVarSet cand_kvs) ; [Bool] default_tvs <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM (Bool -> Bool -> TcTyVar -> TcM Bool default_one Bool poly_kinds Bool False) (DTyVarSet -> [TcTyVar] dVarSetElems (DTyVarSet cand_tvs DTyVarSet -> DTyVarSet -> DTyVarSet `minusDVarSet` DTyVarSet cand_kvs)) ; let some_default :: Bool some_default = forall (t :: * -> *). Foldable t => t Bool -> Bool or [Bool] default_kvs Bool -> Bool -> Bool || forall (t :: * -> *). Foldable t => t Bool -> Bool or [Bool] default_tvs ; case () of () _ | Bool some_default -> [Type] -> TcM [Type] simplify_cand [Type] candidates | Bool any_promoted -> forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM Type -> TcM Type TcM.zonkTcType [Type] candidates | Bool otherwise -> forall (m :: * -> *) a. Monad m => a -> m a return [Type] candidates } where default_one :: Bool -> Bool -> TcTyVar -> TcM Bool default_one Bool poly_kinds Bool is_kind_var TcTyVar tv | Bool -> Bool not (TcTyVar -> Bool isMetaTyVar TcTyVar tv) = forall (m :: * -> *) a. Monad m => a -> m a return Bool False | TcTyVar tv TcTyVar -> VarSet -> Bool `elemVarSet` VarSet mono_tvs = forall (m :: * -> *) a. Monad m => a -> m a return Bool False | Bool otherwise = Bool -> TcTyVar -> TcM Bool defaultTyVar (Bool -> Bool not Bool poly_kinds Bool -> Bool -> Bool && Bool is_kind_var) TcTyVar tv simplify_cand :: [Type] -> TcM [Type] simplify_cand [Type] candidates = do { [CtEvidence] clone_wanteds <- CtOrigin -> [Type] -> TcM [CtEvidence] newWanteds CtOrigin DefaultOrigin [Type] candidates ; WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples } <- forall a. TcLevel -> TcM a -> TcM a setTcLevel TcLevel rhs_tclvl forall a b. (a -> b) -> a -> b $ [CtEvidence] -> TcM WantedConstraints simplifyWantedsTcM [CtEvidence] clone_wanteds -- Discard evidence; simples is fully zonked ; let new_candidates :: [Type] new_candidates = Cts -> [Type] ctsPreds Cts simples ; String -> SDoc -> TcM () traceTc String "Simplified after defaulting" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "Before:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] candidates , String -> SDoc text String "After:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] new_candidates ] ; forall (m :: * -> *) a. Monad m => a -> m a return [Type] new_candidates } ------------------ decideQuantifiedTyVars :: [(Name,TcType)] -- Annotated theta and (name,tau) pairs -> [TcIdSigInst] -- Partial signatures -> [PredType] -- Candidates, zonked -> TcM [TyVar] -- Fix what tyvars we are going to quantify over, and quantify them decideQuantifiedTyVars :: [(Name, Type)] -> [TcIdSigInst] -> [Type] -> TcM [TcTyVar] decideQuantifiedTyVars [(Name, Type)] name_taus [TcIdSigInst] psigs [Type] candidates = do { -- Why psig_tys? We try to quantify over everything free in here -- See Note [Quantification and partial signatures] -- Wrinkles 2 and 3 ; [Type] psig_tv_tys <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM TcTyVar -> TcM Type TcM.zonkTcTyVar [ TcTyVar tv | TcIdSigInst sig <- [TcIdSigInst] psigs , (Name _,Bndr TcTyVar tv Specificity _) <- TcIdSigInst -> [(Name, InvisTVBinder)] sig_inst_skols TcIdSigInst sig ] ; [Type] psig_theta <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM Type -> TcM Type TcM.zonkTcType [ Type pred | TcIdSigInst sig <- [TcIdSigInst] psigs , Type pred <- TcIdSigInst -> [Type] sig_inst_theta TcIdSigInst sig ] ; [Type] tau_tys <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM (Type -> TcM Type TcM.zonkTcType forall b c a. (b -> c) -> (a -> b) -> a -> c . forall a b. (a, b) -> b snd) [(Name, Type)] name_taus ; let -- Try to quantify over variables free in these types psig_tys :: [Type] psig_tys = [Type] psig_tv_tys forall a. [a] -> [a] -> [a] ++ [Type] psig_theta seed_tys :: [Type] seed_tys = [Type] psig_tys forall a. [a] -> [a] -> [a] ++ [Type] tau_tys -- Now "grow" those seeds to find ones reachable via 'candidates' grown_tcvs :: VarSet grown_tcvs = [Type] -> VarSet -> VarSet growThetaTyVars [Type] candidates ([Type] -> VarSet tyCoVarsOfTypes [Type] seed_tys) -- Now we have to classify them into kind variables and type variables -- (sigh) just for the benefit of -XNoPolyKinds; see quantifyTyVars -- -- Keep the psig_tys first, so that candidateQTyVarsOfTypes produces -- them in that order, so that the final qtvs quantifies in the same -- order as the partial signatures do (#13524) ; dv :: CandidatesQTvs dv@DV {dv_kvs :: CandidatesQTvs -> DTyVarSet dv_kvs = DTyVarSet cand_kvs, dv_tvs :: CandidatesQTvs -> DTyVarSet dv_tvs = DTyVarSet cand_tvs} <- [Type] -> TcM CandidatesQTvs candidateQTyVarsOfTypes forall a b. (a -> b) -> a -> b $ [Type] psig_tys forall a. [a] -> [a] -> [a] ++ [Type] candidates forall a. [a] -> [a] -> [a] ++ [Type] tau_tys ; let pick :: DTyVarSet -> DTyVarSet pick = (DTyVarSet -> VarSet -> DTyVarSet `dVarSetIntersectVarSet` VarSet grown_tcvs) dvs_plus :: CandidatesQTvs dvs_plus = CandidatesQTvs dv { dv_kvs :: DTyVarSet dv_kvs = DTyVarSet -> DTyVarSet pick DTyVarSet cand_kvs, dv_tvs :: DTyVarSet dv_tvs = DTyVarSet -> DTyVarSet pick DTyVarSet cand_tvs } ; String -> SDoc -> TcM () traceTc String "decideQuantifiedTyVars" ([SDoc] -> SDoc vcat [ String -> SDoc text String "tau_tys =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] tau_tys , String -> SDoc text String "candidates =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] candidates , String -> SDoc text String "cand_kvs =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr DTyVarSet cand_kvs , String -> SDoc text String "cand_tvs =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr DTyVarSet cand_tvs , String -> SDoc text String "tau_tys =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] tau_tys , String -> SDoc text String "seed_tys =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [Type] seed_tys , String -> SDoc text String "seed_tcvs =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr ([Type] -> VarSet tyCoVarsOfTypes [Type] seed_tys) , String -> SDoc text String "grown_tcvs =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr VarSet grown_tcvs , String -> SDoc text String "dvs =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr CandidatesQTvs dvs_plus]) ; CandidatesQTvs -> TcM [TcTyVar] quantifyTyVars CandidatesQTvs dvs_plus } ------------------ growThetaTyVars :: ThetaType -> TyCoVarSet -> TyCoVarSet -- See Note [Growing the tau-tvs using constraints] growThetaTyVars :: [Type] -> VarSet -> VarSet growThetaTyVars [Type] theta VarSet tcvs | forall (t :: * -> *) a. Foldable t => t a -> Bool null [Type] theta = VarSet tcvs | Bool otherwise = (VarSet -> VarSet) -> VarSet -> VarSet transCloVarSet VarSet -> VarSet mk_next VarSet seed_tcvs where seed_tcvs :: VarSet seed_tcvs = VarSet tcvs VarSet -> VarSet -> VarSet `unionVarSet` [Type] -> VarSet tyCoVarsOfTypes [Type] ips ([Type] ips, [Type] non_ips) = forall a. (a -> Bool) -> [a] -> ([a], [a]) partition Type -> Bool isIPLikePred [Type] theta -- See Note [Inheriting implicit parameters] in GHC.Tc.Utils.TcType mk_next :: VarSet -> VarSet -- Maps current set to newly-grown ones mk_next :: VarSet -> VarSet mk_next VarSet so_far = forall (t :: * -> *) a b. Foldable t => (a -> b -> b) -> b -> t a -> b foldr (VarSet -> Type -> VarSet -> VarSet grow_one VarSet so_far) VarSet emptyVarSet [Type] non_ips grow_one :: VarSet -> Type -> VarSet -> VarSet grow_one VarSet so_far Type pred VarSet tcvs | VarSet pred_tcvs VarSet -> VarSet -> Bool `intersectsVarSet` VarSet so_far = VarSet tcvs VarSet -> VarSet -> VarSet `unionVarSet` VarSet pred_tcvs | Bool otherwise = VarSet tcvs where pred_tcvs :: VarSet pred_tcvs = Type -> VarSet tyCoVarsOfType Type pred {- Note [Promote monomorphic tyvars] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Promote any type variables that are free in the environment. Eg f :: forall qtvs. bound_theta => zonked_tau The free vars of f's type become free in the envt, and hence will show up whenever 'f' is called. They may currently at rhs_tclvl, but they had better be unifiable at the outer_tclvl! Example: envt mentions alpha[1] tau_ty = beta[2] -> beta[2] constraints = alpha ~ [beta] we don't quantify over beta (since it is fixed by envt) so we must promote it! The inferred type is just f :: beta -> beta NB: promoteTyVarSet ignores coercion variables Note [Quantification and partial signatures] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When choosing type variables to quantify, the basic plan is to quantify over all type variables that are * free in the tau_tvs, and * not forced to be monomorphic (mono_tvs), for example by being free in the environment. However, in the case of a partial type signature, be doing inference *in the presence of a type signature*. For example: f :: _ -> a f x = ... or g :: (Eq _a) => _b -> _b In both cases we use plan InferGen, and hence call simplifyInfer. But those 'a' variables are skolems (actually TyVarTvs), and we should be sure to quantify over them. This leads to several wrinkles: * Wrinkle 1. In the case of a type error f :: _ -> Maybe a f x = True && x The inferred type of 'f' is f :: Bool -> Bool, but there's a left-over error of form (HoleCan (Maybe a ~ Bool)). The error-reporting machine expects to find a binding site for the skolem 'a', so we add it to the quantified tyvars. * Wrinkle 2. Consider the partial type signature f :: (Eq _) => Int -> Int f x = x In normal cases that makes sense; e.g. g :: Eq _a => _a -> _a g x = x where the signature makes the type less general than it could be. But for 'f' we must therefore quantify over the user-annotated constraints, to get f :: forall a. Eq a => Int -> Int (thereby correctly triggering an ambiguity error later). If we don't we'll end up with a strange open type f :: Eq alpha => Int -> Int which isn't ambiguous but is still very wrong. Bottom line: Try to quantify over any variable free in psig_theta, just like the tau-part of the type. * Wrinkle 3 (#13482). Also consider f :: forall a. _ => Int -> Int f x = if (undefined :: a) == undefined then x else 0 Here we get an (Eq a) constraint, but it's not mentioned in the psig_theta nor the type of 'f'. But we still want to quantify over 'a' even if the monomorphism restriction is on. * Wrinkle 4 (#14479) foo :: Num a => a -> a foo xxx = g xxx where g :: forall b. Num b => _ -> b g y = xxx + y In the signature for 'g', we cannot quantify over 'b' because it turns out to get unified with 'a', which is free in g's environment. So we carefully refrain from bogusly quantifying, in GHC.Tc.Solver.decideMonoTyVars. We report the error later, in GHC.Tc.Gen.Bind.chooseInferredQuantifiers. Note [Growing the tau-tvs using constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (growThetaTyVars insts tvs) is the result of extending the set of tyvars, tvs, using all conceivable links from pred E.g. tvs = {a}, preds = {H [a] b, K (b,Int) c, Eq e} Then growThetaTyVars preds tvs = {a,b,c} Notice that growThetaTyVars is conservative if v might be fixed by vs => v `elem` grow(vs,C) Note [Quantification with errors] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If we find that the RHS of the definition has some absolutely-insoluble constraints (including especially "variable not in scope"), we * Abandon all attempts to find a context to quantify over, and instead make the function fully-polymorphic in whatever type we have found * Return a flag from simplifyInfer, indicating that we found an insoluble constraint. This flag is used to suppress the ambiguity check for the inferred type, which may well be bogus, and which tends to obscure the real error. This fix feels a bit clunky, but I failed to come up with anything better. Reasons: - Avoid downstream errors - Do not perform an ambiguity test on a bogus type, which might well fail spuriously, thereby obfuscating the original insoluble error. #14000 is an example I tried an alternative approach: simply failM, after emitting the residual implication constraint; the exception will be caught in GHC.Tc.Gen.Bind.tcPolyBinds, which gives all the binders in the group the type (forall a. a). But that didn't work with -fdefer-type-errors, because the recovery from failM emits no code at all, so there is no function to run! But -fdefer-type-errors aspires to produce a runnable program. NB that we must include *derived* errors in the check for insolubles. Example: (a::*) ~ Int# We get an insoluble derived error *~#, and we don't want to discard it before doing the isInsolubleWC test! (#8262) Note [Default while Inferring] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Our current plan is that defaulting only happens at simplifyTop and not simplifyInfer. This may lead to some insoluble deferred constraints. Example: instance D g => C g Int b constraint inferred = (forall b. 0 => C gamma alpha b) /\ Num alpha type inferred = gamma -> gamma Now, if we try to default (alpha := Int) we will be able to refine the implication to (forall b. 0 => C gamma Int b) which can then be simplified further to (forall b. 0 => D gamma) Finally, we /can/ approximate this implication with (D gamma) and infer the quantified type: forall g. D g => g -> g Instead what will currently happen is that we will get a quantified type (forall g. g -> g) and an implication: forall g. 0 => (forall b. 0 => C g alpha b) /\ Num alpha Which, even if the simplifyTop defaults (alpha := Int) we will still be left with an unsolvable implication: forall g. 0 => (forall b. 0 => D g) The concrete example would be: h :: C g a s => g -> a -> ST s a f (x::gamma) = (\_ -> x) (runST (h x (undefined::alpha)) + 1) But it is quite tedious to do defaulting and resolve the implication constraints, and we have not observed code breaking because of the lack of defaulting in inference, so we don't do it for now. Note [Minimize by Superclasses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we quantify over a constraint, in simplifyInfer we need to quantify over a constraint that is minimal in some sense: For instance, if the final wanted constraint is (Eq alpha, Ord alpha), we'd like to quantify over Ord alpha, because we can just get Eq alpha from superclass selection from Ord alpha. This minimization is what mkMinimalBySCs does. Then, simplifyInfer uses the minimal constraint to check the original wanted. Note [Avoid unnecessary constraint simplification] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -------- NB NB NB (Jun 12) ------------- This note not longer applies; see the notes with #4361. But I'm leaving it in here so we remember the issue.) ---------------------------------------- When inferring the type of a let-binding, with simplifyInfer, try to avoid unnecessarily simplifying class constraints. Doing so aids sharing, but it also helps with delicate situations like instance C t => C [t] where .. f :: C [t] => .... f x = let g y = ...(constraint C [t])... in ... When inferring a type for 'g', we don't want to apply the instance decl, because then we can't satisfy (C t). So we just notice that g isn't quantified over 't' and partition the constraints before simplifying. This only half-works, but then let-generalisation only half-works. ********************************************************************************* * * * Main Simplifier * * * *********************************************************************************** -} simplifyWantedsTcM :: [CtEvidence] -> TcM WantedConstraints -- Solve the specified Wanted constraints -- Discard the evidence binds -- Discards all Derived stuff in result -- Postcondition: fully zonked simplifyWantedsTcM :: [CtEvidence] -> TcM WantedConstraints simplifyWantedsTcM [CtEvidence] wanted = do { String -> SDoc -> TcM () traceTc String "simplifyWantedsTcM {" (forall a. Outputable a => a -> SDoc ppr [CtEvidence] wanted) ; (WantedConstraints result, EvBindMap _) <- forall a. TcS a -> TcM (a, EvBindMap) runTcS (WantedConstraints -> TcS WantedConstraints solveWantedsAndDrop ([CtEvidence] -> WantedConstraints mkSimpleWC [CtEvidence] wanted)) ; WantedConstraints result <- WantedConstraints -> TcM WantedConstraints TcM.zonkWC WantedConstraints result ; String -> SDoc -> TcM () traceTc String "simplifyWantedsTcM }" (forall a. Outputable a => a -> SDoc ppr WantedConstraints result) ; forall (m :: * -> *) a. Monad m => a -> m a return WantedConstraints result } solveWantedsAndDrop :: WantedConstraints -> TcS WantedConstraints -- Since solveWanteds returns the residual WantedConstraints, -- it should always be called within a runTcS or something similar, -- Result is not zonked solveWantedsAndDrop :: WantedConstraints -> TcS WantedConstraints solveWantedsAndDrop WantedConstraints wanted = do { WantedConstraints wc <- WantedConstraints -> TcS WantedConstraints solveWanteds WantedConstraints wanted ; forall (m :: * -> *) a. Monad m => a -> m a return (WantedConstraints -> WantedConstraints dropDerivedWC WantedConstraints wc) } solveWanteds :: WantedConstraints -> TcS WantedConstraints -- so that the inert set doesn't mindlessly propagate. -- NB: wc_simples may be wanted /or/ derived now solveWanteds :: WantedConstraints -> TcS WantedConstraints solveWanteds wc :: WantedConstraints wc@(WC { wc_holes :: WantedConstraints -> Bag Hole wc_holes = Bag Hole holes }) = do { TcLevel cur_lvl <- TcS TcLevel TcS.getTcLevel ; String -> SDoc -> TcS () traceTcS String "solveWanteds {" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "Level =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr TcLevel cur_lvl , forall a. Outputable a => a -> SDoc ppr WantedConstraints wc ] ; DynFlags dflags <- forall (m :: * -> *). HasDynFlags m => m DynFlags getDynFlags ; WantedConstraints solved_wc <- Int -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints simplify_loop Int 0 (DynFlags -> IntWithInf solverIterations DynFlags dflags) Bool True WantedConstraints wc ; Bag Hole holes' <- Bag Hole -> TcS (Bag Hole) simplifyHoles Bag Hole holes ; let final_wc :: WantedConstraints final_wc = WantedConstraints solved_wc { wc_holes :: Bag Hole wc_holes = Bag Hole holes' } ; EvBindsVar ev_binds_var <- TcS EvBindsVar getTcEvBindsVar ; EvBindMap bb <- EvBindsVar -> TcS EvBindMap TcS.getTcEvBindsMap EvBindsVar ev_binds_var ; String -> SDoc -> TcS () traceTcS String "solveWanteds }" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "final wc =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr WantedConstraints final_wc , String -> SDoc text String "current evbinds =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr (EvBindMap -> Bag EvBind evBindMapBinds EvBindMap bb) ] ; forall (m :: * -> *) a. Monad m => a -> m a return WantedConstraints final_wc } simplify_loop :: Int -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints -- Do a round of solving, and call maybe_simplify_again to iterate -- The 'definitely_redo_implications' flags is False if the only reason we -- are iterating is that we have added some new Derived superclasses (from Wanteds) -- hoping for fundeps to help us; see Note [Superclass iteration] -- -- Does not affect wc_holes at all; reason: wc_holes never affects anything -- else, so we do them once, at the end in solveWanteds simplify_loop :: Int -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints simplify_loop Int n IntWithInf limit Bool definitely_redo_implications wc :: WantedConstraints wc@(WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implics }) = do { SDoc -> TcS () csTraceTcS forall a b. (a -> b) -> a -> b $ String -> SDoc text String "simplify_loop iteration=" SDoc -> SDoc -> SDoc <> Int -> SDoc int Int n SDoc -> SDoc -> SDoc <+> (SDoc -> SDoc parens forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc hsep [ String -> SDoc text String "definitely_redo =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr Bool definitely_redo_implications SDoc -> SDoc -> SDoc <> SDoc comma , Int -> SDoc int (forall a. Bag a -> Int lengthBag Cts simples) SDoc -> SDoc -> SDoc <+> String -> SDoc text String "simples to solve" ]) ; String -> SDoc -> TcS () traceTcS String "simplify_loop: wc =" (forall a. Outputable a => a -> SDoc ppr WantedConstraints wc) ; (Int unifs1, WantedConstraints wc1) <- forall a. TcS a -> TcS (Int, a) reportUnifications forall a b. (a -> b) -> a -> b $ -- See Note [Superclass iteration] Cts -> TcS WantedConstraints solveSimpleWanteds Cts simples -- Any insoluble constraints are in 'simples' and so get rewritten -- See Note [Rewrite insolubles] in GHC.Tc.Solver.Monad ; WantedConstraints wc2 <- if Bool -> Bool not Bool definitely_redo_implications -- See Note [Superclass iteration] Bool -> Bool -> Bool && Int unifs1 forall a. Eq a => a -> a -> Bool == Int 0 -- for this conditional Bool -> Bool -> Bool && forall a. Bag a -> Bool isEmptyBag (WantedConstraints -> Bag Implication wc_impl WantedConstraints wc1) then forall (m :: * -> *) a. Monad m => a -> m a return (WantedConstraints wc { wc_simple :: Cts wc_simple = WantedConstraints -> Cts wc_simple WantedConstraints wc1 }) -- Short cut else do { Bag Implication implics2 <- Bag Implication -> TcS (Bag Implication) solveNestedImplications forall a b. (a -> b) -> a -> b $ Bag Implication implics forall a. Bag a -> Bag a -> Bag a `unionBags` (WantedConstraints -> Bag Implication wc_impl WantedConstraints wc1) ; forall (m :: * -> *) a. Monad m => a -> m a return (WantedConstraints wc { wc_simple :: Cts wc_simple = WantedConstraints -> Cts wc_simple WantedConstraints wc1 , wc_impl :: Bag Implication wc_impl = Bag Implication implics2 }) } ; Bool unif_happened <- TcS Bool resetUnificationFlag -- Note [The Unification Level Flag] in GHC.Tc.Solver.Monad ; Int -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints maybe_simplify_again (Int nforall a. Num a => a -> a -> a +Int 1) IntWithInf limit Bool unif_happened WantedConstraints wc2 } maybe_simplify_again :: Int -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints maybe_simplify_again :: Int -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints maybe_simplify_again Int n IntWithInf limit Bool unif_happened wc :: WantedConstraints wc@(WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples }) | Int n Int -> IntWithInf -> Bool `intGtLimit` IntWithInf limit = do { -- Add an error (not a warning) if we blow the limit, -- Typically if we blow the limit we are going to report some other error -- (an unsolved constraint), and we don't want that error to suppress -- the iteration limit warning! SDoc -> TcS () addErrTcS (SDoc -> Int -> SDoc -> SDoc hang (String -> SDoc text String "solveWanteds: too many iterations" SDoc -> SDoc -> SDoc <+> SDoc -> SDoc parens (String -> SDoc text String "limit =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr IntWithInf limit)) Int 2 ([SDoc] -> SDoc vcat [ String -> SDoc text String "Unsolved:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr WantedConstraints wc , String -> SDoc text String "Set limit with -fconstraint-solver-iterations=n; n=0 for no limit" ])) ; forall (m :: * -> *) a. Monad m => a -> m a return WantedConstraints wc } | Bool unif_happened = Int -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints simplify_loop Int n IntWithInf limit Bool True WantedConstraints wc | WantedConstraints -> Bool superClassesMightHelp WantedConstraints wc = -- We still have unsolved goals, and apparently no way to solve them, -- so try expanding superclasses at this level, both Given and Wanted do { [Ct] pending_given <- TcS [Ct] getPendingGivenScs ; let ([Ct] pending_wanted, Cts simples1) = Cts -> ([Ct], Cts) getPendingWantedScs Cts simples ; if forall (t :: * -> *) a. Foldable t => t a -> Bool null [Ct] pending_given Bool -> Bool -> Bool && forall (t :: * -> *) a. Foldable t => t a -> Bool null [Ct] pending_wanted then forall (m :: * -> *) a. Monad m => a -> m a return WantedConstraints wc -- After all, superclasses did not help else do { [Ct] new_given <- [Ct] -> TcS [Ct] makeSuperClasses [Ct] pending_given ; [Ct] new_wanted <- [Ct] -> TcS [Ct] makeSuperClasses [Ct] pending_wanted ; [Ct] -> TcS () solveSimpleGivens [Ct] new_given -- Add the new Givens to the inert set ; Int -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints simplify_loop Int n IntWithInf limit (Bool -> Bool not (forall (t :: * -> *) a. Foldable t => t a -> Bool null [Ct] pending_given)) forall a b. (a -> b) -> a -> b $ WantedConstraints wc { wc_simple :: Cts wc_simple = Cts simples1 forall a. Bag a -> Bag a -> Bag a `unionBags` forall a. [a] -> Bag a listToBag [Ct] new_wanted } } } -- (not (null pending_given)): see Note [Superclass iteration] | Bool otherwise = forall (m :: * -> *) a. Monad m => a -> m a return WantedConstraints wc {- Note [Superclass iteration] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this implication constraint forall a. [W] d: C Int beta forall b. blah where class D a b | a -> b class D a b => C a b We will expand d's superclasses, giving [D] D Int beta, in the hope of geting fundeps to unify beta. Doing so is usually fruitless (no useful fundeps), and if so it seems a pity to waste time iterating the implications (forall b. blah) (If we add new Given superclasses it's a different matter: it's really worth looking at the implications.) Hence the definitely_redo_implications flag to simplify_loop. It's usually True, but False in the case where the only reason to iterate is new Derived superclasses. In that case we check whether the new Deriveds actually led to any new unifications, and iterate the implications only if so. -} solveNestedImplications :: Bag Implication -> TcS (Bag Implication) -- Precondition: the TcS inerts may contain unsolved simples which have -- to be converted to givens before we go inside a nested implication. solveNestedImplications :: Bag Implication -> TcS (Bag Implication) solveNestedImplications Bag Implication implics | forall a. Bag a -> Bool isEmptyBag Bag Implication implics = forall (m :: * -> *) a. Monad m => a -> m a return (forall a. Bag a emptyBag) | Bool otherwise = do { String -> SDoc -> TcS () traceTcS String "solveNestedImplications starting {" SDoc empty ; Bag (Maybe Implication) unsolved_implics <- forall (m :: * -> *) a b. Monad m => (a -> m b) -> Bag a -> m (Bag b) mapBagM Implication -> TcS (Maybe Implication) solveImplication Bag Implication implics -- ... and we are back in the original TcS inerts -- Notice that the original includes the _insoluble_simples so it was safe to ignore -- them in the beginning of this function. ; String -> SDoc -> TcS () traceTcS String "solveNestedImplications end }" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "unsolved_implics =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr Bag (Maybe Implication) unsolved_implics ] ; forall (m :: * -> *) a. Monad m => a -> m a return (forall a. Bag (Maybe a) -> Bag a catBagMaybes Bag (Maybe Implication) unsolved_implics) } solveImplication :: Implication -- Wanted -> TcS (Maybe Implication) -- Simplified implication (empty or singleton) -- Precondition: The TcS monad contains an empty worklist and given-only inerts -- which after trying to solve this implication we must restore to their original value solveImplication :: Implication -> TcS (Maybe Implication) solveImplication imp :: Implication imp@(Implic { ic_tclvl :: Implication -> TcLevel ic_tclvl = TcLevel tclvl , ic_binds :: Implication -> EvBindsVar ic_binds = EvBindsVar ev_binds_var , ic_given :: Implication -> [TcTyVar] ic_given = [TcTyVar] given_ids , ic_wanted :: Implication -> WantedConstraints ic_wanted = WantedConstraints wanteds , ic_info :: Implication -> SkolemInfo ic_info = SkolemInfo info , ic_status :: Implication -> ImplicStatus ic_status = ImplicStatus status }) | ImplicStatus -> Bool isSolvedStatus ImplicStatus status = forall (m :: * -> *) a. Monad m => a -> m a return (forall a. a -> Maybe a Just Implication imp) -- Do nothing | Bool otherwise -- Even for IC_Insoluble it is worth doing more work -- The insoluble stuff might be in one sub-implication -- and other unsolved goals in another; and we want to -- solve the latter as much as possible = do { InertSet inerts <- TcS InertSet getTcSInerts ; String -> SDoc -> TcS () traceTcS String "solveImplication {" (forall a. Outputable a => a -> SDoc ppr Implication imp SDoc -> SDoc -> SDoc $$ String -> SDoc text String "Inerts" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr InertSet inerts) -- commented out; see `where` clause below -- ; when debugIsOn check_tc_level -- Solve the nested constraints ; (HasGivenEqs has_given_eqs, Cts given_insols, WantedConstraints residual_wanted) <- forall a. EvBindsVar -> TcLevel -> TcS a -> TcS a nestImplicTcS EvBindsVar ev_binds_var TcLevel tclvl forall a b. (a -> b) -> a -> b $ do { let loc :: CtLoc loc = TcLevel -> SkolemInfo -> TcLclEnv -> CtLoc mkGivenLoc TcLevel tclvl SkolemInfo info (Implication -> TcLclEnv ic_env Implication imp) givens :: [Ct] givens = CtLoc -> [TcTyVar] -> [Ct] mkGivens CtLoc loc [TcTyVar] given_ids ; [Ct] -> TcS () solveSimpleGivens [Ct] givens ; WantedConstraints residual_wanted <- WantedConstraints -> TcS WantedConstraints solveWanteds WantedConstraints wanteds -- solveWanteds, *not* solveWantedsAndDrop, because -- we want to retain derived equalities so we can float -- them out in floatEqualities. ; (HasGivenEqs has_eqs, Cts given_insols) <- TcLevel -> TcS (HasGivenEqs, Cts) getHasGivenEqs TcLevel tclvl -- Call getHasGivenEqs /after/ solveWanteds, because -- solveWanteds can augment the givens, via expandSuperClasses, -- to reveal given superclass equalities ; forall (m :: * -> *) a. Monad m => a -> m a return (HasGivenEqs has_eqs, Cts given_insols, WantedConstraints residual_wanted) } ; String -> SDoc -> TcS () traceTcS String "solveImplication 2" (forall a. Outputable a => a -> SDoc ppr Cts given_insols SDoc -> SDoc -> SDoc $$ forall a. Outputable a => a -> SDoc ppr WantedConstraints residual_wanted) ; let final_wanted :: WantedConstraints final_wanted = WantedConstraints residual_wanted WantedConstraints -> Cts -> WantedConstraints `addInsols` Cts given_insols -- Don't lose track of the insoluble givens, -- which signal unreachable code; put them in ic_wanted ; Maybe Implication res_implic <- Implication -> TcS (Maybe Implication) setImplicationStatus (Implication imp { ic_given_eqs :: HasGivenEqs ic_given_eqs = HasGivenEqs has_given_eqs , ic_wanted :: WantedConstraints ic_wanted = WantedConstraints final_wanted }) ; EvBindMap evbinds <- EvBindsVar -> TcS EvBindMap TcS.getTcEvBindsMap EvBindsVar ev_binds_var ; VarSet tcvs <- EvBindsVar -> TcS VarSet TcS.getTcEvTyCoVars EvBindsVar ev_binds_var ; String -> SDoc -> TcS () traceTcS String "solveImplication end }" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "has_given_eqs =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr HasGivenEqs has_given_eqs , String -> SDoc text String "res_implic =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr Maybe Implication res_implic , String -> SDoc text String "implication evbinds =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr (EvBindMap -> Bag EvBind evBindMapBinds EvBindMap evbinds) , String -> SDoc text String "implication tvcs =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr VarSet tcvs ] ; forall (m :: * -> *) a. Monad m => a -> m a return Maybe Implication res_implic } -- TcLevels must be strictly increasing (see (ImplicInv) in -- Note [TcLevel invariants] in GHC.Tc.Utils.TcType), -- and in fact I think they should always increase one level at a time. -- Though sensible, this check causes lots of testsuite failures. It is -- remaining commented out for now. {- check_tc_level = do { cur_lvl <- TcS.getTcLevel ; MASSERT2( tclvl == pushTcLevel cur_lvl , text "Cur lvl =" <+> ppr cur_lvl $$ text "Imp lvl =" <+> ppr tclvl ) } -} ---------------------- setImplicationStatus :: Implication -> TcS (Maybe Implication) -- Finalise the implication returned from solveImplication: -- * Set the ic_status field -- * Trim the ic_wanted field to remove Derived constraints -- Precondition: the ic_status field is not already IC_Solved -- Return Nothing if we can discard the implication altogether setImplicationStatus :: Implication -> TcS (Maybe Implication) setImplicationStatus implic :: Implication implic@(Implic { ic_status :: Implication -> ImplicStatus ic_status = ImplicStatus status , ic_info :: Implication -> SkolemInfo ic_info = SkolemInfo info , ic_wanted :: Implication -> WantedConstraints ic_wanted = WantedConstraints wc , ic_given :: Implication -> [TcTyVar] ic_given = [TcTyVar] givens }) | ASSERT2( not (isSolvedStatus status ), ppr info ) -- Precondition: we only set the status if it is not already solved Bool -> Bool not (WantedConstraints -> Bool isSolvedWC WantedConstraints pruned_wc) = do { String -> SDoc -> TcS () traceTcS String "setImplicationStatus(not-all-solved) {" (forall a. Outputable a => a -> SDoc ppr Implication implic) ; Implication implic <- Implication -> TcS Implication neededEvVars Implication implic ; let new_status :: ImplicStatus new_status | WantedConstraints -> Bool insolubleWC WantedConstraints pruned_wc = ImplicStatus IC_Insoluble | Bool otherwise = ImplicStatus IC_Unsolved new_implic :: Implication new_implic = Implication implic { ic_status :: ImplicStatus ic_status = ImplicStatus new_status , ic_wanted :: WantedConstraints ic_wanted = WantedConstraints pruned_wc } ; String -> SDoc -> TcS () traceTcS String "setImplicationStatus(not-all-solved) }" (forall a. Outputable a => a -> SDoc ppr Implication new_implic) ; forall (m :: * -> *) a. Monad m => a -> m a return forall a b. (a -> b) -> a -> b $ forall a. a -> Maybe a Just Implication new_implic } | Bool otherwise -- Everything is solved -- Set status to IC_Solved, -- and compute the dead givens and outer needs -- See Note [Tracking redundant constraints] = do { String -> SDoc -> TcS () traceTcS String "setImplicationStatus(all-solved) {" (forall a. Outputable a => a -> SDoc ppr Implication implic) ; implic :: Implication implic@(Implic { ic_need_inner :: Implication -> VarSet ic_need_inner = VarSet need_inner , ic_need_outer :: Implication -> VarSet ic_need_outer = VarSet need_outer }) <- Implication -> TcS Implication neededEvVars Implication implic ; Bool bad_telescope <- Implication -> TcS Bool checkBadTelescope Implication implic ; let ([TcTyVar] used_givens, [TcTyVar] unused_givens) | SkolemInfo -> Bool warnRedundantGivens SkolemInfo info = forall a. (a -> Bool) -> [a] -> ([a], [a]) partition (TcTyVar -> VarSet -> Bool `elemVarSet` VarSet need_inner) [TcTyVar] givens | Bool otherwise = ([TcTyVar] givens, []) -- None to report minimal_used_givens :: [TcTyVar] minimal_used_givens = forall a. (a -> Type) -> [a] -> [a] mkMinimalBySCs TcTyVar -> Type evVarPred [TcTyVar] used_givens is_minimal :: TcTyVar -> Bool is_minimal = (TcTyVar -> VarSet -> Bool `elemVarSet` [TcTyVar] -> VarSet mkVarSet [TcTyVar] minimal_used_givens) warn_givens :: [TcTyVar] warn_givens | Bool -> Bool not (forall (t :: * -> *) a. Foldable t => t a -> Bool null [TcTyVar] unused_givens) = [TcTyVar] unused_givens | SkolemInfo -> Bool warnRedundantGivens SkolemInfo info = forall a. (a -> Bool) -> [a] -> [a] filterOut TcTyVar -> Bool is_minimal [TcTyVar] used_givens | Bool otherwise = [] discard_entire_implication :: Bool discard_entire_implication -- Can we discard the entire implication? = forall (t :: * -> *) a. Foldable t => t a -> Bool null [TcTyVar] warn_givens -- No warning from this implication Bool -> Bool -> Bool && Bool -> Bool not Bool bad_telescope Bool -> Bool -> Bool && WantedConstraints -> Bool isEmptyWC WantedConstraints pruned_wc -- No live children Bool -> Bool -> Bool && VarSet -> Bool isEmptyVarSet VarSet need_outer -- No needed vars to pass up to parent final_status :: ImplicStatus final_status | Bool bad_telescope = ImplicStatus IC_BadTelescope | Bool otherwise = IC_Solved { ics_dead :: [TcTyVar] ics_dead = [TcTyVar] warn_givens } final_implic :: Implication final_implic = Implication implic { ic_status :: ImplicStatus ic_status = ImplicStatus final_status , ic_wanted :: WantedConstraints ic_wanted = WantedConstraints pruned_wc } ; String -> SDoc -> TcS () traceTcS String "setImplicationStatus(all-solved) }" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "discard:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr Bool discard_entire_implication , String -> SDoc text String "new_implic:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr Implication final_implic ] ; forall (m :: * -> *) a. Monad m => a -> m a return forall a b. (a -> b) -> a -> b $ if Bool discard_entire_implication then forall a. Maybe a Nothing else forall a. a -> Maybe a Just Implication final_implic } where WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implics, wc_holes :: WantedConstraints -> Bag Hole wc_holes = Bag Hole holes } = WantedConstraints wc pruned_simples :: Cts pruned_simples = Cts -> Cts dropDerivedSimples Cts simples pruned_implics :: Bag Implication pruned_implics = forall a. (a -> Bool) -> Bag a -> Bag a filterBag Implication -> Bool keep_me Bag Implication implics pruned_wc :: WantedConstraints pruned_wc = WC { wc_simple :: Cts wc_simple = Cts pruned_simples , wc_impl :: Bag Implication wc_impl = Bag Implication pruned_implics , wc_holes :: Bag Hole wc_holes = Bag Hole holes } -- do not prune holes; these should be reported keep_me :: Implication -> Bool keep_me :: Implication -> Bool keep_me Implication ic | IC_Solved { ics_dead :: ImplicStatus -> [TcTyVar] ics_dead = [TcTyVar] dead_givens } <- Implication -> ImplicStatus ic_status Implication ic -- Fully solved , forall (t :: * -> *) a. Foldable t => t a -> Bool null [TcTyVar] dead_givens -- No redundant givens to report , forall a. Bag a -> Bool isEmptyBag (WantedConstraints -> Bag Implication wc_impl (Implication -> WantedConstraints ic_wanted Implication ic)) -- And no children that might have things to report = Bool False -- Tnen we don't need to keep it | Bool otherwise = Bool True -- Otherwise, keep it checkBadTelescope :: Implication -> TcS Bool -- True <=> the skolems form a bad telescope -- See Note [Checking telescopes] in GHC.Tc.Types.Constraint checkBadTelescope :: Implication -> TcS Bool checkBadTelescope (Implic { ic_info :: Implication -> SkolemInfo ic_info = SkolemInfo info , ic_skols :: Implication -> [TcTyVar] ic_skols = [TcTyVar] skols }) | SkolemInfo -> Bool checkTelescopeSkol SkolemInfo info = do{ [TcTyVar] skols <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM TcTyVar -> TcS TcTyVar TcS.zonkTyCoVarKind [TcTyVar] skols ; forall (m :: * -> *) a. Monad m => a -> m a return (VarSet -> [TcTyVar] -> Bool go VarSet emptyVarSet (forall a. [a] -> [a] reverse [TcTyVar] skols))} | Bool otherwise = forall (m :: * -> *) a. Monad m => a -> m a return Bool False where go :: TyVarSet -- skolems that appear *later* than the current ones -> [TcTyVar] -- ordered skolems, in reverse order -> Bool -- True <=> there is an out-of-order skolem go :: VarSet -> [TcTyVar] -> Bool go VarSet _ [] = Bool False go VarSet later_skols (TcTyVar one_skol : [TcTyVar] earlier_skols) | Type -> VarSet tyCoVarsOfType (TcTyVar -> Type tyVarKind TcTyVar one_skol) VarSet -> VarSet -> Bool `intersectsVarSet` VarSet later_skols = Bool True | Bool otherwise = VarSet -> [TcTyVar] -> Bool go (VarSet later_skols VarSet -> TcTyVar -> VarSet `extendVarSet` TcTyVar one_skol) [TcTyVar] earlier_skols warnRedundantGivens :: SkolemInfo -> Bool warnRedundantGivens :: SkolemInfo -> Bool warnRedundantGivens (SigSkol UserTypeCtxt ctxt Type _ [(Name, TcTyVar)] _) = case UserTypeCtxt ctxt of FunSigCtxt Name _ Bool warn_redundant -> Bool warn_redundant UserTypeCtxt ExprSigCtxt -> Bool True UserTypeCtxt _ -> Bool False -- To think about: do we want to report redundant givens for -- pattern synonyms, PatSynSigSkol? c.f #9953, comment:21. warnRedundantGivens (InstSkol {}) = Bool True warnRedundantGivens SkolemInfo _ = Bool False neededEvVars :: Implication -> TcS Implication -- Find all the evidence variables that are "needed", -- and delete dead evidence bindings -- See Note [Tracking redundant constraints] -- See Note [Delete dead Given evidence bindings] -- -- - Start from initial_seeds (from nested implications) -- -- - Add free vars of RHS of all Wanted evidence bindings -- and coercion variables accumulated in tcvs (all Wanted) -- -- - Generate 'needed', the needed set of EvVars, by doing transitive -- closure through Given bindings -- e.g. Needed {a,b} -- Given a = sc_sel a2 -- Then a2 is needed too -- -- - Prune out all Given bindings that are not needed -- -- - From the 'needed' set, delete ev_bndrs, the binders of the -- evidence bindings, to give the final needed variables -- neededEvVars :: Implication -> TcS Implication neededEvVars implic :: Implication implic@(Implic { ic_given :: Implication -> [TcTyVar] ic_given = [TcTyVar] givens , ic_binds :: Implication -> EvBindsVar ic_binds = EvBindsVar ev_binds_var , ic_wanted :: Implication -> WantedConstraints ic_wanted = WC { wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implics } , ic_need_inner :: Implication -> VarSet ic_need_inner = VarSet old_needs }) = do { EvBindMap ev_binds <- EvBindsVar -> TcS EvBindMap TcS.getTcEvBindsMap EvBindsVar ev_binds_var ; VarSet tcvs <- EvBindsVar -> TcS VarSet TcS.getTcEvTyCoVars EvBindsVar ev_binds_var ; let seeds1 :: VarSet seeds1 = forall (t :: * -> *) a b. Foldable t => (a -> b -> b) -> b -> t a -> b foldr Implication -> VarSet -> VarSet add_implic_seeds VarSet old_needs Bag Implication implics seeds2 :: VarSet seeds2 = forall a. (EvBind -> a -> a) -> a -> EvBindMap -> a nonDetStrictFoldEvBindMap EvBind -> VarSet -> VarSet add_wanted VarSet seeds1 EvBindMap ev_binds -- It's OK to use a non-deterministic fold here -- because add_wanted is commutative seeds3 :: VarSet seeds3 = VarSet seeds2 VarSet -> VarSet -> VarSet `unionVarSet` VarSet tcvs need_inner :: VarSet need_inner = EvBindMap -> VarSet -> VarSet findNeededEvVars EvBindMap ev_binds VarSet seeds3 live_ev_binds :: EvBindMap live_ev_binds = (EvBind -> Bool) -> EvBindMap -> EvBindMap filterEvBindMap (VarSet -> EvBind -> Bool needed_ev_bind VarSet need_inner) EvBindMap ev_binds need_outer :: VarSet need_outer = VarSet -> EvBindMap -> VarSet varSetMinusEvBindMap VarSet need_inner EvBindMap live_ev_binds VarSet -> [TcTyVar] -> VarSet `delVarSetList` [TcTyVar] givens ; EvBindsVar -> EvBindMap -> TcS () TcS.setTcEvBindsMap EvBindsVar ev_binds_var EvBindMap live_ev_binds -- See Note [Delete dead Given evidence bindings] ; String -> SDoc -> TcS () traceTcS String "neededEvVars" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "old_needs:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr VarSet old_needs , String -> SDoc text String "seeds3:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr VarSet seeds3 , String -> SDoc text String "tcvs:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr VarSet tcvs , String -> SDoc text String "ev_binds:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr EvBindMap ev_binds , String -> SDoc text String "live_ev_binds:" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr EvBindMap live_ev_binds ] ; forall (m :: * -> *) a. Monad m => a -> m a return (Implication implic { ic_need_inner :: VarSet ic_need_inner = VarSet need_inner , ic_need_outer :: VarSet ic_need_outer = VarSet need_outer }) } where add_implic_seeds :: Implication -> VarSet -> VarSet add_implic_seeds (Implic { ic_need_outer :: Implication -> VarSet ic_need_outer = VarSet needs }) VarSet acc = VarSet needs VarSet -> VarSet -> VarSet `unionVarSet` VarSet acc needed_ev_bind :: VarSet -> EvBind -> Bool needed_ev_bind VarSet needed (EvBind { eb_lhs :: EvBind -> TcTyVar eb_lhs = TcTyVar ev_var , eb_is_given :: EvBind -> Bool eb_is_given = Bool is_given }) | Bool is_given = TcTyVar ev_var TcTyVar -> VarSet -> Bool `elemVarSet` VarSet needed | Bool otherwise = Bool True -- Keep all wanted bindings add_wanted :: EvBind -> VarSet -> VarSet add_wanted :: EvBind -> VarSet -> VarSet add_wanted (EvBind { eb_is_given :: EvBind -> Bool eb_is_given = Bool is_given, eb_rhs :: EvBind -> EvTerm eb_rhs = EvTerm rhs }) VarSet needs | Bool is_given = VarSet needs -- Add the rhs vars of the Wanted bindings only | Bool otherwise = EvTerm -> VarSet evVarsOfTerm EvTerm rhs VarSet -> VarSet -> VarSet `unionVarSet` VarSet needs ------------------------------------------------- simplifyHoles :: Bag Hole -> TcS (Bag Hole) simplifyHoles :: Bag Hole -> TcS (Bag Hole) simplifyHoles = forall (m :: * -> *) a b. Monad m => (a -> m b) -> Bag a -> m (Bag b) mapBagM Hole -> TcS Hole simpl_hole where simpl_hole :: Hole -> TcS Hole -- See Note [Do not simplify ConstraintHoles] simpl_hole :: Hole -> TcS Hole simpl_hole h :: Hole h@(Hole { hole_sort :: Hole -> HoleSort hole_sort = HoleSort ConstraintHole }) = forall (m :: * -> *) a. Monad m => a -> m a return Hole h -- other wildcards should be simplified for printing -- we must do so here, and not in the error-message generation -- code, because we have all the givens already set up simpl_hole h :: Hole h@(Hole { hole_ty :: Hole -> Type hole_ty = Type ty, hole_loc :: Hole -> CtLoc hole_loc = CtLoc loc }) = do { Type ty' <- CtLoc -> Type -> TcS Type rewriteType CtLoc loc Type ty ; forall (m :: * -> *) a. Monad m => a -> m a return (Hole h { hole_ty :: Type hole_ty = Type ty' }) } {- Note [Delete dead Given evidence bindings] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ As a result of superclass expansion, we speculatively generate evidence bindings for Givens. E.g. f :: (a ~ b) => a -> b -> Bool f x y = ... We'll have [G] d1 :: (a~b) and we'll speculatively generate the evidence binding [G] d2 :: (a ~# b) = sc_sel d Now d2 is available for solving. But it may not be needed! Usually such dead superclass selections will eventually be dropped as dead code, but: * It won't always be dropped (#13032). In the case of an unlifted-equality superclass like d2 above, we generate case heq_sc d1 of d2 -> ... and we can't (in general) drop that case expression in case d1 is bottom. So it's technically unsound to have added it in the first place. * Simply generating all those extra superclasses can generate lots of code that has to be zonked, only to be discarded later. Better not to generate it in the first place. Moreover, if we simplify this implication more than once (e.g. because we can't solve it completely on the first iteration of simpl_looop), we'll generate all the same bindings AGAIN! Easy solution: take advantage of the work we are doing to track dead (unused) Givens, and use it to prune the Given bindings too. This is all done by neededEvVars. This led to a remarkable 25% overall compiler allocation decrease in test T12227. But we don't get to discard all redundant equality superclasses, alas; see #15205. Note [Do not simplify ConstraintHoles] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Before printing the inferred value for a type hole (a _ wildcard in a partial type signature), we simplify it w.r.t. any Givens. This makes for an easier-to-understand diagnostic for the user. However, we do not wish to do this for extra-constraint holes. Here is the example for why (partial-sigs/should_compile/T12844): bar :: _ => FooData rngs bar = foo data FooData rngs class Foo xs where foo :: (Head xs ~ '(r,r')) => FooData xs type family Head (xs :: [k]) where Head (x ': xs) = x GHC correctly infers that the extra-constraints wildcard on `bar` should be (Head rngs ~ '(r, r'), Foo rngs). It then adds this constraint as a Given on the implication constraint for `bar`. (This implication is emitted by emitResidualConstraints.) The Hole for the _ is stored within the implication's WantedConstraints. When simplifyHoles is called, that constraint is already assumed as a Given. Simplifying with respect to it turns it into ('(r, r') ~ '(r, r'), Foo rngs), which is disastrous. Furthermore, there is no need to simplify here: extra-constraints wildcards are filled in with the output of the solver, in chooseInferredQuantifiers (choose_psig_context), so they are already simplified. (Contrast to normal type holes, which are just bound to a meta-variable.) Avoiding the poor output is simple: just don't simplify extra-constraints wildcards. This is the only reason we need to track ConstraintHole separately from TypeHole in HoleSort. See also Note [Extra-constraint holes in partial type signatures] in GHC.Tc.Gen.HsType. Note [Tracking redundant constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ With Opt_WarnRedundantConstraints, GHC can report which constraints of a type signature (or instance declaration) are redundant, and can be omitted. Here is an overview of how it works. This is all tested in typecheck/should_compile/T20602 (among others). ----- What is a redundant constraint? * The things that can be redundant are precisely the Given constraints of an implication. * A constraint can be redundant in two different ways: a) It is not needed by the Wanted constraints covered by the implication E.g. f :: Eq a => a -> Bool f x = True -- Equality not used b) It is implied by other givens. E.g. f :: (Eq a, Ord a) => blah -- Eq a unnecessary g :: (Eq a, a~b, Eq b) => blah -- Either Eq a or Eq b unnecessary * To find (a) we need to know which evidence bindings are 'wanted'; hence the eb_is_given field on an EvBind. * To find (b), we use mkMinimalBySCs on the Givens to see if any are unnecessary. ----- How tracking works * When two Givens are the same, we drop the evidence for the one that requires more superclass selectors. This is done according to Note [Replacement vs keeping] in GHC.Tc.Solver.Interact. * The ic_need fields of an Implic records in-scope (given) evidence variables bound by the context, that were needed to solve this implication (so far). See the declaration of Implication. * When the constraint solver finishes solving all the wanteds in an implication, it sets its status to IC_Solved - The ics_dead field, of IC_Solved, records the subset of this implication's ic_given that are redundant (not needed). * We compute which evidence variables are needed by an implication in setImplicationStatus. A variable is needed if a) it is free in the RHS of a Wanted EvBind, b) it is free in the RHS of an EvBind whose LHS is needed, or c) it is in the ics_need of a nested implication. * After computing which variables are needed, we then look at the remaining variables for internal redundancies. This is case (b) from above. This is also done in setImplicationStatus. Note that we only look for case (b) if case (a) shows up empty, as exemplified below. * We need to be careful not to discard an implication prematurely, even one that is fully solved, because we might thereby forget which variables it needs, and hence wrongly report a constraint as redundant. But we can discard it once its free vars have been incorporated into its parent; or if it simply has no free vars. This careful discarding is also handled in setImplicationStatus. * Examples: f, g, h :: (Eq a, Ord a) => a -> Bool f x = x == x g x = x > x h x = x == x && x > x All three will discover that they have two [G] Eq a constraints: one as given and one extracted from the Ord a constraint. They will both discard the latter, as noted above and in Note [Replacement vs keeping] in GHC.Tc.Solver.Interact. The body of f uses the [G] Eq a, but not the [G] Ord a. It will report a redundant Ord a using the logic for case (a). The body of g uses the [G] Ord a, but not the [G] Eq a. It will report a redundant Eq a using the logic for case (a). The body of h uses both [G] Ord a and [G] Eq a. Case (a) will thus come up with nothing redundant. But then, the case (b) check will discover that Eq a is redundant and report this. If we did case (b) even when case (a) reports something, then we would report both constraints as redundant for f, which is terrible. ----- Reporting redundant constraints * GHC.Tc.Errors does the actual warning, in warnRedundantConstraints. * We don't report redundant givens for *every* implication; only for those which reply True to GHC.Tc.Solver.warnRedundantGivens: - For example, in a class declaration, the default method *can* use the class constraint, but it certainly doesn't *have* to, and we don't want to report an error there. - More subtly, in a function definition f :: (Ord a, Ord a, Ix a) => a -> a f x = rhs we do an ambiguity check on the type (which would find that one of the Ord a constraints was redundant), and then we check that the definition has that type (which might find that both are redundant). We don't want to report the same error twice, so we disable it for the ambiguity check. Hence using two different FunSigCtxts, one with the warn-redundant field set True, and the other set False in - GHC.Tc.Gen.Bind.tcSpecPrag - GHC.Tc.Gen.Bind.tcTySig This decision is taken in setImplicationStatus, rather than GHC.Tc.Errors so that we can discard implication constraints that we don't need. So ics_dead consists only of the *reportable* redundant givens. ----- Shortcomings Consider j :: (Eq a, a ~ b) => a -> Bool j x = x == x k :: (Eq a, b ~ a) => a -> Bool k x = x == x Currently (Nov 2021), j issues no warning, while k says that b ~ a is redundant. This is because j uses the a ~ b constraint to rewrite everything to be in terms of b, while k does none of that. This is ridiculous, but I (Richard E) don't see a good fix. -} -- | Like 'defaultTyVar', but in the TcS monad. defaultTyVarTcS :: TcTyVar -> TcS Bool defaultTyVarTcS :: TcTyVar -> TcS Bool defaultTyVarTcS TcTyVar the_tv | TcTyVar -> Bool isRuntimeRepVar TcTyVar the_tv , Bool -> Bool not (TcTyVar -> Bool isTyVarTyVar TcTyVar the_tv) -- TyVarTvs should only be unified with a tyvar -- never with a type; c.f. GHC.Tc.Utils.TcMType.defaultTyVar -- and Note [Inferring kinds for type declarations] in GHC.Tc.TyCl = do { String -> SDoc -> TcS () traceTcS String "defaultTyVarTcS RuntimeRep" (forall a. Outputable a => a -> SDoc ppr TcTyVar the_tv) ; TcTyVar -> Type -> TcS () unifyTyVar TcTyVar the_tv Type liftedRepTy ; forall (m :: * -> *) a. Monad m => a -> m a return Bool True } | TcTyVar -> Bool isMultiplicityVar TcTyVar the_tv , Bool -> Bool not (TcTyVar -> Bool isTyVarTyVar TcTyVar the_tv) -- TyVarTvs should only be unified with a tyvar -- never with a type; c.f. TcMType.defaultTyVar -- See Note [Kind generalisation and SigTvs] = do { String -> SDoc -> TcS () traceTcS String "defaultTyVarTcS Multiplicity" (forall a. Outputable a => a -> SDoc ppr TcTyVar the_tv) ; TcTyVar -> Type -> TcS () unifyTyVar TcTyVar the_tv Type manyDataConTy ; forall (m :: * -> *) a. Monad m => a -> m a return Bool True } | Bool otherwise = forall (m :: * -> *) a. Monad m => a -> m a return Bool False -- the common case approximateWC :: Bool -> WantedConstraints -> Cts -- Postcondition: Wanted or Derived Cts -- See Note [ApproximateWC] -- See Note [floatKindEqualities vs approximateWC] approximateWC :: Bool -> WantedConstraints -> Cts approximateWC Bool float_past_equalities WantedConstraints wc = VarSet -> WantedConstraints -> Cts float_wc VarSet emptyVarSet WantedConstraints wc where float_wc :: TcTyCoVarSet -> WantedConstraints -> Cts float_wc :: VarSet -> WantedConstraints -> Cts float_wc VarSet trapping_tvs (WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implics }) = forall a. (a -> Bool) -> Bag a -> Bag a filterBag (VarSet -> Ct -> Bool is_floatable VarSet trapping_tvs) Cts simples forall a. Bag a -> Bag a -> Bag a `unionBags` forall a b. (a -> Bag b) -> Bag a -> Bag b concatMapBag (VarSet -> Implication -> Cts float_implic VarSet trapping_tvs) Bag Implication implics float_implic :: TcTyCoVarSet -> Implication -> Cts float_implic :: VarSet -> Implication -> Cts float_implic VarSet trapping_tvs Implication imp | Bool float_past_equalities Bool -> Bool -> Bool || Implication -> HasGivenEqs ic_given_eqs Implication imp forall a. Eq a => a -> a -> Bool /= HasGivenEqs MaybeGivenEqs = VarSet -> WantedConstraints -> Cts float_wc VarSet new_trapping_tvs (Implication -> WantedConstraints ic_wanted Implication imp) | Bool otherwise -- Take care with equalities = Cts emptyCts -- See (1) under Note [ApproximateWC] where new_trapping_tvs :: VarSet new_trapping_tvs = VarSet trapping_tvs VarSet -> [TcTyVar] -> VarSet `extendVarSetList` Implication -> [TcTyVar] ic_skols Implication imp is_floatable :: VarSet -> Ct -> Bool is_floatable VarSet skol_tvs Ct ct | Ct -> Bool isGivenCt Ct ct = Bool False | Ct -> Bool insolubleEqCt Ct ct = Bool False | Bool otherwise = Ct -> VarSet tyCoVarsOfCt Ct ct VarSet -> VarSet -> Bool `disjointVarSet` VarSet skol_tvs {- Note [ApproximateWC] ~~~~~~~~~~~~~~~~~~~~~~~ approximateWC takes a constraint, typically arising from the RHS of a let-binding whose type we are *inferring*, and extracts from it some *simple* constraints that we might plausibly abstract over. Of course the top-level simple constraints are plausible, but we also float constraints out from inside, if they are not captured by skolems. The same function is used when doing type-class defaulting (see the call to applyDefaultingRules) to extract constraints that might be defaulted. There is one caveat: 1. When inferring most-general types (in simplifyInfer), we do *not* float anything out if the implication binds equality constraints, because that defeats the OutsideIn story. Consider data T a where TInt :: T Int MkT :: T a f TInt = 3::Int We get the implication (a ~ Int => res ~ Int), where so far we've decided f :: T a -> res We don't want to float (res~Int) out because then we'll infer f :: T a -> Int which is only on of the possible types. (GHC 7.6 accidentally *did* float out of such implications, which meant it would happily infer non-principal types.) HOWEVER (#12797) in findDefaultableGroups we are not worried about the most-general type; and we /do/ want to float out of equalities. Hence the boolean flag to approximateWC. ------ Historical note ----------- There used to be a second caveat, driven by #8155 2. We do not float out an inner constraint that shares a type variable (transitively) with one that is trapped by a skolem. Eg forall a. F a ~ beta, Integral beta We don't want to float out (Integral beta). Doing so would be bad when defaulting, because then we'll default beta:=Integer, and that makes the error message much worse; we'd get Can't solve F a ~ Integer rather than Can't solve Integral (F a) Moreover, floating out these "contaminated" constraints doesn't help when generalising either. If we generalise over (Integral b), we still can't solve the retained implication (forall a. F a ~ b). Indeed, arguably that too would be a harder error to understand. But this transitive closure stuff gives rise to a complex rule for when defaulting actually happens, and one that was never documented. Moreover (#12923), the more complex rule is sometimes NOT what you want. So I simply removed the extra code to implement the contamination stuff. There was zero effect on the testsuite (not even #8155). ------ End of historical note ----------- Note [DefaultTyVar] ~~~~~~~~~~~~~~~~~~~ defaultTyVar is used on any un-instantiated meta type variables to default any RuntimeRep variables to LiftedRep. This is important to ensure that instance declarations match. For example consider instance Show (a->b) foo x = show (\_ -> True) Then we'll get a constraint (Show (p ->q)) where p has kind (TYPE r), and that won't match the tcTypeKind (*) in the instance decl. See tests tc217 and tc175. We look only at touchable type variables. No further constraints are going to affect these type variables, so it's time to do it by hand. However we aren't ready to default them fully to () or whatever, because the type-class defaulting rules have yet to run. An alternate implementation would be to emit a derived constraint setting the RuntimeRep variable to LiftedRep, but this seems unnecessarily indirect. Note [Promote _and_ default when inferring] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we are inferring a type, we simplify the constraint, and then use approximateWC to produce a list of candidate constraints. Then we MUST a) Promote any meta-tyvars that have been floated out by approximateWC, to restore invariant (WantedInv) described in Note [TcLevel invariants] in GHC.Tc.Utils.TcType. b) Default the kind of any meta-tyvars that are not mentioned in in the environment. To see (b), suppose the constraint is (C ((a :: OpenKind) -> Int)), and we have an instance (C ((x:*) -> Int)). The instance doesn't match -- but it should! If we don't solve the constraint, we'll stupidly quantify over (C (a->Int)) and, worse, in doing so skolemiseQuantifiedTyVar will quantify over (b:*) instead of (a:OpenKind), which can lead to disaster; see #7332. #7641 is a simpler example. Note [Promoting unification variables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we float an equality out of an implication we must "promote" free unification variables of the equality, in order to maintain Invariant (WantedInv) from Note [TcLevel invariants] in GHC.Tc.Types.TcType. This is absolutely necessary. Consider the following example. We start with two implications and a class with a functional dependency. class C x y | x -> y instance C [a] [a] (I1) [untch=beta]forall b. 0 => F Int ~ [beta] (I2) [untch=beta]forall c. 0 => F Int ~ [[alpha]] /\ C beta [c] We float (F Int ~ [beta]) out of I1, and we float (F Int ~ [[alpha]]) out of I2. They may react to yield that (beta := [alpha]) which can then be pushed inwards the leftover of I2 to get (C [alpha] [a]) which, using the FunDep, will mean that (alpha := a). In the end we will have the skolem 'b' escaping in the untouchable beta! Concrete example is in indexed_types/should_fail/ExtraTcsUntch.hs: class C x y | x -> y where op :: x -> y -> () instance C [a] [a] type family F a :: * h :: F Int -> () h = undefined data TEx where TEx :: a -> TEx f (x::beta) = let g1 :: forall b. b -> () g1 _ = h [x] g2 z = case z of TEx y -> (h [[undefined]], op x [y]) in (g1 '3', g2 undefined) ********************************************************************************* * * * Defaulting and disambiguation * * * ********************************************************************************* -} applyDefaultingRules :: WantedConstraints -> TcS Bool -- True <=> I did some defaulting, by unifying a meta-tyvar -- Input WantedConstraints are not necessarily zonked applyDefaultingRules :: WantedConstraints -> TcS Bool applyDefaultingRules WantedConstraints wanteds | WantedConstraints -> Bool isEmptyWC WantedConstraints wanteds = forall (m :: * -> *) a. Monad m => a -> m a return Bool False | Bool otherwise = do { info :: ([Type], (Bool, Bool)) info@([Type] default_tys, (Bool, Bool) _) <- TcS ([Type], (Bool, Bool)) getDefaultInfo ; WantedConstraints wanteds <- WantedConstraints -> TcS WantedConstraints TcS.zonkWC WantedConstraints wanteds ; let groups :: [(TcTyVar, [Ct])] groups = ([Type], (Bool, Bool)) -> WantedConstraints -> [(TcTyVar, [Ct])] findDefaultableGroups ([Type], (Bool, Bool)) info WantedConstraints wanteds ; String -> SDoc -> TcS () traceTcS String "applyDefaultingRules {" forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text String "wanteds =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr WantedConstraints wanteds , String -> SDoc text String "groups =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr [(TcTyVar, [Ct])] groups , String -> SDoc text String "info =" SDoc -> SDoc -> SDoc <+> forall a. Outputable a => a -> SDoc ppr ([Type], (Bool, Bool)) info ] ; [Bool] something_happeneds <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM ([Type] -> (TcTyVar, [Ct]) -> TcS Bool disambigGroup [Type] default_tys) [(TcTyVar, [Ct])] groups ; String -> SDoc -> TcS () traceTcS String "applyDefaultingRules }" (forall a. Outputable a => a -> SDoc ppr [Bool] something_happeneds) ; forall (m :: * -> *) a. Monad m => a -> m a return (forall (t :: * -> *). Foldable t => t Bool -> Bool or [Bool] something_happeneds) } findDefaultableGroups :: ( [Type] , (Bool,Bool) ) -- (Overloaded strings, extended default rules) -> WantedConstraints -- Unsolved (wanted or derived) -> [(TyVar, [Ct])] findDefaultableGroups :: ([Type], (Bool, Bool)) -> WantedConstraints -> [(TcTyVar, [Ct])] findDefaultableGroups ([Type] default_tys, (Bool ovl_strings, Bool extended_defaults)) WantedConstraints wanteds | forall (t :: * -> *) a. Foldable t => t a -> Bool null [Type] default_tys = [] | Bool otherwise = [ (TcTyVar tv, forall a b. (a -> b) -> [a] -> [b] map forall a b c. (a, b, c) -> a fstOf3 [(Ct, Class, TcTyVar)] group) | group' :: NonEmpty (Ct, Class, TcTyVar) group'@((Ct _,Class _,TcTyVar tv) :| [(Ct, Class, TcTyVar)] _) <- [NonEmpty (Ct, Class, TcTyVar)] unary_groups , let group :: [(Ct, Class, TcTyVar)] group = forall (t :: * -> *) a. Foldable t => t a -> [a] toList NonEmpty (Ct, Class, TcTyVar) group' , TcTyVar -> Bool defaultable_tyvar TcTyVar tv , [Class] -> Bool defaultable_classes (forall a b. (a -> b) -> [a] -> [b] map forall a b c. (a, b, c) -> b sndOf3 [(Ct, Class, TcTyVar)] group) ] where simples :: Cts simples = Bool -> WantedConstraints -> Cts approximateWC Bool True WantedConstraints wanteds ([(Ct, Class, TcTyVar)] unaries, [Ct] non_unaries) = forall a b c. (a -> Either b c) -> [a] -> ([b], [c]) partitionWith Ct -> Either (Ct, Class, TcTyVar) Ct find_unary (forall a. Bag a -> [a] bagToList Cts simples) unary_groups :: [NonEmpty (Ct, Class, TcTyVar)] unary_groups = forall a. (a -> a -> Ordering) -> [a] -> [NonEmpty a] equivClasses forall {a} {a} {b} {a} {b}. Ord a => (a, b, a) -> (a, b, a) -> Ordering cmp_tv [(Ct, Class, TcTyVar)] unaries unary_groups :: [NonEmpty (Ct, Class, TcTyVar)] -- (C tv) constraints unaries :: [(Ct, Class, TcTyVar)] -- (C tv) constraints non_unaries :: [Ct] -- and *other* constraints -- Finds unary type-class constraints -- But take account of polykinded classes like Typeable, -- which may look like (Typeable * (a:*)) (#8931) find_unary :: Ct -> Either (Ct, Class, TyVar) Ct find_unary :: Ct -> Either (Ct, Class, TcTyVar) Ct find_unary Ct cc | Just (Class cls,[Type] tys) <- Type -> Maybe (Class, [Type]) getClassPredTys_maybe (Ct -> Type ctPred Ct cc) , [Type ty] <- TyCon -> [Type] -> [Type] filterOutInvisibleTypes (Class -> TyCon classTyCon Class cls) [Type] tys -- Ignore invisible arguments for this purpose , Just TcTyVar tv <- Type -> Maybe TcTyVar tcGetTyVar_maybe Type ty , TcTyVar -> Bool isMetaTyVar TcTyVar tv -- We might have runtime-skolems in GHCi, and -- we definitely don't want to try to assign to those! = forall a b. a -> Either a b Left (Ct cc, Class cls, TcTyVar tv) find_unary Ct cc = forall a b. b -> Either a b Right Ct cc -- Non unary or non dictionary bad_tvs :: TcTyCoVarSet -- TyVars mentioned by non-unaries bad_tvs :: VarSet bad_tvs = forall a. (a -> VarSet) -> [a] -> VarSet mapUnionVarSet Ct -> VarSet tyCoVarsOfCt [Ct] non_unaries cmp_tv :: (a, b, a) -> (a, b, a) -> Ordering cmp_tv (a _,b _,a tv1) (a _,b _,a tv2) = a tv1 forall a. Ord a => a -> a -> Ordering `compare` a tv2 defaultable_tyvar :: TcTyVar -> Bool defaultable_tyvar :: TcTyVar -> Bool defaultable_tyvar TcTyVar tv = let b1 :: Bool b1 = TcTyVar -> Bool isTyConableTyVar TcTyVar tv -- Note [Avoiding spurious errors] b2 :: Bool b2 = Bool -> Bool not (TcTyVar tv TcTyVar -> VarSet -> Bool `elemVarSet` VarSet bad_tvs) in Bool b1 Bool -> Bool -> Bool && (Bool b2 Bool -> Bool -> Bool || Bool extended_defaults) -- Note [Multi-parameter defaults] defaultable_classes :: [Class] -> Bool defaultable_classes :: [Class] -> Bool defaultable_classes [Class] clss | Bool extended_defaults = forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool any (Bool -> Class -> Bool isInteractiveClass Bool ovl_strings) [Class] clss | Bool otherwise = forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool all Class -> Bool is_std_class [Class] clss Bool -> Bool -> Bool && (forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool any (Bool -> Class -> Bool isNumClass Bool ovl_strings) [Class] clss) -- is_std_class adds IsString to the standard numeric classes, -- when -foverloaded-strings is enabled is_std_class :: Class -> Bool is_std_class Class cls = Class -> Bool isStandardClass Class cls Bool -> Bool -> Bool || (Bool ovl_strings Bool -> Bool -> Bool && (Class cls forall a. Uniquable a => a -> Unique -> Bool `hasKey` Unique isStringClassKey)) ------------------------------ disambigGroup :: [Type] -- The default types -> (TcTyVar, [Ct]) -- All classes of the form (C a) -- sharing same type variable -> TcS Bool -- True <=> something happened, reflected in ty_binds disambigGroup :: [Type] -> (TcTyVar, [Ct]) -> TcS Bool disambigGroup [] (TcTyVar, [Ct]) _ = forall (m :: * -> *) a. Monad m => a -> m a return Bool False disambigGroup (Type default_ty:[Type] default_tys) group :: (TcTyVar, [Ct]) group@(TcTyVar the_tv, [Ct] wanteds) = do { String -> SDoc -> TcS () traceTcS String "disambigGroup {" ([SDoc] -> SDoc vcat [ forall a. Outputable a => a -> SDoc ppr Type default_ty, forall a. Outputable a => a -> SDoc ppr TcTyVar the_tv, forall a. Outputable a => a -> SDoc ppr [Ct] wanteds ]) ; EvBindsVar fake_ev_binds_var <- TcS EvBindsVar TcS.newTcEvBinds ; TcLevel tclvl <- TcS TcLevel TcS.getTcLevel ; Bool success <- forall a. EvBindsVar -> TcLevel -> TcS a -> TcS a nestImplicTcS EvBindsVar fake_ev_binds_var (TcLevel -> TcLevel pushTcLevel TcLevel tclvl) TcS Bool try_group ; if Bool success then -- Success: record the type variable binding, and return do { TcTyVar -> Type -> TcS () unifyTyVar TcTyVar the_tv Type default_ty ; forall a. TcM a -> TcS a wrapWarnTcS forall a b. (a -> b) -> a -> b $ [Ct] -> Type -> TcM () warnDefaulting [Ct] wanteds Type default_ty ; String -> SDoc -> TcS () traceTcS String "disambigGroup succeeded }" (forall a. Outputable a => a -> SDoc ppr Type default_ty) ; forall (m :: * -> *) a. Monad m => a -> m a return Bool True } else -- Failure: try with the next type do { String -> SDoc -> TcS () traceTcS String "disambigGroup failed, will try other default types }" (forall a. Outputable a => a -> SDoc ppr Type default_ty) ; [Type] -> (TcTyVar, [Ct]) -> TcS Bool disambigGroup [Type] default_tys (TcTyVar, [Ct]) group } } where try_group :: TcS Bool try_group | Just TCvSubst subst <- Maybe TCvSubst mb_subst = do { TcLclEnv lcl_env <- TcS TcLclEnv TcS.getLclEnv ; TcLevel tc_lvl <- TcS TcLevel TcS.getTcLevel ; let loc :: CtLoc loc = TcLevel -> SkolemInfo -> TcLclEnv -> CtLoc mkGivenLoc TcLevel tc_lvl SkolemInfo UnkSkol TcLclEnv lcl_env ; [CtEvidence] wanted_evs <- forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM (CtLoc -> Type -> TcS CtEvidence newWantedEvVarNC CtLoc loc forall b c a. (b -> c) -> (a -> b) -> a -> c . HasCallStack => TCvSubst -> Type -> Type substTy TCvSubst subst forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> Type ctPred) [Ct] wanteds ; forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b fmap WantedConstraints -> Bool isEmptyWC forall a b. (a -> b) -> a -> b $ Cts -> TcS WantedConstraints solveSimpleWanteds forall a b. (a -> b) -> a -> b $ forall a. [a] -> Bag a listToBag forall a b. (a -> b) -> a -> b $ forall a b. (a -> b) -> [a] -> [b] map CtEvidence -> Ct mkNonCanonical [CtEvidence] wanted_evs } | Bool otherwise = forall (m :: * -> *) a. Monad m => a -> m a return Bool False the_ty :: Type the_ty = TcTyVar -> Type mkTyVarTy TcTyVar the_tv mb_subst :: Maybe TCvSubst mb_subst = Type -> Type -> Maybe TCvSubst tcMatchTyKi Type the_ty Type default_ty -- Make sure the kinds match too; hence this call to tcMatchTyKi -- E.g. suppose the only constraint was (Typeable k (a::k)) -- With the addition of polykinded defaulting we also want to reject -- ill-kinded defaulting attempts like (Eq []) or (Foldable Int) here. -- In interactive mode, or with -XExtendedDefaultRules, -- we default Show a to Show () to avoid graututious errors on "show []" isInteractiveClass :: Bool -- -XOverloadedStrings? -> Class -> Bool isInteractiveClass :: Bool -> Class -> Bool isInteractiveClass Bool ovl_strings Class cls = Bool -> Class -> Bool isNumClass Bool ovl_strings Class cls Bool -> Bool -> Bool || (Class -> Unique classKey Class cls forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool `elem` [Unique] interactiveClassKeys) -- isNumClass adds IsString to the standard numeric classes, -- when -foverloaded-strings is enabled isNumClass :: Bool -- -XOverloadedStrings? -> Class -> Bool isNumClass :: Bool -> Class -> Bool isNumClass Bool ovl_strings Class cls = Class -> Bool isNumericClass Class cls Bool -> Bool -> Bool || (Bool ovl_strings Bool -> Bool -> Bool && (Class cls forall a. Uniquable a => a -> Unique -> Bool `hasKey` Unique isStringClassKey)) {- Note [Avoiding spurious errors] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When doing the unification for defaulting, we check for skolem type variables, and simply don't default them. For example: f = (*) -- Monomorphic g :: Num a => a -> a g x = f x x Here, we get a complaint when checking the type signature for g, that g isn't polymorphic enough; but then we get another one when dealing with the (Num a) context arising from f's definition; we try to unify a with Int (to default it), but find that it's already been unified with the rigid variable from g's type sig. Note [Multi-parameter defaults] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ With -XExtendedDefaultRules, we default only based on single-variable constraints, but do not exclude from defaulting any type variables which also appear in multi-variable constraints. This means that the following will default properly: default (Integer, Double) class A b (c :: Symbol) where a :: b -> Proxy c instance A Integer c where a _ = Proxy main = print (a 5 :: Proxy "5") Note that if we change the above instance ("instance A Integer") to "instance A Double", we get an error: No instance for (A Integer "5") This is because the first defaulted type (Integer) has successfully satisfied its single-parameter constraints (in this case Num). -}