{- This module defines types and simple operations over constraints, as used in the type-checker and constraint solver. -} {-# LANGUAGE CPP, GeneralizedNewtypeDeriving #-} module Constraint ( -- QCInst QCInst(..), isPendingScInst, -- Canonical constraints Xi, Ct(..), Cts, emptyCts, andCts, andManyCts, pprCts, singleCt, listToCts, ctsElts, consCts, snocCts, extendCtsList, isEmptyCts, isCTyEqCan, isCFunEqCan, isPendingScDict, superClassesMightHelp, getPendingWantedScs, isCDictCan_Maybe, isCFunEqCan_maybe, isCNonCanonical, isWantedCt, isDerivedCt, isGivenCt, isHoleCt, isOutOfScopeCt, isExprHoleCt, isTypeHoleCt, isUserTypeErrorCt, getUserTypeErrorMsg, ctEvidence, ctLoc, setCtLoc, ctPred, ctFlavour, ctEqRel, ctOrigin, ctEvId, mkTcEqPredLikeEv, mkNonCanonical, mkNonCanonicalCt, mkGivens, mkIrredCt, mkInsolubleCt, ctEvPred, ctEvLoc, ctEvOrigin, ctEvEqRel, ctEvExpr, ctEvTerm, ctEvCoercion, ctEvEvId, tyCoVarsOfCt, tyCoVarsOfCts, tyCoVarsOfCtList, tyCoVarsOfCtsList, WantedConstraints(..), insolubleWC, emptyWC, isEmptyWC, isSolvedWC, andWC, unionsWC, mkSimpleWC, mkImplicWC, addInsols, insolublesOnly, addSimples, addImplics, tyCoVarsOfWC, dropDerivedWC, dropDerivedSimples, tyCoVarsOfWCList, insolubleCt, insolubleEqCt, isDroppableCt, insolubleImplic, arisesFromGivens, Implication(..), implicationPrototype, ImplicStatus(..), isInsolubleStatus, isSolvedStatus, SubGoalDepth, initialSubGoalDepth, maxSubGoalDepth, bumpSubGoalDepth, subGoalDepthExceeded, CtLoc(..), ctLocSpan, ctLocEnv, ctLocLevel, ctLocOrigin, ctLocTypeOrKind_maybe, ctLocDepth, bumpCtLocDepth, isGivenLoc, setCtLocOrigin, updateCtLocOrigin, setCtLocEnv, setCtLocSpan, pprCtLoc, -- CtEvidence CtEvidence(..), TcEvDest(..), mkKindLoc, toKindLoc, mkGivenLoc, isWanted, isGiven, isDerived, isGivenOrWDeriv, ctEvRole, wrapType, wrapTypeWithImplication, CtFlavour(..), ShadowInfo(..), ctEvFlavour, CtFlavourRole, ctEvFlavourRole, ctFlavourRole, eqCanRewrite, eqCanRewriteFR, eqMayRewriteFR, eqCanDischargeFR, funEqCanDischarge, funEqCanDischargeF, -- Pretty printing pprEvVarTheta, pprEvVars, pprEvVarWithType, -- holes Hole(..), holeOcc, ) where #include "HsVersions.h" import GhcPrelude import {-# SOURCE #-} TcRnTypes ( TcLclEnv, setLclEnvTcLevel, getLclEnvTcLevel , setLclEnvLoc, getLclEnvLoc ) import GHC.Hs.Expr ( UnboundVar(..), unboundVarOcc ) import Predicate import Type import Coercion import Class import TyCon import Var import Id import TcType import TcEvidence import TcOrigin import CoreSyn import TyCoPpr import OccName import FV import VarSet import DynFlags import BasicTypes import Outputable import SrcLoc import Bag import Util import Control.Monad ( msum ) {- ************************************************************************ * * * Canonical constraints * * * * These are the constraints the low-level simplifier works with * * * ************************************************************************ -} -- The syntax of xi (ξ) types: -- xi ::= a | T xis | xis -> xis | ... | forall a. tau -- Two important notes: -- (i) No type families, unless we are under a ForAll -- (ii) Note that xi types can contain unexpanded type synonyms; -- however, the (transitive) expansions of those type synonyms -- will not contain any type functions, unless we are under a ForAll. -- We enforce the structure of Xi types when we flatten (TcCanonical) type Xi = Type -- In many comments, "xi" ranges over Xi type Cts = Bag Ct data Ct -- Atomic canonical constraints = CDictCan { -- e.g. Num xi Ct -> CtEvidence cc_ev :: CtEvidence, -- See Note [Ct/evidence invariant] Ct -> Class cc_class :: Class, Ct -> [Xi] cc_tyargs :: [Xi], -- cc_tyargs are function-free, hence Xi Ct -> Bool cc_pend_sc :: Bool -- See Note [The superclass story] in TcCanonical -- True <=> (a) cc_class has superclasses -- (b) we have not (yet) added those -- superclasses as Givens } | CIrredCan { -- These stand for yet-unusable predicates cc_ev :: CtEvidence, -- See Note [Ct/evidence invariant] Ct -> Bool cc_insol :: Bool -- True <=> definitely an error, can never be solved -- False <=> might be soluble -- For the might-be-soluble case, the ctev_pred of the evidence is -- of form (tv xi1 xi2 ... xin) with a tyvar at the head -- or (tv1 ~ ty2) where the CTyEqCan kind invariant fails -- or (F tys ~ ty) where the CFunEqCan kind invariant fails -- See Note [CIrredCan constraints] -- The definitely-insoluble case is for things like -- Int ~ Bool tycons don't match -- a ~ [a] occurs check } | CTyEqCan { -- tv ~ rhs -- Invariants: -- * See Note [inert_eqs: the inert equalities] in TcSMonad -- * tv not in tvs(rhs) (occurs check) -- * If tv is a TauTv, then rhs has no foralls -- (this avoids substituting a forall for the tyvar in other types) -- * tcTypeKind ty `tcEqKind` tcTypeKind tv; Note [Ct kind invariant] -- * rhs may have at most one top-level cast -- * rhs (perhaps under the one cast) is *almost function-free*, -- See Note [Almost function-free] -- * If the equality is representational, rhs has no top-level newtype -- See Note [No top-level newtypes on RHS of representational -- equalities] in TcCanonical -- * If rhs (perhaps under the cast) is also a tv, then it is oriented -- to give best chance of -- unification happening; eg if rhs is touchable then lhs is too cc_ev :: CtEvidence, -- See Note [Ct/evidence invariant] Ct -> TcTyVar cc_tyvar :: TcTyVar, Ct -> Xi cc_rhs :: TcType, -- Not necessarily function-free (hence not Xi) -- See invariants above Ct -> EqRel cc_eq_rel :: EqRel -- INVARIANT: cc_eq_rel = ctEvEqRel cc_ev } | CFunEqCan { -- F xis ~ fsk -- Invariants: -- * isTypeFamilyTyCon cc_fun -- * tcTypeKind (F xis) = tyVarKind fsk; Note [Ct kind invariant] -- * always Nominal role cc_ev :: CtEvidence, -- See Note [Ct/evidence invariant] Ct -> TyCon cc_fun :: TyCon, -- A type function cc_tyargs :: [Xi], -- cc_tyargs are function-free (hence Xi) -- Either under-saturated or exactly saturated -- *never* over-saturated (because if so -- we should have decomposed) Ct -> TcTyVar cc_fsk :: TcTyVar -- [G] always a FlatSkolTv -- [W], [WD], or [D] always a FlatMetaTv -- See Note [The flattening story] in TcFlatten } | CNonCanonical { -- See Note [NonCanonical Semantics] in TcSMonad cc_ev :: CtEvidence } | CHoleCan { -- See Note [Hole constraints] -- Treated as an "insoluble" constraint -- See Note [Insoluble constraints] cc_ev :: CtEvidence, Ct -> Hole cc_hole :: Hole } | CQuantCan QCInst -- A quantified constraint -- NB: I expect to make more of the cases in Ct -- look like this, with the payload in an -- auxiliary type ------------ data QCInst -- A much simplified version of ClsInst -- See Note [Quantified constraints] in TcCanonical = QCI { QCInst -> CtEvidence qci_ev :: CtEvidence -- Always of type forall tvs. context => ty -- Always Given , QCInst -> [TcTyVar] qci_tvs :: [TcTyVar] -- The tvs , QCInst -> Xi qci_pred :: TcPredType -- The ty , QCInst -> Bool qci_pend_sc :: Bool -- Same as cc_pend_sc flag in CDictCan -- Invariant: True => qci_pred is a ClassPred } instance Outputable QCInst where ppr :: QCInst -> SDoc ppr (QCI { qci_ev :: QCInst -> CtEvidence qci_ev = CtEvidence ev }) = CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence ev ------------ -- | An expression or type hole data Hole = ExprHole UnboundVar -- ^ Either an out-of-scope variable or a "true" hole in an -- expression (TypedHoles) | TypeHole OccName -- ^ A hole in a type (PartialTypeSignatures) instance Outputable Hole where ppr :: Hole -> SDoc ppr (ExprHole UnboundVar ub) = UnboundVar -> SDoc forall a. Outputable a => a -> SDoc ppr UnboundVar ub ppr (TypeHole OccName occ) = String -> SDoc text String "TypeHole" SDoc -> SDoc -> SDoc <> SDoc -> SDoc parens (OccName -> SDoc forall a. Outputable a => a -> SDoc ppr OccName occ) holeOcc :: Hole -> OccName holeOcc :: Hole -> OccName holeOcc (ExprHole UnboundVar uv) = UnboundVar -> OccName unboundVarOcc UnboundVar uv holeOcc (TypeHole OccName occ) = OccName occ {- Note [Hole constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~ CHoleCan constraints are used for two kinds of holes, distinguished by cc_hole: * For holes in expressions (includings variables not in scope) e.g. f x = g _ x * For holes in type signatures e.g. f :: _ -> _ f x = [x,True] Note [CIrredCan constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ CIrredCan constraints are used for constraints that are "stuck" - we can't solve them (yet) - we can't use them to solve other constraints - but they may become soluble if we substitute for some of the type variables in the constraint Example 1: (c Int), where c :: * -> Constraint. We can't do anything with this yet, but if later c := Num, *then* we can solve it Example 2: a ~ b, where a :: *, b :: k, where k is a kind variable We don't want to use this to substitute 'b' for 'a', in case 'k' is subsequently unifed with (say) *->*, because then we'd have ill-kinded types floating about. Rather we want to defer using the equality altogether until 'k' get resolved. Note [Ct/evidence invariant] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If ct :: Ct, then extra fields of 'ct' cache precisely the ctev_pred field of (cc_ev ct), and is fully rewritten wrt the substitution. Eg for CDictCan, ctev_pred (cc_ev ct) = (cc_class ct) (cc_tyargs ct) This holds by construction; look at the unique place where CDictCan is built (in TcCanonical). In contrast, the type of the evidence *term* (ctev_dest / ctev_evar) in the evidence may *not* be fully zonked; we are careful not to look at it during constraint solving. See Note [Evidence field of CtEvidence]. Note [Ct kind invariant] ~~~~~~~~~~~~~~~~~~~~~~~~ CTyEqCan and CFunEqCan both require that the kind of the lhs matches the kind of the rhs. This is necessary because both constraints are used for substitutions during solving. If the kinds differed, then the substitution would take a well-kinded type to an ill-kinded one. Note [Almost function-free] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ A type is *almost function-free* if it has no type functions (something that responds True to isTypeFamilyTyCon), except (possibly) * under a forall, or * in a coercion (either in a CastTy or a CercionTy) The RHS of a CTyEqCan must be almost function-free. This is for two reasons: 1. There cannot be a top-level function. If there were, the equality should really be a CFunEqCan, not a CTyEqCan. 2. Nested functions aren't too bad, on the other hand. However, consider this scenario: type family F a = r | r -> a [D] F ty1 ~ fsk1 [D] F ty2 ~ fsk2 [D] fsk1 ~ [G Int] [D] fsk2 ~ [G Bool] type instance G Int = Char type instance G Bool = Char If it was the case that fsk1 = fsk2, then we could unifty ty1 and ty2 -- good! They don't look equal -- but if we aggressively reduce that G Int and G Bool they would become equal. The "almost function free" makes sure that these redexes are exposed. Note that this equality does *not* depend on casts or coercions, and so skipping these forms is OK. In addition, the result of a type family cannot be a polytype, so skipping foralls is OK, too. We skip foralls because we want the output of the flattener to be almost function-free. See Note [Flattening under a forall] in TcFlatten. As I (Richard E) write this, it is unclear if the scenario pictured above can happen -- I would expect the G Int and G Bool to be reduced. But perhaps it can arise somehow, and maintaining almost function-free is cheap. Historical note: CTyEqCans used to require only condition (1) above: that no type family was at the top of an RHS. But work on #16512 suggested that the injectivity checks were not complete, and adding the requirement that functions do not appear even in a nested fashion was easy (it was already true, but unenforced). The almost-function-free property is checked by isAlmostFunctionFree in TcType. The flattener (in TcFlatten) produces types that are almost function-free. -} mkNonCanonical :: CtEvidence -> Ct mkNonCanonical :: CtEvidence -> Ct mkNonCanonical CtEvidence ev = CNonCanonical :: CtEvidence -> Ct CNonCanonical { cc_ev :: CtEvidence cc_ev = CtEvidence ev } mkNonCanonicalCt :: Ct -> Ct mkNonCanonicalCt :: Ct -> Ct mkNonCanonicalCt Ct ct = CNonCanonical :: CtEvidence -> Ct CNonCanonical { cc_ev :: CtEvidence cc_ev = Ct -> CtEvidence cc_ev Ct ct } mkIrredCt :: CtEvidence -> Ct mkIrredCt :: CtEvidence -> Ct mkIrredCt CtEvidence ev = CIrredCan :: CtEvidence -> Bool -> Ct CIrredCan { cc_ev :: CtEvidence cc_ev = CtEvidence ev, cc_insol :: Bool cc_insol = Bool False } mkInsolubleCt :: CtEvidence -> Ct mkInsolubleCt :: CtEvidence -> Ct mkInsolubleCt CtEvidence ev = CIrredCan :: CtEvidence -> Bool -> Ct CIrredCan { cc_ev :: CtEvidence cc_ev = CtEvidence ev, cc_insol :: Bool cc_insol = Bool True } mkGivens :: CtLoc -> [EvId] -> [Ct] mkGivens :: CtLoc -> [TcTyVar] -> [Ct] mkGivens CtLoc loc [TcTyVar] ev_ids = (TcTyVar -> Ct) -> [TcTyVar] -> [Ct] forall a b. (a -> b) -> [a] -> [b] map TcTyVar -> Ct mk [TcTyVar] ev_ids where mk :: TcTyVar -> Ct mk TcTyVar ev_id = CtEvidence -> Ct mkNonCanonical (CtGiven :: Xi -> TcTyVar -> CtLoc -> CtEvidence CtGiven { ctev_evar :: TcTyVar ctev_evar = TcTyVar ev_id , ctev_pred :: Xi ctev_pred = TcTyVar -> Xi evVarPred TcTyVar ev_id , ctev_loc :: CtLoc ctev_loc = CtLoc loc }) ctEvidence :: Ct -> CtEvidence ctEvidence :: Ct -> CtEvidence ctEvidence (CQuantCan (QCI { qci_ev :: QCInst -> CtEvidence qci_ev = CtEvidence ev })) = CtEvidence ev ctEvidence Ct ct = Ct -> CtEvidence cc_ev Ct ct ctLoc :: Ct -> CtLoc ctLoc :: Ct -> CtLoc ctLoc = CtEvidence -> CtLoc ctEvLoc (CtEvidence -> CtLoc) -> (Ct -> CtEvidence) -> Ct -> CtLoc forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> CtEvidence ctEvidence setCtLoc :: Ct -> CtLoc -> Ct setCtLoc :: Ct -> CtLoc -> Ct setCtLoc Ct ct CtLoc loc = Ct ct { cc_ev :: CtEvidence cc_ev = (Ct -> CtEvidence cc_ev Ct ct) { ctev_loc :: CtLoc ctev_loc = CtLoc loc } } ctOrigin :: Ct -> CtOrigin ctOrigin :: Ct -> CtOrigin ctOrigin = CtLoc -> CtOrigin ctLocOrigin (CtLoc -> CtOrigin) -> (Ct -> CtLoc) -> Ct -> CtOrigin forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> CtLoc ctLoc ctPred :: Ct -> PredType -- See Note [Ct/evidence invariant] ctPred :: Ct -> Xi ctPred Ct ct = CtEvidence -> Xi ctEvPred (Ct -> CtEvidence ctEvidence Ct ct) ctEvId :: Ct -> EvVar -- The evidence Id for this Ct ctEvId :: Ct -> TcTyVar ctEvId Ct ct = CtEvidence -> TcTyVar ctEvEvId (Ct -> CtEvidence ctEvidence Ct ct) -- | Makes a new equality predicate with the same role as the given -- evidence. mkTcEqPredLikeEv :: CtEvidence -> TcType -> TcType -> TcType mkTcEqPredLikeEv :: CtEvidence -> Xi -> Xi -> Xi mkTcEqPredLikeEv CtEvidence ev = case Xi -> EqRel predTypeEqRel Xi pred of EqRel NomEq -> Xi -> Xi -> Xi mkPrimEqPred EqRel ReprEq -> Xi -> Xi -> Xi mkReprPrimEqPred where pred :: Xi pred = CtEvidence -> Xi ctEvPred CtEvidence ev -- | Get the flavour of the given 'Ct' ctFlavour :: Ct -> CtFlavour ctFlavour :: Ct -> CtFlavour ctFlavour = CtEvidence -> CtFlavour ctEvFlavour (CtEvidence -> CtFlavour) -> (Ct -> CtEvidence) -> Ct -> CtFlavour forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> CtEvidence ctEvidence -- | Get the equality relation for the given 'Ct' ctEqRel :: Ct -> EqRel ctEqRel :: Ct -> EqRel ctEqRel = CtEvidence -> EqRel ctEvEqRel (CtEvidence -> EqRel) -> (Ct -> CtEvidence) -> Ct -> EqRel forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> CtEvidence ctEvidence instance Outputable Ct where ppr :: Ct -> SDoc ppr Ct ct = CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr (Ct -> CtEvidence ctEvidence Ct ct) SDoc -> SDoc -> SDoc <+> SDoc -> SDoc parens SDoc pp_sort where pp_sort :: SDoc pp_sort = case Ct ct of CTyEqCan {} -> String -> SDoc text String "CTyEqCan" CFunEqCan {} -> String -> SDoc text String "CFunEqCan" CNonCanonical {} -> String -> SDoc text String "CNonCanonical" CDictCan { cc_pend_sc :: Ct -> Bool cc_pend_sc = Bool pend_sc } | Bool pend_sc -> String -> SDoc text String "CDictCan(psc)" | Bool otherwise -> String -> SDoc text String "CDictCan" CIrredCan { cc_insol :: Ct -> Bool cc_insol = Bool insol } | Bool insol -> String -> SDoc text String "CIrredCan(insol)" | Bool otherwise -> String -> SDoc text String "CIrredCan(sol)" CHoleCan { cc_hole :: Ct -> Hole cc_hole = Hole hole } -> String -> SDoc text String "CHoleCan:" SDoc -> SDoc -> SDoc <+> Hole -> SDoc forall a. Outputable a => a -> SDoc ppr Hole hole CQuantCan (QCI { qci_pend_sc :: QCInst -> Bool qci_pend_sc = Bool pend_sc }) | Bool pend_sc -> String -> SDoc text String "CQuantCan(psc)" | Bool otherwise -> String -> SDoc text String "CQuantCan" {- ************************************************************************ * * Simple functions over evidence variables * * ************************************************************************ -} ---------------- Getting free tyvars ------------------------- -- | Returns free variables of constraints as a non-deterministic set tyCoVarsOfCt :: Ct -> TcTyCoVarSet tyCoVarsOfCt :: Ct -> TcTyCoVarSet tyCoVarsOfCt = FV -> TcTyCoVarSet fvVarSet (FV -> TcTyCoVarSet) -> (Ct -> FV) -> Ct -> TcTyCoVarSet forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> FV tyCoFVsOfCt -- | Returns free variables of constraints as a deterministically ordered. -- list. See Note [Deterministic FV] in FV. tyCoVarsOfCtList :: Ct -> [TcTyCoVar] tyCoVarsOfCtList :: Ct -> [TcTyVar] tyCoVarsOfCtList = FV -> [TcTyVar] fvVarList (FV -> [TcTyVar]) -> (Ct -> FV) -> Ct -> [TcTyVar] forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> FV tyCoFVsOfCt -- | Returns free variables of constraints as a composable FV computation. -- See Note [Deterministic FV] in FV. tyCoFVsOfCt :: Ct -> FV tyCoFVsOfCt :: Ct -> FV tyCoFVsOfCt (CTyEqCan { cc_tyvar :: Ct -> TcTyVar cc_tyvar = TcTyVar tv, cc_rhs :: Ct -> Xi cc_rhs = Xi xi }) = Xi -> FV tyCoFVsOfType Xi xi FV -> FV -> FV `unionFV` TcTyVar -> FV FV.unitFV TcTyVar tv FV -> FV -> FV `unionFV` Xi -> FV tyCoFVsOfType (TcTyVar -> Xi tyVarKind TcTyVar tv) tyCoFVsOfCt (CFunEqCan { cc_tyargs :: Ct -> [Xi] cc_tyargs = [Xi] tys, cc_fsk :: Ct -> TcTyVar cc_fsk = TcTyVar fsk }) = [Xi] -> FV tyCoFVsOfTypes [Xi] tys FV -> FV -> FV `unionFV` TcTyVar -> FV FV.unitFV TcTyVar fsk FV -> FV -> FV `unionFV` Xi -> FV tyCoFVsOfType (TcTyVar -> Xi tyVarKind TcTyVar fsk) tyCoFVsOfCt (CDictCan { cc_tyargs :: Ct -> [Xi] cc_tyargs = [Xi] tys }) = [Xi] -> FV tyCoFVsOfTypes [Xi] tys tyCoFVsOfCt Ct ct = Xi -> FV tyCoFVsOfType (Ct -> Xi ctPred Ct ct) -- | Returns free variables of a bag of constraints as a non-deterministic -- set. See Note [Deterministic FV] in FV. tyCoVarsOfCts :: Cts -> TcTyCoVarSet tyCoVarsOfCts :: Cts -> TcTyCoVarSet tyCoVarsOfCts = FV -> TcTyCoVarSet fvVarSet (FV -> TcTyCoVarSet) -> (Cts -> FV) -> Cts -> TcTyCoVarSet forall b c a. (b -> c) -> (a -> b) -> a -> c . Cts -> FV tyCoFVsOfCts -- | Returns free variables of a bag of constraints as a deterministically -- odered list. See Note [Deterministic FV] in FV. tyCoVarsOfCtsList :: Cts -> [TcTyCoVar] tyCoVarsOfCtsList :: Cts -> [TcTyVar] tyCoVarsOfCtsList = FV -> [TcTyVar] fvVarList (FV -> [TcTyVar]) -> (Cts -> FV) -> Cts -> [TcTyVar] forall b c a. (b -> c) -> (a -> b) -> a -> c . Cts -> FV tyCoFVsOfCts -- | Returns free variables of a bag of constraints as a composable FV -- computation. See Note [Deterministic FV] in FV. tyCoFVsOfCts :: Cts -> FV tyCoFVsOfCts :: Cts -> FV tyCoFVsOfCts = (Ct -> FV -> FV) -> FV -> Cts -> FV forall (t :: * -> *) a b. Foldable t => (a -> b -> b) -> b -> t a -> b foldr (FV -> FV -> FV unionFV (FV -> FV -> FV) -> (Ct -> FV) -> Ct -> FV -> FV forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> FV tyCoFVsOfCt) FV emptyFV -- | Returns free variables of WantedConstraints as a non-deterministic -- set. See Note [Deterministic FV] in FV. tyCoVarsOfWC :: WantedConstraints -> TyCoVarSet -- Only called on *zonked* things, hence no need to worry about flatten-skolems tyCoVarsOfWC :: WantedConstraints -> TcTyCoVarSet tyCoVarsOfWC = FV -> TcTyCoVarSet fvVarSet (FV -> TcTyCoVarSet) -> (WantedConstraints -> FV) -> WantedConstraints -> TcTyCoVarSet forall b c a. (b -> c) -> (a -> b) -> a -> c . WantedConstraints -> FV tyCoFVsOfWC -- | Returns free variables of WantedConstraints as a deterministically -- ordered list. See Note [Deterministic FV] in FV. tyCoVarsOfWCList :: WantedConstraints -> [TyCoVar] -- Only called on *zonked* things, hence no need to worry about flatten-skolems tyCoVarsOfWCList :: WantedConstraints -> [TcTyVar] tyCoVarsOfWCList = FV -> [TcTyVar] fvVarList (FV -> [TcTyVar]) -> (WantedConstraints -> FV) -> WantedConstraints -> [TcTyVar] forall b c a. (b -> c) -> (a -> b) -> a -> c . WantedConstraints -> FV tyCoFVsOfWC -- | Returns free variables of WantedConstraints as a composable FV -- computation. See Note [Deterministic FV] in FV. tyCoFVsOfWC :: WantedConstraints -> FV -- Only called on *zonked* things, hence no need to worry about flatten-skolems tyCoFVsOfWC :: WantedConstraints -> FV tyCoFVsOfWC (WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simple, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implic }) = Cts -> FV tyCoFVsOfCts Cts simple FV -> FV -> FV `unionFV` (Implication -> FV) -> Bag Implication -> FV forall a. (a -> FV) -> Bag a -> FV tyCoFVsOfBag Implication -> FV tyCoFVsOfImplic Bag Implication implic -- | Returns free variables of Implication as a composable FV computation. -- See Note [Deterministic FV] in FV. tyCoFVsOfImplic :: Implication -> FV -- Only called on *zonked* things, hence no need to worry about flatten-skolems tyCoFVsOfImplic :: Implication -> FV tyCoFVsOfImplic (Implic { ic_skols :: Implication -> [TcTyVar] ic_skols = [TcTyVar] skols , ic_given :: Implication -> [TcTyVar] ic_given = [TcTyVar] givens , ic_wanted :: Implication -> WantedConstraints ic_wanted = WantedConstraints wanted }) | WantedConstraints -> Bool isEmptyWC WantedConstraints wanted = FV emptyFV | Bool otherwise = [TcTyVar] -> FV -> FV tyCoFVsVarBndrs [TcTyVar] skols (FV -> FV) -> FV -> FV forall a b. (a -> b) -> a -> b $ [TcTyVar] -> FV -> FV tyCoFVsVarBndrs [TcTyVar] givens (FV -> FV) -> FV -> FV forall a b. (a -> b) -> a -> b $ WantedConstraints -> FV tyCoFVsOfWC WantedConstraints wanted tyCoFVsOfBag :: (a -> FV) -> Bag a -> FV tyCoFVsOfBag :: (a -> FV) -> Bag a -> FV tyCoFVsOfBag a -> FV tvs_of = (a -> FV -> FV) -> FV -> Bag a -> FV forall (t :: * -> *) a b. Foldable t => (a -> b -> b) -> b -> t a -> b foldr (FV -> FV -> FV unionFV (FV -> FV -> FV) -> (a -> FV) -> a -> FV -> FV forall b c a. (b -> c) -> (a -> b) -> a -> c . a -> FV tvs_of) FV emptyFV --------------------------- dropDerivedWC :: WantedConstraints -> WantedConstraints -- See Note [Dropping derived constraints] dropDerivedWC :: WantedConstraints -> WantedConstraints dropDerivedWC wc :: WantedConstraints wc@(WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples }) = WantedConstraints wc { wc_simple :: Cts wc_simple = Cts -> Cts dropDerivedSimples Cts simples } -- The wc_impl implications are already (recursively) filtered -------------------------- dropDerivedSimples :: Cts -> Cts -- Drop all Derived constraints, but make [W] back into [WD], -- so that if we re-simplify these constraints we will get all -- the right derived constraints re-generated. Forgetting this -- step led to #12936 dropDerivedSimples :: Cts -> Cts dropDerivedSimples Cts simples = (Ct -> Maybe Ct) -> Cts -> Cts forall a b. (a -> Maybe b) -> Bag a -> Bag b mapMaybeBag Ct -> Maybe Ct dropDerivedCt Cts simples dropDerivedCt :: Ct -> Maybe Ct dropDerivedCt :: Ct -> Maybe Ct dropDerivedCt Ct ct = case CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev of Wanted ShadowInfo WOnly -> Ct -> Maybe Ct forall a. a -> Maybe a Just (Ct ct' { cc_ev :: CtEvidence cc_ev = CtEvidence ev_wd }) Wanted ShadowInfo _ -> Ct -> Maybe Ct forall a. a -> Maybe a Just Ct ct' CtFlavour _ | Ct -> Bool isDroppableCt Ct ct -> Maybe Ct forall a. Maybe a Nothing | Bool otherwise -> Ct -> Maybe Ct forall a. a -> Maybe a Just Ct ct where ev :: CtEvidence ev = Ct -> CtEvidence ctEvidence Ct ct ev_wd :: CtEvidence ev_wd = CtEvidence ev { ctev_nosh :: ShadowInfo ctev_nosh = ShadowInfo WDeriv } ct' :: Ct ct' = Ct -> Ct setPendingScDict Ct ct -- See Note [Resetting cc_pend_sc] {- Note [Resetting cc_pend_sc] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we discard Derived constraints, in dropDerivedSimples, we must set the cc_pend_sc flag to True, so that if we re-process this CDictCan we will re-generate its derived superclasses. Otherwise we might miss some fundeps. #13662 showed this up. See Note [The superclass story] in TcCanonical. -} isDroppableCt :: Ct -> Bool isDroppableCt :: Ct -> Bool isDroppableCt Ct ct = CtEvidence -> Bool isDerived CtEvidence ev Bool -> Bool -> Bool && Bool -> Bool not Bool keep_deriv -- Drop only derived constraints, and then only if they -- obey Note [Dropping derived constraints] where ev :: CtEvidence ev = Ct -> CtEvidence ctEvidence Ct ct loc :: CtLoc loc = CtEvidence -> CtLoc ctEvLoc CtEvidence ev orig :: CtOrigin orig = CtLoc -> CtOrigin ctLocOrigin CtLoc loc keep_deriv :: Bool keep_deriv = case Ct ct of CHoleCan {} -> Bool True CIrredCan { cc_insol :: Ct -> Bool cc_insol = Bool insoluble } -> Bool -> Bool keep_eq Bool insoluble Ct _ -> Bool -> Bool keep_eq Bool False keep_eq :: Bool -> Bool keep_eq Bool definitely_insoluble | CtOrigin -> Bool isGivenOrigin CtOrigin orig -- Arising only from givens = Bool definitely_insoluble -- Keep only definitely insoluble | Bool otherwise = case CtOrigin orig of KindEqOrigin {} -> Bool True -- See Note [Dropping derived constraints] -- See Note [Dropping derived constraints] -- For fundeps, drop wanted/wanted interactions FunDepOrigin2 {} -> Bool True -- Top-level/Wanted FunDepOrigin1 Xi _ CtOrigin orig1 RealSrcSpan _ Xi _ CtOrigin orig2 RealSrcSpan _ | Bool g1 Bool -> Bool -> Bool || Bool g2 -> Bool True -- Given/Wanted errors: keep all | Bool otherwise -> Bool False -- Wanted/Wanted errors: discard where g1 :: Bool g1 = CtOrigin -> Bool isGivenOrigin CtOrigin orig1 g2 :: Bool g2 = CtOrigin -> Bool isGivenOrigin CtOrigin orig2 CtOrigin _ -> Bool False arisesFromGivens :: Ct -> Bool arisesFromGivens :: Ct -> Bool arisesFromGivens Ct ct = case Ct -> CtEvidence ctEvidence Ct ct of CtGiven {} -> Bool True CtWanted {} -> Bool False CtDerived { ctev_loc :: CtEvidence -> CtLoc ctev_loc = CtLoc loc } -> CtLoc -> Bool isGivenLoc CtLoc loc isGivenLoc :: CtLoc -> Bool isGivenLoc :: CtLoc -> Bool isGivenLoc CtLoc loc = CtOrigin -> Bool isGivenOrigin (CtLoc -> CtOrigin ctLocOrigin CtLoc loc) {- Note [Dropping derived constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In general we discard derived constraints at the end of constraint solving; see dropDerivedWC. For example * Superclasses: if we have an unsolved [W] (Ord a), we don't want to complain about an unsolved [D] (Eq a) as well. * If we have [W] a ~ Int, [W] a ~ Bool, improvement will generate [D] Int ~ Bool, and we don't want to report that because it's incomprehensible. That is why we don't rewrite wanteds with wanteds! * We might float out some Wanteds from an implication, leaving behind their insoluble Deriveds. For example: forall a[2]. [W] alpha[1] ~ Int [W] alpha[1] ~ Bool [D] Int ~ Bool The Derived is insoluble, but we very much want to drop it when floating out. But (tiresomely) we do keep *some* Derived constraints: * Type holes are derived constraints, because they have no evidence and we want to keep them, so we get the error report * Insoluble kind equalities (e.g. [D] * ~ (* -> *)), with KindEqOrigin, may arise from a type equality a ~ Int#, say. See Note [Equalities with incompatible kinds] in TcCanonical. Keeping these around produces better error messages, in practice. E.g., test case dependent/should_fail/T11471 * We keep most derived equalities arising from functional dependencies - Given/Given interactions (subset of FunDepOrigin1): The definitely-insoluble ones reflect unreachable code. Others not-definitely-insoluble ones like [D] a ~ Int do not reflect unreachable code; indeed if fundeps generated proofs, it'd be a useful equality. See #14763. So we discard them. - Given/Wanted interacGiven or Wanted interacting with an instance declaration (FunDepOrigin2) - Given/Wanted interactions (FunDepOrigin1); see #9612 - But for Wanted/Wanted interactions we do /not/ want to report an error (#13506). Consider [W] C Int Int, [W] C Int Bool, with a fundep on class C. We don't want to report an insoluble Int~Bool; c.f. "wanteds do not rewrite wanteds". To distinguish these cases we use the CtOrigin. NB: we keep *all* derived insolubles under some circumstances: * They are looked at by simplifyInfer, to decide whether to generalise. Example: [W] a ~ Int, [W] a ~ Bool We get [D] Int ~ Bool, and indeed the constraints are insoluble, and we want simplifyInfer to see that, even though we don't ultimately want to generate an (inexplicable) error message from it ************************************************************************ * * CtEvidence The "flavor" of a canonical constraint * * ************************************************************************ -} isWantedCt :: Ct -> Bool isWantedCt :: Ct -> Bool isWantedCt = CtEvidence -> Bool isWanted (CtEvidence -> Bool) -> (Ct -> CtEvidence) -> Ct -> Bool forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> CtEvidence ctEvidence isGivenCt :: Ct -> Bool isGivenCt :: Ct -> Bool isGivenCt = CtEvidence -> Bool isGiven (CtEvidence -> Bool) -> (Ct -> CtEvidence) -> Ct -> Bool forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> CtEvidence ctEvidence isDerivedCt :: Ct -> Bool isDerivedCt :: Ct -> Bool isDerivedCt = CtEvidence -> Bool isDerived (CtEvidence -> Bool) -> (Ct -> CtEvidence) -> Ct -> Bool forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> CtEvidence ctEvidence isCTyEqCan :: Ct -> Bool isCTyEqCan :: Ct -> Bool isCTyEqCan (CTyEqCan {}) = Bool True isCTyEqCan (CFunEqCan {}) = Bool False isCTyEqCan Ct _ = Bool False isCDictCan_Maybe :: Ct -> Maybe Class isCDictCan_Maybe :: Ct -> Maybe Class isCDictCan_Maybe (CDictCan {cc_class :: Ct -> Class cc_class = Class cls }) = Class -> Maybe Class forall a. a -> Maybe a Just Class cls isCDictCan_Maybe Ct _ = Maybe Class forall a. Maybe a Nothing isCFunEqCan_maybe :: Ct -> Maybe (TyCon, [Type]) isCFunEqCan_maybe :: Ct -> Maybe (TyCon, [Xi]) isCFunEqCan_maybe (CFunEqCan { cc_fun :: Ct -> TyCon cc_fun = TyCon tc, cc_tyargs :: Ct -> [Xi] cc_tyargs = [Xi] xis }) = (TyCon, [Xi]) -> Maybe (TyCon, [Xi]) forall a. a -> Maybe a Just (TyCon tc, [Xi] xis) isCFunEqCan_maybe Ct _ = Maybe (TyCon, [Xi]) forall a. Maybe a Nothing isCFunEqCan :: Ct -> Bool isCFunEqCan :: Ct -> Bool isCFunEqCan (CFunEqCan {}) = Bool True isCFunEqCan Ct _ = Bool False isCNonCanonical :: Ct -> Bool isCNonCanonical :: Ct -> Bool isCNonCanonical (CNonCanonical {}) = Bool True isCNonCanonical Ct _ = Bool False isHoleCt:: Ct -> Bool isHoleCt :: Ct -> Bool isHoleCt (CHoleCan {}) = Bool True isHoleCt Ct _ = Bool False isOutOfScopeCt :: Ct -> Bool -- We treat expression holes representing out-of-scope variables a bit -- differently when it comes to error reporting isOutOfScopeCt :: Ct -> Bool isOutOfScopeCt (CHoleCan { cc_hole :: Ct -> Hole cc_hole = ExprHole (OutOfScope {}) }) = Bool True isOutOfScopeCt Ct _ = Bool False isExprHoleCt :: Ct -> Bool isExprHoleCt :: Ct -> Bool isExprHoleCt (CHoleCan { cc_hole :: Ct -> Hole cc_hole = ExprHole {} }) = Bool True isExprHoleCt Ct _ = Bool False isTypeHoleCt :: Ct -> Bool isTypeHoleCt :: Ct -> Bool isTypeHoleCt (CHoleCan { cc_hole :: Ct -> Hole cc_hole = TypeHole {} }) = Bool True isTypeHoleCt Ct _ = Bool False {- Note [Custom type errors in constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When GHC reports a type-error about an unsolved-constraint, we check to see if the constraint contains any custom-type errors, and if so we report them. Here are some examples of constraints containing type errors: TypeError msg -- The actual constraint is a type error TypError msg ~ Int -- Some type was supposed to be Int, but ended up -- being a type error instead Eq (TypeError msg) -- A class constraint is stuck due to a type error F (TypeError msg) ~ a -- A type function failed to evaluate due to a type err It is also possible to have constraints where the type error is nested deeper, for example see #11990, and also: Eq (F (TypeError msg)) -- Here the type error is nested under a type-function -- call, which failed to evaluate because of it, -- and so the `Eq` constraint was unsolved. -- This may happen when one function calls another -- and the called function produced a custom type error. -} -- | A constraint is considered to be a custom type error, if it contains -- custom type errors anywhere in it. -- See Note [Custom type errors in constraints] getUserTypeErrorMsg :: Ct -> Maybe Type getUserTypeErrorMsg :: Ct -> Maybe Xi getUserTypeErrorMsg Ct ct = Xi -> Maybe Xi findUserTypeError (Ct -> Xi ctPred Ct ct) where findUserTypeError :: Xi -> Maybe Xi findUserTypeError Xi t = [Maybe Xi] -> Maybe Xi forall (t :: * -> *) (m :: * -> *) a. (Foldable t, MonadPlus m) => t (m a) -> m a msum ( Xi -> Maybe Xi userTypeError_maybe Xi t Maybe Xi -> [Maybe Xi] -> [Maybe Xi] forall a. a -> [a] -> [a] : (Xi -> Maybe Xi) -> [Xi] -> [Maybe Xi] forall a b. (a -> b) -> [a] -> [b] map Xi -> Maybe Xi findUserTypeError (Xi -> [Xi] subTys Xi t) ) subTys :: Xi -> [Xi] subTys Xi t = case Xi -> (Xi, [Xi]) splitAppTys Xi t of (Xi t,[]) -> case HasDebugCallStack => Xi -> Maybe (TyCon, [Xi]) Xi -> Maybe (TyCon, [Xi]) splitTyConApp_maybe Xi t of Maybe (TyCon, [Xi]) Nothing -> [] Just (TyCon _,[Xi] ts) -> [Xi] ts (Xi t,[Xi] ts) -> Xi t Xi -> [Xi] -> [Xi] forall a. a -> [a] -> [a] : [Xi] ts isUserTypeErrorCt :: Ct -> Bool isUserTypeErrorCt :: Ct -> Bool isUserTypeErrorCt Ct ct = case Ct -> Maybe Xi getUserTypeErrorMsg Ct ct of Just Xi _ -> Bool True Maybe Xi _ -> Bool False isPendingScDict :: Ct -> Maybe Ct -- Says whether this is a CDictCan with cc_pend_sc is True, -- AND if so flips the flag isPendingScDict :: Ct -> Maybe Ct isPendingScDict ct :: Ct ct@(CDictCan { cc_pend_sc :: Ct -> Bool cc_pend_sc = Bool True }) = Ct -> Maybe Ct forall a. a -> Maybe a Just (Ct ct { cc_pend_sc :: Bool cc_pend_sc = Bool False }) isPendingScDict Ct _ = Maybe Ct forall a. Maybe a Nothing isPendingScInst :: QCInst -> Maybe QCInst -- Same as isPrendinScDict, but for QCInsts isPendingScInst :: QCInst -> Maybe QCInst isPendingScInst qci :: QCInst qci@(QCI { qci_pend_sc :: QCInst -> Bool qci_pend_sc = Bool True }) = QCInst -> Maybe QCInst forall a. a -> Maybe a Just (QCInst qci { qci_pend_sc :: Bool qci_pend_sc = Bool False }) isPendingScInst QCInst _ = Maybe QCInst forall a. Maybe a Nothing setPendingScDict :: Ct -> Ct -- Set the cc_pend_sc flag to True setPendingScDict :: Ct -> Ct setPendingScDict ct :: Ct ct@(CDictCan { cc_pend_sc :: Ct -> Bool cc_pend_sc = Bool False }) = Ct ct { cc_pend_sc :: Bool cc_pend_sc = Bool True } setPendingScDict Ct ct = Ct ct superClassesMightHelp :: WantedConstraints -> Bool -- ^ True if taking superclasses of givens, or of wanteds (to perhaps -- expose more equalities or functional dependencies) might help to -- solve this constraint. See Note [When superclasses help] superClassesMightHelp :: WantedConstraints -> Bool superClassesMightHelp (WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implics }) = (Ct -> Bool) -> Cts -> Bool forall a. (a -> Bool) -> Bag a -> Bool anyBag Ct -> Bool might_help_ct Cts simples Bool -> Bool -> Bool || (Implication -> Bool) -> Bag Implication -> Bool forall a. (a -> Bool) -> Bag a -> Bool anyBag Implication -> Bool might_help_implic Bag Implication implics where might_help_implic :: Implication -> Bool might_help_implic Implication ic | ImplicStatus IC_Unsolved <- Implication -> ImplicStatus ic_status Implication ic = WantedConstraints -> Bool superClassesMightHelp (Implication -> WantedConstraints ic_wanted Implication ic) | Bool otherwise = Bool False might_help_ct :: Ct -> Bool might_help_ct Ct ct = Ct -> Bool isWantedCt Ct ct Bool -> Bool -> Bool && Bool -> Bool not (Ct -> Bool is_ip Ct ct) is_ip :: Ct -> Bool is_ip (CDictCan { cc_class :: Ct -> Class cc_class = Class cls }) = Class -> Bool isIPClass Class cls is_ip Ct _ = Bool False getPendingWantedScs :: Cts -> ([Ct], Cts) getPendingWantedScs :: Cts -> ([Ct], Cts) getPendingWantedScs Cts simples = ([Ct] -> Ct -> ([Ct], Ct)) -> [Ct] -> Cts -> ([Ct], Cts) forall acc x y. (acc -> x -> (acc, y)) -> acc -> Bag x -> (acc, Bag y) mapAccumBagL [Ct] -> Ct -> ([Ct], Ct) get [] Cts simples where get :: [Ct] -> Ct -> ([Ct], Ct) get [Ct] acc Ct ct | Just Ct ct' <- Ct -> Maybe Ct isPendingScDict Ct ct = (Ct ct'Ct -> [Ct] -> [Ct] forall a. a -> [a] -> [a] :[Ct] acc, Ct ct') | Bool otherwise = ([Ct] acc, Ct ct) {- Note [When superclasses help] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ First read Note [The superclass story] in TcCanonical. We expand superclasses and iterate only if there is at unsolved wanted for which expansion of superclasses (e.g. from given constraints) might actually help. The function superClassesMightHelp tells if doing this superclass expansion might help solve this constraint. Note that * We look inside implications; maybe it'll help to expand the Givens at level 2 to help solve an unsolved Wanted buried inside an implication. E.g. forall a. Ord a => forall b. [W] Eq a * Superclasses help only for Wanted constraints. Derived constraints are not really "unsolved" and we certainly don't want them to trigger superclass expansion. This was a good part of the loop in #11523 * Even for Wanted constraints, we say "no" for implicit parameters. we have [W] ?x::ty, expanding superclasses won't help: - Superclasses can't be implicit parameters - If we have a [G] ?x:ty2, then we'll have another unsolved [D] ty ~ ty2 (from the functional dependency) which will trigger superclass expansion. It's a bit of a special case, but it's easy to do. The runtime cost is low because the unsolved set is usually empty anyway (errors aside), and the first non-imlicit-parameter will terminate the search. The special case is worth it (#11480, comment:2) because it applies to CallStack constraints, which aren't type errors. If we have f :: (C a) => blah f x = ...undefined... we'll get a CallStack constraint. If that's the only unsolved constraint it'll eventually be solved by defaulting. So we don't want to emit warnings about hitting the simplifier's iteration limit. A CallStack constraint really isn't an unsolved constraint; it can always be solved by defaulting. -} singleCt :: Ct -> Cts singleCt :: Ct -> Cts singleCt = Ct -> Cts forall a. a -> Bag a unitBag andCts :: Cts -> Cts -> Cts andCts :: Cts -> Cts -> Cts andCts = Cts -> Cts -> Cts forall a. Bag a -> Bag a -> Bag a unionBags listToCts :: [Ct] -> Cts listToCts :: [Ct] -> Cts listToCts = [Ct] -> Cts forall a. [a] -> Bag a listToBag ctsElts :: Cts -> [Ct] ctsElts :: Cts -> [Ct] ctsElts = Cts -> [Ct] forall a. Bag a -> [a] bagToList consCts :: Ct -> Cts -> Cts consCts :: Ct -> Cts -> Cts consCts = Ct -> Cts -> Cts forall a. a -> Bag a -> Bag a consBag snocCts :: Cts -> Ct -> Cts snocCts :: Cts -> Ct -> Cts snocCts = Cts -> Ct -> Cts forall a. Bag a -> a -> Bag a snocBag extendCtsList :: Cts -> [Ct] -> Cts extendCtsList :: Cts -> [Ct] -> Cts extendCtsList Cts cts [Ct] xs | [Ct] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [Ct] xs = Cts cts | Bool otherwise = Cts cts Cts -> Cts -> Cts forall a. Bag a -> Bag a -> Bag a `unionBags` [Ct] -> Cts forall a. [a] -> Bag a listToBag [Ct] xs andManyCts :: [Cts] -> Cts andManyCts :: [Cts] -> Cts andManyCts = [Cts] -> Cts forall a. [Bag a] -> Bag a unionManyBags emptyCts :: Cts emptyCts :: Cts emptyCts = Cts forall a. Bag a emptyBag isEmptyCts :: Cts -> Bool isEmptyCts :: Cts -> Bool isEmptyCts = Cts -> Bool forall a. Bag a -> Bool isEmptyBag pprCts :: Cts -> SDoc pprCts :: Cts -> SDoc pprCts Cts cts = [SDoc] -> SDoc vcat ((Ct -> SDoc) -> [Ct] -> [SDoc] forall a b. (a -> b) -> [a] -> [b] map Ct -> SDoc forall a. Outputable a => a -> SDoc ppr (Cts -> [Ct] forall a. Bag a -> [a] bagToList Cts cts)) {- ************************************************************************ * * Wanted constraints These are forced to be in TcRnTypes because TcLclEnv mentions WantedConstraints WantedConstraint mentions CtLoc CtLoc mentions ErrCtxt ErrCtxt mentions TcM * * v%************************************************************************ -} data WantedConstraints = WC { WantedConstraints -> Cts wc_simple :: Cts -- Unsolved constraints, all wanted , WantedConstraints -> Bag Implication wc_impl :: Bag Implication } emptyWC :: WantedConstraints emptyWC :: WantedConstraints emptyWC = WC :: Cts -> Bag Implication -> WantedConstraints WC { wc_simple :: Cts wc_simple = Cts forall a. Bag a emptyBag, wc_impl :: Bag Implication wc_impl = Bag Implication forall a. Bag a emptyBag } mkSimpleWC :: [CtEvidence] -> WantedConstraints mkSimpleWC :: [CtEvidence] -> WantedConstraints mkSimpleWC [CtEvidence] cts = WC :: Cts -> Bag Implication -> WantedConstraints WC { wc_simple :: Cts wc_simple = [Ct] -> Cts forall a. [a] -> Bag a listToBag ((CtEvidence -> Ct) -> [CtEvidence] -> [Ct] forall a b. (a -> b) -> [a] -> [b] map CtEvidence -> Ct mkNonCanonical [CtEvidence] cts) , wc_impl :: Bag Implication wc_impl = Bag Implication forall a. Bag a emptyBag } mkImplicWC :: Bag Implication -> WantedConstraints mkImplicWC :: Bag Implication -> WantedConstraints mkImplicWC Bag Implication implic = WC :: Cts -> Bag Implication -> WantedConstraints WC { wc_simple :: Cts wc_simple = Cts forall a. Bag a emptyBag, wc_impl :: Bag Implication wc_impl = Bag Implication implic } isEmptyWC :: WantedConstraints -> Bool isEmptyWC :: WantedConstraints -> Bool isEmptyWC (WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts f, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication i }) = Cts -> Bool forall a. Bag a -> Bool isEmptyBag Cts f Bool -> Bool -> Bool && Bag Implication -> Bool forall a. Bag a -> Bool isEmptyBag Bag Implication i -- | Checks whether a the given wanted constraints are solved, i.e. -- that there are no simple constraints left and all the implications -- are solved. isSolvedWC :: WantedConstraints -> Bool isSolvedWC :: WantedConstraints -> Bool isSolvedWC WC {wc_simple :: WantedConstraints -> Cts wc_simple = Cts wc_simple, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication wc_impl} = Cts -> Bool forall a. Bag a -> Bool isEmptyBag Cts wc_simple Bool -> Bool -> Bool && (Implication -> Bool) -> Bag Implication -> Bool forall a. (a -> Bool) -> Bag a -> Bool allBag (ImplicStatus -> Bool isSolvedStatus (ImplicStatus -> Bool) -> (Implication -> ImplicStatus) -> Implication -> Bool forall b c a. (b -> c) -> (a -> b) -> a -> c . Implication -> ImplicStatus ic_status) Bag Implication wc_impl andWC :: WantedConstraints -> WantedConstraints -> WantedConstraints andWC :: WantedConstraints -> WantedConstraints -> WantedConstraints andWC (WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts f1, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication i1 }) (WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts f2, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication i2 }) = WC :: Cts -> Bag Implication -> WantedConstraints WC { wc_simple :: Cts wc_simple = Cts f1 Cts -> Cts -> Cts forall a. Bag a -> Bag a -> Bag a `unionBags` Cts f2 , wc_impl :: Bag Implication wc_impl = Bag Implication i1 Bag Implication -> Bag Implication -> Bag Implication forall a. Bag a -> Bag a -> Bag a `unionBags` Bag Implication i2 } unionsWC :: [WantedConstraints] -> WantedConstraints unionsWC :: [WantedConstraints] -> WantedConstraints unionsWC = (WantedConstraints -> WantedConstraints -> WantedConstraints) -> WantedConstraints -> [WantedConstraints] -> WantedConstraints forall (t :: * -> *) a b. Foldable t => (a -> b -> b) -> b -> t a -> b foldr WantedConstraints -> WantedConstraints -> WantedConstraints andWC WantedConstraints emptyWC addSimples :: WantedConstraints -> Bag Ct -> WantedConstraints addSimples :: WantedConstraints -> Cts -> WantedConstraints addSimples WantedConstraints wc Cts cts = WantedConstraints wc { wc_simple :: Cts wc_simple = WantedConstraints -> Cts wc_simple WantedConstraints wc Cts -> Cts -> Cts forall a. Bag a -> Bag a -> Bag a `unionBags` Cts cts } -- Consider: Put the new constraints at the front, so they get solved first addImplics :: WantedConstraints -> Bag Implication -> WantedConstraints addImplics :: WantedConstraints -> Bag Implication -> WantedConstraints addImplics WantedConstraints wc Bag Implication implic = WantedConstraints wc { wc_impl :: Bag Implication wc_impl = WantedConstraints -> Bag Implication wc_impl WantedConstraints wc Bag Implication -> Bag Implication -> Bag Implication forall a. Bag a -> Bag a -> Bag a `unionBags` Bag Implication implic } addInsols :: WantedConstraints -> Bag Ct -> WantedConstraints addInsols :: WantedConstraints -> Cts -> WantedConstraints addInsols WantedConstraints wc Cts cts = WantedConstraints wc { wc_simple :: Cts wc_simple = WantedConstraints -> Cts wc_simple WantedConstraints wc Cts -> Cts -> Cts forall a. Bag a -> Bag a -> Bag a `unionBags` Cts cts } insolublesOnly :: WantedConstraints -> WantedConstraints -- Keep only the definitely-insoluble constraints insolublesOnly :: WantedConstraints -> WantedConstraints insolublesOnly (WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implics }) = WC :: Cts -> Bag Implication -> WantedConstraints WC { wc_simple :: Cts wc_simple = (Ct -> Bool) -> Cts -> Cts forall a. (a -> Bool) -> Bag a -> Bag a filterBag Ct -> Bool insolubleCt Cts simples , wc_impl :: Bag Implication wc_impl = (Implication -> Implication) -> Bag Implication -> Bag Implication forall a b. (a -> b) -> Bag a -> Bag b mapBag Implication -> Implication implic_insols_only Bag Implication implics } where implic_insols_only :: Implication -> Implication implic_insols_only Implication implic = Implication implic { ic_wanted :: WantedConstraints ic_wanted = WantedConstraints -> WantedConstraints insolublesOnly (Implication -> WantedConstraints ic_wanted Implication implic) } isSolvedStatus :: ImplicStatus -> Bool isSolvedStatus :: ImplicStatus -> Bool isSolvedStatus (IC_Solved {}) = Bool True isSolvedStatus ImplicStatus _ = Bool False isInsolubleStatus :: ImplicStatus -> Bool isInsolubleStatus :: ImplicStatus -> Bool isInsolubleStatus ImplicStatus IC_Insoluble = Bool True isInsolubleStatus ImplicStatus IC_BadTelescope = Bool True isInsolubleStatus ImplicStatus _ = Bool False insolubleImplic :: Implication -> Bool insolubleImplic :: Implication -> Bool insolubleImplic Implication ic = ImplicStatus -> Bool isInsolubleStatus (Implication -> ImplicStatus ic_status Implication ic) insolubleWC :: WantedConstraints -> Bool insolubleWC :: WantedConstraints -> Bool insolubleWC (WC { wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implics, wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples }) = (Ct -> Bool) -> Cts -> Bool forall a. (a -> Bool) -> Bag a -> Bool anyBag Ct -> Bool insolubleCt Cts simples Bool -> Bool -> Bool || (Implication -> Bool) -> Bag Implication -> Bool forall a. (a -> Bool) -> Bag a -> Bool anyBag Implication -> Bool insolubleImplic Bag Implication implics insolubleCt :: Ct -> Bool -- Definitely insoluble, in particular /excluding/ type-hole constraints -- Namely: a) an equality constraint -- b) that is insoluble -- c) and does not arise from a Given insolubleCt :: Ct -> Bool insolubleCt Ct ct | Ct -> Bool isHoleCt Ct ct = Ct -> Bool isOutOfScopeCt Ct ct -- See Note [Insoluble holes] | Bool -> Bool not (Ct -> Bool insolubleEqCt Ct ct) = Bool False | Ct -> Bool arisesFromGivens Ct ct = Bool False -- See Note [Given insolubles] | Bool otherwise = Bool True insolubleEqCt :: Ct -> Bool -- Returns True of /equality/ constraints -- that are /definitely/ insoluble -- It won't detect some definite errors like -- F a ~ T (F a) -- where F is a type family, which actually has an occurs check -- -- The function is tuned for application /after/ constraint solving -- i.e. assuming canonicalisation has been done -- E.g. It'll reply True for a ~ [a] -- but False for [a] ~ a -- and -- True for Int ~ F a Int -- but False for Maybe Int ~ F a Int Int -- (where F is an arity-1 type function) insolubleEqCt :: Ct -> Bool insolubleEqCt (CIrredCan { cc_insol :: Ct -> Bool cc_insol = Bool insol }) = Bool insol insolubleEqCt Ct _ = Bool False instance Outputable WantedConstraints where ppr :: WantedConstraints -> SDoc ppr (WC {wc_simple :: WantedConstraints -> Cts wc_simple = Cts s, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication i}) = String -> SDoc text String "WC" SDoc -> SDoc -> SDoc <+> SDoc -> SDoc braces ([SDoc] -> SDoc vcat [ SDoc -> Cts -> SDoc forall a. Outputable a => SDoc -> Bag a -> SDoc ppr_bag (String -> SDoc text String "wc_simple") Cts s , SDoc -> Bag Implication -> SDoc forall a. Outputable a => SDoc -> Bag a -> SDoc ppr_bag (String -> SDoc text String "wc_impl") Bag Implication i ]) ppr_bag :: Outputable a => SDoc -> Bag a -> SDoc ppr_bag :: SDoc -> Bag a -> SDoc ppr_bag SDoc doc Bag a bag | Bag a -> Bool forall a. Bag a -> Bool isEmptyBag Bag a bag = SDoc empty | Bool otherwise = SDoc -> Int -> SDoc -> SDoc hang (SDoc doc SDoc -> SDoc -> SDoc <+> SDoc equals) Int 2 ((a -> SDoc -> SDoc) -> SDoc -> Bag a -> SDoc forall (t :: * -> *) a b. Foldable t => (a -> b -> b) -> b -> t a -> b foldr (SDoc -> SDoc -> SDoc ($$) (SDoc -> SDoc -> SDoc) -> (a -> SDoc) -> a -> SDoc -> SDoc forall b c a. (b -> c) -> (a -> b) -> a -> c . a -> SDoc forall a. Outputable a => a -> SDoc ppr) SDoc empty Bag a bag) {- Note [Given insolubles] ~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider (#14325, comment:) class (a~b) => C a b foo :: C a c => a -> c foo x = x hm3 :: C (f b) b => b -> f b hm3 x = foo x In the RHS of hm3, from the [G] C (f b) b we get the insoluble [G] f b ~# b. Then we also get an unsolved [W] C b (f b). Residual implication looks like forall b. C (f b) b => [G] f b ~# b [W] C f (f b) We do /not/ want to set the implication status to IC_Insoluble, because that'll suppress reports of [W] C b (f b). But we may not report the insoluble [G] f b ~# b either (see Note [Given errors] in TcErrors), so we may fail to report anything at all! Yikes. The same applies to Derived constraints that /arise from/ Givens. E.g. f :: (C Int [a]) => blah where a fundep means we get [D] Int ~ [a] By the same reasoning we must not suppress other errors (#15767) Bottom line: insolubleWC (called in TcSimplify.setImplicationStatus) should ignore givens even if they are insoluble. Note [Insoluble holes] ~~~~~~~~~~~~~~~~~~~~~~ Hole constraints that ARE NOT treated as truly insoluble: a) type holes, arising from PartialTypeSignatures, b) "true" expression holes arising from TypedHoles An "expression hole" or "type hole" constraint isn't really an error at all; it's a report saying "_ :: Int" here. But an out-of-scope variable masquerading as expression holes IS treated as truly insoluble, so that it trumps other errors during error reporting. Yuk! ************************************************************************ * * Implication constraints * * ************************************************************************ -} data Implication = Implic { -- Invariants for a tree of implications: -- see TcType Note [TcLevel and untouchable type variables] Implication -> TcLevel ic_tclvl :: TcLevel, -- TcLevel of unification variables -- allocated /inside/ this implication Implication -> [TcTyVar] ic_skols :: [TcTyVar], -- Introduced skolems Implication -> SkolemInfo ic_info :: SkolemInfo, -- See Note [Skolems in an implication] -- See Note [Shadowing in a constraint] Implication -> Maybe SDoc ic_telescope :: Maybe SDoc, -- User-written telescope, if there is one -- See Note [Checking telescopes] Implication -> [TcTyVar] ic_given :: [EvVar], -- Given evidence variables -- (order does not matter) -- See Invariant (GivenInv) in TcType Implication -> Bool ic_no_eqs :: Bool, -- True <=> ic_givens have no equalities, for sure -- False <=> ic_givens might have equalities Implication -> Bool ic_warn_inaccessible :: Bool, -- True <=> -Winaccessible-code is enabled -- at construction. See -- Note [Avoid -Winaccessible-code when deriving] -- in TcInstDcls Implication -> TcLclEnv ic_env :: TcLclEnv, -- Records the TcLClEnv at the time of creation. -- -- The TcLclEnv gives the source location -- and error context for the implication, and -- hence for all the given evidence variables. Implication -> WantedConstraints ic_wanted :: WantedConstraints, -- The wanteds -- See Invariang (WantedInf) in TcType Implication -> EvBindsVar ic_binds :: EvBindsVar, -- Points to the place to fill in the -- abstraction and bindings. -- The ic_need fields keep track of which Given evidence -- is used by this implication or its children -- NB: including stuff used by nested implications that have since -- been discarded -- See Note [Needed evidence variables] Implication -> TcTyCoVarSet ic_need_inner :: VarSet, -- Includes all used Given evidence Implication -> TcTyCoVarSet ic_need_outer :: VarSet, -- Includes only the free Given evidence -- i.e. ic_need_inner after deleting -- (a) givens (b) binders of ic_binds Implication -> ImplicStatus ic_status :: ImplicStatus } implicationPrototype :: Implication implicationPrototype :: Implication implicationPrototype = Implic :: TcLevel -> [TcTyVar] -> SkolemInfo -> Maybe SDoc -> [TcTyVar] -> Bool -> Bool -> TcLclEnv -> WantedConstraints -> EvBindsVar -> TcTyCoVarSet -> TcTyCoVarSet -> ImplicStatus -> Implication Implic { -- These fields must be initialised ic_tclvl :: TcLevel ic_tclvl = String -> TcLevel forall a. String -> a panic String "newImplic:tclvl" , ic_binds :: EvBindsVar ic_binds = String -> EvBindsVar forall a. String -> a panic String "newImplic:binds" , ic_info :: SkolemInfo ic_info = String -> SkolemInfo forall a. String -> a panic String "newImplic:info" , ic_env :: TcLclEnv ic_env = String -> TcLclEnv forall a. String -> a panic String "newImplic:env" , ic_warn_inaccessible :: Bool ic_warn_inaccessible = String -> Bool forall a. String -> a panic String "newImplic:warn_inaccessible" -- The rest have sensible default values , ic_skols :: [TcTyVar] ic_skols = [] , ic_telescope :: Maybe SDoc ic_telescope = Maybe SDoc forall a. Maybe a Nothing , ic_given :: [TcTyVar] ic_given = [] , ic_wanted :: WantedConstraints ic_wanted = WantedConstraints emptyWC , ic_no_eqs :: Bool ic_no_eqs = Bool False , ic_status :: ImplicStatus ic_status = ImplicStatus IC_Unsolved , ic_need_inner :: TcTyCoVarSet ic_need_inner = TcTyCoVarSet emptyVarSet , ic_need_outer :: TcTyCoVarSet ic_need_outer = TcTyCoVarSet emptyVarSet } data ImplicStatus = IC_Solved -- All wanteds in the tree are solved, all the way down { ImplicStatus -> [TcTyVar] ics_dead :: [EvVar] } -- Subset of ic_given that are not needed -- See Note [Tracking redundant constraints] in TcSimplify | IC_Insoluble -- At least one insoluble constraint in the tree | IC_BadTelescope -- solved, but the skolems in the telescope are out of -- dependency order | IC_Unsolved -- Neither of the above; might go either way instance Outputable Implication where ppr :: Implication -> SDoc ppr (Implic { ic_tclvl :: Implication -> TcLevel ic_tclvl = TcLevel tclvl, ic_skols :: Implication -> [TcTyVar] ic_skols = [TcTyVar] skols , ic_given :: Implication -> [TcTyVar] ic_given = [TcTyVar] given, ic_no_eqs :: Implication -> Bool ic_no_eqs = Bool no_eqs , ic_wanted :: Implication -> WantedConstraints ic_wanted = WantedConstraints wanted, ic_status :: Implication -> ImplicStatus ic_status = ImplicStatus status , ic_binds :: Implication -> EvBindsVar ic_binds = EvBindsVar binds , ic_need_inner :: Implication -> TcTyCoVarSet ic_need_inner = TcTyCoVarSet need_in, ic_need_outer :: Implication -> TcTyCoVarSet ic_need_outer = TcTyCoVarSet need_out , ic_info :: Implication -> SkolemInfo ic_info = SkolemInfo info }) = SDoc -> Int -> SDoc -> SDoc hang (String -> SDoc text String "Implic" SDoc -> SDoc -> SDoc <+> SDoc lbrace) Int 2 ([SDoc] -> SDoc sep [ String -> SDoc text String "TcLevel =" SDoc -> SDoc -> SDoc <+> TcLevel -> SDoc forall a. Outputable a => a -> SDoc ppr TcLevel tclvl , String -> SDoc text String "Skolems =" SDoc -> SDoc -> SDoc <+> [TcTyVar] -> SDoc pprTyVars [TcTyVar] skols , String -> SDoc text String "No-eqs =" SDoc -> SDoc -> SDoc <+> Bool -> SDoc forall a. Outputable a => a -> SDoc ppr Bool no_eqs , String -> SDoc text String "Status =" SDoc -> SDoc -> SDoc <+> ImplicStatus -> SDoc forall a. Outputable a => a -> SDoc ppr ImplicStatus status , SDoc -> Int -> SDoc -> SDoc hang (String -> SDoc text String "Given =") Int 2 ([TcTyVar] -> SDoc pprEvVars [TcTyVar] given) , SDoc -> Int -> SDoc -> SDoc hang (String -> SDoc text String "Wanted =") Int 2 (WantedConstraints -> SDoc forall a. Outputable a => a -> SDoc ppr WantedConstraints wanted) , String -> SDoc text String "Binds =" SDoc -> SDoc -> SDoc <+> EvBindsVar -> SDoc forall a. Outputable a => a -> SDoc ppr EvBindsVar binds , SDoc -> SDoc whenPprDebug (String -> SDoc text String "Needed inner =" SDoc -> SDoc -> SDoc <+> TcTyCoVarSet -> SDoc forall a. Outputable a => a -> SDoc ppr TcTyCoVarSet need_in) , SDoc -> SDoc whenPprDebug (String -> SDoc text String "Needed outer =" SDoc -> SDoc -> SDoc <+> TcTyCoVarSet -> SDoc forall a. Outputable a => a -> SDoc ppr TcTyCoVarSet need_out) , SkolemInfo -> SDoc pprSkolInfo SkolemInfo info ] SDoc -> SDoc -> SDoc <+> SDoc rbrace) instance Outputable ImplicStatus where ppr :: ImplicStatus -> SDoc ppr ImplicStatus IC_Insoluble = String -> SDoc text String "Insoluble" ppr ImplicStatus IC_BadTelescope = String -> SDoc text String "Bad telescope" ppr ImplicStatus IC_Unsolved = String -> SDoc text String "Unsolved" ppr (IC_Solved { ics_dead :: ImplicStatus -> [TcTyVar] ics_dead = [TcTyVar] dead }) = String -> SDoc text String "Solved" SDoc -> SDoc -> SDoc <+> (SDoc -> SDoc braces (String -> SDoc text String "Dead givens =" SDoc -> SDoc -> SDoc <+> [TcTyVar] -> SDoc forall a. Outputable a => a -> SDoc ppr [TcTyVar] dead)) {- Note [Checking telescopes] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When kind-checking a /user-written/ type, we might have a "bad telescope" like this one: data SameKind :: forall k. k -> k -> Type type Foo :: forall a k (b :: k). SameKind a b -> Type The kind of 'a' mentions 'k' which is bound after 'a'. Oops. Knowing this means that unification etc must have happened, so it's convenient to detect it in the constraint solver: * We make a single implication constraint when kind-checking the 'forall' in Foo's kind, something like forall a k (b::k). { wanted constraints } * Having solved {wanted}, before discarding the now-solved implication, the costraint solver checks the dependency order of the skolem variables (ic_skols). This is done in setImplicationStatus. * This check is only necessary if the implication was born from a user-written signature. If, say, it comes from checking a pattern match that binds existentials, where the type of the data constructor is known to be valid (it in tcConPat), no need for the check. So the check is done if and only if ic_telescope is (Just blah). * If ic_telesope is (Just d), the d::SDoc displays the original, user-written type variables. * Be careful /NOT/ to discard an implication with non-Nothing ic_telescope, even if ic_wanted is empty. We must give the constraint solver a chance to make that bad-telesope test! Hence the extra guard in emitResidualTvConstraint; see #16247 See also TcHsType Note [Keeping scoped variables in order: Explicit] Note [Needed evidence variables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Th ic_need_evs field holds the free vars of ic_binds, and all the ic_binds in nested implications. * Main purpose: if one of the ic_givens is not mentioned in here, it is redundant. * solveImplication may drop an implication altogether if it has no remaining 'wanteds'. But we still track the free vars of its evidence binds, even though it has now disappeared. Note [Shadowing in a constraint] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We assume NO SHADOWING in a constraint. Specifically * The unification variables are all implicitly quantified at top level, and are all unique * The skolem variables bound in ic_skols are all freah when the implication is created. So we can safely substitute. For example, if we have forall a. a~Int => ...(forall b. ...a...)... we can push the (a~Int) constraint inwards in the "givens" without worrying that 'b' might clash. Note [Skolems in an implication] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The skolems in an implication are not there to perform a skolem escape check. That happens because all the environment variables are in the untouchables, and therefore cannot be unified with anything at all, let alone the skolems. Instead, ic_skols is used only when considering floating a constraint outside the implication in TcSimplify.floatEqualities or TcSimplify.approximateImplications Note [Insoluble constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Some of the errors that we get during canonicalization are best reported when all constraints have been simplified as much as possible. For instance, assume that during simplification the following constraints arise: [Wanted] F alpha ~ uf1 [Wanted] beta ~ uf1 beta When canonicalizing the wanted (beta ~ uf1 beta), if we eagerly fail we will simply see a message: 'Can't construct the infinite type beta ~ uf1 beta' and the user has no idea what the uf1 variable is. Instead our plan is that we will NOT fail immediately, but: (1) Record the "frozen" error in the ic_insols field (2) Isolate the offending constraint from the rest of the inerts (3) Keep on simplifying/canonicalizing At the end, we will hopefully have substituted uf1 := F alpha, and we will be able to report a more informative error: 'Can't construct the infinite type beta ~ F alpha beta' Insoluble constraints *do* include Derived constraints. For example, a functional dependency might give rise to [D] Int ~ Bool, and we must report that. If insolubles did not contain Deriveds, reportErrors would never see it. ************************************************************************ * * Pretty printing * * ************************************************************************ -} pprEvVars :: [EvVar] -> SDoc -- Print with their types pprEvVars :: [TcTyVar] -> SDoc pprEvVars [TcTyVar] ev_vars = [SDoc] -> SDoc vcat ((TcTyVar -> SDoc) -> [TcTyVar] -> [SDoc] forall a b. (a -> b) -> [a] -> [b] map TcTyVar -> SDoc pprEvVarWithType [TcTyVar] ev_vars) pprEvVarTheta :: [EvVar] -> SDoc pprEvVarTheta :: [TcTyVar] -> SDoc pprEvVarTheta [TcTyVar] ev_vars = [Xi] -> SDoc pprTheta ((TcTyVar -> Xi) -> [TcTyVar] -> [Xi] forall a b. (a -> b) -> [a] -> [b] map TcTyVar -> Xi evVarPred [TcTyVar] ev_vars) pprEvVarWithType :: EvVar -> SDoc pprEvVarWithType :: TcTyVar -> SDoc pprEvVarWithType TcTyVar v = TcTyVar -> SDoc forall a. Outputable a => a -> SDoc ppr TcTyVar v SDoc -> SDoc -> SDoc <+> SDoc dcolon SDoc -> SDoc -> SDoc <+> Xi -> SDoc pprType (TcTyVar -> Xi evVarPred TcTyVar v) -- | Wraps the given type with the constraints (via ic_given) in the given -- implication, according to the variables mentioned (via ic_skols) -- in the implication, but taking care to only wrap those variables -- that are mentioned in the type or the implication. wrapTypeWithImplication :: Type -> Implication -> Type wrapTypeWithImplication :: Xi -> Implication -> Xi wrapTypeWithImplication Xi ty Implication impl = Xi -> [TcTyVar] -> [Xi] -> Xi wrapType Xi ty [TcTyVar] mentioned_skols [Xi] givens where givens :: [Xi] givens = (TcTyVar -> Xi) -> [TcTyVar] -> [Xi] forall a b. (a -> b) -> [a] -> [b] map TcTyVar -> Xi idType ([TcTyVar] -> [Xi]) -> [TcTyVar] -> [Xi] forall a b. (a -> b) -> a -> b $ Implication -> [TcTyVar] ic_given Implication impl skols :: [TcTyVar] skols = Implication -> [TcTyVar] ic_skols Implication impl freeVars :: TcTyCoVarSet freeVars = FV -> TcTyCoVarSet fvVarSet (FV -> TcTyCoVarSet) -> FV -> TcTyCoVarSet forall a b. (a -> b) -> a -> b $ [Xi] -> FV tyCoFVsOfTypes (Xi tyXi -> [Xi] -> [Xi] forall a. a -> [a] -> [a] :[Xi] givens) mentioned_skols :: [TcTyVar] mentioned_skols = (TcTyVar -> Bool) -> [TcTyVar] -> [TcTyVar] forall a. (a -> Bool) -> [a] -> [a] filter (TcTyVar -> TcTyCoVarSet -> Bool `elemVarSet` TcTyCoVarSet freeVars) [TcTyVar] skols wrapType :: Type -> [TyVar] -> [PredType] -> Type wrapType :: Xi -> [TcTyVar] -> [Xi] -> Xi wrapType Xi ty [TcTyVar] skols [Xi] givens = [TcTyVar] -> Xi -> Xi mkSpecForAllTys [TcTyVar] skols (Xi -> Xi) -> Xi -> Xi forall a b. (a -> b) -> a -> b $ [Xi] -> Xi -> Xi mkPhiTy [Xi] givens Xi ty {- ************************************************************************ * * CtEvidence * * ************************************************************************ Note [Evidence field of CtEvidence] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ During constraint solving we never look at the type of ctev_evar/ctev_dest; instead we look at the ctev_pred field. The evtm/evar field may be un-zonked. Note [Bind new Givens immediately] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For Givens we make new EvVars and bind them immediately. Two main reasons: * Gain sharing. E.g. suppose we start with g :: C a b, where class D a => C a b class (E a, F a) => D a If we generate all g's superclasses as separate EvTerms we might get selD1 (selC1 g) :: E a selD2 (selC1 g) :: F a selC1 g :: D a which we could do more economically as: g1 :: D a = selC1 g g2 :: E a = selD1 g1 g3 :: F a = selD2 g1 * For *coercion* evidence we *must* bind each given: class (a~b) => C a b where .... f :: C a b => .... Then in f's Givens we have g:(C a b) and the superclass sc(g,0):a~b. But that superclass selector can't (yet) appear in a coercion (see evTermCoercion), so the easy thing is to bind it to an Id. So a Given has EvVar inside it rather than (as previously) an EvTerm. -} -- | A place for type-checking evidence to go after it is generated. -- Wanted equalities are always HoleDest; other wanteds are always -- EvVarDest. data TcEvDest = EvVarDest EvVar -- ^ bind this var to the evidence -- EvVarDest is always used for non-type-equalities -- e.g. class constraints | HoleDest CoercionHole -- ^ fill in this hole with the evidence -- HoleDest is always used for type-equalities -- See Note [Coercion holes] in TyCoRep data CtEvidence = CtGiven -- Truly given, not depending on subgoals { CtEvidence -> Xi ctev_pred :: TcPredType -- See Note [Ct/evidence invariant] , CtEvidence -> TcTyVar ctev_evar :: EvVar -- See Note [Evidence field of CtEvidence] , CtEvidence -> CtLoc ctev_loc :: CtLoc } | CtWanted -- Wanted goal { ctev_pred :: TcPredType -- See Note [Ct/evidence invariant] , CtEvidence -> TcEvDest ctev_dest :: TcEvDest , CtEvidence -> ShadowInfo ctev_nosh :: ShadowInfo -- See Note [Constraint flavours] , ctev_loc :: CtLoc } | CtDerived -- A goal that we don't really have to solve and can't -- immediately rewrite anything other than a derived -- (there's no evidence!) but if we do manage to solve -- it may help in solving other goals. { ctev_pred :: TcPredType , ctev_loc :: CtLoc } ctEvPred :: CtEvidence -> TcPredType -- The predicate of a flavor ctEvPred :: CtEvidence -> Xi ctEvPred = CtEvidence -> Xi ctev_pred ctEvLoc :: CtEvidence -> CtLoc ctEvLoc :: CtEvidence -> CtLoc ctEvLoc = CtEvidence -> CtLoc ctev_loc ctEvOrigin :: CtEvidence -> CtOrigin ctEvOrigin :: CtEvidence -> CtOrigin ctEvOrigin = CtLoc -> CtOrigin ctLocOrigin (CtLoc -> CtOrigin) -> (CtEvidence -> CtLoc) -> CtEvidence -> CtOrigin forall b c a. (b -> c) -> (a -> b) -> a -> c . CtEvidence -> CtLoc ctEvLoc -- | Get the equality relation relevant for a 'CtEvidence' ctEvEqRel :: CtEvidence -> EqRel ctEvEqRel :: CtEvidence -> EqRel ctEvEqRel = Xi -> EqRel predTypeEqRel (Xi -> EqRel) -> (CtEvidence -> Xi) -> CtEvidence -> EqRel forall b c a. (b -> c) -> (a -> b) -> a -> c . CtEvidence -> Xi ctEvPred -- | Get the role relevant for a 'CtEvidence' ctEvRole :: CtEvidence -> Role ctEvRole :: CtEvidence -> Role ctEvRole = EqRel -> Role eqRelRole (EqRel -> Role) -> (CtEvidence -> EqRel) -> CtEvidence -> Role forall b c a. (b -> c) -> (a -> b) -> a -> c . CtEvidence -> EqRel ctEvEqRel ctEvTerm :: CtEvidence -> EvTerm ctEvTerm :: CtEvidence -> EvTerm ctEvTerm CtEvidence ev = EvExpr -> EvTerm EvExpr (CtEvidence -> EvExpr ctEvExpr CtEvidence ev) ctEvExpr :: CtEvidence -> EvExpr ctEvExpr :: CtEvidence -> EvExpr ctEvExpr ev :: CtEvidence ev@(CtWanted { ctev_dest :: CtEvidence -> TcEvDest ctev_dest = HoleDest CoercionHole _ }) = Coercion -> EvExpr forall b. Coercion -> Expr b Coercion (Coercion -> EvExpr) -> Coercion -> EvExpr forall a b. (a -> b) -> a -> b $ HasDebugCallStack => CtEvidence -> Coercion CtEvidence -> Coercion ctEvCoercion CtEvidence ev ctEvExpr CtEvidence ev = TcTyVar -> EvExpr evId (CtEvidence -> TcTyVar ctEvEvId CtEvidence ev) ctEvCoercion :: HasDebugCallStack => CtEvidence -> TcCoercion ctEvCoercion :: CtEvidence -> Coercion ctEvCoercion (CtGiven { ctev_evar :: CtEvidence -> TcTyVar ctev_evar = TcTyVar ev_id }) = TcTyVar -> Coercion mkTcCoVarCo TcTyVar ev_id ctEvCoercion (CtWanted { ctev_dest :: CtEvidence -> TcEvDest ctev_dest = TcEvDest dest }) | HoleDest CoercionHole hole <- TcEvDest dest = -- ctEvCoercion is only called on type equalities -- and they always have HoleDests CoercionHole -> Coercion mkHoleCo CoercionHole hole ctEvCoercion CtEvidence ev = String -> SDoc -> Coercion forall a. HasCallStack => String -> SDoc -> a pprPanic String "ctEvCoercion" (CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence ev) ctEvEvId :: CtEvidence -> EvVar ctEvEvId :: CtEvidence -> TcTyVar ctEvEvId (CtWanted { ctev_dest :: CtEvidence -> TcEvDest ctev_dest = EvVarDest TcTyVar ev }) = TcTyVar ev ctEvEvId (CtWanted { ctev_dest :: CtEvidence -> TcEvDest ctev_dest = HoleDest CoercionHole h }) = CoercionHole -> TcTyVar coHoleCoVar CoercionHole h ctEvEvId (CtGiven { ctev_evar :: CtEvidence -> TcTyVar ctev_evar = TcTyVar ev }) = TcTyVar ev ctEvEvId ctev :: CtEvidence ctev@(CtDerived {}) = String -> SDoc -> TcTyVar forall a. HasCallStack => String -> SDoc -> a pprPanic String "ctEvId:" (CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence ctev) instance Outputable TcEvDest where ppr :: TcEvDest -> SDoc ppr (HoleDest CoercionHole h) = String -> SDoc text String "hole" SDoc -> SDoc -> SDoc <> CoercionHole -> SDoc forall a. Outputable a => a -> SDoc ppr CoercionHole h ppr (EvVarDest TcTyVar ev) = TcTyVar -> SDoc forall a. Outputable a => a -> SDoc ppr TcTyVar ev instance Outputable CtEvidence where ppr :: CtEvidence -> SDoc ppr CtEvidence ev = CtFlavour -> SDoc forall a. Outputable a => a -> SDoc ppr (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev) SDoc -> SDoc -> SDoc <+> SDoc pp_ev SDoc -> SDoc -> SDoc <+> SDoc -> SDoc braces (SubGoalDepth -> SDoc forall a. Outputable a => a -> SDoc ppr (CtLoc -> SubGoalDepth ctl_depth (CtEvidence -> CtLoc ctEvLoc CtEvidence ev))) SDoc -> SDoc -> SDoc <> SDoc dcolon -- Show the sub-goal depth too SDoc -> SDoc -> SDoc <+> Xi -> SDoc forall a. Outputable a => a -> SDoc ppr (CtEvidence -> Xi ctEvPred CtEvidence ev) where pp_ev :: SDoc pp_ev = case CtEvidence ev of CtGiven { ctev_evar :: CtEvidence -> TcTyVar ctev_evar = TcTyVar v } -> TcTyVar -> SDoc forall a. Outputable a => a -> SDoc ppr TcTyVar v CtWanted {ctev_dest :: CtEvidence -> TcEvDest ctev_dest = TcEvDest d } -> TcEvDest -> SDoc forall a. Outputable a => a -> SDoc ppr TcEvDest d CtDerived {} -> String -> SDoc text String "_" isWanted :: CtEvidence -> Bool isWanted :: CtEvidence -> Bool isWanted (CtWanted {}) = Bool True isWanted CtEvidence _ = Bool False isGiven :: CtEvidence -> Bool isGiven :: CtEvidence -> Bool isGiven (CtGiven {}) = Bool True isGiven CtEvidence _ = Bool False isDerived :: CtEvidence -> Bool isDerived :: CtEvidence -> Bool isDerived (CtDerived {}) = Bool True isDerived CtEvidence _ = Bool False {- %************************************************************************ %* * CtFlavour %* * %************************************************************************ Note [Constraint flavours] ~~~~~~~~~~~~~~~~~~~~~~~~~~ Constraints come in four flavours: * [G] Given: we have evidence * [W] Wanted WOnly: we want evidence * [D] Derived: any solution must satisfy this constraint, but we don't need evidence for it. Examples include: - superclasses of [W] class constraints - equalities arising from functional dependencies or injectivity * [WD] Wanted WDeriv: a single constraint that represents both [W] and [D] We keep them paired as one both for efficiency, and because when we have a finite map F tys -> CFunEqCan, it's inconvenient to have two CFunEqCans in the range The ctev_nosh field of a Wanted distinguishes between [W] and [WD] Wanted constraints are born as [WD], but are split into [W] and its "shadow" [D] in TcSMonad.maybeEmitShadow. See Note [The improvement story and derived shadows] in TcSMonad -} data CtFlavour -- See Note [Constraint flavours] = Given | Wanted ShadowInfo | Derived deriving CtFlavour -> CtFlavour -> Bool (CtFlavour -> CtFlavour -> Bool) -> (CtFlavour -> CtFlavour -> Bool) -> Eq CtFlavour forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a /= :: CtFlavour -> CtFlavour -> Bool $c/= :: CtFlavour -> CtFlavour -> Bool == :: CtFlavour -> CtFlavour -> Bool $c== :: CtFlavour -> CtFlavour -> Bool Eq data ShadowInfo = WDeriv -- [WD] This Wanted constraint has no Derived shadow, -- so it behaves like a pair of a Wanted and a Derived | WOnly -- [W] It has a separate derived shadow -- See Note [The improvement story and derived shadows] in TcSMonad deriving( ShadowInfo -> ShadowInfo -> Bool (ShadowInfo -> ShadowInfo -> Bool) -> (ShadowInfo -> ShadowInfo -> Bool) -> Eq ShadowInfo forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a /= :: ShadowInfo -> ShadowInfo -> Bool $c/= :: ShadowInfo -> ShadowInfo -> Bool == :: ShadowInfo -> ShadowInfo -> Bool $c== :: ShadowInfo -> ShadowInfo -> Bool Eq ) isGivenOrWDeriv :: CtFlavour -> Bool isGivenOrWDeriv :: CtFlavour -> Bool isGivenOrWDeriv CtFlavour Given = Bool True isGivenOrWDeriv (Wanted ShadowInfo WDeriv) = Bool True isGivenOrWDeriv CtFlavour _ = Bool False instance Outputable CtFlavour where ppr :: CtFlavour -> SDoc ppr CtFlavour Given = String -> SDoc text String "[G]" ppr (Wanted ShadowInfo WDeriv) = String -> SDoc text String "[WD]" ppr (Wanted ShadowInfo WOnly) = String -> SDoc text String "[W]" ppr CtFlavour Derived = String -> SDoc text String "[D]" ctEvFlavour :: CtEvidence -> CtFlavour ctEvFlavour :: CtEvidence -> CtFlavour ctEvFlavour (CtWanted { ctev_nosh :: CtEvidence -> ShadowInfo ctev_nosh = ShadowInfo nosh }) = ShadowInfo -> CtFlavour Wanted ShadowInfo nosh ctEvFlavour (CtGiven {}) = CtFlavour Given ctEvFlavour (CtDerived {}) = CtFlavour Derived -- | Whether or not one 'Ct' can rewrite another is determined by its -- flavour and its equality relation. See also -- Note [Flavours with roles] in TcSMonad type CtFlavourRole = (CtFlavour, EqRel) -- | Extract the flavour, role, and boxity from a 'CtEvidence' ctEvFlavourRole :: CtEvidence -> CtFlavourRole ctEvFlavourRole :: CtEvidence -> CtFlavourRole ctEvFlavourRole CtEvidence ev = (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev, CtEvidence -> EqRel ctEvEqRel CtEvidence ev) -- | Extract the flavour and role from a 'Ct' ctFlavourRole :: Ct -> CtFlavourRole -- Uses short-cuts to role for special cases ctFlavourRole :: Ct -> CtFlavourRole ctFlavourRole (CDictCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev }) = (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev, EqRel NomEq) ctFlavourRole (CTyEqCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev, cc_eq_rel :: Ct -> EqRel cc_eq_rel = EqRel eq_rel }) = (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev, EqRel eq_rel) ctFlavourRole (CFunEqCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev }) = (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev, EqRel NomEq) ctFlavourRole (CHoleCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev }) = (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev, EqRel NomEq) -- NomEq: CHoleCans can be rewritten by -- by nominal equalities but empahatically -- not by representational equalities ctFlavourRole Ct ct = CtEvidence -> CtFlavourRole ctEvFlavourRole (Ct -> CtEvidence ctEvidence Ct ct) {- Note [eqCanRewrite] ~~~~~~~~~~~~~~~~~~~~~~ (eqCanRewrite ct1 ct2) holds if the constraint ct1 (a CTyEqCan of form tv ~ ty) can be used to rewrite ct2. It must satisfy the properties of a can-rewrite relation, see Definition [Can-rewrite relation] in TcSMonad. With the solver handling Coercible constraints like equality constraints, the rewrite conditions must take role into account, never allowing a representational equality to rewrite a nominal one. Note [Wanteds do not rewrite Wanteds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We don't allow Wanteds to rewrite Wanteds, because that can give rise to very confusing type error messages. A good example is #8450. Here's another f :: a -> Bool f x = ( [x,'c'], [x,True] ) `seq` True Here we get [W] a ~ Char [W] a ~ Bool but we do not want to complain about Bool ~ Char! Note [Deriveds do rewrite Deriveds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ However we DO allow Deriveds to rewrite Deriveds, because that's how improvement works; see Note [The improvement story] in TcInteract. However, for now at least I'm only letting (Derived,NomEq) rewrite (Derived,NomEq) and not doing anything for ReprEq. If we have eqCanRewriteFR (Derived, NomEq) (Derived, _) = True then we lose property R2 of Definition [Can-rewrite relation] in TcSMonad R2. If f1 >= f, and f2 >= f, then either f1 >= f2 or f2 >= f1 Consider f1 = (Given, ReprEq) f2 = (Derived, NomEq) f = (Derived, ReprEq) I thought maybe we could never get Derived ReprEq constraints, but we can; straight from the Wanteds during improvement. And from a Derived ReprEq we could conceivably get a Derived NomEq improvement (by decomposing a type constructor with Nomninal role), and hence unify. -} eqCanRewrite :: EqRel -> EqRel -> Bool eqCanRewrite :: EqRel -> EqRel -> Bool eqCanRewrite EqRel NomEq EqRel _ = Bool True eqCanRewrite EqRel ReprEq EqRel ReprEq = Bool True eqCanRewrite EqRel ReprEq EqRel NomEq = Bool False eqCanRewriteFR :: CtFlavourRole -> CtFlavourRole -> Bool -- Can fr1 actually rewrite fr2? -- Very important function! -- See Note [eqCanRewrite] -- See Note [Wanteds do not rewrite Wanteds] -- See Note [Deriveds do rewrite Deriveds] eqCanRewriteFR :: CtFlavourRole -> CtFlavourRole -> Bool eqCanRewriteFR (CtFlavour Given, EqRel r1) (CtFlavour _, EqRel r2) = EqRel -> EqRel -> Bool eqCanRewrite EqRel r1 EqRel r2 eqCanRewriteFR (Wanted ShadowInfo WDeriv, EqRel NomEq) (CtFlavour Derived, EqRel NomEq) = Bool True eqCanRewriteFR (CtFlavour Derived, EqRel NomEq) (CtFlavour Derived, EqRel NomEq) = Bool True eqCanRewriteFR CtFlavourRole _ CtFlavourRole _ = Bool False eqMayRewriteFR :: CtFlavourRole -> CtFlavourRole -> Bool -- Is it /possible/ that fr1 can rewrite fr2? -- This is used when deciding which inerts to kick out, -- at which time a [WD] inert may be split into [W] and [D] eqMayRewriteFR :: CtFlavourRole -> CtFlavourRole -> Bool eqMayRewriteFR (Wanted ShadowInfo WDeriv, EqRel NomEq) (Wanted ShadowInfo WDeriv, EqRel NomEq) = Bool True eqMayRewriteFR (CtFlavour Derived, EqRel NomEq) (Wanted ShadowInfo WDeriv, EqRel NomEq) = Bool True eqMayRewriteFR CtFlavourRole fr1 CtFlavourRole fr2 = CtFlavourRole -> CtFlavourRole -> Bool eqCanRewriteFR CtFlavourRole fr1 CtFlavourRole fr2 ----------------- {- Note [funEqCanDischarge] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have two CFunEqCans with the same LHS: (x1:F ts ~ f1) `funEqCanDischarge` (x2:F ts ~ f2) Can we drop x2 in favour of x1, either unifying f2 (if it's a flatten meta-var) or adding a new Given (f1 ~ f2), if x2 is a Given? Answer: yes if funEqCanDischarge is true. -} funEqCanDischarge :: CtEvidence -> CtEvidence -> ( SwapFlag -- NotSwapped => lhs can discharge rhs -- Swapped => rhs can discharge lhs , Bool) -- True <=> upgrade non-discharded one -- from [W] to [WD] -- See Note [funEqCanDischarge] funEqCanDischarge :: CtEvidence -> CtEvidence -> (SwapFlag, Bool) funEqCanDischarge CtEvidence ev1 CtEvidence ev2 = ASSERT2( ctEvEqRel ev1 == NomEq, ppr ev1 ) ASSERT2( ctEvEqRel ev2 == NomEq, ppr ev2 ) -- CFunEqCans are all Nominal, hence asserts CtFlavour -> CtFlavour -> (SwapFlag, Bool) funEqCanDischargeF (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev1) (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev2) funEqCanDischargeF :: CtFlavour -> CtFlavour -> (SwapFlag, Bool) funEqCanDischargeF :: CtFlavour -> CtFlavour -> (SwapFlag, Bool) funEqCanDischargeF CtFlavour Given CtFlavour _ = (SwapFlag NotSwapped, Bool False) funEqCanDischargeF CtFlavour _ CtFlavour Given = (SwapFlag IsSwapped, Bool False) funEqCanDischargeF (Wanted ShadowInfo WDeriv) CtFlavour _ = (SwapFlag NotSwapped, Bool False) funEqCanDischargeF CtFlavour _ (Wanted ShadowInfo WDeriv) = (SwapFlag IsSwapped, Bool True) funEqCanDischargeF (Wanted ShadowInfo WOnly) (Wanted ShadowInfo WOnly) = (SwapFlag NotSwapped, Bool False) funEqCanDischargeF (Wanted ShadowInfo WOnly) CtFlavour Derived = (SwapFlag NotSwapped, Bool True) funEqCanDischargeF CtFlavour Derived (Wanted ShadowInfo WOnly) = (SwapFlag IsSwapped, Bool True) funEqCanDischargeF CtFlavour Derived CtFlavour Derived = (SwapFlag NotSwapped, Bool False) {- Note [eqCanDischarge] ~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have two identical CTyEqCan equality constraints (i.e. both LHS and RHS are the same) (x1:a~t) `eqCanDischarge` (xs:a~t) Can we just drop x2 in favour of x1? Answer: yes if eqCanDischarge is true. Note that we do /not/ allow Wanted to discharge Derived. We must keep both. Why? Because the Derived may rewrite other Deriveds in the model whereas the Wanted cannot. However a Wanted can certainly discharge an identical Wanted. So eqCanDischarge does /not/ define a can-rewrite relation in the sense of Definition [Can-rewrite relation] in TcSMonad. We /do/ say that a [W] can discharge a [WD]. In evidence terms it certainly can, and the /caller/ arranges that the otherwise-lost [D] is spat out as a new Derived. -} eqCanDischargeFR :: CtFlavourRole -> CtFlavourRole -> Bool -- See Note [eqCanDischarge] eqCanDischargeFR :: CtFlavourRole -> CtFlavourRole -> Bool eqCanDischargeFR (CtFlavour f1,EqRel r1) (CtFlavour f2, EqRel r2) = EqRel -> EqRel -> Bool eqCanRewrite EqRel r1 EqRel r2 Bool -> Bool -> Bool && CtFlavour -> CtFlavour -> Bool eqCanDischargeF CtFlavour f1 CtFlavour f2 eqCanDischargeF :: CtFlavour -> CtFlavour -> Bool eqCanDischargeF :: CtFlavour -> CtFlavour -> Bool eqCanDischargeF CtFlavour Given CtFlavour _ = Bool True eqCanDischargeF (Wanted ShadowInfo _) (Wanted ShadowInfo _) = Bool True eqCanDischargeF (Wanted ShadowInfo WDeriv) CtFlavour Derived = Bool True eqCanDischargeF CtFlavour Derived CtFlavour Derived = Bool True eqCanDischargeF CtFlavour _ CtFlavour _ = Bool False {- ************************************************************************ * * SubGoalDepth * * ************************************************************************ Note [SubGoalDepth] ~~~~~~~~~~~~~~~~~~~ The 'SubGoalDepth' takes care of stopping the constraint solver from looping. The counter starts at zero and increases. It includes dictionary constraints, equality simplification, and type family reduction. (Why combine these? Because it's actually quite easy to mistake one for another, in sufficiently involved scenarios, like ConstraintKinds.) The flag -freduction-depth=n fixes the maximium level. * The counter includes the depth of type class instance declarations. Example: [W] d{7} : Eq [Int] That is d's dictionary-constraint depth is 7. If we use the instance $dfEqList :: Eq a => Eq [a] to simplify it, we get d{7} = $dfEqList d'{8} where d'{8} : Eq Int, and d' has depth 8. For civilised (decidable) instance declarations, each increase of depth removes a type constructor from the type, so the depth never gets big; i.e. is bounded by the structural depth of the type. * The counter also increments when resolving equalities involving type functions. Example: Assume we have a wanted at depth 7: [W] d{7} : F () ~ a If there is a type function equation "F () = Int", this would be rewritten to [W] d{8} : Int ~ a and remembered as having depth 8. Again, without UndecidableInstances, this counter is bounded, but without it can resolve things ad infinitum. Hence there is a maximum level. * Lastly, every time an equality is rewritten, the counter increases. Again, rewriting an equality constraint normally makes progress, but it's possible the "progress" is just the reduction of an infinitely-reducing type family. Hence we need to track the rewrites. When compiling a program requires a greater depth, then GHC recommends turning off this check entirely by setting -freduction-depth=0. This is because the exact number that works is highly variable, and is likely to change even between minor releases. Because this check is solely to prevent infinite compilation times, it seems safe to disable it when a user has ascertained that their program doesn't loop at the type level. -} -- | See Note [SubGoalDepth] newtype SubGoalDepth = SubGoalDepth Int deriving (SubGoalDepth -> SubGoalDepth -> Bool (SubGoalDepth -> SubGoalDepth -> Bool) -> (SubGoalDepth -> SubGoalDepth -> Bool) -> Eq SubGoalDepth forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a /= :: SubGoalDepth -> SubGoalDepth -> Bool $c/= :: SubGoalDepth -> SubGoalDepth -> Bool == :: SubGoalDepth -> SubGoalDepth -> Bool $c== :: SubGoalDepth -> SubGoalDepth -> Bool Eq, Eq SubGoalDepth Eq SubGoalDepth -> (SubGoalDepth -> SubGoalDepth -> Ordering) -> (SubGoalDepth -> SubGoalDepth -> Bool) -> (SubGoalDepth -> SubGoalDepth -> Bool) -> (SubGoalDepth -> SubGoalDepth -> Bool) -> (SubGoalDepth -> SubGoalDepth -> Bool) -> (SubGoalDepth -> SubGoalDepth -> SubGoalDepth) -> (SubGoalDepth -> SubGoalDepth -> SubGoalDepth) -> Ord SubGoalDepth SubGoalDepth -> SubGoalDepth -> Bool SubGoalDepth -> SubGoalDepth -> Ordering SubGoalDepth -> SubGoalDepth -> SubGoalDepth forall a. Eq a -> (a -> a -> Ordering) -> (a -> a -> Bool) -> (a -> a -> Bool) -> (a -> a -> Bool) -> (a -> a -> Bool) -> (a -> a -> a) -> (a -> a -> a) -> Ord a min :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth $cmin :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth max :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth $cmax :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth >= :: SubGoalDepth -> SubGoalDepth -> Bool $c>= :: SubGoalDepth -> SubGoalDepth -> Bool > :: SubGoalDepth -> SubGoalDepth -> Bool $c> :: SubGoalDepth -> SubGoalDepth -> Bool <= :: SubGoalDepth -> SubGoalDepth -> Bool $c<= :: SubGoalDepth -> SubGoalDepth -> Bool < :: SubGoalDepth -> SubGoalDepth -> Bool $c< :: SubGoalDepth -> SubGoalDepth -> Bool compare :: SubGoalDepth -> SubGoalDepth -> Ordering $ccompare :: SubGoalDepth -> SubGoalDepth -> Ordering $cp1Ord :: Eq SubGoalDepth Ord, Rational -> SubGoalDepth -> SDoc SubGoalDepth -> SDoc (SubGoalDepth -> SDoc) -> (Rational -> SubGoalDepth -> SDoc) -> Outputable SubGoalDepth forall a. (a -> SDoc) -> (Rational -> a -> SDoc) -> Outputable a pprPrec :: Rational -> SubGoalDepth -> SDoc $cpprPrec :: Rational -> SubGoalDepth -> SDoc ppr :: SubGoalDepth -> SDoc $cppr :: SubGoalDepth -> SDoc Outputable) initialSubGoalDepth :: SubGoalDepth initialSubGoalDepth :: SubGoalDepth initialSubGoalDepth = Int -> SubGoalDepth SubGoalDepth Int 0 bumpSubGoalDepth :: SubGoalDepth -> SubGoalDepth bumpSubGoalDepth :: SubGoalDepth -> SubGoalDepth bumpSubGoalDepth (SubGoalDepth Int n) = Int -> SubGoalDepth SubGoalDepth (Int n Int -> Int -> Int forall a. Num a => a -> a -> a + Int 1) maxSubGoalDepth :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth maxSubGoalDepth :: SubGoalDepth -> SubGoalDepth -> SubGoalDepth maxSubGoalDepth (SubGoalDepth Int n) (SubGoalDepth Int m) = Int -> SubGoalDepth SubGoalDepth (Int n Int -> Int -> Int forall a. Ord a => a -> a -> a `max` Int m) subGoalDepthExceeded :: DynFlags -> SubGoalDepth -> Bool subGoalDepthExceeded :: DynFlags -> SubGoalDepth -> Bool subGoalDepthExceeded DynFlags dflags (SubGoalDepth Int d) = Int -> IntWithInf mkIntWithInf Int d IntWithInf -> IntWithInf -> Bool forall a. Ord a => a -> a -> Bool > DynFlags -> IntWithInf reductionDepth DynFlags dflags {- ************************************************************************ * * CtLoc * * ************************************************************************ The 'CtLoc' gives information about where a constraint came from. This is important for decent error message reporting because dictionaries don't appear in the original source code. type will evolve... -} data CtLoc = CtLoc { CtLoc -> CtOrigin ctl_origin :: CtOrigin , CtLoc -> TcLclEnv ctl_env :: TcLclEnv , CtLoc -> Maybe TypeOrKind ctl_t_or_k :: Maybe TypeOrKind -- OK if we're not sure , CtLoc -> SubGoalDepth ctl_depth :: !SubGoalDepth } -- The TcLclEnv includes particularly -- source location: tcl_loc :: RealSrcSpan -- context: tcl_ctxt :: [ErrCtxt] -- binder stack: tcl_bndrs :: TcBinderStack -- level: tcl_tclvl :: TcLevel mkKindLoc :: TcType -> TcType -- original *types* being compared -> CtLoc -> CtLoc mkKindLoc :: Xi -> Xi -> CtLoc -> CtLoc mkKindLoc Xi s1 Xi s2 CtLoc loc = CtLoc -> CtOrigin -> CtLoc setCtLocOrigin (CtLoc -> CtLoc toKindLoc CtLoc loc) (Xi -> Maybe Xi -> CtOrigin -> Maybe TypeOrKind -> CtOrigin KindEqOrigin Xi s1 (Xi -> Maybe Xi forall a. a -> Maybe a Just Xi s2) (CtLoc -> CtOrigin ctLocOrigin CtLoc loc) (CtLoc -> Maybe TypeOrKind ctLocTypeOrKind_maybe CtLoc loc)) -- | Take a CtLoc and moves it to the kind level toKindLoc :: CtLoc -> CtLoc toKindLoc :: CtLoc -> CtLoc toKindLoc CtLoc loc = CtLoc loc { ctl_t_or_k :: Maybe TypeOrKind ctl_t_or_k = TypeOrKind -> Maybe TypeOrKind forall a. a -> Maybe a Just TypeOrKind KindLevel } mkGivenLoc :: TcLevel -> SkolemInfo -> TcLclEnv -> CtLoc mkGivenLoc :: TcLevel -> SkolemInfo -> TcLclEnv -> CtLoc mkGivenLoc TcLevel tclvl SkolemInfo skol_info TcLclEnv env = CtLoc :: CtOrigin -> TcLclEnv -> Maybe TypeOrKind -> SubGoalDepth -> CtLoc CtLoc { ctl_origin :: CtOrigin ctl_origin = SkolemInfo -> CtOrigin GivenOrigin SkolemInfo skol_info , ctl_env :: TcLclEnv ctl_env = TcLclEnv -> TcLevel -> TcLclEnv setLclEnvTcLevel TcLclEnv env TcLevel tclvl , ctl_t_or_k :: Maybe TypeOrKind ctl_t_or_k = Maybe TypeOrKind forall a. Maybe a Nothing -- this only matters for error msgs , ctl_depth :: SubGoalDepth ctl_depth = SubGoalDepth initialSubGoalDepth } ctLocEnv :: CtLoc -> TcLclEnv ctLocEnv :: CtLoc -> TcLclEnv ctLocEnv = CtLoc -> TcLclEnv ctl_env ctLocLevel :: CtLoc -> TcLevel ctLocLevel :: CtLoc -> TcLevel ctLocLevel CtLoc loc = TcLclEnv -> TcLevel getLclEnvTcLevel (CtLoc -> TcLclEnv ctLocEnv CtLoc loc) ctLocDepth :: CtLoc -> SubGoalDepth ctLocDepth :: CtLoc -> SubGoalDepth ctLocDepth = CtLoc -> SubGoalDepth ctl_depth ctLocOrigin :: CtLoc -> CtOrigin ctLocOrigin :: CtLoc -> CtOrigin ctLocOrigin = CtLoc -> CtOrigin ctl_origin ctLocSpan :: CtLoc -> RealSrcSpan ctLocSpan :: CtLoc -> RealSrcSpan ctLocSpan (CtLoc { ctl_env :: CtLoc -> TcLclEnv ctl_env = TcLclEnv lcl}) = TcLclEnv -> RealSrcSpan getLclEnvLoc TcLclEnv lcl ctLocTypeOrKind_maybe :: CtLoc -> Maybe TypeOrKind ctLocTypeOrKind_maybe :: CtLoc -> Maybe TypeOrKind ctLocTypeOrKind_maybe = CtLoc -> Maybe TypeOrKind ctl_t_or_k setCtLocSpan :: CtLoc -> RealSrcSpan -> CtLoc setCtLocSpan :: CtLoc -> RealSrcSpan -> CtLoc setCtLocSpan ctl :: CtLoc ctl@(CtLoc { ctl_env :: CtLoc -> TcLclEnv ctl_env = TcLclEnv lcl }) RealSrcSpan loc = CtLoc -> TcLclEnv -> CtLoc setCtLocEnv CtLoc ctl (TcLclEnv -> RealSrcSpan -> TcLclEnv setLclEnvLoc TcLclEnv lcl RealSrcSpan loc) bumpCtLocDepth :: CtLoc -> CtLoc bumpCtLocDepth :: CtLoc -> CtLoc bumpCtLocDepth loc :: CtLoc loc@(CtLoc { ctl_depth :: CtLoc -> SubGoalDepth ctl_depth = SubGoalDepth d }) = CtLoc loc { ctl_depth :: SubGoalDepth ctl_depth = SubGoalDepth -> SubGoalDepth bumpSubGoalDepth SubGoalDepth d } setCtLocOrigin :: CtLoc -> CtOrigin -> CtLoc setCtLocOrigin :: CtLoc -> CtOrigin -> CtLoc setCtLocOrigin CtLoc ctl CtOrigin orig = CtLoc ctl { ctl_origin :: CtOrigin ctl_origin = CtOrigin orig } updateCtLocOrigin :: CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc updateCtLocOrigin :: CtLoc -> (CtOrigin -> CtOrigin) -> CtLoc updateCtLocOrigin ctl :: CtLoc ctl@(CtLoc { ctl_origin :: CtLoc -> CtOrigin ctl_origin = CtOrigin orig }) CtOrigin -> CtOrigin upd = CtLoc ctl { ctl_origin :: CtOrigin ctl_origin = CtOrigin -> CtOrigin upd CtOrigin orig } setCtLocEnv :: CtLoc -> TcLclEnv -> CtLoc setCtLocEnv :: CtLoc -> TcLclEnv -> CtLoc setCtLocEnv CtLoc ctl TcLclEnv env = CtLoc ctl { ctl_env :: TcLclEnv ctl_env = TcLclEnv env } pprCtLoc :: CtLoc -> SDoc -- "arising from ... at ..." -- Not an instance of Outputable because of the "arising from" prefix pprCtLoc :: CtLoc -> SDoc pprCtLoc (CtLoc { ctl_origin :: CtLoc -> CtOrigin ctl_origin = CtOrigin o, ctl_env :: CtLoc -> TcLclEnv ctl_env = TcLclEnv lcl}) = [SDoc] -> SDoc sep [ CtOrigin -> SDoc pprCtOrigin CtOrigin o , String -> SDoc text String "at" SDoc -> SDoc -> SDoc <+> RealSrcSpan -> SDoc forall a. Outputable a => a -> SDoc ppr (TcLclEnv -> RealSrcSpan getLclEnvLoc TcLclEnv lcl)]