{-# LANGUAGE CPP, DeriveFunctor, TypeFamilies #-} -- Type definitions for the constraint solver module TcSMonad ( -- The work list WorkList(..), isEmptyWorkList, emptyWorkList, extendWorkListNonEq, extendWorkListCt, extendWorkListCts, extendWorkListEq, extendWorkListFunEq, appendWorkList, extendWorkListImplic, selectNextWorkItem, workListSize, workListWantedCount, getWorkList, updWorkListTcS, -- The TcS monad TcS, runTcS, runTcSDeriveds, runTcSWithEvBinds, failTcS, warnTcS, addErrTcS, runTcSEqualities, nestTcS, nestImplicTcS, setEvBindsTcS, checkConstraintsTcS, checkTvConstraintsTcS, runTcPluginTcS, addUsedGRE, addUsedGREs, keepAlive, matchGlobalInst, TcM.ClsInstResult(..), QCInst(..), -- Tracing etc panicTcS, traceTcS, traceFireTcS, bumpStepCountTcS, csTraceTcS, wrapErrTcS, wrapWarnTcS, -- Evidence creation and transformation MaybeNew(..), freshGoals, isFresh, getEvExpr, newTcEvBinds, newNoTcEvBinds, newWantedEq, newWantedEq_SI, emitNewWantedEq, newWanted, newWanted_SI, newWantedEvVar, newWantedNC, newWantedEvVarNC, newDerivedNC, newBoundEvVarId, unifyTyVar, unflattenFmv, reportUnifications, setEvBind, setWantedEq, setWantedEvTerm, setEvBindIfWanted, newEvVar, newGivenEvVar, newGivenEvVars, emitNewDeriveds, emitNewDerivedEq, checkReductionDepth, getSolvedDicts, setSolvedDicts, getInstEnvs, getFamInstEnvs, -- Getting the environments getTopEnv, getGblEnv, getLclEnv, getTcEvBindsVar, getTcLevel, getTcEvTyCoVars, getTcEvBindsMap, setTcEvBindsMap, tcLookupClass, tcLookupId, -- Inerts InertSet(..), InertCans(..), updInertTcS, updInertCans, updInertDicts, updInertIrreds, getNoGivenEqs, setInertCans, getInertEqs, getInertCans, getInertGivens, getInertInsols, getTcSInerts, setTcSInerts, matchableGivens, prohibitedSuperClassSolve, mightMatchLater, getUnsolvedInerts, removeInertCts, getPendingGivenScs, addInertCan, insertFunEq, addInertForAll, emitWorkNC, emitWork, isImprovable, -- The Model kickOutAfterUnification, -- Inert Safe Haskell safe-overlap failures addInertSafehask, insertSafeOverlapFailureTcS, updInertSafehask, getSafeOverlapFailures, -- Inert CDictCans DictMap, emptyDictMap, lookupInertDict, findDictsByClass, addDict, addDictsByClass, delDict, foldDicts, filterDicts, findDict, -- Inert CTyEqCans EqualCtList, findTyEqs, foldTyEqs, isInInertEqs, lookupInertTyVar, -- Inert solved dictionaries addSolvedDict, lookupSolvedDict, -- Irreds foldIrreds, -- The flattening cache lookupFlatCache, extendFlatCache, newFlattenSkolem, -- Flatten skolems dischargeFunEq, pprKicked, -- Inert CFunEqCans updInertFunEqs, findFunEq, findFunEqsByTyCon, instDFunType, -- Instantiation -- MetaTyVars newFlexiTcSTy, instFlexi, instFlexiX, cloneMetaTyVar, demoteUnfilledFmv, tcInstSkolTyVarsX, TcLevel, isFilledMetaTyVar_maybe, isFilledMetaTyVar, zonkTyCoVarsAndFV, zonkTcType, zonkTcTypes, zonkTcTyVar, zonkCo, zonkTyCoVarsAndFVList, zonkSimples, zonkWC, zonkTyCoVarKind, -- References newTcRef, readTcRef, writeTcRef, updTcRef, -- Misc getDefaultInfo, getDynFlags, getGlobalRdrEnvTcS, matchFam, matchFamTcM, checkWellStagedDFun, pprEq -- Smaller utils, re-exported from TcM -- TODO (DV): these are only really used in the -- instance matcher in TcSimplify. I am wondering -- if the whole instance matcher simply belongs -- here ) where #include "HsVersions.h" import GhcPrelude import HscTypes import qualified Inst as TcM import InstEnv import FamInst import FamInstEnv import qualified TcRnMonad as TcM import qualified TcMType as TcM import qualified ClsInst as TcM( matchGlobalInst, ClsInstResult(..) ) import qualified TcEnv as TcM ( checkWellStaged, tcGetDefaultTys, tcLookupClass, tcLookupId, topIdLvl ) import ClsInst( InstanceWhat(..), safeOverlap, instanceReturnsDictCon ) import TcType import DynFlags import Type import Coercion import Unify import TcEvidence import Class import TyCon import TcErrors ( solverDepthErrorTcS ) import Name import Module ( HasModule, getModule ) import RdrName ( GlobalRdrEnv, GlobalRdrElt ) import qualified RnEnv as TcM import Var import VarEnv import VarSet import Outputable import Bag import UniqSupply import Util import TcRnTypes import TcOrigin import Constraint import Predicate import Unique import UniqFM import UniqDFM import Maybes import CoreMap import Control.Monad import qualified Control.Monad.Fail as MonadFail import MonadUtils import Data.IORef import Data.List ( partition, mapAccumL ) #if defined(DEBUG) import Digraph import UniqSet #endif {- ************************************************************************ * * * Worklists * * Canonical and non-canonical constraints that the simplifier has to * * work on. Including their simplification depths. * * * * * ************************************************************************ Note [WorkList priorities] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ A WorkList contains canonical and non-canonical items (of all flavors). Notice that each Ct now has a simplification depth. We may consider using this depth for prioritization as well in the future. As a simple form of priority queue, our worklist separates out * equalities (wl_eqs); see Note [Prioritise equalities] * type-function equalities (wl_funeqs) * all the rest (wl_rest) Note [Prioritise equalities] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It's very important to process equalities /first/: * (Efficiency) The general reason to do so is that if we process a class constraint first, we may end up putting it into the inert set and then kicking it out later. That's extra work compared to just doing the equality first. * (Avoiding fundep iteration) As #14723 showed, it's possible to get non-termination if we - Emit the Derived fundep equalities for a class constraint, generating some fresh unification variables. - That leads to some unification - Which kicks out the class constraint - Which isn't solved (because there are still some more Derived equalities in the work-list), but generates yet more fundeps Solution: prioritise derived equalities over class constraints * (Class equalities) We need to prioritise equalities even if they are hidden inside a class constraint; see Note [Prioritise class equalities] * (Kick-out) We want to apply this priority scheme to kicked-out constraints too (see the call to extendWorkListCt in kick_out_rewritable E.g. a CIrredCan can be a hetero-kinded (t1 ~ t2), which may become homo-kinded when kicked out, and hence we want to prioritise it. * (Derived equalities) Originally we tried to postpone processing Derived equalities, in the hope that we might never need to deal with them at all; but in fact we must process Derived equalities eagerly, partly for the (Efficiency) reason, and more importantly for (Avoiding fundep iteration). Note [Prioritise class equalities] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We prioritise equalities in the solver (see selectWorkItem). But class constraints like (a ~ b) and (a ~~ b) are actually equalities too; see Note [The equality types story] in TysPrim. Failing to prioritise these is inefficient (more kick-outs etc). But, worse, it can prevent us spotting a "recursive knot" among Wanted constraints. See comment:10 of #12734 for a worked-out example. So we arrange to put these particular class constraints in the wl_eqs. NB: since we do not currently apply the substitution to the inert_solved_dicts, the knot-tying still seems a bit fragile. But this makes it better. -} -- See Note [WorkList priorities] data WorkList = WL { wl_eqs :: [Ct] -- CTyEqCan, CDictCan, CIrredCan -- Given, Wanted, and Derived -- Contains both equality constraints and their -- class-level variants (a~b) and (a~~b); -- See Note [Prioritise equalities] -- See Note [Prioritise class equalities] , wl_funeqs :: [Ct] , wl_rest :: [Ct] , wl_implics :: Bag Implication -- See Note [Residual implications] } appendWorkList :: WorkList -> WorkList -> WorkList appendWorkList (WL { wl_eqs = eqs1, wl_funeqs = funeqs1, wl_rest = rest1 , wl_implics = implics1 }) (WL { wl_eqs = eqs2, wl_funeqs = funeqs2, wl_rest = rest2 , wl_implics = implics2 }) = WL { wl_eqs = eqs1 ++ eqs2 , wl_funeqs = funeqs1 ++ funeqs2 , wl_rest = rest1 ++ rest2 , wl_implics = implics1 `unionBags` implics2 } workListSize :: WorkList -> Int workListSize (WL { wl_eqs = eqs, wl_funeqs = funeqs, wl_rest = rest }) = length eqs + length funeqs + length rest workListWantedCount :: WorkList -> Int -- Count the things we need to solve -- excluding the insolubles (c.f. inert_count) workListWantedCount (WL { wl_eqs = eqs, wl_rest = rest }) = count isWantedCt eqs + count is_wanted rest where is_wanted ct | CIrredCan { cc_ev = ev, cc_insol = insol } <- ct = not insol && isWanted ev | otherwise = isWantedCt ct extendWorkListEq :: Ct -> WorkList -> WorkList extendWorkListEq ct wl = wl { wl_eqs = ct : wl_eqs wl } extendWorkListFunEq :: Ct -> WorkList -> WorkList extendWorkListFunEq ct wl = wl { wl_funeqs = ct : wl_funeqs wl } extendWorkListNonEq :: Ct -> WorkList -> WorkList -- Extension by non equality extendWorkListNonEq ct wl = wl { wl_rest = ct : wl_rest wl } extendWorkListDeriveds :: [CtEvidence] -> WorkList -> WorkList extendWorkListDeriveds evs wl = extendWorkListCts (map mkNonCanonical evs) wl extendWorkListImplic :: Bag Implication -> WorkList -> WorkList extendWorkListImplic implics wl = wl { wl_implics = implics `unionBags` wl_implics wl } extendWorkListCt :: Ct -> WorkList -> WorkList -- Agnostic extendWorkListCt ct wl = case classifyPredType (ctPred ct) of EqPred NomEq ty1 _ | Just tc <- tcTyConAppTyCon_maybe ty1 , isTypeFamilyTyCon tc -> extendWorkListFunEq ct wl EqPred {} -> extendWorkListEq ct wl ClassPred cls _ -- See Note [Prioritise class equalities] | isEqPredClass cls -> extendWorkListEq ct wl _ -> extendWorkListNonEq ct wl extendWorkListCts :: [Ct] -> WorkList -> WorkList -- Agnostic extendWorkListCts cts wl = foldr extendWorkListCt wl cts isEmptyWorkList :: WorkList -> Bool isEmptyWorkList (WL { wl_eqs = eqs, wl_funeqs = funeqs , wl_rest = rest, wl_implics = implics }) = null eqs && null rest && null funeqs && isEmptyBag implics emptyWorkList :: WorkList emptyWorkList = WL { wl_eqs = [], wl_rest = [] , wl_funeqs = [], wl_implics = emptyBag } selectWorkItem :: WorkList -> Maybe (Ct, WorkList) -- See Note [Prioritise equalities] selectWorkItem wl@(WL { wl_eqs = eqs, wl_funeqs = feqs , wl_rest = rest }) | ct:cts <- eqs = Just (ct, wl { wl_eqs = cts }) | ct:fes <- feqs = Just (ct, wl { wl_funeqs = fes }) | ct:cts <- rest = Just (ct, wl { wl_rest = cts }) | otherwise = Nothing getWorkList :: TcS WorkList getWorkList = do { wl_var <- getTcSWorkListRef ; wrapTcS (TcM.readTcRef wl_var) } selectNextWorkItem :: TcS (Maybe Ct) -- Pick which work item to do next -- See Note [Prioritise equalities] selectNextWorkItem = do { wl_var <- getTcSWorkListRef ; wl <- readTcRef wl_var ; case selectWorkItem wl of { Nothing -> return Nothing ; Just (ct, new_wl) -> do { -- checkReductionDepth (ctLoc ct) (ctPred ct) -- This is done by TcInteract.chooseInstance ; writeTcRef wl_var new_wl ; return (Just ct) } } } -- Pretty printing instance Outputable WorkList where ppr (WL { wl_eqs = eqs, wl_funeqs = feqs , wl_rest = rest, wl_implics = implics }) = text "WL" <+> (braces $ vcat [ ppUnless (null eqs) $ text "Eqs =" <+> vcat (map ppr eqs) , ppUnless (null feqs) $ text "Funeqs =" <+> vcat (map ppr feqs) , ppUnless (null rest) $ text "Non-eqs =" <+> vcat (map ppr rest) , ppUnless (isEmptyBag implics) $ ifPprDebug (text "Implics =" <+> vcat (map ppr (bagToList implics))) (text "(Implics omitted)") ]) {- ********************************************************************* * * InertSet: the inert set * * * * ********************************************************************* -} data InertSet = IS { inert_cans :: InertCans -- Canonical Given, Wanted, Derived -- Sometimes called "the inert set" , inert_fsks :: [(TcTyVar, TcType)] -- A list of (fsk, ty) pairs; we add one element when we flatten -- a function application in a Given constraint, creating -- a new fsk in newFlattenSkolem. When leaving a nested scope, -- unflattenGivens unifies fsk := ty -- -- We could also get this info from inert_funeqs, filtered by -- level, but it seems simpler and more direct to capture the -- fsk as we generate them. , inert_flat_cache :: ExactFunEqMap (TcCoercion, TcType, CtFlavour) -- See Note [Type family equations] -- If F tys :-> (co, rhs, flav), -- then co :: F tys ~ rhs -- flav is [G] or [WD] -- -- Just a hash-cons cache for use when flattening only -- These include entirely un-processed goals, so don't use -- them to solve a top-level goal, else you may end up solving -- (w:F ty ~ a) by setting w:=w! We just use the flat-cache -- when allocating a new flatten-skolem. -- Not necessarily inert wrt top-level equations (or inert_cans) -- NB: An ExactFunEqMap -- this doesn't match via loose types! , inert_solved_dicts :: DictMap CtEvidence -- All Wanteds, of form ev :: C t1 .. tn -- See Note [Solved dictionaries] -- and Note [Do not add superclasses of solved dictionaries] } instance Outputable InertSet where ppr (IS { inert_cans = ics , inert_fsks = ifsks , inert_solved_dicts = solved_dicts }) = vcat [ ppr ics , text "Inert fsks =" <+> ppr ifsks , ppUnless (null dicts) $ text "Solved dicts =" <+> vcat (map ppr dicts) ] where dicts = bagToList (dictsToBag solved_dicts) emptyInertCans :: InertCans emptyInertCans = IC { inert_count = 0 , inert_eqs = emptyDVarEnv , inert_dicts = emptyDicts , inert_safehask = emptyDicts , inert_funeqs = emptyFunEqs , inert_insts = [] , inert_irreds = emptyCts } emptyInert :: InertSet emptyInert = IS { inert_cans = emptyInertCans , inert_fsks = [] , inert_flat_cache = emptyExactFunEqs , inert_solved_dicts = emptyDictMap } {- Note [Solved dictionaries] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we apply a top-level instance declaration, we add the "solved" dictionary to the inert_solved_dicts. In general, we use it to avoid creating a new EvVar when we have a new goal that we have solved in the past. But in particular, we can use it to create *recursive* dictionaries. The simplest, degnerate case is instance C [a] => C [a] where ... If we have [W] d1 :: C [x] then we can apply the instance to get d1 = $dfCList d [W] d2 :: C [x] Now 'd1' goes in inert_solved_dicts, and we can solve d2 directly from d1. d1 = $dfCList d d2 = d1 See Note [Example of recursive dictionaries] VERY IMPORTANT INVARIANT: (Solved Dictionary Invariant) Every member of the inert_solved_dicts is the result of applying an instance declaration that "takes a step" An instance "takes a step" if it has the form dfunDList d1 d2 = MkD (...) (...) (...) That is, the dfun is lazy in its arguments, and guarantees to immediately return a dictionary constructor. NB: all dictionary data constructors are lazy in their arguments. This property is crucial to ensure that all dictionaries are non-bottom, which in turn ensures that the whole "recursive dictionary" idea works at all, even if we get something like rec { d = dfunDList d dx } See Note [Recursive superclasses] in TcInstDcls. Reason: - All instances, except two exceptions listed below, "take a step" in the above sense - Exception 1: local quantified constraints have no such guarantee; indeed, adding a "solved dictionary" when appling a quantified constraint led to the ability to define unsafeCoerce in #17267. - Exception 2: the magic built-in instace for (~) has no such guarantee. It behaves as if we had class (a ~# b) => (a ~ b) where {} instance (a ~# b) => (a ~ b) where {} The "dfun" for the instance is strict in the coercion. Anyway there's no point in recording a "solved dict" for (t1 ~ t2); it's not going to allow a recursive dictionary to be constructed. Ditto (~~) and Coercible. THEREFORE we only add a "solved dictionary" - when applying an instance declaration - subject to Exceptions 1 and 2 above In implementation terms - TcSMonad.addSolvedDict adds a new solved dictionary, conditional on the kind of instance - It is only called when applying an instance decl, in TcInteract.doTopReactDict - ClsInst.InstanceWhat says what kind of instance was used to solve the constraint. In particular * LocalInstance identifies quantified constraints * BuiltinEqInstance identifies the strange built-in instances for equality. - ClsInst.instanceReturnsDictCon says which kind of instance guarantees to return a dictionary constructor Other notes about solved dictionaries * See also Note [Do not add superclasses of solved dictionaries] * The inert_solved_dicts field is not rewritten by equalities, so it may get out of date. * The inert_solved_dicts are all Wanteds, never givens * We only cache dictionaries from top-level instances, not from local quantified constraints. Reason: if we cached the latter we'd need to purge the cache when bringing new quantified constraints into scope, because quantified constraints "shadow" top-level instances. Note [Do not add superclasses of solved dictionaries] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Every member of inert_solved_dicts is the result of applying a dictionary function, NOT of applying superclass selection to anything. Consider class Ord a => C a where instance Ord [a] => C [a] where ... Suppose we are trying to solve [G] d1 : Ord a [W] d2 : C [a] Then we'll use the instance decl to give [G] d1 : Ord a Solved: d2 : C [a] = $dfCList d3 [W] d3 : Ord [a] We must not add d4 : Ord [a] to the 'solved' set (by taking the superclass of d2), otherwise we'll use it to solve d3, without ever using d1, which would be a catastrophe. Solution: when extending the solved dictionaries, do not add superclasses. That's why each element of the inert_solved_dicts is the result of applying a dictionary function. Note [Example of recursive dictionaries] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ --- Example 1 data D r = ZeroD | SuccD (r (D r)); instance (Eq (r (D r))) => Eq (D r) where ZeroD == ZeroD = True (SuccD a) == (SuccD b) = a == b _ == _ = False; equalDC :: D [] -> D [] -> Bool; equalDC = (==); We need to prove (Eq (D [])). Here's how we go: [W] d1 : Eq (D []) By instance decl of Eq (D r): [W] d2 : Eq [D []] where d1 = dfEqD d2 By instance decl of Eq [a]: [W] d3 : Eq (D []) where d2 = dfEqList d3 d1 = dfEqD d2 Now this wanted can interact with our "solved" d1 to get: d3 = d1 -- Example 2: This code arises in the context of "Scrap Your Boilerplate with Class" class Sat a class Data ctx a instance Sat (ctx Char) => Data ctx Char -- dfunData1 instance (Sat (ctx [a]), Data ctx a) => Data ctx [a] -- dfunData2 class Data Maybe a => Foo a instance Foo t => Sat (Maybe t) -- dfunSat instance Data Maybe a => Foo a -- dfunFoo1 instance Foo a => Foo [a] -- dfunFoo2 instance Foo [Char] -- dfunFoo3 Consider generating the superclasses of the instance declaration instance Foo a => Foo [a] So our problem is this [G] d0 : Foo t [W] d1 : Data Maybe [t] -- Desired superclass We may add the given in the inert set, along with its superclasses Inert: [G] d0 : Foo t [G] d01 : Data Maybe t -- Superclass of d0 WorkList [W] d1 : Data Maybe [t] Solve d1 using instance dfunData2; d1 := dfunData2 d2 d3 Inert: [G] d0 : Foo t [G] d01 : Data Maybe t -- Superclass of d0 Solved: d1 : Data Maybe [t] WorkList: [W] d2 : Sat (Maybe [t]) [W] d3 : Data Maybe t Now, we may simplify d2 using dfunSat; d2 := dfunSat d4 Inert: [G] d0 : Foo t [G] d01 : Data Maybe t -- Superclass of d0 Solved: d1 : Data Maybe [t] d2 : Sat (Maybe [t]) WorkList: [W] d3 : Data Maybe t [W] d4 : Foo [t] Now, we can just solve d3 from d01; d3 := d01 Inert [G] d0 : Foo t [G] d01 : Data Maybe t -- Superclass of d0 Solved: d1 : Data Maybe [t] d2 : Sat (Maybe [t]) WorkList [W] d4 : Foo [t] Now, solve d4 using dfunFoo2; d4 := dfunFoo2 d5 Inert [G] d0 : Foo t [G] d01 : Data Maybe t -- Superclass of d0 Solved: d1 : Data Maybe [t] d2 : Sat (Maybe [t]) d4 : Foo [t] WorkList: [W] d5 : Foo t Now, d5 can be solved! d5 := d0 Result d1 := dfunData2 d2 d3 d2 := dfunSat d4 d3 := d01 d4 := dfunFoo2 d5 d5 := d0 -} {- ********************************************************************* * * InertCans: the canonical inerts * * * * ********************************************************************* -} data InertCans -- See Note [Detailed InertCans Invariants] for more = IC { inert_eqs :: InertEqs -- See Note [inert_eqs: the inert equalities] -- All CTyEqCans; index is the LHS tyvar -- Domain = skolems and untouchables; a touchable would be unified , inert_funeqs :: FunEqMap Ct -- All CFunEqCans; index is the whole family head type. -- All Nominal (that's an invarint of all CFunEqCans) -- LHS is fully rewritten (modulo eqCanRewrite constraints) -- wrt inert_eqs -- Can include all flavours, [G], [W], [WD], [D] -- See Note [Type family equations] , inert_dicts :: DictMap Ct -- Dictionaries only -- All fully rewritten (modulo flavour constraints) -- wrt inert_eqs , inert_insts :: [QCInst] , inert_safehask :: DictMap Ct -- Failed dictionary resolution due to Safe Haskell overlapping -- instances restriction. We keep this separate from inert_dicts -- as it doesn't cause compilation failure, just safe inference -- failure. -- -- ^ See Note [Safe Haskell Overlapping Instances Implementation] -- in TcSimplify , inert_irreds :: Cts -- Irreducible predicates that cannot be made canonical, -- and which don't interact with others (e.g. (c a)) -- and insoluble predicates (e.g. Int ~ Bool, or a ~ [a]) , inert_count :: Int -- Number of Wanted goals in -- inert_eqs, inert_dicts, inert_safehask, inert_irreds -- Does not include insolubles -- When non-zero, keep trying to solve } type InertEqs = DTyVarEnv EqualCtList type EqualCtList = [Ct] -- See Note [EqualCtList invariants] {- Note [Detailed InertCans Invariants] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The InertCans represents a collection of constraints with the following properties: * All canonical * No two dictionaries with the same head * No two CIrreds with the same type * Family equations inert wrt top-level family axioms * Dictionaries have no matching top-level instance * Given family or dictionary constraints don't mention touchable unification variables * Non-CTyEqCan constraints are fully rewritten with respect to the CTyEqCan equalities (modulo canRewrite of course; eg a wanted cannot rewrite a given) * CTyEqCan equalities: see Note [inert_eqs: the inert equalities] Note [EqualCtList invariants] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * All are equalities * All these equalities have the same LHS * The list is never empty * No element of the list can rewrite any other * Derived before Wanted From the fourth invariant it follows that the list is - A single [G], or - Zero or one [D] or [WD], followd by any number of [W] The Wanteds can't rewrite anything which is why we put them last Note [Type family equations] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Type-family equations, CFunEqCans, of form (ev : F tys ~ ty), live in three places * The work-list, of course * The inert_funeqs are un-solved but fully processed, and in the InertCans. They can be [G], [W], [WD], or [D]. * The inert_flat_cache. This is used when flattening, to get maximal sharing. Everthing in the inert_flat_cache is [G] or [WD] It contains lots of things that are still in the work-list. E.g Suppose we have (w1: F (G a) ~ Int), and (w2: H (G a) ~ Int) in the work list. Then we flatten w1, dumping (w3: G a ~ f1) in the work list. Now if we flatten w2 before we get to w3, we still want to share that (G a). Because it contains work-list things, DO NOT use the flat cache to solve a top-level goal. Eg in the above example we don't want to solve w3 using w3 itself! The CFunEqCan Ownership Invariant: * Each [G/W/WD] CFunEqCan has a distinct fsk or fmv It "owns" that fsk/fmv, in the sense that: - reducing a [W/WD] CFunEqCan fills in the fmv - unflattening a [W/WD] CFunEqCan fills in the fmv (in both cases unless an occurs-check would result) * In contrast a [D] CFunEqCan does not "own" its fmv: - reducing a [D] CFunEqCan does not fill in the fmv; it just generates an equality - unflattening ignores [D] CFunEqCans altogether Note [inert_eqs: the inert equalities] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Definition [Can-rewrite relation] A "can-rewrite" relation between flavours, written f1 >= f2, is a binary relation with the following properties (R1) >= is transitive (R2) If f1 >= f, and f2 >= f, then either f1 >= f2 or f2 >= f1 Lemma. If f1 >= f then f1 >= f1 Proof. By property (R2), with f1=f2 Definition [Generalised substitution] A "generalised substitution" S is a set of triples (a -f-> t), where a is a type variable t is a type f is a flavour such that (WF1) if (a -f1-> t1) in S (a -f2-> t2) in S then neither (f1 >= f2) nor (f2 >= f1) hold (WF2) if (a -f-> t) is in S, then t /= a Definition [Applying a generalised substitution] If S is a generalised substitution S(f,a) = t, if (a -fs-> t) in S, and fs >= f = a, otherwise Application extends naturally to types S(f,t), modulo roles. See Note [Flavours with roles]. Theorem: S(f,a) is well defined as a function. Proof: Suppose (a -f1-> t1) and (a -f2-> t2) are both in S, and f1 >= f and f2 >= f Then by (R2) f1 >= f2 or f2 >= f1, which contradicts (WF1) Notation: repeated application. S^0(f,t) = t S^(n+1)(f,t) = S(f, S^n(t)) Definition: inert generalised substitution A generalised substitution S is "inert" iff (IG1) there is an n such that for every f,t, S^n(f,t) = S^(n+1)(f,t) By (IG1) we define S*(f,t) to be the result of exahaustively applying S(f,_) to t. ---------------------------------------------------------------- Our main invariant: the inert CTyEqCans should be an inert generalised substitution ---------------------------------------------------------------- Note that inertness is not the same as idempotence. To apply S to a type, you may have to apply it recursive. But inertness does guarantee that this recursive use will terminate. Note [Extending the inert equalities] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Main Theorem [Stability under extension] Suppose we have a "work item" a -fw-> t and an inert generalised substitution S, THEN the extended substitution T = S+(a -fw-> t) is an inert generalised substitution PROVIDED (T1) S(fw,a) = a -- LHS of work-item is a fixpoint of S(fw,_) (T2) S(fw,t) = t -- RHS of work-item is a fixpoint of S(fw,_) (T3) a not in t -- No occurs check in the work item AND, for every (b -fs-> s) in S: (K0) not (fw >= fs) Reason: suppose we kick out (a -fs-> s), and add (a -fw-> t) to the inert set. The latter can't rewrite the former, so the kick-out achieved nothing OR { (K1) not (a = b) Reason: if fw >= fs, WF1 says we can't have both a -fw-> t and a -fs-> s AND (K2): guarantees inertness of the new substitution { (K2a) not (fs >= fs) OR (K2b) fs >= fw OR (K2d) a not in s } AND (K3) See Note [K3: completeness of solving] { (K3a) If the role of fs is nominal: s /= a (K3b) If the role of fs is representational: s is not of form (a t1 .. tn) } } Conditions (T1-T3) are established by the canonicaliser Conditions (K1-K3) are established by TcSMonad.kickOutRewritable The idea is that * (T1-2) are guaranteed by exhaustively rewriting the work-item with S(fw,_). * T3 is guaranteed by a simple occurs-check on the work item. This is done during canonicalisation, in canEqTyVar; (invariant: a CTyEqCan never has an occurs check). * (K1-3) are the "kick-out" criteria. (As stated, they are really the "keep" criteria.) If the current inert S contains a triple that does not satisfy (K1-3), then we remove it from S by "kicking it out", and re-processing it. * Note that kicking out is a Bad Thing, because it means we have to re-process a constraint. The less we kick out, the better. TODO: Make sure that kicking out really *is* a Bad Thing. We've assumed this but haven't done the empirical study to check. * Assume we have G>=G, G>=W and that's all. Then, when performing a unification we add a new given a -G-> ty. But doing so does NOT require us to kick out an inert wanted that mentions a, because of (K2a). This is a common case, hence good not to kick out. * Lemma (L2): if not (fw >= fw), then K0 holds and we kick out nothing Proof: using Definition [Can-rewrite relation], fw can't rewrite anything and so K0 holds. Intuitively, since fw can't rewrite anything, adding it cannot cause any loops This is a common case, because Wanteds cannot rewrite Wanteds. It's used to avoid even looking for constraint to kick out. * Lemma (L1): The conditions of the Main Theorem imply that there is no (a -fs-> t) in S, s.t. (fs >= fw). Proof. Suppose the contrary (fs >= fw). Then because of (T1), S(fw,a)=a. But since fs>=fw, S(fw,a) = s, hence s=a. But now we have (a -fs-> a) in S, which contradicts (WF2). * The extended substitution satisfies (WF1) and (WF2) - (K1) plus (L1) guarantee that the extended substitution satisfies (WF1). - (T3) guarantees (WF2). * (K2) is about inertness. Intuitively, any infinite chain T^0(f,t), T^1(f,t), T^2(f,T).... must pass through the new work item infinitely often, since the substitution without the work item is inert; and must pass through at least one of the triples in S infinitely often. - (K2a): if not(fs>=fs) then there is no f that fs can rewrite (fs>=f), and hence this triple never plays a role in application S(f,a). It is always safe to extend S with such a triple. (NB: we could strengten K1) in this way too, but see K3. - (K2b): If this holds then, by (T2), b is not in t. So applying the work item does not generate any new opportunities for applying S - (K2c): If this holds, we can't pass through this triple infinitely often, because if we did then fs>=f, fw>=f, hence by (R2) * either fw>=fs, contradicting K2c * or fs>=fw; so by the argument in K2b we can't have a loop - (K2d): if a not in s, we hae no further opportunity to apply the work item, similar to (K2b) NB: Dimitrios has a PDF that does this in more detail Key lemma to make it watertight. Under the conditions of the Main Theorem, forall f st fw >= f, a is not in S^k(f,t), for any k Also, consider roles more carefully. See Note [Flavours with roles] Note [K3: completeness of solving] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (K3) is not necessary for the extended substitution to be inert. In fact K1 could be made stronger by saying ... then (not (fw >= fs) or not (fs >= fs)) But it's not enough for S to be inert; we also want completeness. That is, we want to be able to solve all soluble wanted equalities. Suppose we have work-item b -G-> a inert-item a -W-> b Assuming (G >= W) but not (W >= W), this fulfills all the conditions, so we could extend the inerts, thus: inert-items b -G-> a a -W-> b But if we kicked-out the inert item, we'd get work-item a -W-> b inert-item b -G-> a Then rewrite the work-item gives us (a -W-> a), which is soluble via Refl. So we add one more clause to the kick-out criteria Another way to understand (K3) is that we treat an inert item a -f-> b in the same way as b -f-> a So if we kick out one, we should kick out the other. The orientation is somewhat accidental. When considering roles, we also need the second clause (K3b). Consider work-item c -G/N-> a inert-item a -W/R-> b c The work-item doesn't get rewritten by the inert, because (>=) doesn't hold. But we don't kick out the inert item because not (W/R >= W/R). So we just add the work item. But then, consider if we hit the following: work-item b -G/N-> Id inert-items a -W/R-> b c c -G/N-> a where newtype Id x = Id x For similar reasons, if we only had (K3a), we wouldn't kick the representational inert out. And then, we'd miss solving the inert, which now reduced to reflexivity. The solution here is to kick out representational inerts whenever the tyvar of a work item is "exposed", where exposed means being at the head of the top-level application chain (a t1 .. tn). See TcType.isTyVarHead. This is encoded in (K3b). Beware: if we make this test succeed too often, we kick out too much, and the solver might loop. Consider (#14363) work item: [G] a ~R f b inert item: [G] b ~R f a In GHC 8.2 the completeness tests more aggressive, and kicked out the inert item; but no rewriting happened and there was an infinite loop. All we need is to have the tyvar at the head. Note [Flavours with roles] ~~~~~~~~~~~~~~~~~~~~~~~~~~ The system described in Note [inert_eqs: the inert equalities] discusses an abstract set of flavours. In GHC, flavours have two components: the flavour proper, taken from {Wanted, Derived, Given} and the equality relation (often called role), taken from {NomEq, ReprEq}. When substituting w.r.t. the inert set, as described in Note [inert_eqs: the inert equalities], we must be careful to respect all components of a flavour. For example, if we have inert set: a -G/R-> Int b -G/R-> Bool type role T nominal representational and we wish to compute S(W/R, T a b), the correct answer is T a Bool, NOT T Int Bool. The reason is that T's first parameter has a nominal role, and thus rewriting a to Int in T a b is wrong. Indeed, this non-congruence of substitution means that the proof in Note [The inert equalities] may need to be revisited, but we don't think that the end conclusion is wrong. -} instance Outputable InertCans where ppr (IC { inert_eqs = eqs , inert_funeqs = funeqs, inert_dicts = dicts , inert_safehask = safehask, inert_irreds = irreds , inert_insts = insts , inert_count = count }) = braces $ vcat [ ppUnless (isEmptyDVarEnv eqs) $ text "Equalities:" <+> pprCts (foldDVarEnv (\eqs rest -> listToBag eqs `andCts` rest) emptyCts eqs) , ppUnless (isEmptyTcAppMap funeqs) $ text "Type-function equalities =" <+> pprCts (funEqsToBag funeqs) , ppUnless (isEmptyTcAppMap dicts) $ text "Dictionaries =" <+> pprCts (dictsToBag dicts) , ppUnless (isEmptyTcAppMap safehask) $ text "Safe Haskell unsafe overlap =" <+> pprCts (dictsToBag safehask) , ppUnless (isEmptyCts irreds) $ text "Irreds =" <+> pprCts irreds , ppUnless (null insts) $ text "Given instances =" <+> vcat (map ppr insts) , text "Unsolved goals =" <+> int count ] {- ********************************************************************* * * Shadow constraints and improvement * * ************************************************************************ Note [The improvement story and derived shadows] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Because Wanteds cannot rewrite Wanteds (see Note [Wanteds do not rewrite Wanteds] in Constraint), we may miss some opportunities for solving. Here's a classic example (indexed-types/should_fail/T4093a) Ambiguity check for f: (Foo e ~ Maybe e) => Foo e We get [G] Foo e ~ Maybe e [W] Foo e ~ Foo ee -- ee is a unification variable [W] Foo ee ~ Maybe ee Flatten: [G] Foo e ~ fsk [G] fsk ~ Maybe e -- (A) [W] Foo ee ~ fmv [W] fmv ~ fsk -- (B) From Foo e ~ Foo ee [W] fmv ~ Maybe ee --> rewrite (B) with (A) [W] Foo ee ~ fmv [W] fmv ~ Maybe e [W] fmv ~ Maybe ee But now we appear to be stuck, since we don't rewrite Wanteds with Wanteds. This is silly because we can see that ee := e is the only solution. The basic plan is * generate Derived constraints that shadow Wanted constraints * allow Derived to rewrite Derived * in order to cause some unifications to take place * that in turn solve the original Wanteds The ONLY reason for all these Derived equalities is to tell us how to unify a variable: that is, what Mark Jones calls "improvement". The same idea is sometimes also called "saturation"; find all the equalities that must hold in any solution. Or, equivalently, you can think of the derived shadows as implementing the "model": a non-idempotent but no-occurs-check substitution, reflecting *all* *Nominal* equalities (a ~N ty) that are not immediately soluble by unification. More specifically, here's how it works (Oct 16): * Wanted constraints are born as [WD]; this behaves like a [W] and a [D] paired together. * When we are about to add a [WD] to the inert set, if it can be rewritten by a [D] a ~ ty, then we split it into [W] and [D], putting the latter into the work list (see maybeEmitShadow). In the example above, we get to the point where we are stuck: [WD] Foo ee ~ fmv [WD] fmv ~ Maybe e [WD] fmv ~ Maybe ee But now when [WD] fmv ~ Maybe ee is about to be added, we'll split it into [W] and [D], since the inert [WD] fmv ~ Maybe e can rewrite it. Then: work item: [D] fmv ~ Maybe ee inert: [W] fmv ~ Maybe ee [WD] fmv ~ Maybe e -- (C) [WD] Foo ee ~ fmv See Note [Splitting WD constraints]. Now the work item is rewritten by (C) and we soon get ee := e. Additional notes: * The derived shadow equalities live in inert_eqs, along with the Givens and Wanteds; see Note [EqualCtList invariants]. * We make Derived shadows only for Wanteds, not Givens. So we have only [G], not [GD] and [G] plus splitting. See Note [Add derived shadows only for Wanteds] * We also get Derived equalities from functional dependencies and type-function injectivity; see calls to unifyDerived. * This splitting business applies to CFunEqCans too; and then we do apply type-function reductions to the [D] CFunEqCan. See Note [Reduction for Derived CFunEqCans] * It's worth having [WD] rather than just [W] and [D] because * efficiency: silly to process the same thing twice * inert_funeqs, inert_dicts is a finite map keyed by the type; it's inconvenient for it to map to TWO constraints Note [Splitting WD constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We are about to add a [WD] constraint to the inert set; and we know that the inert set has fully rewritten it. Should we split it into [W] and [D], and put the [D] in the work list for further work? * CDictCan (C tys) or CFunEqCan (F tys ~ fsk): Yes if the inert set could rewrite tys to make the class constraint, or type family, fire. That is, yes if the inert_eqs intersects with the free vars of tys. For this test we use (anyRewritableTyVar True) which ignores casts and coercions in tys, because rewriting the casts or coercions won't make the thing fire more often. * CTyEqCan (a ~ ty): Yes if the inert set could rewrite 'a' or 'ty'. We need to check both 'a' and 'ty' against the inert set: - Inert set contains [D] a ~ ty2 Then we want to put [D] a ~ ty in the worklist, so we'll get [D] ty ~ ty2 with consequent good things - Inert set contains [D] b ~ a, where b is in ty. We can't just add [WD] a ~ ty[b] to the inert set, because that breaks the inert-set invariants. If we tried to canonicalise another [D] constraint mentioning 'a', we'd get an infinite loop Moreover we must use (anyRewritableTyVar False) for the RHS, because even tyvars in the casts and coercions could give an infinite loop if we don't expose it * CIrredCan: Yes if the inert set can rewrite the constraint. We used to think splitting irreds was unnecessary, but see Note [Splitting Irred WD constraints] * Others: nothing is gained by splitting. Note [Splitting Irred WD constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Splitting Irred constraints can make a difference. Here is the scenario: a[sk] :: F v -- F is a type family beta :: alpha work item: [WD] a ~ beta This is heterogeneous, so we try flattening the kinds. co :: F v ~ fmv [WD] (a |> co) ~ beta This is still hetero, so we emit a kind equality and make the work item an inert Irred. work item: [D] fmv ~ alpha inert: [WD] (a |> co) ~ beta (CIrredCan) Can't make progress on the work item. Add to inert set. This kicks out the old inert, because a [D] can rewrite a [WD]. work item: [WD] (a |> co) ~ beta inert: [D] fmv ~ alpha (CTyEqCan) Can't make progress on this work item either (although GHC tries by decomposing the cast and reflattening... but that doesn't make a difference), which is still hetero. Emit a new kind equality and add to inert set. But, critically, we split the Irred. work list: [D] fmv ~ alpha (CTyEqCan) [D] (a |> co) ~ beta (CIrred) -- this one was split off inert: [W] (a |> co) ~ beta [D] fmv ~ alpha We quickly solve the first work item, as it's the same as an inert. work item: [D] (a |> co) ~ beta inert: [W] (a |> co) ~ beta [D] fmv ~ alpha We decompose the cast, yielding [D] a ~ beta We then flatten the kinds. The lhs kind is F v, which flattens to fmv which then rewrites to alpha. co' :: F v ~ alpha [D] (a |> co') ~ beta Now this equality is homo-kinded. So we swizzle it around to [D] beta ~ (a |> co') and set beta := a |> co', and go home happy. If we don't split the Irreds, we loop. This is all dangerously subtle. This is triggered by test case typecheck/should_compile/SplitWD. Note [Examples of how Derived shadows helps completeness] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #10009, a very nasty example: f :: (UnF (F b) ~ b) => F b -> () g :: forall a. (UnF (F a) ~ a) => a -> () g _ = f (undefined :: F a) For g we get [G] UnF (F a) ~ a [WD] UnF (F beta) ~ beta [WD] F a ~ F beta Flatten: [G] g1: F a ~ fsk1 fsk1 := F a [G] g2: UnF fsk1 ~ fsk2 fsk2 := UnF fsk1 [G] g3: fsk2 ~ a [WD] w1: F beta ~ fmv1 [WD] w2: UnF fmv1 ~ fmv2 [WD] w3: fmv2 ~ beta [WD] w4: fmv1 ~ fsk1 -- From F a ~ F beta using flat-cache -- and re-orient to put meta-var on left Rewrite w2 with w4: [D] d1: UnF fsk1 ~ fmv2 React that with g2: [D] d2: fmv2 ~ fsk2 React that with w3: [D] beta ~ fsk2 and g3: [D] beta ~ a -- Hooray beta := a And that is enough to solve everything Note [Add derived shadows only for Wanteds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We only add shadows for Wanted constraints. That is, we have [WD] but not [GD]; and maybeEmitShaodw looks only at [WD] constraints. It does just possibly make sense ot add a derived shadow for a Given. If we created a Derived shadow of a Given, it could be rewritten by other Deriveds, and that could, conceivably, lead to a useful unification. But (a) I have been unable to come up with an example of this happening (b) see #12660 for how adding the derived shadows of a Given led to an infinite loop. (c) It's unlikely that rewriting derived Givens will lead to a unification because Givens don't mention touchable unification variables For (b) there may be other ways to solve the loop, but simply reraining from adding derived shadows of Givens is particularly simple. And it's more efficient too! Still, here's one possible reason for adding derived shadows for Givens. Consider work-item [G] a ~ [b], inerts has [D] b ~ a. If we added the derived shadow (into the work list) [D] a ~ [b] When we process it, we'll rewrite to a ~ [a] and get an occurs check. Without it we'll miss the occurs check (reporting inaccessible code); but that's probably OK. Note [Keep CDictCan shadows as CDictCan] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have class C a => D a b and [G] D a b, [G] C a in the inert set. Now we insert [D] b ~ c. We want to kick out a derived shadow for [D] D a b, so we can rewrite it with the new constraint, and perhaps get instance reduction or other consequences. BUT we do not want to kick out a *non-canonical* (D a b). If we did, we would do this: - rewrite it to [D] D a c, with pend_sc = True - use expandSuperClasses to add C a - go round again, which solves C a from the givens This loop goes on for ever and triggers the simpl_loop limit. Solution: kick out the CDictCan which will have pend_sc = False, because we've already added its superclasses. So we won't re-add them. If we forget the pend_sc flag, our cunning scheme for avoiding generating superclasses repeatedly will fail. See #11379 for a case of this. Note [Do not do improvement for WOnly] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We do improvement between two constraints (e.g. for injectivity or functional dependencies) only if both are "improvable". And we improve a constraint wrt the top-level instances only if it is improvable. Improvable: [G] [WD] [D} Not improvable: [W] Reasons: * It's less work: fewer pairs to compare * Every [W] has a shadow [D] so nothing is lost * Consider [WD] C Int b, where 'b' is a skolem, and class C a b | a -> b instance C Int Bool We'll do a fundep on it and emit [D] b ~ Bool That will kick out constraint [WD] C Int b Then we'll split it to [W] C Int b (keep in inert) and [D] C Int b (in work list) When processing the latter we'll rewrite it to [D] C Int Bool At that point it would be /stupid/ to interact it with the inert [W] C Int b in the inert set; after all, it's the very constraint from which the [D] C Int Bool was split! We can avoid this by not doing improvement on [W] constraints. This came up in #12860. -} maybeEmitShadow :: InertCans -> Ct -> TcS Ct -- See Note [The improvement story and derived shadows] maybeEmitShadow ics ct | let ev = ctEvidence ct , CtWanted { ctev_pred = pred, ctev_loc = loc , ctev_nosh = WDeriv } <- ev , shouldSplitWD (inert_eqs ics) ct = do { traceTcS "Emit derived shadow" (ppr ct) ; let derived_ev = CtDerived { ctev_pred = pred , ctev_loc = loc } shadow_ct = ct { cc_ev = derived_ev } -- Te shadow constraint keeps the canonical shape. -- This just saves work, but is sometimes important; -- see Note [Keep CDictCan shadows as CDictCan] ; emitWork [shadow_ct] ; let ev' = ev { ctev_nosh = WOnly } ct' = ct { cc_ev = ev' } -- Record that it now has a shadow -- This is /the/ place we set the flag to WOnly ; return ct' } | otherwise = return ct shouldSplitWD :: InertEqs -> Ct -> Bool -- Precondition: 'ct' is [WD], and is inert -- True <=> we should split ct ito [W] and [D] because -- the inert_eqs can make progress on the [D] -- See Note [Splitting WD constraints] shouldSplitWD inert_eqs (CFunEqCan { cc_tyargs = tys }) = should_split_match_args inert_eqs tys -- We don't need to split if the tv is the RHS fsk shouldSplitWD inert_eqs (CDictCan { cc_tyargs = tys }) = should_split_match_args inert_eqs tys -- NB True: ignore coercions -- See Note [Splitting WD constraints] shouldSplitWD inert_eqs (CTyEqCan { cc_tyvar = tv, cc_rhs = ty , cc_eq_rel = eq_rel }) = tv `elemDVarEnv` inert_eqs || anyRewritableTyVar False eq_rel (canRewriteTv inert_eqs) ty -- NB False: do not ignore casts and coercions -- See Note [Splitting WD constraints] shouldSplitWD inert_eqs (CIrredCan { cc_ev = ev }) = anyRewritableTyVar False (ctEvEqRel ev) (canRewriteTv inert_eqs) (ctEvPred ev) shouldSplitWD _ _ = False -- No point in splitting otherwise should_split_match_args :: InertEqs -> [TcType] -> Bool -- True if the inert_eqs can rewrite anything in the argument -- types, ignoring casts and coercions should_split_match_args inert_eqs tys = any (anyRewritableTyVar True NomEq (canRewriteTv inert_eqs)) tys -- NB True: ignore casts coercions -- See Note [Splitting WD constraints] canRewriteTv :: InertEqs -> EqRel -> TyVar -> Bool canRewriteTv inert_eqs eq_rel tv | Just (ct : _) <- lookupDVarEnv inert_eqs tv , CTyEqCan { cc_eq_rel = eq_rel1 } <- ct = eq_rel1 `eqCanRewrite` eq_rel | otherwise = False isImprovable :: CtEvidence -> Bool -- See Note [Do not do improvement for WOnly] isImprovable (CtWanted { ctev_nosh = WOnly }) = False isImprovable _ = True {- ********************************************************************* * * Inert equalities * * ********************************************************************* -} addTyEq :: InertEqs -> TcTyVar -> Ct -> InertEqs addTyEq old_eqs tv ct = extendDVarEnv_C add_eq old_eqs tv [ct] where add_eq old_eqs _ | isWantedCt ct , (eq1 : eqs) <- old_eqs = eq1 : ct : eqs | otherwise = ct : old_eqs foldTyEqs :: (Ct -> b -> b) -> InertEqs -> b -> b foldTyEqs k eqs z = foldDVarEnv (\cts z -> foldr k z cts) z eqs findTyEqs :: InertCans -> TyVar -> EqualCtList findTyEqs icans tv = lookupDVarEnv (inert_eqs icans) tv `orElse` [] delTyEq :: InertEqs -> TcTyVar -> TcType -> InertEqs delTyEq m tv t = modifyDVarEnv (filter (not . isThisOne)) m tv where isThisOne (CTyEqCan { cc_rhs = t1 }) = eqType t t1 isThisOne _ = False lookupInertTyVar :: InertEqs -> TcTyVar -> Maybe TcType lookupInertTyVar ieqs tv = case lookupDVarEnv ieqs tv of Just (CTyEqCan { cc_rhs = rhs, cc_eq_rel = NomEq } : _ ) -> Just rhs _ -> Nothing {- ********************************************************************* * * Inert instances: inert_insts * * ********************************************************************* -} addInertForAll :: QCInst -> TcS () -- Add a local Given instance, typically arising from a type signature addInertForAll new_qci = updInertCans $ \ics -> ics { inert_insts = add_qci (inert_insts ics) } where add_qci :: [QCInst] -> [QCInst] -- See Note [Do not add duplicate quantified instances] add_qci qcis | any same_qci qcis = qcis | otherwise = new_qci : qcis same_qci old_qci = tcEqType (ctEvPred (qci_ev old_qci)) (ctEvPred (qci_ev new_qci)) {- Note [Do not add duplicate quantified instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this (#15244): f :: (C g, D g) => .... class S g => C g where ... class S g => D g where ... class (forall a. Eq a => Eq (g a)) => S g where ... Then in f's RHS there are two identical quantified constraints available, one via the superclasses of C and one via the superclasses of D. The two are identical, and it seems wrong to reject the program because of that. But without doing duplicate-elimination we will have two matching QCInsts when we try to solve constraints arising from f's RHS. The simplest thing is simply to eliminate duplicattes, which we do here. -} {- ********************************************************************* * * Adding an inert * * ************************************************************************ Note [Adding an equality to the InertCans] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When adding an equality to the inerts: * Split [WD] into [W] and [D] if the inerts can rewrite the latter; done by maybeEmitShadow. * Kick out any constraints that can be rewritten by the thing we are adding. Done by kickOutRewritable. * Note that unifying a:=ty, is like adding [G] a~ty; just use kickOutRewritable with Nominal, Given. See kickOutAfterUnification. Note [Kicking out CFunEqCan for fundeps] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider: New: [D] fmv1 ~ fmv2 Inert: [W] F alpha ~ fmv1 [W] F beta ~ fmv2 where F is injective. The new (derived) equality certainly can't rewrite the inerts. But we *must* kick out the first one, to get: New: [W] F alpha ~ fmv1 Inert: [W] F beta ~ fmv2 [D] fmv1 ~ fmv2 and now improvement will discover [D] alpha ~ beta. This is important; eg in #9587. So in kickOutRewritable we look at all the tyvars of the CFunEqCan, including the fsk. -} addInertCan :: Ct -> TcS () -- Constraints *other than* equalities -- Precondition: item /is/ canonical -- See Note [Adding an equality to the InertCans] addInertCan ct = do { traceTcS "insertInertCan {" $ text "Trying to insert new inert item:" <+> ppr ct ; ics <- getInertCans ; ct <- maybeEmitShadow ics ct ; ics <- maybeKickOut ics ct ; setInertCans (add_item ics ct) ; traceTcS "addInertCan }" $ empty } maybeKickOut :: InertCans -> Ct -> TcS InertCans -- For a CTyEqCan, kick out any inert that can be rewritten by the CTyEqCan maybeKickOut ics ct | CTyEqCan { cc_tyvar = tv, cc_ev = ev, cc_eq_rel = eq_rel } <- ct = do { (_, ics') <- kickOutRewritable (ctEvFlavour ev, eq_rel) tv ics ; return ics' } | otherwise = return ics add_item :: InertCans -> Ct -> InertCans add_item ics item@(CFunEqCan { cc_fun = tc, cc_tyargs = tys }) = ics { inert_funeqs = insertFunEq (inert_funeqs ics) tc tys item } add_item ics item@(CTyEqCan { cc_tyvar = tv, cc_ev = ev }) = ics { inert_eqs = addTyEq (inert_eqs ics) tv item , inert_count = bumpUnsolvedCount ev (inert_count ics) } add_item ics@(IC { inert_irreds = irreds, inert_count = count }) item@(CIrredCan { cc_ev = ev, cc_insol = insoluble }) = ics { inert_irreds = irreds `Bag.snocBag` item , inert_count = if insoluble then count -- inert_count does not include insolubles else bumpUnsolvedCount ev count } add_item ics item@(CDictCan { cc_ev = ev, cc_class = cls, cc_tyargs = tys }) = ics { inert_dicts = addDict (inert_dicts ics) cls tys item , inert_count = bumpUnsolvedCount ev (inert_count ics) } add_item _ item = pprPanic "upd_inert set: can't happen! Inserting " $ ppr item -- Can't be CNonCanonical, CHoleCan, -- because they only land in inert_irreds bumpUnsolvedCount :: CtEvidence -> Int -> Int bumpUnsolvedCount ev n | isWanted ev = n+1 | otherwise = n ----------------------------------------- kickOutRewritable :: CtFlavourRole -- Flavour/role of the equality that -- is being added to the inert set -> TcTyVar -- The new equality is tv ~ ty -> InertCans -> TcS (Int, InertCans) kickOutRewritable new_fr new_tv ics = do { let (kicked_out, ics') = kick_out_rewritable new_fr new_tv ics n_kicked = workListSize kicked_out ; unless (n_kicked == 0) $ do { updWorkListTcS (appendWorkList kicked_out) ; csTraceTcS $ hang (text "Kick out, tv =" <+> ppr new_tv) 2 (vcat [ text "n-kicked =" <+> int n_kicked , text "kicked_out =" <+> ppr kicked_out , text "Residual inerts =" <+> ppr ics' ]) } ; return (n_kicked, ics') } kick_out_rewritable :: CtFlavourRole -- Flavour/role of the equality that -- is being added to the inert set -> TcTyVar -- The new equality is tv ~ ty -> InertCans -> (WorkList, InertCans) -- See Note [kickOutRewritable] kick_out_rewritable new_fr new_tv ics@(IC { inert_eqs = tv_eqs , inert_dicts = dictmap , inert_safehask = safehask , inert_funeqs = funeqmap , inert_irreds = irreds , inert_insts = old_insts , inert_count = n }) | not (new_fr `eqMayRewriteFR` new_fr) = (emptyWorkList, ics) -- If new_fr can't rewrite itself, it can't rewrite -- anything else, so no need to kick out anything. -- (This is a common case: wanteds can't rewrite wanteds) -- Lemma (L2) in Note [Extending the inert equalities] | otherwise = (kicked_out, inert_cans_in) where inert_cans_in = IC { inert_eqs = tv_eqs_in , inert_dicts = dicts_in , inert_safehask = safehask -- ?? , inert_funeqs = feqs_in , inert_irreds = irs_in , inert_insts = insts_in , inert_count = n - workListWantedCount kicked_out } kicked_out :: WorkList -- NB: use extendWorkList to ensure that kicked-out equalities get priority -- See Note [Prioritise equalities] (Kick-out). -- The irreds may include non-canonical (hetero-kinded) equality -- constraints, which perhaps may have become soluble after new_tv -- is substituted; ditto the dictionaries, which may include (a~b) -- or (a~~b) constraints. kicked_out = foldr extendWorkListCt (emptyWorkList { wl_eqs = tv_eqs_out , wl_funeqs = feqs_out }) ((dicts_out `andCts` irs_out) `extendCtsList` insts_out) (tv_eqs_out, tv_eqs_in) = foldDVarEnv kick_out_eqs ([], emptyDVarEnv) tv_eqs (feqs_out, feqs_in) = partitionFunEqs kick_out_ct funeqmap -- See Note [Kicking out CFunEqCan for fundeps] (dicts_out, dicts_in) = partitionDicts kick_out_ct dictmap (irs_out, irs_in) = partitionBag kick_out_ct irreds -- Kick out even insolubles: See Note [Rewrite insolubles] -- Of course we must kick out irreducibles like (c a), in case -- we can rewrite 'c' to something more useful -- Kick-out for inert instances -- See Note [Quantified constraints] in TcCanonical insts_out :: [Ct] insts_in :: [QCInst] (insts_out, insts_in) | fr_may_rewrite (Given, NomEq) -- All the insts are Givens = partitionWith kick_out_qci old_insts | otherwise = ([], old_insts) kick_out_qci qci | let ev = qci_ev qci , fr_can_rewrite_ty NomEq (ctEvPred (qci_ev qci)) = Left (mkNonCanonical ev) | otherwise = Right qci (_, new_role) = new_fr fr_can_rewrite_ty :: EqRel -> Type -> Bool fr_can_rewrite_ty role ty = anyRewritableTyVar False role fr_can_rewrite_tv ty fr_can_rewrite_tv :: EqRel -> TyVar -> Bool fr_can_rewrite_tv role tv = new_role `eqCanRewrite` role && tv == new_tv fr_may_rewrite :: CtFlavourRole -> Bool fr_may_rewrite fs = new_fr `eqMayRewriteFR` fs -- Can the new item rewrite the inert item? kick_out_ct :: Ct -> Bool -- Kick it out if the new CTyEqCan can rewrite the inert one -- See Note [kickOutRewritable] kick_out_ct ct | let fs@(_,role) = ctFlavourRole ct = fr_may_rewrite fs && fr_can_rewrite_ty role (ctPred ct) -- False: ignore casts and coercions -- NB: this includes the fsk of a CFunEqCan. It can't -- actually be rewritten, but we need to kick it out -- so we get to take advantage of injectivity -- See Note [Kicking out CFunEqCan for fundeps] kick_out_eqs :: EqualCtList -> ([Ct], DTyVarEnv EqualCtList) -> ([Ct], DTyVarEnv EqualCtList) kick_out_eqs eqs (acc_out, acc_in) = (eqs_out ++ acc_out, case eqs_in of [] -> acc_in (eq1:_) -> extendDVarEnv acc_in (cc_tyvar eq1) eqs_in) where (eqs_out, eqs_in) = partition kick_out_eq eqs -- Implements criteria K1-K3 in Note [Extending the inert equalities] kick_out_eq (CTyEqCan { cc_tyvar = tv, cc_rhs = rhs_ty , cc_ev = ev, cc_eq_rel = eq_rel }) | not (fr_may_rewrite fs) = False -- Keep it in the inert set if the new thing can't rewrite it -- Below here (fr_may_rewrite fs) is True | tv == new_tv = True -- (K1) | kick_out_for_inertness = True | kick_out_for_completeness = True | otherwise = False where fs = (ctEvFlavour ev, eq_rel) kick_out_for_inertness = (fs `eqMayRewriteFR` fs) -- (K2a) && not (fs `eqMayRewriteFR` new_fr) -- (K2b) && fr_can_rewrite_ty eq_rel rhs_ty -- (K2d) -- (K2c) is guaranteed by the first guard of keep_eq kick_out_for_completeness = case eq_rel of NomEq -> rhs_ty `eqType` mkTyVarTy new_tv ReprEq -> isTyVarHead new_tv rhs_ty kick_out_eq ct = pprPanic "keep_eq" (ppr ct) kickOutAfterUnification :: TcTyVar -> TcS Int kickOutAfterUnification new_tv = do { ics <- getInertCans ; (n_kicked, ics2) <- kickOutRewritable (Given,NomEq) new_tv ics -- Given because the tv := xi is given; NomEq because -- only nominal equalities are solved by unification ; setInertCans ics2 ; return n_kicked } {- Note [kickOutRewritable] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ See also Note [inert_eqs: the inert equalities]. When we add a new inert equality (a ~N ty) to the inert set, we must kick out any inert items that could be rewritten by the new equality, to maintain the inert-set invariants. - We want to kick out an existing inert constraint if a) the new constraint can rewrite the inert one b) 'a' is free in the inert constraint (so that it *will*) rewrite it if we kick it out. For (b) we use tyCoVarsOfCt, which returns the type variables /and the kind variables/ that are directly visible in the type. Hence we will have exposed all the rewriting we care about to make the most precise kinds visible for matching classes etc. No need to kick out constraints that mention type variables whose kinds contain this variable! - A Derived equality can kick out [D] constraints in inert_eqs, inert_dicts, inert_irreds etc. - We don't kick out constraints from inert_solved_dicts, and inert_solved_funeqs optimistically. But when we lookup we have to take the substitution into account Note [Rewrite insolubles] ~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have an insoluble alpha ~ [alpha], which is insoluble because an occurs check. And then we unify alpha := [Int]. Then we really want to rewrite the insoluble to [Int] ~ [[Int]]. Now it can be decomposed. Otherwise we end up with a "Can't match [Int] ~ [[Int]]" which is true, but a bit confusing because the outer type constructors match. Similarly, if we have a CHoleCan, we'd like to rewrite it with any Givens, to give as informative an error messasge as possible (#12468, #11325). Hence: * In the main simlifier loops in TcSimplify (solveWanteds, simpl_loop), we feed the insolubles in solveSimpleWanteds, so that they get rewritten (albeit not solved). * We kick insolubles out of the inert set, if they can be rewritten (see TcSMonad.kick_out_rewritable) * We rewrite those insolubles in TcCanonical. See Note [Make sure that insolubles are fully rewritten] -} -------------- addInertSafehask :: InertCans -> Ct -> InertCans addInertSafehask ics item@(CDictCan { cc_class = cls, cc_tyargs = tys }) = ics { inert_safehask = addDict (inert_dicts ics) cls tys item } addInertSafehask _ item = pprPanic "addInertSafehask: can't happen! Inserting " $ ppr item insertSafeOverlapFailureTcS :: InstanceWhat -> Ct -> TcS () -- See Note [Safe Haskell Overlapping Instances Implementation] in TcSimplify insertSafeOverlapFailureTcS what item | safeOverlap what = return () | otherwise = updInertCans (\ics -> addInertSafehask ics item) getSafeOverlapFailures :: TcS Cts -- See Note [Safe Haskell Overlapping Instances Implementation] in TcSimplify getSafeOverlapFailures = do { IC { inert_safehask = safehask } <- getInertCans ; return $ foldDicts consCts safehask emptyCts } -------------- addSolvedDict :: InstanceWhat -> CtEvidence -> Class -> [Type] -> TcS () -- Conditionally add a new item in the solved set of the monad -- See Note [Solved dictionaries] addSolvedDict what item cls tys | isWanted item , instanceReturnsDictCon what = do { traceTcS "updSolvedSetTcs:" $ ppr item ; updInertTcS $ \ ics -> ics { inert_solved_dicts = addDict (inert_solved_dicts ics) cls tys item } } | otherwise = return () getSolvedDicts :: TcS (DictMap CtEvidence) getSolvedDicts = do { ics <- getTcSInerts; return (inert_solved_dicts ics) } setSolvedDicts :: DictMap CtEvidence -> TcS () setSolvedDicts solved_dicts = updInertTcS $ \ ics -> ics { inert_solved_dicts = solved_dicts } {- ********************************************************************* * * Other inert-set operations * * ********************************************************************* -} updInertTcS :: (InertSet -> InertSet) -> TcS () -- Modify the inert set with the supplied function updInertTcS upd_fn = do { is_var <- getTcSInertsRef ; wrapTcS (do { curr_inert <- TcM.readTcRef is_var ; TcM.writeTcRef is_var (upd_fn curr_inert) }) } getInertCans :: TcS InertCans getInertCans = do { inerts <- getTcSInerts; return (inert_cans inerts) } setInertCans :: InertCans -> TcS () setInertCans ics = updInertTcS $ \ inerts -> inerts { inert_cans = ics } updRetInertCans :: (InertCans -> (a, InertCans)) -> TcS a -- Modify the inert set with the supplied function updRetInertCans upd_fn = do { is_var <- getTcSInertsRef ; wrapTcS (do { inerts <- TcM.readTcRef is_var ; let (res, cans') = upd_fn (inert_cans inerts) ; TcM.writeTcRef is_var (inerts { inert_cans = cans' }) ; return res }) } updInertCans :: (InertCans -> InertCans) -> TcS () -- Modify the inert set with the supplied function updInertCans upd_fn = updInertTcS $ \ inerts -> inerts { inert_cans = upd_fn (inert_cans inerts) } updInertDicts :: (DictMap Ct -> DictMap Ct) -> TcS () -- Modify the inert set with the supplied function updInertDicts upd_fn = updInertCans $ \ ics -> ics { inert_dicts = upd_fn (inert_dicts ics) } updInertSafehask :: (DictMap Ct -> DictMap Ct) -> TcS () -- Modify the inert set with the supplied function updInertSafehask upd_fn = updInertCans $ \ ics -> ics { inert_safehask = upd_fn (inert_safehask ics) } updInertFunEqs :: (FunEqMap Ct -> FunEqMap Ct) -> TcS () -- Modify the inert set with the supplied function updInertFunEqs upd_fn = updInertCans $ \ ics -> ics { inert_funeqs = upd_fn (inert_funeqs ics) } updInertIrreds :: (Cts -> Cts) -> TcS () -- Modify the inert set with the supplied function updInertIrreds upd_fn = updInertCans $ \ ics -> ics { inert_irreds = upd_fn (inert_irreds ics) } getInertEqs :: TcS (DTyVarEnv EqualCtList) getInertEqs = do { inert <- getInertCans; return (inert_eqs inert) } getInertInsols :: TcS Cts -- Returns insoluble equality constraints -- specifically including Givens getInertInsols = do { inert <- getInertCans ; return (filterBag insolubleEqCt (inert_irreds inert)) } getInertGivens :: TcS [Ct] -- Returns the Given constraints in the inert set, -- with type functions *not* unflattened getInertGivens = do { inerts <- getInertCans ; let all_cts = foldDicts (:) (inert_dicts inerts) $ foldFunEqs (:) (inert_funeqs inerts) $ concat (dVarEnvElts (inert_eqs inerts)) ; return (filter isGivenCt all_cts) } getPendingGivenScs :: TcS [Ct] -- Find all inert Given dictionaries, or quantified constraints, -- whose cc_pend_sc flag is True -- and that belong to the current level -- Set their cc_pend_sc flag to False in the inert set, and return that Ct getPendingGivenScs = do { lvl <- getTcLevel ; updRetInertCans (get_sc_pending lvl) } get_sc_pending :: TcLevel -> InertCans -> ([Ct], InertCans) get_sc_pending this_lvl ic@(IC { inert_dicts = dicts, inert_insts = insts }) = ASSERT2( all isGivenCt sc_pending, ppr sc_pending ) -- When getPendingScDics is called, -- there are never any Wanteds in the inert set (sc_pending, ic { inert_dicts = dicts', inert_insts = insts' }) where sc_pending = sc_pend_insts ++ sc_pend_dicts sc_pend_dicts = foldDicts get_pending dicts [] dicts' = foldr add dicts sc_pend_dicts (sc_pend_insts, insts') = mapAccumL get_pending_inst [] insts get_pending :: Ct -> [Ct] -> [Ct] -- Get dicts with cc_pend_sc = True -- but flipping the flag get_pending dict dicts | Just dict' <- isPendingScDict dict , belongs_to_this_level (ctEvidence dict) = dict' : dicts | otherwise = dicts add :: Ct -> DictMap Ct -> DictMap Ct add ct@(CDictCan { cc_class = cls, cc_tyargs = tys }) dicts = addDict dicts cls tys ct add ct _ = pprPanic "getPendingScDicts" (ppr ct) get_pending_inst :: [Ct] -> QCInst -> ([Ct], QCInst) get_pending_inst cts qci@(QCI { qci_ev = ev }) | Just qci' <- isPendingScInst qci , belongs_to_this_level ev = (CQuantCan qci' : cts, qci') | otherwise = (cts, qci) belongs_to_this_level ev = ctLocLevel (ctEvLoc ev) == this_lvl -- We only want Givens from this level; see (3a) in -- Note [The superclass story] in TcCanonical getUnsolvedInerts :: TcS ( Bag Implication , Cts -- Tyvar eqs: a ~ ty , Cts -- Fun eqs: F a ~ ty , Cts ) -- All others -- Return all the unsolved [Wanted] or [Derived] constraints -- -- Post-condition: the returned simple constraints are all fully zonked -- (because they come from the inert set) -- the unsolved implics may not be getUnsolvedInerts = do { IC { inert_eqs = tv_eqs , inert_funeqs = fun_eqs , inert_irreds = irreds , inert_dicts = idicts } <- getInertCans ; let unsolved_tv_eqs = foldTyEqs add_if_unsolved tv_eqs emptyCts unsolved_fun_eqs = foldFunEqs add_if_wanted fun_eqs emptyCts unsolved_irreds = Bag.filterBag is_unsolved irreds unsolved_dicts = foldDicts add_if_unsolved idicts emptyCts unsolved_others = unsolved_irreds `unionBags` unsolved_dicts ; implics <- getWorkListImplics ; traceTcS "getUnsolvedInerts" $ vcat [ text " tv eqs =" <+> ppr unsolved_tv_eqs , text "fun eqs =" <+> ppr unsolved_fun_eqs , text "others =" <+> ppr unsolved_others , text "implics =" <+> ppr implics ] ; return ( implics, unsolved_tv_eqs, unsolved_fun_eqs, unsolved_others) } where add_if_unsolved :: Ct -> Cts -> Cts add_if_unsolved ct cts | is_unsolved ct = ct `consCts` cts | otherwise = cts is_unsolved ct = not (isGivenCt ct) -- Wanted or Derived -- For CFunEqCans we ignore the Derived ones, and keep -- only the Wanteds for flattening. The Derived ones -- share a unification variable with the corresponding -- Wanted, so we definitely don't want to participate -- in unflattening -- See Note [Type family equations] add_if_wanted ct cts | isWantedCt ct = ct `consCts` cts | otherwise = cts isInInertEqs :: DTyVarEnv EqualCtList -> TcTyVar -> TcType -> Bool -- True if (a ~N ty) is in the inert set, in either Given or Wanted isInInertEqs eqs tv rhs = case lookupDVarEnv eqs tv of Nothing -> False Just cts -> any (same_pred rhs) cts where same_pred rhs ct | CTyEqCan { cc_rhs = rhs2, cc_eq_rel = eq_rel } <- ct , NomEq <- eq_rel , rhs `eqType` rhs2 = True | otherwise = False getNoGivenEqs :: TcLevel -- TcLevel of this implication -> [TcTyVar] -- Skolems of this implication -> TcS ( Bool -- True <=> definitely no residual given equalities , Cts ) -- Insoluble equalities arising from givens -- See Note [When does an implication have given equalities?] getNoGivenEqs tclvl skol_tvs = do { inerts@(IC { inert_eqs = ieqs, inert_irreds = irreds }) <- getInertCans ; let has_given_eqs = foldr ((||) . ct_given_here) False irreds || anyDVarEnv eqs_given_here ieqs insols = filterBag insolubleEqCt irreds -- Specifically includes ones that originated in some -- outer context but were refined to an insoluble by -- a local equality; so do /not/ add ct_given_here. ; traceTcS "getNoGivenEqs" $ vcat [ if has_given_eqs then text "May have given equalities" else text "No given equalities" , text "Skols:" <+> ppr skol_tvs , text "Inerts:" <+> ppr inerts , text "Insols:" <+> ppr insols] ; return (not has_given_eqs, insols) } where eqs_given_here :: EqualCtList -> Bool eqs_given_here [ct@(CTyEqCan { cc_tyvar = tv })] -- Givens are always a sigleton = not (skolem_bound_here tv) && ct_given_here ct eqs_given_here _ = False ct_given_here :: Ct -> Bool -- True for a Given bound by the current implication, -- i.e. the current level ct_given_here ct = isGiven ev && tclvl == ctLocLevel (ctEvLoc ev) where ev = ctEvidence ct skol_tv_set = mkVarSet skol_tvs skolem_bound_here tv -- See Note [Let-bound skolems] = case tcTyVarDetails tv of SkolemTv {} -> tv `elemVarSet` skol_tv_set _ -> False -- | Returns Given constraints that might, -- potentially, match the given pred. This is used when checking to see if a -- Given might overlap with an instance. See Note [Instance and Given overlap] -- in TcInteract. matchableGivens :: CtLoc -> PredType -> InertSet -> Cts matchableGivens loc_w pred_w (IS { inert_cans = inert_cans }) = filterBag matchable_given all_relevant_givens where -- just look in class constraints and irreds. matchableGivens does get called -- for ~R constraints, but we don't need to look through equalities, because -- canonical equalities are used for rewriting. We'll only get caught by -- non-canonical -- that is, irreducible -- equalities. all_relevant_givens :: Cts all_relevant_givens | Just (clas, _) <- getClassPredTys_maybe pred_w = findDictsByClass (inert_dicts inert_cans) clas `unionBags` inert_irreds inert_cans | otherwise = inert_irreds inert_cans matchable_given :: Ct -> Bool matchable_given ct | CtGiven { ctev_loc = loc_g, ctev_pred = pred_g } <- ctEvidence ct = mightMatchLater pred_g loc_g pred_w loc_w | otherwise = False mightMatchLater :: TcPredType -> CtLoc -> TcPredType -> CtLoc -> Bool mightMatchLater given_pred given_loc wanted_pred wanted_loc = not (prohibitedSuperClassSolve given_loc wanted_loc) && isJust (tcUnifyTys bind_meta_tv [given_pred] [wanted_pred]) where bind_meta_tv :: TcTyVar -> BindFlag -- Any meta tyvar may be unified later, so we treat it as -- bindable when unifying with givens. That ensures that we -- conservatively assume that a meta tyvar might get unified with -- something that matches the 'given', until demonstrated -- otherwise. More info in Note [Instance and Given overlap] -- in TcInteract bind_meta_tv tv | isMetaTyVar tv , not (isFskTyVar tv) = BindMe | otherwise = Skolem prohibitedSuperClassSolve :: CtLoc -> CtLoc -> Bool -- See Note [Solving superclass constraints] in TcInstDcls prohibitedSuperClassSolve from_loc solve_loc | GivenOrigin (InstSC given_size) <- ctLocOrigin from_loc , ScOrigin wanted_size <- ctLocOrigin solve_loc = given_size >= wanted_size | otherwise = False {- Note [Unsolved Derived equalities] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In getUnsolvedInerts, we return a derived equality from the inert_eqs because it is a candidate for floating out of this implication. We only float equalities with a meta-tyvar on the left, so we only pull those out here. Note [When does an implication have given equalities?] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider an implication beta => alpha ~ Int where beta is a unification variable that has already been unified to () in an outer scope. Then we can float the (alpha ~ Int) out just fine. So when deciding whether the givens contain an equality, we should canonicalise first, rather than just looking at the original givens (#8644). So we simply look at the inert, canonical Givens and see if there are any equalities among them, the calculation of has_given_eqs. There are some wrinkles: * We must know which ones are bound in *this* implication and which are bound further out. We can find that out from the TcLevel of the Given, which is itself recorded in the tcl_tclvl field of the TcLclEnv stored in the Given (ev_given_here). What about interactions between inner and outer givens? - Outer given is rewritten by an inner given, then there must have been an inner given equality, hence the “given-eq” flag will be true anyway. - Inner given rewritten by outer, retains its level (ie. The inner one) * We must take account of *potential* equalities, like the one above: beta => ...blah... If we still don't know what beta is, we conservatively treat it as potentially becoming an equality. Hence including 'irreds' in the calculation or has_given_eqs. * When flattening givens, we generate Given equalities like <F [a]> : F [a] ~ f, with Refl evidence, and we *don't* want those to count as an equality in the givens! After all, the entire flattening business is just an internal matter, and the evidence does not mention any of the 'givens' of this implication. So we do not treat inert_funeqs as a 'given equality'. * See Note [Let-bound skolems] for another wrinkle * We do *not* need to worry about representational equalities, because these do not affect the ability to float constraints. Note [Let-bound skolems] ~~~~~~~~~~~~~~~~~~~~~~~~ If * the inert set contains a canonical Given CTyEqCan (a ~ ty) and * 'a' is a skolem bound in this very implication, then: a) The Given is pretty much a let-binding, like f :: (a ~ b->c) => a -> a Here the equality constraint is like saying let a = b->c in ... It is not adding any new, local equality information, and hence can be ignored by has_given_eqs b) 'a' will have been completely substituted out in the inert set, so we can safely discard it. Notably, it doesn't need to be returned as part of 'fsks' For an example, see #9211. See also TcUnify Note [Deeper level on the left] for how we ensure that the right variable is on the left of the equality when both are tyvars. You might wonder whether the skokem really needs to be bound "in the very same implication" as the equuality constraint. (c.f. #15009) Consider this: data S a where MkS :: (a ~ Int) => S a g :: forall a. S a -> a -> blah g x y = let h = \z. ( z :: Int , case x of MkS -> [y,z]) in ... From the type signature for `g`, we get `y::a` . Then when when we encounter the `\z`, we'll assign `z :: alpha[1]`, say. Next, from the body of the lambda we'll get [W] alpha[1] ~ Int -- From z::Int [W] forall[2]. (a ~ Int) => [W] alpha[1] ~ a -- From [y,z] Now, suppose we decide to float `alpha ~ a` out of the implication and then unify `alpha := a`. Now we are stuck! But if treat `alpha ~ Int` first, and unify `alpha := Int`, all is fine. But we absolutely cannot float that equality or we will get stuck. -} removeInertCts :: [Ct] -> InertCans -> InertCans -- ^ Remove inert constraints from the 'InertCans', for use when a -- typechecker plugin wishes to discard a given. removeInertCts cts icans = foldl' removeInertCt icans cts removeInertCt :: InertCans -> Ct -> InertCans removeInertCt is ct = case ct of CDictCan { cc_class = cl, cc_tyargs = tys } -> is { inert_dicts = delDict (inert_dicts is) cl tys } CFunEqCan { cc_fun = tf, cc_tyargs = tys } -> is { inert_funeqs = delFunEq (inert_funeqs is) tf tys } CTyEqCan { cc_tyvar = x, cc_rhs = ty } -> is { inert_eqs = delTyEq (inert_eqs is) x ty } CQuantCan {} -> panic "removeInertCt: CQuantCan" CIrredCan {} -> panic "removeInertCt: CIrredEvCan" CNonCanonical {} -> panic "removeInertCt: CNonCanonical" CHoleCan {} -> panic "removeInertCt: CHoleCan" lookupFlatCache :: TyCon -> [Type] -> TcS (Maybe (TcCoercion, TcType, CtFlavour)) lookupFlatCache fam_tc tys = do { IS { inert_flat_cache = flat_cache , inert_cans = IC { inert_funeqs = inert_funeqs } } <- getTcSInerts ; return (firstJusts [lookup_inerts inert_funeqs, lookup_flats flat_cache]) } where lookup_inerts inert_funeqs | Just (CFunEqCan { cc_ev = ctev, cc_fsk = fsk, cc_tyargs = xis }) <- findFunEq inert_funeqs fam_tc tys , tys `eqTypes` xis -- The lookup might find a near-match; see -- Note [Use loose types in inert set] = Just (ctEvCoercion ctev, mkTyVarTy fsk, ctEvFlavour ctev) | otherwise = Nothing lookup_flats flat_cache = findExactFunEq flat_cache fam_tc tys lookupInInerts :: CtLoc -> TcPredType -> TcS (Maybe CtEvidence) -- Is this exact predicate type cached in the solved or canonicals of the InertSet? lookupInInerts loc pty | ClassPred cls tys <- classifyPredType pty = do { inerts <- getTcSInerts ; return (lookupSolvedDict inerts loc cls tys `mplus` lookupInertDict (inert_cans inerts) loc cls tys) } | otherwise -- NB: No caching for equalities, IPs, holes, or errors = return Nothing -- | Look up a dictionary inert. NB: the returned 'CtEvidence' might not -- match the input exactly. Note [Use loose types in inert set]. lookupInertDict :: InertCans -> CtLoc -> Class -> [Type] -> Maybe CtEvidence lookupInertDict (IC { inert_dicts = dicts }) loc cls tys = case findDict dicts loc cls tys of Just ct -> Just (ctEvidence ct) _ -> Nothing -- | Look up a solved inert. NB: the returned 'CtEvidence' might not -- match the input exactly. See Note [Use loose types in inert set]. lookupSolvedDict :: InertSet -> CtLoc -> Class -> [Type] -> Maybe CtEvidence -- Returns just if exactly this predicate type exists in the solved. lookupSolvedDict (IS { inert_solved_dicts = solved }) loc cls tys = case findDict solved loc cls tys of Just ev -> Just ev _ -> Nothing {- ********************************************************************* * * Irreds * * ********************************************************************* -} foldIrreds :: (Ct -> b -> b) -> Cts -> b -> b foldIrreds k irreds z = foldr k z irreds {- ********************************************************************* * * TcAppMap * * ************************************************************************ Note [Use loose types in inert set] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Say we know (Eq (a |> c1)) and we need (Eq (a |> c2)). One is clearly solvable from the other. So, we do lookup in the inert set using loose types, which omit the kind-check. We must be careful when using the result of a lookup because it may not match the requested info exactly! -} type TcAppMap a = UniqDFM (ListMap LooseTypeMap a) -- Indexed by tycon then the arg types, using "loose" matching, where -- we don't require kind equality. This allows, for example, (a |> co) -- to match (a). -- See Note [Use loose types in inert set] -- Used for types and classes; hence UniqDFM -- See Note [foldTM determinism] for why we use UniqDFM here isEmptyTcAppMap :: TcAppMap a -> Bool isEmptyTcAppMap m = isNullUDFM m emptyTcAppMap :: TcAppMap a emptyTcAppMap = emptyUDFM findTcApp :: TcAppMap a -> Unique -> [Type] -> Maybe a findTcApp m u tys = do { tys_map <- lookupUDFM m u ; lookupTM tys tys_map } delTcApp :: TcAppMap a -> Unique -> [Type] -> TcAppMap a delTcApp m cls tys = adjustUDFM (deleteTM tys) m cls insertTcApp :: TcAppMap a -> Unique -> [Type] -> a -> TcAppMap a insertTcApp m cls tys ct = alterUDFM alter_tm m cls where alter_tm mb_tm = Just (insertTM tys ct (mb_tm `orElse` emptyTM)) -- mapTcApp :: (a->b) -> TcAppMap a -> TcAppMap b -- mapTcApp f = mapUDFM (mapTM f) filterTcAppMap :: (Ct -> Bool) -> TcAppMap Ct -> TcAppMap Ct filterTcAppMap f m = mapUDFM do_tm m where do_tm tm = foldTM insert_mb tm emptyTM insert_mb ct tm | f ct = insertTM tys ct tm | otherwise = tm where tys = case ct of CFunEqCan { cc_tyargs = tys } -> tys CDictCan { cc_tyargs = tys } -> tys _ -> pprPanic "filterTcAppMap" (ppr ct) tcAppMapToBag :: TcAppMap a -> Bag a tcAppMapToBag m = foldTcAppMap consBag m emptyBag foldTcAppMap :: (a -> b -> b) -> TcAppMap a -> b -> b foldTcAppMap k m z = foldUDFM (foldTM k) z m {- ********************************************************************* * * DictMap * * ********************************************************************* -} {- Note [Tuples hiding implicit parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider f,g :: (?x::Int, C a) => a -> a f v = let ?x = 4 in g v The call to 'g' gives rise to a Wanted constraint (?x::Int, C a). We must /not/ solve this from the Given (?x::Int, C a), because of the intervening binding for (?x::Int). #14218. We deal with this by arranging that we always fail when looking up a tuple constraint that hides an implicit parameter. Not that this applies * both to the inert_dicts (lookupInertDict) * and to the solved_dicts (looukpSolvedDict) An alternative would be not to extend these sets with such tuple constraints, but it seemed more direct to deal with the lookup. Note [Solving CallStack constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose f :: HasCallStack => blah. Then * Each call to 'f' gives rise to [W] s1 :: IP "callStack" CallStack -- CtOrigin = OccurrenceOf f with a CtOrigin that says "OccurrenceOf f". Remember that HasCallStack is just shorthand for IP "callStack CallStack See Note [Overview of implicit CallStacks] in TcEvidence * We cannonicalise such constraints, in TcCanonical.canClassNC, by pushing the call-site info on the stack, and changing the CtOrigin to record that has been done. Bind: s1 = pushCallStack <site-info> s2 [W] s2 :: IP "callStack" CallStack -- CtOrigin = IPOccOrigin * Then, and only then, we can solve the constraint from an enclosing Given. So we must be careful /not/ to solve 's1' from the Givens. Again, we ensure this by arranging that findDict always misses when looking up souch constraints. -} type DictMap a = TcAppMap a emptyDictMap :: DictMap a emptyDictMap = emptyTcAppMap findDict :: DictMap a -> CtLoc -> Class -> [Type] -> Maybe a findDict m loc cls tys | isCTupleClass cls , any hasIPPred tys -- See Note [Tuples hiding implicit parameters] = Nothing | Just {} <- isCallStackPred cls tys , OccurrenceOf {} <- ctLocOrigin loc = Nothing -- See Note [Solving CallStack constraints] | otherwise = findTcApp m (getUnique cls) tys findDictsByClass :: DictMap a -> Class -> Bag a findDictsByClass m cls | Just tm <- lookupUDFM m cls = foldTM consBag tm emptyBag | otherwise = emptyBag delDict :: DictMap a -> Class -> [Type] -> DictMap a delDict m cls tys = delTcApp m (getUnique cls) tys addDict :: DictMap a -> Class -> [Type] -> a -> DictMap a addDict m cls tys item = insertTcApp m (getUnique cls) tys item addDictsByClass :: DictMap Ct -> Class -> Bag Ct -> DictMap Ct addDictsByClass m cls items = addToUDFM m cls (foldr add emptyTM items) where add ct@(CDictCan { cc_tyargs = tys }) tm = insertTM tys ct tm add ct _ = pprPanic "addDictsByClass" (ppr ct) filterDicts :: (Ct -> Bool) -> DictMap Ct -> DictMap Ct filterDicts f m = filterTcAppMap f m partitionDicts :: (Ct -> Bool) -> DictMap Ct -> (Bag Ct, DictMap Ct) partitionDicts f m = foldTcAppMap k m (emptyBag, emptyDicts) where k ct (yeses, noes) | f ct = (ct `consBag` yeses, noes) | otherwise = (yeses, add ct noes) add ct@(CDictCan { cc_class = cls, cc_tyargs = tys }) m = addDict m cls tys ct add ct _ = pprPanic "partitionDicts" (ppr ct) dictsToBag :: DictMap a -> Bag a dictsToBag = tcAppMapToBag foldDicts :: (a -> b -> b) -> DictMap a -> b -> b foldDicts = foldTcAppMap emptyDicts :: DictMap a emptyDicts = emptyTcAppMap {- ********************************************************************* * * FunEqMap * * ********************************************************************* -} type FunEqMap a = TcAppMap a -- A map whose key is a (TyCon, [Type]) pair emptyFunEqs :: TcAppMap a emptyFunEqs = emptyTcAppMap findFunEq :: FunEqMap a -> TyCon -> [Type] -> Maybe a findFunEq m tc tys = findTcApp m (getUnique tc) tys funEqsToBag :: FunEqMap a -> Bag a funEqsToBag m = foldTcAppMap consBag m emptyBag findFunEqsByTyCon :: FunEqMap a -> TyCon -> [a] -- Get inert function equation constraints that have the given tycon -- in their head. Not that the constraints remain in the inert set. -- We use this to check for derived interactions with built-in type-function -- constructors. findFunEqsByTyCon m tc | Just tm <- lookupUDFM m tc = foldTM (:) tm [] | otherwise = [] foldFunEqs :: (a -> b -> b) -> FunEqMap a -> b -> b foldFunEqs = foldTcAppMap -- mapFunEqs :: (a -> b) -> FunEqMap a -> FunEqMap b -- mapFunEqs = mapTcApp -- filterFunEqs :: (Ct -> Bool) -> FunEqMap Ct -> FunEqMap Ct -- filterFunEqs = filterTcAppMap insertFunEq :: FunEqMap a -> TyCon -> [Type] -> a -> FunEqMap a insertFunEq m tc tys val = insertTcApp m (getUnique tc) tys val partitionFunEqs :: (Ct -> Bool) -> FunEqMap Ct -> ([Ct], FunEqMap Ct) -- Optimise for the case where the predicate is false -- partitionFunEqs is called only from kick-out, and kick-out usually -- kicks out very few equalities, so we want to optimise for that case partitionFunEqs f m = (yeses, foldr del m yeses) where yeses = foldTcAppMap k m [] k ct yeses | f ct = ct : yeses | otherwise = yeses del (CFunEqCan { cc_fun = tc, cc_tyargs = tys }) m = delFunEq m tc tys del ct _ = pprPanic "partitionFunEqs" (ppr ct) delFunEq :: FunEqMap a -> TyCon -> [Type] -> FunEqMap a delFunEq m tc tys = delTcApp m (getUnique tc) tys ------------------------------ type ExactFunEqMap a = UniqFM (ListMap TypeMap a) emptyExactFunEqs :: ExactFunEqMap a emptyExactFunEqs = emptyUFM findExactFunEq :: ExactFunEqMap a -> TyCon -> [Type] -> Maybe a findExactFunEq m tc tys = do { tys_map <- lookupUFM m (getUnique tc) ; lookupTM tys tys_map } insertExactFunEq :: ExactFunEqMap a -> TyCon -> [Type] -> a -> ExactFunEqMap a insertExactFunEq m tc tys val = alterUFM alter_tm m (getUnique tc) where alter_tm mb_tm = Just (insertTM tys val (mb_tm `orElse` emptyTM)) {- ************************************************************************ * * * The TcS solver monad * * * ************************************************************************ Note [The TcS monad] ~~~~~~~~~~~~~~~~~~~~ The TcS monad is a weak form of the main Tc monad All you can do is * fail * allocate new variables * fill in evidence variables Filling in a dictionary evidence variable means to create a binding for it, so TcS carries a mutable location where the binding can be added. This is initialised from the innermost implication constraint. -} data TcSEnv = TcSEnv { tcs_ev_binds :: EvBindsVar, tcs_unified :: IORef Int, -- The number of unification variables we have filled -- The important thing is whether it is non-zero tcs_count :: IORef Int, -- Global step count tcs_inerts :: IORef InertSet, -- Current inert set -- The main work-list and the flattening worklist -- See Note [Work list priorities] and tcs_worklist :: IORef WorkList -- Current worklist } --------------- newtype TcS a = TcS { unTcS :: TcSEnv -> TcM a } deriving (Functor) instance Applicative TcS where pure x = TcS (\_ -> return x) (<*>) = ap instance Monad TcS where #if !MIN_VERSION_base(4,13,0) fail = MonadFail.fail #endif m >>= k = TcS (\ebs -> unTcS m ebs >>= \r -> unTcS (k r) ebs) instance MonadFail.MonadFail TcS where fail err = TcS (\_ -> fail err) instance MonadUnique TcS where getUniqueSupplyM = wrapTcS getUniqueSupplyM instance HasModule TcS where getModule = wrapTcS getModule instance MonadThings TcS where lookupThing n = wrapTcS (lookupThing n) -- Basic functionality -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ wrapTcS :: TcM a -> TcS a -- Do not export wrapTcS, because it promotes an arbitrary TcM to TcS, -- and TcS is supposed to have limited functionality wrapTcS = TcS . const -- a TcM action will not use the TcEvBinds wrapErrTcS :: TcM a -> TcS a -- The thing wrapped should just fail -- There's no static check; it's up to the user -- Having a variant for each error message is too painful wrapErrTcS = wrapTcS wrapWarnTcS :: TcM a -> TcS a -- The thing wrapped should just add a warning, or no-op -- There's no static check; it's up to the user wrapWarnTcS = wrapTcS failTcS, panicTcS :: SDoc -> TcS a warnTcS :: WarningFlag -> SDoc -> TcS () addErrTcS :: SDoc -> TcS () failTcS = wrapTcS . TcM.failWith warnTcS flag = wrapTcS . TcM.addWarn (Reason flag) addErrTcS = wrapTcS . TcM.addErr panicTcS doc = pprPanic "TcCanonical" doc traceTcS :: String -> SDoc -> TcS () traceTcS herald doc = wrapTcS (TcM.traceTc herald doc) runTcPluginTcS :: TcPluginM a -> TcS a runTcPluginTcS m = wrapTcS . runTcPluginM m =<< getTcEvBindsVar instance HasDynFlags TcS where getDynFlags = wrapTcS getDynFlags getGlobalRdrEnvTcS :: TcS GlobalRdrEnv getGlobalRdrEnvTcS = wrapTcS TcM.getGlobalRdrEnv bumpStepCountTcS :: TcS () bumpStepCountTcS = TcS $ \env -> do { let ref = tcs_count env ; n <- TcM.readTcRef ref ; TcM.writeTcRef ref (n+1) } csTraceTcS :: SDoc -> TcS () csTraceTcS doc = wrapTcS $ csTraceTcM (return doc) traceFireTcS :: CtEvidence -> SDoc -> TcS () -- Dump a rule-firing trace traceFireTcS ev doc = TcS $ \env -> csTraceTcM $ do { n <- TcM.readTcRef (tcs_count env) ; tclvl <- TcM.getTcLevel ; return (hang (text "Step" <+> int n <> brackets (text "l:" <> ppr tclvl <> comma <> text "d:" <> ppr (ctLocDepth (ctEvLoc ev))) <+> doc <> colon) 4 (ppr ev)) } csTraceTcM :: TcM SDoc -> TcM () -- Constraint-solver tracing, -ddump-cs-trace csTraceTcM mk_doc = do { dflags <- getDynFlags ; when ( dopt Opt_D_dump_cs_trace dflags || dopt Opt_D_dump_tc_trace dflags ) ( do { msg <- mk_doc ; TcM.traceTcRn Opt_D_dump_cs_trace msg }) } runTcS :: TcS a -- What to run -> TcM (a, EvBindMap) runTcS tcs = do { ev_binds_var <- TcM.newTcEvBinds ; res <- runTcSWithEvBinds ev_binds_var tcs ; ev_binds <- TcM.getTcEvBindsMap ev_binds_var ; return (res, ev_binds) } -- | This variant of 'runTcS' will keep solving, even when only Deriveds -- are left around. It also doesn't return any evidence, as callers won't -- need it. runTcSDeriveds :: TcS a -> TcM a runTcSDeriveds tcs = do { ev_binds_var <- TcM.newTcEvBinds ; runTcSWithEvBinds ev_binds_var tcs } -- | This can deal only with equality constraints. runTcSEqualities :: TcS a -> TcM a runTcSEqualities thing_inside = do { ev_binds_var <- TcM.newNoTcEvBinds ; runTcSWithEvBinds ev_binds_var thing_inside } runTcSWithEvBinds :: EvBindsVar -> TcS a -> TcM a runTcSWithEvBinds ev_binds_var tcs = do { unified_var <- TcM.newTcRef 0 ; step_count <- TcM.newTcRef 0 ; inert_var <- TcM.newTcRef emptyInert ; wl_var <- TcM.newTcRef emptyWorkList ; let env = TcSEnv { tcs_ev_binds = ev_binds_var , tcs_unified = unified_var , tcs_count = step_count , tcs_inerts = inert_var , tcs_worklist = wl_var } -- Run the computation ; res <- unTcS tcs env ; count <- TcM.readTcRef step_count ; when (count > 0) $ csTraceTcM $ return (text "Constraint solver steps =" <+> int count) ; unflattenGivens inert_var #if defined(DEBUG) ; ev_binds <- TcM.getTcEvBindsMap ev_binds_var ; checkForCyclicBinds ev_binds #endif ; return res } ---------------------------- #if defined(DEBUG) checkForCyclicBinds :: EvBindMap -> TcM () checkForCyclicBinds ev_binds_map | null cycles = return () | null coercion_cycles = TcM.traceTc "Cycle in evidence binds" $ ppr cycles | otherwise = pprPanic "Cycle in coercion bindings" $ ppr coercion_cycles where ev_binds = evBindMapBinds ev_binds_map cycles :: [[EvBind]] cycles = [c | CyclicSCC c <- stronglyConnCompFromEdgedVerticesUniq edges] coercion_cycles = [c | c <- cycles, any is_co_bind c] is_co_bind (EvBind { eb_lhs = b }) = isEqPrimPred (varType b) edges :: [ Node EvVar EvBind ] edges = [ DigraphNode bind bndr (nonDetEltsUniqSet (evVarsOfTerm rhs)) | bind@(EvBind { eb_lhs = bndr, eb_rhs = rhs}) <- bagToList ev_binds ] -- It's OK to use nonDetEltsUFM here as -- stronglyConnCompFromEdgedVertices is still deterministic even -- if the edges are in nondeterministic order as explained in -- Note [Deterministic SCC] in Digraph. #endif ---------------------------- setEvBindsTcS :: EvBindsVar -> TcS a -> TcS a setEvBindsTcS ref (TcS thing_inside) = TcS $ \ env -> thing_inside (env { tcs_ev_binds = ref }) nestImplicTcS :: EvBindsVar -> TcLevel -> TcS a -> TcS a nestImplicTcS ref inner_tclvl (TcS thing_inside) = TcS $ \ TcSEnv { tcs_unified = unified_var , tcs_inerts = old_inert_var , tcs_count = count } -> do { inerts <- TcM.readTcRef old_inert_var ; let nest_inert = emptyInert { inert_cans = inert_cans inerts , inert_solved_dicts = inert_solved_dicts inerts } -- See Note [Do not inherit the flat cache] ; new_inert_var <- TcM.newTcRef nest_inert ; new_wl_var <- TcM.newTcRef emptyWorkList ; let nest_env = TcSEnv { tcs_ev_binds = ref , tcs_unified = unified_var , tcs_count = count , tcs_inerts = new_inert_var , tcs_worklist = new_wl_var } ; res <- TcM.setTcLevel inner_tclvl $ thing_inside nest_env ; unflattenGivens new_inert_var #if defined(DEBUG) -- Perform a check that the thing_inside did not cause cycles ; ev_binds <- TcM.getTcEvBindsMap ref ; checkForCyclicBinds ev_binds #endif ; return res } {- Note [Do not inherit the flat cache] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We do not want to inherit the flat cache when processing nested implications. Consider a ~ F b, forall c. b~Int => blah If we have F b ~ fsk in the flat-cache, and we push that into the nested implication, we might miss that F b can be rewritten to F Int, and hence perhpas solve it. Moreover, the fsk from outside is flattened out after solving the outer level, but and we don't do that flattening recursively. -} nestTcS :: TcS a -> TcS a -- Use the current untouchables, augmenting the current -- evidence bindings, and solved dictionaries -- But have no effect on the InertCans, or on the inert_flat_cache -- (we want to inherit the latter from processing the Givens) nestTcS (TcS thing_inside) = TcS $ \ env@(TcSEnv { tcs_inerts = inerts_var }) -> do { inerts <- TcM.readTcRef inerts_var ; new_inert_var <- TcM.newTcRef inerts ; new_wl_var <- TcM.newTcRef emptyWorkList ; let nest_env = env { tcs_inerts = new_inert_var , tcs_worklist = new_wl_var } ; res <- thing_inside nest_env ; new_inerts <- TcM.readTcRef new_inert_var -- we want to propogate the safe haskell failures ; let old_ic = inert_cans inerts new_ic = inert_cans new_inerts nxt_ic = old_ic { inert_safehask = inert_safehask new_ic } ; TcM.writeTcRef inerts_var -- See Note [Propagate the solved dictionaries] (inerts { inert_solved_dicts = inert_solved_dicts new_inerts , inert_cans = nxt_ic }) ; return res } checkTvConstraintsTcS :: SkolemInfo -> [TcTyVar] -- Skolems -> TcS (result, Cts) -> TcS result -- Just like TcUnify.checkTvConstraints, but -- - In the TcS monnad -- - The thing-inside should not put things in the work-list -- Instead, it returns the Wanted constraints it needs -- - No 'givens', and no TcEvBinds; this is type-level constraints only checkTvConstraintsTcS skol_info skol_tvs (TcS thing_inside) = TcS $ \ tcs_env -> do { let wl_panic = pprPanic "TcSMonad.buildImplication" $ ppr skol_info $$ ppr skol_tvs -- This panic checks that the thing-inside -- does not emit any work-list constraints new_tcs_env = tcs_env { tcs_worklist = wl_panic } ; (new_tclvl, (res, wanteds)) <- TcM.pushTcLevelM $ thing_inside new_tcs_env ; unless (null wanteds) $ do { ev_binds_var <- TcM.newNoTcEvBinds ; imp <- TcM.newImplication ; let wc = emptyWC { wc_simple = wanteds } imp' = imp { ic_tclvl = new_tclvl , ic_skols = skol_tvs , ic_wanted = wc , ic_binds = ev_binds_var , ic_info = skol_info } -- Add the implication to the work-list ; TcM.updTcRef (tcs_worklist tcs_env) (extendWorkListImplic (unitBag imp')) } ; return res } checkConstraintsTcS :: SkolemInfo -> [TcTyVar] -- Skolems -> [EvVar] -- Givens -> TcS (result, Cts) -> TcS (result, TcEvBinds) -- Just like checkConstraintsTcS, but -- - In the TcS monnad -- - The thing-inside should not put things in the work-list -- Instead, it returns the Wanted constraints it needs -- - I did not bother to put in the fast-path for -- empty-skols/empty-givens, or for empty-wanteds, because -- this function is used only for "quantified constraints" in -- with both tests are pretty much guaranteed to fail checkConstraintsTcS skol_info skol_tvs given (TcS thing_inside) = TcS $ \ tcs_env -> do { let wl_panic = pprPanic "TcSMonad.buildImplication" $ ppr skol_info $$ ppr skol_tvs -- This panic checks that the thing-inside -- does not emit any work-list constraints new_tcs_env = tcs_env { tcs_worklist = wl_panic } ; (new_tclvl, (res, wanteds)) <- TcM.pushTcLevelM $ thing_inside new_tcs_env ; ev_binds_var <- TcM.newTcEvBinds ; imp <- TcM.newImplication ; let wc = emptyWC { wc_simple = wanteds } imp' = imp { ic_tclvl = new_tclvl , ic_skols = skol_tvs , ic_given = given , ic_wanted = wc , ic_binds = ev_binds_var , ic_info = skol_info } -- Add the implication to the work-list ; TcM.updTcRef (tcs_worklist tcs_env) (extendWorkListImplic (unitBag imp')) ; return (res, TcEvBinds ev_binds_var) } {- Note [Propagate the solved dictionaries] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It's really quite important that nestTcS does not discard the solved dictionaries from the thing_inside. Consider Eq [a] forall b. empty => Eq [a] We solve the simple (Eq [a]), under nestTcS, and then turn our attention to the implications. It's definitely fine to use the solved dictionaries on the inner implications, and it can make a signficant performance difference if you do so. -} -- Getters and setters of TcEnv fields -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- Getter of inerts and worklist getTcSInertsRef :: TcS (IORef InertSet) getTcSInertsRef = TcS (return . tcs_inerts) getTcSWorkListRef :: TcS (IORef WorkList) getTcSWorkListRef = TcS (return . tcs_worklist) getTcSInerts :: TcS InertSet getTcSInerts = getTcSInertsRef >>= readTcRef setTcSInerts :: InertSet -> TcS () setTcSInerts ics = do { r <- getTcSInertsRef; writeTcRef r ics } getWorkListImplics :: TcS (Bag Implication) getWorkListImplics = do { wl_var <- getTcSWorkListRef ; wl_curr <- readTcRef wl_var ; return (wl_implics wl_curr) } updWorkListTcS :: (WorkList -> WorkList) -> TcS () updWorkListTcS f = do { wl_var <- getTcSWorkListRef ; updTcRef wl_var f } emitWorkNC :: [CtEvidence] -> TcS () emitWorkNC evs | null evs = return () | otherwise = emitWork (map mkNonCanonical evs) emitWork :: [Ct] -> TcS () emitWork cts = do { traceTcS "Emitting fresh work" (vcat (map ppr cts)) ; updWorkListTcS (extendWorkListCts cts) } newTcRef :: a -> TcS (TcRef a) newTcRef x = wrapTcS (TcM.newTcRef x) readTcRef :: TcRef a -> TcS a readTcRef ref = wrapTcS (TcM.readTcRef ref) writeTcRef :: TcRef a -> a -> TcS () writeTcRef ref val = wrapTcS (TcM.writeTcRef ref val) updTcRef :: TcRef a -> (a->a) -> TcS () updTcRef ref upd_fn = wrapTcS (TcM.updTcRef ref upd_fn) getTcEvBindsVar :: TcS EvBindsVar getTcEvBindsVar = TcS (return . tcs_ev_binds) getTcLevel :: TcS TcLevel getTcLevel = wrapTcS TcM.getTcLevel getTcEvTyCoVars :: EvBindsVar -> TcS TyCoVarSet getTcEvTyCoVars ev_binds_var = wrapTcS $ TcM.getTcEvTyCoVars ev_binds_var getTcEvBindsMap :: EvBindsVar -> TcS EvBindMap getTcEvBindsMap ev_binds_var = wrapTcS $ TcM.getTcEvBindsMap ev_binds_var setTcEvBindsMap :: EvBindsVar -> EvBindMap -> TcS () setTcEvBindsMap ev_binds_var binds = wrapTcS $ TcM.setTcEvBindsMap ev_binds_var binds unifyTyVar :: TcTyVar -> TcType -> TcS () -- Unify a meta-tyvar with a type -- We keep track of how many unifications have happened in tcs_unified, -- -- We should never unify the same variable twice! unifyTyVar tv ty = ASSERT2( isMetaTyVar tv, ppr tv ) TcS $ \ env -> do { TcM.traceTc "unifyTyVar" (ppr tv <+> text ":=" <+> ppr ty) ; TcM.writeMetaTyVar tv ty ; TcM.updTcRef (tcs_unified env) (+1) } reportUnifications :: TcS a -> TcS (Int, a) reportUnifications (TcS thing_inside) = TcS $ \ env -> do { inner_unified <- TcM.newTcRef 0 ; res <- thing_inside (env { tcs_unified = inner_unified }) ; n_unifs <- TcM.readTcRef inner_unified ; TcM.updTcRef (tcs_unified env) (+ n_unifs) ; return (n_unifs, res) } getDefaultInfo :: TcS ([Type], (Bool, Bool)) getDefaultInfo = wrapTcS TcM.tcGetDefaultTys -- Just get some environments needed for instance looking up and matching -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ getInstEnvs :: TcS InstEnvs getInstEnvs = wrapTcS $ TcM.tcGetInstEnvs getFamInstEnvs :: TcS (FamInstEnv, FamInstEnv) getFamInstEnvs = wrapTcS $ FamInst.tcGetFamInstEnvs getTopEnv :: TcS HscEnv getTopEnv = wrapTcS $ TcM.getTopEnv getGblEnv :: TcS TcGblEnv getGblEnv = wrapTcS $ TcM.getGblEnv getLclEnv :: TcS TcLclEnv getLclEnv = wrapTcS $ TcM.getLclEnv tcLookupClass :: Name -> TcS Class tcLookupClass c = wrapTcS $ TcM.tcLookupClass c tcLookupId :: Name -> TcS Id tcLookupId n = wrapTcS $ TcM.tcLookupId n -- Setting names as used (used in the deriving of Coercible evidence) -- Too hackish to expose it to TcS? In that case somehow extract the used -- constructors from the result of solveInteract addUsedGREs :: [GlobalRdrElt] -> TcS () addUsedGREs gres = wrapTcS $ TcM.addUsedGREs gres addUsedGRE :: Bool -> GlobalRdrElt -> TcS () addUsedGRE warn_if_deprec gre = wrapTcS $ TcM.addUsedGRE warn_if_deprec gre keepAlive :: Name -> TcS () keepAlive = wrapTcS . TcM.keepAlive -- Various smaller utilities [TODO, maybe will be absorbed in the instance matcher] -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ checkWellStagedDFun :: CtLoc -> InstanceWhat -> PredType -> TcS () -- Check that we do not try to use an instance before it is available. E.g. -- instance Eq T where ... -- f x = $( ... (\(p::T) -> p == p)... ) -- Here we can't use the equality function from the instance in the splice checkWellStagedDFun loc what pred | TopLevInstance { iw_dfun_id = dfun_id } <- what , let bind_lvl = TcM.topIdLvl dfun_id , bind_lvl > impLevel = wrapTcS $ TcM.setCtLocM loc $ do { use_stage <- TcM.getStage ; TcM.checkWellStaged pp_thing bind_lvl (thLevel use_stage) } | otherwise = return () -- Fast path for common case where pp_thing = text "instance for" <+> quotes (ppr pred) pprEq :: TcType -> TcType -> SDoc pprEq ty1 ty2 = pprParendType ty1 <+> char '~' <+> pprParendType ty2 isFilledMetaTyVar_maybe :: TcTyVar -> TcS (Maybe Type) isFilledMetaTyVar_maybe tv = wrapTcS (TcM.isFilledMetaTyVar_maybe tv) isFilledMetaTyVar :: TcTyVar -> TcS Bool isFilledMetaTyVar tv = wrapTcS (TcM.isFilledMetaTyVar tv) zonkTyCoVarsAndFV :: TcTyCoVarSet -> TcS TcTyCoVarSet zonkTyCoVarsAndFV tvs = wrapTcS (TcM.zonkTyCoVarsAndFV tvs) zonkTyCoVarsAndFVList :: [TcTyCoVar] -> TcS [TcTyCoVar] zonkTyCoVarsAndFVList tvs = wrapTcS (TcM.zonkTyCoVarsAndFVList tvs) zonkCo :: Coercion -> TcS Coercion zonkCo = wrapTcS . TcM.zonkCo zonkTcType :: TcType -> TcS TcType zonkTcType ty = wrapTcS (TcM.zonkTcType ty) zonkTcTypes :: [TcType] -> TcS [TcType] zonkTcTypes tys = wrapTcS (TcM.zonkTcTypes tys) zonkTcTyVar :: TcTyVar -> TcS TcType zonkTcTyVar tv = wrapTcS (TcM.zonkTcTyVar tv) zonkSimples :: Cts -> TcS Cts zonkSimples cts = wrapTcS (TcM.zonkSimples cts) zonkWC :: WantedConstraints -> TcS WantedConstraints zonkWC wc = wrapTcS (TcM.zonkWC wc) zonkTyCoVarKind :: TcTyCoVar -> TcS TcTyCoVar zonkTyCoVarKind tv = wrapTcS (TcM.zonkTyCoVarKind tv) {- ********************************************************************* * * * Flatten skolems * * * ********************************************************************* -} newFlattenSkolem :: CtFlavour -> CtLoc -> TyCon -> [TcType] -- F xis -> TcS (CtEvidence, Coercion, TcTyVar) -- [G/WD] x:: F xis ~ fsk newFlattenSkolem flav loc tc xis = do { stuff@(ev, co, fsk) <- new_skolem ; let fsk_ty = mkTyVarTy fsk ; extendFlatCache tc xis (co, fsk_ty, ctEvFlavour ev) ; return stuff } where fam_ty = mkTyConApp tc xis new_skolem | Given <- flav = do { fsk <- wrapTcS (TcM.newFskTyVar fam_ty) -- Extend the inert_fsks list, for use by unflattenGivens ; updInertTcS $ \is -> is { inert_fsks = (fsk, fam_ty) : inert_fsks is } -- Construct the Refl evidence ; let pred = mkPrimEqPred fam_ty (mkTyVarTy fsk) co = mkNomReflCo fam_ty ; ev <- newGivenEvVar loc (pred, evCoercion co) ; return (ev, co, fsk) } | otherwise -- Generate a [WD] for both Wanted and Derived -- See Note [No Derived CFunEqCans] = do { fmv <- wrapTcS (TcM.newFmvTyVar fam_ty) ; (ev, hole_co) <- newWantedEq loc Nominal fam_ty (mkTyVarTy fmv) ; return (ev, hole_co, fmv) } ---------------------------- unflattenGivens :: IORef InertSet -> TcM () -- Unflatten all the fsks created by flattening types in Given -- constraints. We must be sure to do this, else we end up with -- flatten-skolems buried in any residual Wanteds -- -- NB: this is the /only/ way that a fsk (MetaDetails = FlatSkolTv) -- is filled in. Nothing else does so. -- -- It's here (rather than in TcFlatten) because the Right Places -- to call it are in runTcSWithEvBinds/nestImplicTcS, where it -- is nicely paired with the creation an empty inert_fsks list. unflattenGivens inert_var = do { inerts <- TcM.readTcRef inert_var ; TcM.traceTc "unflattenGivens" (ppr (inert_fsks inerts)) ; mapM_ flatten_one (inert_fsks inerts) } where flatten_one (fsk, ty) = TcM.writeMetaTyVar fsk ty ---------------------------- extendFlatCache :: TyCon -> [Type] -> (TcCoercion, TcType, CtFlavour) -> TcS () extendFlatCache tc xi_args stuff@(_, ty, fl) | isGivenOrWDeriv fl -- Maintain the invariant that inert_flat_cache -- only has [G] and [WD] CFunEqCans = do { dflags <- getDynFlags ; when (gopt Opt_FlatCache dflags) $ do { traceTcS "extendFlatCache" (vcat [ ppr tc <+> ppr xi_args , ppr fl, ppr ty ]) -- 'co' can be bottom, in the case of derived items ; updInertTcS $ \ is@(IS { inert_flat_cache = fc }) -> is { inert_flat_cache = insertExactFunEq fc tc xi_args stuff } } } | otherwise = return () ---------------------------- unflattenFmv :: TcTyVar -> TcType -> TcS () -- Fill a flatten-meta-var, simply by unifying it. -- This does NOT count as a unification in tcs_unified. unflattenFmv tv ty = ASSERT2( isMetaTyVar tv, ppr tv ) TcS $ \ _ -> do { TcM.traceTc "unflattenFmv" (ppr tv <+> text ":=" <+> ppr ty) ; TcM.writeMetaTyVar tv ty } ---------------------------- demoteUnfilledFmv :: TcTyVar -> TcS () -- If a flatten-meta-var is still un-filled, -- turn it into an ordinary meta-var demoteUnfilledFmv fmv = wrapTcS $ do { is_filled <- TcM.isFilledMetaTyVar fmv ; unless is_filled $ do { tv_ty <- TcM.newFlexiTyVarTy (tyVarKind fmv) ; TcM.writeMetaTyVar fmv tv_ty } } ----------------------------- dischargeFunEq :: CtEvidence -> TcTyVar -> TcCoercion -> TcType -> TcS () -- (dischargeFunEq tv co ty) -- Preconditions -- - ev :: F tys ~ tv is a CFunEqCan -- - tv is a FlatMetaTv of FlatSkolTv -- - co :: F tys ~ xi -- - fmv/fsk `notElem` xi -- - fmv not filled (for Wanteds) -- - xi is flattened (and obeys Note [Almost function-free] in TcRnTypes) -- -- Then for [W] or [WD], we actually fill in the fmv: -- set fmv := xi, -- set ev := co -- kick out any inert things that are now rewritable -- -- For [D], we instead emit an equality that must ultimately hold -- [D] xi ~ fmv -- Does not evaluate 'co' if 'ev' is Derived -- -- For [G], emit this equality -- [G] (sym ev; co) :: fsk ~ xi -- See TcFlatten Note [The flattening story], -- especially "Ownership of fsk/fmv" dischargeFunEq (CtGiven { ctev_evar = old_evar, ctev_loc = loc }) fsk co xi = do { new_ev <- newGivenEvVar loc ( new_pred, evCoercion new_co ) ; emitWorkNC [new_ev] } where new_pred = mkPrimEqPred (mkTyVarTy fsk) xi new_co = mkTcSymCo (mkTcCoVarCo old_evar) `mkTcTransCo` co dischargeFunEq ev@(CtWanted { ctev_dest = dest }) fmv co xi = ASSERT2( not (fmv `elemVarSet` tyCoVarsOfType xi), ppr ev $$ ppr fmv $$ ppr xi ) do { setWantedEvTerm dest (evCoercion co) ; unflattenFmv fmv xi ; n_kicked <- kickOutAfterUnification fmv ; traceTcS "dischargeFmv" (ppr fmv <+> equals <+> ppr xi $$ pprKicked n_kicked) } dischargeFunEq (CtDerived { ctev_loc = loc }) fmv _co xi = emitNewDerivedEq loc Nominal xi (mkTyVarTy fmv) -- FunEqs are always at Nominal role pprKicked :: Int -> SDoc pprKicked 0 = empty pprKicked n = parens (int n <+> text "kicked out") {- ********************************************************************* * * * Instantiation etc. * * ********************************************************************* -} -- Instantiations -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ instDFunType :: DFunId -> [DFunInstType] -> TcS ([TcType], TcThetaType) instDFunType dfun_id inst_tys = wrapTcS $ TcM.instDFunType dfun_id inst_tys newFlexiTcSTy :: Kind -> TcS TcType newFlexiTcSTy knd = wrapTcS (TcM.newFlexiTyVarTy knd) cloneMetaTyVar :: TcTyVar -> TcS TcTyVar cloneMetaTyVar tv = wrapTcS (TcM.cloneMetaTyVar tv) instFlexi :: [TKVar] -> TcS TCvSubst instFlexi = instFlexiX emptyTCvSubst instFlexiX :: TCvSubst -> [TKVar] -> TcS TCvSubst instFlexiX subst tvs = wrapTcS (foldlM instFlexiHelper subst tvs) instFlexiHelper :: TCvSubst -> TKVar -> TcM TCvSubst instFlexiHelper subst tv = do { uniq <- TcM.newUnique ; details <- TcM.newMetaDetails TauTv ; let name = setNameUnique (tyVarName tv) uniq kind = substTyUnchecked subst (tyVarKind tv) ty' = mkTyVarTy (mkTcTyVar name kind details) ; TcM.traceTc "instFlexi" (ppr ty') ; return (extendTvSubst subst tv ty') } matchGlobalInst :: DynFlags -> Bool -- True <=> caller is the short-cut solver -- See Note [Shortcut solving: overlap] -> Class -> [Type] -> TcS TcM.ClsInstResult matchGlobalInst dflags short_cut cls tys = wrapTcS (TcM.matchGlobalInst dflags short_cut cls tys) tcInstSkolTyVarsX :: TCvSubst -> [TyVar] -> TcS (TCvSubst, [TcTyVar]) tcInstSkolTyVarsX subst tvs = wrapTcS $ TcM.tcInstSkolTyVarsX subst tvs -- Creating and setting evidence variables and CtFlavors -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ data MaybeNew = Fresh CtEvidence | Cached EvExpr isFresh :: MaybeNew -> Bool isFresh (Fresh {}) = True isFresh (Cached {}) = False freshGoals :: [MaybeNew] -> [CtEvidence] freshGoals mns = [ ctev | Fresh ctev <- mns ] getEvExpr :: MaybeNew -> EvExpr getEvExpr (Fresh ctev) = ctEvExpr ctev getEvExpr (Cached evt) = evt setEvBind :: EvBind -> TcS () setEvBind ev_bind = do { evb <- getTcEvBindsVar ; wrapTcS $ TcM.addTcEvBind evb ev_bind } -- | Mark variables as used filling a coercion hole useVars :: CoVarSet -> TcS () useVars co_vars = do { ev_binds_var <- getTcEvBindsVar ; let ref = ebv_tcvs ev_binds_var ; wrapTcS $ do { tcvs <- TcM.readTcRef ref ; let tcvs' = tcvs `unionVarSet` co_vars ; TcM.writeTcRef ref tcvs' } } -- | Equalities only setWantedEq :: TcEvDest -> Coercion -> TcS () setWantedEq (HoleDest hole) co = do { useVars (coVarsOfCo co) ; wrapTcS $ TcM.fillCoercionHole hole co } setWantedEq (EvVarDest ev) _ = pprPanic "setWantedEq" (ppr ev) -- | Good for both equalities and non-equalities setWantedEvTerm :: TcEvDest -> EvTerm -> TcS () setWantedEvTerm (HoleDest hole) tm | Just co <- evTermCoercion_maybe tm = do { useVars (coVarsOfCo co) ; wrapTcS $ TcM.fillCoercionHole hole co } | otherwise = do { let co_var = coHoleCoVar hole ; setEvBind (mkWantedEvBind co_var tm) ; wrapTcS $ TcM.fillCoercionHole hole (mkTcCoVarCo co_var) } setWantedEvTerm (EvVarDest ev_id) tm = setEvBind (mkWantedEvBind ev_id tm) setEvBindIfWanted :: CtEvidence -> EvTerm -> TcS () setEvBindIfWanted ev tm = case ev of CtWanted { ctev_dest = dest } -> setWantedEvTerm dest tm _ -> return () newTcEvBinds :: TcS EvBindsVar newTcEvBinds = wrapTcS TcM.newTcEvBinds newNoTcEvBinds :: TcS EvBindsVar newNoTcEvBinds = wrapTcS TcM.newNoTcEvBinds newEvVar :: TcPredType -> TcS EvVar newEvVar pred = wrapTcS (TcM.newEvVar pred) newGivenEvVar :: CtLoc -> (TcPredType, EvTerm) -> TcS CtEvidence -- Make a new variable of the given PredType, -- immediately bind it to the given term -- and return its CtEvidence -- See Note [Bind new Givens immediately] in Constraint newGivenEvVar loc (pred, rhs) = do { new_ev <- newBoundEvVarId pred rhs ; return (CtGiven { ctev_pred = pred, ctev_evar = new_ev, ctev_loc = loc }) } -- | Make a new 'Id' of the given type, bound (in the monad's EvBinds) to the -- given term newBoundEvVarId :: TcPredType -> EvTerm -> TcS EvVar newBoundEvVarId pred rhs = do { new_ev <- newEvVar pred ; setEvBind (mkGivenEvBind new_ev rhs) ; return new_ev } newGivenEvVars :: CtLoc -> [(TcPredType, EvTerm)] -> TcS [CtEvidence] newGivenEvVars loc pts = mapM (newGivenEvVar loc) pts emitNewWantedEq :: CtLoc -> Role -> TcType -> TcType -> TcS Coercion -- | Emit a new Wanted equality into the work-list emitNewWantedEq loc role ty1 ty2 = do { (ev, co) <- newWantedEq loc role ty1 ty2 ; updWorkListTcS (extendWorkListEq (mkNonCanonical ev)) ; return co } -- | Make a new equality CtEvidence newWantedEq :: CtLoc -> Role -> TcType -> TcType -> TcS (CtEvidence, Coercion) newWantedEq = newWantedEq_SI WDeriv newWantedEq_SI :: ShadowInfo -> CtLoc -> Role -> TcType -> TcType -> TcS (CtEvidence, Coercion) newWantedEq_SI si loc role ty1 ty2 = do { hole <- wrapTcS $ TcM.newCoercionHole pty ; traceTcS "Emitting new coercion hole" (ppr hole <+> dcolon <+> ppr pty) ; return ( CtWanted { ctev_pred = pty, ctev_dest = HoleDest hole , ctev_nosh = si , ctev_loc = loc} , mkHoleCo hole ) } where pty = mkPrimEqPredRole role ty1 ty2 -- no equalities here. Use newWantedEq instead newWantedEvVarNC :: CtLoc -> TcPredType -> TcS CtEvidence newWantedEvVarNC = newWantedEvVarNC_SI WDeriv newWantedEvVarNC_SI :: ShadowInfo -> CtLoc -> TcPredType -> TcS CtEvidence -- Don't look up in the solved/inerts; we know it's not there newWantedEvVarNC_SI si loc pty = do { new_ev <- newEvVar pty ; traceTcS "Emitting new wanted" (ppr new_ev <+> dcolon <+> ppr pty $$ pprCtLoc loc) ; return (CtWanted { ctev_pred = pty, ctev_dest = EvVarDest new_ev , ctev_nosh = si , ctev_loc = loc })} newWantedEvVar :: CtLoc -> TcPredType -> TcS MaybeNew newWantedEvVar = newWantedEvVar_SI WDeriv newWantedEvVar_SI :: ShadowInfo -> CtLoc -> TcPredType -> TcS MaybeNew -- For anything except ClassPred, this is the same as newWantedEvVarNC newWantedEvVar_SI si loc pty = do { mb_ct <- lookupInInerts loc pty ; case mb_ct of Just ctev | not (isDerived ctev) -> do { traceTcS "newWantedEvVar/cache hit" $ ppr ctev ; return $ Cached (ctEvExpr ctev) } _ -> do { ctev <- newWantedEvVarNC_SI si loc pty ; return (Fresh ctev) } } newWanted :: CtLoc -> PredType -> TcS MaybeNew -- Deals with both equalities and non equalities. Tries to look -- up non-equalities in the cache newWanted = newWanted_SI WDeriv newWanted_SI :: ShadowInfo -> CtLoc -> PredType -> TcS MaybeNew newWanted_SI si loc pty | Just (role, ty1, ty2) <- getEqPredTys_maybe pty = Fresh . fst <$> newWantedEq_SI si loc role ty1 ty2 | otherwise = newWantedEvVar_SI si loc pty -- deals with both equalities and non equalities. Doesn't do any cache lookups. newWantedNC :: CtLoc -> PredType -> TcS CtEvidence newWantedNC loc pty | Just (role, ty1, ty2) <- getEqPredTys_maybe pty = fst <$> newWantedEq loc role ty1 ty2 | otherwise = newWantedEvVarNC loc pty emitNewDeriveds :: CtLoc -> [TcPredType] -> TcS () emitNewDeriveds loc preds | null preds = return () | otherwise = do { evs <- mapM (newDerivedNC loc) preds ; traceTcS "Emitting new deriveds" (ppr evs) ; updWorkListTcS (extendWorkListDeriveds evs) } emitNewDerivedEq :: CtLoc -> Role -> TcType -> TcType -> TcS () -- Create new equality Derived and put it in the work list -- There's no caching, no lookupInInerts emitNewDerivedEq loc role ty1 ty2 = do { ev <- newDerivedNC loc (mkPrimEqPredRole role ty1 ty2) ; traceTcS "Emitting new derived equality" (ppr ev $$ pprCtLoc loc) ; updWorkListTcS (extendWorkListEq (mkNonCanonical ev)) } -- Very important: put in the wl_eqs -- See Note [Prioritise equalities] (Avoiding fundep iteration) newDerivedNC :: CtLoc -> TcPredType -> TcS CtEvidence newDerivedNC loc pred = do { -- checkReductionDepth loc pred ; return (CtDerived { ctev_pred = pred, ctev_loc = loc }) } -- --------- Check done in TcInteract.selectNewWorkItem???? --------- -- | Checks if the depth of the given location is too much. Fails if -- it's too big, with an appropriate error message. checkReductionDepth :: CtLoc -> TcType -- ^ type being reduced -> TcS () checkReductionDepth loc ty = do { dflags <- getDynFlags ; when (subGoalDepthExceeded dflags (ctLocDepth loc)) $ wrapErrTcS $ solverDepthErrorTcS loc ty } matchFam :: TyCon -> [Type] -> TcS (Maybe (CoercionN, TcType)) -- Given (F tys) return (ty, co), where co :: F tys ~N ty matchFam tycon args = wrapTcS $ matchFamTcM tycon args matchFamTcM :: TyCon -> [Type] -> TcM (Maybe (CoercionN, TcType)) -- Given (F tys) return (ty, co), where co :: F tys ~N ty matchFamTcM tycon args = do { fam_envs <- FamInst.tcGetFamInstEnvs ; let match_fam_result = reduceTyFamApp_maybe fam_envs Nominal tycon args ; TcM.traceTc "matchFamTcM" $ vcat [ text "Matching:" <+> ppr (mkTyConApp tycon args) , ppr_res match_fam_result ] ; return match_fam_result } where ppr_res Nothing = text "Match failed" ppr_res (Just (co,ty)) = hang (text "Match succeeded:") 2 (vcat [ text "Rewrites to:" <+> ppr ty , text "Coercion:" <+> ppr co ]) {- Note [Residual implications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The wl_implics in the WorkList are the residual implication constraints that are generated while solving or canonicalising the current worklist. Specifically, when canonicalising (forall a. t1 ~ forall a. t2) from which we get the implication (forall a. t1 ~ t2) See TcSMonad.deferTcSForAllEq -}