-- | Solving Class constraints CDictCan module GHC.Tc.Solver.Dict ( doTopReactDict, checkInstanceOK, matchLocalInst, chooseInstance ) where import GHC.Prelude import GHC.Tc.Errors.Types import GHC.Tc.Utils.TcType import GHC.Tc.Instance.FunDeps import GHC.Tc.Types.Evidence import GHC.Tc.Types.Constraint import GHC.Tc.Types.Origin import GHC.Tc.Solver.InertSet import GHC.Tc.Solver.Monad import GHC.Builtin.Names ( coercibleTyConKey, heqTyConKey, eqTyConKey ) import GHC.Core.Type as Type import GHC.Core.InstEnv ( DFunInstType, Coherence(..) ) import GHC.Core.Class import GHC.Core.Predicate import GHC.Core.Unify ( ruleMatchTyKiX ) import GHC.Types.Var import GHC.Types.Var.Set import GHC.Types.SrcLoc import GHC.Types.Var.Env import GHC.Types.Unique( hasKey ) import GHC.Utils.Outputable import GHC.Utils.Panic import GHC.Utils.Misc import GHC.Driver.Session import qualified GHC.LanguageExtensions as LangExt import Data.Maybe ( listToMaybe, mapMaybe ) {- ******************************************************************* * * Top-level reaction for class constraints (CDictCan) * * **********************************************************************-} doTopReactDict :: InertSet -> Ct -> TcS (StopOrContinue Ct) -- Try to use type-class instance declarations to simplify the constraint doTopReactDict inerts work_item@(CDictCan { cc_ev = ev, cc_class = cls , cc_tyargs = xis }) | isGiven ev -- Never use instances for Given constraints = continueWith work_item -- See Note [No Given/Given fundeps] | Just solved_ev <- lookupSolvedDict inerts dict_loc cls xis -- Cached = do { setEvBindIfWanted ev IsCoherent (ctEvTerm solved_ev) ; stopWith ev "Dict/Top (cached)" } | otherwise -- Wanted, but not cached = do { dflags <- getDynFlags ; lkup_res <- matchClassInst dflags inerts cls xis dict_loc ; case lkup_res of OneInst { cir_what = what } -> do { insertSafeOverlapFailureTcS what work_item ; addSolvedDict what ev cls xis ; chooseInstance ev lkup_res } _ -> -- NoInstance or NotSure -- We didn't solve it; so try functional dependencies with -- the instance environment do { doTopFundepImprovement work_item ; tryLastResortProhibitedSuperclass inerts work_item } } where dict_loc = ctEvLoc ev doTopReactDict _ w = pprPanic "doTopReactDict" (ppr w) -- | As a last resort, we TEMPORARILY allow a prohibited superclass solve, -- emitting a loud warning when doing so: we might be creating non-terminating -- evidence (as we are in T22912 for example). -- -- See Note [Migrating away from loopy superclass solving] in GHC.Tc.TyCl.Instance. tryLastResortProhibitedSuperclass :: InertSet -> Ct -> TcS (StopOrContinue Ct) tryLastResortProhibitedSuperclass inerts work_item@(CDictCan { cc_ev = ev_w, cc_class = cls, cc_tyargs = xis }) | let loc_w = ctEvLoc ev_w orig_w = ctLocOrigin loc_w , ScOrigin _ NakedSc <- orig_w -- work_item is definitely Wanted , Just ct_i <- lookupInertDict (inert_cans inerts) loc_w cls xis , let ev_i = ctEvidence ct_i , isGiven ev_i = do { setEvBindIfWanted ev_w IsCoherent (ctEvTerm ev_i) ; ctLocWarnTcS loc_w $ TcRnLoopySuperclassSolve loc_w (ctPred work_item) ; return $ Stop ev_w (text "Loopy superclass") } tryLastResortProhibitedSuperclass _ work_item = continueWith work_item chooseInstance :: CtEvidence -> ClsInstResult -> TcS (StopOrContinue Ct) chooseInstance work_item (OneInst { cir_new_theta = theta , cir_what = what , cir_mk_ev = mk_ev , cir_coherence = coherence }) = do { traceTcS "doTopReact/found instance for" $ ppr work_item ; deeper_loc <- checkInstanceOK loc what pred ; checkReductionDepth deeper_loc pred ; assertPprM (getTcEvBindsVar >>= return . not . isCoEvBindsVar) (ppr work_item) ; evc_vars <- mapM (newWanted deeper_loc (ctEvRewriters work_item)) theta ; setEvBindIfWanted work_item coherence (mk_ev (map getEvExpr evc_vars)) ; emitWorkNC (freshGoals evc_vars) ; stopWith work_item "Dict/Top (solved wanted)" } where pred = ctEvPred work_item loc = ctEvLoc work_item chooseInstance work_item lookup_res = pprPanic "chooseInstance" (ppr work_item $$ ppr lookup_res) checkInstanceOK :: CtLoc -> InstanceWhat -> TcPredType -> TcS CtLoc -- Check that it's OK to use this instance: -- (a) the use is well staged in the Template Haskell sense -- Returns the CtLoc to used for sub-goals -- Probably also want to call checkReductionDepth checkInstanceOK loc what pred = do { checkWellStagedDFun loc what pred ; return deeper_loc } where deeper_loc = zap_origin (bumpCtLocDepth loc) origin = ctLocOrigin loc zap_origin loc -- After applying an instance we can set ScOrigin to -- NotNakedSc, so that prohibitedSuperClassSolve never fires -- See Note [Solving superclass constraints] in -- GHC.Tc.TyCl.Instance, (sc1). | ScOrigin what _ <- origin = setCtLocOrigin loc (ScOrigin what NotNakedSc) | otherwise = loc matchClassInst :: DynFlags -> InertSet -> Class -> [Type] -> CtLoc -> TcS ClsInstResult matchClassInst dflags inerts clas tys loc -- First check whether there is an in-scope Given that could -- match this constraint. In that case, do not use any instance -- whether top level, or local quantified constraints. -- See Note [Instance and Given overlap] | not (xopt LangExt.IncoherentInstances dflags) , not (naturallyCoherentClass clas) , not (noMatchableGivenDicts inerts loc clas tys) = do { traceTcS "Delaying instance application" $ vcat [ text "Work item=" <+> pprClassPred clas tys ] ; return NotSure } | otherwise = do { traceTcS "matchClassInst" $ text "pred =" <+> ppr pred <+> char '{' ; local_res <- matchLocalInst pred loc ; case local_res of OneInst {} -> -- See Note [Local instances and incoherence] do { traceTcS "} matchClassInst local match" $ ppr local_res ; return local_res } NotSure -> -- In the NotSure case for local instances -- we don't want to try global instances do { traceTcS "} matchClassInst local not sure" empty ; return local_res } NoInstance -- No local instances, so try global ones -> do { global_res <- matchGlobalInst dflags False clas tys ; traceTcS "} matchClassInst global result" $ ppr global_res ; return global_res } } where pred = mkClassPred clas tys -- | If a class is "naturally coherent", then we needn't worry at all, in any -- way, about overlapping/incoherent instances. Just solve the thing! -- See Note [Naturally coherent classes] -- See also Note [The equality types story] in GHC.Builtin.Types.Prim. naturallyCoherentClass :: Class -> Bool naturallyCoherentClass cls = isCTupleClass cls || cls `hasKey` heqTyConKey || cls `hasKey` eqTyConKey || cls `hasKey` coercibleTyConKey {- Note [Instance and Given overlap] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Example, from the OutsideIn(X) paper: instance P x => Q [x] instance (x ~ y) => R y [x] wob :: forall a b. (Q [b], R b a) => a -> Int g :: forall a. Q [a] => [a] -> Int g x = wob x From 'g' we get the implication constraint: forall a. Q [a] => (Q [beta], R beta [a]) If we react (Q [beta]) with its top-level axiom, we end up with a (P beta), which we have no way of discharging. On the other hand, if we react R beta [a] with the top-level we get (beta ~ a), which is solvable and can help us rewrite (Q [beta]) to (Q [a]) which is now solvable by the given Q [a]. The partial solution is that: In matchClassInst (and thus in topReact), we return a matching instance only when there is no Given in the inerts which is unifiable to this particular dictionary. We treat any meta-tyvar as "unifiable" for this purpose, *including* untouchable ones. But not skolems like 'a' in the implication constraint above. The end effect is that, much as we do for overlapping instances, we delay choosing a class instance if there is a possibility of another instance OR a given to match our constraint later on. This fixes tickets #4981 and #5002. Other notes: * The check is done *first*, so that it also covers classes with built-in instance solving, such as - constraint tuples - natural numbers - Typeable * See also Note [What might equal later?] in GHC.Tc.Solver.InertSet. * The given-overlap problem is arguably not easy to appear in practice due to our aggressive prioritization of equality solving over other constraints, but it is possible. I've added a test case in typecheck/should-compile/GivenOverlapping.hs * Another "live" example is #10195; another is #10177. * We ignore the overlap problem if -XIncoherentInstances is in force: see #6002 for a worked-out example where this makes a difference. * Moreover notice that our goals here are different than the goals of the top-level overlapping checks. There we are interested in validating the following principle: If we inline a function f at a site where the same global instance environment is available as the instance environment at the definition site of f then we should get the same behaviour. But for the Given Overlap check our goal is just related to completeness of constraint solving. * The solution is only a partial one. Consider the above example with g :: forall a. Q [a] => [a] -> Int g x = let v = wob x in v and suppose we have -XNoMonoLocalBinds, so that we attempt to find the most general type for 'v'. When generalising v's type we'll simplify its Q [alpha] constraint, but we don't have Q [a] in the 'givens', so we will use the instance declaration after all. #11948 was a case in point. All of this is disgustingly delicate, so to discourage people from writing simplifiable class givens, we warn about signatures that contain them; see GHC.Tc.Validity Note [Simplifiable given constraints]. Note [Naturally coherent classes] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A few built-in classes are "naturally coherent". This term means that the "instance" for the class is bidirectional with its superclass(es). For example, consider (~~), which behaves as if it was defined like this: class a ~# b => a ~~ b instance a ~# b => a ~~ b (See Note [The equality types story] in GHC.Builtin.Types.Prim.) Faced with [W] t1 ~~ t2, it's always OK to reduce it to [W] t1 ~# t2, without worrying about Note [Instance and Given overlap]. Why? Because if we had [G] s1 ~~ s2, then we'd get the superclass [G] s1 ~# s2, and so the reduction of the [W] constraint does not risk losing any solutions. On the other hand, it can be fatal to /fail/ to reduce such equalities, on the grounds of Note [Instance and Given overlap], because many good things flow from [W] t1 ~# t2. The same reasoning applies to * (~~) heqTyCon * (~) eqTyCon * Coercible coercibleTyCon And less obviously to: * Tuple classes. For reasons described in GHC.Tc.Solver.Types Note [Tuples hiding implicit parameters], we may have a constraint [W] (?x::Int, C a) with an exactly-matching Given constraint. We must decompose this tuple and solve the components separately, otherwise we won't solve it at all! It is perfectly safe to decompose it, because again the superclasses invert the instance; e.g. class (c1, c2) => (% c1, c2 %) instance (c1, c2) => (% c1, c2 %) Example in #14218 Examples: T5853, T10432, T5315, T9222, T2627b, T3028b PS: the term "naturally coherent" doesn't really seem helpful. Perhaps "invertible" or something? I left it for now though. Note [Local instances and incoherence] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider f :: forall b c. (Eq b, forall a. Eq a => Eq (c a)) => c b -> Bool f x = x==x We get [W] Eq (c b), and we must use the local instance to solve it. BUT that wanted also unifies with the top-level Eq [a] instance, and Eq (Maybe a) etc. We want the local instance to "win", otherwise we can't solve the wanted at all. So we mark it as Incohherent. According to Note [Rules for instance lookup] in GHC.Core.InstEnv, that'll make it win even if there are other instances that unify. Moreover this is not a hack! The evidence for this local instance will be constructed by GHC at a call site... from the very instances that unify with it here. It is not like an incoherent user-written instance which might have utterly different behaviour. Consider f :: Eq a => blah. If we have [W] Eq a, we certainly get it from the Eq a context, without worrying that there are lots of top-level instances that unify with [W] Eq a! We'll use those instances to build evidence to pass to f. That's just the nullary case of what's happening here. -} matchLocalInst :: TcPredType -> CtLoc -> TcS ClsInstResult -- Look up the predicate in Given quantified constraints, -- which are effectively just local instance declarations. matchLocalInst pred loc = do { inerts@(IS { inert_cans = ics }) <- getTcSInerts ; case match_local_inst inerts (inert_insts ics) of { ([], []) -> do { traceTcS "No local instance for" (ppr pred) ; return NoInstance } ; (matches, unifs) -> do { matches <- mapM mk_instDFun matches ; unifs <- mapM mk_instDFun unifs -- See Note [Use only the best matching quantified constraint] ; case dominatingMatch matches of { Just (dfun_id, tys, theta) | all ((theta `impliedBySCs`) . thdOf3) unifs -> do { let result = OneInst { cir_new_theta = theta , cir_mk_ev = evDFunApp dfun_id tys , cir_coherence = IsCoherent , cir_what = LocalInstance } ; traceTcS "Best local instance found:" $ vcat [ text "pred:" <+> ppr pred , text "result:" <+> ppr result , text "matches:" <+> ppr matches , text "unifs:" <+> ppr unifs ] ; return result } ; mb_best -> do { traceTcS "Multiple local instances; not committing to any" $ vcat [ text "pred:" <+> ppr pred , text "matches:" <+> ppr matches , text "unifs:" <+> ppr unifs , text "best_match:" <+> ppr mb_best ] ; return NotSure }}}}} where pred_tv_set = tyCoVarsOfType pred mk_instDFun :: (CtEvidence, [DFunInstType]) -> TcS InstDFun mk_instDFun (ev, tys) = let dfun_id = ctEvEvId ev in do { (tys, theta) <- instDFunType (ctEvEvId ev) tys ; return (dfun_id, tys, theta) } -- Compute matching and unifying local instances match_local_inst :: InertSet -> [QCInst] -> ( [(CtEvidence, [DFunInstType])] , [(CtEvidence, [DFunInstType])] ) match_local_inst _inerts [] = ([], []) match_local_inst inerts (qci@(QCI { qci_tvs = qtvs , qci_pred = qpred , qci_ev = qev }) :qcis) | let in_scope = mkInScopeSet (qtv_set `unionVarSet` pred_tv_set) , Just tv_subst <- ruleMatchTyKiX qtv_set (mkRnEnv2 in_scope) emptyTvSubstEnv qpred pred , let match = (qev, map (lookupVarEnv tv_subst) qtvs) = (match:matches, unifs) | otherwise = assertPpr (disjointVarSet qtv_set (tyCoVarsOfType pred)) (ppr qci $$ ppr pred) -- ASSERT: unification relies on the -- quantified variables being fresh (matches, this_unif `combine` unifs) where qloc = ctEvLoc qev qtv_set = mkVarSet qtvs (matches, unifs) = match_local_inst inerts qcis this_unif | Just subst <- mightEqualLater inerts qpred qloc pred loc = Just (qev, map (lookupTyVar subst) qtvs) | otherwise = Nothing combine Nothing us = us combine (Just u) us = u : us -- | Instance dictionary function and type. type InstDFun = (DFunId, [TcType], TcThetaType) -- | Try to find a local quantified instance that dominates all others, -- i.e. which has a weaker instance context than all the others. -- -- See Note [Use only the best matching quantified constraint]. dominatingMatch :: [InstDFun] -> Maybe InstDFun dominatingMatch matches = listToMaybe $ mapMaybe (uncurry go) (holes matches) -- listToMaybe: arbitrarily pick any one context that is weaker than -- all others, e.g. so that we can handle [Eq a, Num a] vs [Num a, Eq a] -- (see test case T22223). where go :: InstDFun -> [InstDFun] -> Maybe InstDFun go this [] = Just this go this@(_,_,this_theta) ((_,_,other_theta):others) | this_theta `impliedBySCs` other_theta = go this others | otherwise = Nothing -- | Whether a collection of constraints is implied by another collection, -- according to a simple superclass check. -- -- See Note [When does a quantified instance dominate another?]. impliedBySCs :: TcThetaType -> TcThetaType -> Bool impliedBySCs c1 c2 = all in_c2 c1 where in_c2 :: TcPredType -> Bool in_c2 pred = any (pred `tcEqType`) c2_expanded c2_expanded :: [TcPredType] -- Includes all superclasses c2_expanded = [ q | p <- c2, q <- p : transSuperClasses p ] {- Note [When does a quantified instance dominate another?] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When matching local quantified instances, it's useful to be able to pick the one with the weakest precondition, e.g. if one has both [G] d1: forall a b. ( Eq a, Num b, C a b ) => D a b [G] d2: forall a . C a Int => D a Int [W] {w}: D a Int Then it makes sense to use d2 to solve w, as doing so we end up with a strictly weaker proof obligation of `C a Int`, compared to `(Eq a, Num Int, C a Int)` were we to use d1. In theory, to compute whether one context implies another, we would need to recursively invoke the constraint solver. This is expensive, so we instead do a simple check using superclasses, implemented in impliedBySCs. Examples: - [Eq a] is implied by [Ord a] - [Ord a] is not implied by [Eq a], - any context is implied by itself, - the empty context is implied by any context. Note [Use only the best matching quantified constraint] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider (#20582) the ambiguity check for (forall a. Ord (m a), forall a. Semigroup a => Eq (m a)) => m Int Because of eager expansion of given superclasses, we get [G] d1: forall a. Ord (m a) [G] d2: forall a. Eq (m a) [G] d3: forall a. Semigroup a => Eq (m a) [W] {w1}: forall a. Ord (m a) [W] {w2}: forall a. Semigroup a => Eq (m a) The first wanted is solved straightforwardly. But the second wanted matches *two* local instances: d2 and d3. Our general rule around multiple local instances is that we refuse to commit to any of them. However, that means that our type fails the ambiguity check. That's bad: the type is perfectly fine. (This actually came up in the wild, in the streamly library.) The solution is to prefer local instances which are easier to prove, meaning that they have a weaker precondition. In this case, the empty context of d2 is a weaker constraint than the "Semigroup a" context of d3, so we prefer using it when proving w2. This allows us to pass the ambiguity check here. Our criterion for solving a Wanted by matching local quantified instances is thus as follows: - There is a matching local quantified instance that dominates all others matches, in the sense of [When does a quantified instance dominate another?]. Any such match do, we pick it arbitrarily (the T22223 example below says why). - This local quantified instance also dominates all the unifiers, as we wouldn't want to commit to a single match when we might have multiple, genuinely different matches after further unification takes place. Some other examples: #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 ... Here, in f's RHS, there are two identical quantified constraints available, one via the superclasses of C and one via the superclasses of D. Given that each implies the other, we pick one arbitrarily. #22216: class Eq a class Eq a => Ord a class (forall b. Eq b => Eq (f b)) => Eq1 f class (Eq1 f, forall b. Ord b => Ord (f b)) => Ord1 f Suppose we have [G] d1: Ord1 f [G] d2: Eq a [W] {w}: Eq (f a) Superclass expansion of d1 gives us: [G] d3 : Eq1 f [G] d4 : forall b. Ord b => Ord (f b) expanding d4 and d5 gives us, respectively: [G] d5 : forall b. Eq b => Eq (f b) [G] d6 : forall b. Ord b => Eq (f b) Now we have two matching local instances that we could use when solving the Wanted. However, it's obviously silly to use d6, given that d5 provides us with as much information, with a strictly weaker precondition. So we pick d5 to solve w. If we chose d6, we would get [W] Ord a, which in this case we can't solve. #22223: [G] forall a b. (Eq a, Ord b) => C a b [G] forall a b. (Ord b, Eq a) => C a b [W] C x y Here we should be free to pick either quantified constraint, as they are equivalent up to re-ordering of the constraints in the context. See also Note [Do not add duplicate quantified instances] in GHC.Tc.Solver.Monad. Test cases: typecheck/should_compile/T20582 quantified-constraints/T15244 quantified-constraints/T22216{a,b,c,d,e} quantified-constraints/T22223 Historical note: a previous solution was to instead pick the local instance with the least superclass depth (see Note [Replacement vs keeping]), but that doesn't work for the example from #22216. ************************************************************************ * * * Functional dependencies, instantiation of equations * * ************************************************************************ When we spot an equality arising from a functional dependency, we now use that equality (a "wanted") to rewrite the work-item constraint right away. This avoids two dangers Danger 1: If we send the original constraint on down the pipeline it may react with an instance declaration, and in delicate situations (when a Given overlaps with an instance) that may produce new insoluble goals: see #4952 Danger 2: If we don't rewrite the constraint, it may re-react with the same thing later, and produce the same equality again --> termination worries. To achieve this required some refactoring of GHC.Tc.Instance.FunDeps (nicer now!). Note [FunDep and implicit parameter reactions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Currently, our story of interacting two dictionaries (or a dictionary and top-level instances) for functional dependencies, and implicit parameters, is that we simply produce new Wanted equalities. So for example class D a b | a -> b where ... Inert: d1 :g D Int Bool WorkItem: d2 :w D Int alpha We generate the extra work item cv :w alpha ~ Bool where 'cv' is currently unused. However, this new item can perhaps be spontaneously solved to become given and react with d2, discharging it in favour of a new constraint d2' thus: d2' :w D Int Bool d2 := d2' |> D Int cv Now d2' can be discharged from d1 We could be more aggressive and try to *immediately* solve the dictionary using those extra equalities. If that were the case with the same inert set and work item we might discard d2 directly: cv :w alpha ~ Bool d2 := d1 |> D Int cv But in general it's a bit painful to figure out the necessary coercion, so we just take the first approach. Here is a better example. Consider: class C a b c | a -> b And: [Given] d1 : C T Int Char [Wanted] d2 : C T beta Int In this case, it's *not even possible* to solve the wanted immediately. So we should simply output the functional dependency and add this guy [but NOT its superclasses] back in the worklist. Even worse: [Given] d1 : C T Int beta [Wanted] d2: C T beta Int Then it is solvable, but its very hard to detect this on the spot. It's exactly the same with implicit parameters, except that the "aggressive" approach would be much easier to implement. Note [Fundeps with instances, and equality orientation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This Note describes a delicate interaction that constrains the orientation of equalities. This one is about fundeps, but the /exact/ same thing arises for type-family injectivity constraints: see Note [Improvement orientation]. doTopFundepImprovement compares the constraint with all the instance declarations, to see if we can produce any equalities. E.g class C2 a b | a -> b instance C Int Bool Then the constraint (C Int ty) generates the equality [W] ty ~ Bool. There is a nasty corner in #19415 which led to the typechecker looping: class C s t b | s -> t instance ... => C (T kx x) (T ky y) Int T :: forall k. k -> Type work_item: dwrk :: C (T @ka (a::ka)) (T @kb0 (b0::kb0)) Char where kb0, b0 are unification vars ==> {doTopFundepImprovement: compare work_item with instance, generate /fresh/ unification variables kfresh0, yfresh0, emit a new Wanted, and add dwrk to inert set} Suppose we emit this new Wanted from the fundep: [W] T kb0 (b0::kb0) ~ T kfresh0 (yfresh0::kfresh0) ==> {solve that equality kb0 := kfresh0, b0 := yfresh0} Now kick out dwrk, since it mentions kb0 But now we are back to the start! Loop! NB1: This example relies on an instance that does not satisfy the coverage condition (although it may satisfy the weak coverage condition), and hence whose fundeps generate fresh unification variables. Not satisfying the coverage condition is known to lead to termination trouble, but in this case it's plain silly. NB2: In this example, the third parameter to C ensures that the instance doesn't actually match the Wanted, so we can't use it to solve the Wanted We solve the problem by (#21703): carefully orienting the new Wanted so that all the freshly-generated unification variables are on the LHS. Thus we emit [W] T kfresh0 (yfresh0::kfresh0) ~ T kb0 (b0::kb0) and /NOT/ [W] T kb0 (b0::kb0) ~ T kfresh0 (yfresh0::kfresh0) Now we'll unify kfresh0:=kb0, yfresh0:=b0, and all is well. The general idea is that we want to preferentially eliminate those freshly-generated unification variables, rather than unifying older variables, which causes kick-out etc. Keeping younger variables on the left also gives very minor improvement in the compiler performance by having less kick-outs and allocations (-0.1% on average). Indeed Historical Note [Eliminate younger unification variables] in GHC.Tc.Utils.Unify describes an earlier attempt to do so systematically, apparently now in abeyance. But this is is a delicate solution. We must take care to /preserve/ orientation during solving. Wrinkles: (W1) We start with [W] T kfresh0 (yfresh0::kfresh0) ~ T kb0 (b0::kb0) Decompose to [W] kfresh0 ~ kb0 [W] (yfresh0::kfresh0) ~ (b0::kb0) Preserve orientiation when decomposing!! (W2) Suppose we happen to tackle the second Wanted from (W1) first. Then in canEqCanLHSHetero we emit a /kind/ equality, as well as a now-homogeneous type equality [W] kco : kfresh0 ~ kb0 [W] (yfresh0::kfresh0) ~ (b0::kb0) |> (sym kco) Preserve orientation in canEqCanLHSHetero!! (Failing to preserve orientation here was the immediate cause of #21703.) (W3) There is a potential interaction with the swapping done by GHC.Tc.Utils.Unify.swapOverTyVars. We think it's fine, but it's a slight worry. See especially Note [TyVar/TyVar orientation] in that module. The trouble is that "preserving orientation" is a rather global invariant, and sometimes we definitely do want to swap (e.g. Int ~ alpha), so we don't even have a precise statement of what the invariant is. The advantage of the preserve-orientation plan is that it is extremely cheap to implement, and apparently works beautifully. --- Alternative plan (1) --- Rather than have an ill-defined invariant, another possiblity is to elminate those fresh unification variables at birth, when generating the new fundep-inspired equalities. The key idea is to call `instFlexiX` in `emitFunDepWanteds` on only those type variables that are guaranteed to give us some progress. This means we have to locally (without calling emitWanteds) identify the type variables that do not give us any progress. In the above example, we _know_ that emitting the two wanteds `kco` and `co` is fruitless. Q: How do we identify such no-ops? 1. Generate a matching substitution from LHS to RHS ɸ = [kb0 :-> k0, b0 :-> y0] 2. Call `instFlexiX` on only those type variables that do not appear in the domain of ɸ ɸ' = instFlexiX ɸ (tvs - domain ɸ) 3. Apply ɸ' on LHS and then call emitWanteds unifyWanteds ... (subst ɸ' LHS) RHS Why will this work? The matching substitution ɸ will be a best effort substitution that gives us all the easy solutions. It can be generated with modified version of `Core/Unify.unify_tys` where we run it in a matching mode and never generate `SurelyApart` and always return a `MaybeApart Subst` instead. The same alternative plan would work for type-family injectivity constraints: see Note [Improvement orientation]. --- End of Alternative plan (1) --- --- Alternative plan (2) --- We could have a new flavour of TcTyVar (like `TauTv`, `TyVarTv` etc; see GHC.Tc.Utils.TcType.MetaInfo) for the fresh unification variables introduced by functional dependencies. Say `FunDepTv`. Then in GHC.Tc.Utils.Unify.swapOverTyVars we could arrange to keep a `FunDepTv` on the left if possible. Looks possible, but it's one more complication. --- End of Alternative plan (2) --- --- Historical note: Failed Alternative Plan (3) --- Previously we used a flag `cc_fundeps` in `CDictCan`. It would flip to False once we used a fun dep to hint the solver to break and to stop emitting more wanteds. This solution was not complete, and caused a failures while trying to solve for transitive functional dependencies (test case: T21703) -- End of Historical note: Failed Alternative Plan (3) -- Note [Weird fundeps] ~~~~~~~~~~~~~~~~~~~~ Consider class Het a b | a -> b where het :: m (f c) -> a -> m b class GHet (a :: * -> *) (b :: * -> *) | a -> b instance GHet (K a) (K [a]) instance Het a b => GHet (K a) (K b) The two instances don't actually conflict on their fundeps, although it's pretty strange. So they are both accepted. Now try [W] GHet (K Int) (K Bool) This triggers fundeps from both instance decls; [W] K Bool ~ K [a] [W] K Bool ~ K beta And there's a risk of complaining about Bool ~ [a]. But in fact the Wanted matches the second instance, so we never get as far as the fundeps. #7875 is a case in point. -} doTopFundepImprovement :: Ct -> TcS () -- Try to functional-dependency improvement between the constraint -- and the top-level instance declarations -- See Note [Fundeps with instances, and equality orientation] -- See also Note [Weird fundeps] doTopFundepImprovement work_item@(CDictCan { cc_ev = ev, cc_class = cls , cc_tyargs = xis }) = do { traceTcS "try_fundeps" (ppr work_item) ; instEnvs <- getInstEnvs ; let fundep_eqns = improveFromInstEnv instEnvs mk_ct_loc cls xis ; emitFunDepWanteds (ctEvRewriters ev) fundep_eqns } where dict_pred = mkClassPred cls xis dict_loc = ctEvLoc ev dict_origin = ctLocOrigin dict_loc mk_ct_loc :: PredType -- From instance decl -> SrcSpan -- also from instance deol -> (CtLoc, RewriterSet) mk_ct_loc inst_pred inst_loc = ( dict_loc { ctl_origin = FunDepOrigin2 dict_pred dict_origin inst_pred inst_loc } , emptyRewriterSet ) doTopFundepImprovement work_item = pprPanic "doTopFundepImprovement" (ppr work_item)