{-# LANGUAGE CPP #-} module TcInteract ( solveSimpleGivens, -- Solves [Ct] solveSimpleWanteds, -- Solves Cts ) where #include "HsVersions.h" import GhcPrelude import BasicTypes ( SwapFlag(..), isSwapped, infinity, IntWithInf, intGtLimit ) import TcCanonical import TcFlatten import TcUnify( canSolveByUnification ) import VarSet import Type import InstEnv( DFunInstType ) import CoAxiom( sfInteractTop, sfInteractInert ) import Var import TcType import PrelNames ( coercibleTyConKey, heqTyConKey, eqTyConKey, ipClassKey ) import CoAxiom ( TypeEqn, CoAxiom(..), CoAxBranch(..), fromBranches ) import Class import TyCon import FunDeps import FamInst import ClsInst( InstanceWhat(..), safeOverlap ) import FamInstEnv import Unify ( tcUnifyTyWithTFs, ruleMatchTyKiX ) import TcEvidence import Outputable import TcRnTypes import TcSMonad import Bag import MonadUtils ( concatMapM, foldlM ) import CoreSyn import Data.List( partition, deleteFirstsBy ) import SrcLoc import VarEnv import Control.Monad import Maybes( isJust ) import Pair (Pair(..)) import Unique( hasKey ) import DynFlags import Util import qualified GHC.LanguageExtensions as LangExt import Control.Monad.Trans.Class import Control.Monad.Trans.Maybe {- ********************************************************************** * * * Main Interaction Solver * * * ********************************************************************** Note [Basic Simplifier Plan] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1. Pick an element from the WorkList if there exists one with depth less than our context-stack depth. 2. Run it down the 'stage' pipeline. Stages are: - canonicalization - inert reactions - spontaneous reactions - top-level intreactions Each stage returns a StopOrContinue and may have sideffected the inerts or worklist. The threading of the stages is as follows: - If (Stop) is returned by a stage then we start again from Step 1. - If (ContinueWith ct) is returned by a stage, we feed 'ct' on to the next stage in the pipeline. 4. If the element has survived (i.e. ContinueWith x) the last stage then we add him in the inerts and jump back to Step 1. If in Step 1 no such element exists, we have exceeded our context-stack depth and will simply fail. Note [Unflatten after solving the simple wanteds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We unflatten after solving the wc_simples of an implication, and before attempting to float. This means that * The fsk/fmv flatten-skolems only survive during solveSimples. We don't need to worry about them across successive passes over the constraint tree. (E.g. we don't need the old ic_fsk field of an implication. * When floating an equality outwards, we don't need to worry about floating its associated flattening constraints. * Another tricky case becomes easy: Trac #4935 type instance F True a b = a type instance F False a b = b [w] F c a b ~ gamma (c ~ True) => a ~ gamma (c ~ False) => b ~ gamma Obviously this is soluble with gamma := F c a b, and unflattening will do exactly that after solving the simple constraints and before attempting the implications. Before, when we were not unflattening, we had to push Wanted funeqs in as new givens. Yuk! Another example that becomes easy: indexed_types/should_fail/T7786 [W] BuriedUnder sub k Empty ~ fsk [W] Intersect fsk inv ~ s [w] xxx[1] ~ s [W] forall[2] . (xxx[1] ~ Empty) => Intersect (BuriedUnder sub k Empty) inv ~ Empty Note [Running plugins on unflattened wanteds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There is an annoying mismatch between solveSimpleGivens and solveSimpleWanteds, because the latter needs to fiddle with the inert set, unflatten and zonk the wanteds. It passes the zonked wanteds to runTcPluginsWanteds, which produces a replacement set of wanteds, some additional insolubles and a flag indicating whether to go round the loop again. If so, prepareInertsForImplications is used to remove the previous wanteds (which will still be in the inert set). Note that prepareInertsForImplications will discard the insolubles, so we must keep track of them separately. -} solveSimpleGivens :: [Ct] -> TcS () solveSimpleGivens :: [Ct] -> TcS () solveSimpleGivens givens :: [Ct] givens | [Ct] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [Ct] givens -- Shortcut for common case = () -> TcS () forall (m :: * -> *) a. Monad m => a -> m a return () | Bool otherwise = do { String -> SDoc -> TcS () traceTcS "solveSimpleGivens {" ([Ct] -> SDoc forall a. Outputable a => a -> SDoc ppr [Ct] givens) ; [Ct] -> TcS () go [Ct] givens ; String -> SDoc -> TcS () traceTcS "End solveSimpleGivens }" SDoc empty } where go :: [Ct] -> TcS () go givens :: [Ct] givens = do { Cts -> TcS () solveSimples ([Ct] -> Cts forall a. [a] -> Bag a listToBag [Ct] givens) ; [Ct] new_givens <- TcS [Ct] runTcPluginsGiven ; Bool -> TcS () -> TcS () forall (f :: * -> *). Applicative f => Bool -> f () -> f () when ([Ct] -> Bool forall a. [a] -> Bool notNull [Ct] new_givens) (TcS () -> TcS ()) -> TcS () -> TcS () forall a b. (a -> b) -> a -> b $ [Ct] -> TcS () go [Ct] new_givens } solveSimpleWanteds :: Cts -> TcS WantedConstraints -- NB: 'simples' may contain /derived/ equalities, floated -- out from a nested implication. So don't discard deriveds! -- The result is not necessarily zonked solveSimpleWanteds :: Cts -> TcS WantedConstraints solveSimpleWanteds simples :: Cts simples = do { String -> SDoc -> TcS () traceTcS "solveSimpleWanteds {" (Cts -> SDoc forall a. Outputable a => a -> SDoc ppr Cts simples) ; DynFlags dflags <- TcS DynFlags forall (m :: * -> *). HasDynFlags m => m DynFlags getDynFlags ; (n :: Int n,wc :: WantedConstraints wc) <- Int -> IntWithInf -> WantedConstraints -> TcS (Int, WantedConstraints) go 1 (DynFlags -> IntWithInf solverIterations DynFlags dflags) (WantedConstraints emptyWC { wc_simple :: Cts wc_simple = Cts simples }) ; String -> SDoc -> TcS () traceTcS "solveSimpleWanteds end }" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text "iterations =" SDoc -> SDoc -> SDoc <+> Int -> SDoc forall a. Outputable a => a -> SDoc ppr Int n , String -> SDoc text "residual =" SDoc -> SDoc -> SDoc <+> WantedConstraints -> SDoc forall a. Outputable a => a -> SDoc ppr WantedConstraints wc ] ; WantedConstraints -> TcS WantedConstraints forall (m :: * -> *) a. Monad m => a -> m a return WantedConstraints wc } where go :: Int -> IntWithInf -> WantedConstraints -> TcS (Int, WantedConstraints) go :: Int -> IntWithInf -> WantedConstraints -> TcS (Int, WantedConstraints) go n :: Int n limit :: IntWithInf limit wc :: WantedConstraints wc | Int n Int -> IntWithInf -> Bool `intGtLimit` IntWithInf limit = SDoc -> TcS (Int, WantedConstraints) forall a. SDoc -> TcS a failTcS (SDoc -> Int -> SDoc -> SDoc hang (String -> SDoc text "solveSimpleWanteds: too many iterations" SDoc -> SDoc -> SDoc <+> SDoc -> SDoc parens (String -> SDoc text "limit =" SDoc -> SDoc -> SDoc <+> IntWithInf -> SDoc forall a. Outputable a => a -> SDoc ppr IntWithInf limit)) 2 ([SDoc] -> SDoc vcat [ String -> SDoc text "Set limit with -fconstraint-solver-iterations=n; n=0 for no limit" , String -> SDoc text "Simples =" SDoc -> SDoc -> SDoc <+> Cts -> SDoc forall a. Outputable a => a -> SDoc ppr Cts simples , String -> SDoc text "WC =" SDoc -> SDoc -> SDoc <+> WantedConstraints -> SDoc forall a. Outputable a => a -> SDoc ppr WantedConstraints wc ])) | Cts -> Bool forall a. Bag a -> Bool isEmptyBag (WantedConstraints -> Cts wc_simple WantedConstraints wc) = (Int, WantedConstraints) -> TcS (Int, WantedConstraints) forall (m :: * -> *) a. Monad m => a -> m a return (Int n,WantedConstraints wc) | Bool otherwise = do { -- Solve (unif_count :: Int unif_count, wc1 :: WantedConstraints wc1) <- WantedConstraints -> TcS (Int, WantedConstraints) solve_simple_wanteds WantedConstraints wc -- Run plugins ; (rerun_plugin :: Bool rerun_plugin, wc2 :: WantedConstraints wc2) <- WantedConstraints -> TcS (Bool, WantedConstraints) runTcPluginsWanted WantedConstraints wc1 -- See Note [Running plugins on unflattened wanteds] ; if Int unif_count Int -> Int -> Bool forall a. Eq a => a -> a -> Bool == 0 Bool -> Bool -> Bool && Bool -> Bool not Bool rerun_plugin then (Int, WantedConstraints) -> TcS (Int, WantedConstraints) forall (m :: * -> *) a. Monad m => a -> m a return (Int n, WantedConstraints wc2) -- Done else do { String -> SDoc -> TcS () traceTcS "solveSimple going round again:" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ Int -> SDoc forall a. Outputable a => a -> SDoc ppr Int unif_count SDoc -> SDoc -> SDoc $$ Bool -> SDoc forall a. Outputable a => a -> SDoc ppr Bool rerun_plugin ; Int -> IntWithInf -> WantedConstraints -> TcS (Int, WantedConstraints) go (Int nInt -> Int -> Int forall a. Num a => a -> a -> a +1) IntWithInf limit WantedConstraints wc2 } } -- Loop solve_simple_wanteds :: WantedConstraints -> TcS (Int, WantedConstraints) -- Try solving these constraints -- Affects the unification state (of course) but not the inert set -- The result is not necessarily zonked solve_simple_wanteds :: WantedConstraints -> TcS (Int, WantedConstraints) solve_simple_wanteds (WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples1, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implics1 }) = TcS (Int, WantedConstraints) -> TcS (Int, WantedConstraints) forall a. TcS a -> TcS a nestTcS (TcS (Int, WantedConstraints) -> TcS (Int, WantedConstraints)) -> TcS (Int, WantedConstraints) -> TcS (Int, WantedConstraints) forall a b. (a -> b) -> a -> b $ do { Cts -> TcS () solveSimples Cts simples1 ; (implics2 :: Bag Implication implics2, tv_eqs :: Cts tv_eqs, fun_eqs :: Cts fun_eqs, others :: Cts others) <- TcS (Bag Implication, Cts, Cts, Cts) getUnsolvedInerts ; (unif_count :: Int unif_count, unflattened_eqs :: Cts unflattened_eqs) <- TcS Cts -> TcS (Int, Cts) forall a. TcS a -> TcS (Int, a) reportUnifications (TcS Cts -> TcS (Int, Cts)) -> TcS Cts -> TcS (Int, Cts) forall a b. (a -> b) -> a -> b $ Cts -> Cts -> TcS Cts unflattenWanteds Cts tv_eqs Cts fun_eqs -- See Note [Unflatten after solving the simple wanteds] ; (Int, WantedConstraints) -> TcS (Int, WantedConstraints) forall (m :: * -> *) a. Monad m => a -> m a return ( Int unif_count , WC :: Cts -> Bag Implication -> WantedConstraints WC { wc_simple :: Cts wc_simple = Cts others Cts -> Cts -> Cts `andCts` Cts unflattened_eqs , wc_impl :: Bag Implication wc_impl = Bag Implication implics1 Bag Implication -> Bag Implication -> Bag Implication forall a. Bag a -> Bag a -> Bag a `unionBags` Bag Implication implics2 }) } {- Note [The solveSimpleWanteds loop] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Solving a bunch of simple constraints is done in a loop, (the 'go' loop of 'solveSimpleWanteds'): 1. Try to solve them; unflattening may lead to improvement that was not exploitable during solving 2. Try the plugin 3. If step 1 did improvement during unflattening; or if the plugin wants to run again, go back to step 1 Non-obviously, improvement can also take place during the unflattening that takes place in step (1). See TcFlatten, See Note [Unflattening can force the solver to iterate] -} -- The main solver loop implements Note [Basic Simplifier Plan] --------------------------------------------------------------- solveSimples :: Cts -> TcS () -- Returns the final InertSet in TcS -- Has no effect on work-list or residual-implications -- The constraints are initially examined in left-to-right order solveSimples :: Cts -> TcS () solveSimples cts :: Cts cts = {-# SCC "solveSimples" #-} do { (WorkList -> WorkList) -> TcS () updWorkListTcS (\wl :: WorkList wl -> (Ct -> WorkList -> WorkList) -> WorkList -> Cts -> WorkList forall a r. (a -> r -> r) -> r -> Bag a -> r foldrBag Ct -> WorkList -> WorkList extendWorkListCt WorkList wl Cts cts) ; TcS () solve_loop } where solve_loop :: TcS () solve_loop = {-# SCC "solve_loop" #-} do { Maybe Ct sel <- TcS (Maybe Ct) selectNextWorkItem ; case Maybe Ct sel of Nothing -> () -> TcS () forall (m :: * -> *) a. Monad m => a -> m a return () Just ct :: Ct ct -> do { [(String, SimplifierStage)] -> Ct -> TcS () runSolverPipeline [(String, SimplifierStage)] thePipeline Ct ct ; TcS () solve_loop } } -- | Extract the (inert) givens and invoke the plugins on them. -- Remove solved givens from the inert set and emit insolubles, but -- return new work produced so that 'solveSimpleGivens' can feed it back -- into the main solver. runTcPluginsGiven :: TcS [Ct] runTcPluginsGiven :: TcS [Ct] runTcPluginsGiven = do { [TcPluginSolver] plugins <- TcS [TcPluginSolver] getTcPlugins ; if [TcPluginSolver] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [TcPluginSolver] plugins then [Ct] -> TcS [Ct] forall (m :: * -> *) a. Monad m => a -> m a return [] else do { [Ct] givens <- TcS [Ct] getInertGivens ; if [Ct] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [Ct] givens then [Ct] -> TcS [Ct] forall (m :: * -> *) a. Monad m => a -> m a return [] else do { TcPluginProgress p <- [TcPluginSolver] -> SplitCts -> TcS TcPluginProgress runTcPlugins [TcPluginSolver] plugins ([Ct] givens,[],[]) ; let (solved_givens :: [Ct] solved_givens, _, _) = TcPluginProgress -> ([Ct], [Ct], [(EvTerm, Ct)]) pluginSolvedCts TcPluginProgress p insols :: [Ct] insols = TcPluginProgress -> [Ct] pluginBadCts TcPluginProgress p ; (InertCans -> InertCans) -> TcS () updInertCans ([Ct] -> InertCans -> InertCans removeInertCts [Ct] solved_givens) ; (Cts -> Cts) -> TcS () updInertIrreds (\irreds :: Cts irreds -> Cts -> [Ct] -> Cts extendCtsList Cts irreds [Ct] insols) ; [Ct] -> TcS [Ct] forall (m :: * -> *) a. Monad m => a -> m a return (TcPluginProgress -> [Ct] pluginNewCts TcPluginProgress p) } } } -- | Given a bag of (flattened, zonked) wanteds, invoke the plugins on -- them and produce an updated bag of wanteds (possibly with some new -- work) and a bag of insolubles. The boolean indicates whether -- 'solveSimpleWanteds' should feed the updated wanteds back into the -- main solver. runTcPluginsWanted :: WantedConstraints -> TcS (Bool, WantedConstraints) runTcPluginsWanted :: WantedConstraints -> TcS (Bool, WantedConstraints) runTcPluginsWanted wc :: WantedConstraints wc@(WC { wc_simple :: WantedConstraints -> Cts wc_simple = Cts simples1, wc_impl :: WantedConstraints -> Bag Implication wc_impl = Bag Implication implics1 }) | Cts -> Bool forall a. Bag a -> Bool isEmptyBag Cts simples1 = (Bool, WantedConstraints) -> TcS (Bool, WantedConstraints) forall (m :: * -> *) a. Monad m => a -> m a return (Bool False, WantedConstraints wc) | Bool otherwise = do { [TcPluginSolver] plugins <- TcS [TcPluginSolver] getTcPlugins ; if [TcPluginSolver] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [TcPluginSolver] plugins then (Bool, WantedConstraints) -> TcS (Bool, WantedConstraints) forall (m :: * -> *) a. Monad m => a -> m a return (Bool False, WantedConstraints wc) else do { [Ct] given <- TcS [Ct] getInertGivens ; Cts simples1 <- Cts -> TcS Cts zonkSimples Cts simples1 -- Plugin requires zonked inputs ; let (wanted :: [Ct] wanted, derived :: [Ct] derived) = (Ct -> Bool) -> [Ct] -> ([Ct], [Ct]) forall a. (a -> Bool) -> [a] -> ([a], [a]) partition Ct -> Bool isWantedCt (Cts -> [Ct] forall a. Bag a -> [a] bagToList Cts simples1) ; TcPluginProgress p <- [TcPluginSolver] -> SplitCts -> TcS TcPluginProgress runTcPlugins [TcPluginSolver] plugins ([Ct] given, [Ct] derived, [Ct] wanted) ; let (_, _, solved_wanted :: [(EvTerm, Ct)] solved_wanted) = TcPluginProgress -> ([Ct], [Ct], [(EvTerm, Ct)]) pluginSolvedCts TcPluginProgress p (_, unsolved_derived :: [Ct] unsolved_derived, unsolved_wanted :: [Ct] unsolved_wanted) = TcPluginProgress -> SplitCts pluginInputCts TcPluginProgress p new_wanted :: [Ct] new_wanted = TcPluginProgress -> [Ct] pluginNewCts TcPluginProgress p insols :: [Ct] insols = TcPluginProgress -> [Ct] pluginBadCts TcPluginProgress p -- SLPJ: I'm deeply suspicious of this -- ; updInertCans (removeInertCts $ solved_givens ++ solved_deriveds) ; ((EvTerm, Ct) -> TcS ()) -> [(EvTerm, Ct)] -> TcS () forall (t :: * -> *) (m :: * -> *) a b. (Foldable t, Monad m) => (a -> m b) -> t a -> m () mapM_ (EvTerm, Ct) -> TcS () setEv [(EvTerm, Ct)] solved_wanted ; (Bool, WantedConstraints) -> TcS (Bool, WantedConstraints) forall (m :: * -> *) a. Monad m => a -> m a return ( [Ct] -> Bool forall a. [a] -> Bool notNull (TcPluginProgress -> [Ct] pluginNewCts TcPluginProgress p) , WC :: Cts -> Bag Implication -> WantedConstraints WC { wc_simple :: Cts wc_simple = [Ct] -> Cts forall a. [a] -> Bag a listToBag [Ct] new_wanted Cts -> Cts -> Cts `andCts` [Ct] -> Cts forall a. [a] -> Bag a listToBag [Ct] unsolved_wanted Cts -> Cts -> Cts `andCts` [Ct] -> Cts forall a. [a] -> Bag a listToBag [Ct] unsolved_derived Cts -> Cts -> Cts `andCts` [Ct] -> Cts forall a. [a] -> Bag a listToBag [Ct] insols , wc_impl :: Bag Implication wc_impl = Bag Implication implics1 } ) } } where setEv :: (EvTerm,Ct) -> TcS () setEv :: (EvTerm, Ct) -> TcS () setEv (ev :: EvTerm ev,ct :: Ct ct) = case Ct -> CtEvidence ctEvidence Ct ct of CtWanted { ctev_dest :: CtEvidence -> TcEvDest ctev_dest = TcEvDest dest } -> TcEvDest -> EvTerm -> TcS () setWantedEvTerm TcEvDest dest EvTerm ev _ -> String -> TcS () forall a. String -> a panic "runTcPluginsWanted.setEv: attempt to solve non-wanted!" -- | A triple of (given, derived, wanted) constraints to pass to plugins type SplitCts = ([Ct], [Ct], [Ct]) -- | A solved triple of constraints, with evidence for wanteds type SolvedCts = ([Ct], [Ct], [(EvTerm,Ct)]) -- | Represents collections of constraints generated by typechecker -- plugins data TcPluginProgress = TcPluginProgress { TcPluginProgress -> SplitCts pluginInputCts :: SplitCts -- ^ Original inputs to the plugins with solved/bad constraints -- removed, but otherwise unmodified , TcPluginProgress -> ([Ct], [Ct], [(EvTerm, Ct)]) pluginSolvedCts :: SolvedCts -- ^ Constraints solved by plugins , TcPluginProgress -> [Ct] pluginBadCts :: [Ct] -- ^ Constraints reported as insoluble by plugins , TcPluginProgress -> [Ct] pluginNewCts :: [Ct] -- ^ New constraints emitted by plugins } getTcPlugins :: TcS [TcPluginSolver] getTcPlugins :: TcS [TcPluginSolver] getTcPlugins = do { TcGblEnv tcg_env <- TcS TcGblEnv getGblEnv; [TcPluginSolver] -> TcS [TcPluginSolver] forall (m :: * -> *) a. Monad m => a -> m a return (TcGblEnv -> [TcPluginSolver] tcg_tc_plugins TcGblEnv tcg_env) } -- | Starting from a triple of (given, derived, wanted) constraints, -- invoke each of the typechecker plugins in turn and return -- -- * the remaining unmodified constraints, -- * constraints that have been solved, -- * constraints that are insoluble, and -- * new work. -- -- Note that new work generated by one plugin will not be seen by -- other plugins on this pass (but the main constraint solver will be -- re-invoked and they will see it later). There is no check that new -- work differs from the original constraints supplied to the plugin: -- the plugin itself should perform this check if necessary. runTcPlugins :: [TcPluginSolver] -> SplitCts -> TcS TcPluginProgress runTcPlugins :: [TcPluginSolver] -> SplitCts -> TcS TcPluginProgress runTcPlugins plugins :: [TcPluginSolver] plugins all_cts :: SplitCts all_cts = (TcPluginProgress -> TcPluginSolver -> TcS TcPluginProgress) -> TcPluginProgress -> [TcPluginSolver] -> TcS TcPluginProgress forall (t :: * -> *) (m :: * -> *) b a. (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b foldM TcPluginProgress -> TcPluginSolver -> TcS TcPluginProgress do_plugin TcPluginProgress initialProgress [TcPluginSolver] plugins where do_plugin :: TcPluginProgress -> TcPluginSolver -> TcS TcPluginProgress do_plugin :: TcPluginProgress -> TcPluginSolver -> TcS TcPluginProgress do_plugin p :: TcPluginProgress p solver :: TcPluginSolver solver = do TcPluginResult result <- TcPluginM TcPluginResult -> TcS TcPluginResult forall a. TcPluginM a -> TcS a runTcPluginTcS (TcPluginSolver -> SplitCts -> TcPluginM TcPluginResult forall a b c d. (a -> b -> c -> d) -> (a, b, c) -> d uncurry3 TcPluginSolver solver (TcPluginProgress -> SplitCts pluginInputCts TcPluginProgress p)) TcPluginProgress -> TcS TcPluginProgress forall (m :: * -> *) a. Monad m => a -> m a return (TcPluginProgress -> TcS TcPluginProgress) -> TcPluginProgress -> TcS TcPluginProgress forall a b. (a -> b) -> a -> b $ TcPluginProgress -> TcPluginResult -> TcPluginProgress progress TcPluginProgress p TcPluginResult result progress :: TcPluginProgress -> TcPluginResult -> TcPluginProgress progress :: TcPluginProgress -> TcPluginResult -> TcPluginProgress progress p :: TcPluginProgress p (TcPluginContradiction bad_cts :: [Ct] bad_cts) = TcPluginProgress p { pluginInputCts :: SplitCts pluginInputCts = [Ct] -> SplitCts -> SplitCts discard [Ct] bad_cts (TcPluginProgress -> SplitCts pluginInputCts TcPluginProgress p) , pluginBadCts :: [Ct] pluginBadCts = [Ct] bad_cts [Ct] -> [Ct] -> [Ct] forall a. [a] -> [a] -> [a] ++ TcPluginProgress -> [Ct] pluginBadCts TcPluginProgress p } progress p :: TcPluginProgress p (TcPluginOk solved_cts :: [(EvTerm, Ct)] solved_cts new_cts :: [Ct] new_cts) = TcPluginProgress p { pluginInputCts :: SplitCts pluginInputCts = [Ct] -> SplitCts -> SplitCts discard (((EvTerm, Ct) -> Ct) -> [(EvTerm, Ct)] -> [Ct] forall a b. (a -> b) -> [a] -> [b] map (EvTerm, Ct) -> Ct forall a b. (a, b) -> b snd [(EvTerm, Ct)] solved_cts) (TcPluginProgress -> SplitCts pluginInputCts TcPluginProgress p) , pluginSolvedCts :: ([Ct], [Ct], [(EvTerm, Ct)]) pluginSolvedCts = [(EvTerm, Ct)] -> ([Ct], [Ct], [(EvTerm, Ct)]) -> ([Ct], [Ct], [(EvTerm, Ct)]) add [(EvTerm, Ct)] solved_cts (TcPluginProgress -> ([Ct], [Ct], [(EvTerm, Ct)]) pluginSolvedCts TcPluginProgress p) , pluginNewCts :: [Ct] pluginNewCts = [Ct] new_cts [Ct] -> [Ct] -> [Ct] forall a. [a] -> [a] -> [a] ++ TcPluginProgress -> [Ct] pluginNewCts TcPluginProgress p } initialProgress :: TcPluginProgress initialProgress = SplitCts -> ([Ct], [Ct], [(EvTerm, Ct)]) -> [Ct] -> [Ct] -> TcPluginProgress TcPluginProgress SplitCts all_cts ([], [], []) [] [] discard :: [Ct] -> SplitCts -> SplitCts discard :: [Ct] -> SplitCts -> SplitCts discard cts :: [Ct] cts (xs :: [Ct] xs, ys :: [Ct] ys, zs :: [Ct] zs) = ([Ct] xs [Ct] -> [Ct] -> [Ct] `without` [Ct] cts, [Ct] ys [Ct] -> [Ct] -> [Ct] `without` [Ct] cts, [Ct] zs [Ct] -> [Ct] -> [Ct] `without` [Ct] cts) without :: [Ct] -> [Ct] -> [Ct] without :: [Ct] -> [Ct] -> [Ct] without = (Ct -> Ct -> Bool) -> [Ct] -> [Ct] -> [Ct] forall a. (a -> a -> Bool) -> [a] -> [a] -> [a] deleteFirstsBy Ct -> Ct -> Bool eqCt eqCt :: Ct -> Ct -> Bool eqCt :: Ct -> Ct -> Bool eqCt c :: Ct c c' :: Ct c' = Ct -> CtFlavour ctFlavour Ct c CtFlavour -> CtFlavour -> Bool forall a. Eq a => a -> a -> Bool == Ct -> CtFlavour ctFlavour Ct c' Bool -> Bool -> Bool && Ct -> PredType ctPred Ct c HasDebugCallStack => PredType -> PredType -> Bool PredType -> PredType -> Bool `tcEqType` Ct -> PredType ctPred Ct c' add :: [(EvTerm,Ct)] -> SolvedCts -> SolvedCts add :: [(EvTerm, Ct)] -> ([Ct], [Ct], [(EvTerm, Ct)]) -> ([Ct], [Ct], [(EvTerm, Ct)]) add xs :: [(EvTerm, Ct)] xs scs :: ([Ct], [Ct], [(EvTerm, Ct)]) scs = (([Ct], [Ct], [(EvTerm, Ct)]) -> (EvTerm, Ct) -> ([Ct], [Ct], [(EvTerm, Ct)])) -> ([Ct], [Ct], [(EvTerm, Ct)]) -> [(EvTerm, Ct)] -> ([Ct], [Ct], [(EvTerm, Ct)]) forall (t :: * -> *) b a. Foldable t => (b -> a -> b) -> b -> t a -> b foldl' ([Ct], [Ct], [(EvTerm, Ct)]) -> (EvTerm, Ct) -> ([Ct], [Ct], [(EvTerm, Ct)]) addOne ([Ct], [Ct], [(EvTerm, Ct)]) scs [(EvTerm, Ct)] xs addOne :: SolvedCts -> (EvTerm,Ct) -> SolvedCts addOne :: ([Ct], [Ct], [(EvTerm, Ct)]) -> (EvTerm, Ct) -> ([Ct], [Ct], [(EvTerm, Ct)]) addOne (givens :: [Ct] givens, deriveds :: [Ct] deriveds, wanteds :: [(EvTerm, Ct)] wanteds) (ev :: EvTerm ev,ct :: Ct ct) = case Ct -> CtEvidence ctEvidence Ct ct of CtGiven {} -> (Ct ctCt -> [Ct] -> [Ct] forall a. a -> [a] -> [a] :[Ct] givens, [Ct] deriveds, [(EvTerm, Ct)] wanteds) CtDerived{} -> ([Ct] givens, Ct ctCt -> [Ct] -> [Ct] forall a. a -> [a] -> [a] :[Ct] deriveds, [(EvTerm, Ct)] wanteds) CtWanted {} -> ([Ct] givens, [Ct] deriveds, (EvTerm ev,Ct ct)(EvTerm, Ct) -> [(EvTerm, Ct)] -> [(EvTerm, Ct)] forall a. a -> [a] -> [a] :[(EvTerm, Ct)] wanteds) type WorkItem = Ct type SimplifierStage = WorkItem -> TcS (StopOrContinue Ct) runSolverPipeline :: [(String,SimplifierStage)] -- The pipeline -> WorkItem -- The work item -> TcS () -- Run this item down the pipeline, leaving behind new work and inerts runSolverPipeline :: [(String, SimplifierStage)] -> Ct -> TcS () runSolverPipeline pipeline :: [(String, SimplifierStage)] pipeline workItem :: Ct workItem = do { WorkList wl <- TcS WorkList getWorkList ; InertSet inerts <- TcS InertSet getTcSInerts ; TcLevel tclevel <- TcS TcLevel getTcLevel ; String -> SDoc -> TcS () traceTcS "----------------------------- " SDoc empty ; String -> SDoc -> TcS () traceTcS "Start solver pipeline {" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text "tclevel =" SDoc -> SDoc -> SDoc <+> TcLevel -> SDoc forall a. Outputable a => a -> SDoc ppr TcLevel tclevel , String -> SDoc text "work item =" SDoc -> SDoc -> SDoc <+> Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct workItem , String -> SDoc text "inerts =" SDoc -> SDoc -> SDoc <+> InertSet -> SDoc forall a. Outputable a => a -> SDoc ppr InertSet inerts , String -> SDoc text "rest of worklist =" SDoc -> SDoc -> SDoc <+> WorkList -> SDoc forall a. Outputable a => a -> SDoc ppr WorkList wl ] ; TcS () bumpStepCountTcS -- One step for each constraint processed ; StopOrContinue Ct final_res <- [(String, SimplifierStage)] -> StopOrContinue Ct -> TcS (StopOrContinue Ct) run_pipeline [(String, SimplifierStage)] pipeline (Ct -> StopOrContinue Ct forall a. a -> StopOrContinue a ContinueWith Ct workItem) ; case StopOrContinue Ct final_res of Stop ev :: CtEvidence ev s :: SDoc s -> do { CtEvidence -> SDoc -> TcS () traceFireTcS CtEvidence ev SDoc s ; String -> SDoc -> TcS () traceTcS "End solver pipeline (discharged) }" SDoc empty ; () -> TcS () forall (m :: * -> *) a. Monad m => a -> m a return () } ContinueWith ct :: Ct ct -> do { Ct -> TcS () addInertCan Ct ct ; CtEvidence -> SDoc -> TcS () traceFireTcS (Ct -> CtEvidence ctEvidence Ct ct) (String -> SDoc text "Kept as inert") ; String -> SDoc -> TcS () traceTcS "End solver pipeline (kept as inert) }" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ (String -> SDoc text "final_item =" SDoc -> SDoc -> SDoc <+> Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct ct) } } where run_pipeline :: [(String,SimplifierStage)] -> StopOrContinue Ct -> TcS (StopOrContinue Ct) run_pipeline :: [(String, SimplifierStage)] -> StopOrContinue Ct -> TcS (StopOrContinue Ct) run_pipeline [] res :: StopOrContinue Ct res = StopOrContinue Ct -> TcS (StopOrContinue Ct) forall (m :: * -> *) a. Monad m => a -> m a return StopOrContinue Ct res run_pipeline _ (Stop ev :: CtEvidence ev s :: SDoc s) = StopOrContinue Ct -> TcS (StopOrContinue Ct) forall (m :: * -> *) a. Monad m => a -> m a return (CtEvidence -> SDoc -> StopOrContinue Ct forall a. CtEvidence -> SDoc -> StopOrContinue a Stop CtEvidence ev SDoc s) run_pipeline ((stg_name :: String stg_name,stg :: SimplifierStage stg):stgs :: [(String, SimplifierStage)] stgs) (ContinueWith ct :: Ct ct) = do { String -> SDoc -> TcS () traceTcS ("runStage " String -> String -> String forall a. [a] -> [a] -> [a] ++ String stg_name String -> String -> String forall a. [a] -> [a] -> [a] ++ " {") (String -> SDoc text "workitem = " SDoc -> SDoc -> SDoc <+> Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct ct) ; StopOrContinue Ct res <- SimplifierStage stg Ct ct ; String -> SDoc -> TcS () traceTcS ("end stage " String -> String -> String forall a. [a] -> [a] -> [a] ++ String stg_name String -> String -> String forall a. [a] -> [a] -> [a] ++ " }") SDoc empty ; [(String, SimplifierStage)] -> StopOrContinue Ct -> TcS (StopOrContinue Ct) run_pipeline [(String, SimplifierStage)] stgs StopOrContinue Ct res } {- Example 1: Inert: {c ~ d, F a ~ t, b ~ Int, a ~ ty} (all given) Reagent: a ~ [b] (given) React with (c~d) ==> IR (ContinueWith (a~[b])) True [] React with (F a ~ t) ==> IR (ContinueWith (a~[b])) False [F [b] ~ t] React with (b ~ Int) ==> IR (ContinueWith (a~[Int]) True [] Example 2: Inert: {c ~w d, F a ~g t, b ~w Int, a ~w ty} Reagent: a ~w [b] React with (c ~w d) ==> IR (ContinueWith (a~[b])) True [] React with (F a ~g t) ==> IR (ContinueWith (a~[b])) True [] (can't rewrite given with wanted!) etc. Example 3: Inert: {a ~ Int, F Int ~ b} (given) Reagent: F a ~ b (wanted) React with (a ~ Int) ==> IR (ContinueWith (F Int ~ b)) True [] React with (F Int ~ b) ==> IR Stop True [] -- after substituting we re-canonicalize and get nothing -} thePipeline :: [(String,SimplifierStage)] thePipeline :: [(String, SimplifierStage)] thePipeline = [ ("canonicalization", SimplifierStage TcCanonical.canonicalize) , ("interact with inerts", SimplifierStage interactWithInertsStage) , ("top-level reactions", SimplifierStage topReactionsStage) ] {- ********************************************************************************* * * The interact-with-inert Stage * * ********************************************************************************* Note [The Solver Invariant] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ We always add Givens first. So you might think that the solver has the invariant If the work-item is Given, then the inert item must Given But this isn't quite true. Suppose we have, c1: [W] beta ~ [alpha], c2 : [W] blah, c3 :[W] alpha ~ Int After processing the first two, we get c1: [G] beta ~ [alpha], c2 : [W] blah Now, c3 does not interact with the given c1, so when we spontaneously solve c3, we must re-react it with the inert set. So we can attempt a reaction between inert c2 [W] and work-item c3 [G]. It *is* true that [Solver Invariant] If the work-item is Given, AND there is a reaction then the inert item must Given or, equivalently, If the work-item is Given, and the inert item is Wanted/Derived then there is no reaction -} -- Interaction result of WorkItem <~> Ct interactWithInertsStage :: WorkItem -> TcS (StopOrContinue Ct) -- Precondition: if the workitem is a CTyEqCan then it will not be able to -- react with anything at this stage. interactWithInertsStage :: SimplifierStage interactWithInertsStage wi :: Ct wi = do { InertSet inerts <- TcS InertSet getTcSInerts ; let ics :: InertCans ics = InertSet -> InertCans inert_cans InertSet inerts ; case Ct wi of CTyEqCan {} -> InertCans -> SimplifierStage interactTyVarEq InertCans ics Ct wi CFunEqCan {} -> InertCans -> SimplifierStage interactFunEq InertCans ics Ct wi CIrredCan {} -> InertCans -> SimplifierStage interactIrred InertCans ics Ct wi CDictCan {} -> InertCans -> SimplifierStage interactDict InertCans ics Ct wi _ -> String -> SDoc -> TcS (StopOrContinue Ct) forall a. HasCallStack => String -> SDoc -> a pprPanic "interactWithInerts" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct wi) } -- CHoleCan are put straight into inert_frozen, so never get here -- CNonCanonical have been canonicalised data InteractResult = KeepInert -- Keep the inert item, and solve the work item from it -- (if the latter is Wanted; just discard it if not) | KeepWork -- Keep the work item, and solve the intert item from it instance Outputable InteractResult where ppr :: InteractResult -> SDoc ppr KeepInert = String -> SDoc text "keep inert" ppr KeepWork = String -> SDoc text "keep work-item" solveOneFromTheOther :: CtEvidence -- Inert -> CtEvidence -- WorkItem -> TcS InteractResult -- Precondition: -- * inert and work item represent evidence for the /same/ predicate -- -- We can always solve one from the other: even if both are wanted, -- although we don't rewrite wanteds with wanteds, we can combine -- two wanteds into one by solving one from the other solveOneFromTheOther :: CtEvidence -> CtEvidence -> TcS InteractResult solveOneFromTheOther ev_i :: CtEvidence ev_i ev_w :: CtEvidence ev_w | CtEvidence -> Bool isDerived CtEvidence ev_w -- Work item is Derived; just discard it = InteractResult -> TcS InteractResult forall (m :: * -> *) a. Monad m => a -> m a return InteractResult KeepInert | CtEvidence -> Bool isDerived CtEvidence ev_i -- The inert item is Derived, we can just throw it away, = InteractResult -> TcS InteractResult forall (m :: * -> *) a. Monad m => a -> m a return InteractResult KeepWork -- The ev_w is inert wrt earlier inert-set items, -- so it's safe to continue on from this point | CtWanted { ctev_loc :: CtEvidence -> CtLoc ctev_loc = CtLoc loc_w } <- CtEvidence ev_w , CtLoc -> CtLoc -> Bool prohibitedSuperClassSolve (CtEvidence -> CtLoc ctEvLoc CtEvidence ev_i) CtLoc loc_w = -- inert must be Given do { String -> SDoc -> TcS () traceTcS "prohibitedClassSolve1" (CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence ev_i SDoc -> SDoc -> SDoc $$ CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence ev_w) ; InteractResult -> TcS InteractResult forall (m :: * -> *) a. Monad m => a -> m a return InteractResult KeepWork } | CtWanted {} <- CtEvidence ev_w -- Inert is Given or Wanted = InteractResult -> TcS InteractResult forall (m :: * -> *) a. Monad m => a -> m a return InteractResult KeepInert -- From here on the work-item is Given | CtWanted { ctev_loc :: CtEvidence -> CtLoc ctev_loc = CtLoc loc_i } <- CtEvidence ev_i , CtLoc -> CtLoc -> Bool prohibitedSuperClassSolve (CtEvidence -> CtLoc ctEvLoc CtEvidence ev_w) CtLoc loc_i = do { String -> SDoc -> TcS () traceTcS "prohibitedClassSolve2" (CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence ev_i SDoc -> SDoc -> SDoc $$ CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence ev_w) ; InteractResult -> TcS InteractResult forall (m :: * -> *) a. Monad m => a -> m a return InteractResult KeepInert } -- Just discard the un-usable Given -- This never actually happens because -- Givens get processed first | CtWanted {} <- CtEvidence ev_i = InteractResult -> TcS InteractResult forall (m :: * -> *) a. Monad m => a -> m a return InteractResult KeepWork -- From here on both are Given -- See Note [Replacement vs keeping] | TcLevel lvl_i TcLevel -> TcLevel -> Bool forall a. Eq a => a -> a -> Bool == TcLevel lvl_w = do { EvBindsVar ev_binds_var <- TcS EvBindsVar getTcEvBindsVar ; EvBindMap binds <- EvBindsVar -> TcS EvBindMap getTcEvBindsMap EvBindsVar ev_binds_var ; InteractResult -> TcS InteractResult forall (m :: * -> *) a. Monad m => a -> m a return (EvBindMap -> InteractResult same_level_strategy EvBindMap binds) } | Bool otherwise -- Both are Given, levels differ = InteractResult -> TcS InteractResult forall (m :: * -> *) a. Monad m => a -> m a return InteractResult different_level_strategy where pred :: PredType pred = CtEvidence -> PredType ctEvPred CtEvidence ev_i loc_i :: CtLoc loc_i = CtEvidence -> CtLoc ctEvLoc CtEvidence ev_i loc_w :: CtLoc loc_w = CtEvidence -> CtLoc ctEvLoc CtEvidence ev_w lvl_i :: TcLevel lvl_i = CtLoc -> TcLevel ctLocLevel CtLoc loc_i lvl_w :: TcLevel lvl_w = CtLoc -> TcLevel ctLocLevel CtLoc loc_w ev_id_i :: EvVar ev_id_i = CtEvidence -> EvVar ctEvEvId CtEvidence ev_i ev_id_w :: EvVar ev_id_w = CtEvidence -> EvVar ctEvEvId CtEvidence ev_w different_level_strategy :: InteractResult different_level_strategy -- Both Given | PredType -> Bool isIPPred PredType pred, TcLevel lvl_w TcLevel -> TcLevel -> Bool forall a. Ord a => a -> a -> Bool > TcLevel lvl_i = InteractResult KeepWork | TcLevel lvl_w TcLevel -> TcLevel -> Bool forall a. Ord a => a -> a -> Bool < TcLevel lvl_i = InteractResult KeepWork | Bool otherwise = InteractResult KeepInert same_level_strategy :: EvBindMap -> InteractResult same_level_strategy binds :: EvBindMap binds -- Both Given | GivenOrigin (InstSC s_i :: IntWithInf s_i) <- CtLoc -> CtOrigin ctLocOrigin CtLoc loc_i = case CtLoc -> CtOrigin ctLocOrigin CtLoc loc_w of GivenOrigin (InstSC s_w :: IntWithInf s_w) | IntWithInf s_w IntWithInf -> IntWithInf -> Bool forall a. Ord a => a -> a -> Bool < IntWithInf s_i -> InteractResult KeepWork | Bool otherwise -> InteractResult KeepInert _ -> InteractResult KeepWork | GivenOrigin (InstSC {}) <- CtLoc -> CtOrigin ctLocOrigin CtLoc loc_w = InteractResult KeepInert | EvBindMap -> EvVar -> Bool has_binding EvBindMap binds EvVar ev_id_w , Bool -> Bool not (EvBindMap -> EvVar -> Bool has_binding EvBindMap binds EvVar ev_id_i) , Bool -> Bool not (EvVar ev_id_i EvVar -> VarSet -> Bool `elemVarSet` EvBindMap -> VarSet -> VarSet findNeededEvVars EvBindMap binds (EvVar -> VarSet unitVarSet EvVar ev_id_w)) = InteractResult KeepWork | Bool otherwise = InteractResult KeepInert has_binding :: EvBindMap -> EvVar -> Bool has_binding binds :: EvBindMap binds ev_id :: EvVar ev_id = Maybe EvBind -> Bool forall a. Maybe a -> Bool isJust (EvBindMap -> EvVar -> Maybe EvBind lookupEvBind EvBindMap binds EvVar ev_id) {- Note [Replacement vs keeping] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we have two Given constraints both of type (C tys), say, which should we keep? More subtle than you might think! * Constraints come from different levels (different_level_strategy) - For implicit parameters we want to keep the innermost (deepest) one, so that it overrides the outer one. See Note [Shadowing of Implicit Parameters] - For everything else, we want to keep the outermost one. Reason: that makes it more likely that the inner one will turn out to be unused, and can be reported as redundant. See Note [Tracking redundant constraints] in TcSimplify. It transpires that using the outermost one is reponsible for an 8% performance improvement in nofib cryptarithm2, compared to just rolling the dice. I didn't investigate why. * Constraints coming from the same level (i.e. same implication) (a) Always get rid of InstSC ones if possible, since they are less useful for solving. If both are InstSC, choose the one with the smallest TypeSize See Note [Solving superclass constraints] in TcInstDcls (b) Keep the one that has a non-trivial evidence binding. Example: f :: (Eq a, Ord a) => blah then we may find [G] d3 :: Eq a [G] d2 :: Eq a with bindings d3 = sc_sel (d1::Ord a) We want to discard d2 in favour of the superclass selection from the Ord dictionary. Why? See Note [Tracking redundant constraints] in TcSimplify again. (c) But don't do (b) if the evidence binding depends transitively on the one without a binding. Example (with RecursiveSuperClasses) class C a => D a class D a => C a Inert: d1 :: C a, d2 :: D a Binds: d3 = sc_sel d2, d2 = sc_sel d1 Work item: d3 :: C a Then it'd be ridiculous to replace d1 with d3 in the inert set! Hence the findNeedEvVars test. See Trac #14774. * Finally, when there is still a choice, use KeepInert rather than KeepWork, for two reasons: - to avoid unnecessary munging of the inert set. - to cut off superclass loops; see Note [Superclass loops] in TcCanonical Doing the depth-check for implicit parameters, rather than making the work item always override, is important. Consider data T a where { T1 :: (?x::Int) => T Int; T2 :: T a } f :: (?x::a) => T a -> Int f T1 = ?x f T2 = 3 We have a [G] (?x::a) in the inert set, and at the pattern match on T1 we add two new givens in the work-list: [G] (?x::Int) [G] (a ~ Int) Now consider these steps - process a~Int, kicking out (?x::a) - process (?x::Int), the inner given, adding to inert set - process (?x::a), the outer given, overriding the inner given Wrong! The depth-check ensures that the inner implicit parameter wins. (Actually I think that the order in which the work-list is processed means that this chain of events won't happen, but that's very fragile.) ********************************************************************************* * * interactIrred * * ********************************************************************************* Note [Multiple matching irreds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You might think that it's impossible to have multiple irreds all match the work item; after all, interactIrred looks for matches and solves one from the other. However, note that interacting insoluble, non-droppable irreds does not do this matching. We thus might end up with several insoluble, non-droppable, matching irreds in the inert set. When another irred comes along that we have not yet labeled insoluble, we can find multiple matches. These multiple matches cause no harm, but it would be wrong to ASSERT that they aren't there (as we once had done). This problem can be tickled by typecheck/should_compile/holes. -} -- Two pieces of irreducible evidence: if their types are *exactly identical* -- we can rewrite them. We can never improve using this: -- if we want ty1 :: Constraint and have ty2 :: Constraint it clearly does not -- mean that (ty1 ~ ty2) interactIrred :: InertCans -> Ct -> TcS (StopOrContinue Ct) interactIrred :: InertCans -> SimplifierStage interactIrred inerts :: InertCans inerts workItem :: Ct workItem@(CIrredCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev_w, cc_insol :: Ct -> Bool cc_insol = Bool insoluble }) | Bool insoluble -- For insolubles, don't allow the constaint to be dropped -- which can happen with solveOneFromTheOther, so that -- we get distinct error messages with -fdefer-type-errors -- See Note [Do not add duplicate derived insolubles] , Bool -> Bool not (Ct -> Bool isDroppableCt Ct workItem) = SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct workItem | let (matching_irreds :: Bag (Ct, SwapFlag) matching_irreds, others :: Cts others) = Cts -> CtEvidence -> (Bag (Ct, SwapFlag), Cts) findMatchingIrreds (InertCans -> Cts inert_irreds InertCans inerts) CtEvidence ev_w , ((ct_i :: Ct ct_i, swap :: SwapFlag swap) : _rest :: [(Ct, SwapFlag)] _rest) <- Bag (Ct, SwapFlag) -> [(Ct, SwapFlag)] forall a. Bag a -> [a] bagToList Bag (Ct, SwapFlag) matching_irreds -- See Note [Multiple matching irreds] , let ev_i :: CtEvidence ev_i = Ct -> CtEvidence ctEvidence Ct ct_i = do { InteractResult what_next <- CtEvidence -> CtEvidence -> TcS InteractResult solveOneFromTheOther CtEvidence ev_i CtEvidence ev_w ; String -> SDoc -> TcS () traceTcS "iteractIrred" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct workItem SDoc -> SDoc -> SDoc $$ InteractResult -> SDoc forall a. Outputable a => a -> SDoc ppr InteractResult what_next SDoc -> SDoc -> SDoc $$ Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct ct_i) ; case InteractResult what_next of KeepInert -> do { CtEvidence -> EvTerm -> TcS () setEvBindIfWanted CtEvidence ev_w (SwapFlag -> CtEvidence -> EvTerm swap_me SwapFlag swap CtEvidence ev_i) ; StopOrContinue Ct -> TcS (StopOrContinue Ct) forall (m :: * -> *) a. Monad m => a -> m a return (CtEvidence -> SDoc -> StopOrContinue Ct forall a. CtEvidence -> SDoc -> StopOrContinue a Stop CtEvidence ev_w (String -> SDoc text "Irred equal" SDoc -> SDoc -> SDoc <+> SDoc -> SDoc parens (InteractResult -> SDoc forall a. Outputable a => a -> SDoc ppr InteractResult what_next))) } KeepWork -> do { CtEvidence -> EvTerm -> TcS () setEvBindIfWanted CtEvidence ev_i (SwapFlag -> CtEvidence -> EvTerm swap_me SwapFlag swap CtEvidence ev_w) ; (Cts -> Cts) -> TcS () updInertIrreds (\_ -> Cts others) ; SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct workItem } } | Bool otherwise = SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct workItem where swap_me :: SwapFlag -> CtEvidence -> EvTerm swap_me :: SwapFlag -> CtEvidence -> EvTerm swap_me swap :: SwapFlag swap ev :: CtEvidence ev = case SwapFlag swap of NotSwapped -> CtEvidence -> EvTerm ctEvTerm CtEvidence ev IsSwapped -> TcCoercion -> EvTerm evCoercion (TcCoercion -> TcCoercion mkTcSymCo (EvTerm -> TcCoercion evTermCoercion (CtEvidence -> EvTerm ctEvTerm CtEvidence ev))) interactIrred _ wi :: Ct wi = String -> SDoc -> TcS (StopOrContinue Ct) forall a. HasCallStack => String -> SDoc -> a pprPanic "interactIrred" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct wi) findMatchingIrreds :: Cts -> CtEvidence -> (Bag (Ct, SwapFlag), Bag Ct) findMatchingIrreds :: Cts -> CtEvidence -> (Bag (Ct, SwapFlag), Cts) findMatchingIrreds irreds :: Cts irreds ev :: CtEvidence ev | EqPred eq_rel1 :: EqRel eq_rel1 lty1 :: PredType lty1 rty1 :: PredType rty1 <- PredType -> PredTree classifyPredType PredType pred -- See Note [Solving irreducible equalities] = (Ct -> Either (Ct, SwapFlag) Ct) -> Cts -> (Bag (Ct, SwapFlag), Cts) forall a b c. (a -> Either b c) -> Bag a -> (Bag b, Bag c) partitionBagWith (EqRel -> PredType -> PredType -> Ct -> Either (Ct, SwapFlag) Ct match_eq EqRel eq_rel1 PredType lty1 PredType rty1) Cts irreds | Bool otherwise = (Ct -> Either (Ct, SwapFlag) Ct) -> Cts -> (Bag (Ct, SwapFlag), Cts) forall a b c. (a -> Either b c) -> Bag a -> (Bag b, Bag c) partitionBagWith Ct -> Either (Ct, SwapFlag) Ct match_non_eq Cts irreds where pred :: PredType pred = CtEvidence -> PredType ctEvPred CtEvidence ev match_non_eq :: Ct -> Either (Ct, SwapFlag) Ct match_non_eq ct :: Ct ct | Ct -> PredType ctPred Ct ct PredType -> PredType -> Bool `tcEqTypeNoKindCheck` PredType pred = (Ct, SwapFlag) -> Either (Ct, SwapFlag) Ct forall a b. a -> Either a b Left (Ct ct, SwapFlag NotSwapped) | Bool otherwise = Ct -> Either (Ct, SwapFlag) Ct forall a b. b -> Either a b Right Ct ct match_eq :: EqRel -> PredType -> PredType -> Ct -> Either (Ct, SwapFlag) Ct match_eq eq_rel1 :: EqRel eq_rel1 lty1 :: PredType lty1 rty1 :: PredType rty1 ct :: Ct ct | EqPred eq_rel2 :: EqRel eq_rel2 lty2 :: PredType lty2 rty2 :: PredType rty2 <- PredType -> PredTree classifyPredType (Ct -> PredType ctPred Ct ct) , EqRel eq_rel1 EqRel -> EqRel -> Bool forall a. Eq a => a -> a -> Bool == EqRel eq_rel2 , Just swap :: SwapFlag swap <- PredType -> PredType -> PredType -> PredType -> Maybe SwapFlag match_eq_help PredType lty1 PredType rty1 PredType lty2 PredType rty2 = (Ct, SwapFlag) -> Either (Ct, SwapFlag) Ct forall a b. a -> Either a b Left (Ct ct, SwapFlag swap) | Bool otherwise = Ct -> Either (Ct, SwapFlag) Ct forall a b. b -> Either a b Right Ct ct match_eq_help :: PredType -> PredType -> PredType -> PredType -> Maybe SwapFlag match_eq_help lty1 :: PredType lty1 rty1 :: PredType rty1 lty2 :: PredType lty2 rty2 :: PredType rty2 | PredType lty1 PredType -> PredType -> Bool `tcEqTypeNoKindCheck` PredType lty2, PredType rty1 PredType -> PredType -> Bool `tcEqTypeNoKindCheck` PredType rty2 = SwapFlag -> Maybe SwapFlag forall a. a -> Maybe a Just SwapFlag NotSwapped | PredType lty1 PredType -> PredType -> Bool `tcEqTypeNoKindCheck` PredType rty2, PredType rty1 PredType -> PredType -> Bool `tcEqTypeNoKindCheck` PredType lty2 = SwapFlag -> Maybe SwapFlag forall a. a -> Maybe a Just SwapFlag IsSwapped | Bool otherwise = Maybe SwapFlag forall a. Maybe a Nothing {- Note [Solving irreducible equalities] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider (Trac #14333) [G] a b ~R# c d [W] c d ~R# a b Clearly we should be able to solve this! Even though the constraints are not decomposable. We solve this when looking up the work-item in the irreducible constraints to look for an identical one. When doing this lookup, findMatchingIrreds spots the equality case, and matches either way around. It has to return a swap-flag so we can generate evidence that is the right way round too. Note [Do not add duplicate derived insolubles] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In general we *must* add an insoluble (Int ~ Bool) even if there is one such there already, because they may come from distinct call sites. Not only do we want an error message for each, but with -fdefer-type-errors we must generate evidence for each. But for *derived* insolubles, we only want to report each one once. Why? (a) A constraint (C r s t) where r -> s, say, may generate the same fundep equality many times, as the original constraint is successively rewritten. (b) Ditto the successive iterations of the main solver itself, as it traverses the constraint tree. See example below. Also for *given* insolubles we may get repeated errors, as we repeatedly traverse the constraint tree. These are relatively rare anyway, so removing duplicates seems ok. (Alternatively we could take the SrcLoc into account.) Note that the test does not need to be particularly efficient because it is only used if the program has a type error anyway. Example of (b): assume a top-level class and instance declaration: class D a b | a -> b instance D [a] [a] Assume we have started with an implication: forall c. Eq c => { wc_simple = D [c] c [W] } which we have simplified to: forall c. Eq c => { wc_simple = D [c] c [W] (c ~ [c]) [D] } For some reason, e.g. because we floated an equality somewhere else, we might try to re-solve this implication. If we do not do a dropDerivedWC, then we will end up trying to solve the following constraints the second time: (D [c] c) [W] (c ~ [c]) [D] which will result in two Deriveds to end up in the insoluble set: wc_simple = D [c] c [W] (c ~ [c]) [D], (c ~ [c]) [D] -} {- ********************************************************************************* * * interactDict * * ********************************************************************************* Note [Shortcut solving] ~~~~~~~~~~~~~~~~~~~~~~~ When we interact a [W] constraint with a [G] constraint that solves it, there is a possibility that we could produce better code if instead we solved from a top-level instance declaration (See #12791, #5835). For example: class M a b where m :: a -> b type C a b = (Num a, M a b) f :: C Int b => b -> Int -> Int f _ x = x + 1 The body of `f` requires a [W] `Num Int` instance. We could solve this constraint from the givens because we have `C Int b` and that provides us a solution for `Num Int`. This would let us produce core like the following (with -O2): f :: forall b. C Int b => b -> Int -> Int f = \ (@ b) ($d(%,%) :: C Int b) _ (eta1 :: Int) -> + @ Int (GHC.Classes.$p1(%,%) @ (Num Int) @ (M Int b) $d(%,%)) eta1 A.f1 This is bad! We could do /much/ better if we solved [W] `Num Int` directly from the instance that we have in scope: f :: forall b. C Int b => b -> Int -> Int f = \ (@ b) _ _ (x :: Int) -> case x of { GHC.Types.I# x1 -> GHC.Types.I# (GHC.Prim.+# x1 1#) } ** NB: It is important to emphasize that all this is purely an optimization: ** exactly the same programs should typecheck with or without this ** procedure. Solving fully ~~~~~~~~~~~~~ There is a reason why the solver does not simply try to solve such constraints with top-level instances. If the solver finds a relevant instance declaration in scope, that instance may require a context that can't be solved for. A good example of this is: f :: Ord [a] => ... f x = ..Need Eq [a]... If we have instance `Eq a => Eq [a]` in scope and we tried to use it, we would be left with the obligation to solve the constraint Eq a, which we cannot. So we must be conservative in our attempt to use an instance declaration to solve the [W] constraint we're interested in. Our rule is that we try to solve all of the instance's subgoals recursively all at once. Precisely: We only attempt to solve constraints of the form `C1, ... Cm => C t1 ... t n`, where all the Ci are themselves class constraints of the form `C1', ... Cm' => C' t1' ... tn'` and we only succeed if the entire tree of constraints is solvable from instances. An example that succeeds: class Eq a => C a b | b -> a where m :: b -> a f :: C [Int] b => b -> Bool f x = m x == [] We solve for `Eq [Int]`, which requires `Eq Int`, which we also have. This produces the following core: f :: forall b. C [Int] b => b -> Bool f = \ (@ b) ($dC :: C [Int] b) (x :: b) -> GHC.Classes.$fEq[]_$s$c== (m @ [Int] @ b $dC x) (GHC.Types.[] @ Int) An example that fails: class Eq a => C a b | b -> a where m :: b -> a f :: C [a] b => b -> Bool f x = m x == [] Which, because solving `Eq [a]` demands `Eq a` which we cannot solve, produces: f :: forall a b. C [a] b => b -> Bool f = \ (@ a) (@ b) ($dC :: C [a] b) (eta :: b) -> == @ [a] (A.$p1C @ [a] @ b $dC) (m @ [a] @ b $dC eta) (GHC.Types.[] @ a) Note [Shortcut solving: type families] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have (Trac #13943) class Take (n :: Nat) where ... instance {-# OVERLAPPING #-} Take 0 where .. instance {-# OVERLAPPABLE #-} (Take (n - 1)) => Take n where .. And we have [W] Take 3. That only matches one instance so we get [W] Take (3-1). Really we should now flatten to reduce the (3-1) to 2, and so on -- but that is reproducing yet more of the solver. Sigh. For now, we just give up (remember all this is just an optimisation). But we must not just naively try to lookup (Take (3-1)) in the InstEnv, or it'll (wrongly) appear not to match (Take 0) and get a unique match on the (Take n) instance. That leads immediately to an infinite loop. Hence the check that 'preds' have no type families (isTyFamFree). Note [Shortcut solving: incoherence] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This optimization relies on coherence of dictionaries to be correct. When we cannot assume coherence because of IncoherentInstances then this optimization can change the behavior of the user's code. The following four modules produce a program whose output would change depending on whether we apply this optimization when IncoherentInstances is in effect: ######### {-# LANGUAGE MultiParamTypeClasses #-} module A where class A a where int :: a -> Int class A a => C a b where m :: b -> a -> a ######### {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} module B where import A instance A a where int _ = 1 instance C a [b] where m _ = id ######### {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, FlexibleContexts #-} {-# LANGUAGE IncoherentInstances #-} module C where import A instance A Int where int _ = 2 instance C Int [Int] where m _ = id intC :: C Int a => a -> Int -> Int intC _ x = int x ######### module Main where import A import B import C main :: IO () main = print (intC [] (0::Int)) The output of `main` if we avoid the optimization under the effect of IncoherentInstances is `1`. If we were to do the optimization, the output of `main` would be `2`. Note [Shortcut try_solve_from_instance] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The workhorse of the short-cut solver is try_solve_from_instance :: (EvBindMap, DictMap CtEvidence) -> CtEvidence -- Solve this -> MaybeT TcS (EvBindMap, DictMap CtEvidence) Note that: * The CtEvidence is the goal to be solved * The MaybeT anages early failure if we find a subgoal that cannot be solved from instances. * The (EvBindMap, DictMap CtEvidence) is an accumulating purely-functional state that allows try_solve_from_instance to augmennt the evidence bindings and inert_solved_dicts as it goes. If it succeeds, we commit all these bindings and solved dicts to the main TcS InertSet. If not, we abandon it all entirely. Passing along the solved_dicts important for two reasons: * We need to be able to handle recursive super classes. The solved_dicts state ensures that we remember what we have already tried to solve to avoid looping. * As Trac #15164 showed, it can be important to exploit sharing between goals. E.g. To solve G we may need G1 and G2. To solve G1 we may need H; and to solve G2 we may need H. If we don't spot this sharing we may solve H twice; and if this pattern repeats we may get exponentially bad behaviour. -} interactDict :: InertCans -> Ct -> TcS (StopOrContinue Ct) interactDict :: InertCans -> SimplifierStage interactDict inerts :: InertCans inerts workItem :: Ct workItem@(CDictCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev_w, cc_class :: Ct -> Class cc_class = Class cls, cc_tyargs :: Ct -> [PredType] cc_tyargs = [PredType] tys }) | Just ev_i :: CtEvidence ev_i <- InertCans -> CtLoc -> Class -> [PredType] -> Maybe CtEvidence lookupInertDict InertCans inerts (CtEvidence -> CtLoc ctEvLoc CtEvidence ev_w) Class cls [PredType] tys = -- There is a matching dictionary in the inert set do { -- First to try to solve it /completely/ from top level instances -- See Note [Shortcut solving] DynFlags dflags <- TcS DynFlags forall (m :: * -> *). HasDynFlags m => m DynFlags getDynFlags ; Bool short_cut_worked <- DynFlags -> CtEvidence -> CtEvidence -> TcS Bool shortCutSolver DynFlags dflags CtEvidence ev_w CtEvidence ev_i ; if Bool short_cut_worked then CtEvidence -> String -> TcS (StopOrContinue Ct) forall a. CtEvidence -> String -> TcS (StopOrContinue a) stopWith CtEvidence ev_w "interactDict/solved from instance" else do { -- Ths short-cut solver didn't fire, so we -- solve ev_w from the matching inert ev_i we found InteractResult what_next <- CtEvidence -> CtEvidence -> TcS InteractResult solveOneFromTheOther CtEvidence ev_i CtEvidence ev_w ; String -> SDoc -> TcS () traceTcS "lookupInertDict" (InteractResult -> SDoc forall a. Outputable a => a -> SDoc ppr InteractResult what_next) ; case InteractResult what_next of KeepInert -> do { CtEvidence -> EvTerm -> TcS () setEvBindIfWanted CtEvidence ev_w (CtEvidence -> EvTerm ctEvTerm CtEvidence ev_i) ; StopOrContinue Ct -> TcS (StopOrContinue Ct) forall (m :: * -> *) a. Monad m => a -> m a return (StopOrContinue Ct -> TcS (StopOrContinue Ct)) -> StopOrContinue Ct -> TcS (StopOrContinue Ct) forall a b. (a -> b) -> a -> b $ CtEvidence -> SDoc -> StopOrContinue Ct forall a. CtEvidence -> SDoc -> StopOrContinue a Stop CtEvidence ev_w (String -> SDoc text "Dict equal" SDoc -> SDoc -> SDoc <+> SDoc -> SDoc parens (InteractResult -> SDoc forall a. Outputable a => a -> SDoc ppr InteractResult what_next)) } KeepWork -> do { CtEvidence -> EvTerm -> TcS () setEvBindIfWanted CtEvidence ev_i (CtEvidence -> EvTerm ctEvTerm CtEvidence ev_w) ; (DictMap Ct -> DictMap Ct) -> TcS () updInertDicts ((DictMap Ct -> DictMap Ct) -> TcS ()) -> (DictMap Ct -> DictMap Ct) -> TcS () forall a b. (a -> b) -> a -> b $ \ ds :: DictMap Ct ds -> DictMap Ct -> Class -> [PredType] -> DictMap Ct forall a. DictMap a -> Class -> [PredType] -> DictMap a delDict DictMap Ct ds Class cls [PredType] tys ; SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct workItem } } } | Class cls Class -> Unique -> Bool forall a. Uniquable a => a -> Unique -> Bool `hasKey` Unique ipClassKey , CtEvidence -> Bool isGiven CtEvidence ev_w = InertCans -> SimplifierStage interactGivenIP InertCans inerts Ct workItem | Bool otherwise = do { InertCans -> CtEvidence -> Class -> TcS () addFunDepWork InertCans inerts CtEvidence ev_w Class cls ; SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct workItem } interactDict _ wi :: Ct wi = String -> SDoc -> TcS (StopOrContinue Ct) forall a. HasCallStack => String -> SDoc -> a pprPanic "interactDict" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct wi) -- See Note [Shortcut solving] shortCutSolver :: DynFlags -> CtEvidence -- Work item -> CtEvidence -- Inert we want to try to replace -> TcS Bool -- True <=> success shortCutSolver :: DynFlags -> CtEvidence -> CtEvidence -> TcS Bool shortCutSolver dflags :: DynFlags dflags ev_w :: CtEvidence ev_w ev_i :: CtEvidence ev_i | CtEvidence -> Bool isWanted CtEvidence ev_w Bool -> Bool -> Bool && CtEvidence -> Bool isGiven CtEvidence ev_i -- We are about to solve a [W] constraint from a [G] constraint. We take -- a moment to see if we can get a better solution using an instance. -- Note that we only do this for the sake of performance. Exactly the same -- programs should typecheck regardless of whether we take this step or -- not. See Note [Shortcut solving] Bool -> Bool -> Bool && Bool -> Bool not (Extension -> DynFlags -> Bool xopt Extension LangExt.IncoherentInstances DynFlags dflags) -- If IncoherentInstances is on then we cannot rely on coherence of proofs -- in order to justify this optimization: The proof provided by the -- [G] constraint's superclass may be different from the top-level proof. -- See Note [Shortcut solving: incoherence] Bool -> Bool -> Bool && GeneralFlag -> DynFlags -> Bool gopt GeneralFlag Opt_SolveConstantDicts DynFlags dflags -- Enabled by the -fsolve-constant-dicts flag = do { EvBindsVar ev_binds_var <- TcS EvBindsVar getTcEvBindsVar ; EvBindMap ev_binds <- ASSERT2( not (isCoEvBindsVar ev_binds_var ), ppr ev_w ) EvBindsVar -> TcS EvBindMap getTcEvBindsMap EvBindsVar ev_binds_var ; DictMap CtEvidence solved_dicts <- TcS (DictMap CtEvidence) getSolvedDicts ; Maybe (EvBindMap, DictMap CtEvidence) mb_stuff <- MaybeT TcS (EvBindMap, DictMap CtEvidence) -> TcS (Maybe (EvBindMap, DictMap CtEvidence)) forall (m :: * -> *) a. MaybeT m a -> m (Maybe a) runMaybeT (MaybeT TcS (EvBindMap, DictMap CtEvidence) -> TcS (Maybe (EvBindMap, DictMap CtEvidence))) -> MaybeT TcS (EvBindMap, DictMap CtEvidence) -> TcS (Maybe (EvBindMap, DictMap CtEvidence)) forall a b. (a -> b) -> a -> b $ (EvBindMap, DictMap CtEvidence) -> CtEvidence -> MaybeT TcS (EvBindMap, DictMap CtEvidence) try_solve_from_instance (EvBindMap ev_binds, DictMap CtEvidence solved_dicts) CtEvidence ev_w ; case Maybe (EvBindMap, DictMap CtEvidence) mb_stuff of Nothing -> Bool -> TcS Bool forall (m :: * -> *) a. Monad m => a -> m a return Bool False Just (ev_binds' :: EvBindMap ev_binds', solved_dicts' :: DictMap CtEvidence solved_dicts') -> do { EvBindsVar -> EvBindMap -> TcS () setTcEvBindsMap EvBindsVar ev_binds_var EvBindMap ev_binds' ; DictMap CtEvidence -> TcS () setSolvedDicts DictMap CtEvidence solved_dicts' ; Bool -> TcS Bool forall (m :: * -> *) a. Monad m => a -> m a return Bool True } } | Bool otherwise = Bool -> TcS Bool forall (m :: * -> *) a. Monad m => a -> m a return Bool False where -- This `CtLoc` is used only to check the well-staged condition of any -- candidate DFun. Our subgoals all have the same stage as our root -- [W] constraint so it is safe to use this while solving them. loc_w :: CtLoc loc_w = CtEvidence -> CtLoc ctEvLoc CtEvidence ev_w try_solve_from_instance -- See Note [Shortcut try_solve_from_instance] :: (EvBindMap, DictMap CtEvidence) -> CtEvidence -> MaybeT TcS (EvBindMap, DictMap CtEvidence) try_solve_from_instance :: (EvBindMap, DictMap CtEvidence) -> CtEvidence -> MaybeT TcS (EvBindMap, DictMap CtEvidence) try_solve_from_instance (ev_binds :: EvBindMap ev_binds, solved_dicts :: DictMap CtEvidence solved_dicts) ev :: CtEvidence ev | let pred :: PredType pred = CtEvidence -> PredType ctEvPred CtEvidence ev loc :: CtLoc loc = CtEvidence -> CtLoc ctEvLoc CtEvidence ev , ClassPred cls :: Class cls tys :: [PredType] tys <- PredType -> PredTree classifyPredType PredType pred = do { ClsInstResult inst_res <- TcS ClsInstResult -> MaybeT TcS ClsInstResult forall (t :: (* -> *) -> * -> *) (m :: * -> *) a. (MonadTrans t, Monad m) => m a -> t m a lift (TcS ClsInstResult -> MaybeT TcS ClsInstResult) -> TcS ClsInstResult -> MaybeT TcS ClsInstResult forall a b. (a -> b) -> a -> b $ DynFlags -> Bool -> Class -> [PredType] -> TcS ClsInstResult matchGlobalInst DynFlags dflags Bool True Class cls [PredType] tys ; case ClsInstResult inst_res of OneInst { cir_new_theta :: ClsInstResult -> [PredType] cir_new_theta = [PredType] preds , cir_mk_ev :: ClsInstResult -> [EvExpr] -> EvTerm cir_mk_ev = [EvExpr] -> EvTerm mk_ev , cir_what :: ClsInstResult -> InstanceWhat cir_what = InstanceWhat what } | InstanceWhat -> Bool safeOverlap InstanceWhat what , (PredType -> Bool) -> [PredType] -> Bool forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool all PredType -> Bool isTyFamFree [PredType] preds -- Note [Shortcut solving: type families] -> do { let solved_dicts' :: DictMap CtEvidence solved_dicts' = DictMap CtEvidence -> Class -> [PredType] -> CtEvidence -> DictMap CtEvidence forall a. DictMap a -> Class -> [PredType] -> a -> DictMap a addDict DictMap CtEvidence solved_dicts Class cls [PredType] tys CtEvidence ev -- solved_dicts': it is important that we add our goal -- to the cache before we solve! Otherwise we may end -- up in a loop while solving recursive dictionaries. ; TcS () -> MaybeT TcS () forall (t :: (* -> *) -> * -> *) (m :: * -> *) a. (MonadTrans t, Monad m) => m a -> t m a lift (TcS () -> MaybeT TcS ()) -> TcS () -> MaybeT TcS () forall a b. (a -> b) -> a -> b $ String -> SDoc -> TcS () traceTcS "shortCutSolver: found instance" ([PredType] -> SDoc forall a. Outputable a => a -> SDoc ppr [PredType] preds) ; CtLoc loc' <- TcS CtLoc -> MaybeT TcS CtLoc forall (t :: (* -> *) -> * -> *) (m :: * -> *) a. (MonadTrans t, Monad m) => m a -> t m a lift (TcS CtLoc -> MaybeT TcS CtLoc) -> TcS CtLoc -> MaybeT TcS CtLoc forall a b. (a -> b) -> a -> b $ CtLoc -> InstanceWhat -> PredType -> TcS CtLoc checkInstanceOK CtLoc loc InstanceWhat what PredType pred ; [MaybeNew] evc_vs <- (PredType -> MaybeT TcS MaybeNew) -> [PredType] -> MaybeT TcS [MaybeNew] forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM (CtLoc -> DictMap CtEvidence -> PredType -> MaybeT TcS MaybeNew new_wanted_cached CtLoc loc' DictMap CtEvidence solved_dicts') [PredType] preds -- Emit work for subgoals but use our local cache -- so we can solve recursive dictionaries. ; let ev_tm :: EvTerm ev_tm = [EvExpr] -> EvTerm mk_ev ((MaybeNew -> EvExpr) -> [MaybeNew] -> [EvExpr] forall a b. (a -> b) -> [a] -> [b] map MaybeNew -> EvExpr getEvExpr [MaybeNew] evc_vs) ev_binds' :: EvBindMap ev_binds' = EvBindMap -> EvBind -> EvBindMap extendEvBinds EvBindMap ev_binds (EvBind -> EvBindMap) -> EvBind -> EvBindMap forall a b. (a -> b) -> a -> b $ EvVar -> EvTerm -> EvBind mkWantedEvBind (CtEvidence -> EvVar ctEvEvId CtEvidence ev) EvTerm ev_tm ; ((EvBindMap, DictMap CtEvidence) -> CtEvidence -> MaybeT TcS (EvBindMap, DictMap CtEvidence)) -> (EvBindMap, DictMap CtEvidence) -> [CtEvidence] -> MaybeT TcS (EvBindMap, DictMap CtEvidence) forall (m :: * -> *) a b. Monad m => (a -> b -> m a) -> a -> [b] -> m a foldlM (EvBindMap, DictMap CtEvidence) -> CtEvidence -> MaybeT TcS (EvBindMap, DictMap CtEvidence) try_solve_from_instance (EvBindMap ev_binds', DictMap CtEvidence solved_dicts') ([MaybeNew] -> [CtEvidence] freshGoals [MaybeNew] evc_vs) } _ -> MaybeT TcS (EvBindMap, DictMap CtEvidence) forall (m :: * -> *) a. MonadPlus m => m a mzero } | Bool otherwise = MaybeT TcS (EvBindMap, DictMap CtEvidence) forall (m :: * -> *) a. MonadPlus m => m a mzero -- Use a local cache of solved dicts while emitting EvVars for new work -- We bail out of the entire computation if we need to emit an EvVar for -- a subgoal that isn't a ClassPred. new_wanted_cached :: CtLoc -> DictMap CtEvidence -> TcPredType -> MaybeT TcS MaybeNew new_wanted_cached :: CtLoc -> DictMap CtEvidence -> PredType -> MaybeT TcS MaybeNew new_wanted_cached loc :: CtLoc loc cache :: DictMap CtEvidence cache pty :: PredType pty | ClassPred cls :: Class cls tys :: [PredType] tys <- PredType -> PredTree classifyPredType PredType pty = TcS MaybeNew -> MaybeT TcS MaybeNew forall (t :: (* -> *) -> * -> *) (m :: * -> *) a. (MonadTrans t, Monad m) => m a -> t m a lift (TcS MaybeNew -> MaybeT TcS MaybeNew) -> TcS MaybeNew -> MaybeT TcS MaybeNew forall a b. (a -> b) -> a -> b $ case DictMap CtEvidence -> CtLoc -> Class -> [PredType] -> Maybe CtEvidence forall a. DictMap a -> CtLoc -> Class -> [PredType] -> Maybe a findDict DictMap CtEvidence cache CtLoc loc_w Class cls [PredType] tys of Just ctev :: CtEvidence ctev -> MaybeNew -> TcS MaybeNew forall (m :: * -> *) a. Monad m => a -> m a return (MaybeNew -> TcS MaybeNew) -> MaybeNew -> TcS MaybeNew forall a b. (a -> b) -> a -> b $ EvExpr -> MaybeNew Cached (CtEvidence -> EvExpr ctEvExpr CtEvidence ctev) Nothing -> CtEvidence -> MaybeNew Fresh (CtEvidence -> MaybeNew) -> TcS CtEvidence -> TcS MaybeNew forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b <$> CtLoc -> PredType -> TcS CtEvidence newWantedNC CtLoc loc PredType pty | Bool otherwise = MaybeT TcS MaybeNew forall (m :: * -> *) a. MonadPlus m => m a mzero addFunDepWork :: InertCans -> CtEvidence -> Class -> TcS () -- Add derived constraints from type-class functional dependencies. addFunDepWork :: InertCans -> CtEvidence -> Class -> TcS () addFunDepWork inerts :: InertCans inerts work_ev :: CtEvidence work_ev cls :: Class cls | CtEvidence -> Bool isImprovable CtEvidence work_ev = (Ct -> TcS ()) -> Cts -> TcS () forall (m :: * -> *) a b. Monad m => (a -> m b) -> Bag a -> m () mapBagM_ Ct -> TcS () add_fds (DictMap Ct -> Class -> Cts forall a. DictMap a -> Class -> Bag a findDictsByClass (InertCans -> DictMap Ct inert_dicts InertCans inerts) Class cls) -- No need to check flavour; fundeps work between -- any pair of constraints, regardless of flavour -- Importantly we don't throw workitem back in the -- worklist because this can cause loops (see #5236) | Bool otherwise = () -> TcS () forall (m :: * -> *) a. Monad m => a -> m a return () where work_pred :: PredType work_pred = CtEvidence -> PredType ctEvPred CtEvidence work_ev work_loc :: CtLoc work_loc = CtEvidence -> CtLoc ctEvLoc CtEvidence work_ev add_fds :: Ct -> TcS () add_fds inert_ct :: Ct inert_ct | CtEvidence -> Bool isImprovable CtEvidence inert_ev = do { String -> SDoc -> TcS () traceTcS "addFunDepWork" ([SDoc] -> SDoc vcat [ CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence work_ev , CtLoc -> SDoc pprCtLoc CtLoc work_loc, Bool -> SDoc forall a. Outputable a => a -> SDoc ppr (CtLoc -> Bool isGivenLoc CtLoc work_loc) , CtLoc -> SDoc pprCtLoc CtLoc inert_loc, Bool -> SDoc forall a. Outputable a => a -> SDoc ppr (CtLoc -> Bool isGivenLoc CtLoc inert_loc) , CtLoc -> SDoc pprCtLoc CtLoc derived_loc, Bool -> SDoc forall a. Outputable a => a -> SDoc ppr (CtLoc -> Bool isGivenLoc CtLoc derived_loc) ]) ; [FunDepEqn CtLoc] -> TcS () emitFunDepDeriveds ([FunDepEqn CtLoc] -> TcS ()) -> [FunDepEqn CtLoc] -> TcS () forall a b. (a -> b) -> a -> b $ CtLoc -> PredType -> PredType -> [FunDepEqn CtLoc] forall loc. loc -> PredType -> PredType -> [FunDepEqn loc] improveFromAnother CtLoc derived_loc PredType inert_pred PredType work_pred -- We don't really rewrite tys2, see below _rewritten_tys2, so that's ok -- NB: We do create FDs for given to report insoluble equations that arise -- from pairs of Givens, and also because of floating when we approximate -- implications. The relevant test is: typecheck/should_fail/FDsFromGivens.hs } | Bool otherwise = () -> TcS () forall (m :: * -> *) a. Monad m => a -> m a return () where inert_ev :: CtEvidence inert_ev = Ct -> CtEvidence ctEvidence Ct inert_ct inert_pred :: PredType inert_pred = CtEvidence -> PredType ctEvPred CtEvidence inert_ev inert_loc :: CtLoc inert_loc = CtEvidence -> CtLoc ctEvLoc CtEvidence inert_ev derived_loc :: CtLoc derived_loc = CtLoc work_loc { ctl_depth :: SubGoalDepth ctl_depth = CtLoc -> SubGoalDepth ctl_depth CtLoc work_loc SubGoalDepth -> SubGoalDepth -> SubGoalDepth `maxSubGoalDepth` CtLoc -> SubGoalDepth ctl_depth CtLoc inert_loc , ctl_origin :: CtOrigin ctl_origin = PredType -> CtLoc -> PredType -> CtLoc -> CtOrigin FunDepOrigin1 PredType work_pred CtLoc work_loc PredType inert_pred CtLoc inert_loc } {- ********************************************************************** * * Implicit parameters * * ********************************************************************** -} interactGivenIP :: InertCans -> Ct -> TcS (StopOrContinue Ct) -- Work item is Given (?x:ty) -- See Note [Shadowing of Implicit Parameters] interactGivenIP :: InertCans -> SimplifierStage interactGivenIP inerts :: InertCans inerts workItem :: Ct workItem@(CDictCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev, cc_class :: Ct -> Class cc_class = Class cls , cc_tyargs :: Ct -> [PredType] cc_tyargs = tys :: [PredType] tys@(ip_str :: PredType ip_str:_) }) = do { (InertCans -> InertCans) -> TcS () updInertCans ((InertCans -> InertCans) -> TcS ()) -> (InertCans -> InertCans) -> TcS () forall a b. (a -> b) -> a -> b $ \cans :: InertCans cans -> InertCans cans { inert_dicts :: DictMap Ct inert_dicts = DictMap Ct -> Class -> [PredType] -> Ct -> DictMap Ct forall a. DictMap a -> Class -> [PredType] -> a -> DictMap a addDict DictMap Ct filtered_dicts Class cls [PredType] tys Ct workItem } ; CtEvidence -> String -> TcS (StopOrContinue Ct) forall a. CtEvidence -> String -> TcS (StopOrContinue a) stopWith CtEvidence ev "Given IP" } where dicts :: DictMap Ct dicts = InertCans -> DictMap Ct inert_dicts InertCans inerts ip_dicts :: Cts ip_dicts = DictMap Ct -> Class -> Cts forall a. DictMap a -> Class -> Bag a findDictsByClass DictMap Ct dicts Class cls other_ip_dicts :: Cts other_ip_dicts = (Ct -> Bool) -> Cts -> Cts forall a. (a -> Bool) -> Bag a -> Bag a filterBag (Bool -> Bool not (Bool -> Bool) -> (Ct -> Bool) -> Ct -> Bool forall b c a. (b -> c) -> (a -> b) -> a -> c . Ct -> Bool is_this_ip) Cts ip_dicts filtered_dicts :: DictMap Ct filtered_dicts = DictMap Ct -> Class -> Cts -> DictMap Ct addDictsByClass DictMap Ct dicts Class cls Cts other_ip_dicts -- Pick out any Given constraints for the same implicit parameter is_this_ip :: Ct -> Bool is_this_ip (CDictCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev, cc_tyargs :: Ct -> [PredType] cc_tyargs = ip_str' :: PredType ip_str':_ }) = CtEvidence -> Bool isGiven CtEvidence ev Bool -> Bool -> Bool && PredType ip_str HasDebugCallStack => PredType -> PredType -> Bool PredType -> PredType -> Bool `tcEqType` PredType ip_str' is_this_ip _ = Bool False interactGivenIP _ wi :: Ct wi = String -> SDoc -> TcS (StopOrContinue Ct) forall a. HasCallStack => String -> SDoc -> a pprPanic "interactGivenIP" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct wi) {- Note [Shadowing of Implicit Parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider the following example: f :: (?x :: Char) => Char f = let ?x = 'a' in ?x The "let ?x = ..." generates an implication constraint of the form: ?x :: Char => ?x :: Char Furthermore, the signature for `f` also generates an implication constraint, so we end up with the following nested implication: ?x :: Char => (?x :: Char => ?x :: Char) Note that the wanted (?x :: Char) constraint may be solved in two incompatible ways: either by using the parameter from the signature, or by using the local definition. Our intention is that the local definition should "shadow" the parameter of the signature, and we implement this as follows: when we add a new *given* implicit parameter to the inert set, it replaces any existing givens for the same implicit parameter. Similarly, consider f :: (?x::a) => Bool -> a g v = let ?x::Int = 3 in (f v, let ?x::Bool = True in f v) This should probably be well typed, with g :: Bool -> (Int, Bool) So the inner binding for ?x::Bool *overrides* the outer one. All this works for the normal cases but it has an odd side effect in some pathological programs like this: -- This is accepted, the second parameter shadows f1 :: (?x :: Int, ?x :: Char) => Char f1 = ?x -- This is rejected, the second parameter shadows f2 :: (?x :: Int, ?x :: Char) => Int f2 = ?x Both of these are actually wrong: when we try to use either one, we'll get two incompatible wanted constraints (?x :: Int, ?x :: Char), which would lead to an error. I can think of two ways to fix this: 1. Simply disallow multiple constraints for the same implicit parameter---this is never useful, and it can be detected completely syntactically. 2. Move the shadowing machinery to the location where we nest implications, and add some code here that will produce an error if we get multiple givens for the same implicit parameter. ********************************************************************** * * interactFunEq * * ********************************************************************** -} interactFunEq :: InertCans -> Ct -> TcS (StopOrContinue Ct) -- Try interacting the work item with the inert set interactFunEq :: InertCans -> SimplifierStage interactFunEq inerts :: InertCans inerts work_item :: Ct work_item@(CFunEqCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev, cc_fun :: Ct -> TyCon cc_fun = TyCon tc , cc_tyargs :: Ct -> [PredType] cc_tyargs = [PredType] args, cc_fsk :: Ct -> EvVar cc_fsk = EvVar fsk }) | Just inert_ct :: Ct inert_ct@(CFunEqCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev_i , cc_fsk :: Ct -> EvVar cc_fsk = EvVar fsk_i }) <- DictMap Ct -> TyCon -> [PredType] -> Maybe Ct forall a. FunEqMap a -> TyCon -> [PredType] -> Maybe a findFunEq (InertCans -> DictMap Ct inert_funeqs InertCans inerts) TyCon tc [PredType] args , pr :: (SwapFlag, Bool) pr@(swap_flag :: SwapFlag swap_flag, upgrade_flag :: Bool upgrade_flag) <- CtEvidence ev_i CtEvidence -> CtEvidence -> (SwapFlag, Bool) `funEqCanDischarge` CtEvidence ev = do { String -> SDoc -> TcS () traceTcS "reactFunEq (rewrite inert item):" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text "work_item =" SDoc -> SDoc -> SDoc <+> Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct work_item , String -> SDoc text "inertItem=" SDoc -> SDoc -> SDoc <+> CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence ev_i , String -> SDoc text "(swap_flag, upgrade)" SDoc -> SDoc -> SDoc <+> (SwapFlag, Bool) -> SDoc forall a. Outputable a => a -> SDoc ppr (SwapFlag, Bool) pr ] ; if SwapFlag -> Bool isSwapped SwapFlag swap_flag then do { -- Rewrite inert using work-item let work_item' :: Ct work_item' | Bool upgrade_flag = Ct -> Ct upgradeWanted Ct work_item | Bool otherwise = Ct work_item ; (DictMap Ct -> DictMap Ct) -> TcS () updInertFunEqs ((DictMap Ct -> DictMap Ct) -> TcS ()) -> (DictMap Ct -> DictMap Ct) -> TcS () forall a b. (a -> b) -> a -> b $ \ feqs :: DictMap Ct feqs -> DictMap Ct -> TyCon -> [PredType] -> Ct -> DictMap Ct forall a. FunEqMap a -> TyCon -> [PredType] -> a -> FunEqMap a insertFunEq DictMap Ct feqs TyCon tc [PredType] args Ct work_item' -- Do the updInertFunEqs before the reactFunEq, so that -- we don't kick out the inertItem as well as consuming it! ; CtEvidence -> EvVar -> CtEvidence -> EvVar -> TcS () reactFunEq CtEvidence ev EvVar fsk CtEvidence ev_i EvVar fsk_i ; CtEvidence -> String -> TcS (StopOrContinue Ct) forall a. CtEvidence -> String -> TcS (StopOrContinue a) stopWith CtEvidence ev "Work item rewrites inert" } else do { -- Rewrite work-item using inert ; Bool -> TcS () -> TcS () forall (f :: * -> *). Applicative f => Bool -> f () -> f () when Bool upgrade_flag (TcS () -> TcS ()) -> TcS () -> TcS () forall a b. (a -> b) -> a -> b $ (DictMap Ct -> DictMap Ct) -> TcS () updInertFunEqs ((DictMap Ct -> DictMap Ct) -> TcS ()) -> (DictMap Ct -> DictMap Ct) -> TcS () forall a b. (a -> b) -> a -> b $ \ feqs :: DictMap Ct feqs -> DictMap Ct -> TyCon -> [PredType] -> Ct -> DictMap Ct forall a. FunEqMap a -> TyCon -> [PredType] -> a -> FunEqMap a insertFunEq DictMap Ct feqs TyCon tc [PredType] args (Ct -> Ct upgradeWanted Ct inert_ct) ; CtEvidence -> EvVar -> CtEvidence -> EvVar -> TcS () reactFunEq CtEvidence ev_i EvVar fsk_i CtEvidence ev EvVar fsk ; CtEvidence -> String -> TcS (StopOrContinue Ct) forall a. CtEvidence -> String -> TcS (StopOrContinue a) stopWith CtEvidence ev "Inert rewrites work item" } } | Bool otherwise -- Try improvement = do { CtEvidence -> InertCans -> TyCon -> [PredType] -> EvVar -> TcS () improveLocalFunEqs CtEvidence ev InertCans inerts TyCon tc [PredType] args EvVar fsk ; SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct work_item } interactFunEq _ work_item :: Ct work_item = String -> SDoc -> TcS (StopOrContinue Ct) forall a. HasCallStack => String -> SDoc -> a pprPanic "interactFunEq" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct work_item) upgradeWanted :: Ct -> Ct -- We are combining a [W] F tys ~ fmv1 and [D] F tys ~ fmv2 -- so upgrade the [W] to [WD] before putting it in the inert set upgradeWanted :: Ct -> Ct upgradeWanted ct :: Ct ct = Ct ct { cc_ev :: CtEvidence cc_ev = CtEvidence -> CtEvidence upgrade_ev (Ct -> CtEvidence cc_ev Ct ct) } where upgrade_ev :: CtEvidence -> CtEvidence upgrade_ev ev :: CtEvidence ev = ASSERT2( isWanted ev, ppr ct ) CtEvidence ev { ctev_nosh :: ShadowInfo ctev_nosh = ShadowInfo WDeriv } improveLocalFunEqs :: CtEvidence -> InertCans -> TyCon -> [TcType] -> TcTyVar -> TcS () -- Generate derived improvement equalities, by comparing -- the current work item with inert CFunEqs -- E.g. x + y ~ z, x + y' ~ z => [D] y ~ y' -- -- See Note [FunDep and implicit parameter reactions] improveLocalFunEqs :: CtEvidence -> InertCans -> TyCon -> [PredType] -> EvVar -> TcS () improveLocalFunEqs work_ev :: CtEvidence work_ev inerts :: InertCans inerts fam_tc :: TyCon fam_tc args :: [PredType] args fsk :: EvVar fsk | CtEvidence -> Bool isGiven CtEvidence work_ev -- See Note [No FunEq improvement for Givens] Bool -> Bool -> Bool || Bool -> Bool not (CtEvidence -> Bool isImprovable CtEvidence work_ev) = () -> TcS () forall (m :: * -> *) a. Monad m => a -> m a return () | Bool -> Bool not ([FunDepEqn CtLoc] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [FunDepEqn CtLoc] improvement_eqns) = do { String -> SDoc -> TcS () traceTcS "interactFunEq improvements: " (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text "Eqns:" SDoc -> SDoc -> SDoc <+> [FunDepEqn CtLoc] -> SDoc forall a. Outputable a => a -> SDoc ppr [FunDepEqn CtLoc] improvement_eqns , String -> SDoc text "Candidates:" SDoc -> SDoc -> SDoc <+> [Ct] -> SDoc forall a. Outputable a => a -> SDoc ppr [Ct] funeqs_for_tc , String -> SDoc text "Inert eqs:" SDoc -> SDoc -> SDoc <+> InertEqs -> SDoc forall a. Outputable a => a -> SDoc ppr InertEqs ieqs ] ; [FunDepEqn CtLoc] -> TcS () emitFunDepDeriveds [FunDepEqn CtLoc] improvement_eqns } | Bool otherwise = () -> TcS () forall (m :: * -> *) a. Monad m => a -> m a return () where ieqs :: InertEqs ieqs = InertCans -> InertEqs inert_eqs InertCans inerts funeqs :: DictMap Ct funeqs = InertCans -> DictMap Ct inert_funeqs InertCans inerts funeqs_for_tc :: [Ct] funeqs_for_tc = DictMap Ct -> TyCon -> [Ct] forall a. FunEqMap a -> TyCon -> [a] findFunEqsByTyCon DictMap Ct funeqs TyCon fam_tc rhs :: PredType rhs = InertEqs -> EvVar -> PredType lookupFlattenTyVar InertEqs ieqs EvVar fsk work_loc :: CtLoc work_loc = CtEvidence -> CtLoc ctEvLoc CtEvidence work_ev work_pred :: PredType work_pred = CtEvidence -> PredType ctEvPred CtEvidence work_ev fam_inj_info :: Injectivity fam_inj_info = TyCon -> Injectivity tyConInjectivityInfo TyCon fam_tc -------------------- improvement_eqns :: [FunDepEqn CtLoc] improvement_eqns :: [FunDepEqn CtLoc] improvement_eqns | Just ops :: BuiltInSynFamily ops <- TyCon -> Maybe BuiltInSynFamily isBuiltInSynFamTyCon_maybe TyCon fam_tc = -- Try built-in families, notably for arithmethic (Ct -> [FunDepEqn CtLoc]) -> [Ct] -> [FunDepEqn CtLoc] forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b] concatMap (BuiltInSynFamily -> Ct -> [FunDepEqn CtLoc] do_one_built_in BuiltInSynFamily ops) [Ct] funeqs_for_tc | Injective injective_args :: [Bool] injective_args <- Injectivity fam_inj_info = -- Try improvement from type families with injectivity annotations (Ct -> [FunDepEqn CtLoc]) -> [Ct] -> [FunDepEqn CtLoc] forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b] concatMap ([Bool] -> Ct -> [FunDepEqn CtLoc] do_one_injective [Bool] injective_args) [Ct] funeqs_for_tc | Bool otherwise = [] -------------------- do_one_built_in :: BuiltInSynFamily -> Ct -> [FunDepEqn CtLoc] do_one_built_in ops :: BuiltInSynFamily ops (CFunEqCan { cc_tyargs :: Ct -> [PredType] cc_tyargs = [PredType] iargs, cc_fsk :: Ct -> EvVar cc_fsk = EvVar ifsk, cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence inert_ev }) = CtEvidence -> [TypeEqn] -> [FunDepEqn CtLoc] mk_fd_eqns CtEvidence inert_ev (BuiltInSynFamily -> [PredType] -> PredType -> [PredType] -> PredType -> [TypeEqn] sfInteractInert BuiltInSynFamily ops [PredType] args PredType rhs [PredType] iargs (InertEqs -> EvVar -> PredType lookupFlattenTyVar InertEqs ieqs EvVar ifsk)) do_one_built_in _ _ = String -> SDoc -> [FunDepEqn CtLoc] forall a. HasCallStack => String -> SDoc -> a pprPanic "interactFunEq 1" (TyCon -> SDoc forall a. Outputable a => a -> SDoc ppr TyCon fam_tc) -------------------- -- See Note [Type inference for type families with injectivity] do_one_injective :: [Bool] -> Ct -> [FunDepEqn CtLoc] do_one_injective inj_args :: [Bool] inj_args (CFunEqCan { cc_tyargs :: Ct -> [PredType] cc_tyargs = [PredType] inert_args , cc_fsk :: Ct -> EvVar cc_fsk = EvVar ifsk, cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence inert_ev }) | CtEvidence -> Bool isImprovable CtEvidence inert_ev , PredType rhs HasDebugCallStack => PredType -> PredType -> Bool PredType -> PredType -> Bool `tcEqType` InertEqs -> EvVar -> PredType lookupFlattenTyVar InertEqs ieqs EvVar ifsk = CtEvidence -> [TypeEqn] -> [FunDepEqn CtLoc] mk_fd_eqns CtEvidence inert_ev ([TypeEqn] -> [FunDepEqn CtLoc]) -> [TypeEqn] -> [FunDepEqn CtLoc] forall a b. (a -> b) -> a -> b $ [ PredType -> PredType -> TypeEqn forall a. a -> a -> Pair a Pair PredType arg PredType iarg | (arg :: PredType arg, iarg :: PredType iarg, True) <- [PredType] -> [PredType] -> [Bool] -> [(PredType, PredType, Bool)] forall a b c. [a] -> [b] -> [c] -> [(a, b, c)] zip3 [PredType] args [PredType] inert_args [Bool] inj_args ] | Bool otherwise = [] do_one_injective _ _ = String -> SDoc -> [FunDepEqn CtLoc] forall a. HasCallStack => String -> SDoc -> a pprPanic "interactFunEq 2" (TyCon -> SDoc forall a. Outputable a => a -> SDoc ppr TyCon fam_tc) -------------------- mk_fd_eqns :: CtEvidence -> [TypeEqn] -> [FunDepEqn CtLoc] mk_fd_eqns :: CtEvidence -> [TypeEqn] -> [FunDepEqn CtLoc] mk_fd_eqns inert_ev :: CtEvidence inert_ev eqns :: [TypeEqn] eqns | [TypeEqn] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [TypeEqn] eqns = [] | Bool otherwise = [ FDEqn :: forall loc. [EvVar] -> [TypeEqn] -> PredType -> PredType -> loc -> FunDepEqn loc FDEqn { fd_qtvs :: [EvVar] fd_qtvs = [], fd_eqs :: [TypeEqn] fd_eqs = [TypeEqn] eqns , fd_pred1 :: PredType fd_pred1 = PredType work_pred , fd_pred2 :: PredType fd_pred2 = CtEvidence -> PredType ctEvPred CtEvidence inert_ev , fd_loc :: CtLoc fd_loc = CtLoc loc } ] where inert_loc :: CtLoc inert_loc = CtEvidence -> CtLoc ctEvLoc CtEvidence inert_ev loc :: CtLoc loc = CtLoc inert_loc { ctl_depth :: SubGoalDepth ctl_depth = CtLoc -> SubGoalDepth ctl_depth CtLoc inert_loc SubGoalDepth -> SubGoalDepth -> SubGoalDepth `maxSubGoalDepth` CtLoc -> SubGoalDepth ctl_depth CtLoc work_loc } ------------- reactFunEq :: CtEvidence -> TcTyVar -- From this :: F args1 ~ fsk1 -> CtEvidence -> TcTyVar -- Solve this :: F args2 ~ fsk2 -> TcS () reactFunEq :: CtEvidence -> EvVar -> CtEvidence -> EvVar -> TcS () reactFunEq from_this :: CtEvidence from_this fsk1 :: EvVar fsk1 solve_this :: CtEvidence solve_this fsk2 :: EvVar fsk2 = do { String -> SDoc -> TcS () traceTcS "reactFunEq" ([SDoc] -> SDoc vcat [CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence from_this, EvVar -> SDoc forall a. Outputable a => a -> SDoc ppr EvVar fsk1, CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence solve_this, EvVar -> SDoc forall a. Outputable a => a -> SDoc ppr EvVar fsk2]) ; CtEvidence -> EvVar -> TcCoercion -> PredType -> TcS () dischargeFunEq CtEvidence solve_this EvVar fsk2 (HasDebugCallStack => CtEvidence -> TcCoercion CtEvidence -> TcCoercion ctEvCoercion CtEvidence from_this) (EvVar -> PredType mkTyVarTy EvVar fsk1) ; String -> SDoc -> TcS () traceTcS "reactFunEq done" (CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence from_this SDoc -> SDoc -> SDoc $$ EvVar -> SDoc forall a. Outputable a => a -> SDoc ppr EvVar fsk1 SDoc -> SDoc -> SDoc $$ CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence solve_this SDoc -> SDoc -> SDoc $$ EvVar -> SDoc forall a. Outputable a => a -> SDoc ppr EvVar fsk2) } {- Note [Type inference for type families with injectivity] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have a type family with an injectivity annotation: type family F a b = r | r -> b Then if we have two CFunEqCan constraints for F with the same RHS F s1 t1 ~ rhs F s2 t2 ~ rhs then we can use the injectivity to get a new Derived constraint on the injective argument [D] t1 ~ t2 That in turn can help GHC solve constraints that would otherwise require guessing. For example, consider the ambiguity check for f :: F Int b -> Int We get the constraint [W] F Int b ~ F Int beta where beta is a unification variable. Injectivity lets us pick beta ~ b. Injectivity information is also used at the call sites. For example: g = f True gives rise to [W] F Int b ~ Bool from which we can derive b. This requires looking at the defining equations of a type family, ie. finding equation with a matching RHS (Bool in this example) and infering values of type variables (b in this example) from the LHS patterns of the matching equation. For closed type families we have to perform additional apartness check for the selected equation to check that the selected is guaranteed to fire for given LHS arguments. These new constraints are simply *Derived* constraints; they have no evidence. We could go further and offer evidence from decomposing injective type-function applications, but that would require new evidence forms, and an extension to FC, so we don't do that right now (Dec 14). See also Note [Injective type families] in TyCon Note [Cache-caused loops] ~~~~~~~~~~~~~~~~~~~~~~~~~ It is very dangerous to cache a rewritten wanted family equation as 'solved' in our solved cache (which is the default behaviour or xCtEvidence), because the interaction may not be contributing towards a solution. Here is an example: Initial inert set: [W] g1 : F a ~ beta1 Work item: [W] g2 : F a ~ beta2 The work item will react with the inert yielding the _same_ inert set plus: (i) Will set g2 := g1 `cast` g3 (ii) Will add to our solved cache that [S] g2 : F a ~ beta2 (iii) Will emit [W] g3 : beta1 ~ beta2 Now, the g3 work item will be spontaneously solved to [G] g3 : beta1 ~ beta2 and then it will react the item in the inert ([W] g1 : F a ~ beta1). So it will set g1 := g ; sym g3 and what is g? Well it would ideally be a new goal of type (F a ~ beta2) but remember that we have this in our solved cache, and it is ... g2! In short we created the evidence loop: g2 := g1 ; g3 g3 := refl g1 := g2 ; sym g3 To avoid this situation we do not cache as solved any workitems (or inert) which did not really made a 'step' towards proving some goal. Solved's are just an optimization so we don't lose anything in terms of completeness of solving. Note [Efficient Orientation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we are interacting two FunEqCans with the same LHS: (inert) ci :: (F ty ~ xi_i) (work) cw :: (F ty ~ xi_w) We prefer to keep the inert (else we pass the work item on down the pipeline, which is a bit silly). If we keep the inert, we will (a) discharge 'cw' (b) produce a new equality work-item (xi_w ~ xi_i) Notice the orientation (xi_w ~ xi_i) NOT (xi_i ~ xi_w): new_work :: xi_w ~ xi_i cw := ci ; sym new_work Why? Consider the simplest case when xi1 is a type variable. If we generate xi1~xi2, porcessing that constraint will kick out 'ci'. If we generate xi2~xi1, there is less chance of that happening. Of course it can and should still happen if xi1=a, xi1=Int, say. But we want to avoid it happening needlessly. Similarly, if we *can't* keep the inert item (because inert is Wanted, and work is Given, say), we prefer to orient the new equality (xi_i ~ xi_w). Note [Carefully solve the right CFunEqCan] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ---- OLD COMMENT, NOW NOT NEEDED ---- because we now allow multiple ---- wanted FunEqs with the same head Consider the constraints c1 :: F Int ~ a -- Arising from an application line 5 c2 :: F Int ~ Bool -- Arising from an application line 10 Suppose that 'a' is a unification variable, arising only from flattening. So there is no error on line 5; it's just a flattening variable. But there is (or might be) an error on line 10. Two ways to combine them, leaving either (Plan A) c1 :: F Int ~ a -- Arising from an application line 5 c3 :: a ~ Bool -- Arising from an application line 10 or (Plan B) c2 :: F Int ~ Bool -- Arising from an application line 10 c4 :: a ~ Bool -- Arising from an application line 5 Plan A will unify c3, leaving c1 :: F Int ~ Bool as an error on the *totally innocent* line 5. An example is test SimpleFail16 where the expected/actual message comes out backwards if we use the wrong plan. The second is the right thing to do. Hence the isMetaTyVarTy test when solving pairwise CFunEqCan. ********************************************************************** * * interactTyVarEq * * ********************************************************************** -} inertsCanDischarge :: InertCans -> TcTyVar -> TcType -> CtFlavourRole -> Maybe ( CtEvidence -- The evidence for the inert , SwapFlag -- Whether we need mkSymCo , Bool) -- True <=> keep a [D] version -- of the [WD] constraint inertsCanDischarge :: InertCans -> EvVar -> PredType -> CtFlavourRole -> Maybe (CtEvidence, SwapFlag, Bool) inertsCanDischarge inerts :: InertCans inerts tv :: EvVar tv rhs :: PredType rhs fr :: CtFlavourRole fr | (ev_i :: CtEvidence ev_i : _) <- [ CtEvidence ev_i | CTyEqCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev_i, cc_rhs :: Ct -> PredType cc_rhs = PredType rhs_i , cc_eq_rel :: Ct -> EqRel cc_eq_rel = EqRel eq_rel } <- InertCans -> EvVar -> [Ct] findTyEqs InertCans inerts EvVar tv , (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev_i, EqRel eq_rel) CtFlavourRole -> CtFlavourRole -> Bool `eqCanDischargeFR` CtFlavourRole fr , PredType rhs_i HasDebugCallStack => PredType -> PredType -> Bool PredType -> PredType -> Bool `tcEqType` PredType rhs ] = -- Inert: a ~ ty -- Work item: a ~ ty (CtEvidence, SwapFlag, Bool) -> Maybe (CtEvidence, SwapFlag, Bool) forall a. a -> Maybe a Just (CtEvidence ev_i, SwapFlag NotSwapped, CtEvidence -> Bool keep_deriv CtEvidence ev_i) | Just tv_rhs :: EvVar tv_rhs <- PredType -> Maybe EvVar getTyVar_maybe PredType rhs , (ev_i :: CtEvidence ev_i : _) <- [ CtEvidence ev_i | CTyEqCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev_i, cc_rhs :: Ct -> PredType cc_rhs = PredType rhs_i , cc_eq_rel :: Ct -> EqRel cc_eq_rel = EqRel eq_rel } <- InertCans -> EvVar -> [Ct] findTyEqs InertCans inerts EvVar tv_rhs , (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev_i, EqRel eq_rel) CtFlavourRole -> CtFlavourRole -> Bool `eqCanDischargeFR` CtFlavourRole fr , PredType rhs_i HasDebugCallStack => PredType -> PredType -> Bool PredType -> PredType -> Bool `tcEqType` EvVar -> PredType mkTyVarTy EvVar tv ] = -- Inert: a ~ b -- Work item: b ~ a (CtEvidence, SwapFlag, Bool) -> Maybe (CtEvidence, SwapFlag, Bool) forall a. a -> Maybe a Just (CtEvidence ev_i, SwapFlag IsSwapped, CtEvidence -> Bool keep_deriv CtEvidence ev_i) | Bool otherwise = Maybe (CtEvidence, SwapFlag, Bool) forall a. Maybe a Nothing where keep_deriv :: CtEvidence -> Bool keep_deriv ev_i :: CtEvidence ev_i | Wanted WOnly <- CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev_i -- inert is [W] , (Wanted WDeriv, _) <- CtFlavourRole fr -- work item is [WD] = Bool True -- Keep a derived verison of the work item | Bool otherwise = Bool False -- Work item is fully discharged interactTyVarEq :: InertCans -> Ct -> TcS (StopOrContinue Ct) -- CTyEqCans are always consumed, so always returns Stop interactTyVarEq :: InertCans -> SimplifierStage interactTyVarEq inerts :: InertCans inerts workItem :: Ct workItem@(CTyEqCan { cc_tyvar :: Ct -> EvVar cc_tyvar = EvVar tv , cc_rhs :: Ct -> PredType cc_rhs = PredType rhs , cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev , cc_eq_rel :: Ct -> EqRel cc_eq_rel = EqRel eq_rel }) | Just (ev_i :: CtEvidence ev_i, swapped :: SwapFlag swapped, keep_deriv :: Bool keep_deriv) <- InertCans -> EvVar -> PredType -> CtFlavourRole -> Maybe (CtEvidence, SwapFlag, Bool) inertsCanDischarge InertCans inerts EvVar tv PredType rhs (CtEvidence -> CtFlavour ctEvFlavour CtEvidence ev, EqRel eq_rel) = do { CtEvidence -> EvTerm -> TcS () setEvBindIfWanted CtEvidence ev (EvTerm -> TcS ()) -> EvTerm -> TcS () forall a b. (a -> b) -> a -> b $ TcCoercion -> EvTerm evCoercion (SwapFlag -> TcCoercion -> TcCoercion maybeSym SwapFlag swapped (TcCoercion -> TcCoercion) -> TcCoercion -> TcCoercion forall a b. (a -> b) -> a -> b $ Role -> Role -> TcCoercion -> TcCoercion tcDowngradeRole (EqRel -> Role eqRelRole EqRel eq_rel) (CtEvidence -> Role ctEvRole CtEvidence ev_i) (HasDebugCallStack => CtEvidence -> TcCoercion CtEvidence -> TcCoercion ctEvCoercion CtEvidence ev_i)) ; let deriv_ev :: CtEvidence deriv_ev = CtDerived :: PredType -> CtLoc -> CtEvidence CtDerived { ctev_pred :: PredType ctev_pred = CtEvidence -> PredType ctEvPred CtEvidence ev , ctev_loc :: CtLoc ctev_loc = CtEvidence -> CtLoc ctEvLoc CtEvidence ev } ; Bool -> TcS () -> TcS () forall (f :: * -> *). Applicative f => Bool -> f () -> f () when Bool keep_deriv (TcS () -> TcS ()) -> TcS () -> TcS () forall a b. (a -> b) -> a -> b $ [Ct] -> TcS () emitWork [Ct workItem { cc_ev :: CtEvidence cc_ev = CtEvidence deriv_ev }] -- As a Derived it might not be fully rewritten, -- so we emit it as new work ; CtEvidence -> String -> TcS (StopOrContinue Ct) forall a. CtEvidence -> String -> TcS (StopOrContinue a) stopWith CtEvidence ev "Solved from inert" } | EqRel ReprEq <- EqRel eq_rel -- See Note [Do not unify representational equalities] = do { String -> SDoc -> TcS () traceTcS "Not unifying representational equality" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct workItem) ; SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct workItem } | CtEvidence -> Bool isGiven CtEvidence ev -- See Note [Touchables and givens] = SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct workItem | Bool otherwise = do { TcLevel tclvl <- TcS TcLevel getTcLevel ; if TcLevel -> EvVar -> PredType -> Bool canSolveByUnification TcLevel tclvl EvVar tv PredType rhs then do { CtEvidence -> EvVar -> PredType -> TcS () solveByUnification CtEvidence ev EvVar tv PredType rhs ; Int n_kicked <- EvVar -> TcS Int kickOutAfterUnification EvVar tv ; StopOrContinue Ct -> TcS (StopOrContinue Ct) forall (m :: * -> *) a. Monad m => a -> m a return (CtEvidence -> SDoc -> StopOrContinue Ct forall a. CtEvidence -> SDoc -> StopOrContinue a Stop CtEvidence ev (String -> SDoc text "Solved by unification" SDoc -> SDoc -> SDoc <+> Int -> SDoc pprKicked Int n_kicked)) } else SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct workItem } interactTyVarEq _ wi :: Ct wi = String -> SDoc -> TcS (StopOrContinue Ct) forall a. HasCallStack => String -> SDoc -> a pprPanic "interactTyVarEq" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct wi) solveByUnification :: CtEvidence -> TcTyVar -> Xi -> TcS () -- Solve with the identity coercion -- Precondition: kind(xi) equals kind(tv) -- Precondition: CtEvidence is Wanted or Derived -- Precondition: CtEvidence is nominal -- Returns: workItem where -- workItem = the new Given constraint -- -- NB: No need for an occurs check here, because solveByUnification always -- arises from a CTyEqCan, a *canonical* constraint. Its invariants -- say that in (a ~ xi), the type variable a does not appear in xi. -- See TcRnTypes.Ct invariants. -- -- Post: tv is unified (by side effect) with xi; -- we often write tv := xi solveByUnification :: CtEvidence -> EvVar -> PredType -> TcS () solveByUnification wd :: CtEvidence wd tv :: EvVar tv xi :: PredType xi = do { let tv_ty :: PredType tv_ty = EvVar -> PredType mkTyVarTy EvVar tv ; String -> SDoc -> TcS () traceTcS "Sneaky unification:" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [String -> SDoc text "Unifies:" SDoc -> SDoc -> SDoc <+> EvVar -> SDoc forall a. Outputable a => a -> SDoc ppr EvVar tv SDoc -> SDoc -> SDoc <+> String -> SDoc text ":=" SDoc -> SDoc -> SDoc <+> PredType -> SDoc forall a. Outputable a => a -> SDoc ppr PredType xi, String -> SDoc text "Coercion:" SDoc -> SDoc -> SDoc <+> PredType -> PredType -> SDoc pprEq PredType tv_ty PredType xi, String -> SDoc text "Left Kind is:" SDoc -> SDoc -> SDoc <+> PredType -> SDoc forall a. Outputable a => a -> SDoc ppr (HasDebugCallStack => PredType -> PredType PredType -> PredType tcTypeKind PredType tv_ty), String -> SDoc text "Right Kind is:" SDoc -> SDoc -> SDoc <+> PredType -> SDoc forall a. Outputable a => a -> SDoc ppr (HasDebugCallStack => PredType -> PredType PredType -> PredType tcTypeKind PredType xi) ] ; EvVar -> PredType -> TcS () unifyTyVar EvVar tv PredType xi ; CtEvidence -> EvTerm -> TcS () setEvBindIfWanted CtEvidence wd (TcCoercion -> EvTerm evCoercion (PredType -> TcCoercion mkTcNomReflCo PredType xi)) } {- Note [Avoid double unifications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The spontaneous solver has to return a given which mentions the unified unification variable *on the left* of the equality. Here is what happens if not: Original wanted: (a ~ alpha), (alpha ~ Int) We spontaneously solve the first wanted, without changing the order! given : a ~ alpha [having unified alpha := a] Now the second wanted comes along, but he cannot rewrite the given, so we simply continue. At the end we spontaneously solve that guy, *reunifying* [alpha := Int] We avoid this problem by orienting the resulting given so that the unification variable is on the left. [Note that alternatively we could attempt to enforce this at canonicalization] See also Note [No touchables as FunEq RHS] in TcSMonad; avoiding double unifications is the main reason we disallow touchable unification variables as RHS of type family equations: F xis ~ alpha. Note [Do not unify representational equalities] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider [W] alpha ~R# b where alpha is touchable. Should we unify alpha := b? Certainly not! Unifying forces alpha and be to be the same; but they only need to be representationally equal types. For example, we might have another constraint [W] alpha ~# N b where newtype N b = MkN b and we want to get alpha := N b. See also Trac #15144, which was caused by unifying a representational equality (in the unflattener). ************************************************************************ * * * 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 Trac #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 FunDeps.hs (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 Derived 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 :d 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, but that requires those equalities to carry evidence and derived do not carry evidence. If that were the case with the same inert set and work item we might dischard 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 [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; [D] K Bool ~ K [a] [D] 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. Trac #7875 is a case in point. -} emitFunDepDeriveds :: [FunDepEqn CtLoc] -> TcS () -- See Note [FunDep and implicit parameter reactions] emitFunDepDeriveds :: [FunDepEqn CtLoc] -> TcS () emitFunDepDeriveds fd_eqns :: [FunDepEqn CtLoc] fd_eqns = (FunDepEqn CtLoc -> TcS ()) -> [FunDepEqn CtLoc] -> TcS () forall (t :: * -> *) (m :: * -> *) a b. (Foldable t, Monad m) => (a -> m b) -> t a -> m () mapM_ FunDepEqn CtLoc -> TcS () do_one_FDEqn [FunDepEqn CtLoc] fd_eqns where do_one_FDEqn :: FunDepEqn CtLoc -> TcS () do_one_FDEqn (FDEqn { fd_qtvs :: forall loc. FunDepEqn loc -> [EvVar] fd_qtvs = [EvVar] tvs, fd_eqs :: forall loc. FunDepEqn loc -> [TypeEqn] fd_eqs = [TypeEqn] eqs, fd_loc :: forall loc. FunDepEqn loc -> loc fd_loc = CtLoc loc }) | [EvVar] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [EvVar] tvs -- Common shortcut = do { String -> SDoc -> TcS () traceTcS "emitFunDepDeriveds 1" (SubGoalDepth -> SDoc forall a. Outputable a => a -> SDoc ppr (CtLoc -> SubGoalDepth ctl_depth CtLoc loc) SDoc -> SDoc -> SDoc $$ [TypeEqn] -> SDoc forall a. Outputable a => a -> SDoc ppr [TypeEqn] eqs SDoc -> SDoc -> SDoc $$ Bool -> SDoc forall a. Outputable a => a -> SDoc ppr (CtLoc -> Bool isGivenLoc CtLoc loc)) ; (TypeEqn -> TcS ()) -> [TypeEqn] -> TcS () forall (t :: * -> *) (m :: * -> *) a b. (Foldable t, Monad m) => (a -> m b) -> t a -> m () mapM_ (CtLoc -> Role -> TypeEqn -> TcS () unifyDerived CtLoc loc Role Nominal) [TypeEqn] eqs } | Bool otherwise = do { String -> SDoc -> TcS () traceTcS "emitFunDepDeriveds 2" (SubGoalDepth -> SDoc forall a. Outputable a => a -> SDoc ppr (CtLoc -> SubGoalDepth ctl_depth CtLoc loc) SDoc -> SDoc -> SDoc $$ [EvVar] -> SDoc forall a. Outputable a => a -> SDoc ppr [EvVar] tvs SDoc -> SDoc -> SDoc $$ [TypeEqn] -> SDoc forall a. Outputable a => a -> SDoc ppr [TypeEqn] eqs) ; TCvSubst subst <- [EvVar] -> TcS TCvSubst instFlexi [EvVar] tvs -- Takes account of kind substitution ; (TypeEqn -> TcS ()) -> [TypeEqn] -> TcS () forall (t :: * -> *) (m :: * -> *) a b. (Foldable t, Monad m) => (a -> m b) -> t a -> m () mapM_ (CtLoc -> TCvSubst -> TypeEqn -> TcS () do_one_eq CtLoc loc TCvSubst subst) [TypeEqn] eqs } do_one_eq :: CtLoc -> TCvSubst -> TypeEqn -> TcS () do_one_eq loc :: CtLoc loc subst :: TCvSubst subst (Pair ty1 :: PredType ty1 ty2 :: PredType ty2) = CtLoc -> Role -> TypeEqn -> TcS () unifyDerived CtLoc loc Role Nominal (TypeEqn -> TcS ()) -> TypeEqn -> TcS () forall a b. (a -> b) -> a -> b $ PredType -> PredType -> TypeEqn forall a. a -> a -> Pair a Pair (TCvSubst -> PredType -> PredType Type.substTyUnchecked TCvSubst subst PredType ty1) (TCvSubst -> PredType -> PredType Type.substTyUnchecked TCvSubst subst PredType ty2) {- ********************************************************************** * * The top-reaction Stage * * ********************************************************************** -} topReactionsStage :: WorkItem -> TcS (StopOrContinue Ct) -- The work item does not react with the inert set, -- so try interaction with top-level instances. Note: topReactionsStage :: SimplifierStage topReactionsStage work_item :: Ct work_item = do { String -> SDoc -> TcS () traceTcS "doTopReact" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct work_item) ; case Ct work_item of CDictCan {} -> do { InertSet inerts <- TcS InertSet getTcSInerts ; InertSet -> SimplifierStage doTopReactDict InertSet inerts Ct work_item } CFunEqCan {} -> SimplifierStage doTopReactFunEq Ct work_item CIrredCan {} -> SimplifierStage doTopReactOther Ct work_item CTyEqCan {} -> SimplifierStage doTopReactOther Ct work_item _ -> -- Any other work item does not react with any top-level equations SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct work_item } -------------------- doTopReactOther :: Ct -> TcS (StopOrContinue Ct) -- Try local quantified constraints for -- CTyEqCan e.g. (a ~# ty) -- and CIrredCan e.g. (c a) -- -- Why equalities? See TcCanonical -- Note [Equality superclasses in quantified constraints] doTopReactOther :: SimplifierStage doTopReactOther work_item :: Ct work_item | CtEvidence -> Bool isGiven CtEvidence ev = SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct work_item | EqPred eq_rel :: EqRel eq_rel t1 :: PredType t1 t2 :: PredType t2 <- PredType -> PredTree classifyPredType PredType pred = -- See Note [Looking up primitive equalities in quantified constraints] case EqRel -> PredType -> PredType -> Maybe (Class, [PredType]) boxEqPred EqRel eq_rel PredType t1 PredType t2 of Nothing -> SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct work_item Just (cls :: Class cls, tys :: [PredType] tys) -> do { ClsInstResult res <- PredType -> CtLoc -> TcS ClsInstResult matchLocalInst (Class -> [PredType] -> PredType mkClassPred Class cls [PredType] tys) CtLoc loc ; case ClsInstResult res of OneInst { cir_mk_ev :: ClsInstResult -> [EvExpr] -> EvTerm cir_mk_ev = [EvExpr] -> EvTerm mk_ev } -> Ct -> ClsInstResult -> TcS (StopOrContinue Ct) chooseInstance Ct work_item (ClsInstResult res { cir_mk_ev :: [EvExpr] -> EvTerm cir_mk_ev = Class -> [PredType] -> ([EvExpr] -> EvTerm) -> [EvExpr] -> EvTerm forall t. Class -> [PredType] -> (t -> EvTerm) -> t -> EvTerm mk_eq_ev Class cls [PredType] tys [EvExpr] -> EvTerm mk_ev }) where _ -> SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct work_item } | Bool otherwise = do { ClsInstResult res <- PredType -> CtLoc -> TcS ClsInstResult matchLocalInst PredType pred CtLoc loc ; case ClsInstResult res of OneInst {} -> Ct -> ClsInstResult -> TcS (StopOrContinue Ct) chooseInstance Ct work_item ClsInstResult res _ -> SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct work_item } where ev :: CtEvidence ev = Ct -> CtEvidence ctEvidence Ct work_item loc :: CtLoc loc = CtEvidence -> CtLoc ctEvLoc CtEvidence ev pred :: PredType pred = CtEvidence -> PredType ctEvPred CtEvidence ev mk_eq_ev :: Class -> [PredType] -> (t -> EvTerm) -> t -> EvTerm mk_eq_ev cls :: Class cls tys :: [PredType] tys mk_ev :: t -> EvTerm mk_ev evs :: t evs = case (t -> EvTerm mk_ev t evs) of EvExpr e :: EvExpr e -> EvExpr -> EvTerm EvExpr (EvVar -> EvExpr forall b. EvVar -> Expr b Var EvVar sc_id EvExpr -> [PredType] -> EvExpr forall b. Expr b -> [PredType] -> Expr b `mkTyApps` [PredType] tys EvExpr -> EvExpr -> EvExpr forall b. Expr b -> Expr b -> Expr b `App` EvExpr e) ev :: EvTerm ev -> String -> SDoc -> EvTerm forall a. HasCallStack => String -> SDoc -> a pprPanic "mk_eq_ev" (EvTerm -> SDoc forall a. Outputable a => a -> SDoc ppr EvTerm ev) where [sc_id :: EvVar sc_id] = Class -> [EvVar] classSCSelIds Class cls {- Note [Looking up primitive equalities in quantified constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For equalities (a ~# b) look up (a ~ b), and then do a superclass selection. This avoids having to support quantified constraints whose kind is not Constraint, such as (forall a. F a ~# b) See * Note [Evidence for quantified constraints] in Type * Note [Equality superclasses in quantified constraints] in TcCanonical -} -------------------- doTopReactFunEq :: Ct -> TcS (StopOrContinue Ct) doTopReactFunEq :: SimplifierStage doTopReactFunEq work_item :: Ct work_item@(CFunEqCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence old_ev, cc_fun :: Ct -> TyCon cc_fun = TyCon fam_tc , cc_tyargs :: Ct -> [PredType] cc_tyargs = [PredType] args, cc_fsk :: Ct -> EvVar cc_fsk = EvVar fsk }) | EvVar fsk EvVar -> VarSet -> Bool `elemVarSet` [PredType] -> VarSet tyCoVarsOfTypes [PredType] args = TcS (StopOrContinue Ct) no_reduction -- See Note [FunEq occurs-check principle] | Bool otherwise -- Note [Reduction for Derived CFunEqCans] = do { Maybe (TcCoercion, PredType) match_res <- TyCon -> [PredType] -> TcS (Maybe (TcCoercion, PredType)) matchFam TyCon fam_tc [PredType] args -- Look up in top-level instances, or built-in axiom -- See Note [MATCHING-SYNONYMS] ; case Maybe (TcCoercion, PredType) match_res of Nothing -> TcS (StopOrContinue Ct) no_reduction Just match_info :: (TcCoercion, PredType) match_info -> CtEvidence -> EvVar -> (TcCoercion, PredType) -> TcS (StopOrContinue Ct) reduce_top_fun_eq CtEvidence old_ev EvVar fsk (TcCoercion, PredType) match_info } where no_reduction :: TcS (StopOrContinue Ct) no_reduction = do { CtEvidence -> TyCon -> [PredType] -> EvVar -> TcS () improveTopFunEqs CtEvidence old_ev TyCon fam_tc [PredType] args EvVar fsk ; SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct work_item } doTopReactFunEq w :: Ct w = String -> SDoc -> TcS (StopOrContinue Ct) forall a. HasCallStack => String -> SDoc -> a pprPanic "doTopReactFunEq" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct w) reduce_top_fun_eq :: CtEvidence -> TcTyVar -> (TcCoercion, TcType) -> TcS (StopOrContinue Ct) -- We have found an applicable top-level axiom: use it to reduce -- Precondition: fsk is not free in rhs_ty reduce_top_fun_eq :: CtEvidence -> EvVar -> (TcCoercion, PredType) -> TcS (StopOrContinue Ct) reduce_top_fun_eq old_ev :: CtEvidence old_ev fsk :: EvVar fsk (ax_co :: TcCoercion ax_co, rhs_ty :: PredType rhs_ty) | Bool -> Bool not (CtEvidence -> Bool isDerived CtEvidence old_ev) -- Precondition of shortCutReduction , Just (tc :: TyCon tc, tc_args :: [PredType] tc_args) <- HasCallStack => PredType -> Maybe (TyCon, [PredType]) PredType -> Maybe (TyCon, [PredType]) tcSplitTyConApp_maybe PredType rhs_ty , TyCon -> Bool isTypeFamilyTyCon TyCon tc , [PredType] tc_args [PredType] -> Int -> Bool forall a. [a] -> Int -> Bool `lengthIs` TyCon -> Int tyConArity TyCon tc -- Short-cut = -- RHS is another type-family application -- Try shortcut; see Note [Top-level reductions for type functions] do { CtEvidence -> EvVar -> TcCoercion -> TyCon -> [PredType] -> TcS () shortCutReduction CtEvidence old_ev EvVar fsk TcCoercion ax_co TyCon tc [PredType] tc_args ; CtEvidence -> String -> TcS (StopOrContinue Ct) forall a. CtEvidence -> String -> TcS (StopOrContinue a) stopWith CtEvidence old_ev "Fun/Top (shortcut)" } | Bool otherwise = ASSERT2( not (fsk `elemVarSet` tyCoVarsOfType rhs_ty) , ppr old_ev $$ ppr rhs_ty ) -- Guaranteed by Note [FunEq occurs-check principle] do { CtEvidence -> EvVar -> TcCoercion -> PredType -> TcS () dischargeFunEq CtEvidence old_ev EvVar fsk TcCoercion ax_co PredType rhs_ty ; String -> SDoc -> TcS () traceTcS "doTopReactFunEq" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text "old_ev:" SDoc -> SDoc -> SDoc <+> CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence old_ev , Int -> SDoc -> SDoc nest 2 (String -> SDoc text ":=") SDoc -> SDoc -> SDoc <+> TcCoercion -> SDoc forall a. Outputable a => a -> SDoc ppr TcCoercion ax_co ] ; CtEvidence -> String -> TcS (StopOrContinue Ct) forall a. CtEvidence -> String -> TcS (StopOrContinue a) stopWith CtEvidence old_ev "Fun/Top" } improveTopFunEqs :: CtEvidence -> TyCon -> [TcType] -> TcTyVar -> TcS () -- See Note [FunDep and implicit parameter reactions] improveTopFunEqs :: CtEvidence -> TyCon -> [PredType] -> EvVar -> TcS () improveTopFunEqs ev :: CtEvidence ev fam_tc :: TyCon fam_tc args :: [PredType] args fsk :: EvVar fsk | CtEvidence -> Bool isGiven CtEvidence ev -- See Note [No FunEq improvement for Givens] Bool -> Bool -> Bool || Bool -> Bool not (CtEvidence -> Bool isImprovable CtEvidence ev) = () -> TcS () forall (m :: * -> *) a. Monad m => a -> m a return () | Bool otherwise = do { InertEqs ieqs <- TcS InertEqs getInertEqs ; (FamInstEnv, FamInstEnv) fam_envs <- TcS (FamInstEnv, FamInstEnv) getFamInstEnvs ; [TypeEqn] eqns <- (FamInstEnv, FamInstEnv) -> TyCon -> [PredType] -> PredType -> TcS [TypeEqn] improve_top_fun_eqs (FamInstEnv, FamInstEnv) fam_envs TyCon fam_tc [PredType] args (InertEqs -> EvVar -> PredType lookupFlattenTyVar InertEqs ieqs EvVar fsk) ; String -> SDoc -> TcS () traceTcS "improveTopFunEqs" ([SDoc] -> SDoc vcat [ TyCon -> SDoc forall a. Outputable a => a -> SDoc ppr TyCon fam_tc SDoc -> SDoc -> SDoc <+> [PredType] -> SDoc forall a. Outputable a => a -> SDoc ppr [PredType] args SDoc -> SDoc -> SDoc <+> EvVar -> SDoc forall a. Outputable a => a -> SDoc ppr EvVar fsk , [TypeEqn] -> SDoc forall a. Outputable a => a -> SDoc ppr [TypeEqn] eqns ]) ; (TypeEqn -> TcS ()) -> [TypeEqn] -> TcS () forall (t :: * -> *) (m :: * -> *) a b. (Foldable t, Monad m) => (a -> m b) -> t a -> m () mapM_ (CtLoc -> Role -> TypeEqn -> TcS () unifyDerived CtLoc loc Role Nominal) [TypeEqn] eqns } where loc :: CtLoc loc = CtEvidence -> CtLoc ctEvLoc CtEvidence ev -- ToDo: this location is wrong; it should be FunDepOrigin2 -- See Trac #14778 improve_top_fun_eqs :: FamInstEnvs -> TyCon -> [TcType] -> TcType -> TcS [TypeEqn] improve_top_fun_eqs :: (FamInstEnv, FamInstEnv) -> TyCon -> [PredType] -> PredType -> TcS [TypeEqn] improve_top_fun_eqs fam_envs :: (FamInstEnv, FamInstEnv) fam_envs fam_tc :: TyCon fam_tc args :: [PredType] args rhs_ty :: PredType rhs_ty | Just ops :: BuiltInSynFamily ops <- TyCon -> Maybe BuiltInSynFamily isBuiltInSynFamTyCon_maybe TyCon fam_tc = [TypeEqn] -> TcS [TypeEqn] forall (m :: * -> *) a. Monad m => a -> m a return (BuiltInSynFamily -> [PredType] -> PredType -> [TypeEqn] sfInteractTop BuiltInSynFamily ops [PredType] args PredType rhs_ty) -- see Note [Type inference for type families with injectivity] | TyCon -> Bool isOpenTypeFamilyTyCon TyCon fam_tc , Injective injective_args :: [Bool] injective_args <- TyCon -> Injectivity tyConInjectivityInfo TyCon fam_tc , let fam_insts :: [FamInst] fam_insts = (FamInstEnv, FamInstEnv) -> TyCon -> [FamInst] lookupFamInstEnvByTyCon (FamInstEnv, FamInstEnv) fam_envs TyCon fam_tc = -- it is possible to have several compatible equations in an open type -- family but we only want to derive equalities from one such equation. do { let improvs :: [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] improvs = [FamInst] -> (FamInst -> [EvVar]) -> (FamInst -> [PredType]) -> (FamInst -> PredType) -> (FamInst -> Maybe CoAxBranch) -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] forall a. [a] -> (a -> [EvVar]) -> (a -> [PredType]) -> (a -> PredType) -> (a -> Maybe CoAxBranch) -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] buildImprovementData [FamInst] fam_insts FamInst -> [EvVar] fi_tvs FamInst -> [PredType] fi_tys FamInst -> PredType fi_rhs (Maybe CoAxBranch -> FamInst -> Maybe CoAxBranch forall a b. a -> b -> a const Maybe CoAxBranch forall a. Maybe a Nothing) ; String -> SDoc -> TcS () traceTcS "improve_top_fun_eqs2" ([([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] -> SDoc forall a. Outputable a => a -> SDoc ppr [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] improvs) ; (([PredType], TCvSubst, [EvVar], Maybe CoAxBranch) -> TcS [TypeEqn]) -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] -> TcS [TypeEqn] forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b] concatMapM ([Bool] -> ([PredType], TCvSubst, [EvVar], Maybe CoAxBranch) -> TcS [TypeEqn] injImproveEqns [Bool] injective_args) ([([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] -> TcS [TypeEqn]) -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] -> TcS [TypeEqn] forall a b. (a -> b) -> a -> b $ Int -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] forall a. Int -> [a] -> [a] take 1 [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] improvs } | Just ax :: CoAxiom Branched ax <- TyCon -> Maybe (CoAxiom Branched) isClosedSynFamilyTyConWithAxiom_maybe TyCon fam_tc , Injective injective_args :: [Bool] injective_args <- TyCon -> Injectivity tyConInjectivityInfo TyCon fam_tc = (([PredType], TCvSubst, [EvVar], Maybe CoAxBranch) -> TcS [TypeEqn]) -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] -> TcS [TypeEqn] forall (m :: * -> *) a b. Monad m => (a -> m [b]) -> [a] -> m [b] concatMapM ([Bool] -> ([PredType], TCvSubst, [EvVar], Maybe CoAxBranch) -> TcS [TypeEqn] injImproveEqns [Bool] injective_args) ([([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] -> TcS [TypeEqn]) -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] -> TcS [TypeEqn] forall a b. (a -> b) -> a -> b $ [CoAxBranch] -> (CoAxBranch -> [EvVar]) -> (CoAxBranch -> [PredType]) -> (CoAxBranch -> PredType) -> (CoAxBranch -> Maybe CoAxBranch) -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] forall a. [a] -> (a -> [EvVar]) -> (a -> [PredType]) -> (a -> PredType) -> (a -> Maybe CoAxBranch) -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] buildImprovementData (Branches Branched -> [CoAxBranch] forall (br :: BranchFlag). Branches br -> [CoAxBranch] fromBranches (CoAxiom Branched -> Branches Branched forall (br :: BranchFlag). CoAxiom br -> Branches br co_ax_branches CoAxiom Branched ax)) CoAxBranch -> [EvVar] cab_tvs CoAxBranch -> [PredType] cab_lhs CoAxBranch -> PredType cab_rhs CoAxBranch -> Maybe CoAxBranch forall a. a -> Maybe a Just | Bool otherwise = [TypeEqn] -> TcS [TypeEqn] forall (m :: * -> *) a. Monad m => a -> m a return [] where buildImprovementData :: [a] -- axioms for a TF (FamInst or CoAxBranch) -> (a -> [TyVar]) -- get bound tyvars of an axiom -> (a -> [Type]) -- get LHS of an axiom -> (a -> Type) -- get RHS of an axiom -> (a -> Maybe CoAxBranch) -- Just => apartness check required -> [( [Type], TCvSubst, [TyVar], Maybe CoAxBranch )] -- Result: -- ( [arguments of a matching axiom] -- , RHS-unifying substitution -- , axiom variables without substitution -- , Maybe matching axiom [Nothing - open TF, Just - closed TF ] ) buildImprovementData :: [a] -> (a -> [EvVar]) -> (a -> [PredType]) -> (a -> PredType) -> (a -> Maybe CoAxBranch) -> [([PredType], TCvSubst, [EvVar], Maybe CoAxBranch)] buildImprovementData axioms :: [a] axioms axiomTVs :: a -> [EvVar] axiomTVs axiomLHS :: a -> [PredType] axiomLHS axiomRHS :: a -> PredType axiomRHS wrap :: a -> Maybe CoAxBranch wrap = [ ([PredType] ax_args, TCvSubst subst, [EvVar] unsubstTvs, a -> Maybe CoAxBranch wrap a axiom) | a axiom <- [a] axioms , let ax_args :: [PredType] ax_args = a -> [PredType] axiomLHS a axiom ax_rhs :: PredType ax_rhs = a -> PredType axiomRHS a axiom ax_tvs :: [EvVar] ax_tvs = a -> [EvVar] axiomTVs a axiom , Just subst :: TCvSubst subst <- [Bool -> PredType -> PredType -> Maybe TCvSubst tcUnifyTyWithTFs Bool False PredType ax_rhs PredType rhs_ty] , let notInSubst :: EvVar -> Bool notInSubst tv :: EvVar tv = Bool -> Bool not (EvVar tv EvVar -> VarEnv PredType -> Bool forall a. EvVar -> VarEnv a -> Bool `elemVarEnv` TCvSubst -> VarEnv PredType getTvSubstEnv TCvSubst subst) unsubstTvs :: [EvVar] unsubstTvs = (EvVar -> Bool) -> [EvVar] -> [EvVar] forall a. (a -> Bool) -> [a] -> [a] filter (EvVar -> Bool notInSubst (EvVar -> Bool) -> (EvVar -> Bool) -> EvVar -> Bool forall (f :: * -> *). Applicative f => f Bool -> f Bool -> f Bool <&&> EvVar -> Bool isTyVar) [EvVar] ax_tvs ] -- The order of unsubstTvs is important; it must be -- in telescope order e.g. (k:*) (a:k) injImproveEqns :: [Bool] -> ([Type], TCvSubst, [TyCoVar], Maybe CoAxBranch) -> TcS [TypeEqn] injImproveEqns :: [Bool] -> ([PredType], TCvSubst, [EvVar], Maybe CoAxBranch) -> TcS [TypeEqn] injImproveEqns inj_args :: [Bool] inj_args (ax_args :: [PredType] ax_args, subst :: TCvSubst subst, unsubstTvs :: [EvVar] unsubstTvs, cabr :: Maybe CoAxBranch cabr) = do { TCvSubst subst <- TCvSubst -> [EvVar] -> TcS TCvSubst instFlexiX TCvSubst subst [EvVar] unsubstTvs -- If the current substitution bind [k -> *], and -- one of the un-substituted tyvars is (a::k), we'd better -- be sure to apply the current substitution to a's kind. -- Hence instFlexiX. Trac #13135 was an example. ; [TypeEqn] -> TcS [TypeEqn] forall (m :: * -> *) a. Monad m => a -> m a return [ PredType -> PredType -> TypeEqn forall a. a -> a -> Pair a Pair (TCvSubst -> PredType -> PredType substTyUnchecked TCvSubst subst PredType ax_arg) PredType arg -- NB: the ax_arg part is on the left -- see Note [Improvement orientation] | case Maybe CoAxBranch cabr of Just cabr' :: CoAxBranch cabr' -> [PredType] -> CoAxBranch -> Bool apartnessCheck (HasCallStack => TCvSubst -> [PredType] -> [PredType] TCvSubst -> [PredType] -> [PredType] substTys TCvSubst subst [PredType] ax_args) CoAxBranch cabr' _ -> Bool True , (ax_arg :: PredType ax_arg, arg :: PredType arg, True) <- [PredType] -> [PredType] -> [Bool] -> [(PredType, PredType, Bool)] forall a b c. [a] -> [b] -> [c] -> [(a, b, c)] zip3 [PredType] ax_args [PredType] args [Bool] inj_args ] } shortCutReduction :: CtEvidence -> TcTyVar -> TcCoercion -> TyCon -> [TcType] -> TcS () -- See Note [Top-level reductions for type functions] -- Previously, we flattened the tc_args here, but there's no need to do so. -- And, if we did, this function would have all the complication of -- TcCanonical.canCFunEqCan. See Note [canCFunEqCan] shortCutReduction :: CtEvidence -> EvVar -> TcCoercion -> TyCon -> [PredType] -> TcS () shortCutReduction old_ev :: CtEvidence old_ev fsk :: EvVar fsk ax_co :: TcCoercion ax_co fam_tc :: TyCon fam_tc tc_args :: [PredType] tc_args = ASSERT( ctEvEqRel old_ev == NomEq) -- ax_co :: F args ~ G tc_args -- old_ev :: F args ~ fsk do { CtEvidence new_ev <- case CtEvidence -> CtFlavour ctEvFlavour CtEvidence old_ev of Given -> CtLoc -> (PredType, EvTerm) -> TcS CtEvidence newGivenEvVar CtLoc deeper_loc ( PredType -> PredType -> PredType mkPrimEqPred (TyCon -> [PredType] -> PredType mkTyConApp TyCon fam_tc [PredType] tc_args) (EvVar -> PredType mkTyVarTy EvVar fsk) , TcCoercion -> EvTerm evCoercion (TcCoercion -> TcCoercion mkTcSymCo TcCoercion ax_co TcCoercion -> TcCoercion -> TcCoercion `mkTcTransCo` HasDebugCallStack => CtEvidence -> TcCoercion CtEvidence -> TcCoercion ctEvCoercion CtEvidence old_ev) ) Wanted {} -> do { (new_ev :: CtEvidence new_ev, new_co :: TcCoercion new_co) <- CtLoc -> Role -> PredType -> PredType -> TcS (CtEvidence, TcCoercion) newWantedEq CtLoc deeper_loc Role Nominal (TyCon -> [PredType] -> PredType mkTyConApp TyCon fam_tc [PredType] tc_args) (EvVar -> PredType mkTyVarTy EvVar fsk) ; TcEvDest -> TcCoercion -> TcS () setWantedEq (CtEvidence -> TcEvDest ctev_dest CtEvidence old_ev) (TcCoercion -> TcS ()) -> TcCoercion -> TcS () forall a b. (a -> b) -> a -> b $ TcCoercion ax_co TcCoercion -> TcCoercion -> TcCoercion `mkTcTransCo` TcCoercion new_co ; CtEvidence -> TcS CtEvidence forall (m :: * -> *) a. Monad m => a -> m a return CtEvidence new_ev } Derived -> String -> SDoc -> TcS CtEvidence forall a. HasCallStack => String -> SDoc -> a pprPanic "shortCutReduction" (CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence old_ev) ; let new_ct :: Ct new_ct = CFunEqCan :: CtEvidence -> TyCon -> [PredType] -> EvVar -> Ct CFunEqCan { cc_ev :: CtEvidence cc_ev = CtEvidence new_ev, cc_fun :: TyCon cc_fun = TyCon fam_tc , cc_tyargs :: [PredType] cc_tyargs = [PredType] tc_args, cc_fsk :: EvVar cc_fsk = EvVar fsk } ; (WorkList -> WorkList) -> TcS () updWorkListTcS (Ct -> WorkList -> WorkList extendWorkListFunEq Ct new_ct) } where deeper_loc :: CtLoc deeper_loc = CtLoc -> CtLoc bumpCtLocDepth (CtEvidence -> CtLoc ctEvLoc CtEvidence old_ev) {- Note [Top-level reductions for type functions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ c.f. Note [The flattening story] in TcFlatten Suppose we have a CFunEqCan F tys ~ fmv/fsk, and a matching axiom. Here is what we do, in four cases: * Wanteds: general firing rule (work item) [W] x : F tys ~ fmv instantiate axiom: ax_co : F tys ~ rhs Then: Discharge fmv := rhs Discharge x := ax_co ; sym x2 This is *the* way that fmv's get unified; even though they are "untouchable". NB: Given Note [FunEq occurs-check principle], fmv does not appear in tys, and hence does not appear in the instantiated RHS. So the unification can't make an infinite type. * Wanteds: short cut firing rule Applies when the RHS of the axiom is another type-function application (work item) [W] x : F tys ~ fmv instantiate axiom: ax_co : F tys ~ G rhs_tys It would be a waste to create yet another fmv for (G rhs_tys). Instead (shortCutReduction): - Flatten rhs_tys (cos : rhs_tys ~ rhs_xis) - Add G rhs_xis ~ fmv to flat cache (note: the same old fmv) - New canonical wanted [W] x2 : G rhs_xis ~ fmv (CFunEqCan) - Discharge x := ax_co ; G cos ; x2 * Givens: general firing rule (work item) [G] g : F tys ~ fsk instantiate axiom: ax_co : F tys ~ rhs Now add non-canonical given (since rhs is not flat) [G] (sym g ; ax_co) : fsk ~ rhs (Non-canonical) * Givens: short cut firing rule Applies when the RHS of the axiom is another type-function application (work item) [G] g : F tys ~ fsk instantiate axiom: ax_co : F tys ~ G rhs_tys It would be a waste to create yet another fsk for (G rhs_tys). Instead (shortCutReduction): - Flatten rhs_tys: flat_cos : tys ~ flat_tys - Add new Canonical given [G] (sym (G flat_cos) ; co ; g) : G flat_tys ~ fsk (CFunEqCan) Note [FunEq occurs-check principle] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I have spent a lot of time finding a good way to deal with CFunEqCan constraints like F (fuv, a) ~ fuv where flatten-skolem occurs on the LHS. Now in principle we might may progress by doing a reduction, but in practice its hard to find examples where it is useful, and easy to find examples where we fall into an infinite reduction loop. A rule that works very well is this: *** FunEq occurs-check principle *** Do not reduce a CFunEqCan F tys ~ fsk if fsk appears free in tys Instead we treat it as stuck. Examples: * Trac #5837 has [G] a ~ TF (a,Int), with an instance type instance TF (a,b) = (TF a, TF b) This readily loops when solving givens. But with the FunEq occurs check principle, it rapidly gets stuck which is fine. * Trac #12444 is a good example, explained in comment:2. We have type instance F (Succ x) = Succ (F x) [W] alpha ~ Succ (F alpha) If we allow the reduction to happen, we get an infinite loop Note [Cached solved FunEqs] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ When trying to solve, say (FunExpensive big-type ~ ty), it's important to see if we have reduced (FunExpensive big-type) before, lest we simply repeat it. Hence the lookup in inert_solved_funeqs. Moreover we must use `funEqCanDischarge` because both uses might (say) be Wanteds, and we *still* want to save the re-computation. Note [MATCHING-SYNONYMS] ~~~~~~~~~~~~~~~~~~~~~~~~ When trying to match a dictionary (D tau) to a top-level instance, or a type family equation (F taus_1 ~ tau_2) to a top-level family instance, we do *not* need to expand type synonyms because the matcher will do that for us. Note [Improvement orientation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A very delicate point is the orientation of derived equalities arising from injectivity improvement (Trac #12522). Suppse we have type family F x = t | t -> x type instance F (a, Int) = (Int, G a) where G is injective; and wanted constraints [W] TF (alpha, beta) ~ fuv [W] fuv ~ (Int, <some type>) The injectivity will give rise to derived constraints [D] gamma1 ~ alpha [D] Int ~ beta The fresh unification variable gamma1 comes from the fact that we can only do "partial improvement" here; see Section 5.2 of "Injective type families for Haskell" (HS'15). Now, it's very important to orient the equations this way round, so that the fresh unification variable will be eliminated in favour of alpha. If we instead had [D] alpha ~ gamma1 then we would unify alpha := gamma1; and kick out the wanted constraint. But when we grough it back in, it'd look like [W] TF (gamma1, beta) ~ fuv and exactly the same thing would happen again! Infinite loop. This all seems fragile, and it might seem more robust to avoid introducing gamma1 in the first place, in the case where the actual argument (alpha, beta) partly matches the improvement template. But that's a bit tricky, esp when we remember that the kinds much match too; so it's easier to let the normal machinery handle it. Instead we are careful to orient the new derived equality with the template on the left. Delicate, but it works. Note [No FunEq improvement for Givens] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We don't do improvements (injectivity etc) for Givens. Why? * It generates Derived constraints on skolems, which don't do us much good, except perhaps identify inaccessible branches. (They'd be perfectly valid though.) * For type-nat stuff the derived constraints include type families; e.g. (a < b), (b < c) ==> a < c If we generate a Derived for this, we'll generate a Derived/Wanted CFunEqCan; and, since the same InertCans (after solving Givens) are used for each iteration, that massively confused the unflattening step (TcFlatten.unflatten). In fact it led to some infinite loops: indexed-types/should_compile/T10806 indexed-types/should_compile/T10507 polykinds/T10742 Note [Reduction for Derived CFunEqCans] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You may wonder if it's important to use top-level instances to simplify [D] CFunEqCan's. But it is. Here's an example (T10226). type instance F Int = Int type instance FInv Int = Int Suppose we have to solve [WD] FInv (F alpha) ~ alpha [WD] F alpha ~ Int --> flatten [WD] F alpha ~ fuv0 [WD] FInv fuv0 ~ fuv1 -- (A) [WD] fuv1 ~ alpha [WD] fuv0 ~ Int -- (B) --> Rewwrite (A) with (B), splitting it [WD] F alpha ~ fuv0 [W] FInv fuv0 ~ fuv1 [D] FInv Int ~ fuv1 -- (C) [WD] fuv1 ~ alpha [WD] fuv0 ~ Int --> Reduce (C) with top-level instance **** This is the key step *** [WD] F alpha ~ fuv0 [W] FInv fuv0 ~ fuv1 [D] fuv1 ~ Int -- (D) [WD] fuv1 ~ alpha -- (E) [WD] fuv0 ~ Int --> Rewrite (D) with (E) [WD] F alpha ~ fuv0 [W] FInv fuv0 ~ fuv1 [D] alpha ~ Int -- (F) [WD] fuv1 ~ alpha [WD] fuv0 ~ Int --> unify (F) alpha := Int, and that solves it Another example is indexed-types/should_compile/T10634 -} {- ******************************************************************* * * 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 :: InertSet -> SimplifierStage doTopReactDict inerts :: InertSet inerts work_item :: Ct work_item@(CDictCan { cc_ev :: Ct -> CtEvidence cc_ev = CtEvidence ev, cc_class :: Ct -> Class cc_class = Class cls , cc_tyargs :: Ct -> [PredType] cc_tyargs = [PredType] xis }) | CtEvidence -> Bool isGiven CtEvidence ev -- Never use instances for Given constraints = do { TcS () try_fundep_improvement ; SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct work_item } | Just solved_ev :: CtEvidence solved_ev <- InertSet -> CtLoc -> Class -> [PredType] -> Maybe CtEvidence lookupSolvedDict InertSet inerts CtLoc dict_loc Class cls [PredType] xis -- Cached = do { CtEvidence -> EvTerm -> TcS () setEvBindIfWanted CtEvidence ev (CtEvidence -> EvTerm ctEvTerm CtEvidence solved_ev) ; CtEvidence -> String -> TcS (StopOrContinue Ct) forall a. CtEvidence -> String -> TcS (StopOrContinue a) stopWith CtEvidence ev "Dict/Top (cached)" } | Bool otherwise -- Wanted or Derived, but not cached = do { DynFlags dflags <- TcS DynFlags forall (m :: * -> *). HasDynFlags m => m DynFlags getDynFlags ; ClsInstResult lkup_res <- DynFlags -> InertSet -> Class -> [PredType] -> CtLoc -> TcS ClsInstResult matchClassInst DynFlags dflags InertSet inerts Class cls [PredType] xis CtLoc dict_loc ; case ClsInstResult lkup_res of OneInst { cir_what :: ClsInstResult -> InstanceWhat cir_what = InstanceWhat what } -> do { Bool -> TcS () -> TcS () forall (f :: * -> *). Applicative f => Bool -> f () -> f () unless (InstanceWhat -> Bool safeOverlap InstanceWhat what) (TcS () -> TcS ()) -> TcS () -> TcS () forall a b. (a -> b) -> a -> b $ Ct -> TcS () insertSafeOverlapFailureTcS Ct work_item ; Bool -> TcS () -> TcS () forall (f :: * -> *). Applicative f => Bool -> f () -> f () when (CtEvidence -> Bool isWanted CtEvidence ev) (TcS () -> TcS ()) -> TcS () -> TcS () forall a b. (a -> b) -> a -> b $ CtEvidence -> Class -> [PredType] -> TcS () addSolvedDict CtEvidence ev Class cls [PredType] xis ; Ct -> ClsInstResult -> TcS (StopOrContinue Ct) chooseInstance Ct work_item ClsInstResult lkup_res } _ -> -- NoInstance or NotSure do { Bool -> TcS () -> TcS () forall (f :: * -> *). Applicative f => Bool -> f () -> f () when (CtEvidence -> Bool isImprovable CtEvidence ev) (TcS () -> TcS ()) -> TcS () -> TcS () forall a b. (a -> b) -> a -> b $ TcS () try_fundep_improvement ; SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct work_item } } where dict_pred :: PredType dict_pred = Class -> [PredType] -> PredType mkClassPred Class cls [PredType] xis dict_loc :: CtLoc dict_loc = CtEvidence -> CtLoc ctEvLoc CtEvidence ev dict_origin :: CtOrigin dict_origin = CtLoc -> CtOrigin ctLocOrigin CtLoc dict_loc -- We didn't solve it; so try functional dependencies with -- the instance environment, and return -- See also Note [Weird fundeps] try_fundep_improvement :: TcS () try_fundep_improvement = do { String -> SDoc -> TcS () traceTcS "try_fundeps" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct work_item) ; InstEnvs instEnvs <- TcS InstEnvs getInstEnvs ; [FunDepEqn CtLoc] -> TcS () emitFunDepDeriveds ([FunDepEqn CtLoc] -> TcS ()) -> [FunDepEqn CtLoc] -> TcS () forall a b. (a -> b) -> a -> b $ InstEnvs -> (PredType -> SrcSpan -> CtLoc) -> PredType -> [FunDepEqn CtLoc] forall loc. InstEnvs -> (PredType -> SrcSpan -> loc) -> PredType -> [FunDepEqn loc] improveFromInstEnv InstEnvs instEnvs PredType -> SrcSpan -> CtLoc mk_ct_loc PredType dict_pred } mk_ct_loc :: PredType -- From instance decl -> SrcSpan -- also from instance deol -> CtLoc mk_ct_loc :: PredType -> SrcSpan -> CtLoc mk_ct_loc inst_pred :: PredType inst_pred inst_loc :: SrcSpan inst_loc = CtLoc dict_loc { ctl_origin :: CtOrigin ctl_origin = PredType -> CtOrigin -> PredType -> SrcSpan -> CtOrigin FunDepOrigin2 PredType dict_pred CtOrigin dict_origin PredType inst_pred SrcSpan inst_loc } doTopReactDict _ w :: Ct w = String -> SDoc -> TcS (StopOrContinue Ct) forall a. HasCallStack => String -> SDoc -> a pprPanic "doTopReactDict" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct w) chooseInstance :: Ct -> ClsInstResult -> TcS (StopOrContinue Ct) chooseInstance :: Ct -> ClsInstResult -> TcS (StopOrContinue Ct) chooseInstance work_item :: Ct work_item (OneInst { cir_new_theta :: ClsInstResult -> [PredType] cir_new_theta = [PredType] theta , cir_what :: ClsInstResult -> InstanceWhat cir_what = InstanceWhat what , cir_mk_ev :: ClsInstResult -> [EvExpr] -> EvTerm cir_mk_ev = [EvExpr] -> EvTerm mk_ev }) = do { String -> SDoc -> TcS () traceTcS "doTopReact/found instance for" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ CtEvidence -> SDoc forall a. Outputable a => a -> SDoc ppr CtEvidence ev ; CtLoc deeper_loc <- CtLoc -> InstanceWhat -> PredType -> TcS CtLoc checkInstanceOK CtLoc loc InstanceWhat what PredType pred ; if CtEvidence -> Bool isDerived CtEvidence ev then CtLoc -> [PredType] -> TcS (StopOrContinue Ct) forall a. CtLoc -> [PredType] -> TcS (StopOrContinue a) finish_derived CtLoc deeper_loc [PredType] theta else CtLoc -> [PredType] -> ([EvExpr] -> EvTerm) -> TcS (StopOrContinue Ct) finish_wanted CtLoc deeper_loc [PredType] theta [EvExpr] -> EvTerm mk_ev } where ev :: CtEvidence ev = Ct -> CtEvidence ctEvidence Ct work_item pred :: PredType pred = CtEvidence -> PredType ctEvPred CtEvidence ev loc :: CtLoc loc = CtEvidence -> CtLoc ctEvLoc CtEvidence ev finish_wanted :: CtLoc -> [TcPredType] -> ([EvExpr] -> EvTerm) -> TcS (StopOrContinue Ct) -- Precondition: evidence term matches the predicate workItem finish_wanted :: CtLoc -> [PredType] -> ([EvExpr] -> EvTerm) -> TcS (StopOrContinue Ct) finish_wanted loc :: CtLoc loc theta :: [PredType] theta mk_ev :: [EvExpr] -> EvTerm mk_ev = do { EvBindsVar evb <- TcS EvBindsVar getTcEvBindsVar ; if EvBindsVar -> Bool isCoEvBindsVar EvBindsVar evb then -- See Note [Instances in no-evidence implications] SimplifierStage forall a. a -> TcS (StopOrContinue a) continueWith Ct work_item else do { [MaybeNew] evc_vars <- (PredType -> TcS MaybeNew) -> [PredType] -> TcS [MaybeNew] forall (t :: * -> *) (m :: * -> *) a b. (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b) mapM (CtLoc -> PredType -> TcS MaybeNew newWanted CtLoc loc) [PredType] theta ; CtEvidence -> EvTerm -> TcS () setEvBindIfWanted CtEvidence ev ([EvExpr] -> EvTerm mk_ev ((MaybeNew -> EvExpr) -> [MaybeNew] -> [EvExpr] forall a b. (a -> b) -> [a] -> [b] map MaybeNew -> EvExpr getEvExpr [MaybeNew] evc_vars)) ; [CtEvidence] -> TcS () emitWorkNC ([MaybeNew] -> [CtEvidence] freshGoals [MaybeNew] evc_vars) ; CtEvidence -> String -> TcS (StopOrContinue Ct) forall a. CtEvidence -> String -> TcS (StopOrContinue a) stopWith CtEvidence ev "Dict/Top (solved wanted)" } } finish_derived :: CtLoc -> [PredType] -> TcS (StopOrContinue a) finish_derived loc :: CtLoc loc theta :: [PredType] theta = -- Use type-class instances for Deriveds, in the hope -- of generating some improvements -- C.f. Example 3 of Note [The improvement story] -- It's easy because no evidence is involved do { CtLoc -> [PredType] -> TcS () emitNewDeriveds CtLoc loc [PredType] theta ; String -> SDoc -> TcS () traceTcS "finish_derived" (SubGoalDepth -> SDoc forall a. Outputable a => a -> SDoc ppr (CtLoc -> SubGoalDepth ctl_depth CtLoc loc)) ; CtEvidence -> String -> TcS (StopOrContinue a) forall a. CtEvidence -> String -> TcS (StopOrContinue a) stopWith CtEvidence ev "Dict/Top (solved derived)" } chooseInstance work_item :: Ct work_item lookup_res :: ClsInstResult lookup_res = String -> SDoc -> TcS (StopOrContinue Ct) forall a. HasCallStack => String -> SDoc -> a pprPanic "chooseInstance" (Ct -> SDoc forall a. Outputable a => a -> SDoc ppr Ct work_item SDoc -> SDoc -> SDoc $$ ClsInstResult -> SDoc forall a. Outputable a => a -> SDoc ppr ClsInstResult lookup_res) checkInstanceOK :: CtLoc -> InstanceWhat -> TcPredType -> TcS CtLoc -- Check that it's OK to use this insstance: -- (a) the use is well staged in the Template Haskell sense -- (b) we have not recursed too deep -- Returns the CtLoc to used for sub-goals checkInstanceOK :: CtLoc -> InstanceWhat -> PredType -> TcS CtLoc checkInstanceOK loc :: CtLoc loc what :: InstanceWhat what pred :: PredType pred = do { CtLoc -> InstanceWhat -> PredType -> TcS () checkWellStagedDFun CtLoc loc InstanceWhat what PredType pred ; CtLoc -> PredType -> TcS () checkReductionDepth CtLoc deeper_loc PredType pred ; CtLoc -> TcS CtLoc forall (m :: * -> *) a. Monad m => a -> m a return CtLoc deeper_loc } where deeper_loc :: CtLoc deeper_loc = CtLoc -> CtLoc zap_origin (CtLoc -> CtLoc bumpCtLocDepth CtLoc loc) origin :: CtOrigin origin = CtLoc -> CtOrigin ctLocOrigin CtLoc loc zap_origin :: CtLoc -> CtLoc zap_origin loc :: CtLoc loc -- After applying an instance we can set ScOrigin to -- infinity, so that prohibitedSuperClassSolve never fires | ScOrigin {} <- CtOrigin origin = CtLoc -> CtOrigin -> CtLoc setCtLocOrigin CtLoc loc (IntWithInf -> CtOrigin ScOrigin IntWithInf infinity) | Bool otherwise = CtLoc loc {- Note [Instances in no-evidence implications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In Trac #15290 we had [G] forall p q. Coercible p q => Coercible (m p) (m q)) [W] forall <no-ev> a. m (Int, IntStateT m a) ~R# m (Int, StateT Int m a) The Given is an ordinary quantified constraint; the Wanted is an implication equality that arises from [W] (forall a. t1) ~R# (forall a. t2) But because the (t1 ~R# t2) is solved "inside a type" (under that forall a) we can't generate any term evidence. So we can't actually use that lovely quantified constraint. Alas! This test arranges to ignore the instance-based solution under these (rare) circumstances. It's sad, but I really don't see what else we can do. -} matchClassInst :: DynFlags -> InertSet -> Class -> [Type] -> CtLoc -> TcS ClsInstResult matchClassInst :: DynFlags -> InertSet -> Class -> [PredType] -> CtLoc -> TcS ClsInstResult matchClassInst dflags :: DynFlags dflags inerts :: InertSet inerts clas :: Class clas tys :: [PredType] tys loc :: CtLoc 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. -- ee Note [Instance and Given overlap] | Bool -> Bool not (Extension -> DynFlags -> Bool xopt Extension LangExt.IncoherentInstances DynFlags dflags) , Bool -> Bool not (Class -> Bool naturallyCoherentClass Class clas) , let matchable_givens :: Cts matchable_givens = CtLoc -> PredType -> InertSet -> Cts matchableGivens CtLoc loc PredType pred InertSet inerts , Bool -> Bool not (Cts -> Bool forall a. Bag a -> Bool isEmptyBag Cts matchable_givens) = do { String -> SDoc -> TcS () traceTcS "Delaying instance application" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc vcat [ String -> SDoc text "Work item=" SDoc -> SDoc -> SDoc <+> Class -> [PredType] -> SDoc pprClassPred Class clas [PredType] tys , String -> SDoc text "Potential matching givens:" SDoc -> SDoc -> SDoc <+> Cts -> SDoc forall a. Outputable a => a -> SDoc ppr Cts matchable_givens ] ; ClsInstResult -> TcS ClsInstResult forall (m :: * -> *) a. Monad m => a -> m a return ClsInstResult NotSure } | Bool otherwise = do { String -> SDoc -> TcS () traceTcS "matchClassInst" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ String -> SDoc text "pred =" SDoc -> SDoc -> SDoc <+> PredType -> SDoc forall a. Outputable a => a -> SDoc ppr PredType pred SDoc -> SDoc -> SDoc <+> Char -> SDoc char '{' ; ClsInstResult local_res <- PredType -> CtLoc -> TcS ClsInstResult matchLocalInst PredType pred CtLoc loc ; case ClsInstResult local_res of OneInst {} -> -- See Note [Local instances and incoherence] do { String -> SDoc -> TcS () traceTcS "} matchClassInst local match" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ ClsInstResult -> SDoc forall a. Outputable a => a -> SDoc ppr ClsInstResult local_res ; ClsInstResult -> TcS ClsInstResult forall (m :: * -> *) a. Monad m => a -> m a return ClsInstResult local_res } NotSure -> -- In the NotSure case for local instances -- we don't want to try global instances do { String -> SDoc -> TcS () traceTcS "} matchClassInst local not sure" SDoc empty ; ClsInstResult -> TcS ClsInstResult forall (m :: * -> *) a. Monad m => a -> m a return ClsInstResult local_res } NoInstance -- No local instances, so try global ones -> do { ClsInstResult global_res <- DynFlags -> Bool -> Class -> [PredType] -> TcS ClsInstResult matchGlobalInst DynFlags dflags Bool False Class clas [PredType] tys ; String -> SDoc -> TcS () traceTcS "} matchClassInst global result" (SDoc -> TcS ()) -> SDoc -> TcS () forall a b. (a -> b) -> a -> b $ ClsInstResult -> SDoc forall a. Outputable a => a -> SDoc ppr ClsInstResult global_res ; ClsInstResult -> TcS ClsInstResult forall (m :: * -> *) a. Monad m => a -> m a return ClsInstResult global_res } } where pred :: PredType pred = Class -> [PredType] -> PredType mkClassPred Class clas [PredType] 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 class story] in TysPrim. naturallyCoherentClass :: Class -> Bool naturallyCoherentClass :: Class -> Bool naturallyCoherentClass cls :: Class cls = Class -> Bool isCTupleClass Class cls Bool -> Bool -> Bool || Class cls Class -> Unique -> Bool forall a. Uniquable a => a -> Unique -> Bool `hasKey` Unique heqTyConKey Bool -> Bool -> Bool || Class cls Class -> Unique -> Bool forall a. Uniquable a => a -> Unique -> Bool `hasKey` Unique eqTyConKey Bool -> Bool -> Bool || Class cls Class -> Unique -> Bool forall a. Uniquable a => a -> Unique -> Bool `hasKey` Unique 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 impliation 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 Trac #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 * Flatten-skolems: we do not treat a flatten-skolem as unifiable for this purpose. E.g. f :: Eq (F a) => [a] -> [a] f xs = ....(xs==xs)..... Here we get [W] Eq [a], and we don't want to refrain from solving it because of the given (Eq (F a)) constraint! * 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 Trac #10195; another is #10177. * We ignore the overlap problem if -XIncoherentInstances is in force: see Trac #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. Trac #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 TcValidity 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 TysPrim.) 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 TcSMonad 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 Trac #14218 Exammples: 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 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. Consdider 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 :: PredType -> CtLoc -> TcS ClsInstResult matchLocalInst pred :: PredType pred loc :: CtLoc loc = do { InertCans ics <- TcS InertCans getInertCans ; case [QCInst] -> ([(CtEvidence, [DFunInstType])], Bool) match_local_inst (InertCans -> [QCInst] inert_insts InertCans ics) of ([], False) -> ClsInstResult -> TcS ClsInstResult forall (m :: * -> *) a. Monad m => a -> m a return ClsInstResult NoInstance ([(dfun_ev :: CtEvidence dfun_ev, inst_tys :: [DFunInstType] inst_tys)], unifs :: Bool unifs) | Bool -> Bool not Bool unifs -> do { let dfun_id :: EvVar dfun_id = CtEvidence -> EvVar ctEvEvId CtEvidence dfun_ev ; (tys :: [PredType] tys, theta :: [PredType] theta) <- EvVar -> [DFunInstType] -> TcS ([PredType], [PredType]) instDFunType EvVar dfun_id [DFunInstType] inst_tys ; ClsInstResult -> TcS ClsInstResult forall (m :: * -> *) a. Monad m => a -> m a return (ClsInstResult -> TcS ClsInstResult) -> ClsInstResult -> TcS ClsInstResult forall a b. (a -> b) -> a -> b $ OneInst :: [PredType] -> ([EvExpr] -> EvTerm) -> InstanceWhat -> ClsInstResult OneInst { cir_new_theta :: [PredType] cir_new_theta = [PredType] theta , cir_mk_ev :: [EvExpr] -> EvTerm cir_mk_ev = EvVar -> [PredType] -> [EvExpr] -> EvTerm evDFunApp EvVar dfun_id [PredType] tys , cir_what :: InstanceWhat cir_what = InstanceWhat LocalInstance } } _ -> ClsInstResult -> TcS ClsInstResult forall (m :: * -> *) a. Monad m => a -> m a return ClsInstResult NotSure } where pred_tv_set :: VarSet pred_tv_set = PredType -> VarSet tyCoVarsOfType PredType pred match_local_inst :: [QCInst] -> ( [(CtEvidence, [DFunInstType])] , Bool ) -- True <=> Some unify but do not match match_local_inst :: [QCInst] -> ([(CtEvidence, [DFunInstType])], Bool) match_local_inst [] = ([], Bool False) match_local_inst (qci :: QCInst qci@(QCI { qci_tvs :: QCInst -> [EvVar] qci_tvs = [EvVar] qtvs, qci_pred :: QCInst -> PredType qci_pred = PredType qpred , qci_ev :: QCInst -> CtEvidence qci_ev = CtEvidence ev }) : qcis :: [QCInst] qcis) | let in_scope :: InScopeSet in_scope = VarSet -> InScopeSet mkInScopeSet (VarSet qtv_set VarSet -> VarSet -> VarSet `unionVarSet` VarSet pred_tv_set) , Just tv_subst :: VarEnv PredType tv_subst <- VarSet -> RnEnv2 -> VarEnv PredType -> PredType -> PredType -> Maybe (VarEnv PredType) ruleMatchTyKiX VarSet qtv_set (InScopeSet -> RnEnv2 mkRnEnv2 InScopeSet in_scope) VarEnv PredType emptyTvSubstEnv PredType qpred PredType pred , let match :: (CtEvidence, [DFunInstType]) match = (CtEvidence ev, (EvVar -> DFunInstType) -> [EvVar] -> [DFunInstType] forall a b. (a -> b) -> [a] -> [b] map (VarEnv PredType -> EvVar -> DFunInstType forall a. VarEnv a -> EvVar -> Maybe a lookupVarEnv VarEnv PredType tv_subst) [EvVar] qtvs) = ((CtEvidence, [DFunInstType]) match(CtEvidence, [DFunInstType]) -> [(CtEvidence, [DFunInstType])] -> [(CtEvidence, [DFunInstType])] forall a. a -> [a] -> [a] :[(CtEvidence, [DFunInstType])] matches, Bool unif) | Bool otherwise = ASSERT2( disjointVarSet qtv_set (tyCoVarsOfType pred) , ppr qci $$ ppr pred ) -- ASSERT: unification relies on the -- quantified variables being fresh ([(CtEvidence, [DFunInstType])] matches, Bool unif Bool -> Bool -> Bool || Bool this_unif) where qtv_set :: VarSet qtv_set = [EvVar] -> VarSet mkVarSet [EvVar] qtvs this_unif :: Bool this_unif = PredType -> CtLoc -> PredType -> CtLoc -> Bool mightMatchLater PredType qpred (CtEvidence -> CtLoc ctEvLoc CtEvidence ev) PredType pred CtLoc loc (matches :: [(CtEvidence, [DFunInstType])] matches, unif :: Bool unif) = [QCInst] -> ([(CtEvidence, [DFunInstType])], Bool) match_local_inst [QCInst] qcis