{- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 Type subsumption and unification -} {-# LANGUAGE CPP, MultiWayIf, TupleSections #-} module TcUnify ( -- Full-blown subsumption tcWrapResult, tcWrapResultO, tcSkolemise, tcSkolemiseET, tcSubTypeHR, tcSubType, tcSubTypeO, tcSubType_NC, tcSubTypeDS, tcSubTypeDS_O, tcSubTypeDS_NC, tcSubTypeDS_NC_O, tcSubTypeET, tcSubTypeET_NC, checkConstraints, buildImplicationFor, -- Various unifications unifyType_, unifyType, unifyTheta, unifyKind, noThing, uType, unifyExpType, -------------------------------- -- Holes tcInfer, matchExpectedListTy, matchExpectedPArrTy, matchExpectedTyConApp, matchExpectedAppTy, matchExpectedFunTys, matchActualFunTys, matchActualFunTysPart, matchExpectedFunKind, wrapFunResCoercion ) where #include "HsVersions.h" import HsSyn import TyCoRep import TcMType import TcRnMonad import TcType import Type import Coercion import TcEvidence import Name ( isSystemName ) import Inst import TyCon import TysWiredIn import Var import VarSet import VarEnv import ErrUtils import DynFlags import BasicTypes import Name ( Name ) import Bag import Util import Outputable import FastString import Control.Monad import Control.Arrow ( second ) {- ************************************************************************ * * matchExpected functions * * ************************************************************************ Note [Herald for matchExpectedFunTys] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The 'herald' always looks like: "The equation(s) for 'f' have" "The abstraction (\x.e) takes" "The section (+ x) expects" "The function 'f' is applied to" This is used to construct a message of form The abstraction `\Just 1 -> ...' takes two arguments but its type `Maybe a -> a' has only one The equation(s) for `f' have two arguments but its type `Maybe a -> a' has only one The section `(f 3)' requires 'f' to take two arguments but its type `Int -> Int' has only one The function 'f' is applied to two arguments but its type `Int -> Int' has only one Note [matchExpectedFunTys] ~~~~~~~~~~~~~~~~~~~~~~~~~~ matchExpectedFunTys checks that a sigma has the form of an n-ary function. It passes the decomposed type to the thing_inside, and returns a wrapper to coerce between the two types It's used wherever a language construct must have a functional type, namely: A lambda expression A function definition An operator section This function must be written CPS'd because it needs to fill in the ExpTypes produced for arguments before it can fill in the ExpType passed in. -} -- Use this one when you have an "expected" type. matchExpectedFunTys :: SDoc -- See Note [Herald for matchExpectedFunTys] -> Arity -> ExpRhoType -- deeply skolemised -> ([ExpSigmaType] -> ExpRhoType -> TcM a) -- must fill in these ExpTypes here -> TcM (a, HsWrapper) -- If matchExpectedFunTys n ty = (_, wrap) -- then wrap : (t1 -> ... -> tn -> ty_r) "->" ty, -- where [t1, ..., tn], ty_r are passed to the thing_inside matchExpectedFunTys herald arity orig_ty thing_inside = case orig_ty of Check ty -> go [] arity ty _ -> defer [] arity orig_ty where go acc_arg_tys 0 ty = do { result <- thing_inside (reverse acc_arg_tys) (mkCheckExpType ty) ; return (result, idHsWrapper) } go acc_arg_tys n ty | Just ty' <- coreView ty = go acc_arg_tys n ty' go acc_arg_tys n (ForAllTy (Anon arg_ty) res_ty) = ASSERT( not (isPredTy arg_ty) ) do { (result, wrap_res) <- go (mkCheckExpType arg_ty : acc_arg_tys) (n-1) res_ty ; return ( result , mkWpFun idHsWrapper wrap_res arg_ty res_ty ) } go acc_arg_tys n ty@(TyVarTy tv) | ASSERT( isTcTyVar tv) isMetaTyVar tv = do { cts <- readMetaTyVar tv ; case cts of Indirect ty' -> go acc_arg_tys n ty' Flexi -> defer acc_arg_tys n (mkCheckExpType ty) } -- In all other cases we bale out into ordinary unification -- However unlike the meta-tyvar case, we are sure that the -- number of arguments doesn't match arity of the original -- type, so we can add a bit more context to the error message -- (cf Trac #7869). -- -- It is not always an error, because specialized type may have -- different arity, for example: -- -- > f1 = f2 'a' -- > f2 :: Monad m => m Bool -- > f2 = undefined -- -- But in that case we add specialized type into error context -- anyway, because it may be useful. See also Trac #9605. go acc_arg_tys n ty = addErrCtxtM mk_ctxt $ defer acc_arg_tys n (mkCheckExpType ty) ------------ defer acc_arg_tys n fun_ty = do { more_arg_tys <- replicateM n newOpenInferExpType ; res_ty <- newOpenInferExpType ; result <- thing_inside (reverse acc_arg_tys ++ more_arg_tys) res_ty ; more_arg_tys <- mapM readExpType more_arg_tys ; res_ty <- readExpType res_ty ; let unif_fun_ty = mkFunTys more_arg_tys res_ty ; wrap <- tcSubTypeDS GenSigCtxt noThing unif_fun_ty fun_ty ; return (result, wrap) } ------------ mk_ctxt :: TidyEnv -> TcM (TidyEnv, MsgDoc) mk_ctxt env = do { (env', ty) <- zonkTidyTcType env orig_tc_ty ; let (args, _) = tcSplitFunTys ty n_actual = length args (env'', orig_ty') = tidyOpenType env' orig_tc_ty ; return ( env'' , mk_fun_tys_msg orig_ty' ty n_actual arity herald) } where orig_tc_ty = checkingExpType "matchExpectedFunTys" orig_ty -- this is safe b/c we're called from "go" -- Like 'matchExpectedFunTys', but used when you have an "actual" type, -- for example in function application matchActualFunTys :: Outputable a => SDoc -- See Note [Herald for matchExpectedFunTys] -> CtOrigin -> Maybe a -- the thing with type TcSigmaType -> Arity -> TcSigmaType -> TcM (HsWrapper, [TcSigmaType], TcSigmaType) -- If matchActualFunTys n ty = (wrap, [t1,..,tn], ty_r) -- then wrap : ty "->" (t1 -> ... -> tn -> ty_r) matchActualFunTys herald ct_orig mb_thing arity ty = matchActualFunTysPart herald ct_orig mb_thing arity ty [] arity -- | Variant of 'matchActualFunTys' that works when supplied only part -- (that is, to the right of some arrows) of the full function type matchActualFunTysPart :: Outputable a => SDoc -- See Note [Herald for matchExpectedFunTys] -> CtOrigin -> Maybe a -- the thing with type TcSigmaType -> Arity -> TcSigmaType -> [TcSigmaType] -- reversed args. See (*) below. -> Arity -- overall arity of the function, for errs -> TcM (HsWrapper, [TcSigmaType], TcSigmaType) matchActualFunTysPart herald ct_orig mb_thing arity orig_ty orig_old_args full_arity = go arity orig_old_args orig_ty -- Does not allocate unnecessary meta variables: if the input already is -- a function, we just take it apart. Not only is this efficient, -- it's important for higher rank: the argument might be of form -- (forall a. ty) -> other -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd -- hide the forall inside a meta-variable -- (*) Sometimes it's necessary to call matchActualFunTys with only part -- (that is, to the right of some arrows) of the type of the function in -- question. (See TcExpr.tcArgs.) This argument is the reversed list of -- arguments already seen (that is, not part of the TcSigmaType passed -- in elsewhere). where -- This function has a bizarre mechanic: it accumulates arguments on -- the way down and also builds an argument list on the way up. Why: -- 1. The returns args list and the accumulated args list might be different. -- The accumulated args include all the arg types for the function, -- including those from before this function was called. The returned -- list should include only those arguments produced by this call of -- matchActualFunTys -- -- 2. The HsWrapper can be built only on the way up. It seems (more) -- bizarre to build the HsWrapper but not the arg_tys. -- -- Refactoring is welcome. go :: Arity -> [TcSigmaType] -- accumulator of arguments (reversed) -> TcSigmaType -- the remainder of the type as we're processing -> TcM (HsWrapper, [TcSigmaType], TcSigmaType) go 0 _ ty = return (idHsWrapper, [], ty) go n acc_args ty | not (null tvs && null theta) = do { (wrap1, rho) <- topInstantiate ct_orig ty ; (wrap2, arg_tys, res_ty) <- go n acc_args rho ; return (wrap2 <.> wrap1, arg_tys, res_ty) } where (tvs, theta, _) = tcSplitSigmaTy ty go n acc_args ty | Just ty' <- coreView ty = go n acc_args ty' go n acc_args (ForAllTy (Anon arg_ty) res_ty) = ASSERT( not (isPredTy arg_ty) ) do { (wrap_res, tys, ty_r) <- go (n-1) (arg_ty : acc_args) res_ty ; return ( mkWpFun idHsWrapper wrap_res arg_ty ty_r , arg_ty : tys, ty_r ) } go n acc_args ty@(TyVarTy tv) | ASSERT( isTcTyVar tv) isMetaTyVar tv = do { cts <- readMetaTyVar tv ; case cts of Indirect ty' -> go n acc_args ty' Flexi -> defer n ty } -- In all other cases we bale out into ordinary unification -- However unlike the meta-tyvar case, we are sure that the -- number of arguments doesn't match arity of the original -- type, so we can add a bit more context to the error message -- (cf Trac #7869). -- -- It is not always an error, because specialized type may have -- different arity, for example: -- -- > f1 = f2 'a' -- > f2 :: Monad m => m Bool -- > f2 = undefined -- -- But in that case we add specialized type into error context -- anyway, because it may be useful. See also Trac #9605. go n acc_args ty = addErrCtxtM (mk_ctxt (reverse acc_args) ty) $ defer n ty ------------ defer n fun_ty = do { arg_tys <- replicateM n newOpenFlexiTyVarTy ; res_ty <- newOpenFlexiTyVarTy ; let unif_fun_ty = mkFunTys arg_tys res_ty ; co <- unifyType mb_thing fun_ty unif_fun_ty ; return (mkWpCastN co, arg_tys, res_ty) } ------------ mk_ctxt :: [TcSigmaType] -> TcSigmaType -> TidyEnv -> TcM (TidyEnv, MsgDoc) mk_ctxt arg_tys res_ty env = do { let ty = mkFunTys arg_tys res_ty ; (env1, zonked) <- zonkTidyTcType env ty -- zonking might change # of args ; let (zonked_args, _) = tcSplitFunTys zonked n_actual = length zonked_args (env2, unzonked) = tidyOpenType env1 ty ; return ( env2 , mk_fun_tys_msg unzonked zonked n_actual full_arity herald) } mk_fun_tys_msg :: TcType -- the full type passed in (unzonked) -> TcType -- the full type passed in (zonked) -> Arity -- the # of args found -> Arity -- the # of args wanted -> SDoc -- overall herald -> SDoc mk_fun_tys_msg full_ty ty n_args full_arity herald = herald <+> speakNOf full_arity (text "argument") <> comma $$ if n_args == full_arity then text "its type is" <+> quotes (pprType full_ty) <> comma $$ text "it is specialized to" <+> quotes (pprType ty) else sep [text "but its type" <+> quotes (pprType ty), if n_args == 0 then text "has none" else text "has only" <+> speakN n_args] ---------------------- matchExpectedListTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType) -- Special case for lists matchExpectedListTy exp_ty = do { (co, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty ; return (co, elt_ty) } ---------------------- matchExpectedPArrTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType) -- Special case for parrs matchExpectedPArrTy exp_ty = do { (co, [elt_ty]) <- matchExpectedTyConApp parrTyCon exp_ty ; return (co, elt_ty) } --------------------- matchExpectedTyConApp :: TyCon -- T :: forall kv1 ... kvm. k1 -> ... -> kn -> * -> TcRhoType -- orig_ty -> TcM (TcCoercionN, -- T k1 k2 k3 a b c ~N orig_ty [TcSigmaType]) -- Element types, k1 k2 k3 a b c -- It's used for wired-in tycons, so we call checkWiredInTyCon -- Precondition: never called with FunTyCon -- Precondition: input type :: * -- Postcondition: (T k1 k2 k3 a b c) is well-kinded matchExpectedTyConApp tc orig_ty = go orig_ty where go ty | Just ty' <- coreView ty = go ty' go ty@(TyConApp tycon args) | tc == tycon -- Common case = return (mkTcNomReflCo ty, args) go (TyVarTy tv) | ASSERT( isTcTyVar tv) isMetaTyVar tv = do { cts <- readMetaTyVar tv ; case cts of Indirect ty -> go ty Flexi -> defer } go _ = defer -- If the common case does not occur, instantiate a template -- T k1 .. kn t1 .. tm, and unify with the original type -- Doing it this way ensures that the types we return are -- kind-compatible with T. For example, suppose we have -- matchExpectedTyConApp T (f Maybe) -- where data T a = MkT a -- Then we don't want to instantate T's data constructors with -- (a::*) ~ Maybe -- because that'll make types that are utterly ill-kinded. -- This happened in Trac #7368 defer = do { (_subst, args) <- tcInstBinders (tyConBinders tc) ; co <- unifyType noThing (mkTyConApp tc args) orig_ty ; return (co, args) } ---------------------- matchExpectedAppTy :: TcRhoType -- orig_ty -> TcM (TcCoercion, -- m a ~N orig_ty (TcSigmaType, TcSigmaType)) -- Returns m, a -- If the incoming type is a mutable type variable of kind k, then -- matchExpectedAppTy returns a new type variable (m: * -> k); note the *. matchExpectedAppTy orig_ty = go orig_ty where go ty | Just ty' <- coreView ty = go ty' | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty = return (mkTcNomReflCo orig_ty, (fun_ty, arg_ty)) go (TyVarTy tv) | ASSERT( isTcTyVar tv) isMetaTyVar tv = do { cts <- readMetaTyVar tv ; case cts of Indirect ty -> go ty Flexi -> defer } go _ = defer -- Defer splitting by generating an equality constraint defer = do { ty1 <- newFlexiTyVarTy kind1 ; ty2 <- newFlexiTyVarTy kind2 ; co <- unifyType noThing (mkAppTy ty1 ty2) orig_ty ; return (co, (ty1, ty2)) } orig_kind = typeKind orig_ty kind1 = mkFunTy liftedTypeKind orig_kind kind2 = liftedTypeKind -- m :: * -> k -- arg type :: * {- ************************************************************************ * * Subsumption checking * * ************************************************************************ Note [Subsumption checking: tcSubType] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ All the tcSubType calls have the form tcSubType actual_ty expected_ty which checks actual_ty <= expected_ty That is, that a value of type actual_ty is acceptable in a place expecting a value of type expected_ty. I.e. that actual ty is more polymorphic than expected_ty It returns a coercion function co_fn :: actual_ty ~ expected_ty which takes an HsExpr of type actual_ty into one of type expected_ty. These functions do not actually check for subsumption. They check if expected_ty is an appropriate annotation to use for something of type actual_ty. This difference matters when thinking about visible type application. For example, forall a. a -> forall b. b -> b DOES NOT SUBSUME forall a b. a -> b -> b because the type arguments appear in a different order. (Neither does it work the other way around.) BUT, these types are appropriate annotations for one another. Because the user directs annotations, it's OK if some arguments shuffle around -- after all, it's what the user wants. Bottom line: none of this changes with visible type application. There are a number of wrinkles (below). Notice that Wrinkle 1 and 2 both require eta-expansion, which technically may increase termination. We just put up with this, in exchange for getting more predictable type inference. Wrinkle 1: Note [Deep skolemisation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We want (forall a. Int -> a -> a) <= (Int -> forall a. a->a) (see section 4.6 of "Practical type inference for higher rank types") So we must deeply-skolemise the RHS before we instantiate the LHS. That is why tc_sub_type starts with a call to tcSkolemise (which does the deep skolemisation), and then calls the DS variant (which assumes that expected_ty is deeply skolemised) Wrinkle 2: Note [Co/contra-variance of subsumption checking] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider g :: (Int -> Int) -> Int f1 :: (forall a. a -> a) -> Int f1 = g f2 :: (forall a. a -> a) -> Int f2 x = g x f2 will typecheck, and it would be odd/fragile if f1 did not. But f1 will only typecheck if we have that (Int->Int) -> Int <= (forall a. a->a) -> Int And that is only true if we do the full co/contravariant thing in the subsumption check. That happens in the FunTy case of tcSubTypeDS_NC_O, and is the sole reason for the WpFun form of HsWrapper. Another powerful reason for doing this co/contra stuff is visible in Trac #9569, involving instantiation of constraint variables, and again involving eta-expansion. Wrinkle 3: Note [Higher rank types] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider tc150: f y = \ (x::forall a. a->a). blah The following happens: * We will infer the type of the RHS, ie with a res_ty = alpha. * Then the lambda will split alpha := beta -> gamma. * And then we'll check tcSubType IsSwapped beta (forall a. a->a) So it's important that we unify beta := forall a. a->a, rather than skolemising the type. -} -- | Call this variant when you are in a higher-rank situation and -- you know the right-hand type is deeply skolemised. tcSubTypeHR :: Outputable a => CtOrigin -- ^ of the actual type -> Maybe a -- ^ If present, it has type ty_actual -> TcSigmaType -> ExpRhoType -> TcM HsWrapper tcSubTypeHR orig = tcSubTypeDS_NC_O orig GenSigCtxt tcSubType :: Outputable a => UserTypeCtxt -> Maybe a -- ^ If present, it has type ty_actual -> TcSigmaType -> ExpSigmaType -> TcM HsWrapper -- Checks that actual <= expected -- Returns HsWrapper :: actual ~ expected tcSubType ctxt maybe_thing ty_actual ty_expected = tcSubTypeO origin ctxt ty_actual ty_expected where origin = TypeEqOrigin { uo_actual = ty_actual , uo_expected = ty_expected , uo_thing = mkErrorThing <$> maybe_thing } -- | This is like 'tcSubType' but accepts an 'ExpType' as the /actual/ type. -- You probably want this only when looking at patterns, never expressions. tcSubTypeET :: CtOrigin -> ExpSigmaType -> TcSigmaType -> TcM HsWrapper tcSubTypeET orig ty_actual ty_expected = uExpTypeX orig ty_expected ty_actual (return . mkWpCastN . mkTcSymCo) (\ty_a -> tcSubTypeO orig GenSigCtxt ty_a (mkCheckExpType ty_expected)) -- | This is like 'tcSubType' but accepts an 'ExpType' as the /actual/ type. -- You probably want this only when looking at patterns, never expressions. -- Does not add context. tcSubTypeET_NC :: UserTypeCtxt -> ExpSigmaType -> TcSigmaType -> TcM HsWrapper tcSubTypeET_NC _ ty_actual@(Infer {}) ty_expected = mkWpCastN . mkTcSymCo <$> unifyExpType noThing ty_expected ty_actual tcSubTypeET_NC ctxt (Check ty_actual) ty_expected = tc_sub_type orig orig ctxt ty_actual ty_expected' where ty_expected' = mkCheckExpType ty_expected orig = TypeEqOrigin { uo_actual = ty_actual , uo_expected = ty_expected' , uo_thing = Nothing } tcSubTypeO :: CtOrigin -- ^ of the actual type -> UserTypeCtxt -- ^ of the expected type -> TcSigmaType -> ExpSigmaType -> TcM HsWrapper tcSubTypeO origin ctxt ty_actual ty_expected = addSubTypeCtxt ty_actual ty_expected $ do { traceTc "tcSubType" (vcat [ pprCtOrigin origin , pprUserTypeCtxt ctxt , ppr ty_actual , ppr ty_expected ]) ; tc_sub_type eq_orig origin ctxt ty_actual ty_expected } where eq_orig | TypeEqOrigin {} <- origin = origin | otherwise = TypeEqOrigin { uo_actual = ty_actual , uo_expected = ty_expected , uo_thing = Nothing } tcSubTypeDS :: Outputable a => UserTypeCtxt -> Maybe a -- ^ has type ty_actual -> TcSigmaType -> ExpRhoType -> TcM HsWrapper -- Just like tcSubType, but with the additional precondition that -- ty_expected is deeply skolemised (hence "DS") tcSubTypeDS ctxt m_expr ty_actual ty_expected = addSubTypeCtxt ty_actual ty_expected $ tcSubTypeDS_NC ctxt m_expr ty_actual ty_expected -- | Like 'tcSubTypeDS', but takes a 'CtOrigin' to use when instantiating -- the "actual" type tcSubTypeDS_O :: Outputable a => CtOrigin -> UserTypeCtxt -> Maybe a -> TcSigmaType -> ExpRhoType -> TcM HsWrapper tcSubTypeDS_O orig ctxt maybe_thing ty_actual ty_expected = addSubTypeCtxt ty_actual ty_expected $ do { traceTc "tcSubTypeDS_O" (vcat [ pprCtOrigin orig , pprUserTypeCtxt ctxt , ppr ty_actual , ppr ty_expected ]) ; tcSubTypeDS_NC_O orig ctxt maybe_thing ty_actual ty_expected } addSubTypeCtxt :: TcType -> ExpType -> TcM a -> TcM a addSubTypeCtxt ty_actual ty_expected thing_inside | isRhoTy ty_actual -- If there is no polymorphism involved, the , isRhoExpTy ty_expected -- TypeEqOrigin stuff (added by the _NC functions) = thing_inside -- gives enough context by itself | otherwise = addErrCtxtM mk_msg thing_inside where mk_msg tidy_env = do { (tidy_env, ty_actual) <- zonkTidyTcType tidy_env ty_actual -- might not be filled if we're debugging. ugh. ; mb_ty_expected <- readExpType_maybe ty_expected ; (tidy_env, ty_expected) <- case mb_ty_expected of Just ty -> second mkCheckExpType <$> zonkTidyTcType tidy_env ty Nothing -> return (tidy_env, ty_expected) ; ty_expected <- readExpType ty_expected ; (tidy_env, ty_expected) <- zonkTidyTcType tidy_env ty_expected ; let msg = vcat [ hang (text "When checking that:") 4 (ppr ty_actual) , nest 2 (hang (text "is more polymorphic than:") 2 (ppr ty_expected)) ] ; return (tidy_env, msg) } --------------- -- The "_NC" variants do not add a typechecker-error context; -- the caller is assumed to do that tcSubType_NC :: UserTypeCtxt -> TcSigmaType -> ExpSigmaType -> TcM HsWrapper tcSubType_NC ctxt ty_actual ty_expected = do { traceTc "tcSubType_NC" (vcat [pprUserTypeCtxt ctxt, ppr ty_actual, ppr ty_expected]) ; tc_sub_type origin origin ctxt ty_actual ty_expected } where origin = TypeEqOrigin { uo_actual = ty_actual , uo_expected = ty_expected , uo_thing = Nothing } tcSubTypeDS_NC :: Outputable a => UserTypeCtxt -> Maybe a -- ^ If present, this has type ty_actual -> TcSigmaType -> ExpRhoType -> TcM HsWrapper tcSubTypeDS_NC ctxt maybe_thing ty_actual ty_expected = do { traceTc "tcSubTypeDS_NC" (vcat [pprUserTypeCtxt ctxt, ppr ty_actual, ppr ty_expected]) ; tcSubTypeDS_NC_O origin ctxt maybe_thing ty_actual ty_expected } where origin = TypeEqOrigin { uo_actual = ty_actual , uo_expected = ty_expected , uo_thing = mkErrorThing <$> maybe_thing } tcSubTypeDS_NC_O :: Outputable a => CtOrigin -- origin used for instantiation only -> UserTypeCtxt -> Maybe a -> TcSigmaType -> ExpRhoType -> TcM HsWrapper -- Just like tcSubType, but with the additional precondition that -- ty_expected is deeply skolemised tcSubTypeDS_NC_O inst_orig ctxt m_thing ty_actual et = uExpTypeX eq_orig ty_actual et (return . mkWpCastN) (tc_sub_type_ds eq_orig inst_orig ctxt ty_actual) where eq_orig = TypeEqOrigin { uo_actual = ty_actual, uo_expected = et , uo_thing = mkErrorThing <$> m_thing } --------------- tc_sub_type :: CtOrigin -- origin used when calling uType -> CtOrigin -- origin used when instantiating -> UserTypeCtxt -> TcSigmaType -> ExpSigmaType -> TcM HsWrapper tc_sub_type eq_orig inst_orig ctxt ty_actual et = uExpTypeX eq_orig ty_actual et (return . mkWpCastN) (tc_sub_tc_type eq_orig inst_orig ctxt ty_actual) tc_sub_tc_type :: CtOrigin -- used when calling uType -> CtOrigin -- used when instantiating -> UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper tc_sub_tc_type eq_orig inst_orig ctxt ty_actual ty_expected | Just tv_actual <- tcGetTyVar_maybe ty_actual -- See Note [Higher rank types] = do { lookup_res <- lookupTcTyVar tv_actual ; case lookup_res of Filled ty_actual' -> tc_sub_tc_type eq_orig inst_orig ctxt ty_actual' ty_expected -- It's tempting to see if tv_actual can unify with a polytype -- and, if so, call uType; otherwise, skolemise first. But this -- is wrong, because skolemising will bump the TcLevel and the -- unification will fail anyway. -- It's also tempting to call uUnfilledVar directly, but calling -- uType seems safer in the presence of possible refactoring -- later. Unfilled _ -> mkWpCastN <$> uType eq_orig TypeLevel ty_actual ty_expected } | otherwise -- See Note [Deep skolemisation] = do { (sk_wrap, inner_wrap) <- tcSkolemise ctxt ty_expected $ \ _ sk_rho -> tc_sub_type_ds eq_orig inst_orig ctxt ty_actual sk_rho ; return (sk_wrap <.> inner_wrap) } --------------- tc_sub_type_ds :: CtOrigin -- used when calling uType -> CtOrigin -- used when instantiating -> UserTypeCtxt -> TcSigmaType -> TcRhoType -> TcM HsWrapper -- Just like tcSubType, but with the additional precondition that -- ty_expected is deeply skolemised tc_sub_type_ds eq_orig inst_orig ctxt ty_actual ty_expected = go ty_actual ty_expected where go ty_a ty_e | Just ty_a' <- coreView ty_a = go ty_a' ty_e | Just ty_e' <- coreView ty_e = go ty_a ty_e' go (TyVarTy tv_a) ty_e = do { lookup_res <- lookupTcTyVar tv_a ; case lookup_res of Filled ty_a' -> do { traceTc "tcSubTypeDS_NC_O following filled act meta-tyvar:" (ppr tv_a <+> text "-->" <+> ppr ty_a') ; tc_sub_type_ds eq_orig inst_orig ctxt ty_a' ty_e } Unfilled _ -> unify } go ty_a (TyVarTy tv_e) = do { dflags <- getDynFlags ; tclvl <- getTcLevel ; lookup_res <- lookupTcTyVar tv_e ; case lookup_res of Filled ty_e' -> do { traceTc "tcSubTypeDS_NC_O following filled exp meta-tyvar:" (ppr tv_e <+> text "-->" <+> ppr ty_e') ; tc_sub_tc_type eq_orig inst_orig ctxt ty_a ty_e' } Unfilled details | canUnifyWithPolyType dflags details && isTouchableMetaTyVar tclvl tv_e -- don't want skolems here -> unify -- We've avoided instantiating ty_actual just in case ty_expected is -- polymorphic. But we've now assiduously determined that it is *not* -- polymorphic. So instantiate away. This is needed for e.g. test -- typecheck/should_compile/T4284. | otherwise -> inst_and_unify } go (ForAllTy (Anon act_arg) act_res) (ForAllTy (Anon exp_arg) exp_res) | not (isPredTy act_arg) , not (isPredTy exp_arg) = -- See Note [Co/contra-variance of subsumption checking] do { res_wrap <- tc_sub_type_ds eq_orig inst_orig ctxt act_res exp_res ; arg_wrap <- tc_sub_tc_type eq_orig (GivenOrigin (SigSkol GenSigCtxt (mkCheckExpType exp_arg))) ctxt exp_arg act_arg ; return (mkWpFun arg_wrap res_wrap exp_arg exp_res) } -- arg_wrap :: exp_arg ~ act_arg -- res_wrap :: act-res ~ exp_res go ty_a ty_e | let (tvs, theta, _) = tcSplitSigmaTy ty_a , not (null tvs && null theta) = do { (in_wrap, in_rho) <- topInstantiate inst_orig ty_a ; body_wrap <- tc_sub_type_ds (eq_orig { uo_actual = in_rho , uo_expected = mkCheckExpType ty_expected }) inst_orig ctxt in_rho ty_e ; return (body_wrap <.> in_wrap) } | otherwise -- Revert to unification = inst_and_unify -- It's still possible that ty_actual has nested foralls. Instantiate -- these, as there's no way unification will succeed with them in. -- See typecheck/should_compile/T11305 for an example of when this -- is important. The problem is that we're checking something like -- a -> forall b. b -> b <= alpha beta gamma -- where we end up with alpha := (->) inst_and_unify = do { (wrap, rho_a) <- deeplyInstantiate inst_orig ty_actual -- if we haven't recurred through an arrow, then -- the eq_orig will list ty_actual. In this case, -- we want to update the origin to reflect the -- instantiation. If we *have* recurred through -- an arrow, it's better not to update. ; let eq_orig' = case eq_orig of TypeEqOrigin { uo_actual = orig_ty_actual } | orig_ty_actual `tcEqType` ty_actual , not (isIdHsWrapper wrap) -> eq_orig { uo_actual = rho_a } _ -> eq_orig ; cow <- uType eq_orig' TypeLevel rho_a ty_expected ; return (mkWpCastN cow <.> wrap) } -- use versions without synonyms expanded unify = mkWpCastN <$> uType eq_orig TypeLevel ty_actual ty_expected ----------------- -- needs both un-type-checked (for origins) and type-checked (for wrapping) -- expressions tcWrapResult :: HsExpr Name -> HsExpr TcId -> TcSigmaType -> ExpRhoType -> TcM (HsExpr TcId) tcWrapResult rn_expr = tcWrapResultO (exprCtOrigin rn_expr) -- | Sometimes we don't have a @HsExpr Name@ to hand, and this is more -- convenient. tcWrapResultO :: CtOrigin -> HsExpr TcId -> TcSigmaType -> ExpRhoType -> TcM (HsExpr TcId) tcWrapResultO orig expr actual_ty res_ty = do { traceTc "tcWrapResult" (vcat [ text "Actual: " <+> ppr actual_ty , text "Expected:" <+> ppr res_ty ]) ; cow <- tcSubTypeDS_NC_O orig GenSigCtxt (Just expr) actual_ty res_ty ; return (mkHsWrap cow expr) } ----------------------------------- wrapFunResCoercion :: [TcType] -- Type of args -> HsWrapper -- HsExpr a -> HsExpr b -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b) wrapFunResCoercion arg_tys co_fn_res | isIdHsWrapper co_fn_res = return idHsWrapper | null arg_tys = return co_fn_res | otherwise = do { arg_ids <- newSysLocalIds (fsLit "sub") arg_tys ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpEvVarApps arg_ids) } ----------------------------------- -- | Infer a type using a fresh ExpType -- See also Note [ExpType] in TcMType tcInfer :: (ExpRhoType -> TcM a) -> TcM (a, TcType) tcInfer tc_check = do { res_ty <- newOpenInferExpType ; result <- tc_check res_ty ; res_ty <- readExpType res_ty ; return (result, res_ty) } {- ************************************************************************ * * \subsection{Generalisation} * * ************************************************************************ -} -- | Take an "expected type" and strip off quantifiers to expose the -- type underneath, binding the new skolems for the @thing_inside@. -- The returned 'HsWrapper' has type @specific_ty -> expected_ty@. tcSkolemise :: UserTypeCtxt -> TcSigmaType -> ([TcTyVar] -> TcType -> TcM result) -- ^ These are only ever used for scoped type variables. -> TcM (HsWrapper, result) -- ^ The expression has type: spec_ty -> expected_ty tcSkolemise ctxt expected_ty thing_inside -- We expect expected_ty to be a forall-type -- If not, the call is a no-op = do { traceTc "tcSkolemise" Outputable.empty ; (wrap, tvs', given, rho') <- deeplySkolemise expected_ty ; lvl <- getTcLevel ; when debugIsOn $ traceTc "tcSkolemise" $ vcat [ ppr lvl, text "expected_ty" <+> ppr expected_ty, text "inst tyvars" <+> ppr tvs', text "given" <+> ppr given, text "inst type" <+> ppr rho' ] -- Generally we must check that the "forall_tvs" havn't been constrained -- The interesting bit here is that we must include the free variables -- of the expected_ty. Here's an example: -- runST (newVar True) -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool)) -- for (newVar True), with s fresh. Then we unify with the runST's arg type -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool. -- So now s' isn't unconstrained because it's linked to a. -- -- However [Oct 10] now that the untouchables are a range of -- TcTyVars, all this is handled automatically with no need for -- extra faffing around -- Use the *instantiated* type in the SkolemInfo -- so that the names of displayed type variables line up ; let skol_info = SigSkol ctxt (mkCheckExpType $ mkFunTys (map varType given) rho') ; (ev_binds, result) <- checkConstraints skol_info tvs' given $ thing_inside tvs' rho' ; return (wrap <.> mkWpLet ev_binds, result) } -- The ev_binds returned by checkConstraints is very -- often empty, in which case mkWpLet is a no-op -- | Variant of 'tcSkolemise' that takes an ExpType tcSkolemiseET :: UserTypeCtxt -> ExpSigmaType -> (ExpRhoType -> TcM result) -> TcM (HsWrapper, result) tcSkolemiseET _ et@(Infer {}) thing_inside = (idHsWrapper, ) <$> thing_inside et tcSkolemiseET ctxt (Check ty) thing_inside = tcSkolemise ctxt ty $ \_ -> thing_inside . mkCheckExpType checkConstraints :: SkolemInfo -> [TcTyVar] -- Skolems -> [EvVar] -- Given -> TcM result -> TcM (TcEvBinds, result) checkConstraints skol_info skol_tvs given thing_inside = do { (implics, ev_binds, result) <- buildImplication skol_info skol_tvs given thing_inside ; emitImplications implics ; return (ev_binds, result) } buildImplication :: SkolemInfo -> [TcTyVar] -- Skolems -> [EvVar] -- Given -> TcM result -> TcM (Bag Implication, TcEvBinds, result) buildImplication skol_info skol_tvs given thing_inside = do { tc_lvl <- getTcLevel ; deferred_type_errors <- goptM Opt_DeferTypeErrors <||> goptM Opt_DeferTypedHoles ; if null skol_tvs && null given && (not deferred_type_errors || not (isTopTcLevel tc_lvl)) then do { res <- thing_inside ; return (emptyBag, emptyTcEvBinds, res) } -- Fast path. We check every function argument with -- tcPolyExpr, which uses tcSkolemise and hence checkConstraints. -- But with the solver producing unlifted equalities, we need -- to have an EvBindsVar for them when they might be deferred to -- runtime. Otherwise, they end up as top-level unlifted bindings, -- which are verboten. See also Note [Deferred errors for coercion holes] -- in TcErrors. else do { (tclvl, wanted, result) <- pushLevelAndCaptureConstraints thing_inside ; (implics, ev_binds) <- buildImplicationFor tclvl skol_info skol_tvs given wanted ; return (implics, ev_binds, result) }} buildImplicationFor :: TcLevel -> SkolemInfo -> [TcTyVar] -> [EvVar] -> WantedConstraints -> TcM (Bag Implication, TcEvBinds) buildImplicationFor tclvl skol_info skol_tvs given wanted | isEmptyWC wanted && null given -- Optimisation : if there are no wanteds, and no givens -- don't generate an implication at all. -- Reason for the (null given): we don't want to lose -- the "inaccessible alternative" error check = return (emptyBag, emptyTcEvBinds) | otherwise = ASSERT2( all isTcTyVar skol_tvs, ppr skol_tvs ) ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs ) do { ev_binds_var <- newTcEvBinds ; env <- getLclEnv ; let implic = Implic { ic_tclvl = tclvl , ic_skols = skol_tvs , ic_no_eqs = False , ic_given = given , ic_wanted = wanted , ic_status = IC_Unsolved , ic_binds = Just ev_binds_var , ic_env = env , ic_info = skol_info } ; return (unitBag implic, TcEvBinds ev_binds_var) } {- ************************************************************************ * * Boxy unification * * ************************************************************************ The exported functions are all defined as versions of some non-exported generic functions. -} -- | Unify two types, discarding a resultant coercion. Any constraints -- generated will still need to be solved, however. unifyType_ :: Outputable a => Maybe a -- ^ If present, has type 'ty1' -> TcTauType -> TcTauType -> TcM () unifyType_ thing ty1 ty2 = void $ unifyType thing ty1 ty2 unifyType :: Outputable a => Maybe a -- ^ If present, has type 'ty1' -> TcTauType -> TcTauType -> TcM TcCoercionN -- Actual and expected types -- Returns a coercion : ty1 ~ ty2 unifyType thing ty1 ty2 = uType origin TypeLevel ty1 ty2 where origin = TypeEqOrigin { uo_actual = ty1, uo_expected = mkCheckExpType ty2 , uo_thing = mkErrorThing <$> thing } -- | Variant of 'unifyType' that takes an 'ExpType' as its second type unifyExpType :: Outputable a => Maybe a -> TcTauType -> ExpType -> TcM TcCoercionN unifyExpType mb_thing ty1 ty2 = uExpType ty_orig ty1 ty2 where ty_orig = TypeEqOrigin { uo_actual = ty1 , uo_expected = ty2 , uo_thing = mkErrorThing <$> mb_thing } -- | Use this instead of 'Nothing' when calling 'unifyType' without -- a good "thing" (where the "thing" has the "actual" type passed in) -- This has an 'Outputable' instance, avoiding amgiguity problems. noThing :: Maybe (HsExpr Name) noThing = Nothing unifyKind :: Outputable a => Maybe a -> TcKind -> TcKind -> TcM CoercionN unifyKind thing ty1 ty2 = uType origin KindLevel ty1 ty2 where origin = TypeEqOrigin { uo_actual = ty1, uo_expected = mkCheckExpType ty2 , uo_thing = mkErrorThing <$> thing } --------------- unifyPred :: PredType -> PredType -> TcM TcCoercionN -- Actual and expected types unifyPred = unifyType noThing --------------- unifyTheta :: TcThetaType -> TcThetaType -> TcM [TcCoercionN] -- Actual and expected types unifyTheta theta1 theta2 = do { checkTc (equalLength theta1 theta2) (vcat [text "Contexts differ in length", nest 2 $ parens $ text "Use RelaxedPolyRec to allow this"]) ; zipWithM unifyPred theta1 theta2 } {- %************************************************************************ %* * uType and friends %* * %************************************************************************ uType is the heart of the unifier. -} uExpType :: CtOrigin -> TcType -> ExpType -> TcM CoercionN uExpType orig ty1 et = uExpTypeX orig ty1 et return $ uType orig TypeLevel ty1 -- | Tries to unify with an ExpType. If the ExpType is filled in, calls the first -- continuation with the produced coercion. Otherwise, calls the second -- continuation. This can happen either with a Check or with an untouchable -- ExpType that reverts to a tau-type. See Note [TcLevel of ExpType] uExpTypeX :: CtOrigin -> TcType -> ExpType -> (TcCoercionN -> TcM a) -- Infer case, co :: TcType ~N ExpType -> (TcType -> TcM a) -- Check / untouchable case -> TcM a uExpTypeX orig ty1 et@(Infer _ tc_lvl ki _) coercion_cont type_cont = do { cur_lvl <- getTcLevel ; if cur_lvl `sameDepthAs` tc_lvl then do { ki_co <- uType kind_orig KindLevel (typeKind ty1) ki ; writeExpType et (ty1 `mkCastTy` ki_co) ; coercion_cont $ mkTcNomReflCo ty1 `mkTcCoherenceRightCo` ki_co } else do { traceTc "Preventing writing to untouchable ExpType" empty ; tau <- expTypeToType et -- See Note [TcLevel of ExpType] ; type_cont tau }} where kind_orig = KindEqOrigin ty1 Nothing orig (Just TypeLevel) uExpTypeX _ _ (Check ty2) _ type_cont = type_cont ty2 ------------ uType, uType_defer :: CtOrigin -> TypeOrKind -> TcType -- ty1 is the *actual* type -> TcType -- ty2 is the *expected* type -> TcM Coercion -------------- -- It is always safe to defer unification to the main constraint solver -- See Note [Deferred unification] uType_defer origin t_or_k ty1 ty2 = do { hole <- newCoercionHole ; loc <- getCtLocM origin (Just t_or_k) ; emitSimple $ mkNonCanonical $ CtWanted { ctev_dest = HoleDest hole , ctev_pred = mkPrimEqPred ty1 ty2 , ctev_loc = loc } -- Error trace only -- NB. do *not* call mkErrInfo unless tracing is on, because -- it is hugely expensive (#5631) ; whenDOptM Opt_D_dump_tc_trace $ do { ctxt <- getErrCtxt ; doc <- mkErrInfo emptyTidyEnv ctxt ; traceTc "utype_defer" (vcat [ppr hole, ppr ty1, ppr ty2, pprCtOrigin origin, doc]) } ; return (mkHoleCo hole Nominal ty1 ty2) } -------------- uType origin t_or_k orig_ty1 orig_ty2 = do { tclvl <- getTcLevel ; traceTc "u_tys " $ vcat [ text "tclvl" <+> ppr tclvl , sep [ ppr orig_ty1, text "~", ppr orig_ty2] , pprCtOrigin origin] ; co <- go orig_ty1 orig_ty2 ; if isReflCo co then traceTc "u_tys yields no coercion" Outputable.empty else traceTc "u_tys yields coercion:" (ppr co) ; return co } where go :: TcType -> TcType -> TcM Coercion -- The arguments to 'go' are always semantically identical -- to orig_ty{1,2} except for looking through type synonyms -- Variables; go for uVar -- Note that we pass in *original* (before synonym expansion), -- so that type variables tend to get filled in with -- the most informative version of the type go (TyVarTy tv1) ty2 = do { lookup_res <- lookupTcTyVar tv1 ; case lookup_res of Filled ty1 -> do { traceTc "found filled tyvar" (ppr tv1 <+> text ":->" <+> ppr ty1) ; go ty1 ty2 } Unfilled ds1 -> uUnfilledVar origin t_or_k NotSwapped tv1 ds1 ty2 } go ty1 (TyVarTy tv2) = do { lookup_res <- lookupTcTyVar tv2 ; case lookup_res of Filled ty2 -> do { traceTc "found filled tyvar" (ppr tv2 <+> text ":->" <+> ppr ty2) ; go ty1 ty2 } Unfilled ds2 -> uUnfilledVar origin t_or_k IsSwapped tv2 ds2 ty1 } -- See Note [Expanding synonyms during unification] go ty1@(TyConApp tc1 []) (TyConApp tc2 []) | tc1 == tc2 = return $ mkReflCo Nominal ty1 -- See Note [Expanding synonyms during unification] -- -- Also NB that we recurse to 'go' so that we don't push a -- new item on the origin stack. As a result if we have -- type Foo = Int -- and we try to unify Foo ~ Bool -- we'll end up saying "can't match Foo with Bool" -- rather than "can't match "Int with Bool". See Trac #4535. go ty1 ty2 | Just ty1' <- coreView ty1 = go ty1' ty2 | Just ty2' <- coreView ty2 = go ty1 ty2' go (CastTy t1 co1) t2 = do { co_tys <- go t1 t2 ; return (mkCoherenceLeftCo co_tys co1) } go t1 (CastTy t2 co2) = do { co_tys <- go t1 t2 ; return (mkCoherenceRightCo co_tys co2) } -- Functions (or predicate functions) just check the two parts go (ForAllTy (Anon fun1) arg1) (ForAllTy (Anon fun2) arg2) = do { co_l <- uType origin t_or_k fun1 fun2 ; co_r <- uType origin t_or_k arg1 arg2 ; return $ mkFunCo Nominal co_l co_r } -- Always defer if a type synonym family (type function) -- is involved. (Data families behave rigidly.) go ty1@(TyConApp tc1 _) ty2 | isTypeFamilyTyCon tc1 = defer ty1 ty2 go ty1 ty2@(TyConApp tc2 _) | isTypeFamilyTyCon tc2 = defer ty1 ty2 go (TyConApp tc1 tys1) (TyConApp tc2 tys2) -- See Note [Mismatched type lists and application decomposition] | tc1 == tc2, length tys1 == length tys2 = ASSERT2( isGenerativeTyCon tc1 Nominal, ppr tc1 ) do { cos <- zipWithM (uType origin t_or_k) tys1 tys2 ; return $ mkTyConAppCo Nominal tc1 cos } go (LitTy m) ty@(LitTy n) | m == n = return $ mkNomReflCo ty -- See Note [Care with type applications] -- Do not decompose FunTy against App; -- it's often a type error, so leave it for the constraint solver go (AppTy s1 t1) (AppTy s2 t2) = go_app s1 t1 s2 t2 go (AppTy s1 t1) (TyConApp tc2 ts2) | Just (ts2', t2') <- snocView ts2 = ASSERT( mightBeUnsaturatedTyCon tc2 ) go_app s1 t1 (TyConApp tc2 ts2') t2' go (TyConApp tc1 ts1) (AppTy s2 t2) | Just (ts1', t1') <- snocView ts1 = ASSERT( mightBeUnsaturatedTyCon tc1 ) go_app (TyConApp tc1 ts1') t1' s2 t2 go (CoercionTy co1) (CoercionTy co2) = do { let ty1 = coercionType co1 ty2 = coercionType co2 ; kco <- uType (KindEqOrigin orig_ty1 (Just orig_ty2) origin (Just t_or_k)) KindLevel ty1 ty2 ; return $ mkProofIrrelCo Nominal kco co1 co2 } -- Anything else fails -- E.g. unifying for-all types, which is relative unusual go ty1 ty2 = defer ty1 ty2 ------------------ defer ty1 ty2 -- See Note [Check for equality before deferring] | ty1 `tcEqType` ty2 = return (mkNomReflCo ty1) | otherwise = uType_defer origin t_or_k ty1 ty2 ------------------ go_app s1 t1 s2 t2 = do { co_s <- uType origin t_or_k s1 s2 ; co_t <- uType origin t_or_k t1 t2 ; return $ mkAppCo co_s co_t } {- Note [Check for equality before deferring] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Particularly in ambiguity checks we can get equalities like (ty ~ ty). If ty involves a type function we may defer, which isn't very sensible. An egregious example of this was in test T9872a, which has a type signature Proxy :: Proxy (Solutions Cubes) Doing the ambiguity check on this signature generates the equality Solutions Cubes ~ Solutions Cubes and currently the constraint solver normalises both sides at vast cost. This little short-cut in 'defer' helps quite a bit. Note [Care with type applications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Note: type applications need a bit of care! They can match FunTy and TyConApp, so use splitAppTy_maybe NB: we've already dealt with type variables and Notes, so if one type is an App the other one jolly well better be too Note [Mismatched type lists and application decomposition] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we find two TyConApps, you might think that the argument lists are guaranteed equal length. But they aren't. Consider matching w (T x) ~ Foo (T x y) We do match (w ~ Foo) first, but in some circumstances we simply create a deferred constraint; and then go ahead and match (T x ~ T x y). This came up in Trac #3950. So either (a) either we must check for identical argument kinds when decomposing applications, (b) or we must be prepared for ill-kinded unification sub-problems Currently we adopt (b) since it seems more robust -- no need to maintain a global invariant. Note [Expanding synonyms during unification] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We expand synonyms during unification, but: * We expand *after* the variable case so that we tend to unify variables with un-expanded type synonym. This just makes it more likely that the inferred types will mention type synonyms understandable to the user * We expand *before* the TyConApp case. For example, if we have type Phantom a = Int and are unifying Phantom Int ~ Phantom Char it is *wrong* to unify Int and Char. * The problem case immediately above can happen only with arguments to the tycon. So we check for nullary tycons *before* expanding. This is particularly helpful when checking (* ~ *), because * is now a type synonym. Note [Deferred Unification] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically, and yet its consistency is undetermined. Previously, there was no way to still make it consistent. So a mismatch error was issued. Now these unifications are deferred until constraint simplification, where type family instances and given equations may (or may not) establish the consistency. Deferred unifications are of the form F ... ~ ... or x ~ ... where F is a type function and x is a type variable. E.g. id :: x ~ y => x -> y id e = e involves the unification x = y. It is deferred until we bring into account the context x ~ y to establish that it holds. If available, we defer original types (rather than those where closed type synonyms have already been expanded via tcCoreView). This is, as usual, to improve error messages. ************************************************************************ * * uVar and friends * * ************************************************************************ @uVar@ is called when at least one of the types being unified is a variable. It does {\em not} assume that the variable is a fixed point of the substitution; rather, notice that @uVar@ (defined below) nips back into @uTys@ if it turns out that the variable is already bound. -} uUnfilledVar :: CtOrigin -> TypeOrKind -> SwapFlag -> TcTyVar -> TcTyVarDetails -- Tyvar 1 -> TcTauType -- Type 2 -> TcM Coercion -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar -- It might be a skolem, or untouchable, or meta uUnfilledVar origin t_or_k swapped tv1 details1 (TyVarTy tv2) | tv1 == tv2 -- Same type variable => no-op = return (mkNomReflCo (mkTyVarTy tv1)) | otherwise -- Distinct type variables = do { lookup2 <- lookupTcTyVar tv2 ; case lookup2 of Filled ty2' -> uUnfilledVar origin t_or_k swapped tv1 details1 ty2' Unfilled details2 -> uUnfilledVars origin t_or_k swapped tv1 details1 tv2 details2 } uUnfilledVar origin t_or_k swapped tv1 details1 non_var_ty2 -- ty2 is not a type variable = case details1 of MetaTv { mtv_ref = ref1 } -> do { dflags <- getDynFlags ; mb_ty2' <- checkTauTvUpdate dflags origin t_or_k tv1 non_var_ty2 ; case mb_ty2' of Just (ty2', co_k) -> maybe_sym swapped <$> updateMeta tv1 ref1 ty2' co_k Nothing -> do { traceTc "Occ/type-family defer" (ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1) $$ ppr non_var_ty2 $$ ppr (typeKind non_var_ty2)) ; defer } } _other -> do { traceTc "Skolem defer" (ppr tv1); defer } -- Skolems of all sorts where defer = unSwap swapped (uType_defer origin t_or_k) (mkTyVarTy tv1) non_var_ty2 -- Occurs check or an untouchable: just defer -- NB: occurs check isn't necessarily fatal: -- eg tv1 occured in type family parameter ---------------- uUnfilledVars :: CtOrigin -> TypeOrKind -> SwapFlag -> TcTyVar -> TcTyVarDetails -- Tyvar 1 -> TcTyVar -> TcTyVarDetails -- Tyvar 2 -> TcM Coercion -- Invarant: The type variables are distinct, -- Neither is filled in yet uUnfilledVars origin t_or_k swapped tv1 details1 tv2 details2 = do { traceTc "uUnfilledVars for" (ppr tv1 <+> text "and" <+> ppr tv2) ; traceTc "uUnfilledVars" ( text "trying to unify" <+> ppr k1 <+> text "with" <+> ppr k2) ; co_k <- uType kind_origin KindLevel k1 k2 ; let no_swap ref = maybe_sym swapped <$> updateMeta tv1 ref ty2 (mkSymCo co_k) do_swap ref = maybe_sym (flipSwap swapped) <$> updateMeta tv2 ref ty1 co_k ; case (details1, details2) of { ( MetaTv { mtv_info = i1, mtv_ref = ref1 } , MetaTv { mtv_info = i2, mtv_ref = ref2 } ) | nicer_to_update_tv1 tv1 i1 i2 -> no_swap ref1 | otherwise -> do_swap ref2 ; (MetaTv { mtv_ref = ref1 }, _) -> no_swap ref1 ; (_, MetaTv { mtv_ref = ref2 }) -> do_swap ref2 -- Can't do it in-place, so defer -- This happens for skolems of all sorts ; _ -> do { traceTc "deferring because I can't find a meta-tyvar:" (pprTcTyVarDetails details1 <+> pprTcTyVarDetails details2) ; unSwap swapped (uType_defer origin t_or_k) ty1 ty2 } } } where k1 = tyVarKind tv1 k2 = tyVarKind tv2 ty1 = mkTyVarTy tv1 ty2 = mkTyVarTy tv2 kind_origin = KindEqOrigin ty1 (Just ty2) origin (Just t_or_k) -- | apply sym iff swapped maybe_sym :: SwapFlag -> Coercion -> Coercion maybe_sym IsSwapped = mkSymCo maybe_sym NotSwapped = id nicer_to_update_tv1 :: TcTyVar -> MetaInfo -> MetaInfo -> Bool nicer_to_update_tv1 _ _ SigTv = True nicer_to_update_tv1 _ SigTv _ = False -- Try not to update SigTvs; and try to update sys-y type -- variables in preference to ones gotten (say) by -- instantiating a polymorphic function with a user-written -- type sig nicer_to_update_tv1 tv1 _ _ = isSystemName (Var.varName tv1) ---------------- checkTauTvUpdate :: DynFlags -> CtOrigin -> TypeOrKind -> TcTyVar -- tv :: k1 -> TcType -- ty :: k2 -> TcM (Maybe ( TcType -- possibly-expanded ty , Coercion )) -- :: k2 ~N k1 -- (checkTauTvUpdate tv ty) -- We are about to update the TauTv tv with ty. -- Check (a) that tv doesn't occur in ty (occurs check) -- (b) that kind(ty) matches kind(tv) -- -- We have two possible outcomes: -- (1) Return the type to update the type variable with, -- [we know the update is ok] -- (2) Return Nothing, -- [the update might be dodgy] -- -- Note that "Nothing" does not mean "definite error". For example -- type family F a -- type instance F Int = Int -- consider -- a ~ F a -- This is perfectly reasonable, if we later get a ~ Int. For now, though, -- we return Nothing, leaving it to the later constraint simplifier to -- sort matters out. checkTauTvUpdate dflags origin t_or_k tv ty | SigTv <- info = ASSERT( not (isTyVarTy ty) ) return Nothing | otherwise = do { ty <- zonkTcType ty ; co_k <- uType kind_origin KindLevel (typeKind ty) (tyVarKind tv) ; if | defer_me ty -> -- Quick test -- Failed quick test so try harder case occurCheckExpand dflags tv ty of OC_OK ty2 | defer_me ty2 -> return Nothing | otherwise -> return (Just (ty2, co_k)) _ -> return Nothing | otherwise -> return (Just (ty, co_k)) } where kind_origin = KindEqOrigin (mkTyVarTy tv) (Just ty) origin (Just t_or_k) details = tcTyVarDetails tv info = mtv_info details impredicative = canUnifyWithPolyType dflags details defer_me :: TcType -> Bool -- Checks for (a) occurrence of tv -- (b) type family applications -- (c) foralls -- See Note [Prevent unification with type families] -- See Note [Refactoring hazard: checkTauTvUpdate] defer_me (LitTy {}) = False defer_me (TyVarTy tv') = tv == tv' || defer_me (tyVarKind tv') defer_me (TyConApp tc tys) = isTypeFamilyTyCon tc || any defer_me tys || not (impredicative || isTauTyCon tc) defer_me (ForAllTy bndr t) = defer_me (binderType bndr) || defer_me t || (isNamedBinder bndr && not impredicative) defer_me (AppTy fun arg) = defer_me fun || defer_me arg defer_me (CastTy ty co) = defer_me ty || defer_me_co co defer_me (CoercionTy co) = defer_me_co co -- We don't really care if there are type families in a coercion, -- but we still can't have an occurs-check failure defer_me_co co = tv `elemVarSet` tyCoVarsOfCo co {- Note [Prevent unification with type families] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We prevent unification with type families because of an uneasy compromise. It's perfectly sound to unify with type families, and it even improves the error messages in the testsuite. It also modestly improves performance, at least in some cases. But it's disastrous for test case perf/compiler/T3064. Here is the problem: Suppose we have (F ty) where we also have [G] F ty ~ a. What do we do? Do we reduce F? Or do we use the given? Hard to know what's best. GHC reduces. This is a disaster for T3064, where the type's size spirals out of control during reduction. (We're not helped by the fact that the flattener re-flattens all the arguments every time around.) If we prevent unification with type families, then the solver happens to use the equality before expanding the type family. It would be lovely in the future to revisit this problem and remove this extra, unnecessary check. But we retain it for now as it seems to work better in practice. Note [Refactoring hazard: checkTauTvUpdate] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I (Richard E.) have a sad story about refactoring this code, retained here to prevent others (or a future me!) from falling into the same traps. It all started with #11407, which was caused by the fact that the TyVarTy case of defer_me didn't look in the kind. But it seemed reasonable to simply remove the defer_me check instead. It referred to two Notes (since removed) that were out of date, and the fast_check code in occurCheckExpand seemed to do just about the same thing as defer_me. The one piece that defer_me did that wasn't repeated by occurCheckExpand was the type-family check. (See Note [Prevent unification with type families].) So I checked the result of occurCheckExpand for any type family occurrences and deferred if there were any. This was done in commit e9bf7bb5cc9fb3f87dd05111aa23da76b86a8967 . This approach turned out not to be performant, because the expanded type was bigger than the original type, and tyConsOfType looks through type synonyms. So it then struck me that we could dispense with the defer_me check entirely. This simplified the code nicely, and it cut the allocations in T5030 by half. But, as documented in Note [Prevent unification with type families], this destroyed performance in T3064. Regardless, I missed this regression and the change was committed as 3f5d1a13f112f34d992f6b74656d64d95a3f506d . Bottom lines: * defer_me is back, but now fixed w.r.t. #11407. * Tread carefully before you start to refactor here. There can be lots of hard-to-predict consequences. Note [Type synonyms and the occur check] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Generally speaking we try to update a variable with type synonyms not expanded, which improves later error messages, unless looking inside a type synonym may help resolve a spurious occurs check error. Consider: type A a = () f :: (A a -> a -> ()) -> () f = \ _ -> () x :: () x = f (\ x p -> p x) We will eventually get a constraint of the form t ~ A t. The ok function above will properly expand the type (A t) to just (), which is ok to be unified with t. If we had unified with the original type A t, we would lead the type checker into an infinite loop. Hence, if the occurs check fails for a type synonym application, then (and *only* then), the ok function expands the synonym to detect opportunities for occurs check success using the underlying definition of the type synonym. The same applies later on in the constraint interaction code; see TcInteract, function @occ_check_ok@. Note [Non-TcTyVars in TcUnify] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Because the same code is now shared between unifying types and unifying kinds, we sometimes will see proper TyVars floating around the unifier. Example (from test case polykinds/PolyKinds12): type family Apply (f :: k1 -> k2) (x :: k1) :: k2 type instance Apply g y = g y When checking the instance declaration, we first *kind-check* the LHS and RHS, discovering that the instance really should be type instance Apply k3 k4 (g :: k3 -> k4) (y :: k3) = g y During this kind-checking, all the tyvars will be TcTyVars. Then, however, as a second pass, we desugar the RHS (which is done in functions prefixed with "tc" in TcTyClsDecls"). By this time, all the kind-vars are proper TyVars, not TcTyVars, get some kind unification must happen. Thus, we always check if a TyVar is a TcTyVar before asking if it's a meta-tyvar. This used to not be necessary for type-checking (that is, before * :: *) because expressions get desugared via an algorithm separate from type-checking (with wrappers, etc.). Types get desugared very differently, causing this wibble in behavior seen here. -} data LookupTyVarResult -- The result of a lookupTcTyVar call = Unfilled TcTyVarDetails -- SkolemTv or virgin MetaTv | Filled TcType lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult lookupTcTyVar tyvar | MetaTv { mtv_ref = ref } <- details = do { meta_details <- readMutVar ref ; case meta_details of Indirect ty -> return (Filled ty) Flexi -> do { is_touchable <- isTouchableTcM tyvar -- Note [Unifying untouchables] ; if is_touchable then return (Unfilled details) else return (Unfilled vanillaSkolemTv) } } | otherwise = return (Unfilled details) where details = tcTyVarDetails tyvar -- | Fill in a meta-tyvar updateMeta :: TcTyVar -- ^ tv to fill in, tv :: k1 -> TcRef MetaDetails -- ^ ref to tv's metadetails -> TcType -- ^ ty2 :: k2 -> Coercion -- ^ kind_co :: k2 ~N k1 -> TcM Coercion -- ^ :: tv ~N ty2 (= ty2 |> kind_co ~N ty2) updateMeta tv1 ref1 ty2 kind_co = do { let ty2_refl = mkNomReflCo ty2 (ty2', co) = ( ty2 `mkCastTy` kind_co , mkCoherenceLeftCo ty2_refl kind_co ) ; writeMetaTyVarRef tv1 ref1 ty2' ; return co } {- Note [Unifying untouchables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We treat an untouchable type variable as if it was a skolem. That ensures it won't unify with anything. It's a slight had, because we return a made-up TcTyVarDetails, but I think it works smoothly. -} -- | Breaks apart a function kind into its pieces. matchExpectedFunKind :: Arity -- ^ # of args remaining, only for errors -> TcType -- ^ type, only for errors -> TcKind -- ^ function kind -> TcM (Coercion, TcKind, TcKind) -- ^ co :: old_kind ~ arg -> res matchExpectedFunKind num_args_remaining ty = go where go k | Just k' <- coreView k = go k' go k@(TyVarTy kvar) | isTcTyVar kvar, isMetaTyVar kvar = do { maybe_kind <- readMetaTyVar kvar ; case maybe_kind of Indirect fun_kind -> go fun_kind Flexi -> defer k } go k@(ForAllTy (Anon arg) res) = return (mkNomReflCo k, arg, res) go other = defer other defer k = do { arg_kind <- newMetaKindVar ; res_kind <- newMetaKindVar ; let new_fun = mkFunTy arg_kind res_kind thing = mkTypeErrorThingArgs ty num_args_remaining origin = TypeEqOrigin { uo_actual = k , uo_expected = mkCheckExpType new_fun , uo_thing = Just thing } ; co <- uType origin KindLevel k new_fun ; return (co, arg_kind, res_kind) } where