{- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 -} {-# LANGUAGE MultiWayIf #-} -- | Functions for inferring (and simplifying) the context for derived instances. module GHC.Tc.Deriv.Infer ( inferConstraints , simplifyInstanceContexts ) where import GHC.Prelude import GHC.Tc.Deriv.Utils import GHC.Tc.Utils.Env import GHC.Tc.Deriv.Generate import GHC.Tc.Deriv.Functor import GHC.Tc.Deriv.Generics import GHC.Tc.Utils.TcMType import GHC.Tc.Utils.Monad import GHC.Tc.Types.Origin import GHC.Tc.Types.Constraint import GHC.Tc.Utils.TcType import GHC.Tc.Solver import GHC.Tc.Solver.Monad ( runTcS ) import GHC.Tc.Validity (validDerivPred) import GHC.Tc.Utils.Unify (buildImplicationFor) import GHC.Tc.Zonk.TcType ( zonkWC ) import GHC.Tc.Zonk.Env ( ZonkFlexi(..), initZonkEnv ) import GHC.Core.Class import GHC.Core.DataCon import GHC.Core.TyCon import GHC.Core.TyCo.Ppr (pprTyVars) import GHC.Core.Type import GHC.Core.Predicate import GHC.Core.Unify (tcUnifyTy) import GHC.Data.Pair import GHC.Builtin.Names import GHC.Builtin.Types (typeToTypeKind) import GHC.Utils.Error import GHC.Utils.Outputable import GHC.Utils.Panic import GHC.Utils.Misc import GHC.Types.Basic import GHC.Types.Var import GHC.Data.Bag import Control.Monad import Control.Monad.Trans.Class (lift) import Control.Monad.Trans.Reader (ask) import Data.Function (on) import Data.Functor.Classes (liftEq) import Data.List (sortBy) import Data.Maybe ---------------------- inferConstraints :: DerivSpecMechanism -> DerivM (ThetaSpec, [TyVar], [TcType], DerivSpecMechanism) -- inferConstraints figures out the constraints needed for the -- instance declaration generated by a 'deriving' clause on a -- data type declaration. It also returns the new in-scope type -- variables and instance types, in case they were changed due to -- the presence of functor-like constraints. -- See Note [Inferring the instance context] -- e.g. inferConstraints -- C Int (T [a]) -- Class and inst_tys -- :RTList a -- Rep tycon and its arg tys -- where T [a] ~R :RTList a -- -- Generate a sufficiently large set of constraints that typechecking the -- generated method definitions should succeed. This set will be simplified -- before being used in the instance declaration inferConstraints :: DerivSpecMechanism -> DerivM (ThetaSpec, [TyVar], [PredType], DerivSpecMechanism) inferConstraints DerivSpecMechanism mechanism = do { DerivEnv { denv_tvs = tvs , denv_cls = main_cls , denv_inst_tys = inst_tys } <- ReaderT DerivEnv (IOEnv (Env TcGblEnv TcLclEnv)) DerivEnv forall (m :: * -> *) r. Monad m => ReaderT r m r ask ; wildcard <- isStandaloneWildcardDeriv ; let infer_constraints :: DerivM (ThetaSpec, [TyVar], [TcType], DerivSpecMechanism) infer_constraints = case DerivSpecMechanism mechanism of DerivSpecStock{dsm_stock_dit :: DerivSpecMechanism -> DerivInstTys dsm_stock_dit = DerivInstTys dit} -> do (thetas, tvs, inst_tys, dit') <- DerivInstTys -> DerivM (ThetaSpec, [TyVar], [PredType], DerivInstTys) inferConstraintsStock DerivInstTys dit pure ( thetas, tvs, inst_tys , mechanism{dsm_stock_dit = dit'} ) DerivSpecMechanism DerivSpecAnyClass -> DerivM ThetaSpec -> DerivM (ThetaSpec, [TyVar], [PredType], DerivSpecMechanism) infer_constraints_simple DerivM ThetaSpec inferConstraintsAnyclass DerivSpecNewtype { dsm_newtype_dit :: DerivSpecMechanism -> DerivInstTys dsm_newtype_dit = DerivInstTys{dit_cls_tys :: DerivInstTys -> [PredType] dit_cls_tys = [PredType] cls_tys} , dsm_newtype_rep_ty :: DerivSpecMechanism -> PredType dsm_newtype_rep_ty = PredType rep_ty } -> DerivM ThetaSpec -> DerivM (ThetaSpec, [TyVar], [PredType], DerivSpecMechanism) infer_constraints_simple (DerivM ThetaSpec -> DerivM (ThetaSpec, [TyVar], [PredType], DerivSpecMechanism)) -> DerivM ThetaSpec -> DerivM (ThetaSpec, [TyVar], [PredType], DerivSpecMechanism) forall a b. (a -> b) -> a -> b $ [PredType] -> PredType -> DerivM ThetaSpec inferConstraintsCoerceBased [PredType] cls_tys PredType rep_ty DerivSpecVia { dsm_via_cls_tys :: DerivSpecMechanism -> [PredType] dsm_via_cls_tys = [PredType] cls_tys , dsm_via_ty :: DerivSpecMechanism -> PredType dsm_via_ty = PredType via_ty } -> DerivM ThetaSpec -> DerivM (ThetaSpec, [TyVar], [PredType], DerivSpecMechanism) infer_constraints_simple (DerivM ThetaSpec -> DerivM (ThetaSpec, [TyVar], [PredType], DerivSpecMechanism)) -> DerivM ThetaSpec -> DerivM (ThetaSpec, [TyVar], [PredType], DerivSpecMechanism) forall a b. (a -> b) -> a -> b $ [PredType] -> PredType -> DerivM ThetaSpec inferConstraintsCoerceBased [PredType] cls_tys PredType via_ty -- Most deriving strategies do not need to do anything special to -- the type variables and arguments to the class in the derived -- instance, so they can pass through unchanged. The exception to -- this rule is stock deriving. See -- Note [Inferring the instance context]. infer_constraints_simple :: DerivM ThetaSpec -> DerivM (ThetaSpec, [TyVar], [TcType], DerivSpecMechanism) infer_constraints_simple DerivM ThetaSpec infer_thetas = do thetas <- DerivM ThetaSpec infer_thetas pure (thetas, tvs, inst_tys, mechanism) -- Constraints arising from superclasses -- See Note [Superclasses of derived instance] cls_tvs = Class -> [TyVar] classTyVars Class main_cls sc_constraints = Bool -> SDoc -> ThetaSpec -> ThetaSpec forall a. HasCallStack => Bool -> SDoc -> a -> a assertPpr ([TyVar] -> [PredType] -> Bool forall a b. [a] -> [b] -> Bool equalLength [TyVar] cls_tvs [PredType] inst_tys) (Class -> SDoc forall a. Outputable a => a -> SDoc ppr Class main_cls SDoc -> SDoc -> SDoc forall doc. IsLine doc => doc -> doc -> doc <+> [PredType] -> SDoc forall a. Outputable a => a -> SDoc ppr [PredType] inst_tys) (ThetaSpec -> ThetaSpec) -> ThetaSpec -> ThetaSpec forall a b. (a -> b) -> a -> b $ CtOrigin -> TypeOrKind -> [PredType] -> ThetaSpec mkDirectThetaSpec (Bool -> CtOrigin mkDerivOrigin Bool wildcard) TypeOrKind TypeLevel (HasDebugCallStack => Subst -> [PredType] -> [PredType] Subst -> [PredType] -> [PredType] substTheta Subst cls_subst (Class -> [PredType] classSCTheta Class main_cls)) cls_subst = Bool -> Subst -> Subst forall a. HasCallStack => Bool -> a -> a assert ([TyVar] -> [PredType] -> Bool forall a b. [a] -> [b] -> Bool equalLength [TyVar] cls_tvs [PredType] inst_tys) (Subst -> Subst) -> Subst -> Subst forall a b. (a -> b) -> a -> b $ [TyVar] -> [PredType] -> Subst HasDebugCallStack => [TyVar] -> [PredType] -> Subst zipTvSubst [TyVar] cls_tvs [PredType] inst_tys ; (inferred_constraints, tvs', inst_tys', mechanism') <- infer_constraints ; lift $ traceTc "inferConstraints" $ vcat [ ppr main_cls <+> ppr inst_tys' , ppr inferred_constraints ] ; return ( sc_constraints ++ inferred_constraints , tvs', inst_tys', mechanism' ) } -- | Like 'inferConstraints', but used only in the case of the @stock@ deriving -- strategy. The constraints are inferred by inspecting the fields of each data -- constructor. In this example: -- -- > data Foo = MkFoo Int Char deriving Show -- -- We would infer the following constraints ('ThetaSpec's): -- -- > (Show Int, Show Char) -- -- Note that this function also returns the type variables ('TyVar's) and -- class arguments ('TcType's) for the resulting instance. This is because -- when deriving 'Functor'-like classes, we must sometimes perform kind -- substitutions to ensure the resulting instance is well kinded, which may -- affect the type variables and class arguments. In this example: -- -- > newtype Compose (f :: k -> Type) (g :: Type -> k) (a :: Type) = -- > Compose (f (g a)) deriving stock Functor -- -- We must unify @k@ with @Type@ in order for the resulting 'Functor' instance -- to be well kinded, so we return @[]@/@[Type, f, g]@ for the -- 'TyVar's/'TcType's, /not/ @[k]@/@[k, f, g]@. -- See Note [Inferring the instance context]. inferConstraintsStock :: DerivInstTys -> DerivM (ThetaSpec, [TyVar], [TcType], DerivInstTys) inferConstraintsStock :: DerivInstTys -> DerivM (ThetaSpec, [TyVar], [PredType], DerivInstTys) inferConstraintsStock dit :: DerivInstTys dit@(DerivInstTys { dit_cls_tys :: DerivInstTys -> [PredType] dit_cls_tys = [PredType] cls_tys , dit_tc :: DerivInstTys -> TyCon dit_tc = TyCon tc , dit_tc_args :: DerivInstTys -> [PredType] dit_tc_args = [PredType] tc_args , dit_rep_tc :: DerivInstTys -> TyCon dit_rep_tc = TyCon rep_tc , dit_rep_tc_args :: DerivInstTys -> [PredType] dit_rep_tc_args = [PredType] rep_tc_args }) = do DerivEnv { denv_tvs = tvs , denv_cls = main_cls , denv_inst_tys = inst_tys } <- ReaderT DerivEnv (IOEnv (Env TcGblEnv TcLclEnv)) DerivEnv forall (m :: * -> *) r. Monad m => ReaderT r m r ask wildcard <- isStandaloneWildcardDeriv let inst_ty = TyCon -> [PredType] -> PredType mkTyConApp TyCon tc [PredType] tc_args tc_binders = TyCon -> [TyConBinder] tyConBinders TyCon rep_tc choose_level TyConBinder bndr | TyConBinder -> Bool isNamedTyConBinder TyConBinder bndr = TypeOrKind KindLevel | Bool otherwise = TypeOrKind TypeLevel t_or_ks = (TyConBinder -> TypeOrKind) -> [TyConBinder] -> [TypeOrKind] forall a b. (a -> b) -> [a] -> [b] map TyConBinder -> TypeOrKind choose_level [TyConBinder] tc_binders [TypeOrKind] -> [TypeOrKind] -> [TypeOrKind] forall a. [a] -> [a] -> [a] ++ TypeOrKind -> [TypeOrKind] forall a. a -> [a] repeat TypeOrKind TypeLevel -- want to report *kind* errors when possible -- Constraints arising from the arguments of each constructor con_arg_constraints :: ([TyVar] -> CtOrigin -> TypeOrKind -> Type -> [(ThetaSpec, Maybe Subst)]) -> (ThetaSpec, [TyVar], [TcType], DerivInstTys) con_arg_constraints [TyVar] -> CtOrigin -> TypeOrKind -> PredType -> [(ThetaSpec, Maybe Subst)] get_arg_constraints = let -- Constraints from the fields of each data constructor. ([ThetaSpec] predss, [Maybe Subst] mbSubsts) = [(ThetaSpec, Maybe Subst)] -> ([ThetaSpec], [Maybe Subst]) forall a b. [(a, b)] -> ([a], [b]) unzip [ (ThetaSpec, Maybe Subst) preds_and_mbSubst | DataCon data_con <- TyCon -> [DataCon] tyConDataCons TyCon rep_tc , (Int arg_n, TypeOrKind arg_t_or_k, PredType arg_ty) <- [Int] -> [TypeOrKind] -> [PredType] -> [(Int, TypeOrKind, PredType)] forall a b c. [a] -> [b] -> [c] -> [(a, b, c)] zip3 [Int 1..] [TypeOrKind] t_or_ks ([PredType] -> [(Int, TypeOrKind, PredType)]) -> [PredType] -> [(Int, TypeOrKind, PredType)] forall a b. (a -> b) -> a -> b $ DataCon -> DerivInstTys -> [PredType] derivDataConInstArgTys DataCon data_con DerivInstTys dit -- No constraints for unlifted types -- See Note [Deriving and unboxed types] , Bool -> Bool not (HasDebugCallStack => PredType -> Bool PredType -> Bool isUnliftedType PredType arg_ty) , let orig :: CtOrigin orig = DataCon -> Int -> Bool -> CtOrigin DerivOriginDC DataCon data_con Int arg_n Bool wildcard , (ThetaSpec, Maybe Subst) preds_and_mbSubst <- [TyVar] -> CtOrigin -> TypeOrKind -> PredType -> [(ThetaSpec, Maybe Subst)] get_arg_constraints (DataCon -> [TyVar] dataConUnivTyVars DataCon data_con) CtOrigin orig TypeOrKind arg_t_or_k PredType arg_ty ] -- Stupid constraints from DatatypeContexts. Note that we -- must gather these constraints from the data constructors, -- not from the parent type constructor, as the latter could -- lead to redundant constraints due to thinning. -- See Note [The stupid context] in GHC.Core.DataCon. stupid_theta :: [PredType] stupid_theta = [ [TyVar] -> [PredType] -> PredType -> PredType HasDebugCallStack => [TyVar] -> [PredType] -> PredType -> PredType substTyWith (DataCon -> [TyVar] dataConUnivTyVars DataCon data_con) (DataCon -> [PredType] -> [PredType] dataConInstUnivs DataCon data_con [PredType] rep_tc_args) PredType stupid_pred | DataCon data_con <- TyCon -> [DataCon] tyConDataCons TyCon rep_tc , PredType stupid_pred <- DataCon -> [PredType] dataConStupidTheta DataCon data_con ] preds :: ThetaSpec preds = [ThetaSpec] -> ThetaSpec forall (t :: * -> *) a. Foldable t => t [a] -> [a] concat [ThetaSpec] predss -- If the constraints require a subtype to be of kind -- (* -> *) (which is the case for functor-like -- constraints), then we explicitly unify the subtype's -- kinds with (* -> *). -- See Note [Inferring the instance context] subst :: Subst subst = (Subst -> Subst -> Subst) -> Subst -> [Subst] -> Subst forall b a. (b -> a -> b) -> b -> [a] -> b forall (t :: * -> *) b a. Foldable t => (b -> a -> b) -> b -> t a -> b foldl' Subst -> Subst -> Subst composeTCvSubst Subst emptySubst ([Maybe Subst] -> [Subst] forall a. [Maybe a] -> [a] catMaybes [Maybe Subst] mbSubsts) unmapped_tvs :: [TyVar] unmapped_tvs = (TyVar -> Bool) -> [TyVar] -> [TyVar] forall a. (a -> Bool) -> [a] -> [a] filter (\TyVar v -> TyVar v TyVar -> Subst -> Bool `notElemSubst` Subst subst Bool -> Bool -> Bool && Bool -> Bool not (TyVar v TyVar -> Subst -> Bool `isInScope` Subst subst)) [TyVar] tvs (Subst subst', [TyVar] _) = HasDebugCallStack => Subst -> [TyVar] -> (Subst, [TyVar]) Subst -> [TyVar] -> (Subst, [TyVar]) substTyVarBndrs Subst subst [TyVar] unmapped_tvs stupid_theta_origin :: ThetaSpec stupid_theta_origin = CtOrigin -> TypeOrKind -> [PredType] -> ThetaSpec mkDirectThetaSpec CtOrigin deriv_origin TypeOrKind TypeLevel (HasDebugCallStack => Subst -> [PredType] -> [PredType] Subst -> [PredType] -> [PredType] substTheta Subst subst' [PredType] stupid_theta) preds' :: ThetaSpec preds' = (PredSpec -> PredSpec) -> ThetaSpec -> ThetaSpec forall a b. (a -> b) -> [a] -> [b] map (HasDebugCallStack => Subst -> PredSpec -> PredSpec Subst -> PredSpec -> PredSpec substPredSpec Subst subst') ThetaSpec preds inst_tys' :: [PredType] inst_tys' = HasDebugCallStack => Subst -> [PredType] -> [PredType] Subst -> [PredType] -> [PredType] substTys Subst subst' [PredType] inst_tys dit' :: DerivInstTys dit' = Subst -> DerivInstTys -> DerivInstTys substDerivInstTys Subst subst' DerivInstTys dit tvs' :: [TyVar] tvs' = [PredType] -> [TyVar] tyCoVarsOfTypesWellScoped [PredType] inst_tys' in ( ThetaSpec stupid_theta_origin ThetaSpec -> ThetaSpec -> ThetaSpec forall a. [a] -> [a] -> [a] ++ ThetaSpec preds' , [TyVar] tvs', [PredType] inst_tys', DerivInstTys dit' ) is_generic = Class main_cls Class -> Unique -> Bool forall a. Uniquable a => a -> Unique -> Bool `hasKey` Unique genClassKey is_generic1 = Class main_cls Class -> Unique -> Bool forall a. Uniquable a => a -> Unique -> Bool `hasKey` Unique gen1ClassKey -- is_functor_like: see Note [Inferring the instance context] is_functor_like = HasDebugCallStack => PredType -> PredType PredType -> PredType typeKind PredType inst_ty HasDebugCallStack => PredType -> PredType -> Bool PredType -> PredType -> Bool `tcEqKind` PredType typeToTypeKind Bool -> Bool -> Bool || Bool is_generic1 get_gen1_constraints :: Class -> [TyVar] -- The universally quantified type variables for the -- data constructor -> CtOrigin -> TypeOrKind -> Type -> [(ThetaSpec, Maybe Subst)] get_gen1_constraints Class functor_cls [TyVar] dc_univs CtOrigin orig TypeOrKind t_or_k PredType ty = CtOrigin -> TypeOrKind -> Class -> [PredType] -> [(ThetaSpec, Maybe Subst)] mk_functor_like_constraints CtOrigin orig TypeOrKind t_or_k Class functor_cls ([PredType] -> [(ThetaSpec, Maybe Subst)]) -> [PredType] -> [(ThetaSpec, Maybe Subst)] forall a b. (a -> b) -> a -> b $ TyVar -> PredType -> [PredType] get_gen1_constrained_tys TyVar last_dc_univ PredType ty where -- If we are deriving an instance of 'Generic1' and have made -- it this far, then there should be at least one universal type -- variable, making this use of 'last' safe. last_dc_univ :: TyVar last_dc_univ = Bool -> TyVar -> TyVar forall a. HasCallStack => Bool -> a -> a assert (Bool -> Bool not ([TyVar] -> Bool forall a. [a] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [TyVar] dc_univs)) (TyVar -> TyVar) -> TyVar -> TyVar forall a b. (a -> b) -> a -> b $ [TyVar] -> TyVar forall a. HasCallStack => [a] -> a last [TyVar] dc_univs get_std_constrained_tys :: [TyVar] -- The universally quantified type variables for the -- data constructor -> CtOrigin -> TypeOrKind -> Type -> [(ThetaSpec, Maybe Subst)] get_std_constrained_tys [TyVar] dc_univs CtOrigin orig TypeOrKind t_or_k PredType ty | Bool is_functor_like = CtOrigin -> TypeOrKind -> Class -> [PredType] -> [(ThetaSpec, Maybe Subst)] mk_functor_like_constraints CtOrigin orig TypeOrKind t_or_k Class main_cls ([PredType] -> [(ThetaSpec, Maybe Subst)]) -> [PredType] -> [(ThetaSpec, Maybe Subst)] forall a b. (a -> b) -> a -> b $ TyVar -> PredType -> [PredType] deepSubtypesContaining TyVar last_dc_univ PredType ty | Bool otherwise = [( [CtOrigin -> TypeOrKind -> Class -> PredType -> PredSpec mk_cls_pred CtOrigin orig TypeOrKind t_or_k Class main_cls PredType ty] , Maybe Subst forall a. Maybe a Nothing )] where -- If 'is_functor_like' holds, then there should be at least one -- universal type variable, making this use of 'last' safe. last_dc_univ :: TyVar last_dc_univ = Bool -> TyVar -> TyVar forall a. HasCallStack => Bool -> a -> a assert (Bool -> Bool not ([TyVar] -> Bool forall a. [a] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [TyVar] dc_univs)) (TyVar -> TyVar) -> TyVar -> TyVar forall a b. (a -> b) -> a -> b $ [TyVar] -> TyVar forall a. HasCallStack => [a] -> a last [TyVar] dc_univs mk_functor_like_constraints :: CtOrigin -> TypeOrKind -> Class -> [Type] -> [(ThetaSpec, Maybe Subst)] -- 'cls' is usually main_cls (Functor or Traversable etc), but if -- main_cls = Generic1, then 'cls' can be Functor; see -- get_gen1_constraints -- -- For each type, generate two constraints, -- [cls ty, kind(ty) ~ (*->*)], and a kind substitution that results -- from unifying kind(ty) with * -> *. If the unification is -- successful, it will ensure that the resulting instance is well -- kinded. If not, the second constraint will result in an error -- message which points out the kind mismatch. -- See Note [Inferring the instance context] mk_functor_like_constraints CtOrigin orig TypeOrKind t_or_k Class cls = (PredType -> (ThetaSpec, Maybe Subst)) -> [PredType] -> [(ThetaSpec, Maybe Subst)] forall a b. (a -> b) -> [a] -> [b] map ((PredType -> (ThetaSpec, Maybe Subst)) -> [PredType] -> [(ThetaSpec, Maybe Subst)]) -> (PredType -> (ThetaSpec, Maybe Subst)) -> [PredType] -> [(ThetaSpec, Maybe Subst)] forall a b. (a -> b) -> a -> b $ \PredType ty -> let ki :: PredType ki = HasDebugCallStack => PredType -> PredType PredType -> PredType typeKind PredType ty in ( [ CtOrigin -> TypeOrKind -> Class -> PredType -> PredSpec mk_cls_pred CtOrigin orig TypeOrKind t_or_k Class cls PredType ty , SimplePredSpec { sps_pred :: PredType sps_pred = PredType -> PredType -> PredType mkPrimEqPred PredType ki PredType typeToTypeKind , sps_origin :: CtOrigin sps_origin = CtOrigin orig , sps_type_or_kind :: TypeOrKind sps_type_or_kind = TypeOrKind KindLevel } ] , PredType -> PredType -> Maybe Subst tcUnifyTy PredType ki PredType typeToTypeKind ) -- Extra Data constraints -- The Data class (only) requires that for -- instance (...) => Data (T t1 t2) -- IF t1:*, t2:* -- THEN (Data t1, Data t2) are among the (...) constraints -- Reason: when the IF holds, we generate a method -- dataCast2 f = gcast2 f -- and we need the Data constraints to typecheck the method extra_constraints | Class main_cls Class -> Unique -> Bool forall a. Uniquable a => a -> Unique -> Bool `hasKey` Unique dataClassKey , (PredType -> Bool) -> [PredType] -> Bool forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool all (PredType -> Bool isLiftedTypeKind (PredType -> Bool) -> (PredType -> PredType) -> PredType -> Bool forall b c a. (b -> c) -> (a -> b) -> a -> c . HasDebugCallStack => PredType -> PredType PredType -> PredType typeKind) [PredType] rep_tc_args = [ CtOrigin -> TypeOrKind -> Class -> PredType -> PredSpec mk_cls_pred CtOrigin deriv_origin TypeOrKind t_or_k Class main_cls PredType ty | (TypeOrKind t_or_k, PredType ty) <- [TypeOrKind] -> [PredType] -> [(TypeOrKind, PredType)] forall a b. [a] -> [b] -> [(a, b)] zip [TypeOrKind] t_or_ks [PredType] rep_tc_args] | Bool otherwise = [] mk_cls_pred CtOrigin orig TypeOrKind t_or_k Class cls PredType ty -- Don't forget to apply to cls_tys' too = SimplePredSpec { sps_pred :: PredType sps_pred = Class -> [PredType] -> PredType mkClassPred Class cls ([PredType] cls_tys' [PredType] -> [PredType] -> [PredType] forall a. [a] -> [a] -> [a] ++ [PredType ty]) , sps_origin :: CtOrigin sps_origin = CtOrigin orig , sps_type_or_kind :: TypeOrKind sps_type_or_kind = TypeOrKind t_or_k } cls_tys' | Bool is_generic1 = [] -- In the awkward Generic1 case, cls_tys' should be -- empty, since we are applying the class Functor. | Bool otherwise = [PredType] cls_tys deriv_origin = Bool -> CtOrigin mkDerivOrigin Bool wildcard if -- Generic constraints are easy | is_generic -> return ([], tvs, inst_tys, dit) -- Generic1 needs Functor -- See Note [Getting base classes] | is_generic1 -> assert (tyConTyVars rep_tc `lengthExceeds` 0) $ -- Generic1 has a single kind variable assert (cls_tys `lengthIs` 1) $ do { functorClass <- lift $ tcLookupClass functorClassName ; pure $ con_arg_constraints $ get_gen1_constraints functorClass } -- The others are a bit more complicated | otherwise -> do { let (arg_constraints, tvs', inst_tys', dit') = con_arg_constraints get_std_constrained_tys ; lift $ traceTc "inferConstraintsStock" $ vcat [ ppr main_cls <+> ppr inst_tys' , ppr arg_constraints ] ; return ( extra_constraints ++ arg_constraints , tvs', inst_tys', dit' ) } -- | Like 'inferConstraints', but used only in the case of @DeriveAnyClass@, -- which gathers its constraints based on the type signatures of the class's -- methods instead of the types of the data constructor's field. -- -- See Note [Gathering and simplifying constraints for DeriveAnyClass] -- for an explanation of how these constraints are used to determine the -- derived instance context. inferConstraintsAnyclass :: DerivM ThetaSpec inferConstraintsAnyclass :: DerivM ThetaSpec inferConstraintsAnyclass = do { DerivEnv { denv_cls = cls , denv_inst_tys = inst_tys } <- ReaderT DerivEnv (IOEnv (Env TcGblEnv TcLclEnv)) DerivEnv forall (m :: * -> *) r. Monad m => ReaderT r m r ask ; let gen_dms = [ (TyVar sel_id, PredType dm_ty) | (TyVar sel_id, Just (Name _, GenericDM PredType dm_ty)) <- Class -> [(TyVar, DefMethInfo)] classOpItems Class cls ] ; wildcard <- isStandaloneWildcardDeriv ; let meth_pred :: (Id, Type) -> PredSpec -- (Id,Type) are the selector Id and the generic default method type -- NB: the latter is /not/ quantified over the class variables -- See Note [Gathering and simplifying constraints for DeriveAnyClass] meth_pred (TyVar sel_id, PredType gen_dm_ty) = let ([TyVar] sel_tvs, PredType _cls_pred, PredType meth_ty) = PredType -> ([TyVar], PredType, PredType) tcSplitMethodTy (TyVar -> PredType varType TyVar sel_id) meth_ty' :: PredType meth_ty' = [TyVar] -> [PredType] -> PredType -> PredType HasDebugCallStack => [TyVar] -> [PredType] -> PredType -> PredType substTyWith [TyVar] sel_tvs [PredType] inst_tys PredType meth_ty gen_dm_ty' :: PredType gen_dm_ty' = [TyVar] -> [PredType] -> PredType -> PredType HasDebugCallStack => [TyVar] -> [PredType] -> PredType -> PredType substTyWith [TyVar] sel_tvs [PredType] inst_tys PredType gen_dm_ty in -- This is the only place where a SubTypePredSpec is -- constructed instead of a SimplePredSpec. See -- Note [Gathering and simplifying constraints for DeriveAnyClass] -- for a more in-depth explanation. SubTypePredSpec { stps_ty_actual :: PredType stps_ty_actual = PredType gen_dm_ty' , stps_ty_expected :: PredType stps_ty_expected = PredType meth_ty' , stps_origin :: CtOrigin stps_origin = Bool -> CtOrigin mkDerivOrigin Bool wildcard } ; pure $ map meth_pred gen_dms } -- Like 'inferConstraints', but used only for @GeneralizedNewtypeDeriving@ and -- @DerivingVia@. Since both strategies generate code involving 'coerce', the -- inferred constraints set up the scaffolding needed to typecheck those uses -- of 'coerce'. In this example: -- -- > newtype Age = MkAge Int deriving newtype Num -- -- We would infer the following constraints ('ThetaSpec'): -- -- > (Num Int, Coercible Age Int) inferConstraintsCoerceBased :: [Type] -> Type -> DerivM ThetaSpec inferConstraintsCoerceBased :: [PredType] -> PredType -> DerivM ThetaSpec inferConstraintsCoerceBased [PredType] cls_tys PredType rep_ty = do DerivEnv { denv_tvs = tvs , denv_cls = cls , denv_inst_tys = inst_tys } <- ReaderT DerivEnv (IOEnv (Env TcGblEnv TcLclEnv)) DerivEnv forall (m :: * -> *) r. Monad m => ReaderT r m r ask sa_wildcard <- isStandaloneWildcardDeriv let -- The following functions are polymorphic over the representation -- type, since we might either give it the underlying type of a -- newtype (for GeneralizedNewtypeDeriving) or a @via@ type -- (for DerivingVia). rep_tys PredType ty = [PredType] cls_tys [PredType] -> [PredType] -> [PredType] forall a. [a] -> [a] -> [a] ++ [PredType ty] rep_pred PredType ty = Class -> [PredType] -> PredType mkClassPred Class cls (PredType -> [PredType] rep_tys PredType ty) rep_pred_o PredType ty = SimplePredSpec { sps_pred :: PredType sps_pred = PredType -> PredType rep_pred PredType ty , sps_origin :: CtOrigin sps_origin = CtOrigin deriv_origin , sps_type_or_kind :: TypeOrKind sps_type_or_kind = TypeOrKind TypeLevel } -- rep_pred is the representation dictionary, from where -- we are going to get all the methods for the final -- dictionary deriv_origin = Bool -> CtOrigin mkDerivOrigin Bool sa_wildcard -- Next we collect constraints for the class methods -- If there are no methods, we don't need any constraints -- Otherwise we need (C rep_ty), for the representation methods, -- and constraints to coerce each individual method meth_preds :: Type -> ThetaSpec meth_preds PredType ty | [TyVar] -> Bool forall a. [a] -> Bool forall (t :: * -> *) a. Foldable t => t a -> Bool null [TyVar] meths = [] -- No methods => no constraints -- (#12814) | Bool otherwise = PredType -> PredSpec rep_pred_o PredType ty PredSpec -> ThetaSpec -> ThetaSpec forall a. a -> [a] -> [a] : PredType -> ThetaSpec coercible_constraints PredType ty meths = Class -> [TyVar] classMethods Class cls coercible_constraints PredType ty = [ SimplePredSpec { sps_pred :: PredType sps_pred = PredType -> PredType -> PredType mkReprPrimEqPred PredType t1 PredType t2 , sps_origin :: CtOrigin sps_origin = TyVar -> PredType -> PredType -> Bool -> CtOrigin DerivOriginCoerce TyVar meth PredType t1 PredType t2 Bool sa_wildcard , sps_type_or_kind :: TypeOrKind sps_type_or_kind = TypeOrKind TypeLevel } | TyVar meth <- [TyVar] meths , let (Pair PredType t1 PredType t2) = Class -> [TyVar] -> [PredType] -> PredType -> TyVar -> Pair PredType mkCoerceClassMethEqn Class cls [TyVar] tvs [PredType] inst_tys PredType ty TyVar meth ] pure (meth_preds rep_ty) {- Note [Inferring the instance context] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There are two sorts of 'deriving', as represented by the two constructors for DerivContext: * InferContext mb_wildcard: This can either be: - The deriving clause for a data type. (e.g, data T a = T1 a deriving( Eq )) In this case, mb_wildcard = Nothing. - A standalone declaration with an extra-constraints wildcard (e.g., deriving instance _ => Eq (Foo a)) In this case, mb_wildcard = Just loc, where loc is the location of the extra-constraints wildcard. Here we must infer an instance context, and generate instance declaration instance Eq a => Eq (T a) where ... * SupplyContext theta: standalone deriving deriving instance Eq a => Eq (T a) Here we only need to fill in the bindings; the instance context (theta) is user-supplied For the InferContext case, we must figure out the instance context (inferConstraintsStock). Suppose we are inferring the instance context for C t1 .. tn (T s1 .. sm) There are two cases * (T s1 .. sm) :: * (the normal case) Then we behave like Eq and guess (C t1 .. tn t) for each data constructor arg of type t. More details below. * (T s1 .. sm) :: * -> * (the functor-like case) Then we behave like Functor. In both cases we produce a bunch of un-simplified constraints and them simplify them in simplifyInstanceContexts; see Note [Simplifying the instance context]. In the functor-like case, we may need to unify some kind variables with * in order for the generated instance to be well-kinded. An example from #10524: newtype Compose (f :: k2 -> *) (g :: k1 -> k2) (a :: k1) = Compose (f (g a)) deriving Functor Earlier in the deriving pipeline, GHC unifies the kind of Compose f g (k1 -> *) with the kind of Functor's argument (* -> *), so k1 := *. But this alone isn't enough, since k2 wasn't unified with *: instance (Functor (f :: k2 -> *), Functor (g :: * -> k2)) => Functor (Compose f g) where ... The two Functor constraints are ill-kinded. To ensure this doesn't happen, we: 1. Collect all of a datatype's subtypes which require functor-like constraints. 2. For each subtype, create a substitution by unifying the subtype's kind with (* -> *). 3. Compose all the substitutions into one, then apply that substitution to all of the in-scope type variables and the instance types. Note [Getting base classes] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Functor and Typeable are defined in package 'base', and that is not available when compiling 'ghc-prim'. So we must be careful that 'deriving' for stuff in ghc-prim does not use Functor or Typeable implicitly via these lookups. Note [Deriving and unboxed types] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We have some special hacks to support things like data T = MkT Int# deriving ( Show ) Specifically, we use GHC.Tc.Deriv.Generate.box to box the Int# into an Int (which we know how to show), and append a '#'. Parentheses are not required for unboxed values (`MkT -3#` is a valid expression). Note [Superclasses of derived instance] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In general, a derived instance decl needs the superclasses of the derived class too. So if we have data T a = ...deriving( Ord ) then the initial context for Ord (T a) should include Eq (T a). Often this is redundant; we'll also generate an Ord constraint for each constructor argument, and that will probably generate enough constraints to make the Eq (T a) constraint be satisfied too. But not always; consider: data S a = S instance Eq (S a) instance Ord (S a) data T a = MkT (S a) deriving( Ord ) instance Num a => Eq (T a) The derived instance for (Ord (T a)) must have a (Num a) constraint! Similarly consider: data T a = MkT deriving( Data ) Here there *is* no argument field, but we must nevertheless generate a context for the Data instances: instance Typeable a => Data (T a) where ... ************************************************************************ * * Finding the fixed point of deriving equations * * ************************************************************************ Note [Simplifying the instance context] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider data T a b = C1 (Foo a) (Bar b) | C2 Int (T b a) | C3 (T a a) deriving (Eq) We want to come up with an instance declaration of the form instance (Ping a, Pong b, ...) => Eq (T a b) where x == y = ... It is pretty easy, albeit tedious, to fill in the code "...". The trick is to figure out what the context for the instance decl is, namely Ping, Pong and friends. Let's call the context reqd for the T instance of class C at types (a,b, ...) C (T a b). Thus: Eq (T a b) = (Ping a, Pong b, ...) Now we can get a (recursive) equation from the data decl. This part is done by inferConstraintsStock. Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1 u Eq (T b a) u Eq Int -- From C2 u Eq (T a a) -- From C3 Foo and Bar may have explicit instances for Eq, in which case we can just substitute for them. Alternatively, either or both may have their Eq instances given by deriving clauses, in which case they form part of the system of equations. Now all we need do is simplify and solve the equations, iterating to find the least fixpoint. This is done by simplifyInstanceConstraints. Notice that the order of the arguments can switch around, as here in the recursive calls to T. Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b. We start with: Eq (T a b) = {} -- The empty set Next iteration: Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1 u Eq (T b a) u Eq Int -- From C2 u Eq (T a a) -- From C3 After simplification: = Eq a u Ping b u {} u {} u {} = Eq a u Ping b Next iteration: Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1 u Eq (T b a) u Eq Int -- From C2 u Eq (T a a) -- From C3 After simplification: = Eq a u Ping b u (Eq b u Ping a) u (Eq a u Ping a) = Eq a u Ping b u Eq b u Ping a The next iteration gives the same result, so this is the fixpoint. We need to make a canonical form of the RHS to ensure convergence. We do this by simplifying the RHS to a form in which - the classes constrain only tyvars - the list is sorted by tyvar (major key) and then class (minor key) - no duplicates, of course Note [Deterministic simplifyInstanceContexts] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Canonicalisation uses nonDetCmpType which is nondeterministic. Sorting with nonDetCmpType puts the returned lists in a nondeterministic order. If we were to return them, we'd get class constraints in nondeterministic order. Consider: data ADT a b = Z a b deriving Eq The generated code could be either: instance (Eq a, Eq b) => Eq (Z a b) where Or: instance (Eq b, Eq a) => Eq (Z a b) where To prevent the order from being nondeterministic we only canonicalize when comparing and return them in the same order as simplifyDeriv returned them. See also Note [nonDetCmpType nondeterminism] -} simplifyInstanceContexts :: [DerivSpec ThetaSpec] -> TcM [DerivSpec ThetaType] -- Used only for deriving clauses or standalone deriving with an -- extra-constraints wildcard (InferContext) -- See Note [Simplifying the instance context] simplifyInstanceContexts :: [DerivSpec ThetaSpec] -> TcM [DerivSpec [PredType]] simplifyInstanceContexts [] = [DerivSpec [PredType]] -> TcM [DerivSpec [PredType]] forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a forall (m :: * -> *) a. Monad m => a -> m a return [] simplifyInstanceContexts [DerivSpec ThetaSpec] infer_specs = do { String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) () traceTc String "simplifyInstanceContexts" (SDoc -> IOEnv (Env TcGblEnv TcLclEnv) ()) -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) () forall a b. (a -> b) -> a -> b $ [SDoc] -> SDoc forall doc. IsDoc doc => [doc] -> doc vcat ((DerivSpec ThetaSpec -> SDoc) -> [DerivSpec ThetaSpec] -> [SDoc] forall a b. (a -> b) -> [a] -> [b] map DerivSpec ThetaSpec -> SDoc forall theta. Outputable theta => DerivSpec theta -> SDoc pprDerivSpec [DerivSpec ThetaSpec] infer_specs) ; final_specs <- Int -> [[PredType]] -> TcM [DerivSpec [PredType]] iterate_deriv Int 1 [[PredType]] initial_solutions -- After simplification finishes, zonk the TcTyVars as described -- in Note [Overlap and deriving]. ; initZonkEnv DefaultFlexi $ traverse zonkDerivSpec final_specs } where ------------------------------------------------------------------ -- The initial solutions for the equations claim that each -- instance has an empty context; this solution is certainly -- in canonical form. initial_solutions :: [ThetaType] initial_solutions :: [[PredType]] initial_solutions = [ [] | DerivSpec ThetaSpec _ <- [DerivSpec ThetaSpec] infer_specs ] ------------------------------------------------------------------ -- iterate_deriv calculates the next batch of solutions, -- compares it with the current one; finishes if they are the -- same, otherwise recurses with the new solutions. -- It fails if any iteration fails iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec ThetaType] iterate_deriv :: Int -> [[PredType]] -> TcM [DerivSpec [PredType]] iterate_deriv Int n [[PredType]] current_solns | Int n Int -> Int -> Bool forall a. Ord a => a -> a -> Bool > Int 20 -- Looks as if we are in an infinite loop -- This can happen if we have -XUndecidableInstances -- (See GHC.Tc.Solver.tcSimplifyDeriv.) = String -> SDoc -> TcM [DerivSpec [PredType]] forall a. HasCallStack => String -> SDoc -> a pprPanic String "solveDerivEqns: probable loop" ([SDoc] -> SDoc forall doc. IsDoc doc => [doc] -> doc vcat ((DerivSpec ThetaSpec -> SDoc) -> [DerivSpec ThetaSpec] -> [SDoc] forall a b. (a -> b) -> [a] -> [b] map DerivSpec ThetaSpec -> SDoc forall theta. Outputable theta => DerivSpec theta -> SDoc pprDerivSpec [DerivSpec ThetaSpec] infer_specs) SDoc -> SDoc -> SDoc forall doc. IsDoc doc => doc -> doc -> doc $$ [[PredType]] -> SDoc forall a. Outputable a => a -> SDoc ppr [[PredType]] current_solns) | Bool otherwise = do { -- Extend the inst info from the explicit instance decls -- with the current set of solutions, and simplify each RHS inst_specs <- ([PredType] -> DerivSpec ThetaSpec -> IOEnv (Env TcGblEnv TcLclEnv) ClsInst) -> [[PredType]] -> [DerivSpec ThetaSpec] -> IOEnv (Env TcGblEnv TcLclEnv) [ClsInst] forall (m :: * -> *) a b c. Applicative m => (a -> b -> m c) -> [a] -> [b] -> m [c] zipWithM (\[PredType] soln -> DerivSpec [PredType] -> IOEnv (Env TcGblEnv TcLclEnv) ClsInst newDerivClsInst (DerivSpec [PredType] -> IOEnv (Env TcGblEnv TcLclEnv) ClsInst) -> (DerivSpec ThetaSpec -> DerivSpec [PredType]) -> DerivSpec ThetaSpec -> IOEnv (Env TcGblEnv TcLclEnv) ClsInst forall b c a. (b -> c) -> (a -> b) -> a -> c . [PredType] -> DerivSpec ThetaSpec -> DerivSpec [PredType] forall theta' theta. theta' -> DerivSpec theta -> DerivSpec theta' setDerivSpecTheta [PredType] soln) [[PredType]] current_solns [DerivSpec ThetaSpec] infer_specs ; new_solns <- checkNoErrs $ extendLocalInstEnv inst_specs $ mapM simplifyDeriv infer_specs ; if (current_solns `eqSolution` new_solns) then return [ setDerivSpecTheta soln spec | (spec, soln) <- zip infer_specs current_solns ] else iterate_deriv (n+1) new_solns } eqSolution :: [[PredType]] -> [[PredType]] -> Bool eqSolution = (([PredType] -> [PredType] -> Bool) -> [[PredType]] -> [[PredType]] -> Bool forall a b. (a -> b -> Bool) -> [a] -> [b] -> Bool forall (f :: * -> *) a b. Eq1 f => (a -> b -> Bool) -> f a -> f b -> Bool liftEq (([PredType] -> [PredType] -> Bool) -> [[PredType]] -> [[PredType]] -> Bool) -> ((PredType -> PredType -> Bool) -> [PredType] -> [PredType] -> Bool) -> (PredType -> PredType -> Bool) -> [[PredType]] -> [[PredType]] -> Bool forall b c a. (b -> c) -> (a -> b) -> a -> c . (PredType -> PredType -> Bool) -> [PredType] -> [PredType] -> Bool forall a b. (a -> b -> Bool) -> [a] -> [b] -> Bool forall (f :: * -> *) a b. Eq1 f => (a -> b -> Bool) -> f a -> f b -> Bool liftEq) HasCallStack => PredType -> PredType -> Bool PredType -> PredType -> Bool eqType ([[PredType]] -> [[PredType]] -> Bool) -> ([[PredType]] -> [[PredType]]) -> [[PredType]] -> [[PredType]] -> Bool forall b c a. (b -> b -> c) -> (a -> b) -> a -> a -> c `on` [[PredType]] -> [[PredType]] canSolution -- Canonicalise for comparison -- See Note [Deterministic simplifyInstanceContexts] canSolution :: [[PredType]] -> [[PredType]] canSolution = ([PredType] -> [PredType]) -> [[PredType]] -> [[PredType]] forall a b. (a -> b) -> [a] -> [b] map ((PredType -> PredType -> Ordering) -> [PredType] -> [PredType] forall a. (a -> a -> Ordering) -> [a] -> [a] sortBy PredType -> PredType -> Ordering nonDetCmpType) derivInstCtxt :: PredType -> SDoc derivInstCtxt :: PredType -> SDoc derivInstCtxt PredType pred = String -> SDoc forall doc. IsLine doc => String -> doc text String "When deriving the instance for" SDoc -> SDoc -> SDoc forall doc. IsLine doc => doc -> doc -> doc <+> SDoc -> SDoc forall doc. IsLine doc => doc -> doc parens (PredType -> SDoc forall a. Outputable a => a -> SDoc ppr PredType pred) {- *********************************************************************************** * * * Simplify derived constraints * * *********************************************************************************** -} -- | Given @instance (wanted) => C inst_ty@, simplify 'wanted' as much -- as possible. Fail if not possible. simplifyDeriv :: DerivSpec ThetaSpec -> TcM ThetaType -- ^ Needed constraints (after simplification), -- i.e. @['PredType']@. simplifyDeriv :: DerivSpec ThetaSpec -> IOEnv (Env TcGblEnv TcLclEnv) [PredType] simplifyDeriv (DS { ds_loc :: forall theta. DerivSpec theta -> SrcSpan ds_loc = SrcSpan loc, ds_tvs :: forall theta. DerivSpec theta -> [TyVar] ds_tvs = [TyVar] tvs , ds_cls :: forall theta. DerivSpec theta -> Class ds_cls = Class clas, ds_tys :: forall theta. DerivSpec theta -> [PredType] ds_tys = [PredType] inst_tys, ds_theta :: forall theta. DerivSpec theta -> theta ds_theta = ThetaSpec deriv_rhs , ds_skol_info :: forall theta. DerivSpec theta -> SkolemInfo ds_skol_info = SkolemInfo skol_info, ds_user_ctxt :: forall theta. DerivSpec theta -> UserTypeCtxt ds_user_ctxt = UserTypeCtxt user_ctxt }) = SrcSpan -> IOEnv (Env TcGblEnv TcLclEnv) [PredType] -> IOEnv (Env TcGblEnv TcLclEnv) [PredType] forall a. SrcSpan -> TcRn a -> TcRn a setSrcSpan SrcSpan loc (IOEnv (Env TcGblEnv TcLclEnv) [PredType] -> IOEnv (Env TcGblEnv TcLclEnv) [PredType]) -> IOEnv (Env TcGblEnv TcLclEnv) [PredType] -> IOEnv (Env TcGblEnv TcLclEnv) [PredType] forall a b. (a -> b) -> a -> b $ SDoc -> IOEnv (Env TcGblEnv TcLclEnv) [PredType] -> IOEnv (Env TcGblEnv TcLclEnv) [PredType] forall a. SDoc -> TcM a -> TcM a addErrCtxt (PredType -> SDoc derivInstCtxt (Class -> [PredType] -> PredType mkClassPred Class clas [PredType] inst_tys)) (IOEnv (Env TcGblEnv TcLclEnv) [PredType] -> IOEnv (Env TcGblEnv TcLclEnv) [PredType]) -> IOEnv (Env TcGblEnv TcLclEnv) [PredType] -> IOEnv (Env TcGblEnv TcLclEnv) [PredType] forall a b. (a -> b) -> a -> b $ do { -- See [STEP DAC BUILD] -- Generate the implication constraints, one for each method, to solve -- with the skolemized variables. Start "one level down" because -- we are going to wrap the result in an implication with tvs, -- in step [DAC RESIDUAL] ; (tc_lvl, wanteds) <- UserTypeCtxt -> ThetaSpec -> TcM (TcLevel, WantedConstraints) captureThetaSpecConstraints UserTypeCtxt user_ctxt ThetaSpec deriv_rhs ; traceTc "simplifyDeriv inputs" $ vcat [ pprTyVars tvs $$ ppr deriv_rhs $$ ppr wanteds, ppr skol_info ] -- See [STEP DAC SOLVE] -- Simplify the constraints, starting at the same level at which -- they are generated (c.f. the call to runTcSWithEvBinds in -- simplifyInfer) ; (solved_wanteds, _) <- setTcLevel tc_lvl $ runTcS $ solveWanteds wanteds -- It's not yet zonked! Obviously zonk it before peering at it ; solved_wanteds <- liftZonkM $ zonkWC solved_wanteds -- See [STEP DAC HOIST] -- From the simplified constraints extract a subset 'good' that will -- become the context 'min_theta' for the derived instance. ; let residual_simple = Bool -> WantedConstraints -> Cts approximateWC Bool True WantedConstraints solved_wanteds head_size = Class -> [PredType] -> PatersonSize pSizeClassPred Class clas [PredType] inst_tys good = (Ct -> Maybe PredType) -> Cts -> Bag PredType forall a b. (a -> Maybe b) -> Bag a -> Bag b mapMaybeBag Ct -> Maybe PredType get_good Cts residual_simple -- Returns @Just p@ (where @p@ is the type of the Ct) if a Ct is -- suitable to be inferred in the context of a derived instance. -- Returns @Nothing@ if the Ct is too exotic. -- See (VD2) in Note [Valid 'deriving' predicate] in -- GHC.Tc.Validity for what constitutes an exotic constraint. get_good :: Ct -> Maybe PredType get_good Ct ct | PatersonSize -> PredType -> Bool validDerivPred PatersonSize head_size PredType p = PredType -> Maybe PredType forall a. a -> Maybe a Just PredType p | Bool otherwise = Maybe PredType forall a. Maybe a Nothing where p :: PredType p = Ct -> PredType ctPred Ct ct ; traceTc "simplifyDeriv outputs" $ vcat [ ppr tvs, ppr residual_simple, ppr good ] -- Return the good unsolved constraints (unskolemizing on the way out.) ; let min_theta = (PredType -> PredType) -> [PredType] -> [PredType] forall a. (a -> PredType) -> [a] -> [a] mkMinimalBySCs PredType -> PredType forall a. a -> a id (Bag PredType -> [PredType] forall a. Bag a -> [a] bagToList Bag PredType good) -- An important property of mkMinimalBySCs (used above) is that in -- addition to removing constraints that are made redundant by -- superclass relationships, it also removes _duplicate_ -- constraints. -- See Note [Gathering and simplifying constraints for -- DeriveAnyClass] -- See [STEP DAC RESIDUAL] -- Ensure that min_theta is enough to solve /all/ the constraints in -- solved_wanteds, by solving the implication constraint -- -- forall tvs. min_theta => solved_wanteds ; min_theta_vars <- mapM newEvVar min_theta ; (leftover_implic, _) <- buildImplicationFor tc_lvl (getSkolemInfo skol_info) tvs min_theta_vars solved_wanteds -- This call to simplifyTop is purely for error reporting -- See Note [Error reporting for deriving clauses] -- See also Note [Valid 'deriving' predicate] in GHC.Tc.Validity, as this -- line of code catches "exotic" constraints like the ones described in -- (VD2) of that Note. ; simplifyTopImplic leftover_implic ; traceTc "GHC.Tc.Deriv" (ppr deriv_rhs $$ ppr min_theta) -- Claim: the result instance declaration is guaranteed valid -- Hence no need to call: -- checkValidInstance tyvars theta clas inst_tys -- See Note [Valid 'deriving' predicate] in GHC.Tc.Validity ; return min_theta } {- Note [Overlap and deriving] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider some overlapping instances: instance Show a => Show [a] where .. instance Show [Char] where ... Now a data type with deriving: data T a = MkT [a] deriving( Show ) We want to get the derived instance instance Show [a] => Show (T a) where... and NOT instance Show a => Show (T a) where... so that the (Show (T Char)) instance does the Right Thing It's very like the situation when we're inferring the type of a function f x = show [x] and we want to infer f :: Show [a] => a -> String As a result, we use vanilla, non-overlappable skolems when inferring the context for the derived instances. Hence, we instantiate the type variables using tcInstSkolTyVars, not tcInstSuperSkolTyVars. We do this skolemisation in GHC.Tc.Deriv.mkEqnHelp, a function which occurs very early in the deriving pipeline, so that by the time GHC needs to infer the instance context, all of the types in the computed DerivSpec have been skolemised appropriately. After the instance context inference has completed, GHC zonks the TcTyVars in the DerivSpec to ensure that types like a[sk:1] do not appear in -ddump-deriv output. All of this is only needed when inferring an instance context, i.e., the InferContext case. For the SupplyContext case, we don't bother skolemising at all. Note [Gathering and simplifying constraints for DeriveAnyClass] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DeriveAnyClass works quite differently from stock and newtype deriving in the way it gathers and simplifies constraints to be used in a derived instance's context. Stock and newtype deriving gather constraints by looking at the data constructors of the data type for which we are deriving an instance. But DeriveAnyClass doesn't need to know about a data type's definition at all! To see why, consider this example of DeriveAnyClass: class Foo a where bar :: forall b. Ix b => a -> b -> String default bar :: (Show a, Ix c) => a -> c -> String bar x y = show x ++ show (range (y,y)) baz :: Eq a => a -> a -> Bool default baz :: (Ord a, Show a) => a -> a -> Bool baz x y = compare x y == EQ Because 'bar' and 'baz' have default signatures, this generates a top-level definition for these generic default methods $gdm_bar :: forall a. Foo a => forall c. (Show a, Ix c) => a -> c -> String $gdm_bar x y = show x ++ show (range (y,y)) (and similarly for baz). Now consider a 'deriving' clause data Maybe s = ... deriving anyclass Foo This derives an instance of the form: instance (CX) => Foo (Maybe s) where bar = $gdm_bar baz = $gdm_baz Now it is GHC's job to fill in a suitable instance context (CX). If GHC were typechecking the binding bar = $gdm_bar it would * skolemise the expected type of bar * instantiate the type of $gdm_bar with meta-type variables * build an implication constraint [STEP DAC BUILD] So that's what we do. Fortunately, there is already functionality within GHC to that does all of the above—namely, tcSubTypeSigma. In the example above, we want to use tcSubTypeSigma to check the following subtyping relation: forall c. (Show a, Ix c) => Maybe s -> c -> String -- actual type <= forall b. (Ix b) => Maybe s -> b -> String -- expected type That is, we check that the type of $gdm_bar (the actual type) is more polymorphic than the type of bar (the expected type). We use SubTypePredSpec, a special form of PredSpec that is only used by DeriveAnyClass, to store the actual and expected types. (Aside: having a separate SubTypePredSpec is not strictly necessary, as we could theoretically construct this implication constraint by hand and store it in a SimplePredSpec. In fact, GHC used to do this. However, this is easier said than done, and there were numerous subtle bugs that resulted from getting this step wrong, such as #20719. Ultimately, we decided that special-casing a PredSpec specifically for DeriveAnyClass was worth it.) tcSubTypeSigma will ultimately spit out an implication constraint, which will look something like this (call it C1): forall[2] b. Ix b => (Show (Maybe s), Ix cc, Maybe s -> b -> String ~ Maybe s -> cc -> String) Here: * The level of this forall constraint is forall[2], because we are later going to wrap it in a forall[1] in [STEP DAC RESIDUAL] * The 'b' comes from the quantified type variable in the expected type of bar. The 'cc' is a unification variable that comes from instantiating the quantified type variable 'c' in $gdm_bar's type. The finer details of skolemisation and metavariable instantiation are handled behind the scenes by tcSubTypeSigma. * It is important that `b` be distinct from `cc`. In this example, this is clearly the case, but it is not always so obvious when the type variables are hidden behind type synonyms. Suppose the example were written like this, for example: type Method a = forall b. Ix b => a -> b -> String class Foo a where bar :: Method a default bar :: Show a => Method a bar = ... Both method signatures quantify a `b` once the `Method` type synonym is expanded. To ensure that GHC doesn't confuse the two `b`s during typechecking, tcSubTypeSigma instantiates the `b` in the original signature with a fresh skolem and the `b` in the default signature with a fresh unification variable. Doing so prevents #20719 from happening. * The (Ix b) constraint comes from the context of bar's type. The (Show (Maybe s)) and (Ix cc) constraints come from the context of $gdm_bar's type. * The equality constraint (Maybe s -> b -> String) ~ (Maybe s -> cc -> String) comes from marrying up the instantiated type of $gdm_bar with the specified type of bar. Notice that the type variables from the instance, 's' in this case, are global to this constraint. Note that it is vital that we instantiate the `c` in $gdm_bar's type with a new unification variable for each iteration of simplifyDeriv. If we re-use the same unification variable across multiple iterations, then bad things can happen, such as #14933. Similarly for 'baz', tcSubTypeSigma gives the constraint C2 forall[2]. Eq (Maybe s) => (Ord a, Show a, Maybe s -> Maybe s -> Bool ~ Maybe s -> Maybe s -> Bool) In this case baz has no local quantification, so the implication constraint has no local skolems and there are no unification variables. [STEP DAC SOLVE] We can combine these two implication constraints into a single constraint (C1, C2), and simplify, unifying cc:=b, to get: forall[2] b. Ix b => Show a /\ forall[2]. Eq (Maybe s) => (Ord a, Show a) [STEP DAC HOIST] Let's call that (C1', C2'). Now we need to hoist the unsolved constraints out of the implications to become our candidate for (CX). That is done by approximateWC, which will return: (Show a, Ord a, Show a) Now we can use mkMinimalBySCs to remove superclasses and duplicates, giving (Show a, Ord a) And that's what GHC uses for CX. [STEP DAC RESIDUAL] In this case we have solved all the leftover constraints, but what if we don't? Simple! We just form the final residual constraint forall[1] s. CX => (C1',C2') and simplify that. In simple cases it'll succeed easily, because CX literally contains the constraints in C1', C2', but if there is anything more complicated it will be reported in a civilised way. Note [Error reporting for deriving clauses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A surprisingly tricky aspect of deriving to get right is reporting sensible error messages. In particular, if simplifyDeriv reaches a constraint that it cannot solve, which might include: 1. Insoluble constraints 2. "Exotic" constraints (See Note [Valid 'deriving' predicate] in GHC.Tc.Validity) Then we report an error immediately in simplifyDeriv. Another possible choice is to punt and let another part of the typechecker (e.g., simplifyInstanceContexts) catch the errors. But this tends to lead to worse error messages, so we do it directly in simplifyDeriv. simplifyDeriv checks for errors in a clever way. If the deriving machinery infers the context (Foo a)--that is, if this instance is to be generated: instance Foo a => ... Then we form an implication of the form: forall a. Foo a => <residual_wanted_constraints> And pass it to the simplifier. If the context (Foo a) is enough to discharge all the constraints in <residual_wanted_constraints>, then everything is hunky-dory. But if <residual_wanted_constraints> contains, say, an insoluble constraint, then (Foo a) won't be able to solve it, causing GHC to error. -}