{-# LANGUAGE CPP #-} module ClsInst ( matchGlobalInst, ClsInstResult(..), InstanceWhat(..), safeOverlap, AssocInstInfo(..), isNotAssociated ) where #include "HsVersions.h" import GhcPrelude import TcEnv import TcRnMonad import TcType import TcMType import TcEvidence import TcTypeableValidity import RnEnv( addUsedGRE ) import RdrName( lookupGRE_FieldLabel ) import InstEnv import Inst( instDFunType ) import FamInst( tcGetFamInstEnvs, tcInstNewTyCon_maybe, tcLookupDataFamInst ) import TysWiredIn import TysPrim( eqPrimTyCon, eqReprPrimTyCon ) import PrelNames import Id import Type import MkCore ( mkStringExprFS, mkNaturalExpr ) import Name ( Name ) import VarEnv ( VarEnv ) import DataCon import TyCon import Class import DynFlags import Outputable import Util( splitAtList, fstOf3 ) import Data.Maybe {- ******************************************************************* * * A helper for associated types within class instance declarations * * **********************************************************************-} -- | Extra information about the parent instance declaration, needed -- when type-checking associated types. The 'Class' is the enclosing -- class, the [TyVar] are the /scoped/ type variable of the instance decl. -- The @VarEnv Type@ maps class variables to their instance types. data AssocInstInfo = NotAssociated | InClsInst { ai_class :: Class , ai_tyvars :: [TyVar] -- ^ The /scoped/ tyvars of the instance -- Why scoped? See bind_me in -- TcValidity.checkConsistentFamInst , ai_inst_env :: VarEnv Type -- ^ Maps /class/ tyvars to their instance types -- See Note [Matching in the consistent-instantation check] } isNotAssociated :: AssocInstInfo -> Bool isNotAssociated NotAssociated = True isNotAssociated (InClsInst {}) = False {- ******************************************************************* * * Class lookup * * **********************************************************************-} -- | Indicates if Instance met the Safe Haskell overlapping instances safety -- check. -- -- See Note [Safe Haskell Overlapping Instances] in TcSimplify -- See Note [Safe Haskell Overlapping Instances Implementation] in TcSimplify type SafeOverlapping = Bool data ClsInstResult = NoInstance -- Definitely no instance | OneInst { cir_new_theta :: [TcPredType] , cir_mk_ev :: [EvExpr] -> EvTerm , cir_what :: InstanceWhat } | NotSure -- Multiple matches and/or one or more unifiers data InstanceWhat = BuiltinInstance | LocalInstance | TopLevInstance { iw_dfun_id :: DFunId , iw_safe_over :: SafeOverlapping } instance Outputable ClsInstResult where ppr NoInstance = text "NoInstance" ppr NotSure = text "NotSure" ppr (OneInst { cir_new_theta = ev , cir_what = what }) = text "OneInst" <+> vcat [ppr ev, ppr what] instance Outputable InstanceWhat where ppr BuiltinInstance = text "built-in instance" ppr LocalInstance = text "locally-quantified instance" ppr (TopLevInstance { iw_safe_over = so }) = text "top-level instance" <+> (text $ if so then "[safe]" else "[unsafe]") safeOverlap :: InstanceWhat -> Bool safeOverlap (TopLevInstance { iw_safe_over = so }) = so safeOverlap _ = True matchGlobalInst :: DynFlags -> Bool -- True <=> caller is the short-cut solver -- See Note [Shortcut solving: overlap] -> Class -> [Type] -> TcM ClsInstResult matchGlobalInst dflags short_cut clas tys | cls_name == knownNatClassName = matchKnownNat dflags short_cut clas tys | cls_name == knownSymbolClassName = matchKnownSymbol dflags short_cut clas tys | isCTupleClass clas = matchCTuple clas tys | cls_name == typeableClassName = matchTypeable clas tys | clas `hasKey` heqTyConKey = matchHeteroEquality tys | clas `hasKey` eqTyConKey = matchHomoEquality tys | clas `hasKey` coercibleTyConKey = matchCoercible tys | cls_name == hasFieldClassName = matchHasField dflags short_cut clas tys | otherwise = matchInstEnv dflags short_cut clas tys where cls_name = className clas {- ******************************************************************** * * Looking in the instance environment * * ***********************************************************************-} matchInstEnv :: DynFlags -> Bool -> Class -> [Type] -> TcM ClsInstResult matchInstEnv dflags short_cut_solver clas tys = do { instEnvs <- tcGetInstEnvs ; let safeOverlapCheck = safeHaskell dflags `elem` [Sf_Safe, Sf_Trustworthy] (matches, unify, unsafeOverlaps) = lookupInstEnv True instEnvs clas tys safeHaskFail = safeOverlapCheck && not (null unsafeOverlaps) ; traceTc "matchInstEnv" $ vcat [ text "goal:" <+> ppr clas <+> ppr tys , text "matches:" <+> ppr matches , text "unify:" <+> ppr unify ] ; case (matches, unify, safeHaskFail) of -- Nothing matches ([], [], _) -> do { traceTc "matchClass not matching" (ppr pred) ; return NoInstance } -- A single match (& no safe haskell failure) ([(ispec, inst_tys)], [], False) | short_cut_solver -- Called from the short-cut solver , isOverlappable ispec -- If the instance has OVERLAPPABLE or OVERLAPS or INCOHERENT -- then don't let the short-cut solver choose it, because a -- later instance might overlap it. Trac #14434 is an example -- See Note [Shortcut solving: overlap] -> do { traceTc "matchClass: ignoring overlappable" (ppr pred) ; return NotSure } | otherwise -> do { let dfun_id = instanceDFunId ispec ; traceTc "matchClass success" $ vcat [text "dict" <+> ppr pred, text "witness" <+> ppr dfun_id <+> ppr (idType dfun_id) ] -- Record that this dfun is needed ; match_one (null unsafeOverlaps) dfun_id inst_tys } -- More than one matches (or Safe Haskell fail!). Defer any -- reactions of a multitude until we learn more about the reagent _ -> do { traceTc "matchClass multiple matches, deferring choice" $ vcat [text "dict" <+> ppr pred, text "matches" <+> ppr matches] ; return NotSure } } where pred = mkClassPred clas tys match_one :: SafeOverlapping -> DFunId -> [DFunInstType] -> TcM ClsInstResult -- See Note [DFunInstType: instantiating types] in InstEnv match_one so dfun_id mb_inst_tys = do { traceTc "match_one" (ppr dfun_id $$ ppr mb_inst_tys) ; (tys, theta) <- instDFunType dfun_id mb_inst_tys ; traceTc "match_one 2" (ppr dfun_id $$ ppr tys $$ ppr theta) ; return $ OneInst { cir_new_theta = theta , cir_mk_ev = evDFunApp dfun_id tys , cir_what = TopLevInstance { iw_dfun_id = dfun_id , iw_safe_over = so } } } {- Note [Shortcut solving: overlap] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have instance {-# OVERLAPPABLE #-} C a where ... and we are typechecking f :: C a => a -> a f = e -- Gives rise to [W] C a We don't want to solve the wanted constraint with the overlappable instance; rather we want to use the supplied (C a)! That was the whole point of it being overlappable! Trac #14434 wwas an example. Alas even if the instance has no overlap flag, thus instance C a where ... there is nothing to stop it being overlapped. GHC provides no way to declare an instance as "final" so it can't be overlapped. But really only final instances are OK for short-cut solving. Sigh. Trac #15135 was a puzzling example. -} {- ******************************************************************** * * Class lookup for CTuples * * ***********************************************************************-} matchCTuple :: Class -> [Type] -> TcM ClsInstResult matchCTuple clas tys -- (isCTupleClass clas) holds = return (OneInst { cir_new_theta = tys , cir_mk_ev = tuple_ev , cir_what = BuiltinInstance }) -- The dfun *is* the data constructor! where data_con = tyConSingleDataCon (classTyCon clas) tuple_ev = evDFunApp (dataConWrapId data_con) tys {- ******************************************************************** * * Class lookup for Literals * * ***********************************************************************-} {- Note [KnownNat & KnownSymbol and EvLit] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A part of the type-level literals implementation are the classes "KnownNat" and "KnownSymbol", which provide a "smart" constructor for defining singleton values. Here is the key stuff from GHC.TypeLits class KnownNat (n :: Nat) where natSing :: SNat n newtype SNat (n :: Nat) = SNat Integer Conceptually, this class has infinitely many instances: instance KnownNat 0 where natSing = SNat 0 instance KnownNat 1 where natSing = SNat 1 instance KnownNat 2 where natSing = SNat 2 ... In practice, we solve `KnownNat` predicates in the type-checker (see typecheck/TcInteract.hs) because we can't have infinitely many instances. The evidence (aka "dictionary") for `KnownNat` is of the form `EvLit (EvNum n)`. We make the following assumptions about dictionaries in GHC: 1. The "dictionary" for classes with a single method---like `KnownNat`---is a newtype for the type of the method, so using a evidence amounts to a coercion, and 2. Newtypes use the same representation as their definition types. So, the evidence for `KnownNat` is just a value of the representation type, wrapped in two newtype constructors: one to make it into a `SNat` value, and another to make it into a `KnownNat` dictionary. Also note that `natSing` and `SNat` are never actually exposed from the library---they are just an implementation detail. Instead, users see a more convenient function, defined in terms of `natSing`: natVal :: KnownNat n => proxy n -> Integer The reason we don't use this directly in the class is that it is simpler and more efficient to pass around an integer rather than an entire function, especially when the `KnowNat` evidence is packaged up in an existential. The story for kind `Symbol` is analogous: * class KnownSymbol * newtype SSymbol * Evidence: a Core literal (e.g. mkNaturalExpr) Note [Fabricating Evidence for Literals in Backpack] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Let `T` be a type of kind `Nat`. When solving for a purported instance of `KnownNat T`, ghc tries to resolve the type `T` to an integer `n`, in which case the evidence `EvLit (EvNum n)` is generated on the fly. It might appear that this is sufficient as users cannot define their own instances of `KnownNat`. However, for backpack module this would not work (see issue #15379). Consider the signature `Abstract` > signature Abstract where > data T :: Nat > instance KnownNat T and a module `Util` that depends on it: > module Util where > import Abstract > printT :: IO () > printT = do print $ natVal (Proxy :: Proxy T) Clearly, we need to "use" the dictionary associated with `KnownNat T` in the module `Util`, but it is too early for the compiler to produce a real dictionary as we still have not fixed what `T` is. Only when we mixin a concrete module > module Concrete where > type T = 42 do we really get hold of the underlying integer. So the strategy that we follow is the following 1. If T is indeed available as a type alias for an integer constant, generate the dictionary on the fly, failing which 2. Look up the type class environment for the evidence. Finally actual code gets generate for Util only when a module like Concrete gets "mixed-in" in place of the signature Abstract. As a result all things, including the typeclass instances, in Concrete gets reexported. So `KnownNat` gets resolved the normal way post-Backpack. A similar generation works for `KnownSymbol` as well -} matchKnownNat :: DynFlags -> Bool -- True <=> caller is the short-cut solver -- See Note [Shortcut solving: overlap] -> Class -> [Type] -> TcM ClsInstResult matchKnownNat _ _ clas [ty] -- clas = KnownNat | Just n <- isNumLitTy ty = do et <- mkNaturalExpr n makeLitDict clas ty et matchKnownNat df sc clas tys = matchInstEnv df sc clas tys -- See Note [Fabricating Evidence for Literals in Backpack] for why -- this lookup into the instance environment is required. matchKnownSymbol :: DynFlags -> Bool -- True <=> caller is the short-cut solver -- See Note [Shortcut solving: overlap] -> Class -> [Type] -> TcM ClsInstResult matchKnownSymbol _ _ clas [ty] -- clas = KnownSymbol | Just s <- isStrLitTy ty = do et <- mkStringExprFS s makeLitDict clas ty et matchKnownSymbol df sc clas tys = matchInstEnv df sc clas tys -- See Note [Fabricating Evidence for Literals in Backpack] for why -- this lookup into the instance environment is required. makeLitDict :: Class -> Type -> EvExpr -> TcM ClsInstResult -- makeLitDict adds a coercion that will convert the literal into a dictionary -- of the appropriate type. See Note [KnownNat & KnownSymbol and EvLit] -- in TcEvidence. The coercion happens in 2 steps: -- -- Integer -> SNat n -- representation of literal to singleton -- SNat n -> KnownNat n -- singleton to dictionary -- -- The process is mirrored for Symbols: -- String -> SSymbol n -- SSymbol n -> KnownSymbol n makeLitDict clas ty et | Just (_, co_dict) <- tcInstNewTyCon_maybe (classTyCon clas) [ty] -- co_dict :: KnownNat n ~ SNat n , [ meth ] <- classMethods clas , Just tcRep <- tyConAppTyCon_maybe -- SNat $ funResultTy -- SNat n $ dropForAlls -- KnownNat n => SNat n $ idType meth -- forall n. KnownNat n => SNat n , Just (_, co_rep) <- tcInstNewTyCon_maybe tcRep [ty] -- SNat n ~ Integer , let ev_tm = mkEvCast et (mkTcSymCo (mkTcTransCo co_dict co_rep)) = return $ OneInst { cir_new_theta = [] , cir_mk_ev = \_ -> ev_tm , cir_what = BuiltinInstance } | otherwise = pprPanic "makeLitDict" $ text "Unexpected evidence for" <+> ppr (className clas) $$ vcat (map (ppr . idType) (classMethods clas)) {- ******************************************************************** * * Class lookup for Typeable * * ***********************************************************************-} -- | Assumes that we've checked that this is the 'Typeable' class, -- and it was applied to the correct argument. matchTypeable :: Class -> [Type] -> TcM ClsInstResult matchTypeable clas [k,t] -- clas = Typeable -- For the first two cases, See Note [No Typeable for polytypes or qualified types] | isForAllTy k = return NoInstance -- Polytype | isJust (tcSplitPredFunTy_maybe t) = return NoInstance -- Qualified type -- Now cases that do work | k `eqType` typeNatKind = doTyLit knownNatClassName t | k `eqType` typeSymbolKind = doTyLit knownSymbolClassName t | tcIsConstraintKind t = doTyConApp clas t constraintKindTyCon [] | Just (arg,ret) <- splitFunTy_maybe t = doFunTy clas t arg ret | Just (tc, ks) <- splitTyConApp_maybe t -- See Note [Typeable (T a b c)] , onlyNamedBndrsApplied tc ks = doTyConApp clas t tc ks | Just (f,kt) <- splitAppTy_maybe t = doTyApp clas t f kt matchTypeable _ _ = return NoInstance -- | Representation for a type @ty@ of the form @arg -> ret@. doFunTy :: Class -> Type -> Type -> Type -> TcM ClsInstResult doFunTy clas ty arg_ty ret_ty = return $ OneInst { cir_new_theta = preds , cir_mk_ev = mk_ev , cir_what = BuiltinInstance } where preds = map (mk_typeable_pred clas) [arg_ty, ret_ty] mk_ev [arg_ev, ret_ev] = evTypeable ty $ EvTypeableTrFun (EvExpr arg_ev) (EvExpr ret_ev) mk_ev _ = panic "TcInteract.doFunTy" -- | Representation for type constructor applied to some kinds. -- 'onlyNamedBndrsApplied' has ensured that this application results in a type -- of monomorphic kind (e.g. all kind variables have been instantiated). doTyConApp :: Class -> Type -> TyCon -> [Kind] -> TcM ClsInstResult doTyConApp clas ty tc kind_args | tyConIsTypeable tc = return $ OneInst { cir_new_theta = (map (mk_typeable_pred clas) kind_args) , cir_mk_ev = mk_ev , cir_what = BuiltinInstance } | otherwise = return NoInstance where mk_ev kinds = evTypeable ty $ EvTypeableTyCon tc (map EvExpr kinds) -- | Representation for TyCon applications of a concrete kind. We just use the -- kind itself, but first we must make sure that we've instantiated all kind- -- polymorphism, but no more. onlyNamedBndrsApplied :: TyCon -> [KindOrType] -> Bool onlyNamedBndrsApplied tc ks = all isNamedTyConBinder used_bndrs && not (any isNamedTyConBinder leftover_bndrs) where bndrs = tyConBinders tc (used_bndrs, leftover_bndrs) = splitAtList ks bndrs doTyApp :: Class -> Type -> Type -> KindOrType -> TcM ClsInstResult -- Representation for an application of a type to a type-or-kind. -- This may happen when the type expression starts with a type variable. -- Example (ignoring kind parameter): -- Typeable (f Int Char) --> -- (Typeable (f Int), Typeable Char) --> -- (Typeable f, Typeable Int, Typeable Char) --> (after some simp. steps) -- Typeable f doTyApp clas ty f tk | isForAllTy (tcTypeKind f) = return NoInstance -- We can't solve until we know the ctr. | otherwise = return $ OneInst { cir_new_theta = map (mk_typeable_pred clas) [f, tk] , cir_mk_ev = mk_ev , cir_what = BuiltinInstance } where mk_ev [t1,t2] = evTypeable ty $ EvTypeableTyApp (EvExpr t1) (EvExpr t2) mk_ev _ = panic "doTyApp" -- Emit a `Typeable` constraint for the given type. mk_typeable_pred :: Class -> Type -> PredType mk_typeable_pred clas ty = mkClassPred clas [ tcTypeKind ty, ty ] -- Typeable is implied by KnownNat/KnownSymbol. In the case of a type literal -- we generate a sub-goal for the appropriate class. -- See Note [Typeable for Nat and Symbol] doTyLit :: Name -> Type -> TcM ClsInstResult doTyLit kc t = do { kc_clas <- tcLookupClass kc ; let kc_pred = mkClassPred kc_clas [ t ] mk_ev [ev] = evTypeable t $ EvTypeableTyLit (EvExpr ev) mk_ev _ = panic "doTyLit" ; return (OneInst { cir_new_theta = [kc_pred] , cir_mk_ev = mk_ev , cir_what = BuiltinInstance }) } {- Note [Typeable (T a b c)] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For type applications we always decompose using binary application, via doTyApp, until we get to a *kind* instantiation. Example Proxy :: forall k. k -> * To solve Typeable (Proxy (* -> *) Maybe) we - First decompose with doTyApp, to get (Typeable (Proxy (* -> *))) and Typeable Maybe - Then solve (Typeable (Proxy (* -> *))) with doTyConApp If we attempt to short-cut by solving it all at once, via doTyConApp (this note is sadly truncated FIXME) Note [No Typeable for polytypes or qualified types] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We do not support impredicative typeable, such as Typeable (forall a. a->a) Typeable (Eq a => a -> a) Typeable (() => Int) Typeable (((),()) => Int) See Trac #9858. For forall's the case is clear: we simply don't have a TypeRep for them. For qualified but not polymorphic types, like (Eq a => a -> a), things are murkier. But: * We don't need a TypeRep for these things. TypeReps are for monotypes only. * Perhaps we could treat `=>` as another type constructor for `Typeable` purposes, and thus support things like `Eq Int => Int`, however, at the current state of affairs this would be an odd exception as no other class works with impredicative types. For now we leave it off, until we have a better story for impredicativity. Note [Typeable for Nat and Symbol] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We have special Typeable instances for Nat and Symbol. Roughly we have this instance, implemented here by doTyLit: instance KnownNat n => Typeable (n :: Nat) where typeRep = typeNatTypeRep @n where Data.Typeable.Internals.typeNatTypeRep :: KnownNat a => TypeRep a Ultimately typeNatTypeRep uses 'natSing' from KnownNat to get a runtime value 'n'; it turns it into a string with 'show' and uses that to whiz up a TypeRep TyCon for 'n', with mkTypeLitTyCon. See #10348. Because of this rule it's inadvisable (see #15322) to have a constraint f :: (Typeable (n :: Nat)) => blah in a function signature; it gives rise to overlap problems just as if you'd written f :: Eq [a] => blah -} {- ******************************************************************** * * Class lookup for lifted equality * * ***********************************************************************-} -- See also Note [The equality types story] in TysPrim matchHeteroEquality :: [Type] -> TcM ClsInstResult -- Solves (t1 ~~ t2) matchHeteroEquality args = return (OneInst { cir_new_theta = [ mkTyConApp eqPrimTyCon args ] , cir_mk_ev = evDataConApp heqDataCon args , cir_what = BuiltinInstance }) matchHomoEquality :: [Type] -> TcM ClsInstResult -- Solves (t1 ~ t2) matchHomoEquality args@[k,t1,t2] = return (OneInst { cir_new_theta = [ mkTyConApp eqPrimTyCon [k,k,t1,t2] ] , cir_mk_ev = evDataConApp eqDataCon args , cir_what = BuiltinInstance }) matchHomoEquality args = pprPanic "matchHomoEquality" (ppr args) -- See also Note [The equality types story] in TysPrim matchCoercible :: [Type] -> TcM ClsInstResult matchCoercible args@[k, t1, t2] = return (OneInst { cir_new_theta = [ mkTyConApp eqReprPrimTyCon args' ] , cir_mk_ev = evDataConApp coercibleDataCon args , cir_what = BuiltinInstance }) where args' = [k, k, t1, t2] matchCoercible args = pprPanic "matchLiftedCoercible" (ppr args) {- ******************************************************************** * * Class lookup for overloaded record fields * * ***********************************************************************-} {- Note [HasField instances] ~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have data T y = MkT { foo :: [y] } and `foo` is in scope. Then GHC will automatically solve a constraint like HasField "foo" (T Int) b by emitting a new wanted T alpha -> [alpha] ~# T Int -> b and building a HasField dictionary out of the selector function `foo`, appropriately cast. The HasField class is defined (in GHC.Records) thus: class HasField (x :: k) r a | x r -> a where getField :: r -> a Since this is a one-method class, it is represented as a newtype. Hence we can solve `HasField "foo" (T Int) b` by taking an expression of type `T Int -> b` and casting it using the newtype coercion. Note that foo :: forall y . T y -> [y] so the expression we construct is foo @alpha |> co where co :: (T alpha -> [alpha]) ~# HasField "foo" (T Int) b is built from co1 :: (T alpha -> [alpha]) ~# (T Int -> b) which is the new wanted, and co2 :: (T Int -> b) ~# HasField "foo" (T Int) b which can be derived from the newtype coercion. If `foo` is not in scope, or has a higher-rank or existentially quantified type, then the constraint is not solved automatically, but may be solved by a user-supplied HasField instance. Similarly, if we encounter a HasField constraint where the field is not a literal string, or does not belong to the type, then we fall back on the normal constraint solver behaviour. -} -- See Note [HasField instances] matchHasField :: DynFlags -> Bool -> Class -> [Type] -> TcM ClsInstResult matchHasField dflags short_cut clas tys = do { fam_inst_envs <- tcGetFamInstEnvs ; rdr_env <- getGlobalRdrEnv ; case tys of -- We are matching HasField {k} x r a... [_k_ty, x_ty, r_ty, a_ty] -- x should be a literal string | Just x <- isStrLitTy x_ty -- r should be an applied type constructor , Just (tc, args) <- tcSplitTyConApp_maybe r_ty -- use representation tycon (if data family); it has the fields , let r_tc = fstOf3 (tcLookupDataFamInst fam_inst_envs tc args) -- x should be a field of r , Just fl <- lookupTyConFieldLabel x r_tc -- the field selector should be in scope , Just gre <- lookupGRE_FieldLabel rdr_env fl -> do { sel_id <- tcLookupId (flSelector fl) ; (tv_prs, preds, sel_ty) <- tcInstType newMetaTyVars sel_id -- The first new wanted constraint equates the actual -- type of the selector with the type (r -> a) within -- the HasField x r a dictionary. The preds will -- typically be empty, but if the datatype has a -- "stupid theta" then we have to include it here. ; let theta = mkPrimEqPred sel_ty (mkFunTy r_ty a_ty) : preds -- Use the equality proof to cast the selector Id to -- type (r -> a), then use the newtype coercion to cast -- it to a HasField dictionary. mk_ev (ev1:evs) = evSelector sel_id tvs evs `evCast` co where co = mkTcSubCo (evTermCoercion (EvExpr ev1)) `mkTcTransCo` mkTcSymCo co2 mk_ev [] = panic "matchHasField.mk_ev" Just (_, co2) = tcInstNewTyCon_maybe (classTyCon clas) tys tvs = mkTyVarTys (map snd tv_prs) -- The selector must not be "naughty" (i.e. the field -- cannot have an existentially quantified type), and -- it must not be higher-rank. ; if not (isNaughtyRecordSelector sel_id) && isTauTy sel_ty then do { addUsedGRE True gre ; return OneInst { cir_new_theta = theta , cir_mk_ev = mk_ev , cir_what = BuiltinInstance } } else matchInstEnv dflags short_cut clas tys } _ -> matchInstEnv dflags short_cut clas tys }