X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Ftypecheck%2FTcGenDeriv.lhs;h=5e4a31a1e2705b0d19ab3fa2cd5ce934f9bd90bc;hb=8655d6ca41df4aa77a559d4067ad3815797b9803;hp=4c07ff5e0a481125803e2dc3efa0803bd6850ebb;hpb=0c9c303faa01623df27c4ecab339527034425cb0;p=ghc-hetmet.git diff --git a/ghc/compiler/typecheck/TcGenDeriv.lhs b/ghc/compiler/typecheck/TcGenDeriv.lhs index 4c07ff5..5e4a31a 100644 --- a/ghc/compiler/typecheck/TcGenDeriv.lhs +++ b/ghc/compiler/typecheck/TcGenDeriv.lhs @@ -17,6 +17,8 @@ module TcGenDeriv ( gen_Ord_binds, gen_Read_binds, gen_Show_binds, + gen_Data_binds, + gen_Typeable_binds, gen_tag_n_con_monobind, con2tag_RDR, tag2con_RDR, maxtag_RDR, @@ -49,23 +51,22 @@ import Name ( getOccString, getOccName, getSrcLoc, occNameString, ) import HscTypes ( FixityEnv, lookupFixity ) -import PrelInfo -- Lots of Names +import PrelNames -- Lots of Names import PrimOp -- Lots of Names import SrcLoc ( generatedSrcLoc, SrcLoc ) import TyCon ( TyCon, isNewTyCon, tyConDataCons, isEnumerationTyCon, - maybeTyConSingleCon, tyConFamilySize + maybeTyConSingleCon, tyConFamilySize, tyConTyVars ) import TcType ( isUnLiftedType, tcEqType, Type ) -import TysPrim ( charPrimTy, intPrimTy, wordPrimTy, addrPrimTy, - floatPrimTy, doublePrimTy - ) +import TysPrim ( charPrimTy, intPrimTy, wordPrimTy, addrPrimTy, floatPrimTy, doublePrimTy ) +import TysWiredIn ( charDataCon, intDataCon, floatDataCon, doubleDataCon, wordDataCon ) import Util ( zipWithEqual, isSingleton, zipWith3Equal, nOfThem, zipEqual ) import Panic ( panic, assertPanic ) -import Maybes ( maybeToBool ) import Char ( ord, isAlpha ) import Constants import List ( partition, intersperse ) +import Outputable import FastString import OccName \end{code} @@ -210,7 +211,7 @@ gen_Eq_binds tycon nested_eq_expr tys as bs = foldl1 and_Expr (zipWith3Equal "nested_eq" nested_eq tys as bs) where - nested_eq ty a b = HsPar (eq_Expr ty (HsVar a) (HsVar b)) + nested_eq ty a b = HsPar (eq_Expr tycon ty (HsVar a) (HsVar b)) \end{code} %************************************************************************ @@ -367,11 +368,11 @@ gen_Ord_binds tycon tys_needed = dataConOrigArgTys data_con nested_compare_expr [ty] [a] [b] - = careful_compare_Case ty eqTag_Expr (HsVar a) (HsVar b) + = careful_compare_Case tycon ty eqTag_Expr (HsVar a) (HsVar b) nested_compare_expr (ty:tys) (a:as) (b:bs) = let eq_expr = nested_compare_expr tys as bs - in careful_compare_Case ty eq_expr (HsVar a) (HsVar b) + in careful_compare_Case tycon ty eq_expr (HsVar a) (HsVar b) default_rhs | null nullary_cons = impossible_Expr -- Keep desugarer from complaining about -- inexhaustive patterns @@ -511,8 +512,8 @@ gen_Bounded_binds tycon tycon_loc = getSrcLoc tycon ----- enum-flavored: --------------------------- - min_bound_enum = mk_easy_FunMonoBind tycon_loc minBound_RDR [] [] (HsVar data_con_1_RDR) - max_bound_enum = mk_easy_FunMonoBind tycon_loc maxBound_RDR [] [] (HsVar data_con_N_RDR) + min_bound_enum = mkVarMonoBind tycon_loc minBound_RDR (HsVar data_con_1_RDR) + max_bound_enum = mkVarMonoBind tycon_loc maxBound_RDR (HsVar data_con_N_RDR) data_con_1 = head data_cons data_con_N = last data_cons @@ -522,9 +523,9 @@ gen_Bounded_binds tycon ----- single-constructor-flavored: ------------- arity = dataConSourceArity data_con_1 - min_bound_1con = mk_easy_FunMonoBind tycon_loc minBound_RDR [] [] $ + min_bound_1con = mkVarMonoBind tycon_loc minBound_RDR $ mkHsVarApps data_con_1_RDR (nOfThem arity minBound_RDR) - max_bound_1con = mk_easy_FunMonoBind tycon_loc maxBound_RDR [] [] $ + max_bound_1con = mkVarMonoBind tycon_loc maxBound_RDR $ mkHsVarApps data_con_1_RDR (nOfThem arity maxBound_RDR) \end{code} @@ -652,10 +653,9 @@ gen_Ix_binds tycon data_con = case maybeTyConSingleCon tycon of -- just checking... Nothing -> panic "get_Ix_binds" - Just dc -> if (any isUnLiftedType (dataConOrigArgTys dc)) then - error ("ERROR: Can't derive Ix for a single-constructor type with primitive argument types: "++tycon_str) - else - dc + Just dc | any isUnLiftedType (dataConOrigArgTys dc) + -> pprPanic "Can't derive Ix for a single-constructor type with primitive argument types:" (ppr tycon) + | otherwise -> dc con_arity = dataConSourceArity data_con data_con_RDR = getRdrName data_con @@ -770,17 +770,17 @@ gen_Read_binds get_fixity tycon where ----------------------------------------------------------------------- default_binds - = mk_easy_FunMonoBind loc readList_RDR [] [] (HsVar readListDefault_RDR) + = mkVarMonoBind loc readList_RDR (HsVar readListDefault_RDR) `AndMonoBinds` - mk_easy_FunMonoBind loc readListPrec_RDR [] [] (HsVar readListPrecDefault_RDR) + mkVarMonoBind loc readListPrec_RDR (HsVar readListPrecDefault_RDR) ----------------------------------------------------------------------- loc = getSrcLoc tycon data_cons = tyConDataCons tycon (nullary_cons, non_nullary_cons) = partition isNullaryDataCon data_cons - read_prec = mk_easy_FunMonoBind loc readPrec_RDR [] [] - (HsApp (HsVar parens_RDR) read_cons) + read_prec = mkVarMonoBind loc readPrec_RDR + (HsApp (HsVar parens_RDR) read_cons) read_cons = foldr1 mk_alt (read_nullary_cons ++ read_non_nullary_cons) read_non_nullary_cons = map read_non_nullary_con non_nullary_cons @@ -806,13 +806,13 @@ gen_Read_binds get_fixity tycon prefix_stmts -- T a b c = [bindLex (ident_pat (data_con_str data_con))] - ++ map read_arg as_needed + ++ read_args ++ [result_stmt data_con as_needed] infix_stmts -- a %% b - = [read_arg a1, + = [read_a1, bindLex (symbol_pat (data_con_str data_con)), - read_arg a2, + read_a2, result_stmt data_con [a1,a2]] lbl_stmts -- T { f1 = a, f2 = b } @@ -830,7 +830,9 @@ gen_Read_binds get_fixity tycon dc_nm = getName data_con is_infix = isDataSymOcc (getOccName dc_nm) as_needed = take con_arity as_RDRs - (a1:a2:_) = as_needed + read_args = zipWithEqual "gen_Read_binds" read_arg as_needed (dataConOrigArgTys data_con) + (read_a1:read_a2:_) = read_args + (a1:a2:_) = as_needed prec = getPrec is_infix get_fixity dc_nm ------------------------------------------------------------------------ @@ -848,17 +850,20 @@ gen_Read_binds get_fixity tycon data_con_str con = mkHsString (occNameUserString (getOccName con)) read_punc c = bindLex (punc_pat c) - read_arg a = BindStmt (VarPat a) (mkHsVarApps step_RDR [readPrec_RDR]) loc + read_arg a ty + | isUnLiftedType ty = pprPanic "Error in deriving:" (text "Can't read unlifted types yet:" <+> ppr ty) + | otherwise = BindStmt (VarPat a) (mkHsVarApps step_RDR [readPrec_RDR]) loc read_field lbl a = read_lbl lbl ++ [read_punc "=", BindStmt (VarPat a) (mkHsVarApps reset_RDR [readPrec_RDR]) loc] -- When reading field labels we might encounter - -- a = 3 + -- a = 3 + -- _a = 3 -- or (#) = 4 -- Note the parens! - read_lbl lbl | isAlpha (head lbl_str) + read_lbl lbl | is_id_start (head lbl_str) = [bindLex (ident_pat lbl_lit)] | otherwise = [read_punc "(", @@ -867,6 +872,7 @@ gen_Read_binds get_fixity tycon where lbl_str = occNameUserString (getOccName (fieldLabelName lbl)) lbl_lit = mkHsString lbl_str + is_id_start c = isAlpha c || c == '_' \end{code} @@ -907,7 +913,7 @@ gen_Show_binds get_fixity tycon where tycon_loc = getSrcLoc tycon ----------------------------------------------------------------------- - show_list = mk_easy_FunMonoBind tycon_loc showList_RDR [] [] + show_list = mkVarMonoBind tycon_loc showList_RDR (HsApp (HsVar showList___RDR) (HsPar (HsApp (HsVar showsPrec_RDR) (mkHsIntLit 0)))) ----------------------------------------------------------------------- shows_prec = mk_FunMonoBind tycon_loc showsPrec_RDR (map pats_etc (tyConDataCons tycon)) @@ -924,6 +930,7 @@ gen_Show_binds get_fixity tycon data_con_RDR = getRdrName data_con con_arity = dataConSourceArity data_con bs_needed = take con_arity bs_RDRs + arg_tys = dataConOrigArgTys data_con -- Correspond 1-1 with bs_needed con_pat = mkConPat data_con_RDR bs_needed nullary_con = con_arity == 0 labels = dataConFieldLabels data_con @@ -948,16 +955,13 @@ gen_Show_binds get_fixity tycon where occ_nm = getOccName (fieldLabelName l) nm = occNameUserString occ_nm - is_op = isSymOcc occ_nm -- Legal, but rare. - the_name - | is_op = '(':nm ++ ")" - | otherwise = nm + the_name | is_op = '(':nm ++ ")" + | otherwise = nm - show_args = [ mkHsApps showsPrec_RDR [HsLit (HsInt arg_prec), HsVar b] - | b <- bs_needed ] + show_args = zipWith show_arg bs_needed arg_tys (show_arg1:show_arg2:_) = show_args - show_prefix_args = intersperse (HsVar showSpace_RDR) show_args + show_prefix_args = intersperse (HsVar showSpace_RDR) show_args -- Assumption for record syntax: no of fields == no of labelled fields -- (and in same order) @@ -967,6 +971,12 @@ gen_Show_binds get_fixity tycon | (lbl,arg) <- zipEqual "gen_Show_binds" labels show_args ] + -- Generates (showsPrec p x) for argument x, but it also boxes + -- the argument first if necessary. Note that this prints unboxed + -- things without any '#' decorations; could change that if need be + show_arg b arg_ty = mkHsApps showsPrec_RDR [HsLit (HsInt arg_prec), + box_if_necy "Show" tycon (HsVar b) arg_ty] + -- Fixity stuff is_infix = isDataSymOcc dc_occ_nm con_prec_plus_one = 1 + getPrec is_infix get_fixity dc_nm @@ -1003,6 +1013,111 @@ isLRAssoc get_fixity nm = %************************************************************************ %* * +\subsection{Typeable} +%* * +%************************************************************************ + +From the data type + + data T a b = .... + +we generate + + instance (Typeable a, Typeable b) => Typeable (T a b) where + typeOf _ = mkTypeRep (mkTyConRep "T") + [typeOf (undefined::a), + typeOf (undefined::b)] + +Notice the use of lexically scoped type variables. + +\begin{code} +gen_Typeable_binds :: TyCon -> RdrNameMonoBinds +gen_Typeable_binds tycon + = mk_easy_FunMonoBind tycon_loc typeOf_RDR [wildPat] [] + (mkHsApps mkTypeRep_RDR [tycon_rep, arg_reps]) + where + tycon_loc = getSrcLoc tycon + tyvars = tyConTyVars tycon + tycon_rep = HsVar mkTyConRep_RDR `HsApp` HsLit (mkHsString (showSDoc (ppr tycon))) + arg_reps = ExplicitList placeHolderType (map mk tyvars) + mk tyvar = HsApp (HsVar typeOf_RDR) + (ExprWithTySig (HsVar undefined_RDR) + (HsTyVar (getRdrName tyvar))) +\end{code} + + + +%************************************************************************ +%* * +\subsection{Data} +%* * +%************************************************************************ + +From the data type + + data T a b = T1 a b | T2 + +we generate + + instance (Data a, Data b) => Data (T a b) where + gfoldl k z (T1 a b) = z T `k` a `k` b + gfoldl k z T2 = z T2 + -- ToDo: add gmapT,Q,M, gfoldr + + gunfold k z (Constr "T1") = k (k (z T1)) + gunfold k z (Constr "T2") = z T2 + + conOf (T1 _ _) = Constr "T1" + conOf T2 = Constr "T2" + + consOf _ = [Constr "T1", Constr "T2"] + +ToDo: generate auxiliary bindings for the Constrs? + +\begin{code} +gen_Data_binds :: TyCon -> RdrNameMonoBinds +gen_Data_binds tycon + = andMonoBindList [gfoldl_bind, gunfold_bind, conOf_bind, consOf_bind] + where + tycon_loc = getSrcLoc tycon + data_cons = tyConDataCons tycon + + ------------ gfoldl + gfoldl_bind = mk_FunMonoBind tycon_loc gfoldl_RDR (map gfoldl_eqn data_cons) + gfoldl_eqn con = ([VarPat k_RDR, VarPat z_RDR, mkConPat con_name as_needed], + foldl mk_k_app (HsVar z_RDR `HsApp` HsVar con_name) as_needed) + where + con_name :: RdrName + con_name = getRdrName con + as_needed = take (dataConSourceArity con) as_RDRs + mk_k_app e v = HsPar (mkHsOpApp e k_RDR (HsVar v)) + + ------------ gunfold + gunfold_bind = mk_FunMonoBind tycon_loc gunfold_RDR (map gunfold_eqn data_cons) + gunfold_eqn con = ([VarPat k_RDR, VarPat z_RDR, + ConPatIn constr_RDR (PrefixCon [LitPat (mk_constr_string con)])], + apN (dataConSourceArity con) + (\e -> HsVar k_RDR `HsApp` e) + (z_Expr `HsApp` HsVar (getRdrName con))) + mk_constr_string con = mkHsString (occNameUserString (getOccName con)) + + ------------ conOf + conOf_bind = mk_FunMonoBind tycon_loc conOf_RDR (map conOf_eqn data_cons) + conOf_eqn con = ([mkWildConPat con], mk_constr con) + + ------------ consOf + consOf_bind = mk_easy_FunMonoBind tycon_loc consOf_RDR [wildPat] [] + (ExplicitList placeHolderType (map mk_constr data_cons)) + mk_constr con = HsVar constr_RDR `HsApp` (HsLit (mk_constr_string con)) + + +apN :: Int -> (a -> a) -> a -> a +apN 0 k z = z +apN n k z = apN (n-1) k (k z) +\end{code} + +%************************************************************************ +%* * \subsection{Generating extra binds (@con2tag@ and @tag2con@)} %* * %************************************************************************ @@ -1030,21 +1145,34 @@ gen_tag_n_con_monobind gen_tag_n_con_monobind (rdr_name, tycon, GenCon2Tag) | lots_of_constructors - = mk_FunMonoBind (getSrcLoc tycon) rdr_name - [([VarPat a_RDR], HsApp getTag_Expr a_Expr)] + = mk_FunMonoBind loc rdr_name [([], get_tag_rhs)] | otherwise - = mk_FunMonoBind (getSrcLoc tycon) rdr_name (map mk_stuff (tyConDataCons tycon)) + = mk_FunMonoBind loc rdr_name (map mk_stuff (tyConDataCons tycon)) where + loc = getSrcLoc tycon + + -- Give a signature to the bound variable, so + -- that the case expression generated by getTag is + -- monomorphic. In the push-enter model we get better code. + get_tag_rhs = ExprWithTySig + (HsLam (mk_match loc [VarPat a_RDR] + (HsApp getTag_Expr a_Expr) + EmptyBinds)) + (HsForAllTy Nothing [] con2tag_ty) + -- Nothing => implicit quantification + + con2tag_ty = foldl HsAppTy (HsTyVar (getRdrName tycon)) + [HsTyVar (getRdrName tv) | tv <- tyConTyVars tycon] + `HsFunTy` + HsTyVar (getRdrName intPrimTyConName) + lots_of_constructors = tyConFamilySize tycon > mAX_FAMILY_SIZE_FOR_VEC_RETURNS mk_stuff :: DataCon -> ([RdrNamePat], RdrNameHsExpr) - mk_stuff var - = ([pat], HsLit (HsIntPrim (toInteger ((dataConTag var) - fIRST_TAG)))) - where - pat = ConPatIn var_RDR (PrefixCon (nOfThem (dataConSourceArity var) wildPat)) - var_RDR = getRdrName var + mk_stuff con = ([mkWildConPat con], + HsLit (HsIntPrim (toInteger ((dataConTag con) - fIRST_TAG)))) gen_tag_n_con_monobind (rdr_name, tycon, GenTag2Con) = mk_FunMonoBind (getSrcLoc tycon) rdr_name @@ -1053,8 +1181,8 @@ gen_tag_n_con_monobind (rdr_name, tycon, GenTag2Con) (HsTyVar (getRdrName tycon)))] gen_tag_n_con_monobind (rdr_name, tycon, GenMaxTag) - = mk_easy_FunMonoBind (getSrcLoc tycon) - rdr_name [] [] (HsApp (HsVar mkInt_RDR) (HsLit (HsIntPrim max_tag))) + = mkVarMonoBind (getSrcLoc tycon) rdr_name + (HsApp (HsVar mkInt_RDR) (HsLit (HsIntPrim max_tag))) where max_tag = case (tyConDataCons tycon) of data_cons -> toInteger ((length data_cons) - fIRST_TAG) @@ -1082,6 +1210,9 @@ multi-clause definitions; it generates: \end{verbatim} \begin{code} +mkVarMonoBind :: SrcLoc -> RdrName -> RdrNameHsExpr -> RdrNameMonoBinds +mkVarMonoBind loc var rhs = mk_easy_FunMonoBind loc var [] [] rhs + mk_easy_FunMonoBind :: SrcLoc -> RdrName -> [RdrNamePat] -> [RdrNameMonoBinds] -> RdrNameHsExpr -> RdrNameMonoBinds @@ -1090,7 +1221,7 @@ mk_easy_FunMonoBind loc fun pats binds expr = FunMonoBind fun False{-not infix-} [mk_easy_Match loc pats binds expr] loc mk_easy_Match loc pats binds expr - = mk_match loc pats expr (mkMonoBind (andMonoBindList binds) [] Recursive) + = mk_match loc pats expr (mkMonoBind Recursive (andMonoBindList binds)) -- The renamer expects everything in its input to be a -- "recursive" MonoBinds, and it is its job to sort things out -- from there. @@ -1123,6 +1254,7 @@ mkHsChar c = HsChar (ord c) mkConPat con vars = ConPatIn con (PrefixCon (map VarPat vars)) mkNullaryConPat con = ConPatIn con (PrefixCon []) +mkWildConPat con = ConPatIn (getRdrName con) (PrefixCon (nOfThem (dataConSourceArity con) wildPat)) \end{code} ToDo: Better SrcLocs. @@ -1133,7 +1265,8 @@ compare_gen_Case :: -> RdrNameHsExpr -> RdrNameHsExpr -> RdrNameHsExpr careful_compare_Case :: -- checks for primitive types... - Type + TyCon -- The tycon we are deriving for + -> Type -> RdrNameHsExpr -- What to do for equality -> RdrNameHsExpr -> RdrNameHsExpr -> RdrNameHsExpr @@ -1150,24 +1283,41 @@ compare_gen_Case eq a b -- General case mkSimpleMatch [mkNullaryConPat gtTag_RDR] gtTag_Expr placeHolderType generatedSrcLoc] generatedSrcLoc -careful_compare_Case ty eq a b - | not (isUnLiftedType ty) = - compare_gen_Case eq a b - | otherwise = - -- we have to do something special for primitive things... - HsIf (genOpApp a relevant_eq_op b) - eq - (HsIf (genOpApp a relevant_lt_op b) ltTag_Expr gtTag_Expr generatedSrcLoc) - generatedSrcLoc +careful_compare_Case tycon ty eq a b + | not (isUnLiftedType ty) + = compare_gen_Case eq a b + | otherwise -- We have to do something special for primitive things... + = HsIf (genOpApp a relevant_eq_op b) + eq + (HsIf (genOpApp a relevant_lt_op b) ltTag_Expr gtTag_Expr generatedSrcLoc) + generatedSrcLoc where - relevant_eq_op = assoc_ty_id eq_op_tbl ty - relevant_lt_op = assoc_ty_id lt_op_tbl ty - -assoc_ty_id tyids ty - = if null res then panic "assoc_ty" - else head res + relevant_eq_op = assoc_ty_id "Ord" tycon eq_op_tbl ty + relevant_lt_op = assoc_ty_id "Ord" tycon lt_op_tbl ty + + +box_if_necy :: String -- The class involved + -> TyCon -- The tycon involved + -> RdrNameHsExpr -- The argument + -> Type -- The argument type + -> RdrNameHsExpr -- Boxed version of the arg +box_if_necy cls_str tycon arg arg_ty + | isUnLiftedType arg_ty = HsApp (HsVar box_con) arg + | otherwise = arg where - res = [id | (ty',id) <- tyids, ty `tcEqType` ty'] + box_con = assoc_ty_id cls_str tycon box_con_tbl arg_ty + +assoc_ty_id :: String -- The class involved + -> TyCon -- The tycon involved + -> [(Type,a)] -- The table + -> Type -- The type + -> a -- The result of the lookup +assoc_ty_id cls_str tycon tbl ty + | null res = pprPanic "Error in deriving:" (text "Can't derive" <+> text cls_str <+> + text "for primitive type" <+> ppr ty) + | otherwise = head res + where + res = [id | (ty',id) <- tbl, ty `tcEqType` ty'] eq_op_tbl = [(charPrimTy, eqChar_RDR) @@ -1187,6 +1337,15 @@ lt_op_tbl = ,(doublePrimTy, ltDouble_RDR) ] +box_con_tbl = + [(charPrimTy, getRdrName charDataCon) + ,(intPrimTy, getRdrName intDataCon) + ,(wordPrimTy, getRdrName wordDataCon) + ,(addrPrimTy, addrDataCon_RDR) + ,(floatPrimTy, getRdrName floatDataCon) + ,(doublePrimTy, getRdrName doubleDataCon) + ] + ----------------------------------------------------------------------- and_Expr, append_Expr :: RdrNameHsExpr -> RdrNameHsExpr -> RdrNameHsExpr @@ -1196,14 +1355,14 @@ append_Expr a b = genOpApp a append_RDR b ----------------------------------------------------------------------- -eq_Expr :: Type -> RdrNameHsExpr -> RdrNameHsExpr -> RdrNameHsExpr -eq_Expr ty a b = genOpApp a eq_op b +eq_Expr :: TyCon -> Type -> RdrNameHsExpr -> RdrNameHsExpr -> RdrNameHsExpr +eq_Expr tycon ty a b = genOpApp a eq_op b where eq_op | not (isUnLiftedType ty) = eq_RDR | otherwise = -- we have to do something special for primitive things... - assoc_ty_id eq_op_tbl ty + assoc_ty_id "Eq" tycon eq_op_tbl ty \end{code} @@ -1293,6 +1452,9 @@ a_RDR = varUnqual FSLIT("a") b_RDR = varUnqual FSLIT("b") c_RDR = varUnqual FSLIT("c") d_RDR = varUnqual FSLIT("d") +e_RDR = varUnqual FSLIT("e") +k_RDR = varUnqual FSLIT("k") +z_RDR = varUnqual FSLIT("z") :: RdrName ah_RDR = varUnqual FSLIT("a#") bh_RDR = varUnqual FSLIT("b#") ch_RDR = varUnqual FSLIT("c#") @@ -1309,6 +1471,7 @@ a_Expr = HsVar a_RDR b_Expr = HsVar b_RDR c_Expr = HsVar c_RDR d_Expr = HsVar d_RDR +z_Expr = HsVar z_RDR ltTag_Expr = HsVar ltTag_RDR eqTag_Expr = HsVar eqTag_RDR gtTag_Expr = HsVar gtTag_RDR