2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcClassDcl]{Typechecking class declarations}
7 module TcClassDcl ( kcClassDecl, tcClassDecl1, tcClassDecls2, mkImplicitClassBinds,
8 tcMethodBind, checkFromThisClass
11 #include "HsVersions.h"
13 import HsSyn ( HsDecl(..), TyClDecl(..), Sig(..), MonoBinds(..),
14 InPat(..), HsBinds(..), GRHSs(..),
15 HsExpr(..), HsLit(..), HsType(..), HsPred(..),
16 pprHsClassAssertion, unguardedRHS,
17 andMonoBinds, andMonoBindList, getTyVarName,
18 isClassDecl, isClassOpSig, isPragSig, collectMonoBinders
20 import HsPragmas ( ClassPragmas(..) )
21 import BasicTypes ( NewOrData(..), TopLevelFlag(..), RecFlag(..) )
22 import RnHsSyn ( RenamedTyClDecl, RenamedClassPragmas,
23 RenamedClassOpSig, RenamedMonoBinds,
24 RenamedContext, RenamedHsDecl, RenamedSig
26 import TcHsSyn ( TcMonoBinds, idsToMonoBinds )
28 import Inst ( Inst, InstOrigin(..), LIE, emptyLIE, plusLIE, plusLIEs, newDicts, newMethod )
29 import TcEnv ( TcId, ValueEnv, TcTyThing(..), tcAddImportedIdInfo,
30 tcLookupClass, tcLookupTy, tcExtendTyVarEnvForMeths, tcExtendGlobalTyVars,
33 import TcBinds ( tcBindWithSigs, tcSpecSigs )
34 import TcTyDecls ( mkNewTyConRep )
35 import TcUnify ( unifyKinds )
37 import TcMonoType ( kcHsType, tcHsTopType, tcExtendTopTyVarScope,
38 tcContext, checkSigTyVars, sigCtxt, mkTcSig
40 import TcSimplify ( tcSimplifyAndCheck, bindInstsOfLocalFuns )
41 import TcType ( TcType, TcTyVar, tcInstTyVars, zonkTcTyVarBndr, tcGetTyVar )
42 import TcInstUtil ( classDataCon )
43 import PrelInfo ( nO_METHOD_BINDING_ERROR_ID )
44 import FieldLabel ( firstFieldLabelTag )
45 import Bag ( unionManyBags, bagToList )
46 import Class ( mkClass, classBigSig, classSelIds, Class, ClassOpItem )
47 import CmdLineOpts ( opt_GlasgowExts, opt_WarnMissingMethods )
48 import MkId ( mkDictSelId, mkDataConId, mkDataConWrapId, mkDefaultMethodId )
49 import DataCon ( mkDataCon, dataConId, dataConWrapId, notMarkedStrict )
50 import Id ( Id, setInlinePragma, idUnfolding, idType, idName )
51 import CoreUnfold ( unfoldingTemplate )
53 import Name ( Name, nameOccName, isLocallyDefined, NamedThing(..) )
54 import NameSet ( emptyNameSet )
56 import Type ( Type, ThetaType, ClassContext,
57 mkFunTy, mkTyVarTy, mkTyVarTys, mkDictTy, mkDictTys,
58 mkSigmaTy, mkForAllTys, mkClassPred, classesOfPreds,
59 boxedTypeKind, mkArrowKind
61 import Var ( tyVarKind, TyVar )
62 import VarSet ( mkVarSet, emptyVarSet )
63 import TyCon ( AlgTyConFlavour(..), mkClassTyCon )
64 import Unique ( Unique, Uniquable(..) )
66 import Maybes ( seqMaybe )
67 import FiniteMap ( lookupWithDefaultFM )
74 Every class implicitly declares a new data type, corresponding to dictionaries
75 of that class. So, for example:
77 class (D a) => C a where
79 op2 :: forall b. Ord b => a -> b -> b
81 would implicitly declare
83 data CDict a = CDict (D a)
85 (forall b. Ord b => a -> b -> b)
87 (We could use a record decl, but that means changing more of the existing apparatus.
90 For classes with just one superclass+method, we use a newtype decl instead:
93 op :: forallb. a -> b -> b
97 newtype CDict a = CDict (forall b. a -> b -> b)
99 Now DictTy in Type is just a form of type synomym:
100 DictTy c t = TyConTy CDict `AppTy` t
102 Death to "ExpandingDicts".
105 %************************************************************************
107 \subsection{Kind checking}
109 %************************************************************************
112 kcClassDecl (ClassDecl context class_name
113 tyvar_names fundeps class_sigs def_methods pragmas
115 = -- CHECK ARITY 1 FOR HASKELL 1.4
116 checkTc (opt_GlasgowExts || length tyvar_names == 1)
117 (classArityErr class_name) `thenTc_`
119 -- Get the (mutable) class kind
120 tcLookupTy class_name `thenNF_Tc` \ (kind, _, _) ->
122 -- Make suitable tyvars and do kind checking
123 -- The net effect is to mutate the class kind
124 tcExtendTopTyVarScope kind tyvar_names $ \ _ _ ->
125 tcContext context `thenTc_`
126 mapTc kc_sig the_class_sigs `thenTc_`
130 the_class_sigs = filter isClassOpSig class_sigs
132 kc_sig (ClassOpSig _ _ _ op_ty loc) = tcAddSrcLoc loc (kcHsType op_ty)
136 %************************************************************************
138 \subsection{Type checking}
140 %************************************************************************
143 tcClassDecl1 rec_env rec_inst_mapper rec_vrcs
144 (ClassDecl context class_name
145 tyvar_names fundeps class_sigs def_methods pragmas
146 tycon_name datacon_name datacon_wkr_name sc_sel_names src_loc)
147 = -- LOOK THINGS UP IN THE ENVIRONMENT
148 tcLookupTy class_name `thenTc` \ (class_kind, _, AClass rec_class) ->
149 tcExtendTopTyVarScope class_kind tyvar_names $ \ tyvars _ ->
150 -- The class kind is by now immutable
153 -- traceTc (text "tcClassCtxt" <+> ppr class_name) `thenTc_`
154 tcClassContext class_name rec_class tyvars context sc_sel_names
155 `thenTc` \ (sc_theta, sc_tys, sc_sel_ids) ->
156 -- traceTc (text "tcClassCtxt done" <+> ppr class_name) `thenTc_`
158 -- CHECK THE FUNCTIONAL DEPENDENCIES,
159 tcFundeps fundeps `thenTc` \ fds ->
161 -- CHECK THE CLASS SIGNATURES,
162 mapTc (tcClassSig rec_env rec_class tyvars)
163 (filter isClassOpSig class_sigs)
164 `thenTc` \ sig_stuff ->
166 -- MAKE THE CLASS OBJECT ITSELF
168 (op_tys, op_items) = unzip sig_stuff
169 rec_class_inst_env = rec_inst_mapper rec_class
170 clas = mkClass class_name tyvars fds
171 sc_theta sc_sel_ids op_items
175 dict_component_tys = sc_tys ++ op_tys
176 new_or_data = case dict_component_tys of
177 [_] -> NewTyCon (mkNewTyConRep tycon)
180 dict_con = mkDataCon datacon_name
181 [notMarkedStrict | _ <- dict_component_tys]
182 [{- No labelled fields -}]
185 [{-No existential tyvars-}] [{-Or context-}]
187 tycon dict_con_id dict_wrap_id
189 dict_con_id = mkDataConId datacon_wkr_name dict_con
190 dict_wrap_id = mkDataConWrapId dict_con
192 argvrcs = lookupWithDefaultFM rec_vrcs (pprPanic "tcClassDecl1: argvrcs:" $
196 tycon = mkClassTyCon tycon_name
200 dict_con -- Constructors
201 clas -- Yes! It's a dictionary
208 tcFundeps = mapTc tc_fundep
210 mapTc tc_fd_tyvar us `thenTc` \ us' ->
211 mapTc tc_fd_tyvar vs `thenTc` \ vs' ->
214 tcLookupTy v `thenTc` \(_, _, thing) ->
216 ATyVar tv -> returnTc tv
217 -- ZZ else should fail more gracefully
221 tcClassContext :: Name -> Class -> [TyVar]
222 -> RenamedContext -- class context
223 -> [Name] -- Names for superclass selectors
224 -> TcM s (ClassContext, -- the superclass context
225 [Type], -- types of the superclass dictionaries
226 [Id]) -- superclass selector Ids
228 tcClassContext class_name rec_class rec_tyvars context sc_sel_names
229 = -- Check the context.
230 -- The renamer has already checked that the context mentions
231 -- only the type variable of the class decl.
233 -- For std Haskell check that the context constrains only tyvars
234 (if opt_GlasgowExts then
237 mapTc check_constraint context
240 tcContext context `thenTc` \ sc_theta ->
243 sc_theta' = classesOfPreds sc_theta
244 sc_tys = mkDictTys sc_theta'
245 sc_sel_ids = zipWithEqual "tcClassContext" mk_super_id sc_sel_names sc_tys
248 returnTc (sc_theta', sc_tys, sc_sel_ids)
251 rec_tyvar_tys = mkTyVarTys rec_tyvars
253 mk_super_id name dict_ty
254 = mkDictSelId name rec_class ty
256 ty = mkForAllTys rec_tyvars $
257 mkFunTy (mkDictTy rec_class rec_tyvar_tys) dict_ty
259 check_constraint (HsPClass c tys) = checkTc (all is_tyvar tys)
260 (superClassErr class_name (c, tys))
262 is_tyvar (MonoTyVar _) = True
263 is_tyvar other = False
266 tcClassSig :: ValueEnv -- Knot tying only!
267 -> Class -- ...ditto...
268 -> [TyVar] -- The class type variable, used for error check only
270 -> TcM s (Type, -- Type of the method
271 ClassOpItem) -- Selector Id, default-method Id, True if explicit default binding
274 tcClassSig rec_env rec_clas rec_clas_tyvars
275 (ClassOpSig op_name dm_name explicit_dm
277 = tcAddSrcLoc src_loc $
279 -- Check the type signature. NB that the envt *already has*
280 -- bindings for the type variables; see comments in TcTyAndClassDcls.
282 -- NB: Renamer checks that the class type variable is mentioned in local_ty,
283 -- and that it is not constrained by theta
284 -- traceTc (text "tcClassSig" <+> ppr op_name) `thenTc_`
285 tcHsTopType op_ty `thenTc` \ local_ty ->
287 global_ty = mkSigmaTy rec_clas_tyvars
288 [mkClassPred rec_clas (mkTyVarTys rec_clas_tyvars)]
291 -- Build the selector id and default method id
292 sel_id = mkDictSelId op_name rec_clas global_ty
293 dm_id = mkDefaultMethodId dm_name rec_clas global_ty
294 final_dm_id = tcAddImportedIdInfo rec_env dm_id
296 -- traceTc (text "tcClassSig done" <+> ppr op_name) `thenTc_`
297 returnTc (local_ty, (sel_id, final_dm_id, explicit_dm))
301 %************************************************************************
303 \subsection[ClassDcl-pass2]{Class decls pass 2: default methods}
305 %************************************************************************
307 The purpose of pass 2 is
310 to beat on the explicitly-provided default-method decls (if any),
311 using them to produce a complete set of default-method decls.
312 (Omitted ones elicit an error message.)
314 to produce a definition for the selector function for each method
315 and superclass dictionary.
318 Pass~2 only applies to locally-defined class declarations.
320 The function @tcClassDecls2@ just arranges to apply @tcClassDecl2@ to
321 each local class decl.
324 tcClassDecls2 :: [RenamedHsDecl]
325 -> NF_TcM s (LIE, TcMonoBinds)
329 (returnNF_Tc (emptyLIE, EmptyMonoBinds))
330 [tcClassDecl2 cls_decl | TyClD cls_decl <- decls, isClassDecl cls_decl]
332 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
333 tc2 `thenNF_Tc` \ (lie2, binds2) ->
334 returnNF_Tc (lie1 `plusLIE` lie2,
335 binds1 `AndMonoBinds` binds2)
338 @tcClassDecl2@ is the business end of things.
341 tcClassDecl2 :: RenamedTyClDecl -- The class declaration
342 -> NF_TcM s (LIE, TcMonoBinds)
344 tcClassDecl2 (ClassDecl context class_name
345 tyvar_names _ class_sigs default_binds pragmas _ _ _ _ src_loc)
347 | not (isLocallyDefined class_name)
348 = returnNF_Tc (emptyLIE, EmptyMonoBinds)
350 | otherwise -- It is locally defined
351 = recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
352 tcAddSrcLoc src_loc $
353 tcLookupClass class_name `thenNF_Tc` \ clas ->
354 tcDefaultMethodBinds clas default_binds class_sigs
358 mkImplicitClassBinds :: [Class] -> NF_TcM s ([Id], TcMonoBinds)
359 mkImplicitClassBinds classes
360 = returnNF_Tc (concat cls_ids_s, andMonoBindList binds_s)
361 -- The selector binds are already in the selector Id's unfoldings
363 (cls_ids_s, binds_s) = unzip (map mk_implicit classes)
365 mk_implicit clas = (all_cls_ids, binds)
367 dict_con = classDataCon clas
368 all_cls_ids = dataConId dict_con : cls_ids
369 cls_ids = dataConWrapId dict_con : classSelIds clas
371 -- The wrapper and selectors get bindings, the worker does not
372 binds | isLocallyDefined clas = idsToMonoBinds cls_ids
373 | otherwise = EmptyMonoBinds
376 %************************************************************************
378 \subsection[Default methods]{Default methods}
380 %************************************************************************
382 The default methods for a class are each passed a dictionary for the
383 class, so that they get access to the other methods at the same type.
384 So, given the class decl
388 op2 :: Ord b => a -> b -> b -> b
391 op2 x y z = if (op1 x) && (y < z) then y else z
393 we get the default methods:
395 defm.Foo.op1 :: forall a. Foo a => a -> Bool
396 defm.Foo.op1 = /\a -> \dfoo -> \x -> True
398 defm.Foo.op2 :: forall a. Foo a => forall b. Ord b => a -> b -> b -> b
399 defm.Foo.op2 = /\ a -> \ dfoo -> /\ b -> \ dord -> \x y z ->
400 if (op1 a dfoo x) && (< b dord y z) then y else z
403 When we come across an instance decl, we may need to use the default
406 instance Foo Int where {}
410 const.Foo.Int.op1 :: Int -> Bool
411 const.Foo.Int.op1 = defm.Foo.op1 Int dfun.Foo.Int
413 const.Foo.Int.op2 :: forall b. Ord b => Int -> b -> b -> b
414 const.Foo.Int.op2 = defm.Foo.op2 Int dfun.Foo.Int
416 dfun.Foo.Int :: Foo Int
417 dfun.Foo.Int = (const.Foo.Int.op1, const.Foo.Int.op2)
419 Notice that, as with method selectors above, we assume that dictionary
420 application is curried, so there's no need to mention the Ord dictionary
421 in const.Foo.Int.op2 (or the type variable).
424 instance Foo a => Foo [a] where {}
426 dfun.Foo.List :: forall a. Foo a -> Foo [a]
428 = /\ a -> \ dfoo_a ->
430 op1 = defm.Foo.op1 [a] dfoo_list
431 op2 = defm.Foo.op2 [a] dfoo_list
432 dfoo_list = (op1, op2)
442 -> TcM s (LIE, TcMonoBinds)
444 tcDefaultMethodBinds clas default_binds sigs
445 = -- Check that the default bindings come from this class
446 checkFromThisClass clas op_items default_binds `thenNF_Tc_`
448 -- Do each default method separately
449 -- For Hugs compatibility we make a default-method for every
450 -- class op, regardless of whether or not the programmer supplied an
451 -- explicit default decl for the class. GHC will actually never
452 -- call the default method for such operations, because it'll whip up
453 -- a more-informative default method at each instance decl.
454 mapAndUnzipTc tc_dm op_items `thenTc` \ (defm_binds, const_lies) ->
456 returnTc (plusLIEs const_lies, andMonoBindList defm_binds)
458 prags = filter isPragSig sigs
460 (tyvars, _, _, op_items) = classBigSig clas
462 origin = ClassDeclOrigin
464 -- We make a separate binding for each default method.
465 -- At one time I used a single AbsBinds for all of them, thus
466 -- AbsBind [d] [dm1, dm2, dm3] { dm1 = ...; dm2 = ...; dm3 = ... }
467 -- But that desugars into
468 -- ds = \d -> (..., ..., ...)
469 -- dm1 = \d -> case ds d of (a,b,c) -> a
470 -- And since ds is big, it doesn't get inlined, so we don't get good
471 -- default methods. Better to make separate AbsBinds for each
473 tc_dm op_item@(_, dm_id, _)
474 = tcInstTyVars tyvars `thenNF_Tc` \ (clas_tyvars, inst_tys, _) ->
476 theta = [(mkClassPred clas inst_tys)]
478 newDicts origin theta `thenNF_Tc` \ (this_dict, [this_dict_id]) ->
480 avail_insts = this_dict
482 tcExtendTyVarEnvForMeths tyvars clas_tyvars (
483 tcMethodBind clas origin clas_tyvars inst_tys theta
484 default_binds prags False
486 ) `thenTc` \ (defm_bind, insts_needed, (_, local_dm_id)) ->
488 tcAddErrCtxt (defltMethCtxt clas) $
490 -- tcMethodBind has checked that the class_tyvars havn't
491 -- been unified with each other or another type, but we must
492 -- still zonk them before passing them to tcSimplifyAndCheck
493 mapNF_Tc zonkTcTyVarBndr clas_tyvars `thenNF_Tc` \ clas_tyvars' ->
497 (ptext SLIT("class") <+> ppr clas)
498 (mkVarSet clas_tyvars')
500 insts_needed `thenTc` \ (const_lie, dict_binds) ->
506 [(clas_tyvars', dm_id, local_dm_id)]
507 emptyNameSet -- No inlines (yet)
508 (dict_binds `andMonoBinds` defm_bind)
510 returnTc (full_bind, const_lie)
514 checkFromThisClass :: Class -> [ClassOpItem] -> RenamedMonoBinds -> NF_TcM s ()
515 checkFromThisClass clas op_items mono_binds
516 = mapNF_Tc check_from_this_class bndrs `thenNF_Tc_`
519 check_from_this_class (bndr, loc)
520 | nameOccName bndr `elem` sel_names = returnNF_Tc ()
521 | otherwise = tcAddSrcLoc loc $
522 addErrTc (badMethodErr bndr clas)
523 sel_names = [getOccName sel_id | (sel_id,_,_) <- op_items]
524 bndrs = bagToList (collectMonoBinders mono_binds)
528 @tcMethodBind@ is used to type-check both default-method and
529 instance-decl method declarations. We must type-check methods one at a
530 time, because their signatures may have different contexts and
537 -> [TcTyVar] -- Instantiated type variables for the
538 -- enclosing class/instance decl.
539 -- They'll be signature tyvars, and we
540 -- want to check that they don't get bound
541 -> [TcType] -- Instance types
542 -> TcThetaType -- Available theta; this could be used to check
543 -- the method signature, but actually that's done by
544 -- the caller; here, it's just used for the error message
545 -> RenamedMonoBinds -- Method binding (pick the right one from in here)
546 -> [RenamedSig] -- Pramgas (just for this one)
547 -> Bool -- True <=> This method is from an instance declaration
548 -> ClassOpItem -- The method selector and default-method Id
549 -> TcM s (TcMonoBinds, LIE, (LIE, TcId))
551 tcMethodBind clas origin inst_tyvars inst_tys inst_theta
552 meth_binds prags is_inst_decl
553 (sel_id, dm_id, explicit_dm)
554 = tcGetSrcLoc `thenNF_Tc` \ loc ->
556 newMethod origin sel_id inst_tys `thenNF_Tc` \ meth@(_, meth_id) ->
557 mkTcSig meth_id loc `thenNF_Tc` \ sig_info ->
560 meth_name = idName meth_id
561 maybe_user_bind = find_bind meth_name meth_binds
563 no_user_bind = case maybe_user_bind of {Nothing -> True; other -> False}
565 meth_bind = case maybe_user_bind of
567 Nothing -> mk_default_bind meth_name loc
569 meth_prags = find_prags meth_name prags
572 -- Warn if no method binding, only if -fwarn-missing-methods
573 warnTc (is_inst_decl && opt_WarnMissingMethods && no_user_bind && not explicit_dm)
574 (omittedMethodWarn sel_id clas) `thenNF_Tc_`
576 -- Check the bindings; first add inst_tyvars to the envt
577 -- so that we don't quantify over them in nested places
578 -- The *caller* put the class/inst decl tyvars into the envt
579 tcExtendGlobalTyVars (mkVarSet inst_tyvars) (
580 tcAddErrCtxt (methodCtxt sel_id) $
581 tcBindWithSigs NotTopLevel meth_bind
582 [sig_info] meth_prags NonRecursive
583 ) `thenTc` \ (binds, insts, _) ->
586 tcExtendLocalValEnv [(meth_name, meth_id)] (
587 tcSpecSigs meth_prags
588 ) `thenTc` \ (prag_binds1, prag_lie) ->
590 -- The prag_lie for a SPECIALISE pragma will mention the function
591 -- itself, so we have to simplify them away right now lest they float
593 bindInstsOfLocalFuns prag_lie [meth_id] `thenTc` \ (prag_lie', prag_binds2) ->
596 -- Now check that the instance type variables
597 -- (or, in the case of a class decl, the class tyvars)
598 -- have not been unified with anything in the environment
599 tcAddErrCtxtM (sigCtxt sig_msg inst_tyvars inst_theta (idType meth_id)) $
600 checkSigTyVars inst_tyvars emptyVarSet `thenTc_`
602 returnTc (binds `AndMonoBinds` prag_binds1 `AndMonoBinds` prag_binds2,
603 insts `plusLIE` prag_lie',
606 sig_msg = ptext SLIT("When checking the expected type for class method") <+> ppr sel_name
608 sel_name = idName sel_id
610 -- The renamer just puts the selector ID as the binder in the method binding
611 -- but we must use the method name; so we substitute it here. Crude but simple.
612 find_bind meth_name (FunMonoBind op_name fix matches loc)
613 | op_name == sel_name = Just (FunMonoBind meth_name fix matches loc)
614 find_bind meth_name (PatMonoBind (VarPatIn op_name) grhss loc)
615 | op_name == sel_name = Just (PatMonoBind (VarPatIn meth_name) grhss loc)
616 find_bind meth_name (AndMonoBinds b1 b2)
617 = find_bind meth_name b1 `seqMaybe` find_bind meth_name b2
618 find_bind meth_name other = Nothing -- Default case
621 -- Find the prags for this method, and replace the
622 -- selector name with the method name
623 find_prags meth_name [] = []
624 find_prags meth_name (SpecSig name ty loc : prags)
625 | name == sel_name = SpecSig meth_name ty loc : find_prags meth_name prags
626 find_prags meth_name (InlineSig name phase loc : prags)
627 | name == sel_name = InlineSig meth_name phase loc : find_prags meth_name prags
628 find_prags meth_name (NoInlineSig name phase loc : prags)
629 | name == sel_name = NoInlineSig meth_name phase loc : find_prags meth_name prags
630 find_prags meth_name (prag:prags) = find_prags meth_name prags
632 mk_default_bind local_meth_name loc
633 = PatMonoBind (VarPatIn local_meth_name)
634 (GRHSs (unguardedRHS (default_expr loc) loc) EmptyBinds Nothing)
638 | explicit_dm = HsVar (getName dm_id) -- There's a default method
639 | otherwise = error_expr loc -- No default method
641 error_expr loc = HsApp (HsVar (getName nO_METHOD_BINDING_ERROR_ID))
642 (HsLit (HsString (_PK_ (error_msg loc))))
644 error_msg loc = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
650 classArityErr class_name
651 = ptext SLIT("Too many parameters for class") <+> quotes (ppr class_name)
653 superClassErr class_name sc
654 = ptext SLIT("Illegal superclass constraint") <+> quotes (pprHsClassAssertion sc)
655 <+> ptext SLIT("in declaration for class") <+> quotes (ppr class_name)
657 defltMethCtxt class_name
658 = ptext SLIT("When checking the default methods for class") <+> quotes (ppr class_name)
661 = ptext SLIT("In the definition for method") <+> quotes (ppr sel_id)
663 badMethodErr bndr clas
664 = hsep [ptext SLIT("Class"), quotes (ppr clas),
665 ptext SLIT("does not have a method"), quotes (ppr bndr)]
667 omittedMethodWarn sel_id clas
668 = sep [ptext SLIT("No explicit method nor default method for") <+> quotes (ppr sel_id),
669 ptext SLIT("in an instance declaration for") <+> quotes (ppr clas)]