2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
4 \section[TcInstDecls]{Typechecking instance declarations}
12 #include "HsVersions.h"
14 import HsSyn ( HsDecl(..), InstDecl(..),
15 HsBinds(..), MonoBinds(..), GRHSsAndBinds(..), GRHS(..),
16 HsExpr(..), InPat(..), HsLit(..), Sig(..),
18 collectMonoBinders, andMonoBinds
20 import HsBinds ( sigsForMe )
21 import RnHsSyn ( RenamedHsBinds, RenamedMonoBinds,
22 RenamedInstDecl, RenamedHsExpr,
23 RenamedSig, RenamedHsDecl
25 import TcHsSyn ( TcMonoBinds, TcIdOcc(..), TcIdBndr,
26 maybeBoxedPrimType, tcIdType
29 import TcBinds ( tcPragmaSigs )
30 import TcClassDcl ( tcMethodBind, badMethodErr )
32 import RnMonad ( RnNameSupply )
33 import Inst ( Inst, InstOrigin(..),
34 newDicts, LIE, emptyLIE, plusLIE, plusLIEs )
35 import TcDeriv ( tcDeriving )
36 import TcEnv ( tcExtendGlobalValEnv, tcAddImportedIdInfo )
37 import TcInstUtil ( InstInfo(..), mkInstanceRelatedIds, classDataCon )
38 import TcKind ( TcKind, unifyKind )
39 import TcMonoType ( tcHsType )
40 import TcSimplify ( tcSimplifyAndCheck )
41 import TcType ( TcType, TcTyVar, TcTyVarSet,
42 zonkSigTyVar, tcInstSigTyVars, tcInstType, tcInstTheta
45 import Bag ( emptyBag, unitBag, unionBags, unionManyBags,
46 foldBag, bagToList, Bag
48 import CmdLineOpts ( opt_GlasgowExts, opt_WarnMissingMethods )
49 import Class ( classBigSig, Class )
50 import Id ( isNullaryDataCon, dataConArgTys, replaceIdInfo, idName, Id )
51 import Maybes ( maybeToBool, seqMaybe, catMaybes )
52 import Name ( nameOccName, mkLocalName,
53 isLocallyDefined, Module,
56 import PrelVals ( nO_METHOD_BINDING_ERROR_ID, eRROR_ID )
57 import PprType ( pprParendType, pprConstraint )
58 import SrcLoc ( SrcLoc, noSrcLoc )
59 import TyCon ( isSynTyCon, isDataTyCon, tyConDerivings )
60 import Type ( Type, ThetaType, isUnpointedType,
61 splitSigmaTy, isTyVarTy, mkSigmaTy,
62 splitTyConApp_maybe, splitDictTy_maybe,
63 splitAlgTyConApp_maybe, splitRhoTy,
66 import TyVar ( zipTyVarEnv, mkTyVarSet, tyVarSetToList, TyVar )
67 import TysPrim ( byteArrayPrimTyCon, mutableByteArrayPrimTyCon )
68 import TysWiredIn ( stringTy )
69 import Unique ( Unique, cCallableClassKey, cReturnableClassKey, Uniquable(..) )
73 Typechecking instance declarations is done in two passes. The first
74 pass, made by @tcInstDecls1@, collects information to be used in the
77 This pre-processed info includes the as-yet-unprocessed bindings
78 inside the instance declaration. These are type-checked in the second
79 pass, when the class-instance envs and GVE contain all the info from
80 all the instance and value decls. Indeed that's the reason we need
81 two passes over the instance decls.
84 Here is the overall algorithm.
85 Assume that we have an instance declaration
87 instance c => k (t tvs) where b
91 $LIE_c$ is the LIE for the context of class $c$
93 $betas_bar$ is the free variables in the class method type, excluding the
96 $LIE_cop$ is the LIE constraining a particular class method
98 $tau_cop$ is the tau type of a class method
100 $LIE_i$ is the LIE for the context of instance $i$
102 $X$ is the instance constructor tycon
104 $gammas_bar$ is the set of type variables of the instance
106 $LIE_iop$ is the LIE for a particular class method instance
108 $tau_iop$ is the tau type for this instance of a class method
110 $alpha$ is the class variable
112 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
114 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
117 ToDo: Update the list above with names actually in the code.
121 First, make the LIEs for the class and instance contexts, which means
122 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
123 and make LIElistI and LIEI.
125 Then process each method in turn.
127 order the instance methods according to the ordering of the class methods
129 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
131 Create final dictionary function from bindings generated already
133 df = lambda inst_tyvars
140 in <op1,op2,...,opn,sd1,...,sdm>
142 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
143 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
147 tcInstDecls1 :: TcEnv s -- Contains IdInfo for dfun ids
149 -> Module -- module name for deriving
150 -> RnNameSupply -- for renaming derivings
151 -> TcM s (Bag InstInfo,
155 tcInstDecls1 unf_env decls mod_name rn_name_supply
156 = -- Do the ordinary instance declarations
157 mapNF_Tc (tcInstDecl1 unf_env mod_name)
158 [inst_decl | InstD inst_decl <- decls] `thenNF_Tc` \ inst_info_bags ->
160 decl_inst_info = unionManyBags inst_info_bags
162 -- Handle "derived" instances; note that we only do derivings
163 -- for things in this module; we ignore deriving decls from
165 tcDeriving mod_name rn_name_supply decl_inst_info
166 `thenTc` \ (deriv_inst_info, deriv_binds, ddump_deriv) ->
169 full_inst_info = deriv_inst_info `unionBags` decl_inst_info
171 returnTc (full_inst_info, deriv_binds, ddump_deriv)
174 tcInstDecl1 :: TcEnv s -> Module -> RenamedInstDecl -> NF_TcM s (Bag InstInfo)
176 tcInstDecl1 unf_env mod_name (InstDecl poly_ty binds uprags (Just dfun_name) src_loc)
177 = -- Prime error recovery, set source location
178 recoverNF_Tc (returnNF_Tc emptyBag) $
179 tcAddSrcLoc src_loc $
181 -- Type-check all the stuff before the "where"
182 tcHsType poly_ty `thenTc` \ poly_ty' ->
184 (tyvars, theta, dict_ty) = splitSigmaTy poly_ty'
185 (clas, inst_tys) = case splitDictTy_maybe dict_ty of
186 Nothing -> pprPanic "tcInstDecl1" (ppr poly_ty)
190 -- Check for respectable instance type
191 scrutiniseInstanceType clas inst_tys `thenTc_`
193 -- Make the dfun id and constant-method ids
195 (dfun_id, dfun_theta) = mkInstanceRelatedIds dfun_name
196 clas tyvars inst_tys theta
197 -- Add info from interface file
198 final_dfun_id = tcAddImportedIdInfo unf_env dfun_id
200 returnTc (unitBag (InstInfo clas tyvars inst_tys theta
201 dfun_theta final_dfun_id
202 binds src_loc uprags))
206 %************************************************************************
208 \subsection{Type-checking instance declarations, pass 2}
210 %************************************************************************
213 tcInstDecls2 :: Bag InstInfo
214 -> NF_TcM s (LIE s, TcMonoBinds s)
216 tcInstDecls2 inst_decls
217 = foldBag combine tcInstDecl2 (returnNF_Tc (emptyLIE, EmptyMonoBinds)) inst_decls
219 combine tc1 tc2 = tc1 `thenNF_Tc` \ (lie1, binds1) ->
220 tc2 `thenNF_Tc` \ (lie2, binds2) ->
221 returnNF_Tc (lie1 `plusLIE` lie2,
222 binds1 `AndMonoBinds` binds2)
226 ======= New documentation starts here (Sept 92) ==============
228 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
229 the dictionary function for this instance declaration. For example
231 instance Foo a => Foo [a] where
235 might generate something like
237 dfun.Foo.List dFoo_a = let op1 x = ...
243 HOWEVER, if the instance decl has no context, then it returns a
244 bigger @HsBinds@ with declarations for each method. For example
246 instance Foo [a] where
252 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
253 const.Foo.op1.List a x = ...
254 const.Foo.op2.List a y = ...
256 This group may be mutually recursive, because (for example) there may
257 be no method supplied for op2 in which case we'll get
259 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
261 that is, the default method applied to the dictionary at this type.
263 What we actually produce in either case is:
265 AbsBinds [a] [dfun_theta_dicts]
266 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
267 { d = (sd1,sd2, ..., op1, op2, ...)
272 The "maybe" says that we only ask AbsBinds to make global constant methods
273 if the dfun_theta is empty.
276 For an instance declaration, say,
278 instance (C1 a, C2 b) => C (T a b) where
281 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
282 function whose type is
284 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
286 Notice that we pass it the superclass dictionaries at the instance type; this
287 is the ``Mark Jones optimisation''. The stuff before the "=>" here
288 is the @dfun_theta@ below.
290 First comes the easy case of a non-local instance decl.
293 tcInstDecl2 :: InstInfo -> NF_TcM s (LIE s, TcMonoBinds s)
295 tcInstDecl2 (InstInfo clas inst_tyvars inst_tys
296 inst_decl_theta dfun_theta
299 | not (isLocallyDefined dfun_id)
300 = returnNF_Tc (emptyLIE, EmptyMonoBinds)
303 -- I deleted this "optimisation" because when importing these
304 -- instance decls the renamer would look for the dfun bindings and they weren't there.
305 -- This would be fixable, but it seems simpler just to produce a tiny void binding instead,
306 -- even though it's never used.
308 -- This case deals with CCallable etc, which don't need any bindings
310 = returnNF_Tc (emptyLIE, EmptyBinds)
314 = -- Prime error recovery
315 recoverNF_Tc (returnNF_Tc (emptyLIE, EmptyMonoBinds)) $
318 -- Get the class signature
320 origin = InstanceDeclOrigin
322 sc_theta, sc_sel_ids,
323 op_sel_ids, defm_ids) = classBigSig clas
326 -- Instantiate the instance decl with tc-style type variables
327 tcInstSigTyVars inst_tyvars `thenNF_Tc` \ (inst_tyvars', _, tenv) ->
328 mapNF_Tc (tcInstType tenv) inst_tys `thenNF_Tc` \ inst_tys' ->
329 tcInstTheta tenv dfun_theta `thenNF_Tc` \ dfun_theta' ->
330 tcInstTheta tenv inst_decl_theta `thenNF_Tc` \ inst_decl_theta' ->
332 -- Instantiate the super-class context with inst_tys
333 tcInstTheta (zipTyVarEnv class_tyvars inst_tys') sc_theta `thenNF_Tc` \ sc_theta' ->
335 -- Create dictionary Ids from the specified instance contexts.
336 newDicts origin sc_theta' `thenNF_Tc` \ (sc_dicts, sc_dict_ids) ->
337 newDicts origin dfun_theta' `thenNF_Tc` \ (dfun_arg_dicts, dfun_arg_dicts_ids) ->
338 newDicts origin inst_decl_theta' `thenNF_Tc` \ (inst_decl_dicts, _) ->
339 newDicts origin [(clas,inst_tys')] `thenNF_Tc` \ (this_dict, [this_dict_id]) ->
341 -- Check that all the method bindings come from this class
343 check_from_this_class (bndr, loc)
344 | nameOccName bndr `elem` sel_names = returnNF_Tc ()
345 | otherwise = tcAddSrcLoc loc $
346 addErrTc (badMethodErr bndr clas)
347 sel_names = map getOccName op_sel_ids
348 bndrs = bagToList (collectMonoBinders monobinds)
350 mapNF_Tc check_from_this_class bndrs `thenNF_Tc_`
352 tcExtendGlobalValEnv (catMaybes defm_ids) (
354 -- Default-method Ids may be mentioned in synthesised RHSs
355 mapAndUnzip3Tc (tcInstMethodBind clas inst_tys' inst_tyvars' monobinds uprags)
356 (op_sel_ids `zip` defm_ids)
357 ) `thenTc` \ (method_binds_s, insts_needed_s, meth_lies_w_ids) ->
359 -- Deal with SPECIALISE instance pragmas
361 dfun_prags = [Sig (idName dfun_id) ty loc | SpecInstSig ty loc <- uprags]
363 tcExtendGlobalValEnv [dfun_id] (
364 tcPragmaSigs dfun_prags
365 ) `thenTc` \ (prag_info_fn, prag_binds, prag_lie) ->
367 -- Check the overloading constraints of the methods and superclasses
368 mapNF_Tc zonkSigTyVar inst_tyvars' `thenNF_Tc` \ zonked_inst_tyvars ->
371 inst_tyvars_set = mkTyVarSet zonked_inst_tyvars
373 (meth_lies, meth_ids) = unzip meth_lies_w_ids
375 -- These insts are in scope; quite a few, eh?
376 avail_insts = this_dict `plusLIE`
377 dfun_arg_dicts `plusLIE`
379 unionManyBags meth_lies
381 methods_lie = plusLIEs insts_needed_s
384 -- Check that we *could* construct the superclass dictionaries,
385 -- even though we are *actually* going to pass the superclass dicts in;
386 -- the check ensures that the caller will never have a problem building
388 tcAddErrCtxt superClassCtxt (
390 (ptext SLIT("instance declaration context"))
391 inst_tyvars_set -- Local tyvars
392 inst_decl_dicts -- The instance dictionaries available
393 sc_dicts -- The superclass dicationaries reqd
395 -- Ignore the result; we're only doing
396 -- this to make sure it can be done.
398 -- Ditto method bindings
399 tcAddErrCtxt methodCtxt (
401 (ptext SLIT("instance declaration context"))
402 inst_tyvars_set -- Local tyvars
407 -- Now do the simplification again, this time to get the
408 -- bindings; this time we use an enhanced "avails"
409 -- Ignore errors because they come from the *previous* tcSimplifys
412 (ptext SLIT("instance declaration context"))
414 dfun_arg_dicts -- NB! Don't include this_dict here, else the sc_dicts
415 -- get bound by just selecting from this_dict!!
416 (sc_dicts `plusLIE` methods_lie `plusLIE` prag_lie)
417 ) `thenTc` \ (const_lie, lie_binds) ->
420 -- Create the result bindings
422 dict_constr = classDataCon clas
423 scs_and_meths = sc_dict_ids ++ meth_ids
427 = -- Blatant special case for CCallable, CReturnable
428 -- If the dictionary is empty then we should never
429 -- select anything from it, so we make its RHS just
430 -- emit an error message. This in turn means that we don't
431 -- mention the constructor, which doesn't exist for CCallable, CReturnable
432 -- Hardly beautiful, but only three extra lines.
433 HsApp (TyApp (HsVar (RealId eRROR_ID)) [tcIdType this_dict_id])
434 (HsLitOut (HsString msg) stringTy)
436 | otherwise -- The common case
437 = foldl HsApp (TyApp (HsVar (RealId dict_constr)) inst_tys')
438 (map HsVar (sc_dict_ids ++ meth_ids))
439 -- We don't produce a binding for the dict_constr; instead we
440 -- rely on the simplifier to unfold this saturated application
442 msg = _PK_ ("Compiler error: bad dictionary " ++ showSDoc (ppr clas))
444 dict_bind = VarMonoBind this_dict_id dict_rhs
445 method_binds = andMonoBinds method_binds_s
447 final_dfun_id = replaceIdInfo dfun_id (prag_info_fn (idName dfun_id))
453 [(inst_tyvars', RealId final_dfun_id, this_dict_id)]
454 (lie_binds `AndMonoBinds`
455 method_binds `AndMonoBinds`
456 prag_binds `AndMonoBinds`
460 main_bind `AndMonoBinds` prag_binds)
464 %************************************************************************
466 \subsection{Processing each method}
468 %************************************************************************
473 -> [TcType s] -- Instance types
474 -> [TcTyVar s] -- and their free (sig) tyvars
475 -> RenamedMonoBinds -- Method binding
476 -> [RenamedSig] -- Pragmas
477 -> (Id, Maybe Id) -- Selector id and default-method id
478 -> TcM s (TcMonoBinds s, LIE s, (LIE s, TcIdOcc s))
480 tcInstMethodBind clas inst_tys inst_tyvars meth_binds prags (sel_id, maybe_dm_id)
481 = tcGetSrcLoc `thenNF_Tc` \ loc ->
482 tcGetUnique `thenNF_Tc` \ uniq ->
484 sel_name = idName sel_id
485 meth_occ = getOccName sel_name
486 default_meth_name = mkLocalName uniq meth_occ loc
487 maybe_meth_bind = find meth_occ meth_binds
488 the_meth_bind = case maybe_meth_bind of
490 Nothing -> mk_default_bind default_meth_name loc
491 meth_prags = sigsForMe (== sel_name) prags
494 -- Warn if no method binding, only if -fwarn-missing-methods
496 warnTc (opt_WarnMissingMethods &&
497 not (maybeToBool maybe_meth_bind) &&
498 not (maybeToBool maybe_dm_id))
499 (omittedMethodWarn sel_id clas) `thenNF_Tc_`
501 -- Typecheck the method binding
502 tcMethodBind clas origin inst_tys inst_tyvars sel_id the_meth_bind meth_prags
504 origin = InstanceDeclOrigin -- Poor
506 find occ EmptyMonoBinds = Nothing
507 find occ (AndMonoBinds b1 b2) = find occ b1 `seqMaybe` find occ b2
509 find occ b@(FunMonoBind op_name _ _ _) | nameOccName op_name == occ = Just b
510 | otherwise = Nothing
511 find occ b@(PatMonoBind (VarPatIn op_name) _ _) | nameOccName op_name == occ = Just b
512 | otherwise = Nothing
513 find occ other = panic "Urk! Bad instance method binding"
516 mk_default_bind local_meth_name loc
517 = PatMonoBind (VarPatIn local_meth_name)
518 (GRHSsAndBindsIn (unguardedRHS (default_expr loc) loc) EmptyBinds)
522 = case maybe_dm_id of
523 Just dm_id -> HsVar (getName dm_id) -- There's a default method
524 Nothing -> error_expr loc -- No default method
527 = HsApp (HsVar (getName nO_METHOD_BINDING_ERROR_ID))
528 (HsLit (HsString (_PK_ (error_msg loc))))
530 error_msg loc = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
535 %************************************************************************
537 \subsection{Checking for a decent instance type}
539 %************************************************************************
541 @scrutiniseInstanceType@ checks the type {\em and} its syntactic constraints:
542 it must normally look like: @instance Foo (Tycon a b c ...) ...@
544 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
545 flag is on, or (2)~the instance is imported (they must have been
546 compiled elsewhere). In these cases, we let them go through anyway.
548 We can also have instances for functions: @instance Foo (a -> b) ...@.
551 scrutiniseInstanceType clas inst_taus
552 | -- CCALL CHECK (a).... urgh!
553 -- To verify that a user declaration of a CCallable/CReturnable
554 -- instance is OK, we must be able to see the constructor(s)
555 -- of the instance type (see next guard.)
557 -- We flag this separately to give a more precise error msg.
559 (uniqueOf clas == cCallableClassKey || uniqueOf clas == cReturnableClassKey)
560 && is_alg_tycon_app && not constructors_visible
561 = failWithTc (invisibleDataConPrimCCallErr clas first_inst_tau)
564 -- A user declaration of a CCallable/CReturnable instance
565 -- must be for a "boxed primitive" type.
566 (uniqueOf clas == cCallableClassKey && not (ccallable_type first_inst_tau)) ||
567 (uniqueOf clas == cReturnableClassKey && not (creturnable_type first_inst_tau))
568 = failWithTc (nonBoxedPrimCCallErr clas first_inst_tau)
571 -- It is obviously illegal to have an explicit instance
572 -- for something that we are also planning to `derive'
573 | maybeToBool alg_tycon_app_maybe && clas `elem` (tyConDerivings alg_tycon)
574 = failWithTc (derivingWhenInstanceExistsErr clas first_inst_tau)
575 -- Kind check will have ensured inst_taus is of length 1
577 -- WITH HASKELL 1.4, MUST HAVE C (T a b c)
578 | not opt_GlasgowExts
579 && not (length inst_taus == 1 &&
580 maybeToBool maybe_tycon_app && -- Yes, there's a type constuctor
581 not (isSynTyCon tycon) && -- ...but not a synonym
582 all isTyVarTy arg_tys && -- Applied to type variables
583 length (tyVarSetToList (tyVarsOfTypes arg_tys)) == length arg_tys
584 -- This last condition checks that all the type variables are distinct
586 = failWithTc (instTypeErr clas inst_taus
587 (text "the instance type must be of form (T a b c)" $$
588 text "where T is not a synonym, and a,b,c are distinct type variables")
595 (first_inst_tau : _) = inst_taus
597 -- Stuff for algebraic or -> type
598 maybe_tycon_app = splitTyConApp_maybe first_inst_tau
599 Just (tycon, arg_tys) = maybe_tycon_app
601 -- Stuff for an *algebraic* data type
602 alg_tycon_app_maybe = splitAlgTyConApp_maybe first_inst_tau
603 -- The "Alg" part looks through synonyms
604 is_alg_tycon_app = maybeToBool alg_tycon_app_maybe
605 Just (alg_tycon, _, data_cons) = alg_tycon_app_maybe
607 constructors_visible = not (null data_cons)
610 -- These conditions come directly from what the DsCCall is capable of.
611 -- Totally grotesque. Green card should solve this.
613 ccallable_type ty = isUnpointedType ty || -- Allow CCallable Int# etc
614 maybeToBool (maybeBoxedPrimType ty) || -- Ditto Int etc
618 byte_arr_thing = case splitAlgTyConApp_maybe ty of
619 Just (tycon, ty_args, [data_con]) | isDataTyCon tycon ->
620 length data_con_arg_tys == 2 &&
621 maybeToBool maybe_arg2_tycon &&
622 (arg2_tycon == byteArrayPrimTyCon ||
623 arg2_tycon == mutableByteArrayPrimTyCon)
625 data_con_arg_tys = dataConArgTys data_con ty_args
626 (data_con_arg_ty1 : data_con_arg_ty2 : _) = data_con_arg_tys
627 maybe_arg2_tycon = splitTyConApp_maybe data_con_arg_ty2
628 Just (arg2_tycon,_) = maybe_arg2_tycon
632 creturnable_type ty = maybeToBool (maybeBoxedPrimType ty) ||
633 -- Or, a data type with a single nullary constructor
634 case (splitAlgTyConApp_maybe ty) of
635 Just (tycon, tys_applied, [data_con])
636 -> isNullaryDataCon data_con
642 instTypeErr clas tys msg
643 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes (pprConstraint clas tys),
647 derivingWhenInstanceExistsErr clas tycon
648 = hang (hsep [ptext SLIT("Deriving class"),
650 ptext SLIT("type"), quotes (ppr tycon)])
651 4 (ptext SLIT("when an explicit instance exists"))
653 nonBoxedPrimCCallErr clas inst_ty
654 = hang (ptext SLIT("Unacceptable instance type for ccall-ish class"))
655 4 (hsep [ ptext SLIT("class"), ppr clas, ptext SLIT("type"),
658 omittedMethodWarn sel_id clas
659 = sep [ptext SLIT("No explicit method nor default method for") <+> quotes (ppr sel_id),
660 ptext SLIT("in an instance declaration for") <+> quotes (ppr clas)]
663 Declaring CCallable & CReturnable instances in a module different
664 from where the type was defined. Caused by importing data type
665 abstractly (either programmatically or by the renamer being over-eager
668 invisibleDataConPrimCCallErr clas inst_ty
669 = hang (hsep [ptext SLIT("Constructors for"), quotes (ppr inst_ty),
670 ptext SLIT("not visible when checking"),
671 quotes (ppr clas), ptext SLIT("instance")])
672 4 (hsep [text "(Try either importing", ppr inst_ty,
673 text "non-abstractly or compile using -fno-prune-tydecls ..)"])
675 methodCtxt = ptext SLIT("When checking the methods of an instance declaration")
676 superClassCtxt = ptext SLIT("When checking the superclasses of an instance declaration")