2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcInstDecls]{Typechecking instance declarations}
7 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
9 #include "HsVersions.h"
12 import TcBinds ( mkPragFun, tcPrags, badBootDeclErr )
13 import TcClassDcl ( tcMethodBind, mkMethodBind, badMethodErr,
14 tcClassDecl2, getGenericInstances )
16 import TcMType ( tcSkolSigType, checkValidInstance, checkValidInstHead )
17 import TcType ( mkClassPred, tcSplitSigmaTy, tcSplitDFunHead, mkTyVarTys,
18 SkolemInfo(InstSkol), tcSplitDFunTy )
19 import Inst ( tcInstClassOp, newDicts, instToId, showLIE,
20 getOverlapFlag, tcExtendLocalInstEnv )
21 import InstEnv ( mkLocalInstance, instanceDFunId )
22 import TcDeriv ( tcDeriving )
23 import TcEnv ( InstInfo(..), InstBindings(..),
24 newDFunName, tcExtendIdEnv
26 import TcHsType ( kcHsSigType, tcHsKindedType )
27 import TcUnify ( checkSigTyVars )
28 import TcSimplify ( tcSimplifyCheck, tcSimplifySuperClasses )
29 import Type ( zipOpenTvSubst, substTheta, substTys, mkTyConApp, mkTyVarTy )
30 import Coercion ( mkAppCoercion, mkAppsCoercion )
31 import TyCon ( TyCon, newTyConCo )
32 import DataCon ( classDataCon, dataConTyCon )
33 import Class ( classBigSig )
34 import Var ( TyVar, Id, idName, idType )
35 import Id ( mkSysLocal )
36 import UniqSupply ( uniqsFromSupply )
37 import MkId ( mkDictFunId )
38 import Name ( Name, getSrcLoc )
39 import Maybe ( catMaybes )
40 import SrcLoc ( noSrcSpan, srcLocSpan, unLoc, noLoc, Located(..), srcSpanStart )
41 import ListSetOps ( minusList )
44 import BasicTypes ( Activation( AlwaysActive ), InlineSpec(..) )
48 Typechecking instance declarations is done in two passes. The first
49 pass, made by @tcInstDecls1@, collects information to be used in the
52 This pre-processed info includes the as-yet-unprocessed bindings
53 inside the instance declaration. These are type-checked in the second
54 pass, when the class-instance envs and GVE contain all the info from
55 all the instance and value decls. Indeed that's the reason we need
56 two passes over the instance decls.
58 Here is the overall algorithm.
59 Assume that we have an instance declaration
61 instance c => k (t tvs) where b
65 $LIE_c$ is the LIE for the context of class $c$
67 $betas_bar$ is the free variables in the class method type, excluding the
70 $LIE_cop$ is the LIE constraining a particular class method
72 $tau_cop$ is the tau type of a class method
74 $LIE_i$ is the LIE for the context of instance $i$
76 $X$ is the instance constructor tycon
78 $gammas_bar$ is the set of type variables of the instance
80 $LIE_iop$ is the LIE for a particular class method instance
82 $tau_iop$ is the tau type for this instance of a class method
84 $alpha$ is the class variable
86 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
88 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
91 ToDo: Update the list above with names actually in the code.
95 First, make the LIEs for the class and instance contexts, which means
96 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
97 and make LIElistI and LIEI.
99 Then process each method in turn.
101 order the instance methods according to the ordering of the class methods
103 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
105 Create final dictionary function from bindings generated already
107 df = lambda inst_tyvars
114 in <op1,op2,...,opn,sd1,...,sdm>
116 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
117 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
121 %************************************************************************
123 \subsection{Extracting instance decls}
125 %************************************************************************
127 Gather up the instance declarations from their various sources
130 tcInstDecls1 -- Deal with both source-code and imported instance decls
131 :: [LTyClDecl Name] -- For deriving stuff
132 -> [LInstDecl Name] -- Source code instance decls
133 -> TcM (TcGblEnv, -- The full inst env
134 [InstInfo], -- Source-code instance decls to process;
135 -- contains all dfuns for this module
136 HsValBinds Name) -- Supporting bindings for derived instances
138 tcInstDecls1 tycl_decls inst_decls
140 -- Stop if addInstInfos etc discovers any errors
141 -- (they recover, so that we get more than one error each round)
143 -- (1) Do the ordinary instance declarations
144 mappM tcLocalInstDecl1 inst_decls `thenM` \ local_inst_infos ->
147 local_inst_info = catMaybes local_inst_infos
148 clas_decls = filter (isClassDecl.unLoc) tycl_decls
150 -- (2) Instances from generic class declarations
151 getGenericInstances clas_decls `thenM` \ generic_inst_info ->
153 -- Next, construct the instance environment so far, consisting of
154 -- a) local instance decls
155 -- b) generic instances
156 addInsts local_inst_info $
157 addInsts generic_inst_info $
159 -- (3) Compute instances from "deriving" clauses;
160 -- This stuff computes a context for the derived instance decl, so it
161 -- needs to know about all the instances possible; hence inst_env4
162 tcDeriving tycl_decls `thenM` \ (deriv_inst_info, deriv_binds) ->
163 addInsts deriv_inst_info $
165 getGblEnv `thenM` \ gbl_env ->
167 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
170 addInsts :: [InstInfo] -> TcM a -> TcM a
171 addInsts infos thing_inside
172 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
176 tcLocalInstDecl1 :: LInstDecl Name
177 -> TcM (Maybe InstInfo) -- Nothing if there was an error
178 -- A source-file instance declaration
179 -- Type-check all the stuff before the "where"
181 -- We check for respectable instance type, and context
182 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
183 -- !!!TODO: Handle the `ats' parameter!!! -=chak
184 = -- Prime error recovery, set source location
185 recoverM (returnM Nothing) $
187 addErrCtxt (instDeclCtxt1 poly_ty) $
189 do { is_boot <- tcIsHsBoot
190 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
193 -- Typecheck the instance type itself. We can't use
194 -- tcHsSigType, because it's not a valid user type.
195 ; kinded_ty <- kcHsSigType poly_ty
196 ; poly_ty' <- tcHsKindedType kinded_ty
197 ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
199 ; (clas, inst_tys) <- checkValidInstHead tau
200 ; checkValidInstance tyvars theta clas inst_tys
202 ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
203 ; overlap_flag <- getOverlapFlag
204 ; let dfun = mkDictFunId dfun_name tyvars theta clas inst_tys
205 ispec = mkLocalInstance dfun overlap_flag
207 ; return (Just (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags })) }
211 %************************************************************************
213 \subsection{Type-checking instance declarations, pass 2}
215 %************************************************************************
218 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
219 -> TcM (LHsBinds Id, TcLclEnv)
220 -- (a) From each class declaration,
221 -- generate any default-method bindings
222 -- (b) From each instance decl
223 -- generate the dfun binding
225 tcInstDecls2 tycl_decls inst_decls
226 = do { -- (a) Default methods from class decls
227 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
228 filter (isClassDecl.unLoc) tycl_decls
229 ; tcExtendIdEnv (concat dm_ids_s) $ do
231 -- (b) instance declarations
232 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
235 ; let binds = unionManyBags dm_binds_s `unionBags`
236 unionManyBags inst_binds_s
237 ; tcl_env <- getLclEnv -- Default method Ids in here
238 ; returnM (binds, tcl_env) }
241 ======= New documentation starts here (Sept 92) ==============
243 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
244 the dictionary function for this instance declaration. For example
246 instance Foo a => Foo [a] where
250 might generate something like
252 dfun.Foo.List dFoo_a = let op1 x = ...
258 HOWEVER, if the instance decl has no context, then it returns a
259 bigger @HsBinds@ with declarations for each method. For example
261 instance Foo [a] where
267 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
268 const.Foo.op1.List a x = ...
269 const.Foo.op2.List a y = ...
271 This group may be mutually recursive, because (for example) there may
272 be no method supplied for op2 in which case we'll get
274 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
276 that is, the default method applied to the dictionary at this type.
278 What we actually produce in either case is:
280 AbsBinds [a] [dfun_theta_dicts]
281 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
282 { d = (sd1,sd2, ..., op1, op2, ...)
287 The "maybe" says that we only ask AbsBinds to make global constant methods
288 if the dfun_theta is empty.
291 For an instance declaration, say,
293 instance (C1 a, C2 b) => C (T a b) where
296 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
297 function whose type is
299 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
301 Notice that we pass it the superclass dictionaries at the instance type; this
302 is the ``Mark Jones optimisation''. The stuff before the "=>" here
303 is the @dfun_theta@ below.
305 First comes the easy case of a non-local instance decl.
309 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
310 -- Returns a binding for the dfun
313 -- Derived newtype instances
315 -- We need to make a copy of the dictionary we are deriving from
316 -- because we may need to change some of the superclass dictionaries
317 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
319 -- In the case of a newtype, things are rather easy
320 -- class Show a => Foo a b where ...
321 -- newtype T a = MkT (Tree [a]) deriving( Foo Int )
322 -- The newtype gives an FC axiom looking like
323 -- axiom CoT a :: Tree [a] = T a
325 -- So all need is to generate a binding looking like
326 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
327 -- dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
328 -- case df `cast` (Foo Int (CoT a)) of
329 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
331 tcInstDecl2 (InstInfo { iSpec = ispec,
332 iBinds = NewTypeDerived tycon rep_tys })
333 = do { let dfun_id = instanceDFunId ispec
334 rigid_info = InstSkol dfun_id
335 origin = SigOrigin rigid_info
336 inst_ty = idType dfun_id
337 maybe_co_con = newTyConCo tycon
338 ; (tvs, theta, inst_head) <- tcSkolSigType rigid_info inst_ty
339 ; dicts <- newDicts origin theta
340 ; uniqs <- newUniqueSupply
341 ; let (rep_dict_id:sc_dict_ids) = map instToId dicts
342 -- (Here, we are relying on the order of dictionary
343 -- arguments built by NewTypeDerived in TcDeriv.)
345 wrap_fn = CoTyLams tvs <.> CoLams sc_dict_ids
347 coerced_rep_dict = mkHsCoerce (co_fn tvs cls_tycon) (HsVar rep_dict_id)
348 mk_located a = L noSrcSpan a
349 body | null sc_dict_ids = coerced_rep_dict
350 | otherwise = HsCase (mk_located coerced_rep_dict) $
351 MatchGroup [the_match] inst_head
352 the_match = mkSimpleMatch [the_pat] the_rhs
353 op_ids = zipWith (mkSysLocal FSLIT("op"))
354 (uniqsFromSupply uniqs) op_tys
355 the_pat = mk_located $ ConPatOut { pat_con = mk_located cls_data_con, pat_tvs = [],
356 pat_dicts = sc_dict_ids,
357 pat_binds = emptyLHsBinds,
358 pat_args = PrefixCon (map nlVarPat op_ids),
360 (cls, op_tys) = tcSplitDFunHead inst_head
361 cls_data_con = classDataCon cls
362 cls_tycon = dataConTyCon cls_data_con
364 the_rhs = mkHsConApp (cls_data_con) (mkTyVarTys tvs) (map HsVar (sc_dict_ids ++ op_ids))
366 ; return (unitBag (mk_located $ VarBind (dfun_id) (mk_located (mkHsCoerce wrap_fn body)))) }
368 co_fn :: [TyVar] -> TyCon -> ExprCoFn
369 co_fn tvs cls_tycon | Just co_con <- newTyConCo tycon
370 = ExprCoFn (mkAppCoercion (mkAppsCoercion (mkTyConApp cls_tycon []) rep_tys)
371 (mkTyConApp co_con (map mkTyVarTy tvs)))
375 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
377 dfun_id = instanceDFunId ispec
378 rigid_info = InstSkol dfun_id
379 inst_ty = idType dfun_id
381 -- Prime error recovery
382 recoverM (returnM emptyLHsBinds) $
383 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
384 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
386 -- Instantiate the instance decl with skolem constants
387 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
388 -- These inst_tyvars' scope over the 'where' part
389 -- Those tyvars are inside the dfun_id's type, which is a bit
390 -- bizarre, but OK so long as you realise it!
392 (clas, inst_tys') = tcSplitDFunHead inst_head'
393 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
395 -- Instantiate the super-class context with inst_tys
396 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
397 origin = SigOrigin rigid_info
399 -- Create dictionary Ids from the specified instance contexts.
400 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
401 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
402 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
403 -- Default-method Ids may be mentioned in synthesised RHSs,
404 -- but they'll already be in the environment.
406 -- Typecheck the methods
407 let -- These insts are in scope; quite a few, eh?
408 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
410 tcMethods origin clas inst_tyvars'
411 dfun_theta' inst_tys' avail_insts
412 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
414 -- Figure out bindings for the superclass context
415 -- Don't include this_dict in the 'givens', else
416 -- sc_dicts get bound by just selecting from this_dict!!
417 addErrCtxt superClassCtxt
418 (tcSimplifySuperClasses inst_tyvars'
420 sc_dicts) `thenM` \ sc_binds ->
422 -- It's possible that the superclass stuff might unified one
423 -- of the inst_tyavars' with something in the envt
424 checkSigTyVars inst_tyvars' `thenM_`
426 -- Deal with 'SPECIALISE instance' pragmas
427 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
429 -- Create the result bindings
431 dict_constr = classDataCon clas
432 scs_and_meths = map instToId sc_dicts ++ meth_ids
433 this_dict_id = instToId this_dict
434 inline_prag | null dfun_arg_dicts = []
435 | otherwise = [InlinePrag (Inline AlwaysActive True)]
436 -- Always inline the dfun; this is an experimental decision
437 -- because it makes a big performance difference sometimes.
438 -- Often it means we can do the method selection, and then
439 -- inline the method as well. Marcin's idea; see comments below.
441 -- BUT: don't inline it if it's a constant dictionary;
442 -- we'll get all the benefit without inlining, and we get
443 -- a **lot** of code duplication if we inline it
445 -- See Note [Inline dfuns] below
448 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
449 -- We don't produce a binding for the dict_constr; instead we
450 -- rely on the simplifier to unfold this saturated application
451 -- We do this rather than generate an HsCon directly, because
452 -- it means that the special cases (e.g. dictionary with only one
453 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
454 -- than needing to be repeated here.
456 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
457 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
459 main_bind = noLoc $ AbsBinds
461 (map instToId dfun_arg_dicts)
462 [(inst_tyvars', dfun_id, this_dict_id,
463 inline_prag ++ prags)]
466 showLIE (text "instance") `thenM_`
467 returnM (unitBag main_bind)
470 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
471 avail_insts op_items monobinds uprags
472 = -- Check that all the method bindings come from this class
474 sel_names = [idName sel_id | (sel_id, _) <- op_items]
475 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
477 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
479 -- Make the method bindings
481 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
483 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
485 -- And type check them
486 -- It's really worth making meth_insts available to the tcMethodBind
487 -- Consider instance Monad (ST s) where
488 -- {-# INLINE (>>) #-}
489 -- (>>) = ...(>>=)...
490 -- If we don't include meth_insts, we end up with bindings like this:
491 -- rec { dict = MkD then bind ...
492 -- then = inline_me (... (GHC.Base.>>= dict) ...)
494 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
495 -- and (b) the inline_me prevents us inlining the >>= selector, which
496 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
497 -- is not inlined across modules. Rather ironic since this does not
498 -- happen without the INLINE pragma!
500 -- Solution: make meth_insts available, so that 'then' refers directly
501 -- to the local 'bind' rather than going via the dictionary.
503 -- BUT WATCH OUT! If the method type mentions the class variable, then
504 -- this optimisation is not right. Consider
508 -- instance C Int where
510 -- The occurrence of 'op' on the rhs gives rise to a constraint
512 -- The trouble is that the 'meth_inst' for op, which is 'available', also
513 -- looks like 'op at Int'. But they are not the same.
515 prag_fn = mkPragFun uprags
516 all_insts = avail_insts ++ catMaybes meth_insts
517 sig_fn n = Just [] -- No scoped type variables, but every method has
518 -- a type signature, in effect, so that we check
519 -- the method has the right type
520 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
521 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
524 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
526 returnM (meth_ids, unionManyBags meth_binds_s)
530 ------------------------------
531 [Inline dfuns] Inlining dfuns unconditionally
532 ------------------------------
534 The code above unconditionally inlines dict funs. Here's why.
535 Consider this program:
537 test :: Int -> Int -> Bool
538 test x y = (x,y) == (y,x) || test y x
539 -- Recursive to avoid making it inline.
541 This needs the (Eq (Int,Int)) instance. If we inline that dfun
542 the code we end up with is good:
545 \r -> case ==# [ww ww1] of wild {
546 PrelBase.False -> Test.$wtest ww1 ww;
548 case ==# [ww1 ww] of wild1 {
549 PrelBase.False -> Test.$wtest ww1 ww;
550 PrelBase.True -> PrelBase.True [];
553 Test.test = \r [w w1]
556 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
559 If we don't inline the dfun, the code is not nearly as good:
561 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
562 PrelBase.:DEq tpl1 tpl2 -> tpl2;
567 let { y = PrelBase.I#! [ww1]; } in
568 let { x = PrelBase.I#! [ww]; } in
569 let { sat_slx = PrelTup.(,)! [y x]; } in
570 let { sat_sly = PrelTup.(,)! [x y];
572 case == sat_sly sat_slx of wild {
573 PrelBase.False -> Test.$wtest ww1 ww;
574 PrelBase.True -> PrelBase.True [];
581 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
584 Why doesn't GHC inline $fEq? Because it looks big:
586 PrelTup.zdfEqZ1T{-rcX-}
587 = \ @ a{-reT-} :: * @ b{-reS-} :: *
588 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
589 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
591 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
592 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
594 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
595 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
597 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
598 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
599 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
601 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
603 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
605 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
606 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
610 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
611 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
612 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
613 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
615 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
617 and it's not as bad as it seems, because it's further dramatically
618 simplified: only zeze2 is extracted and its body is simplified.
621 %************************************************************************
623 \subsection{Error messages}
625 %************************************************************************
628 instDeclCtxt1 hs_inst_ty
629 = inst_decl_ctxt (case unLoc hs_inst_ty of
630 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
631 HsPredTy pred -> ppr pred
632 other -> ppr hs_inst_ty) -- Don't expect this
633 instDeclCtxt2 dfun_ty
634 = inst_decl_ctxt (ppr (mkClassPred cls tys))
636 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
638 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
640 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")