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,
18 SkolemInfo(InstSkol), tcSplitDFunTy, mkFunTy )
19 import Inst ( newDictBndr, newDictBndrs, 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 ( tcSimplifySuperClasses )
29 import Type ( zipOpenTvSubst, substTheta, mkTyConApp, mkTyVarTy )
30 import Coercion ( mkAppCoercion, mkAppsCoercion )
31 import TyCon ( TyCon, newTyConCo )
32 import DataCon ( classDataCon, dataConTyCon, dataConInstArgTys )
33 import Class ( classBigSig )
34 import Var ( TyVar, Id, idName, idType )
35 import Id ( mkSysLocal )
36 import UniqSupply ( uniqsFromSupply, splitUniqSupply )
37 import MkId ( mkDictFunId )
38 import Name ( Name, getSrcLoc )
39 import Maybe ( catMaybes )
40 import SrcLoc ( 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
312 ------------------------
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 ; inst_loc <- getInstLoc origin
338 ; (tvs, theta, inst_head) <- tcSkolSigType rigid_info inst_ty
339 ; dicts <- newDictBndrs inst_loc theta
340 ; uniqs <- newUniqueSupply
341 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head
342 ; this_dict <- newDictBndr inst_loc (mkClassPred cls rep_tys)
343 ; let (rep_dict_id:sc_dict_ids)
344 | null dicts = [instToId this_dict]
345 | otherwise = map instToId dicts
347 -- (Here, we are relying on the order of dictionary
348 -- arguments built by NewTypeDerived in TcDeriv.)
350 wrap_fn = mkCoTyLams tvs <.> mkCoLams (rep_dict_id:sc_dict_ids)
352 coerced_rep_dict = mkHsCoerce (co_fn tvs cls_tycon) (HsVar rep_dict_id)
354 body | null sc_dict_ids = coerced_rep_dict
355 | otherwise = HsCase (noLoc coerced_rep_dict) $
356 MatchGroup [the_match] (mkFunTy in_dict_ty inst_head)
357 in_dict_ty = mkTyConApp cls_tycon cls_inst_tys
359 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
360 the_rhs = mkHsConApp cls_data_con cls_inst_tys (map HsVar (sc_dict_ids ++ op_ids))
362 (uniqs1, uniqs2) = splitUniqSupply uniqs
364 op_ids = zipWith (mkSysLocal FSLIT("op"))
365 (uniqsFromSupply uniqs1) op_tys
367 dict_ids = zipWith (mkSysLocal FSLIT("dict"))
368 (uniqsFromSupply uniqs2) (map idType sc_dict_ids)
370 the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
371 pat_dicts = dict_ids,
372 pat_binds = emptyLHsBinds,
373 pat_args = PrefixCon (map nlVarPat op_ids),
376 cls_data_con = classDataCon cls
377 cls_tycon = dataConTyCon cls_data_con
378 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
380 n_dict_args = if length dicts == 0 then 0 else length dicts - 1
381 op_tys = drop n_dict_args cls_arg_tys
383 dict = mkHsCoerce wrap_fn body
384 ; return (unitBag (noLoc $ VarBind dfun_id (noLoc dict))) }
386 co_fn :: [TyVar] -> TyCon -> ExprCoFn
387 co_fn tvs cls_tycon | Just co_con <- newTyConCo tycon
388 = ExprCoFn (mkAppCoercion -- (mkAppsCoercion
389 (mkTyConApp cls_tycon [])
391 (mkTyConApp co_con (map mkTyVarTy tvs)))
395 ------------------------
396 -- Ordinary instances
398 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
400 dfun_id = instanceDFunId ispec
401 rigid_info = InstSkol dfun_id
402 inst_ty = idType dfun_id
404 -- Prime error recovery
405 recoverM (returnM emptyLHsBinds) $
406 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
407 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
409 -- Instantiate the instance decl with skolem constants
410 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
411 -- These inst_tyvars' scope over the 'where' part
412 -- Those tyvars are inside the dfun_id's type, which is a bit
413 -- bizarre, but OK so long as you realise it!
415 (clas, inst_tys') = tcSplitDFunHead inst_head'
416 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
418 -- Instantiate the super-class context with inst_tys
419 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
420 origin = SigOrigin rigid_info
422 -- Create dictionary Ids from the specified instance contexts.
423 getInstLoc InstScOrigin `thenM` \ sc_loc ->
424 newDictBndrs sc_loc sc_theta' `thenM` \ sc_dicts ->
425 getInstLoc origin `thenM` \ inst_loc ->
426 newDictBndrs inst_loc dfun_theta' `thenM` \ dfun_arg_dicts ->
427 newDictBndr inst_loc (mkClassPred clas inst_tys') `thenM` \ this_dict ->
428 -- Default-method Ids may be mentioned in synthesised RHSs,
429 -- but they'll already be in the environment.
431 -- Typecheck the methods
432 let -- These insts are in scope; quite a few, eh?
433 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
435 tcMethods origin clas inst_tyvars'
436 dfun_theta' inst_tys' avail_insts
437 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
439 -- Figure out bindings for the superclass context
440 -- Don't include this_dict in the 'givens', else
441 -- sc_dicts get bound by just selecting from this_dict!!
442 addErrCtxt superClassCtxt
443 (tcSimplifySuperClasses inst_tyvars'
445 sc_dicts) `thenM` \ sc_binds ->
447 -- It's possible that the superclass stuff might unified one
448 -- of the inst_tyavars' with something in the envt
449 checkSigTyVars inst_tyvars' `thenM_`
451 -- Deal with 'SPECIALISE instance' pragmas
452 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
454 -- Create the result bindings
456 dict_constr = classDataCon clas
457 scs_and_meths = map instToId sc_dicts ++ meth_ids
458 this_dict_id = instToId this_dict
459 inline_prag | null dfun_arg_dicts = []
460 | otherwise = [InlinePrag (Inline AlwaysActive True)]
461 -- Always inline the dfun; this is an experimental decision
462 -- because it makes a big performance difference sometimes.
463 -- Often it means we can do the method selection, and then
464 -- inline the method as well. Marcin's idea; see comments below.
466 -- BUT: don't inline it if it's a constant dictionary;
467 -- we'll get all the benefit without inlining, and we get
468 -- a **lot** of code duplication if we inline it
470 -- See Note [Inline dfuns] below
473 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
474 -- We don't produce a binding for the dict_constr; instead we
475 -- rely on the simplifier to unfold this saturated application
476 -- We do this rather than generate an HsCon directly, because
477 -- it means that the special cases (e.g. dictionary with only one
478 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
479 -- than needing to be repeated here.
481 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
482 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
484 main_bind = noLoc $ AbsBinds
486 (map instToId dfun_arg_dicts)
487 [(inst_tyvars', dfun_id, this_dict_id,
488 inline_prag ++ prags)]
491 showLIE (text "instance") `thenM_`
492 returnM (unitBag main_bind)
495 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
496 avail_insts op_items monobinds uprags
497 = -- Check that all the method bindings come from this class
499 sel_names = [idName sel_id | (sel_id, _) <- op_items]
500 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
502 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
504 -- Make the method bindings
506 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
508 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
510 -- And type check them
511 -- It's really worth making meth_insts available to the tcMethodBind
512 -- Consider instance Monad (ST s) where
513 -- {-# INLINE (>>) #-}
514 -- (>>) = ...(>>=)...
515 -- If we don't include meth_insts, we end up with bindings like this:
516 -- rec { dict = MkD then bind ...
517 -- then = inline_me (... (GHC.Base.>>= dict) ...)
519 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
520 -- and (b) the inline_me prevents us inlining the >>= selector, which
521 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
522 -- is not inlined across modules. Rather ironic since this does not
523 -- happen without the INLINE pragma!
525 -- Solution: make meth_insts available, so that 'then' refers directly
526 -- to the local 'bind' rather than going via the dictionary.
528 -- BUT WATCH OUT! If the method type mentions the class variable, then
529 -- this optimisation is not right. Consider
533 -- instance C Int where
535 -- The occurrence of 'op' on the rhs gives rise to a constraint
537 -- The trouble is that the 'meth_inst' for op, which is 'available', also
538 -- looks like 'op at Int'. But they are not the same.
540 prag_fn = mkPragFun uprags
541 all_insts = avail_insts ++ catMaybes meth_insts
542 sig_fn n = Just [] -- No scoped type variables, but every method has
543 -- a type signature, in effect, so that we check
544 -- the method has the right type
545 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
546 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
549 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
551 returnM (meth_ids, unionManyBags meth_binds_s)
555 ------------------------------
556 [Inline dfuns] Inlining dfuns unconditionally
557 ------------------------------
559 The code above unconditionally inlines dict funs. Here's why.
560 Consider this program:
562 test :: Int -> Int -> Bool
563 test x y = (x,y) == (y,x) || test y x
564 -- Recursive to avoid making it inline.
566 This needs the (Eq (Int,Int)) instance. If we inline that dfun
567 the code we end up with is good:
570 \r -> case ==# [ww ww1] of wild {
571 PrelBase.False -> Test.$wtest ww1 ww;
573 case ==# [ww1 ww] of wild1 {
574 PrelBase.False -> Test.$wtest ww1 ww;
575 PrelBase.True -> PrelBase.True [];
578 Test.test = \r [w w1]
581 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
584 If we don't inline the dfun, the code is not nearly as good:
586 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
587 PrelBase.:DEq tpl1 tpl2 -> tpl2;
592 let { y = PrelBase.I#! [ww1]; } in
593 let { x = PrelBase.I#! [ww]; } in
594 let { sat_slx = PrelTup.(,)! [y x]; } in
595 let { sat_sly = PrelTup.(,)! [x y];
597 case == sat_sly sat_slx of wild {
598 PrelBase.False -> Test.$wtest ww1 ww;
599 PrelBase.True -> PrelBase.True [];
606 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
609 Why doesn't GHC inline $fEq? Because it looks big:
611 PrelTup.zdfEqZ1T{-rcX-}
612 = \ @ a{-reT-} :: * @ b{-reS-} :: *
613 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
614 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
616 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
617 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
619 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
620 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
622 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
623 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
624 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
626 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
628 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
630 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
631 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
635 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
636 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
637 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
638 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
640 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
642 and it's not as bad as it seems, because it's further dramatically
643 simplified: only zeze2 is extracted and its body is simplified.
646 %************************************************************************
648 \subsection{Error messages}
650 %************************************************************************
653 instDeclCtxt1 hs_inst_ty
654 = inst_decl_ctxt (case unLoc hs_inst_ty of
655 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
656 HsPredTy pred -> ppr pred
657 other -> ppr hs_inst_ty) -- Don't expect this
658 instDeclCtxt2 dfun_ty
659 = inst_decl_ctxt (ppr (mkClassPred cls tys))
661 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
663 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
665 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")