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"
11 import HsSyn ( InstDecl(..), HsType(..),
12 MonoBinds(..), HsExpr(..), HsLit(..), Sig(..),
13 andMonoBindList, collectMonoBinders,
16 import RnHsSyn ( RenamedHsBinds, RenamedInstDecl, RenamedTyClDecl )
17 import TcHsSyn ( TcMonoBinds, mkHsConApp )
18 import TcBinds ( tcSpecSigs )
19 import TcClassDcl ( tcMethodBind, mkMethodBind, badMethodErr,
20 tcClassDecl2, getGenericInstances )
22 import TcMType ( tcInstType, checkValidTheta, checkValidInstHead, instTypeErr,
23 checkAmbiguity, SourceTyCtxt(..) )
24 import TcType ( mkClassPred, tcSplitForAllTys, tyVarsOfType,
25 tcSplitSigmaTy, getClassPredTys, tcSplitPredTy_maybe, mkTyVarTys,
26 TyVarDetails(..), tcSplitDFunTy
28 import Inst ( InstOrigin(..), tcInstClassOp, newDicts, instToId,
29 showLIE, tcExtendLocalInstEnv )
30 import TcDeriv ( tcDeriving )
31 import TcEnv ( tcExtendGlobalValEnv, tcExtendTyVarEnv2,
32 InstInfo(..), InstBindings(..),
33 newDFunName, tcExtendLocalValEnv
35 import PprType ( pprClassPred )
36 import TcHsType ( kcHsSigType, tcHsKindedType )
37 import TcUnify ( checkSigTyVars )
38 import TcSimplify ( tcSimplifyCheck, tcSimplifyTop )
39 import Subst ( mkTyVarSubst, substTheta, substTy )
40 import DataCon ( classDataCon )
41 import Class ( classBigSig )
42 import Var ( idName, idType )
44 import MkId ( mkDictFunId, rUNTIME_ERROR_ID )
45 import FunDeps ( checkInstFDs )
46 import Name ( getSrcLoc )
47 import NameSet ( unitNameSet, emptyNameSet, nameSetToList )
48 import UnicodeUtil ( stringToUtf8 )
49 import Maybe ( catMaybes )
50 import ListSetOps ( minusList )
55 Typechecking instance declarations is done in two passes. The first
56 pass, made by @tcInstDecls1@, collects information to be used in the
59 This pre-processed info includes the as-yet-unprocessed bindings
60 inside the instance declaration. These are type-checked in the second
61 pass, when the class-instance envs and GVE contain all the info from
62 all the instance and value decls. Indeed that's the reason we need
63 two passes over the instance decls.
66 Here is the overall algorithm.
67 Assume that we have an instance declaration
69 instance c => k (t tvs) where b
73 $LIE_c$ is the LIE for the context of class $c$
75 $betas_bar$ is the free variables in the class method type, excluding the
78 $LIE_cop$ is the LIE constraining a particular class method
80 $tau_cop$ is the tau type of a class method
82 $LIE_i$ is the LIE for the context of instance $i$
84 $X$ is the instance constructor tycon
86 $gammas_bar$ is the set of type variables of the instance
88 $LIE_iop$ is the LIE for a particular class method instance
90 $tau_iop$ is the tau type for this instance of a class method
92 $alpha$ is the class variable
94 $LIE_cop' = LIE_cop [X gammas_bar / alpha, fresh betas_bar]$
96 $tau_cop' = tau_cop [X gammas_bar / alpha, fresh betas_bar]$
99 ToDo: Update the list above with names actually in the code.
103 First, make the LIEs for the class and instance contexts, which means
104 instantiate $thetaC [X inst_tyvars / alpha ]$, yielding LIElistC' and LIEC',
105 and make LIElistI and LIEI.
107 Then process each method in turn.
109 order the instance methods according to the ordering of the class methods
111 express LIEC' in terms of LIEI, yielding $dbinds_super$ or an error
113 Create final dictionary function from bindings generated already
115 df = lambda inst_tyvars
122 in <op1,op2,...,opn,sd1,...,sdm>
124 Here, Bop1 \ldots Bopn bind the methods op1 \ldots opn,
125 and $dbinds_super$ bind the superclass dictionaries sd1 \ldots sdm.
129 %************************************************************************
131 \subsection{Extracting instance decls}
133 %************************************************************************
135 Gather up the instance declarations from their various sources
138 tcInstDecls1 -- Deal with both source-code and imported instance decls
139 :: [RenamedTyClDecl] -- For deriving stuff
140 -> [RenamedInstDecl] -- Source code instance decls
141 -> TcM (TcGblEnv, -- The full inst env
142 [InstInfo], -- Source-code instance decls to process;
143 -- contains all dfuns for this module
144 RenamedHsBinds) -- Supporting bindings for derived instances
146 tcInstDecls1 tycl_decls inst_decls
148 -- Stop if addInstInfos etc discovers any errors
149 -- (they recover, so that we get more than one error each round)
151 -- (1) Do the ordinary instance declarations
152 mappM tcLocalInstDecl1 inst_decls `thenM` \ local_inst_infos ->
155 local_inst_info = catMaybes local_inst_infos
156 clas_decls = filter isClassDecl tycl_decls
158 -- (2) Instances from generic class declarations
159 getGenericInstances clas_decls `thenM` \ generic_inst_info ->
161 -- Next, construct the instance environment so far, consisting of
162 -- a) imported instance decls (from this module)
163 -- b) local instance decls
164 -- c) generic instances
165 addInsts local_inst_info $
166 addInsts generic_inst_info $
168 -- (3) Compute instances from "deriving" clauses;
169 -- This stuff computes a context for the derived instance decl, so it
170 -- needs to know about all the instances possible; hence inst_env4
171 tcDeriving tycl_decls `thenM` \ (deriv_inst_info, deriv_binds) ->
172 addInsts deriv_inst_info $
174 getGblEnv `thenM` \ gbl_env ->
176 generic_inst_info ++ deriv_inst_info ++ local_inst_info,
179 addInsts :: [InstInfo] -> TcM a -> TcM a
180 addInsts infos thing_inside
181 = tcExtendLocalInstEnv (map iDFunId infos) thing_inside
185 tcLocalInstDecl1 :: RenamedInstDecl
186 -> TcM (Maybe InstInfo) -- Nothing if there was an error
187 -- A source-file instance declaration
188 -- Type-check all the stuff before the "where"
190 -- We check for respectable instance type, and context
191 -- but only do this for non-imported instance decls.
192 -- Imported ones should have been checked already, and may indeed
193 -- contain something illegal in normal Haskell, notably
194 -- instance CCallable [Char]
195 tcLocalInstDecl1 decl@(InstDecl poly_ty binds uprags src_loc)
196 = -- Prime error recovery, set source location
197 recoverM (returnM Nothing) $
199 addErrCtxt (instDeclCtxt1 poly_ty) $
201 -- Typecheck the instance type itself. We can't use
202 -- tcHsSigType, because it's not a valid user type.
203 kcHsSigType poly_ty `thenM` \ kinded_ty ->
204 tcHsKindedType kinded_ty `thenM` \ poly_ty' ->
206 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
208 checkValidTheta InstThetaCtxt theta `thenM_`
209 checkAmbiguity tyvars theta (tyVarsOfType tau) `thenM_`
210 checkValidInstHead tau `thenM` \ (clas,inst_tys) ->
211 checkTc (checkInstFDs theta clas inst_tys)
212 (instTypeErr (pprClassPred clas inst_tys) msg) `thenM_`
213 newDFunName clas inst_tys src_loc `thenM` \ dfun_name ->
214 returnM (Just (InstInfo { iDFunId = mkDictFunId dfun_name tyvars theta clas inst_tys,
215 iBinds = VanillaInst binds uprags }))
217 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
221 %************************************************************************
223 \subsection{Type-checking instance declarations, pass 2}
225 %************************************************************************
228 tcInstDecls2 :: [RenamedTyClDecl] -> [InstInfo]
229 -> TcM (TcLclEnv, TcMonoBinds)
230 -- (a) From each class declaration,
231 -- generate any default-method bindings
232 -- (b) From each instance decl
233 -- generate the dfun binding
235 tcInstDecls2 tycl_decls inst_decls
236 = do { -- (a) Default methods from class decls
237 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
238 filter isClassDecl tycl_decls
239 ; tcExtendLocalValEnv (concat dm_ids_s) $ do
241 -- (b) instance declarations
242 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
245 ; tcl_env <- getLclEnv
246 ; returnM (tcl_env, andMonoBindList dm_binds_s `AndMonoBinds`
247 andMonoBindList inst_binds_s) }
250 ======= New documentation starts here (Sept 92) ==============
252 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
253 the dictionary function for this instance declaration. For example
255 instance Foo a => Foo [a] where
259 might generate something like
261 dfun.Foo.List dFoo_a = let op1 x = ...
267 HOWEVER, if the instance decl has no context, then it returns a
268 bigger @HsBinds@ with declarations for each method. For example
270 instance Foo [a] where
276 dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
277 const.Foo.op1.List a x = ...
278 const.Foo.op2.List a y = ...
280 This group may be mutually recursive, because (for example) there may
281 be no method supplied for op2 in which case we'll get
283 const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
285 that is, the default method applied to the dictionary at this type.
287 What we actually produce in either case is:
289 AbsBinds [a] [dfun_theta_dicts]
290 [(dfun.Foo.List, d)] ++ (maybe) [(const.Foo.op1.List, op1), ...]
291 { d = (sd1,sd2, ..., op1, op2, ...)
296 The "maybe" says that we only ask AbsBinds to make global constant methods
297 if the dfun_theta is empty.
300 For an instance declaration, say,
302 instance (C1 a, C2 b) => C (T a b) where
305 where the {\em immediate} superclasses of C are D1, D2, we build a dictionary
306 function whose type is
308 (C1 a, C2 b, D1 (T a b), D2 (T a b)) => C (T a b)
310 Notice that we pass it the superclass dictionaries at the instance type; this
311 is the ``Mark Jones optimisation''. The stuff before the "=>" here
312 is the @dfun_theta@ below.
314 First comes the easy case of a non-local instance decl.
318 tcInstDecl2 :: InstInfo -> TcM TcMonoBinds
320 tcInstDecl2 (InstInfo { iDFunId = dfun_id, iBinds = binds })
321 = -- Prime error recovery
322 recoverM (returnM EmptyMonoBinds) $
323 addSrcLoc (getSrcLoc dfun_id) $
324 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
326 inst_ty = idType dfun_id
327 (inst_tyvars, _) = tcSplitForAllTys inst_ty
328 -- The tyvars of the instance decl scope over the 'where' part
329 -- Those tyvars are inside the dfun_id's type, which is a bit
330 -- bizarre, but OK so long as you realise it!
333 -- Instantiate the instance decl with tc-style type variables
334 tcInstType InstTv inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
336 Just pred = tcSplitPredTy_maybe inst_head'
337 (clas, inst_tys') = getClassPredTys pred
338 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
340 -- Instantiate the super-class context with inst_tys
341 sc_theta' = substTheta (mkTyVarSubst class_tyvars inst_tys') sc_theta
342 origin = InstanceDeclOrigin
344 -- Create dictionary Ids from the specified instance contexts.
345 newDicts origin sc_theta' `thenM` \ sc_dicts ->
346 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
347 newDicts origin [pred] `thenM` \ [this_dict] ->
348 -- Default-method Ids may be mentioned in synthesised RHSs,
349 -- but they'll already be in the environment.
352 -- Typecheck the methods
353 let -- These insts are in scope; quite a few, eh?
354 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
356 tcMethods clas inst_tyvars inst_tyvars'
357 dfun_theta' inst_tys' avail_insts
358 op_items binds `thenM` \ (meth_ids, meth_binds) ->
360 -- Figure out bindings for the superclass context
361 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
362 `thenM` \ (zonked_inst_tyvars, sc_binds_inner, sc_binds_outer) ->
364 -- Deal with 'SPECIALISE instance' pragmas by making them
365 -- look like SPECIALISE pragmas for the dfun
367 uprags = case binds of
368 VanillaInst _ uprags -> uprags
370 spec_prags = [ SpecSig (idName dfun_id) ty loc
371 | SpecInstSig ty loc <- uprags ]
372 xtve = inst_tyvars `zip` inst_tyvars'
374 tcExtendGlobalValEnv [dfun_id] (
375 tcExtendTyVarEnv2 xtve $
376 tcSpecSigs spec_prags
377 ) `thenM` \ prag_binds ->
379 -- Create the result bindings
381 dict_constr = classDataCon clas
382 scs_and_meths = map instToId sc_dicts ++ meth_ids
383 this_dict_id = instToId this_dict
384 inlines | null dfun_arg_dicts = emptyNameSet
385 | otherwise = unitNameSet (idName dfun_id)
386 -- Always inline the dfun; this is an experimental decision
387 -- because it makes a big performance difference sometimes.
388 -- Often it means we can do the method selection, and then
389 -- inline the method as well. Marcin's idea; see comments below.
391 -- BUT: don't inline it if it's a constant dictionary;
392 -- we'll get all the benefit without inlining, and we get
393 -- a **lot** of code duplication if we inline it
395 -- See Note [Inline dfuns] below
399 = -- Blatant special case for CCallable, CReturnable
400 -- If the dictionary is empty then we should never
401 -- select anything from it, so we make its RHS just
402 -- emit an error message. This in turn means that we don't
403 -- mention the constructor, which doesn't exist for CCallable, CReturnable
404 -- Hardly beautiful, but only three extra lines.
405 HsApp (TyApp (HsVar rUNTIME_ERROR_ID) [idType this_dict_id])
406 (HsLit (HsStringPrim (mkFastString (stringToUtf8 msg))))
408 | otherwise -- The common case
409 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
410 -- We don't produce a binding for the dict_constr; instead we
411 -- rely on the simplifier to unfold this saturated application
412 -- We do this rather than generate an HsCon directly, because
413 -- it means that the special cases (e.g. dictionary with only one
414 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
415 -- than needing to be repeated here.
418 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
420 dict_bind = VarMonoBind this_dict_id dict_rhs
421 all_binds = sc_binds_inner `AndMonoBinds` meth_binds `AndMonoBinds` dict_bind
425 (map instToId dfun_arg_dicts)
426 [(inst_tyvars', dfun_id, this_dict_id)]
429 showLIE (text "instance") `thenM_`
430 returnM (main_bind `AndMonoBinds` prag_binds `AndMonoBinds` sc_binds_outer)
433 tcMethods clas inst_tyvars inst_tyvars' dfun_theta' inst_tys'
434 avail_insts op_items (VanillaInst monobinds uprags)
435 = -- Check that all the method bindings come from this class
437 sel_names = [idName sel_id | (sel_id, _) <- op_items]
438 bad_bndrs = collectMonoBinders monobinds `minusList` sel_names
440 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
442 -- Make the method bindings
444 mk_method_bind = mkMethodBind InstanceDeclOrigin clas inst_tys' monobinds
446 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
448 -- And type check them
449 -- It's really worth making meth_insts available to the tcMethodBind
450 -- Consider instance Monad (ST s) where
451 -- {-# INLINE (>>) #-}
452 -- (>>) = ...(>>=)...
453 -- If we don't include meth_insts, we end up with bindings like this:
454 -- rec { dict = MkD then bind ...
455 -- then = inline_me (... (GHC.Base.>>= dict) ...)
457 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
458 -- and (b) the inline_me prevents us inlining the >>= selector, which
459 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
460 -- is not inlined across modules. Rather ironic since this does not
461 -- happen without the INLINE pragma!
463 -- Solution: make meth_insts available, so that 'then' refers directly
464 -- to the local 'bind' rather than going via the dictionary.
466 -- BUT WATCH OUT! If the method type mentions the class variable, then
467 -- this optimisation is not right. Consider
471 -- instance C Int where
473 -- The occurrence of 'op' on the rhs gives rise to a constraint
475 -- The trouble is that the 'meth_inst' for op, which is 'available', also
476 -- looks like 'op at Int'. But they are not the same.
478 all_insts = avail_insts ++ catMaybes meth_insts
479 xtve = inst_tyvars `zip` inst_tyvars'
480 tc_method_bind = tcMethodBind xtve inst_tyvars' dfun_theta' all_insts uprags
482 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
484 returnM ([meth_id | (_,meth_id,_) <- meth_infos],
485 andMonoBindList meth_binds_s)
488 -- Derived newtype instances
489 tcMethods clas inst_tyvars inst_tyvars' dfun_theta' inst_tys'
490 avail_insts op_items (NewTypeDerived rep_tys)
491 = getInstLoc InstanceDeclOrigin `thenM` \ inst_loc ->
492 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
495 (ptext SLIT("newtype derived instance"))
496 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
498 -- I don't think we have to do the checkSigTyVars thing
500 returnM (meth_ids, lie_binds `AndMonoBinds` andMonoBindList meth_binds)
503 do_one inst_loc (sel_id, _)
504 = -- The binding is like "op @ NewTy = op @ RepTy"
505 -- Make the *binder*, like in mkMethodBind
506 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
508 -- Make the *occurrence on the rhs*
509 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
511 meth_id = instToId meth_inst
513 return (meth_id, VarMonoBind meth_id (HsVar (instToId rhs_inst)), rhs_inst)
515 -- Instantiate rep_tys with the relevant type variables
516 rep_tys' = map (substTy subst) rep_tys
517 subst = mkTyVarSubst inst_tyvars (mkTyVarTys inst_tyvars')
520 Note: [Superclass loops]
521 ~~~~~~~~~~~~~~~~~~~~~~~~~
522 We have to be very, very careful when generating superclasses, lest we
523 accidentally build a loop. Here's an example:
527 class S a => C a where { opc :: a -> a }
528 class S b => D b where { opd :: b -> b }
536 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
537 Simplifying, we may well get:
538 $dfCInt = :C ds1 (opd dd)
541 Notice that we spot that we can extract ds1 from dd.
543 Alas! Alack! We can do the same for (instance D Int):
545 $dfDInt = :D ds2 (opc dc)
549 And now we've defined the superclass in terms of itself.
552 Solution: treat the superclass context separately, and simplify it
553 all the way down to nothing on its own. Don't toss any 'free' parts
554 out to be simplified together with other bits of context.
555 Hence the tcSimplifyTop below.
557 At a more basic level, don't include this_dict in the context wrt
558 which we simplify sc_dicts, else sc_dicts get bound by just selecting
562 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
563 = addErrCtxt superClassCtxt $
564 getLIE (tcSimplifyCheck doc inst_tyvars'
566 sc_dicts) `thenM` \ (sc_binds1, sc_lie) ->
568 -- It's possible that the superclass stuff might have done unification
569 checkSigTyVars inst_tyvars' `thenM` \ zonked_inst_tyvars ->
571 -- We must simplify this all the way down
572 -- lest we build superclass loops
573 -- See Note [Superclass loops] above
574 tcSimplifyTop sc_lie `thenM` \ sc_binds2 ->
576 returnM (zonked_inst_tyvars, sc_binds1, sc_binds2)
579 doc = ptext SLIT("instance declaration superclass context")
583 ------------------------------
584 [Inline dfuns] Inlining dfuns unconditionally
585 ------------------------------
587 The code above unconditionally inlines dict funs. Here's why.
588 Consider this program:
590 test :: Int -> Int -> Bool
591 test x y = (x,y) == (y,x) || test y x
592 -- Recursive to avoid making it inline.
594 This needs the (Eq (Int,Int)) instance. If we inline that dfun
595 the code we end up with is good:
598 \r -> case ==# [ww ww1] of wild {
599 PrelBase.False -> Test.$wtest ww1 ww;
601 case ==# [ww1 ww] of wild1 {
602 PrelBase.False -> Test.$wtest ww1 ww;
603 PrelBase.True -> PrelBase.True [];
606 Test.test = \r [w w1]
609 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
612 If we don't inline the dfun, the code is not nearly as good:
614 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
615 PrelBase.:DEq tpl1 tpl2 -> tpl2;
620 let { y = PrelBase.I#! [ww1]; } in
621 let { x = PrelBase.I#! [ww]; } in
622 let { sat_slx = PrelTup.(,)! [y x]; } in
623 let { sat_sly = PrelTup.(,)! [x y];
625 case == sat_sly sat_slx of wild {
626 PrelBase.False -> Test.$wtest ww1 ww;
627 PrelBase.True -> PrelBase.True [];
634 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
637 Why doesn't GHC inline $fEq? Because it looks big:
639 PrelTup.zdfEqZ1T{-rcX-}
640 = \ @ a{-reT-} :: * @ b{-reS-} :: *
641 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
642 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
644 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
645 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
647 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
648 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
650 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
651 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
652 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
654 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
656 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
658 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
659 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
663 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
664 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
665 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
666 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
668 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
670 and it's not as bad as it seems, because it's further dramatically
671 simplified: only zeze2 is extracted and its body is simplified.
674 %************************************************************************
676 \subsection{Error messages}
678 %************************************************************************
681 instDeclCtxt1 hs_inst_ty
682 = inst_decl_ctxt (case hs_inst_ty of
683 HsForAllTy _ _ (HsPredTy pred) -> ppr pred
684 HsPredTy pred -> ppr pred
685 other -> ppr hs_inst_ty) -- Don't expect this
686 instDeclCtxt2 dfun_ty
687 = inst_decl_ctxt (ppr (mkClassPred cls tys))
689 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
691 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
693 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")