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, mkFunTy )
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, dataConInstArgTys )
33 import Class ( classBigSig, classMethods )
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 ( 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 (cls, cls_inst_tys) = tcSplitDFunHead inst_head
342 ; [this_dict] <- newDicts origin [mkClassPred cls rep_tys]
343 ; let (rep_dict_id:sc_dict_ids) =
349 -- (Here, we are relying on the order of dictionary
350 -- arguments built by NewTypeDerived in TcDeriv.)
352 wrap_fn = CoTyLams tvs <.> CoLams (rep_dict_id:sc_dict_ids)
354 coerced_rep_dict = mkHsCoerce (co_fn tvs cls_tycon) (HsVar rep_dict_id)
356 body | null sc_dict_ids = coerced_rep_dict
357 | otherwise = HsCase (noLoc coerced_rep_dict) $
358 MatchGroup [the_match] (mkFunTy in_dict_ty inst_head)
359 in_dict_ty = mkTyConApp cls_tycon cls_inst_tys
361 the_match = mkSimpleMatch [the_pat] the_rhs
363 (uniqs1, uniqs2) = splitUniqSupply uniqs
365 op_ids = zipWith (mkSysLocal FSLIT("op"))
366 (uniqsFromSupply uniqs1) op_tys
368 dict_ids = zipWith (mkSysLocal FSLIT("dict"))
369 (uniqsFromSupply uniqs2) (map idType sc_dict_ids)
372 ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
373 pat_dicts = dict_ids,
374 pat_binds = emptyLHsBinds,
375 pat_args = PrefixCon (map nlVarPat op_ids),
378 cls_data_con = classDataCon cls
379 cls_tycon = dataConTyCon cls_data_con
380 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
382 n_dict_args = if length dicts == 0 then 0 else length dicts - 1
383 op_tys = drop n_dict_args cls_arg_tys
385 the_rhs = mkHsConApp cls_data_con cls_inst_tys (map HsVar (sc_dict_ids ++ op_ids))
386 dict = (mkHsCoerce wrap_fn body)
387 ; return (unitBag (noLoc $ VarBind (dfun_id) (noLoc dict))) }
389 co_fn :: [TyVar] -> TyCon -> ExprCoFn
390 co_fn tvs cls_tycon | Just co_con <- newTyConCo tycon
391 = ExprCoFn (mkAppCoercion -- (mkAppsCoercion
392 (mkTyConApp cls_tycon [])
394 (mkTyConApp co_con (map mkTyVarTy tvs)))
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 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
424 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
425 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
426 -- Default-method Ids may be mentioned in synthesised RHSs,
427 -- but they'll already be in the environment.
429 -- Typecheck the methods
430 let -- These insts are in scope; quite a few, eh?
431 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
433 tcMethods origin clas inst_tyvars'
434 dfun_theta' inst_tys' avail_insts
435 op_items monobinds uprags `thenM` \ (meth_ids, meth_binds) ->
437 -- Figure out bindings for the superclass context
438 -- Don't include this_dict in the 'givens', else
439 -- sc_dicts get bound by just selecting from this_dict!!
440 addErrCtxt superClassCtxt
441 (tcSimplifySuperClasses inst_tyvars'
443 sc_dicts) `thenM` \ sc_binds ->
445 -- It's possible that the superclass stuff might unified one
446 -- of the inst_tyavars' with something in the envt
447 checkSigTyVars inst_tyvars' `thenM_`
449 -- Deal with 'SPECIALISE instance' pragmas
450 tcPrags dfun_id (filter isSpecInstLSig uprags) `thenM` \ prags ->
452 -- Create the result bindings
454 dict_constr = classDataCon clas
455 scs_and_meths = map instToId sc_dicts ++ meth_ids
456 this_dict_id = instToId this_dict
457 inline_prag | null dfun_arg_dicts = []
458 | otherwise = [InlinePrag (Inline AlwaysActive True)]
459 -- Always inline the dfun; this is an experimental decision
460 -- because it makes a big performance difference sometimes.
461 -- Often it means we can do the method selection, and then
462 -- inline the method as well. Marcin's idea; see comments below.
464 -- BUT: don't inline it if it's a constant dictionary;
465 -- we'll get all the benefit without inlining, and we get
466 -- a **lot** of code duplication if we inline it
468 -- See Note [Inline dfuns] below
471 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
472 -- We don't produce a binding for the dict_constr; instead we
473 -- rely on the simplifier to unfold this saturated application
474 -- We do this rather than generate an HsCon directly, because
475 -- it means that the special cases (e.g. dictionary with only one
476 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
477 -- than needing to be repeated here.
479 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
480 all_binds = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
482 main_bind = noLoc $ AbsBinds
484 (map instToId dfun_arg_dicts)
485 [(inst_tyvars', dfun_id, this_dict_id,
486 inline_prag ++ prags)]
489 showLIE (text "instance") `thenM_`
490 returnM (unitBag main_bind)
493 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
494 avail_insts op_items monobinds uprags
495 = -- Check that all the method bindings come from this class
497 sel_names = [idName sel_id | (sel_id, _) <- op_items]
498 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
500 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
502 -- Make the method bindings
504 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
506 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
508 -- And type check them
509 -- It's really worth making meth_insts available to the tcMethodBind
510 -- Consider instance Monad (ST s) where
511 -- {-# INLINE (>>) #-}
512 -- (>>) = ...(>>=)...
513 -- If we don't include meth_insts, we end up with bindings like this:
514 -- rec { dict = MkD then bind ...
515 -- then = inline_me (... (GHC.Base.>>= dict) ...)
517 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
518 -- and (b) the inline_me prevents us inlining the >>= selector, which
519 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
520 -- is not inlined across modules. Rather ironic since this does not
521 -- happen without the INLINE pragma!
523 -- Solution: make meth_insts available, so that 'then' refers directly
524 -- to the local 'bind' rather than going via the dictionary.
526 -- BUT WATCH OUT! If the method type mentions the class variable, then
527 -- this optimisation is not right. Consider
531 -- instance C Int where
533 -- The occurrence of 'op' on the rhs gives rise to a constraint
535 -- The trouble is that the 'meth_inst' for op, which is 'available', also
536 -- looks like 'op at Int'. But they are not the same.
538 prag_fn = mkPragFun uprags
539 all_insts = avail_insts ++ catMaybes meth_insts
540 sig_fn n = Just [] -- No scoped type variables, but every method has
541 -- a type signature, in effect, so that we check
542 -- the method has the right type
543 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts sig_fn prag_fn
544 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
547 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
549 returnM (meth_ids, unionManyBags meth_binds_s)
553 ------------------------------
554 [Inline dfuns] Inlining dfuns unconditionally
555 ------------------------------
557 The code above unconditionally inlines dict funs. Here's why.
558 Consider this program:
560 test :: Int -> Int -> Bool
561 test x y = (x,y) == (y,x) || test y x
562 -- Recursive to avoid making it inline.
564 This needs the (Eq (Int,Int)) instance. If we inline that dfun
565 the code we end up with is good:
568 \r -> case ==# [ww ww1] of wild {
569 PrelBase.False -> Test.$wtest ww1 ww;
571 case ==# [ww1 ww] of wild1 {
572 PrelBase.False -> Test.$wtest ww1 ww;
573 PrelBase.True -> PrelBase.True [];
576 Test.test = \r [w w1]
579 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
582 If we don't inline the dfun, the code is not nearly as good:
584 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
585 PrelBase.:DEq tpl1 tpl2 -> tpl2;
590 let { y = PrelBase.I#! [ww1]; } in
591 let { x = PrelBase.I#! [ww]; } in
592 let { sat_slx = PrelTup.(,)! [y x]; } in
593 let { sat_sly = PrelTup.(,)! [x y];
595 case == sat_sly sat_slx of wild {
596 PrelBase.False -> Test.$wtest ww1 ww;
597 PrelBase.True -> PrelBase.True [];
604 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
607 Why doesn't GHC inline $fEq? Because it looks big:
609 PrelTup.zdfEqZ1T{-rcX-}
610 = \ @ a{-reT-} :: * @ b{-reS-} :: *
611 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
612 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
614 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
615 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
617 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
618 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
620 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
621 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
622 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
624 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
626 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
628 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
629 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
633 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
634 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
635 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
636 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
638 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
640 and it's not as bad as it seems, because it's further dramatically
641 simplified: only zeze2 is extracted and its body is simplified.
644 %************************************************************************
646 \subsection{Error messages}
648 %************************************************************************
651 instDeclCtxt1 hs_inst_ty
652 = inst_decl_ctxt (case unLoc hs_inst_ty of
653 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
654 HsPredTy pred -> ppr pred
655 other -> ppr hs_inst_ty) -- Don't expect this
656 instDeclCtxt2 dfun_ty
657 = inst_decl_ctxt (ppr (mkClassPred cls tys))
659 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
661 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
663 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")