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 ( tcSpecSigs )
13 import TcClassDcl ( tcMethodBind, mkMethodBind, badMethodErr,
14 tcClassDecl2, getGenericInstances )
16 import TcMType ( tcSkolType, checkValidTheta, checkValidInstHead, instTypeErr,
17 checkAmbiguity, SourceTyCtxt(..) )
18 import TcType ( mkClassPred, tcSplitForAllTys, tyVarsOfType,
19 tcSplitSigmaTy, getClassPredTys, tcSplitPredTy_maybe, mkTyVarTys,
20 SkolemInfo(InstSkol), tcSplitDFunTy, pprClassPred )
21 import Inst ( tcInstClassOp, newDicts, instToId, showLIE, tcExtendLocalInstEnv )
22 import TcDeriv ( tcDeriving )
23 import TcEnv ( tcExtendGlobalValEnv, tcExtendTyVarEnv2,
24 InstInfo(..), InstBindings(..),
25 newDFunName, tcExtendIdEnv
27 import TcHsType ( kcHsSigType, tcHsKindedType )
28 import TcUnify ( checkSigTyVars )
29 import TcSimplify ( tcSimplifyCheck, tcSimplifyTop )
30 import Type ( zipTvSubst, substTheta, substTys )
31 import DataCon ( classDataCon )
32 import Class ( classBigSig )
33 import Var ( Id, idName, idType )
34 import MkId ( mkDictFunId, rUNTIME_ERROR_ID )
35 import FunDeps ( checkInstFDs )
36 import Name ( Name, getSrcLoc )
37 import NameSet ( unitNameSet, emptyNameSet, unionNameSets )
38 import UnicodeUtil ( stringToUtf8 )
39 import Maybe ( catMaybes )
40 import SrcLoc ( srcLocSpan, unLoc, noLoc, Located(..), srcSpanStart )
41 import ListSetOps ( minusList )
47 Typechecking instance declarations is done in two passes. The first
48 pass, made by @tcInstDecls1@, collects information to be used in the
51 This pre-processed info includes the as-yet-unprocessed bindings
52 inside the instance declaration. These are type-checked in the second
53 pass, when the class-instance envs and GVE contain all the info from
54 all the instance and value decls. Indeed that's the reason we need
55 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 [HsBindGroup 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 iDFunId 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 -- but only do this for non-imported instance decls.
183 -- Imported ones should have been checked already, and may indeed
184 -- contain something illegal in normal Haskell, notably
185 -- instance CCallable [Char]
186 tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags))
187 = -- Prime error recovery, set source location
188 recoverM (returnM Nothing) $
190 addErrCtxt (instDeclCtxt1 poly_ty) $
192 -- Typecheck the instance type itself. We can't use
193 -- tcHsSigType, because it's not a valid user type.
194 kcHsSigType poly_ty `thenM` \ kinded_ty ->
195 tcHsKindedType kinded_ty `thenM` \ poly_ty' ->
197 (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
199 checkValidTheta InstThetaCtxt theta `thenM_`
200 checkAmbiguity tyvars theta (tyVarsOfType tau) `thenM_`
201 checkValidInstHead tau `thenM` \ (clas,inst_tys) ->
202 checkTc (checkInstFDs theta clas inst_tys)
203 (instTypeErr (pprClassPred clas inst_tys) msg) `thenM_`
204 newDFunName clas inst_tys (srcSpanStart loc) `thenM` \ dfun_name ->
205 returnM (Just (InstInfo { iDFunId = mkDictFunId dfun_name tyvars theta clas inst_tys,
206 iBinds = VanillaInst binds uprags }))
208 msg = parens (ptext SLIT("the instance types do not agree with the functional dependencies of the class"))
212 %************************************************************************
214 \subsection{Type-checking instance declarations, pass 2}
216 %************************************************************************
219 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo]
220 -> TcM (TcLclEnv, LHsBinds Id)
221 -- (a) From each class declaration,
222 -- generate any default-method bindings
223 -- (b) From each instance decl
224 -- generate the dfun binding
226 tcInstDecls2 tycl_decls inst_decls
227 = do { -- (a) Default methods from class decls
228 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
229 filter (isClassDecl.unLoc) tycl_decls
230 ; tcExtendIdEnv (concat dm_ids_s) $ do
232 -- (b) instance declarations
233 ; inst_binds_s <- mappM tcInstDecl2 inst_decls
236 ; tcl_env <- getLclEnv
237 ; returnM (tcl_env, unionManyBags dm_binds_s `unionBags`
238 unionManyBags inst_binds_s) }
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)
311 tcInstDecl2 (InstInfo { iDFunId = dfun_id, iBinds = binds })
312 = -- Prime error recovery
313 recoverM (returnM emptyLHsBinds) $
314 setSrcSpan (srcLocSpan (getSrcLoc dfun_id)) $
315 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
317 rigid_info = InstSkol dfun_id
318 inst_ty = idType dfun_id
319 (inst_tyvars, _) = tcSplitForAllTys inst_ty
320 -- The tyvars of the instance decl scope over the 'where' part
321 -- Those tyvars are inside the dfun_id's type, which is a bit
322 -- bizarre, but OK so long as you realise it!
325 -- Instantiate the instance decl with tc-style type variables
326 tcSkolType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
328 Just pred = tcSplitPredTy_maybe inst_head'
329 (clas, inst_tys') = getClassPredTys pred
330 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
332 -- Instantiate the super-class context with inst_tys
333 sc_theta' = substTheta (zipTvSubst class_tyvars inst_tys') sc_theta
334 origin = SigOrigin rigid_info
336 -- Create dictionary Ids from the specified instance contexts.
337 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
338 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
339 newDicts origin [pred] `thenM` \ [this_dict] ->
340 -- Default-method Ids may be mentioned in synthesised RHSs,
341 -- but they'll already be in the environment.
344 -- Typecheck the methods
345 let -- These insts are in scope; quite a few, eh?
346 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
348 tcMethods origin clas inst_tyvars inst_tyvars'
349 dfun_theta' inst_tys' avail_insts
350 op_items binds `thenM` \ (meth_ids, meth_binds) ->
352 -- Figure out bindings for the superclass context
353 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
354 `thenM` \ (sc_binds_inner, sc_binds_outer) ->
356 -- It's possible that the superclass stuff might have done unification
357 checkSigTyVars inst_tyvars' `thenM_`
359 -- Deal with 'SPECIALISE instance' pragmas by making them
360 -- look like SPECIALISE pragmas for the dfun
362 uprags = case binds of
363 VanillaInst _ uprags -> uprags
365 spec_prags = [ L loc (SpecSig (L loc (idName dfun_id)) ty)
366 | L loc (SpecInstSig ty) <- uprags ]
367 xtve = inst_tyvars `zip` inst_tyvars'
369 tcExtendGlobalValEnv [dfun_id] (
370 tcExtendTyVarEnv2 xtve $
371 tcSpecSigs spec_prags
372 ) `thenM` \ prag_binds ->
374 -- Create the result bindings
376 dict_constr = classDataCon clas
377 scs_and_meths = map instToId sc_dicts ++ meth_ids
378 this_dict_id = instToId this_dict
379 inlines | null dfun_arg_dicts = emptyNameSet
380 | otherwise = unitNameSet (idName dfun_id)
381 -- Always inline the dfun; this is an experimental decision
382 -- because it makes a big performance difference sometimes.
383 -- Often it means we can do the method selection, and then
384 -- inline the method as well. Marcin's idea; see comments below.
386 -- BUT: don't inline it if it's a constant dictionary;
387 -- we'll get all the benefit without inlining, and we get
388 -- a **lot** of code duplication if we inline it
390 -- See Note [Inline dfuns] below
394 = -- Blatant special case for CCallable, CReturnable
395 -- If the dictionary is empty then we should never
396 -- select anything from it, so we make its RHS just
397 -- emit an error message. This in turn means that we don't
398 -- mention the constructor, which doesn't exist for CCallable, CReturnable
399 -- Hardly beautiful, but only three extra lines.
400 nlHsApp (noLoc $ TyApp (nlHsVar rUNTIME_ERROR_ID)
401 [idType this_dict_id])
402 (nlHsLit (HsStringPrim (mkFastString (stringToUtf8 msg))))
404 | otherwise -- The common case
405 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
406 -- We don't produce a binding for the dict_constr; instead we
407 -- rely on the simplifier to unfold this saturated application
408 -- We do this rather than generate an HsCon directly, because
409 -- it means that the special cases (e.g. dictionary with only one
410 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
411 -- than needing to be repeated here.
414 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
416 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
417 all_binds = dict_bind `consBag` (sc_binds_inner `unionBags` meth_binds)
419 main_bind = noLoc $ AbsBinds
421 (map instToId dfun_arg_dicts)
422 [(inst_tyvars', dfun_id, this_dict_id)]
425 showLIE (text "instance") `thenM_`
426 returnM (unitBag main_bind `unionBags`
427 prag_binds `unionBags`
431 tcMethods origin clas inst_tyvars inst_tyvars' dfun_theta' inst_tys'
432 avail_insts op_items (VanillaInst monobinds uprags)
433 = -- Check that all the method bindings come from this class
435 sel_names = [idName sel_id | (sel_id, _) <- op_items]
436 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
438 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
440 -- Make the method bindings
442 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
444 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
446 -- And type check them
447 -- It's really worth making meth_insts available to the tcMethodBind
448 -- Consider instance Monad (ST s) where
449 -- {-# INLINE (>>) #-}
450 -- (>>) = ...(>>=)...
451 -- If we don't include meth_insts, we end up with bindings like this:
452 -- rec { dict = MkD then bind ...
453 -- then = inline_me (... (GHC.Base.>>= dict) ...)
455 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
456 -- and (b) the inline_me prevents us inlining the >>= selector, which
457 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
458 -- is not inlined across modules. Rather ironic since this does not
459 -- happen without the INLINE pragma!
461 -- Solution: make meth_insts available, so that 'then' refers directly
462 -- to the local 'bind' rather than going via the dictionary.
464 -- BUT WATCH OUT! If the method type mentions the class variable, then
465 -- this optimisation is not right. Consider
469 -- instance C Int where
471 -- The occurrence of 'op' on the rhs gives rise to a constraint
473 -- The trouble is that the 'meth_inst' for op, which is 'available', also
474 -- looks like 'op at Int'. But they are not the same.
476 all_insts = avail_insts ++ catMaybes meth_insts
477 xtve = inst_tyvars `zip` inst_tyvars'
478 tc_method_bind = tcMethodBind xtve inst_tyvars' dfun_theta' all_insts uprags
479 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
482 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
484 returnM (meth_ids, unionManyBags meth_binds_s)
487 -- Derived newtype instances
488 tcMethods origin clas inst_tyvars inst_tyvars' dfun_theta' inst_tys'
489 avail_insts op_items (NewTypeDerived rep_tys)
490 = getInstLoc origin `thenM` \ inst_loc ->
491 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
494 (ptext SLIT("newtype derived instance"))
495 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
497 -- I don't think we have to do the checkSigTyVars thing
499 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
502 do_one inst_loc (sel_id, _)
503 = -- The binding is like "op @ NewTy = op @ RepTy"
504 -- Make the *binder*, like in mkMethodBind
505 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
507 -- Make the *occurrence on the rhs*
508 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
510 meth_id = instToId meth_inst
512 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
514 -- Instantiate rep_tys with the relevant type variables
515 rep_tys' = substTys subst rep_tys
516 subst = zipTvSubst inst_tyvars (mkTyVarTys inst_tyvars')
519 Note: [Superclass loops]
520 ~~~~~~~~~~~~~~~~~~~~~~~~~
521 We have to be very, very careful when generating superclasses, lest we
522 accidentally build a loop. Here's an example:
526 class S a => C a where { opc :: a -> a }
527 class S b => D b where { opd :: b -> b }
535 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
536 Simplifying, we may well get:
537 $dfCInt = :C ds1 (opd dd)
540 Notice that we spot that we can extract ds1 from dd.
542 Alas! Alack! We can do the same for (instance D Int):
544 $dfDInt = :D ds2 (opc dc)
548 And now we've defined the superclass in terms of itself.
551 Solution: treat the superclass context separately, and simplify it
552 all the way down to nothing on its own. Don't toss any 'free' parts
553 out to be simplified together with other bits of context.
554 Hence the tcSimplifyTop below.
556 At a more basic level, don't include this_dict in the context wrt
557 which we simplify sc_dicts, else sc_dicts get bound by just selecting
561 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
562 = addErrCtxt superClassCtxt $
563 getLIE (tcSimplifyCheck doc inst_tyvars'
565 sc_dicts) `thenM` \ (sc_binds1, sc_lie) ->
567 -- We must simplify this all the way down
568 -- lest we build superclass loops
569 -- See Note [Superclass loops] above
570 tcSimplifyTop sc_lie `thenM` \ sc_binds2 ->
572 returnM (sc_binds1, sc_binds2)
575 doc = ptext SLIT("instance declaration superclass context")
579 ------------------------------
580 [Inline dfuns] Inlining dfuns unconditionally
581 ------------------------------
583 The code above unconditionally inlines dict funs. Here's why.
584 Consider this program:
586 test :: Int -> Int -> Bool
587 test x y = (x,y) == (y,x) || test y x
588 -- Recursive to avoid making it inline.
590 This needs the (Eq (Int,Int)) instance. If we inline that dfun
591 the code we end up with is good:
594 \r -> case ==# [ww ww1] of wild {
595 PrelBase.False -> Test.$wtest ww1 ww;
597 case ==# [ww1 ww] of wild1 {
598 PrelBase.False -> Test.$wtest ww1 ww;
599 PrelBase.True -> PrelBase.True [];
602 Test.test = \r [w w1]
605 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
608 If we don't inline the dfun, the code is not nearly as good:
610 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
611 PrelBase.:DEq tpl1 tpl2 -> tpl2;
616 let { y = PrelBase.I#! [ww1]; } in
617 let { x = PrelBase.I#! [ww]; } in
618 let { sat_slx = PrelTup.(,)! [y x]; } in
619 let { sat_sly = PrelTup.(,)! [x y];
621 case == sat_sly sat_slx of wild {
622 PrelBase.False -> Test.$wtest ww1 ww;
623 PrelBase.True -> PrelBase.True [];
630 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
633 Why doesn't GHC inline $fEq? Because it looks big:
635 PrelTup.zdfEqZ1T{-rcX-}
636 = \ @ a{-reT-} :: * @ b{-reS-} :: *
637 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
638 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
640 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
641 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
643 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
644 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
646 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
647 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
648 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
650 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
652 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
654 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
655 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
659 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
660 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
661 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
662 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
664 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
666 and it's not as bad as it seems, because it's further dramatically
667 simplified: only zeze2 is extracted and its body is simplified.
670 %************************************************************************
672 \subsection{Error messages}
674 %************************************************************************
677 instDeclCtxt1 hs_inst_ty
678 = inst_decl_ctxt (case unLoc hs_inst_ty of
679 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
680 HsPredTy pred -> ppr pred
681 other -> ppr hs_inst_ty) -- Don't expect this
682 instDeclCtxt2 dfun_ty
683 = inst_decl_ctxt (ppr (mkClassPred cls tys))
685 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
687 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
689 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")