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 ( tcSkolSigType, checkValidTheta, checkValidInstHead, instTypeErr,
17 checkAmbiguity, SourceTyCtxt(..) )
18 import TcType ( mkClassPred, tyVarsOfType,
19 tcSplitSigmaTy, tcSplitDFunHead, mkTyVarTys,
20 SkolemInfo(InstSkol), tcSplitDFunTy, pprClassPred )
21 import Inst ( tcInstClassOp, newDicts, instToId, showLIE, tcExtendLocalInstEnv )
22 import TcDeriv ( tcDeriving )
23 import TcEnv ( tcExtendGlobalValEnv, tcExtendTyVarEnv,
24 InstInfo(..), InstBindings(..),
25 newDFunName, tcExtendIdEnv
27 import TcHsType ( kcHsSigType, tcHsKindedType )
28 import TcUnify ( checkSigTyVars )
29 import TcSimplify ( tcSimplifyCheck, tcSimplifyTop )
30 import Type ( zipOpenTvSubst, 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 )
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 -- Instantiate the instance decl with skolem constants
319 rigid_info = InstSkol dfun_id
320 inst_ty = idType dfun_id
322 tcSkolSigType rigid_info inst_ty `thenM` \ (inst_tyvars', dfun_theta', inst_head') ->
323 -- These inst_tyvars' scope over the 'where' part
324 -- Those tyvars are inside the dfun_id's type, which is a bit
325 -- bizarre, but OK so long as you realise it!
327 (clas, inst_tys') = tcSplitDFunHead inst_head'
328 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
330 -- Instantiate the super-class context with inst_tys
331 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
332 origin = SigOrigin rigid_info
334 -- Create dictionary Ids from the specified instance contexts.
335 newDicts InstScOrigin sc_theta' `thenM` \ sc_dicts ->
336 newDicts origin dfun_theta' `thenM` \ dfun_arg_dicts ->
337 newDicts origin [mkClassPred clas inst_tys'] `thenM` \ [this_dict] ->
338 -- Default-method Ids may be mentioned in synthesised RHSs,
339 -- but they'll already be in the environment.
342 -- Typecheck the methods
343 let -- These insts are in scope; quite a few, eh?
344 avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
346 tcMethods origin clas inst_tyvars'
347 dfun_theta' inst_tys' avail_insts
348 op_items binds `thenM` \ (meth_ids, meth_binds) ->
350 -- Figure out bindings for the superclass context
351 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
352 `thenM` \ (sc_binds_inner, sc_binds_outer) ->
354 -- It's possible that the superclass stuff might have done unification
355 checkSigTyVars inst_tyvars' `thenM_`
357 -- Deal with 'SPECIALISE instance' pragmas by making them
358 -- look like SPECIALISE pragmas for the dfun
360 uprags = case binds of
361 VanillaInst _ uprags -> uprags
363 spec_prags = [ L loc (SpecSig (L loc (idName dfun_id)) ty)
364 | L loc (SpecInstSig ty) <- uprags ]
366 tcExtendGlobalValEnv [dfun_id] (
367 tcExtendTyVarEnv inst_tyvars' $
368 tcSpecSigs spec_prags
369 ) `thenM` \ prag_binds ->
371 -- Create the result bindings
373 dict_constr = classDataCon clas
374 scs_and_meths = map instToId sc_dicts ++ meth_ids
375 this_dict_id = instToId this_dict
376 inlines | null dfun_arg_dicts = emptyNameSet
377 | otherwise = unitNameSet (idName dfun_id)
378 -- Always inline the dfun; this is an experimental decision
379 -- because it makes a big performance difference sometimes.
380 -- Often it means we can do the method selection, and then
381 -- inline the method as well. Marcin's idea; see comments below.
383 -- BUT: don't inline it if it's a constant dictionary;
384 -- we'll get all the benefit without inlining, and we get
385 -- a **lot** of code duplication if we inline it
387 -- See Note [Inline dfuns] below
391 = -- Blatant special case for CCallable, CReturnable
392 -- If the dictionary is empty then we should never
393 -- select anything from it, so we make its RHS just
394 -- emit an error message. This in turn means that we don't
395 -- mention the constructor, which doesn't exist for CCallable, CReturnable
396 -- Hardly beautiful, but only three extra lines.
397 nlHsApp (noLoc $ TyApp (nlHsVar rUNTIME_ERROR_ID)
398 [idType this_dict_id])
399 (nlHsLit (HsStringPrim (mkFastString (stringToUtf8 msg))))
401 | otherwise -- The common case
402 = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
403 -- We don't produce a binding for the dict_constr; instead we
404 -- rely on the simplifier to unfold this saturated application
405 -- We do this rather than generate an HsCon directly, because
406 -- it means that the special cases (e.g. dictionary with only one
407 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
408 -- than needing to be repeated here.
411 msg = "Compiler error: bad dictionary " ++ showSDoc (ppr clas)
413 dict_bind = noLoc (VarBind this_dict_id dict_rhs)
414 all_binds = dict_bind `consBag` (sc_binds_inner `unionBags` meth_binds)
416 main_bind = noLoc $ AbsBinds
418 (map instToId dfun_arg_dicts)
419 [(inst_tyvars', dfun_id, this_dict_id)]
422 showLIE (text "instance") `thenM_`
423 returnM (unitBag main_bind `unionBags`
424 prag_binds `unionBags`
428 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
429 avail_insts op_items (VanillaInst monobinds uprags)
430 = -- Check that all the method bindings come from this class
432 sel_names = [idName sel_id | (sel_id, _) <- op_items]
433 bad_bndrs = collectHsBindBinders monobinds `minusList` sel_names
435 mappM (addErrTc . badMethodErr clas) bad_bndrs `thenM_`
437 -- Make the method bindings
439 mk_method_bind = mkMethodBind origin clas inst_tys' monobinds
441 mapAndUnzipM mk_method_bind op_items `thenM` \ (meth_insts, meth_infos) ->
443 -- And type check them
444 -- It's really worth making meth_insts available to the tcMethodBind
445 -- Consider instance Monad (ST s) where
446 -- {-# INLINE (>>) #-}
447 -- (>>) = ...(>>=)...
448 -- If we don't include meth_insts, we end up with bindings like this:
449 -- rec { dict = MkD then bind ...
450 -- then = inline_me (... (GHC.Base.>>= dict) ...)
452 -- The trouble is that (a) 'then' and 'dict' are mutually recursive,
453 -- and (b) the inline_me prevents us inlining the >>= selector, which
454 -- would unravel the loop. Result: (>>) ends up as a loop breaker, and
455 -- is not inlined across modules. Rather ironic since this does not
456 -- happen without the INLINE pragma!
458 -- Solution: make meth_insts available, so that 'then' refers directly
459 -- to the local 'bind' rather than going via the dictionary.
461 -- BUT WATCH OUT! If the method type mentions the class variable, then
462 -- this optimisation is not right. Consider
466 -- instance C Int where
468 -- The occurrence of 'op' on the rhs gives rise to a constraint
470 -- The trouble is that the 'meth_inst' for op, which is 'available', also
471 -- looks like 'op at Int'. But they are not the same.
473 all_insts = avail_insts ++ catMaybes meth_insts
474 tc_method_bind = tcMethodBind inst_tyvars' dfun_theta' all_insts uprags
475 meth_ids = [meth_id | (_,meth_id,_) <- meth_infos]
478 mapM tc_method_bind meth_infos `thenM` \ meth_binds_s ->
480 returnM (meth_ids, unionManyBags meth_binds_s)
483 -- Derived newtype instances
484 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys'
485 avail_insts op_items (NewTypeDerived rep_tys)
486 = getInstLoc origin `thenM` \ inst_loc ->
487 mapAndUnzip3M (do_one inst_loc) op_items `thenM` \ (meth_ids, meth_binds, rhs_insts) ->
490 (ptext SLIT("newtype derived instance"))
491 inst_tyvars' avail_insts rhs_insts `thenM` \ lie_binds ->
493 -- I don't think we have to do the checkSigTyVars thing
495 returnM (meth_ids, lie_binds `unionBags` listToBag meth_binds)
498 do_one inst_loc (sel_id, _)
499 = -- The binding is like "op @ NewTy = op @ RepTy"
500 -- Make the *binder*, like in mkMethodBind
501 tcInstClassOp inst_loc sel_id inst_tys' `thenM` \ meth_inst ->
503 -- Make the *occurrence on the rhs*
504 tcInstClassOp inst_loc sel_id rep_tys' `thenM` \ rhs_inst ->
506 meth_id = instToId meth_inst
508 return (meth_id, noLoc (VarBind meth_id (nlHsVar (instToId rhs_inst))), rhs_inst)
510 -- Instantiate rep_tys with the relevant type variables
511 -- This looks a bit odd, because inst_tyvars' are the skolemised version
512 -- of the type variables in the instance declaration; but rep_tys doesn't
513 -- have the skolemised version, so we substitute them in here
514 rep_tys' = substTys subst rep_tys
515 subst = zipOpenTvSubst inst_tyvars' (mkTyVarTys inst_tyvars')
518 Note: [Superclass loops]
519 ~~~~~~~~~~~~~~~~~~~~~~~~~
520 We have to be very, very careful when generating superclasses, lest we
521 accidentally build a loop. Here's an example:
525 class S a => C a where { opc :: a -> a }
526 class S b => D b where { opd :: b -> b }
534 From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
535 Simplifying, we may well get:
536 $dfCInt = :C ds1 (opd dd)
539 Notice that we spot that we can extract ds1 from dd.
541 Alas! Alack! We can do the same for (instance D Int):
543 $dfDInt = :D ds2 (opc dc)
547 And now we've defined the superclass in terms of itself.
550 Solution: treat the superclass context separately, and simplify it
551 all the way down to nothing on its own. Don't toss any 'free' parts
552 out to be simplified together with other bits of context.
553 Hence the tcSimplifyTop below.
555 At a more basic level, don't include this_dict in the context wrt
556 which we simplify sc_dicts, else sc_dicts get bound by just selecting
560 tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
561 = addErrCtxt superClassCtxt $
562 getLIE (tcSimplifyCheck doc inst_tyvars'
564 sc_dicts) `thenM` \ (sc_binds1, sc_lie) ->
566 -- We must simplify this all the way down
567 -- lest we build superclass loops
568 -- See Note [Superclass loops] above
569 tcSimplifyTop sc_lie `thenM` \ sc_binds2 ->
571 returnM (sc_binds1, sc_binds2)
574 doc = ptext SLIT("instance declaration superclass context")
578 ------------------------------
579 [Inline dfuns] Inlining dfuns unconditionally
580 ------------------------------
582 The code above unconditionally inlines dict funs. Here's why.
583 Consider this program:
585 test :: Int -> Int -> Bool
586 test x y = (x,y) == (y,x) || test y x
587 -- Recursive to avoid making it inline.
589 This needs the (Eq (Int,Int)) instance. If we inline that dfun
590 the code we end up with is good:
593 \r -> case ==# [ww ww1] of wild {
594 PrelBase.False -> Test.$wtest ww1 ww;
596 case ==# [ww1 ww] of wild1 {
597 PrelBase.False -> Test.$wtest ww1 ww;
598 PrelBase.True -> PrelBase.True [];
601 Test.test = \r [w w1]
604 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
607 If we don't inline the dfun, the code is not nearly as good:
609 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
610 PrelBase.:DEq tpl1 tpl2 -> tpl2;
615 let { y = PrelBase.I#! [ww1]; } in
616 let { x = PrelBase.I#! [ww]; } in
617 let { sat_slx = PrelTup.(,)! [y x]; } in
618 let { sat_sly = PrelTup.(,)! [x y];
620 case == sat_sly sat_slx of wild {
621 PrelBase.False -> Test.$wtest ww1 ww;
622 PrelBase.True -> PrelBase.True [];
629 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
632 Why doesn't GHC inline $fEq? Because it looks big:
634 PrelTup.zdfEqZ1T{-rcX-}
635 = \ @ a{-reT-} :: * @ b{-reS-} :: *
636 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
637 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
639 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
640 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
642 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
643 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
645 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
646 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
647 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
649 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
651 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
653 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
654 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
658 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
659 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
660 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
661 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
663 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
665 and it's not as bad as it seems, because it's further dramatically
666 simplified: only zeze2 is extracted and its body is simplified.
669 %************************************************************************
671 \subsection{Error messages}
673 %************************************************************************
676 instDeclCtxt1 hs_inst_ty
677 = inst_decl_ctxt (case unLoc hs_inst_ty of
678 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
679 HsPredTy pred -> ppr pred
680 other -> ppr hs_inst_ty) -- Don't expect this
681 instDeclCtxt2 dfun_ty
682 = inst_decl_ctxt (ppr (mkClassPred cls tys))
684 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
686 inst_decl_ctxt doc = ptext SLIT("In the instance declaration for") <+> quotes doc
688 superClassCtxt = ptext SLIT("When checking the super-classes of an instance declaration")