2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 TcInstDecls: Typechecking instance declarations
9 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
15 import TcPat( addInlinePrags )
23 import MkCore ( nO_METHOD_BINDING_ERROR_ID )
26 import RnSource ( addTcgDUs )
36 import CoreUtils ( mkPiTypes )
37 import CoreUnfold ( mkDFunUnfolding )
38 import CoreSyn ( Expr(Var), DFunArg(..), CoreExpr )
51 import Maybes ( orElse )
56 #include "HsVersions.h"
59 Typechecking instance declarations is done in two passes. The first
60 pass, made by @tcInstDecls1@, collects information to be used in the
63 This pre-processed info includes the as-yet-unprocessed bindings
64 inside the instance declaration. These are type-checked in the second
65 pass, when the class-instance envs and GVE contain all the info from
66 all the instance and value decls. Indeed that's the reason we need
67 two passes over the instance decls.
70 Note [How instance declarations are translated]
71 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
72 Here is how we translation instance declarations into Core
76 op1, op2 :: Ix b => a -> b -> b
80 {-# INLINE [2] op1 #-}
84 op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b
88 -- Default methods get the 'self' dictionary as argument
89 -- so they can call other methods at the same type
90 -- Default methods get the same type as their method selector
91 $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b
92 $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). <dm-rhs>
93 -- NB: type variables 'a' and 'b' are *both* in scope in <dm-rhs>
94 -- Note [Tricky type variable scoping]
96 -- A top-level definition for each instance method
97 -- Here op1_i, op2_i are the "instance method Ids"
98 -- The INLINE pragma comes from the user pragma
99 {-# INLINE [2] op1_i #-} -- From the instance decl bindings
100 op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
101 op1_i = /\a. \(d:C a).
104 -- Note [Subtle interaction of recursion and overlap]
106 local_op1 :: forall b. Ix b => [a] -> b -> b
108 -- Source code; run the type checker on this
109 -- NB: Type variable 'a' (but not 'b') is in scope in <rhs>
110 -- Note [Tricky type variable scoping]
114 op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
116 -- The dictionary function itself
117 {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions
118 df_i :: forall a. C a -> C [a]
119 df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
120 -- But see Note [Default methods in instances]
121 -- We can't apply the type checker to the default-method call
123 -- Use a RULE to short-circuit applications of the class ops
124 {-# RULE "op1@C[a]" forall a, d:C a.
125 op1 [a] (df_i d) = op1_i a d #-}
127 Note [Instances and loop breakers]
128 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
129 * Note that df_i may be mutually recursive with both op1_i and op2_i.
130 It's crucial that df_i is not chosen as the loop breaker, even
131 though op1_i has a (user-specified) INLINE pragma.
133 * Instead the idea is to inline df_i into op1_i, which may then select
134 methods from the MkC record, and thereby break the recursion with
135 df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at
136 the same type, it won't mention df_i, so there won't be recursion in
139 * If op1_i is marked INLINE by the user there's a danger that we won't
140 inline df_i in it, and that in turn means that (since it'll be a
141 loop-breaker because df_i isn't), op1_i will ironically never be
142 inlined. But this is OK: the recursion breaking happens by way of
143 a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
144 unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
146 Note [ClassOp/DFun selection]
147 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
148 One thing we see a lot is stuff like
150 where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both*
151 'op2' and 'df' to get
152 case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
153 MkD _ op2 _ _ _ -> op2
154 And that will reduce to ($cop2 d1 d2) which is what we wanted.
156 But it's tricky to make this work in practice, because it requires us to
157 inline both 'op2' and 'df'. But neither is keen to inline without having
158 seen the other's result; and it's very easy to get code bloat (from the
159 big intermediate) if you inline a bit too much.
161 Instead we use a cunning trick.
162 * We arrange that 'df' and 'op2' NEVER inline.
164 * We arrange that 'df' is ALWAYS defined in the sylised form
165 df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
167 * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
168 that lists its methods.
170 * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return
171 a suitable constructor application -- inlining df "on the fly" as it
174 * We give the ClassOp 'op2' a BuiltinRule that extracts the right piece
175 iff its argument satisfies exprIsConApp_maybe. This is done in
178 * We make 'df' CONLIKE, so that shared uses stil match; eg
180 in ...(op2 d)...(op1 d)...
182 Note [Single-method classes]
183 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
184 If the class has just one method (or, more accurately, just one element
185 of {superclasses + methods}), then we use a different strategy.
187 class C a where op :: a -> a
188 instance C a => C [a] where op = <blah>
190 We translate the class decl into a newtype, which just gives a
191 top-level axiom. The "constructor" MkC expands to a cast, as does the
194 axiom Co:C a :: C a ~ (a->a)
196 op :: forall a. C a -> (a -> a)
197 op a d = d |> (Co:C a)
199 MkC :: forall a. (a->a) -> C a
200 MkC = /\a.\op. op |> (sym Co:C a)
202 The clever RULE stuff doesn't work now, because ($df a d) isn't
203 a constructor application, so exprIsConApp_maybe won't return
206 Instead, we simply rely on the fact that casts are cheap:
208 $df :: forall a. C a => C [a]
209 {-# INLINE df #-} -- NB: INLINE this
210 $df = /\a. \d. MkC [a] ($cop_list a d)
211 = $cop_list |> forall a. C a -> (sym (Co:C [a]))
213 $cop_list :: forall a. C a => [a] -> [a]
218 we'll inline 'op' and '$df', since both are simply casts, and
221 Why do we use this different strategy? Because otherwise we
222 end up with non-inlined dictionaries that look like
224 which adds an extra indirection to every use, which seems stupid. See
225 Trac #4138 for an example (although the regression reported there
226 wasn't due to the indirction).
228 There is an awkward wrinkle though: we want to be very
230 instance C a => C [a] where
233 then we'll get an INLINE pragma on $cop_list but it's important that
234 $cop_list only inlines when it's applied to *two* arguments (the
235 dictionary and the list argument). So we nust not eta-expand $df
236 above. We ensure that this doesn't happen by putting an INLINE
237 pragma on the dfun itself; after all, it ends up being just a cast.
239 There is one more dark corner to the INLINE story, even more deeply
240 buried. Consider this (Trac #3772):
242 class DeepSeq a => C a where
245 instance C a => C [a] where
248 class DeepSeq a where
249 deepSeq :: a -> b -> b
251 instance DeepSeq a => DeepSeq [a] where
252 {-# INLINE deepSeq #-}
253 deepSeq xs b = foldr deepSeq b xs
255 That gives rise to these defns:
257 $cdeepSeq :: DeepSeq a -> [a] -> b -> b
258 -- User INLINE( 3 args )!
259 $cdeepSeq a (d:DS a) b (x:[a]) (y:b) = ...
261 $fDeepSeq[] :: DeepSeq a -> DeepSeq [a]
262 -- DFun (with auto INLINE pragma)
263 $fDeepSeq[] a d = $cdeepSeq a d |> blah
265 $cp1 a d :: C a => DeepSep [a]
266 -- We don't want to eta-expand this, lest
267 -- $cdeepSeq gets inlined in it!
268 $cp1 a d = $fDeepSep[] a (scsel a d)
270 $fC[] :: C a => C [a]
272 $fC[] a d = MkC ($cp1 a d) ($cgen a d)
274 Here $cp1 is the code that generates the superclass for C [a]. The
275 issue is this: we must not eta-expand $cp1 either, or else $fDeepSeq[]
276 and then $cdeepSeq will inline there, which is definitely wrong. Like
277 on the dfun, we solve this by adding an INLINE pragma to $cp1.
279 Note [Subtle interaction of recursion and overlap]
280 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
282 class C a where { op1,op2 :: a -> a }
283 instance C a => C [a] where
284 op1 x = op2 x ++ op2 x
286 instance C [Int] where
289 When type-checking the C [a] instance, we need a C [a] dictionary (for
290 the call of op2). If we look up in the instance environment, we find
291 an overlap. And in *general* the right thing is to complain (see Note
292 [Overlapping instances] in InstEnv). But in *this* case it's wrong to
293 complain, because we just want to delegate to the op2 of this same
296 Why is this justified? Because we generate a (C [a]) constraint in
297 a context in which 'a' cannot be instantiated to anything that matches
298 other overlapping instances, or else we would not be excecuting this
299 version of op1 in the first place.
301 It might even be a bit disguised:
303 nullFail :: C [a] => [a] -> [a]
304 nullFail x = op2 x ++ op2 x
306 instance C a => C [a] where
309 Precisely this is used in package 'regex-base', module Context.hs.
310 See the overlapping instances for RegexContext, and the fact that they
311 call 'nullFail' just like the example above. The DoCon package also
312 does the same thing; it shows up in module Fraction.hs
314 Conclusion: when typechecking the methods in a C [a] instance, we want to
315 treat the 'a' as an *existential* type variable, in the sense described
316 by Note [Binding when looking up instances]. That is why isOverlappableTyVar
317 responds True to an InstSkol, which is the kind of skolem we use in
321 Note [Tricky type variable scoping]
322 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
325 op1, op2 :: Ix b => a -> b -> b
328 instance C a => C [a]
329 {-# INLINE [2] op1 #-}
332 note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
333 in scope in <rhs>. In particular, we must make sure that 'b' is in
334 scope when typechecking <dm-rhs>. This is achieved by subFunTys,
335 which brings appropriate tyvars into scope. This happens for both
336 <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
337 complained if 'b' is mentioned in <rhs>.
341 %************************************************************************
343 \subsection{Extracting instance decls}
345 %************************************************************************
347 Gather up the instance declarations from their various sources
350 tcInstDecls1 -- Deal with both source-code and imported instance decls
351 :: [LTyClDecl Name] -- For deriving stuff
352 -> [LInstDecl Name] -- Source code instance decls
353 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
354 -> TcM (TcGblEnv, -- The full inst env
355 [InstInfo Name], -- Source-code instance decls to process;
356 -- contains all dfuns for this module
357 HsValBinds Name) -- Supporting bindings for derived instances
359 tcInstDecls1 tycl_decls inst_decls deriv_decls
361 do { -- Stop if addInstInfos etc discovers any errors
362 -- (they recover, so that we get more than one error each
365 -- (1) Do class and family instance declarations
366 ; idx_tycons <- mapAndRecoverM (tcFamInstDecl TopLevel) $
367 filter (isFamInstDecl . unLoc) tycl_decls
368 ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_decls
371 at_tycons_s) = unzip local_info_tycons
372 ; at_idx_tycons = concat at_tycons_s ++ idx_tycons
373 ; clas_decls = filter (isClassDecl . unLoc) tycl_decls
374 ; implicit_things = concatMap implicitTyThings at_idx_tycons
375 ; aux_binds = mkRecSelBinds at_idx_tycons
378 -- (2) Add the tycons of indexed types and their implicit
379 -- tythings to the global environment
380 ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do {
383 -- Next, construct the instance environment so far, consisting
385 -- (a) local instance decls
386 -- (b) local family instance decls
387 ; addInsts local_info $
388 addFamInsts at_idx_tycons $ do {
390 -- (3) Compute instances from "deriving" clauses;
391 -- This stuff computes a context for the derived instance
392 -- decl, so it needs to know about all the instances possible
393 -- NB: class instance declarations can contain derivings as
394 -- part of associated data type declarations
395 failIfErrsM -- If the addInsts stuff gave any errors, don't
396 -- try the deriving stuff, because that may give
398 ; (deriv_inst_info, deriv_binds, deriv_dus, deriv_tys, deriv_ty_insts)
399 <- tcDeriving tycl_decls inst_decls deriv_decls
401 -- Extend the global environment also with the generated datatypes for
402 -- the generic representation
403 ; gbl_env <- addFamInsts (map ATyCon deriv_ty_insts) $
404 tcExtendGlobalEnv (map ATyCon (deriv_tys ++ deriv_ty_insts)) $
405 addInsts deriv_inst_info getGblEnv
406 ; return ( addTcgDUs gbl_env deriv_dus,
407 deriv_inst_info ++ local_info,
408 aux_binds `plusHsValBinds` deriv_binds)
411 addInsts :: [InstInfo Name] -> TcM a -> TcM a
412 addInsts infos thing_inside
413 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
415 addFamInsts :: [TyThing] -> TcM a -> TcM a
416 addFamInsts tycons thing_inside
417 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
419 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
420 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
425 tcLocalInstDecl1 :: LInstDecl Name
426 -> TcM (InstInfo Name, [TyThing])
427 -- A source-file instance declaration
428 -- Type-check all the stuff before the "where"
430 -- We check for respectable instance type, and context
431 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
433 addErrCtxt (instDeclCtxt1 poly_ty) $
435 do { is_boot <- tcIsHsBoot
436 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
439 ; (tyvars, theta, clas, inst_tys) <- tcHsInstHead poly_ty
440 ; checkValidInstance poly_ty tyvars theta clas inst_tys
442 -- Next, process any associated types.
443 ; idx_tycons <- recoverM (return []) $
444 do { idx_tycons <- checkNoErrs $
445 mapAndRecoverM (tcFamInstDecl NotTopLevel) ats
446 ; checkValidAndMissingATs clas (tyvars, inst_tys)
448 ; return idx_tycons }
450 -- Finally, construct the Core representation of the instance.
451 -- (This no longer includes the associated types.)
452 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
453 -- Dfun location is that of instance *header*
454 ; overlap_flag <- getOverlapFlag
455 ; let (eq_theta,dict_theta) = partition isEqPred theta
456 theta' = eq_theta ++ dict_theta
457 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
458 ispec = mkLocalInstance dfun overlap_flag
460 ; return (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags False },
464 -- We pass in the source form and the type checked form of the ATs. We
465 -- really need the source form only to be able to produce more informative
467 checkValidAndMissingATs :: Class
468 -> ([TyVar], [TcType]) -- instance types
469 -> [(LTyClDecl Name, -- source form of AT
470 TyThing)] -- Core form of AT
472 checkValidAndMissingATs clas inst_tys ats
473 = do { -- Issue a warning for each class AT that is not defined in this
475 ; let class_ats = map tyConName (classATs clas)
476 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
477 omitted = filterOut (`elemNameSet` defined_ats) class_ats
478 ; warn <- doptM Opt_WarnMissingMethods
479 ; mapM_ (warnTc warn . omittedATWarn) omitted
481 -- Ensure that all AT indexes that correspond to class parameters
482 -- coincide with the types in the instance head. All remaining
483 -- AT arguments must be variables. Also raise an error for any
484 -- type instances that are not associated with this class.
485 ; mapM_ (checkIndexes clas inst_tys) ats
488 checkIndexes clas inst_tys (hsAT, ATyCon tycon)
489 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
490 = checkIndexes' clas inst_tys hsAT
492 snd . fromJust . tyConFamInst_maybe $ tycon)
493 checkIndexes _ _ _ = panic "checkIndexes"
495 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
496 = let atName = tcdName . unLoc $ hsAT
498 setSrcSpan (getLoc hsAT) $
499 addErrCtxt (atInstCtxt atName) $
500 case find ((atName ==) . tyConName) (classATs clas) of
501 Nothing -> addErrTc $ badATErr clas atName -- not in this class
503 -- The following is tricky! We need to deal with three
504 -- complications: (1) The AT possibly only uses a subset of
505 -- the class parameters as indexes and those it uses may be in
506 -- a different order; (2) the AT may have extra arguments,
507 -- which must be type variables; and (3) variables in AT and
508 -- instance head will be different `Name's even if their
509 -- source lexemes are identical.
511 -- e.g. class C a b c where
512 -- data D b a :: * -> * -- NB (1) b a, omits c
513 -- instance C [x] Bool Char where
514 -- data D Bool [x] v = MkD x [v] -- NB (2) v
515 -- -- NB (3) the x in 'instance C...' have differnt
516 -- -- Names to x's in 'data D...'
518 -- Re (1), `poss' contains a permutation vector to extract the
519 -- class parameters in the right order.
521 -- Re (2), we wrap the (permuted) class parameters in a Maybe
522 -- type and use Nothing for any extra AT arguments. (First
523 -- equation of `checkIndex' below.)
525 -- Re (3), we replace any type variable in the AT parameters
526 -- that has the same source lexeme as some variable in the
527 -- instance types with the instance type variable sharing its
531 -- For *associated* type families, gives the position
532 -- of that 'TyVar' in the class argument list (0-indexed)
533 -- e.g. class C a b c where { type F c a :: *->* }
534 -- Then we get Just [2,0]
535 poss = catMaybes [ tv `elemIndex` classTyVars clas
536 | tv <- tyConTyVars atycon]
537 -- We will get Nothings for the "extra" type
538 -- variables in an associated data type
539 -- e.g. class C a where { data D a :: *->* }
540 -- here D gets arity 2 and has two tyvars
542 relevantInstTys = map (instTys !!) poss
543 instArgs = map Just relevantInstTys ++
544 repeat Nothing -- extra arguments
545 renaming = substSameTyVar atTvs instTvs
547 zipWithM_ checkIndex (substTys renaming atTys) instArgs
549 checkIndex ty Nothing
550 | isTyVarTy ty = return ()
551 | otherwise = addErrTc $ mustBeVarArgErr ty
552 checkIndex ty (Just instTy)
553 | ty `tcEqType` instTy = return ()
554 | otherwise = addErrTc $ wrongATArgErr ty instTy
556 listToNameSet = addListToNameSet emptyNameSet
558 substSameTyVar [] _ = emptyTvSubst
559 substSameTyVar (tv:tvs) replacingTvs =
560 let replacement = case find (tv `sameLexeme`) replacingTvs of
561 Nothing -> mkTyVarTy tv
562 Just rtv -> mkTyVarTy rtv
564 tv1 `sameLexeme` tv2 =
565 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
567 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
571 %************************************************************************
573 Type-checking instance declarations, pass 2
575 %************************************************************************
578 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
580 -- (a) From each class declaration,
581 -- generate any default-method bindings
582 -- (b) From each instance decl
583 -- generate the dfun binding
585 tcInstDecls2 tycl_decls inst_decls
586 = do { -- (a) Default methods from class decls
587 let class_decls = filter (isClassDecl . unLoc) tycl_decls
588 ; dm_binds_s <- mapM tcClassDecl2 class_decls
589 ; let dm_binds = unionManyBags dm_binds_s
591 -- (b) instance declarations
592 ; let dm_ids = collectHsBindsBinders dm_binds
593 -- Add the default method Ids (again)
594 -- See Note [Default methods and instances]
595 ; inst_binds_s <- tcExtendIdEnv dm_ids $
596 mapM tcInstDecl2 inst_decls
599 ; return (dm_binds `unionBags` unionManyBags inst_binds_s) }
602 See Note [Default methods and instances]
603 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
604 The default method Ids are already in the type environment (see Note
605 [Default method Ids and Template Haskell] in TcTyClsDcls), BUT they
606 don't have their InlinePragmas yet. Usually that would not matter,
607 because the simplifier propagates information from binding site to
608 use. But, unusually, when compiling instance decls we *copy* the
609 INLINE pragma from the default method to the method for that
610 particular operation (see Note [INLINE and default methods] below).
612 So right here in tcInstDecl2 we must re-extend the type envt with
613 the default method Ids replete with their INLINE pragmas. Urk.
617 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
618 -- Returns a binding for the dfun
619 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
620 = recoverM (return emptyLHsBinds) $
622 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
623 do { -- Instantiate the instance decl with skolem constants
624 ; (inst_tyvars, dfun_theta, inst_head) <- tcSkolDFunType (idType dfun_id)
625 ; let (clas, inst_tys) = tcSplitDFunHead inst_head
626 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
627 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys) sc_theta
628 n_ty_args = length inst_tyvars
629 n_silent = dfunNSilent dfun_id
630 (silent_theta, orig_theta) = splitAt n_silent dfun_theta
632 ; silent_ev_vars <- mapM newSilentGiven silent_theta
633 ; orig_ev_vars <- newEvVars orig_theta
634 ; let dfun_ev_vars = silent_ev_vars ++ orig_ev_vars
636 ; (sc_dicts, sc_args)
637 <- mapAndUnzipM (tcSuperClass n_ty_args dfun_ev_vars) sc_theta'
639 -- Check that any superclasses gotten from a silent arguemnt
640 -- can be deduced from the originally-specified dfun arguments
641 ; ct_loc <- getCtLoc ScOrigin
642 ; _ <- checkConstraints skol_info inst_tyvars orig_ev_vars $
643 emitFlats $ listToBag $
644 [ mkEvVarX sc ct_loc | sc <- sc_dicts, isSilentEvVar sc ]
646 -- Deal with 'SPECIALISE instance' pragmas
647 -- See Note [SPECIALISE instance pragmas]
648 ; spec_info@(spec_inst_prags,_) <- tcSpecInstPrags dfun_id ibinds
650 -- Typecheck the methods
651 ; (meth_ids, meth_binds)
652 <- tcExtendTyVarEnv inst_tyvars $
653 -- The inst_tyvars scope over the 'where' part
654 -- Those tyvars are inside the dfun_id's type, which is a bit
655 -- bizarre, but OK so long as you realise it!
656 tcInstanceMethods dfun_id clas inst_tyvars dfun_ev_vars
660 -- Create the result bindings
661 ; self_dict <- newEvVar (ClassP clas inst_tys)
662 ; let class_tc = classTyCon clas
663 [dict_constr] = tyConDataCons class_tc
664 dict_bind = mkVarBind self_dict dict_rhs
665 dict_rhs = foldl mk_app inst_constr $
666 map HsVar sc_dicts ++ map (wrapId arg_wrapper) meth_ids
667 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys)
668 (dataConWrapId dict_constr)
669 -- We don't produce a binding for the dict_constr; instead we
670 -- rely on the simplifier to unfold this saturated application
671 -- We do this rather than generate an HsCon directly, because
672 -- it means that the special cases (e.g. dictionary with only one
673 -- member) are dealt with by the common MkId.mkDataConWrapId
674 -- code rather than needing to be repeated here.
676 mk_app :: LHsExpr Id -> HsExpr Id -> LHsExpr Id
677 mk_app fun arg = L loc (HsApp fun (L loc arg))
679 arg_wrapper = mkWpEvVarApps dfun_ev_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars)
681 -- Do not inline the dfun; instead give it a magic DFunFunfolding
682 -- See Note [ClassOp/DFun selection]
683 -- See also note [Single-method classes]
685 | isNewTyCon class_tc
686 = dfun_id `setInlinePragma` alwaysInlinePragma { inl_sat = Just 0 }
688 = dfun_id `setIdUnfolding` mkDFunUnfolding dfun_ty (sc_args ++ meth_args)
689 `setInlinePragma` dfunInlinePragma
690 meth_args = map (DFunPolyArg . Var) meth_ids
692 main_bind = AbsBinds { abs_tvs = inst_tyvars
693 , abs_ev_vars = dfun_ev_vars
694 , abs_exports = [(inst_tyvars, dfun_id_w_fun, self_dict,
695 SpecPrags spec_inst_prags)]
696 , abs_ev_binds = emptyTcEvBinds
697 , abs_binds = unitBag dict_bind }
699 ; return (unitBag (L loc main_bind) `unionBags`
700 listToBag meth_binds)
703 skol_info = InstSkol -- See Note [Subtle interaction of recursion and overlap]
704 dfun_ty = idType dfun_id
705 dfun_id = instanceDFunId ispec
706 loc = getSrcSpan dfun_id
708 ------------------------------
709 tcSuperClass :: Int -> [EvVar] -> PredType -> TcM (EvVar, DFunArg CoreExpr)
710 -- All superclasses should be either
711 -- (a) be one of the arguments to the dfun, of
712 -- (b) be a constant, soluble at top level
713 tcSuperClass n_ty_args ev_vars pred
714 | Just (ev, i) <- find n_ty_args ev_vars
715 = return (ev, DFunLamArg i)
717 = ASSERT2( isEmptyVarSet (tyVarsOfPred pred), ppr pred) -- Constant!
718 do { sc_dict <- emitWanted ScOrigin pred
719 ; return (sc_dict, DFunConstArg (Var sc_dict)) }
722 find i (ev:evs) | pred `tcEqPred` evVarPred ev = Just (ev, i)
723 | otherwise = find (i+1) evs
725 ------------------------------
726 tcSpecInstPrags :: DFunId -> InstBindings Name
727 -> TcM ([Located TcSpecPrag], PragFun)
728 tcSpecInstPrags _ (NewTypeDerived {})
729 = return ([], \_ -> [])
730 tcSpecInstPrags dfun_id (VanillaInst binds uprags _)
731 = do { spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) $
732 filter isSpecInstLSig uprags
733 -- The filter removes the pragmas for methods
734 ; return (spec_inst_prags, mkPragFun uprags binds) }
737 Note [Silent Superclass Arguments]
738 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
739 Consider the following (extreme) situation:
740 class C a => D a where ...
741 instance D [a] => D [a] where ...
742 Although this looks wrong (assume D [a] to prove D [a]), it is only a
743 more extreme case of what happens with recursive dictionaries.
745 To implement the dfun we must generate code for the superclass C [a],
746 which we can get by superclass selection from the supplied argument!
748 dfun :: forall a. D [a] -> D [a]
749 dfun = \d::D [a] -> MkD (scsel d) ..
751 However this means that if we later encounter a situation where
752 we have a [Wanted] dw::D [a] we could solve it thus:
754 Although recursive, this binding would pass the TcSMonadisGoodRecEv
755 check because it appears as guarded. But in reality, it will make a
756 bottom superclass. The trouble is that isGoodRecEv can't "see" the
757 superclass-selection inside dfun.
759 Our solution to this problem is to change the way ‘dfuns’ are created
760 for instances, so that we pass as first arguments to the dfun some
761 ``silent superclass arguments’’, which are the immediate superclasses
762 of the dictionary we are trying to construct. In our example:
763 dfun :: forall a. (C [a], D [a] -> D [a]
764 dfun = \(dc::C [a]) (dd::D [a]) -> DOrd dc ...
768 -----------------------------------------------------------
769 DFun Superclass Invariant
770 ~~~~~~~~~~~~~~~~~~~~~~~~
771 In the body of a DFun, every superclass argument to the
772 returned dictionary is
773 either * one of the arguments of the DFun,
774 or * constant, bound at top level
775 -----------------------------------------------------------
777 This means that no superclass is hidden inside a dfun application, so
778 the counting argument in isGoodRecEv (more dfun calls than superclass
779 selections) works correctly.
781 The extra arguments required to satisfy the DFun Superclass Invariant
782 always come first, and are called the "silent" arguments. DFun types
783 are built (only) by MkId.mkDictFunId, so that is where we decide
784 what silent arguments are to be added.
786 This net effect is that it is safe to treat a dfun application as
787 wrapping a dictionary constructor around its arguments (in particular,
788 a dfun never picks superclasses from the arguments under the dictionary
791 In our example, if we had [Wanted] dw :: D [a] we would get via the instance:
793 [Wanted] (d1 :: C [a])
794 [Wanted] (d2 :: D [a])
795 [Derived] (d :: D [a])
796 [Derived] (scd :: C [a]) scd := scsel d
797 [Derived] (scd2 :: C [a]) scd2 := scsel d2
799 And now, though we *can* solve:
801 we will get an isGoodRecEv failure when we try to solve:
806 Test case SCLoop tests this fix.
808 Note [SPECIALISE instance pragmas]
809 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
812 instance (Ix a, Ix b) => Ix (a,b) where
813 {-# SPECIALISE instance Ix (Int,Int) #-}
816 We do *not* want to make a specialised version of the dictionary
817 function. Rather, we want specialised versions of each method.
818 Thus we should generate something like this:
820 $dfIx :: (Ix a, Ix x) => Ix (a,b)
821 {- DFUN [$crange, ...] -}
822 $dfIx da db = Ix ($crange da db) (...other methods...)
824 $dfIxPair :: (Ix a, Ix x) => Ix (a,b)
825 {- DFUN [$crangePair, ...] -}
826 $dfIxPair = Ix ($crangePair da db) (...other methods...)
828 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
829 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
830 $crange da db = <blah>
832 {-# RULE range ($dfIx da db) = $crange da db #-}
836 * The RULE is unaffected by the specialisation. We don't want to
837 specialise $dfIx, because then it would need a specialised RULE
838 which is a pain. The single RULE works fine at all specialisations.
839 See Note [How instance declarations are translated] above
841 * Instead, we want to specialise the *method*, $crange
843 In practice, rather than faking up a SPECIALISE pragama for each
844 method (which is painful, since we'd have to figure out its
845 specialised type), we call tcSpecPrag *as if* were going to specialise
846 $dfIx -- you can see that in the call to tcSpecInst. That generates a
847 SpecPrag which, as it turns out, can be used unchanged for each method.
848 The "it turns out" bit is delicate, but it works fine!
851 tcSpecInst :: Id -> Sig Name -> TcM TcSpecPrag
852 tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
853 = addErrCtxt (spec_ctxt prag) $
854 do { let name = idName dfun_id
855 ; (tyvars, theta, clas, tys) <- tcHsInstHead hs_ty
856 ; let (_, spec_dfun_ty) = mkDictFunTy tyvars theta clas tys
858 ; co_fn <- tcSubType (SpecPragOrigin name) SpecInstCtxt
859 (idType dfun_id) spec_dfun_ty
860 ; return (SpecPrag dfun_id co_fn defaultInlinePragma) }
862 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
864 tcSpecInst _ _ = panic "tcSpecInst"
867 %************************************************************************
869 Type-checking an instance method
871 %************************************************************************
874 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
875 - Remembering to use fresh Name (the instance method Name) as the binder
876 - Bring the instance method Ids into scope, for the benefit of tcInstSig
877 - Use sig_fn mapping instance method Name -> instance tyvars
879 - Use tcValBinds to do the checking
882 tcInstanceMethods :: DFunId -> Class -> [TcTyVar]
885 -> ([Located TcSpecPrag], PragFun)
888 -> TcM ([Id], [LHsBind Id])
889 -- The returned inst_meth_ids all have types starting
890 -- forall tvs. theta => ...
891 tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys
892 (spec_inst_prags, prag_fn)
893 op_items (VanillaInst binds _ standalone_deriv)
894 = mapAndUnzipM tc_item op_items
896 ----------------------
897 tc_item :: (Id, DefMeth) -> TcM (Id, LHsBind Id)
898 tc_item (sel_id, dm_info)
899 = case findMethodBind (idName sel_id) binds of
900 Just user_bind -> tc_body sel_id standalone_deriv user_bind
901 Nothing -> tc_default sel_id dm_info
903 ----------------------
904 tc_body :: Id -> Bool -> LHsBind Name -> TcM (TcId, LHsBind Id)
905 tc_body sel_id generated_code rn_bind
906 = add_meth_ctxt sel_id generated_code rn_bind $
907 do { (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars
909 ; let prags = prag_fn (idName sel_id)
910 ; meth_id1 <- addInlinePrags meth_id prags
911 ; spec_prags <- tcSpecPrags meth_id1 prags
912 ; bind <- tcInstanceMethodBody InstSkol
914 meth_id1 local_meth_id meth_sig_fn
915 (mk_meth_spec_prags meth_id1 spec_prags)
917 ; return (meth_id1, bind) }
919 ----------------------
920 tc_default :: Id -> DefMeth -> TcM (TcId, LHsBind Id)
922 tc_default sel_id (GenDefMeth dm_name)
923 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id dm_name
924 ; tc_body sel_id False {- Not generated code? -} meth_bind }
926 tc_default sel_id GenDefMeth -- Derivable type classes stuff
927 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id
928 ; tc_body sel_id False {- Not generated code? -} meth_bind }
930 tc_default sel_id NoDefMeth -- No default method at all
931 = do { warnMissingMethod sel_id
932 ; (meth_id, _) <- mkMethIds clas tyvars dfun_ev_vars
934 ; return (meth_id, mkVarBind meth_id $
935 mkLHsWrap lam_wrapper error_rhs) }
937 error_rhs = L loc $ HsApp error_fun error_msg
938 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
939 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
940 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
941 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
942 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_ev_vars
944 tc_default sel_id (DefMeth dm_name) -- A polymorphic default method
945 = do { -- Build the typechecked version directly,
946 -- without calling typecheck_method;
947 -- see Note [Default methods in instances]
948 -- Generate /\as.\ds. let self = df as ds
949 -- in $dm inst_tys self
950 -- The 'let' is necessary only because HsSyn doesn't allow
951 -- you to apply a function to a dictionary *expression*.
953 ; self_dict <- newEvVar (ClassP clas inst_tys)
954 ; let self_ev_bind = EvBind self_dict $
955 EvDFunApp dfun_id (mkTyVarTys tyvars) dfun_ev_vars
957 ; (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars
959 ; dm_id <- tcLookupId dm_name
960 ; let dm_inline_prag = idInlinePragma dm_id
961 rhs = HsWrap (mkWpEvVarApps [self_dict] <.> mkWpTyApps inst_tys) $
964 meth_bind = L loc $ VarBind { var_id = local_meth_id
965 , var_rhs = L loc rhs
966 , var_inline = False }
967 meth_id1 = meth_id `setInlinePragma` dm_inline_prag
968 -- Copy the inline pragma (if any) from the default
969 -- method to this version. Note [INLINE and default methods]
971 bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars
972 , abs_exports = [( tyvars, meth_id1, local_meth_id
973 , mk_meth_spec_prags meth_id1 [])]
974 , abs_ev_binds = EvBinds (unitBag self_ev_bind)
975 , abs_binds = unitBag meth_bind }
976 -- Default methods in an instance declaration can't have their own
977 -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
978 -- currently they are rejected with
979 -- "INLINE pragma lacks an accompanying binding"
981 ; return (meth_id1, L loc bind) }
983 ----------------------
984 mk_meth_spec_prags :: Id -> [LTcSpecPrag] -> TcSpecPrags
985 -- Adapt the SPECIALISE pragmas to work for this method Id
986 -- There are two sources:
987 -- * spec_inst_prags: {-# SPECIALISE instance :: <blah> #-}
988 -- These ones have the dfun inside, but [perhaps surprisingly]
989 -- the correct wrapper
990 -- * spec_prags_for_me: {-# SPECIALISE op :: <blah> #-}
991 mk_meth_spec_prags meth_id spec_prags_for_me
992 = SpecPrags (spec_prags_for_me ++
993 [ L loc (SpecPrag meth_id wrap inl)
994 | L loc (SpecPrag _ wrap inl) <- spec_inst_prags])
996 loc = getSrcSpan dfun_id
997 meth_sig_fn _ = Just ([],loc) -- The 'Just' says "yes, there's a type sig"
998 -- But there are no scoped type variables from local_method_id
999 -- Only the ones from the instance decl itself, which are already
1000 -- in scope. Example:
1001 -- class C a where { op :: forall b. Eq b => ... }
1002 -- instance C [c] where { op = <rhs> }
1003 -- In <rhs>, 'c' is scope but 'b' is not!
1005 -- For instance decls that come from standalone deriving clauses
1006 -- we want to print out the full source code if there's an error
1007 -- because otherwise the user won't see the code at all
1008 add_meth_ctxt sel_id generated_code rn_bind thing
1009 | generated_code = addLandmarkErrCtxt (derivBindCtxt sel_id clas inst_tys rn_bind) thing
1013 tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys
1014 _ op_items (NewTypeDerived coi _)
1017 -- class Show b => Foo a b where
1018 -- op :: a -> b -> b
1019 -- newtype N a = MkN (Tree [a])
1020 -- deriving instance (Show p, Foo Int p) => Foo Int (N p)
1021 -- -- NB: standalone deriving clause means
1022 -- -- that the contex is user-specified
1023 -- Hence op :: forall a b. Foo a b => a -> b -> b
1025 -- We're going to make an instance like
1026 -- instance (Show p, Foo Int p) => Foo Int (N p)
1029 -- $copT :: forall p. (Show p, Foo Int p) => Int -> N p -> N p
1030 -- $copT p (d1:Show p) (d2:Foo Int p)
1031 -- = op Int (Tree [p]) rep_d |> op_co
1033 -- rep_d :: Foo Int (Tree [p]) = ...d1...d2...
1034 -- op_co :: (Int -> Tree [p] -> Tree [p]) ~ (Int -> T p -> T p)
1035 -- We get op_co by substituting [Int/a] and [co/b] in type for op
1036 -- where co : [p] ~ T p
1038 -- Notice that the dictionary bindings "..d1..d2.." must be generated
1039 -- by the constraint solver, since the <context> may be
1042 = do { rep_d_stuff <- checkConstraints InstSkol tyvars dfun_ev_vars $
1043 emitWanted ScOrigin rep_pred
1045 ; mapAndUnzipM (tc_item rep_d_stuff) op_items }
1047 loc = getSrcSpan dfun_id
1049 inst_tvs = fst (tcSplitForAllTys (idType dfun_id))
1050 Just (init_inst_tys, _) = snocView inst_tys
1051 rep_ty = fst (coercionKind co) -- [p]
1052 rep_pred = mkClassPred clas (init_inst_tys ++ [rep_ty])
1055 co = substTyWith inst_tvs (mkTyVarTys tyvars) $
1056 case coi of { IdCo ty -> ty ;
1057 ACo co -> mkSymCoercion co }
1060 tc_item :: (TcEvBinds, EvVar) -> (Id, DefMeth) -> TcM (TcId, LHsBind TcId)
1061 tc_item (rep_ev_binds, rep_d) (sel_id, _)
1062 = do { (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars
1065 ; let meth_rhs = wrapId (mk_op_wrapper sel_id rep_d) sel_id
1066 meth_bind = VarBind { var_id = local_meth_id
1067 , var_rhs = L loc meth_rhs
1068 , var_inline = False }
1070 bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars
1071 , abs_exports = [(tyvars, meth_id,
1072 local_meth_id, noSpecPrags)]
1073 , abs_ev_binds = rep_ev_binds
1074 , abs_binds = unitBag $ L loc meth_bind }
1076 ; return (meth_id, L loc bind) }
1079 mk_op_wrapper :: Id -> EvVar -> HsWrapper
1080 mk_op_wrapper sel_id rep_d
1081 = WpCast (substTyWith sel_tvs (init_inst_tys ++ [co]) local_meth_ty)
1082 <.> WpEvApp (EvId rep_d)
1083 <.> mkWpTyApps (init_inst_tys ++ [rep_ty])
1085 (sel_tvs, sel_rho) = tcSplitForAllTys (idType sel_id)
1086 (_, local_meth_ty) = tcSplitPredFunTy_maybe sel_rho
1087 `orElse` pprPanic "tcInstanceMethods" (ppr sel_id)
1089 ----------------------
1090 mkMethIds :: Class -> [TcTyVar] -> [EvVar] -> [TcType] -> Id -> TcM (TcId, TcId)
1091 mkMethIds clas tyvars dfun_ev_vars inst_tys sel_id
1092 = do { uniq <- newUnique
1093 ; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
1094 ; local_meth_name <- newLocalName sel_name
1095 -- Base the local_meth_name on the selector name, becuase
1096 -- type errors from tcInstanceMethodBody come from here
1098 ; let meth_id = mkLocalId meth_name meth_ty
1099 local_meth_id = mkLocalId local_meth_name local_meth_ty
1100 ; return (meth_id, local_meth_id) }
1102 local_meth_ty = instantiateMethod clas sel_id inst_tys
1103 meth_ty = mkForAllTys tyvars $ mkPiTypes dfun_ev_vars local_meth_ty
1104 sel_name = idName sel_id
1106 ----------------------
1107 wrapId :: HsWrapper -> id -> HsExpr id
1108 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
1110 derivBindCtxt :: Id -> Class -> [Type ] -> LHsBind Name -> SDoc
1111 derivBindCtxt sel_id clas tys _bind
1112 = vcat [ ptext (sLit "When typechecking the code for ") <+> quotes (ppr sel_id)
1113 , nest 2 (ptext (sLit "in a standalone derived instance for")
1114 <+> quotes (pprClassPred clas tys) <> colon)
1115 , nest 2 $ ptext (sLit "To see the code I am typechecking, use -ddump-deriv") ]
1118 -- , nest 2 $ pprSetDepth AllTheWay $ ppr bind ]
1120 warnMissingMethod :: Id -> TcM ()
1121 warnMissingMethod sel_id
1122 = do { warn <- doptM Opt_WarnMissingMethods
1123 ; warnTc (warn -- Warn only if -fwarn-missing-methods
1124 && not (startsWithUnderscore (getOccName sel_id)))
1125 -- Don't warn about _foo methods
1126 (ptext (sLit "No explicit method nor default method for")
1127 <+> quotes (ppr sel_id)) }
1130 Note [Export helper functions]
1131 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1132 We arrange to export the "helper functions" of an instance declaration,
1133 so that they are not subject to preInlineUnconditionally, even if their
1134 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
1135 the dict fun as Ids, not as CoreExprs, so we can't substitute a
1136 non-variable for them.
1138 We could change this by making DFunUnfoldings have CoreExprs, but it
1139 seems a bit simpler this way.
1141 Note [Default methods in instances]
1142 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1149 instance Baz Int Int
1151 From the class decl we get
1153 $dmfoo :: forall v x. Baz v x => x -> x
1156 Notice that the type is ambiguous. That's fine, though. The instance
1159 $dBazIntInt = MkBaz fooIntInt
1160 fooIntInt = $dmfoo Int Int $dBazIntInt
1162 BUT this does mean we must generate the dictionary translation of
1163 fooIntInt directly, rather than generating source-code and
1164 type-checking it. That was the bug in Trac #1061. In any case it's
1165 less work to generate the translated version!
1167 Note [INLINE and default methods]
1168 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1169 Default methods need special case. They are supposed to behave rather like
1170 macros. For exmample
1173 op1, op2 :: Bool -> a -> a
1176 op1 b x = op2 (not b) x
1178 instance Foo Int where
1179 -- op1 via default method
1182 The instance declaration should behave
1184 just as if 'op1' had been defined with the
1185 code, and INLINE pragma, from its original
1188 That is, just as if you'd written
1190 instance Foo Int where
1194 op1 b x = op2 (not b) x
1196 So for the above example we generate:
1199 {-# INLINE $dmop1 #-}
1200 -- $dmop1 has an InlineCompulsory unfolding
1201 $dmop1 d b x = op2 d (not b) x
1203 $fFooInt = MkD $cop1 $cop2
1205 {-# INLINE $cop1 #-}
1206 $cop1 = $dmop1 $fFooInt
1212 * We *copy* any INLINE pragma from the default method $dmop1 to the
1213 instance $cop1. Otherwise we'll just inline the former in the
1214 latter and stop, which isn't what the user expected
1216 * Regardless of its pragma, we give the default method an
1217 unfolding with an InlineCompulsory source. That means
1218 that it'll be inlined at every use site, notably in
1219 each instance declaration, such as $cop1. This inlining
1220 must happen even though
1221 a) $dmop1 is not saturated in $cop1
1222 b) $cop1 itself has an INLINE pragma
1224 It's vital that $dmop1 *is* inlined in this way, to allow the mutual
1225 recursion between $fooInt and $cop1 to be broken
1227 * To communicate the need for an InlineCompulsory to the desugarer
1228 (which makes the Unfoldings), we use the IsDefaultMethod constructor
1232 %************************************************************************
1234 \subsection{Error messages}
1236 %************************************************************************
1239 instDeclCtxt1 :: LHsType Name -> SDoc
1240 instDeclCtxt1 hs_inst_ty
1241 = inst_decl_ctxt (case unLoc hs_inst_ty of
1242 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
1243 HsPredTy pred -> ppr pred
1244 _ -> ppr hs_inst_ty) -- Don't expect this
1245 instDeclCtxt2 :: Type -> SDoc
1246 instDeclCtxt2 dfun_ty
1247 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1249 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
1251 inst_decl_ctxt :: SDoc -> SDoc
1252 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
1254 atInstCtxt :: Name -> SDoc
1255 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
1258 mustBeVarArgErr :: Type -> SDoc
1259 mustBeVarArgErr ty =
1260 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
1261 ptext (sLit "must be variables")
1262 , ptext (sLit "Instead of a variable, found") <+> ppr ty
1265 wrongATArgErr :: Type -> Type -> SDoc
1266 wrongATArgErr ty instTy =
1267 sep [ ptext (sLit "Type indexes must match class instance head")
1268 , ptext (sLit "Found") <+> quotes (ppr ty)
1269 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)