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
24 import RnSource ( addTcgDUs )
34 import CoreUnfold ( mkDFunUnfolding )
35 import CoreSyn ( Expr(Var) )
53 #include "HsVersions.h"
56 Typechecking instance declarations is done in two passes. The first
57 pass, made by @tcInstDecls1@, collects information to be used in the
60 This pre-processed info includes the as-yet-unprocessed bindings
61 inside the instance declaration. These are type-checked in the second
62 pass, when the class-instance envs and GVE contain all the info from
63 all the instance and value decls. Indeed that's the reason we need
64 two passes over the instance decls.
67 Note [How instance declarations are translated]
68 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
69 Here is how we translation instance declarations into Core
73 op1, op2 :: Ix b => a -> b -> b
77 {-# INLINE [2] op1 #-}
81 op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b
85 -- Default methods get the 'self' dictionary as argument
86 -- so they can call other methods at the same type
87 -- Default methods get the same type as their method selector
88 $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b
89 $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). <dm-rhs>
90 -- NB: type variables 'a' and 'b' are *both* in scope in <dm-rhs>
91 -- Note [Tricky type variable scoping]
93 -- A top-level definition for each instance method
94 -- Here op1_i, op2_i are the "instance method Ids"
95 -- The INLINE pragma comes from the user pragma
96 {-# INLINE [2] op1_i #-} -- From the instance decl bindings
97 op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
98 op1_i = /\a. \(d:C a).
101 -- Note [Subtle interaction of recursion and overlap]
103 local_op1 :: forall b. Ix b => [a] -> b -> b
105 -- Source code; run the type checker on this
106 -- NB: Type variable 'a' (but not 'b') is in scope in <rhs>
107 -- Note [Tricky type variable scoping]
111 op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
113 -- The dictionary function itself
114 {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions
115 df_i :: forall a. C a -> C [a]
116 df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
117 -- But see Note [Default methods in instances]
118 -- We can't apply the type checker to the default-method call
120 -- Use a RULE to short-circuit applications of the class ops
121 {-# RULE "op1@C[a]" forall a, d:C a.
122 op1 [a] (df_i d) = op1_i a d #-}
124 Note [Instances and loop breakers]
125 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
126 * Note that df_i may be mutually recursive with both op1_i and op2_i.
127 It's crucial that df_i is not chosen as the loop breaker, even
128 though op1_i has a (user-specified) INLINE pragma.
130 * Instead the idea is to inline df_i into op1_i, which may then select
131 methods from the MkC record, and thereby break the recursion with
132 df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at
133 the same type, it won't mention df_i, so there won't be recursion in
136 * If op1_i is marked INLINE by the user there's a danger that we won't
137 inline df_i in it, and that in turn means that (since it'll be a
138 loop-breaker because df_i isn't), op1_i will ironically never be
139 inlined. But this is OK: the recursion breaking happens by way of
140 a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
141 unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
143 Note [ClassOp/DFun selection]
144 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
145 One thing we see a lot is stuff like
147 where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both*
148 'op2' and 'df' to get
149 case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
150 MkD _ op2 _ _ _ -> op2
151 And that will reduce to ($cop2 d1 d2) which is what we wanted.
153 But it's tricky to make this work in practice, because it requires us to
154 inline both 'op2' and 'df'. But neither is keen to inline without having
155 seen the other's result; and it's very easy to get code bloat (from the
156 big intermediate) if you inline a bit too much.
158 Instead we use a cunning trick.
159 * We arrange that 'df' and 'op2' NEVER inline.
161 * We arrange that 'df' is ALWAYS defined in the sylised form
162 df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
164 * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
165 that lists its methods.
167 * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return
168 a suitable constructor application -- inlining df "on the fly" as it
171 * We give the ClassOp 'op2' a BuiltinRule that extracts the right piece
172 iff its argument satisfies exprIsConApp_maybe. This is done in
175 * We make 'df' CONLIKE, so that shared uses stil match; eg
177 in ...(op2 d)...(op1 d)...
179 Note [Single-method classes]
180 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
181 If the class has just one method (or, more accurately, just one element
182 of {superclasses + methods}), then we still use the *same* strategy
184 class C a where op :: a -> a
185 instance C a => C [a] where op = <blah>
187 We translate the class decl into a newtype, which just gives
190 axiom Co:C a :: C a ~ (a->a)
192 op :: forall a. C a -> (a -> a)
193 op a d = d |> (Co:C a)
195 MkC :: forall a. (a->a) -> C a
196 MkC = /\a.\op. op |> (sym Co:C a)
198 df :: forall a. C a => C [a]
199 {-# NOINLINE df DFun[ $cop_list ] #-}
200 df = /\a. \d. MkD ($cop_list a d)
202 $cop_list :: forall a. C a => a -> a
205 The "constructor" MkD expands to a cast, as does the class-op selector.
206 The RULE works just like for multi-field dictionaries:
207 * (df a d) returns (Just (MkD,..,[$cop_list a d]))
208 to exprIsConApp_Maybe
210 * The RULE for op picks the right result
212 This is a bit of a hack, because (df a d) isn't *really* a constructor
213 application. But it works just fine in this case, exprIsConApp_maybe
214 is otherwise used only when we hit a case expression which will have
215 a real data constructor in it.
217 The biggest reason for doing it this way, apart form uniformity, is
218 that we want to be very careful when we have
219 instance C a => C [a] where
222 then we'll get an INLINE pragma on $cop_list. The danger is that
223 we'll get something like
224 foo = /\a.\d. $cop_list a d
225 and then we'll eta expand, and then we'll inline TOO EARLY. This happened in
226 Trac #3772 and I spent far too long fiddling arond trying to fix it.
227 Look at the test for Trac #3772.
229 Note [Subtle interaction of recursion and overlap]
230 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
232 class C a where { op1,op2 :: a -> a }
233 instance C a => C [a] where
234 op1 x = op2 x ++ op2 x
236 intance C [Int] where
239 When type-checking the C [a] instance, we need a C [a] dictionary (for
240 the call of op2). If we look up in the instance environment, we find
241 an overlap. And in *general* the right thing is to complain (see Note
242 [Overlapping instances] in InstEnv). But in *this* case it's wrong to
243 complain, because we just want to delegate to the op2 of this same
246 Why is this justified? Because we generate a (C [a]) constraint in
247 a context in which 'a' cannot be instantiated to anything that matches
248 other overlapping instances, or else we would not be excecuting this
249 version of op1 in the first place.
251 It might even be a bit disguised:
253 nullFail :: C [a] => [a] -> [a]
254 nullFail x = op2 x ++ op2 x
256 instance C a => C [a] where
259 Precisely this is used in package 'regex-base', module Context.hs.
260 See the overlapping instances for RegexContext, and the fact that they
261 call 'nullFail' just like the example above. The DoCon package also
262 does the same thing; it shows up in module Fraction.hs
264 Conclusion: when typechecking the methods in a C [a] instance, we want
265 to have C [a] available. That is why we have the strange local
266 definition for 'this' in the definition of op1_i in the example above.
267 We can typecheck the defintion of local_op1, and when doing tcSimplifyCheck
268 we supply 'this' as a given dictionary. Only needed, though, if there
269 are some type variables involved; otherwise there can be no overlap and
272 Note [Tricky type variable scoping]
273 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
276 op1, op2 :: Ix b => a -> b -> b
279 instance C a => C [a]
280 {-# INLINE [2] op1 #-}
283 note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
284 in scope in <rhs>. In particular, we must make sure that 'b' is in
285 scope when typechecking <dm-rhs>. This is achieved by subFunTys,
286 which brings appropriate tyvars into scope. This happens for both
287 <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
288 complained if 'b' is mentioned in <rhs>.
292 %************************************************************************
294 \subsection{Extracting instance decls}
296 %************************************************************************
298 Gather up the instance declarations from their various sources
301 tcInstDecls1 -- Deal with both source-code and imported instance decls
302 :: [LTyClDecl Name] -- For deriving stuff
303 -> [LInstDecl Name] -- Source code instance decls
304 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
305 -> TcM (TcGblEnv, -- The full inst env
306 [InstInfo Name], -- Source-code instance decls to process;
307 -- contains all dfuns for this module
308 HsValBinds Name) -- Supporting bindings for derived instances
310 tcInstDecls1 tycl_decls inst_decls deriv_decls
312 do { -- Stop if addInstInfos etc discovers any errors
313 -- (they recover, so that we get more than one error each
316 -- (1) Do class and family instance declarations
317 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
318 ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_decls
319 ; idx_tycons <- mapAndRecoverM tcIdxTyInstDeclTL idxty_decls
322 at_tycons_s) = unzip local_info_tycons
323 ; at_idx_tycons = concat at_tycons_s ++ idx_tycons
324 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
325 ; implicit_things = concatMap implicitTyThings at_idx_tycons
326 ; aux_binds = mkRecSelBinds at_idx_tycons
329 -- (2) Add the tycons of indexed types and their implicit
330 -- tythings to the global environment
331 ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do {
333 -- (3) Instances from generic class declarations
334 ; generic_inst_info <- getGenericInstances clas_decls
336 -- Next, construct the instance environment so far, consisting
338 -- a) local instance decls
339 -- b) generic instances
340 -- c) local family instance decls
341 ; addInsts local_info $
342 addInsts generic_inst_info $
343 addFamInsts at_idx_tycons $ do {
345 -- (4) Compute instances from "deriving" clauses;
346 -- This stuff computes a context for the derived instance
347 -- decl, so it needs to know about all the instances possible
348 -- NB: class instance declarations can contain derivings as
349 -- part of associated data type declarations
350 failIfErrsM -- If the addInsts stuff gave any errors, don't
351 -- try the deriving stuff, becuase that may give
353 ; (deriv_inst_info, deriv_binds, deriv_dus)
354 <- tcDeriving tycl_decls inst_decls deriv_decls
355 ; gbl_env <- addInsts deriv_inst_info getGblEnv
356 ; return ( addTcgDUs gbl_env deriv_dus,
357 generic_inst_info ++ deriv_inst_info ++ local_info,
358 aux_binds `plusHsValBinds` deriv_binds)
361 -- Make sure that toplevel type instance are not for associated types.
362 -- !!!TODO: Need to perform this check for the TyThing of type functions,
364 tcIdxTyInstDeclTL ldecl@(L loc decl) =
365 do { tything <- tcFamInstDecl ldecl
367 when (isAssocFamily tything) $
368 addErr $ assocInClassErr (tcdName decl)
371 isAssocFamily (ATyCon tycon) =
372 case tyConFamInst_maybe tycon of
373 Nothing -> panic "isAssocFamily: no family?!?"
374 Just (fam, _) -> isTyConAssoc fam
375 isAssocFamily _ = panic "isAssocFamily: no tycon?!?"
377 assocInClassErr :: Name -> SDoc
378 assocInClassErr name =
379 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
380 ptext (sLit "must be inside a class instance")
382 addInsts :: [InstInfo Name] -> TcM a -> TcM a
383 addInsts infos thing_inside
384 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
386 addFamInsts :: [TyThing] -> TcM a -> TcM a
387 addFamInsts tycons thing_inside
388 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
390 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
391 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
396 tcLocalInstDecl1 :: LInstDecl Name
397 -> TcM (InstInfo Name, [TyThing])
398 -- A source-file instance declaration
399 -- Type-check all the stuff before the "where"
401 -- We check for respectable instance type, and context
402 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
404 addErrCtxt (instDeclCtxt1 poly_ty) $
406 do { is_boot <- tcIsHsBoot
407 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
410 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
412 -- Now, check the validity of the instance.
413 ; (clas, inst_tys) <- checkValidInstance poly_ty tyvars theta tau
415 -- Next, process any associated types.
416 ; idx_tycons <- recoverM (return []) $
417 do { idx_tycons <- checkNoErrs $ mapAndRecoverM tcFamInstDecl ats
418 ; checkValidAndMissingATs clas (tyvars, inst_tys)
420 ; return idx_tycons }
422 -- Finally, construct the Core representation of the instance.
423 -- (This no longer includes the associated types.)
424 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
425 -- Dfun location is that of instance *header*
426 ; overlap_flag <- getOverlapFlag
427 ; let (eq_theta,dict_theta) = partition isEqPred theta
428 theta' = eq_theta ++ dict_theta
429 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
430 ispec = mkLocalInstance dfun overlap_flag
432 ; return (InstInfo { iSpec = ispec,
433 iBinds = VanillaInst binds uprags False },
437 -- We pass in the source form and the type checked form of the ATs. We
438 -- really need the source form only to be able to produce more informative
440 checkValidAndMissingATs :: Class
441 -> ([TyVar], [TcType]) -- instance types
442 -> [(LTyClDecl Name, -- source form of AT
443 TyThing)] -- Core form of AT
445 checkValidAndMissingATs clas inst_tys ats
446 = do { -- Issue a warning for each class AT that is not defined in this
448 ; let class_ats = map tyConName (classATs clas)
449 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
450 omitted = filterOut (`elemNameSet` defined_ats) class_ats
451 ; warn <- doptM Opt_WarnMissingMethods
452 ; mapM_ (warnTc warn . omittedATWarn) omitted
454 -- Ensure that all AT indexes that correspond to class parameters
455 -- coincide with the types in the instance head. All remaining
456 -- AT arguments must be variables. Also raise an error for any
457 -- type instances that are not associated with this class.
458 ; mapM_ (checkIndexes clas inst_tys) ats
461 checkIndexes clas inst_tys (hsAT, ATyCon tycon)
462 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
463 = checkIndexes' clas inst_tys hsAT
465 snd . fromJust . tyConFamInst_maybe $ tycon)
466 checkIndexes _ _ _ = panic "checkIndexes"
468 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
469 = let atName = tcdName . unLoc $ hsAT
471 setSrcSpan (getLoc hsAT) $
472 addErrCtxt (atInstCtxt atName) $
473 case find ((atName ==) . tyConName) (classATs clas) of
474 Nothing -> addErrTc $ badATErr clas atName -- not in this class
476 case assocTyConArgPoss_maybe atycon of
477 Nothing -> panic "checkIndexes': AT has no args poss?!?"
480 -- The following is tricky! We need to deal with three
481 -- complications: (1) The AT possibly only uses a subset of
482 -- the class parameters as indexes and those it uses may be in
483 -- a different order; (2) the AT may have extra arguments,
484 -- which must be type variables; and (3) variables in AT and
485 -- instance head will be different `Name's even if their
486 -- source lexemes are identical.
488 -- e.g. class C a b c where
489 -- data D b a :: * -> * -- NB (1) b a, omits c
490 -- instance C [x] Bool Char where
491 -- data D Bool [x] v = MkD x [v] -- NB (2) v
492 -- -- NB (3) the x in 'instance C...' have differnt
493 -- -- Names to x's in 'data D...'
495 -- Re (1), `poss' contains a permutation vector to extract the
496 -- class parameters in the right order.
498 -- Re (2), we wrap the (permuted) class parameters in a Maybe
499 -- type and use Nothing for any extra AT arguments. (First
500 -- equation of `checkIndex' below.)
502 -- Re (3), we replace any type variable in the AT parameters
503 -- that has the same source lexeme as some variable in the
504 -- instance types with the instance type variable sharing its
507 let relevantInstTys = map (instTys !!) poss
508 instArgs = map Just relevantInstTys ++
509 repeat Nothing -- extra arguments
510 renaming = substSameTyVar atTvs instTvs
512 zipWithM_ checkIndex (substTys renaming atTys) instArgs
514 checkIndex ty Nothing
515 | isTyVarTy ty = return ()
516 | otherwise = addErrTc $ mustBeVarArgErr ty
517 checkIndex ty (Just instTy)
518 | ty `tcEqType` instTy = return ()
519 | otherwise = addErrTc $ wrongATArgErr ty instTy
521 listToNameSet = addListToNameSet emptyNameSet
523 substSameTyVar [] _ = emptyTvSubst
524 substSameTyVar (tv:tvs) replacingTvs =
525 let replacement = case find (tv `sameLexeme`) replacingTvs of
526 Nothing -> mkTyVarTy tv
527 Just rtv -> mkTyVarTy rtv
529 tv1 `sameLexeme` tv2 =
530 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
532 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
536 %************************************************************************
538 Type-checking instance declarations, pass 2
540 %************************************************************************
543 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
545 -- (a) From each class declaration,
546 -- generate any default-method bindings
547 -- (b) From each instance decl
548 -- generate the dfun binding
550 tcInstDecls2 tycl_decls inst_decls
551 = do { -- (a) Default methods from class decls
552 let class_decls = filter (isClassDecl . unLoc) tycl_decls
553 ; dm_binds_s <- mapM tcClassDecl2 class_decls
555 -- (b) instance declarations
556 ; inst_binds_s <- mapM tcInstDecl2 inst_decls
559 ; return (unionManyBags dm_binds_s `unionBags`
560 unionManyBags inst_binds_s) }
562 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
563 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
564 = recoverM (return emptyLHsBinds) $
566 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
567 tc_inst_decl2 dfun_id ibinds
569 dfun_id = instanceDFunId ispec
570 loc = getSrcSpan dfun_id
575 tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id)
576 -- Returns a binding for the dfun
578 ------------------------
579 -- Derived newtype instances; surprisingly tricky!
581 -- class Show a => Foo a b where ...
582 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
584 -- The newtype gives an FC axiom looking like
585 -- axiom CoN a :: N a ~ Tree [a]
586 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
588 -- So all need is to generate a binding looking like:
589 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
590 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
591 -- case df `cast` (Foo Int (sym (CoN a))) of
592 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
594 -- If there are no superclasses, matters are simpler, because we don't need the case
595 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
597 tc_inst_decl2 dfun_id (NewTypeDerived coi _)
598 = do { let rigid_info = InstSkol
599 origin = SigOrigin rigid_info
600 inst_ty = idType dfun_id
601 inst_tvs = fst (tcSplitForAllTys inst_ty)
602 ; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
603 -- inst_head_ty is a PredType
605 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
606 (class_tyvars, sc_theta, _, _) = classBigSig cls
607 cls_tycon = classTyCon cls
608 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
609 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
613 IdCo -> (last_ty, idHsWrapper)
614 ACo co -> (snd (coercionKind co'), WpCast (mk_full_coercion co'))
616 co' = substTyWith inst_tvs (mkTyVarTys inst_tvs') co
617 -- NB: the free variable of coi are bound by the
618 -- universally quantified variables of the dfun_id
619 -- This is weird, and maybe we should make NewTypeDerived
620 -- carry a type-variable list too; but it works fine
622 -----------------------
624 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
625 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
626 -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
627 -- where rep_ty is the (eta-reduced) type rep of T
628 -- So we just replace T with CoT, and insert a 'sym'
629 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
631 mk_full_coercion co = mkTyConApp cls_tycon
632 (initial_cls_inst_tys ++ [mkSymCoercion co])
633 -- Full coercion : (Foo Int (Tree [a]) ~ Foo Int (N a)
635 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
636 -- In our example, rep_pred is (Foo Int (Tree [a]))
638 ; sc_loc <- getInstLoc InstScOrigin
639 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
640 ; inst_loc <- getInstLoc origin
641 ; dfun_dicts <- newDictBndrs inst_loc theta
642 ; rep_dict <- newDictBndr inst_loc rep_pred
643 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
645 -- Figure out bindings for the superclass context from dfun_dicts
646 -- Don't include this_dict in the 'givens', else
647 -- sc_dicts get bound by just selecting from this_dict!!
648 ; sc_binds <- addErrCtxt superClassCtxt $
649 tcSimplifySuperClasses inst_loc this_dict dfun_dicts
652 -- It's possible that the superclass stuff might unified something
653 -- in the envt with one of the clas_tyvars
654 ; checkSigTyVars inst_tvs'
656 ; let coerced_rep_dict = wrapId wrapper (instToId rep_dict)
658 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
659 ; let dict_bind = mkVarBind (instToId this_dict) (noLoc body)
661 ; return (unitBag $ noLoc $
662 AbsBinds inst_tvs' (map instToVar dfun_dicts)
663 [(inst_tvs', dfun_id, instToId this_dict, noSpecPrags)]
664 (dict_bind `consBag` sc_binds)) }
666 -----------------------
667 -- (make_body C tys scs coreced_rep_dict)
669 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
670 -- But if there are no superclasses, it returns just coerced_rep_dict
671 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
673 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
674 | null sc_dicts -- Case (a)
675 = return coerced_rep_dict
676 | otherwise -- Case (b)
677 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
678 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
679 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
680 pat_dicts = dummy_sc_dict_ids,
681 pat_binds = emptyLHsBinds,
682 pat_args = PrefixCon (map nlVarPat op_ids),
684 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
685 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
686 map HsVar (sc_dict_ids ++ op_ids)
688 -- Warning: this HsCase scrutinises a value with a PredTy, which is
689 -- never otherwise seen in Haskell source code. It'd be
690 -- nicer to generate Core directly!
691 ; return (HsCase (noLoc coerced_rep_dict) $
692 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
694 sc_dict_ids = map instToId sc_dicts
695 pat_ty = mkTyConApp cls_tycon cls_inst_tys
696 cls_data_con = head (tyConDataCons cls_tycon)
697 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
698 op_tys = dropList sc_dict_ids cls_arg_tys
700 ------------------------
701 -- Ordinary instances
703 tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
704 = do { let rigid_info = InstSkol
705 inst_ty = idType dfun_id
706 loc = getSrcSpan dfun_id
708 -- Instantiate the instance decl with skolem constants
709 ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
710 -- These inst_tyvars' scope over the 'where' part
711 -- Those tyvars are inside the dfun_id's type, which is a bit
712 -- bizarre, but OK so long as you realise it!
714 (clas, inst_tys') = tcSplitDFunHead inst_head'
715 (class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas
717 -- Instantiate the super-class context with inst_tys
718 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
719 origin = SigOrigin rigid_info
721 -- Create dictionary Ids from the specified instance contexts.
722 ; inst_loc <- getInstLoc origin
723 ; dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
724 ; this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
725 -- Default-method Ids may be mentioned in synthesised RHSs,
726 -- but they'll already be in the environment.
729 -- Cook up a binding for "this = df d1 .. dn",
730 -- to use in each method binding
731 -- Need to clone the dict in case it is floated out, and
732 -- then clashes with its friends
733 ; cloned_this <- cloneDict this_dict
734 ; let cloned_this_bind = mkVarBind (instToId cloned_this) $
735 L loc $ wrapId app_wrapper dfun_id
736 app_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
737 dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
739 | null inst_tyvars' && null dfun_theta' = (this_dict, emptyBag)
740 | otherwise = (cloned_this, unitBag cloned_this_bind)
742 -- Deal with 'SPECIALISE instance' pragmas
743 -- See Note [SPECIALISE instance pragmas]
744 ; let spec_inst_sigs = filter isSpecInstLSig uprags
745 -- The filter removes the pragmas for methods
746 ; spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) spec_inst_sigs
748 -- Typecheck the methods
749 ; let prag_fn = mkPragFun uprags monobinds
750 tc_meth = tcInstanceMethod loc standalone_deriv
754 prag_fn spec_inst_prags monobinds
756 ; (meth_ids, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
757 mapAndUnzipM tc_meth op_items
759 -- Figure out bindings for the superclass context
760 ; sc_loc <- getInstLoc InstScOrigin
761 ; sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
762 ; let tc_sc = tcSuperClass inst_loc inst_tyvars' dfun_dicts nested_this_pair
763 ; (sc_ids, sc_binds) <- mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts)
765 -- It's possible that the superclass stuff might unified
766 -- something in the envt with one of the inst_tyvars'
767 ; checkSigTyVars inst_tyvars'
769 -- Create the result bindings
770 ; let dict_constr = classDataCon clas
771 this_dict_id = instToId this_dict
772 dict_bind = mkVarBind this_dict_id dict_rhs
773 dict_rhs = foldl mk_app inst_constr sc_meth_ids
774 sc_meth_ids = sc_ids ++ meth_ids
775 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
776 (dataConWrapId dict_constr)
777 -- We don't produce a binding for the dict_constr; instead we
778 -- rely on the simplifier to unfold this saturated application
779 -- We do this rather than generate an HsCon directly, because
780 -- it means that the special cases (e.g. dictionary with only one
781 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
782 -- than needing to be repeated here.
784 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
785 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
786 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
788 -- Do not inline the dfun; instead give it a magic DFunFunfolding
789 -- See Note [ClassOp/DFun selection]
790 -- See also note [Single-method classes]
791 dfun_id_w_fun = dfun_id
792 `setIdUnfolding` mkDFunUnfolding inst_ty (map Var sc_meth_ids)
793 `setInlinePragma` dfunInlinePragma
798 [(inst_tyvars', dfun_id_w_fun, this_dict_id, SpecPrags spec_inst_prags)]
801 ; showLIE (text "instance")
802 ; return (unitBag (L loc main_bind) `unionBags`
803 listToBag meth_binds `unionBags`
808 -- Create the result bindings
809 ; let this_dict_id = instToId this_dict
810 arg_ids = sc_ids ++ meth_ids
811 arg_binds = listToBag meth_binds `unionBags`
814 ; showLIE (text "instance")
815 ; case newTyConCo_maybe (classTyCon clas) of
816 Nothing -- A multi-method class
817 -> return (unitBag (L loc data_bind) `unionBags` arg_binds)
819 data_dfun_id = dfun_id -- Do not inline; instead give it a magic DFunFunfolding
820 -- See Note [ClassOp/DFun selection]
821 `setIdUnfolding` mkDFunUnfolding dict_constr arg_ids
822 `setInlinePragma` dfunInlinePragma
824 data_bind = AbsBinds inst_tyvars' dfun_lam_vars
825 [(inst_tyvars', data_dfun_id, this_dict_id, spec_inst_prags)]
828 dict_bind = mkVarBind this_dict_id dict_rhs
829 dict_rhs = foldl mk_app inst_constr arg_ids
830 dict_constr = classDataCon clas
831 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
832 (dataConWrapId dict_constr)
833 -- We don't produce a binding for the dict_constr; instead we
834 -- rely on the simplifier to unfold this saturated application
835 -- We do this rather than generate an HsCon directly, because
836 -- it means that the special cases (e.g. dictionary with only one
837 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
838 -- than needing to be repeated here.
840 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
841 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
842 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
844 Just the_nt_co -- (Just co) for a single-method class
845 -> return (unitBag (L loc nt_bind) `unionBags` arg_binds)
847 nt_dfun_id = dfun_id -- Just let the dfun inline; see Note [Single-method classes]
848 `setInlinePragma` alwaysInlinePragma
850 local_nt_dfun = setIdType this_dict_id inst_ty -- A bit of a hack, but convenient
852 nt_bind = AbsBinds [] []
853 [([], nt_dfun_id, local_nt_dfun, spec_inst_prags)]
854 (unitBag (mkVarBind local_nt_dfun (L loc (wrapId nt_cast the_meth_id))))
856 the_meth_id = ASSERT( length arg_ids == 1 ) head arg_ids
857 nt_cast = WpCast $ mkPiTypes (inst_tyvars' ++ dfun_lam_vars) $
858 mkSymCoercion (mkTyConApp the_nt_co inst_tys')
861 ------------------------------
862 tcSuperClass :: InstLoc -> [TyVar] -> [Inst]
863 -> (Inst, LHsBinds Id)
864 -> (Id, Inst) -> TcM (Id, LHsBind Id)
865 -- Build a top level decl like
866 -- sc_op = /\a \d. let this = ... in
869 -- The "this" part is just-in-case (discarded if not used)
870 -- See Note [Recursive superclasses]
871 tcSuperClass inst_loc tyvars dicts (this_dict, this_bind)
873 = addErrCtxt superClassCtxt $
874 do { sc_binds <- tcSimplifySuperClasses inst_loc
875 this_dict dicts [sc_dict]
876 -- Don't include this_dict in the 'givens', else
877 -- sc_dicts get bound by just selecting from this_dict!!
880 ; let sc_op_ty = mkSigmaTy tyvars (map dictPred dicts)
881 (mkPredTy (dictPred sc_dict))
882 sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq
884 sc_op_id = mkLocalId sc_op_name sc_op_ty
885 sc_id = instToVar sc_dict
886 sc_op_bind = AbsBinds tyvars
887 (map instToVar dicts)
888 [(tyvars, sc_op_id, sc_id, noSpecPrags)]
889 (this_bind `unionBags` sc_binds)
891 ; return (sc_op_id, noLoc sc_op_bind) }
894 Note [Recursive superclasses]
895 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
896 See Trac #1470 for why we would *like* to add "this_dict" to the
897 available instances here. But we can't do so because then the superclases
898 get satisfied by selection from this_dict, and that leads to an immediate
899 loop. What we need is to add this_dict to Avails without adding its
900 superclasses, and we currently have no way to do that.
902 Note [SPECIALISE instance pragmas]
903 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
906 instance (Ix a, Ix b) => Ix (a,b) where
907 {-# SPECIALISE instance Ix (Int,Int) #-}
910 We do *not* want to make a specialised version of the dictionary
911 function. Rather, we want specialised versions of each method.
912 Thus we should generate something like this:
914 $dfIx :: (Ix a, Ix x) => Ix (a,b)
915 {- DFUN [$crange, ...] -}
916 $dfIx da db = Ix ($crange da db) (...other methods...)
918 $dfIxPair :: (Ix a, Ix x) => Ix (a,b)
919 {- DFUN [$crangePair, ...] -}
920 $dfIxPair = Ix ($crangePair da db) (...other methods...)
922 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
923 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
924 $crange da db = <blah>
926 {-# RULE range ($dfIx da db) = $crange da db #-}
930 * The RULE is unaffected by the specialisation. We don't want to
931 specialise $dfIx, because then it would need a specialised RULE
932 which is a pain. The single RULE works fine at all specialisations.
933 See Note [How instance declarations are translated] above
935 * Instead, we want to specialise the *method*, $crange
937 In practice, rather than faking up a SPECIALISE pragama for each
938 method (which is painful, since we'd have to figure out its
939 specialised type), we call tcSpecPrag *as if* were going to specialise
940 $dfIx -- you can see that in the call to tcSpecInst. That generates a
941 SpecPrag which, as it turns out, can be used unchanged for each method.
942 The "it turns out" bit is delicate, but it works fine!
945 tcSpecInst :: Id -> Sig Name -> TcM TcSpecPrag
946 tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
947 = addErrCtxt (spec_ctxt prag) $
948 do { let name = idName dfun_id
949 ; (tyvars, theta, tau) <- tcHsInstHead hs_ty
950 ; let spec_ty = mkSigmaTy tyvars theta tau
951 ; co_fn <- tcSubExp (SpecPragOrigin name) (idType dfun_id) spec_ty
952 ; return (SpecPrag co_fn defaultInlinePragma) }
954 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
956 tcSpecInst _ _ = panic "tcSpecInst"
959 %************************************************************************
961 Type-checking an instance method
963 %************************************************************************
966 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
967 - Remembering to use fresh Name (the instance method Name) as the binder
968 - Bring the instance method Ids into scope, for the benefit of tcInstSig
969 - Use sig_fn mapping instance method Name -> instance tyvars
971 - Use tcValBinds to do the checking
974 tcInstanceMethod :: SrcSpan -> Bool -> Class -> [TcTyVar] -> [Inst]
976 -> (Inst, LHsBinds Id) -- "This" and its binding
977 -> TcPragFun -- Local prags
978 -> [Located TcSpecPrag] -- Arising from 'SPECLALISE instance'
981 -> TcM (Id, LHsBind Id)
982 -- The returned inst_meth_ids all have types starting
983 -- forall tvs. theta => ...
985 tcInstanceMethod loc standalone_deriv clas tyvars dfun_dicts inst_tys
986 (this_dict, this_dict_bind)
987 prag_fn spec_inst_prags binds_in (sel_id, dm_info)
988 = do { uniq <- newUnique
989 ; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
990 ; local_meth_name <- newLocalName sel_name
991 -- Base the local_meth_name on the selector name, becuase
992 -- type errors from tcInstanceMethodBody come from here
994 ; let local_meth_ty = instantiateMethod clas sel_id inst_tys
995 meth_ty = mkSigmaTy tyvars (map dictPred dfun_dicts) local_meth_ty
996 meth_id = mkLocalId meth_name meth_ty
997 local_meth_id = mkLocalId local_meth_name local_meth_ty
1001 = add_meth_ctxt rn_bind $
1002 do { (meth_id1, spec_prags) <- tcPrags NonRecursive False True
1003 meth_id (prag_fn sel_name)
1004 ; bind <- tcInstanceMethodBody (instLoc this_dict)
1006 ([this_dict], this_dict_bind)
1007 meth_id1 local_meth_id
1009 (SpecPrags (spec_inst_prags ++ spec_prags))
1011 ; return (meth_id1, bind) }
1014 tc_default :: DefMeth -> TcM (Id, LHsBind Id)
1015 -- The user didn't supply a method binding, so we have to make
1016 -- up a default binding, in a way depending on the default-method info
1018 tc_default NoDefMeth -- No default method at all
1019 = do { warnMissingMethod sel_id
1020 ; return (meth_id, mkVarBind meth_id $
1021 mkLHsWrap lam_wrapper error_rhs) }
1023 tc_default GenDefMeth -- Derivable type classes stuff
1024 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id local_meth_name
1025 ; tc_body meth_bind }
1027 tc_default (DefMeth dm_name) -- An polymorphic default method
1028 = do { -- Build the typechecked version directly,
1029 -- without calling typecheck_method;
1030 -- see Note [Default methods in instances]
1031 -- Generate /\as.\ds. let this = df as ds
1032 -- in $dm inst_tys this
1033 -- The 'let' is necessary only because HsSyn doesn't allow
1034 -- you to apply a function to a dictionary *expression*.
1036 ; dm_id <- tcLookupId dm_name
1037 ; let dm_inline_prag = idInlinePragma dm_id
1038 rhs = HsWrap (WpApp (instToId this_dict) <.> mkWpTyApps inst_tys) $
1041 meth_bind = L loc $ VarBind { var_id = local_meth_id
1042 , var_rhs = L loc rhs
1043 , var_inline = False }
1044 meth_id1 = meth_id `setInlinePragma` dm_inline_prag
1045 -- Copy the inline pragma (if any) from the default
1046 -- method to this version. Note [INLINE and default methods]
1048 bind = AbsBinds { abs_tvs = tyvars, abs_dicts = dfun_lam_vars
1049 , abs_exports = [( tyvars, meth_id1, local_meth_id
1050 , SpecPrags spec_inst_prags)]
1051 , abs_binds = this_dict_bind `unionBags` unitBag meth_bind }
1052 -- Default methods in an instance declaration can't have their own
1053 -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
1054 -- currently they are rejected with
1055 -- "INLINE pragma lacks an accompanying binding"
1057 ; return (meth_id1, L loc bind) }
1059 ; case findMethodBind sel_name local_meth_name binds_in of
1060 Just user_bind -> tc_body user_bind -- User-supplied method binding
1061 Nothing -> tc_default dm_info -- None supplied
1064 sel_name = idName sel_id
1066 meth_sig_fn _ = Just [] -- The 'Just' says "yes, there's a type sig"
1067 -- But there are no scoped type variables from local_method_id
1068 -- Only the ones from the instance decl itself, which are already
1069 -- in scope. Example:
1070 -- class C a where { op :: forall b. Eq b => ... }
1071 -- instance C [c] where { op = <rhs> }
1072 -- In <rhs>, 'c' is scope but 'b' is not!
1074 error_rhs = L loc $ HsApp error_fun error_msg
1075 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
1076 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
1077 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
1078 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
1080 dfun_lam_vars = map instToVar dfun_dicts
1081 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_lam_vars
1083 -- For instance decls that come from standalone deriving clauses
1084 -- we want to print out the full source code if there's an error
1085 -- because otherwise the user won't see the code at all
1086 add_meth_ctxt rn_bind thing
1087 | standalone_deriv = addLandmarkErrCtxt (derivBindCtxt clas inst_tys rn_bind) thing
1090 wrapId :: HsWrapper -> id -> HsExpr id
1091 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
1093 derivBindCtxt :: Class -> [Type ] -> LHsBind Name -> SDoc
1094 derivBindCtxt clas tys bind
1095 = vcat [ ptext (sLit "When typechecking a standalone-derived method for")
1096 <+> quotes (pprClassPred clas tys) <> colon
1097 , nest 2 $ pprSetDepth AllTheWay $ ppr bind ]
1099 warnMissingMethod :: Id -> TcM ()
1100 warnMissingMethod sel_id
1101 = do { warn <- doptM Opt_WarnMissingMethods
1102 ; warnTc (warn -- Warn only if -fwarn-missing-methods
1103 && not (startsWithUnderscore (getOccName sel_id)))
1104 -- Don't warn about _foo methods
1105 (ptext (sLit "No explicit method nor default method for")
1106 <+> quotes (ppr sel_id)) }
1109 Note [Export helper functions]
1110 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1111 We arrange to export the "helper functions" of an instance declaration,
1112 so that they are not subject to preInlineUnconditionally, even if their
1113 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
1114 the dict fun as Ids, not as CoreExprs, so we can't substitute a
1115 non-variable for them.
1117 We could change this by making DFunUnfoldings have CoreExprs, but it
1118 seems a bit simpler this way.
1120 Note [Default methods in instances]
1121 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1128 instance Baz Int Int
1130 From the class decl we get
1132 $dmfoo :: forall v x. Baz v x => x -> x
1135 Notice that the type is ambiguous. That's fine, though. The instance
1138 $dBazIntInt = MkBaz fooIntInt
1139 fooIntInt = $dmfoo Int Int $dBazIntInt
1141 BUT this does mean we must generate the dictionary translation of
1142 fooIntInt directly, rather than generating source-code and
1143 type-checking it. That was the bug in Trac #1061. In any case it's
1144 less work to generate the translated version!
1146 Note [INLINE and default methods]
1147 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1148 Default methods need special case. They are supposed to behave rather like
1149 macros. For exmample
1152 op1, op2 :: Bool -> a -> a
1155 op1 b x = op2 (not b) x
1157 instance Foo Int where
1158 -- op1 via default method
1161 The instance declaration should behave
1163 just as if 'op1' had been defined with the
1164 code, and INLINE pragma, from its original
1167 That is, just as if you'd written
1169 instance Foo Int where
1173 op1 b x = op2 (not b) x
1175 So for the above example we generate:
1178 {-# INLINE $dmop1 #-}
1179 -- $dmop1 has an InlineCompulsory unfolding
1180 $dmop1 d b x = op2 d (not b) x
1182 $fFooInt = MkD $cop1 $cop2
1184 {-# INLINE $cop1 #-}
1185 $cop1 = $dmop1 $fFooInt
1191 * We *copy* any INLINE pragma from the default method $dmop1 to the
1192 instance $cop1. Otherwise we'll just inline the former in the
1193 latter and stop, which isn't what the user expected
1195 * Regardless of its pragma, we give the default method an
1196 unfolding with an InlineCompulsory source. That means
1197 that it'll be inlined at every use site, notably in
1198 each instance declaration, such as $cop1. This inlining
1199 must happen even though
1200 a) $dmop1 is not saturated in $cop1
1201 b) $cop1 itself has an INLINE pragma
1203 It's vital that $dmop1 *is* inlined in this way, to allow the mutual
1204 recursion between $fooInt and $cop1 to be broken
1206 * To communicate the need for an InlineCompulsory to the desugarer
1207 (which makes the Unfoldings), we use the IsDefaultMethod constructor
1211 %************************************************************************
1213 \subsection{Error messages}
1215 %************************************************************************
1218 instDeclCtxt1 :: LHsType Name -> SDoc
1219 instDeclCtxt1 hs_inst_ty
1220 = inst_decl_ctxt (case unLoc hs_inst_ty of
1221 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
1222 HsPredTy pred -> ppr pred
1223 _ -> ppr hs_inst_ty) -- Don't expect this
1224 instDeclCtxt2 :: Type -> SDoc
1225 instDeclCtxt2 dfun_ty
1226 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1228 (_,cls,tys) = tcSplitDFunTy dfun_ty
1230 inst_decl_ctxt :: SDoc -> SDoc
1231 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
1233 superClassCtxt :: SDoc
1234 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
1236 atInstCtxt :: Name -> SDoc
1237 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
1240 mustBeVarArgErr :: Type -> SDoc
1241 mustBeVarArgErr ty =
1242 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
1243 ptext (sLit "must be variables")
1244 , ptext (sLit "Instead of a variable, found") <+> ppr ty
1247 wrongATArgErr :: Type -> Type -> SDoc
1248 wrongATArgErr ty instTy =
1249 sep [ ptext (sLit "Type indexes must match class instance head")
1250 , ptext (sLit "Found") <+> quotes (ppr ty)
1251 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)