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 )
52 #include "HsVersions.h"
55 Typechecking instance declarations is done in two passes. The first
56 pass, made by @tcInstDecls1@, collects information to be used in the
59 This pre-processed info includes the as-yet-unprocessed bindings
60 inside the instance declaration. These are type-checked in the second
61 pass, when the class-instance envs and GVE contain all the info from
62 all the instance and value decls. Indeed that's the reason we need
63 two passes over the instance decls.
66 Note [How instance declarations are translated]
67 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
68 Here is how we translation instance declarations into Core
72 op1, op2 :: Ix b => a -> b -> b
76 {-# INLINE [2] op1 #-}
80 op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b
84 -- Default methods get the 'self' dictionary as argument
85 -- so they can call other methods at the same type
86 -- Default methods get the same type as their method selector
87 $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b
88 $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). <dm-rhs>
89 -- NB: type variables 'a' and 'b' are *both* in scope in <dm-rhs>
90 -- Note [Tricky type variable scoping]
92 -- A top-level definition for each instance method
93 -- Here op1_i, op2_i are the "instance method Ids"
94 -- The INLINE pragma comes from the user pragma
95 {-# INLINE [2] op1_i #-} -- From the instance decl bindings
96 op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
97 op1_i = /\a. \(d:C a).
100 -- Note [Subtle interaction of recursion and overlap]
102 local_op1 :: forall b. Ix b => [a] -> b -> b
104 -- Source code; run the type checker on this
105 -- NB: Type variable 'a' (but not 'b') is in scope in <rhs>
106 -- Note [Tricky type variable scoping]
110 op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
112 -- The dictionary function itself
113 {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions
114 df_i :: forall a. C a -> C [a]
115 df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
116 -- But see Note [Default methods in instances]
117 -- We can't apply the type checker to the default-method call
119 -- Use a RULE to short-circuit applications of the class ops
120 {-# RULE "op1@C[a]" forall a, d:C a.
121 op1 [a] (df_i d) = op1_i a d #-}
123 Note [Instances and loop breakers]
124 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
125 * Note that df_i may be mutually recursive with both op1_i and op2_i.
126 It's crucial that df_i is not chosen as the loop breaker, even
127 though op1_i has a (user-specified) INLINE pragma.
129 * Instead the idea is to inline df_i into op1_i, which may then select
130 methods from the MkC record, and thereby break the recursion with
131 df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at
132 the same type, it won't mention df_i, so there won't be recursion in
135 * If op1_i is marked INLINE by the user there's a danger that we won't
136 inline df_i in it, and that in turn means that (since it'll be a
137 loop-breaker because df_i isn't), op1_i will ironically never be
138 inlined. But this is OK: the recursion breaking happens by way of
139 a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
140 unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
142 Note [ClassOp/DFun selection]
143 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
144 One thing we see a lot is stuff like
146 where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both*
147 'op2' and 'df' to get
148 case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
149 MkD _ op2 _ _ _ -> op2
150 And that will reduce to ($cop2 d1 d2) which is what we wanted.
152 But it's tricky to make this work in practice, because it requires us to
153 inline both 'op2' and 'df'. But neither is keen to inline without having
154 seen the other's result; and it's very easy to get code bloat (from the
155 big intermediate) if you inline a bit too much.
157 Instead we use a cunning trick.
158 * We arrange that 'df' and 'op2' NEVER inline.
160 * We arrange that 'df' is ALWAYS defined in the sylised form
161 df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
163 * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
164 that lists its methods.
166 * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return
167 a suitable constructor application -- inlining df "on the fly" as it
170 * We give the ClassOp 'op2' a BuiltinRule that extracts the right piece
171 iff its argument satisfies exprIsConApp_maybe. This is done in
174 * We make 'df' CONLIKE, so that shared uses stil match; eg
176 in ...(op2 d)...(op1 d)...
178 Note [Single-method classes]
179 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
180 If the class has just one method (or, more accurately, just one element
181 of {superclasses + methods}), then we still use the *same* strategy
183 class C a where op :: a -> a
184 instance C a => C [a] where op = <blah>
186 We translate the class decl into a newtype, which just gives
189 axiom Co:C a :: C a ~ (a->a)
191 op :: forall a. C a -> (a -> a)
192 op a d = d |> (Co:C a)
194 MkC :: forall a. (a->a) -> C a
195 MkC = /\a.\op. op |> (sym Co:C a)
197 df :: forall a. C a => C [a]
198 {-# NOINLINE df DFun[ $cop_list ] #-}
199 df = /\a. \d. MkD ($cop_list a d)
201 $cop_list :: forall a. C a => a -> a
204 The "constructor" MkD expands to a cast, as does the class-op selector.
205 The RULE works just like for multi-field dictionaries:
206 * (df a d) returns (Just (MkD,..,[$cop_list a d]))
207 to exprIsConApp_Maybe
209 * The RULE for op picks the right result
211 This is a bit of a hack, because (df a d) isn't *really* a constructor
212 application. But it works just fine in this case, exprIsConApp_maybe
213 is otherwise used only when we hit a case expression which will have
214 a real data constructor in it.
216 The biggest reason for doing it this way, apart form uniformity, is
217 that we want to be very careful when we have
218 instance C a => C [a] where
221 then we'll get an INLINE pragma on $cop_list. The danger is that
222 we'll get something like
223 foo = /\a.\d. $cop_list a d
224 and then we'll eta expand, and then we'll inline TOO EARLY. This happened in
225 Trac #3772 and I spent far too long fiddling arond trying to fix it.
226 Look at the test for Trac #3772.
228 Note [Subtle interaction of recursion and overlap]
229 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
231 class C a where { op1,op2 :: a -> a }
232 instance C a => C [a] where
233 op1 x = op2 x ++ op2 x
235 intance C [Int] where
238 When type-checking the C [a] instance, we need a C [a] dictionary (for
239 the call of op2). If we look up in the instance environment, we find
240 an overlap. And in *general* the right thing is to complain (see Note
241 [Overlapping instances] in InstEnv). But in *this* case it's wrong to
242 complain, because we just want to delegate to the op2 of this same
245 Why is this justified? Because we generate a (C [a]) constraint in
246 a context in which 'a' cannot be instantiated to anything that matches
247 other overlapping instances, or else we would not be excecuting this
248 version of op1 in the first place.
250 It might even be a bit disguised:
252 nullFail :: C [a] => [a] -> [a]
253 nullFail x = op2 x ++ op2 x
255 instance C a => C [a] where
258 Precisely this is used in package 'regex-base', module Context.hs.
259 See the overlapping instances for RegexContext, and the fact that they
260 call 'nullFail' just like the example above. The DoCon package also
261 does the same thing; it shows up in module Fraction.hs
263 Conclusion: when typechecking the methods in a C [a] instance, we want
264 to have C [a] available. That is why we have the strange local
265 definition for 'this' in the definition of op1_i in the example above.
266 We can typecheck the defintion of local_op1, and when doing tcSimplifyCheck
267 we supply 'this' as a given dictionary. Only needed, though, if there
268 are some type variables involved; otherwise there can be no overlap and
271 Note [Tricky type variable scoping]
272 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
275 op1, op2 :: Ix b => a -> b -> b
278 instance C a => C [a]
279 {-# INLINE [2] op1 #-}
282 note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
283 in scope in <rhs>. In particular, we must make sure that 'b' is in
284 scope when typechecking <dm-rhs>. This is achieved by subFunTys,
285 which brings appropriate tyvars into scope. This happens for both
286 <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
287 complained if 'b' is mentioned in <rhs>.
291 %************************************************************************
293 \subsection{Extracting instance decls}
295 %************************************************************************
297 Gather up the instance declarations from their various sources
300 tcInstDecls1 -- Deal with both source-code and imported instance decls
301 :: [LTyClDecl Name] -- For deriving stuff
302 -> [LInstDecl Name] -- Source code instance decls
303 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
304 -> TcM (TcGblEnv, -- The full inst env
305 [InstInfo Name], -- Source-code instance decls to process;
306 -- contains all dfuns for this module
307 HsValBinds Name) -- Supporting bindings for derived instances
309 tcInstDecls1 tycl_decls inst_decls deriv_decls
311 do { -- Stop if addInstInfos etc discovers any errors
312 -- (they recover, so that we get more than one error each
315 -- (1) Do class and family instance declarations
316 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
317 ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_decls
318 ; idx_tycons <- mapAndRecoverM tcIdxTyInstDeclTL idxty_decls
321 at_tycons_s) = unzip local_info_tycons
322 ; at_idx_tycons = concat at_tycons_s ++ idx_tycons
323 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
324 ; implicit_things = concatMap implicitTyThings at_idx_tycons
325 ; aux_binds = mkAuxBinds at_idx_tycons
328 -- (2) Add the tycons of indexed types and their implicit
329 -- tythings to the global environment
330 ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do {
332 -- (3) Instances from generic class declarations
333 ; generic_inst_info <- getGenericInstances clas_decls
335 -- Next, construct the instance environment so far, consisting
337 -- a) local instance decls
338 -- b) generic instances
339 -- c) local family instance decls
340 ; addInsts local_info $
341 addInsts generic_inst_info $
342 addFamInsts at_idx_tycons $ do {
344 -- (4) Compute instances from "deriving" clauses;
345 -- This stuff computes a context for the derived instance
346 -- decl, so it needs to know about all the instances possible
347 -- NB: class instance declarations can contain derivings as
348 -- part of associated data type declarations
349 failIfErrsM -- If the addInsts stuff gave any errors, don't
350 -- try the deriving stuff, becuase that may give
352 ; (deriv_inst_info, deriv_binds, deriv_dus)
353 <- tcDeriving tycl_decls inst_decls deriv_decls
354 ; gbl_env <- addInsts deriv_inst_info getGblEnv
355 ; return ( addTcgDUs gbl_env deriv_dus,
356 generic_inst_info ++ deriv_inst_info ++ local_info,
357 aux_binds `plusHsValBinds` deriv_binds)
360 -- Make sure that toplevel type instance are not for associated types.
361 -- !!!TODO: Need to perform this check for the TyThing of type functions,
363 tcIdxTyInstDeclTL ldecl@(L loc decl) =
364 do { tything <- tcFamInstDecl ldecl
366 when (isAssocFamily tything) $
367 addErr $ assocInClassErr (tcdName decl)
370 isAssocFamily (ATyCon tycon) =
371 case tyConFamInst_maybe tycon of
372 Nothing -> panic "isAssocFamily: no family?!?"
373 Just (fam, _) -> isTyConAssoc fam
374 isAssocFamily _ = panic "isAssocFamily: no tycon?!?"
376 assocInClassErr :: Name -> SDoc
377 assocInClassErr name =
378 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
379 ptext (sLit "must be inside a class instance")
381 addInsts :: [InstInfo Name] -> TcM a -> TcM a
382 addInsts infos thing_inside
383 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
385 addFamInsts :: [TyThing] -> TcM a -> TcM a
386 addFamInsts tycons thing_inside
387 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
389 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
390 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
395 tcLocalInstDecl1 :: LInstDecl Name
396 -> TcM (InstInfo Name, [TyThing])
397 -- A source-file instance declaration
398 -- Type-check all the stuff before the "where"
400 -- We check for respectable instance type, and context
401 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
403 addErrCtxt (instDeclCtxt1 poly_ty) $
405 do { is_boot <- tcIsHsBoot
406 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
409 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
411 -- Now, check the validity of the instance.
412 ; (clas, inst_tys) <- checkValidInstance poly_ty tyvars theta tau
414 -- Next, process any associated types.
415 ; idx_tycons <- recoverM (return []) $
416 do { idx_tycons <- checkNoErrs $ mapAndRecoverM tcFamInstDecl ats
417 ; checkValidAndMissingATs clas (tyvars, inst_tys)
419 ; return idx_tycons }
421 -- Finally, construct the Core representation of the instance.
422 -- (This no longer includes the associated types.)
423 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
424 -- Dfun location is that of instance *header*
425 ; overlap_flag <- getOverlapFlag
426 ; let (eq_theta,dict_theta) = partition isEqPred theta
427 theta' = eq_theta ++ dict_theta
428 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
429 ispec = mkLocalInstance dfun overlap_flag
431 ; return (InstInfo { iSpec = ispec,
432 iBinds = VanillaInst binds uprags False },
436 -- We pass in the source form and the type checked form of the ATs. We
437 -- really need the source form only to be able to produce more informative
439 checkValidAndMissingATs :: Class
440 -> ([TyVar], [TcType]) -- instance types
441 -> [(LTyClDecl Name, -- source form of AT
442 TyThing)] -- Core form of AT
444 checkValidAndMissingATs clas inst_tys ats
445 = do { -- Issue a warning for each class AT that is not defined in this
447 ; let class_ats = map tyConName (classATs clas)
448 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
449 omitted = filterOut (`elemNameSet` defined_ats) class_ats
450 ; warn <- doptM Opt_WarnMissingMethods
451 ; mapM_ (warnTc warn . omittedATWarn) omitted
453 -- Ensure that all AT indexes that correspond to class parameters
454 -- coincide with the types in the instance head. All remaining
455 -- AT arguments must be variables. Also raise an error for any
456 -- type instances that are not associated with this class.
457 ; mapM_ (checkIndexes clas inst_tys) ats
460 checkIndexes clas inst_tys (hsAT, ATyCon tycon)
461 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
462 = checkIndexes' clas inst_tys hsAT
464 snd . fromJust . tyConFamInst_maybe $ tycon)
465 checkIndexes _ _ _ = panic "checkIndexes"
467 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
468 = let atName = tcdName . unLoc $ hsAT
470 setSrcSpan (getLoc hsAT) $
471 addErrCtxt (atInstCtxt atName) $
472 case find ((atName ==) . tyConName) (classATs clas) of
473 Nothing -> addErrTc $ badATErr clas atName -- not in this class
475 case assocTyConArgPoss_maybe atycon of
476 Nothing -> panic "checkIndexes': AT has no args poss?!?"
479 -- The following is tricky! We need to deal with three
480 -- complications: (1) The AT possibly only uses a subset of
481 -- the class parameters as indexes and those it uses may be in
482 -- a different order; (2) the AT may have extra arguments,
483 -- which must be type variables; and (3) variables in AT and
484 -- instance head will be different `Name's even if their
485 -- source lexemes are identical.
487 -- e.g. class C a b c where
488 -- data D b a :: * -> * -- NB (1) b a, omits c
489 -- instance C [x] Bool Char where
490 -- data D Bool [x] v = MkD x [v] -- NB (2) v
491 -- -- NB (3) the x in 'instance C...' have differnt
492 -- -- Names to x's in 'data D...'
494 -- Re (1), `poss' contains a permutation vector to extract the
495 -- class parameters in the right order.
497 -- Re (2), we wrap the (permuted) class parameters in a Maybe
498 -- type and use Nothing for any extra AT arguments. (First
499 -- equation of `checkIndex' below.)
501 -- Re (3), we replace any type variable in the AT parameters
502 -- that has the same source lexeme as some variable in the
503 -- instance types with the instance type variable sharing its
506 let relevantInstTys = map (instTys !!) poss
507 instArgs = map Just relevantInstTys ++
508 repeat Nothing -- extra arguments
509 renaming = substSameTyVar atTvs instTvs
511 zipWithM_ checkIndex (substTys renaming atTys) instArgs
513 checkIndex ty Nothing
514 | isTyVarTy ty = return ()
515 | otherwise = addErrTc $ mustBeVarArgErr ty
516 checkIndex ty (Just instTy)
517 | ty `tcEqType` instTy = return ()
518 | otherwise = addErrTc $ wrongATArgErr ty instTy
520 listToNameSet = addListToNameSet emptyNameSet
522 substSameTyVar [] _ = emptyTvSubst
523 substSameTyVar (tv:tvs) replacingTvs =
524 let replacement = case find (tv `sameLexeme`) replacingTvs of
525 Nothing -> mkTyVarTy tv
526 Just rtv -> mkTyVarTy rtv
528 tv1 `sameLexeme` tv2 =
529 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
531 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
535 %************************************************************************
537 Type-checking instance declarations, pass 2
539 %************************************************************************
542 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
543 -> TcM (LHsBinds Id, TcLclEnv)
544 -- (a) From each class declaration,
545 -- generate any default-method bindings
546 -- (b) From each instance decl
547 -- generate the dfun binding
549 tcInstDecls2 tycl_decls inst_decls
550 = do { -- (a) Default methods from class decls
551 let class_decls = filter (isClassDecl . unLoc) tycl_decls
552 ; (dm_ids_s, dm_binds_s) <- mapAndUnzipM tcClassDecl2 class_decls
554 ; tcExtendIdEnv (concat dm_ids_s) $ do
556 -- (b) instance declarations
557 { inst_binds_s <- mapM tcInstDecl2 inst_decls
560 ; let binds = unionManyBags dm_binds_s `unionBags`
561 unionManyBags inst_binds_s
562 ; tcl_env <- getLclEnv -- Default method Ids in here
563 ; return (binds, tcl_env) } }
565 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
566 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
567 = recoverM (return emptyLHsBinds) $
569 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
570 tc_inst_decl2 dfun_id ibinds
572 dfun_id = instanceDFunId ispec
573 loc = getSrcSpan dfun_id
578 tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id)
579 -- Returns a binding for the dfun
581 ------------------------
582 -- Derived newtype instances; surprisingly tricky!
584 -- class Show a => Foo a b where ...
585 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
587 -- The newtype gives an FC axiom looking like
588 -- axiom CoN a :: N a ~ Tree [a]
589 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
591 -- So all need is to generate a binding looking like:
592 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
593 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
594 -- case df `cast` (Foo Int (sym (CoN a))) of
595 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
597 -- If there are no superclasses, matters are simpler, because we don't need the case
598 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
600 tc_inst_decl2 dfun_id (NewTypeDerived coi _)
601 = do { let rigid_info = InstSkol
602 origin = SigOrigin rigid_info
603 inst_ty = idType dfun_id
604 inst_tvs = fst (tcSplitForAllTys inst_ty)
605 ; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
606 -- inst_head_ty is a PredType
608 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
609 (class_tyvars, sc_theta, _, _) = classBigSig cls
610 cls_tycon = classTyCon cls
611 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
612 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
616 IdCo -> (last_ty, idHsWrapper)
617 ACo co -> (snd (coercionKind co'), WpCast (mk_full_coercion co'))
619 co' = substTyWith inst_tvs (mkTyVarTys inst_tvs') co
620 -- NB: the free variable of coi are bound by the
621 -- universally quantified variables of the dfun_id
622 -- This is weird, and maybe we should make NewTypeDerived
623 -- carry a type-variable list too; but it works fine
625 -----------------------
627 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
628 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
629 -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
630 -- where rep_ty is the (eta-reduced) type rep of T
631 -- So we just replace T with CoT, and insert a 'sym'
632 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
634 mk_full_coercion co = mkTyConApp cls_tycon
635 (initial_cls_inst_tys ++ [mkSymCoercion co])
636 -- Full coercion : (Foo Int (Tree [a]) ~ Foo Int (N a)
638 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
639 -- In our example, rep_pred is (Foo Int (Tree [a]))
641 ; sc_loc <- getInstLoc InstScOrigin
642 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
643 ; inst_loc <- getInstLoc origin
644 ; dfun_dicts <- newDictBndrs inst_loc theta
645 ; rep_dict <- newDictBndr inst_loc rep_pred
646 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
648 -- Figure out bindings for the superclass context from dfun_dicts
649 -- Don't include this_dict in the 'givens', else
650 -- sc_dicts get bound by just selecting from this_dict!!
651 ; sc_binds <- addErrCtxt superClassCtxt $
652 tcSimplifySuperClasses inst_loc this_dict dfun_dicts
655 -- It's possible that the superclass stuff might unified something
656 -- in the envt with one of the clas_tyvars
657 ; checkSigTyVars inst_tvs'
659 ; let coerced_rep_dict = wrapId wrapper (instToId rep_dict)
661 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
662 ; let dict_bind = mkVarBind (instToId this_dict) (noLoc body)
664 ; return (unitBag $ noLoc $
665 AbsBinds inst_tvs' (map instToVar dfun_dicts)
666 [(inst_tvs', dfun_id, instToId this_dict, noSpecPrags)]
667 (dict_bind `consBag` sc_binds)) }
669 -----------------------
670 -- (make_body C tys scs coreced_rep_dict)
672 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
673 -- But if there are no superclasses, it returns just coerced_rep_dict
674 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
676 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
677 | null sc_dicts -- Case (a)
678 = return coerced_rep_dict
679 | otherwise -- Case (b)
680 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
681 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
682 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
683 pat_dicts = dummy_sc_dict_ids,
684 pat_binds = emptyLHsBinds,
685 pat_args = PrefixCon (map nlVarPat op_ids),
687 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
688 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
689 map HsVar (sc_dict_ids ++ op_ids)
691 -- Warning: this HsCase scrutinises a value with a PredTy, which is
692 -- never otherwise seen in Haskell source code. It'd be
693 -- nicer to generate Core directly!
694 ; return (HsCase (noLoc coerced_rep_dict) $
695 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
697 sc_dict_ids = map instToId sc_dicts
698 pat_ty = mkTyConApp cls_tycon cls_inst_tys
699 cls_data_con = head (tyConDataCons cls_tycon)
700 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
701 op_tys = dropList sc_dict_ids cls_arg_tys
703 ------------------------
704 -- Ordinary instances
706 tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
707 = do { let rigid_info = InstSkol
708 inst_ty = idType dfun_id
709 loc = getSrcSpan dfun_id
711 -- Instantiate the instance decl with skolem constants
712 ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
713 -- These inst_tyvars' scope over the 'where' part
714 -- Those tyvars are inside the dfun_id's type, which is a bit
715 -- bizarre, but OK so long as you realise it!
717 (clas, inst_tys') = tcSplitDFunHead inst_head'
718 (class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas
720 -- Instantiate the super-class context with inst_tys
721 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
722 origin = SigOrigin rigid_info
724 -- Create dictionary Ids from the specified instance contexts.
725 ; inst_loc <- getInstLoc origin
726 ; dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
727 ; this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
728 -- Default-method Ids may be mentioned in synthesised RHSs,
729 -- but they'll already be in the environment.
732 -- Cook up a binding for "this = df d1 .. dn",
733 -- to use in each method binding
734 -- Need to clone the dict in case it is floated out, and
735 -- then clashes with its friends
736 ; cloned_this <- cloneDict this_dict
737 ; let cloned_this_bind = mkVarBind (instToId cloned_this) $
738 L loc $ wrapId app_wrapper dfun_id
739 app_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
740 dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
742 | null inst_tyvars' && null dfun_theta' = (this_dict, emptyBag)
743 | otherwise = (cloned_this, unitBag cloned_this_bind)
745 -- Deal with 'SPECIALISE instance' pragmas
746 -- See Note [SPECIALISE instance pragmas]
747 ; let spec_inst_sigs = filter isSpecInstLSig uprags
748 -- The filter removes the pragmas for methods
749 ; spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) spec_inst_sigs
751 -- Typecheck the methods
752 ; let prag_fn = mkPragFun uprags monobinds
753 tc_meth = tcInstanceMethod loc standalone_deriv
757 prag_fn spec_inst_prags monobinds
759 ; (meth_ids, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
760 mapAndUnzipM tc_meth op_items
762 -- Figure out bindings for the superclass context
763 ; sc_loc <- getInstLoc InstScOrigin
764 ; sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
765 ; let tc_sc = tcSuperClass inst_loc inst_tyvars' dfun_dicts nested_this_pair
766 ; (sc_ids, sc_binds) <- mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts)
768 -- It's possible that the superclass stuff might unified
769 -- something in the envt with one of the inst_tyvars'
770 ; checkSigTyVars inst_tyvars'
772 -- Create the result bindings
773 ; let dict_constr = classDataCon clas
774 this_dict_id = instToId this_dict
775 dict_bind = mkVarBind this_dict_id dict_rhs
776 dict_rhs = foldl mk_app inst_constr (sc_ids ++ meth_ids)
777 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
778 (dataConWrapId dict_constr)
779 -- We don't produce a binding for the dict_constr; instead we
780 -- rely on the simplifier to unfold this saturated application
781 -- We do this rather than generate an HsCon directly, because
782 -- it means that the special cases (e.g. dictionary with only one
783 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
784 -- than needing to be repeated here.
786 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
787 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
788 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
790 -- Do not inline the dfun; instead give it a magic DFunFunfolding
791 -- See Note [ClassOp/DFun selection]
792 -- See also note [Single-method classes]
793 dfun_id_w_fun = dfun_id
794 `setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids)
795 `setInlinePragma` dfunInlinePragma
800 [(inst_tyvars', dfun_id_w_fun, this_dict_id, SpecPrags spec_inst_prags)]
803 ; showLIE (text "instance")
804 ; return (unitBag (L loc main_bind) `unionBags`
805 listToBag meth_binds `unionBags`
810 -- Create the result bindings
811 ; let this_dict_id = instToId this_dict
812 arg_ids = sc_ids ++ meth_ids
813 arg_binds = listToBag meth_binds `unionBags`
816 ; showLIE (text "instance")
817 ; case newTyConCo_maybe (classTyCon clas) of
818 Nothing -- A multi-method class
819 -> return (unitBag (L loc data_bind) `unionBags` arg_binds)
821 data_dfun_id = dfun_id -- Do not inline; instead give it a magic DFunFunfolding
822 -- See Note [ClassOp/DFun selection]
823 `setIdUnfolding` mkDFunUnfolding dict_constr arg_ids
824 `setInlinePragma` dfunInlinePragma
826 data_bind = AbsBinds inst_tyvars' dfun_lam_vars
827 [(inst_tyvars', data_dfun_id, this_dict_id, spec_inst_prags)]
830 dict_bind = mkVarBind this_dict_id dict_rhs
831 dict_rhs = foldl mk_app inst_constr arg_ids
832 dict_constr = classDataCon clas
833 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
834 (dataConWrapId dict_constr)
835 -- We don't produce a binding for the dict_constr; instead we
836 -- rely on the simplifier to unfold this saturated application
837 -- We do this rather than generate an HsCon directly, because
838 -- it means that the special cases (e.g. dictionary with only one
839 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
840 -- than needing to be repeated here.
842 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
843 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
844 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
846 Just the_nt_co -- (Just co) for a single-method class
847 -> return (unitBag (L loc nt_bind) `unionBags` arg_binds)
849 nt_dfun_id = dfun_id -- Just let the dfun inline; see Note [Single-method classes]
850 `setInlinePragma` alwaysInlinePragma
852 local_nt_dfun = setIdType this_dict_id inst_ty -- A bit of a hack, but convenient
854 nt_bind = AbsBinds [] []
855 [([], nt_dfun_id, local_nt_dfun, spec_inst_prags)]
856 (unitBag (mkVarBind local_nt_dfun (L loc (wrapId nt_cast the_meth_id))))
858 the_meth_id = ASSERT( length arg_ids == 1 ) head arg_ids
859 nt_cast = WpCast $ mkPiTypes (inst_tyvars' ++ dfun_lam_vars) $
860 mkSymCoercion (mkTyConApp the_nt_co inst_tys')
863 ------------------------------
864 tcSuperClass :: InstLoc -> [TyVar] -> [Inst]
865 -> (Inst, LHsBinds Id)
866 -> (Id, Inst) -> TcM (Id, LHsBind Id)
867 -- Build a top level decl like
868 -- sc_op = /\a \d. let this = ... in
871 -- The "this" part is just-in-case (discarded if not used)
872 -- See Note [Recursive superclasses]
873 tcSuperClass inst_loc tyvars dicts (this_dict, this_bind)
875 = addErrCtxt superClassCtxt $
876 do { sc_binds <- tcSimplifySuperClasses inst_loc
877 this_dict dicts [sc_dict]
878 -- Don't include this_dict in the 'givens', else
879 -- sc_dicts get bound by just selecting from this_dict!!
882 ; let sc_op_ty = mkSigmaTy tyvars (map dictPred dicts)
883 (mkPredTy (dictPred sc_dict))
884 sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq
886 sc_op_id = mkLocalId sc_op_name sc_op_ty
887 sc_id = instToVar sc_dict
888 sc_op_bind = AbsBinds tyvars
889 (map instToVar dicts)
890 [(tyvars, sc_op_id, sc_id, noSpecPrags)]
891 (this_bind `unionBags` sc_binds)
893 ; return (sc_op_id, noLoc sc_op_bind) }
896 Note [Recursive superclasses]
897 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
898 See Trac #1470 for why we would *like* to add "this_dict" to the
899 available instances here. But we can't do so because then the superclases
900 get satisfied by selection from this_dict, and that leads to an immediate
901 loop. What we need is to add this_dict to Avails without adding its
902 superclasses, and we currently have no way to do that.
904 Note [SPECIALISE instance pragmas]
905 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
908 instance (Ix a, Ix b) => Ix (a,b) where
909 {-# SPECIALISE instance Ix (Int,Int) #-}
912 We do *not* want to make a specialised version of the dictionary
913 function. Rather, we want specialised versions of each method.
914 Thus we should generate something like this:
916 $dfIx :: (Ix a, Ix x) => Ix (a,b)
917 {- DFUN [$crange, ...] -}
918 $dfIx da db = Ix ($crange da db) (...other methods...)
920 $dfIxPair :: (Ix a, Ix x) => Ix (a,b)
921 {- DFUN [$crangePair, ...] -}
922 $dfIxPair = Ix ($crangePair da db) (...other methods...)
924 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
925 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
926 $crange da db = <blah>
928 {-# RULE range ($dfIx da db) = $crange da db #-}
932 * The RULE is unaffected by the specialisation. We don't want to
933 specialise $dfIx, because then it would need a specialised RULE
934 which is a pain. The single RULE works fine at all specialisations.
935 See Note [How instance declarations are translated] above
937 * Instead, we want to specialise the *method*, $crange
939 In practice, rather than faking up a SPECIALISE pragama for each
940 method (which is painful, since we'd have to figure out its
941 specialised type), we call tcSpecPrag *as if* were going to specialise
942 $dfIx -- you can see that in the call to tcSpecInst. That generates a
943 SpecPrag which, as it turns out, can be used unchanged for each method.
944 The "it turns out" bit is delicate, but it works fine!
947 tcSpecInst :: Id -> Sig Name -> TcM TcSpecPrag
948 tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
949 = addErrCtxt (spec_ctxt prag) $
950 do { let name = idName dfun_id
951 ; (tyvars, theta, tau) <- tcHsInstHead hs_ty
952 ; let spec_ty = mkSigmaTy tyvars theta tau
953 ; co_fn <- tcSubExp (SpecPragOrigin name) (idType dfun_id) spec_ty
954 ; return (SpecPrag co_fn defaultInlinePragma) }
956 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
958 tcSpecInst _ _ = panic "tcSpecInst"
961 %************************************************************************
963 Type-checking an instance method
965 %************************************************************************
968 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
969 - Remembering to use fresh Name (the instance method Name) as the binder
970 - Bring the instance method Ids into scope, for the benefit of tcInstSig
971 - Use sig_fn mapping instance method Name -> instance tyvars
973 - Use tcValBinds to do the checking
976 tcInstanceMethod :: SrcSpan -> Bool -> Class -> [TcTyVar] -> [Inst]
978 -> (Inst, LHsBinds Id) -- "This" and its binding
979 -> TcPragFun -- Local prags
980 -> [Located TcSpecPrag] -- Arising from 'SPECLALISE instance'
983 -> TcM (Id, LHsBind Id)
984 -- The returned inst_meth_ids all have types starting
985 -- forall tvs. theta => ...
987 tcInstanceMethod loc standalone_deriv clas tyvars dfun_dicts inst_tys
988 (this_dict, this_dict_bind)
989 prag_fn spec_inst_prags binds_in (sel_id, dm_info)
990 = do { uniq <- newUnique
991 ; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
992 ; local_meth_name <- newLocalName sel_name
993 -- Base the local_meth_name on the selector name, becuase
994 -- type errors from tcInstanceMethodBody come from here
996 ; let local_meth_ty = instantiateMethod clas sel_id inst_tys
997 meth_ty = mkSigmaTy tyvars (map dictPred dfun_dicts) local_meth_ty
998 meth_id = mkLocalId meth_name meth_ty
999 local_meth_id = mkLocalId local_meth_name local_meth_ty
1003 = add_meth_ctxt rn_bind $
1004 do { (meth_id1, spec_prags) <- tcPrags NonRecursive False True
1005 meth_id (prag_fn sel_name)
1006 ; tcInstanceMethodBody (instLoc this_dict)
1008 ([this_dict], this_dict_bind)
1009 meth_id1 local_meth_id
1011 (SpecPrags (spec_inst_prags ++ spec_prags))
1015 tc_default :: DefMeth -> TcM (Id, LHsBind Id)
1016 -- The user didn't supply a method binding, so we have to make
1017 -- up a default binding, in a way depending on the default-method info
1019 tc_default NoDefMeth -- No default method at all
1020 = do { warnMissingMethod sel_id
1021 ; return (meth_id, mkVarBind meth_id $
1022 mkLHsWrap lam_wrapper error_rhs) }
1024 tc_default GenDefMeth -- Derivable type classes stuff
1025 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id local_meth_name
1026 ; tc_body meth_bind }
1028 tc_default (DefMeth dm_name) -- An polymorphic default method
1029 = do { -- Build the typechecked version directly,
1030 -- without calling typecheck_method;
1031 -- see Note [Default methods in instances]
1032 -- Generate /\as.\ds. let this = df as ds
1033 -- in $dm inst_tys this
1034 -- The 'let' is necessary only because HsSyn doesn't allow
1035 -- you to apply a function to a dictionary *expression*.
1037 ; dm_id <- tcLookupId dm_name
1038 ; let dm_inline_prag = idInlinePragma dm_id
1039 rhs = HsWrap (WpApp (instToId this_dict) <.> mkWpTyApps inst_tys) $
1042 meth_bind = L loc $ VarBind { var_id = local_meth_id
1043 , var_rhs = L loc rhs
1044 , var_inline = False }
1045 meth_id1 = meth_id `setInlinePragma` dm_inline_prag
1046 -- Copy the inline pragma (if any) from the default
1047 -- method to this version. Note [INLINE and default methods]
1049 bind = AbsBinds { abs_tvs = tyvars, abs_dicts = dfun_lam_vars
1050 , abs_exports = [( tyvars, meth_id1, local_meth_id
1051 , SpecPrags spec_inst_prags)]
1052 , abs_binds = this_dict_bind `unionBags` unitBag meth_bind }
1053 -- Default methods in an instance declaration can't have their own
1054 -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
1055 -- currently they are rejected with
1056 -- "INLINE pragma lacks an accompanying binding"
1058 ; return (meth_id1, L loc bind) }
1060 ; case findMethodBind sel_name local_meth_name binds_in of
1061 Just user_bind -> tc_body user_bind -- User-supplied method binding
1062 Nothing -> tc_default dm_info -- None supplied
1065 sel_name = idName sel_id
1067 meth_sig_fn _ = Just [] -- The 'Just' says "yes, there's a type sig"
1068 -- But there are no scoped type variables from local_method_id
1069 -- Only the ones from the instance decl itself, which are already
1070 -- in scope. Example:
1071 -- class C a where { op :: forall b. Eq b => ... }
1072 -- instance C [c] where { op = <rhs> }
1073 -- In <rhs>, 'c' is scope but 'b' is not!
1075 error_rhs = L loc $ HsApp error_fun error_msg
1076 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
1077 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
1078 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
1079 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
1081 dfun_lam_vars = map instToVar dfun_dicts
1082 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_lam_vars
1084 -- For instance decls that come from standalone deriving clauses
1085 -- we want to print out the full source code if there's an error
1086 -- because otherwise the user won't see the code at all
1087 add_meth_ctxt rn_bind thing
1088 | standalone_deriv = addLandmarkErrCtxt (derivBindCtxt clas inst_tys rn_bind) thing
1091 wrapId :: HsWrapper -> id -> HsExpr id
1092 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
1094 derivBindCtxt :: Class -> [Type ] -> LHsBind Name -> SDoc
1095 derivBindCtxt clas tys bind
1096 = vcat [ ptext (sLit "When typechecking a standalone-derived method for")
1097 <+> quotes (pprClassPred clas tys) <> colon
1098 , nest 2 $ pprSetDepth AllTheWay $ ppr bind ]
1100 warnMissingMethod :: Id -> TcM ()
1101 warnMissingMethod sel_id
1102 = do { warn <- doptM Opt_WarnMissingMethods
1103 ; warnTc (warn -- Warn only if -fwarn-missing-methods
1104 && not (startsWithUnderscore (getOccName sel_id)))
1105 -- Don't warn about _foo methods
1106 (ptext (sLit "No explicit method nor default method for")
1107 <+> quotes (ppr sel_id)) }
1110 Note [Export helper functions]
1111 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1112 We arrange to export the "helper functions" of an instance declaration,
1113 so that they are not subject to preInlineUnconditionally, even if their
1114 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
1115 the dict fun as Ids, not as CoreExprs, so we can't substitute a
1116 non-variable for them.
1118 We could change this by making DFunUnfoldings have CoreExprs, but it
1119 seems a bit simpler this way.
1121 Note [Default methods in instances]
1122 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1129 instance Baz Int Int
1131 From the class decl we get
1133 $dmfoo :: forall v x. Baz v x => x -> x
1136 Notice that the type is ambiguous. That's fine, though. The instance
1139 $dBazIntInt = MkBaz fooIntInt
1140 fooIntInt = $dmfoo Int Int $dBazIntInt
1142 BUT this does mean we must generate the dictionary translation of
1143 fooIntInt directly, rather than generating source-code and
1144 type-checking it. That was the bug in Trac #1061. In any case it's
1145 less work to generate the translated version!
1147 Note [INLINE and default methods]
1148 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1149 Default methods need special case. They are supposed to behave rather like
1150 macros. For exmample
1153 op1, op2 :: Bool -> a -> a
1156 op1 b x = op2 (not b) x
1158 instance Foo Int where
1159 -- op1 via default method
1162 The instance declaration should behave
1164 just as if 'op1' had been defined with the
1165 code, and INLINE pragma, from its original
1168 That is, just as if you'd written
1170 instance Foo Int where
1174 op1 b x = op2 (not b) x
1176 So for the above example we generate:
1179 {-# INLINE $dmop1 #-}
1180 -- $dmop1 has an InlineCompulsory unfolding
1181 $dmop1 d b x = op2 d (not b) x
1183 $fFooInt = MkD $cop1 $cop2
1185 {-# INLINE $cop1 #-}
1186 $cop1 = $dmop1 $fFooInt
1192 * We *copy* any INLINE pragma from the default method $dmop1 to the
1193 instance $cop1. Otherwise we'll just inline the former in the
1194 latter and stop, which isn't what the user expected
1196 * Regardless of its pragma, we give the default method an
1197 unfolding with an InlineCompulsory source. That means
1198 that it'll be inlined at every use site, notably in
1199 each instance declaration, such as $cop1. This inlining
1200 must happen even though
1201 a) $dmop1 is not saturated in $cop1
1202 b) $cop1 itself has an INLINE pragma
1204 It's vital that $dmop1 *is* inlined in this way, to allow the mutual
1205 recursion between $fooInt and $cop1 to be broken
1207 * To communicate the need for an InlineCompulsory to the desugarer
1208 (which makes the Unfoldings), we use the IsDefaultMethod constructor
1212 %************************************************************************
1214 \subsection{Error messages}
1216 %************************************************************************
1219 instDeclCtxt1 :: LHsType Name -> SDoc
1220 instDeclCtxt1 hs_inst_ty
1221 = inst_decl_ctxt (case unLoc hs_inst_ty of
1222 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
1223 HsPredTy pred -> ppr pred
1224 _ -> ppr hs_inst_ty) -- Don't expect this
1225 instDeclCtxt2 :: Type -> SDoc
1226 instDeclCtxt2 dfun_ty
1227 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1229 (_,cls,tys) = tcSplitDFunTy dfun_ty
1231 inst_decl_ctxt :: SDoc -> SDoc
1232 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
1234 superClassCtxt :: SDoc
1235 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
1237 atInstCtxt :: Name -> SDoc
1238 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
1241 mustBeVarArgErr :: Type -> SDoc
1242 mustBeVarArgErr ty =
1243 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
1244 ptext (sLit "must be variables")
1245 , ptext (sLit "Instead of a variable, found") <+> ppr ty
1248 wrongATArgErr :: Type -> Type -> SDoc
1249 wrongATArgErr ty instTy =
1250 sep [ ptext (sLit "Type indexes must match class instance head")
1251 , ptext (sLit "Found") <+> quotes (ppr ty)
1252 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)