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 ; idx_tycons <- mapAndRecoverM (tcFamInstDecl TopLevel) $
318 filter (isFamInstDecl . unLoc) tycl_decls
319 ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_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 addInsts :: [InstInfo Name] -> TcM a -> TcM a
362 addInsts infos thing_inside
363 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
365 addFamInsts :: [TyThing] -> TcM a -> TcM a
366 addFamInsts tycons thing_inside
367 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
369 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
370 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
375 tcLocalInstDecl1 :: LInstDecl Name
376 -> TcM (InstInfo Name, [TyThing])
377 -- A source-file instance declaration
378 -- Type-check all the stuff before the "where"
380 -- We check for respectable instance type, and context
381 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
383 addErrCtxt (instDeclCtxt1 poly_ty) $
385 do { is_boot <- tcIsHsBoot
386 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
389 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
391 -- Now, check the validity of the instance.
392 ; (clas, inst_tys) <- checkValidInstance poly_ty tyvars theta tau
394 -- Next, process any associated types.
395 ; idx_tycons <- recoverM (return []) $
396 do { idx_tycons <- checkNoErrs $
397 mapAndRecoverM (tcFamInstDecl NotTopLevel) ats
398 ; checkValidAndMissingATs clas (tyvars, inst_tys)
400 ; return idx_tycons }
402 -- Finally, construct the Core representation of the instance.
403 -- (This no longer includes the associated types.)
404 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
405 -- Dfun location is that of instance *header*
406 ; overlap_flag <- getOverlapFlag
407 ; let (eq_theta,dict_theta) = partition isEqPred theta
408 theta' = eq_theta ++ dict_theta
409 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
410 ispec = mkLocalInstance dfun overlap_flag
412 ; return (InstInfo { iSpec = ispec,
413 iBinds = VanillaInst binds uprags False },
417 -- We pass in the source form and the type checked form of the ATs. We
418 -- really need the source form only to be able to produce more informative
420 checkValidAndMissingATs :: Class
421 -> ([TyVar], [TcType]) -- instance types
422 -> [(LTyClDecl Name, -- source form of AT
423 TyThing)] -- Core form of AT
425 checkValidAndMissingATs clas inst_tys ats
426 = do { -- Issue a warning for each class AT that is not defined in this
428 ; let class_ats = map tyConName (classATs clas)
429 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
430 omitted = filterOut (`elemNameSet` defined_ats) class_ats
431 ; warn <- doptM Opt_WarnMissingMethods
432 ; mapM_ (warnTc warn . omittedATWarn) omitted
434 -- Ensure that all AT indexes that correspond to class parameters
435 -- coincide with the types in the instance head. All remaining
436 -- AT arguments must be variables. Also raise an error for any
437 -- type instances that are not associated with this class.
438 ; mapM_ (checkIndexes clas inst_tys) ats
441 checkIndexes clas inst_tys (hsAT, ATyCon tycon)
442 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
443 = checkIndexes' clas inst_tys hsAT
445 snd . fromJust . tyConFamInst_maybe $ tycon)
446 checkIndexes _ _ _ = panic "checkIndexes"
448 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
449 = let atName = tcdName . unLoc $ hsAT
451 setSrcSpan (getLoc hsAT) $
452 addErrCtxt (atInstCtxt atName) $
453 case find ((atName ==) . tyConName) (classATs clas) of
454 Nothing -> addErrTc $ badATErr clas atName -- not in this class
456 case assocTyConArgPoss_maybe atycon of
457 Nothing -> panic "checkIndexes': AT has no args poss?!?"
460 -- The following is tricky! We need to deal with three
461 -- complications: (1) The AT possibly only uses a subset of
462 -- the class parameters as indexes and those it uses may be in
463 -- a different order; (2) the AT may have extra arguments,
464 -- which must be type variables; and (3) variables in AT and
465 -- instance head will be different `Name's even if their
466 -- source lexemes are identical.
468 -- e.g. class C a b c where
469 -- data D b a :: * -> * -- NB (1) b a, omits c
470 -- instance C [x] Bool Char where
471 -- data D Bool [x] v = MkD x [v] -- NB (2) v
472 -- -- NB (3) the x in 'instance C...' have differnt
473 -- -- Names to x's in 'data D...'
475 -- Re (1), `poss' contains a permutation vector to extract the
476 -- class parameters in the right order.
478 -- Re (2), we wrap the (permuted) class parameters in a Maybe
479 -- type and use Nothing for any extra AT arguments. (First
480 -- equation of `checkIndex' below.)
482 -- Re (3), we replace any type variable in the AT parameters
483 -- that has the same source lexeme as some variable in the
484 -- instance types with the instance type variable sharing its
487 let relevantInstTys = map (instTys !!) poss
488 instArgs = map Just relevantInstTys ++
489 repeat Nothing -- extra arguments
490 renaming = substSameTyVar atTvs instTvs
492 zipWithM_ checkIndex (substTys renaming atTys) instArgs
494 checkIndex ty Nothing
495 | isTyVarTy ty = return ()
496 | otherwise = addErrTc $ mustBeVarArgErr ty
497 checkIndex ty (Just instTy)
498 | ty `tcEqType` instTy = return ()
499 | otherwise = addErrTc $ wrongATArgErr ty instTy
501 listToNameSet = addListToNameSet emptyNameSet
503 substSameTyVar [] _ = emptyTvSubst
504 substSameTyVar (tv:tvs) replacingTvs =
505 let replacement = case find (tv `sameLexeme`) replacingTvs of
506 Nothing -> mkTyVarTy tv
507 Just rtv -> mkTyVarTy rtv
509 tv1 `sameLexeme` tv2 =
510 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
512 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
516 %************************************************************************
518 Type-checking instance declarations, pass 2
520 %************************************************************************
523 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
525 -- (a) From each class declaration,
526 -- generate any default-method bindings
527 -- (b) From each instance decl
528 -- generate the dfun binding
530 tcInstDecls2 tycl_decls inst_decls
531 = do { -- (a) Default methods from class decls
532 let class_decls = filter (isClassDecl . unLoc) tycl_decls
533 ; dm_binds_s <- mapM tcClassDecl2 class_decls
535 -- (b) instance declarations
536 ; inst_binds_s <- mapM tcInstDecl2 inst_decls
539 ; return (unionManyBags dm_binds_s `unionBags`
540 unionManyBags inst_binds_s) }
542 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
543 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
544 = recoverM (return emptyLHsBinds) $
546 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
547 tc_inst_decl2 dfun_id ibinds
549 dfun_id = instanceDFunId ispec
550 loc = getSrcSpan dfun_id
555 tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id)
556 -- Returns a binding for the dfun
558 ------------------------
559 -- Derived newtype instances; surprisingly tricky!
561 -- class Show a => Foo a b where ...
562 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
564 -- The newtype gives an FC axiom looking like
565 -- axiom CoN a :: N a ~ Tree [a]
566 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
568 -- So all need is to generate a binding looking like:
569 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
570 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
571 -- case df `cast` (Foo Int (sym (CoN a))) of
572 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
574 -- If there are no superclasses, matters are simpler, because we don't need the case
575 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
577 tc_inst_decl2 dfun_id (NewTypeDerived coi _)
578 = do { let rigid_info = InstSkol
579 origin = SigOrigin rigid_info
580 inst_ty = idType dfun_id
581 inst_tvs = fst (tcSplitForAllTys inst_ty)
582 ; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
583 -- inst_head_ty is a PredType
585 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
586 (class_tyvars, sc_theta, _, _) = classBigSig cls
587 cls_tycon = classTyCon cls
588 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
589 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
593 IdCo -> (last_ty, idHsWrapper)
594 ACo co -> (snd (coercionKind co'), WpCast (mk_full_coercion co'))
596 co' = substTyWith inst_tvs (mkTyVarTys inst_tvs') co
597 -- NB: the free variable of coi are bound by the
598 -- universally quantified variables of the dfun_id
599 -- This is weird, and maybe we should make NewTypeDerived
600 -- carry a type-variable list too; but it works fine
602 -----------------------
604 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
605 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
606 -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
607 -- where rep_ty is the (eta-reduced) type rep of T
608 -- So we just replace T with CoT, and insert a 'sym'
609 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
611 mk_full_coercion co = mkTyConApp cls_tycon
612 (initial_cls_inst_tys ++ [mkSymCoercion co])
613 -- Full coercion : (Foo Int (Tree [a]) ~ Foo Int (N a)
615 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
616 -- In our example, rep_pred is (Foo Int (Tree [a]))
618 ; sc_loc <- getInstLoc InstScOrigin
619 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
620 ; inst_loc <- getInstLoc origin
621 ; dfun_dicts <- newDictBndrs inst_loc theta
622 ; rep_dict <- newDictBndr inst_loc rep_pred
623 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
625 -- Figure out bindings for the superclass context from dfun_dicts
626 -- Don't include this_dict in the 'givens', else
627 -- sc_dicts get bound by just selecting from this_dict!!
628 ; sc_binds <- addErrCtxt superClassCtxt $
629 tcSimplifySuperClasses inst_loc this_dict dfun_dicts
632 -- It's possible that the superclass stuff might unified something
633 -- in the envt with one of the clas_tyvars
634 ; checkSigTyVars inst_tvs'
636 ; let coerced_rep_dict = wrapId wrapper (instToId rep_dict)
638 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
639 ; let dict_bind = mkVarBind (instToId this_dict) (noLoc body)
641 ; return (unitBag $ noLoc $
642 AbsBinds inst_tvs' (map instToVar dfun_dicts)
643 [(inst_tvs', dfun_id, instToId this_dict, noSpecPrags)]
644 (dict_bind `consBag` sc_binds)) }
646 -----------------------
647 -- (make_body C tys scs coreced_rep_dict)
649 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
650 -- But if there are no superclasses, it returns just coerced_rep_dict
651 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
653 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
654 | null sc_dicts -- Case (a)
655 = return coerced_rep_dict
656 | otherwise -- Case (b)
657 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
658 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
659 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
660 pat_dicts = dummy_sc_dict_ids,
661 pat_binds = emptyLHsBinds,
662 pat_args = PrefixCon (map nlVarPat op_ids),
664 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
665 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
666 map HsVar (sc_dict_ids ++ op_ids)
668 -- Warning: this HsCase scrutinises a value with a PredTy, which is
669 -- never otherwise seen in Haskell source code. It'd be
670 -- nicer to generate Core directly!
671 ; return (HsCase (noLoc coerced_rep_dict) $
672 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
674 sc_dict_ids = map instToId sc_dicts
675 pat_ty = mkTyConApp cls_tycon cls_inst_tys
676 cls_data_con = head (tyConDataCons cls_tycon)
677 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
678 op_tys = dropList sc_dict_ids cls_arg_tys
680 ------------------------
681 -- Ordinary instances
683 tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
684 = do { let rigid_info = InstSkol
685 inst_ty = idType dfun_id
686 loc = getSrcSpan dfun_id
688 -- Instantiate the instance decl with skolem constants
689 ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
690 -- These inst_tyvars' scope over the 'where' part
691 -- Those tyvars are inside the dfun_id's type, which is a bit
692 -- bizarre, but OK so long as you realise it!
694 (clas, inst_tys') = tcSplitDFunHead inst_head'
695 (class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas
697 -- Instantiate the super-class context with inst_tys
698 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
699 origin = SigOrigin rigid_info
701 -- Create dictionary Ids from the specified instance contexts.
702 ; inst_loc <- getInstLoc origin
703 ; dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
704 ; this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
705 -- Default-method Ids may be mentioned in synthesised RHSs,
706 -- but they'll already be in the environment.
709 -- Cook up a binding for "this = df d1 .. dn",
710 -- to use in each method binding
711 -- Need to clone the dict in case it is floated out, and
712 -- then clashes with its friends
713 ; cloned_this <- cloneDict this_dict
714 ; let cloned_this_bind = mkVarBind (instToId cloned_this) $
715 L loc $ wrapId app_wrapper dfun_id
716 app_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
717 dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
719 | null inst_tyvars' && null dfun_theta' = (this_dict, emptyBag)
720 | otherwise = (cloned_this, unitBag cloned_this_bind)
722 -- Deal with 'SPECIALISE instance' pragmas
723 -- See Note [SPECIALISE instance pragmas]
724 ; let spec_inst_sigs = filter isSpecInstLSig uprags
725 -- The filter removes the pragmas for methods
726 ; spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) spec_inst_sigs
728 -- Typecheck the methods
729 ; let prag_fn = mkPragFun uprags monobinds
730 tc_meth = tcInstanceMethod loc standalone_deriv
734 prag_fn spec_inst_prags monobinds
736 ; (meth_ids, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
737 mapAndUnzipM tc_meth op_items
739 -- Figure out bindings for the superclass context
740 ; sc_loc <- getInstLoc InstScOrigin
741 ; sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
742 ; let tc_sc = tcSuperClass inst_loc inst_tyvars' dfun_dicts nested_this_pair
743 ; (sc_ids, sc_binds) <- mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts)
745 -- It's possible that the superclass stuff might unified
746 -- something in the envt with one of the inst_tyvars'
747 ; checkSigTyVars inst_tyvars'
749 -- Create the result bindings
750 ; let dict_constr = classDataCon clas
751 this_dict_id = instToId this_dict
752 dict_bind = mkVarBind this_dict_id dict_rhs
753 dict_rhs = foldl mk_app inst_constr sc_meth_ids
754 sc_meth_ids = sc_ids ++ meth_ids
755 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
756 (dataConWrapId dict_constr)
757 -- We don't produce a binding for the dict_constr; instead we
758 -- rely on the simplifier to unfold this saturated application
759 -- We do this rather than generate an HsCon directly, because
760 -- it means that the special cases (e.g. dictionary with only one
761 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
762 -- than needing to be repeated here.
764 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
765 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
766 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
768 -- Do not inline the dfun; instead give it a magic DFunFunfolding
769 -- See Note [ClassOp/DFun selection]
770 -- See also note [Single-method classes]
771 dfun_id_w_fun = dfun_id
772 `setIdUnfolding` mkDFunUnfolding inst_ty (map Var sc_meth_ids)
773 `setInlinePragma` dfunInlinePragma
778 [(inst_tyvars', dfun_id_w_fun, this_dict_id, SpecPrags spec_inst_prags)]
781 ; showLIE (text "instance")
782 ; return (unitBag (L loc main_bind) `unionBags`
783 listToBag meth_binds `unionBags`
788 -- Create the result bindings
789 ; let this_dict_id = instToId this_dict
790 arg_ids = sc_ids ++ meth_ids
791 arg_binds = listToBag meth_binds `unionBags`
794 ; showLIE (text "instance")
795 ; case newTyConCo_maybe (classTyCon clas) of
796 Nothing -- A multi-method class
797 -> return (unitBag (L loc data_bind) `unionBags` arg_binds)
799 data_dfun_id = dfun_id -- Do not inline; instead give it a magic DFunFunfolding
800 -- See Note [ClassOp/DFun selection]
801 `setIdUnfolding` mkDFunUnfolding dict_constr arg_ids
802 `setInlinePragma` dfunInlinePragma
804 data_bind = AbsBinds inst_tyvars' dfun_lam_vars
805 [(inst_tyvars', data_dfun_id, this_dict_id, spec_inst_prags)]
808 dict_bind = mkVarBind this_dict_id dict_rhs
809 dict_rhs = foldl mk_app inst_constr arg_ids
810 dict_constr = classDataCon clas
811 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
812 (dataConWrapId dict_constr)
813 -- We don't produce a binding for the dict_constr; instead we
814 -- rely on the simplifier to unfold this saturated application
815 -- We do this rather than generate an HsCon directly, because
816 -- it means that the special cases (e.g. dictionary with only one
817 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
818 -- than needing to be repeated here.
820 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
821 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
822 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
824 Just the_nt_co -- (Just co) for a single-method class
825 -> return (unitBag (L loc nt_bind) `unionBags` arg_binds)
827 nt_dfun_id = dfun_id -- Just let the dfun inline; see Note [Single-method classes]
828 `setInlinePragma` alwaysInlinePragma
830 local_nt_dfun = setIdType this_dict_id inst_ty -- A bit of a hack, but convenient
832 nt_bind = AbsBinds [] []
833 [([], nt_dfun_id, local_nt_dfun, spec_inst_prags)]
834 (unitBag (mkVarBind local_nt_dfun (L loc (wrapId nt_cast the_meth_id))))
836 the_meth_id = ASSERT( length arg_ids == 1 ) head arg_ids
837 nt_cast = WpCast $ mkPiTypes (inst_tyvars' ++ dfun_lam_vars) $
838 mkSymCoercion (mkTyConApp the_nt_co inst_tys')
841 ------------------------------
842 tcSuperClass :: InstLoc -> [TyVar] -> [Inst]
843 -> (Inst, LHsBinds Id)
844 -> (Id, Inst) -> TcM (Id, LHsBind Id)
845 -- Build a top level decl like
846 -- sc_op = /\a \d. let this = ... in
849 -- The "this" part is just-in-case (discarded if not used)
850 -- See Note [Recursive superclasses]
851 tcSuperClass inst_loc tyvars dicts (this_dict, this_bind)
853 = addErrCtxt superClassCtxt $
854 do { sc_binds <- tcSimplifySuperClasses inst_loc
855 this_dict dicts [sc_dict]
856 -- Don't include this_dict in the 'givens', else
857 -- sc_dicts get bound by just selecting from this_dict!!
860 ; let sc_op_ty = mkSigmaTy tyvars (map dictPred dicts)
861 (mkPredTy (dictPred sc_dict))
862 sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq
864 sc_op_id = mkLocalId sc_op_name sc_op_ty
865 sc_id = instToVar sc_dict
866 sc_op_bind = AbsBinds tyvars
867 (map instToVar dicts)
868 [(tyvars, sc_op_id, sc_id, noSpecPrags)]
869 (this_bind `unionBags` sc_binds)
871 ; return (sc_op_id, noLoc sc_op_bind) }
874 Note [Recursive superclasses]
875 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
876 See Trac #1470 for why we would *like* to add "this_dict" to the
877 available instances here. But we can't do so because then the superclases
878 get satisfied by selection from this_dict, and that leads to an immediate
879 loop. What we need is to add this_dict to Avails without adding its
880 superclasses, and we currently have no way to do that.
882 Note [SPECIALISE instance pragmas]
883 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
886 instance (Ix a, Ix b) => Ix (a,b) where
887 {-# SPECIALISE instance Ix (Int,Int) #-}
890 We do *not* want to make a specialised version of the dictionary
891 function. Rather, we want specialised versions of each method.
892 Thus we should generate something like this:
894 $dfIx :: (Ix a, Ix x) => Ix (a,b)
895 {- DFUN [$crange, ...] -}
896 $dfIx da db = Ix ($crange da db) (...other methods...)
898 $dfIxPair :: (Ix a, Ix x) => Ix (a,b)
899 {- DFUN [$crangePair, ...] -}
900 $dfIxPair = Ix ($crangePair da db) (...other methods...)
902 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
903 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
904 $crange da db = <blah>
906 {-# RULE range ($dfIx da db) = $crange da db #-}
910 * The RULE is unaffected by the specialisation. We don't want to
911 specialise $dfIx, because then it would need a specialised RULE
912 which is a pain. The single RULE works fine at all specialisations.
913 See Note [How instance declarations are translated] above
915 * Instead, we want to specialise the *method*, $crange
917 In practice, rather than faking up a SPECIALISE pragama for each
918 method (which is painful, since we'd have to figure out its
919 specialised type), we call tcSpecPrag *as if* were going to specialise
920 $dfIx -- you can see that in the call to tcSpecInst. That generates a
921 SpecPrag which, as it turns out, can be used unchanged for each method.
922 The "it turns out" bit is delicate, but it works fine!
925 tcSpecInst :: Id -> Sig Name -> TcM TcSpecPrag
926 tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
927 = addErrCtxt (spec_ctxt prag) $
928 do { let name = idName dfun_id
929 ; (tyvars, theta, tau) <- tcHsInstHead hs_ty
930 ; let spec_ty = mkSigmaTy tyvars theta tau
931 ; co_fn <- tcSubExp (SpecPragOrigin name) (idType dfun_id) spec_ty
932 ; return (SpecPrag co_fn defaultInlinePragma) }
934 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
936 tcSpecInst _ _ = panic "tcSpecInst"
939 %************************************************************************
941 Type-checking an instance method
943 %************************************************************************
946 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
947 - Remembering to use fresh Name (the instance method Name) as the binder
948 - Bring the instance method Ids into scope, for the benefit of tcInstSig
949 - Use sig_fn mapping instance method Name -> instance tyvars
951 - Use tcValBinds to do the checking
954 tcInstanceMethod :: SrcSpan -> Bool -> Class -> [TcTyVar] -> [Inst]
956 -> (Inst, LHsBinds Id) -- "This" and its binding
957 -> TcPragFun -- Local prags
958 -> [Located TcSpecPrag] -- Arising from 'SPECLALISE instance'
961 -> TcM (Id, LHsBind Id)
962 -- The returned inst_meth_ids all have types starting
963 -- forall tvs. theta => ...
965 tcInstanceMethod loc standalone_deriv clas tyvars dfun_dicts inst_tys
966 (this_dict, this_dict_bind)
967 prag_fn spec_inst_prags binds_in (sel_id, dm_info)
968 = do { uniq <- newUnique
969 ; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
970 ; local_meth_name <- newLocalName sel_name
971 -- Base the local_meth_name on the selector name, becuase
972 -- type errors from tcInstanceMethodBody come from here
974 ; let local_meth_ty = instantiateMethod clas sel_id inst_tys
975 meth_ty = mkSigmaTy tyvars (map dictPred dfun_dicts) local_meth_ty
976 meth_id = mkLocalId meth_name meth_ty
977 local_meth_id = mkLocalId local_meth_name local_meth_ty
981 = add_meth_ctxt rn_bind $
982 do { (meth_id1, spec_prags) <- tcPrags NonRecursive False True
983 meth_id (prag_fn sel_name)
984 ; bind <- tcInstanceMethodBody (instLoc this_dict)
986 ([this_dict], this_dict_bind)
987 meth_id1 local_meth_id
989 (SpecPrags (spec_inst_prags ++ spec_prags))
991 ; return (meth_id1, bind) }
994 tc_default :: DefMeth -> TcM (Id, LHsBind Id)
995 -- The user didn't supply a method binding, so we have to make
996 -- up a default binding, in a way depending on the default-method info
998 tc_default NoDefMeth -- No default method at all
999 = do { warnMissingMethod sel_id
1000 ; return (meth_id, mkVarBind meth_id $
1001 mkLHsWrap lam_wrapper error_rhs) }
1003 tc_default GenDefMeth -- Derivable type classes stuff
1004 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id local_meth_name
1005 ; tc_body meth_bind }
1007 tc_default (DefMeth dm_name) -- An polymorphic default method
1008 = do { -- Build the typechecked version directly,
1009 -- without calling typecheck_method;
1010 -- see Note [Default methods in instances]
1011 -- Generate /\as.\ds. let this = df as ds
1012 -- in $dm inst_tys this
1013 -- The 'let' is necessary only because HsSyn doesn't allow
1014 -- you to apply a function to a dictionary *expression*.
1016 ; dm_id <- tcLookupId dm_name
1017 ; let dm_inline_prag = idInlinePragma dm_id
1018 rhs = HsWrap (WpApp (instToId this_dict) <.> mkWpTyApps inst_tys) $
1021 meth_bind = L loc $ VarBind { var_id = local_meth_id
1022 , var_rhs = L loc rhs
1023 , var_inline = False }
1024 meth_id1 = meth_id `setInlinePragma` dm_inline_prag
1025 -- Copy the inline pragma (if any) from the default
1026 -- method to this version. Note [INLINE and default methods]
1028 bind = AbsBinds { abs_tvs = tyvars, abs_dicts = dfun_lam_vars
1029 , abs_exports = [( tyvars, meth_id1, local_meth_id
1030 , SpecPrags spec_inst_prags)]
1031 , abs_binds = this_dict_bind `unionBags` unitBag meth_bind }
1032 -- Default methods in an instance declaration can't have their own
1033 -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
1034 -- currently they are rejected with
1035 -- "INLINE pragma lacks an accompanying binding"
1037 ; return (meth_id1, L loc bind) }
1039 ; case findMethodBind sel_name local_meth_name binds_in of
1040 Just user_bind -> tc_body user_bind -- User-supplied method binding
1041 Nothing -> tc_default dm_info -- None supplied
1044 sel_name = idName sel_id
1046 meth_sig_fn _ = Just [] -- The 'Just' says "yes, there's a type sig"
1047 -- But there are no scoped type variables from local_method_id
1048 -- Only the ones from the instance decl itself, which are already
1049 -- in scope. Example:
1050 -- class C a where { op :: forall b. Eq b => ... }
1051 -- instance C [c] where { op = <rhs> }
1052 -- In <rhs>, 'c' is scope but 'b' is not!
1054 error_rhs = L loc $ HsApp error_fun error_msg
1055 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
1056 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
1057 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
1058 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
1060 dfun_lam_vars = map instToVar dfun_dicts
1061 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_lam_vars
1063 -- For instance decls that come from standalone deriving clauses
1064 -- we want to print out the full source code if there's an error
1065 -- because otherwise the user won't see the code at all
1066 add_meth_ctxt rn_bind thing
1067 | standalone_deriv = addLandmarkErrCtxt (derivBindCtxt clas inst_tys rn_bind) thing
1070 wrapId :: HsWrapper -> id -> HsExpr id
1071 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
1073 derivBindCtxt :: Class -> [Type ] -> LHsBind Name -> SDoc
1074 derivBindCtxt clas tys bind
1075 = vcat [ ptext (sLit "When typechecking a standalone-derived method for")
1076 <+> quotes (pprClassPred clas tys) <> colon
1077 , nest 2 $ pprSetDepth AllTheWay $ ppr bind ]
1079 warnMissingMethod :: Id -> TcM ()
1080 warnMissingMethod sel_id
1081 = do { warn <- doptM Opt_WarnMissingMethods
1082 ; warnTc (warn -- Warn only if -fwarn-missing-methods
1083 && not (startsWithUnderscore (getOccName sel_id)))
1084 -- Don't warn about _foo methods
1085 (ptext (sLit "No explicit method nor default method for")
1086 <+> quotes (ppr sel_id)) }
1089 Note [Export helper functions]
1090 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1091 We arrange to export the "helper functions" of an instance declaration,
1092 so that they are not subject to preInlineUnconditionally, even if their
1093 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
1094 the dict fun as Ids, not as CoreExprs, so we can't substitute a
1095 non-variable for them.
1097 We could change this by making DFunUnfoldings have CoreExprs, but it
1098 seems a bit simpler this way.
1100 Note [Default methods in instances]
1101 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1108 instance Baz Int Int
1110 From the class decl we get
1112 $dmfoo :: forall v x. Baz v x => x -> x
1115 Notice that the type is ambiguous. That's fine, though. The instance
1118 $dBazIntInt = MkBaz fooIntInt
1119 fooIntInt = $dmfoo Int Int $dBazIntInt
1121 BUT this does mean we must generate the dictionary translation of
1122 fooIntInt directly, rather than generating source-code and
1123 type-checking it. That was the bug in Trac #1061. In any case it's
1124 less work to generate the translated version!
1126 Note [INLINE and default methods]
1127 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1128 Default methods need special case. They are supposed to behave rather like
1129 macros. For exmample
1132 op1, op2 :: Bool -> a -> a
1135 op1 b x = op2 (not b) x
1137 instance Foo Int where
1138 -- op1 via default method
1141 The instance declaration should behave
1143 just as if 'op1' had been defined with the
1144 code, and INLINE pragma, from its original
1147 That is, just as if you'd written
1149 instance Foo Int where
1153 op1 b x = op2 (not b) x
1155 So for the above example we generate:
1158 {-# INLINE $dmop1 #-}
1159 -- $dmop1 has an InlineCompulsory unfolding
1160 $dmop1 d b x = op2 d (not b) x
1162 $fFooInt = MkD $cop1 $cop2
1164 {-# INLINE $cop1 #-}
1165 $cop1 = $dmop1 $fFooInt
1171 * We *copy* any INLINE pragma from the default method $dmop1 to the
1172 instance $cop1. Otherwise we'll just inline the former in the
1173 latter and stop, which isn't what the user expected
1175 * Regardless of its pragma, we give the default method an
1176 unfolding with an InlineCompulsory source. That means
1177 that it'll be inlined at every use site, notably in
1178 each instance declaration, such as $cop1. This inlining
1179 must happen even though
1180 a) $dmop1 is not saturated in $cop1
1181 b) $cop1 itself has an INLINE pragma
1183 It's vital that $dmop1 *is* inlined in this way, to allow the mutual
1184 recursion between $fooInt and $cop1 to be broken
1186 * To communicate the need for an InlineCompulsory to the desugarer
1187 (which makes the Unfoldings), we use the IsDefaultMethod constructor
1191 %************************************************************************
1193 \subsection{Error messages}
1195 %************************************************************************
1198 instDeclCtxt1 :: LHsType Name -> SDoc
1199 instDeclCtxt1 hs_inst_ty
1200 = inst_decl_ctxt (case unLoc hs_inst_ty of
1201 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
1202 HsPredTy pred -> ppr pred
1203 _ -> ppr hs_inst_ty) -- Don't expect this
1204 instDeclCtxt2 :: Type -> SDoc
1205 instDeclCtxt2 dfun_ty
1206 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1208 (_,cls,tys) = tcSplitDFunTy dfun_ty
1210 inst_decl_ctxt :: SDoc -> SDoc
1211 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
1213 superClassCtxt :: SDoc
1214 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
1216 atInstCtxt :: Name -> SDoc
1217 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
1220 mustBeVarArgErr :: Type -> SDoc
1221 mustBeVarArgErr ty =
1222 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
1223 ptext (sLit "must be variables")
1224 , ptext (sLit "Instead of a variable, found") <+> ppr ty
1227 wrongATArgErr :: Type -> Type -> SDoc
1228 wrongATArgErr ty instTy =
1229 sep [ ptext (sLit "Type indexes must match class instance head")
1230 , ptext (sLit "Found") <+> quotes (ppr ty)
1231 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)