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
534 ; let dm_binds = unionManyBags dm_binds_s
536 -- (b) instance declarations
537 ; let dm_ids = collectHsBindsBinders dm_binds
538 -- Add the default method Ids (again)
539 -- See Note [Default methods and instances]
540 ; inst_binds_s <- tcExtendIdEnv dm_ids $
541 mapM tcInstDecl2 inst_decls
544 ; return (dm_binds `unionBags` unionManyBags inst_binds_s) }
546 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
547 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
548 = recoverM (return emptyLHsBinds) $
550 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
551 tc_inst_decl2 dfun_id ibinds
553 dfun_id = instanceDFunId ispec
554 loc = getSrcSpan dfun_id
557 See Note [Default methods and instances]
558 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
559 The default method Ids are already in the type environment (see Note
560 [Default method Ids and Template Haskell] in TcTyClsDcls), BUT they
561 don't have their InlinePragmas yet. Usually that would not matter,
562 because the simplifier propagates information from binding site to
563 use. But, unusually, when compiling instance decls we *copy* the
564 INLINE pragma from the default method to the method for that
565 particular operation (see Note [INLINE and default methods] below).
567 So right here in tcInstDecl2 we must re-extend the type envt with
568 the default method Ids replete with their INLINE pragmas. Urk.
571 tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id)
572 -- Returns a binding for the dfun
574 ------------------------
575 -- Derived newtype instances; surprisingly tricky!
577 -- class Show a => Foo a b where ...
578 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
580 -- The newtype gives an FC axiom looking like
581 -- axiom CoN a :: N a ~ Tree [a]
582 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
584 -- So all need is to generate a binding looking like:
585 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
586 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
587 -- case df `cast` (Foo Int (sym (CoN a))) of
588 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
590 -- If there are no superclasses, matters are simpler, because we don't need the case
591 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
593 tc_inst_decl2 dfun_id (NewTypeDerived coi _)
594 = do { let rigid_info = InstSkol
595 origin = SigOrigin rigid_info
596 inst_ty = idType dfun_id
597 inst_tvs = fst (tcSplitForAllTys inst_ty)
598 ; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
599 -- inst_head_ty is a PredType
601 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
602 (class_tyvars, sc_theta, _, _) = classBigSig cls
603 cls_tycon = classTyCon cls
604 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
605 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
609 IdCo -> (last_ty, idHsWrapper)
610 ACo co -> (snd (coercionKind co'), WpCast (mk_full_coercion co'))
612 co' = substTyWith inst_tvs (mkTyVarTys inst_tvs') co
613 -- NB: the free variable of coi are bound by the
614 -- universally quantified variables of the dfun_id
615 -- This is weird, and maybe we should make NewTypeDerived
616 -- carry a type-variable list too; but it works fine
618 -----------------------
620 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
621 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
622 -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
623 -- where rep_ty is the (eta-reduced) type rep of T
624 -- So we just replace T with CoT, and insert a 'sym'
625 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
627 mk_full_coercion co = mkTyConApp cls_tycon
628 (initial_cls_inst_tys ++ [mkSymCoercion co])
629 -- Full coercion : (Foo Int (Tree [a]) ~ Foo Int (N a)
631 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
632 -- In our example, rep_pred is (Foo Int (Tree [a]))
634 ; sc_loc <- getInstLoc InstScOrigin
635 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
636 ; inst_loc <- getInstLoc origin
637 ; dfun_dicts <- newDictBndrs inst_loc theta
638 ; rep_dict <- newDictBndr inst_loc rep_pred
639 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
641 -- Figure out bindings for the superclass context from dfun_dicts
642 -- Don't include this_dict in the 'givens', else
643 -- sc_dicts get bound by just selecting from this_dict!!
644 ; sc_binds <- addErrCtxt superClassCtxt $
645 tcSimplifySuperClasses inst_loc this_dict dfun_dicts
648 -- It's possible that the superclass stuff might unified something
649 -- in the envt with one of the clas_tyvars
650 ; checkSigTyVars inst_tvs'
652 ; let coerced_rep_dict = wrapId wrapper (instToId rep_dict)
654 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
655 ; let dict_bind = mkVarBind (instToId this_dict) (noLoc body)
657 ; return (unitBag $ noLoc $
658 AbsBinds inst_tvs' (map instToVar dfun_dicts)
659 [(inst_tvs', dfun_id, instToId this_dict, noSpecPrags)]
660 (dict_bind `consBag` sc_binds)) }
662 -----------------------
663 -- (make_body C tys scs coreced_rep_dict)
665 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
666 -- But if there are no superclasses, it returns just coerced_rep_dict
667 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
669 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
670 | null sc_dicts -- Case (a)
671 = return coerced_rep_dict
672 | otherwise -- Case (b)
673 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
674 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
675 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
676 pat_dicts = dummy_sc_dict_ids,
677 pat_binds = emptyLHsBinds,
678 pat_args = PrefixCon (map nlVarPat op_ids),
680 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
681 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
682 map HsVar (sc_dict_ids ++ op_ids)
684 -- Warning: this HsCase scrutinises a value with a PredTy, which is
685 -- never otherwise seen in Haskell source code. It'd be
686 -- nicer to generate Core directly!
687 ; return (HsCase (noLoc coerced_rep_dict) $
688 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
690 sc_dict_ids = map instToId sc_dicts
691 pat_ty = mkTyConApp cls_tycon cls_inst_tys
692 cls_data_con = head (tyConDataCons cls_tycon)
693 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
694 op_tys = dropList sc_dict_ids cls_arg_tys
696 ------------------------
697 -- Ordinary instances
699 tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
700 = do { let rigid_info = InstSkol
701 inst_ty = idType dfun_id
702 loc = getSrcSpan dfun_id
704 -- Instantiate the instance decl with skolem constants
705 ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
706 -- These inst_tyvars' scope over the 'where' part
707 -- Those tyvars are inside the dfun_id's type, which is a bit
708 -- bizarre, but OK so long as you realise it!
710 (clas, inst_tys') = tcSplitDFunHead inst_head'
711 (class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas
713 -- Instantiate the super-class context with inst_tys
714 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
715 origin = SigOrigin rigid_info
717 -- Create dictionary Ids from the specified instance contexts.
718 ; inst_loc <- getInstLoc origin
719 ; dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
720 ; this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
721 -- Default-method Ids may be mentioned in synthesised RHSs,
722 -- but they'll already be in the environment.
725 -- Cook up a binding for "this = df d1 .. dn",
726 -- to use in each method binding
727 -- Need to clone the dict in case it is floated out, and
728 -- then clashes with its friends
729 ; cloned_this <- cloneDict this_dict
730 ; let cloned_this_bind = mkVarBind (instToId cloned_this) $
731 L loc $ wrapId app_wrapper dfun_id
732 app_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
733 dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
735 | null inst_tyvars' && null dfun_theta' = (this_dict, emptyBag)
736 | otherwise = (cloned_this, unitBag cloned_this_bind)
738 -- Deal with 'SPECIALISE instance' pragmas
739 -- See Note [SPECIALISE instance pragmas]
740 ; let spec_inst_sigs = filter isSpecInstLSig uprags
741 -- The filter removes the pragmas for methods
742 ; spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) spec_inst_sigs
744 -- Typecheck the methods
745 ; let prag_fn = mkPragFun uprags monobinds
746 tc_meth = tcInstanceMethod loc standalone_deriv
750 prag_fn spec_inst_prags monobinds
752 ; (meth_ids, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
753 mapAndUnzipM tc_meth op_items
755 -- Figure out bindings for the superclass context
756 ; sc_loc <- getInstLoc InstScOrigin
757 ; sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
758 ; let tc_sc = tcSuperClass inst_loc inst_tyvars' dfun_dicts nested_this_pair
759 ; (sc_ids, sc_binds) <- mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts)
761 -- It's possible that the superclass stuff might unified
762 -- something in the envt with one of the inst_tyvars'
763 ; checkSigTyVars inst_tyvars'
765 -- Create the result bindings
766 ; let dict_constr = classDataCon clas
767 this_dict_id = instToId this_dict
768 dict_bind = mkVarBind this_dict_id dict_rhs
769 dict_rhs = foldl mk_app inst_constr sc_meth_ids
770 sc_meth_ids = sc_ids ++ meth_ids
771 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
772 (dataConWrapId dict_constr)
773 -- We don't produce a binding for the dict_constr; instead we
774 -- rely on the simplifier to unfold this saturated application
775 -- We do this rather than generate an HsCon directly, because
776 -- it means that the special cases (e.g. dictionary with only one
777 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
778 -- than needing to be repeated here.
780 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
781 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
782 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
784 -- Do not inline the dfun; instead give it a magic DFunFunfolding
785 -- See Note [ClassOp/DFun selection]
786 -- See also note [Single-method classes]
787 dfun_id_w_fun = dfun_id
788 `setIdUnfolding` mkDFunUnfolding inst_ty (map Var sc_meth_ids)
789 `setInlinePragma` dfunInlinePragma
794 [(inst_tyvars', dfun_id_w_fun, this_dict_id, SpecPrags spec_inst_prags)]
797 ; showLIE (text "instance")
798 ; return (unitBag (L loc main_bind) `unionBags`
799 listToBag meth_binds `unionBags`
804 -- Create the result bindings
805 ; let this_dict_id = instToId this_dict
806 arg_ids = sc_ids ++ meth_ids
807 arg_binds = listToBag meth_binds `unionBags`
810 ; showLIE (text "instance")
811 ; case newTyConCo_maybe (classTyCon clas) of
812 Nothing -- A multi-method class
813 -> return (unitBag (L loc data_bind) `unionBags` arg_binds)
815 data_dfun_id = dfun_id -- Do not inline; instead give it a magic DFunFunfolding
816 -- See Note [ClassOp/DFun selection]
817 `setIdUnfolding` mkDFunUnfolding dict_constr arg_ids
818 `setInlinePragma` dfunInlinePragma
820 data_bind = AbsBinds inst_tyvars' dfun_lam_vars
821 [(inst_tyvars', data_dfun_id, this_dict_id, spec_inst_prags)]
824 dict_bind = mkVarBind this_dict_id dict_rhs
825 dict_rhs = foldl mk_app inst_constr arg_ids
826 dict_constr = classDataCon clas
827 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
828 (dataConWrapId dict_constr)
829 -- We don't produce a binding for the dict_constr; instead we
830 -- rely on the simplifier to unfold this saturated application
831 -- We do this rather than generate an HsCon directly, because
832 -- it means that the special cases (e.g. dictionary with only one
833 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
834 -- than needing to be repeated here.
836 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
837 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
838 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
840 Just the_nt_co -- (Just co) for a single-method class
841 -> return (unitBag (L loc nt_bind) `unionBags` arg_binds)
843 nt_dfun_id = dfun_id -- Just let the dfun inline; see Note [Single-method classes]
844 `setInlinePragma` alwaysInlinePragma
846 local_nt_dfun = setIdType this_dict_id inst_ty -- A bit of a hack, but convenient
848 nt_bind = AbsBinds [] []
849 [([], nt_dfun_id, local_nt_dfun, spec_inst_prags)]
850 (unitBag (mkVarBind local_nt_dfun (L loc (wrapId nt_cast the_meth_id))))
852 the_meth_id = ASSERT( length arg_ids == 1 ) head arg_ids
853 nt_cast = WpCast $ mkPiTypes (inst_tyvars' ++ dfun_lam_vars) $
854 mkSymCoercion (mkTyConApp the_nt_co inst_tys')
857 ------------------------------
858 tcSuperClass :: InstLoc -> [TyVar] -> [Inst]
859 -> (Inst, LHsBinds Id)
860 -> (Id, Inst) -> TcM (Id, LHsBind Id)
861 -- Build a top level decl like
862 -- sc_op = /\a \d. let this = ... in
865 -- The "this" part is just-in-case (discarded if not used)
866 -- See Note [Recursive superclasses]
867 tcSuperClass inst_loc tyvars dicts (this_dict, this_bind)
869 = addErrCtxt superClassCtxt $
870 do { sc_binds <- tcSimplifySuperClasses inst_loc
871 this_dict dicts [sc_dict]
872 -- Don't include this_dict in the 'givens', else
873 -- sc_dicts get bound by just selecting from this_dict!!
876 ; let sc_op_ty = mkSigmaTy tyvars (map dictPred dicts)
877 (mkPredTy (dictPred sc_dict))
878 sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq
880 sc_op_id = mkLocalId sc_op_name sc_op_ty
881 sc_id = instToVar sc_dict
882 sc_op_bind = AbsBinds tyvars
883 (map instToVar dicts)
884 [(tyvars, sc_op_id, sc_id, noSpecPrags)]
885 (this_bind `unionBags` sc_binds)
887 ; return (sc_op_id, noLoc sc_op_bind) }
890 Note [Recursive superclasses]
891 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
892 See Trac #1470 for why we would *like* to add "this_dict" to the
893 available instances here. But we can't do so because then the superclases
894 get satisfied by selection from this_dict, and that leads to an immediate
895 loop. What we need is to add this_dict to Avails without adding its
896 superclasses, and we currently have no way to do that.
898 Note [SPECIALISE instance pragmas]
899 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
902 instance (Ix a, Ix b) => Ix (a,b) where
903 {-# SPECIALISE instance Ix (Int,Int) #-}
906 We do *not* want to make a specialised version of the dictionary
907 function. Rather, we want specialised versions of each method.
908 Thus we should generate something like this:
910 $dfIx :: (Ix a, Ix x) => Ix (a,b)
911 {- DFUN [$crange, ...] -}
912 $dfIx da db = Ix ($crange da db) (...other methods...)
914 $dfIxPair :: (Ix a, Ix x) => Ix (a,b)
915 {- DFUN [$crangePair, ...] -}
916 $dfIxPair = Ix ($crangePair da db) (...other methods...)
918 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
919 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
920 $crange da db = <blah>
922 {-# RULE range ($dfIx da db) = $crange da db #-}
926 * The RULE is unaffected by the specialisation. We don't want to
927 specialise $dfIx, because then it would need a specialised RULE
928 which is a pain. The single RULE works fine at all specialisations.
929 See Note [How instance declarations are translated] above
931 * Instead, we want to specialise the *method*, $crange
933 In practice, rather than faking up a SPECIALISE pragama for each
934 method (which is painful, since we'd have to figure out its
935 specialised type), we call tcSpecPrag *as if* were going to specialise
936 $dfIx -- you can see that in the call to tcSpecInst. That generates a
937 SpecPrag which, as it turns out, can be used unchanged for each method.
938 The "it turns out" bit is delicate, but it works fine!
941 tcSpecInst :: Id -> Sig Name -> TcM TcSpecPrag
942 tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
943 = addErrCtxt (spec_ctxt prag) $
944 do { let name = idName dfun_id
945 ; (tyvars, theta, tau) <- tcHsInstHead hs_ty
946 ; let spec_ty = mkSigmaTy tyvars theta tau
947 ; co_fn <- tcSubExp (SpecPragOrigin name) (idType dfun_id) spec_ty
948 ; return (SpecPrag co_fn defaultInlinePragma) }
950 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
952 tcSpecInst _ _ = panic "tcSpecInst"
955 %************************************************************************
957 Type-checking an instance method
959 %************************************************************************
962 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
963 - Remembering to use fresh Name (the instance method Name) as the binder
964 - Bring the instance method Ids into scope, for the benefit of tcInstSig
965 - Use sig_fn mapping instance method Name -> instance tyvars
967 - Use tcValBinds to do the checking
970 tcInstanceMethod :: SrcSpan -> Bool -> Class -> [TcTyVar] -> [Inst]
972 -> (Inst, LHsBinds Id) -- "This" and its binding
973 -> TcPragFun -- Local prags
974 -> [Located TcSpecPrag] -- Arising from 'SPECLALISE instance'
977 -> TcM (Id, LHsBind Id)
978 -- The returned inst_meth_ids all have types starting
979 -- forall tvs. theta => ...
981 tcInstanceMethod loc standalone_deriv clas tyvars dfun_dicts inst_tys
982 (this_dict, this_dict_bind)
983 prag_fn spec_inst_prags binds_in (sel_id, dm_info)
984 = do { uniq <- newUnique
985 ; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
986 ; local_meth_name <- newLocalName sel_name
987 -- Base the local_meth_name on the selector name, becuase
988 -- type errors from tcInstanceMethodBody come from here
990 ; let local_meth_ty = instantiateMethod clas sel_id inst_tys
991 meth_ty = mkSigmaTy tyvars (map dictPred dfun_dicts) local_meth_ty
992 meth_id = mkLocalId meth_name meth_ty
993 local_meth_id = mkLocalId local_meth_name local_meth_ty
997 = add_meth_ctxt rn_bind $
998 do { (meth_id1, spec_prags) <- tcPrags NonRecursive False True
999 meth_id (prag_fn sel_name)
1000 ; bind <- tcInstanceMethodBody (instLoc this_dict)
1002 ([this_dict], this_dict_bind)
1003 meth_id1 local_meth_id
1005 (SpecPrags (spec_inst_prags ++ spec_prags))
1007 ; return (meth_id1, bind) }
1010 tc_default :: DefMeth -> TcM (Id, LHsBind Id)
1011 -- The user didn't supply a method binding, so we have to make
1012 -- up a default binding, in a way depending on the default-method info
1014 tc_default NoDefMeth -- No default method at all
1015 = do { warnMissingMethod sel_id
1016 ; return (meth_id, mkVarBind meth_id $
1017 mkLHsWrap lam_wrapper error_rhs) }
1019 tc_default GenDefMeth -- Derivable type classes stuff
1020 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id local_meth_name
1021 ; tc_body meth_bind }
1023 tc_default (DefMeth dm_name) -- An polymorphic default method
1024 = do { -- Build the typechecked version directly,
1025 -- without calling typecheck_method;
1026 -- see Note [Default methods in instances]
1027 -- Generate /\as.\ds. let this = df as ds
1028 -- in $dm inst_tys this
1029 -- The 'let' is necessary only because HsSyn doesn't allow
1030 -- you to apply a function to a dictionary *expression*.
1032 ; dm_id <- tcLookupId dm_name
1033 ; let dm_inline_prag = idInlinePragma dm_id
1034 rhs = HsWrap (WpApp (instToId this_dict) <.> mkWpTyApps inst_tys) $
1037 meth_bind = L loc $ VarBind { var_id = local_meth_id
1038 , var_rhs = L loc rhs
1039 , var_inline = False }
1040 meth_id1 = meth_id `setInlinePragma` dm_inline_prag
1041 -- Copy the inline pragma (if any) from the default
1042 -- method to this version. Note [INLINE and default methods]
1044 bind = AbsBinds { abs_tvs = tyvars, abs_dicts = dfun_lam_vars
1045 , abs_exports = [( tyvars, meth_id1, local_meth_id
1046 , SpecPrags spec_inst_prags)]
1047 , abs_binds = this_dict_bind `unionBags` unitBag meth_bind }
1048 -- Default methods in an instance declaration can't have their own
1049 -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
1050 -- currently they are rejected with
1051 -- "INLINE pragma lacks an accompanying binding"
1053 ; return (meth_id1, L loc bind) }
1055 ; case findMethodBind sel_name local_meth_name binds_in of
1056 Just user_bind -> tc_body user_bind -- User-supplied method binding
1057 Nothing -> tc_default dm_info -- None supplied
1060 sel_name = idName sel_id
1062 meth_sig_fn _ = Just [] -- The 'Just' says "yes, there's a type sig"
1063 -- But there are no scoped type variables from local_method_id
1064 -- Only the ones from the instance decl itself, which are already
1065 -- in scope. Example:
1066 -- class C a where { op :: forall b. Eq b => ... }
1067 -- instance C [c] where { op = <rhs> }
1068 -- In <rhs>, 'c' is scope but 'b' is not!
1070 error_rhs = L loc $ HsApp error_fun error_msg
1071 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
1072 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
1073 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
1074 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
1076 dfun_lam_vars = map instToVar dfun_dicts
1077 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_lam_vars
1079 -- For instance decls that come from standalone deriving clauses
1080 -- we want to print out the full source code if there's an error
1081 -- because otherwise the user won't see the code at all
1082 add_meth_ctxt rn_bind thing
1083 | standalone_deriv = addLandmarkErrCtxt (derivBindCtxt clas inst_tys rn_bind) thing
1086 wrapId :: HsWrapper -> id -> HsExpr id
1087 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
1089 derivBindCtxt :: Class -> [Type ] -> LHsBind Name -> SDoc
1090 derivBindCtxt clas tys bind
1091 = vcat [ ptext (sLit "When typechecking a standalone-derived method for")
1092 <+> quotes (pprClassPred clas tys) <> colon
1093 , nest 2 $ pprSetDepth AllTheWay $ ppr bind ]
1095 warnMissingMethod :: Id -> TcM ()
1096 warnMissingMethod sel_id
1097 = do { warn <- doptM Opt_WarnMissingMethods
1098 ; warnTc (warn -- Warn only if -fwarn-missing-methods
1099 && not (startsWithUnderscore (getOccName sel_id)))
1100 -- Don't warn about _foo methods
1101 (ptext (sLit "No explicit method nor default method for")
1102 <+> quotes (ppr sel_id)) }
1105 Note [Export helper functions]
1106 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1107 We arrange to export the "helper functions" of an instance declaration,
1108 so that they are not subject to preInlineUnconditionally, even if their
1109 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
1110 the dict fun as Ids, not as CoreExprs, so we can't substitute a
1111 non-variable for them.
1113 We could change this by making DFunUnfoldings have CoreExprs, but it
1114 seems a bit simpler this way.
1116 Note [Default methods in instances]
1117 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1124 instance Baz Int Int
1126 From the class decl we get
1128 $dmfoo :: forall v x. Baz v x => x -> x
1131 Notice that the type is ambiguous. That's fine, though. The instance
1134 $dBazIntInt = MkBaz fooIntInt
1135 fooIntInt = $dmfoo Int Int $dBazIntInt
1137 BUT this does mean we must generate the dictionary translation of
1138 fooIntInt directly, rather than generating source-code and
1139 type-checking it. That was the bug in Trac #1061. In any case it's
1140 less work to generate the translated version!
1142 Note [INLINE and default methods]
1143 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1144 Default methods need special case. They are supposed to behave rather like
1145 macros. For exmample
1148 op1, op2 :: Bool -> a -> a
1151 op1 b x = op2 (not b) x
1153 instance Foo Int where
1154 -- op1 via default method
1157 The instance declaration should behave
1159 just as if 'op1' had been defined with the
1160 code, and INLINE pragma, from its original
1163 That is, just as if you'd written
1165 instance Foo Int where
1169 op1 b x = op2 (not b) x
1171 So for the above example we generate:
1174 {-# INLINE $dmop1 #-}
1175 -- $dmop1 has an InlineCompulsory unfolding
1176 $dmop1 d b x = op2 d (not b) x
1178 $fFooInt = MkD $cop1 $cop2
1180 {-# INLINE $cop1 #-}
1181 $cop1 = $dmop1 $fFooInt
1187 * We *copy* any INLINE pragma from the default method $dmop1 to the
1188 instance $cop1. Otherwise we'll just inline the former in the
1189 latter and stop, which isn't what the user expected
1191 * Regardless of its pragma, we give the default method an
1192 unfolding with an InlineCompulsory source. That means
1193 that it'll be inlined at every use site, notably in
1194 each instance declaration, such as $cop1. This inlining
1195 must happen even though
1196 a) $dmop1 is not saturated in $cop1
1197 b) $cop1 itself has an INLINE pragma
1199 It's vital that $dmop1 *is* inlined in this way, to allow the mutual
1200 recursion between $fooInt and $cop1 to be broken
1202 * To communicate the need for an InlineCompulsory to the desugarer
1203 (which makes the Unfoldings), we use the IsDefaultMethod constructor
1207 %************************************************************************
1209 \subsection{Error messages}
1211 %************************************************************************
1214 instDeclCtxt1 :: LHsType Name -> SDoc
1215 instDeclCtxt1 hs_inst_ty
1216 = inst_decl_ctxt (case unLoc hs_inst_ty of
1217 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
1218 HsPredTy pred -> ppr pred
1219 _ -> ppr hs_inst_ty) -- Don't expect this
1220 instDeclCtxt2 :: Type -> SDoc
1221 instDeclCtxt2 dfun_ty
1222 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1224 (_,cls,tys) = tcSplitDFunTy dfun_ty
1226 inst_decl_ctxt :: SDoc -> SDoc
1227 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
1229 superClassCtxt :: SDoc
1230 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
1232 atInstCtxt :: Name -> SDoc
1233 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
1236 mustBeVarArgErr :: Type -> SDoc
1237 mustBeVarArgErr ty =
1238 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
1239 ptext (sLit "must be variables")
1240 , ptext (sLit "Instead of a variable, found") <+> ppr ty
1243 wrongATArgErr :: Type -> Type -> SDoc
1244 wrongATArgErr ty instTy =
1245 sep [ ptext (sLit "Type indexes must match class instance head")
1246 , ptext (sLit "Found") <+> quotes (ppr ty)
1247 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)