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 RnEnv ( lookupGlobalOccRn )
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 {-# INLINE [2] op1_i #-} -- From the instance decl bindings
95 op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
96 op1_i = /\a. \(d:C a).
99 -- Note [Subtle interaction of recursion and overlap]
101 local_op1 :: forall b. Ix b => [a] -> b -> b
103 -- Source code; run the type checker on this
104 -- NB: Type variable 'a' (but not 'b') is in scope in <rhs>
105 -- Note [Tricky type variable scoping]
109 op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
111 -- The dictionary function itself
112 {-# INLINE df_i #-} -- Always inline dictionary functions
113 df_i :: forall a. C a -> C [a]
114 df_i = /\a. \d:C a. letrec d' = MkC (op1_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 * The dictionary function itself is inlined as vigorously as we
121 possibly can, so that we expose that dictionary constructor to
122 selectors as much as poss. That is why the op_i stuff is in
123 *separate* bindings, so that the df_i binding is small enough
124 to inline. See Note [Inline dfuns unconditionally].
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.
129 Not even once! Else op1_i, op2_i may be inlined into df_i.
131 * Instead the idea is to inline df_i into op1_i, which may then select
132 methods from the MkC record, and thereby break the recursion with
133 df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at
134 the same type, it won't mention df_i, so there won't be recursion in
137 * If op1_i is marked INLINE by the user there's a danger that we won't
138 inline df_i in it, and that in turn means that (since it'll be a
139 loop-breaker because df_i isn't), op1_i will ironically never be
140 inlined. We need to fix this somehow -- perhaps allowing inlining
141 of INLINE functions inside other INLINE functions.
143 Note [Subtle interaction of recursion and overlap]
144 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
146 class C a where { op1,op2 :: a -> a }
147 instance C a => C [a] where
148 op1 x = op2 x ++ op2 x
150 intance C [Int] where
153 When type-checking the C [a] instance, we need a C [a] dictionary (for
154 the call of op2). If we look up in the instance environment, we find
155 an overlap. And in *general* the right thing is to complain (see Note
156 [Overlapping instances] in InstEnv). But in *this* case it's wrong to
157 complain, because we just want to delegate to the op2 of this same
160 Why is this justified? Because we generate a (C [a]) constraint in
161 a context in which 'a' cannot be instantiated to anything that matches
162 other overlapping instances, or else we would not be excecuting this
163 version of op1 in the first place.
165 It might even be a bit disguised:
167 nullFail :: C [a] => [a] -> [a]
168 nullFail x = op2 x ++ op2 x
170 instance C a => C [a] where
173 Precisely this is used in package 'regex-base', module Context.hs.
174 See the overlapping instances for RegexContext, and the fact that they
175 call 'nullFail' just like the example above. The DoCon package also
176 does the same thing; it shows up in module Fraction.hs
178 Conclusion: when typechecking the methods in a C [a] instance, we want
179 to have C [a] available. That is why we have the strange local
180 definition for 'this' in the definition of op1_i in the example above.
181 We can typecheck the defintion of local_op1, and when doing tcSimplifyCheck
182 we supply 'this' as a given dictionary. Only needed, though, if there
183 are some type variales involved; otherwise there can be no overlap and
186 Note [Tricky type variable scoping]
187 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
190 op1, op2 :: Ix b => a -> b -> b
193 instance C a => C [a]
194 {-# INLINE [2] op1 #-}
197 note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
198 in scope in <rhs>. In particular, we must make sure that 'b' is in
199 scope when typechecking <dm-rhs>. This is achieved by subFunTys,
200 which brings appropriate tyvars into scope. This happens for both
201 <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
202 complained if 'b' is mentioned in <rhs>.
204 Note [Inline dfuns unconditionally]
205 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
206 The code above unconditionally inlines dict funs. Here's why.
207 Consider this program:
209 test :: Int -> Int -> Bool
210 test x y = (x,y) == (y,x) || test y x
211 -- Recursive to avoid making it inline.
213 This needs the (Eq (Int,Int)) instance. If we inline that dfun
214 the code we end up with is good:
217 \r -> case ==# [ww ww1] of wild {
218 PrelBase.False -> Test.$wtest ww1 ww;
220 case ==# [ww1 ww] of wild1 {
221 PrelBase.False -> Test.$wtest ww1 ww;
222 PrelBase.True -> PrelBase.True [];
225 Test.test = \r [w w1]
228 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
231 If we don't inline the dfun, the code is not nearly as good:
233 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
234 PrelBase.:DEq tpl1 tpl2 -> tpl2;
239 let { y = PrelBase.I#! [ww1]; } in
240 let { x = PrelBase.I#! [ww]; } in
241 let { sat_slx = PrelTup.(,)! [y x]; } in
242 let { sat_sly = PrelTup.(,)! [x y];
244 case == sat_sly sat_slx of wild {
245 PrelBase.False -> Test.$wtest ww1 ww;
246 PrelBase.True -> PrelBase.True [];
253 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
256 Why didn't GHC inline $fEq in those days? Because it looked big:
258 PrelTup.zdfEqZ1T{-rcX-}
259 = \ @ a{-reT-} :: * @ b{-reS-} :: *
260 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
261 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
263 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
264 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
266 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
267 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
269 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
270 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
271 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
273 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
275 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
277 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
278 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
282 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
283 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
284 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
285 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
287 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
289 and it's not as bad as it seems, because it's further dramatically
290 simplified: only zeze2 is extracted and its body is simplified.
293 %************************************************************************
295 \subsection{Extracting instance decls}
297 %************************************************************************
299 Gather up the instance declarations from their various sources
302 tcInstDecls1 -- Deal with both source-code and imported instance decls
303 :: [LTyClDecl Name] -- For deriving stuff
304 -> [LInstDecl Name] -- Source code instance decls
305 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
306 -> TcM (TcGblEnv, -- The full inst env
307 [InstInfo Name], -- Source-code instance decls to process;
308 -- contains all dfuns for this module
309 HsValBinds Name) -- Supporting bindings for derived instances
311 tcInstDecls1 tycl_decls inst_decls deriv_decls
313 do { -- Stop if addInstInfos etc discovers any errors
314 -- (they recover, so that we get more than one error each
317 -- (1) Do class and family instance declarations
318 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
319 ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_decls
320 ; idx_tycons <- mapAndRecoverM tcIdxTyInstDeclTL idxty_decls
323 at_tycons_s) = unzip local_info_tycons
324 ; at_idx_tycons = concat at_tycons_s ++ idx_tycons
325 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
326 ; implicit_things = concatMap implicitTyThings at_idx_tycons
327 ; aux_binds = mkAuxBinds at_idx_tycons
330 -- (2) Add the tycons of indexed types and their implicit
331 -- tythings to the global environment
332 ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do {
334 -- (3) Instances from generic class declarations
335 ; generic_inst_info <- getGenericInstances clas_decls
337 -- Next, construct the instance environment so far, consisting
339 -- a) local instance decls
340 -- b) generic instances
341 -- c) local family instance decls
342 ; addInsts local_info $ do {
343 ; addInsts generic_inst_info $ do {
344 ; addFamInsts at_idx_tycons $ do {
346 -- (4) Compute instances from "deriving" clauses;
347 -- This stuff computes a context for the derived instance
348 -- decl, so it needs to know about all the instances possible
349 -- NB: class instance declarations can contain derivings as
350 -- part of associated data type declarations
351 failIfErrsM -- If the addInsts stuff gave any errors, don't
352 -- try the deriving stuff, becuase that may give
354 ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_decls
356 ; gbl_env <- addInsts deriv_inst_info getGblEnv
358 generic_inst_info ++ deriv_inst_info ++ local_info,
359 aux_binds `plusHsValBinds` deriv_binds)
362 -- Make sure that toplevel type instance are not for associated types.
363 -- !!!TODO: Need to perform this check for the TyThing of type functions,
365 tcIdxTyInstDeclTL ldecl@(L loc decl) =
366 do { tything <- tcFamInstDecl ldecl
368 when (isAssocFamily tything) $
369 addErr $ assocInClassErr (tcdName decl)
372 isAssocFamily (ATyCon tycon) =
373 case tyConFamInst_maybe tycon of
374 Nothing -> panic "isAssocFamily: no family?!?"
375 Just (fam, _) -> isTyConAssoc fam
376 isAssocFamily _ = panic "isAssocFamily: no tycon?!?"
378 assocInClassErr :: Name -> SDoc
379 assocInClassErr name =
380 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
381 ptext (sLit "must be inside a class instance")
383 addInsts :: [InstInfo Name] -> TcM a -> TcM a
384 addInsts infos thing_inside
385 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
387 addFamInsts :: [TyThing] -> TcM a -> TcM a
388 addFamInsts tycons thing_inside
389 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
391 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
392 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
397 tcLocalInstDecl1 :: LInstDecl Name
398 -> TcM (InstInfo Name, [TyThing])
399 -- A source-file instance declaration
400 -- Type-check all the stuff before the "where"
402 -- We check for respectable instance type, and context
403 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
405 addErrCtxt (instDeclCtxt1 poly_ty) $
407 do { is_boot <- tcIsHsBoot
408 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
411 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
413 -- Now, check the validity of the instance.
414 ; (clas, inst_tys) <- checkValidInstHead tau
415 ; checkValidInstance tyvars theta clas inst_tys
417 -- Next, process any associated types.
418 ; idx_tycons <- recoverM (return []) $
419 do { idx_tycons <- checkNoErrs $ mapAndRecoverM tcFamInstDecl ats
420 ; checkValidAndMissingATs clas (tyvars, inst_tys)
422 ; return idx_tycons }
424 -- Finally, construct the Core representation of the instance.
425 -- (This no longer includes the associated types.)
426 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
427 -- Dfun location is that of instance *header*
428 ; overlap_flag <- getOverlapFlag
429 ; let (eq_theta,dict_theta) = partition isEqPred theta
430 theta' = eq_theta ++ dict_theta
431 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
432 ispec = mkLocalInstance dfun overlap_flag
434 ; return (InstInfo { iSpec = ispec,
435 iBinds = VanillaInst binds uprags },
439 -- We pass in the source form and the type checked form of the ATs. We
440 -- really need the source form only to be able to produce more informative
442 checkValidAndMissingATs :: Class
443 -> ([TyVar], [TcType]) -- instance types
444 -> [(LTyClDecl Name, -- source form of AT
445 TyThing)] -- Core form of AT
447 checkValidAndMissingATs clas inst_tys ats
448 = do { -- Issue a warning for each class AT that is not defined in this
450 ; let class_ats = map tyConName (classATs clas)
451 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
452 omitted = filterOut (`elemNameSet` defined_ats) class_ats
453 ; warn <- doptM Opt_WarnMissingMethods
454 ; mapM_ (warnTc warn . omittedATWarn) omitted
456 -- Ensure that all AT indexes that correspond to class parameters
457 -- coincide with the types in the instance head. All remaining
458 -- AT arguments must be variables. Also raise an error for any
459 -- type instances that are not associated with this class.
460 ; mapM_ (checkIndexes clas inst_tys) ats
463 checkIndexes clas inst_tys (hsAT, ATyCon tycon)
464 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
465 = checkIndexes' clas inst_tys hsAT
467 snd . fromJust . tyConFamInst_maybe $ tycon)
468 checkIndexes _ _ _ = panic "checkIndexes"
470 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
471 = let atName = tcdName . unLoc $ hsAT
473 setSrcSpan (getLoc hsAT) $
474 addErrCtxt (atInstCtxt atName) $
475 case find ((atName ==) . tyConName) (classATs clas) of
476 Nothing -> addErrTc $ badATErr clas atName -- not in this class
478 case assocTyConArgPoss_maybe atycon of
479 Nothing -> panic "checkIndexes': AT has no args poss?!?"
482 -- The following is tricky! We need to deal with three
483 -- complications: (1) The AT possibly only uses a subset of
484 -- the class parameters as indexes and those it uses may be in
485 -- a different order; (2) the AT may have extra arguments,
486 -- which must be type variables; and (3) variables in AT and
487 -- instance head will be different `Name's even if their
488 -- source lexemes are identical.
490 -- e.g. class C a b c where
491 -- data D b a :: * -> * -- NB (1) b a, omits c
492 -- instance C [x] Bool Char where
493 -- data D Bool [x] v = MkD x [v] -- NB (2) v
494 -- -- NB (3) the x in 'instance C...' have differnt
495 -- -- Names to x's in 'data D...'
497 -- Re (1), `poss' contains a permutation vector to extract the
498 -- class parameters in the right order.
500 -- Re (2), we wrap the (permuted) class parameters in a Maybe
501 -- type and use Nothing for any extra AT arguments. (First
502 -- equation of `checkIndex' below.)
504 -- Re (3), we replace any type variable in the AT parameters
505 -- that has the same source lexeme as some variable in the
506 -- instance types with the instance type variable sharing its
509 let relevantInstTys = map (instTys !!) poss
510 instArgs = map Just relevantInstTys ++
511 repeat Nothing -- extra arguments
512 renaming = substSameTyVar atTvs instTvs
514 zipWithM_ checkIndex (substTys renaming atTys) instArgs
516 checkIndex ty Nothing
517 | isTyVarTy ty = return ()
518 | otherwise = addErrTc $ mustBeVarArgErr ty
519 checkIndex ty (Just instTy)
520 | ty `tcEqType` instTy = return ()
521 | otherwise = addErrTc $ wrongATArgErr ty instTy
523 listToNameSet = addListToNameSet emptyNameSet
525 substSameTyVar [] _ = emptyTvSubst
526 substSameTyVar (tv:tvs) replacingTvs =
527 let replacement = case find (tv `sameLexeme`) replacingTvs of
528 Nothing -> mkTyVarTy tv
529 Just rtv -> mkTyVarTy rtv
531 tv1 `sameLexeme` tv2 =
532 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
534 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
538 %************************************************************************
540 Type-checking instance declarations, pass 2
542 %************************************************************************
545 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
546 -> TcM (LHsBinds Id, TcLclEnv)
547 -- (a) From each class declaration,
548 -- generate any default-method bindings
549 -- (b) From each instance decl
550 -- generate the dfun binding
552 tcInstDecls2 tycl_decls inst_decls
553 = do { -- (a) Default methods from class decls
554 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
555 filter (isClassDecl.unLoc) tycl_decls
556 ; tcExtendIdEnv (concat dm_ids_s) $ do
558 -- (b) instance declarations
559 ; inst_binds_s <- mapM tcInstDecl2 inst_decls
562 ; let binds = unionManyBags dm_binds_s `unionBags`
563 unionManyBags inst_binds_s
564 ; tcl_env <- getLclEnv -- Default method Ids in here
565 ; return (binds, tcl_env) }
567 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
568 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
569 = recoverM (return emptyLHsBinds) $
571 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
572 tc_inst_decl2 dfun_id ibinds
574 dfun_id = instanceDFunId ispec
575 loc = getSrcSpan dfun_id
580 tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id)
581 -- Returns a binding for the dfun
583 ------------------------
584 -- Derived newtype instances; surprisingly tricky!
586 -- class Show a => Foo a b where ...
587 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
589 -- The newtype gives an FC axiom looking like
590 -- axiom CoN a :: N a ~ Tree [a]
591 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
593 -- So all need is to generate a binding looking like:
594 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
595 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
596 -- case df `cast` (Foo Int (sym (CoN a))) of
597 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
599 -- If there are no superclasses, matters are simpler, because we don't need the case
600 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
602 tc_inst_decl2 dfun_id (NewTypeDerived coi)
603 = do { let rigid_info = InstSkol
604 origin = SigOrigin rigid_info
605 inst_ty = idType dfun_id
606 ; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
607 -- inst_head_ty is a PredType
609 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
610 (class_tyvars, sc_theta, _, _) = classBigSig cls
611 cls_tycon = classTyCon cls
612 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
613 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
617 IdCo -> (last_ty, idHsWrapper)
618 ACo co -> (snd (coercionKind co), WpCast (mk_full_coercion co))
620 -----------------------
622 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
623 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
624 -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
625 -- where rep_ty is the (eta-reduced) type rep of T
626 -- So we just replace T with CoT, and insert a 'sym'
627 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
629 mk_full_coercion co = mkTyConApp cls_tycon
630 (initial_cls_inst_tys ++ [mkSymCoercion co])
631 -- Full coercion : (Foo Int (Tree [a]) ~ Foo Int (N a)
633 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
634 -- In our example, rep_pred is (Foo Int (Tree [a]))
636 ; sc_loc <- getInstLoc InstScOrigin
637 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
638 ; inst_loc <- getInstLoc origin
639 ; dfun_dicts <- newDictBndrs inst_loc theta
640 ; rep_dict <- newDictBndr inst_loc rep_pred
641 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
643 -- Figure out bindings for the superclass context from dfun_dicts
644 -- Don't include this_dict in the 'givens', else
645 -- sc_dicts get bound by just selecting from this_dict!!
646 ; sc_binds <- addErrCtxt superClassCtxt $
647 tcSimplifySuperClasses inst_loc this_dict dfun_dicts
650 -- It's possible that the superclass stuff might unified something
651 -- in the envt with one of the clas_tyvars
652 ; checkSigTyVars inst_tvs'
654 ; let coerced_rep_dict = wrapId wrapper (instToId rep_dict)
656 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
657 ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body)
659 ; return (unitBag $ noLoc $
660 AbsBinds inst_tvs' (map instToVar dfun_dicts)
661 [(inst_tvs', dfun_id, instToId this_dict, [])]
662 (dict_bind `consBag` sc_binds)) }
664 -----------------------
665 -- (make_body C tys scs coreced_rep_dict)
667 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
668 -- But if there are no superclasses, it returns just coerced_rep_dict
669 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
671 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
672 | null sc_dicts -- Case (a)
673 = return coerced_rep_dict
674 | otherwise -- Case (b)
675 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
676 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
677 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
678 pat_dicts = dummy_sc_dict_ids,
679 pat_binds = emptyLHsBinds,
680 pat_args = PrefixCon (map nlVarPat op_ids),
682 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
683 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
684 map HsVar (sc_dict_ids ++ op_ids)
686 -- Warning: this HsCase scrutinises a value with a PredTy, which is
687 -- never otherwise seen in Haskell source code. It'd be
688 -- nicer to generate Core directly!
689 ; return (HsCase (noLoc coerced_rep_dict) $
690 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
692 sc_dict_ids = map instToId sc_dicts
693 pat_ty = mkTyConApp cls_tycon cls_inst_tys
694 cls_data_con = head (tyConDataCons cls_tycon)
695 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
696 op_tys = dropList sc_dict_ids cls_arg_tys
698 ------------------------
699 -- Ordinary instances
701 tc_inst_decl2 dfun_id (VanillaInst monobinds uprags)
702 = do { let rigid_info = InstSkol
703 inst_ty = idType dfun_id
705 -- Instantiate the instance decl with skolem constants
706 ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
707 -- These inst_tyvars' scope over the 'where' part
708 -- Those tyvars are inside the dfun_id's type, which is a bit
709 -- bizarre, but OK so long as you realise it!
711 (clas, inst_tys') = tcSplitDFunHead inst_head'
712 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
714 -- Instantiate the super-class context with inst_tys
715 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
716 origin = SigOrigin rigid_info
718 -- Create dictionary Ids from the specified instance contexts.
719 ; sc_loc <- getInstLoc InstScOrigin
720 ; sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
721 ; inst_loc <- getInstLoc origin
722 ; dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
723 ; this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
725 -- Default-method Ids may be mentioned in synthesised RHSs,
726 -- but they'll already be in the environment.
728 -- Typecheck the methods
729 ; let this_dict_id = instToId this_dict
730 dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
731 prag_fn = mkPragFun uprags
732 loc = getSrcSpan dfun_id
733 tc_meth = tcInstanceMethod loc clas inst_tyvars'
735 dfun_theta' inst_tys'
738 ; (meth_exprs, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
739 mapAndUnzipM tc_meth op_items
741 -- Figure out bindings for the superclass context
742 -- Don't include this_dict in the 'givens', else
743 -- sc_dicts get bound by just selecting from this_dict!!
744 ; sc_binds <- addErrCtxt superClassCtxt $
745 tcSimplifySuperClasses inst_loc this_dict dfun_dicts sc_dicts
746 -- Note [Recursive superclasses]
748 -- It's possible that the superclass stuff might unified something
749 -- in the envt with one of the inst_tyvars'
750 ; checkSigTyVars inst_tyvars'
752 -- Deal with 'SPECIALISE instance' pragmas
753 ; prags <- tcPrags dfun_id (filter isSpecInstLSig uprags)
755 -- Create the result bindings
756 ; let dict_constr = classDataCon clas
757 inline_prag | null dfun_dicts = []
758 | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
759 -- Always inline the dfun; this is an experimental decision
760 -- because it makes a big performance difference sometimes.
761 -- Often it means we can do the method selection, and then
762 -- inline the method as well. Marcin's idea; see comments below.
764 -- BUT: don't inline it if it's a constant dictionary;
765 -- we'll get all the benefit without inlining, and we get
766 -- a **lot** of code duplication if we inline it
768 -- See Note [Inline dfuns] below
770 sc_dict_vars = map instToVar sc_dicts
771 dict_bind = L loc (VarBind this_dict_id dict_rhs)
772 dict_rhs = foldl (\ f a -> L loc (HsApp f (L loc a))) inst_constr meth_exprs
773 inst_constr = L loc $ wrapId (mkWpApps sc_dict_vars <.> mkWpTyApps inst_tys')
774 (dataConWrapId dict_constr)
775 -- We don't produce a binding for the dict_constr; instead we
776 -- rely on the simplifier to unfold this saturated application
777 -- We do this rather than generate an HsCon directly, because
778 -- it means that the special cases (e.g. dictionary with only one
779 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
780 -- than needing to be repeated here.
783 main_bind = noLoc $ AbsBinds
786 [(inst_tyvars', dfun_id, this_dict_id, inline_prag ++ prags)]
787 (dict_bind `consBag` sc_binds)
789 ; showLIE (text "instance")
790 ; return (main_bind `consBag` unionManyBags meth_binds) }
793 Note [Recursive superclasses]
794 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
795 See Trac #1470 for why we would *like* to add "this_dict" to the
796 available instances here. But we can't do so because then the superclases
797 get satisfied by selection from this_dict, and that leads to an immediate
798 loop. What we need is to add this_dict to Avails without adding its
799 superclasses, and we currently have no way to do that.
802 %************************************************************************
804 Type-checking an instance method
806 %************************************************************************
809 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
810 - Remembering to use fresh Name (the instance method Name) as the binder
811 - Bring the instance method Ids into scope, for the benefit of tcInstSig
812 - Use sig_fn mapping instance method Name -> instance tyvars
814 - Use tcValBinds to do the checking
817 tcInstanceMethod :: SrcSpan -> Class -> [TcTyVar] -> [Inst]
818 -> TcThetaType -> [TcType]
820 -> TcPragFun -> LHsBinds Name
822 -> TcM (HsExpr Id, LHsBinds Id)
823 -- The returned inst_meth_ids all have types starting
824 -- forall tvs. theta => ...
826 tcInstanceMethod loc clas tyvars dfun_dicts theta inst_tys
827 this_dict dfun_id prag_fn binds_in (sel_id, dm_info)
828 = do { cloned_this <- cloneDict this_dict
829 -- Need to clone the dict in case it is floated out, and
830 -- then clashes with its friends
832 ; let local_meth_name = mkInternalName uniq1 sel_occ loc -- Same OccName
833 this_dict_bind = L loc $ VarBind (instToId cloned_this) $
834 L loc $ wrapId meth_wrapper dfun_id
835 mb_this_bind | null tyvars = Nothing
836 | otherwise = Just (cloned_this, this_dict_bind)
837 -- Only need the this_dict stuff if there are type variables
838 -- involved; otherwise overlap is not possible
839 -- See Note [Subtle interaction of recursion and overlap]
841 tc_body rn_bind = do { (meth_id, tc_binds) <- tcInstanceMethodBody
842 InstSkol clas tyvars dfun_dicts theta inst_tys
845 meth_sig_fn meth_prag_fn rn_bind
846 ; return (wrapId meth_wrapper meth_id, tc_binds) }
848 ; case (findMethodBind sel_name local_meth_name binds_in, dm_info) of
849 -- There is a user-supplied method binding, so use it
850 (Just user_bind, _) -> tc_body user_bind
852 -- The user didn't supply a method binding, so we have to make
853 -- up a default binding, in a way depending on the default-method info
855 (Nothing, GenDefMeth) -> do -- Derivable type classes stuff
856 { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id local_meth_name
857 ; tc_body meth_bind }
859 (Nothing, NoDefMeth) -> do -- No default method in the class
860 { warn <- doptM Opt_WarnMissingMethods
861 ; warnTc (warn -- Warn only if -fwarn-missing-methods
862 && reportIfUnused (getOccName sel_id))
863 -- Don't warn about _foo methods
865 ; return (error_rhs, emptyBag) }
867 (Nothing, DefMeth) -> do -- An polymorphic default method
868 { -- Build the typechecked version directly,
869 -- without calling typecheck_method;
870 -- see Note [Default methods in instances]
871 dm_name <- lookupGlobalOccRn (mkDefMethRdrName sel_name)
872 -- Might not be imported, but will be an OrigName
873 ; dm_id <- tcLookupId dm_name
874 ; return (wrapId dm_wrapper dm_id, emptyBag) } }
876 sel_name = idName sel_id
877 sel_occ = nameOccName sel_name
878 this_dict_id = instToId this_dict
880 meth_prag_fn _ = prag_fn sel_name
881 meth_sig_fn _ = Just [] -- The 'Just' says "yes, there's a type sig"
882 -- But there are no scoped type variables from local_method_id
883 -- Only the ones from the instance decl itself, which are already
884 -- in scope. Example:
885 -- class C a where { op :: forall b. Eq b => ... }
886 -- instance C [c] where { op = <rhs> }
887 -- In <rhs>, 'c' is scope but 'b' is not!
889 error_rhs = HsApp error_fun error_msg
890 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
891 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
892 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
893 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
895 dm_wrapper = WpApp this_dict_id <.> mkWpTyApps inst_tys
897 omitted_meth_warn :: SDoc
898 omitted_meth_warn = ptext (sLit "No explicit method nor default method for")
899 <+> quotes (ppr sel_id)
901 dfun_lam_vars = map instToVar dfun_dicts
902 meth_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys tyvars)
905 wrapId :: HsWrapper -> id -> HsExpr id
906 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
909 Note [Default methods in instances]
910 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
919 From the class decl we get
921 $dmfoo :: forall v x. Baz v x => x -> x
923 Notice that the type is ambiguous. That's fine, though. The instance decl generates
925 $dBazIntInt = MkBaz ($dmfoo Int Int $dBazIntInt)
927 BUT this does mean we must generate the dictionary translation directly, rather
928 than generating source-code and type-checking it. That was the bug ing
929 Trac #1061. In any case it's less work to generate the translated version!
932 %************************************************************************
934 \subsection{Error messages}
936 %************************************************************************
939 instDeclCtxt1 :: LHsType Name -> SDoc
940 instDeclCtxt1 hs_inst_ty
941 = inst_decl_ctxt (case unLoc hs_inst_ty of
942 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
943 HsPredTy pred -> ppr pred
944 _ -> ppr hs_inst_ty) -- Don't expect this
945 instDeclCtxt2 :: Type -> SDoc
946 instDeclCtxt2 dfun_ty
947 = inst_decl_ctxt (ppr (mkClassPred cls tys))
949 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
951 inst_decl_ctxt :: SDoc -> SDoc
952 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
954 superClassCtxt :: SDoc
955 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
957 atInstCtxt :: Name -> SDoc
958 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
961 mustBeVarArgErr :: Type -> SDoc
963 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
964 ptext (sLit "must be variables")
965 , ptext (sLit "Instead of a variable, found") <+> ppr ty
968 wrongATArgErr :: Type -> Type -> SDoc
969 wrongATArgErr ty instTy =
970 sep [ ptext (sLit "Type indexes must match class instance head")
971 , ptext (sLit "Found") <+> quotes (ppr ty)
972 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)