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 funcitons 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_tycon = concat at_tycons_s ++ idx_tycons
325 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
326 ; implicit_things = concatMap implicitTyThings at_idx_tycon
329 -- (2) Add the tycons of indexed types and their implicit
330 -- tythings to the global environment
331 ; tcExtendGlobalEnv (at_idx_tycon ++ 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 $ do {
342 ; addInsts generic_inst_info $ do {
343 ; addFamInsts at_idx_tycon $ 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) <- tcDeriving tycl_decls inst_decls
355 ; addInsts deriv_inst_info $ do {
357 ; gbl_env <- getGblEnv
359 generic_inst_info ++ deriv_inst_info ++ local_info,
363 -- Make sure that toplevel type instance are not for associated types.
364 -- !!!TODO: Need to perform this check for the TyThing of type functions,
366 tcIdxTyInstDeclTL ldecl@(L loc decl) =
367 do { tything <- tcFamInstDecl ldecl
369 when (isAssocFamily tything) $
370 addErr $ assocInClassErr (tcdName decl)
373 isAssocFamily (ATyCon tycon) =
374 case tyConFamInst_maybe tycon of
375 Nothing -> panic "isAssocFamily: no family?!?"
376 Just (fam, _) -> isTyConAssoc fam
377 isAssocFamily _ = panic "isAssocFamily: no tycon?!?"
379 assocInClassErr :: Name -> SDoc
380 assocInClassErr name =
381 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
382 ptext (sLit "must be inside a class instance")
384 addInsts :: [InstInfo Name] -> TcM a -> TcM a
385 addInsts infos thing_inside
386 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
388 addFamInsts :: [TyThing] -> TcM a -> TcM a
389 addFamInsts tycons thing_inside
390 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
392 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
393 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
398 tcLocalInstDecl1 :: LInstDecl Name
399 -> TcM (InstInfo Name, [TyThing])
400 -- A source-file instance declaration
401 -- Type-check all the stuff before the "where"
403 -- We check for respectable instance type, and context
404 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
406 addErrCtxt (instDeclCtxt1 poly_ty) $
408 do { is_boot <- tcIsHsBoot
409 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
412 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
414 -- Now, check the validity of the instance.
415 ; (clas, inst_tys) <- checkValidInstHead tau
416 ; checkValidInstance tyvars theta clas inst_tys
418 -- Next, process any associated types.
419 ; idx_tycons <- recoverM (return []) $
420 do { idx_tycons <- checkNoErrs $ mapAndRecoverM tcFamInstDecl ats
421 ; checkValidAndMissingATs clas (tyvars, inst_tys)
423 ; return idx_tycons }
425 -- Finally, construct the Core representation of the instance.
426 -- (This no longer includes the associated types.)
427 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
428 -- Dfun location is that of instance *header*
429 ; overlap_flag <- getOverlapFlag
430 ; let (eq_theta,dict_theta) = partition isEqPred theta
431 theta' = eq_theta ++ dict_theta
432 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
433 ispec = mkLocalInstance dfun overlap_flag
435 ; return (InstInfo { iSpec = ispec,
436 iBinds = VanillaInst binds uprags },
440 -- We pass in the source form and the type checked form of the ATs. We
441 -- really need the source form only to be able to produce more informative
443 checkValidAndMissingATs :: Class
444 -> ([TyVar], [TcType]) -- instance types
445 -> [(LTyClDecl Name, -- source form of AT
446 TyThing)] -- Core form of AT
448 checkValidAndMissingATs clas inst_tys ats
449 = do { -- Issue a warning for each class AT that is not defined in this
451 ; let class_ats = map tyConName (classATs clas)
452 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
453 omitted = filterOut (`elemNameSet` defined_ats) class_ats
454 ; warn <- doptM Opt_WarnMissingMethods
455 ; mapM_ (warnTc warn . omittedATWarn) omitted
457 -- Ensure that all AT indexes that correspond to class parameters
458 -- coincide with the types in the instance head. All remaining
459 -- AT arguments must be variables. Also raise an error for any
460 -- type instances that are not associated with this class.
461 ; mapM_ (checkIndexes clas inst_tys) ats
464 checkIndexes clas inst_tys (hsAT, ATyCon tycon)
465 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
466 = checkIndexes' clas inst_tys hsAT
468 snd . fromJust . tyConFamInst_maybe $ tycon)
469 checkIndexes _ _ _ = panic "checkIndexes"
471 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
472 = let atName = tcdName . unLoc $ hsAT
474 setSrcSpan (getLoc hsAT) $
475 addErrCtxt (atInstCtxt atName) $
476 case find ((atName ==) . tyConName) (classATs clas) of
477 Nothing -> addErrTc $ badATErr clas atName -- not in this class
479 case assocTyConArgPoss_maybe atycon of
480 Nothing -> panic "checkIndexes': AT has no args poss?!?"
483 -- The following is tricky! We need to deal with three
484 -- complications: (1) The AT possibly only uses a subset of
485 -- the class parameters as indexes and those it uses may be in
486 -- a different order; (2) the AT may have extra arguments,
487 -- which must be type variables; and (3) variables in AT and
488 -- instance head will be different `Name's even if their
489 -- source lexemes are identical.
491 -- e.g. class C a b c where
492 -- data D b a :: * -> * -- NB (1) b a, omits c
493 -- instance C [x] Bool Char where
494 -- data D Bool [x] v = MkD x [v] -- NB (2) v
495 -- -- NB (3) the x in 'instance C...' have differnt
496 -- -- Names to x's in 'data D...'
498 -- Re (1), `poss' contains a permutation vector to extract the
499 -- class parameters in the right order.
501 -- Re (2), we wrap the (permuted) class parameters in a Maybe
502 -- type and use Nothing for any extra AT arguments. (First
503 -- equation of `checkIndex' below.)
505 -- Re (3), we replace any type variable in the AT parameters
506 -- that has the same source lexeme as some variable in the
507 -- instance types with the instance type variable sharing its
510 let relevantInstTys = map (instTys !!) poss
511 instArgs = map Just relevantInstTys ++
512 repeat Nothing -- extra arguments
513 renaming = substSameTyVar atTvs instTvs
515 zipWithM_ checkIndex (substTys renaming atTys) instArgs
517 checkIndex ty Nothing
518 | isTyVarTy ty = return ()
519 | otherwise = addErrTc $ mustBeVarArgErr ty
520 checkIndex ty (Just instTy)
521 | ty `tcEqType` instTy = return ()
522 | otherwise = addErrTc $ wrongATArgErr ty instTy
524 listToNameSet = addListToNameSet emptyNameSet
526 substSameTyVar [] _ = emptyTvSubst
527 substSameTyVar (tv:tvs) replacingTvs =
528 let replacement = case find (tv `sameLexeme`) replacingTvs of
529 Nothing -> mkTyVarTy tv
530 Just rtv -> mkTyVarTy rtv
532 tv1 `sameLexeme` tv2 =
533 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
535 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
539 %************************************************************************
541 Type-checking instance declarations, pass 2
543 %************************************************************************
546 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
547 -> TcM (LHsBinds Id, TcLclEnv)
548 -- (a) From each class declaration,
549 -- generate any default-method bindings
550 -- (b) From each instance decl
551 -- generate the dfun binding
553 tcInstDecls2 tycl_decls inst_decls
554 = do { -- (a) Default methods from class decls
555 (dm_binds_s, dm_ids_s) <- mapAndUnzipM tcClassDecl2 $
556 filter (isClassDecl.unLoc) tycl_decls
557 ; tcExtendIdEnv (concat dm_ids_s) $ do
559 -- (b) instance declarations
560 ; inst_binds_s <- mapM tcInstDecl2 inst_decls
563 ; let binds = unionManyBags dm_binds_s `unionBags`
564 unionManyBags inst_binds_s
565 ; tcl_env <- getLclEnv -- Default method Ids in here
566 ; return (binds, tcl_env) }
571 tcInstDecl2 :: InstInfo 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 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived })
594 = do { let dfun_id = instanceDFunId ispec
595 rigid_info = InstSkol
596 origin = SigOrigin rigid_info
597 inst_ty = idType dfun_id
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
606 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
607 (nt_tycon, tc_args) = tcSplitTyConApp last_ty -- Can't fail
608 rep_ty = newTyConInstRhs nt_tycon tc_args
610 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
611 -- In our example, rep_pred is (Foo Int (Tree [a]))
612 the_coercion = make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
613 -- Coercion of kind (Foo Int (Tree [a]) ~ Foo Int (N a)
615 ; sc_loc <- getInstLoc InstScOrigin
616 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
617 ; inst_loc <- getInstLoc origin
618 ; dfun_dicts <- newDictBndrs inst_loc theta
619 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
620 ; rep_dict <- newDictBndr inst_loc rep_pred
622 -- Figure out bindings for the superclass context from dfun_dicts
623 -- Don't include this_dict in the 'givens', else
624 -- sc_dicts get bound by just selecting from this_dict!!
625 ; sc_binds <- addErrCtxt superClassCtxt $
626 tcSimplifySuperClasses inst_loc this_dict dfun_dicts
629 -- It's possible that the superclass stuff might unified something
630 -- in the envt with one of the clas_tyvars
631 ; checkSigTyVars inst_tvs'
633 ; let coerced_rep_dict = wrapId the_coercion (instToId rep_dict)
635 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
636 ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body)
638 ; return (unitBag $ noLoc $
639 AbsBinds inst_tvs' (map instToVar dfun_dicts)
640 [(inst_tvs', dfun_id, instToId this_dict, [])]
641 (dict_bind `consBag` sc_binds)) }
643 -----------------------
645 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
646 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
647 -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
648 -- where rep_ty is the (eta-reduced) type rep of T
649 -- So we just replace T with CoT, and insert a 'sym'
650 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
652 make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
653 | Just co_con <- newTyConCo_maybe nt_tycon
654 , let co = mkSymCoercion (mkTyConApp co_con tc_args)
655 = WpCast (mkTyConApp cls_tycon (initial_cls_inst_tys ++ [co]))
656 | otherwise -- The newtype is transparent; no need for a cast
659 -----------------------
660 -- (make_body C tys scs coreced_rep_dict)
662 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
663 -- But if there are no superclasses, it returns just coerced_rep_dict
664 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
666 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
667 | null sc_dicts -- Case (a)
668 = return coerced_rep_dict
669 | otherwise -- Case (b)
670 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
671 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
672 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
673 pat_dicts = dummy_sc_dict_ids,
674 pat_binds = emptyLHsBinds,
675 pat_args = PrefixCon (map nlVarPat op_ids),
677 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
678 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
679 map HsVar (sc_dict_ids ++ op_ids)
681 -- Warning: this HsCase scrutinises a value with a PredTy, which is
682 -- never otherwise seen in Haskell source code. It'd be
683 -- nicer to generate Core directly!
684 ; return (HsCase (noLoc coerced_rep_dict) $
685 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
687 sc_dict_ids = map instToId sc_dicts
688 pat_ty = mkTyConApp cls_tycon cls_inst_tys
689 cls_data_con = head (tyConDataCons cls_tycon)
690 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
691 op_tys = dropList sc_dict_ids cls_arg_tys
693 ------------------------
694 -- Ordinary instances
696 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
698 dfun_id = instanceDFunId ispec
699 rigid_info = InstSkol
700 inst_ty = idType dfun_id
701 loc = getSrcSpan dfun_id
703 -- Prime error recovery
704 recoverM (return emptyLHsBinds) $
706 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ do
708 -- Instantiate the instance decl with skolem constants
709 (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
710 -- These inst_tyvars' scope over the 'where' part
711 -- Those tyvars are inside the dfun_id's type, which is a bit
712 -- bizarre, but OK so long as you realise it!
714 (clas, inst_tys') = tcSplitDFunHead inst_head'
715 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
717 -- Instantiate the super-class context with inst_tys
718 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
719 origin = SigOrigin rigid_info
721 -- Create dictionary Ids from the specified instance contexts.
722 sc_loc <- getInstLoc InstScOrigin
723 sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
724 inst_loc <- getInstLoc origin
725 dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
726 this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
727 -- Default-method Ids may be mentioned in synthesised RHSs,
728 -- but they'll already be in the environment.
730 -- Typecheck the methods
731 let this_dict_id = instToId this_dict
732 dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
733 prag_fn = mkPragFun uprags
734 tc_meth = tcInstanceMethod loc clas inst_tyvars'
736 dfun_theta' inst_tys'
739 (meth_exprs, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
740 mapAndUnzipM tc_meth op_items
742 -- Figure out bindings for the superclass context
743 -- Don't include this_dict in the 'givens', else
744 -- sc_dicts get bound by just selecting from this_dict!!
745 sc_binds <- addErrCtxt superClassCtxt $
746 tcSimplifySuperClasses inst_loc this_dict dfun_dicts sc_dicts
747 -- Note [Recursive superclasses]
749 -- It's possible that the superclass stuff might unified something
750 -- in the envt with one of the inst_tyvars'
751 checkSigTyVars inst_tyvars'
753 -- Deal with 'SPECIALISE instance' pragmas
754 prags <- tcPrags dfun_id (filter isSpecInstLSig uprags)
756 -- Create the result bindings
758 dict_constr = classDataCon clas
759 inline_prag | null dfun_dicts = []
760 | otherwise = [L loc (InlinePrag (Inline AlwaysActive True))]
761 -- Always inline the dfun; this is an experimental decision
762 -- because it makes a big performance difference sometimes.
763 -- Often it means we can do the method selection, and then
764 -- inline the method as well. Marcin's idea; see comments below.
766 -- BUT: don't inline it if it's a constant dictionary;
767 -- we'll get all the benefit without inlining, and we get
768 -- a **lot** of code duplication if we inline it
770 -- See Note [Inline dfuns] below
772 sc_dict_vars = map instToVar sc_dicts
773 dict_bind = L loc (VarBind this_dict_id dict_rhs)
774 dict_rhs = foldl (\ f a -> L loc (HsApp f (L loc a))) inst_constr meth_exprs
775 inst_constr = L loc $ wrapId (mkWpApps sc_dict_vars <.> mkWpTyApps inst_tys')
776 (dataConWrapId dict_constr)
777 -- We don't produce a binding for the dict_constr; instead we
778 -- rely on the simplifier to unfold this saturated application
779 -- We do this rather than generate an HsCon directly, because
780 -- it means that the special cases (e.g. dictionary with only one
781 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
782 -- than needing to be repeated here.
785 main_bind = noLoc $ AbsBinds
788 [(inst_tyvars', dfun_id, this_dict_id, inline_prag ++ prags)]
789 (dict_bind `consBag` sc_binds)
791 showLIE (text "instance")
792 return (main_bind `consBag` unionManyBags meth_binds)
795 Note [Recursive superclasses]
796 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
797 See Trac #1470 for why we would *like* to add "this_dict" to the
798 available instances here. But we can't do so because then the superclases
799 get satisfied by selection from this_dict, and that leads to an immediate
800 loop. What we need is to add this_dict to Avails without adding its
801 superclasses, and we currently have no way to do that.
804 %************************************************************************
806 Type-checking an instance method
808 %************************************************************************
811 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
812 - Remembering to use fresh Name (the instance method Name) as the binder
813 - Bring the instance method Ids into scope, for the benefit of tcInstSig
814 - Use sig_fn mapping instance method Name -> instance tyvars
816 - Use tcValBinds to do the checking
819 tcInstanceMethod :: SrcSpan -> Class -> [TcTyVar] -> [Inst]
820 -> TcThetaType -> [TcType]
822 -> TcPragFun -> LHsBinds Name
824 -> TcM (HsExpr Id, LHsBinds Id)
825 -- The returned inst_meth_ids all have types starting
826 -- forall tvs. theta => ...
828 tcInstanceMethod loc clas tyvars dfun_dicts theta inst_tys
829 this_dict dfun_id prag_fn binds_in (sel_id, dm_info)
830 = do { cloned_this <- cloneDict this_dict
831 -- Need to clone the dict in case it is floated out, and
832 -- then clashes with its friends
834 ; let local_meth_name = mkInternalName uniq1 sel_occ loc -- Same OccName
835 this_dict_bind = L loc $ VarBind (instToId cloned_this) $
836 L loc $ wrapId meth_wrapper dfun_id
837 mb_this_bind | null tyvars = Nothing
838 | otherwise = Just (cloned_this, this_dict_bind)
839 -- Only need the this_dict stuff if there are type variables
840 -- involved; otherwise overlap is not possible
841 -- See Note [Subtle interaction of recursion and overlap]
843 tc_body rn_bind = do { (meth_id, tc_binds) <- tcInstanceMethodBody
844 InstSkol clas tyvars dfun_dicts theta inst_tys
847 meth_sig_fn meth_prag_fn rn_bind
848 ; return (wrapId meth_wrapper meth_id, tc_binds) }
850 ; case (findMethodBind sel_name local_meth_name binds_in, dm_info) of
851 -- There is a user-supplied method binding, so use it
852 (Just user_bind, _) -> tc_body user_bind
854 -- The user didn't supply a method binding, so we have to make
855 -- up a default binding, in a way depending on the default-method info
857 (Nothing, GenDefMeth) -> do -- Derivable type classes stuff
858 { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id local_meth_name
859 ; tc_body meth_bind }
861 (Nothing, NoDefMeth) -> do -- No default method in the class
862 { warn <- doptM Opt_WarnMissingMethods
863 ; warnTc (warn -- Warn only if -fwarn-missing-methods
864 && reportIfUnused (getOccName sel_id))
865 -- Don't warn about _foo methods
867 ; return (error_rhs, emptyBag) }
869 (Nothing, DefMeth) -> do -- An polymorphic default method
870 { -- Build the typechecked version directly,
871 -- without calling typecheck_method;
872 -- see Note [Default methods in instances]
873 dm_name <- lookupGlobalOccRn (mkDefMethRdrName sel_name)
874 -- Might not be imported, but will be an OrigName
875 ; dm_id <- tcLookupId dm_name
876 ; return (wrapId dm_wrapper dm_id, emptyBag) } }
878 sel_name = idName sel_id
879 sel_occ = nameOccName sel_name
880 this_dict_id = instToId this_dict
882 meth_prag_fn _ = prag_fn sel_name
883 meth_sig_fn _ = Just [] -- The 'Just' says "yes, there's a type sig"
884 -- But there are no scoped type variables from local_method_id
885 -- Only the ones from the instance decl itself, which are already
886 -- in scope. Example:
887 -- class C a where { op :: forall b. Eq b => ... }
888 -- instance C [c] where { op = <rhs> }
889 -- In <rhs>, 'c' is scope but 'b' is not!
891 error_rhs = HsApp error_fun error_msg
892 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
893 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
894 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
895 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
897 dm_wrapper = WpApp this_dict_id <.> mkWpTyApps inst_tys
899 omitted_meth_warn :: SDoc
900 omitted_meth_warn = ptext (sLit "No explicit method nor default method for")
901 <+> quotes (ppr sel_id)
903 dfun_lam_vars = map instToVar dfun_dicts
904 meth_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys tyvars)
907 wrapId :: HsWrapper -> id -> HsExpr id
908 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
911 Note [Default methods in instances]
912 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
921 From the class decl we get
923 $dmfoo :: forall v x. Baz v x => x -> x
925 Notice that the type is ambiguous. That's fine, though. The instance decl generates
927 $dBazIntInt = MkBaz ($dmfoo Int Int $dBazIntInt)
929 BUT this does mean we must generate the dictionary translation directly, rather
930 than generating source-code and type-checking it. That was the bug ing
931 Trac #1061. In any case it's less work to generate the translated version!
934 %************************************************************************
936 \subsection{Error messages}
938 %************************************************************************
941 instDeclCtxt1 :: LHsType Name -> SDoc
942 instDeclCtxt1 hs_inst_ty
943 = inst_decl_ctxt (case unLoc hs_inst_ty of
944 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
945 HsPredTy pred -> ppr pred
946 _ -> ppr hs_inst_ty) -- Don't expect this
947 instDeclCtxt2 :: Type -> SDoc
948 instDeclCtxt2 dfun_ty
949 = inst_decl_ctxt (ppr (mkClassPred cls tys))
951 (_,_,cls,tys) = tcSplitDFunTy dfun_ty
953 inst_decl_ctxt :: SDoc -> SDoc
954 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
956 superClassCtxt :: SDoc
957 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
959 atInstCtxt :: Name -> SDoc
960 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
963 mustBeVarArgErr :: Type -> SDoc
965 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
966 ptext (sLit "must be variables")
967 , ptext (sLit "Instead of a variable, found") <+> ppr ty
970 wrongATArgErr :: Type -> Type -> SDoc
971 wrongATArgErr ty instTy =
972 sep [ ptext (sLit "Type indexes must match class instance head")
973 , ptext (sLit "Found") <+> quotes (ppr ty)
974 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)