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 )
25 import RnSource ( addTcgDUs )
35 import CoreUnfold ( mkDFunUnfolding )
36 import PrelNames ( inlineIdName )
54 #include "HsVersions.h"
57 Typechecking instance declarations is done in two passes. The first
58 pass, made by @tcInstDecls1@, collects information to be used in the
61 This pre-processed info includes the as-yet-unprocessed bindings
62 inside the instance declaration. These are type-checked in the second
63 pass, when the class-instance envs and GVE contain all the info from
64 all the instance and value decls. Indeed that's the reason we need
65 two passes over the instance decls.
68 Note [How instance declarations are translated]
69 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
70 Here is how we translation instance declarations into Core
74 op1, op2 :: Ix b => a -> b -> b
78 {-# INLINE [2] op1 #-}
82 op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b
86 -- Default methods get the 'self' dictionary as argument
87 -- so they can call other methods at the same type
88 -- Default methods get the same type as their method selector
89 $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b
90 $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). <dm-rhs>
91 -- NB: type variables 'a' and 'b' are *both* in scope in <dm-rhs>
92 -- Note [Tricky type variable scoping]
94 -- A top-level definition for each instance method
95 -- Here op1_i, op2_i are the "instance method Ids"
96 -- The INLINE pragma comes from the user pragma
97 {-# INLINE [2] op1_i #-} -- From the instance decl bindings
98 op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
99 op1_i = /\a. \(d:C a).
102 -- Note [Subtle interaction of recursion and overlap]
104 local_op1 :: forall b. Ix b => [a] -> b -> b
106 -- Source code; run the type checker on this
107 -- NB: Type variable 'a' (but not 'b') is in scope in <rhs>
108 -- Note [Tricky type variable scoping]
112 op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
114 -- The dictionary function itself
115 {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions
116 df_i :: forall a. C a -> C [a]
117 df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
118 -- But see Note [Default methods in instances]
119 -- We can't apply the type checker to the default-method call
121 -- Use a RULE to short-circuit applications of the class ops
122 {-# RULE "op1@C[a]" forall a, d:C a.
123 op1 [a] (df_i d) = op1_i a d #-}
125 * The dictionary function itself is inlined as vigorously as we
126 possibly can, so that we expose that dictionary constructor to
127 selectors as much as poss. That is why the op_i stuff is in
128 *separate* bindings, so that the df_i binding is small enough
129 to inline. See Note [Inline dfuns unconditionally].
131 * Note that df_i may be mutually recursive with both op1_i and op2_i.
132 It's crucial that df_i is not chosen as the loop breaker, even
133 though op1_i has a (user-specified) INLINE pragma.
134 Not even once! Else op1_i, op2_i may be inlined into df_i.
136 * Instead the idea is to inline df_i into op1_i, which may then select
137 methods from the MkC record, and thereby break the recursion with
138 df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at
139 the same type, it won't mention df_i, so there won't be recursion in
142 * If op1_i is marked INLINE by the user there's a danger that we won't
143 inline df_i in it, and that in turn means that (since it'll be a
144 loop-breaker because df_i isn't), op1_i will ironically never be
145 inlined. We need to fix this somehow -- perhaps allowing inlining
146 of INLINE functions inside other INLINE functions.
148 Note [Subtle interaction of recursion and overlap]
149 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
151 class C a where { op1,op2 :: a -> a }
152 instance C a => C [a] where
153 op1 x = op2 x ++ op2 x
155 intance C [Int] where
158 When type-checking the C [a] instance, we need a C [a] dictionary (for
159 the call of op2). If we look up in the instance environment, we find
160 an overlap. And in *general* the right thing is to complain (see Note
161 [Overlapping instances] in InstEnv). But in *this* case it's wrong to
162 complain, because we just want to delegate to the op2 of this same
165 Why is this justified? Because we generate a (C [a]) constraint in
166 a context in which 'a' cannot be instantiated to anything that matches
167 other overlapping instances, or else we would not be excecuting this
168 version of op1 in the first place.
170 It might even be a bit disguised:
172 nullFail :: C [a] => [a] -> [a]
173 nullFail x = op2 x ++ op2 x
175 instance C a => C [a] where
178 Precisely this is used in package 'regex-base', module Context.hs.
179 See the overlapping instances for RegexContext, and the fact that they
180 call 'nullFail' just like the example above. The DoCon package also
181 does the same thing; it shows up in module Fraction.hs
183 Conclusion: when typechecking the methods in a C [a] instance, we want
184 to have C [a] available. That is why we have the strange local
185 definition for 'this' in the definition of op1_i in the example above.
186 We can typecheck the defintion of local_op1, and when doing tcSimplifyCheck
187 we supply 'this' as a given dictionary. Only needed, though, if there
188 are some type variables involved; otherwise there can be no overlap and
191 Note [Tricky type variable scoping]
192 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
195 op1, op2 :: Ix b => a -> b -> b
198 instance C a => C [a]
199 {-# INLINE [2] op1 #-}
202 note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
203 in scope in <rhs>. In particular, we must make sure that 'b' is in
204 scope when typechecking <dm-rhs>. This is achieved by subFunTys,
205 which brings appropriate tyvars into scope. This happens for both
206 <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
207 complained if 'b' is mentioned in <rhs>.
209 Note [Inline dfuns unconditionally]
210 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
211 The code above unconditionally inlines dict funs. Here's why.
212 Consider this program:
214 test :: Int -> Int -> Bool
215 test x y = (x,y) == (y,x) || test y x
216 -- Recursive to avoid making it inline.
218 This needs the (Eq (Int,Int)) instance. If we inline that dfun
219 the code we end up with is good:
222 \r -> case ==# [ww ww1] of wild {
223 PrelBase.False -> Test.$wtest ww1 ww;
225 case ==# [ww1 ww] of wild1 {
226 PrelBase.False -> Test.$wtest ww1 ww;
227 PrelBase.True -> PrelBase.True [];
230 Test.test = \r [w w1]
233 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
236 If we don't inline the dfun, the code is not nearly as good:
238 (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
239 PrelBase.:DEq tpl1 tpl2 -> tpl2;
244 let { y = PrelBase.I#! [ww1]; } in
245 let { x = PrelBase.I#! [ww]; } in
246 let { sat_slx = PrelTup.(,)! [y x]; } in
247 let { sat_sly = PrelTup.(,)! [x y];
249 case == sat_sly sat_slx of wild {
250 PrelBase.False -> Test.$wtest ww1 ww;
251 PrelBase.True -> PrelBase.True [];
258 case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
261 Why didn't GHC inline $fEq in those days? Because it looked big:
263 PrelTup.zdfEqZ1T{-rcX-}
264 = \ @ a{-reT-} :: * @ b{-reS-} :: *
265 zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
266 zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
268 zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
269 zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
271 zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
272 zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
274 zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
275 zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
276 ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
278 of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
280 of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
282 (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
283 (zeze{-rf0-} a2{-reZ-} b2{-reY-})
287 a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
288 a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
289 b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
290 PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
292 PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
294 and it's not as bad as it seems, because it's further dramatically
295 simplified: only zeze2 is extracted and its body is simplified.
298 %************************************************************************
300 \subsection{Extracting instance decls}
302 %************************************************************************
304 Gather up the instance declarations from their various sources
307 tcInstDecls1 -- Deal with both source-code and imported instance decls
308 :: [LTyClDecl Name] -- For deriving stuff
309 -> [LInstDecl Name] -- Source code instance decls
310 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
311 -> TcM (TcGblEnv, -- The full inst env
312 [InstInfo Name], -- Source-code instance decls to process;
313 -- contains all dfuns for this module
314 HsValBinds Name) -- Supporting bindings for derived instances
316 tcInstDecls1 tycl_decls inst_decls deriv_decls
318 do { -- Stop if addInstInfos etc discovers any errors
319 -- (they recover, so that we get more than one error each
322 -- (1) Do class and family instance declarations
323 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
324 ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_decls
325 ; idx_tycons <- mapAndRecoverM tcIdxTyInstDeclTL idxty_decls
328 at_tycons_s) = unzip local_info_tycons
329 ; at_idx_tycons = concat at_tycons_s ++ idx_tycons
330 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
331 ; implicit_things = concatMap implicitTyThings at_idx_tycons
332 ; aux_binds = mkAuxBinds at_idx_tycons
335 -- (2) Add the tycons of indexed types and their implicit
336 -- tythings to the global environment
337 ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do {
339 -- (3) Instances from generic class declarations
340 ; generic_inst_info <- getGenericInstances clas_decls
342 -- Next, construct the instance environment so far, consisting
344 -- a) local instance decls
345 -- b) generic instances
346 -- c) local family instance decls
347 ; addInsts local_info $
348 addInsts generic_inst_info $
349 addFamInsts at_idx_tycons $ do {
351 -- (4) Compute instances from "deriving" clauses;
352 -- This stuff computes a context for the derived instance
353 -- decl, so it needs to know about all the instances possible
354 -- NB: class instance declarations can contain derivings as
355 -- part of associated data type declarations
356 failIfErrsM -- If the addInsts stuff gave any errors, don't
357 -- try the deriving stuff, becuase that may give
359 ; (deriv_inst_info, deriv_binds, deriv_dus)
360 <- tcDeriving tycl_decls inst_decls deriv_decls
361 ; gbl_env <- addInsts deriv_inst_info getGblEnv
362 ; return ( addTcgDUs gbl_env deriv_dus,
363 generic_inst_info ++ deriv_inst_info ++ local_info,
364 aux_binds `plusHsValBinds` deriv_binds)
367 -- Make sure that toplevel type instance are not for associated types.
368 -- !!!TODO: Need to perform this check for the TyThing of type functions,
370 tcIdxTyInstDeclTL ldecl@(L loc decl) =
371 do { tything <- tcFamInstDecl ldecl
373 when (isAssocFamily tything) $
374 addErr $ assocInClassErr (tcdName decl)
377 isAssocFamily (ATyCon tycon) =
378 case tyConFamInst_maybe tycon of
379 Nothing -> panic "isAssocFamily: no family?!?"
380 Just (fam, _) -> isTyConAssoc fam
381 isAssocFamily _ = panic "isAssocFamily: no tycon?!?"
383 assocInClassErr :: Name -> SDoc
384 assocInClassErr name =
385 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
386 ptext (sLit "must be inside a class instance")
388 addInsts :: [InstInfo Name] -> TcM a -> TcM a
389 addInsts infos thing_inside
390 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
392 addFamInsts :: [TyThing] -> TcM a -> TcM a
393 addFamInsts tycons thing_inside
394 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
396 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
397 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
402 tcLocalInstDecl1 :: LInstDecl Name
403 -> TcM (InstInfo Name, [TyThing])
404 -- A source-file instance declaration
405 -- Type-check all the stuff before the "where"
407 -- We check for respectable instance type, and context
408 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
410 addErrCtxt (instDeclCtxt1 poly_ty) $
412 do { is_boot <- tcIsHsBoot
413 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
416 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
418 -- Now, check the validity of the instance.
419 ; (clas, inst_tys) <- checkValidInstHead tau
420 ; checkValidInstance tyvars theta clas inst_tys
422 -- Next, process any associated types.
423 ; idx_tycons <- recoverM (return []) $
424 do { idx_tycons <- checkNoErrs $ mapAndRecoverM tcFamInstDecl ats
425 ; checkValidAndMissingATs clas (tyvars, inst_tys)
427 ; return idx_tycons }
429 -- Finally, construct the Core representation of the instance.
430 -- (This no longer includes the associated types.)
431 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
432 -- Dfun location is that of instance *header*
433 ; overlap_flag <- getOverlapFlag
434 ; let (eq_theta,dict_theta) = partition isEqPred theta
435 theta' = eq_theta ++ dict_theta
436 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
437 ispec = mkLocalInstance dfun overlap_flag
439 ; return (InstInfo { iSpec = ispec,
440 iBinds = VanillaInst binds uprags False },
444 -- We pass in the source form and the type checked form of the ATs. We
445 -- really need the source form only to be able to produce more informative
447 checkValidAndMissingATs :: Class
448 -> ([TyVar], [TcType]) -- instance types
449 -> [(LTyClDecl Name, -- source form of AT
450 TyThing)] -- Core form of AT
452 checkValidAndMissingATs clas inst_tys ats
453 = do { -- Issue a warning for each class AT that is not defined in this
455 ; let class_ats = map tyConName (classATs clas)
456 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
457 omitted = filterOut (`elemNameSet` defined_ats) class_ats
458 ; warn <- doptM Opt_WarnMissingMethods
459 ; mapM_ (warnTc warn . omittedATWarn) omitted
461 -- Ensure that all AT indexes that correspond to class parameters
462 -- coincide with the types in the instance head. All remaining
463 -- AT arguments must be variables. Also raise an error for any
464 -- type instances that are not associated with this class.
465 ; mapM_ (checkIndexes clas inst_tys) ats
468 checkIndexes clas inst_tys (hsAT, ATyCon tycon)
469 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
470 = checkIndexes' clas inst_tys hsAT
472 snd . fromJust . tyConFamInst_maybe $ tycon)
473 checkIndexes _ _ _ = panic "checkIndexes"
475 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
476 = let atName = tcdName . unLoc $ hsAT
478 setSrcSpan (getLoc hsAT) $
479 addErrCtxt (atInstCtxt atName) $
480 case find ((atName ==) . tyConName) (classATs clas) of
481 Nothing -> addErrTc $ badATErr clas atName -- not in this class
483 case assocTyConArgPoss_maybe atycon of
484 Nothing -> panic "checkIndexes': AT has no args poss?!?"
487 -- The following is tricky! We need to deal with three
488 -- complications: (1) The AT possibly only uses a subset of
489 -- the class parameters as indexes and those it uses may be in
490 -- a different order; (2) the AT may have extra arguments,
491 -- which must be type variables; and (3) variables in AT and
492 -- instance head will be different `Name's even if their
493 -- source lexemes are identical.
495 -- e.g. class C a b c where
496 -- data D b a :: * -> * -- NB (1) b a, omits c
497 -- instance C [x] Bool Char where
498 -- data D Bool [x] v = MkD x [v] -- NB (2) v
499 -- -- NB (3) the x in 'instance C...' have differnt
500 -- -- Names to x's in 'data D...'
502 -- Re (1), `poss' contains a permutation vector to extract the
503 -- class parameters in the right order.
505 -- Re (2), we wrap the (permuted) class parameters in a Maybe
506 -- type and use Nothing for any extra AT arguments. (First
507 -- equation of `checkIndex' below.)
509 -- Re (3), we replace any type variable in the AT parameters
510 -- that has the same source lexeme as some variable in the
511 -- instance types with the instance type variable sharing its
514 let relevantInstTys = map (instTys !!) poss
515 instArgs = map Just relevantInstTys ++
516 repeat Nothing -- extra arguments
517 renaming = substSameTyVar atTvs instTvs
519 zipWithM_ checkIndex (substTys renaming atTys) instArgs
521 checkIndex ty Nothing
522 | isTyVarTy ty = return ()
523 | otherwise = addErrTc $ mustBeVarArgErr ty
524 checkIndex ty (Just instTy)
525 | ty `tcEqType` instTy = return ()
526 | otherwise = addErrTc $ wrongATArgErr ty instTy
528 listToNameSet = addListToNameSet emptyNameSet
530 substSameTyVar [] _ = emptyTvSubst
531 substSameTyVar (tv:tvs) replacingTvs =
532 let replacement = case find (tv `sameLexeme`) replacingTvs of
533 Nothing -> mkTyVarTy tv
534 Just rtv -> mkTyVarTy rtv
536 tv1 `sameLexeme` tv2 =
537 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
539 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
543 %************************************************************************
545 Type-checking instance declarations, pass 2
547 %************************************************************************
550 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
551 -> TcM (LHsBinds Id, TcLclEnv)
552 -- (a) From each class declaration,
553 -- generate any default-method bindings
554 -- (b) From each instance decl
555 -- generate the dfun binding
557 tcInstDecls2 tycl_decls inst_decls
558 = do { -- (a) Default methods from class decls
559 let class_decls = filter (isClassDecl . unLoc) tycl_decls
560 ; (dm_ids_s, dm_binds_s) <- mapAndUnzipM tcClassDecl2 class_decls
562 ; tcExtendIdEnv (concat dm_ids_s) $ do
564 -- (b) instance declarations
565 { inst_binds_s <- mapM tcInstDecl2 inst_decls
568 ; let binds = unionManyBags dm_binds_s `unionBags`
569 unionManyBags inst_binds_s
570 ; tcl_env <- getLclEnv -- Default method Ids in here
571 ; return (binds, tcl_env) } }
573 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
574 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
575 = recoverM (return emptyLHsBinds) $
577 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
578 tc_inst_decl2 dfun_id ibinds
580 dfun_id = instanceDFunId ispec
581 loc = getSrcSpan dfun_id
586 tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id)
587 -- Returns a binding for the dfun
589 ------------------------
590 -- Derived newtype instances; surprisingly tricky!
592 -- class Show a => Foo a b where ...
593 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
595 -- The newtype gives an FC axiom looking like
596 -- axiom CoN a :: N a ~ Tree [a]
597 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
599 -- So all need is to generate a binding looking like:
600 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
601 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
602 -- case df `cast` (Foo Int (sym (CoN a))) of
603 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
605 -- If there are no superclasses, matters are simpler, because we don't need the case
606 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
608 tc_inst_decl2 dfun_id (NewTypeDerived coi)
609 = do { let rigid_info = InstSkol
610 origin = SigOrigin rigid_info
611 inst_ty = idType dfun_id
612 inst_tvs = fst (tcSplitForAllTys inst_ty)
613 ; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
614 -- inst_head_ty is a PredType
616 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
617 (class_tyvars, sc_theta, _, _) = classBigSig cls
618 cls_tycon = classTyCon cls
619 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
620 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
624 IdCo -> (last_ty, idHsWrapper)
625 ACo co -> (snd (coercionKind co'), WpCast (mk_full_coercion co'))
627 co' = substTyWith inst_tvs (mkTyVarTys inst_tvs') co
628 -- NB: the free variable of coi are bound by the
629 -- universally quantified variables of the dfun_id
630 -- This is weird, and maybe we should make NewTypeDerived
631 -- carry a type-variable list too; but it works fine
633 -----------------------
635 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
636 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
637 -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
638 -- where rep_ty is the (eta-reduced) type rep of T
639 -- So we just replace T with CoT, and insert a 'sym'
640 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
642 mk_full_coercion co = mkTyConApp cls_tycon
643 (initial_cls_inst_tys ++ [mkSymCoercion co])
644 -- Full coercion : (Foo Int (Tree [a]) ~ Foo Int (N a)
646 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
647 -- In our example, rep_pred is (Foo Int (Tree [a]))
649 ; sc_loc <- getInstLoc InstScOrigin
650 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
651 ; inst_loc <- getInstLoc origin
652 ; dfun_dicts <- newDictBndrs inst_loc theta
653 ; rep_dict <- newDictBndr inst_loc rep_pred
654 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
656 -- Figure out bindings for the superclass context from dfun_dicts
657 -- Don't include this_dict in the 'givens', else
658 -- sc_dicts get bound by just selecting from this_dict!!
659 ; sc_binds <- addErrCtxt superClassCtxt $
660 tcSimplifySuperClasses inst_loc this_dict dfun_dicts
663 -- It's possible that the superclass stuff might unified something
664 -- in the envt with one of the clas_tyvars
665 ; checkSigTyVars inst_tvs'
667 ; let coerced_rep_dict = wrapId wrapper (instToId rep_dict)
669 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
670 ; let dict_bind = mkVarBind (instToId this_dict) (noLoc body)
672 ; return (unitBag $ noLoc $
673 AbsBinds inst_tvs' (map instToVar dfun_dicts)
674 [(inst_tvs', dfun_id, instToId this_dict, [])]
675 (dict_bind `consBag` sc_binds)) }
677 -----------------------
678 -- (make_body C tys scs coreced_rep_dict)
680 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
681 -- But if there are no superclasses, it returns just coerced_rep_dict
682 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
684 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
685 | null sc_dicts -- Case (a)
686 = return coerced_rep_dict
687 | otherwise -- Case (b)
688 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
689 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
690 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
691 pat_dicts = dummy_sc_dict_ids,
692 pat_binds = emptyLHsBinds,
693 pat_args = PrefixCon (map nlVarPat op_ids),
695 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
696 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
697 map HsVar (sc_dict_ids ++ op_ids)
699 -- Warning: this HsCase scrutinises a value with a PredTy, which is
700 -- never otherwise seen in Haskell source code. It'd be
701 -- nicer to generate Core directly!
702 ; return (HsCase (noLoc coerced_rep_dict) $
703 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
705 sc_dict_ids = map instToId sc_dicts
706 pat_ty = mkTyConApp cls_tycon cls_inst_tys
707 cls_data_con = head (tyConDataCons cls_tycon)
708 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
709 op_tys = dropList sc_dict_ids cls_arg_tys
711 ------------------------
712 -- Ordinary instances
714 tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
715 = do { let rigid_info = InstSkol
716 inst_ty = idType dfun_id
717 loc = getSrcSpan dfun_id
719 -- Instantiate the instance decl with skolem constants
720 ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
721 -- These inst_tyvars' scope over the 'where' part
722 -- Those tyvars are inside the dfun_id's type, which is a bit
723 -- bizarre, but OK so long as you realise it!
725 (clas, inst_tys') = tcSplitDFunHead inst_head'
726 (class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas
728 -- Instantiate the super-class context with inst_tys
729 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
730 origin = SigOrigin rigid_info
732 -- Create dictionary Ids from the specified instance contexts.
733 ; inst_loc <- getInstLoc origin
734 ; dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
735 ; this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
736 -- Default-method Ids may be mentioned in synthesised RHSs,
737 -- but they'll already be in the environment.
740 -- Cook up a binding for "this = df d1 .. dn",
741 -- to use in each method binding
742 -- Need to clone the dict in case it is floated out, and
743 -- then clashes with its friends
744 ; cloned_this <- cloneDict this_dict
745 ; let cloned_this_bind = mkVarBind (instToId cloned_this) $
746 L loc $ wrapId app_wrapper dfun_id
747 app_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
748 dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
750 | null inst_tyvars' && null dfun_theta' = (this_dict, emptyBag)
751 | otherwise = (cloned_this, unitBag cloned_this_bind)
753 -- Deal with 'SPECIALISE instance' pragmas
754 -- See Note [SPECIALISE instance pragmas]
755 ; let spec_inst_sigs = filter isSpecInstLSig uprags
756 -- The filter removes the pragmas for methods
757 ; spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) spec_inst_sigs
759 -- Typecheck the methods
760 ; let prag_fn = mkPragFun uprags
761 tc_meth = tcInstanceMethod loc standalone_deriv
765 prag_fn spec_inst_prags monobinds
767 ; (meth_ids, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
768 mapAndUnzipM tc_meth op_items
770 -- Figure out bindings for the superclass context
771 ; sc_loc <- getInstLoc InstScOrigin
772 ; sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
773 ; let tc_sc = tcSuperClass inst_loc inst_tyvars' dfun_dicts nested_this_pair
774 ; (sc_ids, sc_binds) <- mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts)
776 -- It's possible that the superclass stuff might unified
777 -- something in the envt with one of the inst_tyvars'
778 ; checkSigTyVars inst_tyvars'
780 -- Create the result bindings
781 ; let dict_constr = classDataCon clas
782 this_dict_id = instToId this_dict
783 dict_bind = mkVarBind this_dict_id dict_rhs
784 dict_rhs = foldl mk_app inst_constr (sc_ids ++ meth_ids)
785 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
786 (dataConWrapId dict_constr)
787 -- We don't produce a binding for the dict_constr; instead we
788 -- rely on the simplifier to unfold this saturated application
789 -- We do this rather than generate an HsCon directly, because
790 -- it means that the special cases (e.g. dictionary with only one
791 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
792 -- than needing to be repeated here.
794 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
795 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
796 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
798 dfun_id_w_fun = dfun_id
799 `setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids)
800 `setInlinePragma` dfunInlinePragma
802 main_bind = noLoc $ AbsBinds
805 [(inst_tyvars', dfun_id_w_fun, this_dict_id, spec_inst_prags)]
808 ; showLIE (text "instance")
809 ; return (unitBag main_bind `unionBags`
810 listToBag meth_binds `unionBags`
811 listToBag sc_binds) }
814 ------------------------------
815 tcSuperClass :: InstLoc -> [TyVar] -> [Inst]
816 -> (Inst, LHsBinds Id)
817 -> (Id, Inst) -> TcM (Id, LHsBind Id)
818 -- Build a top level decl like
819 -- sc_op = /\a \d. let this = ... in
822 -- The "this" part is just-in-case (discarded if not used)
823 -- See Note [Recursive superclasses]
824 tcSuperClass inst_loc tyvars dicts (this_dict, this_bind)
826 = addErrCtxt superClassCtxt $
827 do { sc_binds <- tcSimplifySuperClasses inst_loc
828 this_dict dicts [sc_dict]
829 -- Don't include this_dict in the 'givens', else
830 -- sc_dicts get bound by just selecting from this_dict!!
833 ; let sc_op_ty = mkSigmaTy tyvars (map dictPred dicts)
834 (mkPredTy (dictPred sc_dict))
835 sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq
837 sc_op_id = mkLocalId sc_op_name sc_op_ty
838 sc_id = instToVar sc_dict
839 sc_op_bind = AbsBinds tyvars
840 (map instToVar dicts)
841 [(tyvars, sc_op_id, sc_id, [])]
842 (this_bind `unionBags` sc_binds)
844 ; return (sc_op_id, noLoc sc_op_bind) }
847 Note [Recursive superclasses]
848 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
849 See Trac #1470 for why we would *like* to add "this_dict" to the
850 available instances here. But we can't do so because then the superclases
851 get satisfied by selection from this_dict, and that leads to an immediate
852 loop. What we need is to add this_dict to Avails without adding its
853 superclasses, and we currently have no way to do that.
855 Note [SPECIALISE instance pragmas]
856 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
859 instance (Ix a, Ix b) => Ix (a,b) where
860 {-# SPECIALISE instance Ix (Int,Int) #-}
863 We do *not* want to make a specialised version of the dictionary
864 function. Rather, we want specialised versions of each method.
865 Thus we should generate something like this:
867 $dfIx :: (Ix a, Ix x) => Ix (a,b)
868 {- DFUN [$crange, ...] -}
869 $dfIx da db = Ix ($crange da db) (...other methods...)
871 $dfIxPair :: (Ix a, Ix x) => Ix (a,b)
872 {- DFUN [$crangePair, ...] -}
873 $dfIxPair = Ix ($crangePair da db) (...other methods...)
875 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
876 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
877 $crange da db = <blah>
879 {-# RULE range ($dfIx da db) = $crange da db #-}
883 * The RULE is unaffected by the specialisation. We don't want to
884 specialise $dfIx, because then it would need a specialised RULE
885 which is a pain. The single RULE works fine at all specialisations.
886 See Note [How instance declarations are translated] above
888 * Instead, we want to specialise the *method*, $crange
890 In practice, rather than faking up a SPECIALISE pragama for each
891 method (which is painful, since we'd have to figure out its
892 specialised type), we call tcSpecPrag *as if* were going to specialise
893 $dfIx -- you can see that in the call to tcSpecInst. That generates a
894 SpecPrag which, as it turns out, can be used unchanged for each method.
895 The "it turns out" bit is delicate, but it works fine!
898 tcSpecInst :: Id -> Sig Name -> TcM SpecPrag
899 tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
900 = addErrCtxt (spec_ctxt prag) $
901 do { let name = idName dfun_id
902 ; (tyvars, theta, tau) <- tcHsInstHead hs_ty
903 ; let spec_ty = mkSigmaTy tyvars theta tau
904 ; co_fn <- tcSubExp (SpecPragOrigin name) (idType dfun_id) spec_ty
905 ; return (SpecPrag co_fn defaultInlinePragma) }
907 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
909 tcSpecInst _ _ = panic "tcSpecInst"
912 %************************************************************************
914 Type-checking an instance method
916 %************************************************************************
919 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
920 - Remembering to use fresh Name (the instance method Name) as the binder
921 - Bring the instance method Ids into scope, for the benefit of tcInstSig
922 - Use sig_fn mapping instance method Name -> instance tyvars
924 - Use tcValBinds to do the checking
927 tcInstanceMethod :: SrcSpan -> Bool -> Class -> [TcTyVar] -> [Inst]
929 -> (Inst, LHsBinds Id) -- "This" and its binding
930 -> TcPragFun -- Local prags
931 -> [LSpecPrag] -- Arising from 'SPECLALISE instance'
934 -> TcM (Id, LHsBind Id)
935 -- The returned inst_meth_ids all have types starting
936 -- forall tvs. theta => ...
938 tcInstanceMethod loc standalone_deriv clas tyvars dfun_dicts inst_tys
939 (this_dict, this_dict_bind)
940 prag_fn spec_inst_prags binds_in (sel_id, dm_info)
941 = do { uniq <- newUnique
942 ; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
943 ; local_meth_name <- newLocalName sel_name
944 -- Base the local_meth_name on the selector name, becuase
945 -- type errors from tcInstanceMethodBody come from here
947 ; let local_meth_ty = instantiateMethod clas sel_id inst_tys
948 meth_ty = mkSigmaTy tyvars (map dictPred dfun_dicts) local_meth_ty
949 meth_id = mkLocalId meth_name meth_ty
950 local_meth_id = mkLocalId local_meth_name local_meth_ty
954 = add_meth_ctxt rn_bind $
955 do { (meth_id1, spec_prags) <- tcPrags NonRecursive False True
956 meth_id (prag_fn sel_name)
957 ; tcInstanceMethodBody (instLoc this_dict)
959 ([this_dict], this_dict_bind)
960 meth_id1 local_meth_id
962 (spec_inst_prags ++ spec_prags)
966 tc_default :: DefMeth -> TcM (Id, LHsBind Id)
967 -- The user didn't supply a method binding, so we have to make
968 -- up a default binding, in a way depending on the default-method info
970 tc_default NoDefMeth -- No default method at all
971 = do { warnMissingMethod sel_id
972 ; return (meth_id, mkVarBind meth_id $
973 mkLHsWrap lam_wrapper error_rhs) }
975 tc_default GenDefMeth -- Derivable type classes stuff
976 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id local_meth_name
977 ; tc_body meth_bind }
979 tc_default DefMeth -- An polymorphic default method
980 = do { -- Build the typechecked version directly,
981 -- without calling typecheck_method;
982 -- see Note [Default methods in instances]
983 -- Generate /\as.\ds. let this = df as ds
984 -- in $dm inst_tys this
985 -- The 'let' is necessary only because HsSyn doesn't allow
986 -- you to apply a function to a dictionary *expression*.
987 dm_name <- lookupGlobalOccRn (mkDefMethRdrName sel_name)
988 -- Might not be imported, but will be an OrigName
989 ; dm_id <- tcLookupId dm_name
990 ; inline_id <- tcLookupId inlineIdName
991 ; let dm_inline_prag = idInlinePragma dm_id
992 dm_app = HsWrap (WpApp (instToId this_dict) <.> mkWpTyApps inst_tys) $
994 rhs | isInlinePragma dm_inline_prag -- See Note [INLINE and default methods]
995 = HsApp (L loc (HsWrap (WpTyApp local_meth_ty) (HsVar inline_id)))
999 meth_bind = L loc $ VarBind { var_id = local_meth_id
1000 , var_rhs = L loc rhs
1001 , var_inline = False }
1002 meth_id1 = meth_id `setInlinePragma` dm_inline_prag
1003 -- Copy the inline pragma (if any) from the default
1004 -- method to this version. Note [INLINE and default methods]
1006 bind = AbsBinds { abs_tvs = tyvars, abs_dicts = dfun_lam_vars
1007 , abs_exports = [( tyvars, meth_id1
1008 , local_meth_id, spec_inst_prags)]
1009 , abs_binds = this_dict_bind `unionBags` unitBag meth_bind }
1010 -- Default methods in an instance declaration can't have their own
1011 -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
1012 -- currently they are rejected with
1013 -- "INLINE pragma lacks an accompanying binding"
1015 ; return (meth_id1, L loc bind) }
1017 ; case findMethodBind sel_name local_meth_name binds_in of
1018 Just user_bind -> tc_body user_bind -- User-supplied method binding
1019 Nothing -> tc_default dm_info -- None supplied
1022 sel_name = idName sel_id
1024 meth_sig_fn _ = Just [] -- The 'Just' says "yes, there's a type sig"
1025 -- But there are no scoped type variables from local_method_id
1026 -- Only the ones from the instance decl itself, which are already
1027 -- in scope. Example:
1028 -- class C a where { op :: forall b. Eq b => ... }
1029 -- instance C [c] where { op = <rhs> }
1030 -- In <rhs>, 'c' is scope but 'b' is not!
1032 error_rhs = L loc $ HsApp error_fun error_msg
1033 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
1034 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
1035 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
1036 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
1038 dfun_lam_vars = map instToVar dfun_dicts
1039 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_lam_vars
1041 -- For instance decls that come from standalone deriving clauses
1042 -- we want to print out the full source code if there's an error
1043 -- because otherwise the user won't see the code at all
1044 add_meth_ctxt rn_bind thing
1045 | standalone_deriv = addLandmarkErrCtxt (derivBindCtxt clas inst_tys rn_bind) thing
1048 wrapId :: HsWrapper -> id -> HsExpr id
1049 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
1051 derivBindCtxt :: Class -> [Type ] -> LHsBind Name -> SDoc
1052 derivBindCtxt clas tys bind
1053 = vcat [ ptext (sLit "When typechecking a standalone-derived method for")
1054 <+> quotes (pprClassPred clas tys) <> colon
1055 , nest 2 $ pprSetDepth AllTheWay $ ppr bind ]
1057 warnMissingMethod :: Id -> TcM ()
1058 warnMissingMethod sel_id
1059 = do { warn <- doptM Opt_WarnMissingMethods
1060 ; warnTc (warn -- Warn only if -fwarn-missing-methods
1061 && not (startsWithUnderscore (getOccName sel_id)))
1062 -- Don't warn about _foo methods
1063 (ptext (sLit "No explicit method nor default method for")
1064 <+> quotes (ppr sel_id)) }
1067 Note [Export helper functions]
1068 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1069 We arrange to export the "helper functions" of an instance declaration,
1070 so that they are not subject to preInlineUnconditionally, even if their
1071 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
1072 the dict fun as Ids, not as CoreExprs, so we can't substitute a
1073 non-variable for them.
1075 We could change this by making DFunUnfoldings have CoreExprs, but it
1076 seems a bit simpler this way.
1078 Note [Default methods in instances]
1079 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1086 instance Baz Int Int
1088 From the class decl we get
1090 $dmfoo :: forall v x. Baz v x => x -> x
1093 Notice that the type is ambiguous. That's fine, though. The instance decl generates
1095 $dBazIntInt = MkBaz fooIntInt
1096 fooIntInt = $dmfoo Int Int $dBazIntInt
1098 BUT this does mean we must generate the dictionary translation of
1099 fooIntInt directly, rather than generating source-code and
1100 type-checking it. That was the bug in Trac #1061. In any case it's
1101 less work to generate the translated version!
1103 Note [INLINE and default methods]
1104 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1105 We *copy* any INLINE pragma from the default method to the instance.
1108 op1, op2 :: Bool -> a -> a
1111 op1 b x = op2 (not b) x
1113 instance Foo Int where
1118 {-# INLINE $dmop1 #-}
1119 $dmop1 d b x = op2 d (not b) x
1121 $fFooInt = MkD $cop1 $cop2
1123 {-# INLINE $cop1 #-}
1124 $cop1 = inline $dmop1 $fFooInt
1129 a) We copy $dmop1's inline pragma to $cop1. Otherwise
1130 we'll just inline the former in the latter and stop, which
1131 isn't what the user expected
1133 b) We use the magic 'inline' Id to ensure that $dmop1 really is
1134 inlined in $cop1, even though the latter itself has an INLINE pragma
1135 That is important to allow the mutual recursion between $fooInt and
1138 This is all regrettably delicate.
1141 %************************************************************************
1143 \subsection{Error messages}
1145 %************************************************************************
1148 instDeclCtxt1 :: LHsType Name -> SDoc
1149 instDeclCtxt1 hs_inst_ty
1150 = inst_decl_ctxt (case unLoc hs_inst_ty of
1151 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
1152 HsPredTy pred -> ppr pred
1153 _ -> ppr hs_inst_ty) -- Don't expect this
1154 instDeclCtxt2 :: Type -> SDoc
1155 instDeclCtxt2 dfun_ty
1156 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1158 (_,cls,tys) = tcSplitDFunTy dfun_ty
1160 inst_decl_ctxt :: SDoc -> SDoc
1161 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
1163 superClassCtxt :: SDoc
1164 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
1166 atInstCtxt :: Name -> SDoc
1167 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
1170 mustBeVarArgErr :: Type -> SDoc
1171 mustBeVarArgErr ty =
1172 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
1173 ptext (sLit "must be variables")
1174 , ptext (sLit "Instead of a variable, found") <+> ppr ty
1177 wrongATArgErr :: Type -> Type -> SDoc
1178 wrongATArgErr ty instTy =
1179 sep [ ptext (sLit "Type indexes must match class instance head")
1180 , ptext (sLit "Found") <+> quotes (ppr ty)
1181 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)