2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Handles @deriving@ clauses on @data@ declarations.
9 module TcDeriv ( tcDeriving ) where
11 #include "HsVersions.h"
19 import TcClassDcl( tcAddDeclCtxt ) -- Small helper
20 import TcGenDeriv -- Deriv stuff
59 %************************************************************************
63 %************************************************************************
67 1. Convert the decls (i.e. data/newtype deriving clauses,
68 plus standalone deriving) to [EarlyDerivSpec]
70 2. Infer the missing contexts for the Left DerivSpecs
72 3. Add the derived bindings, generating InstInfos
76 -- DerivSpec is purely local to this module
77 data DerivSpec = DS { ds_loc :: SrcSpan
81 , ds_theta :: ThetaType
85 , ds_tc_args :: [Type]
86 , ds_newtype :: Bool }
87 -- This spec implies a dfun declaration of the form
88 -- df :: forall tvs. theta => C tys
89 -- The Name is the name for the DFun we'll build
90 -- The tyvars bind all the variables in the theta
91 -- For type families, the tycon in
92 -- in ds_tys is the *family* tycon
93 -- in ds_tc, ds_tc_args is the *representation* tycon
94 -- For non-family tycons, both are the same
96 -- ds_newtype = True <=> Newtype deriving
97 -- False <=> Vanilla deriving
102 newtype instance T [a] = MkT (Tree a) deriving( C s )
104 axiom T [a] = :RTList a
105 axiom :RTList a = Tree a
107 DS { ds_tvs = [a,s], ds_cls = C, ds_tys = [s, T [a]]
108 , ds_tc = :RTList, ds_tc_args = [a]
109 , ds_newtype = True }
112 type DerivContext = Maybe ThetaType
113 -- Nothing <=> Vanilla deriving; infer the context of the instance decl
114 -- Just theta <=> Standalone deriving: context supplied by programmer
116 type EarlyDerivSpec = Either DerivSpec DerivSpec
117 -- Left ds => the context for the instance should be inferred
118 -- In this case ds_theta is the list of all the
119 -- constraints needed, such as (Eq [a], Eq a)
120 -- The inference process is to reduce this to a
121 -- simpler form (e.g. Eq a)
123 -- Right ds => the exact context for the instance is supplied
124 -- by the programmer; it is ds_theta
126 pprDerivSpec :: DerivSpec -> SDoc
127 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
128 ds_cls = c, ds_tys = tys, ds_theta = rhs })
129 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
130 <+> equals <+> ppr rhs)
132 instance Outputable DerivSpec where
137 Inferring missing contexts
138 ~~~~~~~~~~~~~~~~~~~~~~~~~~
141 data T a b = C1 (Foo a) (Bar b)
146 [NOTE: See end of these comments for what to do with
147 data (C a, D b) => T a b = ...
150 We want to come up with an instance declaration of the form
152 instance (Ping a, Pong b, ...) => Eq (T a b) where
155 It is pretty easy, albeit tedious, to fill in the code "...". The
156 trick is to figure out what the context for the instance decl is,
157 namely @Ping@, @Pong@ and friends.
159 Let's call the context reqd for the T instance of class C at types
160 (a,b, ...) C (T a b). Thus:
162 Eq (T a b) = (Ping a, Pong b, ...)
164 Now we can get a (recursive) equation from the @data@ decl:
166 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
167 u Eq (T b a) u Eq Int -- From C2
168 u Eq (T a a) -- From C3
170 Foo and Bar may have explicit instances for @Eq@, in which case we can
171 just substitute for them. Alternatively, either or both may have
172 their @Eq@ instances given by @deriving@ clauses, in which case they
173 form part of the system of equations.
175 Now all we need do is simplify and solve the equations, iterating to
176 find the least fixpoint. Notice that the order of the arguments can
177 switch around, as here in the recursive calls to T.
179 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
183 Eq (T a b) = {} -- The empty set
186 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
187 u Eq (T b a) u Eq Int -- From C2
188 u Eq (T a a) -- From C3
190 After simplification:
191 = Eq a u Ping b u {} u {} u {}
196 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
197 u Eq (T b a) u Eq Int -- From C2
198 u Eq (T a a) -- From C3
200 After simplification:
205 = Eq a u Ping b u Eq b u Ping a
207 The next iteration gives the same result, so this is the fixpoint. We
208 need to make a canonical form of the RHS to ensure convergence. We do
209 this by simplifying the RHS to a form in which
211 - the classes constrain only tyvars
212 - the list is sorted by tyvar (major key) and then class (minor key)
213 - no duplicates, of course
215 So, here are the synonyms for the ``equation'' structures:
218 Note [Data decl contexts]
219 ~~~~~~~~~~~~~~~~~~~~~~~~~
222 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
224 We will need an instance decl like:
226 instance (Read a, RealFloat a) => Read (Complex a) where
229 The RealFloat in the context is because the read method for Complex is bound
230 to construct a Complex, and doing that requires that the argument type is
233 But this ain't true for Show, Eq, Ord, etc, since they don't construct
234 a Complex; they only take them apart.
236 Our approach: identify the offending classes, and add the data type
237 context to the instance decl. The "offending classes" are
241 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
242 pattern matching against a constructor from a data type with a context
243 gives rise to the constraints for that context -- or at least the thinned
244 version. So now all classes are "offending".
246 Note [Newtype deriving]
247 ~~~~~~~~~~~~~~~~~~~~~~~
251 newtype T = T Char deriving( C [a] )
253 Notice the free 'a' in the deriving. We have to fill this out to
254 newtype T = T Char deriving( forall a. C [a] )
256 And then translate it to:
257 instance C [a] Char => C [a] T where ...
260 Note [Newtype deriving superclasses]
261 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
262 (See also Trac #1220 for an interesting exchange on newtype
263 deriving and superclasses.)
265 The 'tys' here come from the partial application in the deriving
266 clause. The last arg is the new instance type.
268 We must pass the superclasses; the newtype might be an instance
269 of them in a different way than the representation type
270 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
271 Then the Show instance is not done via isomorphism; it shows
273 The Num instance is derived via isomorphism, but the Show superclass
274 dictionary must the Show instance for Foo, *not* the Show dictionary
275 gotten from the Num dictionary. So we must build a whole new dictionary
276 not just use the Num one. The instance we want is something like:
277 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
280 There may be a coercion needed which we get from the tycon for the newtype
281 when the dict is constructed in TcInstDcls.tcInstDecl2
284 Note [Unused constructors and deriving clauses]
285 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
286 See Trac #3221. Consider
287 data T = T1 | T2 deriving( Show )
288 Are T1 and T2 unused? Well, no: the deriving clause expands to mention
289 both of them. So we gather defs/uses from deriving just like anything else.
291 %************************************************************************
293 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
295 %************************************************************************
298 tcDeriving :: [LTyClDecl Name] -- All type constructors
299 -> [LInstDecl Name] -- All instance declarations
300 -> [LDerivDecl Name] -- All stand-alone deriving declarations
301 -> TcM ([InstInfo Name] -- The generated "instance decls"
302 ,HsValBinds Name -- Extra generated top-level bindings
304 ,[TyCon] -- Extra generated top-level types
305 ,[TyCon]) -- Extra generated type family instances
307 tcDeriving tycl_decls inst_decls deriv_decls
308 = recoverM (return ([], emptyValBindsOut, emptyDUs, [], [])) $
309 do { -- Fish the "deriving"-related information out of the TcEnv
310 -- And make the necessary "equations".
311 is_boot <- tcIsHsBoot
312 ; traceTc "tcDeriving" (ppr is_boot)
313 ; (early_specs, genericsExtras)
314 <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
315 ; let (repMetaTys, repTyCons, metaInsts) = unzip3 genericsExtras
317 ; overlap_flag <- getOverlapFlag
318 ; let (infer_specs, given_specs) = splitEithers early_specs
319 ; insts1 <- mapM (genInst True overlap_flag) given_specs
321 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
322 inferInstanceContexts overlap_flag infer_specs
324 ; insts2 <- mapM (genInst False overlap_flag) final_specs
326 -- We no longer generate the old generic to/from functions
327 -- from each type declaration, so this is emptyBag
328 ; gen_binds <- return emptyBag -- mkGenericBinds is_boot tycl_decls
330 ; (inst_info, rn_binds, rn_dus)
331 <- renameDeriv is_boot gen_binds (insts1 ++ insts2 ++ concat metaInsts)
334 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
335 (ddump_deriving inst_info rn_binds))
337 ; when (not (null inst_info)) $
338 dumpDerivingInfo (ddump_deriving inst_info rn_binds)
340 ; return ( inst_info, rn_binds, rn_dus
341 , concat (map metaTyCons2TyCons repMetaTys), repTyCons) }
343 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
344 ddump_deriving inst_infos extra_binds
345 = hang (ptext (sLit "Derived instances"))
346 2 (vcat (map (\i -> pprInstInfoDetails i $$ text "") inst_infos)
350 renameDeriv :: Bool -> LHsBinds RdrName
351 -> [(InstInfo RdrName, DerivAuxBinds)]
352 -> TcM ([InstInfo Name], HsValBinds Name, DefUses)
353 renameDeriv is_boot gen_binds insts
354 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
355 -- The inst-info bindings will all be empty, but it's easier to
356 -- just use rn_inst_info to change the type appropriately
357 = do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
358 ; return (rn_inst_infos, emptyValBindsOut, usesOnly (plusFVs fvs)) }
361 = discardWarnings $ -- Discard warnings about unused bindings etc
362 do { (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $ -- Type signatures in patterns
363 -- are used in the generic binds
364 rnTopBinds (ValBindsIn gen_binds [])
365 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
367 -- Generate and rename any extra not-one-inst-decl-specific binds,
368 -- notably "con2tag" and/or "tag2con" functions.
369 -- Bring those names into scope before renaming the instances themselves
370 ; loc <- getSrcSpanM -- Generic loc for shared bindings
371 ; let (aux_binds, aux_sigs) = unzip $ map (genAuxBind loc) $
372 rm_dups [] $ concat deriv_aux_binds
373 aux_val_binds = ValBindsIn (listToBag aux_binds) aux_sigs
374 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
375 ; bindLocalNames (collectHsValBinders rn_aux_lhs) $
376 do { (rn_aux, dus_aux) <- rnTopBindsRHS rn_aux_lhs
377 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
378 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
379 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
382 (inst_infos, deriv_aux_binds) = unzip insts
384 -- Remove duplicate requests for auxilliary bindings
386 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
387 | otherwise = rm_dups (b:acc) bs
390 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
391 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
392 = return ( info { iBinds = NewTypeDerived coi tc }
393 , mkFVs (map dataConName (tyConDataCons tc)))
394 -- See Note [Newtype deriving and unused constructors]
396 rn_inst_info inst_info@(InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
397 = -- Bring the right type variables into
398 -- scope (yuk), and rename the method binds
400 bindLocalNames (map Var.varName tyvars) $
401 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) binds
402 ; let binds' = VanillaInst rn_binds [] standalone_deriv
403 ; return (inst_info { iBinds = binds' }, fvs) }
405 (tyvars,_, clas,_) = instanceHead inst
406 clas_nm = className clas
409 Note [Newtype deriving and unused constructors]
410 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
411 Consider this (see Trac #1954):
414 newtype P a = MkP (IO a) deriving Monad
416 If you compile with -fwarn-unused-binds you do not expect the warning
417 "Defined but not used: data consructor MkP". Yet the newtype deriving
418 code does not explicitly mention MkP, but it should behave as if you
420 instance Monad P where
421 return x = MkP (return x)
424 So we want to signal a user of the data constructor 'MkP'. That's
425 what we do in rn_inst_info, and it's the only reason we have the TyCon
426 stored in NewTypeDerived.
429 %************************************************************************
431 From HsSyn to DerivSpec
433 %************************************************************************
435 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
438 -- Make the "extras" for the generic representation
439 mkGenDerivExtras :: TyCon
440 -> TcRn (MetaTyCons, TyCon, [(InstInfo RdrName, DerivAuxBinds)])
441 mkGenDerivExtras tc = do
442 { (metaTyCons, rep0TyInst) <- genGenericRepExtras tc
443 ; metaInsts <- genDtMeta (tc, metaTyCons)
444 ; return (metaTyCons, rep0TyInst, metaInsts) }
446 makeDerivSpecs :: Bool
450 -> TcM ( [EarlyDerivSpec]
451 , [(MetaTyCons, TyCon, [(InstInfo RdrName, DerivAuxBinds)])])
452 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
453 | is_boot -- No 'deriving' at all in hs-boot files
454 = do { mapM_ add_deriv_err deriv_locs
457 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
458 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
460 -- Generic representation stuff: we might need to add some "extras"
462 ; xDerRep <- getDOpts >>= return . xopt Opt_DeriveGeneric
463 ; generic_extras_deriv <- if not xDerRep
464 -- No extras if the flag is off
467 let allTyNames = [ tcdName d | L _ d <- tycl_decls, isDataDecl d ]
468 -- Select only those types that derive Generic
469 ; let sel_tydata = [ tcdName t | (L _ c, L _ t) <- all_tydata
470 , getClassName c == Just genClassName ]
471 ; let sel_deriv_decls = catMaybes [ getTypeName t
472 | L _ (DerivDecl (L _ t)) <- deriv_decls
473 , getClassName t == Just genClassName ]
474 ; derTyDecls <- mapM tcLookupTyCon $
475 filter (needsExtras xDerRep
476 (sel_tydata ++ sel_deriv_decls)) allTyNames
477 -- We need to generate the extras to add to what has
478 -- already been derived
479 ; mapM mkGenDerivExtras derTyDecls }
482 ; return ( eqns1 ++ eqns2, generic_extras_deriv) }
484 -- We need extras if the flag DeriveGeneric is on and this type is
486 needsExtras xDerRep tydata tc_name = xDerRep && tc_name `elem` tydata
488 -- Extracts the name of the class in the deriving
489 getClassName :: HsType Name -> Maybe Name
490 getClassName (HsPredTy (HsClassP n _)) = Just n
491 getClassName _ = Nothing
493 -- Extracts the name of the type in the deriving
494 getTypeName :: HsType Name -> Maybe Name
495 getTypeName (HsTyVar n) = Just n
496 getTypeName (HsOpTy _ (L _ n) _) = Just n
497 getTypeName (HsPredTy (HsClassP _ [L _ n])) = getTypeName n
498 getTypeName _ = Nothing
500 extractTyDataPreds decls
501 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
503 all_tydata :: [(LHsType Name, LTyClDecl Name)]
504 -- Derived predicate paired with its data type declaration
505 all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
507 deriv_locs = map (getLoc . snd) all_tydata
508 ++ map getLoc deriv_decls
510 add_deriv_err loc = setSrcSpan loc $
511 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
512 2 (ptext (sLit "Use an instance declaration instead")))
514 ------------------------------------------------------------------
515 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
516 -- Standalone deriving declarations
517 -- e.g. deriving instance Show a => Show (T a)
518 -- Rather like tcLocalInstDecl
519 deriveStandalone (L loc (DerivDecl deriv_ty))
521 addErrCtxt (standaloneCtxt deriv_ty) $
522 do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
523 ; (tvs, theta, cls, inst_tys) <- tcHsInstHead deriv_ty
524 ; traceTc "Standalone deriving;" $ vcat
525 [ text "tvs:" <+> ppr tvs
526 , text "theta:" <+> ppr theta
527 , text "cls:" <+> ppr cls
528 , text "tys:" <+> ppr inst_tys ]
529 ; checkValidInstance deriv_ty tvs theta cls inst_tys
530 -- C.f. TcInstDcls.tcLocalInstDecl1
532 ; let cls_tys = take (length inst_tys - 1) inst_tys
533 inst_ty = last inst_tys
534 ; traceTc "Standalone deriving:" $ vcat
535 [ text "class:" <+> ppr cls
536 , text "class types:" <+> ppr cls_tys
537 , text "type:" <+> ppr inst_ty ]
538 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
541 ------------------------------------------------------------------
542 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
543 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
544 tcdTyVars = tv_names,
545 tcdTyPats = ty_pats }))
546 = setSrcSpan loc $ -- Use the location of the 'deriving' item
548 do { (tvs, tc, tc_args) <- get_lhs ty_pats
549 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
550 -- the type variables for the type constructor
552 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
553 -- The "deriv_pred" is a LHsType to take account of the fact that for
554 -- newtype deriving we allow deriving (forall a. C [a]).
556 -- Given data T a b c = ... deriving( C d ),
557 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
558 ; let cls_tyvars = classTyVars cls
559 kind = tyVarKind (last cls_tyvars)
560 (arg_kinds, _) = splitKindFunTys kind
561 n_args_to_drop = length arg_kinds
562 n_args_to_keep = tyConArity tc - n_args_to_drop
563 args_to_drop = drop n_args_to_keep tc_args
564 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
565 inst_ty_kind = typeKind inst_ty
566 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
567 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
568 `minusVarSet` dropped_tvs
570 -- Check that the result really is well-kinded
571 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
572 (derivingKindErr tc cls cls_tys kind)
574 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
575 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
576 (derivingEtaErr cls cls_tys inst_ty)
578 -- (a) The data type can be eta-reduced; eg reject:
579 -- data instance T a a = ... deriving( Monad )
580 -- (b) The type class args do not mention any of the dropped type
582 -- newtype T a s = ... deriving( ST s )
584 -- Type families can't be partially applied
585 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
586 -- Note [Deriving, type families, and partial applications]
587 ; checkTc (not (isFamilyTyCon tc) || n_args_to_drop == 0)
588 (typeFamilyPapErr tc cls cls_tys inst_ty)
590 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
592 -- Tiresomely we must figure out the "lhs", which is awkward for type families
593 -- E.g. data T a b = .. deriving( Eq )
594 -- Here, the lhs is (T a b)
595 -- data instance TF Int b = ... deriving( Eq )
596 -- Here, the lhs is (TF Int b)
597 -- But if we just look up the tycon_name, we get is the *family*
598 -- tycon, but not pattern types -- they are in the *rep* tycon.
599 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
600 ; let tvs = tyConTyVars tc
601 ; return (tvs, tc, mkTyVarTys tvs) }
602 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
603 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
604 ; let (tc, tc_args) = tcSplitTyConApp tc_app
605 ; return (tvs, tc, tc_args) }
608 = panic "derivTyData" -- Caller ensures that only TyData can happen
611 Note [Deriving, type families, and partial applications]
612 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
613 When there are no type families, it's quite easy:
615 newtype S a = MkS [a]
616 -- :CoS :: S ~ [] -- Eta-reduced
618 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
619 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
621 When type familes are involved it's trickier:
624 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
625 -- :RT is the representation type for (T Int a)
626 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
627 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
629 instance Eq [a] => Eq (T Int a) -- easy by coercion
630 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
632 The "???" bit is that we don't build the :CoF thing in eta-reduced form
633 Henc the current typeFamilyPapErr, even though the instance makes sense.
634 After all, we can write it out
635 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
640 mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type] -> Type
641 -> DerivContext -- Just => context supplied (standalone deriving)
642 -- Nothing => context inferred (deriving on data decl)
643 -> TcRn EarlyDerivSpec
644 -- Make the EarlyDerivSpec for an instance
645 -- forall tvs. theta => cls (tys ++ [ty])
646 -- where the 'theta' is optional (that's the Maybe part)
647 -- Assumes that this declaration is well-kinded
649 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
650 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
651 , isAlgTyCon tycon -- Check for functions, primitive types etc
652 = mk_alg_eqn tycon tc_args
654 = failWithTc (derivingThingErr False cls cls_tys tc_app
655 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
658 bale_out msg = failWithTc (derivingThingErr False cls cls_tys tc_app msg)
660 mk_alg_eqn tycon tc_args
661 | className cls `elem` typeableClassNames
662 = do { dflags <- getDOpts
663 ; case checkTypeableConditions (dflags, tycon) of
664 Just err -> bale_out err
665 Nothing -> mk_typeable_eqn orig tvs cls tycon tc_args mtheta }
667 | isDataFamilyTyCon tycon
668 , length tc_args /= tyConArity tycon
669 = bale_out (ptext (sLit "Unsaturated data family application"))
672 = do { (rep_tc, rep_tc_args) <- tcLookupDataFamInst tycon tc_args
673 -- Be careful to test rep_tc here: in the case of families,
674 -- we want to check the instance tycon, not the family tycon
676 -- For standalone deriving (mtheta /= Nothing),
677 -- check that all the data constructors are in scope.
678 ; rdr_env <- getGlobalRdrEnv
679 ; let hidden_data_cons = isAbstractTyCon rep_tc ||
680 any not_in_scope (tyConDataCons rep_tc)
681 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
682 ; unless (isNothing mtheta || not hidden_data_cons)
683 (bale_out (derivingHiddenErr tycon))
686 ; if isDataTyCon rep_tc then
687 mkDataTypeEqn orig dflags tvs cls cls_tys
688 tycon tc_args rep_tc rep_tc_args mtheta
690 mkNewTypeEqn orig dflags tvs cls cls_tys
691 tycon tc_args rep_tc rep_tc_args mtheta }
695 %************************************************************************
699 %************************************************************************
702 mkDataTypeEqn :: CtOrigin
704 -> [Var] -- Universally quantified type variables in the instance
705 -> Class -- Class for which we need to derive an instance
706 -> [Type] -- Other parameters to the class except the last
707 -> TyCon -- Type constructor for which the instance is requested
708 -- (last parameter to the type class)
709 -> [Type] -- Parameters to the type constructor
710 -> TyCon -- rep of the above (for type families)
711 -> [Type] -- rep of the above
712 -> DerivContext -- Context of the instance, for standalone deriving
713 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
715 mkDataTypeEqn orig dflags tvs cls cls_tys
716 tycon tc_args rep_tc rep_tc_args mtheta
717 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
718 -- NB: pass the *representation* tycon to checkSideConditions
719 CanDerive -> go_for_it
720 NonDerivableClass -> bale_out (nonStdErr cls)
721 DerivableClassError msg -> bale_out msg
723 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
724 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
726 mk_data_eqn :: CtOrigin -> [TyVar] -> Class
727 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
728 -> TcM EarlyDerivSpec
729 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
730 = do { dfun_name <- new_dfun_name cls tycon
732 ; let inst_tys = [mkTyConApp tycon tc_args]
733 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
734 spec = DS { ds_loc = loc, ds_orig = orig
735 , ds_name = dfun_name, ds_tvs = tvs
736 , ds_cls = cls, ds_tys = inst_tys
737 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
738 , ds_theta = mtheta `orElse` inferred_constraints
739 , ds_newtype = False }
741 ; return (if isJust mtheta then Right spec -- Specified context
742 else Left spec) } -- Infer context
744 ----------------------
745 mk_typeable_eqn :: CtOrigin -> [TyVar] -> Class
746 -> TyCon -> [TcType] -> DerivContext
747 -> TcM EarlyDerivSpec
748 mk_typeable_eqn orig tvs cls tycon tc_args mtheta
749 -- The Typeable class is special in several ways
750 -- data T a b = ... deriving( Typeable )
752 -- instance Typeable2 T where ...
754 -- 1. There are no constraints in the instance
755 -- 2. There are no type variables either
756 -- 3. The actual class we want to generate isn't necessarily
757 -- Typeable; it depends on the arity of the type
758 | isNothing mtheta -- deriving on a data type decl
759 = do { checkTc (cls `hasKey` typeableClassKey)
760 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
761 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
762 ; mk_typeable_eqn orig tvs real_cls tycon [] (Just []) }
764 | otherwise -- standaone deriving
765 = do { checkTc (null tc_args)
766 (ptext (sLit "Derived typeable instance must be of form (Typeable")
767 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
768 ; dfun_name <- new_dfun_name cls tycon
771 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
772 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
773 , ds_tc = tycon, ds_tc_args = []
774 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
776 ----------------------
777 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
778 -- Generate a sufficiently large set of constraints that typechecking the
779 -- generated method definitions should succeed. This set will be simplified
780 -- before being used in the instance declaration
781 inferConstraints _ cls inst_tys rep_tc rep_tc_args
782 -- Generic constraints are easy
783 | cls `hasKey` genClassKey
785 -- The others are a bit more complicated
787 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
788 stupid_constraints ++ extra_constraints
789 ++ sc_constraints ++ con_arg_constraints
791 -- Constraints arising from the arguments of each constructor
793 = [ mkClassPred cls [arg_ty]
794 | data_con <- tyConDataCons rep_tc,
795 arg_ty <- ASSERT( isVanillaDataCon data_con )
796 get_constrained_tys $
797 dataConInstOrigArgTys data_con all_rep_tc_args,
798 not (isUnLiftedType arg_ty) ]
799 -- No constraints for unlifted types
800 -- Where they are legal we generate specilised function calls
802 -- For functor-like classes, two things are different
803 -- (a) We recurse over argument types to generate constraints
804 -- See Functor examples in TcGenDeriv
805 -- (b) The rep_tc_args will be one short
806 is_functor_like = getUnique cls `elem` functorLikeClassKeys
808 get_constrained_tys :: [Type] -> [Type]
809 get_constrained_tys tys
810 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
813 rep_tc_tvs = tyConTyVars rep_tc
814 last_tv = last rep_tc_tvs
815 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
816 | otherwise = rep_tc_args
818 -- Constraints arising from superclasses
819 -- See Note [Superclasses of derived instance]
820 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
823 -- Stupid constraints
824 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
825 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
827 -- Extra Data constraints
828 -- The Data class (only) requires that for
829 -- instance (...) => Data (T t1 t2)
831 -- THEN (Data t1, Data t2) are among the (...) constraints
832 -- Reason: when the IF holds, we generate a method
833 -- dataCast2 f = gcast2 f
834 -- and we need the Data constraints to typecheck the method
836 | cls `hasKey` dataClassKey
837 , all (isLiftedTypeKind . typeKind) rep_tc_args
838 = [mkClassPred cls [ty] | ty <- rep_tc_args]
842 ------------------------------------------------------------------
843 -- Check side conditions that dis-allow derivability for particular classes
844 -- This is *apart* from the newtype-deriving mechanism
846 -- Here we get the representation tycon in case of family instances as it has
847 -- the data constructors - but we need to be careful to fall back to the
848 -- family tycon (with indexes) in error messages.
850 data DerivStatus = CanDerive
851 | DerivableClassError SDoc -- Standard class, but can't do it
852 | NonDerivableClass -- Non-standard class
854 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
855 checkSideConditions dflags mtheta cls cls_tys rep_tc
856 | Just cond <- sideConditions mtheta cls
857 = case (cond (dflags, rep_tc)) of
858 Just err -> DerivableClassError err -- Class-specific error
859 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
860 -- cls_tys (the type args other than last)
862 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
863 | otherwise = NonDerivableClass -- Not a standard class
865 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
867 checkTypeableConditions :: Condition
868 checkTypeableConditions = checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK
870 nonStdErr :: Class -> SDoc
871 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
873 sideConditions :: DerivContext -> Class -> Maybe Condition
874 sideConditions mtheta cls
875 | cls_key == eqClassKey = Just cond_std
876 | cls_key == ordClassKey = Just cond_std
877 | cls_key == showClassKey = Just cond_std
878 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
879 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
880 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
881 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
882 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
883 cond_std `andCond` cond_noUnliftedArgs)
884 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
885 cond_functorOK True) -- NB: no cond_std!
886 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
887 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
888 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
889 cond_functorOK False)
890 | cls_key == genClassKey = Just (cond_RepresentableOk `andCond`
891 checkFlag Opt_DeriveGeneric)
892 | otherwise = Nothing
894 cls_key = getUnique cls
895 cond_std = cond_stdOK mtheta
897 type Condition = (DynFlags, TyCon) -> Maybe SDoc
898 -- first Bool is whether or not we are allowed to derive Data and Typeable
899 -- second Bool is whether or not we are allowed to derive Functor
900 -- TyCon is the *representation* tycon if the
901 -- data type is an indexed one
904 orCond :: Condition -> Condition -> Condition
907 Nothing -> Nothing -- c1 succeeds
908 Just x -> case c2 tc of -- c1 fails
910 Just y -> Just (x $$ ptext (sLit " or") $$ y)
913 andCond :: Condition -> Condition -> Condition
914 andCond c1 c2 tc = case c1 tc of
915 Nothing -> c2 tc -- c1 succeeds
916 Just x -> Just x -- c1 fails
918 cond_stdOK :: DerivContext -> Condition
919 cond_stdOK (Just _) _
920 = Nothing -- Don't check these conservative conditions for
921 -- standalone deriving; just generate the code
922 -- and let the typechecker handle the result
923 cond_stdOK Nothing (_, rep_tc)
924 | null data_cons = Just (no_cons_why rep_tc $$ suggestion)
925 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
926 | otherwise = Nothing
928 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
929 data_cons = tyConDataCons rep_tc
930 con_whys = mapCatMaybes check_con data_cons
932 check_con :: DataCon -> Maybe SDoc
934 | isVanillaDataCon con
935 , all isTauTy (dataConOrigArgTys con) = Nothing
936 | otherwise = Just (badCon con (ptext (sLit "must have a Haskell-98 type")))
938 no_cons_why :: TyCon -> SDoc
939 no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
940 ptext (sLit "must have at least one data constructor")
942 cond_RepresentableOk :: Condition
943 cond_RepresentableOk (_,t) = canDoGenerics t
945 cond_enumOrProduct :: Condition
946 cond_enumOrProduct = cond_isEnumeration `orCond`
947 (cond_isProduct `andCond` cond_noUnliftedArgs)
949 cond_noUnliftedArgs :: Condition
950 -- For some classes (eg Eq, Ord) we allow unlifted arg types
951 -- by generating specilaised code. For others (eg Data) we don't.
952 cond_noUnliftedArgs (_, tc)
953 | null bad_cons = Nothing
954 | otherwise = Just why
956 bad_cons = [ con | con <- tyConDataCons tc
957 , any isUnLiftedType (dataConOrigArgTys con) ]
958 why = badCon (head bad_cons) (ptext (sLit "must have only arguments of lifted type"))
960 cond_isEnumeration :: Condition
961 cond_isEnumeration (_, rep_tc)
962 | isEnumerationTyCon rep_tc = Nothing
963 | otherwise = Just why
965 why = sep [ quotes (pprSourceTyCon rep_tc) <+>
966 ptext (sLit "must be an enumeration type")
967 , ptext (sLit "(an enumeration consists of one or more nullary, non-GADT constructors)") ]
968 -- See Note [Enumeration types] in TyCon
970 cond_isProduct :: Condition
971 cond_isProduct (_, rep_tc)
972 | isProductTyCon rep_tc = Nothing
973 | otherwise = Just why
975 why = quotes (pprSourceTyCon rep_tc) <+>
976 ptext (sLit "must have precisely one constructor")
978 cond_typeableOK :: Condition
979 -- OK for Typeable class
980 -- Currently: (a) args all of kind *
981 -- (b) 7 or fewer args
982 cond_typeableOK (_, tc)
983 | tyConArity tc > 7 = Just too_many
984 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tc))
986 | otherwise = Nothing
988 too_many = quotes (pprSourceTyCon tc) <+>
989 ptext (sLit "must have 7 or fewer arguments")
990 bad_kind = quotes (pprSourceTyCon tc) <+>
991 ptext (sLit "must only have arguments of kind `*'")
993 functorLikeClassKeys :: [Unique]
994 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
996 cond_functorOK :: Bool -> Condition
997 -- OK for Functor/Foldable/Traversable class
998 -- Currently: (a) at least one argument
999 -- (b) don't use argument contravariantly
1000 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
1001 -- (d) optionally: don't use function types
1002 -- (e) no "stupid context" on data type
1003 cond_functorOK allowFunctions (_, rep_tc)
1005 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
1006 <+> ptext (sLit "must have some type parameters"))
1008 | not (null bad_stupid_theta)
1009 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
1010 <+> ptext (sLit "must not have a class context") <+> pprTheta bad_stupid_theta)
1013 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
1015 tc_tvs = tyConTyVars rep_tc
1016 Just (_, last_tv) = snocView tc_tvs
1017 bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
1018 is_bad pred = last_tv `elemVarSet` tyVarsOfPred pred
1020 data_cons = tyConDataCons rep_tc
1021 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
1023 check_vanilla :: DataCon -> Maybe SDoc
1024 check_vanilla con | isVanillaDataCon con = Nothing
1025 | otherwise = Just (badCon con existential)
1027 ft_check :: DataCon -> FFoldType (Maybe SDoc)
1028 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
1029 , ft_co_var = Just (badCon con covariant)
1030 , ft_fun = \x y -> if allowFunctions then x `mplus` y
1031 else Just (badCon con functions)
1032 , ft_tup = \_ xs -> msum xs
1033 , ft_ty_app = \_ x -> x
1034 , ft_bad_app = Just (badCon con wrong_arg)
1035 , ft_forall = \_ x -> x }
1037 existential = ptext (sLit "must not have existential arguments")
1038 covariant = ptext (sLit "must not use the type variable in a function argument")
1039 functions = ptext (sLit "must not contain function types")
1040 wrong_arg = ptext (sLit "must not use the type variable in an argument other than the last")
1042 checkFlag :: ExtensionFlag -> Condition
1043 checkFlag flag (dflags, _)
1044 | xopt flag dflags = Nothing
1045 | otherwise = Just why
1047 why = ptext (sLit "You need -X") <> text flag_str
1048 <+> ptext (sLit "to derive an instance for this class")
1049 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
1051 other -> pprPanic "checkFlag" (ppr other)
1053 std_class_via_iso :: Class -> Bool
1054 -- These standard classes can be derived for a newtype
1055 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
1056 -- because giving so gives the same results as generating the boilerplate
1057 std_class_via_iso clas
1058 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
1059 -- Not Read/Show because they respect the type
1060 -- Not Enum, because newtypes are never in Enum
1063 non_iso_class :: Class -> Bool
1064 -- *Never* derive Read, Show, Typeable, Data, Generic by isomorphism,
1065 -- even with -XGeneralizedNewtypeDeriving
1067 = classKey cls `elem` ([ readClassKey, showClassKey, dataClassKey
1068 , genClassKey] ++ typeableClassKeys)
1070 typeableClassKeys :: [Unique]
1071 typeableClassKeys = map getUnique typeableClassNames
1073 new_dfun_name :: Class -> TyCon -> TcM Name
1074 new_dfun_name clas tycon -- Just a simple wrapper
1075 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1076 ; newDFunName clas [mkTyConApp tycon []] loc }
1077 -- The type passed to newDFunName is only used to generate
1078 -- a suitable string; hence the empty type arg list
1080 badCon :: DataCon -> SDoc -> SDoc
1081 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1084 Note [Superclasses of derived instance]
1085 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1086 In general, a derived instance decl needs the superclasses of the derived
1087 class too. So if we have
1088 data T a = ...deriving( Ord )
1089 then the initial context for Ord (T a) should include Eq (T a). Often this is
1090 redundant; we'll also generate an Ord constraint for each constructor argument,
1091 and that will probably generate enough constraints to make the Eq (T a) constraint
1092 be satisfied too. But not always; consider:
1098 data T a = MkT (S a) deriving( Ord )
1099 instance Num a => Eq (T a)
1101 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1103 data T a = MkT deriving( Data, Typeable )
1104 Here there *is* no argument field, but we must nevertheless generate
1105 a context for the Data instances:
1106 instance Typable a => Data (T a) where ...
1109 %************************************************************************
1113 %************************************************************************
1116 mkNewTypeEqn :: CtOrigin -> DynFlags -> [Var] -> Class
1117 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1119 -> TcRn EarlyDerivSpec
1120 mkNewTypeEqn orig dflags tvs
1121 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1122 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1123 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1124 = do { traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
1125 ; dfun_name <- new_dfun_name cls tycon
1126 ; loc <- getSrcSpanM
1127 ; let spec = DS { ds_loc = loc, ds_orig = orig
1128 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1129 , ds_cls = cls, ds_tys = inst_tys
1130 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1131 , ds_theta = mtheta `orElse` all_preds
1132 , ds_newtype = True }
1133 ; return (if isJust mtheta then Right spec
1137 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1138 CanDerive -> go_for_it -- Use the standard H98 method
1139 DerivableClassError msg -- Error with standard class
1140 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1141 | otherwise -> bale_out msg
1142 NonDerivableClass -- Must use newtype deriving
1143 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1144 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1145 | otherwise -> bale_out non_std
1147 newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
1148 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1149 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1151 non_std = nonStdErr cls
1152 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1154 -- Here is the plan for newtype derivings. We see
1155 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1156 -- where t is a type,
1157 -- ak+1...an is a suffix of a1..an, and are all tyars
1158 -- ak+1...an do not occur free in t, nor in the s1..sm
1159 -- (C s1 ... sm) is a *partial applications* of class C
1160 -- with the last parameter missing
1161 -- (T a1 .. ak) matches the kind of C's last argument
1162 -- (and hence so does t)
1163 -- The latter kind-check has been done by deriveTyData already,
1164 -- and tc_args are already trimmed
1166 -- We generate the instance
1167 -- instance forall ({a1..ak} u fvs(s1..sm)).
1168 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1169 -- where T a1...ap is the partial application of
1170 -- the LHS of the correct kind and p >= k
1172 -- NB: the variables below are:
1173 -- tc_tvs = [a1, ..., an]
1174 -- tyvars_to_keep = [a1, ..., ak]
1175 -- rep_ty = t ak .. an
1176 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1177 -- tys = [s1, ..., sm]
1180 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1181 -- We generate the instance
1182 -- instance Monad (ST s) => Monad (T s) where
1184 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1185 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1186 -- eta-reduces the representation type, so we know that
1188 -- That's convenient here, because we may have to apply
1189 -- it to fewer than its original complement of arguments
1191 -- Note [Newtype representation]
1192 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1193 -- Need newTyConRhs (*not* a recursive representation finder)
1194 -- to get the representation type. For example
1195 -- newtype B = MkB Int
1196 -- newtype A = MkA B deriving( Num )
1197 -- We want the Num instance of B, *not* the Num instance of Int,
1198 -- when making the Num instance of A!
1199 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1200 rep_tys = cls_tys ++ [rep_inst_ty]
1201 rep_pred = mkClassPred cls rep_tys
1202 -- rep_pred is the representation dictionary, from where
1203 -- we are gong to get all the methods for the newtype
1207 -- Next we figure out what superclass dictionaries to use
1208 -- See Note [Newtype deriving superclasses] above
1210 cls_tyvars = classTyVars cls
1211 dfun_tvs = tyVarsOfTypes inst_tys
1212 inst_ty = mkTyConApp tycon tc_args
1213 inst_tys = cls_tys ++ [inst_ty]
1214 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1217 -- If there are no tyvars, there's no need
1218 -- to abstract over the dictionaries we need
1219 -- Example: newtype T = MkT Int deriving( C )
1220 -- We get the derived instance
1223 -- instance C Int => C T
1224 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1226 -------------------------------------------------------------------
1227 -- Figuring out whether we can only do this newtype-deriving thing
1229 can_derive_via_isomorphism
1230 = not (non_iso_class cls)
1234 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1236 arity_ok = length cls_tys + 1 == classArity cls
1237 -- Well kinded; eg not: newtype T ... deriving( ST )
1238 -- because ST needs *2* type params
1240 -- Check that eta reduction is OK
1241 eta_ok = nt_eta_arity <= length rep_tc_args
1242 -- The newtype can be eta-reduced to match the number
1243 -- of type argument actually supplied
1244 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1245 -- Here the 'b' must be the same in the rep type (S [a] b)
1246 -- And the [a] must not mention 'b'. That's all handled
1249 ats_ok = null (classATs cls)
1250 -- No associated types for the class, because we don't
1251 -- currently generate type 'instance' decls; and cannot do
1252 -- so for 'data' instance decls
1255 = vcat [ ppUnless arity_ok arity_msg
1256 , ppUnless eta_ok eta_msg
1257 , ppUnless ats_ok ats_msg ]
1258 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1259 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1260 ats_msg = ptext (sLit "the class has associated types")
1263 Note [Recursive newtypes]
1264 ~~~~~~~~~~~~~~~~~~~~~~~~~
1265 Newtype deriving works fine, even if the newtype is recursive.
1266 e.g. newtype S1 = S1 [T1 ()]
1267 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1268 Remember, too, that type families are curretly (conservatively) given
1269 a recursive flag, so this also allows newtype deriving to work
1272 We used to exclude recursive types, because we had a rather simple
1273 minded way of generating the instance decl:
1275 instance Eq [A] => Eq A -- Makes typechecker loop!
1276 But now we require a simple context, so it's ok.
1279 %************************************************************************
1281 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1283 %************************************************************************
1285 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1286 terms, which is the final correct RHS for the corresponding original
1290 Each (k,TyVarTy tv) in a solution constrains only a type
1294 The (k,TyVarTy tv) pairs in a solution are canonically
1295 ordered by sorting on type varible, tv, (major key) and then class, k,
1300 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1302 inferInstanceContexts _ [] = return []
1304 inferInstanceContexts oflag infer_specs
1305 = do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
1306 ; iterate_deriv 1 initial_solutions }
1308 ------------------------------------------------------------------
1309 -- The initial solutions for the equations claim that each
1310 -- instance has an empty context; this solution is certainly
1311 -- in canonical form.
1312 initial_solutions :: [ThetaType]
1313 initial_solutions = [ [] | _ <- infer_specs ]
1315 ------------------------------------------------------------------
1316 -- iterate_deriv calculates the next batch of solutions,
1317 -- compares it with the current one; finishes if they are the
1318 -- same, otherwise recurses with the new solutions.
1319 -- It fails if any iteration fails
1320 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1321 iterate_deriv n current_solns
1322 | n > 20 -- Looks as if we are in an infinite loop
1323 -- This can happen if we have -XUndecidableInstances
1324 -- (See TcSimplify.tcSimplifyDeriv.)
1325 = pprPanic "solveDerivEqns: probable loop"
1326 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1328 = do { -- Extend the inst info from the explicit instance decls
1329 -- with the current set of solutions, and simplify each RHS
1330 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1331 current_solns infer_specs
1332 ; new_solns <- checkNoErrs $
1333 extendLocalInstEnv inst_specs $
1334 mapM gen_soln infer_specs
1336 ; if (current_solns == new_solns) then
1337 return [ spec { ds_theta = soln }
1338 | (spec, soln) <- zip infer_specs current_solns ]
1340 iterate_deriv (n+1) new_solns }
1342 ------------------------------------------------------------------
1343 gen_soln :: DerivSpec -> TcM [PredType]
1344 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1345 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1347 addErrCtxt (derivInstCtxt the_pred) $
1348 do { -- Check for a bizarre corner case, when the derived instance decl should
1349 -- have form instance C a b => D (T a) where ...
1350 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1351 -- of problems; in particular, it's hard to compare solutions for
1352 -- equality when finding the fixpoint. Moreover, simplifyDeriv
1353 -- has an assert failure because it finds a TyVar when it expects
1354 -- only TcTyVars. So I just rule it out for now. I'm not
1355 -- even sure how it can arise.
1357 ; let tv_set = mkVarSet tyvars
1358 weird_preds = [pred | pred <- deriv_rhs
1359 , not (tyVarsOfPred pred `subVarSet` tv_set)]
1360 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1362 ; theta <- simplifyDeriv orig the_pred tyvars deriv_rhs
1363 -- checkValidInstance tyvars theta clas inst_tys
1364 -- Not necessary; see Note [Exotic derived instance contexts]
1367 ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
1368 -- Claim: the result instance declaration is guaranteed valid
1369 -- Hence no need to call:
1370 -- checkValidInstance tyvars theta clas inst_tys
1371 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1373 the_pred = mkClassPred clas inst_tys
1375 ------------------------------------------------------------------
1376 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1377 mkInstance overlap_flag theta
1378 (DS { ds_name = dfun_name
1379 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1380 = mkLocalInstance dfun overlap_flag
1382 dfun = mkDictFunId dfun_name tyvars theta clas tys
1385 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1386 -- Add new locally-defined instances; don't bother to check
1387 -- for functional dependency errors -- that'll happen in TcInstDcls
1388 extendLocalInstEnv dfuns thing_inside
1389 = do { env <- getGblEnv
1390 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1391 env' = env { tcg_inst_env = inst_env' }
1392 ; setGblEnv env' thing_inside }
1396 %************************************************************************
1398 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1400 %************************************************************************
1402 After all the trouble to figure out the required context for the
1403 derived instance declarations, all that's left is to chug along to
1404 produce them. They will then be shoved into @tcInstDecls2@, which
1405 will do all its usual business.
1407 There are lots of possibilities for code to generate. Here are
1408 various general remarks.
1413 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1414 ``you-couldn't-do-better-by-hand'' efficient.
1417 Deriving @Show@---also pretty common--- should also be reasonable good code.
1420 Deriving for the other classes isn't that common or that big a deal.
1427 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1430 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1433 We {\em normally} generate code only for the non-defaulted methods;
1434 there are some exceptions for @Eq@ and (especially) @Ord@...
1437 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1438 constructor's numeric (@Int#@) tag. These are generated by
1439 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1440 these is around is given by @hasCon2TagFun@.
1442 The examples under the different sections below will make this
1446 Much less often (really just for deriving @Ix@), we use a
1447 @_tag2con_<tycon>@ function. See the examples.
1450 We use the renamer!!! Reason: we're supposed to be
1451 producing @LHsBinds Name@ for the methods, but that means
1452 producing correctly-uniquified code on the fly. This is entirely
1453 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1454 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1455 the renamer. What a great hack!
1459 -- Generate the InstInfo for the required instance paired with the
1460 -- *representation* tycon for that instance,
1461 -- plus any auxiliary bindings required
1463 -- Representation tycons differ from the tycon in the instance signature in
1464 -- case of instances for indexed families.
1466 genInst :: Bool -- True <=> standalone deriving
1468 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1469 genInst standalone_deriv oflag
1470 spec@(DS { ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1471 , ds_theta = theta, ds_newtype = is_newtype
1472 , ds_name = name, ds_cls = clas })
1474 = return (InstInfo { iSpec = inst_spec
1475 , iBinds = NewTypeDerived co rep_tycon }, [])
1478 = do { fix_env <- getFixityEnv
1479 ; let loc = getSrcSpan name
1480 (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1481 -- In case of a family instance, we need to use the representation
1482 -- tycon (after all, it has the data constructors)
1484 ; return (InstInfo { iSpec = inst_spec
1485 , iBinds = VanillaInst meth_binds [] standalone_deriv }
1488 inst_spec = mkInstance oflag theta spec
1489 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1490 Just co_con -> mkAxInstCo co_con rep_tc_args
1492 -- Not a family => rep_tycon = main tycon
1493 co2 = mkAxInstCo (newTyConCo rep_tycon) rep_tc_args
1494 co = co1 `mkTransCo` co2
1495 id_co = mkReflCo (mkTyConApp rep_tycon rep_tc_args)
1497 -- Example: newtype instance N [a] = N1 (Tree a)
1498 -- deriving instance Eq b => Eq (N [(b,b)])
1499 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1500 -- When dealing with the deriving clause
1501 -- co1 : N [(b,b)] ~ R1:N (b,b)
1502 -- co2 : R1:N (b,b) ~ Tree (b,b)
1503 -- co : N [(b,b)] ~ Tree (b,b)
1505 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1506 genDerivBinds loc fix_env clas tycon
1507 | className clas `elem` typeableClassNames
1508 = (gen_Typeable_binds loc tycon, [])
1511 = case assocMaybe gen_list (getUnique clas) of
1512 Just gen_fn -> gen_fn loc tycon
1513 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1515 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1516 gen_list = [(eqClassKey, gen_Eq_binds)
1517 ,(ordClassKey, gen_Ord_binds)
1518 ,(enumClassKey, gen_Enum_binds)
1519 ,(boundedClassKey, gen_Bounded_binds)
1520 ,(ixClassKey, gen_Ix_binds)
1521 ,(showClassKey, gen_Show_binds fix_env)
1522 ,(readClassKey, gen_Read_binds fix_env)
1523 ,(dataClassKey, gen_Data_binds)
1524 ,(functorClassKey, gen_Functor_binds)
1525 ,(foldableClassKey, gen_Foldable_binds)
1526 ,(traversableClassKey, gen_Traversable_binds)
1527 ,(genClassKey, genGenericBinds)
1531 %************************************************************************
1533 \subsection[TcDeriv-generic-binds]{Bindings for the new generic deriving mechanism}
1535 %************************************************************************
1537 For the generic representation we need to generate:
1539 \item A Generic instance
1540 \item A Rep type instance
1541 \item Many auxiliary datatypes and instances for them (for the meta-information)
1544 @genGenericBinds@ does (1)
1545 @genGenericRepExtras@ does (2) and (3)
1546 @genGenericAll@ does all of them
1549 genGenericBinds :: SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1550 genGenericBinds _ tc = (mkBindsRep tc, [ {- No DerivAuxBinds -} ])
1552 genGenericRepExtras :: TyCon -> TcM (MetaTyCons, TyCon)
1553 genGenericRepExtras tc =
1554 do uniqS <- newUniqueSupply
1556 -- Uniques for everyone
1557 (uniqD:uniqs) = uniqsFromSupply uniqS
1558 (uniqsC,us) = splitAt (length tc_cons) uniqs
1559 uniqsS :: [[Unique]] -- Unique supply for the S datatypes
1560 uniqsS = mkUniqsS tc_arits us
1562 mkUniqsS (n:t) us = case splitAt n us of
1563 (us1,us2) -> us1 : mkUniqsS t us2
1565 tc_name = tyConName tc
1566 tc_cons = tyConDataCons tc
1567 tc_arits = map dataConSourceArity tc_cons
1569 tc_occ = nameOccName tc_name
1570 d_occ = mkGenD tc_occ
1571 c_occ m = mkGenC tc_occ m
1572 s_occ m n = mkGenS tc_occ m n
1573 mod_name = nameModule (tyConName tc)
1574 d_name = mkExternalName uniqD mod_name d_occ wiredInSrcSpan
1575 c_names = [ mkExternalName u mod_name (c_occ m) wiredInSrcSpan
1576 | (u,m) <- zip uniqsC [0..] ]
1577 s_names = [ [ mkExternalName u mod_name (s_occ m n) wiredInSrcSpan
1578 | (u,n) <- zip us [0..] ] | (us,m) <- zip uniqsS [0..] ]
1580 mkTyCon name = ASSERT( isExternalName name )
1581 buildAlgTyCon name [] [] mkAbstractTyConRhs
1582 NonRecursive False NoParentTyCon Nothing
1584 metaDTyCon <- mkTyCon d_name
1585 metaCTyCons <- sequence [ mkTyCon c_name | c_name <- c_names ]
1586 metaSTyCons <- mapM sequence
1588 | s_name <- s_namesC ] | s_namesC <- s_names ]
1590 let metaDts = MetaTyCons metaDTyCon metaCTyCons metaSTyCons
1592 rep0_tycon <- tc_mkRepTyCon tc metaDts
1594 return (metaDts, rep0_tycon)
1596 genGenericAll :: TyCon
1597 -> TcM ((InstInfo RdrName, DerivAuxBinds), MetaTyCons, TyCon)
1599 do (metaDts, rep0_tycon) <- genGenericRepExtras tc
1600 clas <- tcLookupClass genClassName
1601 dfun_name <- new_dfun_name clas tc
1603 mkInstRep = (InstInfo { iSpec = inst, iBinds = binds }
1604 , [ {- No DerivAuxBinds -} ])
1605 inst = mkLocalInstance dfun NoOverlap
1606 binds = VanillaInst (mkBindsRep tc) [] False
1608 tvs = tyConTyVars tc
1609 tc_ty = mkTyConApp tc (mkTyVarTys tvs)
1611 dfun = mkDictFunId dfun_name (tyConTyVars tc) [] clas [tc_ty]
1612 return (mkInstRep, metaDts, rep0_tycon)
1614 genDtMeta :: (TyCon, MetaTyCons) -> TcM [(InstInfo RdrName, DerivAuxBinds)]
1615 genDtMeta (tc,metaDts) =
1616 do dClas <- tcLookupClass datatypeClassName
1617 d_dfun_name <- new_dfun_name dClas tc
1618 cClas <- tcLookupClass constructorClassName
1619 c_dfun_names <- sequence [ new_dfun_name cClas tc | _ <- metaC metaDts ]
1620 sClas <- tcLookupClass selectorClassName
1621 s_dfun_names <- sequence (map sequence [ [ new_dfun_name sClas tc
1623 | x <- metaS metaDts ])
1624 fix_env <- getFixityEnv
1627 (dBinds,cBinds,sBinds) = mkBindsMetaD fix_env tc
1630 d_metaTycon = metaD metaDts
1631 d_inst = mkLocalInstance d_dfun NoOverlap
1632 d_binds = VanillaInst dBinds [] False
1633 d_dfun = mkDictFunId d_dfun_name (tyConTyVars tc) [] dClas
1634 [ mkTyConTy d_metaTycon ]
1635 d_mkInst = (InstInfo { iSpec = d_inst, iBinds = d_binds }, [])
1638 c_metaTycons = metaC metaDts
1639 c_insts = [ mkLocalInstance (c_dfun c ds) NoOverlap
1640 | (c, ds) <- myZip1 c_metaTycons c_dfun_names ]
1641 c_binds = [ VanillaInst c [] False | c <- cBinds ]
1642 c_dfun c dfun_name = mkDictFunId dfun_name (tyConTyVars tc) [] cClas
1644 c_mkInst = [ (InstInfo { iSpec = is, iBinds = bs }, [])
1645 | (is,bs) <- myZip1 c_insts c_binds ]
1648 s_metaTycons = metaS metaDts
1649 s_insts = map (map (\(s,ds) -> mkLocalInstance (s_dfun s ds) NoOverlap))
1650 (myZip2 s_metaTycons s_dfun_names)
1651 s_binds = [ [ VanillaInst s [] False | s <- ss ] | ss <- sBinds ]
1652 s_dfun s dfun_name = mkDictFunId dfun_name (tyConTyVars tc) [] sClas
1654 s_mkInst = map (map (\(is,bs) -> (InstInfo {iSpec=is, iBinds=bs}, [])))
1655 (myZip2 s_insts s_binds)
1657 myZip1 :: [a] -> [b] -> [(a,b)]
1658 myZip1 l1 l2 = ASSERT (length l1 == length l2) zip l1 l2
1660 myZip2 :: [[a]] -> [[b]] -> [[(a,b)]]
1662 ASSERT (and (zipWith (>=) (map length l1) (map length l2)))
1663 [ zip x1 x2 | (x1,x2) <- zip l1 l2 ]
1665 return (d_mkInst : c_mkInst ++ concat s_mkInst)
1669 %************************************************************************
1671 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1673 %************************************************************************
1676 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1677 derivingKindErr tc cls cls_tys cls_kind
1678 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1679 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1680 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1681 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1683 derivingEtaErr :: Class -> [Type] -> Type -> Message
1684 derivingEtaErr cls cls_tys inst_ty
1685 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1686 nest 2 (ptext (sLit "instance (...) =>")
1687 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1689 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1690 typeFamilyPapErr tc cls cls_tys inst_ty
1691 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1692 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1694 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1695 derivingThingErr newtype_deriving clas tys ty why
1696 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1697 2 (quotes (ppr pred))
1698 $$ nest 2 extra) <> colon,
1701 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1703 pred = mkClassPred clas (tys ++ [ty])
1705 derivingHiddenErr :: TyCon -> SDoc
1706 derivingHiddenErr tc
1707 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1708 2 (ptext (sLit "so you cannot derive an instance for it"))
1710 standaloneCtxt :: LHsType Name -> SDoc
1711 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1714 derivInstCtxt :: PredType -> Message
1716 = ptext (sLit "When deriving the instance for") <+> parens (ppr pred)
1718 badDerivedPred :: PredType -> Message
1720 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1721 ptext (sLit "type variables that are not data type parameters"),
1722 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]