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
56 %************************************************************************
60 %************************************************************************
64 1. Convert the decls (i.e. data/newtype deriving clauses,
65 plus standalone deriving) to [EarlyDerivSpec]
67 2. Infer the missing contexts for the Left DerivSpecs
69 3. Add the derived bindings, generating InstInfos
73 -- DerivSpec is purely local to this module
74 data DerivSpec = DS { ds_loc :: SrcSpan
78 , ds_theta :: ThetaType
82 , ds_tc_args :: [Type]
83 , ds_newtype :: Bool }
84 -- This spec implies a dfun declaration of the form
85 -- df :: forall tvs. theta => C tys
86 -- The Name is the name for the DFun we'll build
87 -- The tyvars bind all the variables in the theta
88 -- For type families, the tycon in
89 -- in ds_tys is the *family* tycon
90 -- in ds_tc, ds_tc_args is the *representation* tycon
91 -- For non-family tycons, both are the same
93 -- ds_newtype = True <=> Newtype deriving
94 -- False <=> Vanilla deriving
99 newtype instance T [a] = MkT (Tree a) deriving( C s )
101 axiom T [a] = :RTList a
102 axiom :RTList a = Tree a
104 DS { ds_tvs = [a,s], ds_cls = C, ds_tys = [s, T [a]]
105 , ds_tc = :RTList, ds_tc_args = [a]
106 , ds_newtype = True }
109 type DerivContext = Maybe ThetaType
110 -- Nothing <=> Vanilla deriving; infer the context of the instance decl
111 -- Just theta <=> Standalone deriving: context supplied by programmer
113 type EarlyDerivSpec = Either DerivSpec DerivSpec
114 -- Left ds => the context for the instance should be inferred
115 -- In this case ds_theta is the list of all the
116 -- constraints needed, such as (Eq [a], Eq a)
117 -- The inference process is to reduce this to a
118 -- simpler form (e.g. Eq a)
120 -- Right ds => the exact context for the instance is supplied
121 -- by the programmer; it is ds_theta
123 pprDerivSpec :: DerivSpec -> SDoc
124 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
125 ds_cls = c, ds_tys = tys, ds_theta = rhs })
126 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
127 <+> equals <+> ppr rhs)
131 Inferring missing contexts
132 ~~~~~~~~~~~~~~~~~~~~~~~~~~
135 data T a b = C1 (Foo a) (Bar b)
140 [NOTE: See end of these comments for what to do with
141 data (C a, D b) => T a b = ...
144 We want to come up with an instance declaration of the form
146 instance (Ping a, Pong b, ...) => Eq (T a b) where
149 It is pretty easy, albeit tedious, to fill in the code "...". The
150 trick is to figure out what the context for the instance decl is,
151 namely @Ping@, @Pong@ and friends.
153 Let's call the context reqd for the T instance of class C at types
154 (a,b, ...) C (T a b). Thus:
156 Eq (T a b) = (Ping a, Pong b, ...)
158 Now we can get a (recursive) equation from the @data@ decl:
160 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
161 u Eq (T b a) u Eq Int -- From C2
162 u Eq (T a a) -- From C3
164 Foo and Bar may have explicit instances for @Eq@, in which case we can
165 just substitute for them. Alternatively, either or both may have
166 their @Eq@ instances given by @deriving@ clauses, in which case they
167 form part of the system of equations.
169 Now all we need do is simplify and solve the equations, iterating to
170 find the least fixpoint. Notice that the order of the arguments can
171 switch around, as here in the recursive calls to T.
173 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
177 Eq (T a b) = {} -- The empty set
180 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
181 u Eq (T b a) u Eq Int -- From C2
182 u Eq (T a a) -- From C3
184 After simplification:
185 = Eq a u Ping b u {} u {} u {}
190 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
191 u Eq (T b a) u Eq Int -- From C2
192 u Eq (T a a) -- From C3
194 After simplification:
199 = Eq a u Ping b u Eq b u Ping a
201 The next iteration gives the same result, so this is the fixpoint. We
202 need to make a canonical form of the RHS to ensure convergence. We do
203 this by simplifying the RHS to a form in which
205 - the classes constrain only tyvars
206 - the list is sorted by tyvar (major key) and then class (minor key)
207 - no duplicates, of course
209 So, here are the synonyms for the ``equation'' structures:
212 Note [Data decl contexts]
213 ~~~~~~~~~~~~~~~~~~~~~~~~~
216 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
218 We will need an instance decl like:
220 instance (Read a, RealFloat a) => Read (Complex a) where
223 The RealFloat in the context is because the read method for Complex is bound
224 to construct a Complex, and doing that requires that the argument type is
227 But this ain't true for Show, Eq, Ord, etc, since they don't construct
228 a Complex; they only take them apart.
230 Our approach: identify the offending classes, and add the data type
231 context to the instance decl. The "offending classes" are
235 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
236 pattern matching against a constructor from a data type with a context
237 gives rise to the constraints for that context -- or at least the thinned
238 version. So now all classes are "offending".
240 Note [Newtype deriving]
241 ~~~~~~~~~~~~~~~~~~~~~~~
245 newtype T = T Char deriving( C [a] )
247 Notice the free 'a' in the deriving. We have to fill this out to
248 newtype T = T Char deriving( forall a. C [a] )
250 And then translate it to:
251 instance C [a] Char => C [a] T where ...
254 Note [Newtype deriving superclasses]
255 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
256 (See also Trac #1220 for an interesting exchange on newtype
257 deriving and superclasses.)
259 The 'tys' here come from the partial application in the deriving
260 clause. The last arg is the new instance type.
262 We must pass the superclasses; the newtype might be an instance
263 of them in a different way than the representation type
264 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
265 Then the Show instance is not done via isomorphism; it shows
267 The Num instance is derived via isomorphism, but the Show superclass
268 dictionary must the Show instance for Foo, *not* the Show dictionary
269 gotten from the Num dictionary. So we must build a whole new dictionary
270 not just use the Num one. The instance we want is something like:
271 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
274 There may be a coercion needed which we get from the tycon for the newtype
275 when the dict is constructed in TcInstDcls.tcInstDecl2
278 Note [Unused constructors and deriving clauses]
279 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
280 See Trac #3221. Consider
281 data T = T1 | T2 deriving( Show )
282 Are T1 and T2 unused? Well, no: the deriving clause expands to mention
283 both of them. So we gather defs/uses from deriving just like anything else.
285 %************************************************************************
287 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
289 %************************************************************************
292 tcDeriving :: [LTyClDecl Name] -- All type constructors
293 -> [LInstDecl Name] -- All instance declarations
294 -> [LDerivDecl Name] -- All stand-alone deriving declarations
295 -> TcM ([InstInfo Name], -- The generated "instance decls"
296 HsValBinds Name, -- Extra generated top-level bindings
299 tcDeriving tycl_decls inst_decls deriv_decls
300 = recoverM (return ([], emptyValBindsOut, emptyDUs)) $
301 do { -- Fish the "deriving"-related information out of the TcEnv
302 -- And make the necessary "equations".
303 is_boot <- tcIsHsBoot
304 ; traceTc "tcDeriving" (ppr is_boot)
305 ; early_specs <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
307 ; overlap_flag <- getOverlapFlag
308 ; let (infer_specs, given_specs) = splitEithers early_specs
309 ; insts1 <- mapM (genInst True overlap_flag) given_specs
311 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
312 inferInstanceContexts overlap_flag infer_specs
314 ; insts2 <- mapM (genInst False overlap_flag) final_specs
316 -- Generate the generic to/from functions from each type declaration
317 ; gen_binds <- mkGenericBinds is_boot tycl_decls
318 ; (inst_info, rn_binds, rn_dus) <- renameDeriv is_boot gen_binds (insts1 ++ insts2)
321 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
322 (ddump_deriving inst_info rn_binds))
324 ; return (inst_info, rn_binds, rn_dus) }
326 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
327 ddump_deriving inst_infos extra_binds
328 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
330 renameDeriv :: Bool -> LHsBinds RdrName
331 -> [(InstInfo RdrName, DerivAuxBinds)]
332 -> TcM ([InstInfo Name], HsValBinds Name, DefUses)
333 renameDeriv is_boot gen_binds insts
334 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
335 -- The inst-info bindings will all be empty, but it's easier to
336 -- just use rn_inst_info to change the type appropriately
337 = do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
338 ; return (rn_inst_infos, emptyValBindsOut, usesOnly (plusFVs fvs)) }
341 = discardWarnings $ -- Discard warnings about unused bindings etc
342 do { (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $ -- Type signatures in patterns
343 -- are used in the generic binds
344 rnTopBinds (ValBindsIn gen_binds [])
345 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
347 -- Generate and rename any extra not-one-inst-decl-specific binds,
348 -- notably "con2tag" and/or "tag2con" functions.
349 -- Bring those names into scope before renaming the instances themselves
350 ; loc <- getSrcSpanM -- Generic loc for shared bindings
351 ; let (aux_binds, aux_sigs) = unzip $ map (genAuxBind loc) $
352 rm_dups [] $ concat deriv_aux_binds
353 aux_val_binds = ValBindsIn (listToBag aux_binds) aux_sigs
354 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
355 ; let aux_names = collectHsValBinders rn_aux_lhs
357 ; bindLocalNames aux_names $
358 do { (rn_aux, dus_aux) <- rnTopBindsRHS (mkNameSet aux_names) rn_aux_lhs
359 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
360 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
361 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
364 (inst_infos, deriv_aux_binds) = unzip insts
366 -- Remove duplicate requests for auxilliary bindings
368 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
369 | otherwise = rm_dups (b:acc) bs
372 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
373 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
374 = return ( info { iBinds = NewTypeDerived coi tc }
375 , mkFVs (map dataConName (tyConDataCons tc)))
376 -- See Note [Newtype deriving and unused constructors]
378 rn_inst_info (InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
379 = -- Bring the right type variables into
380 -- scope (yuk), and rename the method binds
382 bindLocalNames (map Var.varName tyvars) $
383 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
384 ; let binds' = VanillaInst rn_binds [] standalone_deriv
385 ; return (InstInfo { iSpec = inst, iBinds = binds' }, fvs) }
387 (tyvars,_, clas,_) = instanceHead inst
388 clas_nm = className clas
390 -----------------------------------------
391 mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
392 mkGenericBinds is_boot tycl_decls
396 = do { tcs <- mapM tcLookupTyCon [ tcdName d
397 | L _ d <- tycl_decls, isDataDecl d ]
398 ; return (unionManyBags [ mkTyConGenericBinds tc
399 | tc <- tcs, tyConHasGenerics tc ]) }
400 -- We are only interested in the data type declarations,
401 -- and then only in the ones whose 'has-generics' flag is on
402 -- The predicate tyConHasGenerics finds both of these
405 Note [Newtype deriving and unused constructors]
406 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
407 Consider this (see Trac #1954):
410 newtype P a = MkP (IO a) deriving Monad
412 If you compile with -fwarn-unused-binds you do not expect the warning
413 "Defined but not used: data consructor MkP". Yet the newtype deriving
414 code does not explicitly mention MkP, but it should behave as if you
416 instance Monad P where
417 return x = MkP (return x)
420 So we want to signal a user of the data constructor 'MkP'. That's
421 what we do in rn_inst_info, and it's the only reason we have the TyCon
422 stored in NewTypeDerived.
425 %************************************************************************
427 From HsSyn to DerivSpec
429 %************************************************************************
431 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
434 makeDerivSpecs :: Bool
438 -> TcM [EarlyDerivSpec]
440 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
441 | is_boot -- No 'deriving' at all in hs-boot files
442 = do { mapM_ add_deriv_err deriv_locs
445 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
446 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
447 ; return (eqns1 ++ eqns2) }
449 extractTyDataPreds decls
450 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
452 all_tydata :: [(LHsType Name, LTyClDecl Name)]
453 -- Derived predicate paired with its data type declaration
454 all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
456 deriv_locs = map (getLoc . snd) all_tydata
457 ++ map getLoc deriv_decls
459 add_deriv_err loc = setSrcSpan loc $
460 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
461 2 (ptext (sLit "Use an instance declaration instead")))
463 ------------------------------------------------------------------
464 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
465 -- Standalone deriving declarations
466 -- e.g. deriving instance Show a => Show (T a)
467 -- Rather like tcLocalInstDecl
468 deriveStandalone (L loc (DerivDecl deriv_ty))
470 addErrCtxt (standaloneCtxt deriv_ty) $
471 do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
472 ; (tvs, theta, tau) <- tcHsInstHead deriv_ty
473 ; traceTc "Standalone deriving;" $ vcat
474 [ text "tvs:" <+> ppr tvs
475 , text "theta:" <+> ppr theta
476 , text "tau:" <+> ppr tau ]
477 ; (cls, inst_tys) <- checkValidInstance deriv_ty tvs theta tau
478 -- C.f. TcInstDcls.tcLocalInstDecl1
480 ; let cls_tys = take (length inst_tys - 1) inst_tys
481 inst_ty = last inst_tys
482 ; traceTc "Standalone deriving:" $ vcat
483 [ text "class:" <+> ppr cls
484 , text "class types:" <+> ppr cls_tys
485 , text "type:" <+> ppr inst_ty ]
486 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
489 ------------------------------------------------------------------
490 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
491 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
492 tcdTyVars = tv_names,
493 tcdTyPats = ty_pats }))
494 = setSrcSpan loc $ -- Use the location of the 'deriving' item
496 do { (tvs, tc, tc_args) <- get_lhs ty_pats
497 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
498 -- the type variables for the type constructor
500 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
501 -- The "deriv_pred" is a LHsType to take account of the fact that for
502 -- newtype deriving we allow deriving (forall a. C [a]).
504 -- Given data T a b c = ... deriving( C d ),
505 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
506 ; let cls_tyvars = classTyVars cls
507 kind = tyVarKind (last cls_tyvars)
508 (arg_kinds, _) = splitKindFunTys kind
509 n_args_to_drop = length arg_kinds
510 n_args_to_keep = tyConArity tc - n_args_to_drop
511 args_to_drop = drop n_args_to_keep tc_args
512 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
513 inst_ty_kind = typeKind inst_ty
514 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
515 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
516 `minusVarSet` dropped_tvs
518 -- Check that the result really is well-kinded
519 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
520 (derivingKindErr tc cls cls_tys kind)
522 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
523 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
524 (derivingEtaErr cls cls_tys inst_ty)
526 -- (a) The data type can be eta-reduced; eg reject:
527 -- data instance T a a = ... deriving( Monad )
528 -- (b) The type class args do not mention any of the dropped type
530 -- newtype T a s = ... deriving( ST s )
532 -- Type families can't be partially applied
533 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
534 -- Note [Deriving, type families, and partial applications]
535 ; checkTc (not (isFamilyTyCon tc) || n_args_to_drop == 0)
536 (typeFamilyPapErr tc cls cls_tys inst_ty)
538 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
540 -- Tiresomely we must figure out the "lhs", which is awkward for type families
541 -- E.g. data T a b = .. deriving( Eq )
542 -- Here, the lhs is (T a b)
543 -- data instance TF Int b = ... deriving( Eq )
544 -- Here, the lhs is (TF Int b)
545 -- But if we just look up the tycon_name, we get is the *family*
546 -- tycon, but not pattern types -- they are in the *rep* tycon.
547 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
548 ; let tvs = tyConTyVars tc
549 ; return (tvs, tc, mkTyVarTys tvs) }
550 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
551 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
552 ; let (tc, tc_args) = tcSplitTyConApp tc_app
553 ; return (tvs, tc, tc_args) }
556 = panic "derivTyData" -- Caller ensures that only TyData can happen
559 Note [Deriving, type families, and partial applications]
560 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
561 When there are no type families, it's quite easy:
563 newtype S a = MkS [a]
564 -- :CoS :: S ~ [] -- Eta-reduced
566 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
567 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
569 When type familes are involved it's trickier:
572 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
573 -- :RT is the representation type for (T Int a)
574 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
575 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
577 instance Eq [a] => Eq (T Int a) -- easy by coercion
578 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
580 The "???" bit is that we don't build the :CoF thing in eta-reduced form
581 Henc the current typeFamilyPapErr, even though the instance makes sense.
582 After all, we can write it out
583 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
588 mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type] -> Type
589 -> DerivContext -- Just => context supplied (standalone deriving)
590 -- Nothing => context inferred (deriving on data decl)
591 -> TcRn EarlyDerivSpec
592 -- Make the EarlyDerivSpec for an instance
593 -- forall tvs. theta => cls (tys ++ [ty])
594 -- where the 'theta' is optional (that's the Maybe part)
595 -- Assumes that this declaration is well-kinded
597 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
598 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
599 , isAlgTyCon tycon -- Check for functions, primitive types etc
600 = do { (rep_tc, rep_tc_args) <- tcLookupDataFamInst tycon tc_args
601 -- Be careful to test rep_tc here: in the case of families,
602 -- we want to check the instance tycon, not the family tycon
604 -- For standalone deriving (mtheta /= Nothing),
605 -- check that all the data constructors are in scope.
606 -- No need for this when deriving Typeable, becuase we don't need
607 -- the constructors for that.
608 ; rdr_env <- getGlobalRdrEnv
609 ; let hidden_data_cons = isAbstractTyCon rep_tc || any not_in_scope (tyConDataCons rep_tc)
610 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
611 ; checkTc (isNothing mtheta ||
612 not hidden_data_cons ||
613 className cls `elem` typeableClassNames)
614 (derivingHiddenErr tycon)
617 ; if isDataTyCon rep_tc then
618 mkDataTypeEqn orig dflags tvs cls cls_tys
619 tycon tc_args rep_tc rep_tc_args mtheta
621 mkNewTypeEqn orig dflags tvs cls cls_tys
622 tycon tc_args rep_tc rep_tc_args mtheta }
624 = failWithTc (derivingThingErr False cls cls_tys tc_app
625 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
629 %************************************************************************
633 %************************************************************************
636 mkDataTypeEqn :: CtOrigin
638 -> [Var] -- Universally quantified type variables in the instance
639 -> Class -- Class for which we need to derive an instance
640 -> [Type] -- Other parameters to the class except the last
641 -> TyCon -- Type constructor for which the instance is requested
642 -- (last parameter to the type class)
643 -> [Type] -- Parameters to the type constructor
644 -> TyCon -- rep of the above (for type families)
645 -> [Type] -- rep of the above
646 -> DerivContext -- Context of the instance, for standalone deriving
647 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
649 mkDataTypeEqn orig dflags tvs cls cls_tys
650 tycon tc_args rep_tc rep_tc_args mtheta
651 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
652 -- NB: pass the *representation* tycon to checkSideConditions
653 CanDerive -> go_for_it
654 NonDerivableClass -> bale_out (nonStdErr cls)
655 DerivableClassError msg -> bale_out msg
657 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
658 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
660 mk_data_eqn, mk_typeable_eqn
661 :: CtOrigin -> [TyVar] -> Class
662 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
663 -> TcM EarlyDerivSpec
664 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
665 | getName cls `elem` typeableClassNames
666 = mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
669 = do { dfun_name <- new_dfun_name cls tycon
671 ; let inst_tys = [mkTyConApp tycon tc_args]
672 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
673 spec = DS { ds_loc = loc, ds_orig = orig
674 , ds_name = dfun_name, ds_tvs = tvs
675 , ds_cls = cls, ds_tys = inst_tys
676 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
677 , ds_theta = mtheta `orElse` inferred_constraints
678 , ds_newtype = False }
680 ; return (if isJust mtheta then Right spec -- Specified context
681 else Left spec) } -- Infer context
683 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
684 -- The Typeable class is special in several ways
685 -- data T a b = ... deriving( Typeable )
687 -- instance Typeable2 T where ...
689 -- 1. There are no constraints in the instance
690 -- 2. There are no type variables either
691 -- 3. The actual class we want to generate isn't necessarily
692 -- Typeable; it depends on the arity of the type
693 | isNothing mtheta -- deriving on a data type decl
694 = do { checkTc (cls `hasKey` typeableClassKey)
695 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
696 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
697 ; mk_typeable_eqn orig tvs real_cls tycon [] rep_tc [] (Just []) }
699 | otherwise -- standaone deriving
700 = do { checkTc (null tc_args)
701 (ptext (sLit "Derived typeable instance must be of form (Typeable")
702 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
703 ; dfun_name <- new_dfun_name cls tycon
706 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
707 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
708 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
709 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
712 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
713 -- Generate a sufficiently large set of constraints that typechecking the
714 -- generated method definitions should succeed. This set will be simplified
715 -- before being used in the instance declaration
716 inferConstraints _ cls inst_tys rep_tc rep_tc_args
717 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
718 stupid_constraints ++ extra_constraints
719 ++ sc_constraints ++ con_arg_constraints
721 -- Constraints arising from the arguments of each constructor
723 = [ mkClassPred cls [arg_ty]
724 | data_con <- tyConDataCons rep_tc,
725 arg_ty <- ASSERT( isVanillaDataCon data_con )
726 get_constrained_tys $
727 dataConInstOrigArgTys data_con all_rep_tc_args,
728 not (isUnLiftedType arg_ty) ]
729 -- No constraints for unlifted types
730 -- Where they are legal we generate specilised function calls
732 -- For functor-like classes, two things are different
733 -- (a) We recurse over argument types to generate constraints
734 -- See Functor examples in TcGenDeriv
735 -- (b) The rep_tc_args will be one short
736 is_functor_like = getUnique cls `elem` functorLikeClassKeys
738 get_constrained_tys :: [Type] -> [Type]
739 get_constrained_tys tys
740 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
743 rep_tc_tvs = tyConTyVars rep_tc
744 last_tv = last rep_tc_tvs
745 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
746 | otherwise = rep_tc_args
748 -- Constraints arising from superclasses
749 -- See Note [Superclasses of derived instance]
750 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
753 -- Stupid constraints
754 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
755 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
757 -- Extra Data constraints
758 -- The Data class (only) requires that for
759 -- instance (...) => Data (T t1 t2)
761 -- THEN (Data t1, Data t2) are among the (...) constraints
762 -- Reason: when the IF holds, we generate a method
763 -- dataCast2 f = gcast2 f
764 -- and we need the Data constraints to typecheck the method
766 | cls `hasKey` dataClassKey
767 , all (isLiftedTypeKind . typeKind) rep_tc_args
768 = [mkClassPred cls [ty] | ty <- rep_tc_args]
772 ------------------------------------------------------------------
773 -- Check side conditions that dis-allow derivability for particular classes
774 -- This is *apart* from the newtype-deriving mechanism
776 -- Here we get the representation tycon in case of family instances as it has
777 -- the data constructors - but we need to be careful to fall back to the
778 -- family tycon (with indexes) in error messages.
780 data DerivStatus = CanDerive
781 | DerivableClassError SDoc -- Standard class, but can't do it
782 | NonDerivableClass -- Non-standard class
784 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
785 checkSideConditions dflags mtheta cls cls_tys rep_tc
786 | Just cond <- sideConditions mtheta cls
787 = case (cond (dflags, rep_tc)) of
788 Just err -> DerivableClassError err -- Class-specific error
789 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
790 -- cls_tys (the type args other than last)
792 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
793 | otherwise = NonDerivableClass -- Not a standard class
795 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
797 nonStdErr :: Class -> SDoc
798 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
800 sideConditions :: DerivContext -> Class -> Maybe Condition
801 sideConditions mtheta cls
802 | cls_key == eqClassKey = Just cond_std
803 | cls_key == ordClassKey = Just cond_std
804 | cls_key == showClassKey = Just cond_std
805 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
806 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
807 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
808 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
809 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
810 cond_std `andCond` cond_noUnliftedArgs)
811 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
812 cond_functorOK True) -- NB: no cond_std!
813 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
814 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
815 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
816 cond_functorOK False)
817 | getName cls `elem` typeableClassNames = Just (checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK)
818 | otherwise = Nothing
820 cls_key = getUnique cls
821 cond_std = cond_stdOK mtheta
823 type Condition = (DynFlags, TyCon) -> Maybe SDoc
824 -- first Bool is whether or not we are allowed to derive Data and Typeable
825 -- second Bool is whether or not we are allowed to derive Functor
826 -- TyCon is the *representation* tycon if the
827 -- data type is an indexed one
830 orCond :: Condition -> Condition -> Condition
833 Nothing -> Nothing -- c1 succeeds
834 Just {} -> c2 tc -- c1 fails, try c2
835 -- orCond produced just one error message, namely from c2
836 -- Getting two can be confusing. For a zero-constructor
837 -- type with a standalone isntance decl, we previously got:
838 -- Can't make a derived instance of `Bounded (Test a)':
839 -- `Test' has no data constructors
841 -- `Test' does not have precisely one constructor
843 andCond :: Condition -> Condition -> Condition
844 andCond c1 c2 tc = case c1 tc of
845 Nothing -> c2 tc -- c1 succeeds
846 Just x -> Just x -- c1 fails
848 cond_stdOK :: DerivContext -> Condition
849 cond_stdOK (Just _) _
850 = Nothing -- Don't check these conservative conditions for
851 -- standalone deriving; just generate the code
852 -- and let the typechecker handle the result
853 cond_stdOK Nothing (_, rep_tc)
854 | null data_cons = Just (no_cons_why rep_tc $$ suggestion)
855 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
856 | otherwise = Nothing
858 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
859 data_cons = tyConDataCons rep_tc
860 con_whys = mapCatMaybes check_con data_cons
862 check_con :: DataCon -> Maybe SDoc
864 | isVanillaDataCon con
865 , all isTauTy (dataConOrigArgTys con) = Nothing
866 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
868 no_cons_why :: TyCon -> SDoc
869 no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
870 ptext (sLit "has no data constructors")
872 cond_enumOrProduct :: Condition
873 cond_enumOrProduct = cond_isEnumeration `orCond`
874 (cond_isProduct `andCond` cond_noUnliftedArgs)
876 cond_noUnliftedArgs :: Condition
877 -- For some classes (eg Eq, Ord) we allow unlifted arg types
878 -- by generating specilaised code. For others (eg Data) we don't.
879 cond_noUnliftedArgs (_, tc)
880 | null bad_cons = Nothing
881 | otherwise = Just why
883 bad_cons = [ con | con <- tyConDataCons tc
884 , any isUnLiftedType (dataConOrigArgTys con) ]
885 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
887 cond_isEnumeration :: Condition
888 cond_isEnumeration (_, rep_tc)
889 | null (tyConDataCons rep_tc) = Just (no_cons_why rep_tc)
890 | isEnumerationTyCon rep_tc = Nothing
891 | otherwise = Just why
893 why = quotes (pprSourceTyCon rep_tc) <+>
894 ptext (sLit "has non-nullary constructors")
896 cond_isProduct :: Condition
897 cond_isProduct (_, rep_tc)
898 | isProductTyCon rep_tc = Nothing
899 | otherwise = Just why
901 why = quotes (pprSourceTyCon rep_tc) <+>
902 ptext (sLit "does not have precisely one constructor")
904 cond_typeableOK :: Condition
905 -- OK for Typeable class
906 -- Currently: (a) args all of kind *
907 -- (b) 7 or fewer args
908 cond_typeableOK (_, rep_tc)
909 | tyConArity rep_tc > 7 = Just too_many
910 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
912 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
913 | otherwise = Nothing
915 too_many = quotes (pprSourceTyCon rep_tc) <+>
916 ptext (sLit "has too many arguments")
917 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
918 ptext (sLit "has arguments of kind other than `*'")
919 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
920 ptext (sLit "is a type family")
923 functorLikeClassKeys :: [Unique]
924 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
926 cond_functorOK :: Bool -> Condition
927 -- OK for Functor/Foldable/Traversable class
928 -- Currently: (a) at least one argument
929 -- (b) don't use argument contravariantly
930 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
931 -- (d) optionally: don't use function types
932 -- (e) no "stupid context" on data type
933 cond_functorOK allowFunctions (dflags, rep_tc)
934 | not (dopt Opt_DeriveFunctor dflags)
935 = Just (ptext (sLit "You need -XDeriveFunctor to derive an instance for this class"))
938 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
939 <+> ptext (sLit "has no parameters"))
941 | not (null bad_stupid_theta)
942 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
943 <+> ptext (sLit "has a class context") <+> pprTheta bad_stupid_theta)
946 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
948 tc_tvs = tyConTyVars rep_tc
949 Just (_, last_tv) = snocView tc_tvs
950 bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
951 is_bad pred = last_tv `elemVarSet` tyVarsOfPred pred
953 data_cons = tyConDataCons rep_tc
954 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
956 check_vanilla :: DataCon -> Maybe SDoc
957 check_vanilla con | isVanillaDataCon con = Nothing
958 | otherwise = Just (badCon con existential)
960 ft_check :: DataCon -> FFoldType (Maybe SDoc)
961 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
962 , ft_co_var = Just (badCon con covariant)
963 , ft_fun = \x y -> if allowFunctions then x `mplus` y
964 else Just (badCon con functions)
965 , ft_tup = \_ xs -> msum xs
966 , ft_ty_app = \_ x -> x
967 , ft_bad_app = Just (badCon con wrong_arg)
968 , ft_forall = \_ x -> x }
970 existential = ptext (sLit "has existential arguments")
971 covariant = ptext (sLit "uses the type variable in a function argument")
972 functions = ptext (sLit "contains function types")
973 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
975 checkFlag :: ExtensionFlag -> Condition
976 checkFlag flag (dflags, _)
977 | dopt flag dflags = Nothing
978 | otherwise = Just why
980 why = ptext (sLit "You need -X") <> text flag_str
981 <+> ptext (sLit "to derive an instance for this class")
982 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
984 other -> pprPanic "checkFlag" (ppr other)
986 std_class_via_iso :: Class -> Bool
987 -- These standard classes can be derived for a newtype
988 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
989 -- because giving so gives the same results as generating the boilerplate
990 std_class_via_iso clas
991 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
992 -- Not Read/Show because they respect the type
993 -- Not Enum, because newtypes are never in Enum
996 non_iso_class :: Class -> Bool
997 -- *Never* derive Read,Show,Typeable,Data by isomorphism,
998 -- even with -XGeneralizedNewtypeDeriving
1000 = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
1003 typeableClassKeys :: [Unique]
1004 typeableClassKeys = map getUnique typeableClassNames
1006 new_dfun_name :: Class -> TyCon -> TcM Name
1007 new_dfun_name clas tycon -- Just a simple wrapper
1008 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1009 ; newDFunName clas [mkTyConApp tycon []] loc }
1010 -- The type passed to newDFunName is only used to generate
1011 -- a suitable string; hence the empty type arg list
1013 badCon :: DataCon -> SDoc -> SDoc
1014 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1017 Note [Superclasses of derived instance]
1018 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1019 In general, a derived instance decl needs the superclasses of the derived
1020 class too. So if we have
1021 data T a = ...deriving( Ord )
1022 then the initial context for Ord (T a) should include Eq (T a). Often this is
1023 redundant; we'll also generate an Ord constraint for each constructor argument,
1024 and that will probably generate enough constraints to make the Eq (T a) constraint
1025 be satisfied too. But not always; consider:
1031 data T a = MkT (S a) deriving( Ord )
1032 instance Num a => Eq (T a)
1034 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1036 data T a = MkT deriving( Data, Typeable )
1037 Here there *is* no argument field, but we must nevertheless generate
1038 a context for the Data instances:
1039 instance Typable a => Data (T a) where ...
1042 %************************************************************************
1046 %************************************************************************
1049 mkNewTypeEqn :: CtOrigin -> DynFlags -> [Var] -> Class
1050 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1052 -> TcRn EarlyDerivSpec
1053 mkNewTypeEqn orig dflags tvs
1054 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1055 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1056 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1057 = do { traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
1058 ; dfun_name <- new_dfun_name cls tycon
1059 ; loc <- getSrcSpanM
1060 ; let spec = DS { ds_loc = loc, ds_orig = orig
1061 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1062 , ds_cls = cls, ds_tys = inst_tys
1063 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1064 , ds_theta = mtheta `orElse` all_preds
1065 , ds_newtype = True }
1066 ; return (if isJust mtheta then Right spec
1070 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1071 CanDerive -> go_for_it -- Use the standard H98 method
1072 DerivableClassError msg -- Error with standard class
1073 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1074 | otherwise -> bale_out msg
1075 NonDerivableClass -- Must use newtype deriving
1076 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1077 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1078 | otherwise -> bale_out non_std
1080 newtype_deriving = dopt Opt_GeneralizedNewtypeDeriving dflags
1081 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1082 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1084 non_std = nonStdErr cls
1085 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1087 -- Here is the plan for newtype derivings. We see
1088 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1089 -- where t is a type,
1090 -- ak+1...an is a suffix of a1..an, and are all tyars
1091 -- ak+1...an do not occur free in t, nor in the s1..sm
1092 -- (C s1 ... sm) is a *partial applications* of class C
1093 -- with the last parameter missing
1094 -- (T a1 .. ak) matches the kind of C's last argument
1095 -- (and hence so does t)
1096 -- The latter kind-check has been done by deriveTyData already,
1097 -- and tc_args are already trimmed
1099 -- We generate the instance
1100 -- instance forall ({a1..ak} u fvs(s1..sm)).
1101 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1102 -- where T a1...ap is the partial application of
1103 -- the LHS of the correct kind and p >= k
1105 -- NB: the variables below are:
1106 -- tc_tvs = [a1, ..., an]
1107 -- tyvars_to_keep = [a1, ..., ak]
1108 -- rep_ty = t ak .. an
1109 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1110 -- tys = [s1, ..., sm]
1113 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1114 -- We generate the instance
1115 -- instance Monad (ST s) => Monad (T s) where
1117 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1118 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1119 -- eta-reduces the representation type, so we know that
1121 -- That's convenient here, because we may have to apply
1122 -- it to fewer than its original complement of arguments
1124 -- Note [Newtype representation]
1125 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1126 -- Need newTyConRhs (*not* a recursive representation finder)
1127 -- to get the representation type. For example
1128 -- newtype B = MkB Int
1129 -- newtype A = MkA B deriving( Num )
1130 -- We want the Num instance of B, *not* the Num instance of Int,
1131 -- when making the Num instance of A!
1132 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1133 rep_tys = cls_tys ++ [rep_inst_ty]
1134 rep_pred = mkClassPred cls rep_tys
1135 -- rep_pred is the representation dictionary, from where
1136 -- we are gong to get all the methods for the newtype
1140 -- Next we figure out what superclass dictionaries to use
1141 -- See Note [Newtype deriving superclasses] above
1143 cls_tyvars = classTyVars cls
1144 dfun_tvs = tyVarsOfTypes inst_tys
1145 inst_ty = mkTyConApp tycon tc_args
1146 inst_tys = cls_tys ++ [inst_ty]
1147 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1150 -- If there are no tyvars, there's no need
1151 -- to abstract over the dictionaries we need
1152 -- Example: newtype T = MkT Int deriving( C )
1153 -- We get the derived instance
1156 -- instance C Int => C T
1157 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1159 -------------------------------------------------------------------
1160 -- Figuring out whether we can only do this newtype-deriving thing
1162 can_derive_via_isomorphism
1163 = not (non_iso_class cls)
1167 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1169 arity_ok = length cls_tys + 1 == classArity cls
1170 -- Well kinded; eg not: newtype T ... deriving( ST )
1171 -- because ST needs *2* type params
1173 -- Check that eta reduction is OK
1174 eta_ok = nt_eta_arity <= length rep_tc_args
1175 -- The newtype can be eta-reduced to match the number
1176 -- of type argument actually supplied
1177 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1178 -- Here the 'b' must be the same in the rep type (S [a] b)
1179 -- And the [a] must not mention 'b'. That's all handled
1182 ats_ok = null (classATs cls)
1183 -- No associated types for the class, because we don't
1184 -- currently generate type 'instance' decls; and cannot do
1185 -- so for 'data' instance decls
1188 = vcat [ ppUnless arity_ok arity_msg
1189 , ppUnless eta_ok eta_msg
1190 , ppUnless ats_ok ats_msg ]
1191 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1192 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1193 ats_msg = ptext (sLit "the class has associated types")
1196 Note [Recursive newtypes]
1197 ~~~~~~~~~~~~~~~~~~~~~~~~~
1198 Newtype deriving works fine, even if the newtype is recursive.
1199 e.g. newtype S1 = S1 [T1 ()]
1200 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1201 Remember, too, that type families are curretly (conservatively) given
1202 a recursive flag, so this also allows newtype deriving to work
1205 We used to exclude recursive types, because we had a rather simple
1206 minded way of generating the instance decl:
1208 instance Eq [A] => Eq A -- Makes typechecker loop!
1209 But now we require a simple context, so it's ok.
1212 %************************************************************************
1214 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1216 %************************************************************************
1218 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1219 terms, which is the final correct RHS for the corresponding original
1223 Each (k,TyVarTy tv) in a solution constrains only a type
1227 The (k,TyVarTy tv) pairs in a solution are canonically
1228 ordered by sorting on type varible, tv, (major key) and then class, k,
1233 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1235 inferInstanceContexts _ [] = return []
1237 inferInstanceContexts oflag infer_specs
1238 = do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
1239 ; iterate_deriv 1 initial_solutions }
1241 ------------------------------------------------------------------
1242 -- The initial solutions for the equations claim that each
1243 -- instance has an empty context; this solution is certainly
1244 -- in canonical form.
1245 initial_solutions :: [ThetaType]
1246 initial_solutions = [ [] | _ <- infer_specs ]
1248 ------------------------------------------------------------------
1249 -- iterate_deriv calculates the next batch of solutions,
1250 -- compares it with the current one; finishes if they are the
1251 -- same, otherwise recurses with the new solutions.
1252 -- It fails if any iteration fails
1253 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1254 iterate_deriv n current_solns
1255 | n > 20 -- Looks as if we are in an infinite loop
1256 -- This can happen if we have -XUndecidableInstances
1257 -- (See TcSimplify.tcSimplifyDeriv.)
1258 = pprPanic "solveDerivEqns: probable loop"
1259 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1261 = do { -- Extend the inst info from the explicit instance decls
1262 -- with the current set of solutions, and simplify each RHS
1263 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1264 current_solns infer_specs
1265 ; new_solns <- checkNoErrs $
1266 extendLocalInstEnv inst_specs $
1267 mapM gen_soln infer_specs
1269 ; if (current_solns == new_solns) then
1270 return [ spec { ds_theta = soln }
1271 | (spec, soln) <- zip infer_specs current_solns ]
1273 iterate_deriv (n+1) new_solns }
1275 ------------------------------------------------------------------
1276 gen_soln :: DerivSpec -> TcM [PredType]
1277 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1278 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1280 addErrCtxt (derivInstCtxt clas inst_tys) $
1281 do { -- Check for a bizarre corner case, when the derived instance decl should
1282 -- have form instance C a b => D (T a) where ...
1283 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1284 -- of problems; in particular, it's hard to compare solutions for
1285 -- equality when finding the fixpoint. Moreover, simplifyDeriv
1286 -- has an assert failure because it finds a TyVar when it expects
1287 -- only TcTyVars. So I just rule it out for now. I'm not
1288 -- even sure how it can arise.
1290 ; let tv_set = mkVarSet tyvars
1291 weird_preds = [pred | pred <- deriv_rhs
1292 , not (tyVarsOfPred pred `subVarSet` tv_set)]
1293 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1295 ; theta <- simplifyDeriv orig tyvars deriv_rhs
1296 -- checkValidInstance tyvars theta clas inst_tys
1297 -- Not necessary; see Note [Exotic derived instance contexts]
1300 ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
1301 -- Claim: the result instance declaration is guaranteed valid
1302 -- Hence no need to call:
1303 -- checkValidInstance tyvars theta clas inst_tys
1304 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1306 ------------------------------------------------------------------
1307 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1308 mkInstance overlap_flag theta
1309 (DS { ds_name = dfun_name
1310 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1311 = mkLocalInstance dfun overlap_flag
1313 dfun = mkDictFunId dfun_name tyvars theta clas tys
1316 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1317 -- Add new locally-defined instances; don't bother to check
1318 -- for functional dependency errors -- that'll happen in TcInstDcls
1319 extendLocalInstEnv dfuns thing_inside
1320 = do { env <- getGblEnv
1321 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1322 env' = env { tcg_inst_env = inst_env' }
1323 ; setGblEnv env' thing_inside }
1327 %************************************************************************
1329 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1331 %************************************************************************
1333 After all the trouble to figure out the required context for the
1334 derived instance declarations, all that's left is to chug along to
1335 produce them. They will then be shoved into @tcInstDecls2@, which
1336 will do all its usual business.
1338 There are lots of possibilities for code to generate. Here are
1339 various general remarks.
1344 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1345 ``you-couldn't-do-better-by-hand'' efficient.
1348 Deriving @Show@---also pretty common--- should also be reasonable good code.
1351 Deriving for the other classes isn't that common or that big a deal.
1358 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1361 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1364 We {\em normally} generate code only for the non-defaulted methods;
1365 there are some exceptions for @Eq@ and (especially) @Ord@...
1368 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1369 constructor's numeric (@Int#@) tag. These are generated by
1370 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1371 these is around is given by @hasCon2TagFun@.
1373 The examples under the different sections below will make this
1377 Much less often (really just for deriving @Ix@), we use a
1378 @_tag2con_<tycon>@ function. See the examples.
1381 We use the renamer!!! Reason: we're supposed to be
1382 producing @LHsBinds Name@ for the methods, but that means
1383 producing correctly-uniquified code on the fly. This is entirely
1384 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1385 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1386 the renamer. What a great hack!
1390 -- Generate the InstInfo for the required instance paired with the
1391 -- *representation* tycon for that instance,
1392 -- plus any auxiliary bindings required
1394 -- Representation tycons differ from the tycon in the instance signature in
1395 -- case of instances for indexed families.
1397 genInst :: Bool -- True <=> standalone deriving
1399 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1400 genInst standalone_deriv oflag spec
1402 = return (InstInfo { iSpec = mkInstance oflag (ds_theta spec) spec
1403 , iBinds = NewTypeDerived co rep_tycon }, [])
1406 = do { let loc = getSrcSpan (ds_name spec)
1407 inst = mkInstance oflag (ds_theta spec) spec
1410 -- In case of a family instance, we need to use the representation
1411 -- tycon (after all, it has the data constructors)
1412 ; fix_env <- getFixityEnv
1413 ; let (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1414 binds = VanillaInst meth_binds [] standalone_deriv
1415 ; return (InstInfo { iSpec = inst, iBinds = binds }, aux_binds)
1418 rep_tycon = ds_tc spec
1419 rep_tc_args = ds_tc_args spec
1420 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1421 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1423 -- Not a family => rep_tycon = main tycon
1424 co2 = case newTyConCo_maybe rep_tycon of
1425 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1426 Nothing -> id_co -- The newtype is transparent; no need for a cast
1427 co = co1 `mkTransCoI` co2
1428 id_co = IdCo (mkTyConApp rep_tycon rep_tc_args)
1430 -- Example: newtype instance N [a] = N1 (Tree a)
1431 -- deriving instance Eq b => Eq (N [(b,b)])
1432 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1433 -- When dealing with the deriving clause
1434 -- co1 : N [(b,b)] ~ R1:N (b,b)
1435 -- co2 : R1:N (b,b) ~ Tree (b,b)
1436 -- co : N [(b,b)] ~ Tree (b,b)
1438 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1439 genDerivBinds loc fix_env clas tycon
1440 | className clas `elem` typeableClassNames
1441 = (gen_Typeable_binds loc tycon, [])
1444 = case assocMaybe gen_list (getUnique clas) of
1445 Just gen_fn -> gen_fn loc tycon
1446 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1448 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1449 gen_list = [(eqClassKey, gen_Eq_binds)
1450 ,(ordClassKey, gen_Ord_binds)
1451 ,(enumClassKey, gen_Enum_binds)
1452 ,(boundedClassKey, gen_Bounded_binds)
1453 ,(ixClassKey, gen_Ix_binds)
1454 ,(showClassKey, gen_Show_binds fix_env)
1455 ,(readClassKey, gen_Read_binds fix_env)
1456 ,(dataClassKey, gen_Data_binds)
1457 ,(functorClassKey, gen_Functor_binds)
1458 ,(foldableClassKey, gen_Foldable_binds)
1459 ,(traversableClassKey, gen_Traversable_binds)
1464 %************************************************************************
1466 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1468 %************************************************************************
1471 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1472 derivingKindErr tc cls cls_tys cls_kind
1473 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1474 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1475 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1476 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1478 derivingEtaErr :: Class -> [Type] -> Type -> Message
1479 derivingEtaErr cls cls_tys inst_ty
1480 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1481 nest 2 (ptext (sLit "instance (...) =>")
1482 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1484 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1485 typeFamilyPapErr tc cls cls_tys inst_ty
1486 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1487 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1489 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1490 derivingThingErr newtype_deriving clas tys ty why
1491 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1492 2 (quotes (ppr pred))
1493 $$ nest 2 extra) <> colon,
1496 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1498 pred = mkClassPred clas (tys ++ [ty])
1500 derivingHiddenErr :: TyCon -> SDoc
1501 derivingHiddenErr tc
1502 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1503 2 (ptext (sLit "so you cannot derive an instance for it"))
1505 standaloneCtxt :: LHsType Name -> SDoc
1506 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1509 derivInstCtxt :: Class -> [Type] -> Message
1510 derivInstCtxt clas inst_tys
1511 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1513 badDerivedPred :: PredType -> Message
1515 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1516 ptext (sLit "type variables that are not data type parameters"),
1517 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]