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 ; bindLocalNames (collectHsValBinders rn_aux_lhs) $
356 do { (rn_aux, dus_aux) <- rnTopBindsRHS rn_aux_lhs
357 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
358 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
359 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
362 (inst_infos, deriv_aux_binds) = unzip insts
364 -- Remove duplicate requests for auxilliary bindings
366 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
367 | otherwise = rm_dups (b:acc) bs
370 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
371 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
372 = return ( info { iBinds = NewTypeDerived coi tc }
373 , mkFVs (map dataConName (tyConDataCons tc)))
374 -- See Note [Newtype deriving and unused constructors]
376 rn_inst_info (InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
377 = -- Bring the right type variables into
378 -- scope (yuk), and rename the method binds
380 bindLocalNames (map Var.varName tyvars) $
381 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
382 ; let binds' = VanillaInst rn_binds [] standalone_deriv
383 ; return (InstInfo { iSpec = inst, iBinds = binds' }, fvs) }
385 (tyvars,_, clas,_) = instanceHead inst
386 clas_nm = className clas
388 -----------------------------------------
389 mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
390 mkGenericBinds is_boot tycl_decls
394 = do { tcs <- mapM tcLookupTyCon [ tcdName d
395 | L _ d <- tycl_decls, isDataDecl d ]
396 ; return (unionManyBags [ mkTyConGenericBinds tc
397 | tc <- tcs, tyConHasGenerics tc ]) }
398 -- We are only interested in the data type declarations,
399 -- and then only in the ones whose 'has-generics' flag is on
400 -- The predicate tyConHasGenerics finds both of these
403 Note [Newtype deriving and unused constructors]
404 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
405 Consider this (see Trac #1954):
408 newtype P a = MkP (IO a) deriving Monad
410 If you compile with -fwarn-unused-binds you do not expect the warning
411 "Defined but not used: data consructor MkP". Yet the newtype deriving
412 code does not explicitly mention MkP, but it should behave as if you
414 instance Monad P where
415 return x = MkP (return x)
418 So we want to signal a user of the data constructor 'MkP'. That's
419 what we do in rn_inst_info, and it's the only reason we have the TyCon
420 stored in NewTypeDerived.
423 %************************************************************************
425 From HsSyn to DerivSpec
427 %************************************************************************
429 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
432 makeDerivSpecs :: Bool
436 -> TcM [EarlyDerivSpec]
438 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
439 | is_boot -- No 'deriving' at all in hs-boot files
440 = do { mapM_ add_deriv_err deriv_locs
443 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
444 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
445 ; return (eqns1 ++ eqns2) }
447 extractTyDataPreds decls
448 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
450 all_tydata :: [(LHsType Name, LTyClDecl Name)]
451 -- Derived predicate paired with its data type declaration
452 all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
454 deriv_locs = map (getLoc . snd) all_tydata
455 ++ map getLoc deriv_decls
457 add_deriv_err loc = setSrcSpan loc $
458 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
459 2 (ptext (sLit "Use an instance declaration instead")))
461 ------------------------------------------------------------------
462 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
463 -- Standalone deriving declarations
464 -- e.g. deriving instance Show a => Show (T a)
465 -- Rather like tcLocalInstDecl
466 deriveStandalone (L loc (DerivDecl deriv_ty))
468 addErrCtxt (standaloneCtxt deriv_ty) $
469 do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
470 ; (tvs, theta, tau) <- tcHsInstHead deriv_ty
471 ; traceTc "Standalone deriving;" $ vcat
472 [ text "tvs:" <+> ppr tvs
473 , text "theta:" <+> ppr theta
474 , text "tau:" <+> ppr tau ]
475 ; (cls, inst_tys) <- checkValidInstance deriv_ty tvs theta tau
476 -- C.f. TcInstDcls.tcLocalInstDecl1
478 ; let cls_tys = take (length inst_tys - 1) inst_tys
479 inst_ty = last inst_tys
480 ; traceTc "Standalone deriving:" $ vcat
481 [ text "class:" <+> ppr cls
482 , text "class types:" <+> ppr cls_tys
483 , text "type:" <+> ppr inst_ty ]
484 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
487 ------------------------------------------------------------------
488 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
489 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
490 tcdTyVars = tv_names,
491 tcdTyPats = ty_pats }))
492 = setSrcSpan loc $ -- Use the location of the 'deriving' item
494 do { (tvs, tc, tc_args) <- get_lhs ty_pats
495 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
496 -- the type variables for the type constructor
498 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
499 -- The "deriv_pred" is a LHsType to take account of the fact that for
500 -- newtype deriving we allow deriving (forall a. C [a]).
502 -- Given data T a b c = ... deriving( C d ),
503 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
504 ; let cls_tyvars = classTyVars cls
505 kind = tyVarKind (last cls_tyvars)
506 (arg_kinds, _) = splitKindFunTys kind
507 n_args_to_drop = length arg_kinds
508 n_args_to_keep = tyConArity tc - n_args_to_drop
509 args_to_drop = drop n_args_to_keep tc_args
510 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
511 inst_ty_kind = typeKind inst_ty
512 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
513 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
514 `minusVarSet` dropped_tvs
516 -- Check that the result really is well-kinded
517 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
518 (derivingKindErr tc cls cls_tys kind)
520 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
521 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
522 (derivingEtaErr cls cls_tys inst_ty)
524 -- (a) The data type can be eta-reduced; eg reject:
525 -- data instance T a a = ... deriving( Monad )
526 -- (b) The type class args do not mention any of the dropped type
528 -- newtype T a s = ... deriving( ST s )
530 -- Type families can't be partially applied
531 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
532 -- Note [Deriving, type families, and partial applications]
533 ; checkTc (not (isFamilyTyCon tc) || n_args_to_drop == 0)
534 (typeFamilyPapErr tc cls cls_tys inst_ty)
536 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
538 -- Tiresomely we must figure out the "lhs", which is awkward for type families
539 -- E.g. data T a b = .. deriving( Eq )
540 -- Here, the lhs is (T a b)
541 -- data instance TF Int b = ... deriving( Eq )
542 -- Here, the lhs is (TF Int b)
543 -- But if we just look up the tycon_name, we get is the *family*
544 -- tycon, but not pattern types -- they are in the *rep* tycon.
545 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
546 ; let tvs = tyConTyVars tc
547 ; return (tvs, tc, mkTyVarTys tvs) }
548 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
549 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
550 ; let (tc, tc_args) = tcSplitTyConApp tc_app
551 ; return (tvs, tc, tc_args) }
554 = panic "derivTyData" -- Caller ensures that only TyData can happen
557 Note [Deriving, type families, and partial applications]
558 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
559 When there are no type families, it's quite easy:
561 newtype S a = MkS [a]
562 -- :CoS :: S ~ [] -- Eta-reduced
564 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
565 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
567 When type familes are involved it's trickier:
570 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
571 -- :RT is the representation type for (T Int a)
572 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
573 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
575 instance Eq [a] => Eq (T Int a) -- easy by coercion
576 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
578 The "???" bit is that we don't build the :CoF thing in eta-reduced form
579 Henc the current typeFamilyPapErr, even though the instance makes sense.
580 After all, we can write it out
581 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
586 mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type] -> Type
587 -> DerivContext -- Just => context supplied (standalone deriving)
588 -- Nothing => context inferred (deriving on data decl)
589 -> TcRn EarlyDerivSpec
590 -- Make the EarlyDerivSpec for an instance
591 -- forall tvs. theta => cls (tys ++ [ty])
592 -- where the 'theta' is optional (that's the Maybe part)
593 -- Assumes that this declaration is well-kinded
595 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
596 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
597 , isAlgTyCon tycon -- Check for functions, primitive types etc
598 = do { (rep_tc, rep_tc_args) <- tcLookupDataFamInst tycon tc_args
599 -- Be careful to test rep_tc here: in the case of families,
600 -- we want to check the instance tycon, not the family tycon
602 -- For standalone deriving (mtheta /= Nothing),
603 -- check that all the data constructors are in scope.
604 -- No need for this when deriving Typeable, becuase we don't need
605 -- the constructors for that.
606 ; rdr_env <- getGlobalRdrEnv
607 ; let hidden_data_cons = isAbstractTyCon rep_tc || any not_in_scope (tyConDataCons rep_tc)
608 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
609 ; checkTc (isNothing mtheta ||
610 not hidden_data_cons ||
611 className cls `elem` typeableClassNames)
612 (derivingHiddenErr tycon)
615 ; if isDataTyCon rep_tc then
616 mkDataTypeEqn orig dflags tvs cls cls_tys
617 tycon tc_args rep_tc rep_tc_args mtheta
619 mkNewTypeEqn orig dflags tvs cls cls_tys
620 tycon tc_args rep_tc rep_tc_args mtheta }
622 = failWithTc (derivingThingErr False cls cls_tys tc_app
623 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
627 %************************************************************************
631 %************************************************************************
634 mkDataTypeEqn :: CtOrigin
636 -> [Var] -- Universally quantified type variables in the instance
637 -> Class -- Class for which we need to derive an instance
638 -> [Type] -- Other parameters to the class except the last
639 -> TyCon -- Type constructor for which the instance is requested
640 -- (last parameter to the type class)
641 -> [Type] -- Parameters to the type constructor
642 -> TyCon -- rep of the above (for type families)
643 -> [Type] -- rep of the above
644 -> DerivContext -- Context of the instance, for standalone deriving
645 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
647 mkDataTypeEqn orig dflags tvs cls cls_tys
648 tycon tc_args rep_tc rep_tc_args mtheta
649 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
650 -- NB: pass the *representation* tycon to checkSideConditions
651 CanDerive -> go_for_it
652 NonDerivableClass -> bale_out (nonStdErr cls)
653 DerivableClassError msg -> bale_out msg
655 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
656 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
658 mk_data_eqn, mk_typeable_eqn
659 :: CtOrigin -> [TyVar] -> Class
660 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
661 -> TcM EarlyDerivSpec
662 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
663 | getName cls `elem` typeableClassNames
664 = mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
667 = do { dfun_name <- new_dfun_name cls tycon
669 ; let inst_tys = [mkTyConApp tycon tc_args]
670 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
671 spec = DS { ds_loc = loc, ds_orig = orig
672 , ds_name = dfun_name, ds_tvs = tvs
673 , ds_cls = cls, ds_tys = inst_tys
674 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
675 , ds_theta = mtheta `orElse` inferred_constraints
676 , ds_newtype = False }
678 ; return (if isJust mtheta then Right spec -- Specified context
679 else Left spec) } -- Infer context
681 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
682 -- The Typeable class is special in several ways
683 -- data T a b = ... deriving( Typeable )
685 -- instance Typeable2 T where ...
687 -- 1. There are no constraints in the instance
688 -- 2. There are no type variables either
689 -- 3. The actual class we want to generate isn't necessarily
690 -- Typeable; it depends on the arity of the type
691 | isNothing mtheta -- deriving on a data type decl
692 = do { checkTc (cls `hasKey` typeableClassKey)
693 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
694 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
695 ; mk_typeable_eqn orig tvs real_cls tycon [] rep_tc [] (Just []) }
697 | otherwise -- standaone deriving
698 = do { checkTc (null tc_args)
699 (ptext (sLit "Derived typeable instance must be of form (Typeable")
700 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
701 ; dfun_name <- new_dfun_name cls tycon
704 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
705 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
706 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
707 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
710 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
711 -- Generate a sufficiently large set of constraints that typechecking the
712 -- generated method definitions should succeed. This set will be simplified
713 -- before being used in the instance declaration
714 inferConstraints _ cls inst_tys rep_tc rep_tc_args
715 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
716 stupid_constraints ++ extra_constraints
717 ++ sc_constraints ++ con_arg_constraints
719 -- Constraints arising from the arguments of each constructor
721 = [ mkClassPred cls [arg_ty]
722 | data_con <- tyConDataCons rep_tc,
723 arg_ty <- ASSERT( isVanillaDataCon data_con )
724 get_constrained_tys $
725 dataConInstOrigArgTys data_con all_rep_tc_args,
726 not (isUnLiftedType arg_ty) ]
727 -- No constraints for unlifted types
728 -- Where they are legal we generate specilised function calls
730 -- For functor-like classes, two things are different
731 -- (a) We recurse over argument types to generate constraints
732 -- See Functor examples in TcGenDeriv
733 -- (b) The rep_tc_args will be one short
734 is_functor_like = getUnique cls `elem` functorLikeClassKeys
736 get_constrained_tys :: [Type] -> [Type]
737 get_constrained_tys tys
738 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
741 rep_tc_tvs = tyConTyVars rep_tc
742 last_tv = last rep_tc_tvs
743 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
744 | otherwise = rep_tc_args
746 -- Constraints arising from superclasses
747 -- See Note [Superclasses of derived instance]
748 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
751 -- Stupid constraints
752 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
753 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
755 -- Extra Data constraints
756 -- The Data class (only) requires that for
757 -- instance (...) => Data (T t1 t2)
759 -- THEN (Data t1, Data t2) are among the (...) constraints
760 -- Reason: when the IF holds, we generate a method
761 -- dataCast2 f = gcast2 f
762 -- and we need the Data constraints to typecheck the method
764 | cls `hasKey` dataClassKey
765 , all (isLiftedTypeKind . typeKind) rep_tc_args
766 = [mkClassPred cls [ty] | ty <- rep_tc_args]
770 ------------------------------------------------------------------
771 -- Check side conditions that dis-allow derivability for particular classes
772 -- This is *apart* from the newtype-deriving mechanism
774 -- Here we get the representation tycon in case of family instances as it has
775 -- the data constructors - but we need to be careful to fall back to the
776 -- family tycon (with indexes) in error messages.
778 data DerivStatus = CanDerive
779 | DerivableClassError SDoc -- Standard class, but can't do it
780 | NonDerivableClass -- Non-standard class
782 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
783 checkSideConditions dflags mtheta cls cls_tys rep_tc
784 | Just cond <- sideConditions mtheta cls
785 = case (cond (dflags, rep_tc)) of
786 Just err -> DerivableClassError err -- Class-specific error
787 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
788 -- cls_tys (the type args other than last)
790 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
791 | otherwise = NonDerivableClass -- Not a standard class
793 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
795 nonStdErr :: Class -> SDoc
796 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
798 sideConditions :: DerivContext -> Class -> Maybe Condition
799 sideConditions mtheta cls
800 | cls_key == eqClassKey = Just cond_std
801 | cls_key == ordClassKey = Just cond_std
802 | cls_key == showClassKey = Just cond_std
803 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
804 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
805 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
806 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
807 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
808 cond_std `andCond` cond_noUnliftedArgs)
809 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
810 cond_functorOK True) -- NB: no cond_std!
811 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
812 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
813 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
814 cond_functorOK False)
815 | getName cls `elem` typeableClassNames = Just (checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK)
816 | otherwise = Nothing
818 cls_key = getUnique cls
819 cond_std = cond_stdOK mtheta
821 type Condition = (DynFlags, TyCon) -> Maybe SDoc
822 -- first Bool is whether or not we are allowed to derive Data and Typeable
823 -- second Bool is whether or not we are allowed to derive Functor
824 -- TyCon is the *representation* tycon if the
825 -- data type is an indexed one
828 orCond :: Condition -> Condition -> Condition
831 Nothing -> Nothing -- c1 succeeds
832 Just x -> case c2 tc of -- c1 fails
834 Just y -> Just (x $$ ptext (sLit " and") $$ y)
837 andCond :: Condition -> Condition -> Condition
838 andCond c1 c2 tc = case c1 tc of
839 Nothing -> c2 tc -- c1 succeeds
840 Just x -> Just x -- c1 fails
842 cond_stdOK :: DerivContext -> Condition
843 cond_stdOK (Just _) _
844 = Nothing -- Don't check these conservative conditions for
845 -- standalone deriving; just generate the code
846 -- and let the typechecker handle the result
847 cond_stdOK Nothing (_, rep_tc)
848 | null data_cons = Just (no_cons_why rep_tc $$ suggestion)
849 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
850 | otherwise = Nothing
852 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
853 data_cons = tyConDataCons rep_tc
854 con_whys = mapCatMaybes check_con data_cons
856 check_con :: DataCon -> Maybe SDoc
858 | isVanillaDataCon con
859 , all isTauTy (dataConOrigArgTys con) = Nothing
860 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
862 no_cons_why :: TyCon -> SDoc
863 no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
864 ptext (sLit "has no data constructors")
866 cond_enumOrProduct :: Condition
867 cond_enumOrProduct = cond_isEnumeration `orCond`
868 (cond_isProduct `andCond` cond_noUnliftedArgs)
870 cond_noUnliftedArgs :: Condition
871 -- For some classes (eg Eq, Ord) we allow unlifted arg types
872 -- by generating specilaised code. For others (eg Data) we don't.
873 cond_noUnliftedArgs (_, tc)
874 | null bad_cons = Nothing
875 | otherwise = Just why
877 bad_cons = [ con | con <- tyConDataCons tc
878 , any isUnLiftedType (dataConOrigArgTys con) ]
879 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
881 cond_isEnumeration :: Condition
882 cond_isEnumeration (_, rep_tc)
883 | isEnumerationTyCon rep_tc = Nothing
884 | otherwise = Just why
886 why = sep [ quotes (pprSourceTyCon rep_tc) <+>
887 ptext (sLit "is not an enumeration type")
888 , nest 2 $ ptext (sLit "(an enumeration consists of one or more nullary constructors)") ]
889 -- See Note [Enumeration types] in TyCon
891 cond_isProduct :: Condition
892 cond_isProduct (_, rep_tc)
893 | isProductTyCon rep_tc = Nothing
894 | otherwise = Just why
896 why = quotes (pprSourceTyCon rep_tc) <+>
897 ptext (sLit "does not have precisely one constructor")
899 cond_typeableOK :: Condition
900 -- OK for Typeable class
901 -- Currently: (a) args all of kind *
902 -- (b) 7 or fewer args
903 cond_typeableOK (_, rep_tc)
904 | tyConArity rep_tc > 7 = Just too_many
905 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
907 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
908 | otherwise = Nothing
910 too_many = quotes (pprSourceTyCon rep_tc) <+>
911 ptext (sLit "has too many arguments")
912 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
913 ptext (sLit "has arguments of kind other than `*'")
914 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
915 ptext (sLit "is a type family")
918 functorLikeClassKeys :: [Unique]
919 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
921 cond_functorOK :: Bool -> Condition
922 -- OK for Functor/Foldable/Traversable class
923 -- Currently: (a) at least one argument
924 -- (b) don't use argument contravariantly
925 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
926 -- (d) optionally: don't use function types
927 -- (e) no "stupid context" on data type
928 cond_functorOK allowFunctions (dflags, rep_tc)
929 | not (xopt Opt_DeriveFunctor dflags)
930 = Just (ptext (sLit "You need -XDeriveFunctor to derive an instance for this class"))
933 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
934 <+> ptext (sLit "has no parameters"))
936 | not (null bad_stupid_theta)
937 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
938 <+> ptext (sLit "has a class context") <+> pprTheta bad_stupid_theta)
941 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
943 tc_tvs = tyConTyVars rep_tc
944 Just (_, last_tv) = snocView tc_tvs
945 bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
946 is_bad pred = last_tv `elemVarSet` tyVarsOfPred pred
948 data_cons = tyConDataCons rep_tc
949 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
951 check_vanilla :: DataCon -> Maybe SDoc
952 check_vanilla con | isVanillaDataCon con = Nothing
953 | otherwise = Just (badCon con existential)
955 ft_check :: DataCon -> FFoldType (Maybe SDoc)
956 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
957 , ft_co_var = Just (badCon con covariant)
958 , ft_fun = \x y -> if allowFunctions then x `mplus` y
959 else Just (badCon con functions)
960 , ft_tup = \_ xs -> msum xs
961 , ft_ty_app = \_ x -> x
962 , ft_bad_app = Just (badCon con wrong_arg)
963 , ft_forall = \_ x -> x }
965 existential = ptext (sLit "has existential arguments")
966 covariant = ptext (sLit "uses the type variable in a function argument")
967 functions = ptext (sLit "contains function types")
968 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
970 checkFlag :: ExtensionFlag -> Condition
971 checkFlag flag (dflags, _)
972 | xopt flag dflags = Nothing
973 | otherwise = Just why
975 why = ptext (sLit "You need -X") <> text flag_str
976 <+> ptext (sLit "to derive an instance for this class")
977 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
979 other -> pprPanic "checkFlag" (ppr other)
981 std_class_via_iso :: Class -> Bool
982 -- These standard classes can be derived for a newtype
983 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
984 -- because giving so gives the same results as generating the boilerplate
985 std_class_via_iso clas
986 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
987 -- Not Read/Show because they respect the type
988 -- Not Enum, because newtypes are never in Enum
991 non_iso_class :: Class -> Bool
992 -- *Never* derive Read,Show,Typeable,Data by isomorphism,
993 -- even with -XGeneralizedNewtypeDeriving
995 = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
998 typeableClassKeys :: [Unique]
999 typeableClassKeys = map getUnique typeableClassNames
1001 new_dfun_name :: Class -> TyCon -> TcM Name
1002 new_dfun_name clas tycon -- Just a simple wrapper
1003 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1004 ; newDFunName clas [mkTyConApp tycon []] loc }
1005 -- The type passed to newDFunName is only used to generate
1006 -- a suitable string; hence the empty type arg list
1008 badCon :: DataCon -> SDoc -> SDoc
1009 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1012 Note [Superclasses of derived instance]
1013 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1014 In general, a derived instance decl needs the superclasses of the derived
1015 class too. So if we have
1016 data T a = ...deriving( Ord )
1017 then the initial context for Ord (T a) should include Eq (T a). Often this is
1018 redundant; we'll also generate an Ord constraint for each constructor argument,
1019 and that will probably generate enough constraints to make the Eq (T a) constraint
1020 be satisfied too. But not always; consider:
1026 data T a = MkT (S a) deriving( Ord )
1027 instance Num a => Eq (T a)
1029 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1031 data T a = MkT deriving( Data, Typeable )
1032 Here there *is* no argument field, but we must nevertheless generate
1033 a context for the Data instances:
1034 instance Typable a => Data (T a) where ...
1037 %************************************************************************
1041 %************************************************************************
1044 mkNewTypeEqn :: CtOrigin -> DynFlags -> [Var] -> Class
1045 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1047 -> TcRn EarlyDerivSpec
1048 mkNewTypeEqn orig dflags tvs
1049 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1050 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1051 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1052 = do { traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
1053 ; dfun_name <- new_dfun_name cls tycon
1054 ; loc <- getSrcSpanM
1055 ; let spec = DS { ds_loc = loc, ds_orig = orig
1056 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1057 , ds_cls = cls, ds_tys = inst_tys
1058 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1059 , ds_theta = mtheta `orElse` all_preds
1060 , ds_newtype = True }
1061 ; return (if isJust mtheta then Right spec
1065 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1066 CanDerive -> go_for_it -- Use the standard H98 method
1067 DerivableClassError msg -- Error with standard class
1068 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1069 | otherwise -> bale_out msg
1070 NonDerivableClass -- Must use newtype deriving
1071 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1072 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1073 | otherwise -> bale_out non_std
1075 newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
1076 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1077 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1079 non_std = nonStdErr cls
1080 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1082 -- Here is the plan for newtype derivings. We see
1083 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1084 -- where t is a type,
1085 -- ak+1...an is a suffix of a1..an, and are all tyars
1086 -- ak+1...an do not occur free in t, nor in the s1..sm
1087 -- (C s1 ... sm) is a *partial applications* of class C
1088 -- with the last parameter missing
1089 -- (T a1 .. ak) matches the kind of C's last argument
1090 -- (and hence so does t)
1091 -- The latter kind-check has been done by deriveTyData already,
1092 -- and tc_args are already trimmed
1094 -- We generate the instance
1095 -- instance forall ({a1..ak} u fvs(s1..sm)).
1096 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1097 -- where T a1...ap is the partial application of
1098 -- the LHS of the correct kind and p >= k
1100 -- NB: the variables below are:
1101 -- tc_tvs = [a1, ..., an]
1102 -- tyvars_to_keep = [a1, ..., ak]
1103 -- rep_ty = t ak .. an
1104 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1105 -- tys = [s1, ..., sm]
1108 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1109 -- We generate the instance
1110 -- instance Monad (ST s) => Monad (T s) where
1112 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1113 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1114 -- eta-reduces the representation type, so we know that
1116 -- That's convenient here, because we may have to apply
1117 -- it to fewer than its original complement of arguments
1119 -- Note [Newtype representation]
1120 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1121 -- Need newTyConRhs (*not* a recursive representation finder)
1122 -- to get the representation type. For example
1123 -- newtype B = MkB Int
1124 -- newtype A = MkA B deriving( Num )
1125 -- We want the Num instance of B, *not* the Num instance of Int,
1126 -- when making the Num instance of A!
1127 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1128 rep_tys = cls_tys ++ [rep_inst_ty]
1129 rep_pred = mkClassPred cls rep_tys
1130 -- rep_pred is the representation dictionary, from where
1131 -- we are gong to get all the methods for the newtype
1135 -- Next we figure out what superclass dictionaries to use
1136 -- See Note [Newtype deriving superclasses] above
1138 cls_tyvars = classTyVars cls
1139 dfun_tvs = tyVarsOfTypes inst_tys
1140 inst_ty = mkTyConApp tycon tc_args
1141 inst_tys = cls_tys ++ [inst_ty]
1142 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1145 -- If there are no tyvars, there's no need
1146 -- to abstract over the dictionaries we need
1147 -- Example: newtype T = MkT Int deriving( C )
1148 -- We get the derived instance
1151 -- instance C Int => C T
1152 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1154 -------------------------------------------------------------------
1155 -- Figuring out whether we can only do this newtype-deriving thing
1157 can_derive_via_isomorphism
1158 = not (non_iso_class cls)
1162 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1164 arity_ok = length cls_tys + 1 == classArity cls
1165 -- Well kinded; eg not: newtype T ... deriving( ST )
1166 -- because ST needs *2* type params
1168 -- Check that eta reduction is OK
1169 eta_ok = nt_eta_arity <= length rep_tc_args
1170 -- The newtype can be eta-reduced to match the number
1171 -- of type argument actually supplied
1172 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1173 -- Here the 'b' must be the same in the rep type (S [a] b)
1174 -- And the [a] must not mention 'b'. That's all handled
1177 ats_ok = null (classATs cls)
1178 -- No associated types for the class, because we don't
1179 -- currently generate type 'instance' decls; and cannot do
1180 -- so for 'data' instance decls
1183 = vcat [ ppUnless arity_ok arity_msg
1184 , ppUnless eta_ok eta_msg
1185 , ppUnless ats_ok ats_msg ]
1186 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1187 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1188 ats_msg = ptext (sLit "the class has associated types")
1191 Note [Recursive newtypes]
1192 ~~~~~~~~~~~~~~~~~~~~~~~~~
1193 Newtype deriving works fine, even if the newtype is recursive.
1194 e.g. newtype S1 = S1 [T1 ()]
1195 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1196 Remember, too, that type families are curretly (conservatively) given
1197 a recursive flag, so this also allows newtype deriving to work
1200 We used to exclude recursive types, because we had a rather simple
1201 minded way of generating the instance decl:
1203 instance Eq [A] => Eq A -- Makes typechecker loop!
1204 But now we require a simple context, so it's ok.
1207 %************************************************************************
1209 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1211 %************************************************************************
1213 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1214 terms, which is the final correct RHS for the corresponding original
1218 Each (k,TyVarTy tv) in a solution constrains only a type
1222 The (k,TyVarTy tv) pairs in a solution are canonically
1223 ordered by sorting on type varible, tv, (major key) and then class, k,
1228 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1230 inferInstanceContexts _ [] = return []
1232 inferInstanceContexts oflag infer_specs
1233 = do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
1234 ; iterate_deriv 1 initial_solutions }
1236 ------------------------------------------------------------------
1237 -- The initial solutions for the equations claim that each
1238 -- instance has an empty context; this solution is certainly
1239 -- in canonical form.
1240 initial_solutions :: [ThetaType]
1241 initial_solutions = [ [] | _ <- infer_specs ]
1243 ------------------------------------------------------------------
1244 -- iterate_deriv calculates the next batch of solutions,
1245 -- compares it with the current one; finishes if they are the
1246 -- same, otherwise recurses with the new solutions.
1247 -- It fails if any iteration fails
1248 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1249 iterate_deriv n current_solns
1250 | n > 20 -- Looks as if we are in an infinite loop
1251 -- This can happen if we have -XUndecidableInstances
1252 -- (See TcSimplify.tcSimplifyDeriv.)
1253 = pprPanic "solveDerivEqns: probable loop"
1254 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1256 = do { -- Extend the inst info from the explicit instance decls
1257 -- with the current set of solutions, and simplify each RHS
1258 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1259 current_solns infer_specs
1260 ; new_solns <- checkNoErrs $
1261 extendLocalInstEnv inst_specs $
1262 mapM gen_soln infer_specs
1264 ; if (current_solns == new_solns) then
1265 return [ spec { ds_theta = soln }
1266 | (spec, soln) <- zip infer_specs current_solns ]
1268 iterate_deriv (n+1) new_solns }
1270 ------------------------------------------------------------------
1271 gen_soln :: DerivSpec -> TcM [PredType]
1272 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1273 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1275 addErrCtxt (derivInstCtxt clas inst_tys) $
1276 do { -- Check for a bizarre corner case, when the derived instance decl should
1277 -- have form instance C a b => D (T a) where ...
1278 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1279 -- of problems; in particular, it's hard to compare solutions for
1280 -- equality when finding the fixpoint. Moreover, simplifyDeriv
1281 -- has an assert failure because it finds a TyVar when it expects
1282 -- only TcTyVars. So I just rule it out for now. I'm not
1283 -- even sure how it can arise.
1285 ; let tv_set = mkVarSet tyvars
1286 weird_preds = [pred | pred <- deriv_rhs
1287 , not (tyVarsOfPred pred `subVarSet` tv_set)]
1288 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1290 ; theta <- simplifyDeriv orig tyvars deriv_rhs
1291 -- checkValidInstance tyvars theta clas inst_tys
1292 -- Not necessary; see Note [Exotic derived instance contexts]
1295 ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
1296 -- Claim: the result instance declaration is guaranteed valid
1297 -- Hence no need to call:
1298 -- checkValidInstance tyvars theta clas inst_tys
1299 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1301 ------------------------------------------------------------------
1302 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1303 mkInstance overlap_flag theta
1304 (DS { ds_name = dfun_name
1305 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1306 = mkLocalInstance dfun overlap_flag
1308 dfun = mkDictFunId dfun_name tyvars theta clas tys
1311 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1312 -- Add new locally-defined instances; don't bother to check
1313 -- for functional dependency errors -- that'll happen in TcInstDcls
1314 extendLocalInstEnv dfuns thing_inside
1315 = do { env <- getGblEnv
1316 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1317 env' = env { tcg_inst_env = inst_env' }
1318 ; setGblEnv env' thing_inside }
1322 %************************************************************************
1324 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1326 %************************************************************************
1328 After all the trouble to figure out the required context for the
1329 derived instance declarations, all that's left is to chug along to
1330 produce them. They will then be shoved into @tcInstDecls2@, which
1331 will do all its usual business.
1333 There are lots of possibilities for code to generate. Here are
1334 various general remarks.
1339 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1340 ``you-couldn't-do-better-by-hand'' efficient.
1343 Deriving @Show@---also pretty common--- should also be reasonable good code.
1346 Deriving for the other classes isn't that common or that big a deal.
1353 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1356 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1359 We {\em normally} generate code only for the non-defaulted methods;
1360 there are some exceptions for @Eq@ and (especially) @Ord@...
1363 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1364 constructor's numeric (@Int#@) tag. These are generated by
1365 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1366 these is around is given by @hasCon2TagFun@.
1368 The examples under the different sections below will make this
1372 Much less often (really just for deriving @Ix@), we use a
1373 @_tag2con_<tycon>@ function. See the examples.
1376 We use the renamer!!! Reason: we're supposed to be
1377 producing @LHsBinds Name@ for the methods, but that means
1378 producing correctly-uniquified code on the fly. This is entirely
1379 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1380 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1381 the renamer. What a great hack!
1385 -- Generate the InstInfo for the required instance paired with the
1386 -- *representation* tycon for that instance,
1387 -- plus any auxiliary bindings required
1389 -- Representation tycons differ from the tycon in the instance signature in
1390 -- case of instances for indexed families.
1392 genInst :: Bool -- True <=> standalone deriving
1394 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1395 genInst standalone_deriv oflag spec
1397 = return (InstInfo { iSpec = mkInstance oflag (ds_theta spec) spec
1398 , iBinds = NewTypeDerived co rep_tycon }, [])
1401 = do { let loc = getSrcSpan (ds_name spec)
1402 inst = mkInstance oflag (ds_theta spec) spec
1405 -- In case of a family instance, we need to use the representation
1406 -- tycon (after all, it has the data constructors)
1407 ; fix_env <- getFixityEnv
1408 ; let (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1409 binds = VanillaInst meth_binds [] standalone_deriv
1410 ; return (InstInfo { iSpec = inst, iBinds = binds }, aux_binds)
1413 rep_tycon = ds_tc spec
1414 rep_tc_args = ds_tc_args spec
1415 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1416 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1418 -- Not a family => rep_tycon = main tycon
1419 co2 = case newTyConCo_maybe rep_tycon of
1420 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1421 Nothing -> id_co -- The newtype is transparent; no need for a cast
1422 co = co1 `mkTransCoI` co2
1423 id_co = IdCo (mkTyConApp rep_tycon rep_tc_args)
1425 -- Example: newtype instance N [a] = N1 (Tree a)
1426 -- deriving instance Eq b => Eq (N [(b,b)])
1427 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1428 -- When dealing with the deriving clause
1429 -- co1 : N [(b,b)] ~ R1:N (b,b)
1430 -- co2 : R1:N (b,b) ~ Tree (b,b)
1431 -- co : N [(b,b)] ~ Tree (b,b)
1433 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1434 genDerivBinds loc fix_env clas tycon
1435 | className clas `elem` typeableClassNames
1436 = (gen_Typeable_binds loc tycon, [])
1439 = case assocMaybe gen_list (getUnique clas) of
1440 Just gen_fn -> gen_fn loc tycon
1441 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1443 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1444 gen_list = [(eqClassKey, gen_Eq_binds)
1445 ,(ordClassKey, gen_Ord_binds)
1446 ,(enumClassKey, gen_Enum_binds)
1447 ,(boundedClassKey, gen_Bounded_binds)
1448 ,(ixClassKey, gen_Ix_binds)
1449 ,(showClassKey, gen_Show_binds fix_env)
1450 ,(readClassKey, gen_Read_binds fix_env)
1451 ,(dataClassKey, gen_Data_binds)
1452 ,(functorClassKey, gen_Functor_binds)
1453 ,(foldableClassKey, gen_Foldable_binds)
1454 ,(traversableClassKey, gen_Traversable_binds)
1459 %************************************************************************
1461 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1463 %************************************************************************
1466 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1467 derivingKindErr tc cls cls_tys cls_kind
1468 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1469 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1470 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1471 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1473 derivingEtaErr :: Class -> [Type] -> Type -> Message
1474 derivingEtaErr cls cls_tys inst_ty
1475 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1476 nest 2 (ptext (sLit "instance (...) =>")
1477 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1479 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1480 typeFamilyPapErr tc cls cls_tys inst_ty
1481 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1482 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1484 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1485 derivingThingErr newtype_deriving clas tys ty why
1486 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1487 2 (quotes (ppr pred))
1488 $$ nest 2 extra) <> colon,
1491 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1493 pred = mkClassPred clas (tys ++ [ty])
1495 derivingHiddenErr :: TyCon -> SDoc
1496 derivingHiddenErr tc
1497 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1498 2 (ptext (sLit "so you cannot derive an instance for it"))
1500 standaloneCtxt :: LHsType Name -> SDoc
1501 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1504 derivInstCtxt :: Class -> [Type] -> Message
1505 derivInstCtxt clas inst_tys
1506 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1508 badDerivedPred :: PredType -> Message
1510 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1511 ptext (sLit "type variables that are not data type parameters"),
1512 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]