2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Handles @deriving@ clauses on @data@ declarations.
9 module TcDeriv ( tcDeriving ) where
11 #include "HsVersions.h"
19 import TcClassDcl( tcAddDeclCtxt ) -- Small helper
20 import TcGenDeriv -- Deriv stuff
59 %************************************************************************
63 %************************************************************************
67 1. Convert the decls (i.e. data/newtype deriving clauses,
68 plus standalone deriving) to [EarlyDerivSpec]
70 2. Infer the missing contexts for the Left DerivSpecs
72 3. Add the derived bindings, generating InstInfos
76 -- DerivSpec is purely local to this module
77 data DerivSpec = DS { ds_loc :: SrcSpan
81 , ds_theta :: ThetaType
85 , ds_tc_args :: [Type]
86 , ds_newtype :: Bool }
87 -- This spec implies a dfun declaration of the form
88 -- df :: forall tvs. theta => C tys
89 -- The Name is the name for the DFun we'll build
90 -- The tyvars bind all the variables in the theta
91 -- For type families, the tycon in
92 -- in ds_tys is the *family* tycon
93 -- in ds_tc, ds_tc_args is the *representation* tycon
94 -- For non-family tycons, both are the same
96 -- ds_newtype = True <=> Newtype deriving
97 -- False <=> Vanilla deriving
102 newtype instance T [a] = MkT (Tree a) deriving( C s )
104 axiom T [a] = :RTList a
105 axiom :RTList a = Tree a
107 DS { ds_tvs = [a,s], ds_cls = C, ds_tys = [s, T [a]]
108 , ds_tc = :RTList, ds_tc_args = [a]
109 , ds_newtype = True }
112 type DerivContext = Maybe ThetaType
113 -- Nothing <=> Vanilla deriving; infer the context of the instance decl
114 -- Just theta <=> Standalone deriving: context supplied by programmer
116 type EarlyDerivSpec = Either DerivSpec DerivSpec
117 -- Left ds => the context for the instance should be inferred
118 -- In this case ds_theta is the list of all the
119 -- constraints needed, such as (Eq [a], Eq a)
120 -- The inference process is to reduce this to a
121 -- simpler form (e.g. Eq a)
123 -- Right ds => the exact context for the instance is supplied
124 -- by the programmer; it is ds_theta
126 pprDerivSpec :: DerivSpec -> SDoc
127 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
128 ds_cls = c, ds_tys = tys, ds_theta = rhs })
129 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
130 <+> equals <+> ppr rhs)
134 Inferring missing contexts
135 ~~~~~~~~~~~~~~~~~~~~~~~~~~
138 data T a b = C1 (Foo a) (Bar b)
143 [NOTE: See end of these comments for what to do with
144 data (C a, D b) => T a b = ...
147 We want to come up with an instance declaration of the form
149 instance (Ping a, Pong b, ...) => Eq (T a b) where
152 It is pretty easy, albeit tedious, to fill in the code "...". The
153 trick is to figure out what the context for the instance decl is,
154 namely @Ping@, @Pong@ and friends.
156 Let's call the context reqd for the T instance of class C at types
157 (a,b, ...) C (T a b). Thus:
159 Eq (T a b) = (Ping a, Pong b, ...)
161 Now we can get a (recursive) equation from the @data@ decl:
163 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
164 u Eq (T b a) u Eq Int -- From C2
165 u Eq (T a a) -- From C3
167 Foo and Bar may have explicit instances for @Eq@, in which case we can
168 just substitute for them. Alternatively, either or both may have
169 their @Eq@ instances given by @deriving@ clauses, in which case they
170 form part of the system of equations.
172 Now all we need do is simplify and solve the equations, iterating to
173 find the least fixpoint. Notice that the order of the arguments can
174 switch around, as here in the recursive calls to T.
176 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
180 Eq (T a b) = {} -- The empty set
183 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
184 u Eq (T b a) u Eq Int -- From C2
185 u Eq (T a a) -- From C3
187 After simplification:
188 = Eq a u Ping b u {} u {} u {}
193 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
194 u Eq (T b a) u Eq Int -- From C2
195 u Eq (T a a) -- From C3
197 After simplification:
202 = Eq a u Ping b u Eq b u Ping a
204 The next iteration gives the same result, so this is the fixpoint. We
205 need to make a canonical form of the RHS to ensure convergence. We do
206 this by simplifying the RHS to a form in which
208 - the classes constrain only tyvars
209 - the list is sorted by tyvar (major key) and then class (minor key)
210 - no duplicates, of course
212 So, here are the synonyms for the ``equation'' structures:
215 Note [Data decl contexts]
216 ~~~~~~~~~~~~~~~~~~~~~~~~~
219 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
221 We will need an instance decl like:
223 instance (Read a, RealFloat a) => Read (Complex a) where
226 The RealFloat in the context is because the read method for Complex is bound
227 to construct a Complex, and doing that requires that the argument type is
230 But this ain't true for Show, Eq, Ord, etc, since they don't construct
231 a Complex; they only take them apart.
233 Our approach: identify the offending classes, and add the data type
234 context to the instance decl. The "offending classes" are
238 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
239 pattern matching against a constructor from a data type with a context
240 gives rise to the constraints for that context -- or at least the thinned
241 version. So now all classes are "offending".
243 Note [Newtype deriving]
244 ~~~~~~~~~~~~~~~~~~~~~~~
248 newtype T = T Char deriving( C [a] )
250 Notice the free 'a' in the deriving. We have to fill this out to
251 newtype T = T Char deriving( forall a. C [a] )
253 And then translate it to:
254 instance C [a] Char => C [a] T where ...
257 Note [Newtype deriving superclasses]
258 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
259 (See also Trac #1220 for an interesting exchange on newtype
260 deriving and superclasses.)
262 The 'tys' here come from the partial application in the deriving
263 clause. The last arg is the new instance type.
265 We must pass the superclasses; the newtype might be an instance
266 of them in a different way than the representation type
267 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
268 Then the Show instance is not done via isomorphism; it shows
270 The Num instance is derived via isomorphism, but the Show superclass
271 dictionary must the Show instance for Foo, *not* the Show dictionary
272 gotten from the Num dictionary. So we must build a whole new dictionary
273 not just use the Num one. The instance we want is something like:
274 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
277 There may be a coercion needed which we get from the tycon for the newtype
278 when the dict is constructed in TcInstDcls.tcInstDecl2
281 Note [Unused constructors and deriving clauses]
282 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
283 See Trac #3221. Consider
284 data T = T1 | T2 deriving( Show )
285 Are T1 and T2 unused? Well, no: the deriving clause expands to mention
286 both of them. So we gather defs/uses from deriving just like anything else.
288 %************************************************************************
290 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
292 %************************************************************************
295 tcDeriving :: [LTyClDecl Name] -- All type constructors
296 -> [LInstDecl Name] -- All instance declarations
297 -> [LDerivDecl Name] -- All stand-alone deriving declarations
298 -> TcM ([InstInfo Name] -- The generated "instance decls"
299 ,HsValBinds Name -- Extra generated top-level bindings
301 ,[TyCon] -- Extra generated top-level types
302 ,[TyCon]) -- Extra generated type family instances
304 tcDeriving tycl_decls inst_decls deriv_decls
305 = recoverM (return ([], emptyValBindsOut, emptyDUs, [], [])) $
306 do { -- Fish the "deriving"-related information out of the TcEnv
307 -- And make the necessary "equations".
308 is_boot <- tcIsHsBoot
309 ; traceTc "tcDeriving" (ppr is_boot)
310 ; early_specs <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
312 ; overlap_flag <- getOverlapFlag
313 ; let (infer_specs, given_specs) = splitEithers early_specs
314 ; insts1 <- mapM (genInst True overlap_flag) given_specs
316 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
317 inferInstanceContexts overlap_flag infer_specs
319 ; insts2 <- mapM (genInst False overlap_flag) final_specs
321 -- Generate the (old) generic to/from functions from each type declaration
322 ; gen_binds <- return emptyBag -- mkGenericBinds is_boot tycl_decls
324 -- Generate the generic Representable0/1 instances from each type declaration
325 ; repInstsMeta <- genGenericRepBinds is_boot tycl_decls
327 ; let repInsts = concat (map (\(a,_,_) -> a) repInstsMeta)
328 repMetaTys = map (\(_,b,_) -> b) repInstsMeta
329 repTyCons = map (\(_,_,c) -> c) repInstsMeta
330 -- Should we extendLocalInstEnv with repInsts?
332 ; (inst_info, rn_binds, rn_dus) <- renameDeriv is_boot gen_binds (insts1 ++ insts2 ++ repInsts)
335 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
336 (ddump_deriving inst_info rn_binds))
338 ; when (not (null inst_info)) $
339 dumpDerivingInfo (ddump_deriving inst_info rn_binds)
340 ; return ( inst_info, rn_binds, rn_dus
341 , concat (map metaTyCons2TyCons repMetaTys), repTyCons) }
343 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
344 ddump_deriving inst_infos extra_binds
345 = hang (ptext (sLit "Derived instances"))
346 2 (vcat (map (\i -> pprInstInfoDetails i $$ text "") inst_infos)
349 renameDeriv :: Bool -> LHsBinds RdrName
350 -> [(InstInfo RdrName, DerivAuxBinds)]
351 -> TcM ([InstInfo Name], HsValBinds Name, DefUses)
352 renameDeriv is_boot gen_binds insts
353 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
354 -- The inst-info bindings will all be empty, but it's easier to
355 -- just use rn_inst_info to change the type appropriately
356 = do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
357 ; return (rn_inst_infos, emptyValBindsOut, usesOnly (plusFVs fvs)) }
360 = discardWarnings $ -- Discard warnings about unused bindings etc
361 do { (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $ -- Type signatures in patterns
362 -- are used in the generic binds
363 rnTopBinds (ValBindsIn gen_binds [])
364 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
366 -- Generate and rename any extra not-one-inst-decl-specific binds,
367 -- notably "con2tag" and/or "tag2con" functions.
368 -- Bring those names into scope before renaming the instances themselves
369 ; loc <- getSrcSpanM -- Generic loc for shared bindings
370 ; let (aux_binds, aux_sigs) = unzip $ map (genAuxBind loc) $
371 rm_dups [] $ concat deriv_aux_binds
372 aux_val_binds = ValBindsIn (listToBag aux_binds) aux_sigs
373 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
374 ; bindLocalNames (collectHsValBinders rn_aux_lhs) $
375 do { (rn_aux, dus_aux) <- rnTopBindsRHS rn_aux_lhs
376 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
377 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
378 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
381 (inst_infos, deriv_aux_binds) = unzip insts
383 -- Remove duplicate requests for auxilliary bindings
385 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
386 | otherwise = rm_dups (b:acc) bs
389 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
390 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
391 = return ( info { iBinds = NewTypeDerived coi tc }
392 , mkFVs (map dataConName (tyConDataCons tc)))
393 -- See Note [Newtype deriving and unused constructors]
395 rn_inst_info inst_info@(InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
396 = -- Bring the right type variables into
397 -- scope (yuk), and rename the method binds
399 bindLocalNames (map Var.varName tyvars) $
400 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
401 ; let binds' = VanillaInst rn_binds [] standalone_deriv
402 ; return (inst_info { iBinds = binds' }, fvs) }
404 (tyvars,_, clas,_) = instanceHead inst
405 clas_nm = className clas
407 -----------------------------------------
409 mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
410 mkGenericBinds is_boot tycl_decls
414 = do { tcs <- mapM tcLookupTyCon [ tcdName d
415 | L _ d <- tycl_decls, isDataDecl d ]
416 ; return (unionManyBags [ mkTyConGenericBinds tc
417 | tc <- tcs, tyConHasGenerics tc ]) }
418 -- We are only interested in the data type declarations,
419 -- and then only in the ones whose 'has-generics' flag is on
420 -- The predicate tyConHasGenerics finds both of these
424 Note [Newtype deriving and unused constructors]
425 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
426 Consider this (see Trac #1954):
429 newtype P a = MkP (IO a) deriving Monad
431 If you compile with -fwarn-unused-binds you do not expect the warning
432 "Defined but not used: data consructor MkP". Yet the newtype deriving
433 code does not explicitly mention MkP, but it should behave as if you
435 instance Monad P where
436 return x = MkP (return x)
439 So we want to signal a user of the data constructor 'MkP'. That's
440 what we do in rn_inst_info, and it's the only reason we have the TyCon
441 stored in NewTypeDerived.
444 %************************************************************************
446 From HsSyn to DerivSpec
448 %************************************************************************
450 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
453 makeDerivSpecs :: Bool
457 -> TcM [EarlyDerivSpec]
459 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
460 | is_boot -- No 'deriving' at all in hs-boot files
461 = do { mapM_ add_deriv_err deriv_locs
464 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
465 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
466 ; return (eqns1 ++ eqns2) }
468 extractTyDataPreds decls
469 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
471 all_tydata :: [(LHsType Name, LTyClDecl Name)]
472 -- Derived predicate paired with its data type declaration
473 all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
475 deriv_locs = map (getLoc . snd) all_tydata
476 ++ map getLoc deriv_decls
478 add_deriv_err loc = setSrcSpan loc $
479 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
480 2 (ptext (sLit "Use an instance declaration instead")))
482 ------------------------------------------------------------------
483 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
484 -- Standalone deriving declarations
485 -- e.g. deriving instance Show a => Show (T a)
486 -- Rather like tcLocalInstDecl
487 deriveStandalone (L loc (DerivDecl deriv_ty))
489 addErrCtxt (standaloneCtxt deriv_ty) $
490 do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
491 ; (tvs, theta, cls, inst_tys) <- tcHsInstHead deriv_ty
492 ; traceTc "Standalone deriving;" $ vcat
493 [ text "tvs:" <+> ppr tvs
494 , text "theta:" <+> ppr theta
495 , text "cls:" <+> ppr cls
496 , text "tys:" <+> ppr inst_tys ]
497 ; checkValidInstance deriv_ty tvs theta cls inst_tys
498 -- C.f. TcInstDcls.tcLocalInstDecl1
500 ; let cls_tys = take (length inst_tys - 1) inst_tys
501 inst_ty = last inst_tys
502 ; traceTc "Standalone deriving:" $ vcat
503 [ text "class:" <+> ppr cls
504 , text "class types:" <+> ppr cls_tys
505 , text "type:" <+> ppr inst_ty ]
506 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
509 ------------------------------------------------------------------
510 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
511 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
512 tcdTyVars = tv_names,
513 tcdTyPats = ty_pats }))
514 = setSrcSpan loc $ -- Use the location of the 'deriving' item
516 do { (tvs, tc, tc_args) <- get_lhs ty_pats
517 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
518 -- the type variables for the type constructor
520 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
521 -- The "deriv_pred" is a LHsType to take account of the fact that for
522 -- newtype deriving we allow deriving (forall a. C [a]).
524 -- Given data T a b c = ... deriving( C d ),
525 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
526 ; let cls_tyvars = classTyVars cls
527 kind = tyVarKind (last cls_tyvars)
528 (arg_kinds, _) = splitKindFunTys kind
529 n_args_to_drop = length arg_kinds
530 n_args_to_keep = tyConArity tc - n_args_to_drop
531 args_to_drop = drop n_args_to_keep tc_args
532 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
533 inst_ty_kind = typeKind inst_ty
534 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
535 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
536 `minusVarSet` dropped_tvs
538 -- Check that the result really is well-kinded
539 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
540 (derivingKindErr tc cls cls_tys kind)
542 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
543 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
544 (derivingEtaErr cls cls_tys inst_ty)
546 -- (a) The data type can be eta-reduced; eg reject:
547 -- data instance T a a = ... deriving( Monad )
548 -- (b) The type class args do not mention any of the dropped type
550 -- newtype T a s = ... deriving( ST s )
552 -- Type families can't be partially applied
553 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
554 -- Note [Deriving, type families, and partial applications]
555 ; checkTc (not (isFamilyTyCon tc) || n_args_to_drop == 0)
556 (typeFamilyPapErr tc cls cls_tys inst_ty)
558 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
560 -- Tiresomely we must figure out the "lhs", which is awkward for type families
561 -- E.g. data T a b = .. deriving( Eq )
562 -- Here, the lhs is (T a b)
563 -- data instance TF Int b = ... deriving( Eq )
564 -- Here, the lhs is (TF Int b)
565 -- But if we just look up the tycon_name, we get is the *family*
566 -- tycon, but not pattern types -- they are in the *rep* tycon.
567 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
568 ; let tvs = tyConTyVars tc
569 ; return (tvs, tc, mkTyVarTys tvs) }
570 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
571 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
572 ; let (tc, tc_args) = tcSplitTyConApp tc_app
573 ; return (tvs, tc, tc_args) }
576 = panic "derivTyData" -- Caller ensures that only TyData can happen
579 Note [Deriving, type families, and partial applications]
580 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
581 When there are no type families, it's quite easy:
583 newtype S a = MkS [a]
584 -- :CoS :: S ~ [] -- Eta-reduced
586 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
587 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
589 When type familes are involved it's trickier:
592 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
593 -- :RT is the representation type for (T Int a)
594 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
595 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
597 instance Eq [a] => Eq (T Int a) -- easy by coercion
598 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
600 The "???" bit is that we don't build the :CoF thing in eta-reduced form
601 Henc the current typeFamilyPapErr, even though the instance makes sense.
602 After all, we can write it out
603 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
608 mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type] -> Type
609 -> DerivContext -- Just => context supplied (standalone deriving)
610 -- Nothing => context inferred (deriving on data decl)
611 -> TcRn EarlyDerivSpec
612 -- Make the EarlyDerivSpec for an instance
613 -- forall tvs. theta => cls (tys ++ [ty])
614 -- where the 'theta' is optional (that's the Maybe part)
615 -- Assumes that this declaration is well-kinded
617 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
618 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
619 , isAlgTyCon tycon -- Check for functions, primitive types etc
620 = mk_alg_eqn tycon tc_args
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")))
626 bale_out msg = failWithTc (derivingThingErr False cls cls_tys tc_app msg)
628 mk_alg_eqn tycon tc_args
629 | className cls `elem` typeableClassNames
630 = do { dflags <- getDOpts
631 ; case checkTypeableConditions (dflags, tycon) of
632 Just err -> bale_out err
633 Nothing -> mk_typeable_eqn orig tvs cls tycon tc_args mtheta }
635 | isDataFamilyTyCon tycon
636 , length tc_args /= tyConArity tycon
637 = bale_out (ptext (sLit "Unsaturated data family application"))
640 = do { (rep_tc, rep_tc_args) <- tcLookupDataFamInst tycon tc_args
641 -- Be careful to test rep_tc here: in the case of families,
642 -- we want to check the instance tycon, not the family tycon
644 -- For standalone deriving (mtheta /= Nothing),
645 -- check that all the data constructors are in scope.
646 ; rdr_env <- getGlobalRdrEnv
647 ; let hidden_data_cons = isAbstractTyCon rep_tc ||
648 any not_in_scope (tyConDataCons rep_tc)
649 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
650 ; unless (isNothing mtheta || not hidden_data_cons)
651 (bale_out (derivingHiddenErr tycon))
654 ; if isDataTyCon rep_tc then
655 mkDataTypeEqn orig dflags tvs cls cls_tys
656 tycon tc_args rep_tc rep_tc_args mtheta
658 mkNewTypeEqn orig dflags tvs cls cls_tys
659 tycon tc_args rep_tc rep_tc_args mtheta }
663 %************************************************************************
667 %************************************************************************
670 mkDataTypeEqn :: CtOrigin
672 -> [Var] -- Universally quantified type variables in the instance
673 -> Class -- Class for which we need to derive an instance
674 -> [Type] -- Other parameters to the class except the last
675 -> TyCon -- Type constructor for which the instance is requested
676 -- (last parameter to the type class)
677 -> [Type] -- Parameters to the type constructor
678 -> TyCon -- rep of the above (for type families)
679 -> [Type] -- rep of the above
680 -> DerivContext -- Context of the instance, for standalone deriving
681 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
683 mkDataTypeEqn orig dflags tvs cls cls_tys
684 tycon tc_args rep_tc rep_tc_args mtheta
685 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
686 -- NB: pass the *representation* tycon to checkSideConditions
687 CanDerive -> go_for_it
688 NonDerivableClass -> bale_out (nonStdErr cls)
689 DerivableClassError msg -> bale_out msg
691 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
692 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
694 mk_data_eqn :: CtOrigin -> [TyVar] -> Class
695 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
696 -> TcM EarlyDerivSpec
697 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
698 = do { dfun_name <- new_dfun_name cls tycon
700 ; let inst_tys = [mkTyConApp tycon tc_args]
701 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
702 spec = DS { ds_loc = loc, ds_orig = orig
703 , ds_name = dfun_name, ds_tvs = tvs
704 , ds_cls = cls, ds_tys = inst_tys
705 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
706 , ds_theta = mtheta `orElse` inferred_constraints
707 , ds_newtype = False }
709 ; return (if isJust mtheta then Right spec -- Specified context
710 else Left spec) } -- Infer context
712 ----------------------
713 mk_typeable_eqn :: CtOrigin -> [TyVar] -> Class
714 -> TyCon -> [TcType] -> DerivContext
715 -> TcM EarlyDerivSpec
716 mk_typeable_eqn orig tvs cls tycon tc_args mtheta
717 -- The Typeable class is special in several ways
718 -- data T a b = ... deriving( Typeable )
720 -- instance Typeable2 T where ...
722 -- 1. There are no constraints in the instance
723 -- 2. There are no type variables either
724 -- 3. The actual class we want to generate isn't necessarily
725 -- Typeable; it depends on the arity of the type
726 | isNothing mtheta -- deriving on a data type decl
727 = do { checkTc (cls `hasKey` typeableClassKey)
728 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
729 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
730 ; mk_typeable_eqn orig tvs real_cls tycon [] (Just []) }
732 | otherwise -- standaone deriving
733 = do { checkTc (null tc_args)
734 (ptext (sLit "Derived typeable instance must be of form (Typeable")
735 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
736 ; dfun_name <- new_dfun_name cls tycon
739 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
740 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
741 , ds_tc = tycon, ds_tc_args = []
742 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
744 ----------------------
745 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
746 -- Generate a sufficiently large set of constraints that typechecking the
747 -- generated method definitions should succeed. This set will be simplified
748 -- before being used in the instance declaration
749 inferConstraints _ cls inst_tys rep_tc rep_tc_args
750 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
751 stupid_constraints ++ extra_constraints
752 ++ sc_constraints ++ con_arg_constraints
754 -- Constraints arising from the arguments of each constructor
756 = [ mkClassPred cls [arg_ty]
757 | data_con <- tyConDataCons rep_tc,
758 arg_ty <- ASSERT( isVanillaDataCon data_con )
759 get_constrained_tys $
760 dataConInstOrigArgTys data_con all_rep_tc_args,
761 not (isUnLiftedType arg_ty) ]
762 -- No constraints for unlifted types
763 -- Where they are legal we generate specilised function calls
765 -- For functor-like classes, two things are different
766 -- (a) We recurse over argument types to generate constraints
767 -- See Functor examples in TcGenDeriv
768 -- (b) The rep_tc_args will be one short
769 is_functor_like = getUnique cls `elem` functorLikeClassKeys
771 get_constrained_tys :: [Type] -> [Type]
772 get_constrained_tys tys
773 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
776 rep_tc_tvs = tyConTyVars rep_tc
777 last_tv = last rep_tc_tvs
778 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
779 | otherwise = rep_tc_args
781 -- Constraints arising from superclasses
782 -- See Note [Superclasses of derived instance]
783 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
786 -- Stupid constraints
787 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
788 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
790 -- Extra Data constraints
791 -- The Data class (only) requires that for
792 -- instance (...) => Data (T t1 t2)
794 -- THEN (Data t1, Data t2) are among the (...) constraints
795 -- Reason: when the IF holds, we generate a method
796 -- dataCast2 f = gcast2 f
797 -- and we need the Data constraints to typecheck the method
799 | cls `hasKey` dataClassKey
800 , all (isLiftedTypeKind . typeKind) rep_tc_args
801 = [mkClassPred cls [ty] | ty <- rep_tc_args]
805 ------------------------------------------------------------------
806 -- Check side conditions that dis-allow derivability for particular classes
807 -- This is *apart* from the newtype-deriving mechanism
809 -- Here we get the representation tycon in case of family instances as it has
810 -- the data constructors - but we need to be careful to fall back to the
811 -- family tycon (with indexes) in error messages.
813 data DerivStatus = CanDerive
814 | DerivableClassError SDoc -- Standard class, but can't do it
815 | NonDerivableClass -- Non-standard class
817 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
818 checkSideConditions dflags mtheta cls cls_tys rep_tc
819 | Just cond <- sideConditions mtheta cls
820 = case (cond (dflags, rep_tc)) of
821 Just err -> DerivableClassError err -- Class-specific error
822 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
823 -- cls_tys (the type args other than last)
825 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
826 | otherwise = NonDerivableClass -- Not a standard class
828 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
830 checkTypeableConditions :: Condition
831 checkTypeableConditions = checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK
833 nonStdErr :: Class -> SDoc
834 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
836 sideConditions :: DerivContext -> Class -> Maybe Condition
837 sideConditions mtheta cls
838 | cls_key == eqClassKey = Just cond_std
839 | cls_key == ordClassKey = Just cond_std
840 | cls_key == showClassKey = Just cond_std
841 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
842 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
843 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
844 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
845 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
846 cond_std `andCond` cond_noUnliftedArgs)
847 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
848 cond_functorOK True) -- NB: no cond_std!
849 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
850 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
851 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
852 cond_functorOK False)
853 | otherwise = Nothing
855 cls_key = getUnique cls
856 cond_std = cond_stdOK mtheta
858 type Condition = (DynFlags, TyCon) -> Maybe SDoc
859 -- first Bool is whether or not we are allowed to derive Data and Typeable
860 -- second Bool is whether or not we are allowed to derive Functor
861 -- TyCon is the *representation* tycon if the
862 -- data type is an indexed one
865 orCond :: Condition -> Condition -> Condition
868 Nothing -> Nothing -- c1 succeeds
869 Just x -> case c2 tc of -- c1 fails
871 Just y -> Just (x $$ ptext (sLit " and") $$ y)
874 andCond :: Condition -> Condition -> Condition
875 andCond c1 c2 tc = case c1 tc of
876 Nothing -> c2 tc -- c1 succeeds
877 Just x -> Just x -- c1 fails
879 cond_stdOK :: DerivContext -> Condition
880 cond_stdOK (Just _) _
881 = Nothing -- Don't check these conservative conditions for
882 -- standalone deriving; just generate the code
883 -- and let the typechecker handle the result
884 cond_stdOK Nothing (_, rep_tc)
885 | null data_cons = Just (no_cons_why rep_tc $$ suggestion)
886 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
887 | otherwise = Nothing
889 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
890 data_cons = tyConDataCons rep_tc
891 con_whys = mapCatMaybes check_con data_cons
893 check_con :: DataCon -> Maybe SDoc
895 | isVanillaDataCon con
896 , all isTauTy (dataConOrigArgTys con) = Nothing
897 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
899 no_cons_why :: TyCon -> SDoc
900 no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
901 ptext (sLit "has no data constructors")
903 cond_enumOrProduct :: Condition
904 cond_enumOrProduct = cond_isEnumeration `orCond`
905 (cond_isProduct `andCond` cond_noUnliftedArgs)
907 cond_noUnliftedArgs :: Condition
908 -- For some classes (eg Eq, Ord) we allow unlifted arg types
909 -- by generating specilaised code. For others (eg Data) we don't.
910 cond_noUnliftedArgs (_, tc)
911 | null bad_cons = Nothing
912 | otherwise = Just why
914 bad_cons = [ con | con <- tyConDataCons tc
915 , any isUnLiftedType (dataConOrigArgTys con) ]
916 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
918 cond_isEnumeration :: Condition
919 cond_isEnumeration (_, rep_tc)
920 | isEnumerationTyCon rep_tc = Nothing
921 | otherwise = Just why
923 why = sep [ quotes (pprSourceTyCon rep_tc) <+>
924 ptext (sLit "is not an enumeration type")
925 , ptext (sLit "(an enumeration consists of one or more nullary, non-GADT constructors)") ]
926 -- See Note [Enumeration types] in TyCon
928 cond_isProduct :: Condition
929 cond_isProduct (_, rep_tc)
930 | isProductTyCon rep_tc = Nothing
931 | otherwise = Just why
933 why = quotes (pprSourceTyCon rep_tc) <+>
934 ptext (sLit "does not have precisely one constructor")
936 cond_typeableOK :: Condition
937 -- OK for Typeable class
938 -- Currently: (a) args all of kind *
939 -- (b) 7 or fewer args
940 cond_typeableOK (_, tc)
941 | tyConArity tc > 7 = Just too_many
942 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tc))
944 | otherwise = Nothing
946 too_many = quotes (pprSourceTyCon tc) <+>
947 ptext (sLit "has too many arguments")
948 bad_kind = quotes (pprSourceTyCon tc) <+>
949 ptext (sLit "has arguments of kind other than `*'")
951 functorLikeClassKeys :: [Unique]
952 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
954 cond_functorOK :: Bool -> Condition
955 -- OK for Functor/Foldable/Traversable class
956 -- Currently: (a) at least one argument
957 -- (b) don't use argument contravariantly
958 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
959 -- (d) optionally: don't use function types
960 -- (e) no "stupid context" on data type
961 cond_functorOK allowFunctions (_, rep_tc)
963 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
964 <+> ptext (sLit "has no parameters"))
966 | not (null bad_stupid_theta)
967 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
968 <+> ptext (sLit "has a class context") <+> pprTheta bad_stupid_theta)
971 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
973 tc_tvs = tyConTyVars rep_tc
974 Just (_, last_tv) = snocView tc_tvs
975 bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
976 is_bad pred = last_tv `elemVarSet` tyVarsOfPred pred
978 data_cons = tyConDataCons rep_tc
979 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
981 check_vanilla :: DataCon -> Maybe SDoc
982 check_vanilla con | isVanillaDataCon con = Nothing
983 | otherwise = Just (badCon con existential)
985 ft_check :: DataCon -> FFoldType (Maybe SDoc)
986 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
987 , ft_co_var = Just (badCon con covariant)
988 , ft_fun = \x y -> if allowFunctions then x `mplus` y
989 else Just (badCon con functions)
990 , ft_tup = \_ xs -> msum xs
991 , ft_ty_app = \_ x -> x
992 , ft_bad_app = Just (badCon con wrong_arg)
993 , ft_forall = \_ x -> x }
995 existential = ptext (sLit "has existential arguments")
996 covariant = ptext (sLit "uses the type variable in a function argument")
997 functions = ptext (sLit "contains function types")
998 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
1000 checkFlag :: ExtensionFlag -> Condition
1001 checkFlag flag (dflags, _)
1002 | xopt flag dflags = Nothing
1003 | otherwise = Just why
1005 why = ptext (sLit "You need -X") <> text flag_str
1006 <+> ptext (sLit "to derive an instance for this class")
1007 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
1009 other -> pprPanic "checkFlag" (ppr other)
1011 std_class_via_iso :: Class -> Bool
1012 -- These standard classes can be derived for a newtype
1013 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
1014 -- because giving so gives the same results as generating the boilerplate
1015 std_class_via_iso clas
1016 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
1017 -- Not Read/Show because they respect the type
1018 -- Not Enum, because newtypes are never in Enum
1021 non_iso_class :: Class -> Bool
1022 -- *Never* derive Read,Show,Typeable,Data by isomorphism,
1023 -- even with -XGeneralizedNewtypeDeriving
1025 = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
1028 typeableClassKeys :: [Unique]
1029 typeableClassKeys = map getUnique typeableClassNames
1031 new_dfun_name :: Class -> TyCon -> TcM Name
1032 new_dfun_name clas tycon -- Just a simple wrapper
1033 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1034 ; newDFunName clas [mkTyConApp tycon []] loc }
1035 -- The type passed to newDFunName is only used to generate
1036 -- a suitable string; hence the empty type arg list
1038 badCon :: DataCon -> SDoc -> SDoc
1039 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1042 Note [Superclasses of derived instance]
1043 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1044 In general, a derived instance decl needs the superclasses of the derived
1045 class too. So if we have
1046 data T a = ...deriving( Ord )
1047 then the initial context for Ord (T a) should include Eq (T a). Often this is
1048 redundant; we'll also generate an Ord constraint for each constructor argument,
1049 and that will probably generate enough constraints to make the Eq (T a) constraint
1050 be satisfied too. But not always; consider:
1056 data T a = MkT (S a) deriving( Ord )
1057 instance Num a => Eq (T a)
1059 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1061 data T a = MkT deriving( Data, Typeable )
1062 Here there *is* no argument field, but we must nevertheless generate
1063 a context for the Data instances:
1064 instance Typable a => Data (T a) where ...
1067 %************************************************************************
1071 %************************************************************************
1074 mkNewTypeEqn :: CtOrigin -> DynFlags -> [Var] -> Class
1075 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1077 -> TcRn EarlyDerivSpec
1078 mkNewTypeEqn orig dflags tvs
1079 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1080 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1081 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1082 = do { traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
1083 ; dfun_name <- new_dfun_name cls tycon
1084 ; loc <- getSrcSpanM
1085 ; let spec = DS { ds_loc = loc, ds_orig = orig
1086 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1087 , ds_cls = cls, ds_tys = inst_tys
1088 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1089 , ds_theta = mtheta `orElse` all_preds
1090 , ds_newtype = True }
1091 ; return (if isJust mtheta then Right spec
1095 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1096 CanDerive -> go_for_it -- Use the standard H98 method
1097 DerivableClassError msg -- Error with standard class
1098 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1099 | otherwise -> bale_out msg
1100 NonDerivableClass -- Must use newtype deriving
1101 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1102 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1103 | otherwise -> bale_out non_std
1105 newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
1106 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1107 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1109 non_std = nonStdErr cls
1110 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1112 -- Here is the plan for newtype derivings. We see
1113 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1114 -- where t is a type,
1115 -- ak+1...an is a suffix of a1..an, and are all tyars
1116 -- ak+1...an do not occur free in t, nor in the s1..sm
1117 -- (C s1 ... sm) is a *partial applications* of class C
1118 -- with the last parameter missing
1119 -- (T a1 .. ak) matches the kind of C's last argument
1120 -- (and hence so does t)
1121 -- The latter kind-check has been done by deriveTyData already,
1122 -- and tc_args are already trimmed
1124 -- We generate the instance
1125 -- instance forall ({a1..ak} u fvs(s1..sm)).
1126 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1127 -- where T a1...ap is the partial application of
1128 -- the LHS of the correct kind and p >= k
1130 -- NB: the variables below are:
1131 -- tc_tvs = [a1, ..., an]
1132 -- tyvars_to_keep = [a1, ..., ak]
1133 -- rep_ty = t ak .. an
1134 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1135 -- tys = [s1, ..., sm]
1138 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1139 -- We generate the instance
1140 -- instance Monad (ST s) => Monad (T s) where
1142 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1143 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1144 -- eta-reduces the representation type, so we know that
1146 -- That's convenient here, because we may have to apply
1147 -- it to fewer than its original complement of arguments
1149 -- Note [Newtype representation]
1150 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1151 -- Need newTyConRhs (*not* a recursive representation finder)
1152 -- to get the representation type. For example
1153 -- newtype B = MkB Int
1154 -- newtype A = MkA B deriving( Num )
1155 -- We want the Num instance of B, *not* the Num instance of Int,
1156 -- when making the Num instance of A!
1157 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1158 rep_tys = cls_tys ++ [rep_inst_ty]
1159 rep_pred = mkClassPred cls rep_tys
1160 -- rep_pred is the representation dictionary, from where
1161 -- we are gong to get all the methods for the newtype
1165 -- Next we figure out what superclass dictionaries to use
1166 -- See Note [Newtype deriving superclasses] above
1168 cls_tyvars = classTyVars cls
1169 dfun_tvs = tyVarsOfTypes inst_tys
1170 inst_ty = mkTyConApp tycon tc_args
1171 inst_tys = cls_tys ++ [inst_ty]
1172 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1175 -- If there are no tyvars, there's no need
1176 -- to abstract over the dictionaries we need
1177 -- Example: newtype T = MkT Int deriving( C )
1178 -- We get the derived instance
1181 -- instance C Int => C T
1182 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1184 -------------------------------------------------------------------
1185 -- Figuring out whether we can only do this newtype-deriving thing
1187 can_derive_via_isomorphism
1188 = not (non_iso_class cls)
1192 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1194 arity_ok = length cls_tys + 1 == classArity cls
1195 -- Well kinded; eg not: newtype T ... deriving( ST )
1196 -- because ST needs *2* type params
1198 -- Check that eta reduction is OK
1199 eta_ok = nt_eta_arity <= length rep_tc_args
1200 -- The newtype can be eta-reduced to match the number
1201 -- of type argument actually supplied
1202 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1203 -- Here the 'b' must be the same in the rep type (S [a] b)
1204 -- And the [a] must not mention 'b'. That's all handled
1207 ats_ok = null (classATs cls)
1208 -- No associated types for the class, because we don't
1209 -- currently generate type 'instance' decls; and cannot do
1210 -- so for 'data' instance decls
1213 = vcat [ ppUnless arity_ok arity_msg
1214 , ppUnless eta_ok eta_msg
1215 , ppUnless ats_ok ats_msg ]
1216 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1217 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1218 ats_msg = ptext (sLit "the class has associated types")
1221 Note [Recursive newtypes]
1222 ~~~~~~~~~~~~~~~~~~~~~~~~~
1223 Newtype deriving works fine, even if the newtype is recursive.
1224 e.g. newtype S1 = S1 [T1 ()]
1225 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1226 Remember, too, that type families are curretly (conservatively) given
1227 a recursive flag, so this also allows newtype deriving to work
1230 We used to exclude recursive types, because we had a rather simple
1231 minded way of generating the instance decl:
1233 instance Eq [A] => Eq A -- Makes typechecker loop!
1234 But now we require a simple context, so it's ok.
1237 %************************************************************************
1239 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1241 %************************************************************************
1243 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1244 terms, which is the final correct RHS for the corresponding original
1248 Each (k,TyVarTy tv) in a solution constrains only a type
1252 The (k,TyVarTy tv) pairs in a solution are canonically
1253 ordered by sorting on type varible, tv, (major key) and then class, k,
1258 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1260 inferInstanceContexts _ [] = return []
1262 inferInstanceContexts oflag infer_specs
1263 = do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
1264 ; iterate_deriv 1 initial_solutions }
1266 ------------------------------------------------------------------
1267 -- The initial solutions for the equations claim that each
1268 -- instance has an empty context; this solution is certainly
1269 -- in canonical form.
1270 initial_solutions :: [ThetaType]
1271 initial_solutions = [ [] | _ <- infer_specs ]
1273 ------------------------------------------------------------------
1274 -- iterate_deriv calculates the next batch of solutions,
1275 -- compares it with the current one; finishes if they are the
1276 -- same, otherwise recurses with the new solutions.
1277 -- It fails if any iteration fails
1278 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1279 iterate_deriv n current_solns
1280 | n > 20 -- Looks as if we are in an infinite loop
1281 -- This can happen if we have -XUndecidableInstances
1282 -- (See TcSimplify.tcSimplifyDeriv.)
1283 = pprPanic "solveDerivEqns: probable loop"
1284 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1286 = do { -- Extend the inst info from the explicit instance decls
1287 -- with the current set of solutions, and simplify each RHS
1288 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1289 current_solns infer_specs
1290 ; new_solns <- checkNoErrs $
1291 extendLocalInstEnv inst_specs $
1292 mapM gen_soln infer_specs
1294 ; if (current_solns == new_solns) then
1295 return [ spec { ds_theta = soln }
1296 | (spec, soln) <- zip infer_specs current_solns ]
1298 iterate_deriv (n+1) new_solns }
1300 ------------------------------------------------------------------
1301 gen_soln :: DerivSpec -> TcM [PredType]
1302 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1303 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1305 addErrCtxt (derivInstCtxt clas inst_tys) $
1306 do { -- Check for a bizarre corner case, when the derived instance decl should
1307 -- have form instance C a b => D (T a) where ...
1308 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1309 -- of problems; in particular, it's hard to compare solutions for
1310 -- equality when finding the fixpoint. Moreover, simplifyDeriv
1311 -- has an assert failure because it finds a TyVar when it expects
1312 -- only TcTyVars. So I just rule it out for now. I'm not
1313 -- even sure how it can arise.
1315 ; let tv_set = mkVarSet tyvars
1316 weird_preds = [pred | pred <- deriv_rhs
1317 , not (tyVarsOfPred pred `subVarSet` tv_set)]
1318 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1320 ; theta <- simplifyDeriv orig tyvars deriv_rhs
1321 -- checkValidInstance tyvars theta clas inst_tys
1322 -- Not necessary; see Note [Exotic derived instance contexts]
1325 ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
1326 -- Claim: the result instance declaration is guaranteed valid
1327 -- Hence no need to call:
1328 -- checkValidInstance tyvars theta clas inst_tys
1329 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1331 ------------------------------------------------------------------
1332 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1333 mkInstance overlap_flag theta
1334 (DS { ds_name = dfun_name
1335 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1336 = mkLocalInstance dfun overlap_flag
1338 dfun = mkDictFunId dfun_name tyvars theta clas tys
1341 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1342 -- Add new locally-defined instances; don't bother to check
1343 -- for functional dependency errors -- that'll happen in TcInstDcls
1344 extendLocalInstEnv dfuns thing_inside
1345 = do { env <- getGblEnv
1346 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1347 env' = env { tcg_inst_env = inst_env' }
1348 ; setGblEnv env' thing_inside }
1352 %************************************************************************
1354 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1356 %************************************************************************
1358 After all the trouble to figure out the required context for the
1359 derived instance declarations, all that's left is to chug along to
1360 produce them. They will then be shoved into @tcInstDecls2@, which
1361 will do all its usual business.
1363 There are lots of possibilities for code to generate. Here are
1364 various general remarks.
1369 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1370 ``you-couldn't-do-better-by-hand'' efficient.
1373 Deriving @Show@---also pretty common--- should also be reasonable good code.
1376 Deriving for the other classes isn't that common or that big a deal.
1383 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1386 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1389 We {\em normally} generate code only for the non-defaulted methods;
1390 there are some exceptions for @Eq@ and (especially) @Ord@...
1393 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1394 constructor's numeric (@Int#@) tag. These are generated by
1395 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1396 these is around is given by @hasCon2TagFun@.
1398 The examples under the different sections below will make this
1402 Much less often (really just for deriving @Ix@), we use a
1403 @_tag2con_<tycon>@ function. See the examples.
1406 We use the renamer!!! Reason: we're supposed to be
1407 producing @LHsBinds Name@ for the methods, but that means
1408 producing correctly-uniquified code on the fly. This is entirely
1409 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1410 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1411 the renamer. What a great hack!
1415 -- Generate the InstInfo for the required instance paired with the
1416 -- *representation* tycon for that instance,
1417 -- plus any auxiliary bindings required
1419 -- Representation tycons differ from the tycon in the instance signature in
1420 -- case of instances for indexed families.
1422 genInst :: Bool -- True <=> standalone deriving
1424 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1425 genInst standalone_deriv oflag
1426 spec@(DS { ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1427 , ds_theta = theta, ds_newtype = is_newtype
1428 , ds_name = name, ds_cls = clas })
1430 = return (InstInfo { iSpec = inst_spec
1431 , iBinds = NewTypeDerived co rep_tycon }, [])
1434 = do { fix_env <- getFixityEnv
1435 ; let loc = getSrcSpan name
1436 (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1437 -- In case of a family instance, we need to use the representation
1438 -- tycon (after all, it has the data constructors)
1440 ; return (InstInfo { iSpec = inst_spec
1441 , iBinds = VanillaInst meth_binds [] standalone_deriv }
1444 inst_spec = mkInstance oflag theta spec
1445 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1446 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1448 -- Not a family => rep_tycon = main tycon
1449 co2 = case newTyConCo_maybe rep_tycon of
1450 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1451 Nothing -> id_co -- The newtype is transparent; no need for a cast
1452 co = co1 `mkTransCoI` co2
1453 id_co = IdCo (mkTyConApp rep_tycon rep_tc_args)
1455 -- Example: newtype instance N [a] = N1 (Tree a)
1456 -- deriving instance Eq b => Eq (N [(b,b)])
1457 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1458 -- When dealing with the deriving clause
1459 -- co1 : N [(b,b)] ~ R1:N (b,b)
1460 -- co2 : R1:N (b,b) ~ Tree (b,b)
1461 -- co : N [(b,b)] ~ Tree (b,b)
1463 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1464 genDerivBinds loc fix_env clas tycon
1465 | className clas `elem` typeableClassNames
1466 = (gen_Typeable_binds loc tycon, [])
1469 = case assocMaybe gen_list (getUnique clas) of
1470 Just gen_fn -> gen_fn loc tycon
1471 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1473 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1474 gen_list = [(eqClassKey, gen_Eq_binds)
1475 ,(ordClassKey, gen_Ord_binds)
1476 ,(enumClassKey, gen_Enum_binds)
1477 ,(boundedClassKey, gen_Bounded_binds)
1478 ,(ixClassKey, gen_Ix_binds)
1479 ,(showClassKey, gen_Show_binds fix_env)
1480 ,(readClassKey, gen_Read_binds fix_env)
1481 ,(dataClassKey, gen_Data_binds)
1482 ,(functorClassKey, gen_Functor_binds)
1483 ,(foldableClassKey, gen_Foldable_binds)
1484 ,(traversableClassKey, gen_Traversable_binds)
1487 -- Generate the binds for the generic representation
1488 genGenericRepBinds :: Bool -> [LTyClDecl Name]
1489 -> TcM [([(InstInfo RdrName, DerivAuxBinds)]
1490 , MetaTyCons, TyCon)]
1491 genGenericRepBinds isBoot tyclDecls
1492 | isBoot = return []
1494 allTyDecls <- mapM tcLookupTyCon [ tcdName d | L _ d <- tyclDecls
1496 let tyDecls = filter tyConHasGenerics allTyDecls
1497 inst1 <- mapM genGenericRepBind tyDecls
1498 let (_repInsts, metaTyCons, _repTys) = unzip3 inst1
1499 metaInsts <- ASSERT (length tyDecls == length metaTyCons)
1500 mapM genDtMeta (zip tyDecls metaTyCons)
1501 return (ASSERT (length inst1 == length metaInsts)
1503 | ((ri, ms, rt), mi) <- zip inst1 metaInsts ])
1505 genGenericRepBind :: TyCon -> TcM ((InstInfo RdrName, DerivAuxBinds)
1506 , MetaTyCons, TyCon)
1507 genGenericRepBind tc =
1508 do clas <- tcLookupClass rep0ClassName
1509 uniqS <- newUniqueSupply
1510 dfun_name <- new_dfun_name clas tc
1512 -- Uniques for everyone
1513 (uniqD:uniqs) = uniqsFromSupply uniqS
1514 (uniqsC,us) = splitAt (length tc_cons) uniqs
1515 uniqsS :: [[Unique]] -- Unique supply for the S datatypes
1516 uniqsS = mkUniqsS tc_arits us
1518 mkUniqsS (n:t) us = case splitAt n us of
1519 (us1,us2) -> us1 : mkUniqsS t us2
1521 tc_name = tyConName tc
1522 tc_cons = tyConDataCons tc
1523 tc_arits = map dataConSourceArity tc_cons
1525 tc_occ = nameOccName tc_name
1526 d_occ = mkGenD tc_occ
1527 c_occ m = mkGenC tc_occ m
1528 s_occ m n = mkGenS tc_occ m n
1529 mod_name = nameModule (tyConName tc)
1530 d_name = mkExternalName uniqD mod_name d_occ wiredInSrcSpan
1531 c_names = [ mkExternalName u mod_name (c_occ m) wiredInSrcSpan
1532 | (u,m) <- zip uniqsC [0..] ]
1533 s_names = [ [ mkExternalName u mod_name (s_occ m n) wiredInSrcSpan
1534 | (u,n) <- zip us [0..] ] | (us,m) <- zip uniqsS [0..] ]
1535 tvs = tyConTyVars tc
1536 tc_ty = mkTyConApp tc (mkTyVarTys tvs)
1538 mkTyCon name = ASSERT( isExternalName name )
1539 buildAlgTyCon name [] [] mkAbstractTyConRhs
1540 NonRecursive False False NoParentTyCon Nothing
1542 metaDTyCon <- mkTyCon d_name
1543 metaCTyCons <- sequence [ mkTyCon c_name | c_name <- c_names ]
1544 metaSTyCons <- mapM sequence
1546 | s_name <- s_namesC ] | s_namesC <- s_names ]
1548 let metaDts = MetaTyCons metaDTyCon metaCTyCons metaSTyCons
1550 rep0_tycon <- tc_mkRep0TyCon tc metaDts
1553 mkInstRep0 = (InstInfo { iSpec = inst, iBinds = binds }
1554 , [ {- No DerivAuxBinds -} ])
1555 inst = mkLocalInstance dfun NoOverlap
1556 binds = VanillaInst (mkBindsRep0 tc) [] False
1558 dfun = mkDictFunId dfun_name (tyConTyVars tc) [] clas [tc_ty]
1559 return (mkInstRep0, metaDts, rep0_tycon)
1561 genDtMeta :: (TyCon, MetaTyCons) -> TcM [(InstInfo RdrName, DerivAuxBinds)]
1562 genDtMeta (tc,metaDts) =
1563 do dClas <- tcLookupClass datatypeClassName
1564 d_dfun_name <- new_dfun_name dClas tc
1565 cClas <- tcLookupClass constructorClassName
1566 c_dfun_names <- sequence [ new_dfun_name cClas tc | _ <- metaC metaDts ]
1567 sClas <- tcLookupClass selectorClassName
1568 s_dfun_names <- sequence (map sequence [ [ new_dfun_name sClas tc
1570 | x <- metaS metaDts ])
1571 fix_env <- getFixityEnv
1574 (dBinds,cBinds,sBinds) = mkBindsMetaD fix_env tc
1577 d_metaTycon = metaD metaDts
1578 d_inst = mkLocalInstance d_dfun NoOverlap
1579 d_binds = VanillaInst dBinds [] False
1580 d_dfun = mkDictFunId d_dfun_name (tyConTyVars tc) [] dClas
1581 [ mkTyConTy d_metaTycon ]
1582 d_mkInst = (InstInfo { iSpec = d_inst, iBinds = d_binds }, [])
1585 c_metaTycons = metaC metaDts
1586 c_insts = [ mkLocalInstance (c_dfun c ds) NoOverlap
1587 | (c, ds) <- myZip1 c_metaTycons c_dfun_names ]
1588 c_binds = [ VanillaInst c [] False | c <- cBinds ]
1589 c_dfun c dfun_name = mkDictFunId dfun_name (tyConTyVars tc) [] cClas
1591 c_mkInst = [ (InstInfo { iSpec = is, iBinds = bs }, [])
1592 | (is,bs) <- myZip1 c_insts c_binds ]
1595 s_metaTycons = metaS metaDts
1596 s_insts = map (map (\(s,ds) -> mkLocalInstance (s_dfun s ds) NoOverlap))
1597 (myZip2 s_metaTycons s_dfun_names)
1598 s_binds = [ [ VanillaInst s [] False | s <- ss ] | ss <- sBinds ]
1599 s_dfun s dfun_name = mkDictFunId dfun_name (tyConTyVars tc) [] sClas
1601 s_mkInst = map (map (\(is,bs) -> (InstInfo {iSpec=is, iBinds=bs}, [])))
1602 (myZip2 s_insts s_binds)
1604 myZip1 :: [a] -> [b] -> [(a,b)]
1605 myZip1 l1 l2 = ASSERT (length l1 == length l2) zip l1 l2
1607 myZip2 :: [[a]] -> [[b]] -> [[(a,b)]]
1609 ASSERT (and (zipWith (>=) (map length l1) (map length l2)))
1610 [ zip x1 x2 | (x1,x2) <- zip l1 l2 ]
1612 return (d_mkInst : c_mkInst ++ concat s_mkInst)
1616 %************************************************************************
1618 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1620 %************************************************************************
1623 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1624 derivingKindErr tc cls cls_tys cls_kind
1625 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1626 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1627 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1628 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1630 derivingEtaErr :: Class -> [Type] -> Type -> Message
1631 derivingEtaErr cls cls_tys inst_ty
1632 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1633 nest 2 (ptext (sLit "instance (...) =>")
1634 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1636 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1637 typeFamilyPapErr tc cls cls_tys inst_ty
1638 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1639 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1641 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1642 derivingThingErr newtype_deriving clas tys ty why
1643 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1644 2 (quotes (ppr pred))
1645 $$ nest 2 extra) <> colon,
1648 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1650 pred = mkClassPred clas (tys ++ [ty])
1652 derivingHiddenErr :: TyCon -> SDoc
1653 derivingHiddenErr tc
1654 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1655 2 (ptext (sLit "so you cannot derive an instance for it"))
1657 standaloneCtxt :: LHsType Name -> SDoc
1658 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1661 derivInstCtxt :: Class -> [Type] -> Message
1662 derivInstCtxt clas inst_tys
1663 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1665 badDerivedPred :: PredType -> Message
1667 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1668 ptext (sLit "type variables that are not data type parameters"),
1669 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]