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 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 (inst_info { 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, cls, inst_tys) <- tcHsInstHead deriv_ty
471 ; traceTc "Standalone deriving;" $ vcat
472 [ text "tvs:" <+> ppr tvs
473 , text "theta:" <+> ppr theta
474 , text "cls:" <+> ppr cls
475 , text "tys:" <+> ppr inst_tys ]
476 ; checkValidInstance deriv_ty tvs theta cls inst_tys
477 -- C.f. TcInstDcls.tcLocalInstDecl1
479 ; let cls_tys = take (length inst_tys - 1) inst_tys
480 inst_ty = last inst_tys
481 ; traceTc "Standalone deriving:" $ vcat
482 [ text "class:" <+> ppr cls
483 , text "class types:" <+> ppr cls_tys
484 , text "type:" <+> ppr inst_ty ]
485 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
488 ------------------------------------------------------------------
489 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
490 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
491 tcdTyVars = tv_names,
492 tcdTyPats = ty_pats }))
493 = setSrcSpan loc $ -- Use the location of the 'deriving' item
495 do { (tvs, tc, tc_args) <- get_lhs ty_pats
496 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
497 -- the type variables for the type constructor
499 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
500 -- The "deriv_pred" is a LHsType to take account of the fact that for
501 -- newtype deriving we allow deriving (forall a. C [a]).
503 -- Given data T a b c = ... deriving( C d ),
504 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
505 ; let cls_tyvars = classTyVars cls
506 kind = tyVarKind (last cls_tyvars)
507 (arg_kinds, _) = splitKindFunTys kind
508 n_args_to_drop = length arg_kinds
509 n_args_to_keep = tyConArity tc - n_args_to_drop
510 args_to_drop = drop n_args_to_keep tc_args
511 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
512 inst_ty_kind = typeKind inst_ty
513 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
514 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
515 `minusVarSet` dropped_tvs
517 -- Check that the result really is well-kinded
518 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
519 (derivingKindErr tc cls cls_tys kind)
521 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
522 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
523 (derivingEtaErr cls cls_tys inst_ty)
525 -- (a) The data type can be eta-reduced; eg reject:
526 -- data instance T a a = ... deriving( Monad )
527 -- (b) The type class args do not mention any of the dropped type
529 -- newtype T a s = ... deriving( ST s )
531 -- Type families can't be partially applied
532 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
533 -- Note [Deriving, type families, and partial applications]
534 ; checkTc (not (isFamilyTyCon tc) || n_args_to_drop == 0)
535 (typeFamilyPapErr tc cls cls_tys inst_ty)
537 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
539 -- Tiresomely we must figure out the "lhs", which is awkward for type families
540 -- E.g. data T a b = .. deriving( Eq )
541 -- Here, the lhs is (T a b)
542 -- data instance TF Int b = ... deriving( Eq )
543 -- Here, the lhs is (TF Int b)
544 -- But if we just look up the tycon_name, we get is the *family*
545 -- tycon, but not pattern types -- they are in the *rep* tycon.
546 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
547 ; let tvs = tyConTyVars tc
548 ; return (tvs, tc, mkTyVarTys tvs) }
549 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
550 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
551 ; let (tc, tc_args) = tcSplitTyConApp tc_app
552 ; return (tvs, tc, tc_args) }
555 = panic "derivTyData" -- Caller ensures that only TyData can happen
558 Note [Deriving, type families, and partial applications]
559 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
560 When there are no type families, it's quite easy:
562 newtype S a = MkS [a]
563 -- :CoS :: S ~ [] -- Eta-reduced
565 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
566 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
568 When type familes are involved it's trickier:
571 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
572 -- :RT is the representation type for (T Int a)
573 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
574 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
576 instance Eq [a] => Eq (T Int a) -- easy by coercion
577 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
579 The "???" bit is that we don't build the :CoF thing in eta-reduced form
580 Henc the current typeFamilyPapErr, even though the instance makes sense.
581 After all, we can write it out
582 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
587 mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type] -> Type
588 -> DerivContext -- Just => context supplied (standalone deriving)
589 -- Nothing => context inferred (deriving on data decl)
590 -> TcRn EarlyDerivSpec
591 -- Make the EarlyDerivSpec for an instance
592 -- forall tvs. theta => cls (tys ++ [ty])
593 -- where the 'theta' is optional (that's the Maybe part)
594 -- Assumes that this declaration is well-kinded
596 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
597 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
598 , isAlgTyCon tycon -- Check for functions, primitive types etc
599 = mk_alg_eqn tycon tc_args
601 = failWithTc (derivingThingErr False cls cls_tys tc_app
602 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
605 bale_out msg = failWithTc (derivingThingErr False cls cls_tys tc_app msg)
607 mk_alg_eqn tycon tc_args
608 | className cls `elem` typeableClassNames
609 = do { dflags <- getDOpts
610 ; case checkTypeableConditions (dflags, tycon) of
611 Just err -> bale_out err
612 Nothing -> mk_typeable_eqn orig tvs cls tycon tc_args mtheta }
614 | isDataFamilyTyCon tycon
615 , length tc_args /= tyConArity tycon
616 = bale_out (ptext (sLit "Unsaturated data family application"))
619 = do { (rep_tc, rep_tc_args) <- tcLookupDataFamInst tycon tc_args
620 -- Be careful to test rep_tc here: in the case of families,
621 -- we want to check the instance tycon, not the family tycon
623 -- For standalone deriving (mtheta /= Nothing),
624 -- check that all the data constructors are in scope.
625 ; rdr_env <- getGlobalRdrEnv
626 ; let hidden_data_cons = isAbstractTyCon rep_tc ||
627 any not_in_scope (tyConDataCons rep_tc)
628 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
629 ; unless (isNothing mtheta || not hidden_data_cons)
630 (bale_out (derivingHiddenErr tycon))
633 ; if isDataTyCon rep_tc then
634 mkDataTypeEqn orig dflags tvs cls cls_tys
635 tycon tc_args rep_tc rep_tc_args mtheta
637 mkNewTypeEqn orig dflags tvs cls cls_tys
638 tycon tc_args rep_tc rep_tc_args mtheta }
642 %************************************************************************
646 %************************************************************************
649 mkDataTypeEqn :: CtOrigin
651 -> [Var] -- Universally quantified type variables in the instance
652 -> Class -- Class for which we need to derive an instance
653 -> [Type] -- Other parameters to the class except the last
654 -> TyCon -- Type constructor for which the instance is requested
655 -- (last parameter to the type class)
656 -> [Type] -- Parameters to the type constructor
657 -> TyCon -- rep of the above (for type families)
658 -> [Type] -- rep of the above
659 -> DerivContext -- Context of the instance, for standalone deriving
660 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
662 mkDataTypeEqn orig dflags tvs cls cls_tys
663 tycon tc_args rep_tc rep_tc_args mtheta
664 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
665 -- NB: pass the *representation* tycon to checkSideConditions
666 CanDerive -> go_for_it
667 NonDerivableClass -> bale_out (nonStdErr cls)
668 DerivableClassError msg -> bale_out msg
670 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
671 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
673 mk_data_eqn :: CtOrigin -> [TyVar] -> Class
674 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
675 -> TcM EarlyDerivSpec
676 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
677 = do { dfun_name <- new_dfun_name cls tycon
679 ; let inst_tys = [mkTyConApp tycon tc_args]
680 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
681 spec = DS { ds_loc = loc, ds_orig = orig
682 , ds_name = dfun_name, ds_tvs = tvs
683 , ds_cls = cls, ds_tys = inst_tys
684 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
685 , ds_theta = mtheta `orElse` inferred_constraints
686 , ds_newtype = False }
688 ; return (if isJust mtheta then Right spec -- Specified context
689 else Left spec) } -- Infer context
691 ----------------------
692 mk_typeable_eqn :: CtOrigin -> [TyVar] -> Class
693 -> TyCon -> [TcType] -> DerivContext
694 -> TcM EarlyDerivSpec
695 mk_typeable_eqn orig tvs cls tycon tc_args mtheta
696 -- The Typeable class is special in several ways
697 -- data T a b = ... deriving( Typeable )
699 -- instance Typeable2 T where ...
701 -- 1. There are no constraints in the instance
702 -- 2. There are no type variables either
703 -- 3. The actual class we want to generate isn't necessarily
704 -- Typeable; it depends on the arity of the type
705 | isNothing mtheta -- deriving on a data type decl
706 = do { checkTc (cls `hasKey` typeableClassKey)
707 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
708 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
709 ; mk_typeable_eqn orig tvs real_cls tycon [] (Just []) }
711 | otherwise -- standaone deriving
712 = do { checkTc (null tc_args)
713 (ptext (sLit "Derived typeable instance must be of form (Typeable")
714 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
715 ; dfun_name <- new_dfun_name cls tycon
718 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
719 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
720 , ds_tc = tycon, ds_tc_args = []
721 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
723 ----------------------
724 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
725 -- Generate a sufficiently large set of constraints that typechecking the
726 -- generated method definitions should succeed. This set will be simplified
727 -- before being used in the instance declaration
728 inferConstraints _ cls inst_tys rep_tc rep_tc_args
729 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
730 stupid_constraints ++ extra_constraints
731 ++ sc_constraints ++ con_arg_constraints
733 -- Constraints arising from the arguments of each constructor
735 = [ mkClassPred cls [arg_ty]
736 | data_con <- tyConDataCons rep_tc,
737 arg_ty <- ASSERT( isVanillaDataCon data_con )
738 get_constrained_tys $
739 dataConInstOrigArgTys data_con all_rep_tc_args,
740 not (isUnLiftedType arg_ty) ]
741 -- No constraints for unlifted types
742 -- Where they are legal we generate specilised function calls
744 -- For functor-like classes, two things are different
745 -- (a) We recurse over argument types to generate constraints
746 -- See Functor examples in TcGenDeriv
747 -- (b) The rep_tc_args will be one short
748 is_functor_like = getUnique cls `elem` functorLikeClassKeys
750 get_constrained_tys :: [Type] -> [Type]
751 get_constrained_tys tys
752 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
755 rep_tc_tvs = tyConTyVars rep_tc
756 last_tv = last rep_tc_tvs
757 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
758 | otherwise = rep_tc_args
760 -- Constraints arising from superclasses
761 -- See Note [Superclasses of derived instance]
762 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
765 -- Stupid constraints
766 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
767 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
769 -- Extra Data constraints
770 -- The Data class (only) requires that for
771 -- instance (...) => Data (T t1 t2)
773 -- THEN (Data t1, Data t2) are among the (...) constraints
774 -- Reason: when the IF holds, we generate a method
775 -- dataCast2 f = gcast2 f
776 -- and we need the Data constraints to typecheck the method
778 | cls `hasKey` dataClassKey
779 , all (isLiftedTypeKind . typeKind) rep_tc_args
780 = [mkClassPred cls [ty] | ty <- rep_tc_args]
784 ------------------------------------------------------------------
785 -- Check side conditions that dis-allow derivability for particular classes
786 -- This is *apart* from the newtype-deriving mechanism
788 -- Here we get the representation tycon in case of family instances as it has
789 -- the data constructors - but we need to be careful to fall back to the
790 -- family tycon (with indexes) in error messages.
792 data DerivStatus = CanDerive
793 | DerivableClassError SDoc -- Standard class, but can't do it
794 | NonDerivableClass -- Non-standard class
796 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
797 checkSideConditions dflags mtheta cls cls_tys rep_tc
798 | Just cond <- sideConditions mtheta cls
799 = case (cond (dflags, rep_tc)) of
800 Just err -> DerivableClassError err -- Class-specific error
801 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
802 -- cls_tys (the type args other than last)
804 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
805 | otherwise = NonDerivableClass -- Not a standard class
807 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
809 checkTypeableConditions :: Condition
810 checkTypeableConditions = checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK
812 nonStdErr :: Class -> SDoc
813 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
815 sideConditions :: DerivContext -> Class -> Maybe Condition
816 sideConditions mtheta cls
817 | cls_key == eqClassKey = Just cond_std
818 | cls_key == ordClassKey = Just cond_std
819 | cls_key == showClassKey = Just cond_std
820 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
821 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
822 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
823 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
824 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
825 cond_std `andCond` cond_noUnliftedArgs)
826 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
827 cond_functorOK True) -- NB: no cond_std!
828 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
829 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
830 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
831 cond_functorOK False)
832 | otherwise = Nothing
834 cls_key = getUnique cls
835 cond_std = cond_stdOK mtheta
837 type Condition = (DynFlags, TyCon) -> Maybe SDoc
838 -- first Bool is whether or not we are allowed to derive Data and Typeable
839 -- second Bool is whether or not we are allowed to derive Functor
840 -- TyCon is the *representation* tycon if the
841 -- data type is an indexed one
844 orCond :: Condition -> Condition -> Condition
847 Nothing -> Nothing -- c1 succeeds
848 Just x -> case c2 tc of -- c1 fails
850 Just y -> Just (x $$ ptext (sLit " and") $$ y)
853 andCond :: Condition -> Condition -> Condition
854 andCond c1 c2 tc = case c1 tc of
855 Nothing -> c2 tc -- c1 succeeds
856 Just x -> Just x -- c1 fails
858 cond_stdOK :: DerivContext -> Condition
859 cond_stdOK (Just _) _
860 = Nothing -- Don't check these conservative conditions for
861 -- standalone deriving; just generate the code
862 -- and let the typechecker handle the result
863 cond_stdOK Nothing (_, rep_tc)
864 | null data_cons = Just (no_cons_why rep_tc $$ suggestion)
865 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
866 | otherwise = Nothing
868 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
869 data_cons = tyConDataCons rep_tc
870 con_whys = mapCatMaybes check_con data_cons
872 check_con :: DataCon -> Maybe SDoc
874 | isVanillaDataCon con
875 , all isTauTy (dataConOrigArgTys con) = Nothing
876 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
878 no_cons_why :: TyCon -> SDoc
879 no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
880 ptext (sLit "has no data constructors")
882 cond_enumOrProduct :: Condition
883 cond_enumOrProduct = cond_isEnumeration `orCond`
884 (cond_isProduct `andCond` cond_noUnliftedArgs)
886 cond_noUnliftedArgs :: Condition
887 -- For some classes (eg Eq, Ord) we allow unlifted arg types
888 -- by generating specilaised code. For others (eg Data) we don't.
889 cond_noUnliftedArgs (_, tc)
890 | null bad_cons = Nothing
891 | otherwise = Just why
893 bad_cons = [ con | con <- tyConDataCons tc
894 , any isUnLiftedType (dataConOrigArgTys con) ]
895 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
897 cond_isEnumeration :: Condition
898 cond_isEnumeration (_, rep_tc)
899 | isEnumerationTyCon rep_tc = Nothing
900 | otherwise = Just why
902 why = sep [ quotes (pprSourceTyCon rep_tc) <+>
903 ptext (sLit "is not an enumeration type")
904 , nest 2 $ ptext (sLit "(an enumeration consists of one or more nullary constructors)") ]
905 -- See Note [Enumeration types] in TyCon
907 cond_isProduct :: Condition
908 cond_isProduct (_, rep_tc)
909 | isProductTyCon rep_tc = Nothing
910 | otherwise = Just why
912 why = quotes (pprSourceTyCon rep_tc) <+>
913 ptext (sLit "does not have precisely one constructor")
915 cond_typeableOK :: Condition
916 -- OK for Typeable class
917 -- Currently: (a) args all of kind *
918 -- (b) 7 or fewer args
919 cond_typeableOK (_, tc)
920 | tyConArity tc > 7 = Just too_many
921 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tc))
923 | otherwise = Nothing
925 too_many = quotes (pprSourceTyCon tc) <+>
926 ptext (sLit "has too many arguments")
927 bad_kind = quotes (pprSourceTyCon tc) <+>
928 ptext (sLit "has arguments of kind other than `*'")
930 functorLikeClassKeys :: [Unique]
931 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
933 cond_functorOK :: Bool -> Condition
934 -- OK for Functor/Foldable/Traversable class
935 -- Currently: (a) at least one argument
936 -- (b) don't use argument contravariantly
937 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
938 -- (d) optionally: don't use function types
939 -- (e) no "stupid context" on data type
940 cond_functorOK allowFunctions (_, rep_tc)
942 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
943 <+> ptext (sLit "has no parameters"))
945 | not (null bad_stupid_theta)
946 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
947 <+> ptext (sLit "has a class context") <+> pprTheta bad_stupid_theta)
950 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
952 tc_tvs = tyConTyVars rep_tc
953 Just (_, last_tv) = snocView tc_tvs
954 bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
955 is_bad pred = last_tv `elemVarSet` tyVarsOfPred pred
957 data_cons = tyConDataCons rep_tc
958 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
960 check_vanilla :: DataCon -> Maybe SDoc
961 check_vanilla con | isVanillaDataCon con = Nothing
962 | otherwise = Just (badCon con existential)
964 ft_check :: DataCon -> FFoldType (Maybe SDoc)
965 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
966 , ft_co_var = Just (badCon con covariant)
967 , ft_fun = \x y -> if allowFunctions then x `mplus` y
968 else Just (badCon con functions)
969 , ft_tup = \_ xs -> msum xs
970 , ft_ty_app = \_ x -> x
971 , ft_bad_app = Just (badCon con wrong_arg)
972 , ft_forall = \_ x -> x }
974 existential = ptext (sLit "has existential arguments")
975 covariant = ptext (sLit "uses the type variable in a function argument")
976 functions = ptext (sLit "contains function types")
977 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
979 checkFlag :: ExtensionFlag -> Condition
980 checkFlag flag (dflags, _)
981 | xopt flag dflags = Nothing
982 | otherwise = Just why
984 why = ptext (sLit "You need -X") <> text flag_str
985 <+> ptext (sLit "to derive an instance for this class")
986 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
988 other -> pprPanic "checkFlag" (ppr other)
990 std_class_via_iso :: Class -> Bool
991 -- These standard classes can be derived for a newtype
992 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
993 -- because giving so gives the same results as generating the boilerplate
994 std_class_via_iso clas
995 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
996 -- Not Read/Show because they respect the type
997 -- Not Enum, because newtypes are never in Enum
1000 non_iso_class :: Class -> Bool
1001 -- *Never* derive Read,Show,Typeable,Data by isomorphism,
1002 -- even with -XGeneralizedNewtypeDeriving
1004 = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
1007 typeableClassKeys :: [Unique]
1008 typeableClassKeys = map getUnique typeableClassNames
1010 new_dfun_name :: Class -> TyCon -> TcM Name
1011 new_dfun_name clas tycon -- Just a simple wrapper
1012 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1013 ; newDFunName clas [mkTyConApp tycon []] loc }
1014 -- The type passed to newDFunName is only used to generate
1015 -- a suitable string; hence the empty type arg list
1017 badCon :: DataCon -> SDoc -> SDoc
1018 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1021 Note [Superclasses of derived instance]
1022 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1023 In general, a derived instance decl needs the superclasses of the derived
1024 class too. So if we have
1025 data T a = ...deriving( Ord )
1026 then the initial context for Ord (T a) should include Eq (T a). Often this is
1027 redundant; we'll also generate an Ord constraint for each constructor argument,
1028 and that will probably generate enough constraints to make the Eq (T a) constraint
1029 be satisfied too. But not always; consider:
1035 data T a = MkT (S a) deriving( Ord )
1036 instance Num a => Eq (T a)
1038 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1040 data T a = MkT deriving( Data, Typeable )
1041 Here there *is* no argument field, but we must nevertheless generate
1042 a context for the Data instances:
1043 instance Typable a => Data (T a) where ...
1046 %************************************************************************
1050 %************************************************************************
1053 mkNewTypeEqn :: CtOrigin -> DynFlags -> [Var] -> Class
1054 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1056 -> TcRn EarlyDerivSpec
1057 mkNewTypeEqn orig dflags tvs
1058 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1059 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1060 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1061 = do { traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
1062 ; dfun_name <- new_dfun_name cls tycon
1063 ; loc <- getSrcSpanM
1064 ; let spec = DS { ds_loc = loc, ds_orig = orig
1065 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1066 , ds_cls = cls, ds_tys = inst_tys
1067 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1068 , ds_theta = mtheta `orElse` all_preds
1069 , ds_newtype = True }
1070 ; return (if isJust mtheta then Right spec
1074 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1075 CanDerive -> go_for_it -- Use the standard H98 method
1076 DerivableClassError msg -- Error with standard class
1077 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1078 | otherwise -> bale_out msg
1079 NonDerivableClass -- Must use newtype deriving
1080 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1081 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1082 | otherwise -> bale_out non_std
1084 newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
1085 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1086 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1088 non_std = nonStdErr cls
1089 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1091 -- Here is the plan for newtype derivings. We see
1092 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1093 -- where t is a type,
1094 -- ak+1...an is a suffix of a1..an, and are all tyars
1095 -- ak+1...an do not occur free in t, nor in the s1..sm
1096 -- (C s1 ... sm) is a *partial applications* of class C
1097 -- with the last parameter missing
1098 -- (T a1 .. ak) matches the kind of C's last argument
1099 -- (and hence so does t)
1100 -- The latter kind-check has been done by deriveTyData already,
1101 -- and tc_args are already trimmed
1103 -- We generate the instance
1104 -- instance forall ({a1..ak} u fvs(s1..sm)).
1105 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1106 -- where T a1...ap is the partial application of
1107 -- the LHS of the correct kind and p >= k
1109 -- NB: the variables below are:
1110 -- tc_tvs = [a1, ..., an]
1111 -- tyvars_to_keep = [a1, ..., ak]
1112 -- rep_ty = t ak .. an
1113 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1114 -- tys = [s1, ..., sm]
1117 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1118 -- We generate the instance
1119 -- instance Monad (ST s) => Monad (T s) where
1121 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1122 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1123 -- eta-reduces the representation type, so we know that
1125 -- That's convenient here, because we may have to apply
1126 -- it to fewer than its original complement of arguments
1128 -- Note [Newtype representation]
1129 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1130 -- Need newTyConRhs (*not* a recursive representation finder)
1131 -- to get the representation type. For example
1132 -- newtype B = MkB Int
1133 -- newtype A = MkA B deriving( Num )
1134 -- We want the Num instance of B, *not* the Num instance of Int,
1135 -- when making the Num instance of A!
1136 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1137 rep_tys = cls_tys ++ [rep_inst_ty]
1138 rep_pred = mkClassPred cls rep_tys
1139 -- rep_pred is the representation dictionary, from where
1140 -- we are gong to get all the methods for the newtype
1144 -- Next we figure out what superclass dictionaries to use
1145 -- See Note [Newtype deriving superclasses] above
1147 cls_tyvars = classTyVars cls
1148 dfun_tvs = tyVarsOfTypes inst_tys
1149 inst_ty = mkTyConApp tycon tc_args
1150 inst_tys = cls_tys ++ [inst_ty]
1151 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1154 -- If there are no tyvars, there's no need
1155 -- to abstract over the dictionaries we need
1156 -- Example: newtype T = MkT Int deriving( C )
1157 -- We get the derived instance
1160 -- instance C Int => C T
1161 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1163 -------------------------------------------------------------------
1164 -- Figuring out whether we can only do this newtype-deriving thing
1166 can_derive_via_isomorphism
1167 = not (non_iso_class cls)
1171 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1173 arity_ok = length cls_tys + 1 == classArity cls
1174 -- Well kinded; eg not: newtype T ... deriving( ST )
1175 -- because ST needs *2* type params
1177 -- Check that eta reduction is OK
1178 eta_ok = nt_eta_arity <= length rep_tc_args
1179 -- The newtype can be eta-reduced to match the number
1180 -- of type argument actually supplied
1181 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1182 -- Here the 'b' must be the same in the rep type (S [a] b)
1183 -- And the [a] must not mention 'b'. That's all handled
1186 ats_ok = null (classATs cls)
1187 -- No associated types for the class, because we don't
1188 -- currently generate type 'instance' decls; and cannot do
1189 -- so for 'data' instance decls
1192 = vcat [ ppUnless arity_ok arity_msg
1193 , ppUnless eta_ok eta_msg
1194 , ppUnless ats_ok ats_msg ]
1195 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1196 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1197 ats_msg = ptext (sLit "the class has associated types")
1200 Note [Recursive newtypes]
1201 ~~~~~~~~~~~~~~~~~~~~~~~~~
1202 Newtype deriving works fine, even if the newtype is recursive.
1203 e.g. newtype S1 = S1 [T1 ()]
1204 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1205 Remember, too, that type families are curretly (conservatively) given
1206 a recursive flag, so this also allows newtype deriving to work
1209 We used to exclude recursive types, because we had a rather simple
1210 minded way of generating the instance decl:
1212 instance Eq [A] => Eq A -- Makes typechecker loop!
1213 But now we require a simple context, so it's ok.
1216 %************************************************************************
1218 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1220 %************************************************************************
1222 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1223 terms, which is the final correct RHS for the corresponding original
1227 Each (k,TyVarTy tv) in a solution constrains only a type
1231 The (k,TyVarTy tv) pairs in a solution are canonically
1232 ordered by sorting on type varible, tv, (major key) and then class, k,
1237 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1239 inferInstanceContexts _ [] = return []
1241 inferInstanceContexts oflag infer_specs
1242 = do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
1243 ; iterate_deriv 1 initial_solutions }
1245 ------------------------------------------------------------------
1246 -- The initial solutions for the equations claim that each
1247 -- instance has an empty context; this solution is certainly
1248 -- in canonical form.
1249 initial_solutions :: [ThetaType]
1250 initial_solutions = [ [] | _ <- infer_specs ]
1252 ------------------------------------------------------------------
1253 -- iterate_deriv calculates the next batch of solutions,
1254 -- compares it with the current one; finishes if they are the
1255 -- same, otherwise recurses with the new solutions.
1256 -- It fails if any iteration fails
1257 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1258 iterate_deriv n current_solns
1259 | n > 20 -- Looks as if we are in an infinite loop
1260 -- This can happen if we have -XUndecidableInstances
1261 -- (See TcSimplify.tcSimplifyDeriv.)
1262 = pprPanic "solveDerivEqns: probable loop"
1263 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1265 = do { -- Extend the inst info from the explicit instance decls
1266 -- with the current set of solutions, and simplify each RHS
1267 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1268 current_solns infer_specs
1269 ; new_solns <- checkNoErrs $
1270 extendLocalInstEnv inst_specs $
1271 mapM gen_soln infer_specs
1273 ; if (current_solns == new_solns) then
1274 return [ spec { ds_theta = soln }
1275 | (spec, soln) <- zip infer_specs current_solns ]
1277 iterate_deriv (n+1) new_solns }
1279 ------------------------------------------------------------------
1280 gen_soln :: DerivSpec -> TcM [PredType]
1281 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1282 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1284 addErrCtxt (derivInstCtxt clas inst_tys) $
1285 do { -- Check for a bizarre corner case, when the derived instance decl should
1286 -- have form instance C a b => D (T a) where ...
1287 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1288 -- of problems; in particular, it's hard to compare solutions for
1289 -- equality when finding the fixpoint. Moreover, simplifyDeriv
1290 -- has an assert failure because it finds a TyVar when it expects
1291 -- only TcTyVars. So I just rule it out for now. I'm not
1292 -- even sure how it can arise.
1294 ; let tv_set = mkVarSet tyvars
1295 weird_preds = [pred | pred <- deriv_rhs
1296 , not (tyVarsOfPred pred `subVarSet` tv_set)]
1297 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1299 ; theta <- simplifyDeriv orig tyvars deriv_rhs
1300 -- checkValidInstance tyvars theta clas inst_tys
1301 -- Not necessary; see Note [Exotic derived instance contexts]
1304 ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
1305 -- Claim: the result instance declaration is guaranteed valid
1306 -- Hence no need to call:
1307 -- checkValidInstance tyvars theta clas inst_tys
1308 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1310 ------------------------------------------------------------------
1311 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1312 mkInstance overlap_flag theta
1313 (DS { ds_name = dfun_name
1314 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1315 = mkLocalInstance dfun overlap_flag
1317 dfun = mkDictFunId dfun_name tyvars theta clas tys
1320 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1321 -- Add new locally-defined instances; don't bother to check
1322 -- for functional dependency errors -- that'll happen in TcInstDcls
1323 extendLocalInstEnv dfuns thing_inside
1324 = do { env <- getGblEnv
1325 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1326 env' = env { tcg_inst_env = inst_env' }
1327 ; setGblEnv env' thing_inside }
1331 %************************************************************************
1333 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1335 %************************************************************************
1337 After all the trouble to figure out the required context for the
1338 derived instance declarations, all that's left is to chug along to
1339 produce them. They will then be shoved into @tcInstDecls2@, which
1340 will do all its usual business.
1342 There are lots of possibilities for code to generate. Here are
1343 various general remarks.
1348 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1349 ``you-couldn't-do-better-by-hand'' efficient.
1352 Deriving @Show@---also pretty common--- should also be reasonable good code.
1355 Deriving for the other classes isn't that common or that big a deal.
1362 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1365 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1368 We {\em normally} generate code only for the non-defaulted methods;
1369 there are some exceptions for @Eq@ and (especially) @Ord@...
1372 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1373 constructor's numeric (@Int#@) tag. These are generated by
1374 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1375 these is around is given by @hasCon2TagFun@.
1377 The examples under the different sections below will make this
1381 Much less often (really just for deriving @Ix@), we use a
1382 @_tag2con_<tycon>@ function. See the examples.
1385 We use the renamer!!! Reason: we're supposed to be
1386 producing @LHsBinds Name@ for the methods, but that means
1387 producing correctly-uniquified code on the fly. This is entirely
1388 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1389 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1390 the renamer. What a great hack!
1394 -- Generate the InstInfo for the required instance paired with the
1395 -- *representation* tycon for that instance,
1396 -- plus any auxiliary bindings required
1398 -- Representation tycons differ from the tycon in the instance signature in
1399 -- case of instances for indexed families.
1401 genInst :: Bool -- True <=> standalone deriving
1403 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1404 genInst standalone_deriv oflag
1405 spec@(DS { ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1406 , ds_theta = theta, ds_newtype = is_newtype
1407 , ds_name = name, ds_cls = clas })
1409 = return (InstInfo { iSpec = inst_spec
1410 , iBinds = NewTypeDerived co rep_tycon }, [])
1413 = do { fix_env <- getFixityEnv
1414 ; let loc = getSrcSpan name
1415 (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1416 -- In case of a family instance, we need to use the representation
1417 -- tycon (after all, it has the data constructors)
1419 ; return (InstInfo { iSpec = inst_spec
1420 , iBinds = VanillaInst meth_binds [] standalone_deriv }
1423 inst_spec = mkInstance oflag theta spec
1424 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1425 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1427 -- Not a family => rep_tycon = main tycon
1428 co2 = case newTyConCo_maybe rep_tycon of
1429 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1430 Nothing -> id_co -- The newtype is transparent; no need for a cast
1431 co = co1 `mkTransCoI` co2
1432 id_co = IdCo (mkTyConApp rep_tycon rep_tc_args)
1434 -- Example: newtype instance N [a] = N1 (Tree a)
1435 -- deriving instance Eq b => Eq (N [(b,b)])
1436 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1437 -- When dealing with the deriving clause
1438 -- co1 : N [(b,b)] ~ R1:N (b,b)
1439 -- co2 : R1:N (b,b) ~ Tree (b,b)
1440 -- co : N [(b,b)] ~ Tree (b,b)
1442 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1443 genDerivBinds loc fix_env clas tycon
1444 | className clas `elem` typeableClassNames
1445 = (gen_Typeable_binds loc tycon, [])
1448 = case assocMaybe gen_list (getUnique clas) of
1449 Just gen_fn -> gen_fn loc tycon
1450 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1452 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1453 gen_list = [(eqClassKey, gen_Eq_binds)
1454 ,(ordClassKey, gen_Ord_binds)
1455 ,(enumClassKey, gen_Enum_binds)
1456 ,(boundedClassKey, gen_Bounded_binds)
1457 ,(ixClassKey, gen_Ix_binds)
1458 ,(showClassKey, gen_Show_binds fix_env)
1459 ,(readClassKey, gen_Read_binds fix_env)
1460 ,(dataClassKey, gen_Data_binds)
1461 ,(functorClassKey, gen_Functor_binds)
1462 ,(foldableClassKey, gen_Foldable_binds)
1463 ,(traversableClassKey, gen_Traversable_binds)
1468 %************************************************************************
1470 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1472 %************************************************************************
1475 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1476 derivingKindErr tc cls cls_tys cls_kind
1477 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1478 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1479 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1480 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1482 derivingEtaErr :: Class -> [Type] -> Type -> Message
1483 derivingEtaErr cls cls_tys inst_ty
1484 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1485 nest 2 (ptext (sLit "instance (...) =>")
1486 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1488 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1489 typeFamilyPapErr tc cls cls_tys inst_ty
1490 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1491 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1493 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1494 derivingThingErr newtype_deriving clas tys ty why
1495 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1496 2 (quotes (ppr pred))
1497 $$ nest 2 extra) <> colon,
1500 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1502 pred = mkClassPred clas (tys ++ [ty])
1504 derivingHiddenErr :: TyCon -> SDoc
1505 derivingHiddenErr tc
1506 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1507 2 (ptext (sLit "so you cannot derive an instance for it"))
1509 standaloneCtxt :: LHsType Name -> SDoc
1510 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1513 derivInstCtxt :: Class -> [Type] -> Message
1514 derivInstCtxt clas inst_tys
1515 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1517 badDerivedPred :: PredType -> Message
1519 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1520 ptext (sLit "type variables that are not data type parameters"),
1521 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]