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
72 -- DerivSpec is purely local to this module
73 data DerivSpec = DS { ds_loc :: SrcSpan
74 , ds_orig :: InstOrigin
77 , ds_theta :: ThetaType
81 , ds_tc_args :: [Type]
82 , ds_newtype :: Bool }
83 -- This spec implies a dfun declaration of the form
84 -- df :: forall tvs. theta => C tys
85 -- The Name is the name for the DFun we'll build
86 -- The tyvars bind all the variables in the theta
87 -- For family indexes, the tycon in
88 -- in ds_tys is the *family* tycon
89 -- in ds_tc, ds_tc_args is the *representation* tycon
90 -- For non-family tycons, both are the same
92 -- ds_newtype = True <=> Newtype deriving
93 -- False <=> Vanilla deriving
95 type DerivContext = Maybe ThetaType
96 -- Nothing <=> Vanilla deriving; infer the context of the instance decl
97 -- Just theta <=> Standalone deriving: context supplied by programmer
99 type EarlyDerivSpec = Either DerivSpec DerivSpec
100 -- Left ds => the context for the instance should be inferred
101 -- In this case ds_theta is the list of all the
102 -- constraints needed, such as (Eq [a], Eq a)
103 -- The inference process is to reduce this to a
104 -- simpler form (e.g. Eq a)
106 -- Right ds => the exact context for the instance is supplied
107 -- by the programmer; it is ds_theta
109 pprDerivSpec :: DerivSpec -> SDoc
110 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
111 ds_cls = c, ds_tys = tys, ds_theta = rhs })
112 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
113 <+> equals <+> ppr rhs)
117 Inferring missing contexts
118 ~~~~~~~~~~~~~~~~~~~~~~~~~~
121 data T a b = C1 (Foo a) (Bar b)
126 [NOTE: See end of these comments for what to do with
127 data (C a, D b) => T a b = ...
130 We want to come up with an instance declaration of the form
132 instance (Ping a, Pong b, ...) => Eq (T a b) where
135 It is pretty easy, albeit tedious, to fill in the code "...". The
136 trick is to figure out what the context for the instance decl is,
137 namely @Ping@, @Pong@ and friends.
139 Let's call the context reqd for the T instance of class C at types
140 (a,b, ...) C (T a b). Thus:
142 Eq (T a b) = (Ping a, Pong b, ...)
144 Now we can get a (recursive) equation from the @data@ decl:
146 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
147 u Eq (T b a) u Eq Int -- From C2
148 u Eq (T a a) -- From C3
150 Foo and Bar may have explicit instances for @Eq@, in which case we can
151 just substitute for them. Alternatively, either or both may have
152 their @Eq@ instances given by @deriving@ clauses, in which case they
153 form part of the system of equations.
155 Now all we need do is simplify and solve the equations, iterating to
156 find the least fixpoint. Notice that the order of the arguments can
157 switch around, as here in the recursive calls to T.
159 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
163 Eq (T a b) = {} -- The empty set
166 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
167 u Eq (T b a) u Eq Int -- From C2
168 u Eq (T a a) -- From C3
170 After simplification:
171 = Eq a u Ping b u {} u {} u {}
176 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
177 u Eq (T b a) u Eq Int -- From C2
178 u Eq (T a a) -- From C3
180 After simplification:
185 = Eq a u Ping b u Eq b u Ping a
187 The next iteration gives the same result, so this is the fixpoint. We
188 need to make a canonical form of the RHS to ensure convergence. We do
189 this by simplifying the RHS to a form in which
191 - the classes constrain only tyvars
192 - the list is sorted by tyvar (major key) and then class (minor key)
193 - no duplicates, of course
195 So, here are the synonyms for the ``equation'' structures:
198 Note [Data decl contexts]
199 ~~~~~~~~~~~~~~~~~~~~~~~~~
202 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
204 We will need an instance decl like:
206 instance (Read a, RealFloat a) => Read (Complex a) where
209 The RealFloat in the context is because the read method for Complex is bound
210 to construct a Complex, and doing that requires that the argument type is
213 But this ain't true for Show, Eq, Ord, etc, since they don't construct
214 a Complex; they only take them apart.
216 Our approach: identify the offending classes, and add the data type
217 context to the instance decl. The "offending classes" are
221 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
222 pattern matching against a constructor from a data type with a context
223 gives rise to the constraints for that context -- or at least the thinned
224 version. So now all classes are "offending".
226 Note [Newtype deriving]
227 ~~~~~~~~~~~~~~~~~~~~~~~
231 newtype T = T Char deriving( C [a] )
233 Notice the free 'a' in the deriving. We have to fill this out to
234 newtype T = T Char deriving( forall a. C [a] )
236 And then translate it to:
237 instance C [a] Char => C [a] T where ...
240 Note [Newtype deriving superclasses]
241 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
242 (See also Trac #1220 for an interesting exchange on newtype
243 deriving and superclasses.)
245 The 'tys' here come from the partial application in the deriving
246 clause. The last arg is the new instance type.
248 We must pass the superclasses; the newtype might be an instance
249 of them in a different way than the representation type
250 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
251 Then the Show instance is not done via isomorphism; it shows
253 The Num instance is derived via isomorphism, but the Show superclass
254 dictionary must the Show instance for Foo, *not* the Show dictionary
255 gotten from the Num dictionary. So we must build a whole new dictionary
256 not just use the Num one. The instance we want is something like:
257 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
260 There may be a coercion needed which we get from the tycon for the newtype
261 when the dict is constructed in TcInstDcls.tcInstDecl2
264 Note [Unused constructors and deriving clauses]
265 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
266 See Trac #3221. Consider
267 data T = T1 | T2 deriving( Show )
268 Are T1 and T2 unused? Well, no: the deriving clause expands to mention
269 both of them. So we gather defs/uses from deriving just like anything else.
271 %************************************************************************
273 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
275 %************************************************************************
278 tcDeriving :: [LTyClDecl Name] -- All type constructors
279 -> [LInstDecl Name] -- All instance declarations
280 -> [LDerivDecl Name] -- All stand-alone deriving declarations
281 -> TcM ([InstInfo Name], -- The generated "instance decls"
282 HsValBinds Name, -- Extra generated top-level bindings
285 tcDeriving tycl_decls inst_decls deriv_decls
286 = recoverM (return ([], emptyValBindsOut, emptyDUs)) $
287 do { -- Fish the "deriving"-related information out of the TcEnv
288 -- And make the necessary "equations".
289 is_boot <- tcIsHsBoot
290 ; traceTc (text "tcDeriving" <+> ppr is_boot)
291 ; early_specs <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
293 ; overlap_flag <- getOverlapFlag
294 ; let (infer_specs, given_specs) = splitEithers early_specs
295 ; insts1 <- mapM (genInst True overlap_flag) given_specs
297 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
298 inferInstanceContexts overlap_flag infer_specs
300 ; insts2 <- mapM (genInst False overlap_flag) final_specs
302 -- Generate the generic to/from functions from each type declaration
303 ; gen_binds <- mkGenericBinds is_boot tycl_decls
304 ; (inst_info, rn_binds, rn_dus) <- renameDeriv is_boot gen_binds (insts1 ++ insts2)
307 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
308 (ddump_deriving inst_info rn_binds))
310 ; return (inst_info, rn_binds, rn_dus) }
312 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
313 ddump_deriving inst_infos extra_binds
314 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
316 renameDeriv :: Bool -> LHsBinds RdrName
317 -> [(InstInfo RdrName, DerivAuxBinds)]
318 -> TcM ([InstInfo Name], HsValBinds Name, DefUses)
319 renameDeriv is_boot gen_binds insts
320 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
321 -- The inst-info bindings will all be empty, but it's easier to
322 -- just use rn_inst_info to change the type appropriately
323 = do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
324 ; return (rn_inst_infos, emptyValBindsOut, usesOnly (plusFVs fvs)) }
327 = discardWarnings $ -- Discard warnings about unused bindings etc
328 do { (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $ -- Type signatures in patterns
329 -- are used in the generic binds
330 rnTopBinds (ValBindsIn gen_binds [])
331 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
333 -- Generate and rename any extra not-one-inst-decl-specific binds,
334 -- notably "con2tag" and/or "tag2con" functions.
335 -- Bring those names into scope before renaming the instances themselves
336 ; loc <- getSrcSpanM -- Generic loc for shared bindings
337 ; let aux_binds = listToBag $ map (genAuxBind loc) $
338 rm_dups [] $ concat deriv_aux_binds
339 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv (ValBindsIn aux_binds [])
340 ; let aux_names = collectHsValBinders rn_aux_lhs
342 ; bindLocalNames aux_names $
343 do { (rn_aux, dus_aux) <- rnTopBindsRHS (mkNameSet aux_names) rn_aux_lhs
344 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
345 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
346 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
349 (inst_infos, deriv_aux_binds) = unzip insts
351 -- Remove duplicate requests for auxilliary bindings
353 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
354 | otherwise = rm_dups (b:acc) bs
357 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
358 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
359 = return ( info { iBinds = NewTypeDerived coi tc }
360 , mkFVs (map dataConName (tyConDataCons tc)))
361 -- See Note [Newtype deriving and unused constructors]
363 rn_inst_info (InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
364 = -- Bring the right type variables into
365 -- scope (yuk), and rename the method binds
367 bindLocalNames (map Var.varName tyvars) $
368 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
369 ; let binds' = VanillaInst rn_binds [] standalone_deriv
370 ; return (InstInfo { iSpec = inst, iBinds = binds' }, fvs) }
372 (tyvars,_, clas,_) = instanceHead inst
373 clas_nm = className clas
375 -----------------------------------------
376 mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
377 mkGenericBinds is_boot tycl_decls
381 = do { tcs <- mapM tcLookupTyCon [ tcdName d
382 | L _ d <- tycl_decls, isDataDecl d ]
383 ; return (unionManyBags [ mkTyConGenericBinds tc
384 | tc <- tcs, tyConHasGenerics tc ]) }
385 -- We are only interested in the data type declarations,
386 -- and then only in the ones whose 'has-generics' flag is on
387 -- The predicate tyConHasGenerics finds both of these
390 Note [Newtype deriving and unused constructors]
391 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
392 Consider this (see Trac #1954):
395 newtype P a = MkP (IO a) deriving Monad
397 If you compile with -fwarn-unused-binds you do not expect the warning
398 "Defined but not used: data consructor MkP". Yet the newtype deriving
399 code does not explicitly mention MkP, but it should behave as if you
401 instance Monad P where
402 return x = MkP (return x)
405 So we want to signal a user of the data constructor 'MkP'. That's
406 what we do in rn_inst_info, and it's the only reason we have the TyCon
407 stored in NewTypeDerived.
410 %************************************************************************
412 From HsSyn to DerivSpec
414 %************************************************************************
416 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
419 makeDerivSpecs :: Bool
423 -> TcM [EarlyDerivSpec]
425 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
426 | is_boot -- No 'deriving' at all in hs-boot files
427 = do { mapM_ add_deriv_err deriv_locs
430 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
431 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
432 ; return (eqns1 ++ eqns2) }
434 extractTyDataPreds decls
435 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
437 all_tydata :: [(LHsType Name, LTyClDecl Name)]
438 -- Derived predicate paired with its data type declaration
439 all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
441 deriv_locs = map (getLoc . snd) all_tydata
442 ++ map getLoc deriv_decls
444 add_deriv_err loc = setSrcSpan loc $
445 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
446 2 (ptext (sLit "Use an instance declaration instead")))
448 ------------------------------------------------------------------
449 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
450 -- Standalone deriving declarations
451 -- e.g. deriving instance Show a => Show (T a)
452 -- Rather like tcLocalInstDecl
453 deriveStandalone (L loc (DerivDecl deriv_ty))
455 addErrCtxt (standaloneCtxt deriv_ty) $
456 do { traceTc (text "standalone deriving decl for" <+> ppr deriv_ty)
457 ; (tvs, theta, tau) <- tcHsInstHead deriv_ty
458 ; traceTc (text "standalone deriving;"
459 <+> text "tvs:" <+> ppr tvs
460 <+> text "theta:" <+> ppr theta
461 <+> text "tau:" <+> ppr tau)
462 ; (cls, inst_tys) <- checkValidInstance deriv_ty tvs theta tau
463 -- C.f. TcInstDcls.tcLocalInstDecl1
465 ; let cls_tys = take (length inst_tys - 1) inst_tys
466 inst_ty = last inst_tys
467 ; traceTc (text "standalone deriving;"
468 <+> text "class:" <+> ppr cls
469 <+> text "class types:" <+> ppr cls_tys
470 <+> text "type:" <+> ppr inst_ty)
471 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
474 ------------------------------------------------------------------
475 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
476 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
477 tcdTyVars = tv_names,
478 tcdTyPats = ty_pats }))
479 = setSrcSpan loc $ -- Use the location of the 'deriving' item
481 do { (tvs, tc, tc_args) <- get_lhs ty_pats
482 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
483 -- the type variables for the type constructor
485 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
486 -- The "deriv_pred" is a LHsType to take account of the fact that for
487 -- newtype deriving we allow deriving (forall a. C [a]).
489 -- Given data T a b c = ... deriving( C d ),
490 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
491 ; let cls_tyvars = classTyVars cls
492 kind = tyVarKind (last cls_tyvars)
493 (arg_kinds, _) = splitKindFunTys kind
494 n_args_to_drop = length arg_kinds
495 n_args_to_keep = tyConArity tc - n_args_to_drop
496 args_to_drop = drop n_args_to_keep tc_args
497 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
498 inst_ty_kind = typeKind inst_ty
499 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
500 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
501 `minusVarSet` dropped_tvs
503 -- Check that the result really is well-kinded
504 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
505 (derivingKindErr tc cls cls_tys kind)
507 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
508 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
509 (derivingEtaErr cls cls_tys inst_ty)
511 -- (a) The data type can be eta-reduced; eg reject:
512 -- data instance T a a = ... deriving( Monad )
513 -- (b) The type class args do not mention any of the dropped type
515 -- newtype T a s = ... deriving( ST s )
517 -- Type families can't be partially applied
518 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
519 -- Note [Deriving, type families, and partial applications]
520 ; checkTc (not (isOpenTyCon tc) || n_args_to_drop == 0)
521 (typeFamilyPapErr tc cls cls_tys inst_ty)
523 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
525 -- Tiresomely we must figure out the "lhs", which is awkward for type families
526 -- E.g. data T a b = .. deriving( Eq )
527 -- Here, the lhs is (T a b)
528 -- data instance TF Int b = ... deriving( Eq )
529 -- Here, the lhs is (TF Int b)
530 -- But if we just look up the tycon_name, we get is the *family*
531 -- tycon, but not pattern types -- they are in the *rep* tycon.
532 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
533 ; let tvs = tyConTyVars tc
534 ; return (tvs, tc, mkTyVarTys tvs) }
535 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
536 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
537 ; let (tc, tc_args) = tcSplitTyConApp tc_app
538 ; return (tvs, tc, tc_args) }
541 = panic "derivTyData" -- Caller ensures that only TyData can happen
544 Note [Deriving, type families, and partial applications]
545 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
546 When there are no type families, it's quite easy:
548 newtype S a = MkS [a]
549 -- :CoS :: S ~ [] -- Eta-reduced
551 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (coMkS a)) : Eq [a] ~ Eq (S a)
552 instance Monad [] => Monad S -- by coercion sym (Monad coMkS) : Monad [] ~ Monad S
554 When type familes are involved it's trickier:
557 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
558 -- :RT is the representation type for (T Int a)
559 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
560 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
562 instance Eq [a] => Eq (T Int a) -- easy by coercion
563 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
565 The "???" bit is that we don't build the :CoF thing in eta-reduced form
566 Henc the current typeFamilyPapErr, even though the instance makes sense.
567 After all, we can write it out
568 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
573 mkEqnHelp :: InstOrigin -> [TyVar] -> Class -> [Type] -> Type
574 -> DerivContext -- Just => context supplied (standalone deriving)
575 -- Nothing => context inferred (deriving on data decl)
576 -> TcRn EarlyDerivSpec
577 -- Make the EarlyDerivSpec for an instance
578 -- forall tvs. theta => cls (tys ++ [ty])
579 -- where the 'theta' is optional (that's the Maybe part)
580 -- Assumes that this declaration is well-kinded
582 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
583 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
584 , isAlgTyCon tycon -- Check for functions, primitive types etc
585 = do { (rep_tc, rep_tc_args) <- tcLookupFamInstExact tycon tc_args
586 -- Be careful to test rep_tc here: in the case of families,
587 -- we want to check the instance tycon, not the family tycon
589 -- For standalone deriving (mtheta /= Nothing),
590 -- check that all the data constructors are in scope.
591 -- No need for this when deriving Typeable, becuase we don't need
592 -- the constructors for that.
593 ; rdr_env <- getGlobalRdrEnv
594 ; let hidden_data_cons = isAbstractTyCon rep_tc || any not_in_scope (tyConDataCons rep_tc)
595 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
596 ; checkTc (isNothing mtheta ||
597 not hidden_data_cons ||
598 className cls `elem` typeableClassNames)
599 (derivingHiddenErr tycon)
602 ; if isDataTyCon rep_tc then
603 mkDataTypeEqn orig dflags tvs cls cls_tys
604 tycon tc_args rep_tc rep_tc_args mtheta
606 mkNewTypeEqn orig dflags tvs cls cls_tys
607 tycon tc_args rep_tc rep_tc_args mtheta }
609 = failWithTc (derivingThingErr False cls cls_tys tc_app
610 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
613 Note [Looking up family instances for deriving]
614 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
615 tcLookupFamInstExact is an auxiliary lookup wrapper which requires
616 that looked-up family instances exist. If called with a vanilla
617 tycon, the old type application is simply returned.
620 data instance F () = ... deriving Eq
621 data instance F () = ... deriving Eq
622 then tcLookupFamInstExact will be confused by the two matches;
623 but that can't happen because tcInstDecls1 doesn't call tcDeriving
624 if there are any overlaps.
626 There are two other things that might go wrong with the lookup.
627 First, we might see a standalone deriving clause
629 when there is no data instance F () in scope.
631 Note that it's OK to have
632 data instance F [a] = ...
633 deriving Eq (F [(a,b)])
634 where the match is not exact; the same holds for ordinary data types
635 with standalone deriving declrations.
638 tcLookupFamInstExact :: TyCon -> [Type] -> TcM (TyCon, [Type])
639 tcLookupFamInstExact tycon tys
640 | not (isOpenTyCon tycon)
641 = return (tycon, tys)
643 = do { maybeFamInst <- tcLookupFamInst tycon tys
644 ; case maybeFamInst of
645 Nothing -> famInstNotFound tycon tys
646 Just famInst -> return famInst
649 famInstNotFound :: TyCon -> [Type] -> TcM a
650 famInstNotFound tycon tys
651 = failWithTc (ptext (sLit "No family instance for")
652 <+> quotes (pprTypeApp tycon tys))
656 %************************************************************************
660 %************************************************************************
663 mkDataTypeEqn :: InstOrigin
665 -> [Var] -- Universally quantified type variables in the instance
666 -> Class -- Class for which we need to derive an instance
667 -> [Type] -- Other parameters to the class except the last
668 -> TyCon -- Type constructor for which the instance is requested
669 -- (last parameter to the type class)
670 -> [Type] -- Parameters to the type constructor
671 -> TyCon -- rep of the above (for type families)
672 -> [Type] -- rep of the above
673 -> DerivContext -- Context of the instance, for standalone deriving
674 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
676 mkDataTypeEqn orig dflags tvs cls cls_tys
677 tycon tc_args rep_tc rep_tc_args mtheta
678 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
679 -- NB: pass the *representation* tycon to checkSideConditions
680 CanDerive -> go_for_it
681 NonDerivableClass -> bale_out (nonStdErr cls)
682 DerivableClassError msg -> bale_out msg
684 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
685 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
687 mk_data_eqn, mk_typeable_eqn
688 :: InstOrigin -> [TyVar] -> Class
689 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
690 -> TcM EarlyDerivSpec
691 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
692 | getName cls `elem` typeableClassNames
693 = mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
696 = do { dfun_name <- new_dfun_name cls tycon
698 ; let inst_tys = [mkTyConApp tycon tc_args]
699 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
700 spec = DS { ds_loc = loc, ds_orig = orig
701 , ds_name = dfun_name, ds_tvs = tvs
702 , ds_cls = cls, ds_tys = inst_tys
703 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
704 , ds_theta = mtheta `orElse` inferred_constraints
705 , ds_newtype = False }
707 ; return (if isJust mtheta then Right spec -- Specified context
708 else Left spec) } -- Infer context
710 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
711 -- The Typeable class is special in several ways
712 -- data T a b = ... deriving( Typeable )
714 -- instance Typeable2 T where ...
716 -- 1. There are no constraints in the instance
717 -- 2. There are no type variables either
718 -- 3. The actual class we want to generate isn't necessarily
719 -- Typeable; it depends on the arity of the type
720 | isNothing mtheta -- deriving on a data type decl
721 = do { checkTc (cls `hasKey` typeableClassKey)
722 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
723 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
724 ; mk_typeable_eqn orig tvs real_cls tycon [] rep_tc [] (Just []) }
726 | otherwise -- standaone deriving
727 = do { checkTc (null tc_args)
728 (ptext (sLit "Derived typeable instance must be of form (Typeable")
729 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
730 ; dfun_name <- new_dfun_name cls tycon
733 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
734 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
735 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
736 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
739 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
740 -- Generate a sufficiently large set of constraints that typechecking the
741 -- generated method definitions should succeed. This set will be simplified
742 -- before being used in the instance declaration
743 inferConstraints _ cls inst_tys rep_tc rep_tc_args
744 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
745 stupid_constraints ++ extra_constraints
746 ++ sc_constraints ++ con_arg_constraints
748 -- Constraints arising from the arguments of each constructor
750 = [ mkClassPred cls [arg_ty]
751 | data_con <- tyConDataCons rep_tc,
752 arg_ty <- ASSERT( isVanillaDataCon data_con )
753 get_constrained_tys $
754 dataConInstOrigArgTys data_con all_rep_tc_args,
755 not (isUnLiftedType arg_ty) ]
756 -- No constraints for unlifted types
757 -- Where they are legal we generate specilised function calls
759 -- For functor-like classes, two things are different
760 -- (a) We recurse over argument types to generate constraints
761 -- See Functor examples in TcGenDeriv
762 -- (b) The rep_tc_args will be one short
763 is_functor_like = getUnique cls `elem` functorLikeClassKeys
765 get_constrained_tys :: [Type] -> [Type]
766 get_constrained_tys tys
767 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
770 rep_tc_tvs = tyConTyVars rep_tc
771 last_tv = last rep_tc_tvs
772 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
773 | otherwise = rep_tc_args
775 -- Constraints arising from superclasses
776 -- See Note [Superclasses of derived instance]
777 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
780 -- Stupid constraints
781 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
782 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
784 -- Extra Data constraints
785 -- The Data class (only) requires that for
786 -- instance (...) => Data (T t1 t2)
788 -- THEN (Data t1, Data t2) are among the (...) constraints
789 -- Reason: when the IF holds, we generate a method
790 -- dataCast2 f = gcast2 f
791 -- and we need the Data constraints to typecheck the method
793 | cls `hasKey` dataClassKey
794 , all (isLiftedTypeKind . typeKind) rep_tc_args
795 = [mkClassPred cls [ty] | ty <- rep_tc_args]
799 ------------------------------------------------------------------
800 -- Check side conditions that dis-allow derivability for particular classes
801 -- This is *apart* from the newtype-deriving mechanism
803 -- Here we get the representation tycon in case of family instances as it has
804 -- the data constructors - but we need to be careful to fall back to the
805 -- family tycon (with indexes) in error messages.
807 data DerivStatus = CanDerive
808 | DerivableClassError SDoc -- Standard class, but can't do it
809 | NonDerivableClass -- Non-standard class
811 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
812 checkSideConditions dflags mtheta cls cls_tys rep_tc
813 | Just cond <- sideConditions mtheta cls
814 = case (cond (dflags, rep_tc)) of
815 Just err -> DerivableClassError err -- Class-specific error
816 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
817 -- cls_tys (the type args other than last)
819 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
820 | otherwise = NonDerivableClass -- Not a standard class
822 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
824 nonStdErr :: Class -> SDoc
825 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
827 sideConditions :: DerivContext -> Class -> Maybe Condition
828 sideConditions mtheta cls
829 | cls_key == eqClassKey = Just cond_std
830 | cls_key == ordClassKey = Just cond_std
831 | cls_key == showClassKey = Just cond_std
832 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
833 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
834 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
835 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
836 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
837 cond_std `andCond` cond_noUnliftedArgs)
838 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
839 cond_functorOK True) -- NB: no cond_std!
840 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
841 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
842 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
843 cond_functorOK False)
844 | getName cls `elem` typeableClassNames = Just (checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK)
845 | otherwise = Nothing
847 cls_key = getUnique cls
848 cond_std = cond_stdOK mtheta
850 type Condition = (DynFlags, TyCon) -> Maybe SDoc
851 -- first Bool is whether or not we are allowed to derive Data and Typeable
852 -- second Bool is whether or not we are allowed to derive Functor
853 -- TyCon is the *representation* tycon if the
854 -- data type is an indexed one
857 orCond :: Condition -> Condition -> Condition
860 Nothing -> Nothing -- c1 succeeds
861 Just x -> case c2 tc of -- c1 fails
863 Just y -> Just (x $$ ptext (sLit " and") $$ y)
866 andCond :: Condition -> Condition -> Condition
867 andCond c1 c2 tc = case c1 tc of
868 Nothing -> c2 tc -- c1 succeeds
869 Just x -> Just x -- c1 fails
871 cond_stdOK :: DerivContext -> Condition
872 cond_stdOK (Just _) _
873 = Nothing -- Don't check these conservative conditions for
874 -- standalone deriving; just generate the code
875 cond_stdOK Nothing (_, rep_tc)
876 | null data_cons = Just (no_cons_why $$ suggestion)
877 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
878 | otherwise = Nothing
880 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
881 data_cons = tyConDataCons rep_tc
882 no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
883 ptext (sLit "has no data constructors")
885 con_whys = mapCatMaybes check_con data_cons
887 check_con :: DataCon -> Maybe SDoc
889 | isVanillaDataCon con
890 , all isTauTy (dataConOrigArgTys con) = Nothing
891 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
893 cond_enumOrProduct :: Condition
894 cond_enumOrProduct = cond_isEnumeration `orCond`
895 (cond_isProduct `andCond` cond_noUnliftedArgs)
897 cond_noUnliftedArgs :: Condition
898 -- For some classes (eg Eq, Ord) we allow unlifted arg types
899 -- by generating specilaised code. For others (eg Data) we don't.
900 cond_noUnliftedArgs (_, tc)
901 | null bad_cons = Nothing
902 | otherwise = Just why
904 bad_cons = [ con | con <- tyConDataCons tc
905 , any isUnLiftedType (dataConOrigArgTys con) ]
906 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
908 cond_isEnumeration :: Condition
909 cond_isEnumeration (_, rep_tc)
910 | isEnumerationTyCon rep_tc = Nothing
911 | otherwise = Just why
913 why = quotes (pprSourceTyCon rep_tc) <+>
914 ptext (sLit "has non-nullary constructors")
916 cond_isProduct :: Condition
917 cond_isProduct (_, rep_tc)
918 | isProductTyCon rep_tc = Nothing
919 | otherwise = Just why
921 why = quotes (pprSourceTyCon rep_tc) <+>
922 ptext (sLit "has more than one constructor")
924 cond_typeableOK :: Condition
925 -- OK for Typeable class
926 -- Currently: (a) args all of kind *
927 -- (b) 7 or fewer args
928 cond_typeableOK (_, rep_tc)
929 | tyConArity rep_tc > 7 = Just too_many
930 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
932 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
933 | otherwise = Nothing
935 too_many = quotes (pprSourceTyCon rep_tc) <+>
936 ptext (sLit "has too many arguments")
937 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
938 ptext (sLit "has arguments of kind other than `*'")
939 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
940 ptext (sLit "is a type family")
943 functorLikeClassKeys :: [Unique]
944 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
946 cond_functorOK :: Bool -> Condition
947 -- OK for Functor class
948 -- Currently: (a) at least one argument
949 -- (b) don't use argument contravariantly
950 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
951 -- (d) optionally: don't use function types
952 cond_functorOK allowFunctions (dflags, rep_tc)
953 | not (dopt Opt_DeriveFunctor dflags)
954 = Just (ptext (sLit "You need -XDeriveFunctor to derive an instance for this class"))
956 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
958 data_cons = tyConDataCons rep_tc
959 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
961 check_vanilla :: DataCon -> Maybe SDoc
962 check_vanilla con | isVanillaDataCon con = Nothing
963 | otherwise = Just (badCon con existential)
965 ft_check :: DataCon -> FFoldType (Maybe SDoc)
966 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
967 , ft_co_var = Just (badCon con covariant)
968 , ft_fun = \x y -> if allowFunctions then x `mplus` y
969 else Just (badCon con functions)
970 , ft_tup = \_ xs -> msum xs
971 , ft_ty_app = \_ x -> x
972 , ft_bad_app = Just (badCon con wrong_arg)
973 , ft_forall = \_ x -> x }
975 existential = ptext (sLit "has existential arguments")
976 covariant = ptext (sLit "uses the type variable in a function argument")
977 functions = ptext (sLit "contains function types")
978 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
980 checkFlag :: DynFlag -> Condition
981 checkFlag flag (dflags, _)
982 | dopt flag dflags = Nothing
983 | otherwise = Just why
985 why = ptext (sLit "You need -X") <> text flag_str
986 <+> ptext (sLit "to derive an instance for this class")
987 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
989 other -> pprPanic "checkFlag" (ppr other)
991 std_class_via_iso :: Class -> Bool
992 -- These standard classes can be derived for a newtype
993 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
994 -- because giving so gives the same results as generating the boilerplate
995 std_class_via_iso clas
996 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
997 -- Not Read/Show because they respect the type
998 -- Not Enum, because newtypes are never in Enum
1001 non_iso_class :: Class -> Bool
1002 -- *Never* derive Read,Show,Typeable,Data by isomorphism,
1003 -- even with -XGeneralizedNewtypeDeriving
1005 = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
1008 typeableClassKeys :: [Unique]
1009 typeableClassKeys = map getUnique typeableClassNames
1011 new_dfun_name :: Class -> TyCon -> TcM Name
1012 new_dfun_name clas tycon -- Just a simple wrapper
1013 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1014 ; newDFunName clas [mkTyConApp tycon []] loc }
1015 -- The type passed to newDFunName is only used to generate
1016 -- a suitable string; hence the empty type arg list
1018 badCon :: DataCon -> SDoc -> SDoc
1019 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1022 Note [Superclasses of derived instance]
1023 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1024 In general, a derived instance decl needs the superclasses of the derived
1025 class too. So if we have
1026 data T a = ...deriving( Ord )
1027 then the initial context for Ord (T a) should include Eq (T a). Often this is
1028 redundant; we'll also generate an Ord constraint for each constructor argument,
1029 and that will probably generate enough constraints to make the Eq (T a) constraint
1030 be satisfied too. But not always; consider:
1036 data T a = MkT (S a) deriving( Ord )
1037 instance Num a => Eq (T a)
1039 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1041 data T a = MkT deriving( Data, Typeable )
1042 Here there *is* no argument field, but we must nevertheless generate
1043 a context for the Data instances:
1044 instance Typable a => Data (T a) where ...
1047 %************************************************************************
1051 %************************************************************************
1054 mkNewTypeEqn :: InstOrigin -> DynFlags -> [Var] -> Class
1055 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1057 -> TcRn EarlyDerivSpec
1058 mkNewTypeEqn orig dflags tvs
1059 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1060 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1061 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1062 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
1063 ; dfun_name <- new_dfun_name cls tycon
1064 ; loc <- getSrcSpanM
1065 ; let spec = DS { ds_loc = loc, ds_orig = orig
1066 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1067 , ds_cls = cls, ds_tys = inst_tys
1068 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1069 , ds_theta = mtheta `orElse` all_preds
1070 , ds_newtype = True }
1071 ; return (if isJust mtheta then Right spec
1075 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1076 CanDerive -> go_for_it -- Use the standard H98 method
1077 DerivableClassError msg -- Error with standard class
1078 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1079 | otherwise -> bale_out msg
1080 NonDerivableClass -- Must use newtype deriving
1081 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1082 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1083 | otherwise -> bale_out non_std
1085 newtype_deriving = dopt Opt_GeneralizedNewtypeDeriving dflags
1086 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1087 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1089 non_std = nonStdErr cls
1090 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1092 -- Here is the plan for newtype derivings. We see
1093 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1094 -- where t is a type,
1095 -- ak+1...an is a suffix of a1..an, and are all tyars
1096 -- ak+1...an do not occur free in t, nor in the s1..sm
1097 -- (C s1 ... sm) is a *partial applications* of class C
1098 -- with the last parameter missing
1099 -- (T a1 .. ak) matches the kind of C's last argument
1100 -- (and hence so does t)
1101 -- The latter kind-check has been done by deriveTyData already,
1102 -- and tc_args are already trimmed
1104 -- We generate the instance
1105 -- instance forall ({a1..ak} u fvs(s1..sm)).
1106 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1107 -- where T a1...ap is the partial application of
1108 -- the LHS of the correct kind and p >= k
1110 -- NB: the variables below are:
1111 -- tc_tvs = [a1, ..., an]
1112 -- tyvars_to_keep = [a1, ..., ak]
1113 -- rep_ty = t ak .. an
1114 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1115 -- tys = [s1, ..., sm]
1118 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1119 -- We generate the instance
1120 -- instance Monad (ST s) => Monad (T s) where
1122 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1123 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1124 -- eta-reduces the representation type, so we know that
1126 -- That's convenient here, because we may have to apply
1127 -- it to fewer than its original complement of arguments
1129 -- Note [Newtype representation]
1130 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1131 -- Need newTyConRhs (*not* a recursive representation finder)
1132 -- to get the representation type. For example
1133 -- newtype B = MkB Int
1134 -- newtype A = MkA B deriving( Num )
1135 -- We want the Num instance of B, *not* the Num instance of Int,
1136 -- when making the Num instance of A!
1137 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1138 rep_tys = cls_tys ++ [rep_inst_ty]
1139 rep_pred = mkClassPred cls rep_tys
1140 -- rep_pred is the representation dictionary, from where
1141 -- we are gong to get all the methods for the newtype
1145 -- Next we figure out what superclass dictionaries to use
1146 -- See Note [Newtype deriving superclasses] above
1148 cls_tyvars = classTyVars cls
1149 dfun_tvs = tyVarsOfTypes inst_tys
1150 inst_ty = mkTyConApp tycon tc_args
1151 inst_tys = cls_tys ++ [inst_ty]
1152 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1155 -- If there are no tyvars, there's no need
1156 -- to abstract over the dictionaries we need
1157 -- Example: newtype T = MkT Int deriving( C )
1158 -- We get the derived instance
1161 -- instance C Int => C T
1162 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1164 -------------------------------------------------------------------
1165 -- Figuring out whether we can only do this newtype-deriving thing
1167 can_derive_via_isomorphism
1168 = not (non_iso_class cls)
1172 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1174 arity_ok = length cls_tys + 1 == classArity cls
1175 -- Well kinded; eg not: newtype T ... deriving( ST )
1176 -- because ST needs *2* type params
1178 -- Check that eta reduction is OK
1179 eta_ok = nt_eta_arity <= length rep_tc_args
1180 -- The newtype can be eta-reduced to match the number
1181 -- of type argument actually supplied
1182 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1183 -- Here the 'b' must be the same in the rep type (S [a] b)
1184 -- And the [a] must not mention 'b'. That's all handled
1187 ats_ok = null (classATs cls)
1188 -- No associated types for the class, because we don't
1189 -- currently generate type 'instance' decls; and cannot do
1190 -- so for 'data' instance decls
1193 = vcat [ ppUnless arity_ok arity_msg
1194 , ppUnless eta_ok eta_msg
1195 , ppUnless ats_ok ats_msg ]
1196 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1197 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1198 ats_msg = ptext (sLit "the class has associated types")
1201 Note [Recursive newtypes]
1202 ~~~~~~~~~~~~~~~~~~~~~~~~~
1203 Newtype deriving works fine, even if the newtype is recursive.
1204 e.g. newtype S1 = S1 [T1 ()]
1205 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1206 Remember, too, that type families are curretly (conservatively) given
1207 a recursive flag, so this also allows newtype deriving to work
1210 We used to exclude recursive types, because we had a rather simple
1211 minded way of generating the instance decl:
1213 instance Eq [A] => Eq A -- Makes typechecker loop!
1214 But now we require a simple context, so it's ok.
1217 %************************************************************************
1219 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1221 %************************************************************************
1223 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1224 terms, which is the final correct RHS for the corresponding original
1228 Each (k,TyVarTy tv) in a solution constrains only a type
1232 The (k,TyVarTy tv) pairs in a solution are canonically
1233 ordered by sorting on type varible, tv, (major key) and then class, k,
1238 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1240 inferInstanceContexts _ [] = return []
1242 inferInstanceContexts oflag infer_specs
1243 = do { traceTc (text "inferInstanceContexts" <+> vcat (map pprDerivSpec infer_specs))
1244 ; iterate_deriv 1 initial_solutions }
1246 ------------------------------------------------------------------
1247 -- The initial solutions for the equations claim that each
1248 -- instance has an empty context; this solution is certainly
1249 -- in canonical form.
1250 initial_solutions :: [ThetaType]
1251 initial_solutions = [ [] | _ <- infer_specs ]
1253 ------------------------------------------------------------------
1254 -- iterate_deriv calculates the next batch of solutions,
1255 -- compares it with the current one; finishes if they are the
1256 -- same, otherwise recurses with the new solutions.
1257 -- It fails if any iteration fails
1258 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1259 iterate_deriv n current_solns
1260 | n > 20 -- Looks as if we are in an infinite loop
1261 -- This can happen if we have -XUndecidableInstances
1262 -- (See TcSimplify.tcSimplifyDeriv.)
1263 = pprPanic "solveDerivEqns: probable loop"
1264 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1266 = do { -- Extend the inst info from the explicit instance decls
1267 -- with the current set of solutions, and simplify each RHS
1268 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1269 current_solns infer_specs
1270 ; new_solns <- checkNoErrs $
1271 extendLocalInstEnv inst_specs $
1272 mapM gen_soln infer_specs
1274 ; if (current_solns == new_solns) then
1275 return [ spec { ds_theta = soln }
1276 | (spec, soln) <- zip infer_specs current_solns ]
1278 iterate_deriv (n+1) new_solns }
1280 ------------------------------------------------------------------
1281 gen_soln :: DerivSpec -> TcM [PredType]
1282 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1283 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1285 addErrCtxt (derivInstCtxt clas inst_tys) $
1286 do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
1287 -- checkValidInstance tyvars theta clas inst_tys
1288 -- Not necessary; see Note [Exotic derived instance contexts]
1291 -- Check for a bizarre corner case, when the derived instance decl should
1292 -- have form instance C a b => D (T a) where ...
1293 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1294 -- of problems; in particular, it's hard to compare solutions for
1295 -- equality when finding the fixpoint. So I just rule it out for now.
1296 ; let tv_set = mkVarSet tyvars
1297 weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
1298 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1300 ; traceTc (text "TcDeriv" <+> (ppr deriv_rhs $$ ppr theta))
1301 -- Claim: the result instance declaration is guaranteed valid
1302 -- Hence no need to call:
1303 -- checkValidInstance tyvars theta clas inst_tys
1304 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1306 ------------------------------------------------------------------
1307 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1308 mkInstance overlap_flag theta
1309 (DS { ds_name = dfun_name
1310 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1311 = mkLocalInstance dfun overlap_flag
1313 dfun = mkDictFunId dfun_name tyvars theta clas tys
1316 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1317 -- Add new locally-defined instances; don't bother to check
1318 -- for functional dependency errors -- that'll happen in TcInstDcls
1319 extendLocalInstEnv dfuns thing_inside
1320 = do { env <- getGblEnv
1321 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1322 env' = env { tcg_inst_env = inst_env' }
1323 ; setGblEnv env' thing_inside }
1327 %************************************************************************
1329 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1331 %************************************************************************
1333 After all the trouble to figure out the required context for the
1334 derived instance declarations, all that's left is to chug along to
1335 produce them. They will then be shoved into @tcInstDecls2@, which
1336 will do all its usual business.
1338 There are lots of possibilities for code to generate. Here are
1339 various general remarks.
1344 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1345 ``you-couldn't-do-better-by-hand'' efficient.
1348 Deriving @Show@---also pretty common--- should also be reasonable good code.
1351 Deriving for the other classes isn't that common or that big a deal.
1358 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1361 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1364 We {\em normally} generate code only for the non-defaulted methods;
1365 there are some exceptions for @Eq@ and (especially) @Ord@...
1368 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1369 constructor's numeric (@Int#@) tag. These are generated by
1370 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1371 these is around is given by @hasCon2TagFun@.
1373 The examples under the different sections below will make this
1377 Much less often (really just for deriving @Ix@), we use a
1378 @_tag2con_<tycon>@ function. See the examples.
1381 We use the renamer!!! Reason: we're supposed to be
1382 producing @LHsBinds Name@ for the methods, but that means
1383 producing correctly-uniquified code on the fly. This is entirely
1384 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1385 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1386 the renamer. What a great hack!
1390 -- Generate the InstInfo for the required instance paired with the
1391 -- *representation* tycon for that instance,
1392 -- plus any auxiliary bindings required
1394 -- Representation tycons differ from the tycon in the instance signature in
1395 -- case of instances for indexed families.
1397 genInst :: Bool -- True <=> standalone deriving
1399 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1400 genInst standalone_deriv oflag spec
1402 = return (InstInfo { iSpec = mkInstance oflag (ds_theta spec) spec
1403 , iBinds = NewTypeDerived co rep_tycon }, [])
1406 = do { let loc = getSrcSpan (ds_name spec)
1407 inst = mkInstance oflag (ds_theta spec) spec
1410 -- In case of a family instance, we need to use the representation
1411 -- tycon (after all, it has the data constructors)
1412 ; fix_env <- getFixityEnv
1413 ; let (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1414 binds = VanillaInst meth_binds [] standalone_deriv
1415 ; return (InstInfo { iSpec = inst, iBinds = binds }, aux_binds)
1418 rep_tycon = ds_tc spec
1419 rep_tc_args = ds_tc_args spec
1420 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1422 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1423 co2 = case newTyConCo_maybe rep_tycon of
1424 Nothing -> IdCo -- The newtype is transparent; no need for a cast
1425 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1426 co = co1 `mkTransCoI` co2
1428 -- Example: newtype instance N [a] = N1 (Tree a)
1429 -- deriving instance Eq b => Eq (N [(b,b)])
1430 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1431 -- When dealing with the deriving clause
1432 -- co1 : N [(b,b)] ~ R1:N (b,b)
1433 -- co2 : R1:N (b,b) ~ Tree (b,b)
1434 -- co : N [(b,b)] ~ Tree (b,b)
1436 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1437 genDerivBinds loc fix_env clas tycon
1438 | className clas `elem` typeableClassNames
1439 = (gen_Typeable_binds loc tycon, [])
1442 = case assocMaybe gen_list (getUnique clas) of
1443 Just gen_fn -> gen_fn loc tycon
1444 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1446 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1447 gen_list = [(eqClassKey, gen_Eq_binds)
1448 ,(ordClassKey, gen_Ord_binds)
1449 ,(enumClassKey, gen_Enum_binds)
1450 ,(boundedClassKey, gen_Bounded_binds)
1451 ,(ixClassKey, gen_Ix_binds)
1452 ,(showClassKey, gen_Show_binds fix_env)
1453 ,(readClassKey, gen_Read_binds fix_env)
1454 ,(dataClassKey, gen_Data_binds)
1455 ,(functorClassKey, gen_Functor_binds)
1456 ,(foldableClassKey, gen_Foldable_binds)
1457 ,(traversableClassKey, gen_Traversable_binds)
1462 %************************************************************************
1464 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1466 %************************************************************************
1469 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1470 derivingKindErr tc cls cls_tys cls_kind
1471 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1472 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1473 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1474 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1476 derivingEtaErr :: Class -> [Type] -> Type -> Message
1477 derivingEtaErr cls cls_tys inst_ty
1478 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1479 nest 2 (ptext (sLit "instance (...) =>")
1480 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1482 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1483 typeFamilyPapErr tc cls cls_tys inst_ty
1484 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1485 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1487 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1488 derivingThingErr newtype_deriving clas tys ty why
1489 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1490 2 (quotes (ppr pred))
1491 $$ nest 2 extra) <> colon,
1494 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1496 pred = mkClassPred clas (tys ++ [ty])
1498 derivingHiddenErr :: TyCon -> SDoc
1499 derivingHiddenErr tc
1500 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1501 2 (ptext (sLit "so you cannot derive an instance for it"))
1503 standaloneCtxt :: LHsType Name -> SDoc
1504 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1507 derivInstCtxt :: Class -> [Type] -> Message
1508 derivInstCtxt clas inst_tys
1509 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1511 badDerivedPred :: PredType -> Message
1513 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1514 ptext (sLit "type variables that are not data type parameters"),
1515 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]