2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Handles @deriving@ clauses on @data@ declarations.
9 module TcDeriv ( tcDeriving ) where
11 #include "HsVersions.h"
19 import TcClassDcl( tcAddDeclCtxt ) -- Small helper
20 import TcGenDeriv -- Deriv stuff
56 %************************************************************************
60 %************************************************************************
64 1. Convert the decls (i.e. data/newtype deriving clauses,
65 plus standalone deriving) to [EarlyDerivSpec]
67 2. Infer the missing contexts for the Left DerivSpecs
69 3. Add the derived bindings, generating InstInfos
73 -- DerivSpec is purely local to this module
74 data DerivSpec = DS { ds_loc :: SrcSpan
78 , ds_theta :: ThetaType
82 , ds_tc_args :: [Type]
83 , ds_newtype :: Bool }
84 -- This spec implies a dfun declaration of the form
85 -- df :: forall tvs. theta => C tys
86 -- The Name is the name for the DFun we'll build
87 -- The tyvars bind all the variables in the theta
88 -- For type families, the tycon in
89 -- in ds_tys is the *family* tycon
90 -- in ds_tc, ds_tc_args is the *representation* tycon
91 -- For non-family tycons, both are the same
93 -- ds_newtype = True <=> Newtype deriving
94 -- False <=> Vanilla deriving
99 newtype instance T [a] = MkT (Tree a) deriving( C s )
101 axiom T [a] = :RTList a
102 axiom :RTList a = Tree a
104 DS { ds_tvs = [a,s], ds_cls = C, ds_tys = [s, T [a]]
105 , ds_tc = :RTList, ds_tc_args = [a]
106 , ds_newtype = True }
109 type DerivContext = Maybe ThetaType
110 -- Nothing <=> Vanilla deriving; infer the context of the instance decl
111 -- Just theta <=> Standalone deriving: context supplied by programmer
113 type EarlyDerivSpec = Either DerivSpec DerivSpec
114 -- Left ds => the context for the instance should be inferred
115 -- In this case ds_theta is the list of all the
116 -- constraints needed, such as (Eq [a], Eq a)
117 -- The inference process is to reduce this to a
118 -- simpler form (e.g. Eq a)
120 -- Right ds => the exact context for the instance is supplied
121 -- by the programmer; it is ds_theta
123 pprDerivSpec :: DerivSpec -> SDoc
124 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
125 ds_cls = c, ds_tys = tys, ds_theta = rhs })
126 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
127 <+> equals <+> ppr rhs)
131 Inferring missing contexts
132 ~~~~~~~~~~~~~~~~~~~~~~~~~~
135 data T a b = C1 (Foo a) (Bar b)
140 [NOTE: See end of these comments for what to do with
141 data (C a, D b) => T a b = ...
144 We want to come up with an instance declaration of the form
146 instance (Ping a, Pong b, ...) => Eq (T a b) where
149 It is pretty easy, albeit tedious, to fill in the code "...". The
150 trick is to figure out what the context for the instance decl is,
151 namely @Ping@, @Pong@ and friends.
153 Let's call the context reqd for the T instance of class C at types
154 (a,b, ...) C (T a b). Thus:
156 Eq (T a b) = (Ping a, Pong b, ...)
158 Now we can get a (recursive) equation from the @data@ decl:
160 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
161 u Eq (T b a) u Eq Int -- From C2
162 u Eq (T a a) -- From C3
164 Foo and Bar may have explicit instances for @Eq@, in which case we can
165 just substitute for them. Alternatively, either or both may have
166 their @Eq@ instances given by @deriving@ clauses, in which case they
167 form part of the system of equations.
169 Now all we need do is simplify and solve the equations, iterating to
170 find the least fixpoint. Notice that the order of the arguments can
171 switch around, as here in the recursive calls to T.
173 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
177 Eq (T a b) = {} -- The empty set
180 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
181 u Eq (T b a) u Eq Int -- From C2
182 u Eq (T a a) -- From C3
184 After simplification:
185 = Eq a u Ping b u {} u {} u {}
190 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
191 u Eq (T b a) u Eq Int -- From C2
192 u Eq (T a a) -- From C3
194 After simplification:
199 = Eq a u Ping b u Eq b u Ping a
201 The next iteration gives the same result, so this is the fixpoint. We
202 need to make a canonical form of the RHS to ensure convergence. We do
203 this by simplifying the RHS to a form in which
205 - the classes constrain only tyvars
206 - the list is sorted by tyvar (major key) and then class (minor key)
207 - no duplicates, of course
209 So, here are the synonyms for the ``equation'' structures:
212 Note [Data decl contexts]
213 ~~~~~~~~~~~~~~~~~~~~~~~~~
216 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
218 We will need an instance decl like:
220 instance (Read a, RealFloat a) => Read (Complex a) where
223 The RealFloat in the context is because the read method for Complex is bound
224 to construct a Complex, and doing that requires that the argument type is
227 But this ain't true for Show, Eq, Ord, etc, since they don't construct
228 a Complex; they only take them apart.
230 Our approach: identify the offending classes, and add the data type
231 context to the instance decl. The "offending classes" are
235 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
236 pattern matching against a constructor from a data type with a context
237 gives rise to the constraints for that context -- or at least the thinned
238 version. So now all classes are "offending".
240 Note [Newtype deriving]
241 ~~~~~~~~~~~~~~~~~~~~~~~
245 newtype T = T Char deriving( C [a] )
247 Notice the free 'a' in the deriving. We have to fill this out to
248 newtype T = T Char deriving( forall a. C [a] )
250 And then translate it to:
251 instance C [a] Char => C [a] T where ...
254 Note [Newtype deriving superclasses]
255 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
256 (See also Trac #1220 for an interesting exchange on newtype
257 deriving and superclasses.)
259 The 'tys' here come from the partial application in the deriving
260 clause. The last arg is the new instance type.
262 We must pass the superclasses; the newtype might be an instance
263 of them in a different way than the representation type
264 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
265 Then the Show instance is not done via isomorphism; it shows
267 The Num instance is derived via isomorphism, but the Show superclass
268 dictionary must the Show instance for Foo, *not* the Show dictionary
269 gotten from the Num dictionary. So we must build a whole new dictionary
270 not just use the Num one. The instance we want is something like:
271 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
274 There may be a coercion needed which we get from the tycon for the newtype
275 when the dict is constructed in TcInstDcls.tcInstDecl2
278 Note [Unused constructors and deriving clauses]
279 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
280 See Trac #3221. Consider
281 data T = T1 | T2 deriving( Show )
282 Are T1 and T2 unused? Well, no: the deriving clause expands to mention
283 both of them. So we gather defs/uses from deriving just like anything else.
285 %************************************************************************
287 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
289 %************************************************************************
292 tcDeriving :: [LTyClDecl Name] -- All type constructors
293 -> [LInstDecl Name] -- All instance declarations
294 -> [LDerivDecl Name] -- All stand-alone deriving declarations
295 -> TcM ([InstInfo Name], -- The generated "instance decls"
296 HsValBinds Name, -- Extra generated top-level bindings
299 tcDeriving tycl_decls inst_decls deriv_decls
300 = recoverM (return ([], emptyValBindsOut, emptyDUs)) $
301 do { -- Fish the "deriving"-related information out of the TcEnv
302 -- And make the necessary "equations".
303 is_boot <- tcIsHsBoot
304 ; traceTc "tcDeriving" (ppr is_boot)
305 ; early_specs <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
307 ; overlap_flag <- getOverlapFlag
308 ; let (infer_specs, given_specs) = splitEithers early_specs
309 ; insts1 <- mapM (genInst True overlap_flag) given_specs
311 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
312 inferInstanceContexts overlap_flag infer_specs
314 ; insts2 <- mapM (genInst False overlap_flag) final_specs
316 -- Generate the generic to/from functions from each type declaration
317 ; gen_binds <- mkGenericBinds is_boot tycl_decls
318 ; (inst_info, rn_binds, rn_dus) <- renameDeriv is_boot gen_binds (insts1 ++ insts2)
321 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
322 (ddump_deriving inst_info rn_binds))
324 ; return (inst_info, rn_binds, rn_dus) }
326 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
327 ddump_deriving inst_infos extra_binds
328 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
330 renameDeriv :: Bool -> LHsBinds RdrName
331 -> [(InstInfo RdrName, DerivAuxBinds)]
332 -> TcM ([InstInfo Name], HsValBinds Name, DefUses)
333 renameDeriv is_boot gen_binds insts
334 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
335 -- The inst-info bindings will all be empty, but it's easier to
336 -- just use rn_inst_info to change the type appropriately
337 = do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
338 ; return (rn_inst_infos, emptyValBindsOut, usesOnly (plusFVs fvs)) }
341 = discardWarnings $ -- Discard warnings about unused bindings etc
342 do { (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $ -- Type signatures in patterns
343 -- are used in the generic binds
344 rnTopBinds (ValBindsIn gen_binds [])
345 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
347 -- Generate and rename any extra not-one-inst-decl-specific binds,
348 -- notably "con2tag" and/or "tag2con" functions.
349 -- Bring those names into scope before renaming the instances themselves
350 ; loc <- getSrcSpanM -- Generic loc for shared bindings
351 ; let (aux_binds, aux_sigs) = unzip $ map (genAuxBind loc) $
352 rm_dups [] $ concat deriv_aux_binds
353 aux_val_binds = ValBindsIn (listToBag aux_binds) aux_sigs
354 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
355 ; bindLocalNames (collectHsValBinders rn_aux_lhs) $
356 do { (rn_aux, dus_aux) <- rnTopBindsRHS rn_aux_lhs
357 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
358 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
359 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
362 (inst_infos, deriv_aux_binds) = unzip insts
364 -- Remove duplicate requests for auxilliary bindings
366 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
367 | otherwise = rm_dups (b:acc) bs
370 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
371 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
372 = return ( info { iBinds = NewTypeDerived coi tc }
373 , mkFVs (map dataConName (tyConDataCons tc)))
374 -- See Note [Newtype deriving and unused constructors]
376 rn_inst_info (InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
377 = -- Bring the right type variables into
378 -- scope (yuk), and rename the method binds
380 bindLocalNames (map Var.varName tyvars) $
381 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
382 ; let binds' = VanillaInst rn_binds [] standalone_deriv
383 ; return (InstInfo { iSpec = inst, iBinds = binds' }, fvs) }
385 (tyvars,_, clas,_) = instanceHead inst
386 clas_nm = className clas
388 -----------------------------------------
389 mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
390 mkGenericBinds is_boot tycl_decls
394 = do { tcs <- mapM tcLookupTyCon [ tcdName d
395 | L _ d <- tycl_decls, isDataDecl d ]
396 ; return (unionManyBags [ mkTyConGenericBinds tc
397 | tc <- tcs, tyConHasGenerics tc ]) }
398 -- We are only interested in the data type declarations,
399 -- and then only in the ones whose 'has-generics' flag is on
400 -- The predicate tyConHasGenerics finds both of these
403 Note [Newtype deriving and unused constructors]
404 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
405 Consider this (see Trac #1954):
408 newtype P a = MkP (IO a) deriving Monad
410 If you compile with -fwarn-unused-binds you do not expect the warning
411 "Defined but not used: data consructor MkP". Yet the newtype deriving
412 code does not explicitly mention MkP, but it should behave as if you
414 instance Monad P where
415 return x = MkP (return x)
418 So we want to signal a user of the data constructor 'MkP'. That's
419 what we do in rn_inst_info, and it's the only reason we have the TyCon
420 stored in NewTypeDerived.
423 %************************************************************************
425 From HsSyn to DerivSpec
427 %************************************************************************
429 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
432 makeDerivSpecs :: Bool
436 -> TcM [EarlyDerivSpec]
438 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
439 | is_boot -- No 'deriving' at all in hs-boot files
440 = do { mapM_ add_deriv_err deriv_locs
443 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
444 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
445 ; return (eqns1 ++ eqns2) }
447 extractTyDataPreds decls
448 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
450 all_tydata :: [(LHsType Name, LTyClDecl Name)]
451 -- Derived predicate paired with its data type declaration
452 all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
454 deriv_locs = map (getLoc . snd) all_tydata
455 ++ map getLoc deriv_decls
457 add_deriv_err loc = setSrcSpan loc $
458 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
459 2 (ptext (sLit "Use an instance declaration instead")))
461 ------------------------------------------------------------------
462 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
463 -- Standalone deriving declarations
464 -- e.g. deriving instance Show a => Show (T a)
465 -- Rather like tcLocalInstDecl
466 deriveStandalone (L loc (DerivDecl deriv_ty))
468 addErrCtxt (standaloneCtxt deriv_ty) $
469 do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
470 ; (tvs, theta, tau) <- tcHsInstHead deriv_ty
471 ; traceTc "Standalone deriving;" $ vcat
472 [ text "tvs:" <+> ppr tvs
473 , text "theta:" <+> ppr theta
474 , text "tau:" <+> ppr tau ]
475 ; (cls, inst_tys) <- checkValidInstance deriv_ty tvs theta tau
476 -- C.f. TcInstDcls.tcLocalInstDecl1
478 ; let cls_tys = take (length inst_tys - 1) inst_tys
479 inst_ty = last inst_tys
480 ; traceTc "Standalone deriving:" $ vcat
481 [ text "class:" <+> ppr cls
482 , text "class types:" <+> ppr cls_tys
483 , text "type:" <+> ppr inst_ty ]
484 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
487 ------------------------------------------------------------------
488 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
489 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
490 tcdTyVars = tv_names,
491 tcdTyPats = ty_pats }))
492 = setSrcSpan loc $ -- Use the location of the 'deriving' item
494 do { (tvs, tc, tc_args) <- get_lhs ty_pats
495 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
496 -- the type variables for the type constructor
498 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
499 -- The "deriv_pred" is a LHsType to take account of the fact that for
500 -- newtype deriving we allow deriving (forall a. C [a]).
502 -- Given data T a b c = ... deriving( C d ),
503 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
504 ; let cls_tyvars = classTyVars cls
505 kind = tyVarKind (last cls_tyvars)
506 (arg_kinds, _) = splitKindFunTys kind
507 n_args_to_drop = length arg_kinds
508 n_args_to_keep = tyConArity tc - n_args_to_drop
509 args_to_drop = drop n_args_to_keep tc_args
510 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
511 inst_ty_kind = typeKind inst_ty
512 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
513 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
514 `minusVarSet` dropped_tvs
516 -- Check that the result really is well-kinded
517 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
518 (derivingKindErr tc cls cls_tys kind)
520 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
521 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
522 (derivingEtaErr cls cls_tys inst_ty)
524 -- (a) The data type can be eta-reduced; eg reject:
525 -- data instance T a a = ... deriving( Monad )
526 -- (b) The type class args do not mention any of the dropped type
528 -- newtype T a s = ... deriving( ST s )
530 -- Type families can't be partially applied
531 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
532 -- Note [Deriving, type families, and partial applications]
533 ; checkTc (not (isFamilyTyCon tc) || n_args_to_drop == 0)
534 (typeFamilyPapErr tc cls cls_tys inst_ty)
536 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
538 -- Tiresomely we must figure out the "lhs", which is awkward for type families
539 -- E.g. data T a b = .. deriving( Eq )
540 -- Here, the lhs is (T a b)
541 -- data instance TF Int b = ... deriving( Eq )
542 -- Here, the lhs is (TF Int b)
543 -- But if we just look up the tycon_name, we get is the *family*
544 -- tycon, but not pattern types -- they are in the *rep* tycon.
545 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
546 ; let tvs = tyConTyVars tc
547 ; return (tvs, tc, mkTyVarTys tvs) }
548 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
549 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
550 ; let (tc, tc_args) = tcSplitTyConApp tc_app
551 ; return (tvs, tc, tc_args) }
554 = panic "derivTyData" -- Caller ensures that only TyData can happen
557 Note [Deriving, type families, and partial applications]
558 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
559 When there are no type families, it's quite easy:
561 newtype S a = MkS [a]
562 -- :CoS :: S ~ [] -- Eta-reduced
564 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
565 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
567 When type familes are involved it's trickier:
570 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
571 -- :RT is the representation type for (T Int a)
572 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
573 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
575 instance Eq [a] => Eq (T Int a) -- easy by coercion
576 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
578 The "???" bit is that we don't build the :CoF thing in eta-reduced form
579 Henc the current typeFamilyPapErr, even though the instance makes sense.
580 After all, we can write it out
581 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
586 mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type] -> Type
587 -> DerivContext -- Just => context supplied (standalone deriving)
588 -- Nothing => context inferred (deriving on data decl)
589 -> TcRn EarlyDerivSpec
590 -- Make the EarlyDerivSpec for an instance
591 -- forall tvs. theta => cls (tys ++ [ty])
592 -- where the 'theta' is optional (that's the Maybe part)
593 -- Assumes that this declaration is well-kinded
595 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
596 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
597 , isAlgTyCon tycon -- Check for functions, primitive types etc
598 = mk_alg_eqn tycon tc_args
600 = failWithTc (derivingThingErr False cls cls_tys tc_app
601 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
604 bale_out msg = failWithTc (derivingThingErr False cls cls_tys tc_app msg)
606 mk_alg_eqn tycon tc_args
607 | className cls `elem` typeableClassNames
608 = do { dflags <- getDOpts
609 ; case checkTypeableConditions (dflags, tycon) of
610 Just err -> bale_out err
611 Nothing -> mk_typeable_eqn orig tvs cls tycon tc_args mtheta }
613 | isDataFamilyTyCon tycon
614 , length tc_args /= tyConArity tycon
615 = bale_out (ptext (sLit "Unsaturated data family application"))
618 = do { (rep_tc, rep_tc_args) <- tcLookupDataFamInst tycon tc_args
619 -- Be careful to test rep_tc here: in the case of families,
620 -- we want to check the instance tycon, not the family tycon
622 -- For standalone deriving (mtheta /= Nothing),
623 -- check that all the data constructors are in scope.
624 ; rdr_env <- getGlobalRdrEnv
625 ; let hidden_data_cons = isAbstractTyCon rep_tc ||
626 any not_in_scope (tyConDataCons rep_tc)
627 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
628 ; unless (isNothing mtheta || not hidden_data_cons)
629 (bale_out (derivingHiddenErr tycon))
632 ; if isDataTyCon rep_tc then
633 mkDataTypeEqn orig dflags tvs cls cls_tys
634 tycon tc_args rep_tc rep_tc_args mtheta
636 mkNewTypeEqn orig dflags tvs cls cls_tys
637 tycon tc_args rep_tc rep_tc_args mtheta }
641 %************************************************************************
645 %************************************************************************
648 mkDataTypeEqn :: CtOrigin
650 -> [Var] -- Universally quantified type variables in the instance
651 -> Class -- Class for which we need to derive an instance
652 -> [Type] -- Other parameters to the class except the last
653 -> TyCon -- Type constructor for which the instance is requested
654 -- (last parameter to the type class)
655 -> [Type] -- Parameters to the type constructor
656 -> TyCon -- rep of the above (for type families)
657 -> [Type] -- rep of the above
658 -> DerivContext -- Context of the instance, for standalone deriving
659 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
661 mkDataTypeEqn orig dflags tvs cls cls_tys
662 tycon tc_args rep_tc rep_tc_args mtheta
663 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
664 -- NB: pass the *representation* tycon to checkSideConditions
665 CanDerive -> go_for_it
666 NonDerivableClass -> bale_out (nonStdErr cls)
667 DerivableClassError msg -> bale_out msg
669 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
670 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
672 mk_data_eqn :: CtOrigin -> [TyVar] -> Class
673 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
674 -> TcM EarlyDerivSpec
675 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
676 = do { dfun_name <- new_dfun_name cls tycon
678 ; let inst_tys = [mkTyConApp tycon tc_args]
679 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
680 spec = DS { ds_loc = loc, ds_orig = orig
681 , ds_name = dfun_name, ds_tvs = tvs
682 , ds_cls = cls, ds_tys = inst_tys
683 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
684 , ds_theta = mtheta `orElse` inferred_constraints
685 , ds_newtype = False }
687 ; return (if isJust mtheta then Right spec -- Specified context
688 else Left spec) } -- Infer context
690 ----------------------
691 mk_typeable_eqn :: CtOrigin -> [TyVar] -> Class
692 -> TyCon -> [TcType] -> DerivContext
693 -> TcM EarlyDerivSpec
694 mk_typeable_eqn orig tvs cls tycon tc_args mtheta
695 -- The Typeable class is special in several ways
696 -- data T a b = ... deriving( Typeable )
698 -- instance Typeable2 T where ...
700 -- 1. There are no constraints in the instance
701 -- 2. There are no type variables either
702 -- 3. The actual class we want to generate isn't necessarily
703 -- Typeable; it depends on the arity of the type
704 | isNothing mtheta -- deriving on a data type decl
705 = do { checkTc (cls `hasKey` typeableClassKey)
706 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
707 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
708 ; mk_typeable_eqn orig tvs real_cls tycon [] (Just []) }
710 | otherwise -- standaone deriving
711 = do { checkTc (null tc_args)
712 (ptext (sLit "Derived typeable instance must be of form (Typeable")
713 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
714 ; dfun_name <- new_dfun_name cls tycon
717 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
718 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
719 , ds_tc = tycon, ds_tc_args = []
720 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
722 ----------------------
723 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
724 -- Generate a sufficiently large set of constraints that typechecking the
725 -- generated method definitions should succeed. This set will be simplified
726 -- before being used in the instance declaration
727 inferConstraints _ cls inst_tys rep_tc rep_tc_args
728 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
729 stupid_constraints ++ extra_constraints
730 ++ sc_constraints ++ con_arg_constraints
732 -- Constraints arising from the arguments of each constructor
734 = [ mkClassPred cls [arg_ty]
735 | data_con <- tyConDataCons rep_tc,
736 arg_ty <- ASSERT( isVanillaDataCon data_con )
737 get_constrained_tys $
738 dataConInstOrigArgTys data_con all_rep_tc_args,
739 not (isUnLiftedType arg_ty) ]
740 -- No constraints for unlifted types
741 -- Where they are legal we generate specilised function calls
743 -- For functor-like classes, two things are different
744 -- (a) We recurse over argument types to generate constraints
745 -- See Functor examples in TcGenDeriv
746 -- (b) The rep_tc_args will be one short
747 is_functor_like = getUnique cls `elem` functorLikeClassKeys
749 get_constrained_tys :: [Type] -> [Type]
750 get_constrained_tys tys
751 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
754 rep_tc_tvs = tyConTyVars rep_tc
755 last_tv = last rep_tc_tvs
756 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
757 | otherwise = rep_tc_args
759 -- Constraints arising from superclasses
760 -- See Note [Superclasses of derived instance]
761 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
764 -- Stupid constraints
765 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
766 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
768 -- Extra Data constraints
769 -- The Data class (only) requires that for
770 -- instance (...) => Data (T t1 t2)
772 -- THEN (Data t1, Data t2) are among the (...) constraints
773 -- Reason: when the IF holds, we generate a method
774 -- dataCast2 f = gcast2 f
775 -- and we need the Data constraints to typecheck the method
777 | cls `hasKey` dataClassKey
778 , all (isLiftedTypeKind . typeKind) rep_tc_args
779 = [mkClassPred cls [ty] | ty <- rep_tc_args]
783 ------------------------------------------------------------------
784 -- Check side conditions that dis-allow derivability for particular classes
785 -- This is *apart* from the newtype-deriving mechanism
787 -- Here we get the representation tycon in case of family instances as it has
788 -- the data constructors - but we need to be careful to fall back to the
789 -- family tycon (with indexes) in error messages.
791 data DerivStatus = CanDerive
792 | DerivableClassError SDoc -- Standard class, but can't do it
793 | NonDerivableClass -- Non-standard class
795 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
796 checkSideConditions dflags mtheta cls cls_tys rep_tc
797 | Just cond <- sideConditions mtheta cls
798 = case (cond (dflags, rep_tc)) of
799 Just err -> DerivableClassError err -- Class-specific error
800 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
801 -- cls_tys (the type args other than last)
803 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
804 | otherwise = NonDerivableClass -- Not a standard class
806 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
808 checkTypeableConditions :: Condition
809 checkTypeableConditions = checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK
811 nonStdErr :: Class -> SDoc
812 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
814 sideConditions :: DerivContext -> Class -> Maybe Condition
815 sideConditions mtheta cls
816 | cls_key == eqClassKey = Just cond_std
817 | cls_key == ordClassKey = Just cond_std
818 | cls_key == showClassKey = Just cond_std
819 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
820 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
821 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
822 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
823 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
824 cond_std `andCond` cond_noUnliftedArgs)
825 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
826 cond_functorOK True) -- NB: no cond_std!
827 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
828 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
829 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
830 cond_functorOK False)
831 | otherwise = Nothing
833 cls_key = getUnique cls
834 cond_std = cond_stdOK mtheta
836 type Condition = (DynFlags, TyCon) -> Maybe SDoc
837 -- first Bool is whether or not we are allowed to derive Data and Typeable
838 -- second Bool is whether or not we are allowed to derive Functor
839 -- TyCon is the *representation* tycon if the
840 -- data type is an indexed one
843 orCond :: Condition -> Condition -> Condition
846 Nothing -> Nothing -- c1 succeeds
847 Just x -> case c2 tc of -- c1 fails
849 Just y -> Just (x $$ ptext (sLit " and") $$ y)
852 andCond :: Condition -> Condition -> Condition
853 andCond c1 c2 tc = case c1 tc of
854 Nothing -> c2 tc -- c1 succeeds
855 Just x -> Just x -- c1 fails
857 cond_stdOK :: DerivContext -> Condition
858 cond_stdOK (Just _) _
859 = Nothing -- Don't check these conservative conditions for
860 -- standalone deriving; just generate the code
861 -- and let the typechecker handle the result
862 cond_stdOK Nothing (_, rep_tc)
863 | null data_cons = Just (no_cons_why rep_tc $$ suggestion)
864 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
865 | otherwise = Nothing
867 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
868 data_cons = tyConDataCons rep_tc
869 con_whys = mapCatMaybes check_con data_cons
871 check_con :: DataCon -> Maybe SDoc
873 | isVanillaDataCon con
874 , all isTauTy (dataConOrigArgTys con) = Nothing
875 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
877 no_cons_why :: TyCon -> SDoc
878 no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
879 ptext (sLit "has no data constructors")
881 cond_enumOrProduct :: Condition
882 cond_enumOrProduct = cond_isEnumeration `orCond`
883 (cond_isProduct `andCond` cond_noUnliftedArgs)
885 cond_noUnliftedArgs :: Condition
886 -- For some classes (eg Eq, Ord) we allow unlifted arg types
887 -- by generating specilaised code. For others (eg Data) we don't.
888 cond_noUnliftedArgs (_, tc)
889 | null bad_cons = Nothing
890 | otherwise = Just why
892 bad_cons = [ con | con <- tyConDataCons tc
893 , any isUnLiftedType (dataConOrigArgTys con) ]
894 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
896 cond_isEnumeration :: Condition
897 cond_isEnumeration (_, rep_tc)
898 | isEnumerationTyCon rep_tc = Nothing
899 | otherwise = Just why
901 why = sep [ quotes (pprSourceTyCon rep_tc) <+>
902 ptext (sLit "is not an enumeration type")
903 , nest 2 $ ptext (sLit "(an enumeration consists of one or more nullary constructors)") ]
904 -- See Note [Enumeration types] in TyCon
906 cond_isProduct :: Condition
907 cond_isProduct (_, rep_tc)
908 | isProductTyCon rep_tc = Nothing
909 | otherwise = Just why
911 why = quotes (pprSourceTyCon rep_tc) <+>
912 ptext (sLit "does not have precisely one constructor")
914 cond_typeableOK :: Condition
915 -- OK for Typeable class
916 -- Currently: (a) args all of kind *
917 -- (b) 7 or fewer args
918 cond_typeableOK (_, tc)
919 | tyConArity tc > 7 = Just too_many
920 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tc))
922 | otherwise = Nothing
924 too_many = quotes (pprSourceTyCon tc) <+>
925 ptext (sLit "has too many arguments")
926 bad_kind = quotes (pprSourceTyCon tc) <+>
927 ptext (sLit "has arguments of kind other than `*'")
929 functorLikeClassKeys :: [Unique]
930 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
932 cond_functorOK :: Bool -> Condition
933 -- OK for Functor/Foldable/Traversable class
934 -- Currently: (a) at least one argument
935 -- (b) don't use argument contravariantly
936 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
937 -- (d) optionally: don't use function types
938 -- (e) no "stupid context" on data type
939 cond_functorOK allowFunctions (_, rep_tc)
941 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
942 <+> ptext (sLit "has no parameters"))
944 | not (null bad_stupid_theta)
945 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
946 <+> ptext (sLit "has a class context") <+> pprTheta bad_stupid_theta)
949 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
951 tc_tvs = tyConTyVars rep_tc
952 Just (_, last_tv) = snocView tc_tvs
953 bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
954 is_bad pred = last_tv `elemVarSet` tyVarsOfPred pred
956 data_cons = tyConDataCons rep_tc
957 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
959 check_vanilla :: DataCon -> Maybe SDoc
960 check_vanilla con | isVanillaDataCon con = Nothing
961 | otherwise = Just (badCon con existential)
963 ft_check :: DataCon -> FFoldType (Maybe SDoc)
964 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
965 , ft_co_var = Just (badCon con covariant)
966 , ft_fun = \x y -> if allowFunctions then x `mplus` y
967 else Just (badCon con functions)
968 , ft_tup = \_ xs -> msum xs
969 , ft_ty_app = \_ x -> x
970 , ft_bad_app = Just (badCon con wrong_arg)
971 , ft_forall = \_ x -> x }
973 existential = ptext (sLit "has existential arguments")
974 covariant = ptext (sLit "uses the type variable in a function argument")
975 functions = ptext (sLit "contains function types")
976 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
978 checkFlag :: ExtensionFlag -> Condition
979 checkFlag flag (dflags, _)
980 | xopt flag dflags = Nothing
981 | otherwise = Just why
983 why = ptext (sLit "You need -X") <> text flag_str
984 <+> ptext (sLit "to derive an instance for this class")
985 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
987 other -> pprPanic "checkFlag" (ppr other)
989 std_class_via_iso :: Class -> Bool
990 -- These standard classes can be derived for a newtype
991 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
992 -- because giving so gives the same results as generating the boilerplate
993 std_class_via_iso clas
994 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
995 -- Not Read/Show because they respect the type
996 -- Not Enum, because newtypes are never in Enum
999 non_iso_class :: Class -> Bool
1000 -- *Never* derive Read,Show,Typeable,Data by isomorphism,
1001 -- even with -XGeneralizedNewtypeDeriving
1003 = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
1006 typeableClassKeys :: [Unique]
1007 typeableClassKeys = map getUnique typeableClassNames
1009 new_dfun_name :: Class -> TyCon -> TcM Name
1010 new_dfun_name clas tycon -- Just a simple wrapper
1011 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1012 ; newDFunName clas [mkTyConApp tycon []] loc }
1013 -- The type passed to newDFunName is only used to generate
1014 -- a suitable string; hence the empty type arg list
1016 badCon :: DataCon -> SDoc -> SDoc
1017 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1020 Note [Superclasses of derived instance]
1021 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1022 In general, a derived instance decl needs the superclasses of the derived
1023 class too. So if we have
1024 data T a = ...deriving( Ord )
1025 then the initial context for Ord (T a) should include Eq (T a). Often this is
1026 redundant; we'll also generate an Ord constraint for each constructor argument,
1027 and that will probably generate enough constraints to make the Eq (T a) constraint
1028 be satisfied too. But not always; consider:
1034 data T a = MkT (S a) deriving( Ord )
1035 instance Num a => Eq (T a)
1037 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1039 data T a = MkT deriving( Data, Typeable )
1040 Here there *is* no argument field, but we must nevertheless generate
1041 a context for the Data instances:
1042 instance Typable a => Data (T a) where ...
1045 %************************************************************************
1049 %************************************************************************
1052 mkNewTypeEqn :: CtOrigin -> DynFlags -> [Var] -> Class
1053 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1055 -> TcRn EarlyDerivSpec
1056 mkNewTypeEqn orig dflags tvs
1057 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1058 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1059 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1060 = do { traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
1061 ; dfun_name <- new_dfun_name cls tycon
1062 ; loc <- getSrcSpanM
1063 ; let spec = DS { ds_loc = loc, ds_orig = orig
1064 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1065 , ds_cls = cls, ds_tys = inst_tys
1066 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1067 , ds_theta = mtheta `orElse` all_preds
1068 , ds_newtype = True }
1069 ; return (if isJust mtheta then Right spec
1073 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1074 CanDerive -> go_for_it -- Use the standard H98 method
1075 DerivableClassError msg -- Error with standard class
1076 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1077 | otherwise -> bale_out msg
1078 NonDerivableClass -- Must use newtype deriving
1079 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1080 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1081 | otherwise -> bale_out non_std
1083 newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
1084 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1085 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1087 non_std = nonStdErr cls
1088 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1090 -- Here is the plan for newtype derivings. We see
1091 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1092 -- where t is a type,
1093 -- ak+1...an is a suffix of a1..an, and are all tyars
1094 -- ak+1...an do not occur free in t, nor in the s1..sm
1095 -- (C s1 ... sm) is a *partial applications* of class C
1096 -- with the last parameter missing
1097 -- (T a1 .. ak) matches the kind of C's last argument
1098 -- (and hence so does t)
1099 -- The latter kind-check has been done by deriveTyData already,
1100 -- and tc_args are already trimmed
1102 -- We generate the instance
1103 -- instance forall ({a1..ak} u fvs(s1..sm)).
1104 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1105 -- where T a1...ap is the partial application of
1106 -- the LHS of the correct kind and p >= k
1108 -- NB: the variables below are:
1109 -- tc_tvs = [a1, ..., an]
1110 -- tyvars_to_keep = [a1, ..., ak]
1111 -- rep_ty = t ak .. an
1112 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1113 -- tys = [s1, ..., sm]
1116 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1117 -- We generate the instance
1118 -- instance Monad (ST s) => Monad (T s) where
1120 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1121 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1122 -- eta-reduces the representation type, so we know that
1124 -- That's convenient here, because we may have to apply
1125 -- it to fewer than its original complement of arguments
1127 -- Note [Newtype representation]
1128 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1129 -- Need newTyConRhs (*not* a recursive representation finder)
1130 -- to get the representation type. For example
1131 -- newtype B = MkB Int
1132 -- newtype A = MkA B deriving( Num )
1133 -- We want the Num instance of B, *not* the Num instance of Int,
1134 -- when making the Num instance of A!
1135 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1136 rep_tys = cls_tys ++ [rep_inst_ty]
1137 rep_pred = mkClassPred cls rep_tys
1138 -- rep_pred is the representation dictionary, from where
1139 -- we are gong to get all the methods for the newtype
1143 -- Next we figure out what superclass dictionaries to use
1144 -- See Note [Newtype deriving superclasses] above
1146 cls_tyvars = classTyVars cls
1147 dfun_tvs = tyVarsOfTypes inst_tys
1148 inst_ty = mkTyConApp tycon tc_args
1149 inst_tys = cls_tys ++ [inst_ty]
1150 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1153 -- If there are no tyvars, there's no need
1154 -- to abstract over the dictionaries we need
1155 -- Example: newtype T = MkT Int deriving( C )
1156 -- We get the derived instance
1159 -- instance C Int => C T
1160 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1162 -------------------------------------------------------------------
1163 -- Figuring out whether we can only do this newtype-deriving thing
1165 can_derive_via_isomorphism
1166 = not (non_iso_class cls)
1170 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1172 arity_ok = length cls_tys + 1 == classArity cls
1173 -- Well kinded; eg not: newtype T ... deriving( ST )
1174 -- because ST needs *2* type params
1176 -- Check that eta reduction is OK
1177 eta_ok = nt_eta_arity <= length rep_tc_args
1178 -- The newtype can be eta-reduced to match the number
1179 -- of type argument actually supplied
1180 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1181 -- Here the 'b' must be the same in the rep type (S [a] b)
1182 -- And the [a] must not mention 'b'. That's all handled
1185 ats_ok = null (classATs cls)
1186 -- No associated types for the class, because we don't
1187 -- currently generate type 'instance' decls; and cannot do
1188 -- so for 'data' instance decls
1191 = vcat [ ppUnless arity_ok arity_msg
1192 , ppUnless eta_ok eta_msg
1193 , ppUnless ats_ok ats_msg ]
1194 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1195 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1196 ats_msg = ptext (sLit "the class has associated types")
1199 Note [Recursive newtypes]
1200 ~~~~~~~~~~~~~~~~~~~~~~~~~
1201 Newtype deriving works fine, even if the newtype is recursive.
1202 e.g. newtype S1 = S1 [T1 ()]
1203 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1204 Remember, too, that type families are curretly (conservatively) given
1205 a recursive flag, so this also allows newtype deriving to work
1208 We used to exclude recursive types, because we had a rather simple
1209 minded way of generating the instance decl:
1211 instance Eq [A] => Eq A -- Makes typechecker loop!
1212 But now we require a simple context, so it's ok.
1215 %************************************************************************
1217 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1219 %************************************************************************
1221 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1222 terms, which is the final correct RHS for the corresponding original
1226 Each (k,TyVarTy tv) in a solution constrains only a type
1230 The (k,TyVarTy tv) pairs in a solution are canonically
1231 ordered by sorting on type varible, tv, (major key) and then class, k,
1236 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1238 inferInstanceContexts _ [] = return []
1240 inferInstanceContexts oflag infer_specs
1241 = do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
1242 ; iterate_deriv 1 initial_solutions }
1244 ------------------------------------------------------------------
1245 -- The initial solutions for the equations claim that each
1246 -- instance has an empty context; this solution is certainly
1247 -- in canonical form.
1248 initial_solutions :: [ThetaType]
1249 initial_solutions = [ [] | _ <- infer_specs ]
1251 ------------------------------------------------------------------
1252 -- iterate_deriv calculates the next batch of solutions,
1253 -- compares it with the current one; finishes if they are the
1254 -- same, otherwise recurses with the new solutions.
1255 -- It fails if any iteration fails
1256 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1257 iterate_deriv n current_solns
1258 | n > 20 -- Looks as if we are in an infinite loop
1259 -- This can happen if we have -XUndecidableInstances
1260 -- (See TcSimplify.tcSimplifyDeriv.)
1261 = pprPanic "solveDerivEqns: probable loop"
1262 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1264 = do { -- Extend the inst info from the explicit instance decls
1265 -- with the current set of solutions, and simplify each RHS
1266 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1267 current_solns infer_specs
1268 ; new_solns <- checkNoErrs $
1269 extendLocalInstEnv inst_specs $
1270 mapM gen_soln infer_specs
1272 ; if (current_solns == new_solns) then
1273 return [ spec { ds_theta = soln }
1274 | (spec, soln) <- zip infer_specs current_solns ]
1276 iterate_deriv (n+1) new_solns }
1278 ------------------------------------------------------------------
1279 gen_soln :: DerivSpec -> TcM [PredType]
1280 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1281 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1283 addErrCtxt (derivInstCtxt clas inst_tys) $
1284 do { -- Check for a bizarre corner case, when the derived instance decl should
1285 -- have form instance C a b => D (T a) where ...
1286 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1287 -- of problems; in particular, it's hard to compare solutions for
1288 -- equality when finding the fixpoint. Moreover, simplifyDeriv
1289 -- has an assert failure because it finds a TyVar when it expects
1290 -- only TcTyVars. So I just rule it out for now. I'm not
1291 -- even sure how it can arise.
1293 ; let tv_set = mkVarSet tyvars
1294 weird_preds = [pred | pred <- deriv_rhs
1295 , not (tyVarsOfPred pred `subVarSet` tv_set)]
1296 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1298 ; theta <- simplifyDeriv orig tyvars deriv_rhs
1299 -- checkValidInstance tyvars theta clas inst_tys
1300 -- Not necessary; see Note [Exotic derived instance contexts]
1303 ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
1304 -- Claim: the result instance declaration is guaranteed valid
1305 -- Hence no need to call:
1306 -- checkValidInstance tyvars theta clas inst_tys
1307 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1309 ------------------------------------------------------------------
1310 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1311 mkInstance overlap_flag theta
1312 (DS { ds_name = dfun_name
1313 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1314 = mkLocalInstance dfun overlap_flag
1316 dfun = mkDictFunId dfun_name tyvars theta clas tys
1319 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1320 -- Add new locally-defined instances; don't bother to check
1321 -- for functional dependency errors -- that'll happen in TcInstDcls
1322 extendLocalInstEnv dfuns thing_inside
1323 = do { env <- getGblEnv
1324 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1325 env' = env { tcg_inst_env = inst_env' }
1326 ; setGblEnv env' thing_inside }
1330 %************************************************************************
1332 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1334 %************************************************************************
1336 After all the trouble to figure out the required context for the
1337 derived instance declarations, all that's left is to chug along to
1338 produce them. They will then be shoved into @tcInstDecls2@, which
1339 will do all its usual business.
1341 There are lots of possibilities for code to generate. Here are
1342 various general remarks.
1347 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1348 ``you-couldn't-do-better-by-hand'' efficient.
1351 Deriving @Show@---also pretty common--- should also be reasonable good code.
1354 Deriving for the other classes isn't that common or that big a deal.
1361 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1364 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1367 We {\em normally} generate code only for the non-defaulted methods;
1368 there are some exceptions for @Eq@ and (especially) @Ord@...
1371 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1372 constructor's numeric (@Int#@) tag. These are generated by
1373 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1374 these is around is given by @hasCon2TagFun@.
1376 The examples under the different sections below will make this
1380 Much less often (really just for deriving @Ix@), we use a
1381 @_tag2con_<tycon>@ function. See the examples.
1384 We use the renamer!!! Reason: we're supposed to be
1385 producing @LHsBinds Name@ for the methods, but that means
1386 producing correctly-uniquified code on the fly. This is entirely
1387 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1388 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1389 the renamer. What a great hack!
1393 -- Generate the InstInfo for the required instance paired with the
1394 -- *representation* tycon for that instance,
1395 -- plus any auxiliary bindings required
1397 -- Representation tycons differ from the tycon in the instance signature in
1398 -- case of instances for indexed families.
1400 genInst :: Bool -- True <=> standalone deriving
1402 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1403 genInst standalone_deriv oflag spec
1405 = return (InstInfo { iSpec = mkInstance oflag (ds_theta spec) spec
1406 , iBinds = NewTypeDerived co rep_tycon }, [])
1409 = do { let loc = getSrcSpan (ds_name spec)
1410 inst = mkInstance oflag (ds_theta spec) spec
1413 -- In case of a family instance, we need to use the representation
1414 -- tycon (after all, it has the data constructors)
1415 ; fix_env <- getFixityEnv
1416 ; let (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1417 binds = VanillaInst meth_binds [] standalone_deriv
1418 ; return (InstInfo { iSpec = inst, iBinds = binds }, aux_binds)
1421 rep_tycon = ds_tc spec
1422 rep_tc_args = ds_tc_args spec
1423 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1424 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1426 -- Not a family => rep_tycon = main tycon
1427 co2 = case newTyConCo_maybe rep_tycon of
1428 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1429 Nothing -> id_co -- The newtype is transparent; no need for a cast
1430 co = co1 `mkTransCoI` co2
1431 id_co = IdCo (mkTyConApp rep_tycon rep_tc_args)
1433 -- Example: newtype instance N [a] = N1 (Tree a)
1434 -- deriving instance Eq b => Eq (N [(b,b)])
1435 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1436 -- When dealing with the deriving clause
1437 -- co1 : N [(b,b)] ~ R1:N (b,b)
1438 -- co2 : R1:N (b,b) ~ Tree (b,b)
1439 -- co : N [(b,b)] ~ Tree (b,b)
1441 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1442 genDerivBinds loc fix_env clas tycon
1443 | className clas `elem` typeableClassNames
1444 = (gen_Typeable_binds loc tycon, [])
1447 = case assocMaybe gen_list (getUnique clas) of
1448 Just gen_fn -> gen_fn loc tycon
1449 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1451 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1452 gen_list = [(eqClassKey, gen_Eq_binds)
1453 ,(ordClassKey, gen_Ord_binds)
1454 ,(enumClassKey, gen_Enum_binds)
1455 ,(boundedClassKey, gen_Bounded_binds)
1456 ,(ixClassKey, gen_Ix_binds)
1457 ,(showClassKey, gen_Show_binds fix_env)
1458 ,(readClassKey, gen_Read_binds fix_env)
1459 ,(dataClassKey, gen_Data_binds)
1460 ,(functorClassKey, gen_Functor_binds)
1461 ,(foldableClassKey, gen_Foldable_binds)
1462 ,(traversableClassKey, gen_Traversable_binds)
1467 %************************************************************************
1469 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1471 %************************************************************************
1474 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1475 derivingKindErr tc cls cls_tys cls_kind
1476 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1477 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1478 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1479 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1481 derivingEtaErr :: Class -> [Type] -> Type -> Message
1482 derivingEtaErr cls cls_tys inst_ty
1483 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1484 nest 2 (ptext (sLit "instance (...) =>")
1485 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1487 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1488 typeFamilyPapErr tc cls cls_tys inst_ty
1489 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1490 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1492 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1493 derivingThingErr newtype_deriving clas tys ty why
1494 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1495 2 (quotes (ppr pred))
1496 $$ nest 2 extra) <> colon,
1499 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1501 pred = mkClassPred clas (tys ++ [ty])
1503 derivingHiddenErr :: TyCon -> SDoc
1504 derivingHiddenErr tc
1505 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1506 2 (ptext (sLit "so you cannot derive an instance for it"))
1508 standaloneCtxt :: LHsType Name -> SDoc
1509 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1512 derivInstCtxt :: Class -> [Type] -> Message
1513 derivInstCtxt clas inst_tys
1514 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1516 badDerivedPred :: PredType -> Message
1518 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1519 ptext (sLit "type variables that are not data type parameters"),
1520 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]