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 EarlyDerivSpec = Either DerivSpec DerivSpec
96 -- Left ds => the context for the instance should be inferred
97 -- In this case ds_theta is the list of all the
98 -- constraints needed, such as (Eq [a], Eq a)
99 -- The inference process is to reduce this to a
100 -- simpler form (e.g. Eq a)
102 -- Right ds => the exact context for the instance is supplied
103 -- by the programmer; it is ds_theta
105 pprDerivSpec :: DerivSpec -> SDoc
106 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
107 ds_cls = c, ds_tys = tys, ds_theta = rhs })
108 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
109 <+> equals <+> ppr rhs)
113 Inferring missing contexts
114 ~~~~~~~~~~~~~~~~~~~~~~~~~~
117 data T a b = C1 (Foo a) (Bar b)
122 [NOTE: See end of these comments for what to do with
123 data (C a, D b) => T a b = ...
126 We want to come up with an instance declaration of the form
128 instance (Ping a, Pong b, ...) => Eq (T a b) where
131 It is pretty easy, albeit tedious, to fill in the code "...". The
132 trick is to figure out what the context for the instance decl is,
133 namely @Ping@, @Pong@ and friends.
135 Let's call the context reqd for the T instance of class C at types
136 (a,b, ...) C (T a b). Thus:
138 Eq (T a b) = (Ping a, Pong b, ...)
140 Now we can get a (recursive) equation from the @data@ decl:
142 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
143 u Eq (T b a) u Eq Int -- From C2
144 u Eq (T a a) -- From C3
146 Foo and Bar may have explicit instances for @Eq@, in which case we can
147 just substitute for them. Alternatively, either or both may have
148 their @Eq@ instances given by @deriving@ clauses, in which case they
149 form part of the system of equations.
151 Now all we need do is simplify and solve the equations, iterating to
152 find the least fixpoint. Notice that the order of the arguments can
153 switch around, as here in the recursive calls to T.
155 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
159 Eq (T a b) = {} -- The empty set
162 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
163 u Eq (T b a) u Eq Int -- From C2
164 u Eq (T a a) -- From C3
166 After simplification:
167 = Eq a u Ping b u {} u {} u {}
172 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
173 u Eq (T b a) u Eq Int -- From C2
174 u Eq (T a a) -- From C3
176 After simplification:
181 = Eq a u Ping b u Eq b u Ping a
183 The next iteration gives the same result, so this is the fixpoint. We
184 need to make a canonical form of the RHS to ensure convergence. We do
185 this by simplifying the RHS to a form in which
187 - the classes constrain only tyvars
188 - the list is sorted by tyvar (major key) and then class (minor key)
189 - no duplicates, of course
191 So, here are the synonyms for the ``equation'' structures:
194 Note [Data decl contexts]
195 ~~~~~~~~~~~~~~~~~~~~~~~~~
198 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
200 We will need an instance decl like:
202 instance (Read a, RealFloat a) => Read (Complex a) where
205 The RealFloat in the context is because the read method for Complex is bound
206 to construct a Complex, and doing that requires that the argument type is
209 But this ain't true for Show, Eq, Ord, etc, since they don't construct
210 a Complex; they only take them apart.
212 Our approach: identify the offending classes, and add the data type
213 context to the instance decl. The "offending classes" are
217 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
218 pattern matching against a constructor from a data type with a context
219 gives rise to the constraints for that context -- or at least the thinned
220 version. So now all classes are "offending".
222 Note [Newtype deriving]
223 ~~~~~~~~~~~~~~~~~~~~~~~
227 newtype T = T Char deriving( C [a] )
229 Notice the free 'a' in the deriving. We have to fill this out to
230 newtype T = T Char deriving( forall a. C [a] )
232 And then translate it to:
233 instance C [a] Char => C [a] T where ...
236 Note [Newtype deriving superclasses]
237 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
238 (See also Trac #1220 for an interesting exchange on newtype
239 deriving and superclasses.)
241 The 'tys' here come from the partial application in the deriving
242 clause. The last arg is the new instance type.
244 We must pass the superclasses; the newtype might be an instance
245 of them in a different way than the representation type
246 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
247 Then the Show instance is not done via isomorphism; it shows
249 The Num instance is derived via isomorphism, but the Show superclass
250 dictionary must the Show instance for Foo, *not* the Show dictionary
251 gotten from the Num dictionary. So we must build a whole new dictionary
252 not just use the Num one. The instance we want is something like:
253 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
256 There may be a coercion needed which we get from the tycon for the newtype
257 when the dict is constructed in TcInstDcls.tcInstDecl2
262 %************************************************************************
264 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
266 %************************************************************************
269 tcDeriving :: [LTyClDecl Name] -- All type constructors
270 -> [LInstDecl Name] -- All instance declarations
271 -> [LDerivDecl Name] -- All stand-alone deriving declarations
272 -> TcM ([InstInfo Name], -- The generated "instance decls"
273 HsValBinds Name) -- Extra generated top-level bindings
275 tcDeriving tycl_decls inst_decls deriv_decls
276 = recoverM (return ([], emptyValBindsOut)) $
277 do { -- Fish the "deriving"-related information out of the TcEnv
278 -- And make the necessary "equations".
279 is_boot <- tcIsHsBoot
280 ; traceTc (text "tcDeriving" <+> ppr is_boot)
281 ; early_specs <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
283 ; overlap_flag <- getOverlapFlag
284 ; let (infer_specs, given_specs) = splitEithers early_specs
285 ; insts1 <- mapM (genInst overlap_flag) given_specs
287 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
288 inferInstanceContexts overlap_flag infer_specs
290 ; insts2 <- mapM (genInst overlap_flag) final_specs
292 -- Generate the generic to/from functions from each type declaration
293 ; gen_binds <- mkGenericBinds is_boot
294 ; (inst_info, rn_binds) <- renameDeriv is_boot gen_binds (insts1 ++ insts2)
297 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
298 (ddump_deriving inst_info rn_binds))
300 ; return (inst_info, rn_binds) }
302 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
303 ddump_deriving inst_infos extra_binds
304 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
306 renameDeriv :: Bool -> LHsBinds RdrName
307 -> [(InstInfo RdrName, DerivAuxBinds)]
308 -> TcM ([InstInfo Name], HsValBinds Name)
309 renameDeriv is_boot gen_binds insts
310 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
311 -- The inst-info bindings will all be empty, but it's easier to
312 -- just use rn_inst_info to change the type appropriately
313 = do { rn_inst_infos <- mapM rn_inst_info inst_infos
314 ; return (rn_inst_infos, emptyValBindsOut) }
317 = discardWarnings $ -- Discard warnings about unused bindings etc
318 do { (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $ -- Type signatures in patterns
319 -- are used in the generic binds
320 rnTopBinds (ValBindsIn gen_binds [])
321 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
323 -- Generate and rename any extra not-one-inst-decl-specific binds,
324 -- notably "con2tag" and/or "tag2con" functions.
325 -- Bring those names into scope before renaming the instances themselves
326 ; loc <- getSrcSpanM -- Generic loc for shared bindings
327 ; let aux_binds = listToBag $ map (genAuxBind loc) $
328 rm_dups [] $ concat deriv_aux_binds
329 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv (ValBindsIn aux_binds [])
330 ; let aux_names = map unLoc (collectHsValBinders rn_aux_lhs)
332 ; bindLocalNames aux_names $
333 do { (rn_aux, _dus) <- rnTopBindsRHS (mkNameSet aux_names) rn_aux_lhs
334 ; rn_inst_infos <- mapM rn_inst_info inst_infos
335 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen) } }
338 (inst_infos, deriv_aux_binds) = unzip insts
340 -- Remove duplicate requests for auxilliary bindings
342 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
343 | otherwise = rm_dups (b:acc) bs
346 rn_inst_info (InstInfo { iSpec = inst, iBinds = NewTypeDerived co })
347 = return (InstInfo { iSpec = inst, iBinds = NewTypeDerived co })
349 rn_inst_info (InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs })
350 = -- Bring the right type variables into
351 -- scope (yuk), and rename the method binds
353 bindLocalNames (map Var.varName tyvars) $
354 do { (rn_binds, _fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
355 ; return (InstInfo { iSpec = inst, iBinds = VanillaInst rn_binds [] }) }
357 (tyvars,_,clas,_) = instanceHead inst
358 clas_nm = className clas
360 -----------------------------------------
361 mkGenericBinds :: Bool -> TcM (LHsBinds RdrName)
362 mkGenericBinds is_boot
366 = do { gbl_env <- getGblEnv
367 ; let tcs = typeEnvTyCons (tcg_type_env gbl_env)
368 ; return (unionManyBags [ mkTyConGenericBinds tc |
369 tc <- tcs, tyConHasGenerics tc ]) }
370 -- We are only interested in the data type declarations,
371 -- and then only in the ones whose 'has-generics' flag is on
372 -- The predicate tyConHasGenerics finds both of these
376 %************************************************************************
378 From HsSyn to DerivSpec
380 %************************************************************************
382 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
385 makeDerivSpecs :: Bool
389 -> TcM [EarlyDerivSpec]
391 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
392 | is_boot -- No 'deriving' at all in hs-boot files
393 = do { mapM_ add_deriv_err deriv_locs
396 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
397 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
398 ; return (eqns1 ++ eqns2) }
400 extractTyDataPreds decls
401 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
403 all_tydata :: [(LHsType Name, LTyClDecl Name)]
404 -- Derived predicate paired with its data type declaration
405 all_tydata = extractTyDataPreds tycl_decls ++
406 [ pd -- Traverse assoc data families
407 | L _ (InstDecl _ _ _ ats) <- inst_decls
408 , pd <- extractTyDataPreds ats ]
410 deriv_locs = map (getLoc . snd) all_tydata
411 ++ map getLoc deriv_decls
413 add_deriv_err loc = setSrcSpan loc $
414 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
415 2 (ptext (sLit "Use an instance declaration instead")))
417 ------------------------------------------------------------------
418 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
419 -- Standalone deriving declarations
420 -- e.g. deriving instance Show a => Show (T a)
421 -- Rather like tcLocalInstDecl
422 deriveStandalone (L loc (DerivDecl deriv_ty))
424 addErrCtxt (standaloneCtxt deriv_ty) $
425 do { traceTc (text "standalone deriving decl for" <+> ppr deriv_ty)
426 ; (tvs, theta, tau) <- tcHsInstHead deriv_ty
427 ; traceTc (text "standalone deriving;"
428 <+> text "tvs:" <+> ppr tvs
429 <+> text "theta:" <+> ppr theta
430 <+> text "tau:" <+> ppr tau)
431 ; (cls, inst_tys) <- checkValidInstHead tau
432 ; checkValidInstance tvs theta cls inst_tys
433 -- C.f. TcInstDcls.tcLocalInstDecl1
435 ; let cls_tys = take (length inst_tys - 1) inst_tys
436 inst_ty = last inst_tys
437 ; traceTc (text "standalone deriving;"
438 <+> text "class:" <+> ppr cls
439 <+> text "class types:" <+> ppr cls_tys
440 <+> text "type:" <+> ppr inst_ty)
441 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
444 ------------------------------------------------------------------
445 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
446 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
447 tcdTyVars = tv_names,
448 tcdTyPats = ty_pats }))
449 = setSrcSpan loc $ -- Use the location of the 'deriving' item
451 do { (tvs, tc, tc_args) <- get_lhs ty_pats
452 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
453 -- the type variables for the type constructor
455 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
456 -- The "deriv_pred" is a LHsType to take account of the fact that for
457 -- newtype deriving we allow deriving (forall a. C [a]).
459 -- Given data T a b c = ... deriving( C d ),
460 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
461 ; let cls_tyvars = classTyVars cls
462 kind = tyVarKind (last cls_tyvars)
463 (arg_kinds, _) = splitKindFunTys kind
464 n_args_to_drop = length arg_kinds
465 n_args_to_keep = tyConArity tc - n_args_to_drop
466 args_to_drop = drop n_args_to_keep tc_args
467 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
468 inst_ty_kind = typeKind inst_ty
469 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
470 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
471 `minusVarSet` dropped_tvs
473 -- Check that the result really is well-kinded
474 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
475 (derivingKindErr tc cls cls_tys kind)
477 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
478 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
479 (derivingEtaErr cls cls_tys inst_ty)
481 -- (a) The data type can be eta-reduced; eg reject:
482 -- data instance T a a = ... deriving( Monad )
483 -- (b) The type class args do not mention any of the dropped type
485 -- newtype T a s = ... deriving( ST s )
487 -- Type families can't be partially applied
488 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
489 -- Note [Deriving, type families, and partial applications]
490 ; checkTc (not (isOpenTyCon tc) || n_args_to_drop == 0)
491 (typeFamilyPapErr tc cls cls_tys inst_ty)
493 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
495 -- Tiresomely we must figure out the "lhs", which is awkward for type families
496 -- E.g. data T a b = .. deriving( Eq )
497 -- Here, the lhs is (T a b)
498 -- data instance TF Int b = ... deriving( Eq )
499 -- Here, the lhs is (TF Int b)
500 -- But if we just look up the tycon_name, we get is the *family*
501 -- tycon, but not pattern types -- they are in the *rep* tycon.
502 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
503 ; let tvs = tyConTyVars tc
504 ; return (tvs, tc, mkTyVarTys tvs) }
505 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
506 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
507 ; let (tc, tc_args) = tcSplitTyConApp tc_app
508 ; return (tvs, tc, tc_args) }
511 = panic "derivTyData" -- Caller ensures that only TyData can happen
514 Note [Deriving, type families, and partial applications]
515 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
516 When there are no type families, it's quite easy:
518 newtype S a = MkS [a]
519 -- :CoS :: S ~ [] -- Eta-reduced
521 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (coMkS a)) : Eq [a] ~ Eq (S a)
522 instance Monad [] => Monad S -- by coercion sym (Monad coMkS) : Monad [] ~ Monad S
524 When type familes are involved it's trickier:
527 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
528 -- :RT is the representation type for (T Int a)
529 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
530 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
532 instance Eq [a] => Eq (T Int a) -- easy by coercion
533 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
535 The "???" bit is that we don't build the :CoF thing in eta-reduced form
536 Henc the current typeFamilyPapErr, even though the instance makes sense.
537 After all, we can write it out
538 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
543 mkEqnHelp :: InstOrigin -> [TyVar] -> Class -> [Type] -> Type
544 -> Maybe ThetaType -- Just => context supplied (standalone deriving)
545 -- Nothing => context inferred (deriving on data decl)
546 -> TcRn EarlyDerivSpec
547 -- Make the EarlyDerivSpec for an instance
548 -- forall tvs. theta => cls (tys ++ [ty])
549 -- where the 'theta' is optional (that's the Maybe part)
550 -- Assumes that this declaration is well-kinded
552 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
553 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
554 , isAlgTyCon tycon -- Check for functions, primitive types etc
555 = do { (rep_tc, rep_tc_args) <- tcLookupFamInstExact tycon tc_args
556 -- Be careful to test rep_tc here: in the case of families,
557 -- we want to check the instance tycon, not the family tycon
559 -- For standalone deriving (mtheta /= Nothing),
560 -- check that all the data constructors are in scope.
561 -- No need for this when deriving Typeable, becuase we don't need
562 -- the constructors for that.
563 ; rdr_env <- getGlobalRdrEnv
564 ; let hidden_data_cons = isAbstractTyCon rep_tc || any not_in_scope (tyConDataCons rep_tc)
565 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
566 ; checkTc (isNothing mtheta ||
567 not hidden_data_cons ||
568 className cls `elem` typeableClassNames)
569 (derivingHiddenErr tycon)
572 ; if isDataTyCon rep_tc then
573 mkDataTypeEqn orig dflags tvs cls cls_tys
574 tycon tc_args rep_tc rep_tc_args mtheta
576 mkNewTypeEqn orig dflags tvs cls cls_tys
577 tycon tc_args rep_tc rep_tc_args mtheta }
579 = failWithTc (derivingThingErr cls cls_tys tc_app
580 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
583 Note [Looking up family instances for deriving]
584 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
585 tcLookupFamInstExact is an auxiliary lookup wrapper which requires
586 that looked-up family instances exist. If called with a vanilla
587 tycon, the old type application is simply returned.
590 data instance F () = ... deriving Eq
591 data instance F () = ... deriving Eq
592 then tcLookupFamInstExact will be confused by the two matches;
593 but that can't happen because tcInstDecls1 doesn't call tcDeriving
594 if there are any overlaps.
596 There are two other things that might go wrong with the lookup.
597 First, we might see a standalone deriving clause
599 when there is no data instance F () in scope.
601 Note that it's OK to have
602 data instance F [a] = ...
603 deriving Eq (F [(a,b)])
604 where the match is not exact; the same holds for ordinary data types
605 with standalone deriving declrations.
608 tcLookupFamInstExact :: TyCon -> [Type] -> TcM (TyCon, [Type])
609 tcLookupFamInstExact tycon tys
610 | not (isOpenTyCon tycon)
611 = return (tycon, tys)
613 = do { maybeFamInst <- tcLookupFamInst tycon tys
614 ; case maybeFamInst of
615 Nothing -> famInstNotFound tycon tys
616 Just famInst -> return famInst
619 famInstNotFound :: TyCon -> [Type] -> TcM a
620 famInstNotFound tycon tys
621 = failWithTc (ptext (sLit "No family instance for")
622 <+> quotes (pprTypeApp tycon tys))
626 %************************************************************************
630 %************************************************************************
633 mkDataTypeEqn :: InstOrigin
635 -> [Var] -- Universally quantified type variables in the instance
636 -> Class -- Class for which we need to derive an instance
637 -> [Type] -- Other parameters to the class except the last
638 -> TyCon -- Type constructor for which the instance is requested (last parameter to the type class)
639 -> [Type] -- Parameters to the type constructor
640 -> TyCon -- rep of the above (for type families)
641 -> [Type] -- rep of the above
642 -> Maybe ThetaType -- Context of the instance, for standalone deriving
643 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
645 mkDataTypeEqn orig dflags tvs cls cls_tys
646 tycon tc_args rep_tc rep_tc_args mtheta
647 = case checkSideConditions dflags cls cls_tys rep_tc of
648 -- NB: pass the *representation* tycon to checkSideConditions
649 CanDerive -> mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
650 NonDerivableClass -> bale_out (nonStdErr cls)
651 DerivableClassError msg -> bale_out msg
653 bale_out msg = failWithTc (derivingThingErr cls cls_tys (mkTyConApp tycon tc_args) msg)
655 mk_data_eqn, mk_typeable_eqn
656 :: InstOrigin -> [TyVar] -> Class
657 -> TyCon -> [TcType] -> TyCon -> [TcType] -> Maybe ThetaType
658 -> TcM EarlyDerivSpec
659 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
660 | getName cls `elem` typeableClassNames
661 = mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
664 = do { dfun_name <- new_dfun_name cls tycon
666 ; let ordinary_constraints
667 = [ mkClassPred cls [arg_ty]
668 | data_con <- tyConDataCons rep_tc,
669 arg_ty <- ASSERT( isVanillaDataCon data_con )
670 get_constrained_tys $
672 dataConInstOrigArgTys data_con all_rep_tc_args,
673 not (isUnLiftedType arg_ty) ]
674 -- No constraints for unlifted types
675 -- Where they are legal we generate specilised function calls
677 -- For functor-like classes, two things are different
678 -- (a) We recurse over argument types to generate constraints
679 -- See Functor examples in TcGenDeriv
680 -- (b) The rep_tc_args will be one short
681 is_functor_like = getUnique cls `elem` functorLikeClassKeys
683 get_constrained_tys :: [Type] -> [Type]
684 get_constrained_tys tys
685 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
688 rep_tc_tvs = tyConTyVars rep_tc
689 last_tv = last rep_tc_tvs
690 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
691 | otherwise = rep_tc_args
694 -- See Note [Superclasses of derived instance]
695 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
697 inst_tys = [mkTyConApp tycon tc_args]
698 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
699 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
701 all_constraints = stupid_constraints ++ sc_constraints ++ ordinary_constraints
703 spec = DS { ds_loc = loc, ds_orig = orig
704 , ds_name = dfun_name, ds_tvs = tvs
705 , ds_cls = cls, ds_tys = inst_tys
706 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
707 , ds_theta = mtheta `orElse` all_constraints
708 , ds_newtype = False }
710 ; ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr tycon )
711 return (if isJust mtheta then Right spec -- Specified context
712 else Left spec) } -- Infer context
714 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
715 -- The Typeable class is special in several ways
716 -- data T a b = ... deriving( Typeable )
718 -- instance Typeable2 T where ...
720 -- 1. There are no constraints in the instance
721 -- 2. There are no type variables either
722 -- 3. The actual class we want to generate isn't necessarily
723 -- Typeable; it depends on the arity of the type
724 | isNothing mtheta -- deriving on a data type decl
725 = do { checkTc (cls `hasKey` typeableClassKey)
726 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
727 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
728 ; mk_typeable_eqn orig tvs real_cls tycon [] rep_tc [] (Just []) }
730 | otherwise -- standaone deriving
731 = do { checkTc (null tc_args)
732 (ptext (sLit "Derived typeable instance must be of form (Typeable")
733 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
734 ; dfun_name <- new_dfun_name cls tycon
737 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
738 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
739 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
740 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
743 ------------------------------------------------------------------
744 -- Check side conditions that dis-allow derivability for particular classes
745 -- This is *apart* from the newtype-deriving mechanism
747 -- Here we get the representation tycon in case of family instances as it has
748 -- the data constructors - but we need to be careful to fall back to the
749 -- family tycon (with indexes) in error messages.
751 data DerivStatus = CanDerive
752 | DerivableClassError SDoc -- Standard class, but can't do it
753 | NonDerivableClass -- Non-standard class
755 checkSideConditions :: DynFlags -> Class -> [TcType] -> TyCon -> DerivStatus
756 checkSideConditions dflags cls cls_tys rep_tc
757 | Just cond <- sideConditions cls
758 = case (cond (dflags, rep_tc)) of
759 Just err -> DerivableClassError err -- Class-specific error
760 Nothing | null cls_tys -> CanDerive
761 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
762 | otherwise = NonDerivableClass -- Not a standard class
764 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
766 nonStdErr :: Class -> SDoc
767 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
769 sideConditions :: Class -> Maybe Condition
771 | cls_key == eqClassKey = Just cond_std
772 | cls_key == ordClassKey = Just cond_std
773 | cls_key == showClassKey = Just cond_std
774 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
775 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
776 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
777 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
778 | cls_key == dataClassKey = Just (cond_mayDeriveDataTypeable `andCond` cond_std `andCond` cond_noUnliftedArgs)
779 | cls_key == functorClassKey = Just (cond_std `andCond` cond_functorOK True)
780 | cls_key == foldableClassKey = Just (cond_std `andCond` cond_functorOK False)
781 | cls_key == traversableClassKey = Just (cond_std `andCond` cond_functorOK False)
782 | getName cls `elem` typeableClassNames = Just (cond_mayDeriveDataTypeable `andCond` cond_typeableOK)
783 | otherwise = Nothing
785 cls_key = getUnique cls
787 type Condition = (DynFlags, TyCon) -> Maybe SDoc
788 -- first Bool is whether or not we are allowed to derive Data and Typeable
789 -- second Bool is whether or not we are allowed to derive Functor
790 -- TyCon is the *representation* tycon if the
791 -- data type is an indexed one
794 orCond :: Condition -> Condition -> Condition
797 Nothing -> Nothing -- c1 succeeds
798 Just x -> case c2 tc of -- c1 fails
800 Just y -> Just (x $$ ptext (sLit " and") $$ y)
803 andCond :: Condition -> Condition -> Condition
804 andCond c1 c2 tc = case c1 tc of
805 Nothing -> c2 tc -- c1 succeeds
806 Just x -> Just x -- c1 fails
808 cond_std :: Condition
810 | any (not . isVanillaDataCon) data_cons = Just existential_why
811 | null data_cons = Just no_cons_why
812 | otherwise = Nothing
814 data_cons = tyConDataCons rep_tc
815 no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
816 ptext (sLit "has no data constructors")
817 existential_why = quotes (pprSourceTyCon rep_tc) <+>
818 ptext (sLit "has non-Haskell-98 constructor(s)")
820 cond_enumOrProduct :: Condition
821 cond_enumOrProduct = cond_isEnumeration `orCond`
822 (cond_isProduct `andCond` cond_noUnliftedArgs)
824 cond_noUnliftedArgs :: Condition
825 -- For some classes (eg Eq, Ord) we allow unlifted arg types
826 -- by generating specilaised code. For others (eg Data) we don't.
827 cond_noUnliftedArgs (_, tc)
828 | null bad_cons = Nothing
829 | otherwise = Just why
831 bad_cons = [ con | con <- tyConDataCons tc
832 , any isUnLiftedType (dataConOrigArgTys con) ]
833 why = ptext (sLit "Constructor") <+> quotes (ppr (head bad_cons))
834 <+> ptext (sLit "has arguments of unlifted type")
836 cond_isEnumeration :: Condition
837 cond_isEnumeration (_, rep_tc)
838 | isEnumerationTyCon rep_tc = Nothing
839 | otherwise = Just why
841 why = quotes (pprSourceTyCon rep_tc) <+>
842 ptext (sLit "has non-nullary constructors")
844 cond_isProduct :: Condition
845 cond_isProduct (_, rep_tc)
846 | isProductTyCon rep_tc = Nothing
847 | otherwise = Just why
849 why = quotes (pprSourceTyCon rep_tc) <+>
850 ptext (sLit "has more than one constructor")
852 cond_typeableOK :: Condition
853 -- OK for Typeable class
854 -- Currently: (a) args all of kind *
855 -- (b) 7 or fewer args
856 cond_typeableOK (_, rep_tc)
857 | tyConArity rep_tc > 7 = Just too_many
858 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
860 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
861 | otherwise = Nothing
863 too_many = quotes (pprSourceTyCon rep_tc) <+>
864 ptext (sLit "has too many arguments")
865 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
866 ptext (sLit "has arguments of kind other than `*'")
867 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
868 ptext (sLit "is a type family")
871 functorLikeClassKeys :: [Unique]
872 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
874 cond_functorOK :: Bool -> Condition
875 -- OK for Functor class
876 -- Currently: (a) at least one argument
877 -- (b) don't use argument contravariantly
878 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
879 -- (d) optionally: don't use function types
880 cond_functorOK allowFunctions (dflags, rep_tc)
881 | not (dopt Opt_DeriveFunctor dflags)
882 = Just (ptext (sLit "You need -XDeriveFunctor to derive an instance for this class"))
884 = msum (map check con_types)
886 data_cons = tyConDataCons rep_tc
887 con_types = concatMap dataConOrigArgTys data_cons
888 check = functorLikeTraverse
892 (\x y -> if allowFunctions then x `mplus` y else Just functions)
897 (last (tyConTyVars rep_tc))
898 covariant = quotes (pprSourceTyCon rep_tc) <+>
899 ptext (sLit "uses the type variable in a function argument")
900 functions = quotes (pprSourceTyCon rep_tc) <+>
901 ptext (sLit "contains function types")
902 wrong_arg = quotes (pprSourceTyCon rep_tc) <+>
903 ptext (sLit "uses the type variable in an argument other than the last")
905 cond_mayDeriveDataTypeable :: Condition
906 cond_mayDeriveDataTypeable (dflags, _)
907 | dopt Opt_DeriveDataTypeable dflags = Nothing
908 | otherwise = Just why
910 why = ptext (sLit "You need -XDeriveDataTypeable to derive an instance for this class")
912 std_class_via_iso :: Class -> Bool
913 std_class_via_iso clas -- These standard classes can be derived for a newtype
914 -- using the isomorphism trick *even if no -fglasgow-exts*
915 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
916 -- Not Read/Show because they respect the type
917 -- Not Enum, because newtypes are never in Enum
920 new_dfun_name :: Class -> TyCon -> TcM Name
921 new_dfun_name clas tycon -- Just a simple wrapper
922 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
923 ; newDFunName clas [mkTyConApp tycon []] loc }
924 -- The type passed to newDFunName is only used to generate
925 -- a suitable string; hence the empty type arg list
928 Note [Superclasses of derived instance]
929 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
930 In general, a derived instance decl needs the superclasses of the derived
931 class too. So if we have
932 data T a = ...deriving( Ord )
933 then the initial context for Ord (T a) should include Eq (T a). Often this is
934 redundant; we'll also generate an Ord constraint for each constructor argument,
935 and that will probably generate enough constraints to make the Eq (T a) constraint
936 be satisfied too. But not always; consider:
942 data T a = MkT (S a) deriving( Ord )
943 instance Num a => Eq (T a)
945 The derived instance for (Ord (T a)) must have a (Num a) constraint!
947 data T a = MkT deriving( Data, Typeable )
948 Here there *is* no argument field, but we must nevertheless generate
949 a context for the Data instances:
950 instance Typable a => Data (T a) where ...
953 %************************************************************************
957 %************************************************************************
960 mkNewTypeEqn :: InstOrigin -> DynFlags -> [Var] -> Class
961 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
963 -> TcRn EarlyDerivSpec
964 mkNewTypeEqn orig dflags tvs
965 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
966 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
967 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
968 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
969 ; dfun_name <- new_dfun_name cls tycon
971 ; let spec = DS { ds_loc = loc, ds_orig = orig
972 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
973 , ds_cls = cls, ds_tys = inst_tys
974 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
975 , ds_theta = mtheta `orElse` all_preds
976 , ds_newtype = True }
977 ; return (if isJust mtheta then Right spec
981 = case check_conditions of
982 CanDerive -> mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
983 -- Use the standard H98 method
984 DerivableClassError msg -> bale_out msg -- Error with standard class
985 NonDerivableClass -- Must use newtype deriving
986 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
987 | otherwise -> bale_out non_std_err -- Try newtype deriving!
989 newtype_deriving = dopt Opt_GeneralizedNewtypeDeriving dflags
990 check_conditions = checkSideConditions dflags cls cls_tys rep_tycon
991 bale_out msg = failWithTc (derivingThingErr cls cls_tys inst_ty msg)
993 non_std_err = nonStdErr cls $$
994 ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
996 -- Here is the plan for newtype derivings. We see
997 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
998 -- where t is a type,
999 -- ak+1...an is a suffix of a1..an, and are all tyars
1000 -- ak+1...an do not occur free in t, nor in the s1..sm
1001 -- (C s1 ... sm) is a *partial applications* of class C
1002 -- with the last parameter missing
1003 -- (T a1 .. ak) matches the kind of C's last argument
1004 -- (and hence so does t)
1005 -- The latter kind-check has been done by deriveTyData already,
1006 -- and tc_args are already trimmed
1008 -- We generate the instance
1009 -- instance forall ({a1..ak} u fvs(s1..sm)).
1010 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1011 -- where T a1...ap is the partial application of
1012 -- the LHS of the correct kind and p >= k
1014 -- NB: the variables below are:
1015 -- tc_tvs = [a1, ..., an]
1016 -- tyvars_to_keep = [a1, ..., ak]
1017 -- rep_ty = t ak .. an
1018 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1019 -- tys = [s1, ..., sm]
1022 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1023 -- We generate the instance
1024 -- instance Monad (ST s) => Monad (T s) where
1026 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1027 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1028 -- eta-reduces the representation type, so we know that
1030 -- That's convenient here, because we may have to apply
1031 -- it to fewer than its original complement of arguments
1033 -- Note [Newtype representation]
1034 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1035 -- Need newTyConRhs (*not* a recursive representation finder)
1036 -- to get the representation type. For example
1037 -- newtype B = MkB Int
1038 -- newtype A = MkA B deriving( Num )
1039 -- We want the Num instance of B, *not* the Num instance of Int,
1040 -- when making the Num instance of A!
1041 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1042 rep_tys = cls_tys ++ [rep_inst_ty]
1043 rep_pred = mkClassPred cls rep_tys
1044 -- rep_pred is the representation dictionary, from where
1045 -- we are gong to get all the methods for the newtype
1049 -- Next we figure out what superclass dictionaries to use
1050 -- See Note [Newtype deriving superclasses] above
1052 cls_tyvars = classTyVars cls
1053 dfun_tvs = tyVarsOfTypes inst_tys
1054 inst_ty = mkTyConApp tycon tc_args
1055 inst_tys = cls_tys ++ [inst_ty]
1056 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1059 -- If there are no tyvars, there's no need
1060 -- to abstract over the dictionaries we need
1061 -- Example: newtype T = MkT Int deriving( C )
1062 -- We get the derived instance
1065 -- instance C Int => C T
1066 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1068 -------------------------------------------------------------------
1069 -- Figuring out whether we can only do this newtype-deriving thing
1071 can_derive_via_isomorphism
1072 = not (non_iso_class cls)
1076 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1078 -- Never derive Read,Show,Typeable,Data by isomorphism
1079 non_iso_class cls = className cls `elem` ([readClassName, showClassName, dataClassName] ++
1082 arity_ok = length cls_tys + 1 == classArity cls
1083 -- Well kinded; eg not: newtype T ... deriving( ST )
1084 -- because ST needs *2* type params
1086 -- Check that eta reduction is OK
1087 eta_ok = nt_eta_arity <= length rep_tc_args
1088 -- The newtype can be eta-reduced to match the number
1089 -- of type argument actually supplied
1090 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1091 -- Here the 'b' must be the same in the rep type (S [a] b)
1092 -- And the [a] must not mention 'b'. That's all handled
1095 ats_ok = null (classATs cls)
1096 -- No associated types for the class, because we don't
1097 -- currently generate type 'instance' decls; and cannot do
1098 -- so for 'data' instance decls
1101 = vcat [ ptext (sLit "even with cunning newtype deriving:")
1102 , if arity_ok then empty else arity_msg
1103 , if eta_ok then empty else eta_msg
1104 , if ats_ok then empty else ats_msg ]
1105 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1106 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1107 ats_msg = ptext (sLit "the class has associated types")
1110 Note [Recursive newtypes]
1111 ~~~~~~~~~~~~~~~~~~~~~~~~~
1112 Newtype deriving works fine, even if the newtype is recursive.
1113 e.g. newtype S1 = S1 [T1 ()]
1114 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1115 Remember, too, that type families are curretly (conservatively) given
1116 a recursive flag, so this also allows newtype deriving to work
1119 We used to exclude recursive types, because we had a rather simple
1120 minded way of generating the instance decl:
1122 instance Eq [A] => Eq A -- Makes typechecker loop!
1123 But now we require a simple context, so it's ok.
1126 %************************************************************************
1128 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1130 %************************************************************************
1132 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1133 terms, which is the final correct RHS for the corresponding original
1137 Each (k,TyVarTy tv) in a solution constrains only a type
1141 The (k,TyVarTy tv) pairs in a solution are canonically
1142 ordered by sorting on type varible, tv, (major key) and then class, k,
1147 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1149 inferInstanceContexts _ [] = return []
1151 inferInstanceContexts oflag infer_specs
1152 = do { traceTc (text "inferInstanceContexts" <+> vcat (map pprDerivSpec infer_specs))
1153 ; iterate_deriv 1 initial_solutions }
1155 ------------------------------------------------------------------
1156 -- The initial solutions for the equations claim that each
1157 -- instance has an empty context; this solution is certainly
1158 -- in canonical form.
1159 initial_solutions :: [ThetaType]
1160 initial_solutions = [ [] | _ <- infer_specs ]
1162 ------------------------------------------------------------------
1163 -- iterate_deriv calculates the next batch of solutions,
1164 -- compares it with the current one; finishes if they are the
1165 -- same, otherwise recurses with the new solutions.
1166 -- It fails if any iteration fails
1167 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1168 iterate_deriv n current_solns
1169 | n > 20 -- Looks as if we are in an infinite loop
1170 -- This can happen if we have -XUndecidableInstances
1171 -- (See TcSimplify.tcSimplifyDeriv.)
1172 = pprPanic "solveDerivEqns: probable loop"
1173 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1175 = do { -- Extend the inst info from the explicit instance decls
1176 -- with the current set of solutions, and simplify each RHS
1177 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1178 current_solns infer_specs
1179 ; new_solns <- checkNoErrs $
1180 extendLocalInstEnv inst_specs $
1181 mapM gen_soln infer_specs
1183 ; if (current_solns == new_solns) then
1184 return [ spec { ds_theta = soln }
1185 | (spec, soln) <- zip infer_specs current_solns ]
1187 iterate_deriv (n+1) new_solns }
1189 ------------------------------------------------------------------
1190 gen_soln :: DerivSpec -> TcM [PredType]
1191 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1192 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1194 addErrCtxt (derivInstCtxt clas inst_tys) $
1195 do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
1196 -- checkValidInstance tyvars theta clas inst_tys
1197 -- Not necessary; see Note [Exotic derived instance contexts]
1200 -- Check for a bizarre corner case, when the derived instance decl should
1201 -- have form instance C a b => D (T a) where ...
1202 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1203 -- of problems; in particular, it's hard to compare solutions for
1204 -- equality when finding the fixpoint. So I just rule it out for now.
1205 ; let tv_set = mkVarSet tyvars
1206 weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
1207 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1209 -- Claim: the result instance declaration is guaranteed valid
1210 -- Hence no need to call:
1211 -- checkValidInstance tyvars theta clas inst_tys
1212 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1214 ------------------------------------------------------------------
1215 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1216 mkInstance overlap_flag theta
1217 (DS { ds_name = dfun_name
1218 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1219 = mkLocalInstance dfun overlap_flag
1221 dfun = mkDictFunId dfun_name tyvars theta clas tys
1224 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1225 -- Add new locally-defined instances; don't bother to check
1226 -- for functional dependency errors -- that'll happen in TcInstDcls
1227 extendLocalInstEnv dfuns thing_inside
1228 = do { env <- getGblEnv
1229 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1230 env' = env { tcg_inst_env = inst_env' }
1231 ; setGblEnv env' thing_inside }
1235 %************************************************************************
1237 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1239 %************************************************************************
1241 After all the trouble to figure out the required context for the
1242 derived instance declarations, all that's left is to chug along to
1243 produce them. They will then be shoved into @tcInstDecls2@, which
1244 will do all its usual business.
1246 There are lots of possibilities for code to generate. Here are
1247 various general remarks.
1252 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1253 ``you-couldn't-do-better-by-hand'' efficient.
1256 Deriving @Show@---also pretty common--- should also be reasonable good code.
1259 Deriving for the other classes isn't that common or that big a deal.
1266 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1269 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1272 We {\em normally} generate code only for the non-defaulted methods;
1273 there are some exceptions for @Eq@ and (especially) @Ord@...
1276 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1277 constructor's numeric (@Int#@) tag. These are generated by
1278 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1279 these is around is given by @hasCon2TagFun@.
1281 The examples under the different sections below will make this
1285 Much less often (really just for deriving @Ix@), we use a
1286 @_tag2con_<tycon>@ function. See the examples.
1289 We use the renamer!!! Reason: we're supposed to be
1290 producing @LHsBinds Name@ for the methods, but that means
1291 producing correctly-uniquified code on the fly. This is entirely
1292 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1293 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1294 the renamer. What a great hack!
1298 -- Generate the InstInfo for the required instance paired with the
1299 -- *representation* tycon for that instance,
1300 -- plus any auxiliary bindings required
1302 -- Representation tycons differ from the tycon in the instance signature in
1303 -- case of instances for indexed families.
1305 genInst :: OverlapFlag -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1308 = return (InstInfo { iSpec = mkInstance oflag (ds_theta spec) spec
1309 , iBinds = NewTypeDerived co }, [])
1312 = do { let loc = getSrcSpan (ds_name spec)
1313 inst = mkInstance oflag (ds_theta spec) spec
1316 -- In case of a family instance, we need to use the representation
1317 -- tycon (after all, it has the data constructors)
1318 ; fix_env <- getFixityEnv
1319 ; let (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1321 -- Build the InstInfo
1322 ; return (InstInfo { iSpec = inst,
1323 iBinds = VanillaInst meth_binds [] },
1327 rep_tycon = ds_tc spec
1328 rep_tc_args = ds_tc_args spec
1329 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1331 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1332 co2 = case newTyConCo_maybe rep_tycon of
1333 Nothing -> IdCo -- The newtype is transparent; no need for a cast
1334 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1335 co = co1 `mkTransCoI` co2
1337 -- Example: newtype instance N [a] = N1 (Tree a)
1338 -- deriving instance Eq b => Eq (N [(b,b)])
1339 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1340 -- When dealing with the deriving clause
1341 -- co1 : N [(b,b)] ~ R1:N (b,b)
1342 -- co2 : R1:N (b,b) ~ Tree (b,b)
1344 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1345 genDerivBinds loc fix_env clas tycon
1346 | className clas `elem` typeableClassNames
1347 = (gen_Typeable_binds loc tycon, [])
1350 = case assocMaybe gen_list (getUnique clas) of
1351 Just gen_fn -> gen_fn loc tycon
1352 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1354 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1355 gen_list = [(eqClassKey, gen_Eq_binds)
1356 ,(ordClassKey, gen_Ord_binds)
1357 ,(enumClassKey, gen_Enum_binds)
1358 ,(boundedClassKey, gen_Bounded_binds)
1359 ,(ixClassKey, gen_Ix_binds)
1360 ,(showClassKey, gen_Show_binds fix_env)
1361 ,(readClassKey, gen_Read_binds fix_env)
1362 ,(dataClassKey, gen_Data_binds)
1363 ,(functorClassKey, gen_Functor_binds)
1364 ,(foldableClassKey, gen_Foldable_binds)
1365 ,(traversableClassKey, gen_Traversable_binds)
1370 %************************************************************************
1372 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1374 %************************************************************************
1377 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1378 derivingKindErr tc cls cls_tys cls_kind
1379 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1380 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1381 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1382 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1384 derivingEtaErr :: Class -> [Type] -> Type -> Message
1385 derivingEtaErr cls cls_tys inst_ty
1386 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1387 nest 2 (ptext (sLit "instance (...) =>")
1388 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1390 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1391 typeFamilyPapErr tc cls cls_tys inst_ty
1392 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1393 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1395 derivingThingErr :: Class -> [Type] -> Type -> Message -> Message
1396 derivingThingErr clas tys ty why
1397 = sep [hsep [ptext (sLit "Can't make a derived instance of"),
1399 nest 2 (parens why)]
1401 pred = mkClassPred clas (tys ++ [ty])
1403 derivingHiddenErr :: TyCon -> SDoc
1404 derivingHiddenErr tc
1405 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1406 2 (ptext (sLit "so you cannot derive an instance for it"))
1408 standaloneCtxt :: LHsType Name -> SDoc
1409 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1412 derivInstCtxt :: Class -> [Type] -> Message
1413 derivInstCtxt clas inst_tys
1414 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1416 badDerivedPred :: PredType -> Message
1418 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1419 ptext (sLit "type variables that are not data type parameters"),
1420 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]