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 inst_tys = [mkTyConApp tycon tc_args]
667 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
668 spec = DS { ds_loc = loc, ds_orig = orig
669 , ds_name = dfun_name, ds_tvs = tvs
670 , ds_cls = cls, ds_tys = inst_tys
671 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
672 , ds_theta = mtheta `orElse` inferred_constraints
673 , ds_newtype = False }
675 ; return (if isJust mtheta then Right spec -- Specified context
676 else Left spec) } -- Infer context
678 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
679 -- The Typeable class is special in several ways
680 -- data T a b = ... deriving( Typeable )
682 -- instance Typeable2 T where ...
684 -- 1. There are no constraints in the instance
685 -- 2. There are no type variables either
686 -- 3. The actual class we want to generate isn't necessarily
687 -- Typeable; it depends on the arity of the type
688 | isNothing mtheta -- deriving on a data type decl
689 = do { checkTc (cls `hasKey` typeableClassKey)
690 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
691 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
692 ; mk_typeable_eqn orig tvs real_cls tycon [] rep_tc [] (Just []) }
694 | otherwise -- standaone deriving
695 = do { checkTc (null tc_args)
696 (ptext (sLit "Derived typeable instance must be of form (Typeable")
697 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
698 ; dfun_name <- new_dfun_name cls tycon
701 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
702 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
703 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
704 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
707 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
708 -- Generate a sufficiently large set of constraints that typechecking the
709 -- generated method definitions should succeed. This set will be simplified
710 -- before being used in the instance declaration
711 inferConstraints tvs cls inst_tys rep_tc rep_tc_args
712 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
713 stupid_constraints ++ extra_constraints
714 ++ sc_constraints ++ con_arg_constraints
716 -- Constraints arising from the arguments of each constructor
718 = [ mkClassPred cls [arg_ty]
719 | data_con <- tyConDataCons rep_tc,
720 arg_ty <- ASSERT( isVanillaDataCon data_con )
721 get_constrained_tys $
722 dataConInstOrigArgTys data_con all_rep_tc_args,
723 not (isUnLiftedType arg_ty) ]
724 -- No constraints for unlifted types
725 -- Where they are legal we generate specilised function calls
727 -- For functor-like classes, two things are different
728 -- (a) We recurse over argument types to generate constraints
729 -- See Functor examples in TcGenDeriv
730 -- (b) The rep_tc_args will be one short
731 is_functor_like = getUnique cls `elem` functorLikeClassKeys
733 get_constrained_tys :: [Type] -> [Type]
734 get_constrained_tys tys
735 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
738 rep_tc_tvs = tyConTyVars rep_tc
739 last_tv = last rep_tc_tvs
740 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
741 | otherwise = rep_tc_args
743 -- Constraints arising from superclasses
744 -- See Note [Superclasses of derived instance]
745 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
748 -- Stupid constraints
749 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
750 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
753 -- The Data class (only) requires that for
754 -- instance (...) => Data (T a b)
755 -- then (Data a, Data b) are among the (...) constraints
756 -- Reason: that's what you need to typecheck the method
757 -- dataCast1 f = gcast1 f
759 | cls `hasKey` dataClassKey = [mkClassPred cls [mkTyVarTy tv] | tv <- tvs]
762 ------------------------------------------------------------------
763 -- Check side conditions that dis-allow derivability for particular classes
764 -- This is *apart* from the newtype-deriving mechanism
766 -- Here we get the representation tycon in case of family instances as it has
767 -- the data constructors - but we need to be careful to fall back to the
768 -- family tycon (with indexes) in error messages.
770 data DerivStatus = CanDerive
771 | DerivableClassError SDoc -- Standard class, but can't do it
772 | NonDerivableClass -- Non-standard class
774 checkSideConditions :: DynFlags -> Class -> [TcType] -> TyCon -> DerivStatus
775 checkSideConditions dflags cls cls_tys rep_tc
776 | Just cond <- sideConditions cls
777 = case (cond (dflags, rep_tc)) of
778 Just err -> DerivableClassError err -- Class-specific error
779 Nothing | null cls_tys -> CanDerive
780 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
781 | otherwise = NonDerivableClass -- Not a standard class
783 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
785 nonStdErr :: Class -> SDoc
786 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
788 sideConditions :: Class -> Maybe Condition
790 | cls_key == eqClassKey = Just cond_std
791 | cls_key == ordClassKey = Just cond_std
792 | cls_key == showClassKey = Just cond_std
793 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
794 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
795 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
796 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
797 | cls_key == dataClassKey = Just (cond_mayDeriveDataTypeable `andCond` cond_std `andCond` cond_noUnliftedArgs)
798 | cls_key == functorClassKey = Just (cond_functorOK True) -- NB: no cond_std!
799 | cls_key == foldableClassKey = Just (cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
800 | cls_key == traversableClassKey = Just (cond_functorOK False)
801 | getName cls `elem` typeableClassNames = Just (cond_mayDeriveDataTypeable `andCond` cond_typeableOK)
802 | otherwise = Nothing
804 cls_key = getUnique cls
806 type Condition = (DynFlags, TyCon) -> Maybe SDoc
807 -- first Bool is whether or not we are allowed to derive Data and Typeable
808 -- second Bool is whether or not we are allowed to derive Functor
809 -- TyCon is the *representation* tycon if the
810 -- data type is an indexed one
813 orCond :: Condition -> Condition -> Condition
816 Nothing -> Nothing -- c1 succeeds
817 Just x -> case c2 tc of -- c1 fails
819 Just y -> Just (x $$ ptext (sLit " and") $$ y)
822 andCond :: Condition -> Condition -> Condition
823 andCond c1 c2 tc = case c1 tc of
824 Nothing -> c2 tc -- c1 succeeds
825 Just x -> Just x -- c1 fails
827 cond_std :: Condition
829 | null data_cons = Just no_cons_why
830 | not (null con_whys) = Just (vcat con_whys)
831 | otherwise = Nothing
833 data_cons = tyConDataCons rep_tc
834 no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
835 ptext (sLit "has no data constructors")
837 con_whys = mapCatMaybes check_con data_cons
839 check_con :: DataCon -> Maybe SDoc
841 | isVanillaDataCon con
842 , all isTauTy (dataConOrigArgTys con) = Nothing
843 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
845 cond_enumOrProduct :: Condition
846 cond_enumOrProduct = cond_isEnumeration `orCond`
847 (cond_isProduct `andCond` cond_noUnliftedArgs)
849 cond_noUnliftedArgs :: Condition
850 -- For some classes (eg Eq, Ord) we allow unlifted arg types
851 -- by generating specilaised code. For others (eg Data) we don't.
852 cond_noUnliftedArgs (_, tc)
853 | null bad_cons = Nothing
854 | otherwise = Just why
856 bad_cons = [ con | con <- tyConDataCons tc
857 , any isUnLiftedType (dataConOrigArgTys con) ]
858 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
860 cond_isEnumeration :: Condition
861 cond_isEnumeration (_, rep_tc)
862 | isEnumerationTyCon rep_tc = Nothing
863 | otherwise = Just why
865 why = quotes (pprSourceTyCon rep_tc) <+>
866 ptext (sLit "has non-nullary constructors")
868 cond_isProduct :: Condition
869 cond_isProduct (_, rep_tc)
870 | isProductTyCon rep_tc = Nothing
871 | otherwise = Just why
873 why = quotes (pprSourceTyCon rep_tc) <+>
874 ptext (sLit "has more than one constructor")
876 cond_typeableOK :: Condition
877 -- OK for Typeable class
878 -- Currently: (a) args all of kind *
879 -- (b) 7 or fewer args
880 cond_typeableOK (_, rep_tc)
881 | tyConArity rep_tc > 7 = Just too_many
882 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
884 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
885 | otherwise = Nothing
887 too_many = quotes (pprSourceTyCon rep_tc) <+>
888 ptext (sLit "has too many arguments")
889 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
890 ptext (sLit "has arguments of kind other than `*'")
891 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
892 ptext (sLit "is a type family")
895 functorLikeClassKeys :: [Unique]
896 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
898 cond_functorOK :: Bool -> Condition
899 -- OK for Functor class
900 -- Currently: (a) at least one argument
901 -- (b) don't use argument contravariantly
902 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
903 -- (d) optionally: don't use function types
904 cond_functorOK allowFunctions (dflags, rep_tc)
905 | not (dopt Opt_DeriveFunctor dflags)
906 = Just (ptext (sLit "You need -XDeriveFunctor to derive an instance for this class"))
908 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
910 data_cons = tyConDataCons rep_tc
911 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
913 check_vanilla :: DataCon -> Maybe SDoc
914 check_vanilla con | isVanillaDataCon con = Nothing
915 | otherwise = Just (badCon con existential)
917 ft_check :: DataCon -> FFoldType (Maybe SDoc)
918 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
919 , ft_co_var = Just (badCon con covariant)
920 , ft_fun = \x y -> if allowFunctions then x `mplus` y
921 else Just (badCon con functions)
922 , ft_tup = \_ xs -> msum xs
923 , ft_ty_app = \_ x -> x
924 , ft_bad_app = Just (badCon con wrong_arg)
925 , ft_forall = \_ x -> x }
927 existential = ptext (sLit "has existential arguments")
928 covariant = ptext (sLit "uses the type variable in a function argument")
929 functions = ptext (sLit "contains function types")
930 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
932 cond_mayDeriveDataTypeable :: Condition
933 cond_mayDeriveDataTypeable (dflags, _)
934 | dopt Opt_DeriveDataTypeable dflags = Nothing
935 | otherwise = Just why
937 why = ptext (sLit "You need -XDeriveDataTypeable to derive an instance for this class")
939 std_class_via_iso :: Class -> Bool
940 std_class_via_iso clas -- These standard classes can be derived for a newtype
941 -- using the isomorphism trick *even if no -fglasgow-exts*
942 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
943 -- Not Read/Show because they respect the type
944 -- Not Enum, because newtypes are never in Enum
947 new_dfun_name :: Class -> TyCon -> TcM Name
948 new_dfun_name clas tycon -- Just a simple wrapper
949 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
950 ; newDFunName clas [mkTyConApp tycon []] loc }
951 -- The type passed to newDFunName is only used to generate
952 -- a suitable string; hence the empty type arg list
954 badCon :: DataCon -> SDoc -> SDoc
955 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
958 Note [Superclasses of derived instance]
959 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
960 In general, a derived instance decl needs the superclasses of the derived
961 class too. So if we have
962 data T a = ...deriving( Ord )
963 then the initial context for Ord (T a) should include Eq (T a). Often this is
964 redundant; we'll also generate an Ord constraint for each constructor argument,
965 and that will probably generate enough constraints to make the Eq (T a) constraint
966 be satisfied too. But not always; consider:
972 data T a = MkT (S a) deriving( Ord )
973 instance Num a => Eq (T a)
975 The derived instance for (Ord (T a)) must have a (Num a) constraint!
977 data T a = MkT deriving( Data, Typeable )
978 Here there *is* no argument field, but we must nevertheless generate
979 a context for the Data instances:
980 instance Typable a => Data (T a) where ...
983 %************************************************************************
987 %************************************************************************
990 mkNewTypeEqn :: InstOrigin -> DynFlags -> [Var] -> Class
991 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
993 -> TcRn EarlyDerivSpec
994 mkNewTypeEqn orig dflags tvs
995 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
996 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
997 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
998 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
999 ; dfun_name <- new_dfun_name cls tycon
1000 ; loc <- getSrcSpanM
1001 ; let spec = DS { ds_loc = loc, ds_orig = orig
1002 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1003 , ds_cls = cls, ds_tys = inst_tys
1004 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1005 , ds_theta = mtheta `orElse` all_preds
1006 , ds_newtype = True }
1007 ; return (if isJust mtheta then Right spec
1011 = case check_conditions of
1012 CanDerive -> mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1013 -- Use the standard H98 method
1014 DerivableClassError msg -> bale_out msg -- Error with standard class
1015 NonDerivableClass -- Must use newtype deriving
1016 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1017 | otherwise -> bale_out non_std_err -- Try newtype deriving!
1019 newtype_deriving = dopt Opt_GeneralizedNewtypeDeriving dflags
1020 check_conditions = checkSideConditions dflags cls cls_tys rep_tycon
1021 bale_out msg = failWithTc (derivingThingErr cls cls_tys inst_ty msg)
1023 non_std_err = nonStdErr cls $$
1024 ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1026 -- Here is the plan for newtype derivings. We see
1027 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1028 -- where t is a type,
1029 -- ak+1...an is a suffix of a1..an, and are all tyars
1030 -- ak+1...an do not occur free in t, nor in the s1..sm
1031 -- (C s1 ... sm) is a *partial applications* of class C
1032 -- with the last parameter missing
1033 -- (T a1 .. ak) matches the kind of C's last argument
1034 -- (and hence so does t)
1035 -- The latter kind-check has been done by deriveTyData already,
1036 -- and tc_args are already trimmed
1038 -- We generate the instance
1039 -- instance forall ({a1..ak} u fvs(s1..sm)).
1040 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1041 -- where T a1...ap is the partial application of
1042 -- the LHS of the correct kind and p >= k
1044 -- NB: the variables below are:
1045 -- tc_tvs = [a1, ..., an]
1046 -- tyvars_to_keep = [a1, ..., ak]
1047 -- rep_ty = t ak .. an
1048 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1049 -- tys = [s1, ..., sm]
1052 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1053 -- We generate the instance
1054 -- instance Monad (ST s) => Monad (T s) where
1056 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1057 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1058 -- eta-reduces the representation type, so we know that
1060 -- That's convenient here, because we may have to apply
1061 -- it to fewer than its original complement of arguments
1063 -- Note [Newtype representation]
1064 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1065 -- Need newTyConRhs (*not* a recursive representation finder)
1066 -- to get the representation type. For example
1067 -- newtype B = MkB Int
1068 -- newtype A = MkA B deriving( Num )
1069 -- We want the Num instance of B, *not* the Num instance of Int,
1070 -- when making the Num instance of A!
1071 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1072 rep_tys = cls_tys ++ [rep_inst_ty]
1073 rep_pred = mkClassPred cls rep_tys
1074 -- rep_pred is the representation dictionary, from where
1075 -- we are gong to get all the methods for the newtype
1079 -- Next we figure out what superclass dictionaries to use
1080 -- See Note [Newtype deriving superclasses] above
1082 cls_tyvars = classTyVars cls
1083 dfun_tvs = tyVarsOfTypes inst_tys
1084 inst_ty = mkTyConApp tycon tc_args
1085 inst_tys = cls_tys ++ [inst_ty]
1086 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1089 -- If there are no tyvars, there's no need
1090 -- to abstract over the dictionaries we need
1091 -- Example: newtype T = MkT Int deriving( C )
1092 -- We get the derived instance
1095 -- instance C Int => C T
1096 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1098 -------------------------------------------------------------------
1099 -- Figuring out whether we can only do this newtype-deriving thing
1101 can_derive_via_isomorphism
1102 = not (non_iso_class cls)
1106 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1108 -- Never derive Read,Show,Typeable,Data by isomorphism
1109 non_iso_class cls = className cls `elem` ([readClassName, showClassName, dataClassName] ++
1112 arity_ok = length cls_tys + 1 == classArity cls
1113 -- Well kinded; eg not: newtype T ... deriving( ST )
1114 -- because ST needs *2* type params
1116 -- Check that eta reduction is OK
1117 eta_ok = nt_eta_arity <= length rep_tc_args
1118 -- The newtype can be eta-reduced to match the number
1119 -- of type argument actually supplied
1120 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1121 -- Here the 'b' must be the same in the rep type (S [a] b)
1122 -- And the [a] must not mention 'b'. That's all handled
1125 ats_ok = null (classATs cls)
1126 -- No associated types for the class, because we don't
1127 -- currently generate type 'instance' decls; and cannot do
1128 -- so for 'data' instance decls
1131 = vcat [ ptext (sLit "even with cunning newtype deriving:")
1132 , if arity_ok then empty else arity_msg
1133 , if eta_ok then empty else eta_msg
1134 , if ats_ok then empty else ats_msg ]
1135 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1136 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1137 ats_msg = ptext (sLit "the class has associated types")
1140 Note [Recursive newtypes]
1141 ~~~~~~~~~~~~~~~~~~~~~~~~~
1142 Newtype deriving works fine, even if the newtype is recursive.
1143 e.g. newtype S1 = S1 [T1 ()]
1144 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1145 Remember, too, that type families are curretly (conservatively) given
1146 a recursive flag, so this also allows newtype deriving to work
1149 We used to exclude recursive types, because we had a rather simple
1150 minded way of generating the instance decl:
1152 instance Eq [A] => Eq A -- Makes typechecker loop!
1153 But now we require a simple context, so it's ok.
1156 %************************************************************************
1158 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1160 %************************************************************************
1162 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1163 terms, which is the final correct RHS for the corresponding original
1167 Each (k,TyVarTy tv) in a solution constrains only a type
1171 The (k,TyVarTy tv) pairs in a solution are canonically
1172 ordered by sorting on type varible, tv, (major key) and then class, k,
1177 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1179 inferInstanceContexts _ [] = return []
1181 inferInstanceContexts oflag infer_specs
1182 = do { traceTc (text "inferInstanceContexts" <+> vcat (map pprDerivSpec infer_specs))
1183 ; iterate_deriv 1 initial_solutions }
1185 ------------------------------------------------------------------
1186 -- The initial solutions for the equations claim that each
1187 -- instance has an empty context; this solution is certainly
1188 -- in canonical form.
1189 initial_solutions :: [ThetaType]
1190 initial_solutions = [ [] | _ <- infer_specs ]
1192 ------------------------------------------------------------------
1193 -- iterate_deriv calculates the next batch of solutions,
1194 -- compares it with the current one; finishes if they are the
1195 -- same, otherwise recurses with the new solutions.
1196 -- It fails if any iteration fails
1197 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1198 iterate_deriv n current_solns
1199 | n > 20 -- Looks as if we are in an infinite loop
1200 -- This can happen if we have -XUndecidableInstances
1201 -- (See TcSimplify.tcSimplifyDeriv.)
1202 = pprPanic "solveDerivEqns: probable loop"
1203 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1205 = do { -- Extend the inst info from the explicit instance decls
1206 -- with the current set of solutions, and simplify each RHS
1207 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1208 current_solns infer_specs
1209 ; new_solns <- checkNoErrs $
1210 extendLocalInstEnv inst_specs $
1211 mapM gen_soln infer_specs
1213 ; if (current_solns == new_solns) then
1214 return [ spec { ds_theta = soln }
1215 | (spec, soln) <- zip infer_specs current_solns ]
1217 iterate_deriv (n+1) new_solns }
1219 ------------------------------------------------------------------
1220 gen_soln :: DerivSpec -> TcM [PredType]
1221 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1222 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1224 addErrCtxt (derivInstCtxt clas inst_tys) $
1225 do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
1226 -- checkValidInstance tyvars theta clas inst_tys
1227 -- Not necessary; see Note [Exotic derived instance contexts]
1230 -- Check for a bizarre corner case, when the derived instance decl should
1231 -- have form instance C a b => D (T a) where ...
1232 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1233 -- of problems; in particular, it's hard to compare solutions for
1234 -- equality when finding the fixpoint. So I just rule it out for now.
1235 ; let tv_set = mkVarSet tyvars
1236 weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
1237 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1239 -- Claim: the result instance declaration is guaranteed valid
1240 -- Hence no need to call:
1241 -- checkValidInstance tyvars theta clas inst_tys
1242 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1244 ------------------------------------------------------------------
1245 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1246 mkInstance overlap_flag theta
1247 (DS { ds_name = dfun_name
1248 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1249 = mkLocalInstance dfun overlap_flag
1251 dfun = mkDictFunId dfun_name tyvars theta clas tys
1254 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1255 -- Add new locally-defined instances; don't bother to check
1256 -- for functional dependency errors -- that'll happen in TcInstDcls
1257 extendLocalInstEnv dfuns thing_inside
1258 = do { env <- getGblEnv
1259 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1260 env' = env { tcg_inst_env = inst_env' }
1261 ; setGblEnv env' thing_inside }
1265 %************************************************************************
1267 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1269 %************************************************************************
1271 After all the trouble to figure out the required context for the
1272 derived instance declarations, all that's left is to chug along to
1273 produce them. They will then be shoved into @tcInstDecls2@, which
1274 will do all its usual business.
1276 There are lots of possibilities for code to generate. Here are
1277 various general remarks.
1282 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1283 ``you-couldn't-do-better-by-hand'' efficient.
1286 Deriving @Show@---also pretty common--- should also be reasonable good code.
1289 Deriving for the other classes isn't that common or that big a deal.
1296 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1299 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1302 We {\em normally} generate code only for the non-defaulted methods;
1303 there are some exceptions for @Eq@ and (especially) @Ord@...
1306 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1307 constructor's numeric (@Int#@) tag. These are generated by
1308 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1309 these is around is given by @hasCon2TagFun@.
1311 The examples under the different sections below will make this
1315 Much less often (really just for deriving @Ix@), we use a
1316 @_tag2con_<tycon>@ function. See the examples.
1319 We use the renamer!!! Reason: we're supposed to be
1320 producing @LHsBinds Name@ for the methods, but that means
1321 producing correctly-uniquified code on the fly. This is entirely
1322 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1323 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1324 the renamer. What a great hack!
1328 -- Generate the InstInfo for the required instance paired with the
1329 -- *representation* tycon for that instance,
1330 -- plus any auxiliary bindings required
1332 -- Representation tycons differ from the tycon in the instance signature in
1333 -- case of instances for indexed families.
1335 genInst :: OverlapFlag -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1338 = return (InstInfo { iSpec = mkInstance oflag (ds_theta spec) spec
1339 , iBinds = NewTypeDerived co }, [])
1342 = do { let loc = getSrcSpan (ds_name spec)
1343 inst = mkInstance oflag (ds_theta spec) spec
1346 -- In case of a family instance, we need to use the representation
1347 -- tycon (after all, it has the data constructors)
1348 ; fix_env <- getFixityEnv
1349 ; let (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1351 -- Build the InstInfo
1352 ; return (InstInfo { iSpec = inst,
1353 iBinds = VanillaInst meth_binds [] },
1357 rep_tycon = ds_tc spec
1358 rep_tc_args = ds_tc_args spec
1359 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1361 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1362 co2 = case newTyConCo_maybe rep_tycon of
1363 Nothing -> IdCo -- The newtype is transparent; no need for a cast
1364 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1365 co = co1 `mkTransCoI` co2
1367 -- Example: newtype instance N [a] = N1 (Tree a)
1368 -- deriving instance Eq b => Eq (N [(b,b)])
1369 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1370 -- When dealing with the deriving clause
1371 -- co1 : N [(b,b)] ~ R1:N (b,b)
1372 -- co2 : R1:N (b,b) ~ Tree (b,b)
1374 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1375 genDerivBinds loc fix_env clas tycon
1376 | className clas `elem` typeableClassNames
1377 = (gen_Typeable_binds loc tycon, [])
1380 = case assocMaybe gen_list (getUnique clas) of
1381 Just gen_fn -> gen_fn loc tycon
1382 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1384 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1385 gen_list = [(eqClassKey, gen_Eq_binds)
1386 ,(ordClassKey, gen_Ord_binds)
1387 ,(enumClassKey, gen_Enum_binds)
1388 ,(boundedClassKey, gen_Bounded_binds)
1389 ,(ixClassKey, gen_Ix_binds)
1390 ,(showClassKey, gen_Show_binds fix_env)
1391 ,(readClassKey, gen_Read_binds fix_env)
1392 ,(dataClassKey, gen_Data_binds)
1393 ,(functorClassKey, gen_Functor_binds)
1394 ,(foldableClassKey, gen_Foldable_binds)
1395 ,(traversableClassKey, gen_Traversable_binds)
1400 %************************************************************************
1402 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1404 %************************************************************************
1407 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1408 derivingKindErr tc cls cls_tys cls_kind
1409 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1410 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1411 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1412 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1414 derivingEtaErr :: Class -> [Type] -> Type -> Message
1415 derivingEtaErr cls cls_tys inst_ty
1416 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1417 nest 2 (ptext (sLit "instance (...) =>")
1418 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1420 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1421 typeFamilyPapErr tc cls cls_tys inst_ty
1422 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1423 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1425 derivingThingErr :: Class -> [Type] -> Type -> Message -> Message
1426 derivingThingErr clas tys ty why
1427 = sep [hsep [ptext (sLit "Can't make a derived instance of"),
1429 nest 2 (parens why)]
1431 pred = mkClassPred clas (tys ++ [ty])
1433 derivingHiddenErr :: TyCon -> SDoc
1434 derivingHiddenErr tc
1435 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1436 2 (ptext (sLit "so you cannot derive an instance for it"))
1438 standaloneCtxt :: LHsType Name -> SDoc
1439 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1442 derivInstCtxt :: Class -> [Type] -> Message
1443 derivInstCtxt clas inst_tys
1444 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1446 badDerivedPred :: PredType -> Message
1448 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1449 ptext (sLit "type variables that are not data type parameters"),
1450 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]