2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Handles @deriving@ clauses on @data@ declarations.
9 module TcDeriv ( tcDeriving ) where
11 #include "HsVersions.h"
19 import TcClassDcl( tcAddDeclCtxt ) -- Small helper
20 import TcGenDeriv -- Deriv stuff
56 %************************************************************************
60 %************************************************************************
64 1. Convert the decls (i.e. data/newtype deriving clauses,
65 plus standalone deriving) to [EarlyDerivSpec]
67 2. Infer the missing contexts for the Left DerivSpecs
69 3. Add the derived bindings, generating InstInfos
73 -- DerivSpec is purely local to this module
74 data DerivSpec = DS { ds_loc :: SrcSpan
78 , ds_theta :: ThetaType
82 , ds_tc_args :: [Type]
83 , ds_newtype :: Bool }
84 -- This spec implies a dfun declaration of the form
85 -- df :: forall tvs. theta => C tys
86 -- The Name is the name for the DFun we'll build
87 -- The tyvars bind all the variables in the theta
88 -- For type families, the tycon in
89 -- in ds_tys is the *family* tycon
90 -- in ds_tc, ds_tc_args is the *representation* tycon
91 -- For non-family tycons, both are the same
93 -- ds_newtype = True <=> Newtype deriving
94 -- False <=> Vanilla deriving
99 newtype instance T [a] = MkT (Tree a) deriving( C s )
101 axiom T [a] = :RTList a
102 axiom :RTList a = Tree a
104 DS { ds_tvs = [a,s], ds_cls = C, ds_tys = [s, T [a]]
105 , ds_tc = :RTList, ds_tc_args = [a]
106 , ds_newtype = True }
109 type DerivContext = Maybe ThetaType
110 -- Nothing <=> Vanilla deriving; infer the context of the instance decl
111 -- Just theta <=> Standalone deriving: context supplied by programmer
113 type EarlyDerivSpec = Either DerivSpec DerivSpec
114 -- Left ds => the context for the instance should be inferred
115 -- In this case ds_theta is the list of all the
116 -- constraints needed, such as (Eq [a], Eq a)
117 -- The inference process is to reduce this to a
118 -- simpler form (e.g. Eq a)
120 -- Right ds => the exact context for the instance is supplied
121 -- by the programmer; it is ds_theta
123 pprDerivSpec :: DerivSpec -> SDoc
124 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
125 ds_cls = c, ds_tys = tys, ds_theta = rhs })
126 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
127 <+> equals <+> ppr rhs)
131 Inferring missing contexts
132 ~~~~~~~~~~~~~~~~~~~~~~~~~~
135 data T a b = C1 (Foo a) (Bar b)
140 [NOTE: See end of these comments for what to do with
141 data (C a, D b) => T a b = ...
144 We want to come up with an instance declaration of the form
146 instance (Ping a, Pong b, ...) => Eq (T a b) where
149 It is pretty easy, albeit tedious, to fill in the code "...". The
150 trick is to figure out what the context for the instance decl is,
151 namely @Ping@, @Pong@ and friends.
153 Let's call the context reqd for the T instance of class C at types
154 (a,b, ...) C (T a b). Thus:
156 Eq (T a b) = (Ping a, Pong b, ...)
158 Now we can get a (recursive) equation from the @data@ decl:
160 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
161 u Eq (T b a) u Eq Int -- From C2
162 u Eq (T a a) -- From C3
164 Foo and Bar may have explicit instances for @Eq@, in which case we can
165 just substitute for them. Alternatively, either or both may have
166 their @Eq@ instances given by @deriving@ clauses, in which case they
167 form part of the system of equations.
169 Now all we need do is simplify and solve the equations, iterating to
170 find the least fixpoint. Notice that the order of the arguments can
171 switch around, as here in the recursive calls to T.
173 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
177 Eq (T a b) = {} -- The empty set
180 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
181 u Eq (T b a) u Eq Int -- From C2
182 u Eq (T a a) -- From C3
184 After simplification:
185 = Eq a u Ping b u {} u {} u {}
190 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
191 u Eq (T b a) u Eq Int -- From C2
192 u Eq (T a a) -- From C3
194 After simplification:
199 = Eq a u Ping b u Eq b u Ping a
201 The next iteration gives the same result, so this is the fixpoint. We
202 need to make a canonical form of the RHS to ensure convergence. We do
203 this by simplifying the RHS to a form in which
205 - the classes constrain only tyvars
206 - the list is sorted by tyvar (major key) and then class (minor key)
207 - no duplicates, of course
209 So, here are the synonyms for the ``equation'' structures:
212 Note [Data decl contexts]
213 ~~~~~~~~~~~~~~~~~~~~~~~~~
216 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
218 We will need an instance decl like:
220 instance (Read a, RealFloat a) => Read (Complex a) where
223 The RealFloat in the context is because the read method for Complex is bound
224 to construct a Complex, and doing that requires that the argument type is
227 But this ain't true for Show, Eq, Ord, etc, since they don't construct
228 a Complex; they only take them apart.
230 Our approach: identify the offending classes, and add the data type
231 context to the instance decl. The "offending classes" are
235 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
236 pattern matching against a constructor from a data type with a context
237 gives rise to the constraints for that context -- or at least the thinned
238 version. So now all classes are "offending".
240 Note [Newtype deriving]
241 ~~~~~~~~~~~~~~~~~~~~~~~
245 newtype T = T Char deriving( C [a] )
247 Notice the free 'a' in the deriving. We have to fill this out to
248 newtype T = T Char deriving( forall a. C [a] )
250 And then translate it to:
251 instance C [a] Char => C [a] T where ...
254 Note [Newtype deriving superclasses]
255 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
256 (See also Trac #1220 for an interesting exchange on newtype
257 deriving and superclasses.)
259 The 'tys' here come from the partial application in the deriving
260 clause. The last arg is the new instance type.
262 We must pass the superclasses; the newtype might be an instance
263 of them in a different way than the representation type
264 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
265 Then the Show instance is not done via isomorphism; it shows
267 The Num instance is derived via isomorphism, but the Show superclass
268 dictionary must the Show instance for Foo, *not* the Show dictionary
269 gotten from the Num dictionary. So we must build a whole new dictionary
270 not just use the Num one. The instance we want is something like:
271 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
274 There may be a coercion needed which we get from the tycon for the newtype
275 when the dict is constructed in TcInstDcls.tcInstDecl2
278 Note [Unused constructors and deriving clauses]
279 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
280 See Trac #3221. Consider
281 data T = T1 | T2 deriving( Show )
282 Are T1 and T2 unused? Well, no: the deriving clause expands to mention
283 both of them. So we gather defs/uses from deriving just like anything else.
285 %************************************************************************
287 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
289 %************************************************************************
292 tcDeriving :: [LTyClDecl Name] -- All type constructors
293 -> [LInstDecl Name] -- All instance declarations
294 -> [LDerivDecl Name] -- All stand-alone deriving declarations
295 -> TcM ([InstInfo Name], -- The generated "instance decls"
296 HsValBinds Name, -- Extra generated top-level bindings
299 tcDeriving tycl_decls inst_decls deriv_decls
300 = recoverM (return ([], emptyValBindsOut, emptyDUs)) $
301 do { -- Fish the "deriving"-related information out of the TcEnv
302 -- And make the necessary "equations".
303 is_boot <- tcIsHsBoot
304 ; traceTc "tcDeriving" (ppr is_boot)
305 ; early_specs <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
307 ; overlap_flag <- getOverlapFlag
308 ; let (infer_specs, given_specs) = splitEithers early_specs
309 ; insts1 <- mapM (genInst True overlap_flag) given_specs
311 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
312 inferInstanceContexts overlap_flag infer_specs
314 ; insts2 <- mapM (genInst False overlap_flag) final_specs
316 -- Generate the generic to/from functions from each type declaration
317 ; gen_binds <- mkGenericBinds is_boot tycl_decls
318 ; (inst_info, rn_binds, rn_dus) <- renameDeriv is_boot gen_binds (insts1 ++ insts2)
320 ; when (not (null inst_info)) $
321 dumpDerivingInfo (ddump_deriving inst_info rn_binds)
323 ; return (inst_info, rn_binds, rn_dus) }
325 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
326 ddump_deriving inst_infos extra_binds
327 = hang (ptext (sLit "Derived instances"))
328 2 (vcat (map (\i -> pprInstInfoDetails i $$ text "") inst_infos)
331 renameDeriv :: Bool -> LHsBinds RdrName
332 -> [(InstInfo RdrName, DerivAuxBinds)]
333 -> TcM ([InstInfo Name], HsValBinds Name, DefUses)
334 renameDeriv is_boot gen_binds insts
335 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
336 -- The inst-info bindings will all be empty, but it's easier to
337 -- just use rn_inst_info to change the type appropriately
338 = do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
339 ; return (rn_inst_infos, emptyValBindsOut, usesOnly (plusFVs fvs)) }
342 = discardWarnings $ -- Discard warnings about unused bindings etc
343 do { (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $ -- Type signatures in patterns
344 -- are used in the generic binds
345 rnTopBinds (ValBindsIn gen_binds [])
346 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
348 -- Generate and rename any extra not-one-inst-decl-specific binds,
349 -- notably "con2tag" and/or "tag2con" functions.
350 -- Bring those names into scope before renaming the instances themselves
351 ; loc <- getSrcSpanM -- Generic loc for shared bindings
352 ; let (aux_binds, aux_sigs) = unzip $ map (genAuxBind loc) $
353 rm_dups [] $ concat deriv_aux_binds
354 aux_val_binds = ValBindsIn (listToBag aux_binds) aux_sigs
355 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
356 ; bindLocalNames (collectHsValBinders rn_aux_lhs) $
357 do { (rn_aux, dus_aux) <- rnTopBindsRHS rn_aux_lhs
358 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
359 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
360 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
363 (inst_infos, deriv_aux_binds) = unzip insts
365 -- Remove duplicate requests for auxilliary bindings
367 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
368 | otherwise = rm_dups (b:acc) bs
371 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
372 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
373 = return ( info { iBinds = NewTypeDerived coi tc }
374 , mkFVs (map dataConName (tyConDataCons tc)))
375 -- See Note [Newtype deriving and unused constructors]
377 rn_inst_info inst_info@(InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
378 = -- Bring the right type variables into
379 -- scope (yuk), and rename the method binds
381 bindLocalNames (map Var.varName tyvars) $
382 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
383 ; let binds' = VanillaInst rn_binds [] standalone_deriv
384 ; return (inst_info { iBinds = binds' }, fvs) }
386 (tyvars,_, clas,_) = instanceHead inst
387 clas_nm = className clas
389 -----------------------------------------
390 mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
391 mkGenericBinds is_boot tycl_decls
395 = do { tcs <- mapM tcLookupTyCon [ tcdName d
396 | L _ d <- tycl_decls, isDataDecl d ]
397 ; return (unionManyBags [ mkTyConGenericBinds tc
398 | tc <- tcs, tyConHasGenerics tc ]) }
399 -- We are only interested in the data type declarations,
400 -- and then only in the ones whose 'has-generics' flag is on
401 -- The predicate tyConHasGenerics finds both of these
404 Note [Newtype deriving and unused constructors]
405 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
406 Consider this (see Trac #1954):
409 newtype P a = MkP (IO a) deriving Monad
411 If you compile with -fwarn-unused-binds you do not expect the warning
412 "Defined but not used: data consructor MkP". Yet the newtype deriving
413 code does not explicitly mention MkP, but it should behave as if you
415 instance Monad P where
416 return x = MkP (return x)
419 So we want to signal a user of the data constructor 'MkP'. That's
420 what we do in rn_inst_info, and it's the only reason we have the TyCon
421 stored in NewTypeDerived.
424 %************************************************************************
426 From HsSyn to DerivSpec
428 %************************************************************************
430 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
433 makeDerivSpecs :: Bool
437 -> TcM [EarlyDerivSpec]
439 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
440 | is_boot -- No 'deriving' at all in hs-boot files
441 = do { mapM_ add_deriv_err deriv_locs
444 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
445 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
446 ; return (eqns1 ++ eqns2) }
448 extractTyDataPreds decls
449 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
451 all_tydata :: [(LHsType Name, LTyClDecl Name)]
452 -- Derived predicate paired with its data type declaration
453 all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
455 deriv_locs = map (getLoc . snd) all_tydata
456 ++ map getLoc deriv_decls
458 add_deriv_err loc = setSrcSpan loc $
459 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
460 2 (ptext (sLit "Use an instance declaration instead")))
462 ------------------------------------------------------------------
463 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
464 -- Standalone deriving declarations
465 -- e.g. deriving instance Show a => Show (T a)
466 -- Rather like tcLocalInstDecl
467 deriveStandalone (L loc (DerivDecl deriv_ty))
469 addErrCtxt (standaloneCtxt deriv_ty) $
470 do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
471 ; (tvs, theta, cls, inst_tys) <- tcHsInstHead deriv_ty
472 ; traceTc "Standalone deriving;" $ vcat
473 [ text "tvs:" <+> ppr tvs
474 , text "theta:" <+> ppr theta
475 , text "cls:" <+> ppr cls
476 , text "tys:" <+> ppr inst_tys ]
477 ; checkValidInstance deriv_ty tvs theta cls inst_tys
478 -- C.f. TcInstDcls.tcLocalInstDecl1
480 ; let cls_tys = take (length inst_tys - 1) inst_tys
481 inst_ty = last inst_tys
482 ; traceTc "Standalone deriving:" $ vcat
483 [ text "class:" <+> ppr cls
484 , text "class types:" <+> ppr cls_tys
485 , text "type:" <+> ppr inst_ty ]
486 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
489 ------------------------------------------------------------------
490 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
491 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
492 tcdTyVars = tv_names,
493 tcdTyPats = ty_pats }))
494 = setSrcSpan loc $ -- Use the location of the 'deriving' item
496 do { (tvs, tc, tc_args) <- get_lhs ty_pats
497 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
498 -- the type variables for the type constructor
500 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
501 -- The "deriv_pred" is a LHsType to take account of the fact that for
502 -- newtype deriving we allow deriving (forall a. C [a]).
504 -- Given data T a b c = ... deriving( C d ),
505 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
506 ; let cls_tyvars = classTyVars cls
507 kind = tyVarKind (last cls_tyvars)
508 (arg_kinds, _) = splitKindFunTys kind
509 n_args_to_drop = length arg_kinds
510 n_args_to_keep = tyConArity tc - n_args_to_drop
511 args_to_drop = drop n_args_to_keep tc_args
512 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
513 inst_ty_kind = typeKind inst_ty
514 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
515 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
516 `minusVarSet` dropped_tvs
518 -- Check that the result really is well-kinded
519 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
520 (derivingKindErr tc cls cls_tys kind)
522 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
523 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
524 (derivingEtaErr cls cls_tys inst_ty)
526 -- (a) The data type can be eta-reduced; eg reject:
527 -- data instance T a a = ... deriving( Monad )
528 -- (b) The type class args do not mention any of the dropped type
530 -- newtype T a s = ... deriving( ST s )
532 -- Type families can't be partially applied
533 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
534 -- Note [Deriving, type families, and partial applications]
535 ; checkTc (not (isFamilyTyCon tc) || n_args_to_drop == 0)
536 (typeFamilyPapErr tc cls cls_tys inst_ty)
538 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
540 -- Tiresomely we must figure out the "lhs", which is awkward for type families
541 -- E.g. data T a b = .. deriving( Eq )
542 -- Here, the lhs is (T a b)
543 -- data instance TF Int b = ... deriving( Eq )
544 -- Here, the lhs is (TF Int b)
545 -- But if we just look up the tycon_name, we get is the *family*
546 -- tycon, but not pattern types -- they are in the *rep* tycon.
547 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
548 ; let tvs = tyConTyVars tc
549 ; return (tvs, tc, mkTyVarTys tvs) }
550 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
551 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
552 ; let (tc, tc_args) = tcSplitTyConApp tc_app
553 ; return (tvs, tc, tc_args) }
556 = panic "derivTyData" -- Caller ensures that only TyData can happen
559 Note [Deriving, type families, and partial applications]
560 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
561 When there are no type families, it's quite easy:
563 newtype S a = MkS [a]
564 -- :CoS :: S ~ [] -- Eta-reduced
566 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
567 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
569 When type familes are involved it's trickier:
572 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
573 -- :RT is the representation type for (T Int a)
574 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
575 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
577 instance Eq [a] => Eq (T Int a) -- easy by coercion
578 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
580 The "???" bit is that we don't build the :CoF thing in eta-reduced form
581 Henc the current typeFamilyPapErr, even though the instance makes sense.
582 After all, we can write it out
583 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
588 mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type] -> Type
589 -> DerivContext -- Just => context supplied (standalone deriving)
590 -- Nothing => context inferred (deriving on data decl)
591 -> TcRn EarlyDerivSpec
592 -- Make the EarlyDerivSpec for an instance
593 -- forall tvs. theta => cls (tys ++ [ty])
594 -- where the 'theta' is optional (that's the Maybe part)
595 -- Assumes that this declaration is well-kinded
597 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
598 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
599 , isAlgTyCon tycon -- Check for functions, primitive types etc
600 = mk_alg_eqn tycon tc_args
602 = failWithTc (derivingThingErr False cls cls_tys tc_app
603 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
606 bale_out msg = failWithTc (derivingThingErr False cls cls_tys tc_app msg)
608 mk_alg_eqn tycon tc_args
609 | className cls `elem` typeableClassNames
610 = do { dflags <- getDOpts
611 ; case checkTypeableConditions (dflags, tycon) of
612 Just err -> bale_out err
613 Nothing -> mk_typeable_eqn orig tvs cls tycon tc_args mtheta }
615 | isDataFamilyTyCon tycon
616 , length tc_args /= tyConArity tycon
617 = bale_out (ptext (sLit "Unsaturated data family application"))
620 = do { (rep_tc, rep_tc_args) <- tcLookupDataFamInst tycon tc_args
621 -- Be careful to test rep_tc here: in the case of families,
622 -- we want to check the instance tycon, not the family tycon
624 -- For standalone deriving (mtheta /= Nothing),
625 -- check that all the data constructors are in scope.
626 ; rdr_env <- getGlobalRdrEnv
627 ; let hidden_data_cons = isAbstractTyCon rep_tc ||
628 any not_in_scope (tyConDataCons rep_tc)
629 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
630 ; unless (isNothing mtheta || not hidden_data_cons)
631 (bale_out (derivingHiddenErr tycon))
634 ; if isDataTyCon rep_tc then
635 mkDataTypeEqn orig dflags tvs cls cls_tys
636 tycon tc_args rep_tc rep_tc_args mtheta
638 mkNewTypeEqn orig dflags tvs cls cls_tys
639 tycon tc_args rep_tc rep_tc_args mtheta }
643 %************************************************************************
647 %************************************************************************
650 mkDataTypeEqn :: CtOrigin
652 -> [Var] -- Universally quantified type variables in the instance
653 -> Class -- Class for which we need to derive an instance
654 -> [Type] -- Other parameters to the class except the last
655 -> TyCon -- Type constructor for which the instance is requested
656 -- (last parameter to the type class)
657 -> [Type] -- Parameters to the type constructor
658 -> TyCon -- rep of the above (for type families)
659 -> [Type] -- rep of the above
660 -> DerivContext -- Context of the instance, for standalone deriving
661 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
663 mkDataTypeEqn orig dflags tvs cls cls_tys
664 tycon tc_args rep_tc rep_tc_args mtheta
665 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
666 -- NB: pass the *representation* tycon to checkSideConditions
667 CanDerive -> go_for_it
668 NonDerivableClass -> bale_out (nonStdErr cls)
669 DerivableClassError msg -> bale_out msg
671 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
672 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
674 mk_data_eqn :: CtOrigin -> [TyVar] -> Class
675 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
676 -> TcM EarlyDerivSpec
677 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
678 = do { dfun_name <- new_dfun_name cls tycon
680 ; let inst_tys = [mkTyConApp tycon tc_args]
681 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
682 spec = DS { ds_loc = loc, ds_orig = orig
683 , ds_name = dfun_name, ds_tvs = tvs
684 , ds_cls = cls, ds_tys = inst_tys
685 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
686 , ds_theta = mtheta `orElse` inferred_constraints
687 , ds_newtype = False }
689 ; return (if isJust mtheta then Right spec -- Specified context
690 else Left spec) } -- Infer context
692 ----------------------
693 mk_typeable_eqn :: CtOrigin -> [TyVar] -> Class
694 -> TyCon -> [TcType] -> DerivContext
695 -> TcM EarlyDerivSpec
696 mk_typeable_eqn orig tvs cls tycon tc_args mtheta
697 -- The Typeable class is special in several ways
698 -- data T a b = ... deriving( Typeable )
700 -- instance Typeable2 T where ...
702 -- 1. There are no constraints in the instance
703 -- 2. There are no type variables either
704 -- 3. The actual class we want to generate isn't necessarily
705 -- Typeable; it depends on the arity of the type
706 | isNothing mtheta -- deriving on a data type decl
707 = do { checkTc (cls `hasKey` typeableClassKey)
708 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
709 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
710 ; mk_typeable_eqn orig tvs real_cls tycon [] (Just []) }
712 | otherwise -- standaone deriving
713 = do { checkTc (null tc_args)
714 (ptext (sLit "Derived typeable instance must be of form (Typeable")
715 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
716 ; dfun_name <- new_dfun_name cls tycon
719 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
720 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
721 , ds_tc = tycon, ds_tc_args = []
722 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
724 ----------------------
725 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
726 -- Generate a sufficiently large set of constraints that typechecking the
727 -- generated method definitions should succeed. This set will be simplified
728 -- before being used in the instance declaration
729 inferConstraints _ cls inst_tys rep_tc rep_tc_args
730 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
731 stupid_constraints ++ extra_constraints
732 ++ sc_constraints ++ con_arg_constraints
734 -- Constraints arising from the arguments of each constructor
736 = [ mkClassPred cls [arg_ty]
737 | data_con <- tyConDataCons rep_tc,
738 arg_ty <- ASSERT( isVanillaDataCon data_con )
739 get_constrained_tys $
740 dataConInstOrigArgTys data_con all_rep_tc_args,
741 not (isUnLiftedType arg_ty) ]
742 -- No constraints for unlifted types
743 -- Where they are legal we generate specilised function calls
745 -- For functor-like classes, two things are different
746 -- (a) We recurse over argument types to generate constraints
747 -- See Functor examples in TcGenDeriv
748 -- (b) The rep_tc_args will be one short
749 is_functor_like = getUnique cls `elem` functorLikeClassKeys
751 get_constrained_tys :: [Type] -> [Type]
752 get_constrained_tys tys
753 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
756 rep_tc_tvs = tyConTyVars rep_tc
757 last_tv = last rep_tc_tvs
758 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
759 | otherwise = rep_tc_args
761 -- Constraints arising from superclasses
762 -- See Note [Superclasses of derived instance]
763 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
766 -- Stupid constraints
767 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
768 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
770 -- Extra Data constraints
771 -- The Data class (only) requires that for
772 -- instance (...) => Data (T t1 t2)
774 -- THEN (Data t1, Data t2) are among the (...) constraints
775 -- Reason: when the IF holds, we generate a method
776 -- dataCast2 f = gcast2 f
777 -- and we need the Data constraints to typecheck the method
779 | cls `hasKey` dataClassKey
780 , all (isLiftedTypeKind . typeKind) rep_tc_args
781 = [mkClassPred cls [ty] | ty <- rep_tc_args]
785 ------------------------------------------------------------------
786 -- Check side conditions that dis-allow derivability for particular classes
787 -- This is *apart* from the newtype-deriving mechanism
789 -- Here we get the representation tycon in case of family instances as it has
790 -- the data constructors - but we need to be careful to fall back to the
791 -- family tycon (with indexes) in error messages.
793 data DerivStatus = CanDerive
794 | DerivableClassError SDoc -- Standard class, but can't do it
795 | NonDerivableClass -- Non-standard class
797 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
798 checkSideConditions dflags mtheta cls cls_tys rep_tc
799 | Just cond <- sideConditions mtheta cls
800 = case (cond (dflags, rep_tc)) of
801 Just err -> DerivableClassError err -- Class-specific error
802 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
803 -- cls_tys (the type args other than last)
805 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
806 | otherwise = NonDerivableClass -- Not a standard class
808 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
810 checkTypeableConditions :: Condition
811 checkTypeableConditions = checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK
813 nonStdErr :: Class -> SDoc
814 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
816 sideConditions :: DerivContext -> Class -> Maybe Condition
817 sideConditions mtheta cls
818 | cls_key == eqClassKey = Just cond_std
819 | cls_key == ordClassKey = Just cond_std
820 | cls_key == showClassKey = Just cond_std
821 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
822 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
823 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
824 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
825 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
826 cond_std `andCond` cond_noUnliftedArgs)
827 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
828 cond_functorOK True) -- NB: no cond_std!
829 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
830 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
831 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
832 cond_functorOK False)
833 | otherwise = Nothing
835 cls_key = getUnique cls
836 cond_std = cond_stdOK mtheta
838 type Condition = (DynFlags, TyCon) -> Maybe SDoc
839 -- first Bool is whether or not we are allowed to derive Data and Typeable
840 -- second Bool is whether or not we are allowed to derive Functor
841 -- TyCon is the *representation* tycon if the
842 -- data type is an indexed one
845 orCond :: Condition -> Condition -> Condition
848 Nothing -> Nothing -- c1 succeeds
849 Just x -> case c2 tc of -- c1 fails
851 Just y -> Just (x $$ ptext (sLit " and") $$ y)
854 andCond :: Condition -> Condition -> Condition
855 andCond c1 c2 tc = case c1 tc of
856 Nothing -> c2 tc -- c1 succeeds
857 Just x -> Just x -- c1 fails
859 cond_stdOK :: DerivContext -> Condition
860 cond_stdOK (Just _) _
861 = Nothing -- Don't check these conservative conditions for
862 -- standalone deriving; just generate the code
863 -- and let the typechecker handle the result
864 cond_stdOK Nothing (_, rep_tc)
865 | null data_cons = Just (no_cons_why rep_tc $$ suggestion)
866 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
867 | otherwise = Nothing
869 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
870 data_cons = tyConDataCons rep_tc
871 con_whys = mapCatMaybes check_con data_cons
873 check_con :: DataCon -> Maybe SDoc
875 | isVanillaDataCon con
876 , all isTauTy (dataConOrigArgTys con) = Nothing
877 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
879 no_cons_why :: TyCon -> SDoc
880 no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
881 ptext (sLit "has no data constructors")
883 cond_enumOrProduct :: Condition
884 cond_enumOrProduct = cond_isEnumeration `orCond`
885 (cond_isProduct `andCond` cond_noUnliftedArgs)
887 cond_noUnliftedArgs :: Condition
888 -- For some classes (eg Eq, Ord) we allow unlifted arg types
889 -- by generating specilaised code. For others (eg Data) we don't.
890 cond_noUnliftedArgs (_, tc)
891 | null bad_cons = Nothing
892 | otherwise = Just why
894 bad_cons = [ con | con <- tyConDataCons tc
895 , any isUnLiftedType (dataConOrigArgTys con) ]
896 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
898 cond_isEnumeration :: Condition
899 cond_isEnumeration (_, rep_tc)
900 | isEnumerationTyCon rep_tc = Nothing
901 | otherwise = Just why
903 why = sep [ quotes (pprSourceTyCon rep_tc) <+>
904 ptext (sLit "is not an enumeration type")
905 , ptext (sLit "(an enumeration consists of one or more nullary, non-GADT constructors)") ]
906 -- See Note [Enumeration types] in TyCon
908 cond_isProduct :: Condition
909 cond_isProduct (_, rep_tc)
910 | isProductTyCon rep_tc = Nothing
911 | otherwise = Just why
913 why = quotes (pprSourceTyCon rep_tc) <+>
914 ptext (sLit "does not have precisely one constructor")
916 cond_typeableOK :: Condition
917 -- OK for Typeable class
918 -- Currently: (a) args all of kind *
919 -- (b) 7 or fewer args
920 cond_typeableOK (_, tc)
921 | tyConArity tc > 7 = Just too_many
922 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tc))
924 | otherwise = Nothing
926 too_many = quotes (pprSourceTyCon tc) <+>
927 ptext (sLit "has too many arguments")
928 bad_kind = quotes (pprSourceTyCon tc) <+>
929 ptext (sLit "has arguments of kind other than `*'")
931 functorLikeClassKeys :: [Unique]
932 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
934 cond_functorOK :: Bool -> Condition
935 -- OK for Functor/Foldable/Traversable class
936 -- Currently: (a) at least one argument
937 -- (b) don't use argument contravariantly
938 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
939 -- (d) optionally: don't use function types
940 -- (e) no "stupid context" on data type
941 cond_functorOK allowFunctions (_, rep_tc)
943 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
944 <+> ptext (sLit "has no parameters"))
946 | not (null bad_stupid_theta)
947 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
948 <+> ptext (sLit "has a class context") <+> pprTheta bad_stupid_theta)
951 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
953 tc_tvs = tyConTyVars rep_tc
954 Just (_, last_tv) = snocView tc_tvs
955 bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
956 is_bad pred = last_tv `elemVarSet` tyVarsOfPred pred
958 data_cons = tyConDataCons rep_tc
959 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
961 check_vanilla :: DataCon -> Maybe SDoc
962 check_vanilla con | isVanillaDataCon con = Nothing
963 | otherwise = Just (badCon con existential)
965 ft_check :: DataCon -> FFoldType (Maybe SDoc)
966 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
967 , ft_co_var = Just (badCon con covariant)
968 , ft_fun = \x y -> if allowFunctions then x `mplus` y
969 else Just (badCon con functions)
970 , ft_tup = \_ xs -> msum xs
971 , ft_ty_app = \_ x -> x
972 , ft_bad_app = Just (badCon con wrong_arg)
973 , ft_forall = \_ x -> x }
975 existential = ptext (sLit "has existential arguments")
976 covariant = ptext (sLit "uses the type variable in a function argument")
977 functions = ptext (sLit "contains function types")
978 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
980 checkFlag :: ExtensionFlag -> Condition
981 checkFlag flag (dflags, _)
982 | xopt flag dflags = Nothing
983 | otherwise = Just why
985 why = ptext (sLit "You need -X") <> text flag_str
986 <+> ptext (sLit "to derive an instance for this class")
987 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
989 other -> pprPanic "checkFlag" (ppr other)
991 std_class_via_iso :: Class -> Bool
992 -- These standard classes can be derived for a newtype
993 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
994 -- because giving so gives the same results as generating the boilerplate
995 std_class_via_iso clas
996 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
997 -- Not Read/Show because they respect the type
998 -- Not Enum, because newtypes are never in Enum
1001 non_iso_class :: Class -> Bool
1002 -- *Never* derive Read,Show,Typeable,Data by isomorphism,
1003 -- even with -XGeneralizedNewtypeDeriving
1005 = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
1008 typeableClassKeys :: [Unique]
1009 typeableClassKeys = map getUnique typeableClassNames
1011 new_dfun_name :: Class -> TyCon -> TcM Name
1012 new_dfun_name clas tycon -- Just a simple wrapper
1013 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1014 ; newDFunName clas [mkTyConApp tycon []] loc }
1015 -- The type passed to newDFunName is only used to generate
1016 -- a suitable string; hence the empty type arg list
1018 badCon :: DataCon -> SDoc -> SDoc
1019 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1022 Note [Superclasses of derived instance]
1023 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1024 In general, a derived instance decl needs the superclasses of the derived
1025 class too. So if we have
1026 data T a = ...deriving( Ord )
1027 then the initial context for Ord (T a) should include Eq (T a). Often this is
1028 redundant; we'll also generate an Ord constraint for each constructor argument,
1029 and that will probably generate enough constraints to make the Eq (T a) constraint
1030 be satisfied too. But not always; consider:
1036 data T a = MkT (S a) deriving( Ord )
1037 instance Num a => Eq (T a)
1039 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1041 data T a = MkT deriving( Data, Typeable )
1042 Here there *is* no argument field, but we must nevertheless generate
1043 a context for the Data instances:
1044 instance Typable a => Data (T a) where ...
1047 %************************************************************************
1051 %************************************************************************
1054 mkNewTypeEqn :: CtOrigin -> DynFlags -> [Var] -> Class
1055 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1057 -> TcRn EarlyDerivSpec
1058 mkNewTypeEqn orig dflags tvs
1059 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1060 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1061 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1062 = do { traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
1063 ; dfun_name <- new_dfun_name cls tycon
1064 ; loc <- getSrcSpanM
1065 ; let spec = DS { ds_loc = loc, ds_orig = orig
1066 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1067 , ds_cls = cls, ds_tys = inst_tys
1068 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1069 , ds_theta = mtheta `orElse` all_preds
1070 , ds_newtype = True }
1071 ; return (if isJust mtheta then Right spec
1075 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1076 CanDerive -> go_for_it -- Use the standard H98 method
1077 DerivableClassError msg -- Error with standard class
1078 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1079 | otherwise -> bale_out msg
1080 NonDerivableClass -- Must use newtype deriving
1081 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1082 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1083 | otherwise -> bale_out non_std
1085 newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
1086 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1087 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1089 non_std = nonStdErr cls
1090 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1092 -- Here is the plan for newtype derivings. We see
1093 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1094 -- where t is a type,
1095 -- ak+1...an is a suffix of a1..an, and are all tyars
1096 -- ak+1...an do not occur free in t, nor in the s1..sm
1097 -- (C s1 ... sm) is a *partial applications* of class C
1098 -- with the last parameter missing
1099 -- (T a1 .. ak) matches the kind of C's last argument
1100 -- (and hence so does t)
1101 -- The latter kind-check has been done by deriveTyData already,
1102 -- and tc_args are already trimmed
1104 -- We generate the instance
1105 -- instance forall ({a1..ak} u fvs(s1..sm)).
1106 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1107 -- where T a1...ap is the partial application of
1108 -- the LHS of the correct kind and p >= k
1110 -- NB: the variables below are:
1111 -- tc_tvs = [a1, ..., an]
1112 -- tyvars_to_keep = [a1, ..., ak]
1113 -- rep_ty = t ak .. an
1114 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1115 -- tys = [s1, ..., sm]
1118 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1119 -- We generate the instance
1120 -- instance Monad (ST s) => Monad (T s) where
1122 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1123 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1124 -- eta-reduces the representation type, so we know that
1126 -- That's convenient here, because we may have to apply
1127 -- it to fewer than its original complement of arguments
1129 -- Note [Newtype representation]
1130 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1131 -- Need newTyConRhs (*not* a recursive representation finder)
1132 -- to get the representation type. For example
1133 -- newtype B = MkB Int
1134 -- newtype A = MkA B deriving( Num )
1135 -- We want the Num instance of B, *not* the Num instance of Int,
1136 -- when making the Num instance of A!
1137 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1138 rep_tys = cls_tys ++ [rep_inst_ty]
1139 rep_pred = mkClassPred cls rep_tys
1140 -- rep_pred is the representation dictionary, from where
1141 -- we are gong to get all the methods for the newtype
1145 -- Next we figure out what superclass dictionaries to use
1146 -- See Note [Newtype deriving superclasses] above
1148 cls_tyvars = classTyVars cls
1149 dfun_tvs = tyVarsOfTypes inst_tys
1150 inst_ty = mkTyConApp tycon tc_args
1151 inst_tys = cls_tys ++ [inst_ty]
1152 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1155 -- If there are no tyvars, there's no need
1156 -- to abstract over the dictionaries we need
1157 -- Example: newtype T = MkT Int deriving( C )
1158 -- We get the derived instance
1161 -- instance C Int => C T
1162 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1164 -------------------------------------------------------------------
1165 -- Figuring out whether we can only do this newtype-deriving thing
1167 can_derive_via_isomorphism
1168 = not (non_iso_class cls)
1172 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1174 arity_ok = length cls_tys + 1 == classArity cls
1175 -- Well kinded; eg not: newtype T ... deriving( ST )
1176 -- because ST needs *2* type params
1178 -- Check that eta reduction is OK
1179 eta_ok = nt_eta_arity <= length rep_tc_args
1180 -- The newtype can be eta-reduced to match the number
1181 -- of type argument actually supplied
1182 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1183 -- Here the 'b' must be the same in the rep type (S [a] b)
1184 -- And the [a] must not mention 'b'. That's all handled
1187 ats_ok = null (classATs cls)
1188 -- No associated types for the class, because we don't
1189 -- currently generate type 'instance' decls; and cannot do
1190 -- so for 'data' instance decls
1193 = vcat [ ppUnless arity_ok arity_msg
1194 , ppUnless eta_ok eta_msg
1195 , ppUnless ats_ok ats_msg ]
1196 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1197 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1198 ats_msg = ptext (sLit "the class has associated types")
1201 Note [Recursive newtypes]
1202 ~~~~~~~~~~~~~~~~~~~~~~~~~
1203 Newtype deriving works fine, even if the newtype is recursive.
1204 e.g. newtype S1 = S1 [T1 ()]
1205 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1206 Remember, too, that type families are curretly (conservatively) given
1207 a recursive flag, so this also allows newtype deriving to work
1210 We used to exclude recursive types, because we had a rather simple
1211 minded way of generating the instance decl:
1213 instance Eq [A] => Eq A -- Makes typechecker loop!
1214 But now we require a simple context, so it's ok.
1217 %************************************************************************
1219 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1221 %************************************************************************
1223 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1224 terms, which is the final correct RHS for the corresponding original
1228 Each (k,TyVarTy tv) in a solution constrains only a type
1232 The (k,TyVarTy tv) pairs in a solution are canonically
1233 ordered by sorting on type varible, tv, (major key) and then class, k,
1238 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1240 inferInstanceContexts _ [] = return []
1242 inferInstanceContexts oflag infer_specs
1243 = do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
1244 ; iterate_deriv 1 initial_solutions }
1246 ------------------------------------------------------------------
1247 -- The initial solutions for the equations claim that each
1248 -- instance has an empty context; this solution is certainly
1249 -- in canonical form.
1250 initial_solutions :: [ThetaType]
1251 initial_solutions = [ [] | _ <- infer_specs ]
1253 ------------------------------------------------------------------
1254 -- iterate_deriv calculates the next batch of solutions,
1255 -- compares it with the current one; finishes if they are the
1256 -- same, otherwise recurses with the new solutions.
1257 -- It fails if any iteration fails
1258 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1259 iterate_deriv n current_solns
1260 | n > 20 -- Looks as if we are in an infinite loop
1261 -- This can happen if we have -XUndecidableInstances
1262 -- (See TcSimplify.tcSimplifyDeriv.)
1263 = pprPanic "solveDerivEqns: probable loop"
1264 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1266 = do { -- Extend the inst info from the explicit instance decls
1267 -- with the current set of solutions, and simplify each RHS
1268 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1269 current_solns infer_specs
1270 ; new_solns <- checkNoErrs $
1271 extendLocalInstEnv inst_specs $
1272 mapM gen_soln infer_specs
1274 ; if (current_solns == new_solns) then
1275 return [ spec { ds_theta = soln }
1276 | (spec, soln) <- zip infer_specs current_solns ]
1278 iterate_deriv (n+1) new_solns }
1280 ------------------------------------------------------------------
1281 gen_soln :: DerivSpec -> TcM [PredType]
1282 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1283 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1285 addErrCtxt (derivInstCtxt the_pred) $
1286 do { -- Check for a bizarre corner case, when the derived instance decl should
1287 -- have form instance C a b => D (T a) where ...
1288 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1289 -- of problems; in particular, it's hard to compare solutions for
1290 -- equality when finding the fixpoint. Moreover, simplifyDeriv
1291 -- has an assert failure because it finds a TyVar when it expects
1292 -- only TcTyVars. So I just rule it out for now. I'm not
1293 -- even sure how it can arise.
1295 ; let tv_set = mkVarSet tyvars
1296 weird_preds = [pred | pred <- deriv_rhs
1297 , not (tyVarsOfPred pred `subVarSet` tv_set)]
1298 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1300 ; theta <- simplifyDeriv orig the_pred tyvars deriv_rhs
1301 -- checkValidInstance tyvars theta clas inst_tys
1302 -- Not necessary; see Note [Exotic derived instance contexts]
1305 ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
1306 -- Claim: the result instance declaration is guaranteed valid
1307 -- Hence no need to call:
1308 -- checkValidInstance tyvars theta clas inst_tys
1309 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1311 the_pred = mkClassPred clas inst_tys
1313 ------------------------------------------------------------------
1314 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1315 mkInstance overlap_flag theta
1316 (DS { ds_name = dfun_name
1317 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1318 = mkLocalInstance dfun overlap_flag
1320 dfun = mkDictFunId dfun_name tyvars theta clas tys
1323 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1324 -- Add new locally-defined instances; don't bother to check
1325 -- for functional dependency errors -- that'll happen in TcInstDcls
1326 extendLocalInstEnv dfuns thing_inside
1327 = do { env <- getGblEnv
1328 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1329 env' = env { tcg_inst_env = inst_env' }
1330 ; setGblEnv env' thing_inside }
1334 %************************************************************************
1336 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1338 %************************************************************************
1340 After all the trouble to figure out the required context for the
1341 derived instance declarations, all that's left is to chug along to
1342 produce them. They will then be shoved into @tcInstDecls2@, which
1343 will do all its usual business.
1345 There are lots of possibilities for code to generate. Here are
1346 various general remarks.
1351 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1352 ``you-couldn't-do-better-by-hand'' efficient.
1355 Deriving @Show@---also pretty common--- should also be reasonable good code.
1358 Deriving for the other classes isn't that common or that big a deal.
1365 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1368 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1371 We {\em normally} generate code only for the non-defaulted methods;
1372 there are some exceptions for @Eq@ and (especially) @Ord@...
1375 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1376 constructor's numeric (@Int#@) tag. These are generated by
1377 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1378 these is around is given by @hasCon2TagFun@.
1380 The examples under the different sections below will make this
1384 Much less often (really just for deriving @Ix@), we use a
1385 @_tag2con_<tycon>@ function. See the examples.
1388 We use the renamer!!! Reason: we're supposed to be
1389 producing @LHsBinds Name@ for the methods, but that means
1390 producing correctly-uniquified code on the fly. This is entirely
1391 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1392 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1393 the renamer. What a great hack!
1397 -- Generate the InstInfo for the required instance paired with the
1398 -- *representation* tycon for that instance,
1399 -- plus any auxiliary bindings required
1401 -- Representation tycons differ from the tycon in the instance signature in
1402 -- case of instances for indexed families.
1404 genInst :: Bool -- True <=> standalone deriving
1406 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1407 genInst standalone_deriv oflag
1408 spec@(DS { ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1409 , ds_theta = theta, ds_newtype = is_newtype
1410 , ds_name = name, ds_cls = clas })
1412 = return (InstInfo { iSpec = inst_spec
1413 , iBinds = NewTypeDerived co rep_tycon }, [])
1416 = do { fix_env <- getFixityEnv
1417 ; let loc = getSrcSpan name
1418 (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1419 -- In case of a family instance, we need to use the representation
1420 -- tycon (after all, it has the data constructors)
1422 ; return (InstInfo { iSpec = inst_spec
1423 , iBinds = VanillaInst meth_binds [] standalone_deriv }
1426 inst_spec = mkInstance oflag theta spec
1427 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1428 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1430 -- Not a family => rep_tycon = main tycon
1431 co2 = case newTyConCo_maybe rep_tycon of
1432 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1433 Nothing -> id_co -- The newtype is transparent; no need for a cast
1434 co = co1 `mkTransCoI` co2
1435 id_co = IdCo (mkTyConApp rep_tycon rep_tc_args)
1437 -- Example: newtype instance N [a] = N1 (Tree a)
1438 -- deriving instance Eq b => Eq (N [(b,b)])
1439 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1440 -- When dealing with the deriving clause
1441 -- co1 : N [(b,b)] ~ R1:N (b,b)
1442 -- co2 : R1:N (b,b) ~ Tree (b,b)
1443 -- co : N [(b,b)] ~ Tree (b,b)
1445 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1446 genDerivBinds loc fix_env clas tycon
1447 | className clas `elem` typeableClassNames
1448 = (gen_Typeable_binds loc tycon, [])
1451 = case assocMaybe gen_list (getUnique clas) of
1452 Just gen_fn -> gen_fn loc tycon
1453 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1455 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1456 gen_list = [(eqClassKey, gen_Eq_binds)
1457 ,(ordClassKey, gen_Ord_binds)
1458 ,(enumClassKey, gen_Enum_binds)
1459 ,(boundedClassKey, gen_Bounded_binds)
1460 ,(ixClassKey, gen_Ix_binds)
1461 ,(showClassKey, gen_Show_binds fix_env)
1462 ,(readClassKey, gen_Read_binds fix_env)
1463 ,(dataClassKey, gen_Data_binds)
1464 ,(functorClassKey, gen_Functor_binds)
1465 ,(foldableClassKey, gen_Foldable_binds)
1466 ,(traversableClassKey, gen_Traversable_binds)
1471 %************************************************************************
1473 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1475 %************************************************************************
1478 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1479 derivingKindErr tc cls cls_tys cls_kind
1480 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1481 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1482 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1483 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1485 derivingEtaErr :: Class -> [Type] -> Type -> Message
1486 derivingEtaErr cls cls_tys inst_ty
1487 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1488 nest 2 (ptext (sLit "instance (...) =>")
1489 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1491 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1492 typeFamilyPapErr tc cls cls_tys inst_ty
1493 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1494 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1496 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1497 derivingThingErr newtype_deriving clas tys ty why
1498 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1499 2 (quotes (ppr pred))
1500 $$ nest 2 extra) <> colon,
1503 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1505 pred = mkClassPred clas (tys ++ [ty])
1507 derivingHiddenErr :: TyCon -> SDoc
1508 derivingHiddenErr tc
1509 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1510 2 (ptext (sLit "so you cannot derive an instance for it"))
1512 standaloneCtxt :: LHsType Name -> SDoc
1513 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1516 derivInstCtxt :: PredType -> Message
1518 = ptext (sLit "When deriving the instance for") <+> parens (ppr pred)
1520 badDerivedPred :: PredType -> Message
1522 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1523 ptext (sLit "type variables that are not data type parameters"),
1524 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]