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
59 %************************************************************************
63 %************************************************************************
67 1. Convert the decls (i.e. data/newtype deriving clauses,
68 plus standalone deriving) to [EarlyDerivSpec]
70 2. Infer the missing contexts for the Left DerivSpecs
72 3. Add the derived bindings, generating InstInfos
76 -- DerivSpec is purely local to this module
77 data DerivSpec = DS { ds_loc :: SrcSpan
81 , ds_theta :: ThetaType
85 , ds_tc_args :: [Type]
86 , ds_newtype :: Bool }
87 -- This spec implies a dfun declaration of the form
88 -- df :: forall tvs. theta => C tys
89 -- The Name is the name for the DFun we'll build
90 -- The tyvars bind all the variables in the theta
91 -- For type families, the tycon in
92 -- in ds_tys is the *family* tycon
93 -- in ds_tc, ds_tc_args is the *representation* tycon
94 -- For non-family tycons, both are the same
96 -- ds_newtype = True <=> Newtype deriving
97 -- False <=> Vanilla deriving
102 newtype instance T [a] = MkT (Tree a) deriving( C s )
104 axiom T [a] = :RTList a
105 axiom :RTList a = Tree a
107 DS { ds_tvs = [a,s], ds_cls = C, ds_tys = [s, T [a]]
108 , ds_tc = :RTList, ds_tc_args = [a]
109 , ds_newtype = True }
112 type DerivContext = Maybe ThetaType
113 -- Nothing <=> Vanilla deriving; infer the context of the instance decl
114 -- Just theta <=> Standalone deriving: context supplied by programmer
116 type EarlyDerivSpec = Either DerivSpec DerivSpec
117 -- Left ds => the context for the instance should be inferred
118 -- In this case ds_theta is the list of all the
119 -- constraints needed, such as (Eq [a], Eq a)
120 -- The inference process is to reduce this to a
121 -- simpler form (e.g. Eq a)
123 -- Right ds => the exact context for the instance is supplied
124 -- by the programmer; it is ds_theta
126 pprDerivSpec :: DerivSpec -> SDoc
127 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
128 ds_cls = c, ds_tys = tys, ds_theta = rhs })
129 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
130 <+> equals <+> ppr rhs)
134 Inferring missing contexts
135 ~~~~~~~~~~~~~~~~~~~~~~~~~~
138 data T a b = C1 (Foo a) (Bar b)
143 [NOTE: See end of these comments for what to do with
144 data (C a, D b) => T a b = ...
147 We want to come up with an instance declaration of the form
149 instance (Ping a, Pong b, ...) => Eq (T a b) where
152 It is pretty easy, albeit tedious, to fill in the code "...". The
153 trick is to figure out what the context for the instance decl is,
154 namely @Ping@, @Pong@ and friends.
156 Let's call the context reqd for the T instance of class C at types
157 (a,b, ...) C (T a b). Thus:
159 Eq (T a b) = (Ping a, Pong b, ...)
161 Now we can get a (recursive) equation from the @data@ decl:
163 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
164 u Eq (T b a) u Eq Int -- From C2
165 u Eq (T a a) -- From C3
167 Foo and Bar may have explicit instances for @Eq@, in which case we can
168 just substitute for them. Alternatively, either or both may have
169 their @Eq@ instances given by @deriving@ clauses, in which case they
170 form part of the system of equations.
172 Now all we need do is simplify and solve the equations, iterating to
173 find the least fixpoint. Notice that the order of the arguments can
174 switch around, as here in the recursive calls to T.
176 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
180 Eq (T a b) = {} -- The empty set
183 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
184 u Eq (T b a) u Eq Int -- From C2
185 u Eq (T a a) -- From C3
187 After simplification:
188 = Eq a u Ping b u {} u {} u {}
193 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
194 u Eq (T b a) u Eq Int -- From C2
195 u Eq (T a a) -- From C3
197 After simplification:
202 = Eq a u Ping b u Eq b u Ping a
204 The next iteration gives the same result, so this is the fixpoint. We
205 need to make a canonical form of the RHS to ensure convergence. We do
206 this by simplifying the RHS to a form in which
208 - the classes constrain only tyvars
209 - the list is sorted by tyvar (major key) and then class (minor key)
210 - no duplicates, of course
212 So, here are the synonyms for the ``equation'' structures:
215 Note [Data decl contexts]
216 ~~~~~~~~~~~~~~~~~~~~~~~~~
219 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
221 We will need an instance decl like:
223 instance (Read a, RealFloat a) => Read (Complex a) where
226 The RealFloat in the context is because the read method for Complex is bound
227 to construct a Complex, and doing that requires that the argument type is
230 But this ain't true for Show, Eq, Ord, etc, since they don't construct
231 a Complex; they only take them apart.
233 Our approach: identify the offending classes, and add the data type
234 context to the instance decl. The "offending classes" are
238 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
239 pattern matching against a constructor from a data type with a context
240 gives rise to the constraints for that context -- or at least the thinned
241 version. So now all classes are "offending".
243 Note [Newtype deriving]
244 ~~~~~~~~~~~~~~~~~~~~~~~
248 newtype T = T Char deriving( C [a] )
250 Notice the free 'a' in the deriving. We have to fill this out to
251 newtype T = T Char deriving( forall a. C [a] )
253 And then translate it to:
254 instance C [a] Char => C [a] T where ...
257 Note [Newtype deriving superclasses]
258 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
259 (See also Trac #1220 for an interesting exchange on newtype
260 deriving and superclasses.)
262 The 'tys' here come from the partial application in the deriving
263 clause. The last arg is the new instance type.
265 We must pass the superclasses; the newtype might be an instance
266 of them in a different way than the representation type
267 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
268 Then the Show instance is not done via isomorphism; it shows
270 The Num instance is derived via isomorphism, but the Show superclass
271 dictionary must the Show instance for Foo, *not* the Show dictionary
272 gotten from the Num dictionary. So we must build a whole new dictionary
273 not just use the Num one. The instance we want is something like:
274 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
277 There may be a coercion needed which we get from the tycon for the newtype
278 when the dict is constructed in TcInstDcls.tcInstDecl2
281 Note [Unused constructors and deriving clauses]
282 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
283 See Trac #3221. Consider
284 data T = T1 | T2 deriving( Show )
285 Are T1 and T2 unused? Well, no: the deriving clause expands to mention
286 both of them. So we gather defs/uses from deriving just like anything else.
288 %************************************************************************
290 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
292 %************************************************************************
295 tcDeriving :: [LTyClDecl Name] -- All type constructors
296 -> [LInstDecl Name] -- All instance declarations
297 -> [LDerivDecl Name] -- All stand-alone deriving declarations
298 -> TcM ([InstInfo Name] -- The generated "instance decls"
299 ,HsValBinds Name -- Extra generated top-level bindings
301 ,[TyCon] -- Extra generated top-level types
302 ,[TyCon]) -- Extra generated type family instances
304 tcDeriving tycl_decls inst_decls deriv_decls
305 = recoverM (return ([], emptyValBindsOut, emptyDUs, [], [])) $
306 do { -- Fish the "deriving"-related information out of the TcEnv
307 -- And make the necessary "equations".
308 is_boot <- tcIsHsBoot
309 ; traceTc "tcDeriving" (ppr is_boot)
310 ; (early_specs, genericsExtras)
311 <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
312 ; let (repMetaTys, repTyCons, metaInsts) = unzip3 genericsExtras
314 ; overlap_flag <- getOverlapFlag
315 ; let (infer_specs, given_specs) = splitEithers early_specs
316 ; insts1 <- mapM (genInst True overlap_flag) given_specs
318 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
319 inferInstanceContexts overlap_flag infer_specs
321 ; insts2 <- mapM (genInst False overlap_flag) final_specs
323 -- We no longer generate the old generic to/from functions
324 -- from each type declaration, so this is emptyBag
325 ; gen_binds <- return emptyBag -- mkGenericBinds is_boot tycl_decls
328 -- Generate the generic Representable0 instances
329 -- from each type declaration
330 ; repInstsMeta <- genGenericRepBinds is_boot tycl_decls
332 ; let repInsts = concat (map (\(a,_,_) -> a) repInstsMeta)
333 repMetaTys = map (\(_,b,_) -> b) repInstsMeta
334 repTyCons = map (\(_,_,c) -> c) repInstsMeta
336 ; (inst_info, rn_binds, rn_dus)
337 <- renameDeriv is_boot gen_binds (insts1 ++ insts2 ++ concat metaInsts {- ++ repInsts -})
340 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
341 (ddump_deriving inst_info rn_binds))
343 ; when (not (null inst_info)) $
344 dumpDerivingInfo (ddump_deriving inst_info rn_binds)
346 ; return ( inst_info, rn_binds, rn_dus
347 , concat (map metaTyCons2TyCons repMetaTys), repTyCons) }
349 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
350 ddump_deriving inst_infos extra_binds
351 = hang (ptext (sLit "Derived instances"))
352 2 (vcat (map (\i -> pprInstInfoDetails i $$ text "") inst_infos)
356 renameDeriv :: Bool -> LHsBinds RdrName
357 -> [(InstInfo RdrName, DerivAuxBinds)]
358 -> TcM ([InstInfo Name], HsValBinds Name, DefUses)
359 renameDeriv is_boot gen_binds insts
360 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
361 -- The inst-info bindings will all be empty, but it's easier to
362 -- just use rn_inst_info to change the type appropriately
363 = do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
364 ; return (rn_inst_infos, emptyValBindsOut, usesOnly (plusFVs fvs)) }
367 = discardWarnings $ -- Discard warnings about unused bindings etc
368 do { (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $ -- Type signatures in patterns
369 -- are used in the generic binds
370 rnTopBinds (ValBindsIn gen_binds [])
371 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
373 -- Generate and rename any extra not-one-inst-decl-specific binds,
374 -- notably "con2tag" and/or "tag2con" functions.
375 -- Bring those names into scope before renaming the instances themselves
376 ; loc <- getSrcSpanM -- Generic loc for shared bindings
377 ; let (aux_binds, aux_sigs) = unzip $ map (genAuxBind loc) $
378 rm_dups [] $ concat deriv_aux_binds
379 aux_val_binds = ValBindsIn (listToBag aux_binds) aux_sigs
380 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
381 ; bindLocalNames (collectHsValBinders rn_aux_lhs) $
382 do { (rn_aux, dus_aux) <- rnTopBindsRHS rn_aux_lhs
383 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
384 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
385 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
388 (inst_infos, deriv_aux_binds) = unzip insts
390 -- Remove duplicate requests for auxilliary bindings
392 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
393 | otherwise = rm_dups (b:acc) bs
396 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
397 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
398 = return ( info { iBinds = NewTypeDerived coi tc }
399 , mkFVs (map dataConName (tyConDataCons tc)))
400 -- See Note [Newtype deriving and unused constructors]
402 rn_inst_info inst_info@(InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
403 = -- Bring the right type variables into
404 -- scope (yuk), and rename the method binds
406 bindLocalNames (map Var.varName tyvars) $
407 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
408 ; let binds' = VanillaInst rn_binds [] standalone_deriv
409 ; return (inst_info { iBinds = binds' }, fvs) }
411 (tyvars,_, clas,_) = instanceHead inst
412 clas_nm = className clas
414 -----------------------------------------
416 mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
417 mkGenericBinds is_boot tycl_decls
421 = do { tcs <- mapM tcLookupTyCon [ tcdName d
422 | L _ d <- tycl_decls, isDataDecl d ]
423 ; return (unionManyBags [ mkTyConGenericBinds tc
424 | tc <- tcs, tyConHasGenerics tc ]) }
425 -- We are only interested in the data type declarations,
426 -- and then only in the ones whose 'has-generics' flag is on
427 -- The predicate tyConHasGenerics finds both of these
431 Note [Newtype deriving and unused constructors]
432 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
433 Consider this (see Trac #1954):
436 newtype P a = MkP (IO a) deriving Monad
438 If you compile with -fwarn-unused-binds you do not expect the warning
439 "Defined but not used: data consructor MkP". Yet the newtype deriving
440 code does not explicitly mention MkP, but it should behave as if you
442 instance Monad P where
443 return x = MkP (return x)
446 So we want to signal a user of the data constructor 'MkP'. That's
447 what we do in rn_inst_info, and it's the only reason we have the TyCon
448 stored in NewTypeDerived.
451 %************************************************************************
453 From HsSyn to DerivSpec
455 %************************************************************************
457 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
460 -- Make the EarlyDerivSpec for Representable0
461 mkGenDerivSpec :: TyCon -> TcRn (EarlyDerivSpec)
462 mkGenDerivSpec tc = do
464 ; cls <- tcLookupClass rep0ClassName
465 ; let tc_tvs = tyConTyVars tc
466 ; let tc_app = mkTyConApp tc (mkTyVarTys tc_tvs)
468 ; let mtheta = Just []
469 ; ds <- mkEqnHelp StandAloneDerivOrigin tc_tvs cls cls_tys tc_app mtheta
470 -- JPM TODO: StandAloneDerivOrigin?...
473 -- Make the "extras" for the generic representation
474 mkGenDerivExtras :: TyCon
475 -> TcRn (MetaTyCons, TyCon, [(InstInfo RdrName, DerivAuxBinds)])
476 mkGenDerivExtras tc = do
477 { (metaTyCons, rep0TyInst) <- genGenericRepExtras tc
478 ; metaInsts <- genDtMeta (tc, metaTyCons)
479 ; return (metaTyCons, rep0TyInst, metaInsts) }
481 makeDerivSpecs :: Bool
485 -> TcM ( [EarlyDerivSpec]
486 , [(MetaTyCons, TyCon, [(InstInfo RdrName, DerivAuxBinds)])])
487 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
488 | is_boot -- No 'deriving' at all in hs-boot files
489 = do { mapM_ add_deriv_err deriv_locs
492 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
493 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
494 -- Generate EarlyDerivSpec's for Representable, if asked for
495 ; (xGenerics, xDeriveRepresentable) <- genericsFlags
496 ; let allTyNames = [ tcdName d | L _ d <- tycl_decls, isDataDecl d ]
497 ; allTyDecls <- mapM tcLookupTyCon allTyNames
498 -- Select only those types that derive Representable
499 ; derTyDecls <- mapM tcLookupTyCon $
500 filter (needsExtras all_tydata deriv_decls
501 xDeriveRepresentable) allTyNames
502 -- We need to generate the extras to add to what has
503 -- already been derived
504 ; generic_extras_deriv <- mapM mkGenDerivExtras derTyDecls
505 -- For the remaining types, if Generics is on, we need to
506 -- generate both the instances and the extras
507 ; let remTyDecls = filter (\x -> not (x `elem` derTyDecls)) allTyDecls
508 ; generic_instances <- if xGenerics
509 then mapM mkGenDerivSpec remTyDecls
511 ; generic_extras_flag <- if xGenerics
512 then mapM mkGenDerivExtras remTyDecls
514 -- Merge and return everything
515 ; return ( eqns1 ++ eqns2 ++ generic_instances
516 , generic_extras_deriv ++ generic_extras_flag) }
518 needsExtras all_tydata deriv_decls xDeriveRepresentable tc_name
519 | xDeriveRepresentable
520 -- The flag DeriveGenerics is on, so the types the are
521 -- deriving Representable should get the extras defined
522 && ( tc_name `elem` map (tcdName . unLoc . snd) all_tydata
523 || False) --tc_name `elem` map (unLoc . deriv_type . unLoc) deriv_decls)
524 -- JPM TODO: we should check in deriv_decls too, for now we
525 -- don't accept standalone deriving...
528 -- Don't generate anything
531 extractTyDataPreds decls
532 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
534 all_tydata :: [(LHsType Name, LTyClDecl Name)]
535 -- Derived predicate paired with its data type declaration
536 all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
538 deriv_locs = map (getLoc . snd) all_tydata
539 ++ map getLoc deriv_decls
541 add_deriv_err loc = setSrcSpan loc $
542 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
543 2 (ptext (sLit "Use an instance declaration instead")))
545 genericsFlags :: TcM (Bool, Bool)
546 genericsFlags = do dOpts <- getDOpts
547 return ( xopt Opt_Generics dOpts
548 , xopt Opt_DeriveRepresentable dOpts)
550 ------------------------------------------------------------------
551 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
552 -- Standalone deriving declarations
553 -- e.g. deriving instance Show a => Show (T a)
554 -- Rather like tcLocalInstDecl
555 deriveStandalone (L loc (DerivDecl deriv_ty))
557 addErrCtxt (standaloneCtxt deriv_ty) $
558 do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
559 ; (tvs, theta, cls, inst_tys) <- tcHsInstHead deriv_ty
560 ; traceTc "Standalone deriving;" $ vcat
561 [ text "tvs:" <+> ppr tvs
562 , text "theta:" <+> ppr theta
563 , text "cls:" <+> ppr cls
564 , text "tys:" <+> ppr inst_tys ]
565 ; checkValidInstance deriv_ty tvs theta cls inst_tys
566 -- C.f. TcInstDcls.tcLocalInstDecl1
568 ; let cls_tys = take (length inst_tys - 1) inst_tys
569 inst_ty = last inst_tys
570 ; traceTc "Standalone deriving:" $ vcat
571 [ text "class:" <+> ppr cls
572 , text "class types:" <+> ppr cls_tys
573 , text "type:" <+> ppr inst_ty ]
574 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
577 ------------------------------------------------------------------
578 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
579 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
580 tcdTyVars = tv_names,
581 tcdTyPats = ty_pats }))
582 = setSrcSpan loc $ -- Use the location of the 'deriving' item
584 do { (tvs, tc, tc_args) <- get_lhs ty_pats
585 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
586 -- the type variables for the type constructor
588 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
589 -- The "deriv_pred" is a LHsType to take account of the fact that for
590 -- newtype deriving we allow deriving (forall a. C [a]).
592 -- Given data T a b c = ... deriving( C d ),
593 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
594 ; let cls_tyvars = classTyVars cls
595 kind = tyVarKind (last cls_tyvars)
596 (arg_kinds, _) = splitKindFunTys kind
597 n_args_to_drop = length arg_kinds
598 n_args_to_keep = tyConArity tc - n_args_to_drop
599 args_to_drop = drop n_args_to_keep tc_args
600 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
601 inst_ty_kind = typeKind inst_ty
602 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
603 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
604 `minusVarSet` dropped_tvs
606 -- Check that the result really is well-kinded
607 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
608 (derivingKindErr tc cls cls_tys kind)
610 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
611 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
612 (derivingEtaErr cls cls_tys inst_ty)
614 -- (a) The data type can be eta-reduced; eg reject:
615 -- data instance T a a = ... deriving( Monad )
616 -- (b) The type class args do not mention any of the dropped type
618 -- newtype T a s = ... deriving( ST s )
620 -- Type families can't be partially applied
621 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
622 -- Note [Deriving, type families, and partial applications]
623 ; checkTc (not (isFamilyTyCon tc) || n_args_to_drop == 0)
624 (typeFamilyPapErr tc cls cls_tys inst_ty)
626 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
628 -- Tiresomely we must figure out the "lhs", which is awkward for type families
629 -- E.g. data T a b = .. deriving( Eq )
630 -- Here, the lhs is (T a b)
631 -- data instance TF Int b = ... deriving( Eq )
632 -- Here, the lhs is (TF Int b)
633 -- But if we just look up the tycon_name, we get is the *family*
634 -- tycon, but not pattern types -- they are in the *rep* tycon.
635 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
636 ; let tvs = tyConTyVars tc
637 ; return (tvs, tc, mkTyVarTys tvs) }
638 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
639 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
640 ; let (tc, tc_args) = tcSplitTyConApp tc_app
641 ; return (tvs, tc, tc_args) }
644 = panic "derivTyData" -- Caller ensures that only TyData can happen
647 Note [Deriving, type families, and partial applications]
648 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
649 When there are no type families, it's quite easy:
651 newtype S a = MkS [a]
652 -- :CoS :: S ~ [] -- Eta-reduced
654 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
655 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
657 When type familes are involved it's trickier:
660 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
661 -- :RT is the representation type for (T Int a)
662 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
663 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
665 instance Eq [a] => Eq (T Int a) -- easy by coercion
666 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
668 The "???" bit is that we don't build the :CoF thing in eta-reduced form
669 Henc the current typeFamilyPapErr, even though the instance makes sense.
670 After all, we can write it out
671 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
676 mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type] -> Type
677 -> DerivContext -- Just => context supplied (standalone deriving)
678 -- Nothing => context inferred (deriving on data decl)
679 -> TcRn EarlyDerivSpec
680 -- Make the EarlyDerivSpec for an instance
681 -- forall tvs. theta => cls (tys ++ [ty])
682 -- where the 'theta' is optional (that's the Maybe part)
683 -- Assumes that this declaration is well-kinded
685 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
686 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
687 , isAlgTyCon tycon -- Check for functions, primitive types etc
688 = mk_alg_eqn tycon tc_args
690 = failWithTc (derivingThingErr False cls cls_tys tc_app
691 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
694 bale_out msg = failWithTc (derivingThingErr False cls cls_tys tc_app msg)
696 mk_alg_eqn tycon tc_args
697 | className cls `elem` typeableClassNames
698 = do { dflags <- getDOpts
699 ; case checkTypeableConditions (dflags, tycon) of
700 Just err -> bale_out err
701 Nothing -> mk_typeable_eqn orig tvs cls tycon tc_args mtheta }
703 | isDataFamilyTyCon tycon
704 , length tc_args /= tyConArity tycon
705 = bale_out (ptext (sLit "Unsaturated data family application"))
708 = do { (rep_tc, rep_tc_args) <- tcLookupDataFamInst tycon tc_args
709 -- Be careful to test rep_tc here: in the case of families,
710 -- we want to check the instance tycon, not the family tycon
712 -- For standalone deriving (mtheta /= Nothing),
713 -- check that all the data constructors are in scope.
714 ; rdr_env <- getGlobalRdrEnv
715 ; let hidden_data_cons = isAbstractTyCon rep_tc ||
716 any not_in_scope (tyConDataCons rep_tc)
717 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
718 ; unless (isNothing mtheta || not hidden_data_cons)
719 (bale_out (derivingHiddenErr tycon))
722 ; if isDataTyCon rep_tc then
723 mkDataTypeEqn orig dflags tvs cls cls_tys
724 tycon tc_args rep_tc rep_tc_args mtheta
726 mkNewTypeEqn orig dflags tvs cls cls_tys
727 tycon tc_args rep_tc rep_tc_args mtheta }
731 %************************************************************************
735 %************************************************************************
738 mkDataTypeEqn :: CtOrigin
740 -> [Var] -- Universally quantified type variables in the instance
741 -> Class -- Class for which we need to derive an instance
742 -> [Type] -- Other parameters to the class except the last
743 -> TyCon -- Type constructor for which the instance is requested
744 -- (last parameter to the type class)
745 -> [Type] -- Parameters to the type constructor
746 -> TyCon -- rep of the above (for type families)
747 -> [Type] -- rep of the above
748 -> DerivContext -- Context of the instance, for standalone deriving
749 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
751 mkDataTypeEqn orig dflags tvs cls cls_tys
752 tycon tc_args rep_tc rep_tc_args mtheta
753 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
754 -- NB: pass the *representation* tycon to checkSideConditions
755 CanDerive -> go_for_it
756 NonDerivableClass -> bale_out (nonStdErr cls)
757 DerivableClassError msg -> bale_out msg
759 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
760 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
762 mk_data_eqn :: CtOrigin -> [TyVar] -> Class
763 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
764 -> TcM EarlyDerivSpec
765 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
766 = do { dfun_name <- new_dfun_name cls tycon
768 ; let inst_tys = [mkTyConApp tycon tc_args]
769 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
770 spec = DS { ds_loc = loc, ds_orig = orig
771 , ds_name = dfun_name, ds_tvs = tvs
772 , ds_cls = cls, ds_tys = inst_tys
773 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
774 , ds_theta = mtheta `orElse` inferred_constraints
775 , ds_newtype = False }
777 ; return (if isJust mtheta then Right spec -- Specified context
778 else Left spec) } -- Infer context
780 ----------------------
781 mk_typeable_eqn :: CtOrigin -> [TyVar] -> Class
782 -> TyCon -> [TcType] -> DerivContext
783 -> TcM EarlyDerivSpec
784 mk_typeable_eqn orig tvs cls tycon tc_args mtheta
785 -- The Typeable class is special in several ways
786 -- data T a b = ... deriving( Typeable )
788 -- instance Typeable2 T where ...
790 -- 1. There are no constraints in the instance
791 -- 2. There are no type variables either
792 -- 3. The actual class we want to generate isn't necessarily
793 -- Typeable; it depends on the arity of the type
794 | isNothing mtheta -- deriving on a data type decl
795 = do { checkTc (cls `hasKey` typeableClassKey)
796 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
797 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
798 ; mk_typeable_eqn orig tvs real_cls tycon [] (Just []) }
800 | otherwise -- standaone deriving
801 = do { checkTc (null tc_args)
802 (ptext (sLit "Derived typeable instance must be of form (Typeable")
803 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
804 ; dfun_name <- new_dfun_name cls tycon
807 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
808 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
809 , ds_tc = tycon, ds_tc_args = []
810 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
812 ----------------------
813 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
814 -- Generate a sufficiently large set of constraints that typechecking the
815 -- generated method definitions should succeed. This set will be simplified
816 -- before being used in the instance declaration
817 inferConstraints _ cls inst_tys rep_tc rep_tc_args
818 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
819 stupid_constraints ++ extra_constraints
820 ++ sc_constraints ++ con_arg_constraints
822 -- Constraints arising from the arguments of each constructor
824 = [ mkClassPred cls [arg_ty]
825 | data_con <- tyConDataCons rep_tc,
826 arg_ty <- ASSERT( isVanillaDataCon data_con )
827 get_constrained_tys $
828 dataConInstOrigArgTys data_con all_rep_tc_args,
829 not (isUnLiftedType arg_ty) ]
830 -- No constraints for unlifted types
831 -- Where they are legal we generate specilised function calls
833 -- For functor-like classes, two things are different
834 -- (a) We recurse over argument types to generate constraints
835 -- See Functor examples in TcGenDeriv
836 -- (b) The rep_tc_args will be one short
837 is_functor_like = getUnique cls `elem` functorLikeClassKeys
839 get_constrained_tys :: [Type] -> [Type]
840 get_constrained_tys tys
841 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
844 rep_tc_tvs = tyConTyVars rep_tc
845 last_tv = last rep_tc_tvs
846 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
847 | otherwise = rep_tc_args
849 -- Constraints arising from superclasses
850 -- See Note [Superclasses of derived instance]
851 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
854 -- Stupid constraints
855 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
856 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
858 -- Extra Data constraints
859 -- The Data class (only) requires that for
860 -- instance (...) => Data (T t1 t2)
862 -- THEN (Data t1, Data t2) are among the (...) constraints
863 -- Reason: when the IF holds, we generate a method
864 -- dataCast2 f = gcast2 f
865 -- and we need the Data constraints to typecheck the method
867 | cls `hasKey` dataClassKey
868 , all (isLiftedTypeKind . typeKind) rep_tc_args
869 = [mkClassPred cls [ty] | ty <- rep_tc_args]
873 ------------------------------------------------------------------
874 -- Check side conditions that dis-allow derivability for particular classes
875 -- This is *apart* from the newtype-deriving mechanism
877 -- Here we get the representation tycon in case of family instances as it has
878 -- the data constructors - but we need to be careful to fall back to the
879 -- family tycon (with indexes) in error messages.
881 data DerivStatus = CanDerive
882 | DerivableClassError SDoc -- Standard class, but can't do it
883 | NonDerivableClass -- Non-standard class
885 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
886 checkSideConditions dflags mtheta cls cls_tys rep_tc
887 | Just cond <- sideConditions mtheta cls
888 = case (cond (dflags, rep_tc)) of
889 Just err -> DerivableClassError err -- Class-specific error
890 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
891 -- cls_tys (the type args other than last)
893 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
894 | otherwise = NonDerivableClass -- Not a standard class
896 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
898 checkTypeableConditions :: Condition
899 checkTypeableConditions = checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK
901 nonStdErr :: Class -> SDoc
902 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
904 sideConditions :: DerivContext -> Class -> Maybe Condition
905 sideConditions mtheta cls
906 | cls_key == eqClassKey = Just cond_std
907 | cls_key == ordClassKey = Just cond_std
908 | cls_key == showClassKey = Just cond_std
909 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
910 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
911 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
912 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
913 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
914 cond_std `andCond` cond_noUnliftedArgs)
915 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
916 cond_functorOK True) -- NB: no cond_std!
917 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
918 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
919 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
920 cond_functorOK False)
921 | cls_key == rep0ClassKey = Just (checkFlag Opt_DeriveRepresentable `orCond`
922 checkFlag Opt_Generics)
923 -- JPM TODO: we should use canDoGenerics
924 | otherwise = Nothing
926 cls_key = getUnique cls
927 cond_std = cond_stdOK mtheta
929 type Condition = (DynFlags, TyCon) -> Maybe SDoc
930 -- first Bool is whether or not we are allowed to derive Data and Typeable
931 -- second Bool is whether or not we are allowed to derive Functor
932 -- TyCon is the *representation* tycon if the
933 -- data type is an indexed one
936 orCond :: Condition -> Condition -> Condition
939 Nothing -> Nothing -- c1 succeeds
940 Just x -> case c2 tc of -- c1 fails
942 Just y -> Just (x $$ ptext (sLit " and") $$ y)
945 andCond :: Condition -> Condition -> Condition
946 andCond c1 c2 tc = case c1 tc of
947 Nothing -> c2 tc -- c1 succeeds
948 Just x -> Just x -- c1 fails
950 cond_stdOK :: DerivContext -> Condition
951 cond_stdOK (Just _) _
952 = Nothing -- Don't check these conservative conditions for
953 -- standalone deriving; just generate the code
954 -- and let the typechecker handle the result
955 cond_stdOK Nothing (_, rep_tc)
956 | null data_cons = Just (no_cons_why rep_tc $$ suggestion)
957 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
958 | otherwise = Nothing
960 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
961 data_cons = tyConDataCons rep_tc
962 con_whys = mapCatMaybes check_con data_cons
964 check_con :: DataCon -> Maybe SDoc
966 | isVanillaDataCon con
967 , all isTauTy (dataConOrigArgTys con) = Nothing
968 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
970 no_cons_why :: TyCon -> SDoc
971 no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
972 ptext (sLit "has no data constructors")
974 cond_enumOrProduct :: Condition
975 cond_enumOrProduct = cond_isEnumeration `orCond`
976 (cond_isProduct `andCond` cond_noUnliftedArgs)
978 cond_noUnliftedArgs :: Condition
979 -- For some classes (eg Eq, Ord) we allow unlifted arg types
980 -- by generating specilaised code. For others (eg Data) we don't.
981 cond_noUnliftedArgs (_, tc)
982 | null bad_cons = Nothing
983 | otherwise = Just why
985 bad_cons = [ con | con <- tyConDataCons tc
986 , any isUnLiftedType (dataConOrigArgTys con) ]
987 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
989 cond_isEnumeration :: Condition
990 cond_isEnumeration (_, rep_tc)
991 | isEnumerationTyCon rep_tc = Nothing
992 | otherwise = Just why
994 why = sep [ quotes (pprSourceTyCon rep_tc) <+>
995 ptext (sLit "is not an enumeration type")
996 , ptext (sLit "(an enumeration consists of one or more nullary, non-GADT constructors)") ]
997 -- See Note [Enumeration types] in TyCon
999 cond_isProduct :: Condition
1000 cond_isProduct (_, rep_tc)
1001 | isProductTyCon rep_tc = Nothing
1002 | otherwise = Just why
1004 why = quotes (pprSourceTyCon rep_tc) <+>
1005 ptext (sLit "does not have precisely one constructor")
1007 cond_typeableOK :: Condition
1008 -- OK for Typeable class
1009 -- Currently: (a) args all of kind *
1010 -- (b) 7 or fewer args
1011 cond_typeableOK (_, tc)
1012 | tyConArity tc > 7 = Just too_many
1013 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tc))
1015 | otherwise = Nothing
1017 too_many = quotes (pprSourceTyCon tc) <+>
1018 ptext (sLit "has too many arguments")
1019 bad_kind = quotes (pprSourceTyCon tc) <+>
1020 ptext (sLit "has arguments of kind other than `*'")
1022 functorLikeClassKeys :: [Unique]
1023 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
1025 cond_functorOK :: Bool -> Condition
1026 -- OK for Functor/Foldable/Traversable class
1027 -- Currently: (a) at least one argument
1028 -- (b) don't use argument contravariantly
1029 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
1030 -- (d) optionally: don't use function types
1031 -- (e) no "stupid context" on data type
1032 cond_functorOK allowFunctions (_, rep_tc)
1034 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
1035 <+> ptext (sLit "has no parameters"))
1037 | not (null bad_stupid_theta)
1038 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
1039 <+> ptext (sLit "has a class context") <+> pprTheta bad_stupid_theta)
1042 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
1044 tc_tvs = tyConTyVars rep_tc
1045 Just (_, last_tv) = snocView tc_tvs
1046 bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
1047 is_bad pred = last_tv `elemVarSet` tyVarsOfPred pred
1049 data_cons = tyConDataCons rep_tc
1050 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
1052 check_vanilla :: DataCon -> Maybe SDoc
1053 check_vanilla con | isVanillaDataCon con = Nothing
1054 | otherwise = Just (badCon con existential)
1056 ft_check :: DataCon -> FFoldType (Maybe SDoc)
1057 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
1058 , ft_co_var = Just (badCon con covariant)
1059 , ft_fun = \x y -> if allowFunctions then x `mplus` y
1060 else Just (badCon con functions)
1061 , ft_tup = \_ xs -> msum xs
1062 , ft_ty_app = \_ x -> x
1063 , ft_bad_app = Just (badCon con wrong_arg)
1064 , ft_forall = \_ x -> x }
1066 existential = ptext (sLit "has existential arguments")
1067 covariant = ptext (sLit "uses the type variable in a function argument")
1068 functions = ptext (sLit "contains function types")
1069 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
1071 checkFlag :: ExtensionFlag -> Condition
1072 checkFlag flag (dflags, _)
1073 | xopt flag dflags = Nothing
1074 | otherwise = Just why
1076 why = ptext (sLit "You need -X") <> text flag_str
1077 <+> ptext (sLit "to derive an instance for this class")
1078 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
1080 other -> pprPanic "checkFlag" (ppr other)
1082 std_class_via_iso :: Class -> Bool
1083 -- These standard classes can be derived for a newtype
1084 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
1085 -- because giving so gives the same results as generating the boilerplate
1086 std_class_via_iso clas
1087 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
1088 -- Not Read/Show because they respect the type
1089 -- Not Enum, because newtypes are never in Enum
1092 non_iso_class :: Class -> Bool
1093 -- *Never* derive Read,Show,Typeable,Data by isomorphism,
1094 -- even with -XGeneralizedNewtypeDeriving
1096 = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
1099 typeableClassKeys :: [Unique]
1100 typeableClassKeys = map getUnique typeableClassNames
1102 new_dfun_name :: Class -> TyCon -> TcM Name
1103 new_dfun_name clas tycon -- Just a simple wrapper
1104 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1105 ; newDFunName clas [mkTyConApp tycon []] loc }
1106 -- The type passed to newDFunName is only used to generate
1107 -- a suitable string; hence the empty type arg list
1109 badCon :: DataCon -> SDoc -> SDoc
1110 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1113 Note [Superclasses of derived instance]
1114 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1115 In general, a derived instance decl needs the superclasses of the derived
1116 class too. So if we have
1117 data T a = ...deriving( Ord )
1118 then the initial context for Ord (T a) should include Eq (T a). Often this is
1119 redundant; we'll also generate an Ord constraint for each constructor argument,
1120 and that will probably generate enough constraints to make the Eq (T a) constraint
1121 be satisfied too. But not always; consider:
1127 data T a = MkT (S a) deriving( Ord )
1128 instance Num a => Eq (T a)
1130 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1132 data T a = MkT deriving( Data, Typeable )
1133 Here there *is* no argument field, but we must nevertheless generate
1134 a context for the Data instances:
1135 instance Typable a => Data (T a) where ...
1138 %************************************************************************
1142 %************************************************************************
1145 mkNewTypeEqn :: CtOrigin -> DynFlags -> [Var] -> Class
1146 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1148 -> TcRn EarlyDerivSpec
1149 mkNewTypeEqn orig dflags tvs
1150 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1151 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1152 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1153 = do { traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
1154 ; dfun_name <- new_dfun_name cls tycon
1155 ; loc <- getSrcSpanM
1156 ; let spec = DS { ds_loc = loc, ds_orig = orig
1157 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1158 , ds_cls = cls, ds_tys = inst_tys
1159 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1160 , ds_theta = mtheta `orElse` all_preds
1161 , ds_newtype = True }
1162 ; return (if isJust mtheta then Right spec
1166 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1167 CanDerive -> go_for_it -- Use the standard H98 method
1168 DerivableClassError msg -- Error with standard class
1169 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1170 | otherwise -> bale_out msg
1171 NonDerivableClass -- Must use newtype deriving
1172 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1173 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1174 | otherwise -> bale_out non_std
1176 newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
1177 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1178 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1180 non_std = nonStdErr cls
1181 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1183 -- Here is the plan for newtype derivings. We see
1184 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1185 -- where t is a type,
1186 -- ak+1...an is a suffix of a1..an, and are all tyars
1187 -- ak+1...an do not occur free in t, nor in the s1..sm
1188 -- (C s1 ... sm) is a *partial applications* of class C
1189 -- with the last parameter missing
1190 -- (T a1 .. ak) matches the kind of C's last argument
1191 -- (and hence so does t)
1192 -- The latter kind-check has been done by deriveTyData already,
1193 -- and tc_args are already trimmed
1195 -- We generate the instance
1196 -- instance forall ({a1..ak} u fvs(s1..sm)).
1197 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1198 -- where T a1...ap is the partial application of
1199 -- the LHS of the correct kind and p >= k
1201 -- NB: the variables below are:
1202 -- tc_tvs = [a1, ..., an]
1203 -- tyvars_to_keep = [a1, ..., ak]
1204 -- rep_ty = t ak .. an
1205 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1206 -- tys = [s1, ..., sm]
1209 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1210 -- We generate the instance
1211 -- instance Monad (ST s) => Monad (T s) where
1213 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1214 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1215 -- eta-reduces the representation type, so we know that
1217 -- That's convenient here, because we may have to apply
1218 -- it to fewer than its original complement of arguments
1220 -- Note [Newtype representation]
1221 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1222 -- Need newTyConRhs (*not* a recursive representation finder)
1223 -- to get the representation type. For example
1224 -- newtype B = MkB Int
1225 -- newtype A = MkA B deriving( Num )
1226 -- We want the Num instance of B, *not* the Num instance of Int,
1227 -- when making the Num instance of A!
1228 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1229 rep_tys = cls_tys ++ [rep_inst_ty]
1230 rep_pred = mkClassPred cls rep_tys
1231 -- rep_pred is the representation dictionary, from where
1232 -- we are gong to get all the methods for the newtype
1236 -- Next we figure out what superclass dictionaries to use
1237 -- See Note [Newtype deriving superclasses] above
1239 cls_tyvars = classTyVars cls
1240 dfun_tvs = tyVarsOfTypes inst_tys
1241 inst_ty = mkTyConApp tycon tc_args
1242 inst_tys = cls_tys ++ [inst_ty]
1243 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1246 -- If there are no tyvars, there's no need
1247 -- to abstract over the dictionaries we need
1248 -- Example: newtype T = MkT Int deriving( C )
1249 -- We get the derived instance
1252 -- instance C Int => C T
1253 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1255 -------------------------------------------------------------------
1256 -- Figuring out whether we can only do this newtype-deriving thing
1258 can_derive_via_isomorphism
1259 = not (non_iso_class cls)
1263 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1265 arity_ok = length cls_tys + 1 == classArity cls
1266 -- Well kinded; eg not: newtype T ... deriving( ST )
1267 -- because ST needs *2* type params
1269 -- Check that eta reduction is OK
1270 eta_ok = nt_eta_arity <= length rep_tc_args
1271 -- The newtype can be eta-reduced to match the number
1272 -- of type argument actually supplied
1273 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1274 -- Here the 'b' must be the same in the rep type (S [a] b)
1275 -- And the [a] must not mention 'b'. That's all handled
1278 ats_ok = null (classATs cls)
1279 -- No associated types for the class, because we don't
1280 -- currently generate type 'instance' decls; and cannot do
1281 -- so for 'data' instance decls
1284 = vcat [ ppUnless arity_ok arity_msg
1285 , ppUnless eta_ok eta_msg
1286 , ppUnless ats_ok ats_msg ]
1287 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1288 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1289 ats_msg = ptext (sLit "the class has associated types")
1292 Note [Recursive newtypes]
1293 ~~~~~~~~~~~~~~~~~~~~~~~~~
1294 Newtype deriving works fine, even if the newtype is recursive.
1295 e.g. newtype S1 = S1 [T1 ()]
1296 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1297 Remember, too, that type families are curretly (conservatively) given
1298 a recursive flag, so this also allows newtype deriving to work
1301 We used to exclude recursive types, because we had a rather simple
1302 minded way of generating the instance decl:
1304 instance Eq [A] => Eq A -- Makes typechecker loop!
1305 But now we require a simple context, so it's ok.
1308 %************************************************************************
1310 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1312 %************************************************************************
1314 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1315 terms, which is the final correct RHS for the corresponding original
1319 Each (k,TyVarTy tv) in a solution constrains only a type
1323 The (k,TyVarTy tv) pairs in a solution are canonically
1324 ordered by sorting on type varible, tv, (major key) and then class, k,
1329 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1331 inferInstanceContexts _ [] = return []
1333 inferInstanceContexts oflag infer_specs
1334 = do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
1335 ; iterate_deriv 1 initial_solutions }
1337 ------------------------------------------------------------------
1338 -- The initial solutions for the equations claim that each
1339 -- instance has an empty context; this solution is certainly
1340 -- in canonical form.
1341 initial_solutions :: [ThetaType]
1342 initial_solutions = [ [] | _ <- infer_specs ]
1344 ------------------------------------------------------------------
1345 -- iterate_deriv calculates the next batch of solutions,
1346 -- compares it with the current one; finishes if they are the
1347 -- same, otherwise recurses with the new solutions.
1348 -- It fails if any iteration fails
1349 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1350 iterate_deriv n current_solns
1351 | n > 20 -- Looks as if we are in an infinite loop
1352 -- This can happen if we have -XUndecidableInstances
1353 -- (See TcSimplify.tcSimplifyDeriv.)
1354 = pprPanic "solveDerivEqns: probable loop"
1355 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1357 = do { -- Extend the inst info from the explicit instance decls
1358 -- with the current set of solutions, and simplify each RHS
1359 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1360 current_solns infer_specs
1361 ; new_solns <- checkNoErrs $
1362 extendLocalInstEnv inst_specs $
1363 mapM gen_soln infer_specs
1365 ; if (current_solns == new_solns) then
1366 return [ spec { ds_theta = soln }
1367 | (spec, soln) <- zip infer_specs current_solns ]
1369 iterate_deriv (n+1) new_solns }
1371 ------------------------------------------------------------------
1372 gen_soln :: DerivSpec -> TcM [PredType]
1373 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1374 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1376 addErrCtxt (derivInstCtxt the_pred) $
1377 do { -- Check for a bizarre corner case, when the derived instance decl should
1378 -- have form instance C a b => D (T a) where ...
1379 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1380 -- of problems; in particular, it's hard to compare solutions for
1381 -- equality when finding the fixpoint. Moreover, simplifyDeriv
1382 -- has an assert failure because it finds a TyVar when it expects
1383 -- only TcTyVars. So I just rule it out for now. I'm not
1384 -- even sure how it can arise.
1386 ; let tv_set = mkVarSet tyvars
1387 weird_preds = [pred | pred <- deriv_rhs
1388 , not (tyVarsOfPred pred `subVarSet` tv_set)]
1389 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1391 ; theta <- simplifyDeriv orig the_pred tyvars deriv_rhs
1392 -- checkValidInstance tyvars theta clas inst_tys
1393 -- Not necessary; see Note [Exotic derived instance contexts]
1396 ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
1397 -- Claim: the result instance declaration is guaranteed valid
1398 -- Hence no need to call:
1399 -- checkValidInstance tyvars theta clas inst_tys
1400 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1402 the_pred = mkClassPred clas inst_tys
1404 ------------------------------------------------------------------
1405 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1406 mkInstance overlap_flag theta
1407 (DS { ds_name = dfun_name
1408 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1409 = mkLocalInstance dfun overlap_flag
1411 dfun = mkDictFunId dfun_name tyvars theta clas tys
1414 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1415 -- Add new locally-defined instances; don't bother to check
1416 -- for functional dependency errors -- that'll happen in TcInstDcls
1417 extendLocalInstEnv dfuns thing_inside
1418 = do { env <- getGblEnv
1419 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1420 env' = env { tcg_inst_env = inst_env' }
1421 ; setGblEnv env' thing_inside }
1425 %************************************************************************
1427 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1429 %************************************************************************
1431 After all the trouble to figure out the required context for the
1432 derived instance declarations, all that's left is to chug along to
1433 produce them. They will then be shoved into @tcInstDecls2@, which
1434 will do all its usual business.
1436 There are lots of possibilities for code to generate. Here are
1437 various general remarks.
1442 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1443 ``you-couldn't-do-better-by-hand'' efficient.
1446 Deriving @Show@---also pretty common--- should also be reasonable good code.
1449 Deriving for the other classes isn't that common or that big a deal.
1456 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1459 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1462 We {\em normally} generate code only for the non-defaulted methods;
1463 there are some exceptions for @Eq@ and (especially) @Ord@...
1466 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1467 constructor's numeric (@Int#@) tag. These are generated by
1468 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1469 these is around is given by @hasCon2TagFun@.
1471 The examples under the different sections below will make this
1475 Much less often (really just for deriving @Ix@), we use a
1476 @_tag2con_<tycon>@ function. See the examples.
1479 We use the renamer!!! Reason: we're supposed to be
1480 producing @LHsBinds Name@ for the methods, but that means
1481 producing correctly-uniquified code on the fly. This is entirely
1482 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1483 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1484 the renamer. What a great hack!
1488 -- Generate the InstInfo for the required instance paired with the
1489 -- *representation* tycon for that instance,
1490 -- plus any auxiliary bindings required
1492 -- Representation tycons differ from the tycon in the instance signature in
1493 -- case of instances for indexed families.
1495 genInst :: Bool -- True <=> standalone deriving
1497 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1498 genInst standalone_deriv oflag
1499 spec@(DS { ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1500 , ds_theta = theta, ds_newtype = is_newtype
1501 , ds_name = name, ds_cls = clas })
1503 = return (InstInfo { iSpec = inst_spec
1504 , iBinds = NewTypeDerived co rep_tycon }, [])
1507 = do { fix_env <- getFixityEnv
1508 ; let loc = getSrcSpan name
1509 (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1510 -- In case of a family instance, we need to use the representation
1511 -- tycon (after all, it has the data constructors)
1513 ; return (InstInfo { iSpec = inst_spec
1514 , iBinds = VanillaInst meth_binds [] standalone_deriv }
1517 inst_spec = mkInstance oflag theta spec
1518 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1519 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1521 -- Not a family => rep_tycon = main tycon
1522 co2 = case newTyConCo_maybe rep_tycon of
1523 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1524 Nothing -> id_co -- The newtype is transparent; no need for a cast
1525 co = co1 `mkTransCoI` co2
1526 id_co = IdCo (mkTyConApp rep_tycon rep_tc_args)
1528 -- Example: newtype instance N [a] = N1 (Tree a)
1529 -- deriving instance Eq b => Eq (N [(b,b)])
1530 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1531 -- When dealing with the deriving clause
1532 -- co1 : N [(b,b)] ~ R1:N (b,b)
1533 -- co2 : R1:N (b,b) ~ Tree (b,b)
1534 -- co : N [(b,b)] ~ Tree (b,b)
1536 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1537 genDerivBinds loc fix_env clas tycon
1538 | className clas `elem` typeableClassNames
1539 = (gen_Typeable_binds loc tycon, [])
1542 = case assocMaybe gen_list (getUnique clas) of
1543 Just gen_fn -> gen_fn loc tycon
1544 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1546 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1547 gen_list = [(eqClassKey, gen_Eq_binds)
1548 ,(ordClassKey, gen_Ord_binds)
1549 ,(enumClassKey, gen_Enum_binds)
1550 ,(boundedClassKey, gen_Bounded_binds)
1551 ,(ixClassKey, gen_Ix_binds)
1552 ,(showClassKey, gen_Show_binds fix_env)
1553 ,(readClassKey, gen_Read_binds fix_env)
1554 ,(dataClassKey, gen_Data_binds)
1555 ,(functorClassKey, gen_Functor_binds)
1556 ,(foldableClassKey, gen_Foldable_binds)
1557 ,(traversableClassKey, gen_Traversable_binds)
1558 ,(rep0ClassKey, gen_Rep0_binds)
1562 %************************************************************************
1564 \subsection[TcDeriv-generic-binds]{Bindings for the new generic deriving mechanism}
1566 %************************************************************************
1568 For the generic representation we need to generate:
1570 \item A Representable0 instance
1571 \item A Rep0 type instance
1572 \item Many auxiliary datatypes and instances for them (for the meta-information)
1575 @gen_Rep0_binds@ does (1)
1576 @genGenericRepExtras@ does (2) and (3)
1577 @genGenericRepBind@ does all of them
1581 genGenericRepBinds :: Bool -> [LTyClDecl Name]
1582 -> TcM [([(InstInfo RdrName, DerivAuxBinds)]
1583 , MetaTyCons, TyCon)]
1584 genGenericRepBinds isBoot tyclDecls
1585 | isBoot = return []
1587 allTyDecls <- mapM tcLookupTyCon [ tcdName d | L _ d <- tyclDecls
1589 let tyDecls = filter tyConHasGenerics allTyDecls
1590 inst1 <- mapM genGenericRepBind tyDecls
1591 let (_repInsts, metaTyCons, _repTys) = unzip3 inst1
1592 metaInsts <- ASSERT (length tyDecls == length metaTyCons)
1593 mapM genDtMeta (zip tyDecls metaTyCons)
1594 return (ASSERT (length inst1 == length metaInsts)
1596 | ((ri, ms, rt), mi) <- zip inst1 metaInsts ])
1599 gen_Rep0_binds :: SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1600 gen_Rep0_binds _ tc = (mkBindsRep0 tc, [ {- No DerivAuxBinds -} ])
1602 genGenericRepExtras :: TyCon -> TcM (MetaTyCons, TyCon)
1603 genGenericRepExtras tc =
1604 do uniqS <- newUniqueSupply
1606 -- Uniques for everyone
1607 (uniqD:uniqs) = uniqsFromSupply uniqS
1608 (uniqsC,us) = splitAt (length tc_cons) uniqs
1609 uniqsS :: [[Unique]] -- Unique supply for the S datatypes
1610 uniqsS = mkUniqsS tc_arits us
1612 mkUniqsS (n:t) us = case splitAt n us of
1613 (us1,us2) -> us1 : mkUniqsS t us2
1615 tc_name = tyConName tc
1616 tc_cons = tyConDataCons tc
1617 tc_arits = map dataConSourceArity tc_cons
1619 tc_occ = nameOccName tc_name
1620 d_occ = mkGenD tc_occ
1621 c_occ m = mkGenC tc_occ m
1622 s_occ m n = mkGenS tc_occ m n
1623 mod_name = nameModule (tyConName tc)
1624 d_name = mkExternalName uniqD mod_name d_occ wiredInSrcSpan
1625 c_names = [ mkExternalName u mod_name (c_occ m) wiredInSrcSpan
1626 | (u,m) <- zip uniqsC [0..] ]
1627 s_names = [ [ mkExternalName u mod_name (s_occ m n) wiredInSrcSpan
1628 | (u,n) <- zip us [0..] ] | (us,m) <- zip uniqsS [0..] ]
1630 mkTyCon name = ASSERT( isExternalName name )
1631 buildAlgTyCon name [] [] mkAbstractTyConRhs
1632 NonRecursive False False NoParentTyCon Nothing
1634 metaDTyCon <- mkTyCon d_name
1635 metaCTyCons <- sequence [ mkTyCon c_name | c_name <- c_names ]
1636 metaSTyCons <- mapM sequence
1638 | s_name <- s_namesC ] | s_namesC <- s_names ]
1640 let metaDts = MetaTyCons metaDTyCon metaCTyCons metaSTyCons
1642 rep0_tycon <- tc_mkRep0TyCon tc metaDts
1644 return (metaDts, rep0_tycon)
1646 genGenericRepBind :: TyCon
1647 -> TcM ((InstInfo RdrName, DerivAuxBinds), MetaTyCons, TyCon)
1648 genGenericRepBind tc =
1649 do (metaDts, rep0_tycon) <- genGenericRepExtras tc
1650 clas <- tcLookupClass rep0ClassName
1651 dfun_name <- new_dfun_name clas tc
1653 mkInstRep0 = (InstInfo { iSpec = inst, iBinds = binds }
1654 , [ {- No DerivAuxBinds -} ])
1655 inst = mkLocalInstance dfun NoOverlap
1656 binds = VanillaInst (mkBindsRep0 tc) [] False
1658 tvs = tyConTyVars tc
1659 tc_ty = mkTyConApp tc (mkTyVarTys tvs)
1661 dfun = mkDictFunId dfun_name (tyConTyVars tc) [] clas [tc_ty]
1662 return (mkInstRep0, metaDts, rep0_tycon)
1664 genDtMeta :: (TyCon, MetaTyCons) -> TcM [(InstInfo RdrName, DerivAuxBinds)]
1665 genDtMeta (tc,metaDts) =
1666 do dClas <- tcLookupClass datatypeClassName
1667 d_dfun_name <- new_dfun_name dClas tc
1668 cClas <- tcLookupClass constructorClassName
1669 c_dfun_names <- sequence [ new_dfun_name cClas tc | _ <- metaC metaDts ]
1670 sClas <- tcLookupClass selectorClassName
1671 s_dfun_names <- sequence (map sequence [ [ new_dfun_name sClas tc
1673 | x <- metaS metaDts ])
1674 fix_env <- getFixityEnv
1677 (dBinds,cBinds,sBinds) = mkBindsMetaD fix_env tc
1680 d_metaTycon = metaD metaDts
1681 d_inst = mkLocalInstance d_dfun NoOverlap
1682 d_binds = VanillaInst dBinds [] False
1683 d_dfun = mkDictFunId d_dfun_name (tyConTyVars tc) [] dClas
1684 [ mkTyConTy d_metaTycon ]
1685 d_mkInst = (InstInfo { iSpec = d_inst, iBinds = d_binds }, [])
1688 c_metaTycons = metaC metaDts
1689 c_insts = [ mkLocalInstance (c_dfun c ds) NoOverlap
1690 | (c, ds) <- myZip1 c_metaTycons c_dfun_names ]
1691 c_binds = [ VanillaInst c [] False | c <- cBinds ]
1692 c_dfun c dfun_name = mkDictFunId dfun_name (tyConTyVars tc) [] cClas
1694 c_mkInst = [ (InstInfo { iSpec = is, iBinds = bs }, [])
1695 | (is,bs) <- myZip1 c_insts c_binds ]
1698 s_metaTycons = metaS metaDts
1699 s_insts = map (map (\(s,ds) -> mkLocalInstance (s_dfun s ds) NoOverlap))
1700 (myZip2 s_metaTycons s_dfun_names)
1701 s_binds = [ [ VanillaInst s [] False | s <- ss ] | ss <- sBinds ]
1702 s_dfun s dfun_name = mkDictFunId dfun_name (tyConTyVars tc) [] sClas
1704 s_mkInst = map (map (\(is,bs) -> (InstInfo {iSpec=is, iBinds=bs}, [])))
1705 (myZip2 s_insts s_binds)
1707 myZip1 :: [a] -> [b] -> [(a,b)]
1708 myZip1 l1 l2 = ASSERT (length l1 == length l2) zip l1 l2
1710 myZip2 :: [[a]] -> [[b]] -> [[(a,b)]]
1712 ASSERT (and (zipWith (>=) (map length l1) (map length l2)))
1713 [ zip x1 x2 | (x1,x2) <- zip l1 l2 ]
1715 return (d_mkInst : c_mkInst ++ concat s_mkInst)
1719 %************************************************************************
1721 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1723 %************************************************************************
1726 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1727 derivingKindErr tc cls cls_tys cls_kind
1728 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1729 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1730 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1731 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1733 derivingEtaErr :: Class -> [Type] -> Type -> Message
1734 derivingEtaErr cls cls_tys inst_ty
1735 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1736 nest 2 (ptext (sLit "instance (...) =>")
1737 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1739 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1740 typeFamilyPapErr tc cls cls_tys inst_ty
1741 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1742 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1744 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1745 derivingThingErr newtype_deriving clas tys ty why
1746 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1747 2 (quotes (ppr pred))
1748 $$ nest 2 extra) <> colon,
1751 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1753 pred = mkClassPred clas (tys ++ [ty])
1755 derivingHiddenErr :: TyCon -> SDoc
1756 derivingHiddenErr tc
1757 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1758 2 (ptext (sLit "so you cannot derive an instance for it"))
1760 standaloneCtxt :: LHsType Name -> SDoc
1761 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1764 derivInstCtxt :: PredType -> Message
1766 = ptext (sLit "When deriving the instance for") <+> parens (ppr pred)
1768 badDerivedPred :: PredType -> Message
1770 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1771 ptext (sLit "type variables that are not data type parameters"),
1772 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]