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)
132 instance Outputable DerivSpec where
137 Inferring missing contexts
138 ~~~~~~~~~~~~~~~~~~~~~~~~~~
141 data T a b = C1 (Foo a) (Bar b)
146 [NOTE: See end of these comments for what to do with
147 data (C a, D b) => T a b = ...
150 We want to come up with an instance declaration of the form
152 instance (Ping a, Pong b, ...) => Eq (T a b) where
155 It is pretty easy, albeit tedious, to fill in the code "...". The
156 trick is to figure out what the context for the instance decl is,
157 namely @Ping@, @Pong@ and friends.
159 Let's call the context reqd for the T instance of class C at types
160 (a,b, ...) C (T a b). Thus:
162 Eq (T a b) = (Ping a, Pong b, ...)
164 Now we can get a (recursive) equation from the @data@ decl:
166 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
167 u Eq (T b a) u Eq Int -- From C2
168 u Eq (T a a) -- From C3
170 Foo and Bar may have explicit instances for @Eq@, in which case we can
171 just substitute for them. Alternatively, either or both may have
172 their @Eq@ instances given by @deriving@ clauses, in which case they
173 form part of the system of equations.
175 Now all we need do is simplify and solve the equations, iterating to
176 find the least fixpoint. Notice that the order of the arguments can
177 switch around, as here in the recursive calls to T.
179 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
183 Eq (T a b) = {} -- The empty set
186 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
187 u Eq (T b a) u Eq Int -- From C2
188 u Eq (T a a) -- From C3
190 After simplification:
191 = Eq a u Ping b u {} u {} u {}
196 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
197 u Eq (T b a) u Eq Int -- From C2
198 u Eq (T a a) -- From C3
200 After simplification:
205 = Eq a u Ping b u Eq b u Ping a
207 The next iteration gives the same result, so this is the fixpoint. We
208 need to make a canonical form of the RHS to ensure convergence. We do
209 this by simplifying the RHS to a form in which
211 - the classes constrain only tyvars
212 - the list is sorted by tyvar (major key) and then class (minor key)
213 - no duplicates, of course
215 So, here are the synonyms for the ``equation'' structures:
218 Note [Data decl contexts]
219 ~~~~~~~~~~~~~~~~~~~~~~~~~
222 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
224 We will need an instance decl like:
226 instance (Read a, RealFloat a) => Read (Complex a) where
229 The RealFloat in the context is because the read method for Complex is bound
230 to construct a Complex, and doing that requires that the argument type is
233 But this ain't true for Show, Eq, Ord, etc, since they don't construct
234 a Complex; they only take them apart.
236 Our approach: identify the offending classes, and add the data type
237 context to the instance decl. The "offending classes" are
241 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
242 pattern matching against a constructor from a data type with a context
243 gives rise to the constraints for that context -- or at least the thinned
244 version. So now all classes are "offending".
246 Note [Newtype deriving]
247 ~~~~~~~~~~~~~~~~~~~~~~~
251 newtype T = T Char deriving( C [a] )
253 Notice the free 'a' in the deriving. We have to fill this out to
254 newtype T = T Char deriving( forall a. C [a] )
256 And then translate it to:
257 instance C [a] Char => C [a] T where ...
260 Note [Newtype deriving superclasses]
261 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
262 (See also Trac #1220 for an interesting exchange on newtype
263 deriving and superclasses.)
265 The 'tys' here come from the partial application in the deriving
266 clause. The last arg is the new instance type.
268 We must pass the superclasses; the newtype might be an instance
269 of them in a different way than the representation type
270 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
271 Then the Show instance is not done via isomorphism; it shows
273 The Num instance is derived via isomorphism, but the Show superclass
274 dictionary must the Show instance for Foo, *not* the Show dictionary
275 gotten from the Num dictionary. So we must build a whole new dictionary
276 not just use the Num one. The instance we want is something like:
277 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
280 There may be a coercion needed which we get from the tycon for the newtype
281 when the dict is constructed in TcInstDcls.tcInstDecl2
284 Note [Unused constructors and deriving clauses]
285 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
286 See Trac #3221. Consider
287 data T = T1 | T2 deriving( Show )
288 Are T1 and T2 unused? Well, no: the deriving clause expands to mention
289 both of them. So we gather defs/uses from deriving just like anything else.
291 %************************************************************************
293 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
295 %************************************************************************
298 tcDeriving :: [LTyClDecl Name] -- All type constructors
299 -> [LInstDecl Name] -- All instance declarations
300 -> [LDerivDecl Name] -- All stand-alone deriving declarations
301 -> TcM ([InstInfo Name] -- The generated "instance decls"
302 ,HsValBinds Name -- Extra generated top-level bindings
304 ,[TyCon] -- Extra generated top-level types
305 ,[TyCon]) -- Extra generated type family instances
307 tcDeriving tycl_decls inst_decls deriv_decls
308 = recoverM (return ([], emptyValBindsOut, emptyDUs, [], [])) $
309 do { -- Fish the "deriving"-related information out of the TcEnv
310 -- And make the necessary "equations".
311 is_boot <- tcIsHsBoot
312 ; traceTc "tcDeriving" (ppr is_boot)
313 ; (early_specs, genericsExtras)
314 <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
315 ; let (repMetaTys, repTyCons, metaInsts) = unzip3 genericsExtras
317 ; overlap_flag <- getOverlapFlag
318 ; let (infer_specs, given_specs) = splitEithers early_specs
319 ; insts1 <- mapM (genInst True overlap_flag) given_specs
321 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
322 inferInstanceContexts overlap_flag infer_specs
324 ; insts2 <- mapM (genInst False overlap_flag) final_specs
326 -- We no longer generate the old generic to/from functions
327 -- from each type declaration, so this is emptyBag
328 ; gen_binds <- return emptyBag -- mkGenericBinds is_boot tycl_decls
331 -- Generate the generic Representable0 instances
332 -- from each type declaration
333 ; repInstsMeta <- genGenericRepBinds is_boot tycl_decls
335 ; let repInsts = concat (map (\(a,_,_) -> a) repInstsMeta)
336 repMetaTys = map (\(_,b,_) -> b) repInstsMeta
337 repTyCons = map (\(_,_,c) -> c) repInstsMeta
339 ; (inst_info, rn_binds, rn_dus)
340 <- renameDeriv is_boot gen_binds (insts1 ++ insts2 ++ concat metaInsts {- ++ repInsts -})
343 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
344 (ddump_deriving inst_info rn_binds))
346 ; when (not (null inst_info)) $
347 dumpDerivingInfo (ddump_deriving inst_info rn_binds)
349 ; return ( inst_info, rn_binds, rn_dus
350 , concat (map metaTyCons2TyCons repMetaTys), repTyCons) }
352 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
353 ddump_deriving inst_infos extra_binds
354 = hang (ptext (sLit "Derived instances"))
355 2 (vcat (map (\i -> pprInstInfoDetails i $$ text "") inst_infos)
359 renameDeriv :: Bool -> LHsBinds RdrName
360 -> [(InstInfo RdrName, DerivAuxBinds)]
361 -> TcM ([InstInfo Name], HsValBinds Name, DefUses)
362 renameDeriv is_boot gen_binds insts
363 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
364 -- The inst-info bindings will all be empty, but it's easier to
365 -- just use rn_inst_info to change the type appropriately
366 = do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
367 ; return (rn_inst_infos, emptyValBindsOut, usesOnly (plusFVs fvs)) }
370 = discardWarnings $ -- Discard warnings about unused bindings etc
371 do { (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $ -- Type signatures in patterns
372 -- are used in the generic binds
373 rnTopBinds (ValBindsIn gen_binds [])
374 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
376 -- Generate and rename any extra not-one-inst-decl-specific binds,
377 -- notably "con2tag" and/or "tag2con" functions.
378 -- Bring those names into scope before renaming the instances themselves
379 ; loc <- getSrcSpanM -- Generic loc for shared bindings
380 ; let (aux_binds, aux_sigs) = unzip $ map (genAuxBind loc) $
381 rm_dups [] $ concat deriv_aux_binds
382 aux_val_binds = ValBindsIn (listToBag aux_binds) aux_sigs
383 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
384 ; bindLocalNames (collectHsValBinders rn_aux_lhs) $
385 do { (rn_aux, dus_aux) <- rnTopBindsRHS rn_aux_lhs
386 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
387 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
388 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
391 (inst_infos, deriv_aux_binds) = unzip insts
393 -- Remove duplicate requests for auxilliary bindings
395 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
396 | otherwise = rm_dups (b:acc) bs
399 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
400 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
401 = return ( info { iBinds = NewTypeDerived coi tc }
402 , mkFVs (map dataConName (tyConDataCons tc)))
403 -- See Note [Newtype deriving and unused constructors]
405 rn_inst_info inst_info@(InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
406 = -- Bring the right type variables into
407 -- scope (yuk), and rename the method binds
409 bindLocalNames (map Var.varName tyvars) $
410 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
411 ; let binds' = VanillaInst rn_binds [] standalone_deriv
412 ; return (inst_info { iBinds = binds' }, fvs) }
414 (tyvars,_, clas,_) = instanceHead inst
415 clas_nm = className clas
417 -----------------------------------------
419 mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
420 mkGenericBinds is_boot tycl_decls
424 = do { tcs <- mapM tcLookupTyCon [ tcdName d
425 | L _ d <- tycl_decls, isDataDecl d ]
426 ; return (unionManyBags [ mkTyConGenericBinds tc
427 | tc <- tcs, tyConHasGenerics tc ]) }
428 -- We are only interested in the data type declarations,
429 -- and then only in the ones whose 'has-generics' flag is on
430 -- The predicate tyConHasGenerics finds both of these
434 Note [Newtype deriving and unused constructors]
435 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
436 Consider this (see Trac #1954):
439 newtype P a = MkP (IO a) deriving Monad
441 If you compile with -fwarn-unused-binds you do not expect the warning
442 "Defined but not used: data consructor MkP". Yet the newtype deriving
443 code does not explicitly mention MkP, but it should behave as if you
445 instance Monad P where
446 return x = MkP (return x)
449 So we want to signal a user of the data constructor 'MkP'. That's
450 what we do in rn_inst_info, and it's the only reason we have the TyCon
451 stored in NewTypeDerived.
454 %************************************************************************
456 From HsSyn to DerivSpec
458 %************************************************************************
460 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
463 -- Make the EarlyDerivSpec for Representable0
464 mkGenDerivSpec :: TyCon -> TcRn (EarlyDerivSpec)
465 mkGenDerivSpec tc = do
466 { cls <- tcLookupClass rep0ClassName
467 ; let tc_tvs = tyConTyVars tc
468 ; let tc_app = mkTyConApp tc (mkTyVarTys tc_tvs)
470 ; let mtheta = Just []
471 ; ds <- mkEqnHelp StandAloneDerivOrigin tc_tvs cls cls_tys tc_app mtheta
472 -- JPM TODO: StandAloneDerivOrigin?...
473 ; {- pprTrace "mkGenDerivSpec" (ppr (tc, ds)) $ -} return ds }
475 -- Make the "extras" for the generic representation
476 mkGenDerivExtras :: TyCon
477 -> TcRn (MetaTyCons, TyCon, [(InstInfo RdrName, DerivAuxBinds)])
478 mkGenDerivExtras tc = do
479 { (metaTyCons, rep0TyInst) <- genGenericRepExtras tc
480 ; metaInsts <- genDtMeta (tc, metaTyCons)
481 ; return (metaTyCons, rep0TyInst, metaInsts) }
483 makeDerivSpecs :: Bool
487 -> TcM ( [EarlyDerivSpec]
488 , [(MetaTyCons, TyCon, [(InstInfo RdrName, DerivAuxBinds)])])
489 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
490 | is_boot -- No 'deriving' at all in hs-boot files
491 = do { mapM_ add_deriv_err deriv_locs
494 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
495 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
496 -- Generate EarlyDerivSpec's for Representable, if asked for
497 ; (xGenerics, xDeriveRepresentable) <- genericsFlags
498 ; let allTyNames = [ tcdName d | L _ d <- tycl_decls, isDataDecl d ]
499 ; allTyDecls <- mapM tcLookupTyCon allTyNames
500 -- Select only those types that derive Representable
501 ; let sel_tydata = [ tcdName t | (L _ c, L _ t) <- all_tydata
502 , getClassName c == Just rep0ClassName ]
503 ; let sel_deriv_decls = catMaybes [ getTypeName t
504 | L _ (DerivDecl (L _ t)) <- deriv_decls
505 , getClassName t == Just rep0ClassName ]
506 ; derTyDecls <- mapM tcLookupTyCon $
507 filter (needsExtras xDeriveRepresentable
508 (sel_tydata ++ sel_deriv_decls)) allTyNames
509 -- We need to generate the extras to add to what has
510 -- already been derived
511 ; generic_extras_deriv <- mapM mkGenDerivExtras derTyDecls
512 -- For the remaining types, if Generics is on, we need to
513 -- generate both the instances and the extras, but only for the
514 -- types we can represent.
515 ; let repTyDecls = filter canDoGenerics allTyDecls
516 ; let remTyDecls = filter (\x -> not (x `elem` derTyDecls)) repTyDecls
517 ; generic_instances <- if xGenerics
518 then mapM mkGenDerivSpec remTyDecls
520 ; generic_extras_flag <- if xGenerics
521 then mapM mkGenDerivExtras remTyDecls
523 -- Merge and return everything
524 ; {- pprTrace "allTyDecls" (ppr allTyDecls) $
525 pprTrace "derTyDecls" (ppr derTyDecls) $
526 pprTrace "repTyDecls" (ppr repTyDecls) $
527 pprTrace "remTyDecls" (ppr remTyDecls) $
528 pprTrace "xGenerics" (ppr xGenerics) $
529 pprTrace "xDeriveRep" (ppr xDeriveRepresentable) $
530 pprTrace "all_tydata" (ppr all_tydata) $
531 pprTrace "eqns1" (ppr eqns1) $
532 pprTrace "eqns2" (ppr eqns2) $
534 return ( eqns1 ++ eqns2 ++ generic_instances
535 , generic_extras_deriv ++ generic_extras_flag) }
537 needsExtras xDeriveRepresentable tydata tc_name =
538 -- We need extras if the flag DeriveGenerics is on and this type is
539 -- deriving Representable
540 xDeriveRepresentable && tc_name `elem` tydata
542 -- Extracts the name of the class in the deriving
543 getClassName :: HsType Name -> Maybe Name
544 getClassName (HsPredTy (HsClassP n _)) = Just n
545 getClassName _ = Nothing
547 -- Extracts the name of the type in the deriving
548 getTypeName :: HsType Name -> Maybe Name
549 getTypeName (HsPredTy (HsClassP _ [L _ (HsTyVar n)])) = Just n
550 getTypeName _ = Nothing
552 extractTyDataPreds decls
553 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
555 all_tydata :: [(LHsType Name, LTyClDecl Name)]
556 -- Derived predicate paired with its data type declaration
557 all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
559 deriv_locs = map (getLoc . snd) all_tydata
560 ++ map getLoc deriv_decls
562 add_deriv_err loc = setSrcSpan loc $
563 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
564 2 (ptext (sLit "Use an instance declaration instead")))
566 genericsFlags :: TcM (Bool, Bool)
567 genericsFlags = do dOpts <- getDOpts
568 return ( xopt Opt_Generics dOpts
569 , xopt Opt_DeriveRepresentable dOpts)
571 ------------------------------------------------------------------
572 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
573 -- Standalone deriving declarations
574 -- e.g. deriving instance Show a => Show (T a)
575 -- Rather like tcLocalInstDecl
576 deriveStandalone (L loc (DerivDecl deriv_ty))
578 addErrCtxt (standaloneCtxt deriv_ty) $
579 do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
580 ; (tvs, theta, cls, inst_tys) <- tcHsInstHead deriv_ty
581 ; traceTc "Standalone deriving;" $ vcat
582 [ text "tvs:" <+> ppr tvs
583 , text "theta:" <+> ppr theta
584 , text "cls:" <+> ppr cls
585 , text "tys:" <+> ppr inst_tys ]
586 ; checkValidInstance deriv_ty tvs theta cls inst_tys
587 -- C.f. TcInstDcls.tcLocalInstDecl1
589 ; let cls_tys = take (length inst_tys - 1) inst_tys
590 inst_ty = last inst_tys
591 ; traceTc "Standalone deriving:" $ vcat
592 [ text "class:" <+> ppr cls
593 , text "class types:" <+> ppr cls_tys
594 , text "type:" <+> ppr inst_ty ]
595 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
598 ------------------------------------------------------------------
599 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
600 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
601 tcdTyVars = tv_names,
602 tcdTyPats = ty_pats }))
603 = setSrcSpan loc $ -- Use the location of the 'deriving' item
605 do { (tvs, tc, tc_args) <- get_lhs ty_pats
606 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
607 -- the type variables for the type constructor
609 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
610 -- The "deriv_pred" is a LHsType to take account of the fact that for
611 -- newtype deriving we allow deriving (forall a. C [a]).
613 -- Given data T a b c = ... deriving( C d ),
614 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
615 ; let cls_tyvars = classTyVars cls
616 kind = tyVarKind (last cls_tyvars)
617 (arg_kinds, _) = splitKindFunTys kind
618 n_args_to_drop = length arg_kinds
619 n_args_to_keep = tyConArity tc - n_args_to_drop
620 args_to_drop = drop n_args_to_keep tc_args
621 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
622 inst_ty_kind = typeKind inst_ty
623 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
624 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
625 `minusVarSet` dropped_tvs
627 -- Check that the result really is well-kinded
628 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
629 (derivingKindErr tc cls cls_tys kind)
631 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
632 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
633 (derivingEtaErr cls cls_tys inst_ty)
635 -- (a) The data type can be eta-reduced; eg reject:
636 -- data instance T a a = ... deriving( Monad )
637 -- (b) The type class args do not mention any of the dropped type
639 -- newtype T a s = ... deriving( ST s )
641 -- Type families can't be partially applied
642 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
643 -- Note [Deriving, type families, and partial applications]
644 ; checkTc (not (isFamilyTyCon tc) || n_args_to_drop == 0)
645 (typeFamilyPapErr tc cls cls_tys inst_ty)
647 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
649 -- Tiresomely we must figure out the "lhs", which is awkward for type families
650 -- E.g. data T a b = .. deriving( Eq )
651 -- Here, the lhs is (T a b)
652 -- data instance TF Int b = ... deriving( Eq )
653 -- Here, the lhs is (TF Int b)
654 -- But if we just look up the tycon_name, we get is the *family*
655 -- tycon, but not pattern types -- they are in the *rep* tycon.
656 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
657 ; let tvs = tyConTyVars tc
658 ; return (tvs, tc, mkTyVarTys tvs) }
659 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
660 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
661 ; let (tc, tc_args) = tcSplitTyConApp tc_app
662 ; return (tvs, tc, tc_args) }
665 = panic "derivTyData" -- Caller ensures that only TyData can happen
668 Note [Deriving, type families, and partial applications]
669 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
670 When there are no type families, it's quite easy:
672 newtype S a = MkS [a]
673 -- :CoS :: S ~ [] -- Eta-reduced
675 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
676 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
678 When type familes are involved it's trickier:
681 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
682 -- :RT is the representation type for (T Int a)
683 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
684 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
686 instance Eq [a] => Eq (T Int a) -- easy by coercion
687 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
689 The "???" bit is that we don't build the :CoF thing in eta-reduced form
690 Henc the current typeFamilyPapErr, even though the instance makes sense.
691 After all, we can write it out
692 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
697 mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type] -> Type
698 -> DerivContext -- Just => context supplied (standalone deriving)
699 -- Nothing => context inferred (deriving on data decl)
700 -> TcRn EarlyDerivSpec
701 -- Make the EarlyDerivSpec for an instance
702 -- forall tvs. theta => cls (tys ++ [ty])
703 -- where the 'theta' is optional (that's the Maybe part)
704 -- Assumes that this declaration is well-kinded
706 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
707 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
708 , isAlgTyCon tycon -- Check for functions, primitive types etc
709 = mk_alg_eqn tycon tc_args
711 = failWithTc (derivingThingErr False cls cls_tys tc_app
712 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
715 bale_out msg = failWithTc (derivingThingErr False cls cls_tys tc_app msg)
717 mk_alg_eqn tycon tc_args
718 | className cls `elem` typeableClassNames
719 = do { dflags <- getDOpts
720 ; case checkTypeableConditions (dflags, tycon) of
721 Just err -> bale_out err
722 Nothing -> mk_typeable_eqn orig tvs cls tycon tc_args mtheta }
724 | isDataFamilyTyCon tycon
725 , length tc_args /= tyConArity tycon
726 = bale_out (ptext (sLit "Unsaturated data family application"))
729 = do { (rep_tc, rep_tc_args) <- tcLookupDataFamInst tycon tc_args
730 -- Be careful to test rep_tc here: in the case of families,
731 -- we want to check the instance tycon, not the family tycon
733 -- For standalone deriving (mtheta /= Nothing),
734 -- check that all the data constructors are in scope.
735 ; rdr_env <- getGlobalRdrEnv
736 ; let hidden_data_cons = isAbstractTyCon rep_tc ||
737 any not_in_scope (tyConDataCons rep_tc)
738 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
739 ; unless (isNothing mtheta || not hidden_data_cons)
740 (bale_out (derivingHiddenErr tycon))
743 ; if isDataTyCon rep_tc then
744 mkDataTypeEqn orig dflags tvs cls cls_tys
745 tycon tc_args rep_tc rep_tc_args mtheta
747 mkNewTypeEqn orig dflags tvs cls cls_tys
748 tycon tc_args rep_tc rep_tc_args mtheta }
752 %************************************************************************
756 %************************************************************************
759 mkDataTypeEqn :: CtOrigin
761 -> [Var] -- Universally quantified type variables in the instance
762 -> Class -- Class for which we need to derive an instance
763 -> [Type] -- Other parameters to the class except the last
764 -> TyCon -- Type constructor for which the instance is requested
765 -- (last parameter to the type class)
766 -> [Type] -- Parameters to the type constructor
767 -> TyCon -- rep of the above (for type families)
768 -> [Type] -- rep of the above
769 -> DerivContext -- Context of the instance, for standalone deriving
770 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
772 mkDataTypeEqn orig dflags tvs cls cls_tys
773 tycon tc_args rep_tc rep_tc_args mtheta
774 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
775 -- NB: pass the *representation* tycon to checkSideConditions
776 CanDerive -> go_for_it
777 NonDerivableClass -> bale_out (nonStdErr cls)
778 DerivableClassError msg -> bale_out msg
780 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
781 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
783 mk_data_eqn :: CtOrigin -> [TyVar] -> Class
784 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
785 -> TcM EarlyDerivSpec
786 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
787 = do { dfun_name <- new_dfun_name cls tycon
789 ; let inst_tys = [mkTyConApp tycon tc_args]
790 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
791 spec = DS { ds_loc = loc, ds_orig = orig
792 , ds_name = dfun_name, ds_tvs = tvs
793 , ds_cls = cls, ds_tys = inst_tys
794 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
795 , ds_theta = mtheta `orElse` inferred_constraints
796 , ds_newtype = False }
798 ; return (if isJust mtheta then Right spec -- Specified context
799 else Left spec) } -- Infer context
801 ----------------------
802 mk_typeable_eqn :: CtOrigin -> [TyVar] -> Class
803 -> TyCon -> [TcType] -> DerivContext
804 -> TcM EarlyDerivSpec
805 mk_typeable_eqn orig tvs cls tycon tc_args mtheta
806 -- The Typeable class is special in several ways
807 -- data T a b = ... deriving( Typeable )
809 -- instance Typeable2 T where ...
811 -- 1. There are no constraints in the instance
812 -- 2. There are no type variables either
813 -- 3. The actual class we want to generate isn't necessarily
814 -- Typeable; it depends on the arity of the type
815 | isNothing mtheta -- deriving on a data type decl
816 = do { checkTc (cls `hasKey` typeableClassKey)
817 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
818 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
819 ; mk_typeable_eqn orig tvs real_cls tycon [] (Just []) }
821 | otherwise -- standaone deriving
822 = do { checkTc (null tc_args)
823 (ptext (sLit "Derived typeable instance must be of form (Typeable")
824 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
825 ; dfun_name <- new_dfun_name cls tycon
828 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
829 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
830 , ds_tc = tycon, ds_tc_args = []
831 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
833 ----------------------
834 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
835 -- Generate a sufficiently large set of constraints that typechecking the
836 -- generated method definitions should succeed. This set will be simplified
837 -- before being used in the instance declaration
838 inferConstraints _ cls inst_tys rep_tc rep_tc_args
839 -- Representable0 constraints are easy
840 | cls `hasKey` rep0ClassKey
842 -- The others are a bit more complicated
844 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
845 stupid_constraints ++ extra_constraints
846 ++ sc_constraints ++ con_arg_constraints
848 -- Constraints arising from the arguments of each constructor
850 = [ mkClassPred cls [arg_ty]
851 | data_con <- tyConDataCons rep_tc,
852 arg_ty <- ASSERT( isVanillaDataCon data_con )
853 get_constrained_tys $
854 dataConInstOrigArgTys data_con all_rep_tc_args,
855 not (isUnLiftedType arg_ty) ]
856 -- No constraints for unlifted types
857 -- Where they are legal we generate specilised function calls
859 -- For functor-like classes, two things are different
860 -- (a) We recurse over argument types to generate constraints
861 -- See Functor examples in TcGenDeriv
862 -- (b) The rep_tc_args will be one short
863 is_functor_like = getUnique cls `elem` functorLikeClassKeys
865 get_constrained_tys :: [Type] -> [Type]
866 get_constrained_tys tys
867 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
870 rep_tc_tvs = tyConTyVars rep_tc
871 last_tv = last rep_tc_tvs
872 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
873 | otherwise = rep_tc_args
875 -- Constraints arising from superclasses
876 -- See Note [Superclasses of derived instance]
877 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
880 -- Stupid constraints
881 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
882 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
884 -- Extra Data constraints
885 -- The Data class (only) requires that for
886 -- instance (...) => Data (T t1 t2)
888 -- THEN (Data t1, Data t2) are among the (...) constraints
889 -- Reason: when the IF holds, we generate a method
890 -- dataCast2 f = gcast2 f
891 -- and we need the Data constraints to typecheck the method
893 | cls `hasKey` dataClassKey
894 , all (isLiftedTypeKind . typeKind) rep_tc_args
895 = [mkClassPred cls [ty] | ty <- rep_tc_args]
899 ------------------------------------------------------------------
900 -- Check side conditions that dis-allow derivability for particular classes
901 -- This is *apart* from the newtype-deriving mechanism
903 -- Here we get the representation tycon in case of family instances as it has
904 -- the data constructors - but we need to be careful to fall back to the
905 -- family tycon (with indexes) in error messages.
907 data DerivStatus = CanDerive
908 | DerivableClassError SDoc -- Standard class, but can't do it
909 | NonDerivableClass -- Non-standard class
911 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
912 checkSideConditions dflags mtheta cls cls_tys rep_tc
913 | Just cond <- sideConditions mtheta cls
914 = case (cond (dflags, rep_tc)) of
915 Just err -> DerivableClassError err -- Class-specific error
916 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
917 -- cls_tys (the type args other than last)
919 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
920 | otherwise = NonDerivableClass -- Not a standard class
922 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
924 checkTypeableConditions :: Condition
925 checkTypeableConditions = checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK
927 nonStdErr :: Class -> SDoc
928 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
930 sideConditions :: DerivContext -> Class -> Maybe Condition
931 sideConditions mtheta cls
932 | cls_key == eqClassKey = Just cond_std
933 | cls_key == ordClassKey = Just cond_std
934 | cls_key == showClassKey = Just cond_std
935 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
936 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
937 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
938 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
939 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
940 cond_std `andCond` cond_noUnliftedArgs)
941 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
942 cond_functorOK True) -- NB: no cond_std!
943 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
944 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
945 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
946 cond_functorOK False)
947 | cls_key == rep0ClassKey = Just (cond_RepresentableOk `andCond`
948 (checkFlag Opt_DeriveRepresentable `orCond`
949 checkFlag Opt_Generics))
950 | otherwise = Nothing
952 cls_key = getUnique cls
953 cond_std = cond_stdOK mtheta
955 type Condition = (DynFlags, TyCon) -> Maybe SDoc
956 -- first Bool is whether or not we are allowed to derive Data and Typeable
957 -- second Bool is whether or not we are allowed to derive Functor
958 -- TyCon is the *representation* tycon if the
959 -- data type is an indexed one
962 orCond :: Condition -> Condition -> Condition
965 Nothing -> Nothing -- c1 succeeds
966 Just x -> case c2 tc of -- c1 fails
968 Just y -> Just (x $$ ptext (sLit " and") $$ y)
971 andCond :: Condition -> Condition -> Condition
972 andCond c1 c2 tc = case c1 tc of
973 Nothing -> c2 tc -- c1 succeeds
974 Just x -> Just x -- c1 fails
976 cond_stdOK :: DerivContext -> Condition
977 cond_stdOK (Just _) _
978 = Nothing -- Don't check these conservative conditions for
979 -- standalone deriving; just generate the code
980 -- and let the typechecker handle the result
981 cond_stdOK Nothing (_, rep_tc)
982 | null data_cons = Just (no_cons_why rep_tc $$ suggestion)
983 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
984 | otherwise = Nothing
986 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
987 data_cons = tyConDataCons rep_tc
988 con_whys = mapCatMaybes check_con data_cons
990 check_con :: DataCon -> Maybe SDoc
992 | isVanillaDataCon con
993 , all isTauTy (dataConOrigArgTys con) = Nothing
994 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
996 no_cons_why :: TyCon -> SDoc
997 no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
998 ptext (sLit "has no data constructors")
1000 -- JPM TODO: should give better error message
1001 cond_RepresentableOk :: Condition
1002 cond_RepresentableOk (_,t) | canDoGenerics t = Nothing
1003 | otherwise = Just (ptext (sLit "Cannot derive Representable for type") <+> ppr t)
1005 cond_enumOrProduct :: Condition
1006 cond_enumOrProduct = cond_isEnumeration `orCond`
1007 (cond_isProduct `andCond` cond_noUnliftedArgs)
1009 cond_noUnliftedArgs :: Condition
1010 -- For some classes (eg Eq, Ord) we allow unlifted arg types
1011 -- by generating specilaised code. For others (eg Data) we don't.
1012 cond_noUnliftedArgs (_, tc)
1013 | null bad_cons = Nothing
1014 | otherwise = Just why
1016 bad_cons = [ con | con <- tyConDataCons tc
1017 , any isUnLiftedType (dataConOrigArgTys con) ]
1018 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
1020 cond_isEnumeration :: Condition
1021 cond_isEnumeration (_, rep_tc)
1022 | isEnumerationTyCon rep_tc = Nothing
1023 | otherwise = Just why
1025 why = sep [ quotes (pprSourceTyCon rep_tc) <+>
1026 ptext (sLit "is not an enumeration type")
1027 , ptext (sLit "(an enumeration consists of one or more nullary, non-GADT constructors)") ]
1028 -- See Note [Enumeration types] in TyCon
1030 cond_isProduct :: Condition
1031 cond_isProduct (_, rep_tc)
1032 | isProductTyCon rep_tc = Nothing
1033 | otherwise = Just why
1035 why = quotes (pprSourceTyCon rep_tc) <+>
1036 ptext (sLit "does not have precisely one constructor")
1038 cond_typeableOK :: Condition
1039 -- OK for Typeable class
1040 -- Currently: (a) args all of kind *
1041 -- (b) 7 or fewer args
1042 cond_typeableOK (_, tc)
1043 | tyConArity tc > 7 = Just too_many
1044 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tc))
1046 | otherwise = Nothing
1048 too_many = quotes (pprSourceTyCon tc) <+>
1049 ptext (sLit "has too many arguments")
1050 bad_kind = quotes (pprSourceTyCon tc) <+>
1051 ptext (sLit "has arguments of kind other than `*'")
1053 functorLikeClassKeys :: [Unique]
1054 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
1056 cond_functorOK :: Bool -> Condition
1057 -- OK for Functor/Foldable/Traversable class
1058 -- Currently: (a) at least one argument
1059 -- (b) don't use argument contravariantly
1060 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
1061 -- (d) optionally: don't use function types
1062 -- (e) no "stupid context" on data type
1063 cond_functorOK allowFunctions (_, rep_tc)
1065 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
1066 <+> ptext (sLit "has no parameters"))
1068 | not (null bad_stupid_theta)
1069 = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
1070 <+> ptext (sLit "has a class context") <+> pprTheta bad_stupid_theta)
1073 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
1075 tc_tvs = tyConTyVars rep_tc
1076 Just (_, last_tv) = snocView tc_tvs
1077 bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
1078 is_bad pred = last_tv `elemVarSet` tyVarsOfPred pred
1080 data_cons = tyConDataCons rep_tc
1081 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
1083 check_vanilla :: DataCon -> Maybe SDoc
1084 check_vanilla con | isVanillaDataCon con = Nothing
1085 | otherwise = Just (badCon con existential)
1087 ft_check :: DataCon -> FFoldType (Maybe SDoc)
1088 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
1089 , ft_co_var = Just (badCon con covariant)
1090 , ft_fun = \x y -> if allowFunctions then x `mplus` y
1091 else Just (badCon con functions)
1092 , ft_tup = \_ xs -> msum xs
1093 , ft_ty_app = \_ x -> x
1094 , ft_bad_app = Just (badCon con wrong_arg)
1095 , ft_forall = \_ x -> x }
1097 existential = ptext (sLit "has existential arguments")
1098 covariant = ptext (sLit "uses the type variable in a function argument")
1099 functions = ptext (sLit "contains function types")
1100 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
1102 checkFlag :: ExtensionFlag -> Condition
1103 checkFlag flag (dflags, _)
1104 | xopt flag dflags = Nothing
1105 | otherwise = Just why
1107 why = ptext (sLit "You need -X") <> text flag_str
1108 <+> ptext (sLit "to derive an instance for this class")
1109 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
1111 other -> pprPanic "checkFlag" (ppr other)
1113 std_class_via_iso :: Class -> Bool
1114 -- These standard classes can be derived for a newtype
1115 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
1116 -- because giving so gives the same results as generating the boilerplate
1117 std_class_via_iso clas
1118 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
1119 -- Not Read/Show because they respect the type
1120 -- Not Enum, because newtypes are never in Enum
1123 non_iso_class :: Class -> Bool
1124 -- *Never* derive Read,Show,Typeable,Data,Representable0 by isomorphism,
1125 -- even with -XGeneralizedNewtypeDeriving
1127 = classKey cls `elem` ([ readClassKey, showClassKey, dataClassKey
1128 , rep0ClassKey] ++ typeableClassKeys)
1130 typeableClassKeys :: [Unique]
1131 typeableClassKeys = map getUnique typeableClassNames
1133 new_dfun_name :: Class -> TyCon -> TcM Name
1134 new_dfun_name clas tycon -- Just a simple wrapper
1135 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1136 ; newDFunName clas [mkTyConApp tycon []] loc }
1137 -- The type passed to newDFunName is only used to generate
1138 -- a suitable string; hence the empty type arg list
1140 badCon :: DataCon -> SDoc -> SDoc
1141 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1144 Note [Superclasses of derived instance]
1145 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1146 In general, a derived instance decl needs the superclasses of the derived
1147 class too. So if we have
1148 data T a = ...deriving( Ord )
1149 then the initial context for Ord (T a) should include Eq (T a). Often this is
1150 redundant; we'll also generate an Ord constraint for each constructor argument,
1151 and that will probably generate enough constraints to make the Eq (T a) constraint
1152 be satisfied too. But not always; consider:
1158 data T a = MkT (S a) deriving( Ord )
1159 instance Num a => Eq (T a)
1161 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1163 data T a = MkT deriving( Data, Typeable )
1164 Here there *is* no argument field, but we must nevertheless generate
1165 a context for the Data instances:
1166 instance Typable a => Data (T a) where ...
1169 %************************************************************************
1173 %************************************************************************
1176 mkNewTypeEqn :: CtOrigin -> DynFlags -> [Var] -> Class
1177 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1179 -> TcRn EarlyDerivSpec
1180 mkNewTypeEqn orig dflags tvs
1181 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1182 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1183 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1184 = do { traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
1185 ; dfun_name <- new_dfun_name cls tycon
1186 ; loc <- getSrcSpanM
1187 ; let spec = DS { ds_loc = loc, ds_orig = orig
1188 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1189 , ds_cls = cls, ds_tys = inst_tys
1190 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1191 , ds_theta = mtheta `orElse` all_preds
1192 , ds_newtype = True }
1193 ; return (if isJust mtheta then Right spec
1197 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1198 CanDerive -> go_for_it -- Use the standard H98 method
1199 DerivableClassError msg -- Error with standard class
1200 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1201 | otherwise -> bale_out msg
1202 NonDerivableClass -- Must use newtype deriving
1203 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1204 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1205 | otherwise -> bale_out non_std
1207 newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
1208 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1209 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1211 non_std = nonStdErr cls
1212 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1214 -- Here is the plan for newtype derivings. We see
1215 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1216 -- where t is a type,
1217 -- ak+1...an is a suffix of a1..an, and are all tyars
1218 -- ak+1...an do not occur free in t, nor in the s1..sm
1219 -- (C s1 ... sm) is a *partial applications* of class C
1220 -- with the last parameter missing
1221 -- (T a1 .. ak) matches the kind of C's last argument
1222 -- (and hence so does t)
1223 -- The latter kind-check has been done by deriveTyData already,
1224 -- and tc_args are already trimmed
1226 -- We generate the instance
1227 -- instance forall ({a1..ak} u fvs(s1..sm)).
1228 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1229 -- where T a1...ap is the partial application of
1230 -- the LHS of the correct kind and p >= k
1232 -- NB: the variables below are:
1233 -- tc_tvs = [a1, ..., an]
1234 -- tyvars_to_keep = [a1, ..., ak]
1235 -- rep_ty = t ak .. an
1236 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1237 -- tys = [s1, ..., sm]
1240 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1241 -- We generate the instance
1242 -- instance Monad (ST s) => Monad (T s) where
1244 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1245 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1246 -- eta-reduces the representation type, so we know that
1248 -- That's convenient here, because we may have to apply
1249 -- it to fewer than its original complement of arguments
1251 -- Note [Newtype representation]
1252 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1253 -- Need newTyConRhs (*not* a recursive representation finder)
1254 -- to get the representation type. For example
1255 -- newtype B = MkB Int
1256 -- newtype A = MkA B deriving( Num )
1257 -- We want the Num instance of B, *not* the Num instance of Int,
1258 -- when making the Num instance of A!
1259 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1260 rep_tys = cls_tys ++ [rep_inst_ty]
1261 rep_pred = mkClassPred cls rep_tys
1262 -- rep_pred is the representation dictionary, from where
1263 -- we are gong to get all the methods for the newtype
1267 -- Next we figure out what superclass dictionaries to use
1268 -- See Note [Newtype deriving superclasses] above
1270 cls_tyvars = classTyVars cls
1271 dfun_tvs = tyVarsOfTypes inst_tys
1272 inst_ty = mkTyConApp tycon tc_args
1273 inst_tys = cls_tys ++ [inst_ty]
1274 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1277 -- If there are no tyvars, there's no need
1278 -- to abstract over the dictionaries we need
1279 -- Example: newtype T = MkT Int deriving( C )
1280 -- We get the derived instance
1283 -- instance C Int => C T
1284 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1286 -------------------------------------------------------------------
1287 -- Figuring out whether we can only do this newtype-deriving thing
1289 can_derive_via_isomorphism
1290 = not (non_iso_class cls)
1294 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1296 arity_ok = length cls_tys + 1 == classArity cls
1297 -- Well kinded; eg not: newtype T ... deriving( ST )
1298 -- because ST needs *2* type params
1300 -- Check that eta reduction is OK
1301 eta_ok = nt_eta_arity <= length rep_tc_args
1302 -- The newtype can be eta-reduced to match the number
1303 -- of type argument actually supplied
1304 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1305 -- Here the 'b' must be the same in the rep type (S [a] b)
1306 -- And the [a] must not mention 'b'. That's all handled
1309 ats_ok = null (classATs cls)
1310 -- No associated types for the class, because we don't
1311 -- currently generate type 'instance' decls; and cannot do
1312 -- so for 'data' instance decls
1315 = vcat [ ppUnless arity_ok arity_msg
1316 , ppUnless eta_ok eta_msg
1317 , ppUnless ats_ok ats_msg ]
1318 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1319 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1320 ats_msg = ptext (sLit "the class has associated types")
1323 Note [Recursive newtypes]
1324 ~~~~~~~~~~~~~~~~~~~~~~~~~
1325 Newtype deriving works fine, even if the newtype is recursive.
1326 e.g. newtype S1 = S1 [T1 ()]
1327 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1328 Remember, too, that type families are curretly (conservatively) given
1329 a recursive flag, so this also allows newtype deriving to work
1332 We used to exclude recursive types, because we had a rather simple
1333 minded way of generating the instance decl:
1335 instance Eq [A] => Eq A -- Makes typechecker loop!
1336 But now we require a simple context, so it's ok.
1339 %************************************************************************
1341 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1343 %************************************************************************
1345 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1346 terms, which is the final correct RHS for the corresponding original
1350 Each (k,TyVarTy tv) in a solution constrains only a type
1354 The (k,TyVarTy tv) pairs in a solution are canonically
1355 ordered by sorting on type varible, tv, (major key) and then class, k,
1360 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1362 inferInstanceContexts _ [] = return []
1364 inferInstanceContexts oflag infer_specs
1365 = do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
1366 ; iterate_deriv 1 initial_solutions }
1368 ------------------------------------------------------------------
1369 -- The initial solutions for the equations claim that each
1370 -- instance has an empty context; this solution is certainly
1371 -- in canonical form.
1372 initial_solutions :: [ThetaType]
1373 initial_solutions = [ [] | _ <- infer_specs ]
1375 ------------------------------------------------------------------
1376 -- iterate_deriv calculates the next batch of solutions,
1377 -- compares it with the current one; finishes if they are the
1378 -- same, otherwise recurses with the new solutions.
1379 -- It fails if any iteration fails
1380 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1381 iterate_deriv n current_solns
1382 | n > 20 -- Looks as if we are in an infinite loop
1383 -- This can happen if we have -XUndecidableInstances
1384 -- (See TcSimplify.tcSimplifyDeriv.)
1385 = pprPanic "solveDerivEqns: probable loop"
1386 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1388 = do { -- Extend the inst info from the explicit instance decls
1389 -- with the current set of solutions, and simplify each RHS
1390 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1391 current_solns infer_specs
1392 ; new_solns <- checkNoErrs $
1393 extendLocalInstEnv inst_specs $
1394 mapM gen_soln infer_specs
1396 ; if (current_solns == new_solns) then
1397 return [ spec { ds_theta = soln }
1398 | (spec, soln) <- zip infer_specs current_solns ]
1400 iterate_deriv (n+1) new_solns }
1402 ------------------------------------------------------------------
1403 gen_soln :: DerivSpec -> TcM [PredType]
1404 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1405 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1407 addErrCtxt (derivInstCtxt the_pred) $
1408 do { -- Check for a bizarre corner case, when the derived instance decl should
1409 -- have form instance C a b => D (T a) where ...
1410 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1411 -- of problems; in particular, it's hard to compare solutions for
1412 -- equality when finding the fixpoint. Moreover, simplifyDeriv
1413 -- has an assert failure because it finds a TyVar when it expects
1414 -- only TcTyVars. So I just rule it out for now. I'm not
1415 -- even sure how it can arise.
1417 ; let tv_set = mkVarSet tyvars
1418 weird_preds = [pred | pred <- deriv_rhs
1419 , not (tyVarsOfPred pred `subVarSet` tv_set)]
1420 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1422 ; theta <- simplifyDeriv orig the_pred tyvars deriv_rhs
1423 -- checkValidInstance tyvars theta clas inst_tys
1424 -- Not necessary; see Note [Exotic derived instance contexts]
1427 ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
1428 -- Claim: the result instance declaration is guaranteed valid
1429 -- Hence no need to call:
1430 -- checkValidInstance tyvars theta clas inst_tys
1431 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1433 the_pred = mkClassPred clas inst_tys
1435 ------------------------------------------------------------------
1436 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1437 mkInstance overlap_flag theta
1438 (DS { ds_name = dfun_name
1439 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1440 = mkLocalInstance dfun overlap_flag
1442 dfun = mkDictFunId dfun_name tyvars theta clas tys
1445 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1446 -- Add new locally-defined instances; don't bother to check
1447 -- for functional dependency errors -- that'll happen in TcInstDcls
1448 extendLocalInstEnv dfuns thing_inside
1449 = do { env <- getGblEnv
1450 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1451 env' = env { tcg_inst_env = inst_env' }
1452 ; setGblEnv env' thing_inside }
1456 %************************************************************************
1458 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1460 %************************************************************************
1462 After all the trouble to figure out the required context for the
1463 derived instance declarations, all that's left is to chug along to
1464 produce them. They will then be shoved into @tcInstDecls2@, which
1465 will do all its usual business.
1467 There are lots of possibilities for code to generate. Here are
1468 various general remarks.
1473 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1474 ``you-couldn't-do-better-by-hand'' efficient.
1477 Deriving @Show@---also pretty common--- should also be reasonable good code.
1480 Deriving for the other classes isn't that common or that big a deal.
1487 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1490 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1493 We {\em normally} generate code only for the non-defaulted methods;
1494 there are some exceptions for @Eq@ and (especially) @Ord@...
1497 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1498 constructor's numeric (@Int#@) tag. These are generated by
1499 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1500 these is around is given by @hasCon2TagFun@.
1502 The examples under the different sections below will make this
1506 Much less often (really just for deriving @Ix@), we use a
1507 @_tag2con_<tycon>@ function. See the examples.
1510 We use the renamer!!! Reason: we're supposed to be
1511 producing @LHsBinds Name@ for the methods, but that means
1512 producing correctly-uniquified code on the fly. This is entirely
1513 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1514 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1515 the renamer. What a great hack!
1519 -- Generate the InstInfo for the required instance paired with the
1520 -- *representation* tycon for that instance,
1521 -- plus any auxiliary bindings required
1523 -- Representation tycons differ from the tycon in the instance signature in
1524 -- case of instances for indexed families.
1526 genInst :: Bool -- True <=> standalone deriving
1528 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1529 genInst standalone_deriv oflag
1530 spec@(DS { ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1531 , ds_theta = theta, ds_newtype = is_newtype
1532 , ds_name = name, ds_cls = clas })
1534 = return (InstInfo { iSpec = inst_spec
1535 , iBinds = NewTypeDerived co rep_tycon }, [])
1538 = do { fix_env <- getFixityEnv
1539 ; let loc = getSrcSpan name
1540 (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1541 -- In case of a family instance, we need to use the representation
1542 -- tycon (after all, it has the data constructors)
1544 ; return (InstInfo { iSpec = inst_spec
1545 , iBinds = VanillaInst meth_binds [] standalone_deriv }
1548 inst_spec = mkInstance oflag theta spec
1549 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1550 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1552 -- Not a family => rep_tycon = main tycon
1553 co2 = case newTyConCo_maybe rep_tycon of
1554 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1555 Nothing -> id_co -- The newtype is transparent; no need for a cast
1556 co = co1 `mkTransCoI` co2
1557 id_co = IdCo (mkTyConApp rep_tycon rep_tc_args)
1559 -- Example: newtype instance N [a] = N1 (Tree a)
1560 -- deriving instance Eq b => Eq (N [(b,b)])
1561 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1562 -- When dealing with the deriving clause
1563 -- co1 : N [(b,b)] ~ R1:N (b,b)
1564 -- co2 : R1:N (b,b) ~ Tree (b,b)
1565 -- co : N [(b,b)] ~ Tree (b,b)
1567 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1568 genDerivBinds loc fix_env clas tycon
1569 | className clas `elem` typeableClassNames
1570 = (gen_Typeable_binds loc tycon, [])
1573 = case assocMaybe gen_list (getUnique clas) of
1574 Just gen_fn -> gen_fn loc tycon
1575 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1577 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1578 gen_list = [(eqClassKey, gen_Eq_binds)
1579 ,(ordClassKey, gen_Ord_binds)
1580 ,(enumClassKey, gen_Enum_binds)
1581 ,(boundedClassKey, gen_Bounded_binds)
1582 ,(ixClassKey, gen_Ix_binds)
1583 ,(showClassKey, gen_Show_binds fix_env)
1584 ,(readClassKey, gen_Read_binds fix_env)
1585 ,(dataClassKey, gen_Data_binds)
1586 ,(functorClassKey, gen_Functor_binds)
1587 ,(foldableClassKey, gen_Foldable_binds)
1588 ,(traversableClassKey, gen_Traversable_binds)
1589 ,(rep0ClassKey, gen_Rep0_binds)
1593 %************************************************************************
1595 \subsection[TcDeriv-generic-binds]{Bindings for the new generic deriving mechanism}
1597 %************************************************************************
1599 For the generic representation we need to generate:
1601 \item A Representable0 instance
1602 \item A Rep0 type instance
1603 \item Many auxiliary datatypes and instances for them (for the meta-information)
1606 @gen_Rep0_binds@ does (1)
1607 @genGenericRepExtras@ does (2) and (3)
1608 @genGenericRepBind@ does all of them
1612 genGenericRepBinds :: Bool -> [LTyClDecl Name]
1613 -> TcM [([(InstInfo RdrName, DerivAuxBinds)]
1614 , MetaTyCons, TyCon)]
1615 genGenericRepBinds isBoot tyclDecls
1616 | isBoot = return []
1618 allTyDecls <- mapM tcLookupTyCon [ tcdName d | L _ d <- tyclDecls
1620 let tyDecls = filter tyConHasGenerics allTyDecls
1621 inst1 <- mapM genGenericRepBind tyDecls
1622 let (_repInsts, metaTyCons, _repTys) = unzip3 inst1
1623 metaInsts <- ASSERT (length tyDecls == length metaTyCons)
1624 mapM genDtMeta (zip tyDecls metaTyCons)
1625 return (ASSERT (length inst1 == length metaInsts)
1627 | ((ri, ms, rt), mi) <- zip inst1 metaInsts ])
1630 gen_Rep0_binds :: SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1631 gen_Rep0_binds _ tc = (mkBindsRep0 tc, [ {- No DerivAuxBinds -} ])
1633 genGenericRepExtras :: TyCon -> TcM (MetaTyCons, TyCon)
1634 genGenericRepExtras tc =
1635 do uniqS <- newUniqueSupply
1637 -- Uniques for everyone
1638 (uniqD:uniqs) = uniqsFromSupply uniqS
1639 (uniqsC,us) = splitAt (length tc_cons) uniqs
1640 uniqsS :: [[Unique]] -- Unique supply for the S datatypes
1641 uniqsS = mkUniqsS tc_arits us
1643 mkUniqsS (n:t) us = case splitAt n us of
1644 (us1,us2) -> us1 : mkUniqsS t us2
1646 tc_name = tyConName tc
1647 tc_cons = tyConDataCons tc
1648 tc_arits = map dataConSourceArity tc_cons
1650 tc_occ = nameOccName tc_name
1651 d_occ = mkGenD tc_occ
1652 c_occ m = mkGenC tc_occ m
1653 s_occ m n = mkGenS tc_occ m n
1654 mod_name = nameModule (tyConName tc)
1655 d_name = mkExternalName uniqD mod_name d_occ wiredInSrcSpan
1656 c_names = [ mkExternalName u mod_name (c_occ m) wiredInSrcSpan
1657 | (u,m) <- zip uniqsC [0..] ]
1658 s_names = [ [ mkExternalName u mod_name (s_occ m n) wiredInSrcSpan
1659 | (u,n) <- zip us [0..] ] | (us,m) <- zip uniqsS [0..] ]
1661 mkTyCon name = ASSERT( isExternalName name )
1662 buildAlgTyCon name [] [] mkAbstractTyConRhs
1663 NonRecursive False NoParentTyCon Nothing
1665 metaDTyCon <- mkTyCon d_name
1666 metaCTyCons <- sequence [ mkTyCon c_name | c_name <- c_names ]
1667 metaSTyCons <- mapM sequence
1669 | s_name <- s_namesC ] | s_namesC <- s_names ]
1671 let metaDts = MetaTyCons metaDTyCon metaCTyCons metaSTyCons
1673 rep0_tycon <- tc_mkRep0TyCon tc metaDts
1675 return (metaDts, rep0_tycon)
1677 genGenericRepBind :: TyCon
1678 -> TcM ((InstInfo RdrName, DerivAuxBinds), MetaTyCons, TyCon)
1679 genGenericRepBind tc =
1680 do (metaDts, rep0_tycon) <- genGenericRepExtras tc
1681 clas <- tcLookupClass rep0ClassName
1682 dfun_name <- new_dfun_name clas tc
1684 mkInstRep0 = (InstInfo { iSpec = inst, iBinds = binds }
1685 , [ {- No DerivAuxBinds -} ])
1686 inst = mkLocalInstance dfun NoOverlap
1687 binds = VanillaInst (mkBindsRep0 tc) [] False
1689 tvs = tyConTyVars tc
1690 tc_ty = mkTyConApp tc (mkTyVarTys tvs)
1692 dfun = mkDictFunId dfun_name (tyConTyVars tc) [] clas [tc_ty]
1693 return (mkInstRep0, metaDts, rep0_tycon)
1695 genDtMeta :: (TyCon, MetaTyCons) -> TcM [(InstInfo RdrName, DerivAuxBinds)]
1696 genDtMeta (tc,metaDts) =
1697 do dClas <- tcLookupClass datatypeClassName
1698 d_dfun_name <- new_dfun_name dClas tc
1699 cClas <- tcLookupClass constructorClassName
1700 c_dfun_names <- sequence [ new_dfun_name cClas tc | _ <- metaC metaDts ]
1701 sClas <- tcLookupClass selectorClassName
1702 s_dfun_names <- sequence (map sequence [ [ new_dfun_name sClas tc
1704 | x <- metaS metaDts ])
1705 fix_env <- getFixityEnv
1708 (dBinds,cBinds,sBinds) = mkBindsMetaD fix_env tc
1711 d_metaTycon = metaD metaDts
1712 d_inst = mkLocalInstance d_dfun NoOverlap
1713 d_binds = VanillaInst dBinds [] False
1714 d_dfun = mkDictFunId d_dfun_name (tyConTyVars tc) [] dClas
1715 [ mkTyConTy d_metaTycon ]
1716 d_mkInst = (InstInfo { iSpec = d_inst, iBinds = d_binds }, [])
1719 c_metaTycons = metaC metaDts
1720 c_insts = [ mkLocalInstance (c_dfun c ds) NoOverlap
1721 | (c, ds) <- myZip1 c_metaTycons c_dfun_names ]
1722 c_binds = [ VanillaInst c [] False | c <- cBinds ]
1723 c_dfun c dfun_name = mkDictFunId dfun_name (tyConTyVars tc) [] cClas
1725 c_mkInst = [ (InstInfo { iSpec = is, iBinds = bs }, [])
1726 | (is,bs) <- myZip1 c_insts c_binds ]
1729 s_metaTycons = metaS metaDts
1730 s_insts = map (map (\(s,ds) -> mkLocalInstance (s_dfun s ds) NoOverlap))
1731 (myZip2 s_metaTycons s_dfun_names)
1732 s_binds = [ [ VanillaInst s [] False | s <- ss ] | ss <- sBinds ]
1733 s_dfun s dfun_name = mkDictFunId dfun_name (tyConTyVars tc) [] sClas
1735 s_mkInst = map (map (\(is,bs) -> (InstInfo {iSpec=is, iBinds=bs}, [])))
1736 (myZip2 s_insts s_binds)
1738 myZip1 :: [a] -> [b] -> [(a,b)]
1739 myZip1 l1 l2 = ASSERT (length l1 == length l2) zip l1 l2
1741 myZip2 :: [[a]] -> [[b]] -> [[(a,b)]]
1743 ASSERT (and (zipWith (>=) (map length l1) (map length l2)))
1744 [ zip x1 x2 | (x1,x2) <- zip l1 l2 ]
1746 return (d_mkInst : c_mkInst ++ concat s_mkInst)
1750 %************************************************************************
1752 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1754 %************************************************************************
1757 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1758 derivingKindErr tc cls cls_tys cls_kind
1759 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1760 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1761 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1762 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1764 derivingEtaErr :: Class -> [Type] -> Type -> Message
1765 derivingEtaErr cls cls_tys inst_ty
1766 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1767 nest 2 (ptext (sLit "instance (...) =>")
1768 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1770 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1771 typeFamilyPapErr tc cls cls_tys inst_ty
1772 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1773 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1775 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1776 derivingThingErr newtype_deriving clas tys ty why
1777 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1778 2 (quotes (ppr pred))
1779 $$ nest 2 extra) <> colon,
1782 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1784 pred = mkClassPred clas (tys ++ [ty])
1786 derivingHiddenErr :: TyCon -> SDoc
1787 derivingHiddenErr tc
1788 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1789 2 (ptext (sLit "so you cannot derive an instance for it"))
1791 standaloneCtxt :: LHsType Name -> SDoc
1792 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1795 derivInstCtxt :: PredType -> Message
1797 = ptext (sLit "When deriving the instance for") <+> parens (ppr pred)
1799 badDerivedPred :: PredType -> Message
1801 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1802 ptext (sLit "type variables that are not data type parameters"),
1803 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]