2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Handles @deriving@ clauses on @data@ declarations.
9 module TcDeriv ( tcDeriving ) where
11 #include "HsVersions.h"
19 import TcClassDcl( tcAddDeclCtxt ) -- Small helper
20 import TcGenDeriv -- Deriv stuff
56 %************************************************************************
60 %************************************************************************
64 1. Convert the decls (i.e. data/newtype deriving clauses,
65 plus standalone deriving) to [EarlyDerivSpec]
67 2. Infer the missing contexts for the Left DerivSpecs
69 3. Add the derived bindings, generating InstInfos
72 -- DerivSpec is purely local to this module
73 data DerivSpec = DS { ds_loc :: SrcSpan
74 , ds_orig :: InstOrigin
77 , ds_theta :: ThetaType
81 , ds_tc_args :: [Type]
82 , ds_newtype :: Bool }
83 -- This spec implies a dfun declaration of the form
84 -- df :: forall tvs. theta => C tys
85 -- The Name is the name for the DFun we'll build
86 -- The tyvars bind all the variables in the theta
87 -- For family indexes, the tycon in
88 -- in ds_tys is the *family* tycon
89 -- in ds_tc, ds_tc_args is the *representation* tycon
90 -- For non-family tycons, both are the same
92 -- ds_newtype = True <=> Newtype deriving
93 -- False <=> Vanilla deriving
95 type DerivContext = Maybe ThetaType
96 -- Nothing <=> Vanilla deriving; infer the context of the instance decl
97 -- Just theta <=> Standalone deriving: context supplied by programmer
99 type EarlyDerivSpec = Either DerivSpec DerivSpec
100 -- Left ds => the context for the instance should be inferred
101 -- In this case ds_theta is the list of all the
102 -- constraints needed, such as (Eq [a], Eq a)
103 -- The inference process is to reduce this to a
104 -- simpler form (e.g. Eq a)
106 -- Right ds => the exact context for the instance is supplied
107 -- by the programmer; it is ds_theta
109 pprDerivSpec :: DerivSpec -> SDoc
110 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
111 ds_cls = c, ds_tys = tys, ds_theta = rhs })
112 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
113 <+> equals <+> ppr rhs)
117 Inferring missing contexts
118 ~~~~~~~~~~~~~~~~~~~~~~~~~~
121 data T a b = C1 (Foo a) (Bar b)
126 [NOTE: See end of these comments for what to do with
127 data (C a, D b) => T a b = ...
130 We want to come up with an instance declaration of the form
132 instance (Ping a, Pong b, ...) => Eq (T a b) where
135 It is pretty easy, albeit tedious, to fill in the code "...". The
136 trick is to figure out what the context for the instance decl is,
137 namely @Ping@, @Pong@ and friends.
139 Let's call the context reqd for the T instance of class C at types
140 (a,b, ...) C (T a b). Thus:
142 Eq (T a b) = (Ping a, Pong b, ...)
144 Now we can get a (recursive) equation from the @data@ decl:
146 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
147 u Eq (T b a) u Eq Int -- From C2
148 u Eq (T a a) -- From C3
150 Foo and Bar may have explicit instances for @Eq@, in which case we can
151 just substitute for them. Alternatively, either or both may have
152 their @Eq@ instances given by @deriving@ clauses, in which case they
153 form part of the system of equations.
155 Now all we need do is simplify and solve the equations, iterating to
156 find the least fixpoint. Notice that the order of the arguments can
157 switch around, as here in the recursive calls to T.
159 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
163 Eq (T a b) = {} -- The empty set
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 After simplification:
171 = Eq a u Ping b u {} u {} u {}
176 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
177 u Eq (T b a) u Eq Int -- From C2
178 u Eq (T a a) -- From C3
180 After simplification:
185 = Eq a u Ping b u Eq b u Ping a
187 The next iteration gives the same result, so this is the fixpoint. We
188 need to make a canonical form of the RHS to ensure convergence. We do
189 this by simplifying the RHS to a form in which
191 - the classes constrain only tyvars
192 - the list is sorted by tyvar (major key) and then class (minor key)
193 - no duplicates, of course
195 So, here are the synonyms for the ``equation'' structures:
198 Note [Data decl contexts]
199 ~~~~~~~~~~~~~~~~~~~~~~~~~
202 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
204 We will need an instance decl like:
206 instance (Read a, RealFloat a) => Read (Complex a) where
209 The RealFloat in the context is because the read method for Complex is bound
210 to construct a Complex, and doing that requires that the argument type is
213 But this ain't true for Show, Eq, Ord, etc, since they don't construct
214 a Complex; they only take them apart.
216 Our approach: identify the offending classes, and add the data type
217 context to the instance decl. The "offending classes" are
221 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
222 pattern matching against a constructor from a data type with a context
223 gives rise to the constraints for that context -- or at least the thinned
224 version. So now all classes are "offending".
226 Note [Newtype deriving]
227 ~~~~~~~~~~~~~~~~~~~~~~~
231 newtype T = T Char deriving( C [a] )
233 Notice the free 'a' in the deriving. We have to fill this out to
234 newtype T = T Char deriving( forall a. C [a] )
236 And then translate it to:
237 instance C [a] Char => C [a] T where ...
240 Note [Newtype deriving superclasses]
241 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
242 (See also Trac #1220 for an interesting exchange on newtype
243 deriving and superclasses.)
245 The 'tys' here come from the partial application in the deriving
246 clause. The last arg is the new instance type.
248 We must pass the superclasses; the newtype might be an instance
249 of them in a different way than the representation type
250 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
251 Then the Show instance is not done via isomorphism; it shows
253 The Num instance is derived via isomorphism, but the Show superclass
254 dictionary must the Show instance for Foo, *not* the Show dictionary
255 gotten from the Num dictionary. So we must build a whole new dictionary
256 not just use the Num one. The instance we want is something like:
257 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
260 There may be a coercion needed which we get from the tycon for the newtype
261 when the dict is constructed in TcInstDcls.tcInstDecl2
264 Note [Unused constructors and deriving clauses]
265 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
266 See Trac #3221. Consider
267 data T = T1 | T2 deriving( Show )
268 Are T1 and T2 unused? Well, no: the deriving clause expands to mention
269 both of them. So we gather defs/uses from deriving just like anything else.
271 %************************************************************************
273 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
275 %************************************************************************
278 tcDeriving :: [LTyClDecl Name] -- All type constructors
279 -> [LInstDecl Name] -- All instance declarations
280 -> [LDerivDecl Name] -- All stand-alone deriving declarations
281 -> TcM ([InstInfo Name], -- The generated "instance decls"
282 HsValBinds Name, -- Extra generated top-level bindings
285 tcDeriving tycl_decls inst_decls deriv_decls
286 = recoverM (return ([], emptyValBindsOut, emptyDUs)) $
287 do { -- Fish the "deriving"-related information out of the TcEnv
288 -- And make the necessary "equations".
289 is_boot <- tcIsHsBoot
290 ; traceTc (text "tcDeriving" <+> ppr is_boot)
291 ; early_specs <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
293 ; overlap_flag <- getOverlapFlag
294 ; let (infer_specs, given_specs) = splitEithers early_specs
295 ; insts1 <- mapM (genInst True overlap_flag) given_specs
297 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
298 inferInstanceContexts overlap_flag infer_specs
300 ; insts2 <- mapM (genInst False overlap_flag) final_specs
302 -- Generate the generic to/from functions from each type declaration
303 ; gen_binds <- mkGenericBinds is_boot tycl_decls
304 ; (inst_info, rn_binds, rn_dus) <- renameDeriv is_boot gen_binds (insts1 ++ insts2)
307 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
308 (ddump_deriving inst_info rn_binds))
310 ; return (inst_info, rn_binds, rn_dus) }
312 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
313 ddump_deriving inst_infos extra_binds
314 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
316 renameDeriv :: Bool -> LHsBinds RdrName
317 -> [(InstInfo RdrName, DerivAuxBinds)]
318 -> TcM ([InstInfo Name], HsValBinds Name, DefUses)
319 renameDeriv is_boot gen_binds insts
320 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
321 -- The inst-info bindings will all be empty, but it's easier to
322 -- just use rn_inst_info to change the type appropriately
323 = do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
324 ; return (rn_inst_infos, emptyValBindsOut, usesOnly (plusFVs fvs)) }
327 = discardWarnings $ -- Discard warnings about unused bindings etc
328 do { (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $ -- Type signatures in patterns
329 -- are used in the generic binds
330 rnTopBinds (ValBindsIn gen_binds [])
331 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
333 -- Generate and rename any extra not-one-inst-decl-specific binds,
334 -- notably "con2tag" and/or "tag2con" functions.
335 -- Bring those names into scope before renaming the instances themselves
336 ; loc <- getSrcSpanM -- Generic loc for shared bindings
337 ; let aux_binds = listToBag $ map (genAuxBind loc) $
338 rm_dups [] $ concat deriv_aux_binds
339 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv (ValBindsIn aux_binds [])
340 ; let aux_names = collectHsValBinders rn_aux_lhs
342 ; bindLocalNames aux_names $
343 do { (rn_aux, dus_aux) <- rnTopBindsRHS (mkNameSet aux_names) rn_aux_lhs
344 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
345 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
346 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
349 (inst_infos, deriv_aux_binds) = unzip insts
351 -- Remove duplicate requests for auxilliary bindings
353 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
354 | otherwise = rm_dups (b:acc) bs
357 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
358 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
359 = return ( info { iBinds = NewTypeDerived coi tc }
360 , mkFVs (map dataConName (tyConDataCons tc)))
361 -- See Note [Newtype deriving and unused constructors]
363 rn_inst_info (InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
364 = -- Bring the right type variables into
365 -- scope (yuk), and rename the method binds
367 bindLocalNames (map Var.varName tyvars) $
368 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
369 ; let binds' = VanillaInst rn_binds [] standalone_deriv
370 ; return (InstInfo { iSpec = inst, iBinds = binds' }, fvs) }
372 (tyvars,_, clas,_) = instanceHead inst
373 clas_nm = className clas
375 -----------------------------------------
376 mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
377 mkGenericBinds is_boot tycl_decls
381 = do { tcs <- mapM tcLookupTyCon [ tcdName d
382 | L _ d <- tycl_decls, isDataDecl d ]
383 ; return (unionManyBags [ mkTyConGenericBinds tc
384 | tc <- tcs, tyConHasGenerics tc ]) }
385 -- We are only interested in the data type declarations,
386 -- and then only in the ones whose 'has-generics' flag is on
387 -- The predicate tyConHasGenerics finds both of these
390 Note [Newtype deriving and unused constructors]
391 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
392 Consider this (see Trac #1954):
395 newtype P a = MkP (IO a) deriving Monad
397 If you compile with -fwarn-unused-binds you do not expect the warning
398 "Defined but not used: data consructor MkP". Yet the newtype deriving
399 code does not explicitly mention MkP, but it should behave as if you
401 instance Monad P where
402 return x = MkP (return x)
405 So we want to signal a user of the data constructor 'MkP'. That's
406 what we do in rn_inst_info, and it's the only reason we have the TyCon
407 stored in NewTypeDerived.
410 %************************************************************************
412 From HsSyn to DerivSpec
414 %************************************************************************
416 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
419 makeDerivSpecs :: Bool
423 -> TcM [EarlyDerivSpec]
425 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
426 | is_boot -- No 'deriving' at all in hs-boot files
427 = do { mapM_ add_deriv_err deriv_locs
430 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
431 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
432 ; return (eqns1 ++ eqns2) }
434 extractTyDataPreds decls
435 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
437 all_tydata :: [(LHsType Name, LTyClDecl Name)]
438 -- Derived predicate paired with its data type declaration
439 all_tydata = extractTyDataPreds tycl_decls ++
440 [ pd -- Traverse assoc data families
441 | L _ (InstDecl _ _ _ ats) <- inst_decls
442 , pd <- extractTyDataPreds ats ]
444 deriv_locs = map (getLoc . snd) all_tydata
445 ++ map getLoc deriv_decls
447 add_deriv_err loc = setSrcSpan loc $
448 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
449 2 (ptext (sLit "Use an instance declaration instead")))
451 ------------------------------------------------------------------
452 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
453 -- Standalone deriving declarations
454 -- e.g. deriving instance Show a => Show (T a)
455 -- Rather like tcLocalInstDecl
456 deriveStandalone (L loc (DerivDecl deriv_ty))
458 addErrCtxt (standaloneCtxt deriv_ty) $
459 do { traceTc (text "standalone deriving decl for" <+> ppr deriv_ty)
460 ; (tvs, theta, tau) <- tcHsInstHead deriv_ty
461 ; traceTc (text "standalone deriving;"
462 <+> text "tvs:" <+> ppr tvs
463 <+> text "theta:" <+> ppr theta
464 <+> text "tau:" <+> ppr tau)
465 ; (cls, inst_tys) <- checkValidInstance deriv_ty tvs theta tau
466 -- C.f. TcInstDcls.tcLocalInstDecl1
468 ; let cls_tys = take (length inst_tys - 1) inst_tys
469 inst_ty = last inst_tys
470 ; traceTc (text "standalone deriving;"
471 <+> text "class:" <+> ppr cls
472 <+> text "class types:" <+> ppr cls_tys
473 <+> text "type:" <+> ppr inst_ty)
474 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
477 ------------------------------------------------------------------
478 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
479 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
480 tcdTyVars = tv_names,
481 tcdTyPats = ty_pats }))
482 = setSrcSpan loc $ -- Use the location of the 'deriving' item
484 do { (tvs, tc, tc_args) <- get_lhs ty_pats
485 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
486 -- the type variables for the type constructor
488 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
489 -- The "deriv_pred" is a LHsType to take account of the fact that for
490 -- newtype deriving we allow deriving (forall a. C [a]).
492 -- Given data T a b c = ... deriving( C d ),
493 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
494 ; let cls_tyvars = classTyVars cls
495 kind = tyVarKind (last cls_tyvars)
496 (arg_kinds, _) = splitKindFunTys kind
497 n_args_to_drop = length arg_kinds
498 n_args_to_keep = tyConArity tc - n_args_to_drop
499 args_to_drop = drop n_args_to_keep tc_args
500 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
501 inst_ty_kind = typeKind inst_ty
502 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
503 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
504 `minusVarSet` dropped_tvs
506 -- Check that the result really is well-kinded
507 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
508 (derivingKindErr tc cls cls_tys kind)
510 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
511 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
512 (derivingEtaErr cls cls_tys inst_ty)
514 -- (a) The data type can be eta-reduced; eg reject:
515 -- data instance T a a = ... deriving( Monad )
516 -- (b) The type class args do not mention any of the dropped type
518 -- newtype T a s = ... deriving( ST s )
520 -- Type families can't be partially applied
521 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
522 -- Note [Deriving, type families, and partial applications]
523 ; checkTc (not (isOpenTyCon tc) || n_args_to_drop == 0)
524 (typeFamilyPapErr tc cls cls_tys inst_ty)
526 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
528 -- Tiresomely we must figure out the "lhs", which is awkward for type families
529 -- E.g. data T a b = .. deriving( Eq )
530 -- Here, the lhs is (T a b)
531 -- data instance TF Int b = ... deriving( Eq )
532 -- Here, the lhs is (TF Int b)
533 -- But if we just look up the tycon_name, we get is the *family*
534 -- tycon, but not pattern types -- they are in the *rep* tycon.
535 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
536 ; let tvs = tyConTyVars tc
537 ; return (tvs, tc, mkTyVarTys tvs) }
538 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
539 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
540 ; let (tc, tc_args) = tcSplitTyConApp tc_app
541 ; return (tvs, tc, tc_args) }
544 = panic "derivTyData" -- Caller ensures that only TyData can happen
547 Note [Deriving, type families, and partial applications]
548 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
549 When there are no type families, it's quite easy:
551 newtype S a = MkS [a]
552 -- :CoS :: S ~ [] -- Eta-reduced
554 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (coMkS a)) : Eq [a] ~ Eq (S a)
555 instance Monad [] => Monad S -- by coercion sym (Monad coMkS) : Monad [] ~ Monad S
557 When type familes are involved it's trickier:
560 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
561 -- :RT is the representation type for (T Int a)
562 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
563 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
565 instance Eq [a] => Eq (T Int a) -- easy by coercion
566 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
568 The "???" bit is that we don't build the :CoF thing in eta-reduced form
569 Henc the current typeFamilyPapErr, even though the instance makes sense.
570 After all, we can write it out
571 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
576 mkEqnHelp :: InstOrigin -> [TyVar] -> Class -> [Type] -> Type
577 -> DerivContext -- Just => context supplied (standalone deriving)
578 -- Nothing => context inferred (deriving on data decl)
579 -> TcRn EarlyDerivSpec
580 -- Make the EarlyDerivSpec for an instance
581 -- forall tvs. theta => cls (tys ++ [ty])
582 -- where the 'theta' is optional (that's the Maybe part)
583 -- Assumes that this declaration is well-kinded
585 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
586 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
587 , isAlgTyCon tycon -- Check for functions, primitive types etc
588 = do { (rep_tc, rep_tc_args) <- tcLookupFamInstExact tycon tc_args
589 -- Be careful to test rep_tc here: in the case of families,
590 -- we want to check the instance tycon, not the family tycon
592 -- For standalone deriving (mtheta /= Nothing),
593 -- check that all the data constructors are in scope.
594 -- No need for this when deriving Typeable, becuase we don't need
595 -- the constructors for that.
596 ; rdr_env <- getGlobalRdrEnv
597 ; let hidden_data_cons = isAbstractTyCon rep_tc || any not_in_scope (tyConDataCons rep_tc)
598 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
599 ; checkTc (isNothing mtheta ||
600 not hidden_data_cons ||
601 className cls `elem` typeableClassNames)
602 (derivingHiddenErr tycon)
605 ; if isDataTyCon rep_tc then
606 mkDataTypeEqn orig dflags tvs cls cls_tys
607 tycon tc_args rep_tc rep_tc_args mtheta
609 mkNewTypeEqn orig dflags tvs cls cls_tys
610 tycon tc_args rep_tc rep_tc_args mtheta }
612 = failWithTc (derivingThingErr False cls cls_tys tc_app
613 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
616 Note [Looking up family instances for deriving]
617 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
618 tcLookupFamInstExact is an auxiliary lookup wrapper which requires
619 that looked-up family instances exist. If called with a vanilla
620 tycon, the old type application is simply returned.
623 data instance F () = ... deriving Eq
624 data instance F () = ... deriving Eq
625 then tcLookupFamInstExact will be confused by the two matches;
626 but that can't happen because tcInstDecls1 doesn't call tcDeriving
627 if there are any overlaps.
629 There are two other things that might go wrong with the lookup.
630 First, we might see a standalone deriving clause
632 when there is no data instance F () in scope.
634 Note that it's OK to have
635 data instance F [a] = ...
636 deriving Eq (F [(a,b)])
637 where the match is not exact; the same holds for ordinary data types
638 with standalone deriving declrations.
641 tcLookupFamInstExact :: TyCon -> [Type] -> TcM (TyCon, [Type])
642 tcLookupFamInstExact tycon tys
643 | not (isOpenTyCon tycon)
644 = return (tycon, tys)
646 = do { maybeFamInst <- tcLookupFamInst tycon tys
647 ; case maybeFamInst of
648 Nothing -> famInstNotFound tycon tys
649 Just famInst -> return famInst
652 famInstNotFound :: TyCon -> [Type] -> TcM a
653 famInstNotFound tycon tys
654 = failWithTc (ptext (sLit "No family instance for")
655 <+> quotes (pprTypeApp tycon tys))
659 %************************************************************************
663 %************************************************************************
666 mkDataTypeEqn :: InstOrigin
668 -> [Var] -- Universally quantified type variables in the instance
669 -> Class -- Class for which we need to derive an instance
670 -> [Type] -- Other parameters to the class except the last
671 -> TyCon -- Type constructor for which the instance is requested
672 -- (last parameter to the type class)
673 -> [Type] -- Parameters to the type constructor
674 -> TyCon -- rep of the above (for type families)
675 -> [Type] -- rep of the above
676 -> DerivContext -- Context of the instance, for standalone deriving
677 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
679 mkDataTypeEqn orig dflags tvs cls cls_tys
680 tycon tc_args rep_tc rep_tc_args mtheta
681 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
682 -- NB: pass the *representation* tycon to checkSideConditions
683 CanDerive -> go_for_it
684 NonDerivableClass -> bale_out (nonStdErr cls)
685 DerivableClassError msg -> bale_out msg
687 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
688 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
690 mk_data_eqn, mk_typeable_eqn
691 :: InstOrigin -> [TyVar] -> Class
692 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
693 -> TcM EarlyDerivSpec
694 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
695 | getName cls `elem` typeableClassNames
696 = mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
699 = do { dfun_name <- new_dfun_name cls tycon
701 ; let inst_tys = [mkTyConApp tycon tc_args]
702 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
703 spec = DS { ds_loc = loc, ds_orig = orig
704 , ds_name = dfun_name, ds_tvs = tvs
705 , ds_cls = cls, ds_tys = inst_tys
706 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
707 , ds_theta = mtheta `orElse` inferred_constraints
708 , ds_newtype = False }
710 ; return (if isJust mtheta then Right spec -- Specified context
711 else Left spec) } -- Infer context
713 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
714 -- The Typeable class is special in several ways
715 -- data T a b = ... deriving( Typeable )
717 -- instance Typeable2 T where ...
719 -- 1. There are no constraints in the instance
720 -- 2. There are no type variables either
721 -- 3. The actual class we want to generate isn't necessarily
722 -- Typeable; it depends on the arity of the type
723 | isNothing mtheta -- deriving on a data type decl
724 = do { checkTc (cls `hasKey` typeableClassKey)
725 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
726 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
727 ; mk_typeable_eqn orig tvs real_cls tycon [] rep_tc [] (Just []) }
729 | otherwise -- standaone deriving
730 = do { checkTc (null tc_args)
731 (ptext (sLit "Derived typeable instance must be of form (Typeable")
732 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
733 ; dfun_name <- new_dfun_name cls tycon
736 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
737 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
738 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
739 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
742 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
743 -- Generate a sufficiently large set of constraints that typechecking the
744 -- generated method definitions should succeed. This set will be simplified
745 -- before being used in the instance declaration
746 inferConstraints _ cls inst_tys rep_tc rep_tc_args
747 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
748 stupid_constraints ++ extra_constraints
749 ++ sc_constraints ++ con_arg_constraints
751 -- Constraints arising from the arguments of each constructor
753 = [ mkClassPred cls [arg_ty]
754 | data_con <- tyConDataCons rep_tc,
755 arg_ty <- ASSERT( isVanillaDataCon data_con )
756 get_constrained_tys $
757 dataConInstOrigArgTys data_con all_rep_tc_args,
758 not (isUnLiftedType arg_ty) ]
759 -- No constraints for unlifted types
760 -- Where they are legal we generate specilised function calls
762 -- For functor-like classes, two things are different
763 -- (a) We recurse over argument types to generate constraints
764 -- See Functor examples in TcGenDeriv
765 -- (b) The rep_tc_args will be one short
766 is_functor_like = getUnique cls `elem` functorLikeClassKeys
768 get_constrained_tys :: [Type] -> [Type]
769 get_constrained_tys tys
770 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
773 rep_tc_tvs = tyConTyVars rep_tc
774 last_tv = last rep_tc_tvs
775 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
776 | otherwise = rep_tc_args
778 -- Constraints arising from superclasses
779 -- See Note [Superclasses of derived instance]
780 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
783 -- Stupid constraints
784 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
785 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
787 -- Extra Data constraints
788 -- The Data class (only) requires that for
789 -- instance (...) => Data (T t1 t2)
791 -- THEN (Data t1, Data t2) are among the (...) constraints
792 -- Reason: when the IF holds, we generate a method
793 -- dataCast2 f = gcast2 f
794 -- and we need the Data constraints to typecheck the method
796 | cls `hasKey` dataClassKey
797 , all (isLiftedTypeKind . typeKind) rep_tc_args
798 = [mkClassPred cls [ty] | ty <- rep_tc_args]
802 ------------------------------------------------------------------
803 -- Check side conditions that dis-allow derivability for particular classes
804 -- This is *apart* from the newtype-deriving mechanism
806 -- Here we get the representation tycon in case of family instances as it has
807 -- the data constructors - but we need to be careful to fall back to the
808 -- family tycon (with indexes) in error messages.
810 data DerivStatus = CanDerive
811 | DerivableClassError SDoc -- Standard class, but can't do it
812 | NonDerivableClass -- Non-standard class
814 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
815 checkSideConditions dflags mtheta cls cls_tys rep_tc
816 | Just cond <- sideConditions mtheta cls
817 = case (cond (dflags, rep_tc)) of
818 Just err -> DerivableClassError err -- Class-specific error
819 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
820 -- cls_tys (the type args other than last)
822 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
823 | otherwise = NonDerivableClass -- Not a standard class
825 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
827 nonStdErr :: Class -> SDoc
828 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
830 sideConditions :: DerivContext -> Class -> Maybe Condition
831 sideConditions mtheta cls
832 | cls_key == eqClassKey = Just cond_std
833 | cls_key == ordClassKey = Just cond_std
834 | cls_key == showClassKey = Just cond_std
835 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
836 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
837 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
838 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
839 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
840 cond_std `andCond` cond_noUnliftedArgs)
841 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
842 cond_functorOK True) -- NB: no cond_std!
843 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
844 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
845 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
846 cond_functorOK False)
847 | getName cls `elem` typeableClassNames = Just (checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK)
848 | otherwise = Nothing
850 cls_key = getUnique cls
851 cond_std = cond_stdOK mtheta
853 type Condition = (DynFlags, TyCon) -> Maybe SDoc
854 -- first Bool is whether or not we are allowed to derive Data and Typeable
855 -- second Bool is whether or not we are allowed to derive Functor
856 -- TyCon is the *representation* tycon if the
857 -- data type is an indexed one
860 orCond :: Condition -> Condition -> Condition
863 Nothing -> Nothing -- c1 succeeds
864 Just x -> case c2 tc of -- c1 fails
866 Just y -> Just (x $$ ptext (sLit " and") $$ y)
869 andCond :: Condition -> Condition -> Condition
870 andCond c1 c2 tc = case c1 tc of
871 Nothing -> c2 tc -- c1 succeeds
872 Just x -> Just x -- c1 fails
874 cond_stdOK :: DerivContext -> Condition
875 cond_stdOK (Just _) _
876 = Nothing -- Don't check these conservative conditions for
877 -- standalone deriving; just generate the code
878 cond_stdOK Nothing (_, rep_tc)
879 | null data_cons = Just (no_cons_why $$ suggestion)
880 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
881 | otherwise = Nothing
883 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
884 data_cons = tyConDataCons rep_tc
885 no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
886 ptext (sLit "has no data constructors")
888 con_whys = mapCatMaybes check_con data_cons
890 check_con :: DataCon -> Maybe SDoc
892 | isVanillaDataCon con
893 , all isTauTy (dataConOrigArgTys con) = Nothing
894 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
896 cond_enumOrProduct :: Condition
897 cond_enumOrProduct = cond_isEnumeration `orCond`
898 (cond_isProduct `andCond` cond_noUnliftedArgs)
900 cond_noUnliftedArgs :: Condition
901 -- For some classes (eg Eq, Ord) we allow unlifted arg types
902 -- by generating specilaised code. For others (eg Data) we don't.
903 cond_noUnliftedArgs (_, tc)
904 | null bad_cons = Nothing
905 | otherwise = Just why
907 bad_cons = [ con | con <- tyConDataCons tc
908 , any isUnLiftedType (dataConOrigArgTys con) ]
909 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
911 cond_isEnumeration :: Condition
912 cond_isEnumeration (_, rep_tc)
913 | isEnumerationTyCon rep_tc = Nothing
914 | otherwise = Just why
916 why = quotes (pprSourceTyCon rep_tc) <+>
917 ptext (sLit "has non-nullary constructors")
919 cond_isProduct :: Condition
920 cond_isProduct (_, rep_tc)
921 | isProductTyCon rep_tc = Nothing
922 | otherwise = Just why
924 why = quotes (pprSourceTyCon rep_tc) <+>
925 ptext (sLit "has more than one constructor")
927 cond_typeableOK :: Condition
928 -- OK for Typeable class
929 -- Currently: (a) args all of kind *
930 -- (b) 7 or fewer args
931 cond_typeableOK (_, rep_tc)
932 | tyConArity rep_tc > 7 = Just too_many
933 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
935 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
936 | otherwise = Nothing
938 too_many = quotes (pprSourceTyCon rep_tc) <+>
939 ptext (sLit "has too many arguments")
940 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
941 ptext (sLit "has arguments of kind other than `*'")
942 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
943 ptext (sLit "is a type family")
946 functorLikeClassKeys :: [Unique]
947 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
949 cond_functorOK :: Bool -> Condition
950 -- OK for Functor class
951 -- Currently: (a) at least one argument
952 -- (b) don't use argument contravariantly
953 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
954 -- (d) optionally: don't use function types
955 cond_functorOK allowFunctions (dflags, rep_tc)
956 | not (dopt Opt_DeriveFunctor dflags)
957 = Just (ptext (sLit "You need -XDeriveFunctor to derive an instance for this class"))
959 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
961 data_cons = tyConDataCons rep_tc
962 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
964 check_vanilla :: DataCon -> Maybe SDoc
965 check_vanilla con | isVanillaDataCon con = Nothing
966 | otherwise = Just (badCon con existential)
968 ft_check :: DataCon -> FFoldType (Maybe SDoc)
969 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
970 , ft_co_var = Just (badCon con covariant)
971 , ft_fun = \x y -> if allowFunctions then x `mplus` y
972 else Just (badCon con functions)
973 , ft_tup = \_ xs -> msum xs
974 , ft_ty_app = \_ x -> x
975 , ft_bad_app = Just (badCon con wrong_arg)
976 , ft_forall = \_ x -> x }
978 existential = ptext (sLit "has existential arguments")
979 covariant = ptext (sLit "uses the type variable in a function argument")
980 functions = ptext (sLit "contains function types")
981 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
983 checkFlag :: DynFlag -> Condition
984 checkFlag flag (dflags, _)
985 | dopt flag dflags = Nothing
986 | otherwise = Just why
988 why = ptext (sLit "You need -X") <> text flag_str
989 <+> ptext (sLit "to derive an instance for this class")
990 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
992 other -> pprPanic "checkFlag" (ppr other)
994 std_class_via_iso :: Class -> Bool
995 -- These standard classes can be derived for a newtype
996 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
997 -- because giving so gives the same results as generating the boilerplate
998 std_class_via_iso clas
999 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
1000 -- Not Read/Show because they respect the type
1001 -- Not Enum, because newtypes are never in Enum
1004 non_iso_class :: Class -> Bool
1005 -- *Never* derive Read,Show,Typeable,Data by isomorphism,
1006 -- even with -XGeneralizedNewtypeDeriving
1008 = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
1011 typeableClassKeys :: [Unique]
1012 typeableClassKeys = map getUnique typeableClassNames
1014 new_dfun_name :: Class -> TyCon -> TcM Name
1015 new_dfun_name clas tycon -- Just a simple wrapper
1016 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1017 ; newDFunName clas [mkTyConApp tycon []] loc }
1018 -- The type passed to newDFunName is only used to generate
1019 -- a suitable string; hence the empty type arg list
1021 badCon :: DataCon -> SDoc -> SDoc
1022 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1025 Note [Superclasses of derived instance]
1026 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1027 In general, a derived instance decl needs the superclasses of the derived
1028 class too. So if we have
1029 data T a = ...deriving( Ord )
1030 then the initial context for Ord (T a) should include Eq (T a). Often this is
1031 redundant; we'll also generate an Ord constraint for each constructor argument,
1032 and that will probably generate enough constraints to make the Eq (T a) constraint
1033 be satisfied too. But not always; consider:
1039 data T a = MkT (S a) deriving( Ord )
1040 instance Num a => Eq (T a)
1042 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1044 data T a = MkT deriving( Data, Typeable )
1045 Here there *is* no argument field, but we must nevertheless generate
1046 a context for the Data instances:
1047 instance Typable a => Data (T a) where ...
1050 %************************************************************************
1054 %************************************************************************
1057 mkNewTypeEqn :: InstOrigin -> DynFlags -> [Var] -> Class
1058 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1060 -> TcRn EarlyDerivSpec
1061 mkNewTypeEqn orig dflags tvs
1062 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1063 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1064 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1065 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
1066 ; dfun_name <- new_dfun_name cls tycon
1067 ; loc <- getSrcSpanM
1068 ; let spec = DS { ds_loc = loc, ds_orig = orig
1069 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1070 , ds_cls = cls, ds_tys = inst_tys
1071 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1072 , ds_theta = mtheta `orElse` all_preds
1073 , ds_newtype = True }
1074 ; return (if isJust mtheta then Right spec
1078 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1079 CanDerive -> go_for_it -- Use the standard H98 method
1080 DerivableClassError msg -- Error with standard class
1081 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1082 | otherwise -> bale_out msg
1083 NonDerivableClass -- Must use newtype deriving
1084 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1085 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1086 | otherwise -> bale_out non_std
1088 newtype_deriving = dopt Opt_GeneralizedNewtypeDeriving dflags
1089 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1090 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1092 non_std = nonStdErr cls
1093 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1095 -- Here is the plan for newtype derivings. We see
1096 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1097 -- where t is a type,
1098 -- ak+1...an is a suffix of a1..an, and are all tyars
1099 -- ak+1...an do not occur free in t, nor in the s1..sm
1100 -- (C s1 ... sm) is a *partial applications* of class C
1101 -- with the last parameter missing
1102 -- (T a1 .. ak) matches the kind of C's last argument
1103 -- (and hence so does t)
1104 -- The latter kind-check has been done by deriveTyData already,
1105 -- and tc_args are already trimmed
1107 -- We generate the instance
1108 -- instance forall ({a1..ak} u fvs(s1..sm)).
1109 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1110 -- where T a1...ap is the partial application of
1111 -- the LHS of the correct kind and p >= k
1113 -- NB: the variables below are:
1114 -- tc_tvs = [a1, ..., an]
1115 -- tyvars_to_keep = [a1, ..., ak]
1116 -- rep_ty = t ak .. an
1117 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1118 -- tys = [s1, ..., sm]
1121 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1122 -- We generate the instance
1123 -- instance Monad (ST s) => Monad (T s) where
1125 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1126 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1127 -- eta-reduces the representation type, so we know that
1129 -- That's convenient here, because we may have to apply
1130 -- it to fewer than its original complement of arguments
1132 -- Note [Newtype representation]
1133 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1134 -- Need newTyConRhs (*not* a recursive representation finder)
1135 -- to get the representation type. For example
1136 -- newtype B = MkB Int
1137 -- newtype A = MkA B deriving( Num )
1138 -- We want the Num instance of B, *not* the Num instance of Int,
1139 -- when making the Num instance of A!
1140 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1141 rep_tys = cls_tys ++ [rep_inst_ty]
1142 rep_pred = mkClassPred cls rep_tys
1143 -- rep_pred is the representation dictionary, from where
1144 -- we are gong to get all the methods for the newtype
1148 -- Next we figure out what superclass dictionaries to use
1149 -- See Note [Newtype deriving superclasses] above
1151 cls_tyvars = classTyVars cls
1152 dfun_tvs = tyVarsOfTypes inst_tys
1153 inst_ty = mkTyConApp tycon tc_args
1154 inst_tys = cls_tys ++ [inst_ty]
1155 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1158 -- If there are no tyvars, there's no need
1159 -- to abstract over the dictionaries we need
1160 -- Example: newtype T = MkT Int deriving( C )
1161 -- We get the derived instance
1164 -- instance C Int => C T
1165 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1167 -------------------------------------------------------------------
1168 -- Figuring out whether we can only do this newtype-deriving thing
1170 can_derive_via_isomorphism
1171 = not (non_iso_class cls)
1175 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1177 arity_ok = length cls_tys + 1 == classArity cls
1178 -- Well kinded; eg not: newtype T ... deriving( ST )
1179 -- because ST needs *2* type params
1181 -- Check that eta reduction is OK
1182 eta_ok = nt_eta_arity <= length rep_tc_args
1183 -- The newtype can be eta-reduced to match the number
1184 -- of type argument actually supplied
1185 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1186 -- Here the 'b' must be the same in the rep type (S [a] b)
1187 -- And the [a] must not mention 'b'. That's all handled
1190 ats_ok = null (classATs cls)
1191 -- No associated types for the class, because we don't
1192 -- currently generate type 'instance' decls; and cannot do
1193 -- so for 'data' instance decls
1196 = vcat [ ppUnless arity_ok arity_msg
1197 , ppUnless eta_ok eta_msg
1198 , ppUnless ats_ok ats_msg ]
1199 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1200 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1201 ats_msg = ptext (sLit "the class has associated types")
1204 Note [Recursive newtypes]
1205 ~~~~~~~~~~~~~~~~~~~~~~~~~
1206 Newtype deriving works fine, even if the newtype is recursive.
1207 e.g. newtype S1 = S1 [T1 ()]
1208 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1209 Remember, too, that type families are curretly (conservatively) given
1210 a recursive flag, so this also allows newtype deriving to work
1213 We used to exclude recursive types, because we had a rather simple
1214 minded way of generating the instance decl:
1216 instance Eq [A] => Eq A -- Makes typechecker loop!
1217 But now we require a simple context, so it's ok.
1220 %************************************************************************
1222 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1224 %************************************************************************
1226 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1227 terms, which is the final correct RHS for the corresponding original
1231 Each (k,TyVarTy tv) in a solution constrains only a type
1235 The (k,TyVarTy tv) pairs in a solution are canonically
1236 ordered by sorting on type varible, tv, (major key) and then class, k,
1241 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1243 inferInstanceContexts _ [] = return []
1245 inferInstanceContexts oflag infer_specs
1246 = do { traceTc (text "inferInstanceContexts" <+> vcat (map pprDerivSpec infer_specs))
1247 ; iterate_deriv 1 initial_solutions }
1249 ------------------------------------------------------------------
1250 -- The initial solutions for the equations claim that each
1251 -- instance has an empty context; this solution is certainly
1252 -- in canonical form.
1253 initial_solutions :: [ThetaType]
1254 initial_solutions = [ [] | _ <- infer_specs ]
1256 ------------------------------------------------------------------
1257 -- iterate_deriv calculates the next batch of solutions,
1258 -- compares it with the current one; finishes if they are the
1259 -- same, otherwise recurses with the new solutions.
1260 -- It fails if any iteration fails
1261 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1262 iterate_deriv n current_solns
1263 | n > 20 -- Looks as if we are in an infinite loop
1264 -- This can happen if we have -XUndecidableInstances
1265 -- (See TcSimplify.tcSimplifyDeriv.)
1266 = pprPanic "solveDerivEqns: probable loop"
1267 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1269 = do { -- Extend the inst info from the explicit instance decls
1270 -- with the current set of solutions, and simplify each RHS
1271 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1272 current_solns infer_specs
1273 ; new_solns <- checkNoErrs $
1274 extendLocalInstEnv inst_specs $
1275 mapM gen_soln infer_specs
1277 ; if (current_solns == new_solns) then
1278 return [ spec { ds_theta = soln }
1279 | (spec, soln) <- zip infer_specs current_solns ]
1281 iterate_deriv (n+1) new_solns }
1283 ------------------------------------------------------------------
1284 gen_soln :: DerivSpec -> TcM [PredType]
1285 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1286 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1288 addErrCtxt (derivInstCtxt clas inst_tys) $
1289 do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
1290 -- checkValidInstance tyvars theta clas inst_tys
1291 -- Not necessary; see Note [Exotic derived instance contexts]
1294 -- Check for a bizarre corner case, when the derived instance decl should
1295 -- have form instance C a b => D (T a) where ...
1296 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1297 -- of problems; in particular, it's hard to compare solutions for
1298 -- equality when finding the fixpoint. So I just rule it out for now.
1299 ; let tv_set = mkVarSet tyvars
1300 weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
1301 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1303 ; traceTc (text "TcDeriv" <+> (ppr deriv_rhs $$ ppr theta))
1304 -- Claim: the result instance declaration is guaranteed valid
1305 -- Hence no need to call:
1306 -- checkValidInstance tyvars theta clas inst_tys
1307 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1309 ------------------------------------------------------------------
1310 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1311 mkInstance overlap_flag theta
1312 (DS { ds_name = dfun_name
1313 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1314 = mkLocalInstance dfun overlap_flag
1316 dfun = mkDictFunId dfun_name tyvars theta clas tys
1319 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1320 -- Add new locally-defined instances; don't bother to check
1321 -- for functional dependency errors -- that'll happen in TcInstDcls
1322 extendLocalInstEnv dfuns thing_inside
1323 = do { env <- getGblEnv
1324 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1325 env' = env { tcg_inst_env = inst_env' }
1326 ; setGblEnv env' thing_inside }
1330 %************************************************************************
1332 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1334 %************************************************************************
1336 After all the trouble to figure out the required context for the
1337 derived instance declarations, all that's left is to chug along to
1338 produce them. They will then be shoved into @tcInstDecls2@, which
1339 will do all its usual business.
1341 There are lots of possibilities for code to generate. Here are
1342 various general remarks.
1347 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1348 ``you-couldn't-do-better-by-hand'' efficient.
1351 Deriving @Show@---also pretty common--- should also be reasonable good code.
1354 Deriving for the other classes isn't that common or that big a deal.
1361 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1364 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1367 We {\em normally} generate code only for the non-defaulted methods;
1368 there are some exceptions for @Eq@ and (especially) @Ord@...
1371 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1372 constructor's numeric (@Int#@) tag. These are generated by
1373 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1374 these is around is given by @hasCon2TagFun@.
1376 The examples under the different sections below will make this
1380 Much less often (really just for deriving @Ix@), we use a
1381 @_tag2con_<tycon>@ function. See the examples.
1384 We use the renamer!!! Reason: we're supposed to be
1385 producing @LHsBinds Name@ for the methods, but that means
1386 producing correctly-uniquified code on the fly. This is entirely
1387 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1388 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1389 the renamer. What a great hack!
1393 -- Generate the InstInfo for the required instance paired with the
1394 -- *representation* tycon for that instance,
1395 -- plus any auxiliary bindings required
1397 -- Representation tycons differ from the tycon in the instance signature in
1398 -- case of instances for indexed families.
1400 genInst :: Bool -- True <=> standalone deriving
1402 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1403 genInst standalone_deriv oflag spec
1405 = return (InstInfo { iSpec = mkInstance oflag (ds_theta spec) spec
1406 , iBinds = NewTypeDerived co rep_tycon }, [])
1409 = do { let loc = getSrcSpan (ds_name spec)
1410 inst = mkInstance oflag (ds_theta spec) spec
1413 -- In case of a family instance, we need to use the representation
1414 -- tycon (after all, it has the data constructors)
1415 ; fix_env <- getFixityEnv
1416 ; let (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1417 binds = VanillaInst meth_binds [] standalone_deriv
1418 ; return (InstInfo { iSpec = inst, iBinds = binds }, aux_binds)
1421 rep_tycon = ds_tc spec
1422 rep_tc_args = ds_tc_args spec
1423 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1425 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1426 co2 = case newTyConCo_maybe rep_tycon of
1427 Nothing -> IdCo -- The newtype is transparent; no need for a cast
1428 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1429 co = co1 `mkTransCoI` co2
1431 -- Example: newtype instance N [a] = N1 (Tree a)
1432 -- deriving instance Eq b => Eq (N [(b,b)])
1433 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1434 -- When dealing with the deriving clause
1435 -- co1 : N [(b,b)] ~ R1:N (b,b)
1436 -- co2 : R1:N (b,b) ~ Tree (b,b)
1437 -- co : N [(b,b)] ~ Tree (b,b)
1439 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1440 genDerivBinds loc fix_env clas tycon
1441 | className clas `elem` typeableClassNames
1442 = (gen_Typeable_binds loc tycon, [])
1445 = case assocMaybe gen_list (getUnique clas) of
1446 Just gen_fn -> gen_fn loc tycon
1447 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1449 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1450 gen_list = [(eqClassKey, gen_Eq_binds)
1451 ,(ordClassKey, gen_Ord_binds)
1452 ,(enumClassKey, gen_Enum_binds)
1453 ,(boundedClassKey, gen_Bounded_binds)
1454 ,(ixClassKey, gen_Ix_binds)
1455 ,(showClassKey, gen_Show_binds fix_env)
1456 ,(readClassKey, gen_Read_binds fix_env)
1457 ,(dataClassKey, gen_Data_binds)
1458 ,(functorClassKey, gen_Functor_binds)
1459 ,(foldableClassKey, gen_Foldable_binds)
1460 ,(traversableClassKey, gen_Traversable_binds)
1465 %************************************************************************
1467 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1469 %************************************************************************
1472 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1473 derivingKindErr tc cls cls_tys cls_kind
1474 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1475 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1476 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1477 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1479 derivingEtaErr :: Class -> [Type] -> Type -> Message
1480 derivingEtaErr cls cls_tys inst_ty
1481 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1482 nest 2 (ptext (sLit "instance (...) =>")
1483 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1485 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1486 typeFamilyPapErr tc cls cls_tys inst_ty
1487 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1488 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1490 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1491 derivingThingErr newtype_deriving clas tys ty why
1492 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1493 2 (quotes (ppr pred))
1494 $$ nest 2 extra) <> colon,
1497 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1499 pred = mkClassPred clas (tys ++ [ty])
1501 derivingHiddenErr :: TyCon -> SDoc
1502 derivingHiddenErr tc
1503 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1504 2 (ptext (sLit "so you cannot derive an instance for it"))
1506 standaloneCtxt :: LHsType Name -> SDoc
1507 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1510 derivInstCtxt :: Class -> [Type] -> Message
1511 derivInstCtxt clas inst_tys
1512 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1514 badDerivedPred :: PredType -> Message
1516 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1517 ptext (sLit "type variables that are not data type parameters"),
1518 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]