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, aux_sigs) = unzip $ map (genAuxBind loc) $
338 rm_dups [] $ concat deriv_aux_binds
339 aux_val_binds = ValBindsIn (listToBag aux_binds) aux_sigs
340 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
341 ; let aux_names = collectHsValBinders rn_aux_lhs
343 ; bindLocalNames aux_names $
344 do { (rn_aux, dus_aux) <- rnTopBindsRHS (mkNameSet aux_names) rn_aux_lhs
345 ; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
346 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
347 dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
350 (inst_infos, deriv_aux_binds) = unzip insts
352 -- Remove duplicate requests for auxilliary bindings
354 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
355 | otherwise = rm_dups (b:acc) bs
358 rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
359 rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
360 = return ( info { iBinds = NewTypeDerived coi tc }
361 , mkFVs (map dataConName (tyConDataCons tc)))
362 -- See Note [Newtype deriving and unused constructors]
364 rn_inst_info (InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
365 = -- Bring the right type variables into
366 -- scope (yuk), and rename the method binds
368 bindLocalNames (map Var.varName tyvars) $
369 do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
370 ; let binds' = VanillaInst rn_binds [] standalone_deriv
371 ; return (InstInfo { iSpec = inst, iBinds = binds' }, fvs) }
373 (tyvars,_, clas,_) = instanceHead inst
374 clas_nm = className clas
376 -----------------------------------------
377 mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
378 mkGenericBinds is_boot tycl_decls
382 = do { tcs <- mapM tcLookupTyCon [ tcdName d
383 | L _ d <- tycl_decls, isDataDecl d ]
384 ; return (unionManyBags [ mkTyConGenericBinds tc
385 | tc <- tcs, tyConHasGenerics tc ]) }
386 -- We are only interested in the data type declarations,
387 -- and then only in the ones whose 'has-generics' flag is on
388 -- The predicate tyConHasGenerics finds both of these
391 Note [Newtype deriving and unused constructors]
392 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
393 Consider this (see Trac #1954):
396 newtype P a = MkP (IO a) deriving Monad
398 If you compile with -fwarn-unused-binds you do not expect the warning
399 "Defined but not used: data consructor MkP". Yet the newtype deriving
400 code does not explicitly mention MkP, but it should behave as if you
402 instance Monad P where
403 return x = MkP (return x)
406 So we want to signal a user of the data constructor 'MkP'. That's
407 what we do in rn_inst_info, and it's the only reason we have the TyCon
408 stored in NewTypeDerived.
411 %************************************************************************
413 From HsSyn to DerivSpec
415 %************************************************************************
417 @makeDerivSpecs@ fishes around to find the info about needed derived instances.
420 makeDerivSpecs :: Bool
424 -> TcM [EarlyDerivSpec]
426 makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
427 | is_boot -- No 'deriving' at all in hs-boot files
428 = do { mapM_ add_deriv_err deriv_locs
431 = do { eqns1 <- mapAndRecoverM deriveTyData all_tydata
432 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
433 ; return (eqns1 ++ eqns2) }
435 extractTyDataPreds decls
436 = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
438 all_tydata :: [(LHsType Name, LTyClDecl Name)]
439 -- Derived predicate paired with its data type declaration
440 all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)
442 deriv_locs = map (getLoc . snd) all_tydata
443 ++ map getLoc deriv_decls
445 add_deriv_err loc = setSrcSpan loc $
446 addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
447 2 (ptext (sLit "Use an instance declaration instead")))
449 ------------------------------------------------------------------
450 deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
451 -- Standalone deriving declarations
452 -- e.g. deriving instance Show a => Show (T a)
453 -- Rather like tcLocalInstDecl
454 deriveStandalone (L loc (DerivDecl deriv_ty))
456 addErrCtxt (standaloneCtxt deriv_ty) $
457 do { traceTc (text "standalone deriving decl for" <+> ppr deriv_ty)
458 ; (tvs, theta, tau) <- tcHsInstHead deriv_ty
459 ; traceTc (text "standalone deriving;"
460 <+> text "tvs:" <+> ppr tvs
461 <+> text "theta:" <+> ppr theta
462 <+> text "tau:" <+> ppr tau)
463 ; (cls, inst_tys) <- checkValidInstance deriv_ty tvs theta tau
464 -- C.f. TcInstDcls.tcLocalInstDecl1
466 ; let cls_tys = take (length inst_tys - 1) inst_tys
467 inst_ty = last inst_tys
468 ; traceTc (text "standalone deriving;"
469 <+> text "class:" <+> ppr cls
470 <+> text "class types:" <+> ppr cls_tys
471 <+> text "type:" <+> ppr inst_ty)
472 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
475 ------------------------------------------------------------------
476 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
477 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
478 tcdTyVars = tv_names,
479 tcdTyPats = ty_pats }))
480 = setSrcSpan loc $ -- Use the location of the 'deriving' item
482 do { (tvs, tc, tc_args) <- get_lhs ty_pats
483 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
484 -- the type variables for the type constructor
486 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
487 -- The "deriv_pred" is a LHsType to take account of the fact that for
488 -- newtype deriving we allow deriving (forall a. C [a]).
490 -- Given data T a b c = ... deriving( C d ),
491 -- we want to drop type variables from T so that (C d (T a)) is well-kinded
492 ; let cls_tyvars = classTyVars cls
493 kind = tyVarKind (last cls_tyvars)
494 (arg_kinds, _) = splitKindFunTys kind
495 n_args_to_drop = length arg_kinds
496 n_args_to_keep = tyConArity tc - n_args_to_drop
497 args_to_drop = drop n_args_to_keep tc_args
498 inst_ty = mkTyConApp tc (take n_args_to_keep tc_args)
499 inst_ty_kind = typeKind inst_ty
500 dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
501 univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs)
502 `minusVarSet` dropped_tvs
504 -- Check that the result really is well-kinded
505 ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
506 (derivingKindErr tc cls cls_tys kind)
508 ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a)
509 tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
510 (derivingEtaErr cls cls_tys inst_ty)
512 -- (a) The data type can be eta-reduced; eg reject:
513 -- data instance T a a = ... deriving( Monad )
514 -- (b) The type class args do not mention any of the dropped type
516 -- newtype T a s = ... deriving( ST s )
518 -- Type families can't be partially applied
519 -- e.g. newtype instance T Int a = MkT [a] deriving( Monad )
520 -- Note [Deriving, type families, and partial applications]
521 ; checkTc (not (isOpenTyCon tc) || n_args_to_drop == 0)
522 (typeFamilyPapErr tc cls cls_tys inst_ty)
524 ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
526 -- Tiresomely we must figure out the "lhs", which is awkward for type families
527 -- E.g. data T a b = .. deriving( Eq )
528 -- Here, the lhs is (T a b)
529 -- data instance TF Int b = ... deriving( Eq )
530 -- Here, the lhs is (TF Int b)
531 -- But if we just look up the tycon_name, we get is the *family*
532 -- tycon, but not pattern types -- they are in the *rep* tycon.
533 get_lhs Nothing = do { tc <- tcLookupTyCon tycon_name
534 ; let tvs = tyConTyVars tc
535 ; return (tvs, tc, mkTyVarTys tvs) }
536 get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
537 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
538 ; let (tc, tc_args) = tcSplitTyConApp tc_app
539 ; return (tvs, tc, tc_args) }
542 = panic "derivTyData" -- Caller ensures that only TyData can happen
545 Note [Deriving, type families, and partial applications]
546 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
547 When there are no type families, it's quite easy:
549 newtype S a = MkS [a]
550 -- :CoS :: S ~ [] -- Eta-reduced
552 instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
553 instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
555 When type familes are involved it's trickier:
558 newtype instance T Int a = MkT [a] deriving( Eq, Monad )
559 -- :RT is the representation type for (T Int a)
560 -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced
561 -- :Co:R1T :: :RT ~ [] -- Eta-reduced
563 instance Eq [a] => Eq (T Int a) -- easy by coercion
564 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
566 The "???" bit is that we don't build the :CoF thing in eta-reduced form
567 Henc the current typeFamilyPapErr, even though the instance makes sense.
568 After all, we can write it out
569 instance Monad [] => Monad (T Int) -- only if we can eta reduce???
574 mkEqnHelp :: InstOrigin -> [TyVar] -> Class -> [Type] -> Type
575 -> DerivContext -- Just => context supplied (standalone deriving)
576 -- Nothing => context inferred (deriving on data decl)
577 -> TcRn EarlyDerivSpec
578 -- Make the EarlyDerivSpec for an instance
579 -- forall tvs. theta => cls (tys ++ [ty])
580 -- where the 'theta' is optional (that's the Maybe part)
581 -- Assumes that this declaration is well-kinded
583 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
584 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
585 , isAlgTyCon tycon -- Check for functions, primitive types etc
586 = do { (rep_tc, rep_tc_args) <- tcLookupFamInstExact tycon tc_args
587 -- Be careful to test rep_tc here: in the case of families,
588 -- we want to check the instance tycon, not the family tycon
590 -- For standalone deriving (mtheta /= Nothing),
591 -- check that all the data constructors are in scope.
592 -- No need for this when deriving Typeable, becuase we don't need
593 -- the constructors for that.
594 ; rdr_env <- getGlobalRdrEnv
595 ; let hidden_data_cons = isAbstractTyCon rep_tc || any not_in_scope (tyConDataCons rep_tc)
596 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
597 ; checkTc (isNothing mtheta ||
598 not hidden_data_cons ||
599 className cls `elem` typeableClassNames)
600 (derivingHiddenErr tycon)
603 ; if isDataTyCon rep_tc then
604 mkDataTypeEqn orig dflags tvs cls cls_tys
605 tycon tc_args rep_tc rep_tc_args mtheta
607 mkNewTypeEqn orig dflags tvs cls cls_tys
608 tycon tc_args rep_tc rep_tc_args mtheta }
610 = failWithTc (derivingThingErr False cls cls_tys tc_app
611 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
614 Note [Looking up family instances for deriving]
615 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
616 tcLookupFamInstExact is an auxiliary lookup wrapper which requires
617 that looked-up family instances exist. If called with a vanilla
618 tycon, the old type application is simply returned.
621 data instance F () = ... deriving Eq
622 data instance F () = ... deriving Eq
623 then tcLookupFamInstExact will be confused by the two matches;
624 but that can't happen because tcInstDecls1 doesn't call tcDeriving
625 if there are any overlaps.
627 There are two other things that might go wrong with the lookup.
628 First, we might see a standalone deriving clause
630 when there is no data instance F () in scope.
632 Note that it's OK to have
633 data instance F [a] = ...
634 deriving Eq (F [(a,b)])
635 where the match is not exact; the same holds for ordinary data types
636 with standalone deriving declrations.
639 tcLookupFamInstExact :: TyCon -> [Type] -> TcM (TyCon, [Type])
640 tcLookupFamInstExact tycon tys
641 | not (isOpenTyCon tycon)
642 = return (tycon, tys)
644 = do { maybeFamInst <- tcLookupFamInst tycon tys
645 ; case maybeFamInst of
646 Nothing -> famInstNotFound tycon tys
647 Just famInst -> return famInst
650 famInstNotFound :: TyCon -> [Type] -> TcM a
651 famInstNotFound tycon tys
652 = failWithTc (ptext (sLit "No family instance for")
653 <+> quotes (pprTypeApp tycon tys))
657 %************************************************************************
661 %************************************************************************
664 mkDataTypeEqn :: InstOrigin
666 -> [Var] -- Universally quantified type variables in the instance
667 -> Class -- Class for which we need to derive an instance
668 -> [Type] -- Other parameters to the class except the last
669 -> TyCon -- Type constructor for which the instance is requested
670 -- (last parameter to the type class)
671 -> [Type] -- Parameters to the type constructor
672 -> TyCon -- rep of the above (for type families)
673 -> [Type] -- rep of the above
674 -> DerivContext -- Context of the instance, for standalone deriving
675 -> TcRn EarlyDerivSpec -- Return 'Nothing' if error
677 mkDataTypeEqn orig dflags tvs cls cls_tys
678 tycon tc_args rep_tc rep_tc_args mtheta
679 = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
680 -- NB: pass the *representation* tycon to checkSideConditions
681 CanDerive -> go_for_it
682 NonDerivableClass -> bale_out (nonStdErr cls)
683 DerivableClassError msg -> bale_out msg
685 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
686 bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
688 mk_data_eqn, mk_typeable_eqn
689 :: InstOrigin -> [TyVar] -> Class
690 -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
691 -> TcM EarlyDerivSpec
692 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
693 | getName cls `elem` typeableClassNames
694 = mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
697 = do { dfun_name <- new_dfun_name cls tycon
699 ; let inst_tys = [mkTyConApp tycon tc_args]
700 inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
701 spec = DS { ds_loc = loc, ds_orig = orig
702 , ds_name = dfun_name, ds_tvs = tvs
703 , ds_cls = cls, ds_tys = inst_tys
704 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
705 , ds_theta = mtheta `orElse` inferred_constraints
706 , ds_newtype = False }
708 ; return (if isJust mtheta then Right spec -- Specified context
709 else Left spec) } -- Infer context
711 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
712 -- The Typeable class is special in several ways
713 -- data T a b = ... deriving( Typeable )
715 -- instance Typeable2 T where ...
717 -- 1. There are no constraints in the instance
718 -- 2. There are no type variables either
719 -- 3. The actual class we want to generate isn't necessarily
720 -- Typeable; it depends on the arity of the type
721 | isNothing mtheta -- deriving on a data type decl
722 = do { checkTc (cls `hasKey` typeableClassKey)
723 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
724 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
725 ; mk_typeable_eqn orig tvs real_cls tycon [] rep_tc [] (Just []) }
727 | otherwise -- standaone deriving
728 = do { checkTc (null tc_args)
729 (ptext (sLit "Derived typeable instance must be of form (Typeable")
730 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
731 ; dfun_name <- new_dfun_name cls tycon
734 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
735 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
736 , ds_tc = rep_tc, ds_tc_args = rep_tc_args
737 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
740 inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
741 -- Generate a sufficiently large set of constraints that typechecking the
742 -- generated method definitions should succeed. This set will be simplified
743 -- before being used in the instance declaration
744 inferConstraints _ cls inst_tys rep_tc rep_tc_args
745 = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
746 pprTrace "ic" (ppr rep_tc $$ ppr rep_tc_tvs $$ ppr (tyConStupidTheta rep_tc) $$ ppr stupid_constraints) $
747 stupid_constraints ++ extra_constraints
748 ++ sc_constraints ++ con_arg_constraints
750 -- Constraints arising from the arguments of each constructor
752 = [ mkClassPred cls [arg_ty]
753 | data_con <- tyConDataCons rep_tc,
754 arg_ty <- ASSERT( isVanillaDataCon data_con )
755 get_constrained_tys $
756 dataConInstOrigArgTys data_con all_rep_tc_args,
757 not (isUnLiftedType arg_ty) ]
758 -- No constraints for unlifted types
759 -- Where they are legal we generate specilised function calls
761 -- For functor-like classes, two things are different
762 -- (a) We recurse over argument types to generate constraints
763 -- See Functor examples in TcGenDeriv
764 -- (b) The rep_tc_args will be one short
765 is_functor_like = getUnique cls `elem` functorLikeClassKeys
767 get_constrained_tys :: [Type] -> [Type]
768 get_constrained_tys tys
769 | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
772 rep_tc_tvs = tyConTyVars rep_tc
773 last_tv = last rep_tc_tvs
774 all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
775 | otherwise = rep_tc_args
777 -- Constraints arising from superclasses
778 -- See Note [Superclasses of derived instance]
779 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
782 -- Stupid constraints
783 stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
784 subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
786 -- Extra Data constraints
787 -- The Data class (only) requires that for
788 -- instance (...) => Data (T t1 t2)
790 -- THEN (Data t1, Data t2) are among the (...) constraints
791 -- Reason: when the IF holds, we generate a method
792 -- dataCast2 f = gcast2 f
793 -- and we need the Data constraints to typecheck the method
795 | cls `hasKey` dataClassKey
796 , all (isLiftedTypeKind . typeKind) rep_tc_args
797 = [mkClassPred cls [ty] | ty <- rep_tc_args]
801 ------------------------------------------------------------------
802 -- Check side conditions that dis-allow derivability for particular classes
803 -- This is *apart* from the newtype-deriving mechanism
805 -- Here we get the representation tycon in case of family instances as it has
806 -- the data constructors - but we need to be careful to fall back to the
807 -- family tycon (with indexes) in error messages.
809 data DerivStatus = CanDerive
810 | DerivableClassError SDoc -- Standard class, but can't do it
811 | NonDerivableClass -- Non-standard class
813 checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
814 checkSideConditions dflags mtheta cls cls_tys rep_tc
815 | Just cond <- sideConditions mtheta cls
816 = case (cond (dflags, rep_tc)) of
817 Just err -> DerivableClassError err -- Class-specific error
818 Nothing | null cls_tys -> CanDerive -- All derivable classes are unary, so
819 -- cls_tys (the type args other than last)
821 | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s )
822 | otherwise = NonDerivableClass -- Not a standard class
824 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
826 nonStdErr :: Class -> SDoc
827 nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
829 sideConditions :: DerivContext -> Class -> Maybe Condition
830 sideConditions mtheta cls
831 | cls_key == eqClassKey = Just cond_std
832 | cls_key == ordClassKey = Just cond_std
833 | cls_key == showClassKey = Just cond_std
834 | cls_key == readClassKey = Just (cond_std `andCond` cond_noUnliftedArgs)
835 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
836 | cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct)
837 | cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct)
838 | cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
839 cond_std `andCond` cond_noUnliftedArgs)
840 | cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
841 cond_functorOK True) -- NB: no cond_std!
842 | cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
843 cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
844 | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
845 cond_functorOK False)
846 | getName cls `elem` typeableClassNames = Just (checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK)
847 | otherwise = Nothing
849 cls_key = getUnique cls
850 cond_std = cond_stdOK mtheta
852 type Condition = (DynFlags, TyCon) -> Maybe SDoc
853 -- first Bool is whether or not we are allowed to derive Data and Typeable
854 -- second Bool is whether or not we are allowed to derive Functor
855 -- TyCon is the *representation* tycon if the
856 -- data type is an indexed one
859 orCond :: Condition -> Condition -> Condition
862 Nothing -> Nothing -- c1 succeeds
863 Just x -> case c2 tc of -- c1 fails
865 Just y -> Just (x $$ ptext (sLit " and") $$ y)
868 andCond :: Condition -> Condition -> Condition
869 andCond c1 c2 tc = case c1 tc of
870 Nothing -> c2 tc -- c1 succeeds
871 Just x -> Just x -- c1 fails
873 cond_stdOK :: DerivContext -> Condition
874 cond_stdOK (Just _) _
875 = Nothing -- Don't check these conservative conditions for
876 -- standalone deriving; just generate the code
877 cond_stdOK Nothing (_, rep_tc)
878 | null data_cons = Just (no_cons_why $$ suggestion)
879 | not (null con_whys) = Just (vcat con_whys $$ suggestion)
880 | otherwise = Nothing
882 suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
883 data_cons = tyConDataCons rep_tc
884 no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
885 ptext (sLit "has no data constructors")
887 con_whys = mapCatMaybes check_con data_cons
889 check_con :: DataCon -> Maybe SDoc
891 | isVanillaDataCon con
892 , all isTauTy (dataConOrigArgTys con) = Nothing
893 | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))
895 cond_enumOrProduct :: Condition
896 cond_enumOrProduct = cond_isEnumeration `orCond`
897 (cond_isProduct `andCond` cond_noUnliftedArgs)
899 cond_noUnliftedArgs :: Condition
900 -- For some classes (eg Eq, Ord) we allow unlifted arg types
901 -- by generating specilaised code. For others (eg Data) we don't.
902 cond_noUnliftedArgs (_, tc)
903 | null bad_cons = Nothing
904 | otherwise = Just why
906 bad_cons = [ con | con <- tyConDataCons tc
907 , any isUnLiftedType (dataConOrigArgTys con) ]
908 why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))
910 cond_isEnumeration :: Condition
911 cond_isEnumeration (_, rep_tc)
912 | isEnumerationTyCon rep_tc = Nothing
913 | otherwise = Just why
915 why = quotes (pprSourceTyCon rep_tc) <+>
916 ptext (sLit "has non-nullary constructors")
918 cond_isProduct :: Condition
919 cond_isProduct (_, rep_tc)
920 | isProductTyCon rep_tc = Nothing
921 | otherwise = Just why
923 why = quotes (pprSourceTyCon rep_tc) <+>
924 ptext (sLit "has more than one constructor")
926 cond_typeableOK :: Condition
927 -- OK for Typeable class
928 -- Currently: (a) args all of kind *
929 -- (b) 7 or fewer args
930 cond_typeableOK (_, rep_tc)
931 | tyConArity rep_tc > 7 = Just too_many
932 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
934 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
935 | otherwise = Nothing
937 too_many = quotes (pprSourceTyCon rep_tc) <+>
938 ptext (sLit "has too many arguments")
939 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
940 ptext (sLit "has arguments of kind other than `*'")
941 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
942 ptext (sLit "is a type family")
945 functorLikeClassKeys :: [Unique]
946 functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
948 cond_functorOK :: Bool -> Condition
949 -- OK for Functor class
950 -- Currently: (a) at least one argument
951 -- (b) don't use argument contravariantly
952 -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
953 -- (d) optionally: don't use function types
954 cond_functorOK allowFunctions (dflags, rep_tc)
955 | not (dopt Opt_DeriveFunctor dflags)
956 = Just (ptext (sLit "You need -XDeriveFunctor to derive an instance for this class"))
958 = msum (map check_con data_cons) -- msum picks the first 'Just', if any
960 data_cons = tyConDataCons rep_tc
961 check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
963 check_vanilla :: DataCon -> Maybe SDoc
964 check_vanilla con | isVanillaDataCon con = Nothing
965 | otherwise = Just (badCon con existential)
967 ft_check :: DataCon -> FFoldType (Maybe SDoc)
968 ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
969 , ft_co_var = Just (badCon con covariant)
970 , ft_fun = \x y -> if allowFunctions then x `mplus` y
971 else Just (badCon con functions)
972 , ft_tup = \_ xs -> msum xs
973 , ft_ty_app = \_ x -> x
974 , ft_bad_app = Just (badCon con wrong_arg)
975 , ft_forall = \_ x -> x }
977 existential = ptext (sLit "has existential arguments")
978 covariant = ptext (sLit "uses the type variable in a function argument")
979 functions = ptext (sLit "contains function types")
980 wrong_arg = ptext (sLit "uses the type variable in an argument other than the last")
982 checkFlag :: ExtensionFlag -> Condition
983 checkFlag flag (dflags, _)
984 | dopt flag dflags = Nothing
985 | otherwise = Just why
987 why = ptext (sLit "You need -X") <> text flag_str
988 <+> ptext (sLit "to derive an instance for this class")
989 flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
991 other -> pprPanic "checkFlag" (ppr other)
993 std_class_via_iso :: Class -> Bool
994 -- These standard classes can be derived for a newtype
995 -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
996 -- because giving so gives the same results as generating the boilerplate
997 std_class_via_iso clas
998 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
999 -- Not Read/Show because they respect the type
1000 -- Not Enum, because newtypes are never in Enum
1003 non_iso_class :: Class -> Bool
1004 -- *Never* derive Read,Show,Typeable,Data by isomorphism,
1005 -- even with -XGeneralizedNewtypeDeriving
1007 = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
1010 typeableClassKeys :: [Unique]
1011 typeableClassKeys = map getUnique typeableClassNames
1013 new_dfun_name :: Class -> TyCon -> TcM Name
1014 new_dfun_name clas tycon -- Just a simple wrapper
1015 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
1016 ; newDFunName clas [mkTyConApp tycon []] loc }
1017 -- The type passed to newDFunName is only used to generate
1018 -- a suitable string; hence the empty type arg list
1020 badCon :: DataCon -> SDoc -> SDoc
1021 badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
1024 Note [Superclasses of derived instance]
1025 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1026 In general, a derived instance decl needs the superclasses of the derived
1027 class too. So if we have
1028 data T a = ...deriving( Ord )
1029 then the initial context for Ord (T a) should include Eq (T a). Often this is
1030 redundant; we'll also generate an Ord constraint for each constructor argument,
1031 and that will probably generate enough constraints to make the Eq (T a) constraint
1032 be satisfied too. But not always; consider:
1038 data T a = MkT (S a) deriving( Ord )
1039 instance Num a => Eq (T a)
1041 The derived instance for (Ord (T a)) must have a (Num a) constraint!
1043 data T a = MkT deriving( Data, Typeable )
1044 Here there *is* no argument field, but we must nevertheless generate
1045 a context for the Data instances:
1046 instance Typable a => Data (T a) where ...
1049 %************************************************************************
1053 %************************************************************************
1056 mkNewTypeEqn :: InstOrigin -> DynFlags -> [Var] -> Class
1057 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
1059 -> TcRn EarlyDerivSpec
1060 mkNewTypeEqn orig dflags tvs
1061 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
1062 -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
1063 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
1064 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
1065 ; dfun_name <- new_dfun_name cls tycon
1066 ; loc <- getSrcSpanM
1067 ; let spec = DS { ds_loc = loc, ds_orig = orig
1068 , ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
1069 , ds_cls = cls, ds_tys = inst_tys
1070 , ds_tc = rep_tycon, ds_tc_args = rep_tc_args
1071 , ds_theta = mtheta `orElse` all_preds
1072 , ds_newtype = True }
1073 ; return (if isJust mtheta then Right spec
1077 = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
1078 CanDerive -> go_for_it -- Use the standard H98 method
1079 DerivableClassError msg -- Error with standard class
1080 | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
1081 | otherwise -> bale_out msg
1082 NonDerivableClass -- Must use newtype deriving
1083 | newtype_deriving -> bale_out cant_derive_err -- Too hard, even with newtype deriving
1084 | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
1085 | otherwise -> bale_out non_std
1087 newtype_deriving = dopt Opt_GeneralizedNewtypeDeriving dflags
1088 go_for_it = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
1089 bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
1091 non_std = nonStdErr cls
1092 suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
1094 -- Here is the plan for newtype derivings. We see
1095 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
1096 -- where t is a type,
1097 -- ak+1...an is a suffix of a1..an, and are all tyars
1098 -- ak+1...an do not occur free in t, nor in the s1..sm
1099 -- (C s1 ... sm) is a *partial applications* of class C
1100 -- with the last parameter missing
1101 -- (T a1 .. ak) matches the kind of C's last argument
1102 -- (and hence so does t)
1103 -- The latter kind-check has been done by deriveTyData already,
1104 -- and tc_args are already trimmed
1106 -- We generate the instance
1107 -- instance forall ({a1..ak} u fvs(s1..sm)).
1108 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
1109 -- where T a1...ap is the partial application of
1110 -- the LHS of the correct kind and p >= k
1112 -- NB: the variables below are:
1113 -- tc_tvs = [a1, ..., an]
1114 -- tyvars_to_keep = [a1, ..., ak]
1115 -- rep_ty = t ak .. an
1116 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
1117 -- tys = [s1, ..., sm]
1120 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
1121 -- We generate the instance
1122 -- instance Monad (ST s) => Monad (T s) where
1124 nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
1125 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
1126 -- eta-reduces the representation type, so we know that
1128 -- That's convenient here, because we may have to apply
1129 -- it to fewer than its original complement of arguments
1131 -- Note [Newtype representation]
1132 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1133 -- Need newTyConRhs (*not* a recursive representation finder)
1134 -- to get the representation type. For example
1135 -- newtype B = MkB Int
1136 -- newtype A = MkA B deriving( Num )
1137 -- We want the Num instance of B, *not* the Num instance of Int,
1138 -- when making the Num instance of A!
1139 rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
1140 rep_tys = cls_tys ++ [rep_inst_ty]
1141 rep_pred = mkClassPred cls rep_tys
1142 -- rep_pred is the representation dictionary, from where
1143 -- we are gong to get all the methods for the newtype
1147 -- Next we figure out what superclass dictionaries to use
1148 -- See Note [Newtype deriving superclasses] above
1150 cls_tyvars = classTyVars cls
1151 dfun_tvs = tyVarsOfTypes inst_tys
1152 inst_ty = mkTyConApp tycon tc_args
1153 inst_tys = cls_tys ++ [inst_ty]
1154 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
1157 -- If there are no tyvars, there's no need
1158 -- to abstract over the dictionaries we need
1159 -- Example: newtype T = MkT Int deriving( C )
1160 -- We get the derived instance
1163 -- instance C Int => C T
1164 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
1166 -------------------------------------------------------------------
1167 -- Figuring out whether we can only do this newtype-deriving thing
1169 can_derive_via_isomorphism
1170 = not (non_iso_class cls)
1174 -- && not (isRecursiveTyCon tycon) -- Note [Recursive newtypes]
1176 arity_ok = length cls_tys + 1 == classArity cls
1177 -- Well kinded; eg not: newtype T ... deriving( ST )
1178 -- because ST needs *2* type params
1180 -- Check that eta reduction is OK
1181 eta_ok = nt_eta_arity <= length rep_tc_args
1182 -- The newtype can be eta-reduced to match the number
1183 -- of type argument actually supplied
1184 -- newtype T a b = MkT (S [a] b) deriving( Monad )
1185 -- Here the 'b' must be the same in the rep type (S [a] b)
1186 -- And the [a] must not mention 'b'. That's all handled
1189 ats_ok = null (classATs cls)
1190 -- No associated types for the class, because we don't
1191 -- currently generate type 'instance' decls; and cannot do
1192 -- so for 'data' instance decls
1195 = vcat [ ppUnless arity_ok arity_msg
1196 , ppUnless eta_ok eta_msg
1197 , ppUnless ats_ok ats_msg ]
1198 arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
1199 eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
1200 ats_msg = ptext (sLit "the class has associated types")
1203 Note [Recursive newtypes]
1204 ~~~~~~~~~~~~~~~~~~~~~~~~~
1205 Newtype deriving works fine, even if the newtype is recursive.
1206 e.g. newtype S1 = S1 [T1 ()]
1207 newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
1208 Remember, too, that type families are curretly (conservatively) given
1209 a recursive flag, so this also allows newtype deriving to work
1212 We used to exclude recursive types, because we had a rather simple
1213 minded way of generating the instance decl:
1215 instance Eq [A] => Eq A -- Makes typechecker loop!
1216 But now we require a simple context, so it's ok.
1219 %************************************************************************
1221 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
1223 %************************************************************************
1225 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
1226 terms, which is the final correct RHS for the corresponding original
1230 Each (k,TyVarTy tv) in a solution constrains only a type
1234 The (k,TyVarTy tv) pairs in a solution are canonically
1235 ordered by sorting on type varible, tv, (major key) and then class, k,
1240 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
1242 inferInstanceContexts _ [] = return []
1244 inferInstanceContexts oflag infer_specs
1245 = do { traceTc (text "inferInstanceContexts" <+> vcat (map pprDerivSpec infer_specs))
1246 ; iterate_deriv 1 initial_solutions }
1248 ------------------------------------------------------------------
1249 -- The initial solutions for the equations claim that each
1250 -- instance has an empty context; this solution is certainly
1251 -- in canonical form.
1252 initial_solutions :: [ThetaType]
1253 initial_solutions = [ [] | _ <- infer_specs ]
1255 ------------------------------------------------------------------
1256 -- iterate_deriv calculates the next batch of solutions,
1257 -- compares it with the current one; finishes if they are the
1258 -- same, otherwise recurses with the new solutions.
1259 -- It fails if any iteration fails
1260 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1261 iterate_deriv n current_solns
1262 | n > 20 -- Looks as if we are in an infinite loop
1263 -- This can happen if we have -XUndecidableInstances
1264 -- (See TcSimplify.tcSimplifyDeriv.)
1265 = pprPanic "solveDerivEqns: probable loop"
1266 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1268 = do { -- Extend the inst info from the explicit instance decls
1269 -- with the current set of solutions, and simplify each RHS
1270 let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
1271 current_solns infer_specs
1272 ; new_solns <- checkNoErrs $
1273 extendLocalInstEnv inst_specs $
1274 mapM gen_soln infer_specs
1276 ; if (current_solns == new_solns) then
1277 return [ spec { ds_theta = soln }
1278 | (spec, soln) <- zip infer_specs current_solns ]
1280 iterate_deriv (n+1) new_solns }
1282 ------------------------------------------------------------------
1283 gen_soln :: DerivSpec -> TcM [PredType]
1284 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1285 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1287 addErrCtxt (derivInstCtxt clas inst_tys) $
1288 do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
1289 -- checkValidInstance tyvars theta clas inst_tys
1290 -- Not necessary; see Note [Exotic derived instance contexts]
1293 -- Check for a bizarre corner case, when the derived instance decl should
1294 -- have form instance C a b => D (T a) where ...
1295 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1296 -- of problems; in particular, it's hard to compare solutions for
1297 -- equality when finding the fixpoint. So I just rule it out for now.
1298 ; let tv_set = mkVarSet tyvars
1299 weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
1300 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1302 ; traceTc (text "TcDeriv" <+> (ppr deriv_rhs $$ ppr theta))
1303 -- Claim: the result instance declaration is guaranteed valid
1304 -- Hence no need to call:
1305 -- checkValidInstance tyvars theta clas inst_tys
1306 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1308 ------------------------------------------------------------------
1309 mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1310 mkInstance overlap_flag theta
1311 (DS { ds_name = dfun_name
1312 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1313 = mkLocalInstance dfun overlap_flag
1315 dfun = mkDictFunId dfun_name tyvars theta clas tys
1318 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1319 -- Add new locally-defined instances; don't bother to check
1320 -- for functional dependency errors -- that'll happen in TcInstDcls
1321 extendLocalInstEnv dfuns thing_inside
1322 = do { env <- getGblEnv
1323 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1324 env' = env { tcg_inst_env = inst_env' }
1325 ; setGblEnv env' thing_inside }
1329 %************************************************************************
1331 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1333 %************************************************************************
1335 After all the trouble to figure out the required context for the
1336 derived instance declarations, all that's left is to chug along to
1337 produce them. They will then be shoved into @tcInstDecls2@, which
1338 will do all its usual business.
1340 There are lots of possibilities for code to generate. Here are
1341 various general remarks.
1346 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1347 ``you-couldn't-do-better-by-hand'' efficient.
1350 Deriving @Show@---also pretty common--- should also be reasonable good code.
1353 Deriving for the other classes isn't that common or that big a deal.
1360 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1363 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1366 We {\em normally} generate code only for the non-defaulted methods;
1367 there are some exceptions for @Eq@ and (especially) @Ord@...
1370 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1371 constructor's numeric (@Int#@) tag. These are generated by
1372 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1373 these is around is given by @hasCon2TagFun@.
1375 The examples under the different sections below will make this
1379 Much less often (really just for deriving @Ix@), we use a
1380 @_tag2con_<tycon>@ function. See the examples.
1383 We use the renamer!!! Reason: we're supposed to be
1384 producing @LHsBinds Name@ for the methods, but that means
1385 producing correctly-uniquified code on the fly. This is entirely
1386 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1387 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1388 the renamer. What a great hack!
1392 -- Generate the InstInfo for the required instance paired with the
1393 -- *representation* tycon for that instance,
1394 -- plus any auxiliary bindings required
1396 -- Representation tycons differ from the tycon in the instance signature in
1397 -- case of instances for indexed families.
1399 genInst :: Bool -- True <=> standalone deriving
1401 -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1402 genInst standalone_deriv oflag spec
1404 = return (InstInfo { iSpec = mkInstance oflag (ds_theta spec) spec
1405 , iBinds = NewTypeDerived co rep_tycon }, [])
1408 = do { let loc = getSrcSpan (ds_name spec)
1409 inst = mkInstance oflag (ds_theta spec) spec
1412 -- In case of a family instance, we need to use the representation
1413 -- tycon (after all, it has the data constructors)
1414 ; fix_env <- getFixityEnv
1415 ; let (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
1416 binds = VanillaInst meth_binds [] standalone_deriv
1417 ; return (InstInfo { iSpec = inst, iBinds = binds }, aux_binds)
1420 rep_tycon = ds_tc spec
1421 rep_tc_args = ds_tc_args spec
1422 co1 = case tyConFamilyCoercion_maybe rep_tycon of
1424 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1425 co2 = case newTyConCo_maybe rep_tycon of
1426 Nothing -> IdCo -- The newtype is transparent; no need for a cast
1427 Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
1428 co = co1 `mkTransCoI` co2
1430 -- Example: newtype instance N [a] = N1 (Tree a)
1431 -- deriving instance Eq b => Eq (N [(b,b)])
1432 -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
1433 -- When dealing with the deriving clause
1434 -- co1 : N [(b,b)] ~ R1:N (b,b)
1435 -- co2 : R1:N (b,b) ~ Tree (b,b)
1436 -- co : N [(b,b)] ~ Tree (b,b)
1438 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1439 genDerivBinds loc fix_env clas tycon
1440 | className clas `elem` typeableClassNames
1441 = (gen_Typeable_binds loc tycon, [])
1444 = case assocMaybe gen_list (getUnique clas) of
1445 Just gen_fn -> gen_fn loc tycon
1446 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1448 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1449 gen_list = [(eqClassKey, gen_Eq_binds)
1450 ,(ordClassKey, gen_Ord_binds)
1451 ,(enumClassKey, gen_Enum_binds)
1452 ,(boundedClassKey, gen_Bounded_binds)
1453 ,(ixClassKey, gen_Ix_binds)
1454 ,(showClassKey, gen_Show_binds fix_env)
1455 ,(readClassKey, gen_Read_binds fix_env)
1456 ,(dataClassKey, gen_Data_binds)
1457 ,(functorClassKey, gen_Functor_binds)
1458 ,(foldableClassKey, gen_Foldable_binds)
1459 ,(traversableClassKey, gen_Traversable_binds)
1464 %************************************************************************
1466 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1468 %************************************************************************
1471 derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
1472 derivingKindErr tc cls cls_tys cls_kind
1473 = hang (ptext (sLit "Cannot derive well-kinded instance of form")
1474 <+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
1475 2 (ptext (sLit "Class") <+> quotes (ppr cls)
1476 <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
1478 derivingEtaErr :: Class -> [Type] -> Type -> Message
1479 derivingEtaErr cls cls_tys inst_ty
1480 = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
1481 nest 2 (ptext (sLit "instance (...) =>")
1482 <+> pprClassPred cls (cls_tys ++ [inst_ty]))]
1484 typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
1485 typeFamilyPapErr tc cls cls_tys inst_ty
1486 = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
1487 2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)
1489 derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
1490 derivingThingErr newtype_deriving clas tys ty why
1491 = sep [(hang (ptext (sLit "Can't make a derived instance of"))
1492 2 (quotes (ppr pred))
1493 $$ nest 2 extra) <> colon,
1496 extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
1498 pred = mkClassPred clas (tys ++ [ty])
1500 derivingHiddenErr :: TyCon -> SDoc
1501 derivingHiddenErr tc
1502 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1503 2 (ptext (sLit "so you cannot derive an instance for it"))
1505 standaloneCtxt :: LHsType Name -> SDoc
1506 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1509 derivInstCtxt :: Class -> [Type] -> Message
1510 derivInstCtxt clas inst_tys
1511 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1513 badDerivedPred :: PredType -> Message
1515 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1516 ptext (sLit "type variables that are not data type parameters"),
1517 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]