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
53 %************************************************************************
57 %************************************************************************
61 1. Convert the decls (i.e. data/newtype deriving clauses,
62 plus standalone deriving) to [EarlyDerivSpec]
64 2. Infer the missing contexts for the Left DerivSpecs
66 3. Add the derived bindings, generating InstInfos
69 -- DerivSpec is purely local to this module
70 data DerivSpec = DS { ds_loc :: SrcSpan
71 , ds_orig :: InstOrigin
74 , ds_theta :: ThetaType
77 , ds_newtype :: Bool }
78 -- This spec implies a dfun declaration of the form
79 -- df :: forall tvs. theta => C tys
80 -- The Name is the name for the DFun we'll build
81 -- The tyvars bind all the variables in the theta
82 -- For family indexes, the tycon is the *family* tycon
83 -- (not the representation tycon)
85 -- ds_newtype = True <=> Newtype deriving
86 -- False <=> Vanilla deriving
88 type EarlyDerivSpec = Either DerivSpec DerivSpec
89 -- Left ds => the context for the instance should be inferred
90 -- In this case ds_theta is the list of all the
91 -- constraints needed, such as (Eq [a], Eq a)
92 -- The inference process is to reduce this to a
93 -- simpler form (e.g. Eq a)
95 -- Right ds => the exact context for the instance is supplied
96 -- by the programmer; it is ds_theta
98 pprDerivSpec :: DerivSpec -> SDoc
99 pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
100 ds_cls = c, ds_tys = tys, ds_theta = rhs })
101 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
102 <+> equals <+> ppr rhs)
106 Inferring missing contexts
107 ~~~~~~~~~~~~~~~~~~~~~~~~~~
110 data T a b = C1 (Foo a) (Bar b)
115 [NOTE: See end of these comments for what to do with
116 data (C a, D b) => T a b = ...
119 We want to come up with an instance declaration of the form
121 instance (Ping a, Pong b, ...) => Eq (T a b) where
124 It is pretty easy, albeit tedious, to fill in the code "...". The
125 trick is to figure out what the context for the instance decl is,
126 namely @Ping@, @Pong@ and friends.
128 Let's call the context reqd for the T instance of class C at types
129 (a,b, ...) C (T a b). Thus:
131 Eq (T a b) = (Ping a, Pong b, ...)
133 Now we can get a (recursive) equation from the @data@ decl:
135 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
136 u Eq (T b a) u Eq Int -- From C2
137 u Eq (T a a) -- From C3
139 Foo and Bar may have explicit instances for @Eq@, in which case we can
140 just substitute for them. Alternatively, either or both may have
141 their @Eq@ instances given by @deriving@ clauses, in which case they
142 form part of the system of equations.
144 Now all we need do is simplify and solve the equations, iterating to
145 find the least fixpoint. Notice that the order of the arguments can
146 switch around, as here in the recursive calls to T.
148 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
152 Eq (T a b) = {} -- The empty set
155 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
156 u Eq (T b a) u Eq Int -- From C2
157 u Eq (T a a) -- From C3
159 After simplification:
160 = Eq a u Ping b u {} u {} u {}
165 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
166 u Eq (T b a) u Eq Int -- From C2
167 u Eq (T a a) -- From C3
169 After simplification:
174 = Eq a u Ping b u Eq b u Ping a
176 The next iteration gives the same result, so this is the fixpoint. We
177 need to make a canonical form of the RHS to ensure convergence. We do
178 this by simplifying the RHS to a form in which
180 - the classes constrain only tyvars
181 - the list is sorted by tyvar (major key) and then class (minor key)
182 - no duplicates, of course
184 So, here are the synonyms for the ``equation'' structures:
187 Note [Data decl contexts]
188 ~~~~~~~~~~~~~~~~~~~~~~~~~
191 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
193 We will need an instance decl like:
195 instance (Read a, RealFloat a) => Read (Complex a) where
198 The RealFloat in the context is because the read method for Complex is bound
199 to construct a Complex, and doing that requires that the argument type is
202 But this ain't true for Show, Eq, Ord, etc, since they don't construct
203 a Complex; they only take them apart.
205 Our approach: identify the offending classes, and add the data type
206 context to the instance decl. The "offending classes" are
210 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
211 pattern matching against a constructor from a data type with a context
212 gives rise to the constraints for that context -- or at least the thinned
213 version. So now all classes are "offending".
215 Note [Newtype deriving]
216 ~~~~~~~~~~~~~~~~~~~~~~~
220 newtype T = T Char deriving( C [a] )
222 Notice the free 'a' in the deriving. We have to fill this out to
223 newtype T = T Char deriving( forall a. C [a] )
225 And then translate it to:
226 instance C [a] Char => C [a] T where ...
229 Note [Newtype deriving superclasses]
230 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
231 (See also Trac #1220 for an interesting exchange on newtype
232 deriving and superclasses.)
234 The 'tys' here come from the partial application in the deriving
235 clause. The last arg is the new instance type.
237 We must pass the superclasses; the newtype might be an instance
238 of them in a different way than the representation type
239 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
240 Then the Show instance is not done via isomorphism; it shows
242 The Num instance is derived via isomorphism, but the Show superclass
243 dictionary must the Show instance for Foo, *not* the Show dictionary
244 gotten from the Num dictionary. So we must build a whole new dictionary
245 not just use the Num one. The instance we want is something like:
246 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
249 There may be a coercion needed which we get from the tycon for the newtype
250 when the dict is constructed in TcInstDcls.tcInstDecl2
255 %************************************************************************
257 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
259 %************************************************************************
262 tcDeriving :: [LTyClDecl Name] -- All type constructors
263 -> [LInstDecl Name] -- All instance declarations
264 -> [LDerivDecl Name] -- All stand-alone deriving declarations
265 -> TcM ([InstInfo], -- The generated "instance decls"
266 HsValBinds Name) -- Extra generated top-level bindings
268 tcDeriving tycl_decls inst_decls deriv_decls
269 = recoverM (return ([], emptyValBindsOut)) $
270 do { -- Fish the "deriving"-related information out of the TcEnv
271 -- And make the necessary "equations".
272 ; early_specs <- makeDerivSpecs tycl_decls inst_decls deriv_decls
274 ; overlap_flag <- getOverlapFlag
275 ; let (infer_specs, given_specs) = splitEithers early_specs
276 ; (insts1, aux_binds1) <- mapAndUnzipM (genInst overlap_flag) given_specs
278 ; final_specs <- extendLocalInstEnv (map iSpec insts1) $
279 inferInstanceContexts overlap_flag infer_specs
281 ; (insts2, aux_binds2) <- mapAndUnzipM (genInst overlap_flag) final_specs
283 ; is_boot <- tcIsHsBoot
284 ; rn_binds <- makeAuxBinds is_boot tycl_decls
285 (concat aux_binds1 ++ concat aux_binds2)
287 ; let inst_info = insts1 ++ insts2
290 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
291 (ddump_deriving inst_info rn_binds))
293 ; return (inst_info, rn_binds) }
295 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
296 ddump_deriving inst_infos extra_binds
297 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
299 makeAuxBinds :: Bool -> [LTyClDecl Name] -> DerivAuxBinds -> TcM (HsValBinds Name)
300 makeAuxBinds is_boot tycl_decls deriv_aux_binds
301 | is_boot -- If we are compiling a hs-boot file,
302 -- don't generate any derived bindings
303 = return emptyValBindsOut
306 = do { let aux_binds = listToBag (map genAuxBind (rm_dups [] deriv_aux_binds))
307 -- Generate any extra not-one-inst-decl-specific binds,
308 -- notably "con2tag" and/or "tag2con" functions.
310 -- Generate the generic to/from functions from each type declaration
311 ; gen_binds <- mkGenericBinds tycl_decls
313 -- Rename these extra bindings, discarding warnings about unused bindings etc
314 -- Type signatures in patterns are used in the generic binds
316 setOptM Opt_PatternSignatures $
317 do { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn aux_binds [])
318 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
319 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
321 ; return (rn_deriv `plusHsValBinds` rn_gen) } }
323 -- Remove duplicate requests for auxilliary bindings
325 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
326 | otherwise = rm_dups (b:acc) bs
328 -----------------------------------------
329 mkGenericBinds :: [LTyClDecl Name] -> TcM (LHsBinds RdrName)
330 mkGenericBinds tycl_decls
331 = do { tcs <- mapM tcLookupTyCon
333 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
334 -- We are only interested in the data type declarations
335 ; return (unionManyBags [ mkTyConGenericBinds tc |
336 tc <- tcs, tyConHasGenerics tc ]) }
337 -- And then only in the ones whose 'has-generics' flag is on
341 %************************************************************************
343 From HsSyn to DerivSpec
345 %************************************************************************
347 @makeDerivSpecs@ fishes around to find the info about needed derived
348 instances. Complicating factors:
351 We can only derive @Enum@ if the data type is an enumeration
352 type (all nullary data constructors).
355 We can only derive @Ix@ if the data type is an enumeration {\em
356 or} has just one data constructor (e.g., tuples).
359 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
363 makeDerivSpecs :: [LTyClDecl Name]
366 -> TcM [EarlyDerivSpec]
368 makeDerivSpecs tycl_decls inst_decls deriv_decls
369 = do { eqns1 <- mapAndRecoverM deriveTyData $
370 extractTyDataPreds tycl_decls ++
371 [ pd -- traverse assoc data families
372 | L _ (InstDecl _ _ _ ats) <- inst_decls
373 , pd <- extractTyDataPreds ats ]
374 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
375 ; return (catMaybes (eqns1 ++ eqns2)) }
377 extractTyDataPreds decls =
378 [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
381 ------------------------------------------------------------------
382 deriveStandalone :: LDerivDecl Name -> TcM (Maybe EarlyDerivSpec)
383 -- Standalone deriving declarations
384 -- e.g. deriving instance show a => Show (T a)
385 -- Rather like tcLocalInstDecl
386 deriveStandalone (L loc (DerivDecl deriv_ty))
388 addErrCtxt (standaloneCtxt deriv_ty) $
389 do { traceTc (text "standalone deriving decl for" <+> ppr deriv_ty)
390 ; (tvs, theta, tau) <- tcHsInstHead deriv_ty
391 ; traceTc (text "standalone deriving;"
392 <+> text "tvs:" <+> ppr tvs
393 <+> text "theta:" <+> ppr theta
394 <+> text "tau:" <+> ppr tau)
395 ; (cls, inst_tys) <- checkValidInstHead tau
396 ; let cls_tys = take (length inst_tys - 1) inst_tys
397 inst_ty = last inst_tys
399 ; traceTc (text "standalone deriving;"
400 <+> text "class:" <+> ppr cls
401 <+> text "class types:" <+> ppr cls_tys
402 <+> text "type:" <+> ppr inst_ty)
403 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
406 ------------------------------------------------------------------
407 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM (Maybe EarlyDerivSpec)
408 deriveTyData (deriv_pred, L loc decl@(TyData { tcdLName = L _ tycon_name,
409 tcdTyVars = tv_names,
410 tcdTyPats = ty_pats }))
413 do { let hs_ty_args = ty_pats `orElse` map (nlHsTyVar . hsLTyVarName) tv_names
414 hs_app = nlHsTyConApp tycon_name hs_ty_args
415 -- We get kinding info for the tyvars by typechecking (T a b)
416 -- Hence forming a tycon application and then dis-assembling it
417 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
418 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
419 -- the type variables for the type constructor
420 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
421 -- The "deriv_pred" is a LHsType to take account of the fact that for
422 -- newtype deriving we allow deriving (forall a. C [a]).
423 ; mkEqnHelp DerivOrigin (tvs++deriv_tvs) cls cls_tys tc_app Nothing } }
426 = panic "derivTyData" -- Caller ensures that only TyData can happen
428 ------------------------------------------------------------------
429 mkEqnHelp :: InstOrigin -> [TyVar] -> Class -> [Type] -> Type
430 -> Maybe ThetaType -- Just => context supplied (standalone deriving)
431 -- Nothing => context inferred (deriving on data decl)
432 -> TcRn (Maybe EarlyDerivSpec)
433 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
434 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
436 -- For standalone deriving (mtheta /= Nothing),
437 -- check that all the data constructors are in scope
438 -- By this time we know that the thing is algebraic
439 -- because we've called checkInstHead in derivingStandalone
440 rdr_env <- getGlobalRdrEnv
441 ; let hidden_data_cons = filter not_in_scope (tyConDataCons tycon)
442 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
443 ; checkTc (isNothing mtheta || null hidden_data_cons)
444 (derivingHiddenErr tycon)
446 ; mayDeriveDataTypeable <- doptM Opt_DeriveDataTypeable
447 ; newtype_deriving <- doptM Opt_GeneralizedNewtypeDeriving
449 ; (rep_tc, rep_tc_args) <- tcLookupFamInstExact tycon tc_args
451 -- Be careful to test rep_tc here: in the case of families, we want
452 -- to check the instance tycon, not the family tycon
453 ; if isDataTyCon rep_tc then
454 mkDataTypeEqn orig mayDeriveDataTypeable tvs cls cls_tys
455 tycon tc_args rep_tc rep_tc_args mtheta
457 mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving
459 tycon tc_args rep_tc rep_tc_args mtheta }
461 = baleOut (derivingThingErr cls cls_tys tc_app
462 (ptext (sLit "Last argument of the instance must be a type application")))
464 baleOut :: Message -> TcM (Maybe a)
465 baleOut err = do { addErrTc err; return Nothing }
468 Note [Looking up family instances for deriving]
469 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
470 tcLookupFamInstExact is an auxiliary lookup wrapper which requires
471 that looked-up family instances exist. If called with a vanilla
472 tycon, the old type application is simply returned.
475 data instance F () = ... deriving Eq
476 data instance F () = ... deriving Eq
477 then tcLookupFamInstExact will be confused by the two matches;
478 but that can't happen because tcInstDecls1 doesn't call tcDeriving
479 if there are any overlaps.
481 There are two other things that might go wrong with the lookup.
482 First, we might see a standalone deriving clause
484 when there is no data instance F () in scope.
486 Note that it's OK to have
487 data instance F [a] = ...
488 deriving Eq (F [(a,b)])
489 where the match is not exact; the same holds for ordinary data types
490 with standalone deriving declrations.
493 tcLookupFamInstExact :: TyCon -> [Type] -> TcM (TyCon, [Type])
494 tcLookupFamInstExact tycon tys
495 | not (isOpenTyCon tycon)
496 = return (tycon, tys)
498 = do { maybeFamInst <- tcLookupFamInst tycon tys
499 ; case maybeFamInst of
500 Nothing -> famInstNotFound tycon tys
501 Just famInst -> return famInst
504 famInstNotFound :: TyCon -> [Type] -> TcM a
505 famInstNotFound tycon tys
506 = failWithTc (ptext (sLit "No family instance for")
507 <+> quotes (pprTypeApp tycon (ppr tycon) tys))
511 %************************************************************************
515 %************************************************************************
518 mkDataTypeEqn :: InstOrigin -> Bool -> [Var] -> Class -> [Type]
519 -> TyCon -> [Type] -> TyCon -> [Type] -> Maybe ThetaType
520 -> TcRn (Maybe EarlyDerivSpec) -- Return 'Nothing' if error
522 mkDataTypeEqn orig mayDeriveDataTypeable tvs cls cls_tys
523 tycon tc_args rep_tc rep_tc_args mtheta
524 | Just err <- checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tc
525 -- NB: pass the *representation* tycon to checkSideConditions
526 = baleOut (derivingThingErr cls cls_tys (mkTyConApp tycon tc_args) err)
529 = ASSERT( null cls_tys )
530 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
532 mk_data_eqn, mk_typeable_eqn
533 :: InstOrigin -> [TyVar] -> Class
534 -> TyCon -> [TcType] -> TyCon -> [TcType] -> Maybe ThetaType
535 -> TcM (Maybe EarlyDerivSpec)
536 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
537 | getName cls `elem` typeableClassNames
538 = mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
541 = do { dfun_name <- new_dfun_name cls tycon
543 ; let ordinary_constraints
544 = [ mkClassPred cls [arg_ty]
545 | data_con <- tyConDataCons rep_tc,
546 arg_ty <- ASSERT( isVanillaDataCon data_con )
547 dataConInstOrigArgTys data_con rep_tc_args,
548 not (isUnLiftedType arg_ty) ] -- No constraints for unlifted types?
550 -- See Note [Superclasses of derived instance]
551 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
553 inst_tys = [mkTyConApp tycon tc_args]
555 stupid_subst = zipTopTvSubst (tyConTyVars rep_tc) rep_tc_args
556 stupid_constraints = substTheta stupid_subst (tyConStupidTheta rep_tc)
557 all_constraints = stupid_constraints ++ sc_constraints ++ ordinary_constraints
559 spec = DS { ds_loc = loc, ds_orig = orig
560 , ds_name = dfun_name, ds_tvs = tvs
561 , ds_cls = cls, ds_tys = inst_tys
562 , ds_theta = mtheta `orElse` all_constraints
563 , ds_newtype = False }
565 ; return (if isJust mtheta then Just (Right spec) -- Specified context
566 else Just (Left spec)) } -- Infer context
568 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc _rep_tc_args mtheta
569 -- The Typeable class is special in several ways
570 -- data T a b = ... deriving( Typeable )
572 -- instance Typeable2 T where ...
574 -- 1. There are no constraints in the instance
575 -- 2. There are no type variables either
576 -- 3. The actual class we want to generate isn't necessarily
577 -- Typeable; it depends on the arity of the type
578 | isNothing mtheta -- deriving on a data type decl
579 = do { checkTc (cls `hasKey` typeableClassKey)
580 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
581 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
582 ; mk_typeable_eqn orig tvs real_cls tycon [] rep_tc [] (Just []) }
584 | otherwise -- standaone deriving
585 = do { checkTc (null tc_args)
586 (ptext (sLit "Derived typeable instance must be of form (Typeable")
587 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
588 ; dfun_name <- new_dfun_name cls tycon
590 ; return (Just $ Right $
591 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
592 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
593 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
595 ------------------------------------------------------------------
596 -- Check side conditions that dis-allow derivability for particular classes
597 -- This is *apart* from the newtype-deriving mechanism
599 -- Here we get the representation tycon in case of family instances as it has
600 -- the data constructors - but we need to be careful to fall back to the
601 -- family tycon (with indexes) in error messages.
603 checkSideConditions :: Bool -> Class -> [TcType] -> TyCon -> Maybe SDoc
604 checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tc
606 = Just ty_args_why -- e.g. deriving( Foo s )
608 = case sideConditions cls of
609 Just cond -> cond (mayDeriveDataTypeable, rep_tc)
610 Nothing -> Just non_std_why
612 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
613 non_std_why = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
615 sideConditions :: Class -> Maybe Condition
617 | cls_key == eqClassKey = Just cond_std
618 | cls_key == ordClassKey = Just cond_std
619 | cls_key == readClassKey = Just cond_std
620 | cls_key == showClassKey = Just cond_std
621 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
622 | cls_key == ixClassKey = Just (cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct))
623 | cls_key == boundedClassKey = Just (cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct))
624 | cls_key == dataClassKey = Just (cond_mayDeriveDataTypeable `andCond` cond_std)
625 | getName cls `elem` typeableClassNames = Just (cond_mayDeriveDataTypeable `andCond` cond_typeableOK)
626 | otherwise = Nothing
628 cls_key = getUnique cls
630 type Condition = (Bool, TyCon) -> Maybe SDoc
631 -- Bool is whether or not we are allowed to derive Data and Typeable
632 -- TyCon is the *representation* tycon if the
633 -- data type is an indexed one
636 orCond :: Condition -> Condition -> Condition
639 Nothing -> Nothing -- c1 succeeds
640 Just x -> case c2 tc of -- c1 fails
642 Just y -> Just (x $$ ptext (sLit " and") $$ y)
645 andCond :: Condition -> Condition -> Condition
646 andCond c1 c2 tc = case c1 tc of
647 Nothing -> c2 tc -- c1 succeeds
648 Just x -> Just x -- c1 fails
650 cond_std :: Condition
652 | any (not . isVanillaDataCon) data_cons = Just existential_why
653 | null data_cons = Just no_cons_why
654 | otherwise = Nothing
656 data_cons = tyConDataCons rep_tc
657 no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
658 ptext (sLit "has no data constructors")
659 existential_why = quotes (pprSourceTyCon rep_tc) <+>
660 ptext (sLit "has non-Haskell-98 constructor(s)")
662 cond_isEnumeration :: Condition
663 cond_isEnumeration (_, rep_tc)
664 | isEnumerationTyCon rep_tc = Nothing
665 | otherwise = Just why
667 why = quotes (pprSourceTyCon rep_tc) <+>
668 ptext (sLit "has non-nullary constructors")
670 cond_isProduct :: Condition
671 cond_isProduct (_, rep_tc)
672 | isProductTyCon rep_tc = Nothing
673 | otherwise = Just why
675 why = quotes (pprSourceTyCon rep_tc) <+>
676 ptext (sLit "has more than one constructor")
678 cond_typeableOK :: Condition
679 -- OK for Typeable class
680 -- Currently: (a) args all of kind *
681 -- (b) 7 or fewer args
682 cond_typeableOK (_, rep_tc)
683 | tyConArity rep_tc > 7 = Just too_many
684 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
686 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
687 | otherwise = Nothing
689 too_many = quotes (pprSourceTyCon rep_tc) <+>
690 ptext (sLit "has too many arguments")
691 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
692 ptext (sLit "has arguments of kind other than `*'")
693 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
694 ptext (sLit "is a type family")
696 cond_mayDeriveDataTypeable :: Condition
697 cond_mayDeriveDataTypeable (mayDeriveDataTypeable, _)
698 | mayDeriveDataTypeable = Nothing
699 | otherwise = Just why
701 why = ptext (sLit "You need -XDeriveDataTypeable to derive an instance for this class")
703 std_class_via_iso :: Class -> Bool
704 std_class_via_iso clas -- These standard classes can be derived for a newtype
705 -- using the isomorphism trick *even if no -fglasgow-exts*
706 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
707 -- Not Read/Show because they respect the type
708 -- Not Enum, because newtypes are never in Enum
711 new_dfun_name :: Class -> TyCon -> TcM Name
712 new_dfun_name clas tycon -- Just a simple wrapper
713 = newDFunName clas [mkTyConApp tycon []] (getSrcSpan tycon)
714 -- The type passed to newDFunName is only used to generate
715 -- a suitable string; hence the empty type arg list
718 Note [Superclasses of derived instance]
719 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
720 In general, a derived instance decl needs the superclasses of the derived
721 class too. So if we have
722 data T a = ...deriving( Ord )
723 then the initial context for Ord (T a) should include Eq (T a). Often this is
724 redundant; we'll also generate an Ord constraint for each constructor argument,
725 and that will probably generate enough constraints to make the Eq (T a) constraint
726 be satisfied too. But not always; consider:
732 data T a = MkT (S a) deriving( Ord )
733 instance Num a => Eq (T a)
735 The derived instance for (Ord (T a)) must have a (Num a) constraint!
737 data T a = MkT deriving( Data, Typeable )
738 Here there *is* no argument field, but we must nevertheless generate
739 a context for the Data instances:
740 instance Typable a => Data (T a) where ...
743 %************************************************************************
747 %************************************************************************
750 mkNewTypeEqn :: InstOrigin -> Bool -> Bool -> [Var] -> Class
751 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
753 -> TcRn (Maybe EarlyDerivSpec)
754 mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving tvs
755 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
756 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
757 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
758 ; dfun_name <- new_dfun_name cls tycon
760 ; let spec = DS { ds_loc = loc, ds_orig = orig
761 , ds_name = dfun_name, ds_tvs = dict_tvs
762 , ds_cls = cls, ds_tys = inst_tys
763 , ds_theta = mtheta `orElse` all_preds
764 , ds_newtype = True }
765 ; return (if isJust mtheta then Just (Right spec)
766 else Just (Left spec)) }
768 | isNothing mb_std_err -- Use the standard H98 method
769 = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
771 -- Otherwise we can't derive
772 | newtype_deriving = baleOut cant_derive_err -- Too hard
773 | otherwise = baleOut std_err -- Just complain about being a non-std instance
775 mb_std_err = checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tycon
776 std_err = derivingThingErr cls cls_tys tc_app $
777 vcat [fromJust mb_std_err,
778 ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")]
780 -- Here is the plan for newtype derivings. We see
781 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
782 -- where t is a type,
783 -- ak+1...an is a suffix of a1..an, and are all tyars
784 -- ak+1...an do not occur free in t, nor in the s1..sm
785 -- (C s1 ... sm) is a *partial applications* of class C
786 -- with the last parameter missing
787 -- (T a1 .. ak) matches the kind of C's last argument
788 -- (and hence so does t)
790 -- We generate the instance
791 -- instance forall ({a1..ak} u fvs(s1..sm)).
792 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
793 -- where T a1...ap is the partial application of
794 -- the LHS of the correct kind and p >= k
796 -- NB: the variables below are:
797 -- tc_tvs = [a1, ..., an]
798 -- tyvars_to_keep = [a1, ..., ak]
799 -- rep_ty = t ak .. an
800 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
801 -- tys = [s1, ..., sm]
804 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
805 -- We generate the instance
806 -- instance Monad (ST s) => Monad (T s) where
808 cls_tyvars = classTyVars cls
809 kind = tyVarKind (last cls_tyvars)
810 -- Kind of the thing we want to instance
811 -- e.g. argument kind of Monad, *->*
813 (arg_kinds, _) = splitKindFunTys kind
814 n_args_to_drop = length arg_kinds
815 -- Want to drop 1 arg from (T s a) and (ST s a)
816 -- to get instance Monad (ST s) => Monad (T s)
818 -- Note [Newtype representation]
819 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
820 -- Need newTyConRhs (*not* a recursive representation finder)
821 -- to get the representation type. For example
822 -- newtype B = MkB Int
823 -- newtype A = MkA B deriving( Num )
824 -- We want the Num instance of B, *not* the Num instance of Int,
825 -- when making the Num instance of A!
826 rep_ty = newTyConInstRhs rep_tycon rep_tc_args
827 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
829 n_tyargs_to_keep = tyConArity tycon - n_args_to_drop
830 dropped_tc_args = drop n_tyargs_to_keep tc_args
831 dropped_tvs = tyVarsOfTypes dropped_tc_args
833 n_args_to_keep = length rep_ty_args - n_args_to_drop
834 args_to_drop = drop n_args_to_keep rep_ty_args
835 args_to_keep = take n_args_to_keep rep_ty_args
837 rep_fn' = mkAppTys rep_fn args_to_keep
838 rep_tys = cls_tys ++ [rep_fn']
839 rep_pred = mkClassPred cls rep_tys
840 -- rep_pred is the representation dictionary, from where
841 -- we are gong to get all the methods for the newtype
844 tc_app = mkTyConApp tycon (take n_tyargs_to_keep tc_args)
846 -- Next we figure out what superclass dictionaries to use
847 -- See Note [Newtype deriving superclasses] above
849 inst_tys = cls_tys ++ [tc_app]
850 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
853 -- If there are no tyvars, there's no need
854 -- to abstract over the dictionaries we need
855 -- Example: newtype T = MkT Int deriving( C )
856 -- We get the derived instance
859 -- instance C Int => C T
860 dict_tvs = filterOut (`elemVarSet` dropped_tvs) tvs
861 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
863 -------------------------------------------------------------------
864 -- Figuring out whether we can only do this newtype-deriving thing
866 right_arity = length cls_tys + 1 == classArity cls
868 -- Never derive Read,Show,Typeable,Data this way
869 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
870 can_derive_via_isomorphism
871 = not (getUnique cls `elem` non_iso_classes)
872 && right_arity -- Well kinded;
873 -- eg not: newtype T ... deriving( ST )
874 -- because ST needs *2* type params
875 && n_tyargs_to_keep >= 0 -- Type constructor has right kind:
876 -- eg not: newtype T = T Int deriving( Monad )
877 && n_args_to_keep >= 0 -- Rep type has right kind:
878 -- eg not: newtype T a = T Int deriving( Monad )
879 && eta_ok -- Eta reduction works
880 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
881 -- newtype A = MkA [A]
883 -- instance Eq [A] => Eq A !!
884 -- Here's a recursive newtype that's actually OK
885 -- newtype S1 = S1 [T1 ()]
886 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
887 -- It's currently rejected. Oh well.
888 -- In fact we generate an instance decl that has method of form
889 -- meth @ instTy = meth @ repTy
890 -- (no coerce's). We'd need a coerce if we wanted to handle
891 -- recursive newtypes too
893 -- Check that eta reduction is OK
894 eta_ok = (args_to_drop `tcEqTypes` dropped_tc_args)
895 -- (a) the dropped-off args are identical in the source and rep type
896 -- newtype T a b = MkT (S [a] b) deriving( Monad )
897 -- Here the 'b' must be the same in the rep type (S [a] b)
899 && (tyVarsOfType rep_fn' `disjointVarSet` dropped_tvs)
900 -- (b) the remaining type args do not mention any of the dropped
903 && (tyVarsOfTypes cls_tys `disjointVarSet` dropped_tvs)
904 -- (c) the type class args do not mention any of the dropped type
907 && all isTyVarTy dropped_tc_args
908 -- (d) in case of newtype family instances, the eta-dropped
909 -- arguments must be type variables (not more complex indexes)
911 cant_derive_err = derivingThingErr cls cls_tys tc_app
912 (vcat [ptext (sLit "even with cunning newtype deriving:"),
913 if isRecursiveTyCon tycon then
914 ptext (sLit "the newtype may be recursive")
916 if not right_arity then
917 quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
919 if not (n_tyargs_to_keep >= 0) then
920 ptext (sLit "the type constructor has wrong kind")
921 else if not (n_args_to_keep >= 0) then
922 ptext (sLit "the representation type has wrong kind")
923 else if not eta_ok then
924 ptext (sLit "the eta-reduction property does not hold")
930 %************************************************************************
932 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
934 %************************************************************************
936 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
937 terms, which is the final correct RHS for the corresponding original
941 Each (k,TyVarTy tv) in a solution constrains only a type
945 The (k,TyVarTy tv) pairs in a solution are canonically
946 ordered by sorting on type varible, tv, (major key) and then class, k,
951 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
953 inferInstanceContexts _ [] = return []
955 inferInstanceContexts oflag infer_specs
956 = do { traceTc (text "inferInstanceContexts" <+> vcat (map pprDerivSpec infer_specs))
957 ; iterate_deriv 1 initial_solutions }
959 ------------------------------------------------------------------
960 -- The initial solutions for the equations claim that each
961 -- instance has an empty context; this solution is certainly
962 -- in canonical form.
963 initial_solutions :: [ThetaType]
964 initial_solutions = [ [] | _ <- infer_specs ]
966 ------------------------------------------------------------------
967 -- iterate_deriv calculates the next batch of solutions,
968 -- compares it with the current one; finishes if they are the
969 -- same, otherwise recurses with the new solutions.
970 -- It fails if any iteration fails
971 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
972 iterate_deriv n current_solns
973 | n > 20 -- Looks as if we are in an infinite loop
974 -- This can happen if we have -XUndecidableInstances
975 -- (See TcSimplify.tcSimplifyDeriv.)
976 = pprPanic "solveDerivEqns: probable loop"
977 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
979 = do { -- Extend the inst info from the explicit instance decls
980 -- with the current set of solutions, and simplify each RHS
981 let inst_specs = zipWithEqual "add_solns" (mkInstance2 oflag)
982 current_solns infer_specs
983 ; new_solns <- checkNoErrs $
984 extendLocalInstEnv inst_specs $
985 mapM gen_soln infer_specs
987 ; if (current_solns == new_solns) then
988 return [ spec { ds_theta = soln }
989 | (spec, soln) <- zip infer_specs current_solns ]
991 iterate_deriv (n+1) new_solns }
993 ------------------------------------------------------------------
994 gen_soln :: DerivSpec -> TcM [PredType]
995 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
996 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
998 addErrCtxt (derivInstCtxt clas inst_tys) $
999 do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
1000 -- checkValidInstance tyvars theta clas inst_tys
1001 -- Not necessary; see Note [Exotic derived instance contexts]
1004 -- Check for a bizarre corner case, when the derived instance decl should
1005 -- have form instance C a b => D (T a) where ...
1006 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1007 -- of problems; in particular, it's hard to compare solutions for
1008 -- equality when finding the fixpoint. So I just rule it out for now.
1009 ; let tv_set = mkVarSet tyvars
1010 weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
1011 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1013 -- Claim: the result instance declaration is guaranteed valid
1014 -- Hence no need to call:
1015 -- checkValidInstance tyvars theta clas inst_tys
1016 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1018 ------------------------------------------------------------------
1019 mkInstance1 :: OverlapFlag -> DerivSpec -> Instance
1020 mkInstance1 overlap_flag spec = mkInstance2 overlap_flag (ds_theta spec) spec
1022 mkInstance2 :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1023 mkInstance2 overlap_flag theta
1024 (DS { ds_name = dfun_name
1025 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1026 = mkLocalInstance dfun overlap_flag
1028 dfun = mkDictFunId dfun_name tyvars theta clas tys
1031 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1032 -- Add new locally-defined instances; don't bother to check
1033 -- for functional dependency errors -- that'll happen in TcInstDcls
1034 extendLocalInstEnv dfuns thing_inside
1035 = do { env <- getGblEnv
1036 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1037 env' = env { tcg_inst_env = inst_env' }
1038 ; setGblEnv env' thing_inside }
1042 %************************************************************************
1044 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1046 %************************************************************************
1048 After all the trouble to figure out the required context for the
1049 derived instance declarations, all that's left is to chug along to
1050 produce them. They will then be shoved into @tcInstDecls2@, which
1051 will do all its usual business.
1053 There are lots of possibilities for code to generate. Here are
1054 various general remarks.
1059 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1060 ``you-couldn't-do-better-by-hand'' efficient.
1063 Deriving @Show@---also pretty common--- should also be reasonable good code.
1066 Deriving for the other classes isn't that common or that big a deal.
1073 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1076 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1079 We {\em normally} generate code only for the non-defaulted methods;
1080 there are some exceptions for @Eq@ and (especially) @Ord@...
1083 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1084 constructor's numeric (@Int#@) tag. These are generated by
1085 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1086 these is around is given by @hasCon2TagFun@.
1088 The examples under the different sections below will make this
1092 Much less often (really just for deriving @Ix@), we use a
1093 @_tag2con_<tycon>@ function. See the examples.
1096 We use the renamer!!! Reason: we're supposed to be
1097 producing @LHsBinds Name@ for the methods, but that means
1098 producing correctly-uniquified code on the fly. This is entirely
1099 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1100 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1101 the renamer. What a great hack!
1105 -- Generate the InstInfo for the required instance paired with the
1106 -- *representation* tycon for that instance,
1107 -- plus any auxiliary bindings required
1109 -- Representation tycons differ from the tycon in the instance signature in
1110 -- case of instances for indexed families.
1112 genInst :: OverlapFlag -> DerivSpec -> TcM (InstInfo, DerivAuxBinds)
1115 = return (InstInfo { iSpec = mkInstance1 oflag spec
1116 , iBinds = NewTypeDerived }, [])
1119 = do { fix_env <- getFixityEnv
1121 inst = mkInstance1 oflag spec
1122 (tyvars,_,clas,[ty]) = instanceHead inst
1123 clas_nm = className clas
1124 (visible_tycon, tyArgs) = tcSplitTyConApp ty
1126 -- In case of a family instance, we need to use the representation
1127 -- tycon (after all, it has the data constructors)
1128 ; (tycon, _) <- tcLookupFamInstExact visible_tycon tyArgs
1129 ; let (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
1131 -- Bring the right type variables into
1132 -- scope, and rename the method binds
1133 -- It's a bit yukky that we return *renamed* InstInfo, but
1134 -- *non-renamed* auxiliary bindings
1135 ; (rn_meth_binds, _fvs) <- discardWarnings $
1136 bindLocalNames (map Var.varName tyvars) $
1137 rnMethodBinds clas_nm (\_ -> []) [] meth_binds
1139 -- Build the InstInfo
1140 ; return (InstInfo { iSpec = inst,
1141 iBinds = VanillaInst rn_meth_binds [] },
1145 genDerivBinds :: Class -> FixityEnv -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1146 genDerivBinds clas fix_env tycon
1147 | className clas `elem` typeableClassNames
1148 = (gen_Typeable_binds tycon, [])
1151 = case assocMaybe gen_list (getUnique clas) of
1152 Just gen_fn -> gen_fn tycon
1153 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1155 gen_list :: [(Unique, TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1156 gen_list = [(eqClassKey, gen_Eq_binds)
1157 ,(ordClassKey, gen_Ord_binds)
1158 ,(enumClassKey, gen_Enum_binds)
1159 ,(boundedClassKey, gen_Bounded_binds)
1160 ,(ixClassKey, gen_Ix_binds)
1161 ,(showClassKey, gen_Show_binds fix_env)
1162 ,(readClassKey, gen_Read_binds fix_env)
1163 ,(dataClassKey, gen_Data_binds fix_env)
1168 %************************************************************************
1170 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1172 %************************************************************************
1175 derivingThingErr :: Class -> [Type] -> Type -> Message -> Message
1176 derivingThingErr clas tys ty why
1177 = sep [hsep [ptext (sLit "Can't make a derived instance of"),
1179 nest 2 (parens why)]
1181 pred = mkClassPred clas (tys ++ [ty])
1183 derivingHiddenErr :: TyCon -> SDoc
1184 derivingHiddenErr tc
1185 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1186 2 (ptext (sLit "so you cannot derive an instance for it"))
1188 standaloneCtxt :: LHsType Name -> SDoc
1189 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1192 derivInstCtxt :: Class -> [Type] -> Message
1193 derivInstCtxt clas inst_tys
1194 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1196 badDerivedPred :: PredType -> Message
1198 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1199 ptext (sLit "type variables that are not data type parameters"),
1200 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]