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 Name], -- 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 <- mapM (genInst overlap_flag) given_specs
278 ; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
279 inferInstanceContexts overlap_flag infer_specs
281 ; insts2 <- mapM (genInst overlap_flag) final_specs
283 ; is_boot <- tcIsHsBoot
284 -- Generate the generic to/from functions from each type declaration
285 ; gen_binds <- mkGenericBinds is_boot
286 ; (inst_info, rn_binds) <- renameDeriv is_boot gen_binds (insts1 ++ insts2)
289 ; liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
290 (ddump_deriving inst_info rn_binds))
292 ; return (inst_info, rn_binds) }
294 ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
295 ddump_deriving inst_infos extra_binds
296 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
298 renameDeriv :: Bool -> LHsBinds RdrName
299 -> [(InstInfo RdrName, DerivAuxBinds)]
300 -> TcM ([InstInfo Name], HsValBinds Name)
301 renameDeriv is_boot gen_binds insts
302 | is_boot -- If we are compiling a hs-boot file, don't generate any derived bindings
303 -- The inst-info bindings will all be empty, but it's easier to
304 -- just use rn_inst_info to change the type appropriately
305 = do { rn_inst_infos <- mapM rn_inst_info inst_infos
306 ; return (rn_inst_infos, emptyValBindsOut) }
309 = discardWarnings $ -- Discard warnings about unused bindings etc
310 do { (rn_gen, dus_gen) <- setOptM Opt_PatternSignatures $ -- Type signatures in patterns
311 -- are used in the generic binds
312 rnTopBinds (ValBindsIn gen_binds [])
313 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to be kept alive
315 -- Generate and rename any extra not-one-inst-decl-specific binds,
316 -- notably "con2tag" and/or "tag2con" functions.
317 -- Bring those names into scope before renaming the instances themselves
318 ; loc <- getSrcSpanM -- Generic loc for shared bindings
319 ; let aux_binds = listToBag $ map (genAuxBind loc) $
320 rm_dups [] $ concat deriv_aux_binds
321 ; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv (ValBindsIn aux_binds [])
322 ; let aux_names = map unLoc (collectHsValBinders rn_aux_lhs)
324 ; bindLocalNames aux_names $
325 do { (rn_aux, _dus) <- rnTopBindsRHS aux_names rn_aux_lhs
326 ; rn_inst_infos <- mapM rn_inst_info inst_infos
327 ; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen) } }
330 (inst_infos, deriv_aux_binds) = unzip insts
332 -- Remove duplicate requests for auxilliary bindings
334 rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
335 | otherwise = rm_dups (b:acc) bs
338 rn_inst_info (InstInfo { iSpec = inst, iBinds = NewTypeDerived })
339 = return (InstInfo { iSpec = inst, iBinds = NewTypeDerived })
341 rn_inst_info (InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs })
342 = -- Bring the right type variables into
343 -- scope (yuk), and rename the method binds
345 bindLocalNames (map Var.varName tyvars) $
346 do { (rn_binds, _fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
347 ; return (InstInfo { iSpec = inst, iBinds = VanillaInst rn_binds [] }) }
349 (tyvars,_,clas,_) = instanceHead inst
350 clas_nm = className clas
352 -----------------------------------------
353 mkGenericBinds :: Bool -> TcM (LHsBinds RdrName)
354 mkGenericBinds is_boot
358 = do { gbl_env <- getGblEnv
359 ; let tcs = typeEnvTyCons (tcg_type_env gbl_env)
360 ; return (unionManyBags [ mkTyConGenericBinds tc |
361 tc <- tcs, tyConHasGenerics tc ]) }
362 -- We are only interested in the data type declarations,
363 -- and then only in the ones whose 'has-generics' flag is on
364 -- The predicate tyConHasGenerics finds both of these
368 %************************************************************************
370 From HsSyn to DerivSpec
372 %************************************************************************
374 @makeDerivSpecs@ fishes around to find the info about needed derived
375 instances. Complicating factors:
378 We can only derive @Enum@ if the data type is an enumeration
379 type (all nullary data constructors).
382 We can only derive @Ix@ if the data type is an enumeration {\em
383 or} has just one data constructor (e.g., tuples).
386 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
390 makeDerivSpecs :: [LTyClDecl Name]
393 -> TcM [EarlyDerivSpec]
395 makeDerivSpecs tycl_decls inst_decls deriv_decls
396 = do { eqns1 <- mapAndRecoverM deriveTyData $
397 extractTyDataPreds tycl_decls ++
398 [ pd -- traverse assoc data families
399 | L _ (InstDecl _ _ _ ats) <- inst_decls
400 , pd <- extractTyDataPreds ats ]
401 ; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
402 ; return (catMaybes (eqns1 ++ eqns2)) }
404 extractTyDataPreds decls =
405 [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
408 ------------------------------------------------------------------
409 deriveStandalone :: LDerivDecl Name -> TcM (Maybe EarlyDerivSpec)
410 -- Standalone deriving declarations
411 -- e.g. deriving instance show a => Show (T a)
412 -- Rather like tcLocalInstDecl
413 deriveStandalone (L loc (DerivDecl deriv_ty))
415 addErrCtxt (standaloneCtxt deriv_ty) $
416 do { traceTc (text "standalone deriving decl for" <+> ppr deriv_ty)
417 ; (tvs, theta, tau) <- tcHsInstHead deriv_ty
418 ; traceTc (text "standalone deriving;"
419 <+> text "tvs:" <+> ppr tvs
420 <+> text "theta:" <+> ppr theta
421 <+> text "tau:" <+> ppr tau)
422 ; (cls, inst_tys) <- checkValidInstHead tau
423 ; checkValidInstance tvs theta cls inst_tys
424 -- C.f. TcInstDcls.tcLocalInstDecl1
426 ; let cls_tys = take (length inst_tys - 1) inst_tys
427 inst_ty = last inst_tys
428 ; traceTc (text "standalone deriving;"
429 <+> text "class:" <+> ppr cls
430 <+> text "class types:" <+> ppr cls_tys
431 <+> text "type:" <+> ppr inst_ty)
432 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
435 ------------------------------------------------------------------
436 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM (Maybe EarlyDerivSpec)
437 deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
438 tcdTyVars = tv_names,
439 tcdTyPats = ty_pats }))
440 = setSrcSpan loc $ -- Use the location of the 'deriving' item
442 do { let hs_ty_args = ty_pats `orElse` map (nlHsTyVar . hsLTyVarName) tv_names
443 hs_app = nlHsTyConApp tycon_name hs_ty_args
444 -- We get kinding info for the tyvars by typechecking (T a b)
445 -- Hence forming a tycon application and then dis-assembling it
446 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
447 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
448 -- the type variables for the type constructor
449 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
450 -- The "deriv_pred" is a LHsType to take account of the fact that for
451 -- newtype deriving we allow deriving (forall a. C [a]).
452 ; mkEqnHelp DerivOrigin (tvs++deriv_tvs) cls cls_tys tc_app Nothing } }
455 = panic "derivTyData" -- Caller ensures that only TyData can happen
457 ------------------------------------------------------------------
458 mkEqnHelp :: InstOrigin -> [TyVar] -> Class -> [Type] -> Type
459 -> Maybe ThetaType -- Just => context supplied (standalone deriving)
460 -- Nothing => context inferred (deriving on data decl)
461 -> TcRn (Maybe EarlyDerivSpec)
462 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
463 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
464 , isAlgTyCon tycon -- Check for functions, primitive types etc
465 = do { (rep_tc, rep_tc_args) <- tcLookupFamInstExact tycon tc_args
466 -- Be careful to test rep_tc here: in the case of families,
467 -- we want to check the instance tycon, not the family tycon
469 -- For standalone deriving (mtheta /= Nothing),
470 -- check that all the data constructors are in scope
471 -- By this time we know that the thing is algebraic
472 -- because we've called checkInstHead in derivingStandalone
473 ; rdr_env <- getGlobalRdrEnv
474 ; let hidden_data_cons = isAbstractTyCon rep_tc || any not_in_scope (tyConDataCons rep_tc)
475 not_in_scope dc = null (lookupGRE_Name rdr_env (dataConName dc))
476 ; checkTc (isNothing mtheta || not hidden_data_cons)
477 (derivingHiddenErr tycon)
479 ; mayDeriveDataTypeable <- doptM Opt_DeriveDataTypeable
480 ; newtype_deriving <- doptM Opt_GeneralizedNewtypeDeriving
482 ; if isDataTyCon rep_tc then
483 mkDataTypeEqn orig mayDeriveDataTypeable tvs cls cls_tys
484 tycon tc_args rep_tc rep_tc_args mtheta
486 mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving
488 tycon tc_args rep_tc rep_tc_args mtheta }
490 = baleOut (derivingThingErr cls cls_tys tc_app
491 (ptext (sLit "The last argument of the instance must be a data or newtype application")))
493 baleOut :: Message -> TcM (Maybe a)
494 baleOut err = do { addErrTc err; return Nothing }
497 Note [Looking up family instances for deriving]
498 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
499 tcLookupFamInstExact is an auxiliary lookup wrapper which requires
500 that looked-up family instances exist. If called with a vanilla
501 tycon, the old type application is simply returned.
504 data instance F () = ... deriving Eq
505 data instance F () = ... deriving Eq
506 then tcLookupFamInstExact will be confused by the two matches;
507 but that can't happen because tcInstDecls1 doesn't call tcDeriving
508 if there are any overlaps.
510 There are two other things that might go wrong with the lookup.
511 First, we might see a standalone deriving clause
513 when there is no data instance F () in scope.
515 Note that it's OK to have
516 data instance F [a] = ...
517 deriving Eq (F [(a,b)])
518 where the match is not exact; the same holds for ordinary data types
519 with standalone deriving declrations.
522 tcLookupFamInstExact :: TyCon -> [Type] -> TcM (TyCon, [Type])
523 tcLookupFamInstExact tycon tys
524 | not (isOpenTyCon tycon)
525 = return (tycon, tys)
527 = do { maybeFamInst <- tcLookupFamInst tycon tys
528 ; case maybeFamInst of
529 Nothing -> famInstNotFound tycon tys
530 Just famInst -> return famInst
533 famInstNotFound :: TyCon -> [Type] -> TcM a
534 famInstNotFound tycon tys
535 = failWithTc (ptext (sLit "No family instance for")
536 <+> quotes (pprTypeApp tycon (ppr tycon) tys))
540 %************************************************************************
544 %************************************************************************
547 mkDataTypeEqn :: InstOrigin -> Bool -> [Var] -> Class -> [Type]
548 -> TyCon -> [Type] -> TyCon -> [Type] -> Maybe ThetaType
549 -> TcRn (Maybe EarlyDerivSpec) -- Return 'Nothing' if error
551 mkDataTypeEqn orig mayDeriveDataTypeable tvs cls cls_tys
552 tycon tc_args rep_tc rep_tc_args mtheta
553 | Just err <- checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tc
554 -- NB: pass the *representation* tycon to checkSideConditions
555 = baleOut (derivingThingErr cls cls_tys (mkTyConApp tycon tc_args) err)
558 = ASSERT( null cls_tys )
559 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
561 mk_data_eqn, mk_typeable_eqn
562 :: InstOrigin -> [TyVar] -> Class
563 -> TyCon -> [TcType] -> TyCon -> [TcType] -> Maybe ThetaType
564 -> TcM (Maybe EarlyDerivSpec)
565 mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
566 | getName cls `elem` typeableClassNames
567 = mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
570 = do { dfun_name <- new_dfun_name cls tycon
572 ; let ordinary_constraints
573 = [ mkClassPred cls [arg_ty]
574 | data_con <- tyConDataCons rep_tc,
575 arg_ty <- ASSERT( isVanillaDataCon data_con )
576 dataConInstOrigArgTys data_con rep_tc_args,
577 not (isUnLiftedType arg_ty) ] -- No constraints for unlifted types?
579 -- See Note [Superclasses of derived instance]
580 sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
582 inst_tys = [mkTyConApp tycon tc_args]
584 stupid_subst = zipTopTvSubst (tyConTyVars rep_tc) rep_tc_args
585 stupid_constraints = substTheta stupid_subst (tyConStupidTheta rep_tc)
586 all_constraints = stupid_constraints ++ sc_constraints ++ ordinary_constraints
588 spec = DS { ds_loc = loc, ds_orig = orig
589 , ds_name = dfun_name, ds_tvs = tvs
590 , ds_cls = cls, ds_tys = inst_tys
591 , ds_theta = mtheta `orElse` all_constraints
592 , ds_newtype = False }
594 ; return (if isJust mtheta then Just (Right spec) -- Specified context
595 else Just (Left spec)) } -- Infer context
597 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc _rep_tc_args mtheta
598 -- The Typeable class is special in several ways
599 -- data T a b = ... deriving( Typeable )
601 -- instance Typeable2 T where ...
603 -- 1. There are no constraints in the instance
604 -- 2. There are no type variables either
605 -- 3. The actual class we want to generate isn't necessarily
606 -- Typeable; it depends on the arity of the type
607 | isNothing mtheta -- deriving on a data type decl
608 = do { checkTc (cls `hasKey` typeableClassKey)
609 (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
610 ; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
611 ; mk_typeable_eqn orig tvs real_cls tycon [] rep_tc [] (Just []) }
613 | otherwise -- standaone deriving
614 = do { checkTc (null tc_args)
615 (ptext (sLit "Derived typeable instance must be of form (Typeable")
616 <> int (tyConArity tycon) <+> ppr tycon <> rparen)
617 ; dfun_name <- new_dfun_name cls tycon
619 ; return (Just $ Right $
620 DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
621 , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
622 , ds_theta = mtheta `orElse` [], ds_newtype = False }) }
624 ------------------------------------------------------------------
625 -- Check side conditions that dis-allow derivability for particular classes
626 -- This is *apart* from the newtype-deriving mechanism
628 -- Here we get the representation tycon in case of family instances as it has
629 -- the data constructors - but we need to be careful to fall back to the
630 -- family tycon (with indexes) in error messages.
632 checkSideConditions :: Bool -> Class -> [TcType] -> TyCon -> Maybe SDoc
633 checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tc
635 = Just ty_args_why -- e.g. deriving( Foo s )
637 = case sideConditions cls of
638 Just cond -> cond (mayDeriveDataTypeable, rep_tc)
639 Nothing -> Just non_std_why
641 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
642 non_std_why = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
644 sideConditions :: Class -> Maybe Condition
646 | cls_key == eqClassKey = Just cond_std
647 | cls_key == ordClassKey = Just cond_std
648 | cls_key == readClassKey = Just cond_std
649 | cls_key == showClassKey = Just cond_std
650 | cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
651 | cls_key == ixClassKey = Just (cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct))
652 | cls_key == boundedClassKey = Just (cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct))
653 | cls_key == dataClassKey = Just (cond_mayDeriveDataTypeable `andCond` cond_std)
654 | getName cls `elem` typeableClassNames = Just (cond_mayDeriveDataTypeable `andCond` cond_typeableOK)
655 | otherwise = Nothing
657 cls_key = getUnique cls
659 type Condition = (Bool, TyCon) -> Maybe SDoc
660 -- Bool is whether or not we are allowed to derive Data and Typeable
661 -- TyCon is the *representation* tycon if the
662 -- data type is an indexed one
665 orCond :: Condition -> Condition -> Condition
668 Nothing -> Nothing -- c1 succeeds
669 Just x -> case c2 tc of -- c1 fails
671 Just y -> Just (x $$ ptext (sLit " and") $$ y)
674 andCond :: Condition -> Condition -> Condition
675 andCond c1 c2 tc = case c1 tc of
676 Nothing -> c2 tc -- c1 succeeds
677 Just x -> Just x -- c1 fails
679 cond_std :: Condition
681 | any (not . isVanillaDataCon) data_cons = Just existential_why
682 | null data_cons = Just no_cons_why
683 | otherwise = Nothing
685 data_cons = tyConDataCons rep_tc
686 no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
687 ptext (sLit "has no data constructors")
688 existential_why = quotes (pprSourceTyCon rep_tc) <+>
689 ptext (sLit "has non-Haskell-98 constructor(s)")
691 cond_isEnumeration :: Condition
692 cond_isEnumeration (_, rep_tc)
693 | isEnumerationTyCon rep_tc = Nothing
694 | otherwise = Just why
696 why = quotes (pprSourceTyCon rep_tc) <+>
697 ptext (sLit "has non-nullary constructors")
699 cond_isProduct :: Condition
700 cond_isProduct (_, rep_tc)
701 | isProductTyCon rep_tc = Nothing
702 | otherwise = Just why
704 why = quotes (pprSourceTyCon rep_tc) <+>
705 ptext (sLit "has more than one constructor")
707 cond_typeableOK :: Condition
708 -- OK for Typeable class
709 -- Currently: (a) args all of kind *
710 -- (b) 7 or fewer args
711 cond_typeableOK (_, rep_tc)
712 | tyConArity rep_tc > 7 = Just too_many
713 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
715 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
716 | otherwise = Nothing
718 too_many = quotes (pprSourceTyCon rep_tc) <+>
719 ptext (sLit "has too many arguments")
720 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
721 ptext (sLit "has arguments of kind other than `*'")
722 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
723 ptext (sLit "is a type family")
725 cond_mayDeriveDataTypeable :: Condition
726 cond_mayDeriveDataTypeable (mayDeriveDataTypeable, _)
727 | mayDeriveDataTypeable = Nothing
728 | otherwise = Just why
730 why = ptext (sLit "You need -XDeriveDataTypeable to derive an instance for this class")
732 std_class_via_iso :: Class -> Bool
733 std_class_via_iso clas -- These standard classes can be derived for a newtype
734 -- using the isomorphism trick *even if no -fglasgow-exts*
735 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
736 -- Not Read/Show because they respect the type
737 -- Not Enum, because newtypes are never in Enum
740 new_dfun_name :: Class -> TyCon -> TcM Name
741 new_dfun_name clas tycon -- Just a simple wrapper
742 = do { loc <- getSrcSpanM -- The location of the instance decl, not of the tycon
743 ; newDFunName clas [mkTyConApp tycon []] loc }
744 -- The type passed to newDFunName is only used to generate
745 -- a suitable string; hence the empty type arg list
748 Note [Superclasses of derived instance]
749 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
750 In general, a derived instance decl needs the superclasses of the derived
751 class too. So if we have
752 data T a = ...deriving( Ord )
753 then the initial context for Ord (T a) should include Eq (T a). Often this is
754 redundant; we'll also generate an Ord constraint for each constructor argument,
755 and that will probably generate enough constraints to make the Eq (T a) constraint
756 be satisfied too. But not always; consider:
762 data T a = MkT (S a) deriving( Ord )
763 instance Num a => Eq (T a)
765 The derived instance for (Ord (T a)) must have a (Num a) constraint!
767 data T a = MkT deriving( Data, Typeable )
768 Here there *is* no argument field, but we must nevertheless generate
769 a context for the Data instances:
770 instance Typable a => Data (T a) where ...
773 %************************************************************************
777 %************************************************************************
780 mkNewTypeEqn :: InstOrigin -> Bool -> Bool -> [Var] -> Class
781 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
783 -> TcRn (Maybe EarlyDerivSpec)
784 mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving tvs
785 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
786 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
787 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
788 ; dfun_name <- new_dfun_name cls tycon
790 ; let spec = DS { ds_loc = loc, ds_orig = orig
791 , ds_name = dfun_name, ds_tvs = dict_tvs
792 , ds_cls = cls, ds_tys = inst_tys
793 , ds_theta = mtheta `orElse` all_preds
794 , ds_newtype = True }
795 ; return (if isJust mtheta then Just (Right spec)
796 else Just (Left spec)) }
798 | isNothing mb_std_err -- Use the standard H98 method
799 = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
801 -- Otherwise we can't derive
802 | newtype_deriving = baleOut cant_derive_err -- Too hard
803 | otherwise = baleOut std_err -- Just complain about being a non-std instance
805 mb_std_err = checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tycon
806 std_err = derivingThingErr cls cls_tys tc_app $
807 vcat [fromJust mb_std_err,
808 ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")]
810 -- Here is the plan for newtype derivings. We see
811 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
812 -- where t is a type,
813 -- ak+1...an is a suffix of a1..an, and are all tyars
814 -- ak+1...an do not occur free in t, nor in the s1..sm
815 -- (C s1 ... sm) is a *partial applications* of class C
816 -- with the last parameter missing
817 -- (T a1 .. ak) matches the kind of C's last argument
818 -- (and hence so does t)
820 -- We generate the instance
821 -- instance forall ({a1..ak} u fvs(s1..sm)).
822 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
823 -- where T a1...ap is the partial application of
824 -- the LHS of the correct kind and p >= k
826 -- NB: the variables below are:
827 -- tc_tvs = [a1, ..., an]
828 -- tyvars_to_keep = [a1, ..., ak]
829 -- rep_ty = t ak .. an
830 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
831 -- tys = [s1, ..., sm]
834 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
835 -- We generate the instance
836 -- instance Monad (ST s) => Monad (T s) where
838 cls_tyvars = classTyVars cls
839 kind = tyVarKind (last cls_tyvars)
840 -- Kind of the thing we want to instance
841 -- e.g. argument kind of Monad, *->*
843 (arg_kinds, _) = splitKindFunTys kind
844 n_args_to_drop = length arg_kinds
845 -- Want to drop 1 arg from (T s a) and (ST s a)
846 -- to get instance Monad (ST s) => Monad (T s)
848 -- Note [Newtype representation]
849 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
850 -- Need newTyConRhs (*not* a recursive representation finder)
851 -- to get the representation type. For example
852 -- newtype B = MkB Int
853 -- newtype A = MkA B deriving( Num )
854 -- We want the Num instance of B, *not* the Num instance of Int,
855 -- when making the Num instance of A!
856 rep_ty = newTyConInstRhs rep_tycon rep_tc_args
857 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
859 n_tyargs_to_keep = tyConArity tycon - n_args_to_drop
860 dropped_tc_args = drop n_tyargs_to_keep tc_args
861 dropped_tvs = tyVarsOfTypes dropped_tc_args
863 n_args_to_keep = length rep_ty_args - n_args_to_drop
864 args_to_drop = drop n_args_to_keep rep_ty_args
865 args_to_keep = take n_args_to_keep rep_ty_args
867 rep_fn' = mkAppTys rep_fn args_to_keep
868 rep_tys = cls_tys ++ [rep_fn']
869 rep_pred = mkClassPred cls rep_tys
870 -- rep_pred is the representation dictionary, from where
871 -- we are gong to get all the methods for the newtype
874 tc_app = mkTyConApp tycon (take n_tyargs_to_keep tc_args)
876 -- Next we figure out what superclass dictionaries to use
877 -- See Note [Newtype deriving superclasses] above
879 inst_tys = cls_tys ++ [tc_app]
880 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
883 -- If there are no tyvars, there's no need
884 -- to abstract over the dictionaries we need
885 -- Example: newtype T = MkT Int deriving( C )
886 -- We get the derived instance
889 -- instance C Int => C T
890 dict_tvs = filterOut (`elemVarSet` dropped_tvs) tvs
891 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
893 -------------------------------------------------------------------
894 -- Figuring out whether we can only do this newtype-deriving thing
896 right_arity = length cls_tys + 1 == classArity cls
898 -- Never derive Read,Show,Typeable,Data this way
899 non_iso_class cls = className cls `elem` ([readClassName, showClassName, dataClassName] ++
901 can_derive_via_isomorphism
902 = not (non_iso_class cls)
903 && right_arity -- Well kinded;
904 -- eg not: newtype T ... deriving( ST )
905 -- because ST needs *2* type params
906 && n_tyargs_to_keep >= 0 -- Type constructor has right kind:
907 -- eg not: newtype T = T Int deriving( Monad )
908 && n_args_to_keep >= 0 -- Rep type has right kind:
909 -- eg not: newtype T a = T Int deriving( Monad )
910 && eta_ok -- Eta reduction works
911 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
912 -- newtype A = MkA [A]
914 -- instance Eq [A] => Eq A !!
915 -- Here's a recursive newtype that's actually OK
916 -- newtype S1 = S1 [T1 ()]
917 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
918 -- It's currently rejected. Oh well.
919 -- In fact we generate an instance decl that has method of form
920 -- meth @ instTy = meth @ repTy
921 -- (no coerce's). We'd need a coerce if we wanted to handle
922 -- recursive newtypes too
924 -- Check that eta reduction is OK
925 eta_ok = (args_to_drop `tcEqTypes` dropped_tc_args)
926 -- (a) the dropped-off args are identical in the source and rep type
927 -- newtype T a b = MkT (S [a] b) deriving( Monad )
928 -- Here the 'b' must be the same in the rep type (S [a] b)
930 && (tyVarsOfType rep_fn' `disjointVarSet` dropped_tvs)
931 -- (b) the remaining type args do not mention any of the dropped
934 && (tyVarsOfTypes cls_tys `disjointVarSet` dropped_tvs)
935 -- (c) the type class args do not mention any of the dropped type
938 && all isTyVarTy dropped_tc_args
939 -- (d) in case of newtype family instances, the eta-dropped
940 -- arguments must be type variables (not more complex indexes)
942 cant_derive_err = derivingThingErr cls cls_tys tc_app
943 (vcat [ptext (sLit "even with cunning newtype deriving:"),
944 if isRecursiveTyCon tycon then
945 ptext (sLit "the newtype may be recursive")
947 if not right_arity then
948 quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
950 if not (n_tyargs_to_keep >= 0) then
951 ptext (sLit "the type constructor has wrong kind")
952 else if not (n_args_to_keep >= 0) then
953 ptext (sLit "the representation type has wrong kind")
954 else if not eta_ok then
955 ptext (sLit "the eta-reduction property does not hold")
961 %************************************************************************
963 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
965 %************************************************************************
967 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
968 terms, which is the final correct RHS for the corresponding original
972 Each (k,TyVarTy tv) in a solution constrains only a type
976 The (k,TyVarTy tv) pairs in a solution are canonically
977 ordered by sorting on type varible, tv, (major key) and then class, k,
982 inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]
984 inferInstanceContexts _ [] = return []
986 inferInstanceContexts oflag infer_specs
987 = do { traceTc (text "inferInstanceContexts" <+> vcat (map pprDerivSpec infer_specs))
988 ; iterate_deriv 1 initial_solutions }
990 ------------------------------------------------------------------
991 -- The initial solutions for the equations claim that each
992 -- instance has an empty context; this solution is certainly
993 -- in canonical form.
994 initial_solutions :: [ThetaType]
995 initial_solutions = [ [] | _ <- infer_specs ]
997 ------------------------------------------------------------------
998 -- iterate_deriv calculates the next batch of solutions,
999 -- compares it with the current one; finishes if they are the
1000 -- same, otherwise recurses with the new solutions.
1001 -- It fails if any iteration fails
1002 iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
1003 iterate_deriv n current_solns
1004 | n > 20 -- Looks as if we are in an infinite loop
1005 -- This can happen if we have -XUndecidableInstances
1006 -- (See TcSimplify.tcSimplifyDeriv.)
1007 = pprPanic "solveDerivEqns: probable loop"
1008 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
1010 = do { -- Extend the inst info from the explicit instance decls
1011 -- with the current set of solutions, and simplify each RHS
1012 let inst_specs = zipWithEqual "add_solns" (mkInstance2 oflag)
1013 current_solns infer_specs
1014 ; new_solns <- checkNoErrs $
1015 extendLocalInstEnv inst_specs $
1016 mapM gen_soln infer_specs
1018 ; if (current_solns == new_solns) then
1019 return [ spec { ds_theta = soln }
1020 | (spec, soln) <- zip infer_specs current_solns ]
1022 iterate_deriv (n+1) new_solns }
1024 ------------------------------------------------------------------
1025 gen_soln :: DerivSpec -> TcM [PredType]
1026 gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
1027 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
1029 addErrCtxt (derivInstCtxt clas inst_tys) $
1030 do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
1031 -- checkValidInstance tyvars theta clas inst_tys
1032 -- Not necessary; see Note [Exotic derived instance contexts]
1035 -- Check for a bizarre corner case, when the derived instance decl should
1036 -- have form instance C a b => D (T a) where ...
1037 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
1038 -- of problems; in particular, it's hard to compare solutions for
1039 -- equality when finding the fixpoint. So I just rule it out for now.
1040 ; let tv_set = mkVarSet tyvars
1041 weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
1042 ; mapM_ (addErrTc . badDerivedPred) weird_preds
1044 -- Claim: the result instance declaration is guaranteed valid
1045 -- Hence no need to call:
1046 -- checkValidInstance tyvars theta clas inst_tys
1047 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
1049 ------------------------------------------------------------------
1050 mkInstance1 :: OverlapFlag -> DerivSpec -> Instance
1051 mkInstance1 overlap_flag spec = mkInstance2 overlap_flag (ds_theta spec) spec
1053 mkInstance2 :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
1054 mkInstance2 overlap_flag theta
1055 (DS { ds_name = dfun_name
1056 , ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
1057 = mkLocalInstance dfun overlap_flag
1059 dfun = mkDictFunId dfun_name tyvars theta clas tys
1062 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
1063 -- Add new locally-defined instances; don't bother to check
1064 -- for functional dependency errors -- that'll happen in TcInstDcls
1065 extendLocalInstEnv dfuns thing_inside
1066 = do { env <- getGblEnv
1067 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
1068 env' = env { tcg_inst_env = inst_env' }
1069 ; setGblEnv env' thing_inside }
1073 %************************************************************************
1075 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
1077 %************************************************************************
1079 After all the trouble to figure out the required context for the
1080 derived instance declarations, all that's left is to chug along to
1081 produce them. They will then be shoved into @tcInstDecls2@, which
1082 will do all its usual business.
1084 There are lots of possibilities for code to generate. Here are
1085 various general remarks.
1090 We want derived instances of @Eq@ and @Ord@ (both v common) to be
1091 ``you-couldn't-do-better-by-hand'' efficient.
1094 Deriving @Show@---also pretty common--- should also be reasonable good code.
1097 Deriving for the other classes isn't that common or that big a deal.
1104 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
1107 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
1110 We {\em normally} generate code only for the non-defaulted methods;
1111 there are some exceptions for @Eq@ and (especially) @Ord@...
1114 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
1115 constructor's numeric (@Int#@) tag. These are generated by
1116 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1117 these is around is given by @hasCon2TagFun@.
1119 The examples under the different sections below will make this
1123 Much less often (really just for deriving @Ix@), we use a
1124 @_tag2con_<tycon>@ function. See the examples.
1127 We use the renamer!!! Reason: we're supposed to be
1128 producing @LHsBinds Name@ for the methods, but that means
1129 producing correctly-uniquified code on the fly. This is entirely
1130 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1131 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1132 the renamer. What a great hack!
1136 -- Generate the InstInfo for the required instance paired with the
1137 -- *representation* tycon for that instance,
1138 -- plus any auxiliary bindings required
1140 -- Representation tycons differ from the tycon in the instance signature in
1141 -- case of instances for indexed families.
1143 genInst :: OverlapFlag -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
1146 = return (InstInfo { iSpec = mkInstance1 oflag spec
1147 , iBinds = NewTypeDerived }, [])
1150 = do { let loc = getSrcSpan (ds_name spec)
1151 inst = mkInstance1 oflag spec
1152 (_,_,clas,[ty]) = instanceHead inst
1153 (visible_tycon, tyArgs) = tcSplitTyConApp ty
1155 -- In case of a family instance, we need to use the representation
1156 -- tycon (after all, it has the data constructors)
1157 ; (tycon, _) <- tcLookupFamInstExact visible_tycon tyArgs
1158 ; fix_env <- getFixityEnv
1159 ; let (meth_binds, aux_binds) = genDerivBinds loc fix_env clas tycon
1161 -- Build the InstInfo
1162 ; return (InstInfo { iSpec = inst,
1163 iBinds = VanillaInst meth_binds [] },
1167 genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
1168 genDerivBinds loc fix_env clas tycon
1169 | className clas `elem` typeableClassNames
1170 = (gen_Typeable_binds loc tycon, [])
1173 = case assocMaybe gen_list (getUnique clas) of
1174 Just gen_fn -> gen_fn loc tycon
1175 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1177 gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
1178 gen_list = [(eqClassKey, gen_Eq_binds)
1179 ,(ordClassKey, gen_Ord_binds)
1180 ,(enumClassKey, gen_Enum_binds)
1181 ,(boundedClassKey, gen_Bounded_binds)
1182 ,(ixClassKey, gen_Ix_binds)
1183 ,(showClassKey, gen_Show_binds fix_env)
1184 ,(readClassKey, gen_Read_binds fix_env)
1185 ,(dataClassKey, gen_Data_binds)
1190 %************************************************************************
1192 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1194 %************************************************************************
1197 derivingThingErr :: Class -> [Type] -> Type -> Message -> Message
1198 derivingThingErr clas tys ty why
1199 = sep [hsep [ptext (sLit "Can't make a derived instance of"),
1201 nest 2 (parens why)]
1203 pred = mkClassPred clas (tys ++ [ty])
1205 derivingHiddenErr :: TyCon -> SDoc
1206 derivingHiddenErr tc
1207 = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
1208 2 (ptext (sLit "so you cannot derive an instance for it"))
1210 standaloneCtxt :: LHsType Name -> SDoc
1211 standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
1214 derivInstCtxt :: Class -> [Type] -> Message
1215 derivInstCtxt clas inst_tys
1216 = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1218 badDerivedPred :: PredType -> Message
1220 = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
1221 ptext (sLit "type variables that are not data type parameters"),
1222 nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]