2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Handles @deriving@ clauses on @data@ declarations.
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/CodingStyle#Warnings
16 module TcDeriv ( tcDeriving ) where
18 #include "HsVersions.h"
26 import TcClassDcl( tcAddDeclCtxt ) -- Small helper
27 import TcGenDeriv -- Deriv stuff
59 %************************************************************************
61 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
63 %************************************************************************
67 data T a b = C1 (Foo a) (Bar b)
72 [NOTE: See end of these comments for what to do with
73 data (C a, D b) => T a b = ...
76 We want to come up with an instance declaration of the form
78 instance (Ping a, Pong b, ...) => Eq (T a b) where
81 It is pretty easy, albeit tedious, to fill in the code "...". The
82 trick is to figure out what the context for the instance decl is,
83 namely @Ping@, @Pong@ and friends.
85 Let's call the context reqd for the T instance of class C at types
86 (a,b, ...) C (T a b). Thus:
88 Eq (T a b) = (Ping a, Pong b, ...)
90 Now we can get a (recursive) equation from the @data@ decl:
92 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
93 u Eq (T b a) u Eq Int -- From C2
94 u Eq (T a a) -- From C3
96 Foo and Bar may have explicit instances for @Eq@, in which case we can
97 just substitute for them. Alternatively, either or both may have
98 their @Eq@ instances given by @deriving@ clauses, in which case they
99 form part of the system of equations.
101 Now all we need do is simplify and solve the equations, iterating to
102 find the least fixpoint. Notice that the order of the arguments can
103 switch around, as here in the recursive calls to T.
105 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
109 Eq (T a b) = {} -- The empty set
112 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
113 u Eq (T b a) u Eq Int -- From C2
114 u Eq (T a a) -- From C3
116 After simplification:
117 = Eq a u Ping b u {} u {} u {}
122 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
123 u Eq (T b a) u Eq Int -- From C2
124 u Eq (T a a) -- From C3
126 After simplification:
131 = Eq a u Ping b u Eq b u Ping a
133 The next iteration gives the same result, so this is the fixpoint. We
134 need to make a canonical form of the RHS to ensure convergence. We do
135 this by simplifying the RHS to a form in which
137 - the classes constrain only tyvars
138 - the list is sorted by tyvar (major key) and then class (minor key)
139 - no duplicates, of course
141 So, here are the synonyms for the ``equation'' structures:
144 type DerivRhs = ThetaType
145 type DerivSoln = DerivRhs
146 type DerivEqn = (SrcSpan, InstOrigin, Name, [TyVar], Class, Type, DerivRhs)
147 -- (span, orig, df, tvs, C, ty, rhs)
148 -- implies a dfun declaration of the form
149 -- df :: forall tvs. rhs => C ty
150 -- The Name is the name for the DFun we'll build
151 -- The tyvars bind all the variables in the RHS
152 -- For family indexes, the tycon is the *family* tycon
153 -- (not the representation tycon)
155 pprDerivEqn :: DerivEqn -> SDoc
156 pprDerivEqn (l, _, n, tvs, c, ty, rhs)
157 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr ty]
158 <+> equals <+> ppr rhs)
162 [Data decl contexts] A note about contexts on data decls
163 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
166 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
168 We will need an instance decl like:
170 instance (Read a, RealFloat a) => Read (Complex a) where
173 The RealFloat in the context is because the read method for Complex is bound
174 to construct a Complex, and doing that requires that the argument type is
177 But this ain't true for Show, Eq, Ord, etc, since they don't construct
178 a Complex; they only take them apart.
180 Our approach: identify the offending classes, and add the data type
181 context to the instance decl. The "offending classes" are
185 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
186 pattern matching against a constructor from a data type with a context
187 gives rise to the constraints for that context -- or at least the thinned
188 version. So now all classes are "offending".
195 newtype T = T Char deriving( C [a] )
197 Notice the free 'a' in the deriving. We have to fill this out to
198 newtype T = T Char deriving( forall a. C [a] )
200 And then translate it to:
201 instance C [a] Char => C [a] T where ...
206 %************************************************************************
208 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
210 %************************************************************************
213 tcDeriving :: [LTyClDecl Name] -- All type constructors
214 -> [LInstDecl Name] -- All instance declarations
215 -> [LDerivDecl Name] -- All stand-alone deriving declarations
216 -> TcM ([InstInfo], -- The generated "instance decls"
217 HsValBinds Name) -- Extra generated top-level bindings
219 tcDeriving tycl_decls inst_decls deriv_decls
220 = recoverM (returnM ([], emptyValBindsOut)) $
221 do { -- Fish the "deriving"-related information out of the TcEnv
222 -- and make the necessary "equations".
223 ; (ordinary_eqns, newtype_inst_info)
224 <- makeDerivEqns tycl_decls inst_decls deriv_decls
226 ; (ordinary_inst_info, deriv_binds)
227 <- extendLocalInstEnv (map iSpec newtype_inst_info) $
228 deriveOrdinaryStuff ordinary_eqns
229 -- Add the newtype-derived instances to the inst env
230 -- before tacking the "ordinary" ones
232 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
234 -- If we are compiling a hs-boot file,
235 -- don't generate any derived bindings
236 ; is_boot <- tcIsHsBoot
238 return (inst_info, emptyValBindsOut)
242 -- Generate the generic to/from functions from each type declaration
243 ; gen_binds <- mkGenericBinds tycl_decls
245 -- Rename these extra bindings, discarding warnings about unused bindings etc
246 -- Type signatures in patterns are used in the generic binds
249 setOptM Opt_PatternSignatures $
251 { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn deriv_binds [])
252 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
253 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
255 ; return (rn_deriv `plusHsValBinds` rn_gen) }
259 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
260 (ddump_deriving inst_info rn_binds))
262 ; returnM (inst_info, rn_binds)
265 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
266 ddump_deriving inst_infos extra_binds
267 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
269 -----------------------------------------
270 deriveOrdinaryStuff [] -- Short cut
271 = returnM ([], emptyLHsBinds)
273 deriveOrdinaryStuff eqns
274 = do { -- Take the equation list and solve it, to deliver a list of
275 -- solutions, a.k.a. the contexts for the instance decls
276 -- required for the corresponding equations.
277 overlap_flag <- getOverlapFlag
278 ; inst_specs <- solveDerivEqns overlap_flag eqns
280 -- Generate the InstInfo for each dfun,
281 -- plus any auxiliary bindings it needs
282 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst inst_specs
284 -- Generate any extra not-one-inst-decl-specific binds,
285 -- notably "con2tag" and/or "tag2con" functions.
286 ; extra_binds <- genTaggeryBinds inst_infos
289 ; returnM (map fst inst_infos,
290 unionManyBags (extra_binds : aux_binds_s))
293 -----------------------------------------
294 mkGenericBinds tycl_decls
295 = do { tcs <- mapM tcLookupTyCon
297 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
298 -- We are only interested in the data type declarations
299 ; return (unionManyBags [ mkTyConGenericBinds tc |
300 tc <- tcs, tyConHasGenerics tc ]) }
301 -- And then only in the ones whose 'has-generics' flag is on
305 %************************************************************************
307 \subsection[TcDeriv-eqns]{Forming the equations}
309 %************************************************************************
311 @makeDerivEqns@ fishes around to find the info about needed derived
312 instances. Complicating factors:
315 We can only derive @Enum@ if the data type is an enumeration
316 type (all nullary data constructors).
319 We can only derive @Ix@ if the data type is an enumeration {\em
320 or} has just one data constructor (e.g., tuples).
323 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
326 Note [Newtype deriving superclasses]
327 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
328 The 'tys' here come from the partial application in the deriving
329 clause. The last arg is the new instance type.
331 We must pass the superclasses; the newtype might be an instance
332 of them in a different way than the representation type
333 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
334 Then the Show instance is not done via isomorphism; it shows
336 The Num instance is derived via isomorphism, but the Show superclass
337 dictionary must the Show instance for Foo, *not* the Show dictionary
338 gotten from the Num dictionary. So we must build a whole new dictionary
339 not just use the Num one. The instance we want is something like:
340 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
343 There may be a coercion needed which we get from the tycon for the newtype
344 when the dict is constructed in TcInstDcls.tcInstDecl2
348 makeDerivEqns :: [LTyClDecl Name]
351 -> TcM ([DerivEqn], -- Ordinary derivings
352 [InstInfo]) -- Special newtype derivings
354 makeDerivEqns tycl_decls inst_decls deriv_decls
355 = do { eqns1 <- mapM deriveTyData $
356 extractTyDataPreds tycl_decls ++
357 [ pd -- traverse assoc data families
358 | L _ (InstDecl _ _ _ ats) <- inst_decls
359 , pd <- extractTyDataPreds ats ]
360 ; eqns2 <- mapM deriveStandalone deriv_decls
361 ; return ([eqn | (Just eqn, _) <- eqns1 ++ eqns2],
362 [inst | (_, Just inst) <- eqns1 ++ eqns2]) }
364 extractTyDataPreds decls =
365 [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]
368 ------------------------------------------------------------------
369 deriveStandalone :: LDerivDecl Name -> TcM (Maybe DerivEqn, Maybe InstInfo)
370 -- Standalone deriving declarations
371 -- e.g. deriving instance show a => Show (T a)
372 -- Rather like tcLocalInstDecl
373 deriveStandalone (L loc (DerivDecl deriv_ty))
375 addErrCtxt (standaloneCtxt deriv_ty) $
376 do { traceTc (text "standalone deriving decl for" <+> ppr deriv_ty)
377 ; (tvs, theta, tau) <- tcHsInstHead deriv_ty
378 ; traceTc (text "standalone deriving;"
379 <+> text "tvs:" <+> ppr tvs
380 <+> text "theta:" <+> ppr theta
381 <+> text "tau:" <+> ppr tau)
382 ; (cls, inst_tys) <- checkValidInstHead tau
383 ; let cls_tys = take (length inst_tys - 1) inst_tys
384 inst_ty = last inst_tys
386 ; traceTc (text "standalone deriving;"
387 <+> text "class:" <+> ppr cls
388 <+> text "class types:" <+> ppr cls_tys
389 <+> text "type:" <+> ppr inst_ty)
390 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
393 ------------------------------------------------------------------
394 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
395 deriveTyData (deriv_pred, L loc decl@(TyData { tcdLName = L _ tycon_name,
396 tcdTyVars = tv_names,
397 tcdTyPats = ty_pats }))
400 do { let hs_ty_args = ty_pats `orElse` map (nlHsTyVar . hsLTyVarName) tv_names
401 hs_app = nlHsTyConApp tycon_name hs_ty_args
402 -- We get kinding info for the tyvars by typechecking (T a b)
403 -- Hence forming a tycon application and then dis-assembling it
404 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
405 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
406 -- the type variables for the type constructor
407 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
408 -- The "deriv_pred" is a LHsType to take account of the fact that for
409 -- newtype deriving we allow deriving (forall a. C [a]).
410 ; mkEqnHelp DerivOrigin (tvs++deriv_tvs) cls cls_tys tc_app Nothing } }
411 deriveTyData (deriv_pred, other_decl)
412 = panic "derivTyData" -- Caller ensures that only TyData can happen
414 ------------------------------------------------------------------
415 mkEqnHelp :: InstOrigin -> [TyVar] -> Class -> [Type] -> Type
417 -> TcRn (Maybe DerivEqn, Maybe InstInfo)
418 mkEqnHelp orig tvs cls cls_tys tc_app mtheta
419 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
420 = do { -- Make tc_app saturated, because that's what the
421 -- mkDataTypeEqn things expect
422 -- It might not be saturated in the standalone deriving case
423 -- derive instance Monad (T a)
424 let extra_tvs = dropList tc_args (tyConTyVars tycon)
425 full_tc_args = tc_args ++ mkTyVarTys extra_tvs
426 full_tvs = tvs ++ extra_tvs
428 ; (rep_tc, rep_tc_args) <- tcLookupFamInstExact tycon full_tc_args
430 ; mayDeriveDataTypeable <- doptM Opt_DeriveDataTypeable
431 ; newtype_deriving <- doptM Opt_GeneralizedNewtypeDeriving
432 ; overlap_flag <- getOverlapFlag
434 -- Be careful to test rep_tc here: in the case of families, we want
435 -- to check the instance tycon, not the family tycon
436 ; if isDataTyCon rep_tc then
437 mkDataTypeEqn orig mayDeriveDataTypeable full_tvs cls cls_tys
438 tycon full_tc_args rep_tc rep_tc_args mtheta
440 mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving overlap_flag
442 tycon full_tc_args rep_tc rep_tc_args mtheta }
444 = baleOut (derivingThingErr cls cls_tys tc_app
445 (ptext SLIT("Last argument of the instance must be a type application")))
447 baleOut err = addErrTc err >> returnM (Nothing, Nothing)
450 Auxiliary lookup wrapper which requires that looked up family instances are
451 not type instances. If called with a vanilla tycon, the old type application
455 tcLookupFamInstExact :: TyCon -> [Type] -> TcM (TyCon, [Type])
456 tcLookupFamInstExact tycon tys
457 | not (isOpenTyCon tycon)
458 = return (tycon, tys)
460 = do { maybeFamInst <- tcLookupFamInst tycon tys
461 ; case maybeFamInst of
462 Nothing -> famInstNotFound tycon tys False
463 Just famInst@(_, rep_tys)
464 | not variable_only_subst -> famInstNotFound tycon tys True
465 | otherwise -> return famInst
467 tvs = map (Type.getTyVar
468 "TcDeriv.tcLookupFamInstExact")
470 variable_only_subst = all Type.isTyVarTy rep_tys &&
471 sizeVarSet (mkVarSet tvs) == length tvs
472 -- renaming may have no repetitions
477 %************************************************************************
481 %************************************************************************
484 mkDataTypeEqn :: InstOrigin -> Bool -> [Var] -> Class -> [Type]
485 -> TyCon -> [Type] -> TyCon -> [Type] -> Maybe DerivRhs
486 -> TcRn (Maybe DerivEqn, Maybe InstInfo)
487 mkDataTypeEqn orig mayDeriveDataTypeable tvs cls cls_tys
488 tycon tc_args rep_tc rep_tc_args mtheta
489 | Just err <- checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tc
490 -- NB: pass the *representation* tycon to checkSideConditions
491 = baleOut (derivingThingErr cls cls_tys (mkTyConApp tycon tc_args) err)
494 = ASSERT( null cls_tys )
495 do { loc <- getSrcSpanM
496 ; eqn <- mk_data_eqn loc orig tvs cls tycon tc_args rep_tc
498 ; return (Just eqn, Nothing) }
500 mk_data_eqn :: SrcSpan -> InstOrigin -> [TyVar] -> Class
501 -> TyCon -> [TcType] -> TyCon -> [TcType] -> Maybe DerivRhs
503 mk_data_eqn loc orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
504 | cls `hasKey` typeableClassKey
505 = -- The Typeable class is special in several ways
506 -- data T a b = ... deriving( Typeable )
508 -- instance Typeable2 T where ...
510 -- 1. There are no constraints in the instance
511 -- 2. There are no type variables either
512 -- 3. The actual class we want to generate isn't necessarily
513 -- Typeable; it depends on the arity of the type
514 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
515 ; dfun_name <- new_dfun_name real_clas tycon
516 ; let theta = fromMaybe [] mtheta
517 ; return (loc, orig, dfun_name, [], real_clas, mkTyConApp tycon [], theta)
521 = do { dfun_name <- new_dfun_name cls tycon
522 ; let ordinary_constraints
523 = [ mkClassPred cls [arg_ty]
524 | data_con <- tyConDataCons rep_tc,
525 arg_ty <- ASSERT( isVanillaDataCon data_con )
526 dataConInstOrigArgTys data_con rep_tc_args,
527 not (isUnLiftedType arg_ty) ] -- No constraints for unlifted types?
528 theta = fromMaybe ordinary_constraints mtheta
530 tiresome_subst = zipTopTvSubst (tyConTyVars rep_tc) rep_tc_args
531 stupid_constraints = substTheta tiresome_subst (tyConStupidTheta rep_tc)
532 -- see note [Data decl contexts] above
534 ; return (loc, orig, dfun_name, tvs, cls, mkTyConApp tycon tc_args,
535 stupid_constraints ++ theta)
538 ------------------------------------------------------------------
539 -- Check side conditions that dis-allow derivability for particular classes
540 -- This is *apart* from the newtype-deriving mechanism
542 -- Here we get the representation tycon in case of family instances as it has
543 -- the data constructors - but we need to be careful to fall back to the
544 -- family tycon (with indexes) in error messages.
546 checkSideConditions :: Bool -> Class -> [TcType] -> TyCon -> Maybe SDoc
547 checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tc
549 = Just ty_args_why -- e.g. deriving( Foo s )
551 = case [cond | (key,cond) <- sideConditions, key == getUnique cls] of
552 [] -> Just (non_std_why cls)
553 [cond] -> cond (mayDeriveDataTypeable, rep_tc)
554 other -> pprPanic "checkSideConditions" (ppr cls)
556 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext SLIT("is not a class")
558 non_std_why cls = quotes (ppr cls) <+> ptext SLIT("is not a derivable class")
560 sideConditions :: [(Unique, Condition)]
562 = [ (eqClassKey, cond_std),
563 (ordClassKey, cond_std),
564 (readClassKey, cond_std),
565 (showClassKey, cond_std),
566 (enumClassKey, cond_std `andCond` cond_isEnumeration),
567 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
568 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
569 (typeableClassKey, cond_mayDeriveDataTypeable `andCond` cond_typeableOK),
570 (dataClassKey, cond_mayDeriveDataTypeable `andCond` cond_std)
573 type Condition = (Bool, TyCon) -> Maybe SDoc
574 -- Bool is whether or not we are allowed to derive Data and Typeable
575 -- TyCon is the *representation* tycon if the
576 -- data type is an indexed one
579 orCond :: Condition -> Condition -> Condition
582 Nothing -> Nothing -- c1 succeeds
583 Just x -> case c2 tc of -- c1 fails
585 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
588 andCond c1 c2 tc = case c1 tc of
589 Nothing -> c2 tc -- c1 succeeds
590 Just x -> Just x -- c1 fails
592 cond_std :: Condition
594 | any (not . isVanillaDataCon) data_cons = Just existential_why
595 | null data_cons = Just no_cons_why
596 | otherwise = Nothing
598 data_cons = tyConDataCons rep_tc
599 no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
600 ptext SLIT("has no data constructors")
601 existential_why = quotes (pprSourceTyCon rep_tc) <+>
602 ptext SLIT("has non-Haskell-98 constructor(s)")
604 cond_isEnumeration :: Condition
605 cond_isEnumeration (_, rep_tc)
606 | isEnumerationTyCon rep_tc = Nothing
607 | otherwise = Just why
609 why = quotes (pprSourceTyCon rep_tc) <+>
610 ptext SLIT("has non-nullary constructors")
612 cond_isProduct :: Condition
613 cond_isProduct (_, rep_tc)
614 | isProductTyCon rep_tc = Nothing
615 | otherwise = Just why
617 why = quotes (pprSourceTyCon rep_tc) <+>
618 ptext SLIT("has more than one constructor")
620 cond_typeableOK :: Condition
621 -- OK for Typeable class
622 -- Currently: (a) args all of kind *
623 -- (b) 7 or fewer args
624 cond_typeableOK (_, rep_tc)
625 | tyConArity rep_tc > 7 = Just too_many
626 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
628 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
629 | otherwise = Nothing
631 too_many = quotes (pprSourceTyCon rep_tc) <+>
632 ptext SLIT("has too many arguments")
633 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
634 ptext SLIT("has arguments of kind other than `*'")
635 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
636 ptext SLIT("is a type family")
638 cond_mayDeriveDataTypeable :: Condition
639 cond_mayDeriveDataTypeable (mayDeriveDataTypeable, _)
640 | mayDeriveDataTypeable = Nothing
641 | otherwise = Just why
643 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
645 std_class_via_iso clas -- These standard classes can be derived for a newtype
646 -- using the isomorphism trick *even if no -fglasgow-exts*
647 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
648 -- Not Read/Show because they respect the type
649 -- Not Enum, because newtypes are never in Enum
652 new_dfun_name clas tycon -- Just a simple wrapper
653 = newDFunName clas [mkTyConApp tycon []] (getSrcSpan tycon)
654 -- The type passed to newDFunName is only used to generate
655 -- a suitable string; hence the empty type arg list
659 %************************************************************************
663 %************************************************************************
666 mkNewTypeEqn :: InstOrigin -> Bool -> Bool -> OverlapFlag -> [Var] -> Class
667 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
669 -> TcRn (Maybe DerivEqn, Maybe InstInfo)
670 mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving overlap_flag tvs
671 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
672 | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
673 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
674 ; -- Go ahead and use the isomorphism
675 dfun_name <- new_dfun_name cls tycon
676 ; return (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
677 iBinds = NewTypeDerived ntd_info })) }
679 | isNothing mb_std_err -- Use the standard H98 method
680 = do { loc <- getSrcSpanM
681 ; eqn <- mk_data_eqn loc orig tvs cls tycon tc_args rep_tycon
683 ; return (Just eqn, Nothing) }
685 -- Otherwise we can't derive
686 | newtype_deriving = baleOut cant_derive_err -- Too hard
687 | otherwise = baleOut std_err -- Just complain about being a non-std instance
689 mb_std_err = checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tycon
690 std_err = derivingThingErr cls cls_tys tc_app $
691 vcat [fromJust mb_std_err,
692 ptext SLIT("Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")]
694 -- Here is the plan for newtype derivings. We see
695 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
696 -- where t is a type,
697 -- ak+1...an is a suffix of a1..an, and are all tyars
698 -- ak+1...an do not occur free in t, nor in the s1..sm
699 -- (C s1 ... sm) is a *partial applications* of class C
700 -- with the last parameter missing
701 -- (T a1 .. ak) matches the kind of C's last argument
702 -- (and hence so does t)
704 -- We generate the instance
705 -- instance forall ({a1..ak} u fvs(s1..sm)).
706 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
707 -- where T a1...ap is the partial application of
708 -- the LHS of the correct kind and p >= k
710 -- NB: the variables below are:
711 -- tc_tvs = [a1, ..., an]
712 -- tyvars_to_keep = [a1, ..., ak]
713 -- rep_ty = t ak .. an
714 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
715 -- tys = [s1, ..., sm]
718 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
719 -- We generate the instance
720 -- instance Monad (ST s) => Monad (T s) where
722 cls_tyvars = classTyVars cls
723 kind = tyVarKind (last cls_tyvars)
724 -- Kind of the thing we want to instance
725 -- e.g. argument kind of Monad, *->*
727 (arg_kinds, _) = splitKindFunTys kind
728 n_args_to_drop = length arg_kinds
729 -- Want to drop 1 arg from (T s a) and (ST s a)
730 -- to get instance Monad (ST s) => Monad (T s)
732 -- Note [newtype representation]
733 -- Need newTyConRhs *not* newTyConRep to get the representation
734 -- type, because the latter looks through all intermediate newtypes
736 -- newtype B = MkB Int
737 -- newtype A = MkA B deriving( Num )
738 -- We want the Num instance of B, *not* the Num instance of Int,
739 -- when making the Num instance of A!
740 rep_ty = newTyConInstRhs rep_tycon rep_tc_args
741 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
743 n_tyargs_to_keep = tyConArity tycon - n_args_to_drop
744 dropped_tc_args = drop n_tyargs_to_keep tc_args
745 dropped_tvs = tyVarsOfTypes dropped_tc_args
747 n_args_to_keep = length rep_ty_args - n_args_to_drop
748 args_to_drop = drop n_args_to_keep rep_ty_args
749 args_to_keep = take n_args_to_keep rep_ty_args
751 rep_fn' = mkAppTys rep_fn args_to_keep
752 rep_tys = cls_tys ++ [rep_fn']
753 rep_pred = mkClassPred cls rep_tys
754 -- rep_pred is the representation dictionary, from where
755 -- we are gong to get all the methods for the newtype
758 tc_app = mkTyConApp tycon (take n_tyargs_to_keep tc_args)
760 -- Next we figure out what superclass dictionaries to use
761 -- See Note [Newtype deriving superclasses] above
763 inst_tys = cls_tys ++ [tc_app]
764 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
767 -- If there are no tyvars, there's no need
768 -- to abstract over the dictionaries we need
769 -- Example: newtype T = MkT Int deriving( C )
770 -- We get the derived instance
773 -- instance C Int => C T
774 dict_tvs = filterOut (`elemVarSet` dropped_tvs) tvs
775 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
776 (dict_args, ntd_info) | null dict_tvs = ([], Just all_preds)
777 | otherwise = (all_preds, Nothing)
779 -- Finally! Here's where we build the dictionary Id
780 mk_inst_spec dfun_name = mkLocalInstance dfun overlap_flag
782 dfun = mkDictFunId dfun_name dict_tvs dict_args cls inst_tys
784 -------------------------------------------------------------------
785 -- Figuring out whether we can only do this newtype-deriving thing
787 right_arity = length cls_tys + 1 == classArity cls
789 -- Never derive Read,Show,Typeable,Data this way
790 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
791 can_derive_via_isomorphism
792 = not (getUnique cls `elem` non_iso_classes)
793 && right_arity -- Well kinded;
794 -- eg not: newtype T ... deriving( ST )
795 -- because ST needs *2* type params
796 && n_tyargs_to_keep >= 0 -- Type constructor has right kind:
797 -- eg not: newtype T = T Int deriving( Monad )
798 && n_args_to_keep >= 0 -- Rep type has right kind:
799 -- eg not: newtype T a = T Int deriving( Monad )
800 && eta_ok -- Eta reduction works
801 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
802 -- newtype A = MkA [A]
804 -- instance Eq [A] => Eq A !!
805 -- Here's a recursive newtype that's actually OK
806 -- newtype S1 = S1 [T1 ()]
807 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
808 -- It's currently rejected. Oh well.
809 -- In fact we generate an instance decl that has method of form
810 -- meth @ instTy = meth @ repTy
811 -- (no coerce's). We'd need a coerce if we wanted to handle
812 -- recursive newtypes too
814 -- Check that eta reduction is OK
815 eta_ok = (args_to_drop `tcEqTypes` dropped_tc_args)
816 -- (a) the dropped-off args are identical in the source and rep type
817 -- newtype T a b = MkT (S [a] b) deriving( Monad )
818 -- Here the 'b' must be the same in the rep type (S [a] b)
820 && (tyVarsOfType rep_fn' `disjointVarSet` dropped_tvs)
821 -- (b) the remaining type args do not mention any of the dropped
824 && (tyVarsOfTypes cls_tys `disjointVarSet` dropped_tvs)
825 -- (c) the type class args do not mention any of the dropped type
828 && all isTyVarTy dropped_tc_args
829 -- (d) in case of newtype family instances, the eta-dropped
830 -- arguments must be type variables (not more complex indexes)
832 cant_derive_err = derivingThingErr cls cls_tys tc_app
833 (vcat [ptext SLIT("even with cunning newtype deriving:"),
834 if isRecursiveTyCon tycon then
835 ptext SLIT("the newtype may be recursive")
837 if not right_arity then
838 quotes (ppr (mkClassPred cls cls_tys)) <+> ptext SLIT("does not have arity 1")
840 if not (n_tyargs_to_keep >= 0) then
841 ptext SLIT("the type constructor has wrong kind")
842 else if not (n_args_to_keep >= 0) then
843 ptext SLIT("the representation type has wrong kind")
844 else if not eta_ok then
845 ptext SLIT("the eta-reduction property does not hold")
851 %************************************************************************
853 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
855 %************************************************************************
857 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
858 terms, which is the final correct RHS for the corresponding original
862 Each (k,TyVarTy tv) in a solution constrains only a type
866 The (k,TyVarTy tv) pairs in a solution are canonically
867 ordered by sorting on type varible, tv, (major key) and then class, k,
872 solveDerivEqns :: OverlapFlag
874 -> TcM [Instance]-- Solns in same order as eqns.
875 -- This bunch is Absolutely minimal...
877 solveDerivEqns overlap_flag orig_eqns
878 = do { traceTc (text "solveDerivEqns" <+> vcat (map pprDerivEqn orig_eqns))
879 ; iterateDeriv 1 initial_solutions }
881 -- The initial solutions for the equations claim that each
882 -- instance has an empty context; this solution is certainly
883 -- in canonical form.
884 initial_solutions :: [DerivSoln]
885 initial_solutions = [ [] | _ <- orig_eqns ]
887 ------------------------------------------------------------------
888 -- iterateDeriv calculates the next batch of solutions,
889 -- compares it with the current one; finishes if they are the
890 -- same, otherwise recurses with the new solutions.
891 -- It fails if any iteration fails
892 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
893 iterateDeriv n current_solns
894 | n > 20 -- Looks as if we are in an infinite loop
895 -- This can happen if we have -fallow-undecidable-instances
896 -- (See TcSimplify.tcSimplifyDeriv.)
897 = pprPanic "solveDerivEqns: probable loop"
898 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
901 inst_specs = zipWithEqual "add_solns" mk_inst_spec
902 orig_eqns current_solns
905 -- Extend the inst info from the explicit instance decls
906 -- with the current set of solutions, and simplify each RHS
907 extendLocalInstEnv inst_specs $
908 mappM gen_soln orig_eqns
909 ) `thenM` \ new_solns ->
910 if (current_solns == new_solns) then
913 iterateDeriv (n+1) new_solns
915 ------------------------------------------------------------------
916 gen_soln :: DerivEqn -> TcM [PredType]
917 gen_soln (loc, orig, _, tyvars, clas, inst_ty, deriv_rhs)
919 addErrCtxt (derivInstCtxt clas [inst_ty]) $
920 do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
921 -- checkValidInstance tyvars theta clas [inst_ty]
922 -- Not necessary; see Note [Exotic derived instance contexts]
925 -- Check for a bizarre corner case, when the derived instance decl should
926 -- have form instance C a b => D (T a) where ...
927 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
928 -- of problems; in particular, it's hard to compare solutions for
929 -- equality when finding the fixpoint. So I just rule it out for now.
930 ; let tv_set = mkVarSet tyvars
931 weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
932 ; mapM_ (addErrTc . badDerivedPred) weird_preds
934 -- Claim: the result instance declaration is guaranteed valid
935 -- Hence no need to call:
936 -- checkValidInstance tyvars theta clas inst_tys
937 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
939 ------------------------------------------------------------------
940 mk_inst_spec :: DerivEqn -> DerivSoln -> Instance
941 mk_inst_spec (loc, orig, dfun_name, tyvars, clas, inst_ty, _) theta
942 = mkLocalInstance dfun overlap_flag
944 dfun = mkDictFunId dfun_name tyvars theta clas [inst_ty]
946 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
947 -- Add new locally-defined instances; don't bother to check
948 -- for functional dependency errors -- that'll happen in TcInstDcls
949 extendLocalInstEnv dfuns thing_inside
950 = do { env <- getGblEnv
951 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
952 env' = env { tcg_inst_env = inst_env' }
953 ; setGblEnv env' thing_inside }
957 %************************************************************************
959 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
961 %************************************************************************
963 After all the trouble to figure out the required context for the
964 derived instance declarations, all that's left is to chug along to
965 produce them. They will then be shoved into @tcInstDecls2@, which
966 will do all its usual business.
968 There are lots of possibilities for code to generate. Here are
969 various general remarks.
974 We want derived instances of @Eq@ and @Ord@ (both v common) to be
975 ``you-couldn't-do-better-by-hand'' efficient.
978 Deriving @Show@---also pretty common--- should also be reasonable good code.
981 Deriving for the other classes isn't that common or that big a deal.
988 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
991 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
994 We {\em normally} generate code only for the non-defaulted methods;
995 there are some exceptions for @Eq@ and (especially) @Ord@...
998 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
999 constructor's numeric (@Int#@) tag. These are generated by
1000 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
1001 these is around is given by @hasCon2TagFun@.
1003 The examples under the different sections below will make this
1007 Much less often (really just for deriving @Ix@), we use a
1008 @_tag2con_<tycon>@ function. See the examples.
1011 We use the renamer!!! Reason: we're supposed to be
1012 producing @LHsBinds Name@ for the methods, but that means
1013 producing correctly-uniquified code on the fly. This is entirely
1014 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
1015 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
1016 the renamer. What a great hack!
1020 -- Generate the InstInfo for the required instance paired with the
1021 -- *representation* tycon for that instance,
1022 -- plus any auxiliary bindings required
1024 -- Representation tycons differ from the tycon in the instance signature in
1025 -- case of instances for indexed families.
1027 genInst :: Instance -> TcM ((InstInfo, TyCon), LHsBinds RdrName)
1029 = do { fix_env <- getFixityEnv
1031 (tyvars,_,clas,[ty]) = instanceHead spec
1032 clas_nm = className clas
1033 (visible_tycon, tyArgs) = tcSplitTyConApp ty
1035 -- In case of a family instance, we need to use the representation
1036 -- tycon (after all, it has the data constructors)
1037 ; (tycon, _) <- tcLookupFamInstExact visible_tycon tyArgs
1038 ; let (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
1040 -- Bring the right type variables into
1041 -- scope, and rename the method binds
1042 -- It's a bit yukky that we return *renamed* InstInfo, but
1043 -- *non-renamed* auxiliary bindings
1044 ; (rn_meth_binds, _fvs) <- discardWarnings $
1045 bindLocalNames (map Var.varName tyvars) $
1046 rnMethodBinds clas_nm (\n -> []) [] meth_binds
1048 -- Build the InstInfo
1049 ; return ((InstInfo { iSpec = spec,
1050 iBinds = VanillaInst rn_meth_binds [] }, tycon),
1054 genDerivBinds clas fix_env tycon
1055 | className clas `elem` typeableClassNames
1056 = (gen_Typeable_binds tycon, emptyLHsBinds)
1059 = case assocMaybe gen_list (getUnique clas) of
1060 Just gen_fn -> gen_fn fix_env tycon
1061 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1063 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
1064 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
1065 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
1066 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
1067 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
1068 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
1069 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
1070 ,(showClassKey, no_aux_binds gen_Show_binds)
1071 ,(readClassKey, no_aux_binds gen_Read_binds)
1072 ,(dataClassKey, gen_Data_binds)
1075 -- no_aux_binds is used for generators that don't
1076 -- need to produce any auxiliary bindings
1077 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
1078 ignore_fix_env f fix_env tc = f tc
1082 %************************************************************************
1084 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1086 %************************************************************************
1091 con2tag_Foo :: Foo ... -> Int#
1092 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
1093 maxtag_Foo :: Int -- ditto (NB: not unlifted)
1096 We have a @con2tag@ function for a tycon if:
1099 We're deriving @Eq@ and the tycon has nullary data constructors.
1102 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
1103 (enum type only????)
1106 We have a @tag2con@ function for a tycon if:
1109 We're deriving @Enum@, or @Ix@ (enum type only???)
1112 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
1115 genTaggeryBinds :: [(InstInfo, TyCon)] -> TcM (LHsBinds RdrName)
1116 genTaggeryBinds infos
1117 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
1118 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
1119 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
1121 all_CTs = [ (fst (simpleInstInfoClsTy info), tc)
1122 | (info, tc) <- infos]
1123 all_tycons = map snd all_CTs
1124 (tycons_of_interest, _) = removeDups compare all_tycons
1126 do_con2tag acc_Names tycon
1127 | isDataTyCon tycon &&
1128 ((we_are_deriving eqClassKey tycon
1129 && any isNullarySrcDataCon (tyConDataCons tycon))
1130 || (we_are_deriving ordClassKey tycon
1131 && not (isProductTyCon tycon))
1132 || (we_are_deriving enumClassKey tycon)
1133 || (we_are_deriving ixClassKey tycon))
1135 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
1140 do_tag2con acc_Names tycon
1141 | isDataTyCon tycon &&
1142 (we_are_deriving enumClassKey tycon ||
1143 we_are_deriving ixClassKey tycon
1144 && isEnumerationTyCon tycon)
1145 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
1146 : (maxtag_RDR tycon, tycon, GenMaxTag)
1151 we_are_deriving clas_key tycon
1152 = is_in_eqns clas_key tycon all_CTs
1154 is_in_eqns clas_key tycon [] = False
1155 is_in_eqns clas_key tycon ((c,t):cts)
1156 = (clas_key == classKey c && tycon == t)
1157 || is_in_eqns clas_key tycon cts
1161 derivingThingErr clas tys ty why
1162 = sep [hsep [ptext SLIT("Can't make a derived instance of"),
1164 nest 2 (parens why)]
1166 pred = mkClassPred clas (tys ++ [ty])
1168 standaloneCtxt :: LHsType Name -> SDoc
1169 standaloneCtxt ty = ptext SLIT("In the stand-alone deriving instance for") <+> quotes (ppr ty)
1171 derivInstCtxt clas inst_tys
1172 = ptext SLIT("When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
1175 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
1176 ptext SLIT("type variables that are not data type parameters"),
1177 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]
1179 famInstNotFound tycon tys notExact
1180 = failWithTc (msg <+> quotes (pprTypeApp tycon (ppr tycon) tys))
1182 msg = ptext $ if notExact
1183 then SLIT("No family instance exactly matching")
1184 else SLIT("More than one family instance for")