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
52 %************************************************************************
54 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
56 %************************************************************************
60 data T a b = C1 (Foo a) (Bar b)
65 [NOTE: See end of these comments for what to do with
66 data (C a, D b) => T a b = ...
69 We want to come up with an instance declaration of the form
71 instance (Ping a, Pong b, ...) => Eq (T a b) where
74 It is pretty easy, albeit tedious, to fill in the code "...". The
75 trick is to figure out what the context for the instance decl is,
76 namely @Ping@, @Pong@ and friends.
78 Let's call the context reqd for the T instance of class C at types
79 (a,b, ...) C (T a b). Thus:
81 Eq (T a b) = (Ping a, Pong b, ...)
83 Now we can get a (recursive) equation from the @data@ decl:
85 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
86 u Eq (T b a) u Eq Int -- From C2
87 u Eq (T a a) -- From C3
89 Foo and Bar may have explicit instances for @Eq@, in which case we can
90 just substitute for them. Alternatively, either or both may have
91 their @Eq@ instances given by @deriving@ clauses, in which case they
92 form part of the system of equations.
94 Now all we need do is simplify and solve the equations, iterating to
95 find the least fixpoint. Notice that the order of the arguments can
96 switch around, as here in the recursive calls to T.
98 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
102 Eq (T a b) = {} -- The empty set
105 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
106 u Eq (T b a) u Eq Int -- From C2
107 u Eq (T a a) -- From C3
109 After simplification:
110 = Eq a u Ping b u {} u {} u {}
115 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
116 u Eq (T b a) u Eq Int -- From C2
117 u Eq (T a a) -- From C3
119 After simplification:
124 = Eq a u Ping b u Eq b u Ping a
126 The next iteration gives the same result, so this is the fixpoint. We
127 need to make a canonical form of the RHS to ensure convergence. We do
128 this by simplifying the RHS to a form in which
130 - the classes constrain only tyvars
131 - the list is sorted by tyvar (major key) and then class (minor key)
132 - no duplicates, of course
134 So, here are the synonyms for the ``equation'' structures:
137 type DerivRhs = ThetaType
138 type DerivSoln = DerivRhs
139 type DerivEqn = (SrcSpan, InstOrigin, Name, [TyVar], Class, Type, DerivRhs)
140 -- (span, orig, df, tvs, C, ty, rhs)
141 -- implies a dfun declaration of the form
142 -- df :: forall tvs. rhs => C ty
143 -- The Name is the name for the DFun we'll build
144 -- The tyvars bind all the variables in the RHS
145 -- For family indexes, the tycon is the *family* tycon
146 -- (not the representation tycon)
148 pprDerivEqn :: DerivEqn -> SDoc
149 pprDerivEqn (l, _, n, tvs, c, ty, rhs)
150 = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr ty]
151 <+> equals <+> ppr rhs)
155 [Data decl contexts] A note about contexts on data decls
156 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
159 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
161 We will need an instance decl like:
163 instance (Read a, RealFloat a) => Read (Complex a) where
166 The RealFloat in the context is because the read method for Complex is bound
167 to construct a Complex, and doing that requires that the argument type is
170 But this ain't true for Show, Eq, Ord, etc, since they don't construct
171 a Complex; they only take them apart.
173 Our approach: identify the offending classes, and add the data type
174 context to the instance decl. The "offending classes" are
178 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
179 pattern matching against a constructor from a data type with a context
180 gives rise to the constraints for that context -- or at least the thinned
181 version. So now all classes are "offending".
188 newtype T = T Char deriving( C [a] )
190 Notice the free 'a' in the deriving. We have to fill this out to
191 newtype T = T Char deriving( forall a. C [a] )
193 And then translate it to:
194 instance C [a] Char => C [a] T where ...
199 %************************************************************************
201 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
203 %************************************************************************
206 tcDeriving :: [LTyClDecl Name] -- All type constructors
207 -> [LDerivDecl Name] -- All stand-alone deriving declarations
208 -> TcM ([InstInfo], -- The generated "instance decls"
209 HsValBinds Name) -- Extra generated top-level bindings
211 tcDeriving tycl_decls deriv_decls
212 = recoverM (returnM ([], emptyValBindsOut)) $
213 do { -- Fish the "deriving"-related information out of the TcEnv
214 -- and make the necessary "equations".
215 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns tycl_decls deriv_decls
217 ; (ordinary_inst_info, deriv_binds)
218 <- extendLocalInstEnv (map iSpec newtype_inst_info) $
219 deriveOrdinaryStuff ordinary_eqns
220 -- Add the newtype-derived instances to the inst env
221 -- before tacking the "ordinary" ones
223 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
225 -- If we are compiling a hs-boot file,
226 -- don't generate any derived bindings
227 ; is_boot <- tcIsHsBoot
229 return (inst_info, emptyValBindsOut)
233 -- Generate the generic to/from functions from each type declaration
234 ; gen_binds <- mkGenericBinds tycl_decls
236 -- Rename these extra bindings, discarding warnings about unused bindings etc
237 -- Set -fglasgow exts so that we can have type signatures in patterns,
238 -- which is used in the generic binds
240 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
241 { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn deriv_binds [])
242 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
243 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
245 ; return (rn_deriv `plusHsValBinds` rn_gen) }
249 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
250 (ddump_deriving inst_info rn_binds))
252 ; returnM (inst_info, rn_binds)
255 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
256 ddump_deriving inst_infos extra_binds
257 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
259 -----------------------------------------
260 deriveOrdinaryStuff [] -- Short cut
261 = returnM ([], emptyLHsBinds)
263 deriveOrdinaryStuff eqns
264 = do { -- Take the equation list and solve it, to deliver a list of
265 -- solutions, a.k.a. the contexts for the instance decls
266 -- required for the corresponding equations.
267 overlap_flag <- getOverlapFlag
268 ; inst_specs <- solveDerivEqns overlap_flag eqns
270 -- Generate the InstInfo for each dfun,
271 -- plus any auxiliary bindings it needs
272 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst inst_specs
274 -- Generate any extra not-one-inst-decl-specific binds,
275 -- notably "con2tag" and/or "tag2con" functions.
276 ; extra_binds <- genTaggeryBinds inst_infos
279 ; returnM (map fst inst_infos,
280 unionManyBags (extra_binds : aux_binds_s))
283 -----------------------------------------
284 mkGenericBinds tycl_decls
285 = do { tcs <- mapM tcLookupTyCon
287 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
288 -- We are only interested in the data type declarations
289 ; return (unionManyBags [ mkTyConGenericBinds tc |
290 tc <- tcs, tyConHasGenerics tc ]) }
291 -- And then only in the ones whose 'has-generics' flag is on
295 %************************************************************************
297 \subsection[TcDeriv-eqns]{Forming the equations}
299 %************************************************************************
301 @makeDerivEqns@ fishes around to find the info about needed derived
302 instances. Complicating factors:
305 We can only derive @Enum@ if the data type is an enumeration
306 type (all nullary data constructors).
309 We can only derive @Ix@ if the data type is an enumeration {\em
310 or} has just one data constructor (e.g., tuples).
313 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
316 Note [Newtype deriving superclasses]
317 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
318 The 'tys' here come from the partial application in the deriving
319 clause. The last arg is the new instance type.
321 We must pass the superclasses; the newtype might be an instance
322 of them in a different way than the representation type
323 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
324 Then the Show instance is not done via isomorphism; it shows
326 The Num instance is derived via isomorphism, but the Show superclass
327 dictionary must the Show instance for Foo, *not* the Show dictionary
328 gotten from the Num dictionary. So we must build a whole new dictionary
329 not just use the Num one. The instance we want is something like:
330 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
333 There may be a coercion needed which we get from the tycon for the newtype
334 when the dict is constructed in TcInstDcls.tcInstDecl2
338 makeDerivEqns :: [LTyClDecl Name]
340 -> TcM ([DerivEqn], -- Ordinary derivings
341 [InstInfo]) -- Special newtype derivings
343 makeDerivEqns tycl_decls deriv_decls
344 = do { eqns1 <- mapM deriveTyData $
345 [ (p,d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- tycl_decls
347 ; eqns2 <- mapM deriveStandalone deriv_decls
348 ; return ([eqn | (Just eqn, _) <- eqns1 ++ eqns2],
349 [inst | (_, Just inst) <- eqns1 ++ eqns2]) }
351 ------------------------------------------------------------------
352 deriveStandalone :: LDerivDecl Name -> TcM (Maybe DerivEqn, Maybe InstInfo)
353 -- Standalone deriving declarations
354 -- e.g. derive instance Show T
355 -- Rather like tcLocalInstDecl
356 deriveStandalone (L loc (DerivDecl deriv_ty))
358 addErrCtxt (standaloneCtxt deriv_ty) $
359 do { (tvs, theta, tau) <- tcHsInstHead deriv_ty
360 ; (cls, inst_tys) <- checkValidInstHead tau
361 ; let cls_tys = take (length inst_tys - 1) inst_tys
362 inst_ty = last inst_tys
364 ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty }
366 ------------------------------------------------------------------
367 deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
368 deriveTyData (deriv_pred, L loc decl@(TyData { tcdLName = L _ tycon_name,
369 tcdTyVars = tv_names,
370 tcdTyPats = ty_pats }))
373 do { let hs_ty_args = ty_pats `orElse` map (nlHsTyVar . hsLTyVarName) tv_names
374 hs_app = nlHsTyConApp tycon_name hs_ty_args
375 -- We get kinding info for the tyvars by typechecking (T a b)
376 -- Hence forming a tycon application and then dis-assembling it
377 ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
378 ; tcExtendTyVarEnv tvs $ -- Deriving preds may (now) mention
379 -- the type variables for the type constructor
380 do { (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
381 -- The "deriv_pred" is a LHsType to take account of the fact that for
382 -- newtype deriving we allow deriving (forall a. C [a]).
383 ; mkEqnHelp DerivOrigin (tvs++deriv_tvs) cls cls_tys tc_app } }
384 deriveTyData (deriv_pred, other_decl)
385 = panic "derivTyData" -- Caller ensures that only TyData can happen
387 ------------------------------------------------------------------
388 mkEqnHelp orig tvs cls cls_tys tc_app
389 | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
390 = do { -- Make tc_app saturated, because that's what the
391 -- mkDataTypeEqn things expect
392 -- It might not be saturated in the standalone deriving case
393 -- derive instance Monad (T a)
394 let extra_tvs = dropList tc_args (tyConTyVars tycon)
395 full_tc_args = tc_args ++ mkTyVarTys extra_tvs
396 full_tvs = tvs ++ extra_tvs
398 ; (rep_tc, rep_tc_args) <- tcLookupFamInst tycon full_tc_args
400 ; gla_exts <- doptM Opt_GlasgowExts
401 ; overlap_flag <- getOverlapFlag
403 -- Be careful to test rep_tc here: in the case of families, we want
404 -- to check the instance tycon, not the family tycon
405 ; if isDataTyCon rep_tc then
406 mkDataTypeEqn orig gla_exts full_tvs cls cls_tys
407 tycon full_tc_args rep_tc rep_tc_args
409 mkNewTypeEqn orig gla_exts overlap_flag full_tvs cls cls_tys
410 tycon full_tc_args rep_tc rep_tc_args }
412 = baleOut (derivingThingErr cls cls_tys tc_app
413 (ptext SLIT("Last argument of the instance must be a type application")))
415 baleOut err = addErrTc err >> returnM (Nothing, Nothing)
419 %************************************************************************
423 %************************************************************************
426 mkDataTypeEqn orig gla_exts tvs cls cls_tys tycon tc_args rep_tc rep_tc_args
427 | Just err <- checkSideConditions gla_exts cls cls_tys rep_tc
428 -- NB: pass the *representation* tycon to checkSideConditions
429 = baleOut (derivingThingErr cls cls_tys (mkTyConApp tycon tc_args) err)
432 = ASSERT( null cls_tys )
433 do { loc <- getSrcSpanM
434 ; eqn <- mk_data_eqn loc orig tvs cls tycon tc_args rep_tc rep_tc_args
435 ; return (Just eqn, Nothing) }
437 mk_data_eqn :: SrcSpan -> InstOrigin -> [TyVar] -> Class
438 -> TyCon -> [TcType] -> TyCon -> [TcType] -> TcM DerivEqn
439 mk_data_eqn loc orig tvs cls tycon tc_args rep_tc rep_tc_args
440 | cls `hasKey` typeableClassKey
441 = -- The Typeable class is special in several ways
442 -- data T a b = ... deriving( Typeable )
444 -- instance Typeable2 T where ...
446 -- 1. There are no constraints in the instance
447 -- 2. There are no type variables either
448 -- 3. The actual class we want to generate isn't necessarily
449 -- Typeable; it depends on the arity of the type
450 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
451 ; dfun_name <- new_dfun_name real_clas tycon
452 ; return (loc, orig, dfun_name, [], real_clas, mkTyConApp tycon [], []) }
455 = do { dfun_name <- new_dfun_name cls tycon
456 ; let ordinary_constraints
457 = [ mkClassPred cls [arg_ty]
458 | data_con <- tyConDataCons rep_tc,
459 arg_ty <- dataConInstOrigArgTys data_con rep_tc_args,
460 not (isUnLiftedType arg_ty) ] -- No constraints for unlifted types?
462 tiresome_subst = zipTopTvSubst (tyConTyVars rep_tc) rep_tc_args
463 stupid_constraints = substTheta tiresome_subst (tyConStupidTheta rep_tc)
464 -- see note [Data decl contexts] above
466 ; return (loc, orig, dfun_name, tvs, cls, mkTyConApp tycon tc_args,
467 stupid_constraints ++ ordinary_constraints)
470 ------------------------------------------------------------------
471 -- Check side conditions that dis-allow derivability for particular classes
472 -- This is *apart* from the newtype-deriving mechanism
474 -- Here we get the representation tycon in case of family instances as it has
475 -- the data constructors - but we need to be careful to fall back to the
476 -- family tycon (with indexes) in error messages.
478 checkSideConditions :: Bool -> Class -> [TcType] -> TyCon -> Maybe SDoc
479 checkSideConditions gla_exts cls cls_tys rep_tc
481 = Just ty_args_why -- e.g. deriving( Foo s )
483 = case [cond | (key,cond) <- sideConditions, key == getUnique cls] of
484 [] -> Just (non_std_why cls)
485 [cond] -> cond (gla_exts, rep_tc)
486 other -> pprPanic "checkSideConditions" (ppr cls)
488 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext SLIT("is not a class")
490 non_std_why cls = quotes (ppr cls) <+> ptext SLIT("is not a derivable class")
492 sideConditions :: [(Unique, Condition)]
494 = [ (eqClassKey, cond_std),
495 (ordClassKey, cond_std),
496 (readClassKey, cond_std),
497 (showClassKey, cond_std),
498 (enumClassKey, cond_std `andCond` cond_isEnumeration),
499 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
500 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
501 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
502 (dataClassKey, cond_glaExts `andCond` cond_std)
505 type Condition = (Bool, TyCon) -> Maybe SDoc
506 -- Bool is gla-exts flag
507 -- TyCon is the *representation* tycon if the
508 -- data type is an indexed one
511 orCond :: Condition -> Condition -> Condition
514 Nothing -> Nothing -- c1 succeeds
515 Just x -> case c2 tc of -- c1 fails
517 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
520 andCond c1 c2 tc = case c1 tc of
521 Nothing -> c2 tc -- c1 succeeds
522 Just x -> Just x -- c1 fails
524 cond_std :: Condition
525 cond_std (gla_exts, rep_tc)
526 | any (not . isVanillaDataCon) data_cons = Just existential_why
527 | null data_cons = Just no_cons_why
528 | otherwise = Nothing
530 data_cons = tyConDataCons rep_tc
531 no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
532 ptext SLIT("has no data constructors")
533 existential_why = quotes (pprSourceTyCon rep_tc) <+>
534 ptext SLIT("has non-Haskell-98 constructor(s)")
536 cond_isEnumeration :: Condition
537 cond_isEnumeration (gla_exts, rep_tc)
538 | isEnumerationTyCon rep_tc = Nothing
539 | otherwise = Just why
541 why = quotes (pprSourceTyCon rep_tc) <+>
542 ptext SLIT("has non-nullary constructors")
544 cond_isProduct :: Condition
545 cond_isProduct (gla_exts, rep_tc)
546 | isProductTyCon rep_tc = Nothing
547 | otherwise = Just why
549 why = quotes (pprSourceTyCon rep_tc) <+>
550 ptext SLIT("has more than one constructor")
552 cond_typeableOK :: Condition
553 -- OK for Typeable class
554 -- Currently: (a) args all of kind *
555 -- (b) 7 or fewer args
556 cond_typeableOK (gla_exts, rep_tc)
557 | tyConArity rep_tc > 7 = Just too_many
558 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
560 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
561 | otherwise = Nothing
563 too_many = quotes (pprSourceTyCon rep_tc) <+>
564 ptext SLIT("has too many arguments")
565 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
566 ptext SLIT("has arguments of kind other than `*'")
567 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
568 ptext SLIT("is a type family")
570 cond_glaExts :: Condition
571 cond_glaExts (gla_exts, _rep_tc) | gla_exts = Nothing
572 | otherwise = Just why
574 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
576 std_class_via_iso clas -- These standard classes can be derived for a newtype
577 -- using the isomorphism trick *even if no -fglasgow-exts*
578 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
579 -- Not Read/Show because they respect the type
580 -- Not Enum, because newtypes are never in Enum
583 new_dfun_name clas tycon -- Just a simple wrapper
584 = newDFunName clas [mkTyConApp tycon []] (getSrcSpan tycon)
585 -- The type passed to newDFunName is only used to generate
586 -- a suitable string; hence the empty type arg list
590 %************************************************************************
594 %************************************************************************
597 mkNewTypeEqn orig gla_exts overlap_flag tvs cls cls_tys
599 rep_tycon rep_tc_args
600 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso cls)
601 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
602 ; -- Go ahead and use the isomorphism
603 dfun_name <- new_dfun_name cls tycon
604 ; return (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
605 iBinds = NewTypeDerived ntd_info })) }
607 | isNothing mb_std_err -- Use the standard H98 method
608 = do { loc <- getSrcSpanM
609 ; eqn <- mk_data_eqn loc orig tvs cls tycon tc_args rep_tycon rep_tc_args
610 ; return (Just eqn, Nothing) }
612 -- Otherwise we can't derive
613 | gla_exts = baleOut cant_derive_err -- Too hard
614 | otherwise = baleOut std_err -- Just complain about being a non-std instance
616 mb_std_err = checkSideConditions gla_exts cls cls_tys rep_tycon
617 std_err = derivingThingErr cls cls_tys tc_app $
618 vcat [fromJust mb_std_err,
619 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")]
621 -- Here is the plan for newtype derivings. We see
622 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
623 -- where t is a type,
624 -- ak+1...an is a suffix of a1..an, and are all tyars
625 -- ak+1...an do not occur free in t, nor in the s1..sm
626 -- (C s1 ... sm) is a *partial applications* of class C
627 -- with the last parameter missing
628 -- (T a1 .. ak) matches the kind of C's last argument
629 -- (and hence so does t)
631 -- We generate the instance
632 -- instance forall ({a1..ak} u fvs(s1..sm)).
633 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
634 -- where T a1...ap is the partial application of
635 -- the LHS of the correct kind and p >= k
637 -- NB: the variables below are:
638 -- tc_tvs = [a1, ..., an]
639 -- tyvars_to_keep = [a1, ..., ak]
640 -- rep_ty = t ak .. an
641 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
642 -- tys = [s1, ..., sm]
645 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
646 -- We generate the instance
647 -- instance Monad (ST s) => Monad (T s) where
649 cls_tyvars = classTyVars cls
650 kind = tyVarKind (last cls_tyvars)
651 -- Kind of the thing we want to instance
652 -- e.g. argument kind of Monad, *->*
654 (arg_kinds, _) = splitKindFunTys kind
655 n_args_to_drop = length arg_kinds
656 -- Want to drop 1 arg from (T s a) and (ST s a)
657 -- to get instance Monad (ST s) => Monad (T s)
659 -- Note [newtype representation]
660 -- Need newTyConRhs *not* newTyConRep to get the representation
661 -- type, because the latter looks through all intermediate newtypes
663 -- newtype B = MkB Int
664 -- newtype A = MkA B deriving( Num )
665 -- We want the Num instance of B, *not* the Num instance of Int,
666 -- when making the Num instance of A!
667 rep_ty = newTyConInstRhs rep_tycon rep_tc_args
668 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
670 n_tyargs_to_keep = tyConArity tycon - n_args_to_drop
671 dropped_tc_args = drop n_tyargs_to_keep tc_args
672 dropped_tvs = tyVarsOfTypes dropped_tc_args
674 n_args_to_keep = length rep_ty_args - n_args_to_drop
675 args_to_drop = drop n_args_to_keep rep_ty_args
676 args_to_keep = take n_args_to_keep rep_ty_args
678 rep_fn' = mkAppTys rep_fn args_to_keep
679 rep_tys = cls_tys ++ [rep_fn']
680 rep_pred = mkClassPred cls rep_tys
681 -- rep_pred is the representation dictionary, from where
682 -- we are gong to get all the methods for the newtype
685 tc_app = mkTyConApp tycon (take n_tyargs_to_keep tc_args)
687 -- Next we figure out what superclass dictionaries to use
688 -- See Note [Newtype deriving superclasses] above
690 inst_tys = cls_tys ++ [tc_app]
691 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
694 -- If there are no tyvars, there's no need
695 -- to abstract over the dictionaries we need
696 -- Example: newtype T = MkT Int deriving( C )
697 -- We get the derived instance
700 -- instance C Int => C T
701 dict_tvs = filterOut (`elemVarSet` dropped_tvs) tvs
702 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
703 (dict_args, ntd_info) | null dict_tvs = ([], Just all_preds)
704 | otherwise = (all_preds, Nothing)
706 -- Finally! Here's where we build the dictionary Id
707 mk_inst_spec dfun_name = mkLocalInstance dfun overlap_flag
709 dfun = mkDictFunId dfun_name dict_tvs dict_args cls inst_tys
711 -------------------------------------------------------------------
712 -- Figuring out whether we can only do this newtype-deriving thing
714 right_arity = length cls_tys + 1 == classArity cls
716 -- Never derive Read,Show,Typeable,Data this way
717 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
718 can_derive_via_isomorphism
719 = not (getUnique cls `elem` non_iso_classes)
720 && right_arity -- Well kinded;
721 -- eg not: newtype T ... deriving( ST )
722 -- because ST needs *2* type params
723 && n_tyargs_to_keep >= 0 -- Type constructor has right kind:
724 -- eg not: newtype T = T Int deriving( Monad )
725 && n_args_to_keep >= 0 -- Rep type has right kind:
726 -- eg not: newtype T a = T Int deriving( Monad )
727 && eta_ok -- Eta reduction works
728 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
729 -- newtype A = MkA [A]
731 -- instance Eq [A] => Eq A !!
732 -- Here's a recursive newtype that's actually OK
733 -- newtype S1 = S1 [T1 ()]
734 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
735 -- It's currently rejected. Oh well.
736 -- In fact we generate an instance decl that has method of form
737 -- meth @ instTy = meth @ repTy
738 -- (no coerce's). We'd need a coerce if we wanted to handle
739 -- recursive newtypes too
741 -- Check that eta reduction is OK
742 eta_ok = (args_to_drop `tcEqTypes` dropped_tc_args)
743 -- (a) the dropped-off args are identical in the source and rep type
744 -- newtype T a b = MkT (S [a] b) deriving( Monad )
745 -- Here the 'b' must be the same in the rep type (S [a] b)
747 && (tyVarsOfType rep_fn' `disjointVarSet` dropped_tvs)
748 -- (b) the remaining type args do not mention any of the dropped
751 && (tyVarsOfTypes cls_tys `disjointVarSet` dropped_tvs)
752 -- (c) the type class args do not mention any of the dropped type
755 && all isTyVarTy dropped_tc_args
756 -- (d) in case of newtype family instances, the eta-dropped
757 -- arguments must be type variables (not more complex indexes)
759 cant_derive_err = derivingThingErr cls cls_tys tc_app
760 (vcat [ptext SLIT("even with cunning newtype deriving:"),
761 if isRecursiveTyCon tycon then
762 ptext SLIT("the newtype may be recursive")
764 if not right_arity then
765 quotes (ppr (mkClassPred cls cls_tys)) <+> ptext SLIT("does not have arity 1")
767 if not (n_tyargs_to_keep >= 0) then
768 ptext SLIT("the type constructor has wrong kind")
769 else if not (n_args_to_keep >= 0) then
770 ptext SLIT("the representation type has wrong kind")
771 else if not eta_ok then
772 ptext SLIT("the eta-reduction property does not hold")
778 %************************************************************************
780 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
782 %************************************************************************
784 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
785 terms, which is the final correct RHS for the corresponding original
789 Each (k,TyVarTy tv) in a solution constrains only a type
793 The (k,TyVarTy tv) pairs in a solution are canonically
794 ordered by sorting on type varible, tv, (major key) and then class, k,
799 solveDerivEqns :: OverlapFlag
801 -> TcM [Instance]-- Solns in same order as eqns.
802 -- This bunch is Absolutely minimal...
804 solveDerivEqns overlap_flag orig_eqns
805 = do { traceTc (text "solveDerivEqns" <+> vcat (map pprDerivEqn orig_eqns))
806 ; iterateDeriv 1 initial_solutions }
808 -- The initial solutions for the equations claim that each
809 -- instance has an empty context; this solution is certainly
810 -- in canonical form.
811 initial_solutions :: [DerivSoln]
812 initial_solutions = [ [] | _ <- orig_eqns ]
814 ------------------------------------------------------------------
815 -- iterateDeriv calculates the next batch of solutions,
816 -- compares it with the current one; finishes if they are the
817 -- same, otherwise recurses with the new solutions.
818 -- It fails if any iteration fails
819 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
820 iterateDeriv n current_solns
821 | n > 20 -- Looks as if we are in an infinite loop
822 -- This can happen if we have -fallow-undecidable-instances
823 -- (See TcSimplify.tcSimplifyDeriv.)
824 = pprPanic "solveDerivEqns: probable loop"
825 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
828 inst_specs = zipWithEqual "add_solns" mk_inst_spec
829 orig_eqns current_solns
832 -- Extend the inst info from the explicit instance decls
833 -- with the current set of solutions, and simplify each RHS
834 extendLocalInstEnv inst_specs $
835 mappM gen_soln orig_eqns
836 ) `thenM` \ new_solns ->
837 if (current_solns == new_solns) then
840 iterateDeriv (n+1) new_solns
842 ------------------------------------------------------------------
843 gen_soln :: DerivEqn -> TcM [PredType]
844 gen_soln (loc, orig, _, tyvars, clas, inst_ty, deriv_rhs)
846 do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
847 ; addErrCtxt (derivInstCtxt theta clas [inst_ty]) $
848 do { checkNoErrs (checkValidInstance tyvars theta clas [inst_ty])
849 -- See Note [Deriving context]
850 -- If this fails, don't continue
852 -- Check for a bizarre corner case, when the derived instance decl should
853 -- have form instance C a b => D (T a) where ...
854 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
855 -- of problems; in particular, it's hard to compare solutions for
856 -- equality when finding the fixpoint. So I just rule it out for now.
857 ; let tv_set = mkVarSet tyvars
858 weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
859 ; mapM_ (addErrTc . badDerivedPred) weird_preds
861 -- Claim: the result instance declaration is guaranteed valid
862 -- Hence no need to call:
863 -- checkValidInstance tyvars theta clas inst_tys
864 ; return (sortLe (<=) theta) } } -- Canonicalise before returning the solution
866 ------------------------------------------------------------------
867 mk_inst_spec :: DerivEqn -> DerivSoln -> Instance
868 mk_inst_spec (loc, orig, dfun_name, tyvars, clas, inst_ty, _) theta
869 = mkLocalInstance dfun overlap_flag
871 dfun = mkDictFunId dfun_name tyvars theta clas [inst_ty]
873 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
874 -- Add new locally-defined instances; don't bother to check
875 -- for functional dependency errors -- that'll happen in TcInstDcls
876 extendLocalInstEnv dfuns thing_inside
877 = do { env <- getGblEnv
878 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
879 env' = env { tcg_inst_env = inst_env' }
880 ; setGblEnv env' thing_inside }
883 Note [Deriving context]
884 ~~~~~~~~~~~~~~~~~~~~~~~
885 With -fglasgow-exts, we allow things like (C Int a) in the simplified
886 context for a derived instance declaration, because at a use of this
887 instance, we might know that a=Bool, and have an instance for (C Int
890 We nevertheless insist that each predicate meets the termination
891 conditions. If not, the deriving mechanism generates larger and larger
892 constraints. Example:
894 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
896 Note the lack of a Show instance for Succ. First we'll generate
897 instance (Show (Succ a), Show a) => Show (Seq a)
899 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
900 and so on. Instead we want to complain of no instance for (Show (Succ a)).
903 %************************************************************************
905 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
907 %************************************************************************
909 After all the trouble to figure out the required context for the
910 derived instance declarations, all that's left is to chug along to
911 produce them. They will then be shoved into @tcInstDecls2@, which
912 will do all its usual business.
914 There are lots of possibilities for code to generate. Here are
915 various general remarks.
920 We want derived instances of @Eq@ and @Ord@ (both v common) to be
921 ``you-couldn't-do-better-by-hand'' efficient.
924 Deriving @Show@---also pretty common--- should also be reasonable good code.
927 Deriving for the other classes isn't that common or that big a deal.
934 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
937 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
940 We {\em normally} generate code only for the non-defaulted methods;
941 there are some exceptions for @Eq@ and (especially) @Ord@...
944 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
945 constructor's numeric (@Int#@) tag. These are generated by
946 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
947 these is around is given by @hasCon2TagFun@.
949 The examples under the different sections below will make this
953 Much less often (really just for deriving @Ix@), we use a
954 @_tag2con_<tycon>@ function. See the examples.
957 We use the renamer!!! Reason: we're supposed to be
958 producing @LHsBinds Name@ for the methods, but that means
959 producing correctly-uniquified code on the fly. This is entirely
960 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
961 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
962 the renamer. What a great hack!
966 -- Generate the InstInfo for the required instance paired with the
967 -- *representation* tycon for that instance,
968 -- plus any auxiliary bindings required
970 -- Representation tycons differ from the tycon in the instance signature in
971 -- case of instances for indexed families.
973 genInst :: Instance -> TcM ((InstInfo, TyCon), LHsBinds RdrName)
975 = do { fix_env <- getFixityEnv
977 (tyvars,_,clas,[ty]) = instanceHead spec
978 clas_nm = className clas
979 (visible_tycon, tyArgs) = tcSplitTyConApp ty
981 -- In case of a family instance, we need to use the representation
982 -- tycon (after all, it has the data constructors)
983 ; (tycon, _) <- tcLookupFamInst visible_tycon tyArgs
984 ; let (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
986 -- Bring the right type variables into
987 -- scope, and rename the method binds
988 -- It's a bit yukky that we return *renamed* InstInfo, but
989 -- *non-renamed* auxiliary bindings
990 ; (rn_meth_binds, _fvs) <- discardWarnings $
991 bindLocalNames (map Var.varName tyvars) $
992 rnMethodBinds clas_nm (\n -> []) [] meth_binds
994 -- Build the InstInfo
995 ; return ((InstInfo { iSpec = spec,
996 iBinds = VanillaInst rn_meth_binds [] }, tycon),
1000 genDerivBinds clas fix_env tycon
1001 | className clas `elem` typeableClassNames
1002 = (gen_Typeable_binds tycon, emptyLHsBinds)
1005 = case assocMaybe gen_list (getUnique clas) of
1006 Just gen_fn -> gen_fn fix_env tycon
1007 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1009 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
1010 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
1011 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
1012 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
1013 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
1014 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
1015 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
1016 ,(showClassKey, no_aux_binds gen_Show_binds)
1017 ,(readClassKey, no_aux_binds gen_Read_binds)
1018 ,(dataClassKey, gen_Data_binds)
1021 -- no_aux_binds is used for generators that don't
1022 -- need to produce any auxiliary bindings
1023 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
1024 ignore_fix_env f fix_env tc = f tc
1028 %************************************************************************
1030 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1032 %************************************************************************
1037 con2tag_Foo :: Foo ... -> Int#
1038 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
1039 maxtag_Foo :: Int -- ditto (NB: not unlifted)
1042 We have a @con2tag@ function for a tycon if:
1045 We're deriving @Eq@ and the tycon has nullary data constructors.
1048 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
1049 (enum type only????)
1052 We have a @tag2con@ function for a tycon if:
1055 We're deriving @Enum@, or @Ix@ (enum type only???)
1058 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
1061 genTaggeryBinds :: [(InstInfo, TyCon)] -> TcM (LHsBinds RdrName)
1062 genTaggeryBinds infos
1063 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
1064 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
1065 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
1067 all_CTs = [ (fst (simpleInstInfoClsTy info), tc)
1068 | (info, tc) <- infos]
1069 all_tycons = map snd all_CTs
1070 (tycons_of_interest, _) = removeDups compare all_tycons
1072 do_con2tag acc_Names tycon
1073 | isDataTyCon tycon &&
1074 ((we_are_deriving eqClassKey tycon
1075 && any isNullarySrcDataCon (tyConDataCons tycon))
1076 || (we_are_deriving ordClassKey tycon
1077 && not (isProductTyCon tycon))
1078 || (we_are_deriving enumClassKey tycon)
1079 || (we_are_deriving ixClassKey tycon))
1081 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
1086 do_tag2con acc_Names tycon
1087 | isDataTyCon tycon &&
1088 (we_are_deriving enumClassKey tycon ||
1089 we_are_deriving ixClassKey tycon
1090 && isEnumerationTyCon tycon)
1091 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
1092 : (maxtag_RDR tycon, tycon, GenMaxTag)
1097 we_are_deriving clas_key tycon
1098 = is_in_eqns clas_key tycon all_CTs
1100 is_in_eqns clas_key tycon [] = False
1101 is_in_eqns clas_key tycon ((c,t):cts)
1102 = (clas_key == classKey c && tycon == t)
1103 || is_in_eqns clas_key tycon cts
1107 derivingThingErr clas tys ty why
1108 = sep [hsep [ptext SLIT("Can't make a derived instance of"),
1110 nest 2 (parens why)]
1112 pred = mkClassPred clas (tys ++ [ty])
1114 standaloneCtxt :: LHsType Name -> SDoc
1115 standaloneCtxt ty = ptext SLIT("In the stand-alone deriving instance for") <+> quotes (ppr ty)
1117 derivInstCtxt theta clas inst_tys
1118 = hang (ptext SLIT("In the derived instance:"))
1119 2 (pprThetaArrow theta <+> pprClassPred clas inst_tys)
1120 -- Used for the ...Thetas variants; all top level
1123 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
1124 ptext SLIT("type variables that are not data type parameters"),
1125 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]