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
402 ; if isDataTyCon tycon then
403 mkDataTypeEqn orig gla_exts full_tvs cls cls_tys
404 tycon full_tc_args rep_tc rep_tc_args
406 mkNewTypeEqn orig gla_exts overlap_flag full_tvs cls cls_tys
407 tycon full_tc_args rep_tc rep_tc_args }
409 = baleOut (derivingThingErr cls cls_tys tc_app
410 (ptext SLIT("Last argument of the instance must be a type application")))
412 baleOut err = addErrTc err >> returnM (Nothing, Nothing)
416 %************************************************************************
420 %************************************************************************
423 mkDataTypeEqn orig gla_exts tvs cls cls_tys tycon tc_args rep_tc rep_tc_args
424 | Just err <- checkSideConditions gla_exts cls cls_tys rep_tc
425 -- NB: pass the *representation* tycon to checkSideConditions
426 = baleOut (derivingThingErr cls cls_tys (mkTyConApp tycon tc_args) err)
429 = ASSERT( null cls_tys )
430 do { loc <- getSrcSpanM
431 ; eqn <- mk_data_eqn loc orig tvs cls tycon tc_args rep_tc rep_tc_args
432 ; return (Just eqn, Nothing) }
434 mk_data_eqn :: SrcSpan -> InstOrigin -> [TyVar] -> Class
435 -> TyCon -> [TcType] -> TyCon -> [TcType] -> TcM DerivEqn
436 mk_data_eqn loc orig tvs cls tycon tc_args rep_tc rep_tc_args
437 | cls `hasKey` typeableClassKey
438 = -- The Typeable class is special in several ways
439 -- data T a b = ... deriving( Typeable )
441 -- instance Typeable2 T where ...
443 -- 1. There are no constraints in the instance
444 -- 2. There are no type variables either
445 -- 3. The actual class we want to generate isn't necessarily
446 -- Typeable; it depends on the arity of the type
447 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
448 ; dfun_name <- new_dfun_name real_clas tycon
449 ; return (loc, orig, dfun_name, [], real_clas, mkTyConApp tycon [], []) }
452 = do { dfun_name <- new_dfun_name cls tycon
453 ; let ordinary_constraints
454 = [ mkClassPred cls [arg_ty]
455 | data_con <- tyConDataCons rep_tc,
456 arg_ty <- dataConInstOrigArgTys data_con rep_tc_args,
457 not (isUnLiftedType arg_ty) ] -- No constraints for unlifted types?
459 tiresome_subst = zipTopTvSubst (tyConTyVars rep_tc) rep_tc_args
460 stupid_constraints = substTheta tiresome_subst (tyConStupidTheta rep_tc)
461 -- see note [Data decl contexts] above
463 ; return (loc, orig, dfun_name, tvs, cls, mkTyConApp tycon tc_args,
464 stupid_constraints ++ ordinary_constraints)
467 ------------------------------------------------------------------
468 -- Check side conditions that dis-allow derivability for particular classes
469 -- This is *apart* from the newtype-deriving mechanism
471 -- Here we get the representation tycon in case of family instances as it has
472 -- the data constructors - but we need to be careful to fall back to the
473 -- family tycon (with indexes) in error messages.
475 checkSideConditions :: Bool -> Class -> [TcType] -> TyCon -> Maybe SDoc
476 checkSideConditions gla_exts cls cls_tys rep_tc
478 = Just ty_args_why -- e.g. deriving( Foo s )
480 = case [cond | (key,cond) <- sideConditions, key == getUnique cls] of
481 [] -> Just (non_std_why cls)
482 [cond] -> cond (gla_exts, rep_tc)
483 other -> pprPanic "checkSideConditions" (ppr cls)
485 ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext SLIT("is not a class")
487 non_std_why cls = quotes (ppr cls) <+> ptext SLIT("is not a derivable class")
489 sideConditions :: [(Unique, Condition)]
491 = [ (eqClassKey, cond_std),
492 (ordClassKey, cond_std),
493 (readClassKey, cond_std),
494 (showClassKey, cond_std),
495 (enumClassKey, cond_std `andCond` cond_isEnumeration),
496 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
497 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
498 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
499 (dataClassKey, cond_glaExts `andCond` cond_std)
502 type Condition = (Bool, TyCon) -> Maybe SDoc
503 -- Bool is gla-exts flag
504 -- TyCon is the *representation* tycon if the
505 -- data type is an indexed one
508 orCond :: Condition -> Condition -> Condition
511 Nothing -> Nothing -- c1 succeeds
512 Just x -> case c2 tc of -- c1 fails
514 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
517 andCond c1 c2 tc = case c1 tc of
518 Nothing -> c2 tc -- c1 succeeds
519 Just x -> Just x -- c1 fails
521 cond_std :: Condition
522 cond_std (gla_exts, rep_tc)
523 | any (not . isVanillaDataCon) data_cons = Just existential_why
524 | null data_cons = Just no_cons_why
525 | otherwise = Nothing
527 data_cons = tyConDataCons rep_tc
528 no_cons_why = quotes (pprSourceTyCon rep_tc) <+>
529 ptext SLIT("has no data constructors")
530 existential_why = quotes (pprSourceTyCon rep_tc) <+>
531 ptext SLIT("has non-Haskell-98 constructor(s)")
533 cond_isEnumeration :: Condition
534 cond_isEnumeration (gla_exts, rep_tc)
535 | isEnumerationTyCon rep_tc = Nothing
536 | otherwise = Just why
538 why = quotes (pprSourceTyCon rep_tc) <+>
539 ptext SLIT("has non-nullary constructors")
541 cond_isProduct :: Condition
542 cond_isProduct (gla_exts, rep_tc)
543 | isProductTyCon rep_tc = Nothing
544 | otherwise = Just why
546 why = quotes (pprSourceTyCon rep_tc) <+>
547 ptext SLIT("has more than one constructor")
549 cond_typeableOK :: Condition
550 -- OK for Typeable class
551 -- Currently: (a) args all of kind *
552 -- (b) 7 or fewer args
553 cond_typeableOK (gla_exts, rep_tc)
554 | tyConArity rep_tc > 7 = Just too_many
555 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
557 | isFamInstTyCon rep_tc = Just fam_inst -- no Typable for family insts
558 | otherwise = Nothing
560 too_many = quotes (pprSourceTyCon rep_tc) <+>
561 ptext SLIT("has too many arguments")
562 bad_kind = quotes (pprSourceTyCon rep_tc) <+>
563 ptext SLIT("has arguments of kind other than `*'")
564 fam_inst = quotes (pprSourceTyCon rep_tc) <+>
565 ptext SLIT("is a type family")
567 cond_glaExts :: Condition
568 cond_glaExts (gla_exts, _rep_tc) | gla_exts = Nothing
569 | otherwise = Just why
571 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
573 std_class_via_iso clas -- These standard classes can be derived for a newtype
574 -- using the isomorphism trick *even if no -fglasgow-exts*
575 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
576 -- Not Read/Show because they respect the type
577 -- Not Enum, because newtypes are never in Enum
580 new_dfun_name clas tycon -- Just a simple wrapper
581 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
582 -- The type passed to newDFunName is only used to generate
583 -- a suitable string; hence the empty type arg list
587 %************************************************************************
591 %************************************************************************
594 mkNewTypeEqn orig gla_exts overlap_flag tvs cls cls_tys
596 rep_tycon rep_tc_args
597 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso cls)
598 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
599 ; -- Go ahead and use the isomorphism
600 dfun_name <- new_dfun_name cls tycon
601 ; return (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
602 iBinds = NewTypeDerived ntd_info })) }
604 | isNothing mb_std_err -- Use the standard H98 method
605 = do { loc <- getSrcSpanM
606 ; eqn <- mk_data_eqn loc orig tvs cls tycon tc_args rep_tycon rep_tc_args
607 ; return (Just eqn, Nothing) }
609 -- Otherwise we can't derive
610 | gla_exts = baleOut cant_derive_err -- Too hard
611 | otherwise = baleOut std_err -- Just complain about being a non-std instance
613 mb_std_err = checkSideConditions gla_exts cls cls_tys rep_tycon
614 std_err = derivingThingErr cls cls_tys tc_app $
615 vcat [fromJust mb_std_err,
616 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")]
618 -- Here is the plan for newtype derivings. We see
619 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
620 -- where t is a type,
621 -- ak+1...an is a suffix of a1..an, and are all tyars
622 -- ak+1...an do not occur free in t, nor in the s1..sm
623 -- (C s1 ... sm) is a *partial applications* of class C
624 -- with the last parameter missing
625 -- (T a1 .. ak) matches the kind of C's last argument
626 -- (and hence so does t)
628 -- We generate the instance
629 -- instance forall ({a1..ak} u fvs(s1..sm)).
630 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
631 -- where T a1...ap is the partial application of
632 -- the LHS of the correct kind and p >= k
634 -- NB: the variables below are:
635 -- tc_tvs = [a1, ..., an]
636 -- tyvars_to_keep = [a1, ..., ak]
637 -- rep_ty = t ak .. an
638 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
639 -- tys = [s1, ..., sm]
642 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
643 -- We generate the instance
644 -- instance Monad (ST s) => Monad (T s) where
646 cls_tyvars = classTyVars cls
647 kind = tyVarKind (last cls_tyvars)
648 -- Kind of the thing we want to instance
649 -- e.g. argument kind of Monad, *->*
651 (arg_kinds, _) = splitKindFunTys kind
652 n_args_to_drop = length arg_kinds
653 -- Want to drop 1 arg from (T s a) and (ST s a)
654 -- to get instance Monad (ST s) => Monad (T s)
656 -- Note [newtype representation]
657 -- Need newTyConRhs *not* newTyConRep to get the representation
658 -- type, because the latter looks through all intermediate newtypes
660 -- newtype B = MkB Int
661 -- newtype A = MkA B deriving( Num )
662 -- We want the Num instance of B, *not* the Num instance of Int,
663 -- when making the Num instance of A!
664 rep_ty = newTyConInstRhs rep_tycon rep_tc_args
665 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
667 n_tyargs_to_keep = tyConArity tycon - n_args_to_drop
668 dropped_tc_args = drop n_tyargs_to_keep tc_args
669 dropped_tvs = tyVarsOfTypes dropped_tc_args
671 n_args_to_keep = length rep_ty_args - n_args_to_drop
672 args_to_drop = drop n_args_to_keep rep_ty_args
673 args_to_keep = take n_args_to_keep rep_ty_args
675 rep_fn' = mkAppTys rep_fn args_to_keep
676 rep_tys = cls_tys ++ [rep_fn']
677 rep_pred = mkClassPred cls rep_tys
678 -- rep_pred is the representation dictionary, from where
679 -- we are gong to get all the methods for the newtype
682 tc_app = mkTyConApp tycon (take n_tyargs_to_keep tc_args)
684 -- Next we figure out what superclass dictionaries to use
685 -- See Note [Newtype deriving superclasses] above
687 inst_tys = cls_tys ++ [tc_app]
688 sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
691 -- If there are no tyvars, there's no need
692 -- to abstract over the dictionaries we need
693 -- Example: newtype T = MkT Int deriving( C )
694 -- We get the derived instance
697 -- instance C Int => C T
698 dict_tvs = filterOut (`elemVarSet` dropped_tvs) tvs
699 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
700 (dict_args, ntd_info) | null dict_tvs = ([], Just all_preds)
701 | otherwise = (all_preds, Nothing)
703 -- Finally! Here's where we build the dictionary Id
704 mk_inst_spec dfun_name = mkLocalInstance dfun overlap_flag
706 dfun = mkDictFunId dfun_name dict_tvs dict_args cls inst_tys
708 -------------------------------------------------------------------
709 -- Figuring out whether we can only do this newtype-deriving thing
711 right_arity = length cls_tys + 1 == classArity cls
713 -- Never derive Read,Show,Typeable,Data this way
714 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
715 can_derive_via_isomorphism
716 = not (getUnique cls `elem` non_iso_classes)
717 && right_arity -- Well kinded;
718 -- eg not: newtype T ... deriving( ST )
719 -- because ST needs *2* type params
720 && n_tyargs_to_keep >= 0 -- Type constructor has right kind:
721 -- eg not: newtype T = T Int deriving( Monad )
722 && n_args_to_keep >= 0 -- Rep type has right kind:
723 -- eg not: newtype T a = T Int deriving( Monad )
724 && eta_ok -- Eta reduction works
725 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
726 -- newtype A = MkA [A]
728 -- instance Eq [A] => Eq A !!
729 -- Here's a recursive newtype that's actually OK
730 -- newtype S1 = S1 [T1 ()]
731 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
732 -- It's currently rejected. Oh well.
733 -- In fact we generate an instance decl that has method of form
734 -- meth @ instTy = meth @ repTy
735 -- (no coerce's). We'd need a coerce if we wanted to handle
736 -- recursive newtypes too
738 -- Check that eta reduction is OK
739 eta_ok = (args_to_drop `tcEqTypes` dropped_tc_args)
740 -- (a) the dropped-off args are identical in the source and rep type
741 -- newtype T a b = MkT (S [a] b) deriving( Monad )
742 -- Here the 'b' must be the same in the rep type (S [a] b)
744 && (tyVarsOfType rep_fn' `disjointVarSet` dropped_tvs)
745 -- (b) the remaining type args do not mention any of the dropped
748 && (tyVarsOfTypes cls_tys `disjointVarSet` dropped_tvs)
749 -- (c) the type class args do not mention any of the dropped type
752 && all isTyVarTy dropped_tc_args
753 -- (d) in case of newtype family instances, the eta-dropped
754 -- arguments must be type variables (not more complex indexes)
756 cant_derive_err = derivingThingErr cls cls_tys tc_app
757 (vcat [ptext SLIT("even with cunning newtype deriving:"),
758 if isRecursiveTyCon tycon then
759 ptext SLIT("the newtype may be recursive")
761 if not right_arity then
762 quotes (ppr (mkClassPred cls cls_tys)) <+> ptext SLIT("does not have arity 1")
764 if not (n_tyargs_to_keep >= 0) then
765 ptext SLIT("the type constructor has wrong kind")
766 else if not (n_args_to_keep >= 0) then
767 ptext SLIT("the representation type has wrong kind")
768 else if not eta_ok then
769 ptext SLIT("the eta-reduction property does not hold")
775 %************************************************************************
777 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
779 %************************************************************************
781 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
782 terms, which is the final correct RHS for the corresponding original
786 Each (k,TyVarTy tv) in a solution constrains only a type
790 The (k,TyVarTy tv) pairs in a solution are canonically
791 ordered by sorting on type varible, tv, (major key) and then class, k,
796 solveDerivEqns :: OverlapFlag
798 -> TcM [Instance]-- Solns in same order as eqns.
799 -- This bunch is Absolutely minimal...
801 solveDerivEqns overlap_flag orig_eqns
802 = do { traceTc (text "solveDerivEqns" <+> vcat (map pprDerivEqn orig_eqns))
803 ; iterateDeriv 1 initial_solutions }
805 -- The initial solutions for the equations claim that each
806 -- instance has an empty context; this solution is certainly
807 -- in canonical form.
808 initial_solutions :: [DerivSoln]
809 initial_solutions = [ [] | _ <- orig_eqns ]
811 ------------------------------------------------------------------
812 -- iterateDeriv calculates the next batch of solutions,
813 -- compares it with the current one; finishes if they are the
814 -- same, otherwise recurses with the new solutions.
815 -- It fails if any iteration fails
816 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
817 iterateDeriv n current_solns
818 | n > 20 -- Looks as if we are in an infinite loop
819 -- This can happen if we have -fallow-undecidable-instances
820 -- (See TcSimplify.tcSimplifyDeriv.)
821 = pprPanic "solveDerivEqns: probable loop"
822 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
825 inst_specs = zipWithEqual "add_solns" mk_inst_spec
826 orig_eqns current_solns
829 -- Extend the inst info from the explicit instance decls
830 -- with the current set of solutions, and simplify each RHS
831 extendLocalInstEnv inst_specs $
832 mappM gen_soln orig_eqns
833 ) `thenM` \ new_solns ->
834 if (current_solns == new_solns) then
837 iterateDeriv (n+1) new_solns
839 ------------------------------------------------------------------
840 gen_soln :: DerivEqn -> TcM [PredType]
841 gen_soln (loc, orig, _, tyvars, clas, inst_ty, deriv_rhs)
843 do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
844 ; addErrCtxt (derivInstCtxt theta clas [inst_ty]) $
845 do { checkNoErrs (checkValidInstance tyvars theta clas [inst_ty])
846 -- See Note [Deriving context]
847 -- If this fails, don't continue
849 -- Check for a bizarre corner case, when the derived instance decl should
850 -- have form instance C a b => D (T a) where ...
851 -- Note that 'b' isn't a parameter of T. This gives rise to all sorts
852 -- of problems; in particular, it's hard to compare solutions for
853 -- equality when finding the fixpoint. So I just rule it out for now.
854 ; let tv_set = mkVarSet tyvars
855 weird_preds = [pred | pred <- theta, not (tyVarsOfPred pred `subVarSet` tv_set)]
856 ; mapM_ (addErrTc . badDerivedPred) weird_preds
858 -- Claim: the result instance declaration is guaranteed valid
859 -- Hence no need to call:
860 -- checkValidInstance tyvars theta clas inst_tys
861 ; return (sortLe (<=) theta) } } -- Canonicalise before returning the solution
863 ------------------------------------------------------------------
864 mk_inst_spec :: DerivEqn -> DerivSoln -> Instance
865 mk_inst_spec (loc, orig, dfun_name, tyvars, clas, inst_ty, _) theta
866 = mkLocalInstance dfun overlap_flag
868 dfun = mkDictFunId dfun_name tyvars theta clas [inst_ty]
870 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
871 -- Add new locally-defined instances; don't bother to check
872 -- for functional dependency errors -- that'll happen in TcInstDcls
873 extendLocalInstEnv dfuns thing_inside
874 = do { env <- getGblEnv
875 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
876 env' = env { tcg_inst_env = inst_env' }
877 ; setGblEnv env' thing_inside }
880 Note [Deriving context]
881 ~~~~~~~~~~~~~~~~~~~~~~~
882 With -fglasgow-exts, we allow things like (C Int a) in the simplified
883 context for a derived instance declaration, because at a use of this
884 instance, we might know that a=Bool, and have an instance for (C Int
887 We nevertheless insist that each predicate meets the termination
888 conditions. If not, the deriving mechanism generates larger and larger
889 constraints. Example:
891 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
893 Note the lack of a Show instance for Succ. First we'll generate
894 instance (Show (Succ a), Show a) => Show (Seq a)
896 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
897 and so on. Instead we want to complain of no instance for (Show (Succ a)).
900 %************************************************************************
902 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
904 %************************************************************************
906 After all the trouble to figure out the required context for the
907 derived instance declarations, all that's left is to chug along to
908 produce them. They will then be shoved into @tcInstDecls2@, which
909 will do all its usual business.
911 There are lots of possibilities for code to generate. Here are
912 various general remarks.
917 We want derived instances of @Eq@ and @Ord@ (both v common) to be
918 ``you-couldn't-do-better-by-hand'' efficient.
921 Deriving @Show@---also pretty common--- should also be reasonable good code.
924 Deriving for the other classes isn't that common or that big a deal.
931 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
934 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
937 We {\em normally} generate code only for the non-defaulted methods;
938 there are some exceptions for @Eq@ and (especially) @Ord@...
941 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
942 constructor's numeric (@Int#@) tag. These are generated by
943 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
944 these is around is given by @hasCon2TagFun@.
946 The examples under the different sections below will make this
950 Much less often (really just for deriving @Ix@), we use a
951 @_tag2con_<tycon>@ function. See the examples.
954 We use the renamer!!! Reason: we're supposed to be
955 producing @LHsBinds Name@ for the methods, but that means
956 producing correctly-uniquified code on the fly. This is entirely
957 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
958 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
959 the renamer. What a great hack!
963 -- Generate the InstInfo for the required instance paired with the
964 -- *representation* tycon for that instance,
965 -- plus any auxiliary bindings required
967 -- Representation tycons differ from the tycon in the instance signature in
968 -- case of instances for indexed families.
970 genInst :: Instance -> TcM ((InstInfo, TyCon), LHsBinds RdrName)
972 = do { fix_env <- getFixityEnv
974 (tyvars,_,clas,[ty]) = instanceHead spec
975 clas_nm = className clas
976 (visible_tycon, tyArgs) = tcSplitTyConApp ty
978 -- In case of a family instance, we need to use the representation
979 -- tycon (after all, it has the data constructors)
980 ; (tycon, _) <- tcLookupFamInst visible_tycon tyArgs
981 ; let (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
983 -- Bring the right type variables into
984 -- scope, and rename the method binds
985 -- It's a bit yukky that we return *renamed* InstInfo, but
986 -- *non-renamed* auxiliary bindings
987 ; (rn_meth_binds, _fvs) <- discardWarnings $
988 bindLocalNames (map Var.varName tyvars) $
989 rnMethodBinds clas_nm (\n -> []) [] meth_binds
991 -- Build the InstInfo
992 ; return ((InstInfo { iSpec = spec,
993 iBinds = VanillaInst rn_meth_binds [] }, tycon),
997 genDerivBinds clas fix_env tycon
998 | className clas `elem` typeableClassNames
999 = (gen_Typeable_binds tycon, emptyLHsBinds)
1002 = case assocMaybe gen_list (getUnique clas) of
1003 Just gen_fn -> gen_fn fix_env tycon
1004 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
1006 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
1007 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
1008 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
1009 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
1010 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
1011 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
1012 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
1013 ,(showClassKey, no_aux_binds gen_Show_binds)
1014 ,(readClassKey, no_aux_binds gen_Read_binds)
1015 ,(dataClassKey, gen_Data_binds)
1018 -- no_aux_binds is used for generators that don't
1019 -- need to produce any auxiliary bindings
1020 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
1021 ignore_fix_env f fix_env tc = f tc
1025 %************************************************************************
1027 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
1029 %************************************************************************
1034 con2tag_Foo :: Foo ... -> Int#
1035 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
1036 maxtag_Foo :: Int -- ditto (NB: not unlifted)
1039 We have a @con2tag@ function for a tycon if:
1042 We're deriving @Eq@ and the tycon has nullary data constructors.
1045 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
1046 (enum type only????)
1049 We have a @tag2con@ function for a tycon if:
1052 We're deriving @Enum@, or @Ix@ (enum type only???)
1055 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
1058 genTaggeryBinds :: [(InstInfo, TyCon)] -> TcM (LHsBinds RdrName)
1059 genTaggeryBinds infos
1060 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
1061 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
1062 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
1064 all_CTs = [ (fst (simpleInstInfoClsTy info), tc)
1065 | (info, tc) <- infos]
1066 all_tycons = map snd all_CTs
1067 (tycons_of_interest, _) = removeDups compare all_tycons
1069 do_con2tag acc_Names tycon
1070 | isDataTyCon tycon &&
1071 ((we_are_deriving eqClassKey tycon
1072 && any isNullarySrcDataCon (tyConDataCons tycon))
1073 || (we_are_deriving ordClassKey tycon
1074 && not (isProductTyCon tycon))
1075 || (we_are_deriving enumClassKey tycon)
1076 || (we_are_deriving ixClassKey tycon))
1078 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
1083 do_tag2con acc_Names tycon
1084 | isDataTyCon tycon &&
1085 (we_are_deriving enumClassKey tycon ||
1086 we_are_deriving ixClassKey tycon
1087 && isEnumerationTyCon tycon)
1088 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
1089 : (maxtag_RDR tycon, tycon, GenMaxTag)
1094 we_are_deriving clas_key tycon
1095 = is_in_eqns clas_key tycon all_CTs
1097 is_in_eqns clas_key tycon [] = False
1098 is_in_eqns clas_key tycon ((c,t):cts)
1099 = (clas_key == classKey c && tycon == t)
1100 || is_in_eqns clas_key tycon cts
1104 derivingThingErr clas tys ty why
1105 = sep [hsep [ptext SLIT("Can't make a derived instance of"),
1107 nest 2 (parens why)]
1109 pred = mkClassPred clas (tys ++ [ty])
1111 standaloneCtxt :: LHsType Name -> SDoc
1112 standaloneCtxt ty = ptext SLIT("In the stand-alone deriving instance for") <+> quotes (ppr ty)
1114 derivInstCtxt theta clas inst_tys
1115 = hang (ptext SLIT("In the derived instance:"))
1116 2 (pprThetaArrow theta <+> pprClassPred clas inst_tys)
1117 -- Used for the ...Thetas variants; all top level
1120 = vcat [ptext SLIT("Can't derive instances where the instance context mentions"),
1121 ptext SLIT("type variables that are not data type parameters"),
1122 nest 2 (ptext SLIT("Offending constraint:") <+> ppr pred)]