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 TcGenDeriv -- Deriv stuff
50 %************************************************************************
52 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
54 %************************************************************************
58 data T a b = C1 (Foo a) (Bar b)
63 [NOTE: See end of these comments for what to do with
64 data (C a, D b) => T a b = ...
67 We want to come up with an instance declaration of the form
69 instance (Ping a, Pong b, ...) => Eq (T a b) where
72 It is pretty easy, albeit tedious, to fill in the code "...". The
73 trick is to figure out what the context for the instance decl is,
74 namely @Ping@, @Pong@ and friends.
76 Let's call the context reqd for the T instance of class C at types
77 (a,b, ...) C (T a b). Thus:
79 Eq (T a b) = (Ping a, Pong b, ...)
81 Now we can get a (recursive) equation from the @data@ decl:
83 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
84 u Eq (T b a) u Eq Int -- From C2
85 u Eq (T a a) -- From C3
87 Foo and Bar may have explicit instances for @Eq@, in which case we can
88 just substitute for them. Alternatively, either or both may have
89 their @Eq@ instances given by @deriving@ clauses, in which case they
90 form part of the system of equations.
92 Now all we need do is simplify and solve the equations, iterating to
93 find the least fixpoint. Notice that the order of the arguments can
94 switch around, as here in the recursive calls to T.
96 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
100 Eq (T a b) = {} -- The empty set
103 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
104 u Eq (T b a) u Eq Int -- From C2
105 u Eq (T a a) -- From C3
107 After simplification:
108 = Eq a u Ping b u {} u {} u {}
113 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
114 u Eq (T b a) u Eq Int -- From C2
115 u Eq (T a a) -- From C3
117 After simplification:
122 = Eq a u Ping b u Eq b u Ping a
124 The next iteration gives the same result, so this is the fixpoint. We
125 need to make a canonical form of the RHS to ensure convergence. We do
126 this by simplifying the RHS to a form in which
128 - the classes constrain only tyvars
129 - the list is sorted by tyvar (major key) and then class (minor key)
130 - no duplicates, of course
132 So, here are the synonyms for the ``equation'' structures:
135 type DerivEqn = (SrcSpan, InstOrigin, Name, Class, TyCon, [TyVar], DerivRhs)
136 -- The Name is the name for the DFun we'll build
137 -- The tyvars bind all the variables in the RHS
138 -- For family indexes, the tycon is the representation tycon
140 pprDerivEqn :: DerivEqn -> SDoc
141 pprDerivEqn (l, _, n, c, tc, tvs, rhs)
142 = parens (hsep [ppr l, ppr n, ppr c, ppr origTc, ppr tys] <+> equals <+>
145 (origTc, tys) = tyConOrigHead tc
147 type DerivRhs = ThetaType
148 type DerivSoln = DerivRhs
152 [Data decl contexts] A note about contexts on data decls
153 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
156 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
158 We will need an instance decl like:
160 instance (Read a, RealFloat a) => Read (Complex a) where
163 The RealFloat in the context is because the read method for Complex is bound
164 to construct a Complex, and doing that requires that the argument type is
167 But this ain't true for Show, Eq, Ord, etc, since they don't construct
168 a Complex; they only take them apart.
170 Our approach: identify the offending classes, and add the data type
171 context to the instance decl. The "offending classes" are
175 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
176 pattern matching against a constructor from a data type with a context
177 gives rise to the constraints for that context -- or at least the thinned
178 version. So now all classes are "offending".
185 newtype T = T Char deriving( C [a] )
187 Notice the free 'a' in the deriving. We have to fill this out to
188 newtype T = T Char deriving( forall a. C [a] )
190 And then translate it to:
191 instance C [a] Char => C [a] T where ...
196 %************************************************************************
198 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
200 %************************************************************************
203 tcDeriving :: [LTyClDecl Name] -- All type constructors
204 -> [LDerivDecl Name] -- All stand-alone deriving declarations
205 -> TcM ([InstInfo], -- The generated "instance decls"
206 HsValBinds Name) -- Extra generated top-level bindings
208 tcDeriving tycl_decls deriv_decls
209 = recoverM (returnM ([], emptyValBindsOut)) $
210 do { -- Fish the "deriving"-related information out of the TcEnv
211 -- and make the necessary "equations".
212 overlap_flag <- getOverlapFlag
213 ; (ordinary_eqns, newtype_inst_info)
214 <- makeDerivEqns overlap_flag tycl_decls deriv_decls
216 ; (ordinary_inst_info, deriv_binds)
217 <- extendLocalInstEnv (map iSpec newtype_inst_info) $
218 deriveOrdinaryStuff overlap_flag ordinary_eqns
219 -- Add the newtype-derived instances to the inst env
220 -- before tacking the "ordinary" ones
222 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
224 -- If we are compiling a hs-boot file,
225 -- don't generate any derived bindings
226 ; is_boot <- tcIsHsBoot
228 return (inst_info, emptyValBindsOut)
232 -- Generate the generic to/from functions from each type declaration
233 ; gen_binds <- mkGenericBinds tycl_decls
235 -- Rename these extra bindings, discarding warnings about unused bindings etc
236 -- Set -fglasgow exts so that we can have type signatures in patterns,
237 -- which is used in the generic binds
239 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
240 { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn deriv_binds [])
241 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
242 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
244 ; return (rn_deriv `plusHsValBinds` rn_gen) }
248 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
249 (ddump_deriving inst_info rn_binds))
251 ; returnM (inst_info, rn_binds)
254 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
255 ddump_deriving inst_infos extra_binds
256 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
258 -----------------------------------------
259 deriveOrdinaryStuff overlap_flag [] -- Short cut
260 = returnM ([], emptyLHsBinds)
262 deriveOrdinaryStuff overlap_flag eqns
263 = do { -- Take the equation list and solve it, to deliver a list of
264 -- solutions, a.k.a. the contexts for the instance decls
265 -- required for the corresponding equations.
266 inst_specs <- solveDerivEqns overlap_flag eqns
268 -- Generate the InstInfo for each dfun,
269 -- plus any auxiliary bindings it needs
270 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst inst_specs
272 -- Generate any extra not-one-inst-decl-specific binds,
273 -- notably "con2tag" and/or "tag2con" functions.
274 ; extra_binds <- genTaggeryBinds inst_infos
277 ; returnM (map fst inst_infos,
278 unionManyBags (extra_binds : aux_binds_s))
281 -----------------------------------------
282 mkGenericBinds tycl_decls
283 = do { tcs <- mapM tcLookupTyCon
285 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
286 -- We are only interested in the data type declarations
287 ; return (unionManyBags [ mkTyConGenericBinds tc |
288 tc <- tcs, tyConHasGenerics tc ]) }
289 -- And then only in the ones whose 'has-generics' flag is on
293 %************************************************************************
295 \subsection[TcDeriv-eqns]{Forming the equations}
297 %************************************************************************
299 @makeDerivEqns@ fishes around to find the info about needed derived
300 instances. Complicating factors:
303 We can only derive @Enum@ if the data type is an enumeration
304 type (all nullary data constructors).
307 We can only derive @Ix@ if the data type is an enumeration {\em
308 or} has just one data constructor (e.g., tuples).
311 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
314 Note [Newtype deriving superclasses]
315 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
316 The 'tys' here come from the partial application in the deriving
317 clause. The last arg is the new instance type.
319 We must pass the superclasses; the newtype might be an instance
320 of them in a different way than the representation type
321 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
322 Then the Show instance is not done via isomorphism; it shows
324 The Num instance is derived via isomorphism, but the Show superclass
325 dictionary must the Show instance for Foo, *not* the Show dictionary
326 gotten from the Num dictionary. So we must build a whole new dictionary
327 not just use the Num one. The instance we want is something like:
328 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
331 There may be a coercion needed which we get from the tycon for the newtype
332 when the dict is constructed in TcInstDcls.tcInstDecl2
336 type DerivSpec = (SrcSpan, -- location of the deriving clause
337 InstOrigin, -- deriving at data decl or standalone?
338 NewOrData, -- newtype or data type
339 Name, -- Type constructor for which we derive
340 Maybe [LHsType Name], -- Type indexes if indexed type
341 LHsType Name) -- Class instance to be generated
343 makeDerivEqns :: OverlapFlag
346 -> TcM ([DerivEqn], -- Ordinary derivings
347 [InstInfo]) -- Special newtype derivings
349 makeDerivEqns overlap_flag tycl_decls deriv_decls
350 = do derive_top_level <- mapM top_level_deriv deriv_decls
351 (maybe_ordinaries, maybe_newtypes)
352 <- mapAndUnzipM mk_eqn (derive_data ++ catMaybes derive_top_level)
353 return (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
355 ------------------------------------------------------------------
356 -- Deriving clauses at data declarations
357 derive_data :: [DerivSpec]
358 derive_data = [ (loc, DerivOrigin, nd, tycon, tyPats, pred)
359 | L loc (TyData { tcdND = nd, tcdLName = L _ tycon,
361 tcdDerivs = Just preds }) <- tycl_decls,
364 -- Standalone deriving declarations
365 top_level_deriv :: LDerivDecl Name -> TcM (Maybe DerivSpec)
366 top_level_deriv d@(L loc (DerivDecl inst ty_name)) =
367 recoverM (returnM Nothing) $ setSrcSpan loc $
368 do tycon <- tcLookupLocatedTyCon ty_name
369 let new_or_data = if isNewTyCon tycon then NewType else DataType
370 traceTc (text "Stand-alone deriving:" <+>
371 ppr (new_or_data, unLoc ty_name, inst))
372 return $ Just (loc, StandAloneDerivOrigin, new_or_data,
373 unLoc ty_name, Nothing, inst)
375 ------------------------------------------------------------------
376 -- Derive equation/inst info for one deriving clause (data or standalone)
377 mk_eqn :: DerivSpec -> TcM (Maybe DerivEqn, Maybe InstInfo)
378 -- We swizzle the tyvars and datacons out of the tycon
379 -- to make the rest of the equation
381 -- The "deriv_ty" is a LHsType to take account of the fact that for
382 -- newtype deriving we allow deriving (forall a. C [a]).
384 mk_eqn (loc, orig, new_or_data, tycon_name, mb_tys, hs_deriv_ty)
386 addErrCtxt (derivCtxt tycon_name mb_tys) $
387 do { named_tycon <- tcLookupTyCon tycon_name
389 -- Lookup representation tycon in case of a family instance
390 ; tycon <- case mb_tys of
391 Nothing -> return named_tycon
393 tys <- mapM dsHsType hsTys
394 tcLookupFamInst named_tycon tys
396 -- Enable deriving preds to mention the type variables in the
398 ; tcExtendTyVarEnv (tyConTyVars tycon) $ do
400 { (deriv_tvs, clas, tys) <- tcHsDeriv hs_deriv_ty
401 ; gla_exts <- doptM Opt_GlasgowExts
402 ; mk_eqn_help loc orig gla_exts new_or_data tycon deriv_tvs clas tys
405 ------------------------------------------------------------------
406 -- data/newtype T a = ... deriving( C t1 t2 )
407 -- leads to a call to mk_eqn_help with
408 -- tycon = T, deriv_tvs = ftv(t1,t2), clas = C, tys = [t1,t2]
410 mk_eqn_help loc orig gla_exts DataType tycon deriv_tvs clas tys
411 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
412 = bale_out (derivingThingErr clas tys origTyCon ttys err)
414 = do { eqn <- mkDataTypeEqn loc orig tycon clas
415 ; returnM (Just eqn, Nothing) }
417 (origTyCon, ttys) = tyConOrigHead tycon
419 mk_eqn_help loc orig gla_exts NewType tycon deriv_tvs clas tys
420 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
421 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
422 ; -- Go ahead and use the isomorphism
423 dfun_name <- new_dfun_name clas tycon
424 ; return (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
425 iBinds = NewTypeDerived ntd_info })) }
426 | std_class gla_exts clas
427 = mk_eqn_help loc orig gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
429 | otherwise -- Non-standard instance
430 = bale_out (if gla_exts then
431 cant_derive_err -- Too hard
433 non_std_err) -- Just complain about being a non-std instance
435 -- Here is the plan for newtype derivings. We see
436 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
437 -- where t is a type,
438 -- ak+1...an is a suffix of a1..an
439 -- ak+1...an do not occur free in t, nor in the s1..sm
440 -- (C s1 ... sm) is a *partial applications* of class C
441 -- with the last parameter missing
442 -- (T a1 .. ak) matches the kind of C's last argument
443 -- (and hence so does t)
445 -- We generate the instance
446 -- instance forall ({a1..ak} u fvs(s1..sm)).
447 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
448 -- where T a1...ap is the partial application of
449 -- the LHS of the correct kind and p >= k
451 -- NB: the variables below are:
452 -- tc_tvs = [a1, ..., an]
453 -- tyvars_to_keep = [a1, ..., ak]
454 -- rep_ty = t ak .. an
455 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
456 -- tys = [s1, ..., sm]
459 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
460 -- We generate the instance
461 -- instance Monad (ST s) => Monad (T s) where
463 clas_tyvars = classTyVars clas
464 kind = tyVarKind (last clas_tyvars)
465 -- Kind of the thing we want to instance
466 -- e.g. argument kind of Monad, *->*
468 (arg_kinds, _) = splitKindFunTys kind
469 n_args_to_drop = length arg_kinds
470 -- Want to drop 1 arg from (T s a) and (ST s a)
471 -- to get instance Monad (ST s) => Monad (T s)
473 -- Note [newtype representation]
474 -- Need newTyConRhs *not* newTyConRep to get the representation
475 -- type, because the latter looks through all intermediate newtypes
477 -- newtype B = MkB Int
478 -- newtype A = MkA B deriving( Num )
479 -- We want the Num instance of B, *not* the Num instance of Int,
480 -- when making the Num instance of A!
481 (tc_tvs, rep_ty) = newTyConRhs tycon
482 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
484 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
485 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
486 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
488 n_args_to_keep = length rep_ty_args - n_args_to_drop
489 args_to_drop = drop n_args_to_keep rep_ty_args
490 args_to_keep = take n_args_to_keep rep_ty_args
492 rep_fn' = mkAppTys rep_fn args_to_keep
493 rep_tys = tys ++ [rep_fn']
494 rep_pred = mkClassPred clas rep_tys
495 -- rep_pred is the representation dictionary, from where
496 -- we are gong to get all the methods for the newtype dictionary
498 -- Next we figure out what superclass dictionaries to use
499 -- See Note [Newtype deriving superclasses] above
501 inst_tys = tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)]
502 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
505 -- If there are no tyvars, there's no need
506 -- to abstract over the dictionaries we need
507 -- Example: newtype T = MkT Int deriving( C )
508 -- We get the derived instance
511 -- instance C Int => C T
512 dict_tvs = deriv_tvs ++ tyvars_to_keep
513 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
514 (dict_args, ntd_info) | null dict_tvs = ([], Just all_preds)
515 | otherwise = (all_preds, Nothing)
517 -- Finally! Here's where we build the dictionary Id
518 mk_inst_spec dfun_name = mkLocalInstance dfun overlap_flag
520 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
522 -------------------------------------------------------------------
523 -- Figuring out whether we can only do this newtype-deriving thing
525 right_arity = length tys + 1 == classArity clas
527 -- Never derive Read,Show,Typeable,Data this way
528 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
529 can_derive_via_isomorphism
530 = not (getUnique clas `elem` non_iso_classes)
531 && right_arity -- Well kinded;
532 -- eg not: newtype T ... deriving( ST )
533 -- because ST needs *2* type params
534 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
535 -- eg not: newtype T = T Int deriving( Monad )
536 && n_args_to_keep >= 0 -- Rep type has right kind:
537 -- eg not: newtype T a = T Int deriving( Monad )
538 && eta_ok -- Eta reduction works
539 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
540 -- newtype A = MkA [A]
542 -- instance Eq [A] => Eq A !!
543 -- Here's a recursive newtype that's actually OK
544 -- newtype S1 = S1 [T1 ()]
545 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
546 -- It's currently rejected. Oh well.
547 -- In fact we generate an instance decl that has method of form
548 -- meth @ instTy = meth @ repTy
549 -- (no coerce's). We'd need a coerce if we wanted to handle
550 -- recursive newtypes too
552 -- Check that eta reduction is OK
553 -- (a) the dropped-off args are identical
554 -- (b) the remaining type args do not mention any of teh dropped type variables
555 -- (c) the type class args do not mention any of teh dropped type variables
556 dropped_tvs = mkVarSet tyvars_to_drop
557 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
558 && (tyVarsOfType rep_fn' `disjointVarSet` dropped_tvs)
559 && (tyVarsOfTypes tys `disjointVarSet` dropped_tvs)
561 cant_derive_err = derivingThingErr clas tys tycon (mkTyVarTys tyvars_to_keep)
562 (vcat [ptext SLIT("even with cunning newtype deriving:"),
563 if isRecursiveTyCon tycon then
564 ptext SLIT("the newtype is recursive")
566 if not right_arity then
567 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
569 if not (n_tyvars_to_keep >= 0) then
570 ptext SLIT("the type constructor has wrong kind")
571 else if not (n_args_to_keep >= 0) then
572 ptext SLIT("the representation type has wrong kind")
573 else if not eta_ok then
574 ptext SLIT("the eta-reduction property does not hold")
578 non_std_err = derivingThingErr clas tys tycon (mkTyVarTys tyvars_to_keep)
579 (vcat [non_std_why clas,
580 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
582 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
584 std_class gla_exts clas
585 = key `elem` derivableClassKeys
586 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
590 std_class_via_iso clas -- These standard classes can be derived for a newtype
591 -- using the isomorphism trick *even if no -fglasgow-exts*
592 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
593 -- Not Read/Show because they respect the type
594 -- Not Enum, becuase newtypes are never in Enum
597 new_dfun_name clas tycon -- Just a simple wrapper
598 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
599 -- The type passed to newDFunName is only used to generate
600 -- a suitable string; hence the empty type arg list
602 ------------------------------------------------------------------
603 mkDataTypeEqn :: SrcSpan -> InstOrigin -> TyCon -> Class -> TcM DerivEqn
604 mkDataTypeEqn loc orig tycon clas
605 | clas `hasKey` typeableClassKey
606 = -- The Typeable class is special in several ways
607 -- data T a b = ... deriving( Typeable )
609 -- instance Typeable2 T where ...
611 -- 1. There are no constraints in the instance
612 -- 2. There are no type variables either
613 -- 3. The actual class we want to generate isn't necessarily
614 -- Typeable; it depends on the arity of the type
615 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
616 ; dfun_name <- new_dfun_name real_clas tycon
617 ; return (loc, orig, dfun_name, real_clas, tycon, [], []) }
620 = do { dfun_name <- new_dfun_name clas tycon
621 ; return (loc, orig, dfun_name, clas, tycon, tyvars, constraints)
624 tyvars = tyConTyVars tycon
625 constraints = extra_constraints ++ ordinary_constraints
626 extra_constraints = tyConStupidTheta tycon
627 -- "extra_constraints": see note [Data decl contexts] above
630 = [ mkClassPred clas [arg_ty]
631 | data_con <- tyConDataCons tycon,
632 arg_ty <- dataConInstOrigArgTys data_con (mkTyVarTys tyvars),
633 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
637 ------------------------------------------------------------------
638 -- Check side conditions that dis-allow derivability for particular classes
639 -- This is *apart* from the newtype-deriving mechanism
641 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
642 checkSideConditions gla_exts tycon deriv_tvs clas tys
643 | notNull deriv_tvs || notNull tys
644 = Just ty_args_why -- e.g. deriving( Foo s )
646 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
647 [] -> Just (non_std_why clas)
648 [cond] -> cond (gla_exts, tycon)
649 other -> pprPanic "checkSideConditions" (ppr clas)
651 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
653 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
655 sideConditions :: [(Unique, Condition)]
657 = [ (eqClassKey, cond_std),
658 (ordClassKey, cond_std),
659 (readClassKey, cond_std),
660 (showClassKey, cond_std),
661 (enumClassKey, cond_std `andCond` cond_isEnumeration),
662 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
663 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
664 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
665 (dataClassKey, cond_glaExts `andCond` cond_std)
668 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
670 orCond :: Condition -> Condition -> Condition
673 Nothing -> Nothing -- c1 succeeds
674 Just x -> case c2 tc of -- c1 fails
676 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
679 andCond c1 c2 tc = case c1 tc of
680 Nothing -> c2 tc -- c1 succeeds
681 Just x -> Just x -- c1 fails
683 cond_std :: Condition
684 cond_std (gla_exts, tycon)
685 | any (not . isVanillaDataCon) data_cons = Just existential_why
686 | null data_cons = Just no_cons_why
687 | otherwise = Nothing
689 data_cons = tyConDataCons tycon
690 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
691 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
693 cond_isEnumeration :: Condition
694 cond_isEnumeration (gla_exts, tycon)
695 | isEnumerationTyCon tycon = Nothing
696 | otherwise = Just why
698 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
700 cond_isProduct :: Condition
701 cond_isProduct (gla_exts, tycon)
702 | isProductTyCon tycon = Nothing
703 | otherwise = Just why
705 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
707 cond_typeableOK :: Condition
708 -- OK for Typeable class
709 -- Currently: (a) args all of kind *
710 -- (b) 7 or fewer args
711 cond_typeableOK (gla_exts, tycon)
712 | tyConArity tycon > 7 = Just too_many
713 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tycon))
715 | isFamInstTyCon tycon = Just fam_inst -- no Typable for family insts
716 | otherwise = Nothing
718 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
719 bad_kind = quotes (ppr tycon) <+>
720 ptext SLIT("has arguments of kind other than `*'")
721 fam_inst = quotes (ppr tycon) <+> ptext SLIT("is a type family")
723 cond_glaExts :: Condition
724 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
725 | otherwise = Just why
727 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
730 %************************************************************************
732 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
734 %************************************************************************
736 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
737 terms, which is the final correct RHS for the corresponding original
741 Each (k,TyVarTy tv) in a solution constrains only a type
745 The (k,TyVarTy tv) pairs in a solution are canonically
746 ordered by sorting on type varible, tv, (major key) and then class, k,
751 solveDerivEqns :: OverlapFlag
753 -> TcM [Instance]-- Solns in same order as eqns.
754 -- This bunch is Absolutely minimal...
756 solveDerivEqns overlap_flag orig_eqns
757 = iterateDeriv 1 initial_solutions
759 -- The initial solutions for the equations claim that each
760 -- instance has an empty context; this solution is certainly
761 -- in canonical form.
762 initial_solutions :: [DerivSoln]
763 initial_solutions = [ [] | _ <- orig_eqns ]
765 ------------------------------------------------------------------
766 -- iterateDeriv calculates the next batch of solutions,
767 -- compares it with the current one; finishes if they are the
768 -- same, otherwise recurses with the new solutions.
769 -- It fails if any iteration fails
770 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
771 iterateDeriv n current_solns
772 | n > 20 -- Looks as if we are in an infinite loop
773 -- This can happen if we have -fallow-undecidable-instances
774 -- (See TcSimplify.tcSimplifyDeriv.)
775 = pprPanic "solveDerivEqns: probable loop"
776 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
779 inst_specs = zipWithEqual "add_solns" mk_inst_spec
780 orig_eqns current_solns
783 -- Extend the inst info from the explicit instance decls
784 -- with the current set of solutions, and simplify each RHS
785 extendLocalInstEnv inst_specs $
786 mappM gen_soln orig_eqns
787 ) `thenM` \ new_solns ->
788 if (current_solns == new_solns) then
791 iterateDeriv (n+1) new_solns
793 ------------------------------------------------------------------
794 gen_soln :: DerivEqn -> TcM [PredType]
795 gen_soln (loc, orig, _, clas, tc, tyvars, deriv_rhs)
797 do { let inst_tys = [origHead]
798 ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
799 tcSimplifyDeriv orig tc tyvars deriv_rhs
800 -- Claim: the result instance declaration is guaranteed valid
801 -- Hence no need to call:
802 -- checkValidInstance tyvars theta clas inst_tys
803 ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
805 origHead = uncurry mkTyConApp (tyConOrigHead tc)
807 ------------------------------------------------------------------
808 mk_inst_spec :: DerivEqn -> DerivSoln -> Instance
809 mk_inst_spec (loc, orig, dfun_name, clas, tycon, tyvars, _) theta
810 = mkLocalInstance dfun overlap_flag
812 dfun = mkDictFunId dfun_name tyvars theta clas [origHead]
813 origHead = uncurry mkTyConApp (tyConOrigHead tycon)
815 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
816 -- Add new locally-defined instances; don't bother to check
817 -- for functional dependency errors -- that'll happen in TcInstDcls
818 extendLocalInstEnv dfuns thing_inside
819 = do { env <- getGblEnv
820 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
821 env' = env { tcg_inst_env = inst_env' }
822 ; setGblEnv env' thing_inside }
825 %************************************************************************
827 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
829 %************************************************************************
831 After all the trouble to figure out the required context for the
832 derived instance declarations, all that's left is to chug along to
833 produce them. They will then be shoved into @tcInstDecls2@, which
834 will do all its usual business.
836 There are lots of possibilities for code to generate. Here are
837 various general remarks.
842 We want derived instances of @Eq@ and @Ord@ (both v common) to be
843 ``you-couldn't-do-better-by-hand'' efficient.
846 Deriving @Show@---also pretty common--- should also be reasonable good code.
849 Deriving for the other classes isn't that common or that big a deal.
856 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
859 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
862 We {\em normally} generate code only for the non-defaulted methods;
863 there are some exceptions for @Eq@ and (especially) @Ord@...
866 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
867 constructor's numeric (@Int#@) tag. These are generated by
868 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
869 these is around is given by @hasCon2TagFun@.
871 The examples under the different sections below will make this
875 Much less often (really just for deriving @Ix@), we use a
876 @_tag2con_<tycon>@ function. See the examples.
879 We use the renamer!!! Reason: we're supposed to be
880 producing @LHsBinds Name@ for the methods, but that means
881 producing correctly-uniquified code on the fly. This is entirely
882 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
883 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
884 the renamer. What a great hack!
888 -- Generate the InstInfo for the required instance paired with the
889 -- *representation* tycon for that instance,
890 -- plus any auxiliary bindings required
892 -- Representation tycons differ from the tycon in the instance signature in
893 -- case of instances for indexed families.
895 genInst :: Instance -> TcM ((InstInfo, TyCon), LHsBinds RdrName)
897 = do { fix_env <- getFixityEnv
899 (tyvars,_,clas,[ty]) = instanceHead spec
900 clas_nm = className clas
901 (visible_tycon, tyArgs) = tcSplitTyConApp ty
903 -- In case of a family instance, we need to use the representation
904 -- tycon (after all it has the data constructors)
905 ; tycon <- if isOpenTyCon visible_tycon
906 then tcLookupFamInst visible_tycon tyArgs
907 else return visible_tycon
908 ; let (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
910 -- Bring the right type variables into
911 -- scope, and rename the method binds
912 -- It's a bit yukky that we return *renamed* InstInfo, but
913 -- *non-renamed* auxiliary bindings
914 ; (rn_meth_binds, _fvs) <- discardWarnings $
915 bindLocalNames (map Var.varName tyvars) $
916 rnMethodBinds clas_nm (\n -> []) [] meth_binds
918 -- Build the InstInfo
919 ; return ((InstInfo { iSpec = spec,
920 iBinds = VanillaInst rn_meth_binds [] }, tycon),
924 genDerivBinds clas fix_env tycon
925 | className clas `elem` typeableClassNames
926 = (gen_Typeable_binds tycon, emptyLHsBinds)
929 = case assocMaybe gen_list (getUnique clas) of
930 Just gen_fn -> gen_fn fix_env tycon
931 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
933 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
934 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
935 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
936 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
937 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
938 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
939 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
940 ,(showClassKey, no_aux_binds gen_Show_binds)
941 ,(readClassKey, no_aux_binds gen_Read_binds)
942 ,(dataClassKey, gen_Data_binds)
945 -- no_aux_binds is used for generators that don't
946 -- need to produce any auxiliary bindings
947 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
948 ignore_fix_env f fix_env tc = f tc
952 %************************************************************************
954 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
956 %************************************************************************
961 con2tag_Foo :: Foo ... -> Int#
962 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
963 maxtag_Foo :: Int -- ditto (NB: not unlifted)
966 We have a @con2tag@ function for a tycon if:
969 We're deriving @Eq@ and the tycon has nullary data constructors.
972 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
976 We have a @tag2con@ function for a tycon if:
979 We're deriving @Enum@, or @Ix@ (enum type only???)
982 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
985 genTaggeryBinds :: [(InstInfo, TyCon)] -> TcM (LHsBinds RdrName)
986 genTaggeryBinds infos
987 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
988 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
989 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
991 all_CTs = [ (fst (simpleInstInfoClsTy info), tc)
992 | (info, tc) <- infos]
993 all_tycons = map snd all_CTs
994 (tycons_of_interest, _) = removeDups compare all_tycons
996 do_con2tag acc_Names tycon
997 | isDataTyCon tycon &&
998 ((we_are_deriving eqClassKey tycon
999 && any isNullarySrcDataCon (tyConDataCons tycon))
1000 || (we_are_deriving ordClassKey tycon
1001 && not (isProductTyCon tycon))
1002 || (we_are_deriving enumClassKey tycon)
1003 || (we_are_deriving ixClassKey tycon))
1005 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
1010 do_tag2con acc_Names tycon
1011 | isDataTyCon tycon &&
1012 (we_are_deriving enumClassKey tycon ||
1013 we_are_deriving ixClassKey tycon
1014 && isEnumerationTyCon tycon)
1015 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
1016 : (maxtag_RDR tycon, tycon, GenMaxTag)
1021 we_are_deriving clas_key tycon
1022 = is_in_eqns clas_key tycon all_CTs
1024 is_in_eqns clas_key tycon [] = False
1025 is_in_eqns clas_key tycon ((c,t):cts)
1026 = (clas_key == classKey c && tycon == t)
1027 || is_in_eqns clas_key tycon cts
1031 derivingThingErr clas tys tycon ttys why
1032 = sep [hsep [ptext SLIT("Can't make a derived instance of"),
1034 nest 2 (parens why)]
1036 pred = mkClassPred clas (tys ++ [mkTyConApp tycon ttys])
1038 derivCtxt :: Name -> Maybe [LHsType Name] -> SDoc
1039 derivCtxt tycon mb_tys
1040 = ptext SLIT("When deriving instances for") <+> quotes typeInst
1042 typeInst = case mb_tys of
1043 Nothing -> ppr tycon
1044 Just tys -> ppr tycon <+>
1045 hsep (map (pprParendHsType . unLoc) tys)
1047 derivInstCtxt1 clas inst_tys
1048 = ptext SLIT("When deriving the instance for") <+>
1049 quotes (pprClassPred clas inst_tys)