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"
20 import TcGenDeriv -- Deriv stuff
51 %************************************************************************
53 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
55 %************************************************************************
59 data T a b = C1 (Foo a) (Bar b)
64 [NOTE: See end of these comments for what to do with
65 data (C a, D b) => T a b = ...
68 We want to come up with an instance declaration of the form
70 instance (Ping a, Pong b, ...) => Eq (T a b) where
73 It is pretty easy, albeit tedious, to fill in the code "...". The
74 trick is to figure out what the context for the instance decl is,
75 namely @Ping@, @Pong@ and friends.
77 Let's call the context reqd for the T instance of class C at types
78 (a,b, ...) C (T a b). Thus:
80 Eq (T a b) = (Ping a, Pong b, ...)
82 Now we can get a (recursive) equation from the @data@ decl:
84 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
85 u Eq (T b a) u Eq Int -- From C2
86 u Eq (T a a) -- From C3
88 Foo and Bar may have explicit instances for @Eq@, in which case we can
89 just substitute for them. Alternatively, either or both may have
90 their @Eq@ instances given by @deriving@ clauses, in which case they
91 form part of the system of equations.
93 Now all we need do is simplify and solve the equations, iterating to
94 find the least fixpoint. Notice that the order of the arguments can
95 switch around, as here in the recursive calls to T.
97 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
101 Eq (T a b) = {} -- The empty set
104 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
105 u Eq (T b a) u Eq Int -- From C2
106 u Eq (T a a) -- From C3
108 After simplification:
109 = Eq a u Ping b u {} u {} u {}
114 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
115 u Eq (T b a) u Eq Int -- From C2
116 u Eq (T a a) -- From C3
118 After simplification:
123 = Eq a u Ping b u Eq b u Ping a
125 The next iteration gives the same result, so this is the fixpoint. We
126 need to make a canonical form of the RHS to ensure convergence. We do
127 this by simplifying the RHS to a form in which
129 - the classes constrain only tyvars
130 - the list is sorted by tyvar (major key) and then class (minor key)
131 - no duplicates, of course
133 So, here are the synonyms for the ``equation'' structures:
136 type DerivEqn = (SrcSpan, InstOrigin, Name, Class, TyCon, [TyVar], DerivRhs)
137 -- The Name is the name for the DFun we'll build
138 -- The tyvars bind all the variables in the RHS
140 pprDerivEqn :: DerivEqn -> SDoc
141 pprDerivEqn (l,_,n,c,tc,tvs,rhs)
142 = parens (hsep [ppr l, ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
144 type DerivRhs = ThetaType
145 type DerivSoln = DerivRhs
149 [Data decl contexts] A note about contexts on data decls
150 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
153 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
155 We will need an instance decl like:
157 instance (Read a, RealFloat a) => Read (Complex a) where
160 The RealFloat in the context is because the read method for Complex is bound
161 to construct a Complex, and doing that requires that the argument type is
164 But this ain't true for Show, Eq, Ord, etc, since they don't construct
165 a Complex; they only take them apart.
167 Our approach: identify the offending classes, and add the data type
168 context to the instance decl. The "offending classes" are
172 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
173 pattern matching against a constructor from a data type with a context
174 gives rise to the constraints for that context -- or at least the thinned
175 version. So now all classes are "offending".
182 newtype T = T Char deriving( C [a] )
184 Notice the free 'a' in the deriving. We have to fill this out to
185 newtype T = T Char deriving( forall a. C [a] )
187 And then translate it to:
188 instance C [a] Char => C [a] T where ...
193 %************************************************************************
195 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
197 %************************************************************************
200 tcDeriving :: [LTyClDecl Name] -- All type constructors
201 -> [LDerivDecl Name] -- All stand-alone deriving declarations
202 -> TcM ([InstInfo], -- The generated "instance decls"
203 HsValBinds Name) -- Extra generated top-level bindings
205 tcDeriving tycl_decls deriv_decls
206 = recoverM (returnM ([], emptyValBindsOut)) $
207 do { -- Fish the "deriving"-related information out of the TcEnv
208 -- and make the necessary "equations".
209 overlap_flag <- getOverlapFlag
210 ; (ordinary_eqns, newtype_inst_info)
211 <- makeDerivEqns overlap_flag tycl_decls deriv_decls
213 ; (ordinary_inst_info, deriv_binds)
214 <- extendLocalInstEnv (map iSpec newtype_inst_info) $
215 deriveOrdinaryStuff overlap_flag ordinary_eqns
216 -- Add the newtype-derived instances to the inst env
217 -- before tacking the "ordinary" ones
219 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
221 -- If we are compiling a hs-boot file,
222 -- don't generate any derived bindings
223 ; is_boot <- tcIsHsBoot
225 return (inst_info, emptyValBindsOut)
229 -- Generate the generic to/from functions from each type declaration
230 ; gen_binds <- mkGenericBinds tycl_decls
232 -- Rename these extra bindings, discarding warnings about unused bindings etc
233 -- Set -fglasgow exts so that we can have type signatures in patterns,
234 -- which is used in the generic binds
236 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
237 { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn deriv_binds [])
238 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
239 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
241 ; return (rn_deriv `plusHsValBinds` rn_gen) }
245 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
246 (ddump_deriving inst_info rn_binds))
248 ; returnM (inst_info, rn_binds)
251 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
252 ddump_deriving inst_infos extra_binds
253 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
255 -----------------------------------------
256 deriveOrdinaryStuff overlap_flag [] -- Short cut
257 = returnM ([], emptyLHsBinds)
259 deriveOrdinaryStuff overlap_flag eqns
260 = do { -- Take the equation list and solve it, to deliver a list of
261 -- solutions, a.k.a. the contexts for the instance decls
262 -- required for the corresponding equations.
263 inst_specs <- solveDerivEqns overlap_flag eqns
265 -- Generate the InstInfo for each dfun,
266 -- plus any auxiliary bindings it needs
267 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst inst_specs
269 -- Generate any extra not-one-inst-decl-specific binds,
270 -- notably "con2tag" and/or "tag2con" functions.
271 ; extra_binds <- genTaggeryBinds inst_infos
274 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
277 -----------------------------------------
278 mkGenericBinds tycl_decls
279 = do { tcs <- mapM tcLookupTyCon
281 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
282 -- We are only interested in the data type declarations
283 ; return (unionManyBags [ mkTyConGenericBinds tc |
284 tc <- tcs, tyConHasGenerics tc ]) }
285 -- And then only in the ones whose 'has-generics' flag is on
289 %************************************************************************
291 \subsection[TcDeriv-eqns]{Forming the equations}
293 %************************************************************************
295 @makeDerivEqns@ fishes around to find the info about needed derived
296 instances. Complicating factors:
299 We can only derive @Enum@ if the data type is an enumeration
300 type (all nullary data constructors).
303 We can only derive @Ix@ if the data type is an enumeration {\em
304 or} has just one data constructor (e.g., tuples).
307 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
310 Note [Newtype deriving superclasses]
311 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
312 The 'tys' here come from the partial application in the deriving
313 clause. The last arg is the new instance type.
315 We must pass the superclasses; the newtype might be an instance
316 of them in a different way than the representation type
317 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
318 Then the Show instance is not done via isomorphism; it shows
320 The Num instance is derived via isomorphism, but the Show superclass
321 dictionary must the Show instance for Foo, *not* the Show dictionary
322 gotten from the Num dictionary. So we must build a whole new dictionary
323 not just use the Num one. The instance we want is something like:
324 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
327 There may be a coercion needed which we get from the tycon for the newtype
328 when the dict is constructed in TcInstDcls.tcInstDecl2
332 makeDerivEqns :: OverlapFlag
335 -> TcM ([DerivEqn], -- Ordinary derivings
336 [InstInfo]) -- Special newtype derivings
338 makeDerivEqns overlap_flag tycl_decls deriv_decls
339 = do derive_these_top_level <- mapM top_level_deriv deriv_decls >>= return . catMaybes
340 (maybe_ordinaries, maybe_newtypes)
341 <- mapAndUnzipM mk_eqn (derive_these ++ derive_these_top_level)
342 return (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
344 ------------------------------------------------------------------
345 derive_these :: [(SrcSpan, InstOrigin, NewOrData, Name, LHsType Name)]
346 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
347 derive_these = [ (srcLocSpan (getSrcLoc tycon), DerivOrigin, nd, tycon, pred)
348 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
349 tcdDerivs = Just preds }) <- tycl_decls,
352 top_level_deriv :: LDerivDecl Name -> TcM (Maybe (SrcSpan, InstOrigin, NewOrData, Name, LHsType Name))
353 top_level_deriv d@(L l (DerivDecl inst ty_name)) = recoverM (returnM Nothing) $ setSrcSpan l $
354 do tycon <- tcLookupLocatedTyCon ty_name
355 let new_or_data = if isNewTyCon tycon then NewType else DataType
356 traceTc (text "Stand-alone deriving:" <+> ppr (new_or_data, unLoc ty_name, inst))
357 return $ Just (l, StandAloneDerivOrigin, new_or_data, unLoc ty_name, inst)
359 ------------------------------------------------------------------
360 -- takes (whether newtype or data, name of data type, partially applied type class)
361 mk_eqn :: (SrcSpan, InstOrigin, NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
362 -- We swizzle the tyvars and datacons out of the tycon
363 -- to make the rest of the equation
365 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
366 -- we allow deriving (forall a. C [a]).
368 mk_eqn (loc, orig, new_or_data, tycon_name, hs_deriv_ty)
369 = tcLookupTyCon tycon_name `thenM` \ tycon ->
371 addErrCtxt (derivCtxt tycon) $
372 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
373 -- the type variables for the type constructor
374 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
375 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
376 mk_eqn_help loc orig gla_exts new_or_data tycon deriv_tvs clas tys
378 ------------------------------------------------------------------
379 -- data/newtype T a = ... deriving( C t1 t2 )
380 -- leads to a call to mk_eqn_help with
381 -- tycon = T, deriv_tvs = ftv(t1,t2), clas = C, tys = [t1,t2]
383 mk_eqn_help loc orig gla_exts DataType tycon deriv_tvs clas tys
384 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
385 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
387 = do { eqn <- mkDataTypeEqn loc orig tycon clas
388 ; returnM (Just eqn, Nothing) }
390 mk_eqn_help loc orig gla_exts NewType tycon deriv_tvs clas tys
391 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
392 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
393 ; -- Go ahead and use the isomorphism
394 dfun_name <- new_dfun_name clas tycon
395 ; return (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
396 iBinds = NewTypeDerived ntd_info })) }
397 | std_class gla_exts clas
398 = mk_eqn_help loc orig gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
400 | otherwise -- Non-standard instance
401 = bale_out (if gla_exts then
402 cant_derive_err -- Too hard
404 non_std_err) -- Just complain about being a non-std instance
406 -- Here is the plan for newtype derivings. We see
407 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
408 -- where t is a type,
409 -- ak+1...an is a suffix of a1..an
410 -- ak+1...an do not occur free in t, nor in the s1..sm
411 -- (C s1 ... sm) is a *partial applications* of class C
412 -- with the last parameter missing
413 -- (T a1 .. ak) matches the kind of C's last argument
414 -- (and hence so does t)
416 -- We generate the instance
417 -- instance forall ({a1..ak} u fvs(s1..sm)).
418 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
419 -- where T a1...ap is the partial application of
420 -- the LHS of the correct kind and p >= k
422 -- NB: the variables below are:
423 -- tc_tvs = [a1, ..., an]
424 -- tyvars_to_keep = [a1, ..., ak]
425 -- rep_ty = t ak .. an
426 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
427 -- tys = [s1, ..., sm]
430 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
431 -- We generate the instance
432 -- instance Monad (ST s) => Monad (T s) where
434 clas_tyvars = classTyVars clas
435 kind = tyVarKind (last clas_tyvars)
436 -- Kind of the thing we want to instance
437 -- e.g. argument kind of Monad, *->*
439 (arg_kinds, _) = splitKindFunTys kind
440 n_args_to_drop = length arg_kinds
441 -- Want to drop 1 arg from (T s a) and (ST s a)
442 -- to get instance Monad (ST s) => Monad (T s)
444 -- Note [newtype representation]
445 -- Need newTyConRhs *not* newTyConRep to get the representation
446 -- type, because the latter looks through all intermediate newtypes
448 -- newtype B = MkB Int
449 -- newtype A = MkA B deriving( Num )
450 -- We want the Num instance of B, *not* the Num instance of Int,
451 -- when making the Num instance of A!
452 (tc_tvs, rep_ty) = newTyConRhs tycon
453 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
455 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
456 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
457 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
459 n_args_to_keep = length rep_ty_args - n_args_to_drop
460 args_to_drop = drop n_args_to_keep rep_ty_args
461 args_to_keep = take n_args_to_keep rep_ty_args
463 rep_fn' = mkAppTys rep_fn args_to_keep
464 rep_tys = tys ++ [rep_fn']
465 rep_pred = mkClassPred clas rep_tys
466 -- rep_pred is the representation dictionary, from where
467 -- we are gong to get all the methods for the newtype dictionary
469 -- Next we figure out what superclass dictionaries to use
470 -- See Note [Newtype deriving superclasses] above
472 inst_tys = tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)]
473 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
476 -- If there are no tyvars, there's no need
477 -- to abstract over the dictionaries we need
478 -- Example: newtype T = MkT Int deriving( C )
479 -- We get the derived instance
482 -- instance C Int => C T
483 dict_tvs = deriv_tvs ++ tyvars_to_keep
484 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
485 (dict_args, ntd_info) | null dict_tvs = ([], Just all_preds)
486 | otherwise = (all_preds, Nothing)
488 -- Finally! Here's where we build the dictionary Id
489 mk_inst_spec dfun_name = mkLocalInstance dfun overlap_flag
491 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
493 -------------------------------------------------------------------
494 -- Figuring out whether we can only do this newtype-deriving thing
496 right_arity = length tys + 1 == classArity clas
498 -- Never derive Read,Show,Typeable,Data this way
499 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
500 can_derive_via_isomorphism
501 = not (getUnique clas `elem` non_iso_classes)
502 && right_arity -- Well kinded;
503 -- eg not: newtype T ... deriving( ST )
504 -- because ST needs *2* type params
505 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
506 -- eg not: newtype T = T Int deriving( Monad )
507 && n_args_to_keep >= 0 -- Rep type has right kind:
508 -- eg not: newtype T a = T Int deriving( Monad )
509 && eta_ok -- Eta reduction works
510 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
511 -- newtype A = MkA [A]
513 -- instance Eq [A] => Eq A !!
514 -- Here's a recursive newtype that's actually OK
515 -- newtype S1 = S1 [T1 ()]
516 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
517 -- It's currently rejected. Oh well.
518 -- In fact we generate an instance decl that has method of form
519 -- meth @ instTy = meth @ repTy
520 -- (no coerce's). We'd need a coerce if we wanted to handle
521 -- recursive newtypes too
523 -- Check that eta reduction is OK
524 -- (a) the dropped-off args are identical
525 -- (b) the remaining type args do not mention any of teh dropped type variables
526 -- (c) the type class args do not mention any of teh dropped type variables
527 dropped_tvs = mkVarSet tyvars_to_drop
528 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
529 && (tyVarsOfType rep_fn' `disjointVarSet` dropped_tvs)
530 && (tyVarsOfTypes tys `disjointVarSet` dropped_tvs)
532 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
533 (vcat [ptext SLIT("even with cunning newtype deriving:"),
534 if isRecursiveTyCon tycon then
535 ptext SLIT("the newtype is recursive")
537 if not right_arity then
538 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
540 if not (n_tyvars_to_keep >= 0) then
541 ptext SLIT("the type constructor has wrong kind")
542 else if not (n_args_to_keep >= 0) then
543 ptext SLIT("the representation type has wrong kind")
544 else if not eta_ok then
545 ptext SLIT("the eta-reduction property does not hold")
549 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
550 (vcat [non_std_why clas,
551 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
553 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
555 std_class gla_exts clas
556 = key `elem` derivableClassKeys
557 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
561 std_class_via_iso clas -- These standard classes can be derived for a newtype
562 -- using the isomorphism trick *even if no -fglasgow-exts*
563 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
564 -- Not Read/Show because they respect the type
565 -- Not Enum, becuase newtypes are never in Enum
568 new_dfun_name clas tycon -- Just a simple wrapper
569 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
570 -- The type passed to newDFunName is only used to generate
571 -- a suitable string; hence the empty type arg list
573 ------------------------------------------------------------------
574 mkDataTypeEqn :: SrcSpan -> InstOrigin -> TyCon -> Class -> TcM DerivEqn
575 mkDataTypeEqn loc orig tycon clas
576 | clas `hasKey` typeableClassKey
577 = -- The Typeable class is special in several ways
578 -- data T a b = ... deriving( Typeable )
580 -- instance Typeable2 T where ...
582 -- 1. There are no constraints in the instance
583 -- 2. There are no type variables either
584 -- 3. The actual class we want to generate isn't necessarily
585 -- Typeable; it depends on the arity of the type
586 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
587 ; dfun_name <- new_dfun_name real_clas tycon
588 ; return (loc, orig, dfun_name, real_clas, tycon, [], []) }
591 = do { dfun_name <- new_dfun_name clas tycon
592 ; return (loc, orig, dfun_name, clas, tycon, tyvars, constraints) }
594 tyvars = tyConTyVars tycon
595 constraints = extra_constraints ++ ordinary_constraints
596 extra_constraints = tyConStupidTheta tycon
597 -- "extra_constraints": see note [Data decl contexts] above
600 = [ mkClassPred clas [arg_ty]
601 | data_con <- tyConDataCons tycon,
602 arg_ty <- dataConInstOrigArgTys data_con (map mkTyVarTy (tyConTyVars tycon)),
603 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
607 ------------------------------------------------------------------
608 -- Check side conditions that dis-allow derivability for particular classes
609 -- This is *apart* from the newtype-deriving mechanism
611 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
612 checkSideConditions gla_exts tycon deriv_tvs clas tys
613 | notNull deriv_tvs || notNull tys
614 = Just ty_args_why -- e.g. deriving( Foo s )
616 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
617 [] -> Just (non_std_why clas)
618 [cond] -> cond (gla_exts, tycon)
619 other -> pprPanic "checkSideConditions" (ppr clas)
621 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
623 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
625 sideConditions :: [(Unique, Condition)]
627 = [ (eqClassKey, cond_std),
628 (ordClassKey, cond_std),
629 (readClassKey, cond_std),
630 (showClassKey, cond_std),
631 (enumClassKey, cond_std `andCond` cond_isEnumeration),
632 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
633 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
634 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
635 (dataClassKey, cond_glaExts `andCond` cond_std)
638 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
640 orCond :: Condition -> Condition -> Condition
643 Nothing -> Nothing -- c1 succeeds
644 Just x -> case c2 tc of -- c1 fails
646 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
649 andCond c1 c2 tc = case c1 tc of
650 Nothing -> c2 tc -- c1 succeeds
651 Just x -> Just x -- c1 fails
653 cond_std :: Condition
654 cond_std (gla_exts, tycon)
655 | any (not . isVanillaDataCon) data_cons = Just existential_why
656 | null data_cons = Just no_cons_why
657 | otherwise = Nothing
659 data_cons = tyConDataCons tycon
660 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
661 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
663 cond_isEnumeration :: Condition
664 cond_isEnumeration (gla_exts, tycon)
665 | isEnumerationTyCon tycon = Nothing
666 | otherwise = Just why
668 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
670 cond_isProduct :: Condition
671 cond_isProduct (gla_exts, tycon)
672 | isProductTyCon tycon = Nothing
673 | otherwise = Just why
675 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
677 cond_typeableOK :: Condition
678 -- OK for Typeable class
679 -- Currently: (a) args all of kind *
680 -- (b) 7 or fewer args
681 cond_typeableOK (gla_exts, tycon)
682 | tyConArity tycon > 7 = Just too_many
683 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
684 | otherwise = Nothing
686 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
687 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
689 cond_glaExts :: Condition
690 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
691 | otherwise = Just why
693 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
696 %************************************************************************
698 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
700 %************************************************************************
702 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
703 terms, which is the final correct RHS for the corresponding original
707 Each (k,TyVarTy tv) in a solution constrains only a type
711 The (k,TyVarTy tv) pairs in a solution are canonically
712 ordered by sorting on type varible, tv, (major key) and then class, k,
717 solveDerivEqns :: OverlapFlag
719 -> TcM [Instance]-- Solns in same order as eqns.
720 -- This bunch is Absolutely minimal...
722 solveDerivEqns overlap_flag orig_eqns
723 = iterateDeriv 1 initial_solutions
725 -- The initial solutions for the equations claim that each
726 -- instance has an empty context; this solution is certainly
727 -- in canonical form.
728 initial_solutions :: [DerivSoln]
729 initial_solutions = [ [] | _ <- orig_eqns ]
731 ------------------------------------------------------------------
732 -- iterateDeriv calculates the next batch of solutions,
733 -- compares it with the current one; finishes if they are the
734 -- same, otherwise recurses with the new solutions.
735 -- It fails if any iteration fails
736 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
737 iterateDeriv n current_solns
738 | n > 20 -- Looks as if we are in an infinite loop
739 -- This can happen if we have -fallow-undecidable-instances
740 -- (See TcSimplify.tcSimplifyDeriv.)
741 = pprPanic "solveDerivEqns: probable loop"
742 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
745 inst_specs = zipWithEqual "add_solns" mk_inst_spec
746 orig_eqns current_solns
749 -- Extend the inst info from the explicit instance decls
750 -- with the current set of solutions, and simplify each RHS
751 extendLocalInstEnv inst_specs $
752 mappM gen_soln orig_eqns
753 ) `thenM` \ new_solns ->
754 if (current_solns == new_solns) then
757 iterateDeriv (n+1) new_solns
759 ------------------------------------------------------------------
760 gen_soln :: DerivEqn -> TcM [PredType]
761 gen_soln (loc, orig, _, clas, tc,tyvars,deriv_rhs)
763 do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
764 ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
765 tcSimplifyDeriv orig tc tyvars deriv_rhs
766 ; addErrCtxt (derivInstCtxt2 theta clas inst_tys) $
767 checkValidInstance tyvars theta clas inst_tys
768 ; return (sortLe (<=) theta) } -- Canonicalise before returning the soluction
772 ------------------------------------------------------------------
773 mk_inst_spec :: DerivEqn -> DerivSoln -> Instance
774 mk_inst_spec (loc, orig, dfun_name, clas, tycon, tyvars, _) theta
775 = mkLocalInstance dfun overlap_flag
777 dfun = mkDictFunId dfun_name tyvars theta clas
778 [mkTyConApp tycon (mkTyVarTys tyvars)]
780 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
781 -- Add new locally-defined instances; don't bother to check
782 -- for functional dependency errors -- that'll happen in TcInstDcls
783 extendLocalInstEnv dfuns thing_inside
784 = do { env <- getGblEnv
785 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
786 env' = env { tcg_inst_env = inst_env' }
787 ; setGblEnv env' thing_inside }
790 %************************************************************************
792 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
794 %************************************************************************
796 After all the trouble to figure out the required context for the
797 derived instance declarations, all that's left is to chug along to
798 produce them. They will then be shoved into @tcInstDecls2@, which
799 will do all its usual business.
801 There are lots of possibilities for code to generate. Here are
802 various general remarks.
807 We want derived instances of @Eq@ and @Ord@ (both v common) to be
808 ``you-couldn't-do-better-by-hand'' efficient.
811 Deriving @Show@---also pretty common--- should also be reasonable good code.
814 Deriving for the other classes isn't that common or that big a deal.
821 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
824 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
827 We {\em normally} generate code only for the non-defaulted methods;
828 there are some exceptions for @Eq@ and (especially) @Ord@...
831 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
832 constructor's numeric (@Int#@) tag. These are generated by
833 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
834 these is around is given by @hasCon2TagFun@.
836 The examples under the different sections below will make this
840 Much less often (really just for deriving @Ix@), we use a
841 @_tag2con_<tycon>@ function. See the examples.
844 We use the renamer!!! Reason: we're supposed to be
845 producing @LHsBinds Name@ for the methods, but that means
846 producing correctly-uniquified code on the fly. This is entirely
847 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
848 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
849 the renamer. What a great hack!
853 -- Generate the InstInfo for the required instance,
854 -- plus any auxiliary bindings required
855 genInst :: Instance -> TcM (InstInfo, LHsBinds RdrName)
857 = do { fix_env <- getFixityEnv
859 (tyvars,_,clas,[ty]) = instanceHead spec
860 clas_nm = className clas
861 tycon = tcTyConAppTyCon ty
862 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
864 -- Bring the right type variables into
865 -- scope, and rename the method binds
866 -- It's a bit yukky that we return *renamed* InstInfo, but
867 -- *non-renamed* auxiliary bindings
868 ; (rn_meth_binds, _fvs) <- discardWarnings $
869 bindLocalNames (map Var.varName tyvars) $
870 rnMethodBinds clas_nm (\n -> []) [] meth_binds
872 -- Build the InstInfo
873 ; return (InstInfo { iSpec = spec,
874 iBinds = VanillaInst rn_meth_binds [] },
878 genDerivBinds clas fix_env tycon
879 | className clas `elem` typeableClassNames
880 = (gen_Typeable_binds tycon, emptyLHsBinds)
883 = case assocMaybe gen_list (getUnique clas) of
884 Just gen_fn -> gen_fn fix_env tycon
885 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
887 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
888 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
889 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
890 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
891 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
892 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
893 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
894 ,(showClassKey, no_aux_binds gen_Show_binds)
895 ,(readClassKey, no_aux_binds gen_Read_binds)
896 ,(dataClassKey, gen_Data_binds)
899 -- no_aux_binds is used for generators that don't
900 -- need to produce any auxiliary bindings
901 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
902 ignore_fix_env f fix_env tc = f tc
906 %************************************************************************
908 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
910 %************************************************************************
915 con2tag_Foo :: Foo ... -> Int#
916 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
917 maxtag_Foo :: Int -- ditto (NB: not unlifted)
920 We have a @con2tag@ function for a tycon if:
923 We're deriving @Eq@ and the tycon has nullary data constructors.
926 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
930 We have a @tag2con@ function for a tycon if:
933 We're deriving @Enum@, or @Ix@ (enum type only???)
936 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
939 genTaggeryBinds :: [InstInfo] -> TcM (LHsBinds RdrName)
940 genTaggeryBinds infos
941 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
942 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
943 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
945 all_CTs = [ (cls, tcTyConAppTyCon ty)
947 let (cls,ty) = simpleInstInfoClsTy info ]
948 all_tycons = map snd all_CTs
949 (tycons_of_interest, _) = removeDups compare all_tycons
951 do_con2tag acc_Names tycon
952 | isDataTyCon tycon &&
953 ((we_are_deriving eqClassKey tycon
954 && any isNullarySrcDataCon (tyConDataCons tycon))
955 || (we_are_deriving ordClassKey tycon
956 && not (isProductTyCon tycon))
957 || (we_are_deriving enumClassKey tycon)
958 || (we_are_deriving ixClassKey tycon))
960 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
965 do_tag2con acc_Names tycon
966 | isDataTyCon tycon &&
967 (we_are_deriving enumClassKey tycon ||
968 we_are_deriving ixClassKey tycon
969 && isEnumerationTyCon tycon)
970 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
971 : (maxtag_RDR tycon, tycon, GenMaxTag)
976 we_are_deriving clas_key tycon
977 = is_in_eqns clas_key tycon all_CTs
979 is_in_eqns clas_key tycon [] = False
980 is_in_eqns clas_key tycon ((c,t):cts)
981 = (clas_key == classKey c && tycon == t)
982 || is_in_eqns clas_key tycon cts
986 derivingThingErr clas tys tycon tyvars why
987 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
990 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
992 derivCtxt :: TyCon -> SDoc
994 = ptext SLIT("When deriving instances for") <+> quotes (ppr tycon)
996 derivInstCtxt1 clas inst_tys
997 = ptext SLIT("When deriving the instance for") <+> quotes (pprClassPred clas inst_tys)
999 derivInstCtxt2 theta clas inst_tys
1000 = vcat [ptext SLIT("In the derived instance declaration"),
1001 nest 2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta,
1002 pprClassPred clas inst_tys])]