2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcDeriv]{Deriving}
6 Handles @deriving@ clauses on @data@ declarations.
9 module TcDeriv ( tcDeriving ) where
11 #include "HsVersions.h"
13 import HsSyn ( HsBinds(..), MonoBinds(..), collectMonoBinders )
14 import RdrHsSyn ( RdrNameMonoBinds )
15 import RnHsSyn ( RenamedHsBinds, RenamedMonoBinds )
16 import CmdLineOpts ( opt_D_dump_deriv )
19 import Inst ( InstanceMapper )
20 import TcEnv ( getEnvTyCons )
21 import TcGenDeriv -- Deriv stuff
22 import TcInstUtil ( InstInfo(..), buildInstanceEnvs )
23 import TcSimplify ( tcSimplifyThetas )
25 import RnBinds ( rnMethodBinds, rnTopMonoBinds )
26 import RnEnv ( newDFunName, bindLocatedLocalsRn )
27 import RnMonad ( RnNameSupply,
28 renameSourceCode, thenRn, mapRn, returnRn )
30 import Bag ( Bag, emptyBag, unionBags, listToBag )
31 import Class ( classKey, Class )
32 import ErrUtils ( dumpIfSet, Message, pprBagOfErrors )
33 import MkId ( mkDictFunId )
34 import Id ( mkVanillaId )
35 import DataCon ( dataConArgTys, isNullaryDataCon, isExistentialDataCon )
36 import PrelInfo ( needsDataDeclCtxtClassKeys )
37 import Maybes ( maybeToBool, catMaybes )
38 import Module ( ModuleName )
39 import Name ( isLocallyDefined, getSrcLoc,
43 import RdrName ( RdrName )
44 import RnMonad ( FixityEnv )
45 import SrcLoc ( mkGeneratedSrcLoc, SrcLoc )
46 import TyCon ( tyConTyVars, tyConDataCons, tyConDerivings,
47 tyConTheta, maybeTyConSingleCon, isDataTyCon,
48 isEnumerationTyCon, isAlgTyCon, TyCon
50 import Type ( TauType, mkTyVarTys, mkTyConApp,
51 mkSigmaTy, mkDictTy, isUnboxedType,
52 splitAlgTyConApp, classesToPreds
54 import PprType ( {- instance Outputable Type -} )
55 import TysWiredIn ( voidTy )
57 import Unique -- Keys stuff
58 import Bag ( bagToList )
59 import Util ( zipWithEqual, sortLt, removeDups, assoc, thenCmp )
63 %************************************************************************
65 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
67 %************************************************************************
71 data T a b = C1 (Foo a) (Bar b)
76 [NOTE: See end of these comments for what to do with
77 data (C a, D b) => T a b = ...
80 We want to come up with an instance declaration of the form
82 instance (Ping a, Pong b, ...) => Eq (T a b) where
85 It is pretty easy, albeit tedious, to fill in the code "...". The
86 trick is to figure out what the context for the instance decl is,
87 namely @Ping@, @Pong@ and friends.
89 Let's call the context reqd for the T instance of class C at types
90 (a,b, ...) C (T a b). Thus:
92 Eq (T a b) = (Ping a, Pong b, ...)
94 Now we can get a (recursive) equation from the @data@ decl:
96 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
97 u Eq (T b a) u Eq Int -- From C2
98 u Eq (T a a) -- From C3
100 Foo and Bar may have explicit instances for @Eq@, in which case we can
101 just substitute for them. Alternatively, either or both may have
102 their @Eq@ instances given by @deriving@ clauses, in which case they
103 form part of the system of equations.
105 Now all we need do is simplify and solve the equations, iterating to
106 find the least fixpoint. Notice that the order of the arguments can
107 switch around, as here in the recursive calls to T.
109 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
113 Eq (T a b) = {} -- The empty set
116 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
117 u Eq (T b a) u Eq Int -- From C2
118 u Eq (T a a) -- From C3
120 After simplification:
121 = Eq a u Ping b u {} u {} u {}
126 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
127 u Eq (T b a) u Eq Int -- From C2
128 u Eq (T a a) -- From C3
130 After simplification:
135 = Eq a u Ping b u Eq b u Ping a
137 The next iteration gives the same result, so this is the fixpoint. We
138 need to make a canonical form of the RHS to ensure convergence. We do
139 this by simplifying the RHS to a form in which
141 - the classes constrain only tyvars
142 - the list is sorted by tyvar (major key) and then class (minor key)
143 - no duplicates, of course
145 So, here are the synonyms for the ``equation'' structures:
148 type DerivEqn = (Class, TyCon, [TyVar], DerivRhs)
149 -- The tyvars bind all the variables in the RHS
150 -- NEW: it's convenient to re-use InstInfo
151 -- We'll "panic" out some fields...
153 type DerivRhs = [(Class, [TauType])] -- Same as a ThetaType!
155 type DerivSoln = DerivRhs
159 A note about contexts on data decls
160 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
163 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
165 We will need an instance decl like:
167 instance (Read a, RealFloat a) => Read (Complex a) where
170 The RealFloat in the context is because the read method for Complex is bound
171 to construct a Complex, and doing that requires that the argument type is
174 But this ain't true for Show, Eq, Ord, etc, since they don't construct
175 a Complex; they only take them apart.
177 Our approach: identify the offending classes, and add the data type
178 context to the instance decl. The "offending classes" are
183 %************************************************************************
185 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
187 %************************************************************************
190 tcDeriving :: ModuleName -- name of module under scrutiny
191 -> FixityEnv -- for the deriving code (Show/Read.)
192 -> RnNameSupply -- for "renaming" bits of generated code
193 -> Bag InstInfo -- What we already know about instances
194 -> TcM s (Bag InstInfo, -- The generated "instance decls".
195 RenamedHsBinds) -- Extra generated bindings
197 tcDeriving modname fixs rn_name_supply inst_decl_infos_in
198 = recoverTc (returnTc (emptyBag, EmptyBinds)) $
200 -- Fish the "deriving"-related information out of the TcEnv
201 -- and make the necessary "equations".
202 makeDerivEqns `thenTc` \ eqns ->
204 returnTc (emptyBag, EmptyBinds)
207 -- Take the equation list and solve it, to deliver a list of
208 -- solutions, a.k.a. the contexts for the instance decls
209 -- required for the corresponding equations.
210 solveDerivEqns inst_decl_infos_in eqns `thenTc` \ new_inst_infos ->
212 -- Now augment the InstInfos, adding in the rather boring
213 -- actual-code-to-do-the-methods binds. We may also need to
214 -- generate extra not-one-inst-decl-specific binds, notably
215 -- "con2tag" and/or "tag2con" functions. We do these
218 gen_taggery_Names new_inst_infos `thenTc` \ nm_alist_etc ->
222 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
223 extra_mbinds = foldr AndMonoBinds EmptyMonoBinds extra_mbind_list
224 method_binds_s = map (gen_bind fixs) new_inst_infos
225 mbinders = bagToList (collectMonoBinders extra_mbinds)
227 -- Rename to get RenamedBinds.
228 -- The only tricky bit is that the extra_binds must scope over the
229 -- method bindings for the instances.
230 (dfun_names_w_method_binds, rn_extra_binds)
231 = renameSourceCode modname rn_name_supply (
232 bindLocatedLocalsRn (ptext (SLIT("deriving"))) mbinders $ \ _ ->
233 rnTopMonoBinds extra_mbinds [] `thenRn` \ (rn_extra_binds, _) ->
234 mapRn rn_one method_binds_s `thenRn` \ dfun_names_w_method_binds ->
235 returnRn (dfun_names_w_method_binds, rn_extra_binds)
237 rn_one (cl_nm, tycon_nm, meth_binds)
238 = newDFunName (cl_nm, tycon_nm)
239 mkGeneratedSrcLoc `thenRn` \ dfun_name ->
240 rnMethodBinds meth_binds `thenRn` \ (rn_meth_binds, _) ->
241 returnRn (dfun_name, rn_meth_binds)
243 really_new_inst_infos = zipWith gen_inst_info
245 dfun_names_w_method_binds
247 ddump_deriv = ddump_deriving really_new_inst_infos rn_extra_binds
249 ioToTc (dumpIfSet opt_D_dump_deriv "Derived instances" ddump_deriv) `thenTc_`
251 returnTc (listToBag really_new_inst_infos, rn_extra_binds)
253 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> SDoc
254 ddump_deriving inst_infos extra_binds
255 = vcat (map pp_info inst_infos) $$ ppr extra_binds
257 pp_info (InstInfo clas tvs [ty] inst_decl_theta _ mbinds _ _)
258 = ppr (mkSigmaTy tvs inst_decl_theta' (mkDictTy clas [ty]))
261 where inst_decl_theta' = classesToPreds inst_decl_theta
265 %************************************************************************
267 \subsection[TcDeriv-eqns]{Forming the equations}
269 %************************************************************************
271 @makeDerivEqns@ fishes around to find the info about needed derived
272 instances. Complicating factors:
275 We can only derive @Enum@ if the data type is an enumeration
276 type (all nullary data constructors).
279 We can only derive @Ix@ if the data type is an enumeration {\em
280 or} has just one data constructor (e.g., tuples).
283 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
287 makeDerivEqns :: TcM s [DerivEqn]
290 = tcGetEnv `thenNF_Tc` \ env ->
292 local_data_tycons = filter (\tc -> isLocallyDefined tc && isAlgTyCon tc)
295 think_about_deriving = need_deriving local_data_tycons
296 (derive_these, _) = removeDups cmp_deriv think_about_deriving
297 eqns = map mk_eqn derive_these
299 if null local_data_tycons then
300 returnTc [] -- Bale out now
302 mapTc mk_eqn derive_these `thenTc` \ maybe_eqns ->
303 returnTc (catMaybes maybe_eqns)
305 ------------------------------------------------------------------
306 need_deriving :: [TyCon] -> [(Class, TyCon)]
307 -- find the tycons that have `deriving' clauses;
309 need_deriving tycons_to_consider
310 = foldr (\ tycon acc -> [(clas,tycon) | clas <- tyConDerivings tycon] ++ acc)
314 ------------------------------------------------------------------
315 cmp_deriv :: (Class, TyCon) -> (Class, TyCon) -> Ordering
316 cmp_deriv (c1, t1) (c2, t2)
317 = (c1 `compare` c2) `thenCmp` (t1 `compare` t2)
319 ------------------------------------------------------------------
320 mk_eqn :: (Class, TyCon) -> NF_TcM s (Maybe DerivEqn)
321 -- we swizzle the tyvars and datacons out of the tycon
322 -- to make the rest of the equation
325 = case chk_out clas tycon of
326 Just err -> addErrTc err `thenNF_Tc_`
328 Nothing -> returnNF_Tc (Just (clas, tycon, tyvars, constraints))
330 clas_key = classKey clas
331 tyvars = tyConTyVars tycon -- ToDo: Do we need new tyvars ???
332 tyvar_tys = mkTyVarTys tyvars
333 data_cons = tyConDataCons tycon
335 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
337 -- "extra_constraints": see notes above about contexts on data decls
339 | offensive_class = tyConTheta tycon
342 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
344 mk_constraints data_con
346 | arg_ty <- instd_arg_tys,
347 not (isUnboxedType arg_ty) -- No constraints for unboxed types?
350 instd_arg_tys = dataConArgTys data_con tyvar_tys
352 ------------------------------------------------------------------
353 chk_out :: Class -> TyCon -> Maybe Message
355 | clas `hasKey` enumClassKey && not is_enumeration = bog_out nullary_why
356 | clas `hasKey` boundedClassKey && not is_enumeration_or_single = bog_out single_nullary_why
357 | clas `hasKey` ixClassKey && not is_enumeration_or_single = bog_out single_nullary_why
358 | any isExistentialDataCon (tyConDataCons tycon) = Just (existentialErr clas tycon)
359 | otherwise = Nothing
361 is_enumeration = isEnumerationTyCon tycon
362 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
363 is_enumeration_or_single = is_enumeration || is_single_con
365 single_nullary_why = SLIT("one constructor data type or type with all nullary constructors expected")
366 nullary_why = SLIT("data type with all nullary constructors expected")
368 bog_out why = Just (derivingThingErr clas tycon why)
371 %************************************************************************
373 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
375 %************************************************************************
377 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
378 terms, which is the final correct RHS for the corresponding original
382 Each (k,TyVarTy tv) in a solution constrains only a type
386 The (k,TyVarTy tv) pairs in a solution are canonically
387 ordered by sorting on type varible, tv, (major key) and then class, k,
392 solveDerivEqns :: Bag InstInfo
394 -> TcM s [InstInfo] -- Solns in same order as eqns.
395 -- This bunch is Absolutely minimal...
397 solveDerivEqns inst_decl_infos_in orig_eqns
398 = iterateDeriv initial_solutions
400 -- The initial solutions for the equations claim that each
401 -- instance has an empty context; this solution is certainly
402 -- in canonical form.
403 initial_solutions :: [DerivSoln]
404 initial_solutions = [ [] | _ <- orig_eqns ]
406 ------------------------------------------------------------------
407 -- iterateDeriv calculates the next batch of solutions,
408 -- compares it with the current one; finishes if they are the
409 -- same, otherwise recurses with the new solutions.
410 -- It fails if any iteration fails
411 iterateDeriv :: [DerivSoln] ->TcM s [InstInfo]
412 iterateDeriv current_solns
413 = checkNoErrsTc (iterateOnce current_solns) `thenTc` \ (new_inst_infos, new_solns) ->
414 if (current_solns == new_solns) then
415 returnTc new_inst_infos
417 iterateDeriv new_solns
419 ------------------------------------------------------------------
420 iterateOnce current_solns
421 = -- Extend the inst info from the explicit instance decls
422 -- with the current set of solutions, giving a
424 add_solns inst_decl_infos_in orig_eqns current_solns
425 `thenNF_Tc` \ (new_inst_infos, inst_mapper) ->
427 class_to_inst_env cls = inst_mapper cls
431 listTc [ tcAddErrCtxt (derivCtxt tc) $
432 tcSimplifyThetas class_to_inst_env deriv_rhs
433 | (_,tc,_,deriv_rhs) <- orig_eqns ] `thenTc` \ next_solns ->
435 -- Canonicalise the solutions, so they compare nicely
436 let canonicalised_next_solns
437 = [ sortLt (<) next_soln | next_soln <- next_solns ]
439 returnTc (new_inst_infos, canonicalised_next_solns)
443 add_solns :: Bag InstInfo -- The global, non-derived ones
444 -> [DerivEqn] -> [DerivSoln]
445 -> NF_TcM s ([InstInfo], -- The new, derived ones
447 -- the eqns and solns move "in lockstep"; we have the eqns
448 -- because we need the LHS info for addClassInstance.
450 add_solns inst_infos_in eqns solns
452 = discardErrsTc (buildInstanceEnvs all_inst_infos) `thenNF_Tc` \ inst_mapper ->
453 -- We do the discard-errs so that we don't get repeated error messages
454 -- about duplicate instances.
455 -- They'll appear later, when we do the top-level buildInstanceEnvs.
457 returnNF_Tc (new_inst_infos, inst_mapper)
459 new_inst_infos = zipWithEqual "add_solns" mk_deriv_inst_info eqns solns
461 all_inst_infos = inst_infos_in `unionBags` listToBag new_inst_infos
463 mk_deriv_inst_info (clas, tycon, tyvars, _) theta
464 = InstInfo clas tyvars [mkTyConApp tycon (mkTyVarTys tyvars)]
467 (my_panic "binds") (getSrcLoc tycon)
468 (my_panic "upragmas")
471 = mkVanillaId (getName tycon) dummy_dfun_ty
472 -- The name is getSrcLoc'd in an error message
474 theta' = classesToPreds theta
475 dummy_dfun_ty = mkSigmaTy tyvars theta' voidTy
476 -- All we need from the dfun is its "theta" part, used during
477 -- equation simplification (tcSimplifyThetas). The final
478 -- dfun_id will have the superclass dictionaries as arguments too,
479 -- but that'll be added after the equations are solved. For now,
480 -- it's enough just to make a dummy dfun with the simple theta part.
482 -- The part after the theta is dummied here as voidTy; actually it's
483 -- (C (T a b)), but it doesn't seem worth constructing it.
484 -- We can't leave it as a panic because to get the theta part we
485 -- have to run down the type!
487 my_panic str = panic "add_soln" -- pprPanic ("add_soln:"++str) (hsep [char ':', ppr clas, ppr tycon])
490 %************************************************************************
492 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
494 %************************************************************************
496 After all the trouble to figure out the required context for the
497 derived instance declarations, all that's left is to chug along to
498 produce them. They will then be shoved into @tcInstDecls2@, which
499 will do all its usual business.
501 There are lots of possibilities for code to generate. Here are
502 various general remarks.
507 We want derived instances of @Eq@ and @Ord@ (both v common) to be
508 ``you-couldn't-do-better-by-hand'' efficient.
511 Deriving @Show@---also pretty common--- should also be reasonable good code.
514 Deriving for the other classes isn't that common or that big a deal.
521 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
524 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
527 We {\em normally} generate code only for the non-defaulted methods;
528 there are some exceptions for @Eq@ and (especially) @Ord@...
531 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
532 constructor's numeric (@Int#@) tag. These are generated by
533 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
534 these is around is given by @hasCon2TagFun@.
536 The examples under the different sections below will make this
540 Much less often (really just for deriving @Ix@), we use a
541 @_tag2con_<tycon>@ function. See the examples.
544 We use the renamer!!! Reason: we're supposed to be
545 producing @RenamedMonoBinds@ for the methods, but that means
546 producing correctly-uniquified code on the fly. This is entirely
547 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
548 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
549 the renamer. What a great hack!
553 -- Generate the method bindings for the required instance
554 -- (paired with class name, as we need that when generating dict
556 gen_bind :: FixityEnv -> InstInfo -> ({-class-}OccName, {-tyCon-}OccName, RdrNameMonoBinds)
557 gen_bind fixities (InstInfo clas _ [ty] _ _ _ _ _)
559 = (clas_nm, tycon_nm, EmptyMonoBinds)
560 | clas `hasKey` showClassKey
561 = (clas_nm, tycon_nm, gen_Show_binds fixities tycon)
562 | clas `hasKey` readClassKey
563 = (clas_nm, tycon_nm, gen_Read_binds fixities tycon)
565 = (clas_nm, tycon_nm,
566 assoc "gen_bind:bad derived class"
567 [(eqClassKey, gen_Eq_binds)
568 ,(ordClassKey, gen_Ord_binds)
569 ,(enumClassKey, gen_Enum_binds)
570 ,(boundedClassKey, gen_Bounded_binds)
571 ,(ixClassKey, gen_Ix_binds)
576 clas_nm = nameOccName (getName clas)
577 tycon_nm = nameOccName (getName tycon)
578 from_here = isLocallyDefined tycon
579 (tycon,_,_) = splitAlgTyConApp ty
581 gen_inst_info :: InstInfo
582 -> (Name, RenamedMonoBinds)
583 -> InstInfo -- the gen'd (filled-in) "instance decl"
585 gen_inst_info (InstInfo clas tyvars tys@(ty:_) inst_decl_theta _ _ locn _)
586 (dfun_name, meth_binds)
588 -- Generate the various instance-related Ids
589 InstInfo clas tyvars tys inst_decl_theta
594 dfun_id = mkDictFunId dfun_name clas tyvars tys inst_decl_theta
596 from_here = isLocallyDefined tycon
597 (tycon,_,_) = splitAlgTyConApp ty
601 %************************************************************************
603 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
605 %************************************************************************
610 con2tag_Foo :: Foo ... -> Int#
611 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
612 maxtag_Foo :: Int -- ditto (NB: not unboxed)
615 We have a @con2tag@ function for a tycon if:
618 We're deriving @Eq@ and the tycon has nullary data constructors.
621 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
625 We have a @tag2con@ function for a tycon if:
628 We're deriving @Enum@, or @Ix@ (enum type only???)
631 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
634 gen_taggery_Names :: [InstInfo]
635 -> TcM s [(RdrName, -- for an assoc list
636 TyCon, -- related tycon
639 gen_taggery_Names inst_infos
640 = --pprTrace "gen_taggery:\n" (vcat [hsep [ppr c, ppr t] | (c,t) <- all_CTs]) $
641 foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
642 foldlTc do_tag2con names_so_far tycons_of_interest
644 all_CTs = [ (c, get_tycon ty) | (InstInfo c _ [ty] _ _ _ _ _) <- inst_infos ]
646 get_tycon ty = case splitAlgTyConApp ty of { (tc, _, _) -> tc }
648 all_tycons = map snd all_CTs
649 (tycons_of_interest, _) = removeDups compare all_tycons
651 do_con2tag acc_Names tycon
652 | isDataTyCon tycon &&
653 ((we_are_deriving eqClassKey tycon
654 && any isNullaryDataCon (tyConDataCons tycon))
655 || (we_are_deriving ordClassKey tycon
656 && not (maybeToBool (maybeTyConSingleCon tycon)))
657 || (we_are_deriving enumClassKey tycon)
658 || (we_are_deriving ixClassKey tycon))
660 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
665 do_tag2con acc_Names tycon
666 | isDataTyCon tycon &&
667 (we_are_deriving enumClassKey tycon ||
668 we_are_deriving ixClassKey tycon
669 && isEnumerationTyCon tycon)
670 = returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
671 : (maxtag_RDR tycon, tycon, GenMaxTag)
676 we_are_deriving clas_key tycon
677 = is_in_eqns clas_key tycon all_CTs
679 is_in_eqns clas_key tycon [] = False
680 is_in_eqns clas_key tycon ((c,t):cts)
681 = (clas_key == classKey c && tycon == t)
682 || is_in_eqns clas_key tycon cts
687 derivingThingErr :: Class -> TyCon -> FAST_STRING -> Message
689 derivingThingErr clas tycon why
690 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr clas)],
691 hsep [ptext SLIT("for the type"), quotes (ppr tycon)],
694 existentialErr clas tycon
695 = sep [ptext SLIT("Can't derive any instances for type") <+> quotes (ppr tycon),
696 ptext SLIT("because it has existentially-quantified constructor(s)")]
699 = ptext SLIT("When deriving classes for") <+> quotes (ppr tycon)