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(..), collectLocatedMonoBinders )
14 import RdrHsSyn ( RdrNameMonoBinds )
15 import RnHsSyn ( RenamedHsBinds )
16 import CmdLineOpts ( opt_D_dump_deriv )
19 import TcEnv ( InstEnv, getEnvTyCons, tcSetInstEnv, newDFunName )
20 import TcGenDeriv -- Deriv stuff
21 import TcInstUtil ( InstInfo(..), pprInstInfo, instInfoClass, simpleInstInfoTyCon, buildInstanceEnv )
22 import TcSimplify ( tcSimplifyThetas )
24 import RnBinds ( rnMethodBinds, rnTopMonoBinds )
25 import RnEnv ( bindLocatedLocalsRn )
26 import RnMonad ( RnNameSupply,
27 renameSourceCode, thenRn, mapRn, returnRn )
29 import Bag ( Bag, emptyBag, unionBags, listToBag )
30 import Class ( classKey, Class )
31 import ErrUtils ( dumpIfSet, Message )
32 import MkId ( mkDictFunId )
33 import Id ( mkVanillaId )
34 import DataCon ( dataConArgTys, isNullaryDataCon, isExistentialDataCon )
35 import PrelInfo ( needsDataDeclCtxtClassKeys )
36 import Maybes ( maybeToBool, catMaybes )
37 import Module ( Module )
38 import Name ( isLocallyDefined, getSrcLoc, NamedThing(..) )
39 import RdrName ( RdrName )
40 import RnMonad ( FixityEnv )
42 import TyCon ( tyConTyVars, tyConDataCons, tyConDerivings,
43 tyConTheta, maybeTyConSingleCon, isDataTyCon,
44 isEnumerationTyCon, isAlgTyCon, TyCon
46 import Type ( TauType, mkTyVarTys, mkTyConApp,
47 mkSigmaTy, mkDictTy, isUnboxedType,
48 splitAlgTyConApp, classesToPreds
50 import TysWiredIn ( voidTy )
53 import Bag ( bagToList )
54 import Util ( zipWithEqual, sortLt, thenCmp )
55 import ListSetOps ( removeDups, assoc )
59 %************************************************************************
61 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
63 %************************************************************************
67 data T a b = C1 (Foo a) (Bar b)
72 [NOTE: See end of these comments for what to do with
73 data (C a, D b) => T a b = ...
76 We want to come up with an instance declaration of the form
78 instance (Ping a, Pong b, ...) => Eq (T a b) where
81 It is pretty easy, albeit tedious, to fill in the code "...". The
82 trick is to figure out what the context for the instance decl is,
83 namely @Ping@, @Pong@ and friends.
85 Let's call the context reqd for the T instance of class C at types
86 (a,b, ...) C (T a b). Thus:
88 Eq (T a b) = (Ping a, Pong b, ...)
90 Now we can get a (recursive) equation from the @data@ decl:
92 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
93 u Eq (T b a) u Eq Int -- From C2
94 u Eq (T a a) -- From C3
96 Foo and Bar may have explicit instances for @Eq@, in which case we can
97 just substitute for them. Alternatively, either or both may have
98 their @Eq@ instances given by @deriving@ clauses, in which case they
99 form part of the system of equations.
101 Now all we need do is simplify and solve the equations, iterating to
102 find the least fixpoint. Notice that the order of the arguments can
103 switch around, as here in the recursive calls to T.
105 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
109 Eq (T a b) = {} -- The empty set
112 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
113 u Eq (T b a) u Eq Int -- From C2
114 u Eq (T a a) -- From C3
116 After simplification:
117 = Eq a u Ping b u {} u {} u {}
122 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
123 u Eq (T b a) u Eq Int -- From C2
124 u Eq (T a a) -- From C3
126 After simplification:
131 = Eq a u Ping b u Eq b u Ping a
133 The next iteration gives the same result, so this is the fixpoint. We
134 need to make a canonical form of the RHS to ensure convergence. We do
135 this by simplifying the RHS to a form in which
137 - the classes constrain only tyvars
138 - the list is sorted by tyvar (major key) and then class (minor key)
139 - no duplicates, of course
141 So, here are the synonyms for the ``equation'' structures:
144 type DerivEqn = (Class, TyCon, [TyVar], DerivRhs)
145 -- The tyvars bind all the variables in the RHS
146 -- NEW: it's convenient to re-use InstInfo
147 -- We'll "panic" out some fields...
149 type DerivRhs = [(Class, [TauType])] -- Same as a ThetaType!
151 type DerivSoln = DerivRhs
155 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
179 %************************************************************************
181 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
183 %************************************************************************
186 tcDeriving :: Module -- name of module under scrutiny
187 -> FixityEnv -- for the deriving code (Show/Read.)
188 -> RnNameSupply -- for "renaming" bits of generated code
189 -> Bag InstInfo -- What we already know about instances
190 -> TcM s (Bag InstInfo, -- The generated "instance decls".
191 RenamedHsBinds) -- Extra generated bindings
193 tcDeriving mod fixs rn_name_supply inst_decl_infos_in
194 = recoverTc (returnTc (emptyBag, EmptyBinds)) $
196 -- Fish the "deriving"-related information out of the TcEnv
197 -- and make the necessary "equations".
198 makeDerivEqns `thenTc` \ eqns ->
200 returnTc (emptyBag, EmptyBinds)
203 -- Take the equation list and solve it, to deliver a list of
204 -- solutions, a.k.a. the contexts for the instance decls
205 -- required for the corresponding equations.
206 solveDerivEqns inst_decl_infos_in eqns `thenTc` \ new_inst_infos ->
208 -- Now augment the InstInfos, adding in the rather boring
209 -- actual-code-to-do-the-methods binds. We may also need to
210 -- generate extra not-one-inst-decl-specific binds, notably
211 -- "con2tag" and/or "tag2con" functions. We do these
214 gen_taggery_Names new_inst_infos `thenTc` \ nm_alist_etc ->
218 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
219 extra_mbinds = foldr AndMonoBinds EmptyMonoBinds extra_mbind_list
220 method_binds_s = map (gen_bind fixs) new_inst_infos
221 mbinders = collectLocatedMonoBinders extra_mbinds
223 -- Rename to get RenamedBinds.
224 -- The only tricky bit is that the extra_binds must scope over the
225 -- method bindings for the instances.
226 (rn_method_binds_s, rn_extra_binds)
227 = renameSourceCode mod rn_name_supply (
228 bindLocatedLocalsRn (ptext (SLIT("deriving"))) mbinders $ \ _ ->
229 rnTopMonoBinds extra_mbinds [] `thenRn` \ (rn_extra_binds, _) ->
230 mapRn rn_meths method_binds_s `thenRn` \ rn_method_binds_s ->
231 returnRn (rn_method_binds_s, rn_extra_binds)
234 mapNF_Tc gen_inst_info (new_inst_infos `zip` rn_method_binds_s) `thenNF_Tc` \ really_new_inst_infos ->
236 ioToTc (dumpIfSet opt_D_dump_deriv "Derived instances"
237 (ddump_deriving really_new_inst_infos rn_extra_binds)) `thenTc_`
239 returnTc (listToBag really_new_inst_infos, rn_extra_binds)
241 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> SDoc
242 ddump_deriving inst_infos extra_binds
243 = vcat (map pprInstInfo inst_infos) $$ ppr extra_binds
246 -- Paste the dfun id and method binds into the InstInfo
247 gen_inst_info (InstInfo clas tyvars tys@(ty:_) inst_decl_theta _ _ locn _, meth_binds)
248 = newDFunName mod clas tys locn `thenNF_Tc` \ dfun_name ->
250 dfun_id = mkDictFunId dfun_name clas tyvars tys inst_decl_theta
252 returnNF_Tc (InstInfo clas tyvars tys inst_decl_theta
253 dfun_id meth_binds locn [])
255 rn_meths meths = rnMethodBinds [] meths `thenRn` \ (meths', _) -> returnRn meths'
256 -- Ignore the free vars returned
260 %************************************************************************
262 \subsection[TcDeriv-eqns]{Forming the equations}
264 %************************************************************************
266 @makeDerivEqns@ fishes around to find the info about needed derived
267 instances. Complicating factors:
270 We can only derive @Enum@ if the data type is an enumeration
271 type (all nullary data constructors).
274 We can only derive @Ix@ if the data type is an enumeration {\em
275 or} has just one data constructor (e.g., tuples).
278 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
282 makeDerivEqns :: TcM s [DerivEqn]
285 = tcGetEnv `thenNF_Tc` \ env ->
287 local_data_tycons = filter (\tc -> isLocallyDefined tc && isAlgTyCon tc)
290 think_about_deriving = need_deriving local_data_tycons
291 (derive_these, _) = removeDups cmp_deriv think_about_deriving
293 if null local_data_tycons then
294 returnTc [] -- Bale out now
296 mapTc mk_eqn derive_these `thenTc` \ maybe_eqns ->
297 returnTc (catMaybes maybe_eqns)
299 ------------------------------------------------------------------
300 need_deriving :: [TyCon] -> [(Class, TyCon)]
301 -- find the tycons that have `deriving' clauses;
303 need_deriving tycons_to_consider
304 = foldr (\ tycon acc -> [(clas,tycon) | clas <- tyConDerivings tycon] ++ acc)
308 ------------------------------------------------------------------
309 cmp_deriv :: (Class, TyCon) -> (Class, TyCon) -> Ordering
310 cmp_deriv (c1, t1) (c2, t2)
311 = (c1 `compare` c2) `thenCmp` (t1 `compare` t2)
313 ------------------------------------------------------------------
314 mk_eqn :: (Class, TyCon) -> NF_TcM s (Maybe DerivEqn)
315 -- we swizzle the tyvars and datacons out of the tycon
316 -- to make the rest of the equation
319 = case chk_out clas tycon of
320 Just err -> addErrTc err `thenNF_Tc_`
322 Nothing -> returnNF_Tc (Just (clas, tycon, tyvars, constraints))
324 clas_key = classKey clas
325 tyvars = tyConTyVars tycon -- ToDo: Do we need new tyvars ???
326 tyvar_tys = mkTyVarTys tyvars
327 data_cons = tyConDataCons tycon
329 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
331 -- "extra_constraints": see notes above about contexts on data decls
333 | offensive_class = tyConTheta tycon
336 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
338 mk_constraints data_con
340 | arg_ty <- instd_arg_tys,
341 not (isUnboxedType arg_ty) -- No constraints for unboxed types?
344 instd_arg_tys = dataConArgTys data_con tyvar_tys
346 ------------------------------------------------------------------
347 chk_out :: Class -> TyCon -> Maybe Message
349 | clas `hasKey` enumClassKey && not is_enumeration = bog_out nullary_why
350 | clas `hasKey` boundedClassKey && not is_enumeration_or_single = bog_out single_nullary_why
351 | clas `hasKey` ixClassKey && not is_enumeration_or_single = bog_out single_nullary_why
352 | any isExistentialDataCon (tyConDataCons tycon) = Just (existentialErr clas tycon)
353 | otherwise = Nothing
355 is_enumeration = isEnumerationTyCon tycon
356 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
357 is_enumeration_or_single = is_enumeration || is_single_con
359 single_nullary_why = SLIT("one constructor data type or type with all nullary constructors expected")
360 nullary_why = SLIT("data type with all nullary constructors expected")
362 bog_out why = Just (derivingThingErr clas tycon why)
365 %************************************************************************
367 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
369 %************************************************************************
371 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
372 terms, which is the final correct RHS for the corresponding original
376 Each (k,TyVarTy tv) in a solution constrains only a type
380 The (k,TyVarTy tv) pairs in a solution are canonically
381 ordered by sorting on type varible, tv, (major key) and then class, k,
386 solveDerivEqns :: Bag InstInfo
388 -> TcM s [InstInfo] -- Solns in same order as eqns.
389 -- This bunch is Absolutely minimal...
391 solveDerivEqns inst_decl_infos_in orig_eqns
392 = iterateDeriv initial_solutions
394 -- The initial solutions for the equations claim that each
395 -- instance has an empty context; this solution is certainly
396 -- in canonical form.
397 initial_solutions :: [DerivSoln]
398 initial_solutions = [ [] | _ <- orig_eqns ]
400 ------------------------------------------------------------------
401 -- iterateDeriv calculates the next batch of solutions,
402 -- compares it with the current one; finishes if they are the
403 -- same, otherwise recurses with the new solutions.
404 -- It fails if any iteration fails
405 iterateDeriv :: [DerivSoln] ->TcM s [InstInfo]
406 iterateDeriv current_solns
407 = checkNoErrsTc (iterateOnce current_solns) `thenTc` \ (new_inst_infos, new_solns) ->
408 if (current_solns == new_solns) then
409 returnTc new_inst_infos
411 iterateDeriv new_solns
413 ------------------------------------------------------------------
414 iterateOnce current_solns
415 = -- Extend the inst info from the explicit instance decls
416 -- with the current set of solutions, giving a
418 add_solns inst_decl_infos_in orig_eqns current_solns
419 `thenNF_Tc` \ (new_inst_infos, inst_env) ->
423 tcSetInstEnv inst_env (
424 listTc [ tcAddErrCtxt (derivCtxt tc) $
425 tcSimplifyThetas deriv_rhs
426 | (_,tc,_,deriv_rhs) <- orig_eqns ]
427 ) `thenTc` \ next_solns ->
429 -- Canonicalise the solutions, so they compare nicely
430 let canonicalised_next_solns
431 = [ sortLt (<) next_soln | next_soln <- next_solns ]
433 returnTc (new_inst_infos, canonicalised_next_solns)
437 add_solns :: Bag InstInfo -- The global, non-derived ones
438 -> [DerivEqn] -> [DerivSoln]
439 -> NF_TcM s ([InstInfo], -- The new, derived ones
441 -- the eqns and solns move "in lockstep"; we have the eqns
442 -- because we need the LHS info for addClassInstance.
444 add_solns inst_infos_in eqns solns
446 = discardErrsTc (buildInstanceEnv all_inst_infos) `thenNF_Tc` \ inst_env ->
447 -- We do the discard-errs so that we don't get repeated error messages
448 -- about duplicate instances.
449 -- They'll appear later, when we do the top-level buildInstanceEnv.
451 returnNF_Tc (new_inst_infos, inst_env)
453 new_inst_infos = zipWithEqual "add_solns" mk_deriv_inst_info eqns solns
455 all_inst_infos = inst_infos_in `unionBags` listToBag new_inst_infos
457 mk_deriv_inst_info (clas, tycon, tyvars, _) theta
458 = InstInfo clas tyvars [mkTyConApp tycon (mkTyVarTys tyvars)]
461 (my_panic "binds") (getSrcLoc tycon)
462 (my_panic "upragmas")
465 = mkVanillaId (getName tycon) dummy_dfun_ty
466 -- The name is getSrcLoc'd in an error message
468 theta' = classesToPreds theta
469 dummy_dfun_ty = mkSigmaTy tyvars theta' voidTy
470 -- All we need from the dfun is its "theta" part, used during
471 -- equation simplification (tcSimplifyThetas). The final
472 -- dfun_id will have the superclass dictionaries as arguments too,
473 -- but that'll be added after the equations are solved. For now,
474 -- it's enough just to make a dummy dfun with the simple theta part.
476 -- The part after the theta is dummied here as voidTy; actually it's
477 -- (C (T a b)), but it doesn't seem worth constructing it.
478 -- We can't leave it as a panic because to get the theta part we
479 -- have to run down the type!
481 my_panic str = panic "add_soln" -- pprPanic ("add_soln:"++str) (hsep [char ':', ppr clas, ppr tycon])
484 %************************************************************************
486 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
488 %************************************************************************
490 After all the trouble to figure out the required context for the
491 derived instance declarations, all that's left is to chug along to
492 produce them. They will then be shoved into @tcInstDecls2@, which
493 will do all its usual business.
495 There are lots of possibilities for code to generate. Here are
496 various general remarks.
501 We want derived instances of @Eq@ and @Ord@ (both v common) to be
502 ``you-couldn't-do-better-by-hand'' efficient.
505 Deriving @Show@---also pretty common--- should also be reasonable good code.
508 Deriving for the other classes isn't that common or that big a deal.
515 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
518 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
521 We {\em normally} generate code only for the non-defaulted methods;
522 there are some exceptions for @Eq@ and (especially) @Ord@...
525 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
526 constructor's numeric (@Int#@) tag. These are generated by
527 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
528 these is around is given by @hasCon2TagFun@.
530 The examples under the different sections below will make this
534 Much less often (really just for deriving @Ix@), we use a
535 @_tag2con_<tycon>@ function. See the examples.
538 We use the renamer!!! Reason: we're supposed to be
539 producing @RenamedMonoBinds@ for the methods, but that means
540 producing correctly-uniquified code on the fly. This is entirely
541 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
542 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
543 the renamer. What a great hack!
547 -- Generate the method bindings for the required instance
548 -- (paired with class name, as we need that when generating dict
550 gen_bind :: FixityEnv -> InstInfo -> RdrNameMonoBinds
551 gen_bind fixities inst
552 | not (isLocallyDefined tycon) = EmptyMonoBinds
553 | clas `hasKey` showClassKey = gen_Show_binds fixities tycon
554 | clas `hasKey` readClassKey = gen_Read_binds fixities tycon
556 = assoc "gen_bind:bad derived class"
557 [(eqClassKey, gen_Eq_binds)
558 ,(ordClassKey, gen_Ord_binds)
559 ,(enumClassKey, gen_Enum_binds)
560 ,(boundedClassKey, gen_Bounded_binds)
561 ,(ixClassKey, gen_Ix_binds)
566 clas = instInfoClass inst
567 tycon = simpleInstInfoTyCon inst
571 %************************************************************************
573 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
575 %************************************************************************
580 con2tag_Foo :: Foo ... -> Int#
581 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
582 maxtag_Foo :: Int -- ditto (NB: not unboxed)
585 We have a @con2tag@ function for a tycon if:
588 We're deriving @Eq@ and the tycon has nullary data constructors.
591 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
595 We have a @tag2con@ function for a tycon if:
598 We're deriving @Enum@, or @Ix@ (enum type only???)
601 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
604 gen_taggery_Names :: [InstInfo]
605 -> TcM s [(RdrName, -- for an assoc list
606 TyCon, -- related tycon
609 gen_taggery_Names inst_infos
610 = --pprTrace "gen_taggery:\n" (vcat [hsep [ppr c, ppr t] | (c,t) <- all_CTs]) $
611 foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
612 foldlTc do_tag2con names_so_far tycons_of_interest
614 all_CTs = [ (instInfoClass info, simpleInstInfoTyCon info) | info <- inst_infos ]
616 all_tycons = map snd all_CTs
617 (tycons_of_interest, _) = removeDups compare all_tycons
619 do_con2tag acc_Names tycon
620 | isDataTyCon tycon &&
621 ((we_are_deriving eqClassKey tycon
622 && any isNullaryDataCon (tyConDataCons tycon))
623 || (we_are_deriving ordClassKey tycon
624 && not (maybeToBool (maybeTyConSingleCon tycon)))
625 || (we_are_deriving enumClassKey tycon)
626 || (we_are_deriving ixClassKey tycon))
628 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
633 do_tag2con acc_Names tycon
634 | isDataTyCon tycon &&
635 (we_are_deriving enumClassKey tycon ||
636 we_are_deriving ixClassKey tycon
637 && isEnumerationTyCon tycon)
638 = returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
639 : (maxtag_RDR tycon, tycon, GenMaxTag)
644 we_are_deriving clas_key tycon
645 = is_in_eqns clas_key tycon all_CTs
647 is_in_eqns clas_key tycon [] = False
648 is_in_eqns clas_key tycon ((c,t):cts)
649 = (clas_key == classKey c && tycon == t)
650 || is_in_eqns clas_key tycon cts
655 derivingThingErr :: Class -> TyCon -> FAST_STRING -> Message
657 derivingThingErr clas tycon why
658 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr clas)],
659 hsep [ptext SLIT("for the type"), quotes (ppr tycon)],
662 existentialErr clas tycon
663 = sep [ptext SLIT("Can't derive any instances for type") <+> quotes (ppr tycon),
664 ptext SLIT("because it has existentially-quantified constructor(s)")]
667 = ptext SLIT("When deriving classes for") <+> quotes (ppr tycon)