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, RenamedMonoBinds )
16 import CmdLineOpts ( DynFlag(..), DynFlags )
19 import TcEnv ( TcEnv, tcSetInstEnv, newDFunName, InstInfo(..), pprInstInfo )
20 import TcGenDeriv -- Deriv stuff
21 import InstEnv ( InstEnv, simpleDFunClassTyCon, extendInstEnv )
22 import TcSimplify ( tcSimplifyThetas )
24 import RnBinds ( rnMethodBinds, rnTopMonoBinds )
25 import RnEnv ( bindLocatedLocalsRn )
26 import RnMonad ( --RnNameSupply,
27 renameSourceCode, thenRn, mapRn, returnRn )
28 import HscTypes ( DFunId, PersistentRenamerState )
30 import BasicTypes ( Fixity )
31 import Class ( classKey, Class )
32 import ErrUtils ( dumpIfSet_dyn, Message )
33 import MkId ( mkDictFunId )
34 import DataCon ( dataConArgTys, isNullaryDataCon, isExistentialDataCon )
35 import PrelInfo ( needsDataDeclCtxtClassKeys )
36 import Maybes ( maybeToBool, catMaybes )
37 import Module ( Module )
38 import Name ( Name, isFrom, getSrcLoc )
39 import RdrName ( RdrName )
41 import TyCon ( tyConTyVars, tyConDataCons, tyConDerivings,
42 tyConTheta, maybeTyConSingleCon, isDataTyCon,
43 isEnumerationTyCon, TyCon
45 import Type ( TauType, PredType(..), mkTyVarTys, mkTyConApp, isUnboxedType )
48 import Util ( zipWithEqual, sortLt, thenCmp )
49 import ListSetOps ( removeDups, assoc )
53 %************************************************************************
55 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
57 %************************************************************************
61 data T a b = C1 (Foo a) (Bar b)
66 [NOTE: See end of these comments for what to do with
67 data (C a, D b) => T a b = ...
70 We want to come up with an instance declaration of the form
72 instance (Ping a, Pong b, ...) => Eq (T a b) where
75 It is pretty easy, albeit tedious, to fill in the code "...". The
76 trick is to figure out what the context for the instance decl is,
77 namely @Ping@, @Pong@ and friends.
79 Let's call the context reqd for the T instance of class C at types
80 (a,b, ...) C (T a b). Thus:
82 Eq (T a b) = (Ping a, Pong b, ...)
84 Now we can get a (recursive) equation from the @data@ decl:
86 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
87 u Eq (T b a) u Eq Int -- From C2
88 u Eq (T a a) -- From C3
90 Foo and Bar may have explicit instances for @Eq@, in which case we can
91 just substitute for them. Alternatively, either or both may have
92 their @Eq@ instances given by @deriving@ clauses, in which case they
93 form part of the system of equations.
95 Now all we need do is simplify and solve the equations, iterating to
96 find the least fixpoint. Notice that the order of the arguments can
97 switch around, as here in the recursive calls to T.
99 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
103 Eq (T a b) = {} -- The empty set
106 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
107 u Eq (T b a) u Eq Int -- From C2
108 u Eq (T a a) -- From C3
110 After simplification:
111 = Eq a u Ping b u {} u {} u {}
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:
125 = Eq a u Ping b u Eq b u Ping a
127 The next iteration gives the same result, so this is the fixpoint. We
128 need to make a canonical form of the RHS to ensure convergence. We do
129 this by simplifying the RHS to a form in which
131 - the classes constrain only tyvars
132 - the list is sorted by tyvar (major key) and then class (minor key)
133 - no duplicates, of course
135 So, here are the synonyms for the ``equation'' structures:
138 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
139 -- The Name is the name for the DFun we'll build
140 -- The tyvars bind all the variables in the RHS
142 type DerivRhs = [(Class, [TauType])] -- Same as a ThetaType!
143 --[PredType] -- ... | Class Class [Type==TauType]
145 type DerivSoln = DerivRhs
149 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
173 %************************************************************************
175 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
177 %************************************************************************
180 tcDeriving :: PersistentRenamerState
181 -> Module -- name of module under scrutiny
182 -> InstEnv -- What we already know about instances
183 -> (Name -> Maybe Fixity) -- used in deriving Show and Read
184 -> [TyCon] -- All type constructors
185 -> TcM ([InstInfo], -- The generated "instance decls".
186 RenamedHsBinds) -- Extra generated bindings
188 tcDeriving prs mod inst_env_in get_fixity tycons
189 = recoverTc (returnTc ([], EmptyBinds)) $
191 -- Fish the "deriving"-related information out of the TcEnv
192 -- and make the necessary "equations".
193 makeDerivEqns mod tycons `thenTc` \ eqns ->
195 returnTc ([], EmptyBinds)
198 -- Take the equation list and solve it, to deliver a list of
199 -- solutions, a.k.a. the contexts for the instance decls
200 -- required for the corresponding equations.
201 solveDerivEqns inst_env_in eqns `thenTc` \ new_dfuns ->
203 -- Now augment the InstInfos, adding in the rather boring
204 -- actual-code-to-do-the-methods binds. We may also need to
205 -- generate extra not-one-inst-decl-specific binds, notably
206 -- "con2tag" and/or "tag2con" functions. We do these
209 gen_taggery_Names new_dfuns `thenTc` \ nm_alist_etc ->
211 tcGetEnv `thenNF_Tc` \ env ->
212 getDOptsTc `thenTc` \ dflags ->
214 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
215 extra_mbinds = foldr AndMonoBinds EmptyMonoBinds extra_mbind_list
216 method_binds_s = map (gen_bind get_fixity) new_dfuns
217 mbinders = collectLocatedMonoBinders extra_mbinds
219 -- Rename to get RenamedBinds.
220 -- The only tricky bit is that the extra_binds must scope over the
221 -- method bindings for the instances.
222 (rn_method_binds_s, rn_extra_binds)
223 = renameSourceCode dflags mod prs (
224 bindLocatedLocalsRn (ptext (SLIT("deriving"))) mbinders $ \ _ ->
225 rnTopMonoBinds extra_mbinds [] `thenRn` \ (rn_extra_binds, _) ->
226 mapRn rn_meths method_binds_s `thenRn` \ rn_method_binds_s ->
227 returnRn (rn_method_binds_s, rn_extra_binds)
230 new_inst_infos = zipWith gen_inst_info new_dfuns rn_method_binds_s
233 ioToTc (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
234 (ddump_deriving new_inst_infos rn_extra_binds)) `thenTc_`
236 returnTc (new_inst_infos, rn_extra_binds)
238 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> SDoc
239 ddump_deriving inst_infos extra_binds
240 = vcat (map pprInstInfo inst_infos) $$ ppr extra_binds
243 -- Make a Real dfun instead of the dummy one we have so far
244 gen_inst_info :: DFunId -> RenamedMonoBinds -> InstInfo
245 gen_inst_info dfun binds
246 = InstInfo { iLocal = True, iDFunId = dfun,
247 iBinds = binds, iPrags = [] }
249 rn_meths meths = rnMethodBinds [] meths `thenRn` \ (meths', _) -> returnRn meths'
250 -- Ignore the free vars returned
254 %************************************************************************
256 \subsection[TcDeriv-eqns]{Forming the equations}
258 %************************************************************************
260 @makeDerivEqns@ fishes around to find the info about needed derived
261 instances. Complicating factors:
264 We can only derive @Enum@ if the data type is an enumeration
265 type (all nullary data constructors).
268 We can only derive @Ix@ if the data type is an enumeration {\em
269 or} has just one data constructor (e.g., tuples).
272 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
276 makeDerivEqns :: Module -> [TyCon] -> TcM [DerivEqn]
278 makeDerivEqns this_mod tycons
280 think_about_deriving = need_deriving tycons
281 (derive_these, _) = removeDups cmp_deriv think_about_deriving
283 if null think_about_deriving then
284 returnTc [] -- Bale out now
286 mapTc mk_eqn derive_these `thenTc` \ maybe_eqns ->
287 returnTc (catMaybes maybe_eqns)
289 ------------------------------------------------------------------
290 need_deriving :: [TyCon] -> [(Class, TyCon)]
291 -- find the tycons that have `deriving' clauses;
293 need_deriving tycons_to_consider
294 = [ (clas,tycon) | tycon <- tycons_to_consider,
295 isFrom this_mod tycon,
296 clas <- tyConDerivings tycon ]
298 ------------------------------------------------------------------
299 cmp_deriv :: (Class, TyCon) -> (Class, TyCon) -> Ordering
300 cmp_deriv (c1, t1) (c2, t2)
301 = (c1 `compare` c2) `thenCmp` (t1 `compare` t2)
303 ------------------------------------------------------------------
304 mk_eqn :: (Class, TyCon) -> NF_TcM (Maybe DerivEqn)
305 -- we swizzle the tyvars and datacons out of the tycon
306 -- to make the rest of the equation
309 = case chk_out clas tycon of
310 Just err -> addErrTc err `thenNF_Tc_`
312 Nothing -> newDFunName this_mod clas tyvar_tys locn `thenNF_Tc` \ dfun_name ->
313 returnNF_Tc (Just (dfun_name, clas, tycon, tyvars, constraints))
315 clas_key = classKey clas
316 tyvars = tyConTyVars tycon -- ToDo: Do we need new tyvars ???
317 tyvar_tys = mkTyVarTys tyvars
318 data_cons = tyConDataCons tycon
319 locn = getSrcLoc tycon
321 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
323 -- "extra_constraints": see notes above about contexts on data decls
325 | offensive_class = tyConTheta tycon
328 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
330 mk_constraints data_con
332 | arg_ty <- instd_arg_tys,
333 not (isUnboxedType arg_ty) -- No constraints for unboxed types?
336 instd_arg_tys = dataConArgTys data_con tyvar_tys
338 ------------------------------------------------------------------
339 chk_out :: Class -> TyCon -> Maybe Message
341 | clas `hasKey` enumClassKey && not is_enumeration = bog_out nullary_why
342 | clas `hasKey` boundedClassKey && not is_enumeration_or_single = bog_out single_nullary_why
343 | clas `hasKey` ixClassKey && not is_enumeration_or_single = bog_out single_nullary_why
344 | any isExistentialDataCon (tyConDataCons tycon) = Just (existentialErr clas tycon)
345 | otherwise = Nothing
347 is_enumeration = isEnumerationTyCon tycon
348 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
349 is_enumeration_or_single = is_enumeration || is_single_con
351 single_nullary_why = SLIT("one constructor data type or type with all nullary constructors expected")
352 nullary_why = SLIT("data type with all nullary constructors expected")
354 bog_out why = Just (derivingThingErr clas tycon why)
357 %************************************************************************
359 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
361 %************************************************************************
363 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
364 terms, which is the final correct RHS for the corresponding original
368 Each (k,TyVarTy tv) in a solution constrains only a type
372 The (k,TyVarTy tv) pairs in a solution are canonically
373 ordered by sorting on type varible, tv, (major key) and then class, k,
378 solveDerivEqns :: InstEnv
380 -> TcM [DFunId] -- Solns in same order as eqns.
381 -- This bunch is Absolutely minimal...
383 solveDerivEqns inst_env_in orig_eqns
384 = iterateDeriv initial_solutions
386 -- The initial solutions for the equations claim that each
387 -- instance has an empty context; this solution is certainly
388 -- in canonical form.
389 initial_solutions :: [DerivSoln]
390 initial_solutions = [ [] | _ <- orig_eqns ]
392 ------------------------------------------------------------------
393 -- iterateDeriv calculates the next batch of solutions,
394 -- compares it with the current one; finishes if they are the
395 -- same, otherwise recurses with the new solutions.
396 -- It fails if any iteration fails
397 iterateDeriv :: [DerivSoln] ->TcM [DFunId]
398 iterateDeriv current_solns
399 = checkNoErrsTc (iterateOnce current_solns)
400 `thenTc` \ (new_dfuns, new_solns) ->
401 if (current_solns == new_solns) then
404 iterateDeriv new_solns
406 ------------------------------------------------------------------
407 iterateOnce current_solns
408 = -- Extend the inst info from the explicit instance decls
409 -- with the current set of solutions, giving a
410 getDOptsTc `thenTc` \ dflags ->
411 let (new_dfuns, inst_env) =
412 add_solns dflags inst_env_in orig_eqns current_solns
415 tcSetInstEnv inst_env (
416 listTc [ tcAddErrCtxt (derivCtxt tc) $
417 tcSimplifyThetas deriv_rhs
418 | (_, _,tc,_,deriv_rhs) <- orig_eqns ]
419 ) `thenTc` \ next_solns ->
421 -- Canonicalise the solutions, so they compare nicely
422 let canonicalised_next_solns = [ sortLt (<) next_soln | next_soln <- next_solns ]
424 returnTc (new_dfuns, canonicalised_next_solns)
428 add_solns :: DynFlags
429 -> InstEnv -- The global, non-derived ones
430 -> [DerivEqn] -> [DerivSoln]
431 -> ([DFunId], InstEnv)
432 -- the eqns and solns move "in lockstep"; we have the eqns
433 -- because we need the LHS info for addClassInstance.
435 add_solns dflags inst_env_in eqns solns
436 = (new_dfuns, inst_env)
438 new_dfuns = zipWithEqual "add_solns" mk_deriv_dfun eqns solns
439 (inst_env, _) = extendInstEnv dflags inst_env_in new_dfuns
440 -- Ignore the errors about duplicate instances.
441 -- We don't want repeated error messages
442 -- They'll appear later, when we do the top-level extendInstEnvs
444 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
445 = mkDictFunId dfun_name clas tyvars [mkTyConApp tycon (mkTyVarTys tyvars)]
446 (map pair2PredType theta)
448 pair2PredType (clas, tautypes) = Class clas tautypes
451 %************************************************************************
453 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
455 %************************************************************************
457 After all the trouble to figure out the required context for the
458 derived instance declarations, all that's left is to chug along to
459 produce them. They will then be shoved into @tcInstDecls2@, which
460 will do all its usual business.
462 There are lots of possibilities for code to generate. Here are
463 various general remarks.
468 We want derived instances of @Eq@ and @Ord@ (both v common) to be
469 ``you-couldn't-do-better-by-hand'' efficient.
472 Deriving @Show@---also pretty common--- should also be reasonable good code.
475 Deriving for the other classes isn't that common or that big a deal.
482 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
485 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
488 We {\em normally} generate code only for the non-defaulted methods;
489 there are some exceptions for @Eq@ and (especially) @Ord@...
492 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
493 constructor's numeric (@Int#@) tag. These are generated by
494 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
495 these is around is given by @hasCon2TagFun@.
497 The examples under the different sections below will make this
501 Much less often (really just for deriving @Ix@), we use a
502 @_tag2con_<tycon>@ function. See the examples.
505 We use the renamer!!! Reason: we're supposed to be
506 producing @RenamedMonoBinds@ for the methods, but that means
507 producing correctly-uniquified code on the fly. This is entirely
508 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
509 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
510 the renamer. What a great hack!
514 -- Generate the method bindings for the required instance
515 -- (paired with class name, as we need that when generating dict
517 gen_bind :: (Name -> Maybe Fixity) -> DFunId -> RdrNameMonoBinds
518 gen_bind get_fixity dfun
519 | clas `hasKey` showClassKey = gen_Show_binds get_fixity tycon
520 | clas `hasKey` readClassKey = gen_Read_binds get_fixity tycon
522 = assoc "gen_bind:bad derived class"
523 [(eqClassKey, gen_Eq_binds)
524 ,(ordClassKey, gen_Ord_binds)
525 ,(enumClassKey, gen_Enum_binds)
526 ,(boundedClassKey, gen_Bounded_binds)
527 ,(ixClassKey, gen_Ix_binds)
532 (clas, tycon) = simpleDFunClassTyCon dfun
536 %************************************************************************
538 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
540 %************************************************************************
545 con2tag_Foo :: Foo ... -> Int#
546 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
547 maxtag_Foo :: Int -- ditto (NB: not unboxed)
550 We have a @con2tag@ function for a tycon if:
553 We're deriving @Eq@ and the tycon has nullary data constructors.
556 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
560 We have a @tag2con@ function for a tycon if:
563 We're deriving @Enum@, or @Ix@ (enum type only???)
566 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
569 gen_taggery_Names :: [DFunId]
570 -> TcM [(RdrName, -- for an assoc list
571 TyCon, -- related tycon
574 gen_taggery_Names dfuns
575 = foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
576 foldlTc do_tag2con names_so_far tycons_of_interest
578 all_CTs = map simpleDFunClassTyCon dfuns
579 all_tycons = map snd all_CTs
580 (tycons_of_interest, _) = removeDups compare all_tycons
582 do_con2tag acc_Names tycon
583 | isDataTyCon tycon &&
584 ((we_are_deriving eqClassKey tycon
585 && any isNullaryDataCon (tyConDataCons tycon))
586 || (we_are_deriving ordClassKey tycon
587 && not (maybeToBool (maybeTyConSingleCon tycon)))
588 || (we_are_deriving enumClassKey tycon)
589 || (we_are_deriving ixClassKey tycon))
591 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
596 do_tag2con acc_Names tycon
597 | isDataTyCon tycon &&
598 (we_are_deriving enumClassKey tycon ||
599 we_are_deriving ixClassKey tycon
600 && isEnumerationTyCon tycon)
601 = returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
602 : (maxtag_RDR tycon, tycon, GenMaxTag)
607 we_are_deriving clas_key tycon
608 = is_in_eqns clas_key tycon all_CTs
610 is_in_eqns clas_key tycon [] = False
611 is_in_eqns clas_key tycon ((c,t):cts)
612 = (clas_key == classKey c && tycon == t)
613 || is_in_eqns clas_key tycon cts
617 derivingThingErr :: Class -> TyCon -> FAST_STRING -> Message
619 derivingThingErr clas tycon why
620 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr clas)],
621 hsep [ptext SLIT("for the type"), quotes (ppr tycon)],
624 existentialErr clas tycon
625 = sep [ptext SLIT("Can't derive any instances for type") <+> quotes (ppr tycon),
626 ptext SLIT("because it has existentially-quantified constructor(s)")]
629 = ptext SLIT("When deriving classes for") <+> quotes (ppr tycon)