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(..), TyClDecl(..),
14 collectLocatedMonoBinders )
15 import RdrHsSyn ( RdrNameMonoBinds )
16 import RnHsSyn ( RenamedHsBinds, RenamedMonoBinds, RenamedTyClDecl )
17 import CmdLineOpts ( DynFlag(..), DynFlags )
20 import TcEnv ( TcEnv, tcSetInstEnv, newDFunName, InstInfo(..), pprInstInfo,
21 tcLookupClass, tcLookupTyCon
23 import TcGenDeriv -- Deriv stuff
24 import InstEnv ( InstEnv, simpleDFunClassTyCon, extendInstEnv )
25 import TcSimplify ( tcSimplifyThetas )
27 import RnBinds ( rnMethodBinds, rnTopMonoBinds )
28 import RnEnv ( bindLocatedLocalsRn )
29 import RnMonad ( renameDerivedCode, thenRn, mapRn, returnRn )
30 import HscTypes ( DFunId, PersistentRenamerState )
32 import BasicTypes ( Fixity )
33 import Class ( classKey, Class )
34 import ErrUtils ( dumpIfSet_dyn, Message )
35 import MkId ( mkDictFunId )
36 import DataCon ( dataConArgTys, isNullaryDataCon, isExistentialDataCon )
37 import PrelInfo ( needsDataDeclCtxtClassKeys )
38 import Maybes ( maybeToBool, catMaybes )
39 import Module ( Module )
40 import Name ( Name, getSrcLoc )
41 import RdrName ( RdrName )
43 import TyCon ( tyConTyVars, tyConDataCons,
44 tyConTheta, maybeTyConSingleCon, isDataTyCon,
45 isEnumerationTyCon, TyCon
47 import Type ( TauType, PredType(..), mkTyVarTys, mkTyConApp, isUnboxedType )
50 import Util ( zipWithEqual, sortLt )
51 import ListSetOps ( removeDups, assoc )
56 %************************************************************************
58 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
60 %************************************************************************
64 data T a b = C1 (Foo a) (Bar b)
69 [NOTE: See end of these comments for what to do with
70 data (C a, D b) => T a b = ...
73 We want to come up with an instance declaration of the form
75 instance (Ping a, Pong b, ...) => Eq (T a b) where
78 It is pretty easy, albeit tedious, to fill in the code "...". The
79 trick is to figure out what the context for the instance decl is,
80 namely @Ping@, @Pong@ and friends.
82 Let's call the context reqd for the T instance of class C at types
83 (a,b, ...) C (T a b). Thus:
85 Eq (T a b) = (Ping a, Pong b, ...)
87 Now we can get a (recursive) equation from the @data@ decl:
89 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
90 u Eq (T b a) u Eq Int -- From C2
91 u Eq (T a a) -- From C3
93 Foo and Bar may have explicit instances for @Eq@, in which case we can
94 just substitute for them. Alternatively, either or both may have
95 their @Eq@ instances given by @deriving@ clauses, in which case they
96 form part of the system of equations.
98 Now all we need do is simplify and solve the equations, iterating to
99 find the least fixpoint. Notice that the order of the arguments can
100 switch around, as here in the recursive calls to T.
102 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
106 Eq (T a b) = {} -- The empty set
109 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
110 u Eq (T b a) u Eq Int -- From C2
111 u Eq (T a a) -- From C3
113 After simplification:
114 = Eq a u Ping b u {} u {} u {}
119 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
120 u Eq (T b a) u Eq Int -- From C2
121 u Eq (T a a) -- From C3
123 After simplification:
128 = Eq a u Ping b u Eq b u Ping a
130 The next iteration gives the same result, so this is the fixpoint. We
131 need to make a canonical form of the RHS to ensure convergence. We do
132 this by simplifying the RHS to a form in which
134 - the classes constrain only tyvars
135 - the list is sorted by tyvar (major key) and then class (minor key)
136 - no duplicates, of course
138 So, here are the synonyms for the ``equation'' structures:
141 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
142 -- The Name is the name for the DFun we'll build
143 -- The tyvars bind all the variables in the RHS
145 type DerivRhs = [(Class, [TauType])] -- Same as a ThetaType!
146 --[PredType] -- ... | Class Class [Type==TauType]
148 type DerivSoln = DerivRhs
152 A note about contexts on data decls
153 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
156 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
158 We will need an instance decl like:
160 instance (Read a, RealFloat a) => Read (Complex a) where
163 The RealFloat in the context is because the read method for Complex is bound
164 to construct a Complex, and doing that requires that the argument type is
167 But this ain't true for Show, Eq, Ord, etc, since they don't construct
168 a Complex; they only take them apart.
170 Our approach: identify the offending classes, and add the data type
171 context to the instance decl. The "offending classes" are
176 %************************************************************************
178 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
180 %************************************************************************
183 tcDeriving :: PersistentRenamerState
184 -> Module -- name of module under scrutiny
185 -> InstEnv -- What we already know about instances
186 -> (Name -> Maybe Fixity) -- used in deriving Show and Read
187 -> [RenamedTyClDecl] -- All type constructors
188 -> TcM ([InstInfo], -- The generated "instance decls".
189 RenamedHsBinds) -- Extra generated bindings
191 tcDeriving prs mod inst_env_in get_fixity tycl_decls
192 = recoverTc (returnTc ([], EmptyBinds)) $
194 -- Fish the "deriving"-related information out of the TcEnv
195 -- and make the necessary "equations".
196 makeDerivEqns mod tycl_decls `thenTc` \ eqns ->
198 returnTc ([], EmptyBinds)
201 -- Take the equation list and solve it, to deliver a list of
202 -- solutions, a.k.a. the contexts for the instance decls
203 -- required for the corresponding equations.
204 solveDerivEqns inst_env_in eqns `thenTc` \ new_dfuns ->
206 -- Now augment the InstInfos, adding in the rather boring
207 -- actual-code-to-do-the-methods binds. We may also need to
208 -- generate extra not-one-inst-decl-specific binds, notably
209 -- "con2tag" and/or "tag2con" functions. We do these
212 gen_taggery_Names new_dfuns `thenTc` \ nm_alist_etc ->
214 tcGetEnv `thenNF_Tc` \ env ->
215 getDOptsTc `thenTc` \ dflags ->
217 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
218 extra_mbinds = foldr AndMonoBinds EmptyMonoBinds extra_mbind_list
219 method_binds_s = map (gen_bind get_fixity) new_dfuns
220 mbinders = collectLocatedMonoBinders extra_mbinds
222 -- Rename to get RenamedBinds.
223 -- The only tricky bit is that the extra_binds must scope over the
224 -- method bindings for the instances.
225 (rn_method_binds_s, rn_extra_binds)
226 = renameDerivedCode dflags mod prs (
227 bindLocatedLocalsRn (ptext (SLIT("deriving"))) mbinders $ \ _ ->
228 rnTopMonoBinds extra_mbinds [] `thenRn` \ (rn_extra_binds, _) ->
229 mapRn rn_meths method_binds_s `thenRn` \ rn_method_binds_s ->
230 returnRn (rn_method_binds_s, rn_extra_binds)
233 new_inst_infos = zipWith gen_inst_info new_dfuns rn_method_binds_s
236 ioToTc (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
237 (ddump_deriving new_inst_infos rn_extra_binds)) `thenTc_`
239 returnTc (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 -- Make a Real dfun instead of the dummy one we have so far
247 gen_inst_info :: DFunId -> RenamedMonoBinds -> InstInfo
248 gen_inst_info dfun binds
249 = InstInfo { iLocal = True, iDFunId = dfun,
250 iBinds = binds, iPrags = [] }
252 rn_meths meths = rnMethodBinds [] meths `thenRn` \ (meths', _) -> returnRn meths'
253 -- Ignore the free vars returned
257 %************************************************************************
259 \subsection[TcDeriv-eqns]{Forming the equations}
261 %************************************************************************
263 @makeDerivEqns@ fishes around to find the info about needed derived
264 instances. Complicating factors:
267 We can only derive @Enum@ if the data type is an enumeration
268 type (all nullary data constructors).
271 We can only derive @Ix@ if the data type is an enumeration {\em
272 or} has just one data constructor (e.g., tuples).
275 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
279 makeDerivEqns :: Module -> [RenamedTyClDecl] -> TcM [DerivEqn]
281 makeDerivEqns this_mod tycl_decls
282 = mapTc mk_eqn derive_these `thenTc` \ maybe_eqns ->
283 returnTc (catMaybes maybe_eqns)
285 ------------------------------------------------------------------
286 derive_these :: [(Name, Name)]
287 -- Find the (Class,TyCon) pairs that must be `derived'
288 -- NB: only source-language decls have deriving, no imported ones do
289 derive_these = [ (clas,tycon)
290 | TyData _ _ tycon _ _ _ (Just classes) _ _ _ <- tycl_decls,
291 clas <- nub classes ]
293 ------------------------------------------------------------------
294 mk_eqn :: (Name, Name) -> NF_TcM (Maybe DerivEqn)
295 -- we swizzle the tyvars and datacons out of the tycon
296 -- to make the rest of the equation
298 mk_eqn (clas_name, tycon_name)
299 = tcLookupClass clas_name `thenNF_Tc` \ clas ->
300 tcLookupTyCon tycon_name `thenNF_Tc` \ tycon ->
302 clas_key = classKey clas
303 tyvars = tyConTyVars tycon
304 tyvar_tys = mkTyVarTys tyvars
305 ty = mkTyConApp tycon tyvar_tys
306 data_cons = tyConDataCons tycon
307 locn = getSrcLoc tycon
308 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
310 -- "extra_constraints": see notes above about contexts on data decls
312 | offensive_class = tyConTheta tycon
315 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
317 mk_constraints data_con
319 | arg_ty <- dataConArgTys data_con tyvar_tys,
320 not (isUnboxedType arg_ty) -- No constraints for unboxed types?
323 case chk_out clas tycon of
324 Just err -> addErrTc err `thenNF_Tc_`
326 Nothing -> newDFunName this_mod clas [ty] locn `thenNF_Tc` \ dfun_name ->
327 returnNF_Tc (Just (dfun_name, clas, tycon, tyvars, constraints))
331 ------------------------------------------------------------------
332 chk_out :: Class -> TyCon -> Maybe Message
334 | clas `hasKey` enumClassKey && not is_enumeration = bog_out nullary_why
335 | clas `hasKey` boundedClassKey && not is_enumeration_or_single = bog_out single_nullary_why
336 | clas `hasKey` ixClassKey && not is_enumeration_or_single = bog_out single_nullary_why
337 | any isExistentialDataCon (tyConDataCons tycon) = Just (existentialErr clas tycon)
338 | otherwise = Nothing
340 is_enumeration = isEnumerationTyCon tycon
341 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
342 is_enumeration_or_single = is_enumeration || is_single_con
344 single_nullary_why = SLIT("one constructor data type or type with all nullary constructors expected")
345 nullary_why = SLIT("data type with all nullary constructors expected")
347 bog_out why = Just (derivingThingErr clas tycon why)
350 %************************************************************************
352 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
354 %************************************************************************
356 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
357 terms, which is the final correct RHS for the corresponding original
361 Each (k,TyVarTy tv) in a solution constrains only a type
365 The (k,TyVarTy tv) pairs in a solution are canonically
366 ordered by sorting on type varible, tv, (major key) and then class, k,
371 solveDerivEqns :: InstEnv
373 -> TcM [DFunId] -- Solns in same order as eqns.
374 -- This bunch is Absolutely minimal...
376 solveDerivEqns inst_env_in orig_eqns
377 = iterateDeriv initial_solutions
379 -- The initial solutions for the equations claim that each
380 -- instance has an empty context; this solution is certainly
381 -- in canonical form.
382 initial_solutions :: [DerivSoln]
383 initial_solutions = [ [] | _ <- orig_eqns ]
385 ------------------------------------------------------------------
386 -- iterateDeriv calculates the next batch of solutions,
387 -- compares it with the current one; finishes if they are the
388 -- same, otherwise recurses with the new solutions.
389 -- It fails if any iteration fails
390 iterateDeriv :: [DerivSoln] ->TcM [DFunId]
391 iterateDeriv current_solns
392 = checkNoErrsTc (iterateOnce current_solns)
393 `thenTc` \ (new_dfuns, new_solns) ->
394 if (current_solns == new_solns) then
397 iterateDeriv new_solns
399 ------------------------------------------------------------------
400 iterateOnce current_solns
401 = -- Extend the inst info from the explicit instance decls
402 -- with the current set of solutions, giving a
403 getDOptsTc `thenTc` \ dflags ->
404 let (new_dfuns, inst_env) =
405 add_solns dflags inst_env_in orig_eqns current_solns
408 tcSetInstEnv inst_env (
409 listTc [ tcAddErrCtxt (derivCtxt tc) $
410 tcSimplifyThetas deriv_rhs
411 | (_, _,tc,_,deriv_rhs) <- orig_eqns ]
412 ) `thenTc` \ next_solns ->
414 -- Canonicalise the solutions, so they compare nicely
415 let canonicalised_next_solns = [ sortLt (<) next_soln | next_soln <- next_solns ]
417 returnTc (new_dfuns, canonicalised_next_solns)
421 add_solns :: DynFlags
422 -> InstEnv -- The global, non-derived ones
423 -> [DerivEqn] -> [DerivSoln]
424 -> ([DFunId], InstEnv)
425 -- the eqns and solns move "in lockstep"; we have the eqns
426 -- because we need the LHS info for addClassInstance.
428 add_solns dflags inst_env_in eqns solns
429 = (new_dfuns, inst_env)
431 new_dfuns = zipWithEqual "add_solns" mk_deriv_dfun eqns solns
432 (inst_env, _) = extendInstEnv dflags inst_env_in new_dfuns
433 -- Ignore the errors about duplicate instances.
434 -- We don't want repeated error messages
435 -- They'll appear later, when we do the top-level extendInstEnvs
437 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
438 = mkDictFunId dfun_name clas tyvars [mkTyConApp tycon (mkTyVarTys tyvars)]
439 (map pair2PredType theta)
441 pair2PredType (clas, tautypes) = Class clas tautypes
444 %************************************************************************
446 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
448 %************************************************************************
450 After all the trouble to figure out the required context for the
451 derived instance declarations, all that's left is to chug along to
452 produce them. They will then be shoved into @tcInstDecls2@, which
453 will do all its usual business.
455 There are lots of possibilities for code to generate. Here are
456 various general remarks.
461 We want derived instances of @Eq@ and @Ord@ (both v common) to be
462 ``you-couldn't-do-better-by-hand'' efficient.
465 Deriving @Show@---also pretty common--- should also be reasonable good code.
468 Deriving for the other classes isn't that common or that big a deal.
475 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
478 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
481 We {\em normally} generate code only for the non-defaulted methods;
482 there are some exceptions for @Eq@ and (especially) @Ord@...
485 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
486 constructor's numeric (@Int#@) tag. These are generated by
487 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
488 these is around is given by @hasCon2TagFun@.
490 The examples under the different sections below will make this
494 Much less often (really just for deriving @Ix@), we use a
495 @_tag2con_<tycon>@ function. See the examples.
498 We use the renamer!!! Reason: we're supposed to be
499 producing @RenamedMonoBinds@ for the methods, but that means
500 producing correctly-uniquified code on the fly. This is entirely
501 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
502 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
503 the renamer. What a great hack!
507 -- Generate the method bindings for the required instance
508 -- (paired with class name, as we need that when generating dict
510 gen_bind :: (Name -> Maybe Fixity) -> DFunId -> RdrNameMonoBinds
511 gen_bind get_fixity dfun
512 | clas `hasKey` showClassKey = gen_Show_binds get_fixity tycon
513 | clas `hasKey` readClassKey = gen_Read_binds get_fixity tycon
515 = assoc "gen_bind:bad derived class"
516 [(eqClassKey, gen_Eq_binds)
517 ,(ordClassKey, gen_Ord_binds)
518 ,(enumClassKey, gen_Enum_binds)
519 ,(boundedClassKey, gen_Bounded_binds)
520 ,(ixClassKey, gen_Ix_binds)
525 (clas, tycon) = simpleDFunClassTyCon dfun
529 %************************************************************************
531 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
533 %************************************************************************
538 con2tag_Foo :: Foo ... -> Int#
539 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
540 maxtag_Foo :: Int -- ditto (NB: not unboxed)
543 We have a @con2tag@ function for a tycon if:
546 We're deriving @Eq@ and the tycon has nullary data constructors.
549 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
553 We have a @tag2con@ function for a tycon if:
556 We're deriving @Enum@, or @Ix@ (enum type only???)
559 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
562 gen_taggery_Names :: [DFunId]
563 -> TcM [(RdrName, -- for an assoc list
564 TyCon, -- related tycon
567 gen_taggery_Names dfuns
568 = foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
569 foldlTc do_tag2con names_so_far tycons_of_interest
571 all_CTs = map simpleDFunClassTyCon dfuns
572 all_tycons = map snd all_CTs
573 (tycons_of_interest, _) = removeDups compare all_tycons
575 do_con2tag acc_Names tycon
576 | isDataTyCon tycon &&
577 ((we_are_deriving eqClassKey tycon
578 && any isNullaryDataCon (tyConDataCons tycon))
579 || (we_are_deriving ordClassKey tycon
580 && not (maybeToBool (maybeTyConSingleCon tycon)))
581 || (we_are_deriving enumClassKey tycon)
582 || (we_are_deriving ixClassKey tycon))
584 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
589 do_tag2con acc_Names tycon
590 | isDataTyCon tycon &&
591 (we_are_deriving enumClassKey tycon ||
592 we_are_deriving ixClassKey tycon
593 && isEnumerationTyCon tycon)
594 = returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
595 : (maxtag_RDR tycon, tycon, GenMaxTag)
600 we_are_deriving clas_key tycon
601 = is_in_eqns clas_key tycon all_CTs
603 is_in_eqns clas_key tycon [] = False
604 is_in_eqns clas_key tycon ((c,t):cts)
605 = (clas_key == classKey c && tycon == t)
606 || is_in_eqns clas_key tycon cts
610 derivingThingErr :: Class -> TyCon -> FAST_STRING -> Message
612 derivingThingErr clas tycon why
613 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr clas)],
614 hsep [ptext SLIT("for the type"), quotes (ppr tycon)],
617 existentialErr clas tycon
618 = sep [ptext SLIT("Can't derive any instances for type") <+> quotes (ppr tycon),
619 ptext SLIT("because it has existentially-quantified constructor(s)")]
622 = ptext SLIT("When deriving classes for") <+> quotes (ppr tycon)