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 ( 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 ( className, 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, nameUnique )
41 import RdrName ( RdrName )
43 import TyCon ( tyConTyVars, tyConDataCons,
44 tyConTheta, maybeTyConSingleCon, isDataTyCon,
45 isEnumerationTyCon, TyCon
47 import TcType ( ThetaType, mkTyVarTys, mkTyConApp,
48 isUnLiftedType, mkClassPred )
51 import Util ( zipWithEqual, sortLt )
52 import ListSetOps ( removeDups, assoc )
57 %************************************************************************
59 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
61 %************************************************************************
65 data T a b = C1 (Foo a) (Bar b)
70 [NOTE: See end of these comments for what to do with
71 data (C a, D b) => T a b = ...
74 We want to come up with an instance declaration of the form
76 instance (Ping a, Pong b, ...) => Eq (T a b) where
79 It is pretty easy, albeit tedious, to fill in the code "...". The
80 trick is to figure out what the context for the instance decl is,
81 namely @Ping@, @Pong@ and friends.
83 Let's call the context reqd for the T instance of class C at types
84 (a,b, ...) C (T a b). Thus:
86 Eq (T a b) = (Ping a, Pong b, ...)
88 Now we can get a (recursive) equation from the @data@ decl:
90 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
91 u Eq (T b a) u Eq Int -- From C2
92 u Eq (T a a) -- From C3
94 Foo and Bar may have explicit instances for @Eq@, in which case we can
95 just substitute for them. Alternatively, either or both may have
96 their @Eq@ instances given by @deriving@ clauses, in which case they
97 form part of the system of equations.
99 Now all we need do is simplify and solve the equations, iterating to
100 find the least fixpoint. Notice that the order of the arguments can
101 switch around, as here in the recursive calls to T.
103 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
107 Eq (T a b) = {} -- The empty set
110 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
111 u Eq (T b a) u Eq Int -- From C2
112 u Eq (T a a) -- From C3
114 After simplification:
115 = Eq a u Ping b u {} u {} u {}
120 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
121 u Eq (T b a) u Eq Int -- From C2
122 u Eq (T a a) -- From C3
124 After simplification:
129 = Eq a u Ping b u Eq b u Ping a
131 The next iteration gives the same result, so this is the fixpoint. We
132 need to make a canonical form of the RHS to ensure convergence. We do
133 this by simplifying the RHS to a form in which
135 - the classes constrain only tyvars
136 - the list is sorted by tyvar (major key) and then class (minor key)
137 - no duplicates, of course
139 So, here are the synonyms for the ``equation'' structures:
142 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
143 -- The Name is the name for the DFun we'll build
144 -- The tyvars bind all the variables in the RHS
146 type DerivRhs = ThetaType
147 type DerivSoln = DerivRhs
151 A note about contexts on data decls
152 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
155 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
157 We will need an instance decl like:
159 instance (Read a, RealFloat a) => Read (Complex a) where
162 The RealFloat in the context is because the read method for Complex is bound
163 to construct a Complex, and doing that requires that the argument type is
166 But this ain't true for Show, Eq, Ord, etc, since they don't construct
167 a Complex; they only take them apart.
169 Our approach: identify the offending classes, and add the data type
170 context to the instance decl. The "offending classes" are
175 %************************************************************************
177 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
179 %************************************************************************
182 tcDeriving :: PersistentRenamerState
183 -> Module -- name of module under scrutiny
184 -> InstEnv -- What we already know about instances
185 -> (Name -> Maybe Fixity) -- used in deriving Show and Read
186 -> [RenamedTyClDecl] -- All type constructors
187 -> TcM ([InstInfo], -- The generated "instance decls".
188 RenamedHsBinds) -- Extra generated bindings
190 tcDeriving prs mod inst_env_in get_fixity tycl_decls
191 = recoverTc (returnTc ([], EmptyBinds)) $
193 -- Fish the "deriving"-related information out of the TcEnv
194 -- and make the necessary "equations".
195 makeDerivEqns tycl_decls `thenTc` \ eqns ->
197 returnTc ([], EmptyBinds)
200 -- Take the equation list and solve it, to deliver a list of
201 -- solutions, a.k.a. the contexts for the instance decls
202 -- required for the corresponding equations.
203 solveDerivEqns inst_env_in eqns `thenTc` \ new_dfuns ->
205 -- Now augment the InstInfos, adding in the rather boring
206 -- actual-code-to-do-the-methods binds. We may also need to
207 -- generate extra not-one-inst-decl-specific binds, notably
208 -- "con2tag" and/or "tag2con" functions. We do these
211 gen_taggery_Names new_dfuns `thenTc` \ nm_alist_etc ->
213 tcGetEnv `thenNF_Tc` \ env ->
214 getDOptsTc `thenTc` \ dflags ->
216 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
217 extra_mbinds = foldr AndMonoBinds EmptyMonoBinds extra_mbind_list
218 method_binds_s = map (gen_bind get_fixity) new_dfuns
219 mbinders = collectLocatedMonoBinders extra_mbinds
221 -- Rename to get RenamedBinds.
222 -- The only tricky bit is that the extra_binds must scope over the
223 -- method bindings for the instances.
224 (rn_method_binds_s, rn_extra_binds)
225 = renameDerivedCode dflags mod prs (
226 bindLocatedLocalsRn (ptext (SLIT("deriving"))) mbinders $ \ _ ->
227 rnTopMonoBinds extra_mbinds [] `thenRn` \ (rn_extra_binds, _) ->
228 mapRn rn_meths method_binds_s `thenRn` \ rn_method_binds_s ->
229 returnRn (rn_method_binds_s, rn_extra_binds)
232 new_inst_infos = zipWith gen_inst_info new_dfuns rn_method_binds_s
235 ioToTc (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
236 (ddump_deriving new_inst_infos rn_extra_binds)) `thenTc_`
238 returnTc (new_inst_infos, rn_extra_binds)
240 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> SDoc
241 ddump_deriving inst_infos extra_binds
242 = vcat (map pprInstInfo inst_infos) $$ ppr extra_binds
245 -- Make a Real dfun instead of the dummy one we have so far
246 gen_inst_info :: DFunId -> RenamedMonoBinds -> InstInfo
247 gen_inst_info dfun binds
248 = InstInfo { iDFunId = dfun, iBinds = binds, iPrags = [] }
250 rn_meths (cls, meths) = rnMethodBinds cls [] meths `thenRn` \ (meths', _) ->
251 returnRn meths' -- Ignore the free vars returned
255 %************************************************************************
257 \subsection[TcDeriv-eqns]{Forming the equations}
259 %************************************************************************
261 @makeDerivEqns@ fishes around to find the info about needed derived
262 instances. Complicating factors:
265 We can only derive @Enum@ if the data type is an enumeration
266 type (all nullary data constructors).
269 We can only derive @Ix@ if the data type is an enumeration {\em
270 or} has just one data constructor (e.g., tuples).
273 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
277 makeDerivEqns :: [RenamedTyClDecl] -> TcM [DerivEqn]
279 makeDerivEqns tycl_decls
280 = mapTc mk_eqn derive_these `thenTc` \ maybe_eqns ->
281 returnTc (catMaybes maybe_eqns)
283 ------------------------------------------------------------------
284 derive_these :: [(Name, Name)]
285 -- Find the (Class,TyCon) pairs that must be `derived'
286 -- NB: only source-language decls have deriving, no imported ones do
287 derive_these = [ (clas,tycon)
288 | TyData {tcdName = tycon, tcdDerivs = Just classes} <- tycl_decls,
289 clas <- nub classes ]
291 ------------------------------------------------------------------
292 mk_eqn :: (Name, Name) -> NF_TcM (Maybe DerivEqn)
293 -- we swizzle the tyvars and datacons out of the tycon
294 -- to make the rest of the equation
296 mk_eqn (clas_name, tycon_name)
297 = tcLookupClass clas_name `thenNF_Tc` \ clas ->
298 tcLookupTyCon tycon_name `thenNF_Tc` \ tycon ->
300 clas_key = classKey clas
301 tyvars = tyConTyVars tycon
302 tyvar_tys = mkTyVarTys tyvars
303 ty = mkTyConApp tycon tyvar_tys
304 data_cons = tyConDataCons tycon
305 locn = getSrcLoc tycon
306 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
308 -- "extra_constraints": see notes above about contexts on data decls
310 | offensive_class = tyConTheta tycon
313 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
315 mk_constraints data_con
316 = [ mkClassPred clas [arg_ty]
317 | arg_ty <- dataConArgTys data_con tyvar_tys,
318 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
321 case chk_out clas tycon of
322 Just err -> tcAddSrcLoc (getSrcLoc tycon) $
323 addErrTc err `thenNF_Tc_`
325 Nothing -> newDFunName clas [ty] locn `thenNF_Tc` \ dfun_name ->
326 returnNF_Tc (Just (dfun_name, clas, tycon, tyvars, constraints))
330 ------------------------------------------------------------------
331 chk_out :: Class -> TyCon -> Maybe Message
333 | clas `hasKey` enumClassKey && not is_enumeration = bog_out nullary_why
334 | clas `hasKey` boundedClassKey && not is_enumeration_or_single = bog_out single_nullary_why
335 | clas `hasKey` ixClassKey && not is_enumeration_or_single = bog_out single_nullary_why
336 | null data_cons = bog_out no_cons_why
337 | any isExistentialDataCon data_cons = Just (existentialErr clas tycon)
338 | otherwise = Nothing
340 data_cons = tyConDataCons tycon
341 is_enumeration = isEnumerationTyCon tycon
342 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
343 is_enumeration_or_single = is_enumeration || is_single_con
345 single_nullary_why = SLIT("one constructor data type or type with all nullary constructors expected")
346 nullary_why = SLIT("data type with all nullary constructors expected")
347 no_cons_why = SLIT("type has no data constructors")
349 bog_out why = Just (derivingThingErr clas tycon why)
352 %************************************************************************
354 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
356 %************************************************************************
358 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
359 terms, which is the final correct RHS for the corresponding original
363 Each (k,TyVarTy tv) in a solution constrains only a type
367 The (k,TyVarTy tv) pairs in a solution are canonically
368 ordered by sorting on type varible, tv, (major key) and then class, k,
373 solveDerivEqns :: InstEnv
375 -> TcM [DFunId] -- Solns in same order as eqns.
376 -- This bunch is Absolutely minimal...
378 solveDerivEqns inst_env_in orig_eqns
379 = iterateDeriv initial_solutions
381 -- The initial solutions for the equations claim that each
382 -- instance has an empty context; this solution is certainly
383 -- in canonical form.
384 initial_solutions :: [DerivSoln]
385 initial_solutions = [ [] | _ <- orig_eqns ]
387 ------------------------------------------------------------------
388 -- iterateDeriv calculates the next batch of solutions,
389 -- compares it with the current one; finishes if they are the
390 -- same, otherwise recurses with the new solutions.
391 -- It fails if any iteration fails
392 iterateDeriv :: [DerivSoln] ->TcM [DFunId]
393 iterateDeriv current_solns
394 = checkNoErrsTc (iterateOnce current_solns)
395 `thenTc` \ (new_dfuns, new_solns) ->
396 if (current_solns == new_solns) then
399 iterateDeriv new_solns
401 ------------------------------------------------------------------
402 iterateOnce current_solns
403 = -- Extend the inst info from the explicit instance decls
404 -- with the current set of solutions, giving a
405 getDOptsTc `thenTc` \ dflags ->
406 let (new_dfuns, inst_env) =
407 add_solns dflags inst_env_in orig_eqns current_solns
410 tcSetInstEnv inst_env (
411 listTc [ tcAddSrcLoc (getSrcLoc tc) $
412 tcAddErrCtxt (derivCtxt tc) $
413 tcSimplifyThetas deriv_rhs
414 | (_, _,tc,_,deriv_rhs) <- orig_eqns ]
415 ) `thenTc` \ next_solns ->
417 -- Canonicalise the solutions, so they compare nicely
418 let canonicalised_next_solns = [ sortLt (<) next_soln | next_soln <- next_solns ]
420 returnTc (new_dfuns, canonicalised_next_solns)
424 add_solns :: DynFlags
425 -> InstEnv -- The global, non-derived ones
426 -> [DerivEqn] -> [DerivSoln]
427 -> ([DFunId], InstEnv)
428 -- the eqns and solns move "in lockstep"; we have the eqns
429 -- because we need the LHS info for addClassInstance.
431 add_solns dflags inst_env_in eqns solns
432 = (new_dfuns, inst_env)
434 new_dfuns = zipWithEqual "add_solns" mk_deriv_dfun eqns solns
435 (inst_env, _) = extendInstEnv dflags inst_env_in new_dfuns
436 -- Ignore the errors about duplicate instances.
437 -- We don't want repeated error messages
438 -- They'll appear later, when we do the top-level extendInstEnvs
440 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
441 = mkDictFunId dfun_name clas tyvars
442 [mkTyConApp tycon (mkTyVarTys tyvars)]
446 %************************************************************************
448 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
450 %************************************************************************
452 After all the trouble to figure out the required context for the
453 derived instance declarations, all that's left is to chug along to
454 produce them. They will then be shoved into @tcInstDecls2@, which
455 will do all its usual business.
457 There are lots of possibilities for code to generate. Here are
458 various general remarks.
463 We want derived instances of @Eq@ and @Ord@ (both v common) to be
464 ``you-couldn't-do-better-by-hand'' efficient.
467 Deriving @Show@---also pretty common--- should also be reasonable good code.
470 Deriving for the other classes isn't that common or that big a deal.
477 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
480 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
483 We {\em normally} generate code only for the non-defaulted methods;
484 there are some exceptions for @Eq@ and (especially) @Ord@...
487 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
488 constructor's numeric (@Int#@) tag. These are generated by
489 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
490 these is around is given by @hasCon2TagFun@.
492 The examples under the different sections below will make this
496 Much less often (really just for deriving @Ix@), we use a
497 @_tag2con_<tycon>@ function. See the examples.
500 We use the renamer!!! Reason: we're supposed to be
501 producing @RenamedMonoBinds@ for the methods, but that means
502 producing correctly-uniquified code on the fly. This is entirely
503 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
504 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
505 the renamer. What a great hack!
509 -- Generate the method bindings for the required instance
510 -- (paired with class name, as we need that when renaming
512 gen_bind :: (Name -> Maybe Fixity) -> DFunId -> (Name, RdrNameMonoBinds)
513 gen_bind get_fixity dfun
516 cls_nm = className clas
517 (clas, tycon) = simpleDFunClassTyCon dfun
519 binds = assoc "gen_bind:bad derived class" gen_list
520 (nameUnique cls_nm) tycon
522 gen_list = [(eqClassKey, gen_Eq_binds)
523 ,(ordClassKey, gen_Ord_binds)
524 ,(enumClassKey, gen_Enum_binds)
525 ,(boundedClassKey, gen_Bounded_binds)
526 ,(ixClassKey, gen_Ix_binds)
527 ,(showClassKey, gen_Show_binds get_fixity)
528 ,(readClassKey, gen_Read_binds get_fixity)
533 %************************************************************************
535 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
537 %************************************************************************
542 con2tag_Foo :: Foo ... -> Int#
543 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
544 maxtag_Foo :: Int -- ditto (NB: not unlifted)
547 We have a @con2tag@ function for a tycon if:
550 We're deriving @Eq@ and the tycon has nullary data constructors.
553 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
557 We have a @tag2con@ function for a tycon if:
560 We're deriving @Enum@, or @Ix@ (enum type only???)
563 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
566 gen_taggery_Names :: [DFunId]
567 -> TcM [(RdrName, -- for an assoc list
568 TyCon, -- related tycon
571 gen_taggery_Names dfuns
572 = foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
573 foldlTc do_tag2con names_so_far tycons_of_interest
575 all_CTs = map simpleDFunClassTyCon dfuns
576 all_tycons = map snd all_CTs
577 (tycons_of_interest, _) = removeDups compare all_tycons
579 do_con2tag acc_Names tycon
580 | isDataTyCon tycon &&
581 ((we_are_deriving eqClassKey tycon
582 && any isNullaryDataCon (tyConDataCons tycon))
583 || (we_are_deriving ordClassKey tycon
584 && not (maybeToBool (maybeTyConSingleCon tycon)))
585 || (we_are_deriving enumClassKey tycon)
586 || (we_are_deriving ixClassKey tycon))
588 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
593 do_tag2con acc_Names tycon
594 | isDataTyCon tycon &&
595 (we_are_deriving enumClassKey tycon ||
596 we_are_deriving ixClassKey tycon
597 && isEnumerationTyCon tycon)
598 = returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
599 : (maxtag_RDR tycon, tycon, GenMaxTag)
604 we_are_deriving clas_key tycon
605 = is_in_eqns clas_key tycon all_CTs
607 is_in_eqns clas_key tycon [] = False
608 is_in_eqns clas_key tycon ((c,t):cts)
609 = (clas_key == classKey c && tycon == t)
610 || is_in_eqns clas_key tycon cts
614 derivingThingErr :: Class -> TyCon -> FAST_STRING -> Message
616 derivingThingErr clas tycon why
617 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr clas)],
618 hsep [ptext SLIT("for the type"), quotes (ppr tycon)],
621 existentialErr clas tycon
622 = sep [ptext SLIT("Can't derive any instances for type") <+> quotes (ppr tycon),
623 ptext SLIT("because it has existentially-quantified constructor(s)")]
626 = ptext SLIT("When deriving classes for") <+> quotes (ppr tycon)