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 ( 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 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,
249 iBinds = binds, iPrags = [] }
251 rn_meths meths = rnMethodBinds [] meths `thenRn` \ (meths', _) -> returnRn meths'
252 -- Ignore the free vars returned
256 %************************************************************************
258 \subsection[TcDeriv-eqns]{Forming the equations}
260 %************************************************************************
262 @makeDerivEqns@ fishes around to find the info about needed derived
263 instances. Complicating factors:
266 We can only derive @Enum@ if the data type is an enumeration
267 type (all nullary data constructors).
270 We can only derive @Ix@ if the data type is an enumeration {\em
271 or} has just one data constructor (e.g., tuples).
274 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
278 makeDerivEqns :: [RenamedTyClDecl] -> TcM [DerivEqn]
280 makeDerivEqns tycl_decls
281 = mapTc mk_eqn derive_these `thenTc` \ maybe_eqns ->
282 returnTc (catMaybes maybe_eqns)
284 ------------------------------------------------------------------
285 derive_these :: [(Name, Name)]
286 -- Find the (Class,TyCon) pairs that must be `derived'
287 -- NB: only source-language decls have deriving, no imported ones do
288 derive_these = [ (clas,tycon)
289 | TyData {tcdName = tycon, tcdDerivs = Just classes} <- tycl_decls,
290 clas <- nub classes ]
292 ------------------------------------------------------------------
293 mk_eqn :: (Name, Name) -> NF_TcM (Maybe DerivEqn)
294 -- we swizzle the tyvars and datacons out of the tycon
295 -- to make the rest of the equation
297 mk_eqn (clas_name, tycon_name)
298 = tcLookupClass clas_name `thenNF_Tc` \ clas ->
299 tcLookupTyCon tycon_name `thenNF_Tc` \ tycon ->
301 clas_key = classKey clas
302 tyvars = tyConTyVars tycon
303 tyvar_tys = mkTyVarTys tyvars
304 ty = mkTyConApp tycon tyvar_tys
305 data_cons = tyConDataCons tycon
306 locn = getSrcLoc tycon
307 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
309 -- "extra_constraints": see notes above about contexts on data decls
311 | offensive_class = tyConTheta tycon
314 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
316 mk_constraints data_con
317 = [ mkClassPred clas [arg_ty]
318 | arg_ty <- dataConArgTys data_con tyvar_tys,
319 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
322 case chk_out clas tycon of
323 Just err -> tcAddSrcLoc (getSrcLoc tycon) $
324 addErrTc err `thenNF_Tc_`
326 Nothing -> newDFunName 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 | null data_cons = bog_out no_cons_why
338 | any isExistentialDataCon data_cons = Just (existentialErr clas tycon)
339 | otherwise = Nothing
341 data_cons = tyConDataCons tycon
342 is_enumeration = isEnumerationTyCon tycon
343 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
344 is_enumeration_or_single = is_enumeration || is_single_con
346 single_nullary_why = SLIT("one constructor data type or type with all nullary constructors expected")
347 nullary_why = SLIT("data type with all nullary constructors expected")
348 no_cons_why = SLIT("type has no data constructors")
350 bog_out why = Just (derivingThingErr clas tycon why)
353 %************************************************************************
355 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
357 %************************************************************************
359 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
360 terms, which is the final correct RHS for the corresponding original
364 Each (k,TyVarTy tv) in a solution constrains only a type
368 The (k,TyVarTy tv) pairs in a solution are canonically
369 ordered by sorting on type varible, tv, (major key) and then class, k,
374 solveDerivEqns :: InstEnv
376 -> TcM [DFunId] -- Solns in same order as eqns.
377 -- This bunch is Absolutely minimal...
379 solveDerivEqns inst_env_in orig_eqns
380 = iterateDeriv initial_solutions
382 -- The initial solutions for the equations claim that each
383 -- instance has an empty context; this solution is certainly
384 -- in canonical form.
385 initial_solutions :: [DerivSoln]
386 initial_solutions = [ [] | _ <- orig_eqns ]
388 ------------------------------------------------------------------
389 -- iterateDeriv calculates the next batch of solutions,
390 -- compares it with the current one; finishes if they are the
391 -- same, otherwise recurses with the new solutions.
392 -- It fails if any iteration fails
393 iterateDeriv :: [DerivSoln] ->TcM [DFunId]
394 iterateDeriv current_solns
395 = checkNoErrsTc (iterateOnce current_solns)
396 `thenTc` \ (new_dfuns, new_solns) ->
397 if (current_solns == new_solns) then
400 iterateDeriv new_solns
402 ------------------------------------------------------------------
403 iterateOnce current_solns
404 = -- Extend the inst info from the explicit instance decls
405 -- with the current set of solutions, giving a
406 getDOptsTc `thenTc` \ dflags ->
407 let (new_dfuns, inst_env) =
408 add_solns dflags inst_env_in orig_eqns current_solns
411 tcSetInstEnv inst_env (
412 listTc [ tcAddSrcLoc (getSrcLoc tc) $
413 tcAddErrCtxt (derivCtxt tc) $
414 tcSimplifyThetas deriv_rhs
415 | (_, _,tc,_,deriv_rhs) <- orig_eqns ]
416 ) `thenTc` \ next_solns ->
418 -- Canonicalise the solutions, so they compare nicely
419 let canonicalised_next_solns = [ sortLt (<) next_soln | next_soln <- next_solns ]
421 returnTc (new_dfuns, canonicalised_next_solns)
425 add_solns :: DynFlags
426 -> InstEnv -- The global, non-derived ones
427 -> [DerivEqn] -> [DerivSoln]
428 -> ([DFunId], InstEnv)
429 -- the eqns and solns move "in lockstep"; we have the eqns
430 -- because we need the LHS info for addClassInstance.
432 add_solns dflags inst_env_in eqns solns
433 = (new_dfuns, inst_env)
435 new_dfuns = zipWithEqual "add_solns" mk_deriv_dfun eqns solns
436 (inst_env, _) = extendInstEnv dflags inst_env_in new_dfuns
437 -- Ignore the errors about duplicate instances.
438 -- We don't want repeated error messages
439 -- They'll appear later, when we do the top-level extendInstEnvs
441 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
442 = mkDictFunId dfun_name clas tyvars
443 [mkTyConApp tycon (mkTyVarTys tyvars)]
447 %************************************************************************
449 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
451 %************************************************************************
453 After all the trouble to figure out the required context for the
454 derived instance declarations, all that's left is to chug along to
455 produce them. They will then be shoved into @tcInstDecls2@, which
456 will do all its usual business.
458 There are lots of possibilities for code to generate. Here are
459 various general remarks.
464 We want derived instances of @Eq@ and @Ord@ (both v common) to be
465 ``you-couldn't-do-better-by-hand'' efficient.
468 Deriving @Show@---also pretty common--- should also be reasonable good code.
471 Deriving for the other classes isn't that common or that big a deal.
478 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
481 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
484 We {\em normally} generate code only for the non-defaulted methods;
485 there are some exceptions for @Eq@ and (especially) @Ord@...
488 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
489 constructor's numeric (@Int#@) tag. These are generated by
490 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
491 these is around is given by @hasCon2TagFun@.
493 The examples under the different sections below will make this
497 Much less often (really just for deriving @Ix@), we use a
498 @_tag2con_<tycon>@ function. See the examples.
501 We use the renamer!!! Reason: we're supposed to be
502 producing @RenamedMonoBinds@ for the methods, but that means
503 producing correctly-uniquified code on the fly. This is entirely
504 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
505 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
506 the renamer. What a great hack!
510 -- Generate the method bindings for the required instance
511 -- (paired with class name, as we need that when generating dict
513 gen_bind :: (Name -> Maybe Fixity) -> DFunId -> RdrNameMonoBinds
514 gen_bind get_fixity dfun
515 | clas `hasKey` showClassKey = gen_Show_binds get_fixity tycon
516 | clas `hasKey` readClassKey = gen_Read_binds get_fixity tycon
518 = assoc "gen_bind:bad derived class"
519 [(eqClassKey, gen_Eq_binds)
520 ,(ordClassKey, gen_Ord_binds)
521 ,(enumClassKey, gen_Enum_binds)
522 ,(boundedClassKey, gen_Bounded_binds)
523 ,(ixClassKey, gen_Ix_binds)
528 (clas, tycon) = simpleDFunClassTyCon dfun
532 %************************************************************************
534 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
536 %************************************************************************
541 con2tag_Foo :: Foo ... -> Int#
542 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
543 maxtag_Foo :: Int -- ditto (NB: not unlifted)
546 We have a @con2tag@ function for a tycon if:
549 We're deriving @Eq@ and the tycon has nullary data constructors.
552 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
556 We have a @tag2con@ function for a tycon if:
559 We're deriving @Enum@, or @Ix@ (enum type only???)
562 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
565 gen_taggery_Names :: [DFunId]
566 -> TcM [(RdrName, -- for an assoc list
567 TyCon, -- related tycon
570 gen_taggery_Names dfuns
571 = foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
572 foldlTc do_tag2con names_so_far tycons_of_interest
574 all_CTs = map simpleDFunClassTyCon dfuns
575 all_tycons = map snd all_CTs
576 (tycons_of_interest, _) = removeDups compare all_tycons
578 do_con2tag acc_Names tycon
579 | isDataTyCon tycon &&
580 ((we_are_deriving eqClassKey tycon
581 && any isNullaryDataCon (tyConDataCons tycon))
582 || (we_are_deriving ordClassKey tycon
583 && not (maybeToBool (maybeTyConSingleCon tycon)))
584 || (we_are_deriving enumClassKey tycon)
585 || (we_are_deriving ixClassKey tycon))
587 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
592 do_tag2con acc_Names tycon
593 | isDataTyCon tycon &&
594 (we_are_deriving enumClassKey tycon ||
595 we_are_deriving ixClassKey tycon
596 && isEnumerationTyCon tycon)
597 = returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
598 : (maxtag_RDR tycon, tycon, GenMaxTag)
603 we_are_deriving clas_key tycon
604 = is_in_eqns clas_key tycon all_CTs
606 is_in_eqns clas_key tycon [] = False
607 is_in_eqns clas_key tycon ((c,t):cts)
608 = (clas_key == classKey c && tycon == t)
609 || is_in_eqns clas_key tycon cts
613 derivingThingErr :: Class -> TyCon -> FAST_STRING -> Message
615 derivingThingErr clas tycon why
616 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr clas)],
617 hsep [ptext SLIT("for the type"), quotes (ppr tycon)],
620 existentialErr clas tycon
621 = sep [ptext SLIT("Can't derive any instances for type") <+> quotes (ppr tycon),
622 ptext SLIT("because it has existentially-quantified constructor(s)")]
625 = ptext SLIT("When deriving classes for") <+> quotes (ppr tycon)