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 Type ( TauType, ThetaType, PredType, 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 -> 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 | any isExistentialDataCon (tyConDataCons tycon) = Just (existentialErr clas tycon)
337 | otherwise = Nothing
339 is_enumeration = isEnumerationTyCon tycon
340 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
341 is_enumeration_or_single = is_enumeration || is_single_con
343 single_nullary_why = SLIT("one constructor data type or type with all nullary constructors expected")
344 nullary_why = SLIT("data type with all nullary constructors expected")
346 bog_out why = Just (derivingThingErr clas tycon why)
349 %************************************************************************
351 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
353 %************************************************************************
355 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
356 terms, which is the final correct RHS for the corresponding original
360 Each (k,TyVarTy tv) in a solution constrains only a type
364 The (k,TyVarTy tv) pairs in a solution are canonically
365 ordered by sorting on type varible, tv, (major key) and then class, k,
370 solveDerivEqns :: InstEnv
372 -> TcM [DFunId] -- Solns in same order as eqns.
373 -- This bunch is Absolutely minimal...
375 solveDerivEqns inst_env_in orig_eqns
376 = iterateDeriv initial_solutions
378 -- The initial solutions for the equations claim that each
379 -- instance has an empty context; this solution is certainly
380 -- in canonical form.
381 initial_solutions :: [DerivSoln]
382 initial_solutions = [ [] | _ <- orig_eqns ]
384 ------------------------------------------------------------------
385 -- iterateDeriv calculates the next batch of solutions,
386 -- compares it with the current one; finishes if they are the
387 -- same, otherwise recurses with the new solutions.
388 -- It fails if any iteration fails
389 iterateDeriv :: [DerivSoln] ->TcM [DFunId]
390 iterateDeriv current_solns
391 = checkNoErrsTc (iterateOnce current_solns)
392 `thenTc` \ (new_dfuns, new_solns) ->
393 if (current_solns == new_solns) then
396 iterateDeriv new_solns
398 ------------------------------------------------------------------
399 iterateOnce current_solns
400 = -- Extend the inst info from the explicit instance decls
401 -- with the current set of solutions, giving a
402 getDOptsTc `thenTc` \ dflags ->
403 let (new_dfuns, inst_env) =
404 add_solns dflags inst_env_in orig_eqns current_solns
407 tcSetInstEnv inst_env (
408 listTc [ tcAddSrcLoc (getSrcLoc tc) $
409 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
439 [mkTyConApp tycon (mkTyVarTys tyvars)]
443 %************************************************************************
445 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
447 %************************************************************************
449 After all the trouble to figure out the required context for the
450 derived instance declarations, all that's left is to chug along to
451 produce them. They will then be shoved into @tcInstDecls2@, which
452 will do all its usual business.
454 There are lots of possibilities for code to generate. Here are
455 various general remarks.
460 We want derived instances of @Eq@ and @Ord@ (both v common) to be
461 ``you-couldn't-do-better-by-hand'' efficient.
464 Deriving @Show@---also pretty common--- should also be reasonable good code.
467 Deriving for the other classes isn't that common or that big a deal.
474 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
477 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
480 We {\em normally} generate code only for the non-defaulted methods;
481 there are some exceptions for @Eq@ and (especially) @Ord@...
484 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
485 constructor's numeric (@Int#@) tag. These are generated by
486 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
487 these is around is given by @hasCon2TagFun@.
489 The examples under the different sections below will make this
493 Much less often (really just for deriving @Ix@), we use a
494 @_tag2con_<tycon>@ function. See the examples.
497 We use the renamer!!! Reason: we're supposed to be
498 producing @RenamedMonoBinds@ for the methods, but that means
499 producing correctly-uniquified code on the fly. This is entirely
500 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
501 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
502 the renamer. What a great hack!
506 -- Generate the method bindings for the required instance
507 -- (paired with class name, as we need that when generating dict
509 gen_bind :: (Name -> Maybe Fixity) -> DFunId -> RdrNameMonoBinds
510 gen_bind get_fixity dfun
511 | clas `hasKey` showClassKey = gen_Show_binds get_fixity tycon
512 | clas `hasKey` readClassKey = gen_Read_binds get_fixity tycon
514 = assoc "gen_bind:bad derived class"
515 [(eqClassKey, gen_Eq_binds)
516 ,(ordClassKey, gen_Ord_binds)
517 ,(enumClassKey, gen_Enum_binds)
518 ,(boundedClassKey, gen_Bounded_binds)
519 ,(ixClassKey, gen_Ix_binds)
524 (clas, tycon) = simpleDFunClassTyCon dfun
528 %************************************************************************
530 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
532 %************************************************************************
537 con2tag_Foo :: Foo ... -> Int#
538 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
539 maxtag_Foo :: Int -- ditto (NB: not unlifted)
542 We have a @con2tag@ function for a tycon if:
545 We're deriving @Eq@ and the tycon has nullary data constructors.
548 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
552 We have a @tag2con@ function for a tycon if:
555 We're deriving @Enum@, or @Ix@ (enum type only???)
558 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
561 gen_taggery_Names :: [DFunId]
562 -> TcM [(RdrName, -- for an assoc list
563 TyCon, -- related tycon
566 gen_taggery_Names dfuns
567 = foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
568 foldlTc do_tag2con names_so_far tycons_of_interest
570 all_CTs = map simpleDFunClassTyCon dfuns
571 all_tycons = map snd all_CTs
572 (tycons_of_interest, _) = removeDups compare all_tycons
574 do_con2tag acc_Names tycon
575 | isDataTyCon tycon &&
576 ((we_are_deriving eqClassKey tycon
577 && any isNullaryDataCon (tyConDataCons tycon))
578 || (we_are_deriving ordClassKey tycon
579 && not (maybeToBool (maybeTyConSingleCon tycon)))
580 || (we_are_deriving enumClassKey tycon)
581 || (we_are_deriving ixClassKey tycon))
583 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
588 do_tag2con acc_Names tycon
589 | isDataTyCon tycon &&
590 (we_are_deriving enumClassKey tycon ||
591 we_are_deriving ixClassKey tycon
592 && isEnumerationTyCon tycon)
593 = returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
594 : (maxtag_RDR tycon, tycon, GenMaxTag)
599 we_are_deriving clas_key tycon
600 = is_in_eqns clas_key tycon all_CTs
602 is_in_eqns clas_key tycon [] = False
603 is_in_eqns clas_key tycon ((c,t):cts)
604 = (clas_key == classKey c && tycon == t)
605 || is_in_eqns clas_key tycon cts
609 derivingThingErr :: Class -> TyCon -> FAST_STRING -> Message
611 derivingThingErr clas tycon why
612 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr clas)],
613 hsep [ptext SLIT("for the type"), quotes (ppr tycon)],
616 existentialErr clas tycon
617 = sep [ptext SLIT("Can't derive any instances for type") <+> quotes (ppr tycon),
618 ptext SLIT("because it has existentially-quantified constructor(s)")]
621 = ptext SLIT("When deriving classes for") <+> quotes (ppr tycon)