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, PredType(..), mkTyVarTys, mkTyConApp,
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 = [(Class, [TauType])] -- Same as a ThetaType!
147 --[PredType] -- ... | Class Class [Type==TauType]
149 type DerivSoln = DerivRhs
153 A note about contexts on data decls
154 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
157 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
159 We will need an instance decl like:
161 instance (Read a, RealFloat a) => Read (Complex a) where
164 The RealFloat in the context is because the read method for Complex is bound
165 to construct a Complex, and doing that requires that the argument type is
168 But this ain't true for Show, Eq, Ord, etc, since they don't construct
169 a Complex; they only take them apart.
171 Our approach: identify the offending classes, and add the data type
172 context to the instance decl. The "offending classes" are
177 %************************************************************************
179 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
181 %************************************************************************
184 tcDeriving :: PersistentRenamerState
185 -> Module -- name of module under scrutiny
186 -> InstEnv -- What we already know about instances
187 -> (Name -> Maybe Fixity) -- used in deriving Show and Read
188 -> [RenamedTyClDecl] -- All type constructors
189 -> TcM ([InstInfo], -- The generated "instance decls".
190 RenamedHsBinds) -- Extra generated bindings
192 tcDeriving prs mod inst_env_in get_fixity tycl_decls
193 = recoverTc (returnTc ([], EmptyBinds)) $
195 -- Fish the "deriving"-related information out of the TcEnv
196 -- and make the necessary "equations".
197 makeDerivEqns tycl_decls `thenTc` \ eqns ->
199 returnTc ([], EmptyBinds)
202 -- Take the equation list and solve it, to deliver a list of
203 -- solutions, a.k.a. the contexts for the instance decls
204 -- required for the corresponding equations.
205 solveDerivEqns inst_env_in eqns `thenTc` \ new_dfuns ->
207 -- Now augment the InstInfos, adding in the rather boring
208 -- actual-code-to-do-the-methods binds. We may also need to
209 -- generate extra not-one-inst-decl-specific binds, notably
210 -- "con2tag" and/or "tag2con" functions. We do these
213 gen_taggery_Names new_dfuns `thenTc` \ nm_alist_etc ->
215 tcGetEnv `thenNF_Tc` \ env ->
216 getDOptsTc `thenTc` \ dflags ->
218 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
219 extra_mbinds = foldr AndMonoBinds EmptyMonoBinds extra_mbind_list
220 method_binds_s = map (gen_bind get_fixity) new_dfuns
221 mbinders = collectLocatedMonoBinders extra_mbinds
223 -- Rename to get RenamedBinds.
224 -- The only tricky bit is that the extra_binds must scope over the
225 -- method bindings for the instances.
226 (rn_method_binds_s, rn_extra_binds)
227 = renameDerivedCode dflags mod prs (
228 bindLocatedLocalsRn (ptext (SLIT("deriving"))) mbinders $ \ _ ->
229 rnTopMonoBinds extra_mbinds [] `thenRn` \ (rn_extra_binds, _) ->
230 mapRn rn_meths method_binds_s `thenRn` \ rn_method_binds_s ->
231 returnRn (rn_method_binds_s, rn_extra_binds)
234 new_inst_infos = zipWith gen_inst_info new_dfuns rn_method_binds_s
237 ioToTc (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
238 (ddump_deriving new_inst_infos rn_extra_binds)) `thenTc_`
240 returnTc (new_inst_infos, rn_extra_binds)
242 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> SDoc
243 ddump_deriving inst_infos extra_binds
244 = vcat (map pprInstInfo inst_infos) $$ ppr extra_binds
247 -- Make a Real dfun instead of the dummy one we have so far
248 gen_inst_info :: DFunId -> RenamedMonoBinds -> InstInfo
249 gen_inst_info dfun binds
250 = InstInfo { iDFunId = dfun,
251 iBinds = binds, iPrags = [] }
253 rn_meths meths = rnMethodBinds [] meths `thenRn` \ (meths', _) -> returnRn meths'
254 -- Ignore the free vars returned
258 %************************************************************************
260 \subsection[TcDeriv-eqns]{Forming the equations}
262 %************************************************************************
264 @makeDerivEqns@ fishes around to find the info about needed derived
265 instances. Complicating factors:
268 We can only derive @Enum@ if the data type is an enumeration
269 type (all nullary data constructors).
272 We can only derive @Ix@ if the data type is an enumeration {\em
273 or} has just one data constructor (e.g., tuples).
276 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
280 makeDerivEqns :: [RenamedTyClDecl] -> TcM [DerivEqn]
282 makeDerivEqns tycl_decls
283 = mapTc mk_eqn derive_these `thenTc` \ maybe_eqns ->
284 returnTc (catMaybes maybe_eqns)
286 ------------------------------------------------------------------
287 derive_these :: [(Name, Name)]
288 -- Find the (Class,TyCon) pairs that must be `derived'
289 -- NB: only source-language decls have deriving, no imported ones do
290 derive_these = [ (clas,tycon)
291 | TyData {tcdName = tycon, tcdDerivs = Just classes} <- tycl_decls,
292 clas <- nub classes ]
294 ------------------------------------------------------------------
295 mk_eqn :: (Name, Name) -> NF_TcM (Maybe DerivEqn)
296 -- we swizzle the tyvars and datacons out of the tycon
297 -- to make the rest of the equation
299 mk_eqn (clas_name, tycon_name)
300 = tcLookupClass clas_name `thenNF_Tc` \ clas ->
301 tcLookupTyCon tycon_name `thenNF_Tc` \ tycon ->
303 clas_key = classKey clas
304 tyvars = tyConTyVars tycon
305 tyvar_tys = mkTyVarTys tyvars
306 ty = mkTyConApp tycon tyvar_tys
307 data_cons = tyConDataCons tycon
308 locn = getSrcLoc tycon
309 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
311 -- "extra_constraints": see notes above about contexts on data decls
313 | offensive_class = tyConTheta tycon
316 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
318 mk_constraints data_con
320 | arg_ty <- dataConArgTys data_con tyvar_tys,
321 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
324 case chk_out clas tycon of
325 Just err -> addErrTc err `thenNF_Tc_`
327 Nothing -> newDFunName clas [ty] locn `thenNF_Tc` \ dfun_name ->
328 returnNF_Tc (Just (dfun_name, clas, tycon, tyvars, constraints))
332 ------------------------------------------------------------------
333 chk_out :: Class -> TyCon -> Maybe Message
335 | clas `hasKey` enumClassKey && not is_enumeration = bog_out nullary_why
336 | clas `hasKey` boundedClassKey && not is_enumeration_or_single = bog_out single_nullary_why
337 | clas `hasKey` ixClassKey && not is_enumeration_or_single = bog_out single_nullary_why
338 | any isExistentialDataCon (tyConDataCons tycon) = Just (existentialErr clas tycon)
339 | otherwise = Nothing
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")
348 bog_out why = Just (derivingThingErr clas tycon why)
351 %************************************************************************
353 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
355 %************************************************************************
357 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
358 terms, which is the final correct RHS for the corresponding original
362 Each (k,TyVarTy tv) in a solution constrains only a type
366 The (k,TyVarTy tv) pairs in a solution are canonically
367 ordered by sorting on type varible, tv, (major key) and then class, k,
372 solveDerivEqns :: InstEnv
374 -> TcM [DFunId] -- Solns in same order as eqns.
375 -- This bunch is Absolutely minimal...
377 solveDerivEqns inst_env_in orig_eqns
378 = iterateDeriv initial_solutions
380 -- The initial solutions for the equations claim that each
381 -- instance has an empty context; this solution is certainly
382 -- in canonical form.
383 initial_solutions :: [DerivSoln]
384 initial_solutions = [ [] | _ <- orig_eqns ]
386 ------------------------------------------------------------------
387 -- iterateDeriv calculates the next batch of solutions,
388 -- compares it with the current one; finishes if they are the
389 -- same, otherwise recurses with the new solutions.
390 -- It fails if any iteration fails
391 iterateDeriv :: [DerivSoln] ->TcM [DFunId]
392 iterateDeriv current_solns
393 = checkNoErrsTc (iterateOnce current_solns)
394 `thenTc` \ (new_dfuns, new_solns) ->
395 if (current_solns == new_solns) then
398 iterateDeriv new_solns
400 ------------------------------------------------------------------
401 iterateOnce current_solns
402 = -- Extend the inst info from the explicit instance decls
403 -- with the current set of solutions, giving a
404 getDOptsTc `thenTc` \ dflags ->
405 let (new_dfuns, inst_env) =
406 add_solns dflags inst_env_in orig_eqns current_solns
409 tcSetInstEnv inst_env (
410 listTc [ tcAddErrCtxt (derivCtxt tc) $
411 tcSimplifyThetas deriv_rhs
412 | (_, _,tc,_,deriv_rhs) <- orig_eqns ]
413 ) `thenTc` \ next_solns ->
415 -- Canonicalise the solutions, so they compare nicely
416 let canonicalised_next_solns = [ sortLt (<) next_soln | next_soln <- next_solns ]
418 returnTc (new_dfuns, canonicalised_next_solns)
422 add_solns :: DynFlags
423 -> InstEnv -- The global, non-derived ones
424 -> [DerivEqn] -> [DerivSoln]
425 -> ([DFunId], InstEnv)
426 -- the eqns and solns move "in lockstep"; we have the eqns
427 -- because we need the LHS info for addClassInstance.
429 add_solns dflags inst_env_in eqns solns
430 = (new_dfuns, inst_env)
432 new_dfuns = zipWithEqual "add_solns" mk_deriv_dfun eqns solns
433 (inst_env, _) = extendInstEnv dflags inst_env_in new_dfuns
434 -- Ignore the errors about duplicate instances.
435 -- We don't want repeated error messages
436 -- They'll appear later, when we do the top-level extendInstEnvs
438 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
439 = mkDictFunId dfun_name clas tyvars [mkTyConApp tycon (mkTyVarTys tyvars)]
440 (map pair2PredType theta)
442 pair2PredType (clas, tautypes) = Class clas tautypes
445 %************************************************************************
447 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
449 %************************************************************************
451 After all the trouble to figure out the required context for the
452 derived instance declarations, all that's left is to chug along to
453 produce them. They will then be shoved into @tcInstDecls2@, which
454 will do all its usual business.
456 There are lots of possibilities for code to generate. Here are
457 various general remarks.
462 We want derived instances of @Eq@ and @Ord@ (both v common) to be
463 ``you-couldn't-do-better-by-hand'' efficient.
466 Deriving @Show@---also pretty common--- should also be reasonable good code.
469 Deriving for the other classes isn't that common or that big a deal.
476 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
479 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
482 We {\em normally} generate code only for the non-defaulted methods;
483 there are some exceptions for @Eq@ and (especially) @Ord@...
486 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
487 constructor's numeric (@Int#@) tag. These are generated by
488 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
489 these is around is given by @hasCon2TagFun@.
491 The examples under the different sections below will make this
495 Much less often (really just for deriving @Ix@), we use a
496 @_tag2con_<tycon>@ function. See the examples.
499 We use the renamer!!! Reason: we're supposed to be
500 producing @RenamedMonoBinds@ for the methods, but that means
501 producing correctly-uniquified code on the fly. This is entirely
502 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
503 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
504 the renamer. What a great hack!
508 -- Generate the method bindings for the required instance
509 -- (paired with class name, as we need that when generating dict
511 gen_bind :: (Name -> Maybe Fixity) -> DFunId -> RdrNameMonoBinds
512 gen_bind get_fixity dfun
513 | clas `hasKey` showClassKey = gen_Show_binds get_fixity tycon
514 | clas `hasKey` readClassKey = gen_Read_binds get_fixity tycon
516 = assoc "gen_bind:bad derived class"
517 [(eqClassKey, gen_Eq_binds)
518 ,(ordClassKey, gen_Ord_binds)
519 ,(enumClassKey, gen_Enum_binds)
520 ,(boundedClassKey, gen_Bounded_binds)
521 ,(ixClassKey, gen_Ix_binds)
526 (clas, tycon) = simpleDFunClassTyCon dfun
530 %************************************************************************
532 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
534 %************************************************************************
539 con2tag_Foo :: Foo ... -> Int#
540 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
541 maxtag_Foo :: Int -- ditto (NB: not unlifted)
544 We have a @con2tag@ function for a tycon if:
547 We're deriving @Eq@ and the tycon has nullary data constructors.
550 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
554 We have a @tag2con@ function for a tycon if:
557 We're deriving @Enum@, or @Ix@ (enum type only???)
560 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
563 gen_taggery_Names :: [DFunId]
564 -> TcM [(RdrName, -- for an assoc list
565 TyCon, -- related tycon
568 gen_taggery_Names dfuns
569 = foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
570 foldlTc do_tag2con names_so_far tycons_of_interest
572 all_CTs = map simpleDFunClassTyCon dfuns
573 all_tycons = map snd all_CTs
574 (tycons_of_interest, _) = removeDups compare all_tycons
576 do_con2tag acc_Names tycon
577 | isDataTyCon tycon &&
578 ((we_are_deriving eqClassKey tycon
579 && any isNullaryDataCon (tyConDataCons tycon))
580 || (we_are_deriving ordClassKey tycon
581 && not (maybeToBool (maybeTyConSingleCon tycon)))
582 || (we_are_deriving enumClassKey tycon)
583 || (we_are_deriving ixClassKey tycon))
585 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
590 do_tag2con acc_Names tycon
591 | isDataTyCon tycon &&
592 (we_are_deriving enumClassKey tycon ||
593 we_are_deriving ixClassKey tycon
594 && isEnumerationTyCon tycon)
595 = returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
596 : (maxtag_RDR tycon, tycon, GenMaxTag)
601 we_are_deriving clas_key tycon
602 = is_in_eqns clas_key tycon all_CTs
604 is_in_eqns clas_key tycon [] = False
605 is_in_eqns clas_key tycon ((c,t):cts)
606 = (clas_key == classKey c && tycon == t)
607 || is_in_eqns clas_key tycon cts
611 derivingThingErr :: Class -> TyCon -> FAST_STRING -> Message
613 derivingThingErr clas tycon why
614 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr clas)],
615 hsep [ptext SLIT("for the type"), quotes (ppr tycon)],
618 existentialErr clas tycon
619 = sep [ptext SLIT("Can't derive any instances for type") <+> quotes (ppr tycon),
620 ptext SLIT("because it has existentially-quantified constructor(s)")]
623 = ptext SLIT("When deriving classes for") <+> quotes (ppr tycon)