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(..), collectMonoBinders )
14 import RdrHsSyn ( RdrNameMonoBinds )
15 import RnHsSyn ( RenamedHsBinds, RenamedMonoBinds )
16 import CmdLineOpts ( opt_D_dump_deriv )
19 import TcEnv ( InstEnv, getEnvTyCons, tcSetInstEnv, newDFunName )
20 import TcGenDeriv -- Deriv stuff
21 import TcInstUtil ( InstInfo(..), buildInstanceEnv )
22 import TcSimplify ( tcSimplifyThetas )
24 import RnBinds ( rnMethodBinds, rnTopMonoBinds )
25 import RnEnv ( bindLocatedLocalsRn )
26 import RnMonad ( RnNameSupply,
27 renameSourceCode, thenRn, mapRn, returnRn )
29 import Bag ( Bag, emptyBag, unionBags, listToBag )
30 import Class ( classKey, Class )
31 import ErrUtils ( dumpIfSet, Message, pprBagOfErrors )
32 import MkId ( mkDictFunId )
33 import Id ( mkVanillaId )
34 import DataCon ( dataConArgTys, isNullaryDataCon, isExistentialDataCon )
35 import PrelInfo ( needsDataDeclCtxtClassKeys )
36 import Maybes ( maybeToBool, catMaybes )
37 import Module ( Module )
38 import Name ( isLocallyDefined, getSrcLoc,
42 import RdrName ( RdrName )
43 import RnMonad ( FixityEnv )
44 import SrcLoc ( mkGeneratedSrcLoc, SrcLoc )
45 import TyCon ( tyConTyVars, tyConDataCons, tyConDerivings,
46 tyConTheta, maybeTyConSingleCon, isDataTyCon,
47 isEnumerationTyCon, isAlgTyCon, TyCon
49 import Type ( TauType, mkTyVarTys, mkTyConApp,
50 mkSigmaTy, mkDictTy, isUnboxedType,
51 splitAlgTyConApp, classesToPreds
53 import TysWiredIn ( voidTy )
55 import Unique -- Keys stuff
56 import Bag ( bagToList )
57 import Util ( zipWithEqual, sortLt, removeDups, assoc, thenCmp )
61 %************************************************************************
63 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
65 %************************************************************************
69 data T a b = C1 (Foo a) (Bar b)
74 [NOTE: See end of these comments for what to do with
75 data (C a, D b) => T a b = ...
78 We want to come up with an instance declaration of the form
80 instance (Ping a, Pong b, ...) => Eq (T a b) where
83 It is pretty easy, albeit tedious, to fill in the code "...". The
84 trick is to figure out what the context for the instance decl is,
85 namely @Ping@, @Pong@ and friends.
87 Let's call the context reqd for the T instance of class C at types
88 (a,b, ...) C (T a b). Thus:
90 Eq (T a b) = (Ping a, Pong b, ...)
92 Now we can get a (recursive) equation from the @data@ decl:
94 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
95 u Eq (T b a) u Eq Int -- From C2
96 u Eq (T a a) -- From C3
98 Foo and Bar may have explicit instances for @Eq@, in which case we can
99 just substitute for them. Alternatively, either or both may have
100 their @Eq@ instances given by @deriving@ clauses, in which case they
101 form part of the system of equations.
103 Now all we need do is simplify and solve the equations, iterating to
104 find the least fixpoint. Notice that the order of the arguments can
105 switch around, as here in the recursive calls to T.
107 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
111 Eq (T a b) = {} -- The empty set
114 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
115 u Eq (T b a) u Eq Int -- From C2
116 u Eq (T a a) -- From C3
118 After simplification:
119 = Eq a u Ping b u {} u {} u {}
124 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
125 u Eq (T b a) u Eq Int -- From C2
126 u Eq (T a a) -- From C3
128 After simplification:
133 = Eq a u Ping b u Eq b u Ping a
135 The next iteration gives the same result, so this is the fixpoint. We
136 need to make a canonical form of the RHS to ensure convergence. We do
137 this by simplifying the RHS to a form in which
139 - the classes constrain only tyvars
140 - the list is sorted by tyvar (major key) and then class (minor key)
141 - no duplicates, of course
143 So, here are the synonyms for the ``equation'' structures:
146 type DerivEqn = (Class, TyCon, [TyVar], DerivRhs)
147 -- The tyvars bind all the variables in the RHS
148 -- NEW: it's convenient to re-use InstInfo
149 -- We'll "panic" out some fields...
151 type DerivRhs = [(Class, [TauType])] -- Same as a ThetaType!
153 type DerivSoln = DerivRhs
157 A note about contexts on data decls
158 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
161 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
163 We will need an instance decl like:
165 instance (Read a, RealFloat a) => Read (Complex a) where
168 The RealFloat in the context is because the read method for Complex is bound
169 to construct a Complex, and doing that requires that the argument type is
172 But this ain't true for Show, Eq, Ord, etc, since they don't construct
173 a Complex; they only take them apart.
175 Our approach: identify the offending classes, and add the data type
176 context to the instance decl. The "offending classes" are
181 %************************************************************************
183 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
185 %************************************************************************
188 tcDeriving :: Module -- name of module under scrutiny
189 -> FixityEnv -- for the deriving code (Show/Read.)
190 -> RnNameSupply -- for "renaming" bits of generated code
191 -> Bag InstInfo -- What we already know about instances
192 -> TcM s (Bag InstInfo, -- The generated "instance decls".
193 RenamedHsBinds) -- Extra generated bindings
195 tcDeriving mod fixs rn_name_supply inst_decl_infos_in
196 = recoverTc (returnTc (emptyBag, EmptyBinds)) $
198 -- Fish the "deriving"-related information out of the TcEnv
199 -- and make the necessary "equations".
200 makeDerivEqns `thenTc` \ eqns ->
202 returnTc (emptyBag, EmptyBinds)
205 -- Take the equation list and solve it, to deliver a list of
206 -- solutions, a.k.a. the contexts for the instance decls
207 -- required for the corresponding equations.
208 solveDerivEqns inst_decl_infos_in eqns `thenTc` \ new_inst_infos ->
210 -- Now augment the InstInfos, adding in the rather boring
211 -- actual-code-to-do-the-methods binds. We may also need to
212 -- generate extra not-one-inst-decl-specific binds, notably
213 -- "con2tag" and/or "tag2con" functions. We do these
216 gen_taggery_Names new_inst_infos `thenTc` \ nm_alist_etc ->
220 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
221 extra_mbinds = foldr AndMonoBinds EmptyMonoBinds extra_mbind_list
222 method_binds_s = map (gen_bind fixs) new_inst_infos
223 mbinders = bagToList (collectMonoBinders extra_mbinds)
225 -- Rename to get RenamedBinds.
226 -- The only tricky bit is that the extra_binds must scope over the
227 -- method bindings for the instances.
228 (rn_method_binds_s, rn_extra_binds)
229 = renameSourceCode mod rn_name_supply (
230 bindLocatedLocalsRn (ptext (SLIT("deriving"))) mbinders $ \ _ ->
231 rnTopMonoBinds extra_mbinds [] `thenRn` \ (rn_extra_binds, _) ->
232 mapRn rn_meths method_binds_s `thenRn` \ rn_method_binds_s ->
233 returnRn (rn_method_binds_s, rn_extra_binds)
236 mapNF_Tc gen_inst_info (new_inst_infos `zip` rn_method_binds_s) `thenNF_Tc` \ really_new_inst_infos ->
238 ioToTc (dumpIfSet opt_D_dump_deriv "Derived instances"
239 (ddump_deriving really_new_inst_infos rn_extra_binds)) `thenTc_`
241 returnTc (listToBag really_new_inst_infos, rn_extra_binds)
243 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> SDoc
244 ddump_deriving inst_infos extra_binds
245 = vcat (map pp_info inst_infos) $$ ppr extra_binds
247 pp_info (InstInfo clas tvs [ty] inst_decl_theta _ mbinds _ _)
248 = ppr (mkSigmaTy tvs inst_decl_theta' (mkDictTy clas [ty]))
251 where inst_decl_theta' = classesToPreds inst_decl_theta
253 -- Paste the dfun id and method binds into the InstInfo
254 gen_inst_info (InstInfo clas tyvars tys@(ty:_) inst_decl_theta _ _ locn _, meth_binds)
255 = newDFunName mod clas tys locn `thenNF_Tc` \ dfun_name ->
257 dfun_id = mkDictFunId dfun_name clas tyvars tys inst_decl_theta
259 returnNF_Tc (InstInfo clas tyvars tys inst_decl_theta
260 dfun_id meth_binds locn [])
262 rn_meths meths = rnMethodBinds meths `thenRn` \ (meths', _) -> returnRn meths'
263 -- Ignore the free vars returned
267 %************************************************************************
269 \subsection[TcDeriv-eqns]{Forming the equations}
271 %************************************************************************
273 @makeDerivEqns@ fishes around to find the info about needed derived
274 instances. Complicating factors:
277 We can only derive @Enum@ if the data type is an enumeration
278 type (all nullary data constructors).
281 We can only derive @Ix@ if the data type is an enumeration {\em
282 or} has just one data constructor (e.g., tuples).
285 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
289 makeDerivEqns :: TcM s [DerivEqn]
292 = tcGetEnv `thenNF_Tc` \ env ->
294 local_data_tycons = filter (\tc -> isLocallyDefined tc && isAlgTyCon tc)
297 think_about_deriving = need_deriving local_data_tycons
298 (derive_these, _) = removeDups cmp_deriv think_about_deriving
300 if null local_data_tycons then
301 returnTc [] -- Bale out now
303 mapTc mk_eqn derive_these `thenTc` \ maybe_eqns ->
304 returnTc (catMaybes maybe_eqns)
306 ------------------------------------------------------------------
307 need_deriving :: [TyCon] -> [(Class, TyCon)]
308 -- find the tycons that have `deriving' clauses;
310 need_deriving tycons_to_consider
311 = foldr (\ tycon acc -> [(clas,tycon) | clas <- tyConDerivings tycon] ++ acc)
315 ------------------------------------------------------------------
316 cmp_deriv :: (Class, TyCon) -> (Class, TyCon) -> Ordering
317 cmp_deriv (c1, t1) (c2, t2)
318 = (c1 `compare` c2) `thenCmp` (t1 `compare` t2)
320 ------------------------------------------------------------------
321 mk_eqn :: (Class, TyCon) -> NF_TcM s (Maybe DerivEqn)
322 -- we swizzle the tyvars and datacons out of the tycon
323 -- to make the rest of the equation
326 = case chk_out clas tycon of
327 Just err -> addErrTc err `thenNF_Tc_`
329 Nothing -> returnNF_Tc (Just (clas, tycon, tyvars, constraints))
331 clas_key = classKey clas
332 tyvars = tyConTyVars tycon -- ToDo: Do we need new tyvars ???
333 tyvar_tys = mkTyVarTys tyvars
334 data_cons = tyConDataCons tycon
336 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
338 -- "extra_constraints": see notes above about contexts on data decls
340 | offensive_class = tyConTheta tycon
343 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
345 mk_constraints data_con
347 | arg_ty <- instd_arg_tys,
348 not (isUnboxedType arg_ty) -- No constraints for unboxed types?
351 instd_arg_tys = dataConArgTys data_con tyvar_tys
353 ------------------------------------------------------------------
354 chk_out :: Class -> TyCon -> Maybe Message
356 | clas `hasKey` enumClassKey && not is_enumeration = bog_out nullary_why
357 | clas `hasKey` boundedClassKey && not is_enumeration_or_single = bog_out single_nullary_why
358 | clas `hasKey` ixClassKey && not is_enumeration_or_single = bog_out single_nullary_why
359 | any isExistentialDataCon (tyConDataCons tycon) = Just (existentialErr clas tycon)
360 | otherwise = Nothing
362 is_enumeration = isEnumerationTyCon tycon
363 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
364 is_enumeration_or_single = is_enumeration || is_single_con
366 single_nullary_why = SLIT("one constructor data type or type with all nullary constructors expected")
367 nullary_why = SLIT("data type with all nullary constructors expected")
369 bog_out why = Just (derivingThingErr clas tycon why)
372 %************************************************************************
374 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
376 %************************************************************************
378 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
379 terms, which is the final correct RHS for the corresponding original
383 Each (k,TyVarTy tv) in a solution constrains only a type
387 The (k,TyVarTy tv) pairs in a solution are canonically
388 ordered by sorting on type varible, tv, (major key) and then class, k,
393 solveDerivEqns :: Bag InstInfo
395 -> TcM s [InstInfo] -- Solns in same order as eqns.
396 -- This bunch is Absolutely minimal...
398 solveDerivEqns inst_decl_infos_in orig_eqns
399 = iterateDeriv initial_solutions
401 -- The initial solutions for the equations claim that each
402 -- instance has an empty context; this solution is certainly
403 -- in canonical form.
404 initial_solutions :: [DerivSoln]
405 initial_solutions = [ [] | _ <- orig_eqns ]
407 ------------------------------------------------------------------
408 -- iterateDeriv calculates the next batch of solutions,
409 -- compares it with the current one; finishes if they are the
410 -- same, otherwise recurses with the new solutions.
411 -- It fails if any iteration fails
412 iterateDeriv :: [DerivSoln] ->TcM s [InstInfo]
413 iterateDeriv current_solns
414 = checkNoErrsTc (iterateOnce current_solns) `thenTc` \ (new_inst_infos, new_solns) ->
415 if (current_solns == new_solns) then
416 returnTc new_inst_infos
418 iterateDeriv new_solns
420 ------------------------------------------------------------------
421 iterateOnce current_solns
422 = -- Extend the inst info from the explicit instance decls
423 -- with the current set of solutions, giving a
425 add_solns inst_decl_infos_in orig_eqns current_solns
426 `thenNF_Tc` \ (new_inst_infos, inst_env) ->
430 tcSetInstEnv inst_env (
431 listTc [ tcAddErrCtxt (derivCtxt tc) $
432 tcSimplifyThetas deriv_rhs
433 | (_,tc,_,deriv_rhs) <- orig_eqns ]
434 ) `thenTc` \ next_solns ->
436 -- Canonicalise the solutions, so they compare nicely
437 let canonicalised_next_solns
438 = [ sortLt (<) next_soln | next_soln <- next_solns ]
440 returnTc (new_inst_infos, canonicalised_next_solns)
444 add_solns :: Bag InstInfo -- The global, non-derived ones
445 -> [DerivEqn] -> [DerivSoln]
446 -> NF_TcM s ([InstInfo], -- The new, derived ones
448 -- the eqns and solns move "in lockstep"; we have the eqns
449 -- because we need the LHS info for addClassInstance.
451 add_solns inst_infos_in eqns solns
453 = discardErrsTc (buildInstanceEnv all_inst_infos) `thenNF_Tc` \ inst_env ->
454 -- We do the discard-errs so that we don't get repeated error messages
455 -- about duplicate instances.
456 -- They'll appear later, when we do the top-level buildInstanceEnv.
458 returnNF_Tc (new_inst_infos, inst_env)
460 new_inst_infos = zipWithEqual "add_solns" mk_deriv_inst_info eqns solns
462 all_inst_infos = inst_infos_in `unionBags` listToBag new_inst_infos
464 mk_deriv_inst_info (clas, tycon, tyvars, _) theta
465 = InstInfo clas tyvars [mkTyConApp tycon (mkTyVarTys tyvars)]
468 (my_panic "binds") (getSrcLoc tycon)
469 (my_panic "upragmas")
472 = mkVanillaId (getName tycon) dummy_dfun_ty
473 -- The name is getSrcLoc'd in an error message
475 theta' = classesToPreds theta
476 dummy_dfun_ty = mkSigmaTy tyvars theta' voidTy
477 -- All we need from the dfun is its "theta" part, used during
478 -- equation simplification (tcSimplifyThetas). The final
479 -- dfun_id will have the superclass dictionaries as arguments too,
480 -- but that'll be added after the equations are solved. For now,
481 -- it's enough just to make a dummy dfun with the simple theta part.
483 -- The part after the theta is dummied here as voidTy; actually it's
484 -- (C (T a b)), but it doesn't seem worth constructing it.
485 -- We can't leave it as a panic because to get the theta part we
486 -- have to run down the type!
488 my_panic str = panic "add_soln" -- pprPanic ("add_soln:"++str) (hsep [char ':', ppr clas, ppr tycon])
491 %************************************************************************
493 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
495 %************************************************************************
497 After all the trouble to figure out the required context for the
498 derived instance declarations, all that's left is to chug along to
499 produce them. They will then be shoved into @tcInstDecls2@, which
500 will do all its usual business.
502 There are lots of possibilities for code to generate. Here are
503 various general remarks.
508 We want derived instances of @Eq@ and @Ord@ (both v common) to be
509 ``you-couldn't-do-better-by-hand'' efficient.
512 Deriving @Show@---also pretty common--- should also be reasonable good code.
515 Deriving for the other classes isn't that common or that big a deal.
522 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
525 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
528 We {\em normally} generate code only for the non-defaulted methods;
529 there are some exceptions for @Eq@ and (especially) @Ord@...
532 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
533 constructor's numeric (@Int#@) tag. These are generated by
534 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
535 these is around is given by @hasCon2TagFun@.
537 The examples under the different sections below will make this
541 Much less often (really just for deriving @Ix@), we use a
542 @_tag2con_<tycon>@ function. See the examples.
545 We use the renamer!!! Reason: we're supposed to be
546 producing @RenamedMonoBinds@ for the methods, but that means
547 producing correctly-uniquified code on the fly. This is entirely
548 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
549 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
550 the renamer. What a great hack!
554 -- Generate the method bindings for the required instance
555 -- (paired with class name, as we need that when generating dict
557 gen_bind :: FixityEnv -> InstInfo -> RdrNameMonoBinds
558 gen_bind fixities (InstInfo clas _ [ty] _ _ _ _ _)
559 | not from_here = EmptyMonoBinds
560 | clas `hasKey` showClassKey = gen_Show_binds fixities tycon
561 | clas `hasKey` readClassKey = gen_Read_binds fixities tycon
563 = assoc "gen_bind:bad derived class"
564 [(eqClassKey, gen_Eq_binds)
565 ,(ordClassKey, gen_Ord_binds)
566 ,(enumClassKey, gen_Enum_binds)
567 ,(boundedClassKey, gen_Bounded_binds)
568 ,(ixClassKey, gen_Ix_binds)
573 from_here = isLocallyDefined tycon
574 (tycon,_,_) = splitAlgTyConApp ty
578 %************************************************************************
580 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
582 %************************************************************************
587 con2tag_Foo :: Foo ... -> Int#
588 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
589 maxtag_Foo :: Int -- ditto (NB: not unboxed)
592 We have a @con2tag@ function for a tycon if:
595 We're deriving @Eq@ and the tycon has nullary data constructors.
598 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
602 We have a @tag2con@ function for a tycon if:
605 We're deriving @Enum@, or @Ix@ (enum type only???)
608 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
611 gen_taggery_Names :: [InstInfo]
612 -> TcM s [(RdrName, -- for an assoc list
613 TyCon, -- related tycon
616 gen_taggery_Names inst_infos
617 = --pprTrace "gen_taggery:\n" (vcat [hsep [ppr c, ppr t] | (c,t) <- all_CTs]) $
618 foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
619 foldlTc do_tag2con names_so_far tycons_of_interest
621 all_CTs = [ (c, get_tycon ty) | (InstInfo c _ [ty] _ _ _ _ _) <- inst_infos ]
623 get_tycon ty = case splitAlgTyConApp ty of { (tc, _, _) -> tc }
625 all_tycons = map snd all_CTs
626 (tycons_of_interest, _) = removeDups compare all_tycons
628 do_con2tag acc_Names tycon
629 | isDataTyCon tycon &&
630 ((we_are_deriving eqClassKey tycon
631 && any isNullaryDataCon (tyConDataCons tycon))
632 || (we_are_deriving ordClassKey tycon
633 && not (maybeToBool (maybeTyConSingleCon tycon)))
634 || (we_are_deriving enumClassKey tycon)
635 || (we_are_deriving ixClassKey tycon))
637 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
642 do_tag2con acc_Names tycon
643 | isDataTyCon tycon &&
644 (we_are_deriving enumClassKey tycon ||
645 we_are_deriving ixClassKey tycon
646 && isEnumerationTyCon tycon)
647 = returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
648 : (maxtag_RDR tycon, tycon, GenMaxTag)
653 we_are_deriving clas_key tycon
654 = is_in_eqns clas_key tycon all_CTs
656 is_in_eqns clas_key tycon [] = False
657 is_in_eqns clas_key tycon ((c,t):cts)
658 = (clas_key == classKey c && tycon == t)
659 || is_in_eqns clas_key tycon cts
664 derivingThingErr :: Class -> TyCon -> FAST_STRING -> Message
666 derivingThingErr clas tycon why
667 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr clas)],
668 hsep [ptext SLIT("for the type"), quotes (ppr tycon)],
671 existentialErr clas tycon
672 = sep [ptext SLIT("Can't derive any instances for type") <+> quotes (ppr tycon),
673 ptext SLIT("because it has existentially-quantified constructor(s)")]
676 = ptext SLIT("When deriving classes for") <+> quotes (ppr tycon)