2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
4 \section[TcDeriv]{Deriving}
6 Handles @deriving@ clauses on @data@ declarations.
9 #include "HsVersions.h"
11 module TcDeriv ( tcDeriving ) where
15 import HsSyn ( HsDecl, FixityDecl, Fixity, InstDecl,
16 Sig, HsBinds(..), MonoBinds(..),
17 GRHSsAndBinds, Match, HsExpr, HsLit, InPat,
18 ArithSeqInfo, Fake, HsType,
21 import HsPragmas ( InstancePragmas(..) )
22 import RdrHsSyn ( RdrName, SYN_IE(RdrNameMonoBinds) )
23 import RnHsSyn ( SYN_IE(RenamedHsBinds), SYN_IE(RenamedMonoBinds), SYN_IE(RenamedFixityDecl) )
26 import Inst ( SYN_IE(InstanceMapper) )
27 import TcEnv ( getEnv_TyCons, tcLookupClassByKey )
28 import SpecEnv ( SpecEnv )
29 import TcKind ( TcKind )
30 import TcGenDeriv -- Deriv stuff
31 import TcInstUtil ( InstInfo(..), mkInstanceRelatedIds, buildInstanceEnvs )
32 import TcSimplify ( tcSimplifyThetas )
33 import TcType ( TcIdOcc )
35 import RnBinds ( rnMethodBinds, rnTopMonoBinds )
36 import RnEnv ( newDfunName, bindLocatedLocalsRn )
37 import RnMonad ( SYN_IE(RnM), RnDown, GDown, SDown, RnNameSupply(..),
38 setNameSupplyRn, renameSourceCode, thenRn, mapRn, returnRn )
40 import Bag ( Bag, emptyBag, isEmptyBag, unionBags, listToBag )
41 import Class ( classKey, GenClass, SYN_IE(Class) )
42 import ErrUtils ( addErrLoc, SYN_IE(Error) )
43 import Id ( dataConArgTys, isNullaryDataCon, mkDictFunId )
44 import PrelInfo ( needsDataDeclCtxtClassKeys )
45 import Maybes ( maybeToBool )
46 import Name ( isLocallyDefined, getSrcLoc, ExportFlag(..), Provenance,
47 Name{--O only-}, SYN_IE(Module), NamedThing(..)
49 import Outputable ( PprStyle(..), Outputable(..){-instances e.g., (,)-} )
50 import PprType ( GenType, GenTyVar, GenClass, TyCon )
51 import Pretty ( ($$), vcat, hsep, hcat, parens, empty, (<+>),
52 ptext, char, hang, Doc )
53 import SrcLoc ( mkGeneratedSrcLoc, SrcLoc )
54 import TyCon ( tyConTyVars, tyConDataCons, tyConDerivings,
55 tyConTheta, maybeTyConSingleCon, isDataTyCon,
56 isEnumerationTyCon, isAlgTyCon, TyCon
58 import Type ( GenType(..), SYN_IE(TauType), mkTyVarTys, applyTyCon,
59 mkSigmaTy, mkDictTy, isPrimType, instantiateTy,
60 getAppDataTyCon, getAppTyCon
62 import TysPrim ( voidTy )
63 import TyVar ( GenTyVar, SYN_IE(TyVar) )
64 import UniqFM ( emptyUFM )
65 import Unique -- Keys stuff
66 import Bag ( bagToList )
67 import Util ( zipWithEqual, zipEqual, sortLt, removeDups, assoc,
68 thenCmp, cmpList, panic, panic#, pprPanic, pprPanic#,
69 Ord3(..), assertPanic-- , pprTrace{-ToDo:rm-}
74 %************************************************************************
76 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
78 %************************************************************************
82 data T a b = C1 (Foo a) (Bar b)
87 [NOTE: See end of these comments for what to do with
88 data (C a, D b) => T a b = ...
91 We want to come up with an instance declaration of the form
93 instance (Ping a, Pong b, ...) => Eq (T a b) where
96 It is pretty easy, albeit tedious, to fill in the code "...". The
97 trick is to figure out what the context for the instance decl is,
98 namely @Ping@, @Pong@ and friends.
100 Let's call the context reqd for the T instance of class C at types
101 (a,b, ...) C (T a b). Thus:
103 Eq (T a b) = (Ping a, Pong b, ...)
105 Now we can get a (recursive) equation from the @data@ decl:
107 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
108 u Eq (T b a) u Eq Int -- From C2
109 u Eq (T a a) -- From C3
111 Foo and Bar may have explicit instances for @Eq@, in which case we can
112 just substitute for them. Alternatively, either or both may have
113 their @Eq@ instances given by @deriving@ clauses, in which case they
114 form part of the system of equations.
116 Now all we need do is simplify and solve the equations, iterating to
117 find the least fixpoint. Notice that the order of the arguments can
118 switch around, as here in the recursive calls to T.
120 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
124 Eq (T a b) = {} -- The empty set
127 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
128 u Eq (T b a) u Eq Int -- From C2
129 u Eq (T a a) -- From C3
131 After simplification:
132 = Eq a u Ping b u {} u {} u {}
137 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
138 u Eq (T b a) u Eq Int -- From C2
139 u Eq (T a a) -- From C3
141 After simplification:
146 = Eq a u Ping b u Eq b u Ping a
148 The next iteration gives the same result, so this is the fixpoint. We
149 need to make a canonical form of the RHS to ensure convergence. We do
150 this by simplifying the RHS to a form in which
152 - the classes constrain only tyvars
153 - the list is sorted by tyvar (major key) and then class (minor key)
154 - no duplicates, of course
156 So, here are the synonyms for the ``equation'' structures:
159 type DerivEqn = (Class, TyCon, [TyVar], DerivRhs)
160 -- The tyvars bind all the variables in the RHS
161 -- NEW: it's convenient to re-use InstInfo
162 -- We'll "panic" out some fields...
164 type DerivRhs = [(Class, TauType)] -- Same as a ThetaType!
166 type DerivSoln = DerivRhs
170 A note about contexts on data decls
171 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
174 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
176 We will need an instance decl like:
178 instance (Read a, RealFloat a) => Read (Complex a) where
181 The RealFloat in the context is because the read method for Complex is bound
182 to construct a Complex, and doing that requires that the argument type is
185 But this ain't true for Show, Eq, Ord, etc, since they don't construct
186 a Complex; they only take them apart.
188 Our approach: identify the offending classes, and add the data type
189 context to the instance decl. The "offending classes" are
194 %************************************************************************
196 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
198 %************************************************************************
201 tcDeriving :: Module -- name of module under scrutiny
202 -> RnNameSupply -- for "renaming" bits of generated code
203 -> Bag InstInfo -- What we already know about instances
204 -> TcM s (Bag InstInfo, -- The generated "instance decls".
205 RenamedHsBinds, -- Extra generated bindings
206 PprStyle -> Doc) -- Printable derived instance decls;
207 -- for debugging via -ddump-derivings.
209 tcDeriving modname rn_name_supply inst_decl_infos_in
210 = recoverTc (returnTc (emptyBag, EmptyBinds, \_ -> empty)) $
212 -- Fish the "deriving"-related information out of the TcEnv
213 -- and make the necessary "equations".
214 makeDerivEqns `thenTc` \ eqns ->
216 -- Take the equation list and solve it, to deliver a list of
217 -- solutions, a.k.a. the contexts for the instance decls
218 -- required for the corresponding equations.
219 solveDerivEqns inst_decl_infos_in eqns `thenTc` \ new_inst_infos ->
221 -- Now augment the InstInfos, adding in the rather boring
222 -- actual-code-to-do-the-methods binds. We may also need to
223 -- generate extra not-one-inst-decl-specific binds, notably
224 -- "con2tag" and/or "tag2con" functions. We do these
227 gen_taggery_Names new_inst_infos `thenTc` \ nm_alist_etc ->
231 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
232 extra_mbinds = foldr AndMonoBinds EmptyMonoBinds extra_mbind_list
233 method_binds_s = map gen_bind new_inst_infos
234 mbinders = bagToList (collectMonoBinders extra_mbinds)
236 -- Rename to get RenamedBinds.
237 -- The only tricky bit is that the extra_binds must scope over the
238 -- method bindings for the instances.
239 (dfun_names_w_method_binds, rn_extra_binds)
240 = renameSourceCode modname rn_name_supply (
241 bindLocatedLocalsRn (\_ -> ptext (SLIT("deriving"))) mbinders $ \ _ ->
242 rnTopMonoBinds extra_mbinds [] `thenRn` \ rn_extra_binds ->
243 mapRn rn_one method_binds_s `thenRn` \ dfun_names_w_method_binds ->
244 returnRn (dfun_names_w_method_binds, rn_extra_binds)
246 rn_one meth_binds = newDfunName Nothing mkGeneratedSrcLoc `thenRn` \ dfun_name ->
247 rnMethodBinds meth_binds `thenRn` \ rn_meth_binds ->
248 returnRn (dfun_name, rn_meth_binds)
250 really_new_inst_infos = map (gen_inst_info modname)
251 (new_inst_infos `zip` dfun_names_w_method_binds)
253 ddump_deriv = ddump_deriving really_new_inst_infos rn_extra_binds
255 --pprTrace "derived:\n" (ddump_deriv PprDebug) $
257 returnTc (listToBag really_new_inst_infos,
261 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> (PprStyle -> Doc)
263 ddump_deriving inst_infos extra_binds sty
264 = vcat ((map pp_info inst_infos) ++ [ppr sty extra_binds])
266 pp_info (InstInfo clas tvs ty inst_decl_theta _ _ mbinds _ _)
267 = ($$) (ppr sty (mkSigmaTy tvs inst_decl_theta (mkDictTy clas ty)))
272 %************************************************************************
274 \subsection[TcDeriv-eqns]{Forming the equations}
276 %************************************************************************
278 @makeDerivEqns@ fishes around to find the info about needed derived
279 instances. Complicating factors:
282 We can only derive @Enum@ if the data type is an enumeration
283 type (all nullary data constructors).
286 We can only derive @Ix@ if the data type is an enumeration {\em
287 or} has just one data constructor (e.g., tuples).
290 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
294 makeDerivEqns :: TcM s [DerivEqn]
297 = tcGetEnv `thenNF_Tc` \ env ->
299 local_data_tycons = filter (\tc -> isLocallyDefined tc && isAlgTyCon tc)
302 if null local_data_tycons then
303 -- Bale out now; evalClass may not be loaded if there aren't any
306 tcLookupClassByKey evalClassKey `thenNF_Tc` \ eval_clas ->
308 think_about_deriving = need_deriving eval_clas local_data_tycons
309 (derive_these, _) = removeDups cmp_deriv think_about_deriving
310 eqns = map mk_eqn derive_these
312 mapTc chk_out think_about_deriving `thenTc_`
315 ------------------------------------------------------------------
316 need_deriving :: Class -> [TyCon] -> [(Class, TyCon)]
317 -- find the tycons that have `deriving' clauses;
318 -- we handle the "every datatype in Eval" by
319 -- doing a dummy "deriving" for it.
321 need_deriving eval_clas tycons_to_consider
322 = foldr ( \ tycon acc ->
324 acc_plus = if isLocallyDefined tycon
325 then (eval_clas, tycon) : acc
328 case (tyConDerivings tycon) of
330 cs -> [ (clas,tycon) | clas <- cs ] ++ acc_plus
335 ------------------------------------------------------------------
336 chk_out :: (Class, TyCon) -> TcM s ()
337 chk_out this_one@(clas, tycon)
339 clas_key = classKey clas
341 is_enumeration = isEnumerationTyCon tycon
342 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
344 single_nullary_why = SLIT("one constructor data type or type with all nullary constructors expected")
345 nullary_why = SLIT("data type with all nullary constructors expected")
347 chk_clas clas_uniq clas_str clas_why cond
348 = if (clas_uniq == clas_key)
349 then checkTc cond (derivingThingErr clas_str clas_why tycon)
352 -- Are things OK for deriving Enum (if appropriate)?
353 chk_clas enumClassKey (SLIT("Enum")) nullary_why is_enumeration `thenTc_`
355 -- Are things OK for deriving Bounded (if appropriate)?
356 chk_clas boundedClassKey (SLIT("Bounded")) single_nullary_why
357 (is_enumeration || is_single_con) `thenTc_`
359 -- Are things OK for deriving Ix (if appropriate)?
360 chk_clas ixClassKey (SLIT("Ix.Ix")) single_nullary_why
361 (is_enumeration || is_single_con)
363 ------------------------------------------------------------------
364 cmp_deriv :: (Class, TyCon) -> (Class, TyCon) -> TAG_
365 cmp_deriv (c1, t1) (c2, t2)
366 = (c1 `cmp` c2) `thenCmp` (t1 `cmp` t2)
368 ------------------------------------------------------------------
369 mk_eqn :: (Class, TyCon) -> DerivEqn
370 -- we swizzle the tyvars and datacons out of the tycon
371 -- to make the rest of the equation
374 = (clas, tycon, tyvars, if_not_Eval constraints)
376 clas_key = classKey clas
377 tyvars = tyConTyVars tycon -- ToDo: Do we need new tyvars ???
378 tyvar_tys = mkTyVarTys tyvars
379 data_cons = tyConDataCons tycon
381 if_not_Eval cs = if clas_key == evalClassKey then [] else cs
383 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
385 -- "extra_constraints": see notes above about contexts on data decls
387 | offensive_class = tyConTheta tycon
390 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
392 mk_constraints data_con
394 | arg_ty <- instd_arg_tys,
395 not (isPrimType arg_ty) -- No constraints for primitive types
398 instd_arg_tys = dataConArgTys data_con tyvar_tys
401 %************************************************************************
403 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
405 %************************************************************************
407 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
408 terms, which is the final correct RHS for the corresponding original
412 Each (k,TyVarTy tv) in a solution constrains only a type
416 The (k,TyVarTy tv) pairs in a solution are canonically
417 ordered by sorting on type varible, tv, (major key) and then class, k,
422 solveDerivEqns :: Bag InstInfo
424 -> TcM s [InstInfo] -- Solns in same order as eqns.
425 -- This bunch is Absolutely minimal...
427 solveDerivEqns inst_decl_infos_in orig_eqns
428 = iterateDeriv initial_solutions
430 -- The initial solutions for the equations claim that each
431 -- instance has an empty context; this solution is certainly
432 -- in canonical form.
433 initial_solutions :: [DerivSoln]
434 initial_solutions = [ [] | _ <- orig_eqns ]
436 ------------------------------------------------------------------
437 -- iterateDeriv calculates the next batch of solutions,
438 -- compares it with the current one; finishes if they are the
439 -- same, otherwise recurses with the new solutions.
440 -- It fails if any iteration fails
441 iterateDeriv :: [DerivSoln] ->TcM s [InstInfo]
442 iterateDeriv current_solns
443 = checkNoErrsTc (iterateOnce current_solns) `thenTc` \ (new_inst_infos, new_solns) ->
444 if (current_solns `eq_solns` new_solns) then
445 returnTc new_inst_infos
447 iterateDeriv new_solns
449 ------------------------------------------------------------------
450 iterateOnce current_solns
451 = -- Extend the inst info from the explicit instance decls
452 -- with the current set of solutions, giving a
454 add_solns inst_decl_infos_in orig_eqns current_solns
455 `thenTc` \ (new_inst_infos, inst_mapper) ->
457 class_to_inst_env cls = inst_mapper cls
461 listTc [ tcAddErrCtxt (derivCtxt tc) $
462 tcSimplifyThetas class_to_inst_env [{-Nothing "given"-}] deriv_rhs
463 | (_,tc,_,deriv_rhs) <- orig_eqns ] `thenTc` \ next_solns ->
465 -- Canonicalise the solutions, so they compare nicely
466 let canonicalised_next_solns
467 = [ sortLt lt_rhs next_soln | next_soln <- next_solns ]
469 returnTc (new_inst_infos, canonicalised_next_solns)
471 ------------------------------------------------------------------
472 lt_rhs r1 r2 = case cmp_rhs r1 r2 of { LT_ -> True; _ -> False }
473 eq_solns s1 s2 = case cmp_solns s1 s2 of { EQ_ -> True; _ -> False }
474 cmp_solns s1 s2 = cmpList (cmpList cmp_rhs) s1 s2
475 cmp_rhs (c1, TyVarTy tv1) (c2, TyVarTy tv2)
476 = (tv1 `cmp` tv2) `thenCmp` (c1 `cmp` c2)
478 cmp_rhs other_1 other_2
479 = panic# "tcDeriv:cmp_rhs:" --(hsep [ppr PprDebug other_1, ppr PprDebug other_2])
485 add_solns :: Bag InstInfo -- The global, non-derived ones
486 -> [DerivEqn] -> [DerivSoln]
487 -> TcM s ([InstInfo], -- The new, derived ones
489 -- the eqns and solns move "in lockstep"; we have the eqns
490 -- because we need the LHS info for addClassInstance.
492 add_solns inst_infos_in eqns solns
494 -- ------------------
495 -- OLD: checkErrsTc above now deals with this
496 -- = discardErrsTc (buildInstanceEnvs all_inst_infos `thenTc` \ inst_mapper ->
497 -- We do the discard-errs so that we don't get repeated error messages
498 -- about duplicate instances.
499 -- They'll appear later, when we do the top-level buildInstanceEnvs.
500 -- ------------------
502 = buildInstanceEnvs all_inst_infos `thenTc` \ inst_mapper ->
503 returnTc (new_inst_infos, inst_mapper)
505 new_inst_infos = zipWithEqual "add_solns" mk_deriv_inst_info eqns solns
507 all_inst_infos = inst_infos_in `unionBags` listToBag new_inst_infos
509 mk_deriv_inst_info (clas, tycon, tyvars, _) theta
510 = InstInfo clas tyvars (applyTyCon tycon (mkTyVarTys tyvars))
512 (my_panic "dfun_theta")
516 (my_panic "binds") (getSrcLoc tycon)
517 (my_panic "upragmas")
520 = mkDictFunId (getName tycon) dummy_dfun_ty bottom bottom
521 -- The name is getSrcLoc'd in an error message
523 bottom = panic "dummy_dfun_id"
525 dummy_dfun_ty = mkSigmaTy tyvars theta voidTy
526 -- All we need from the dfun is its "theta" part, used during
527 -- equation simplification (tcSimplifyThetas). The final
528 -- dfun_id will have the superclass dictionaries as arguments too,
529 -- but that'll be added after the equations are solved. For now,
530 -- it's enough just to make a dummy dfun with the simple theta part.
532 -- The part after the theta is dummied here as voidTy; actually it's
533 -- (C (T a b)), but it doesn't seem worth constructing it.
534 -- We can't leave it as a panic because to get the theta part we
535 -- have to run down the type!
537 my_panic str = panic "add_soln" -- pprPanic ("add_soln:"++str) (hsep [char ':', ppr PprDebug clas, ppr PprDebug tycon])
540 %************************************************************************
542 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
544 %************************************************************************
546 After all the trouble to figure out the required context for the
547 derived instance declarations, all that's left is to chug along to
548 produce them. They will then be shoved into @tcInstDecls2@, which
549 will do all its usual business.
551 There are lots of possibilities for code to generate. Here are
552 various general remarks.
557 We want derived instances of @Eq@ and @Ord@ (both v common) to be
558 ``you-couldn't-do-better-by-hand'' efficient.
561 Deriving @Show@---also pretty common--- should also be reasonable good code.
564 Deriving for the other classes isn't that common or that big a deal.
571 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
574 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
577 We {\em normally} generate code only for the non-defaulted methods;
578 there are some exceptions for @Eq@ and (especially) @Ord@...
581 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
582 constructor's numeric (@Int#@) tag. These are generated by
583 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
584 these is around is given by @hasCon2TagFun@.
586 The examples under the different sections below will make this
590 Much less often (really just for deriving @Ix@), we use a
591 @_tag2con_<tycon>@ function. See the examples.
594 We use the renamer!!! Reason: we're supposed to be
595 producing @RenamedMonoBinds@ for the methods, but that means
596 producing correctly-uniquified code on the fly. This is entirely
597 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
598 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
599 the renamer. What a great hack!
603 -- Generate the method bindings for the required instance
604 gen_bind :: InstInfo -> RdrNameMonoBinds
605 gen_bind (InstInfo clas _ ty _ _ _ _ _ _)
609 = assoc "gen_inst_info:bad derived class"
610 [(eqClassKey, gen_Eq_binds)
611 ,(ordClassKey, gen_Ord_binds)
612 ,(enumClassKey, gen_Enum_binds)
613 ,(evalClassKey, gen_Eval_binds)
614 ,(boundedClassKey, gen_Bounded_binds)
615 ,(showClassKey, gen_Show_binds)
616 ,(readClassKey, gen_Read_binds)
617 ,(ixClassKey, gen_Ix_binds)
622 from_here = isLocallyDefined tycon
623 (tycon,_,_) = getAppDataTyCon ty
626 gen_inst_info :: Module -- Module name
627 -> (InstInfo, (Name, RenamedMonoBinds)) -- the main stuff to work on
628 -> InstInfo -- the gen'd (filled-in) "instance decl"
630 gen_inst_info modname
631 (InstInfo clas tyvars ty inst_decl_theta _ _ _ locn _, (dfun_name, meth_binds))
633 -- Generate the various instance-related Ids
634 InstInfo clas tyvars ty inst_decl_theta
639 (dfun_id, dfun_theta) = mkInstanceRelatedIds
644 from_here = isLocallyDefined tycon
645 (tycon,_,_) = getAppDataTyCon ty
649 %************************************************************************
651 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
653 %************************************************************************
658 con2tag_Foo :: Foo ... -> Int#
659 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
660 maxtag_Foo :: Int -- ditto (NB: not unboxed)
663 We have a @con2tag@ function for a tycon if:
666 We're deriving @Eq@ and the tycon has nullary data constructors.
669 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
673 We have a @tag2con@ function for a tycon if:
676 We're deriving @Enum@, or @Ix@ (enum type only???)
679 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
682 gen_taggery_Names :: [InstInfo]
683 -> TcM s [(RdrName, -- for an assoc list
684 TyCon, -- related tycon
687 gen_taggery_Names inst_infos
688 = --pprTrace "gen_taggery:\n" (vcat [hsep [ppr PprDebug c, ppr PprDebug t] | (c,t) <- all_CTs]) $
689 foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
690 foldlTc do_tag2con names_so_far tycons_of_interest
692 all_CTs = [ mk_CT c ty | (InstInfo c _ ty _ _ _ _ _ _) <- inst_infos ]
694 mk_CT c ty = (c, fst (getAppTyCon ty))
696 all_tycons = map snd all_CTs
697 (tycons_of_interest, _) = removeDups cmp all_tycons
699 do_con2tag acc_Names tycon
700 | isDataTyCon tycon &&
701 (we_are_deriving eqClassKey tycon
702 && any isNullaryDataCon (tyConDataCons tycon))
703 || (we_are_deriving ordClassKey tycon
704 && not (maybeToBool (maybeTyConSingleCon tycon)))
705 || (we_are_deriving enumClassKey tycon)
706 || (we_are_deriving ixClassKey tycon)
708 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
713 do_tag2con acc_Names tycon
714 = if (we_are_deriving enumClassKey tycon)
715 || (we_are_deriving ixClassKey tycon)
717 returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
718 : (maxtag_RDR tycon, tycon, GenMaxTag)
723 we_are_deriving clas_key tycon
724 = is_in_eqns clas_key tycon all_CTs
726 is_in_eqns clas_key tycon [] = False
727 is_in_eqns clas_key tycon ((c,t):cts)
728 = (clas_key == classKey c && tycon == t)
729 || is_in_eqns clas_key tycon cts
734 derivingThingErr :: FAST_STRING -> FAST_STRING -> TyCon -> Error
736 derivingThingErr thing why tycon sty
737 = hang (hsep [ptext SLIT("Can't make a derived instance of"), ptext thing])
738 0 (hang (hsep [ptext SLIT("for the type"), ppr sty tycon])
739 0 (parens (ptext why)))
742 = ptext SLIT("When deriving classes for") <+> ppr sty tycon