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) )
24 import TcHsSyn ( TcIdOcc )
27 import Inst ( SYN_IE(InstanceMapper) )
28 import TcEnv ( getEnv_TyCons, tcLookupClassByKey )
29 import SpecEnv ( SpecEnv )
30 import TcKind ( TcKind )
31 import TcGenDeriv -- Deriv stuff
32 import TcInstUtil ( InstInfo(..), mkInstanceRelatedIds, buildInstanceEnvs )
33 import TcSimplify ( tcSimplifyThetas )
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, isEmptyBag, unionBags, listToBag )
41 import Class ( classKey, GenClass, SYN_IE(Class) )
42 import ErrUtils ( pprBagOfErrors, 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)
49 import Outputable ( PprStyle(..), Outputable(..){-instances e.g., (,)-} )
50 import PprType ( GenType, GenTyVar, GenClass, TyCon )
51 import Pretty ( ($$), vcat, hsep, hcat,
52 ptext, text, 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 = -- Fish the "deriving"-related information out of the TcEnv
211 -- and make the necessary "equations".
212 makeDerivEqns `thenTc` \ eqns ->
214 -- Take the equation list and solve it, to deliver a list of
215 -- solutions, a.k.a. the contexts for the instance decls
216 -- required for the corresponding equations.
217 solveDerivEqns inst_decl_infos_in eqns `thenTc` \ new_inst_infos ->
219 -- Now augment the InstInfos, adding in the rather boring
220 -- actual-code-to-do-the-methods binds. We may also need to
221 -- generate extra not-one-inst-decl-specific binds, notably
222 -- "con2tag" and/or "tag2con" functions. We do these
225 gen_taggery_Names new_inst_infos `thenTc` \ nm_alist_etc ->
229 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
230 extra_mbinds = foldr AndMonoBinds EmptyMonoBinds extra_mbind_list
231 method_binds_s = map gen_bind new_inst_infos
232 mbinders = bagToList (collectMonoBinders extra_mbinds)
234 -- Rename to get RenamedBinds.
235 -- The only tricky bit is that the extra_binds must scope over the
236 -- method bindings for the instances.
237 (dfun_names_w_method_binds, rn_extra_binds)
238 = renameSourceCode modname rn_name_supply (
239 bindLocatedLocalsRn (\_ -> text "deriving") mbinders $ \ _ ->
240 rnTopMonoBinds extra_mbinds [] `thenRn` \ rn_extra_binds ->
241 mapRn rn_one method_binds_s `thenRn` \ dfun_names_w_method_binds ->
242 returnRn (dfun_names_w_method_binds, rn_extra_binds)
244 rn_one meth_binds = newDfunName Nothing mkGeneratedSrcLoc `thenRn` \ dfun_name ->
245 rnMethodBinds meth_binds `thenRn` \ rn_meth_binds ->
246 returnRn (dfun_name, rn_meth_binds)
249 mapTc (gen_inst_info modname)
250 (new_inst_infos `zip` dfun_names_w_method_binds) `thenTc` \ really_new_inst_infos ->
252 ddump_deriv = ddump_deriving really_new_inst_infos rn_extra_binds
254 --pprTrace "derived:\n" (ddump_deriv PprDebug) $
256 returnTc (listToBag really_new_inst_infos,
260 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> (PprStyle -> Doc)
262 ddump_deriving inst_infos extra_binds sty
263 = vcat ((map pp_info inst_infos) ++ [ppr sty extra_binds])
265 pp_info (InstInfo clas tvs ty inst_decl_theta _ _ mbinds _ _)
266 = ($$) (ppr sty (mkSigmaTy tvs inst_decl_theta (mkDictTy clas ty)))
271 %************************************************************************
273 \subsection[TcDeriv-eqns]{Forming the equations}
275 %************************************************************************
277 @makeDerivEqns@ fishes around to find the info about needed derived
278 instances. Complicating factors:
281 We can only derive @Enum@ if the data type is an enumeration
282 type (all nullary data constructors).
285 We can only derive @Ix@ if the data type is an enumeration {\em
286 or} has just one data constructor (e.g., tuples).
289 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
293 makeDerivEqns :: TcM s [DerivEqn]
296 = tcGetEnv `thenNF_Tc` \ env ->
298 local_data_tycons = filter (\tc -> isLocallyDefined tc && isAlgTyCon tc)
301 if null local_data_tycons then
302 -- Bale out now; evalClass may not be loaded if there aren't any
305 tcLookupClassByKey evalClassKey `thenNF_Tc` \ eval_clas ->
307 think_about_deriving = need_deriving eval_clas local_data_tycons
308 (derive_these, _) = removeDups cmp_deriv think_about_deriving
309 eqns = map mk_eqn derive_these
311 mapTc chk_out think_about_deriving `thenTc_`
314 ------------------------------------------------------------------
315 need_deriving :: Class -> [TyCon] -> [(Class, TyCon)]
316 -- find the tycons that have `deriving' clauses;
317 -- we handle the "every datatype in Eval" by
318 -- doing a dummy "deriving" for it.
320 need_deriving eval_clas tycons_to_consider
321 = foldr ( \ tycon acc ->
323 acc_plus = if isLocallyDefined tycon
324 then (eval_clas, tycon) : acc
327 case (tyConDerivings tycon) of
329 cs -> [ (clas,tycon) | clas <- cs ] ++ acc_plus
334 ------------------------------------------------------------------
335 chk_out :: (Class, TyCon) -> TcM s ()
336 chk_out this_one@(clas, tycon)
338 clas_key = classKey clas
340 is_enumeration = isEnumerationTyCon tycon
341 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
343 chk_clas clas_uniq clas_str cond
344 = if (clas_uniq == clas_key)
345 then checkTc cond (derivingThingErr clas_str tycon)
348 -- Are things OK for deriving Enum (if appropriate)?
349 chk_clas enumClassKey "Enum" is_enumeration `thenTc_`
351 -- Are things OK for deriving Bounded (if appropriate)?
352 chk_clas boundedClassKey "Bounded"
353 (is_enumeration || is_single_con) `thenTc_`
355 -- Are things OK for deriving Ix (if appropriate)?
356 chk_clas ixClassKey "Ix.Ix" (is_enumeration || is_single_con)
358 ------------------------------------------------------------------
359 cmp_deriv :: (Class, TyCon) -> (Class, TyCon) -> TAG_
360 cmp_deriv (c1, t1) (c2, t2)
361 = (c1 `cmp` c2) `thenCmp` (t1 `cmp` t2)
363 ------------------------------------------------------------------
364 mk_eqn :: (Class, TyCon) -> DerivEqn
365 -- we swizzle the tyvars and datacons out of the tycon
366 -- to make the rest of the equation
369 = (clas, tycon, tyvars, if_not_Eval constraints)
371 clas_key = classKey clas
372 tyvars = tyConTyVars tycon -- ToDo: Do we need new tyvars ???
373 tyvar_tys = mkTyVarTys tyvars
374 data_cons = tyConDataCons tycon
376 if_not_Eval cs = if clas_key == evalClassKey then [] else cs
378 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
380 -- "extra_constraints": see notes above about contexts on data decls
382 | offensive_class = tyConTheta tycon
385 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
387 mk_constraints data_con
389 | arg_ty <- instd_arg_tys,
390 not (isPrimType arg_ty) -- No constraints for primitive types
393 instd_arg_tys = dataConArgTys data_con tyvar_tys
396 %************************************************************************
398 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
400 %************************************************************************
402 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
403 terms, which is the final correct RHS for the corresponding original
407 Each (k,TyVarTy tv) in a solution constrains only a type
411 The (k,TyVarTy tv) pairs in a solution are canonically
412 ordered by sorting on type varible, tv, (major key) and then class, k,
417 solveDerivEqns :: Bag InstInfo
419 -> TcM s [InstInfo] -- Solns in same order as eqns.
420 -- This bunch is Absolutely minimal...
422 solveDerivEqns inst_decl_infos_in orig_eqns
423 = iterateDeriv initial_solutions
425 -- The initial solutions for the equations claim that each
426 -- instance has an empty context; this solution is certainly
427 -- in canonical form.
428 initial_solutions :: [DerivSoln]
429 initial_solutions = [ [] | _ <- orig_eqns ]
431 -- iterateDeriv calculates the next batch of solutions,
432 -- compares it with the current one; finishes if they are the
433 -- same, otherwise recurses with the new solutions.
435 iterateDeriv :: [DerivSoln] ->TcM s [InstInfo]
437 iterateDeriv current_solns
438 = -- Extend the inst info from the explicit instance decls
439 -- with the current set of solutions, giving a
441 add_solns inst_decl_infos_in orig_eqns current_solns
442 `thenTc` \ (new_inst_infos, inst_mapper) ->
444 class_to_inst_env cls = fst (inst_mapper cls)
448 listTc [ tcSimplifyThetas class_to_inst_env [{-Nothing "given"-}] deriv_rhs
449 | (_,_,_,deriv_rhs) <- orig_eqns ] `thenTc` \ next_solns ->
451 -- Canonicalise the solutions, so they compare nicely
452 let canonicalised_next_solns
453 = [ sortLt lt_rhs next_soln | next_soln <- next_solns ] in
455 if (current_solns `eq_solns` canonicalised_next_solns) then
456 returnTc new_inst_infos
458 iterateDeriv canonicalised_next_solns
461 ------------------------------------------------------------------
462 lt_rhs r1 r2 = case cmp_rhs r1 r2 of { LT_ -> True; _ -> False }
463 eq_solns s1 s2 = case cmp_solns s1 s2 of { EQ_ -> True; _ -> False }
464 cmp_solns s1 s2 = cmpList (cmpList cmp_rhs) s1 s2
465 cmp_rhs (c1, TyVarTy tv1) (c2, TyVarTy tv2)
466 = (tv1 `cmp` tv2) `thenCmp` (c1 `cmp` c2)
468 cmp_rhs other_1 other_2
469 = panic# "tcDeriv:cmp_rhs:" --(hsep [ppr PprDebug other_1, ppr PprDebug other_2])
475 add_solns :: Bag InstInfo -- The global, non-derived ones
476 -> [DerivEqn] -> [DerivSoln]
477 -> TcM s ([InstInfo], -- The new, derived ones
479 -- the eqns and solns move "in lockstep"; we have the eqns
480 -- because we need the LHS info for addClassInstance.
482 add_solns inst_infos_in eqns solns
483 = buildInstanceEnvs all_inst_infos `thenTc` \ inst_mapper ->
484 returnTc (new_inst_infos, inst_mapper)
486 new_inst_infos = zipWithEqual "add_solns" mk_deriv_inst_info eqns solns
488 all_inst_infos = inst_infos_in `unionBags` listToBag new_inst_infos
490 mk_deriv_inst_info (clas, tycon, tyvars, _) theta
491 = InstInfo clas tyvars (applyTyCon tycon (mkTyVarTys tyvars))
493 (my_panic "dfun_theta")
497 (my_panic "binds") (getSrcLoc tycon)
498 (my_panic "upragmas")
501 = mkDictFunId bottom dummy_dfun_ty bottom bottom
503 bottom = panic "dummy_dfun_id"
505 dummy_dfun_ty = mkSigmaTy tyvars theta voidTy
506 -- All we need from the dfun is its "theta" part, used during
507 -- equation simplification (tcSimplifyThetas). The final
508 -- dfun_id will have the superclass dictionaries as arguments too,
509 -- but that'll be added after the equations are solved. For now,
510 -- it's enough just to make a dummy dfun with the simple theta part.
512 -- The part after the theta is dummied here as voidTy; actually it's
513 -- (C (T a b)), but it doesn't seem worth constructing it.
514 -- We can't leave it as a panic because to get the theta part we
515 -- have to run down the type!
517 my_panic str = panic "add_soln" -- pprPanic ("add_soln:"++str) (hsep [char ':', ppr PprDebug clas, ppr PprDebug tycon])
520 %************************************************************************
522 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
524 %************************************************************************
526 After all the trouble to figure out the required context for the
527 derived instance declarations, all that's left is to chug along to
528 produce them. They will then be shoved into @tcInstDecls2@, which
529 will do all its usual business.
531 There are lots of possibilities for code to generate. Here are
532 various general remarks.
537 We want derived instances of @Eq@ and @Ord@ (both v common) to be
538 ``you-couldn't-do-better-by-hand'' efficient.
541 Deriving @Show@---also pretty common--- should also be reasonable good code.
544 Deriving for the other classes isn't that common or that big a deal.
551 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
554 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
557 We {\em normally} generate code only for the non-defaulted methods;
558 there are some exceptions for @Eq@ and (especially) @Ord@...
561 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
562 constructor's numeric (@Int#@) tag. These are generated by
563 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
564 these is around is given by @hasCon2TagFun@.
566 The examples under the different sections below will make this
570 Much less often (really just for deriving @Ix@), we use a
571 @_tag2con_<tycon>@ function. See the examples.
574 We use the renamer!!! Reason: we're supposed to be
575 producing @RenamedMonoBinds@ for the methods, but that means
576 producing correctly-uniquified code on the fly. This is entirely
577 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
578 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
579 the renamer. What a great hack!
583 -- Generate the method bindings for the required instance
584 gen_bind :: InstInfo -> RdrNameMonoBinds
585 gen_bind (InstInfo clas _ ty _ _ _ _ _ _)
589 = assoc "gen_inst_info:bad derived class"
590 [(eqClassKey, gen_Eq_binds)
591 ,(ordClassKey, gen_Ord_binds)
592 ,(enumClassKey, gen_Enum_binds)
593 ,(evalClassKey, gen_Eval_binds)
594 ,(boundedClassKey, gen_Bounded_binds)
595 ,(showClassKey, gen_Show_binds)
596 ,(readClassKey, gen_Read_binds)
597 ,(ixClassKey, gen_Ix_binds)
602 from_here = isLocallyDefined tycon
603 (tycon,_,_) = getAppDataTyCon ty
606 gen_inst_info :: Module -- Module name
607 -> (InstInfo, (Name, RenamedMonoBinds)) -- the main stuff to work on
608 -> TcM s InstInfo -- the gen'd (filled-in) "instance decl"
610 gen_inst_info modname
611 (InstInfo clas tyvars ty inst_decl_theta _ _ _ locn _, (dfun_name, meth_binds))
613 -- Generate the various instance-related Ids
618 `thenNF_Tc` \ (dfun_id, dfun_theta) ->
620 returnTc (InstInfo clas tyvars ty inst_decl_theta
625 from_here = isLocallyDefined tycon
626 (tycon,_,_) = getAppDataTyCon ty
630 %************************************************************************
632 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
634 %************************************************************************
639 con2tag_Foo :: Foo ... -> Int#
640 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
641 maxtag_Foo :: Int -- ditto (NB: not unboxed)
644 We have a @con2tag@ function for a tycon if:
647 We're deriving @Eq@ and the tycon has nullary data constructors.
650 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
654 We have a @tag2con@ function for a tycon if:
657 We're deriving @Enum@, or @Ix@ (enum type only???)
660 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
663 gen_taggery_Names :: [InstInfo]
664 -> TcM s [(RdrName, -- for an assoc list
665 TyCon, -- related tycon
668 gen_taggery_Names inst_infos
669 = --pprTrace "gen_taggery:\n" (vcat [hsep [ppr PprDebug c, ppr PprDebug t] | (c,t) <- all_CTs]) $
670 foldlTc do_con2tag [] tycons_of_interest `thenTc` \ names_so_far ->
671 foldlTc do_tag2con names_so_far tycons_of_interest
673 all_CTs = [ mk_CT c ty | (InstInfo c _ ty _ _ _ _ _ _) <- inst_infos ]
675 mk_CT c ty = (c, fst (getAppTyCon ty))
677 all_tycons = map snd all_CTs
678 (tycons_of_interest, _) = removeDups cmp all_tycons
680 do_con2tag acc_Names tycon
681 | isDataTyCon tycon &&
682 (we_are_deriving eqClassKey tycon
683 && any isNullaryDataCon (tyConDataCons tycon))
684 || (we_are_deriving ordClassKey tycon
685 && not (maybeToBool (maybeTyConSingleCon tycon)))
686 || (we_are_deriving enumClassKey tycon)
687 || (we_are_deriving ixClassKey tycon)
689 = returnTc ((con2tag_RDR tycon, tycon, GenCon2Tag)
694 do_tag2con acc_Names tycon
695 = if (we_are_deriving enumClassKey tycon)
696 || (we_are_deriving ixClassKey tycon)
698 returnTc ( (tag2con_RDR tycon, tycon, GenTag2Con)
699 : (maxtag_RDR tycon, tycon, GenMaxTag)
704 we_are_deriving clas_key tycon
705 = is_in_eqns clas_key tycon all_CTs
707 is_in_eqns clas_key tycon [] = False
708 is_in_eqns clas_key tycon ((c,t):cts)
709 = (clas_key == classKey c && tycon == t)
710 || is_in_eqns clas_key tycon cts
715 derivingThingErr :: String -> TyCon -> Error
717 derivingThingErr thing tycon sty
718 = hang (hsep [ptext SLIT("Can't make a derived instance of"), text thing])
719 4 (hsep [ptext SLIT("for the type"), ppr sty tycon])