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, 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)
248 really_new_inst_infos = map (gen_inst_info modname)
249 (new_inst_infos `zip` dfun_names_w_method_binds)
251 ddump_deriv = ddump_deriving really_new_inst_infos rn_extra_binds
253 --pprTrace "derived:\n" (ddump_deriv PprDebug) $
255 returnTc (listToBag really_new_inst_infos,
259 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> (PprStyle -> Doc)
261 ddump_deriving inst_infos extra_binds sty
262 = vcat ((map pp_info inst_infos) ++ [ppr sty extra_binds])
264 pp_info (InstInfo clas tvs ty inst_decl_theta _ _ mbinds _ _)
265 = ($$) (ppr sty (mkSigmaTy tvs inst_decl_theta (mkDictTy clas ty)))
270 %************************************************************************
272 \subsection[TcDeriv-eqns]{Forming the equations}
274 %************************************************************************
276 @makeDerivEqns@ fishes around to find the info about needed derived
277 instances. Complicating factors:
280 We can only derive @Enum@ if the data type is an enumeration
281 type (all nullary data constructors).
284 We can only derive @Ix@ if the data type is an enumeration {\em
285 or} has just one data constructor (e.g., tuples).
288 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
292 makeDerivEqns :: TcM s [DerivEqn]
295 = tcGetEnv `thenNF_Tc` \ env ->
297 local_data_tycons = filter (\tc -> isLocallyDefined tc && isAlgTyCon tc)
300 if null local_data_tycons then
301 -- Bale out now; evalClass may not be loaded if there aren't any
304 tcLookupClassByKey evalClassKey `thenNF_Tc` \ eval_clas ->
306 think_about_deriving = need_deriving eval_clas local_data_tycons
307 (derive_these, _) = removeDups cmp_deriv think_about_deriving
308 eqns = map mk_eqn derive_these
310 mapTc chk_out think_about_deriving `thenTc_`
313 ------------------------------------------------------------------
314 need_deriving :: Class -> [TyCon] -> [(Class, TyCon)]
315 -- find the tycons that have `deriving' clauses;
316 -- we handle the "every datatype in Eval" by
317 -- doing a dummy "deriving" for it.
319 need_deriving eval_clas tycons_to_consider
320 = foldr ( \ tycon acc ->
322 acc_plus = if isLocallyDefined tycon
323 then (eval_clas, tycon) : acc
326 case (tyConDerivings tycon) of
328 cs -> [ (clas,tycon) | clas <- cs ] ++ acc_plus
333 ------------------------------------------------------------------
334 chk_out :: (Class, TyCon) -> TcM s ()
335 chk_out this_one@(clas, tycon)
337 clas_key = classKey clas
339 is_enumeration = isEnumerationTyCon tycon
340 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
342 chk_clas clas_uniq clas_str cond
343 = if (clas_uniq == clas_key)
344 then checkTc cond (derivingThingErr clas_str tycon)
347 -- Are things OK for deriving Enum (if appropriate)?
348 chk_clas enumClassKey "Enum" is_enumeration `thenTc_`
350 -- Are things OK for deriving Bounded (if appropriate)?
351 chk_clas boundedClassKey "Bounded"
352 (is_enumeration || is_single_con) `thenTc_`
354 -- Are things OK for deriving Ix (if appropriate)?
355 chk_clas ixClassKey "Ix.Ix" (is_enumeration || is_single_con)
357 ------------------------------------------------------------------
358 cmp_deriv :: (Class, TyCon) -> (Class, TyCon) -> TAG_
359 cmp_deriv (c1, t1) (c2, t2)
360 = (c1 `cmp` c2) `thenCmp` (t1 `cmp` t2)
362 ------------------------------------------------------------------
363 mk_eqn :: (Class, TyCon) -> DerivEqn
364 -- we swizzle the tyvars and datacons out of the tycon
365 -- to make the rest of the equation
368 = (clas, tycon, tyvars, if_not_Eval constraints)
370 clas_key = classKey clas
371 tyvars = tyConTyVars tycon -- ToDo: Do we need new tyvars ???
372 tyvar_tys = mkTyVarTys tyvars
373 data_cons = tyConDataCons tycon
375 if_not_Eval cs = if clas_key == evalClassKey then [] else cs
377 constraints = extra_constraints ++ concat (map mk_constraints data_cons)
379 -- "extra_constraints": see notes above about contexts on data decls
381 | offensive_class = tyConTheta tycon
384 offensive_class = clas_key `elem` needsDataDeclCtxtClassKeys
386 mk_constraints data_con
388 | arg_ty <- instd_arg_tys,
389 not (isPrimType arg_ty) -- No constraints for primitive types
392 instd_arg_tys = dataConArgTys data_con tyvar_tys
395 %************************************************************************
397 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
399 %************************************************************************
401 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
402 terms, which is the final correct RHS for the corresponding original
406 Each (k,TyVarTy tv) in a solution constrains only a type
410 The (k,TyVarTy tv) pairs in a solution are canonically
411 ordered by sorting on type varible, tv, (major key) and then class, k,
416 solveDerivEqns :: Bag InstInfo
418 -> TcM s [InstInfo] -- Solns in same order as eqns.
419 -- This bunch is Absolutely minimal...
421 solveDerivEqns inst_decl_infos_in orig_eqns
422 = iterateDeriv initial_solutions
424 -- The initial solutions for the equations claim that each
425 -- instance has an empty context; this solution is certainly
426 -- in canonical form.
427 initial_solutions :: [DerivSoln]
428 initial_solutions = [ [] | _ <- orig_eqns ]
430 -- iterateDeriv calculates the next batch of solutions,
431 -- compares it with the current one; finishes if they are the
432 -- same, otherwise recurses with the new solutions.
434 iterateDeriv :: [DerivSoln] ->TcM s [InstInfo]
436 iterateDeriv current_solns
437 = -- Extend the inst info from the explicit instance decls
438 -- with the current set of solutions, giving a
440 add_solns inst_decl_infos_in orig_eqns current_solns
441 `thenTc` \ (new_inst_infos, inst_mapper) ->
443 class_to_inst_env cls = inst_mapper cls
447 listTc [ tcSimplifyThetas class_to_inst_env [{-Nothing "given"-}] deriv_rhs
448 | (_,_,_,deriv_rhs) <- orig_eqns ] `thenTc` \ next_solns ->
450 -- Canonicalise the solutions, so they compare nicely
451 let canonicalised_next_solns
452 = [ sortLt lt_rhs next_soln | next_soln <- next_solns ] in
454 if (current_solns `eq_solns` canonicalised_next_solns) then
455 returnTc new_inst_infos
457 iterateDeriv canonicalised_next_solns
460 ------------------------------------------------------------------
461 lt_rhs r1 r2 = case cmp_rhs r1 r2 of { LT_ -> True; _ -> False }
462 eq_solns s1 s2 = case cmp_solns s1 s2 of { EQ_ -> True; _ -> False }
463 cmp_solns s1 s2 = cmpList (cmpList cmp_rhs) s1 s2
464 cmp_rhs (c1, TyVarTy tv1) (c2, TyVarTy tv2)
465 = (tv1 `cmp` tv2) `thenCmp` (c1 `cmp` c2)
467 cmp_rhs other_1 other_2
468 = panic# "tcDeriv:cmp_rhs:" --(hsep [ppr PprDebug other_1, ppr PprDebug other_2])
474 add_solns :: Bag InstInfo -- The global, non-derived ones
475 -> [DerivEqn] -> [DerivSoln]
476 -> TcM s ([InstInfo], -- The new, derived ones
478 -- the eqns and solns move "in lockstep"; we have the eqns
479 -- because we need the LHS info for addClassInstance.
481 add_solns inst_infos_in eqns solns
482 = discardErrsTc (buildInstanceEnvs all_inst_infos) `thenTc` \ inst_mapper ->
483 -- We do the discard-errs so that we don't get repeated error messages
484 -- about missing or duplicate instances.
485 returnTc (new_inst_infos, inst_mapper)
487 new_inst_infos = zipWithEqual "add_solns" mk_deriv_inst_info eqns solns
489 all_inst_infos = inst_infos_in `unionBags` listToBag new_inst_infos
491 mk_deriv_inst_info (clas, tycon, tyvars, _) theta
492 = InstInfo clas tyvars (applyTyCon tycon (mkTyVarTys tyvars))
494 (my_panic "dfun_theta")
498 (my_panic "binds") (getSrcLoc tycon)
499 (my_panic "upragmas")
502 = mkDictFunId bottom dummy_dfun_ty bottom bottom
504 bottom = panic "dummy_dfun_id"
506 dummy_dfun_ty = mkSigmaTy tyvars theta voidTy
507 -- All we need from the dfun is its "theta" part, used during
508 -- equation simplification (tcSimplifyThetas). The final
509 -- dfun_id will have the superclass dictionaries as arguments too,
510 -- but that'll be added after the equations are solved. For now,
511 -- it's enough just to make a dummy dfun with the simple theta part.
513 -- The part after the theta is dummied here as voidTy; actually it's
514 -- (C (T a b)), but it doesn't seem worth constructing it.
515 -- We can't leave it as a panic because to get the theta part we
516 -- have to run down the type!
518 my_panic str = panic "add_soln" -- pprPanic ("add_soln:"++str) (hsep [char ':', ppr PprDebug clas, ppr PprDebug tycon])
521 %************************************************************************
523 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
525 %************************************************************************
527 After all the trouble to figure out the required context for the
528 derived instance declarations, all that's left is to chug along to
529 produce them. They will then be shoved into @tcInstDecls2@, which
530 will do all its usual business.
532 There are lots of possibilities for code to generate. Here are
533 various general remarks.
538 We want derived instances of @Eq@ and @Ord@ (both v common) to be
539 ``you-couldn't-do-better-by-hand'' efficient.
542 Deriving @Show@---also pretty common--- should also be reasonable good code.
545 Deriving for the other classes isn't that common or that big a deal.
552 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
555 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
558 We {\em normally} generate code only for the non-defaulted methods;
559 there are some exceptions for @Eq@ and (especially) @Ord@...
562 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
563 constructor's numeric (@Int#@) tag. These are generated by
564 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
565 these is around is given by @hasCon2TagFun@.
567 The examples under the different sections below will make this
571 Much less often (really just for deriving @Ix@), we use a
572 @_tag2con_<tycon>@ function. See the examples.
575 We use the renamer!!! Reason: we're supposed to be
576 producing @RenamedMonoBinds@ for the methods, but that means
577 producing correctly-uniquified code on the fly. This is entirely
578 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
579 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
580 the renamer. What a great hack!
584 -- Generate the method bindings for the required instance
585 gen_bind :: InstInfo -> RdrNameMonoBinds
586 gen_bind (InstInfo clas _ ty _ _ _ _ _ _)
590 = assoc "gen_inst_info:bad derived class"
591 [(eqClassKey, gen_Eq_binds)
592 ,(ordClassKey, gen_Ord_binds)
593 ,(enumClassKey, gen_Enum_binds)
594 ,(evalClassKey, gen_Eval_binds)
595 ,(boundedClassKey, gen_Bounded_binds)
596 ,(showClassKey, gen_Show_binds)
597 ,(readClassKey, gen_Read_binds)
598 ,(ixClassKey, gen_Ix_binds)
603 from_here = isLocallyDefined tycon
604 (tycon,_,_) = getAppDataTyCon ty
607 gen_inst_info :: Module -- Module name
608 -> (InstInfo, (Name, RenamedMonoBinds)) -- the main stuff to work on
609 -> InstInfo -- the gen'd (filled-in) "instance decl"
611 gen_inst_info modname
612 (InstInfo clas tyvars ty inst_decl_theta _ _ _ locn _, (dfun_name, meth_binds))
614 -- Generate the various instance-related Ids
615 InstInfo clas tyvars ty inst_decl_theta
620 (dfun_id, dfun_theta) = mkInstanceRelatedIds
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])