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(..),
15 import RdrHsSyn ( RdrNameMonoBinds )
16 import RnHsSyn ( RenamedHsBinds, RenamedMonoBinds, RenamedTyClDecl, RenamedHsPred )
17 import CmdLineOpts ( DynFlag(..) )
20 import TcEnv ( tcExtendTempInstEnv, newDFunName,
21 InstInfo(..), pprInstInfo, InstBindings(..),
22 pprInstInfoDetails, tcLookupTyCon, tcExtendTyVarEnv
24 import TcGenDeriv -- Deriv stuff
25 import InstEnv ( InstEnv, simpleDFunClassTyCon )
26 import TcMonoType ( tcHsPred )
27 import TcSimplify ( tcSimplifyDeriv )
29 import RnBinds ( rnMethodBinds, rnTopMonoBinds )
30 import RnEnv ( bindLocalsFVRn )
31 import TcRnMonad ( thenM, returnM, mapAndUnzipM )
32 import HscTypes ( DFunId )
34 import BasicTypes ( NewOrData(..) )
35 import Class ( className, classArity, classKey, classTyVars, classSCTheta, Class )
36 import Subst ( mkTyVarSubst, substTheta )
37 import ErrUtils ( dumpIfSet_dyn )
38 import MkId ( mkDictFunId )
39 import DataCon ( dataConRepArgTys, dataConOrigArgTys, isNullaryDataCon, isExistentialDataCon )
40 import Maybes ( maybeToBool, catMaybes )
41 import Name ( Name, getSrcLoc, nameUnique )
43 import RdrName ( RdrName )
45 import TyCon ( tyConTyVars, tyConDataCons, tyConArity,
46 tyConTheta, maybeTyConSingleCon, isDataTyCon,
47 isEnumerationTyCon, isRecursiveTyCon, TyCon
49 import TcType ( TcType, ThetaType, mkTyVarTys, mkTyConApp, getClassPredTys_maybe,
50 isUnLiftedType, mkClassPred, tyVarsOfTypes, tcSplitFunTys,
51 tcEqTypes, tcSplitAppTys, mkAppTys )
52 import Var ( TyVar, tyVarKind )
53 import VarSet ( mkVarSet, subVarSet )
55 import Util ( zipWithEqual, sortLt, notNull )
56 import ListSetOps ( removeDups, assoc )
60 %************************************************************************
62 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
64 %************************************************************************
68 data T a b = C1 (Foo a) (Bar b)
73 [NOTE: See end of these comments for what to do with
74 data (C a, D b) => T a b = ...
77 We want to come up with an instance declaration of the form
79 instance (Ping a, Pong b, ...) => Eq (T a b) where
82 It is pretty easy, albeit tedious, to fill in the code "...". The
83 trick is to figure out what the context for the instance decl is,
84 namely @Ping@, @Pong@ and friends.
86 Let's call the context reqd for the T instance of class C at types
87 (a,b, ...) C (T a b). Thus:
89 Eq (T a b) = (Ping a, Pong b, ...)
91 Now we can get a (recursive) equation from the @data@ decl:
93 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
94 u Eq (T b a) u Eq Int -- From C2
95 u Eq (T a a) -- From C3
97 Foo and Bar may have explicit instances for @Eq@, in which case we can
98 just substitute for them. Alternatively, either or both may have
99 their @Eq@ instances given by @deriving@ clauses, in which case they
100 form part of the system of equations.
102 Now all we need do is simplify and solve the equations, iterating to
103 find the least fixpoint. Notice that the order of the arguments can
104 switch around, as here in the recursive calls to T.
106 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
110 Eq (T a b) = {} -- The empty set
113 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
114 u Eq (T b a) u Eq Int -- From C2
115 u Eq (T a a) -- From C3
117 After simplification:
118 = Eq a u Ping b u {} u {} u {}
123 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
124 u Eq (T b a) u Eq Int -- From C2
125 u Eq (T a a) -- From C3
127 After simplification:
132 = Eq a u Ping b u Eq b u Ping a
134 The next iteration gives the same result, so this is the fixpoint. We
135 need to make a canonical form of the RHS to ensure convergence. We do
136 this by simplifying the RHS to a form in which
138 - the classes constrain only tyvars
139 - the list is sorted by tyvar (major key) and then class (minor key)
140 - no duplicates, of course
142 So, here are the synonyms for the ``equation'' structures:
145 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
146 -- The Name is the name for the DFun we'll build
147 -- The tyvars bind all the variables in the RHS
149 pprDerivEqn (n,c,tc,tvs,rhs)
150 = parens (hsep [ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
152 type DerivRhs = ThetaType
153 type DerivSoln = DerivRhs
157 [Data decl contexts] 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
180 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
181 pattern matching against a constructor from a data type with a context
182 gives rise to the constraints for that context -- or at least the thinned
183 version. So now all classes are "offending".
187 %************************************************************************
189 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
191 %************************************************************************
194 tcDeriving :: [RenamedTyClDecl] -- All type constructors
195 -> TcM ([InstInfo], -- The generated "instance decls".
196 RenamedHsBinds, -- Extra generated bindings
197 FreeVars) -- These are free in the generated bindings
199 tcDeriving tycl_decls
200 = recoverM (returnM ([], EmptyBinds, emptyFVs)) $
201 getDOpts `thenM` \ dflags ->
203 -- Fish the "deriving"-related information out of the TcEnv
204 -- and make the necessary "equations".
205 makeDerivEqns tycl_decls `thenM` \ (ordinary_eqns, newtype_inst_info) ->
206 tcExtendTempInstEnv (map iDFunId newtype_inst_info) $
207 -- Add the newtype-derived instances to the inst env
208 -- before tacking the "ordinary" ones
210 deriveOrdinaryStuff ordinary_eqns `thenM` \ (ordinary_inst_info, binds, fvs) ->
212 inst_info = newtype_inst_info ++ ordinary_inst_info
215 ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
216 (ddump_deriving inst_info binds)) `thenM_`
218 returnM (inst_info, binds, fvs)
221 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> SDoc
222 ddump_deriving inst_infos extra_binds
223 = vcat (map ppr_info inst_infos) $$ ppr extra_binds
225 ppr_info inst_info = pprInstInfo inst_info $$
226 nest 4 (pprInstInfoDetails inst_info)
227 -- pprInstInfo doesn't print much: only the type
229 -----------------------------------------
230 deriveOrdinaryStuff [] -- Short cut
231 = returnM ([], EmptyBinds, emptyFVs)
233 deriveOrdinaryStuff eqns
234 = -- Take the equation list and solve it, to deliver a list of
235 -- solutions, a.k.a. the contexts for the instance decls
236 -- required for the corresponding equations.
237 solveDerivEqns eqns `thenM` \ new_dfuns ->
239 -- Now augment the InstInfos, adding in the rather boring
240 -- actual-code-to-do-the-methods binds. We may also need to
241 -- generate extra not-one-inst-decl-specific binds, notably
242 -- "con2tag" and/or "tag2con" functions. We do these
244 gen_taggery_Names new_dfuns `thenM` \ nm_alist_etc ->
247 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
248 extra_mbinds = foldr AndMonoBinds EmptyMonoBinds extra_mbind_list
249 mbinders = collectMonoBinders extra_mbinds
251 mappM gen_bind new_dfuns `thenM` \ method_binds_s ->
253 traceTc (text "tcDeriv" <+> ppr method_binds_s) `thenM_`
254 getModule `thenM` \ this_mod ->
255 initRn (InterfaceMode this_mod) (
256 -- Rename to get RenamedBinds.
257 -- The only tricky bit is that the extra_binds must scope
258 -- over the method bindings for the instances.
259 bindLocalsFVRn (ptext (SLIT("deriving"))) mbinders $ \ _ ->
260 rnTopMonoBinds extra_mbinds [] `thenM` \ (rn_extra_binds, fvs) ->
261 mapAndUnzipM rn_meths method_binds_s `thenM` \ (rn_method_binds_s, fvs_s) ->
262 returnM ((rn_method_binds_s, rn_extra_binds),
263 fvs `plusFV` plusFVs fvs_s)
264 ) `thenM` \ ((rn_method_binds_s, rn_extra_binds), fvs) ->
266 new_inst_infos = zipWith gen_inst_info new_dfuns rn_method_binds_s
268 returnM (new_inst_infos, rn_extra_binds, fvs)
271 -- Make a Real dfun instead of the dummy one we have so far
272 gen_inst_info :: DFunId -> RenamedMonoBinds -> InstInfo
273 gen_inst_info dfun binds
274 = InstInfo { iDFunId = dfun, iBinds = VanillaInst binds [] }
276 rn_meths (cls, meths) = rnMethodBinds cls [] meths
280 %************************************************************************
282 \subsection[TcDeriv-eqns]{Forming the equations}
284 %************************************************************************
286 @makeDerivEqns@ fishes around to find the info about needed derived
287 instances. Complicating factors:
290 We can only derive @Enum@ if the data type is an enumeration
291 type (all nullary data constructors).
294 We can only derive @Ix@ if the data type is an enumeration {\em
295 or} has just one data constructor (e.g., tuples).
298 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
302 makeDerivEqns :: [RenamedTyClDecl]
303 -> TcM ([DerivEqn], -- Ordinary derivings
304 [InstInfo]) -- Special newtype derivings
306 makeDerivEqns tycl_decls
307 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
308 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
310 ------------------------------------------------------------------
311 derive_these :: [(NewOrData, Name, RenamedHsPred)]
312 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
313 -- NB: only source-language decls have deriving, no imported ones do
314 derive_these = [ (nd, tycon, pred)
315 | TyData {tcdND = nd, tcdName = tycon, tcdDerivs = Just preds} <- tycl_decls,
318 ------------------------------------------------------------------
319 mk_eqn :: (NewOrData, Name, RenamedHsPred) -> TcM (Maybe DerivEqn, Maybe InstInfo)
320 -- We swizzle the tyvars and datacons out of the tycon
321 -- to make the rest of the equation
323 mk_eqn (new_or_data, tycon_name, pred)
324 = tcLookupTyCon tycon_name `thenM` \ tycon ->
325 addSrcLoc (getSrcLoc tycon) $
326 addErrCtxt (derivCtxt Nothing tycon) $
327 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
328 -- the type variables for the type constructor
329 tcHsPred pred `thenM` \ pred' ->
330 case getClassPredTys_maybe pred' of
331 Nothing -> bale_out (malformedPredErr tycon pred)
332 Just (clas, tys) -> mk_eqn_help new_or_data tycon clas tys
334 ------------------------------------------------------------------
335 mk_eqn_help DataType tycon clas tys
336 | Just err <- chk_out clas tycon tys
337 = bale_out (derivingThingErr clas tys tycon tyvars err)
339 = new_dfun_name clas tycon `thenM` \ dfun_name ->
340 returnM (Just (dfun_name, clas, tycon, tyvars, constraints), Nothing)
342 tyvars = tyConTyVars tycon
343 data_cons = tyConDataCons tycon
344 constraints = extra_constraints ++
345 [ mkClassPred clas [arg_ty]
346 | data_con <- tyConDataCons tycon,
347 arg_ty <- dataConRepArgTys data_con, -- dataConOrigArgTys???
348 -- Use the same type variables
349 -- as the type constructor,
350 -- hence no need to instantiate
351 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
354 -- "extra_constraints": see note [Data decl contexts] above
355 extra_constraints = tyConTheta tycon
357 mk_eqn_help NewType tycon clas tys
358 = doptM Opt_GlasgowExts `thenM` \ gla_exts ->
359 if can_derive_via_isomorphism && (gla_exts || standard_instance) then
360 -- Go ahead and use the isomorphism
361 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
362 new_dfun_name clas tycon `thenM` \ dfun_name ->
363 returnM (Nothing, Just (InstInfo { iDFunId = mk_dfun dfun_name,
364 iBinds = NewTypeDerived rep_tys }))
366 if standard_instance then
367 mk_eqn_help DataType tycon clas [] -- Go via bale-out route
369 -- Non-standard instance
372 bale_out cant_derive_err
374 -- Just complain about being a non-std instance
377 -- Here is the plan for newtype derivings. We see
378 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
379 -- where aj...an do not occur free in t, and the (C s1 ... sm) is a
380 -- *partial applications* of class C with the last parameter missing
382 -- We generate the instances
383 -- instance C s1 .. sm (t ak...aj) => C s1 .. sm (T a1...aj)
384 -- where T a1...aj is the partial application of the LHS of the correct kind
386 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
387 -- instance Monad (ST s) => Monad (T s) where
388 -- fail = coerce ... (fail @ ST s)
390 clas_tyvars = classTyVars clas
391 kind = tyVarKind (last clas_tyvars)
392 -- Kind of the thing we want to instance
393 -- e.g. argument kind of Monad, *->*
395 (arg_kinds, _) = tcSplitFunTys kind
396 n_args_to_drop = length arg_kinds
397 -- Want to drop 1 arg from (T s a) and (ST s a)
398 -- to get instance Monad (ST s) => Monad (T s)
400 -- Note [newtype representation]
401 -- We must not use newTyConRep to get the representation
402 -- type, because that looks through all intermediate newtypes
403 -- To get the RHS of *this* newtype, just look at the data
404 -- constructor. For example
405 -- newtype B = MkB Int
406 -- newtype A = MkA B deriving( Num )
407 -- We want the Num instance of B, *not* the Num instance of Int,
408 -- when making the Num instance of A!
409 tyvars = tyConTyVars tycon
410 rep_ty = head (dataConOrigArgTys (head (tyConDataCons tycon)))
411 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
413 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
414 tyvars_to_drop = drop n_tyvars_to_keep tyvars
415 tyvars_to_keep = take n_tyvars_to_keep tyvars
417 n_args_to_keep = length rep_ty_args - n_args_to_drop
418 args_to_drop = drop n_args_to_keep rep_ty_args
419 args_to_keep = take n_args_to_keep rep_ty_args
421 rep_tys = tys ++ [mkAppTys rep_fn args_to_keep]
422 rep_pred = mkClassPred clas rep_tys
423 -- rep_pred is the representation dictionary, from where
424 -- we are gong to get all the methods for the newtype dictionary
426 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
427 -- The 'tys' here come from the partial application
428 -- in the deriving clause. The last arg is the new
431 -- We must pass the superclasses; the newtype might be an instance
432 -- of them in a different way than the representation type
433 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
434 -- Then the Show instance is not done via isomprphism; it shows
436 -- The Num instance is derived via isomorphism, but the Show superclass
437 -- dictionary must the Show instance for Foo, *not* the Show dictionary
438 -- gotten from the Num dictionary. So we must build a whole new dictionary
439 -- not just use the Num one. The instance we want is something like:
440 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
443 -- There's no 'corece' needed because after the type checker newtypes
446 sc_theta = substTheta (mkTyVarSubst clas_tyvars inst_tys)
449 -- If there are no tyvars, there's no need
450 -- to abstract over the dictionaries we need
451 dict_args | null tyvars = []
452 | otherwise = rep_pred : sc_theta
454 -- Finally! Here's where we build the dictionary Id
455 mk_dfun dfun_name = mkDictFunId dfun_name tyvars dict_args clas inst_tys
457 -------------------------------------------------------------------
458 -- Figuring out whether we can only do this newtype-deriving thing
460 standard_instance = null tys && classKey clas `elem` derivableClassKeys
462 can_derive_via_isomorphism
463 = not (clas `hasKey` readClassKey) -- Never derive Read,Show this way
464 && not (clas `hasKey` showClassKey)
465 && length tys + 1 == classArity clas -- Well kinded;
466 -- eg not: newtype T ... deriving( ST )
467 -- because ST needs *2* type params
468 && n_tyvars_to_keep >= 0 -- Well kinded;
469 -- eg not: newtype T = T Int deriving( Monad )
470 && n_args_to_keep >= 0 -- Well kinded:
471 -- eg not: newtype T a = T Int deriving( Monad )
472 && eta_ok -- Eta reduction works
473 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
474 -- newtype A = MkA [A]
476 -- instance Eq [A] => Eq A !!
478 -- Check that eta reduction is OK
479 -- (a) the dropped-off args are identical
480 -- (b) the remaining type args mention
481 -- only the remaining type variables
482 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
483 && (tyVarsOfTypes args_to_keep `subVarSet` mkVarSet tyvars_to_keep)
485 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
486 (vcat [ptext SLIT("too hard for cunning newtype deriving"),
487 ptext SLIT("debug info:") <+> ppr n_tyvars_to_keep <+>
488 ppr n_args_to_keep <+> ppr eta_ok <+>
489 ppr (isRecursiveTyCon tycon)
492 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
493 (vcat [non_std_why clas,
494 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
496 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
498 ------------------------------------------------------------------
499 chk_out :: Class -> TyCon -> [TcType] -> Maybe SDoc
500 chk_out clas tycon tys
501 | notNull tys = Just ty_args_why
502 | not (getUnique clas `elem` derivableClassKeys) = Just (non_std_why clas)
503 | clas `hasKey` enumClassKey && not is_enumeration = Just nullary_why
504 | clas `hasKey` boundedClassKey && not is_enumeration_or_single = Just single_nullary_why
505 | clas `hasKey` ixClassKey && not is_enumeration_or_single = Just single_nullary_why
506 | null data_cons = Just no_cons_why
507 | any isExistentialDataCon data_cons = Just existential_why
508 | otherwise = Nothing
510 data_cons = tyConDataCons tycon
511 is_enumeration = isEnumerationTyCon tycon
512 is_single_con = maybeToBool (maybeTyConSingleCon tycon)
513 is_enumeration_or_single = is_enumeration || is_single_con
515 single_nullary_why = ptext SLIT("one constructor data type or type with all nullary constructors expected")
516 nullary_why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
517 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
518 ty_args_why = quotes (ppr pred) <+> ptext SLIT("is not a class")
519 existential_why = quotes (ppr tycon) <+> ptext SLIT("has existentially-quantified constructor(s)")
521 pred = mkClassPred clas tys
523 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
525 new_dfun_name clas tycon -- Just a simple wrapper
526 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
527 -- The type passed to newDFunName is only used to generate
528 -- a suitable string; hence the empty type arg list
531 %************************************************************************
533 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
535 %************************************************************************
537 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
538 terms, which is the final correct RHS for the corresponding original
542 Each (k,TyVarTy tv) in a solution constrains only a type
546 The (k,TyVarTy tv) pairs in a solution are canonically
547 ordered by sorting on type varible, tv, (major key) and then class, k,
552 solveDerivEqns :: [DerivEqn]
553 -> TcM [DFunId] -- Solns in same order as eqns.
554 -- This bunch is Absolutely minimal...
556 solveDerivEqns orig_eqns
557 = iterateDeriv 1 initial_solutions
559 -- The initial solutions for the equations claim that each
560 -- instance has an empty context; this solution is certainly
561 -- in canonical form.
562 initial_solutions :: [DerivSoln]
563 initial_solutions = [ [] | _ <- orig_eqns ]
565 ------------------------------------------------------------------
566 -- iterateDeriv calculates the next batch of solutions,
567 -- compares it with the current one; finishes if they are the
568 -- same, otherwise recurses with the new solutions.
569 -- It fails if any iteration fails
570 iterateDeriv :: Int -> [DerivSoln] ->TcM [DFunId]
571 iterateDeriv n current_solns
572 | n > 20 -- Looks as if we are in an infinite loop
573 -- This can happen if we have -fallow-undecidable-instances
574 -- (See TcSimplify.tcSimplifyDeriv.)
575 = pprPanic "solveDerivEqns: probable loop"
576 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
579 dfuns = zipWithEqual "add_solns" mk_deriv_dfun orig_eqns current_solns
582 -- Extend the inst info from the explicit instance decls
583 -- with the current set of solutions, and simplify each RHS
584 tcExtendTempInstEnv dfuns $
585 mappM gen_soln orig_eqns
586 ) `thenM` \ new_solns ->
587 if (current_solns == new_solns) then
590 iterateDeriv (n+1) new_solns
592 ------------------------------------------------------------------
594 gen_soln (_, clas, tc,tyvars,deriv_rhs)
595 = addSrcLoc (getSrcLoc tc) $
596 addErrCtxt (derivCtxt (Just clas) tc) $
597 tcSimplifyDeriv tyvars deriv_rhs `thenM` \ theta ->
598 returnM (sortLt (<) theta) -- Canonicalise before returning the soluction
600 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
601 = mkDictFunId dfun_name tyvars theta
602 clas [mkTyConApp tycon (mkTyVarTys tyvars)]
605 %************************************************************************
607 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
609 %************************************************************************
611 After all the trouble to figure out the required context for the
612 derived instance declarations, all that's left is to chug along to
613 produce them. They will then be shoved into @tcInstDecls2@, which
614 will do all its usual business.
616 There are lots of possibilities for code to generate. Here are
617 various general remarks.
622 We want derived instances of @Eq@ and @Ord@ (both v common) to be
623 ``you-couldn't-do-better-by-hand'' efficient.
626 Deriving @Show@---also pretty common--- should also be reasonable good code.
629 Deriving for the other classes isn't that common or that big a deal.
636 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
639 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
642 We {\em normally} generate code only for the non-defaulted methods;
643 there are some exceptions for @Eq@ and (especially) @Ord@...
646 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
647 constructor's numeric (@Int#@) tag. These are generated by
648 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
649 these is around is given by @hasCon2TagFun@.
651 The examples under the different sections below will make this
655 Much less often (really just for deriving @Ix@), we use a
656 @_tag2con_<tycon>@ function. See the examples.
659 We use the renamer!!! Reason: we're supposed to be
660 producing @RenamedMonoBinds@ for the methods, but that means
661 producing correctly-uniquified code on the fly. This is entirely
662 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
663 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
664 the renamer. What a great hack!
668 -- Generate the method bindings for the required instance
669 -- (paired with class name, as we need that when renaming
671 gen_bind :: DFunId -> TcM (Name, RdrNameMonoBinds)
673 = getFixityEnv `thenM` \ fix_env ->
674 returnM (cls_nm, gen_binds_fn fix_env cls_nm tycon)
676 cls_nm = className clas
677 (clas, tycon) = simpleDFunClassTyCon dfun
679 gen_binds_fn fix_env cls_nm
680 = assoc "gen_bind:bad derived class"
681 gen_list (nameUnique cls_nm)
683 gen_list = [(eqClassKey, gen_Eq_binds)
684 ,(ordClassKey, gen_Ord_binds)
685 ,(enumClassKey, gen_Enum_binds)
686 ,(boundedClassKey, gen_Bounded_binds)
687 ,(ixClassKey, gen_Ix_binds)
688 ,(showClassKey, gen_Show_binds fix_env)
689 ,(readClassKey, gen_Read_binds fix_env)
694 %************************************************************************
696 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
698 %************************************************************************
703 con2tag_Foo :: Foo ... -> Int#
704 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
705 maxtag_Foo :: Int -- ditto (NB: not unlifted)
708 We have a @con2tag@ function for a tycon if:
711 We're deriving @Eq@ and the tycon has nullary data constructors.
714 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
718 We have a @tag2con@ function for a tycon if:
721 We're deriving @Enum@, or @Ix@ (enum type only???)
724 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
727 gen_taggery_Names :: [DFunId]
728 -> TcM [(RdrName, -- for an assoc list
729 TyCon, -- related tycon
732 gen_taggery_Names dfuns
733 = foldlM do_con2tag [] tycons_of_interest `thenM` \ names_so_far ->
734 foldlM do_tag2con names_so_far tycons_of_interest
736 all_CTs = map simpleDFunClassTyCon dfuns
737 all_tycons = map snd all_CTs
738 (tycons_of_interest, _) = removeDups compare all_tycons
740 do_con2tag acc_Names tycon
741 | isDataTyCon tycon &&
742 ((we_are_deriving eqClassKey tycon
743 && any isNullaryDataCon (tyConDataCons tycon))
744 || (we_are_deriving ordClassKey tycon
745 && not (maybeToBool (maybeTyConSingleCon tycon)))
746 || (we_are_deriving enumClassKey tycon)
747 || (we_are_deriving ixClassKey tycon))
749 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
754 do_tag2con acc_Names tycon
755 | isDataTyCon tycon &&
756 (we_are_deriving enumClassKey tycon ||
757 we_are_deriving ixClassKey tycon
758 && isEnumerationTyCon tycon)
759 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
760 : (maxtag_RDR tycon, tycon, GenMaxTag)
765 we_are_deriving clas_key tycon
766 = is_in_eqns clas_key tycon all_CTs
768 is_in_eqns clas_key tycon [] = False
769 is_in_eqns clas_key tycon ((c,t):cts)
770 = (clas_key == classKey c && tycon == t)
771 || is_in_eqns clas_key tycon cts
775 derivingThingErr clas tys tycon tyvars why
776 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
779 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
781 malformedPredErr tycon pred = ptext SLIT("Illegal deriving item") <+> ppr pred
783 derivCtxt :: Maybe Class -> TyCon -> SDoc
784 derivCtxt maybe_cls tycon
785 = ptext SLIT("When deriving") <+> cls <+> ptext SLIT("for type") <+> quotes (ppr tycon)
787 cls = case maybe_cls of
788 Nothing -> ptext SLIT("instances")
789 Just c -> ptext SLIT("the") <+> quotes (ppr c) <+> ptext SLIT("instance")