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(..), TyClDecl(..), MonoBinds(..),
14 andMonoBindList, collectMonoBinders )
15 import RdrHsSyn ( RdrNameMonoBinds )
16 import RnHsSyn ( RenamedHsBinds, 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 ( simpleDFunClassTyCon )
26 import TcMonoType ( tcHsPred )
27 import TcSimplify ( tcSimplifyDeriv )
29 import RnBinds ( rnMethodBinds, rnTopMonoBinds )
30 import RnEnv ( bindLocalsFV, extendTyVarEnvFVRn )
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 ( dataConOrigArgTys, isNullaryDataCon, isExistentialDataCon )
40 import Maybes ( maybeToBool, catMaybes )
41 import Name ( Name, getSrcLoc )
42 import Unique ( Unique, getUnique )
44 import RdrName ( RdrName )
46 import TyCon ( tyConTyVars, tyConDataCons, tyConArity,
47 tyConTheta, isProductTyCon, isDataTyCon,
48 isEnumerationTyCon, isRecursiveTyCon, TyCon
50 import TcType ( TcType, ThetaType, mkTyVarTy, mkTyVarTys, mkTyConApp,
51 getClassPredTys_maybe,
52 isUnLiftedType, mkClassPred, tyVarsOfTypes, tcSplitFunTys, isTypeKind,
53 tcEqTypes, tcSplitAppTys, mkAppTys, tcSplitDFunTy )
54 import Var ( TyVar, tyVarKind, idType, varName )
55 import VarSet ( mkVarSet, subVarSet )
57 import Util ( zipWithEqual, sortLt, notNull )
58 import ListSetOps ( removeDups, assoc )
62 %************************************************************************
64 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
66 %************************************************************************
70 data T a b = C1 (Foo a) (Bar b)
75 [NOTE: See end of these comments for what to do with
76 data (C a, D b) => T a b = ...
79 We want to come up with an instance declaration of the form
81 instance (Ping a, Pong b, ...) => Eq (T a b) where
84 It is pretty easy, albeit tedious, to fill in the code "...". The
85 trick is to figure out what the context for the instance decl is,
86 namely @Ping@, @Pong@ and friends.
88 Let's call the context reqd for the T instance of class C at types
89 (a,b, ...) C (T a b). Thus:
91 Eq (T a b) = (Ping a, Pong b, ...)
93 Now we can get a (recursive) equation from the @data@ decl:
95 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
96 u Eq (T b a) u Eq Int -- From C2
97 u Eq (T a a) -- From C3
99 Foo and Bar may have explicit instances for @Eq@, in which case we can
100 just substitute for them. Alternatively, either or both may have
101 their @Eq@ instances given by @deriving@ clauses, in which case they
102 form part of the system of equations.
104 Now all we need do is simplify and solve the equations, iterating to
105 find the least fixpoint. Notice that the order of the arguments can
106 switch around, as here in the recursive calls to T.
108 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
112 Eq (T a b) = {} -- The empty set
115 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
116 u Eq (T b a) u Eq Int -- From C2
117 u Eq (T a a) -- From C3
119 After simplification:
120 = Eq a u Ping b u {} u {} u {}
125 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
126 u Eq (T b a) u Eq Int -- From C2
127 u Eq (T a a) -- From C3
129 After simplification:
134 = Eq a u Ping b u Eq b u Ping a
136 The next iteration gives the same result, so this is the fixpoint. We
137 need to make a canonical form of the RHS to ensure convergence. We do
138 this by simplifying the RHS to a form in which
140 - the classes constrain only tyvars
141 - the list is sorted by tyvar (major key) and then class (minor key)
142 - no duplicates, of course
144 So, here are the synonyms for the ``equation'' structures:
147 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
148 -- The Name is the name for the DFun we'll build
149 -- The tyvars bind all the variables in the RHS
151 pprDerivEqn (n,c,tc,tvs,rhs)
152 = parens (hsep [ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
154 type DerivRhs = ThetaType
155 type DerivSoln = DerivRhs
159 [Data decl contexts] A note about contexts on data decls
160 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
163 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
165 We will need an instance decl like:
167 instance (Read a, RealFloat a) => Read (Complex a) where
170 The RealFloat in the context is because the read method for Complex is bound
171 to construct a Complex, and doing that requires that the argument type is
174 But this ain't true for Show, Eq, Ord, etc, since they don't construct
175 a Complex; they only take them apart.
177 Our approach: identify the offending classes, and add the data type
178 context to the instance decl. The "offending classes" are
182 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
183 pattern matching against a constructor from a data type with a context
184 gives rise to the constraints for that context -- or at least the thinned
185 version. So now all classes are "offending".
189 %************************************************************************
191 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
193 %************************************************************************
196 tcDeriving :: [RenamedTyClDecl] -- All type constructors
197 -> TcM ([InstInfo], -- The generated "instance decls".
198 RenamedHsBinds, -- Extra generated bindings
199 FreeVars) -- These are free in the generated bindings
201 tcDeriving tycl_decls
202 = recoverM (returnM ([], EmptyBinds, emptyFVs)) $
203 getDOpts `thenM` \ dflags ->
205 -- Fish the "deriving"-related information out of the TcEnv
206 -- and make the necessary "equations".
207 makeDerivEqns tycl_decls `thenM` \ (ordinary_eqns, newtype_inst_info) ->
208 tcExtendTempInstEnv (map iDFunId newtype_inst_info) $
209 -- Add the newtype-derived instances to the inst env
210 -- before tacking the "ordinary" ones
212 deriveOrdinaryStuff ordinary_eqns `thenM` \ (ordinary_inst_info, binds, fvs) ->
214 inst_info = newtype_inst_info ++ ordinary_inst_info
217 ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
218 (ddump_deriving inst_info binds)) `thenM_`
220 returnM (inst_info, binds, fvs)
223 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> SDoc
224 ddump_deriving inst_infos extra_binds
225 = vcat (map ppr_info inst_infos) $$ ppr extra_binds
227 ppr_info inst_info = pprInstInfo inst_info $$
228 nest 4 (pprInstInfoDetails inst_info)
229 -- pprInstInfo doesn't print much: only the type
231 -----------------------------------------
232 deriveOrdinaryStuff [] -- Short cut
233 = returnM ([], EmptyBinds, emptyFVs)
235 deriveOrdinaryStuff eqns
236 = -- Take the equation list and solve it, to deliver a list of
237 -- solutions, a.k.a. the contexts for the instance decls
238 -- required for the corresponding equations.
239 solveDerivEqns eqns `thenM` \ new_dfuns ->
241 -- Now augment the InstInfos, adding in the rather boring
242 -- actual-code-to-do-the-methods binds. We may also need to
243 -- generate extra not-one-inst-decl-specific binds, notably
244 -- "con2tag" and/or "tag2con" functions. We do these
246 gen_taggery_Names new_dfuns `thenM` \ nm_alist_etc ->
249 extra_mbind_list = map gen_tag_n_con_monobind nm_alist_etc
250 extra_mbinds = andMonoBindList extra_mbind_list
251 mbinders = collectMonoBinders extra_mbinds
253 mappM gen_bind new_dfuns `thenM` \ rdr_name_inst_infos ->
255 traceTc (text "tcDeriv" <+> vcat (map ppr rdr_name_inst_infos)) `thenM_`
256 getModule `thenM` \ this_mod ->
257 initRn (InterfaceMode this_mod) (
258 -- Rename to get RenamedBinds.
259 -- The only tricky bit is that the extra_binds must scope
260 -- over the method bindings for the instances.
261 bindLocalsFV (ptext (SLIT("deriving"))) mbinders $ \ _ ->
262 rnTopMonoBinds extra_mbinds [] `thenM` \ (rn_extra_binds, dus) ->
264 mapAndUnzipM rn_inst_info rdr_name_inst_infos `thenM` \ (pairs, fvs_s) ->
267 (rn_inst_infos, aux_binds_s) = unzip pairs
268 all_binds = rn_extra_binds `ThenBinds` foldr ThenBinds EmptyBinds aux_binds_s
270 returnM ((rn_inst_infos, all_binds),
271 duUses dus `plusFV` plusFVs fvs_s)
272 ) `thenM` \ ((rn_inst_infos, rn_extra_binds), fvs) ->
273 returnM (rn_inst_infos, rn_extra_binds, fvs)
276 rn_inst_info (dfun, (meth_binds, aux_binds))
277 = -- Rename the auxiliary bindings
278 bindLocalsFV (ptext (SLIT("deriving"))) mbinders $ \ _ ->
279 rnTopMonoBinds aux_binds [] `thenM` \ (rn_aux_binds, dus) ->
281 -- Bring the right type variables into scope
282 extendTyVarEnvFVRn (map varName tyvars) $
283 rnMethodBinds (className cls) [] meth_binds `thenM` \ (rn_meth_binds, fvs) ->
285 return ((InstInfo { iDFunId = dfun, iBinds = VanillaInst rn_meth_binds [] },
287 duUses dus `plusFV` fvs)
289 mbinders = collectMonoBinders aux_binds
290 (tyvars, _, cls, _) = tcSplitDFunTy (idType dfun)
294 %************************************************************************
296 \subsection[TcDeriv-eqns]{Forming the equations}
298 %************************************************************************
300 @makeDerivEqns@ fishes around to find the info about needed derived
301 instances. Complicating factors:
304 We can only derive @Enum@ if the data type is an enumeration
305 type (all nullary data constructors).
308 We can only derive @Ix@ if the data type is an enumeration {\em
309 or} has just one data constructor (e.g., tuples).
312 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
316 makeDerivEqns :: [RenamedTyClDecl]
317 -> TcM ([DerivEqn], -- Ordinary derivings
318 [InstInfo]) -- Special newtype derivings
320 makeDerivEqns tycl_decls
321 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
322 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
324 ------------------------------------------------------------------
325 derive_these :: [(NewOrData, Name, RenamedHsPred)]
326 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
327 -- NB: only source-language decls have deriving, no imported ones do
328 derive_these = [ (nd, tycon, pred)
329 | TyData {tcdND = nd, tcdName = tycon, tcdDerivs = Just preds} <- tycl_decls,
332 ------------------------------------------------------------------
333 mk_eqn :: (NewOrData, Name, RenamedHsPred) -> TcM (Maybe DerivEqn, Maybe InstInfo)
334 -- We swizzle the tyvars and datacons out of the tycon
335 -- to make the rest of the equation
337 mk_eqn (new_or_data, tycon_name, pred)
338 = tcLookupTyCon tycon_name `thenM` \ tycon ->
339 addSrcLoc (getSrcLoc tycon) $
340 addErrCtxt (derivCtxt Nothing tycon) $
341 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
342 -- the type variables for the type constructor
343 tcHsPred pred `thenM` \ pred' ->
344 case getClassPredTys_maybe pred' of
345 Nothing -> bale_out (malformedPredErr tycon pred)
346 Just (clas, tys) -> doptM Opt_GlasgowExts `thenM` \ gla_exts ->
347 mk_eqn_help gla_exts new_or_data tycon clas tys
349 ------------------------------------------------------------------
350 mk_eqn_help gla_exts DataType tycon clas tys
351 | Just err <- checkSideConditions gla_exts clas tycon tys
352 = bale_out (derivingThingErr clas tys tycon tyvars err)
354 = new_dfun_name clas tycon `thenM` \ dfun_name ->
355 returnM (Just (dfun_name, clas, tycon, tyvars, constraints), Nothing)
357 tyvars = tyConTyVars tycon
358 data_cons = tyConDataCons tycon
359 constraints = extra_constraints ++ ordinary_constraints
360 -- "extra_constraints": see note [Data decl contexts] above
361 extra_constraints = tyConTheta tycon
364 | clas `hasKey` typeableClassKey -- For the Typeable class, the constraints
365 -- don't involve the constructor ags, only
367 -- e.g. data T a b = ...
369 -- instance (Typeable a, Typable b)
370 -- => Typeable (T a b) where
371 = [mkClassPred clas [mkTyVarTy tv] | tv <- tyvars]
373 = [ mkClassPred clas [arg_ty]
374 | data_con <- tyConDataCons tycon,
375 arg_ty <- dataConOrigArgTys data_con,
376 -- Use the same type variables
377 -- as the type constructor,
378 -- hence no need to instantiate
379 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
382 mk_eqn_help gla_exts NewType tycon clas tys
383 | can_derive_via_isomorphism && (gla_exts || standard_class gla_exts clas)
384 = -- Go ahead and use the isomorphism
385 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
386 new_dfun_name clas tycon `thenM` \ dfun_name ->
387 returnM (Nothing, Just (InstInfo { iDFunId = mk_dfun dfun_name,
388 iBinds = NewTypeDerived rep_tys }))
389 | standard_class gla_exts clas
390 = mk_eqn_help gla_exts DataType tycon clas tys -- Go via bale-out route
392 | otherwise -- Non-standard instance
393 = bale_out (if gla_exts then
394 cant_derive_err -- Too hard
396 non_std_err) -- Just complain about being a non-std instance
398 -- Here is the plan for newtype derivings. We see
399 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
400 -- where aj...an do not occur free in t, and the (C s1 ... sm) is a
401 -- *partial applications* of class C with the last parameter missing
403 -- We generate the instances
404 -- instance C s1 .. sm (t ak...aj) => C s1 .. sm (T a1...aj)
405 -- where T a1...aj is the partial application of the LHS of the correct kind
407 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
408 -- instance Monad (ST s) => Monad (T s) where
409 -- fail = coerce ... (fail @ ST s)
411 clas_tyvars = classTyVars clas
412 kind = tyVarKind (last clas_tyvars)
413 -- Kind of the thing we want to instance
414 -- e.g. argument kind of Monad, *->*
416 (arg_kinds, _) = tcSplitFunTys kind
417 n_args_to_drop = length arg_kinds
418 -- Want to drop 1 arg from (T s a) and (ST s a)
419 -- to get instance Monad (ST s) => Monad (T s)
421 -- Note [newtype representation]
422 -- We must not use newTyConRep to get the representation
423 -- type, because that looks through all intermediate newtypes
424 -- To get the RHS of *this* newtype, just look at the data
425 -- constructor. For example
426 -- newtype B = MkB Int
427 -- newtype A = MkA B deriving( Num )
428 -- We want the Num instance of B, *not* the Num instance of Int,
429 -- when making the Num instance of A!
430 tyvars = tyConTyVars tycon
431 rep_ty = head (dataConOrigArgTys (head (tyConDataCons tycon)))
432 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
434 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
435 tyvars_to_drop = drop n_tyvars_to_keep tyvars
436 tyvars_to_keep = take n_tyvars_to_keep tyvars
438 n_args_to_keep = length rep_ty_args - n_args_to_drop
439 args_to_drop = drop n_args_to_keep rep_ty_args
440 args_to_keep = take n_args_to_keep rep_ty_args
442 rep_tys = tys ++ [mkAppTys rep_fn args_to_keep]
443 rep_pred = mkClassPred clas rep_tys
444 -- rep_pred is the representation dictionary, from where
445 -- we are gong to get all the methods for the newtype dictionary
447 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
448 -- The 'tys' here come from the partial application
449 -- in the deriving clause. The last arg is the new
452 -- We must pass the superclasses; the newtype might be an instance
453 -- of them in a different way than the representation type
454 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
455 -- Then the Show instance is not done via isomprphism; it shows
457 -- The Num instance is derived via isomorphism, but the Show superclass
458 -- dictionary must the Show instance for Foo, *not* the Show dictionary
459 -- gotten from the Num dictionary. So we must build a whole new dictionary
460 -- not just use the Num one. The instance we want is something like:
461 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
464 -- There's no 'corece' needed because after the type checker newtypes
467 sc_theta = substTheta (mkTyVarSubst clas_tyvars inst_tys)
470 -- If there are no tyvars, there's no need
471 -- to abstract over the dictionaries we need
472 dict_args | null tyvars = []
473 | otherwise = rep_pred : sc_theta
475 -- Finally! Here's where we build the dictionary Id
476 mk_dfun dfun_name = mkDictFunId dfun_name tyvars dict_args clas inst_tys
478 -------------------------------------------------------------------
479 -- Figuring out whether we can only do this newtype-deriving thing
481 right_arity = length tys + 1 == classArity clas
483 -- Never derive Read,Show,Typeable,Data this way
484 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
485 can_derive_via_isomorphism
486 = not (getUnique clas `elem` non_iso_classes)
487 && right_arity -- Well kinded;
488 -- eg not: newtype T ... deriving( ST )
489 -- because ST needs *2* type params
490 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
491 -- eg not: newtype T = T Int deriving( Monad )
492 && n_args_to_keep >= 0 -- Rep type has right kind:
493 -- eg not: newtype T a = T Int deriving( Monad )
494 && eta_ok -- Eta reduction works
495 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
496 -- newtype A = MkA [A]
498 -- instance Eq [A] => Eq A !!
500 -- Here's a recursive newtype that's actually OK
501 -- newtype S1 = S1 [T1 ()]
502 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
503 -- It's currently rejected. Oh well.
505 -- Check that eta reduction is OK
506 -- (a) the dropped-off args are identical
507 -- (b) the remaining type args mention
508 -- only the remaining type variables
509 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
510 && (tyVarsOfTypes args_to_keep `subVarSet` mkVarSet tyvars_to_keep)
512 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
513 (vcat [ptext SLIT("even with cunning newtype deriving:"),
514 if isRecursiveTyCon tycon then
515 ptext SLIT("the newtype is recursive")
517 if not right_arity then
518 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
520 if not (n_tyvars_to_keep >= 0) then
521 ptext SLIT("the type constructor has wrong kind")
522 else if not (n_args_to_keep >= 0) then
523 ptext SLIT("the representation type has wrong kind")
524 else if not eta_ok then
525 ptext SLIT("the eta-reduction property does not hold")
529 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
530 (vcat [non_std_why clas,
531 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
533 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
534 standard_class gla_exts clas = key `elem` derivableClassKeys
535 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
542 new_dfun_name clas tycon -- Just a simple wrapper
543 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
544 -- The type passed to newDFunName is only used to generate
545 -- a suitable string; hence the empty type arg list
548 ------------------------------------------------------------------
549 -- Check side conditions that dis-allow derivability for particular classes
550 -- This is *apart* from the newtype-deriving mechanism
552 checkSideConditions :: Bool -> Class -> TyCon -> [TcType] -> Maybe SDoc
553 checkSideConditions gla_exts clas tycon tys
555 = Just ty_args_why -- e.g. deriving( Foo s )
557 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
558 [] -> Just (non_std_why clas)
559 [cond] -> cond (gla_exts, tycon)
560 other -> pprPanic "checkSideConditions" (ppr clas)
562 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
564 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
566 sideConditions :: [(Unique, Condition)]
568 = [ (eqClassKey, cond_std),
569 (ordClassKey, cond_std),
570 (readClassKey, cond_std),
571 (showClassKey, cond_std),
572 (enumClassKey, cond_std `andCond` cond_isEnumeration),
573 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
574 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
575 (typeableClassKey, cond_glaExts `andCond` cond_allTypeKind),
576 (dataClassKey, cond_glaExts `andCond` cond_std)
579 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
581 orCond :: Condition -> Condition -> Condition
584 Nothing -> Nothing -- c1 succeeds
585 Just x -> case c2 tc of -- c1 fails
587 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
590 andCond c1 c2 tc = case c1 tc of
591 Nothing -> c2 tc -- c1 succeeds
592 Just x -> Just x -- c1 fails
594 cond_std :: Condition
595 cond_std (gla_exts, tycon)
596 | any isExistentialDataCon data_cons = Just existential_why
597 | null data_cons = Just no_cons_why
598 | otherwise = Nothing
600 data_cons = tyConDataCons tycon
601 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
602 existential_why = quotes (ppr tycon) <+> ptext SLIT("has existentially-quantified constructor(s)")
604 cond_isEnumeration :: Condition
605 cond_isEnumeration (gla_exts, tycon)
606 | isEnumerationTyCon tycon = Nothing
607 | otherwise = Just why
609 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
611 cond_isProduct :: Condition
612 cond_isProduct (gla_exts, tycon)
613 | isProductTyCon tycon = Nothing
614 | otherwise = Just why
616 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
618 cond_allTypeKind :: Condition
619 cond_allTypeKind (gla_exts, tycon)
620 | all (isTypeKind . tyVarKind) (tyConTyVars tycon) = Nothing
621 | otherwise = Just why
623 why = quotes (ppr tycon) <+> ptext SLIT("is parameterised over arguments of kind other than `*'")
625 cond_glaExts :: Condition
626 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
627 | otherwise = Just why
629 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
632 %************************************************************************
634 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
636 %************************************************************************
638 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
639 terms, which is the final correct RHS for the corresponding original
643 Each (k,TyVarTy tv) in a solution constrains only a type
647 The (k,TyVarTy tv) pairs in a solution are canonically
648 ordered by sorting on type varible, tv, (major key) and then class, k,
653 solveDerivEqns :: [DerivEqn]
654 -> TcM [DFunId] -- Solns in same order as eqns.
655 -- This bunch is Absolutely minimal...
657 solveDerivEqns orig_eqns
658 = iterateDeriv 1 initial_solutions
660 -- The initial solutions for the equations claim that each
661 -- instance has an empty context; this solution is certainly
662 -- in canonical form.
663 initial_solutions :: [DerivSoln]
664 initial_solutions = [ [] | _ <- orig_eqns ]
666 ------------------------------------------------------------------
667 -- iterateDeriv calculates the next batch of solutions,
668 -- compares it with the current one; finishes if they are the
669 -- same, otherwise recurses with the new solutions.
670 -- It fails if any iteration fails
671 iterateDeriv :: Int -> [DerivSoln] ->TcM [DFunId]
672 iterateDeriv n current_solns
673 | n > 20 -- Looks as if we are in an infinite loop
674 -- This can happen if we have -fallow-undecidable-instances
675 -- (See TcSimplify.tcSimplifyDeriv.)
676 = pprPanic "solveDerivEqns: probable loop"
677 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
680 dfuns = zipWithEqual "add_solns" mk_deriv_dfun orig_eqns current_solns
683 -- Extend the inst info from the explicit instance decls
684 -- with the current set of solutions, and simplify each RHS
685 tcExtendTempInstEnv dfuns $
686 mappM gen_soln orig_eqns
687 ) `thenM` \ new_solns ->
688 if (current_solns == new_solns) then
691 iterateDeriv (n+1) new_solns
693 ------------------------------------------------------------------
695 gen_soln (_, clas, tc,tyvars,deriv_rhs)
696 = addSrcLoc (getSrcLoc tc) $
697 addErrCtxt (derivCtxt (Just clas) tc) $
698 tcSimplifyDeriv tyvars deriv_rhs `thenM` \ theta ->
699 returnM (sortLt (<) theta) -- Canonicalise before returning the soluction
701 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
702 = mkDictFunId dfun_name tyvars theta
703 clas [mkTyConApp tycon (mkTyVarTys tyvars)]
706 %************************************************************************
708 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
710 %************************************************************************
712 After all the trouble to figure out the required context for the
713 derived instance declarations, all that's left is to chug along to
714 produce them. They will then be shoved into @tcInstDecls2@, which
715 will do all its usual business.
717 There are lots of possibilities for code to generate. Here are
718 various general remarks.
723 We want derived instances of @Eq@ and @Ord@ (both v common) to be
724 ``you-couldn't-do-better-by-hand'' efficient.
727 Deriving @Show@---also pretty common--- should also be reasonable good code.
730 Deriving for the other classes isn't that common or that big a deal.
737 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
740 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
743 We {\em normally} generate code only for the non-defaulted methods;
744 there are some exceptions for @Eq@ and (especially) @Ord@...
747 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
748 constructor's numeric (@Int#@) tag. These are generated by
749 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
750 these is around is given by @hasCon2TagFun@.
752 The examples under the different sections below will make this
756 Much less often (really just for deriving @Ix@), we use a
757 @_tag2con_<tycon>@ function. See the examples.
760 We use the renamer!!! Reason: we're supposed to be
761 producing @RenamedMonoBinds@ for the methods, but that means
762 producing correctly-uniquified code on the fly. This is entirely
763 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
764 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
765 the renamer. What a great hack!
769 -- Generate the method bindings for the required instance
770 -- (paired with DFunId, as we need that when renaming
772 gen_bind :: DFunId -> TcM (DFunId, (RdrNameMonoBinds, RdrNameMonoBinds))
774 = getFixityEnv `thenM` \ fix_env ->
776 (clas, tycon) = simpleDFunClassTyCon dfun
777 gen_binds_fn = assoc "gen_bind:bad derived class"
778 gen_list (getUnique clas)
780 gen_list = [(eqClassKey, no_aux_binds gen_Eq_binds)
781 ,(ordClassKey, no_aux_binds gen_Ord_binds)
782 ,(enumClassKey, no_aux_binds gen_Enum_binds)
783 ,(boundedClassKey, no_aux_binds gen_Bounded_binds)
784 ,(ixClassKey, no_aux_binds gen_Ix_binds)
785 ,(showClassKey, no_aux_binds (gen_Show_binds fix_env))
786 ,(readClassKey, no_aux_binds (gen_Read_binds fix_env))
787 ,(typeableClassKey,no_aux_binds gen_Typeable_binds)
788 ,(dataClassKey, gen_Data_binds fix_env)
791 -- Used for generators that don't need to produce
792 -- any auxiliary bindings
793 no_aux_binds f tc = (f tc, EmptyMonoBinds)
795 returnM (dfun, gen_binds_fn tycon)
799 %************************************************************************
801 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
803 %************************************************************************
808 con2tag_Foo :: Foo ... -> Int#
809 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
810 maxtag_Foo :: Int -- ditto (NB: not unlifted)
813 We have a @con2tag@ function for a tycon if:
816 We're deriving @Eq@ and the tycon has nullary data constructors.
819 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
823 We have a @tag2con@ function for a tycon if:
826 We're deriving @Enum@, or @Ix@ (enum type only???)
829 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
832 gen_taggery_Names :: [DFunId]
833 -> TcM [(RdrName, -- for an assoc list
834 TyCon, -- related tycon
837 gen_taggery_Names dfuns
838 = foldlM do_con2tag [] tycons_of_interest `thenM` \ names_so_far ->
839 foldlM do_tag2con names_so_far tycons_of_interest
841 all_CTs = map simpleDFunClassTyCon dfuns
842 all_tycons = map snd all_CTs
843 (tycons_of_interest, _) = removeDups compare all_tycons
845 do_con2tag acc_Names tycon
846 | isDataTyCon tycon &&
847 ((we_are_deriving eqClassKey tycon
848 && any isNullaryDataCon (tyConDataCons tycon))
849 || (we_are_deriving ordClassKey tycon
850 && not (isProductTyCon tycon))
851 || (we_are_deriving enumClassKey tycon)
852 || (we_are_deriving ixClassKey tycon))
854 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
859 do_tag2con acc_Names tycon
860 | isDataTyCon tycon &&
861 (we_are_deriving enumClassKey tycon ||
862 we_are_deriving ixClassKey tycon
863 && isEnumerationTyCon tycon)
864 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
865 : (maxtag_RDR tycon, tycon, GenMaxTag)
870 we_are_deriving clas_key tycon
871 = is_in_eqns clas_key tycon all_CTs
873 is_in_eqns clas_key tycon [] = False
874 is_in_eqns clas_key tycon ((c,t):cts)
875 = (clas_key == classKey c && tycon == t)
876 || is_in_eqns clas_key tycon cts
880 derivingThingErr clas tys tycon tyvars why
881 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
884 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
886 malformedPredErr tycon pred = ptext SLIT("Illegal deriving item") <+> ppr pred
888 derivCtxt :: Maybe Class -> TyCon -> SDoc
889 derivCtxt maybe_cls tycon
890 = ptext SLIT("When deriving") <+> cls <+> ptext SLIT("for type") <+> quotes (ppr tycon)
892 cls = case maybe_cls of
893 Nothing -> ptext SLIT("instances")
894 Just c -> ptext SLIT("the") <+> quotes (ppr c) <+> ptext SLIT("instance")