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"
14 import DynFlags ( DynFlag(..) )
16 import Generics ( mkTyConGenericBinds )
18 import TcMType ( checkValidInstance )
19 import TcEnv ( newDFunName, pprInstInfoDetails,
20 InstInfo(..), InstBindings(..), simpleInstInfoClsTy,
21 tcLookupClass, tcLookupTyCon, tcExtendTyVarEnv
23 import TcGenDeriv -- Deriv stuff
24 import InstEnv ( Instance, OverlapFlag, mkLocalInstance, instanceHead, extendInstEnvList )
25 import Inst ( getOverlapFlag )
26 import TcHsType ( tcHsDeriv )
27 import TcSimplify ( tcSimplifyDeriv )
29 import RnBinds ( rnMethodBinds, rnTopBinds )
30 import RnEnv ( bindLocalNames )
31 import HscTypes ( FixityEnv )
33 import Class ( className, classArity, classKey, classTyVars, classSCTheta, Class )
34 import Type ( zipOpenTvSubst, substTheta, pprThetaArrow, pprClassPred, mkTyVarTy )
35 import ErrUtils ( dumpIfSet_dyn )
36 import MkId ( mkDictFunId )
37 import DataCon ( isNullarySrcDataCon, isVanillaDataCon, dataConOrigArgTys, dataConInstOrigArgTys )
38 import Maybes ( catMaybes )
39 import RdrName ( RdrName )
40 import Name ( Name, getSrcLoc )
41 import NameSet ( duDefs )
42 import Kind ( splitKindFunTys )
43 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
44 tyConStupidTheta, isProductTyCon, isDataTyCon, newTyConRhs,
45 isEnumerationTyCon, isRecursiveTyCon, TyCon
47 import TcType ( TcType, ThetaType, mkTyVarTys, mkTyConApp, tcTyConAppTyCon,
48 isUnLiftedType, mkClassPred, tyVarsOfType,
49 isArgTypeKind, tcEqTypes, tcSplitAppTys, mkAppTys )
50 import Var ( TyVar, tyVarKind, varName )
51 import VarSet ( mkVarSet, subVarSet )
53 import SrcLoc ( srcLocSpan, Located(..) )
54 import Util ( zipWithEqual, sortLe, notNull )
55 import ListSetOps ( removeDups, assocMaybe )
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".
190 newtype T = T Char deriving( C [a] )
192 Notice the free 'a' in the deriving. We have to fill this out to
193 newtype T = T Char deriving( forall a. C [a] )
195 And then translate it to:
196 instance C [a] Char => C [a] T where ...
201 %************************************************************************
203 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
205 %************************************************************************
208 tcDeriving :: [LTyClDecl Name] -- All type constructors
209 -> TcM ([InstInfo], -- The generated "instance decls"
210 HsValBinds Name) -- Extra generated top-level bindings
212 tcDeriving tycl_decls
213 = recoverM (returnM ([], emptyValBindsOut)) $
214 do { -- Fish the "deriving"-related information out of the TcEnv
215 -- and make the necessary "equations".
216 overlap_flag <- getOverlapFlag
217 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns overlap_flag tycl_decls
219 ; (ordinary_inst_info, deriv_binds)
220 <- extendLocalInstEnv (map iSpec newtype_inst_info) $
221 deriveOrdinaryStuff overlap_flag ordinary_eqns
222 -- Add the newtype-derived instances to the inst env
223 -- before tacking the "ordinary" ones
225 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
227 -- If we are compiling a hs-boot file,
228 -- don't generate any derived bindings
229 ; is_boot <- tcIsHsBoot
231 return (inst_info, emptyValBindsOut)
235 -- Generate the generic to/from functions from each type declaration
236 ; gen_binds <- mkGenericBinds tycl_decls
238 -- Rename these extra bindings, discarding warnings about unused bindings etc
239 -- Set -fglasgow exts so that we can have type signatures in patterns,
240 -- which is used in the generic binds
242 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
243 { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn deriv_binds [])
244 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
245 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
247 ; return (rn_deriv `plusHsValBinds` rn_gen) }
251 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
252 (ddump_deriving inst_info rn_binds))
254 ; returnM (inst_info, rn_binds)
257 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
258 ddump_deriving inst_infos extra_binds
259 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
261 -----------------------------------------
262 deriveOrdinaryStuff overlap_flag [] -- Short cut
263 = returnM ([], emptyLHsBinds)
265 deriveOrdinaryStuff overlap_flag eqns
266 = do { -- Take the equation list and solve it, to deliver a list of
267 -- solutions, a.k.a. the contexts for the instance decls
268 -- required for the corresponding equations.
269 inst_specs <- solveDerivEqns overlap_flag eqns
271 -- Generate the InstInfo for each dfun,
272 -- plus any auxiliary bindings it needs
273 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst inst_specs
275 -- Generate any extra not-one-inst-decl-specific binds,
276 -- notably "con2tag" and/or "tag2con" functions.
277 ; extra_binds <- genTaggeryBinds inst_infos
280 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
283 -----------------------------------------
284 mkGenericBinds tycl_decls
285 = do { tcs <- mapM tcLookupTyCon
287 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
288 -- We are only interested in the data type declarations
289 ; return (unionManyBags [ mkTyConGenericBinds tc |
290 tc <- tcs, tyConHasGenerics tc ]) }
291 -- And then only in the ones whose 'has-generics' flag is on
295 %************************************************************************
297 \subsection[TcDeriv-eqns]{Forming the equations}
299 %************************************************************************
301 @makeDerivEqns@ fishes around to find the info about needed derived
302 instances. Complicating factors:
305 We can only derive @Enum@ if the data type is an enumeration
306 type (all nullary data constructors).
309 We can only derive @Ix@ if the data type is an enumeration {\em
310 or} has just one data constructor (e.g., tuples).
313 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
317 makeDerivEqns :: OverlapFlag
319 -> TcM ([DerivEqn], -- Ordinary derivings
320 [InstInfo]) -- Special newtype derivings
322 makeDerivEqns overlap_flag tycl_decls
323 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
324 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
326 ------------------------------------------------------------------
327 derive_these :: [(NewOrData, Name, LHsType Name)]
328 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
329 derive_these = [ (nd, tycon, pred)
330 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
331 tcdDerivs = Just preds }) <- tycl_decls,
334 ------------------------------------------------------------------
335 mk_eqn :: (NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
336 -- We swizzle the tyvars and datacons out of the tycon
337 -- to make the rest of the equation
339 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
340 -- we allow deriving (forall a. C [a]).
342 mk_eqn (new_or_data, tycon_name, hs_deriv_ty)
343 = tcLookupTyCon tycon_name `thenM` \ tycon ->
344 setSrcSpan (srcLocSpan (getSrcLoc tycon)) $
345 addErrCtxt (derivCtxt tycon) $
346 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
347 -- the type variables for the type constructor
348 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
349 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
350 mk_eqn_help gla_exts new_or_data tycon deriv_tvs clas tys
352 ------------------------------------------------------------------
353 -- data/newtype T a = ... deriving( C t1 t2 )
354 -- leads to a call to mk_eqn_help with
355 -- tycon = T, deriv_tvs = ftv(t1,t2), clas = C, tys = [t1,t2]
357 mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys
358 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
359 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
361 = do { eqn <- mkDataTypeEqn tycon clas
362 ; returnM (Just eqn, Nothing) }
364 mk_eqn_help gla_exts NewType tycon deriv_tvs clas tys
365 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
366 = -- Go ahead and use the isomorphism
367 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
368 new_dfun_name clas tycon `thenM` \ dfun_name ->
369 returnM (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
370 iBinds = NewTypeDerived rep_tys }))
371 | std_class gla_exts clas
372 = mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
374 | otherwise -- Non-standard instance
375 = bale_out (if gla_exts then
376 cant_derive_err -- Too hard
378 non_std_err) -- Just complain about being a non-std instance
380 -- Here is the plan for newtype derivings. We see
381 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
382 -- where t is a type,
383 -- ak...an is a suffix of a1..an
384 -- ak...an do not occur free in t,
385 -- (C s1 ... sm) is a *partial applications* of class C
386 -- with the last parameter missing
388 -- We generate the instances
389 -- instance C s1 .. sm (t ak...ap) => C s1 .. sm (T a1...ap)
390 -- where T a1...ap is the partial application of the LHS of the correct kind
393 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
394 -- instance Monad (ST s) => Monad (T s) where
395 -- fail = coerce ... (fail @ ST s)
396 -- (Actually we don't need the coerce, because non-rec newtypes are transparent
398 clas_tyvars = classTyVars clas
399 kind = tyVarKind (last clas_tyvars)
400 -- Kind of the thing we want to instance
401 -- e.g. argument kind of Monad, *->*
403 (arg_kinds, _) = splitKindFunTys kind
404 n_args_to_drop = length arg_kinds
405 -- Want to drop 1 arg from (T s a) and (ST s a)
406 -- to get instance Monad (ST s) => Monad (T s)
408 -- Note [newtype representation]
409 -- Need newTyConRhs *not* newTyConRep to get the representation
410 -- type, because the latter looks through all intermediate newtypes
412 -- newtype B = MkB Int
413 -- newtype A = MkA B deriving( Num )
414 -- We want the Num instance of B, *not* the Num instance of Int,
415 -- when making the Num instance of A!
416 (tc_tvs, rep_ty) = newTyConRhs tycon
417 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
419 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
420 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
421 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
423 n_args_to_keep = length rep_ty_args - n_args_to_drop
424 args_to_drop = drop n_args_to_keep rep_ty_args
425 args_to_keep = take n_args_to_keep rep_ty_args
427 rep_fn' = mkAppTys rep_fn args_to_keep
428 rep_tys = tys ++ [rep_fn']
429 rep_pred = mkClassPred clas rep_tys
430 -- rep_pred is the representation dictionary, from where
431 -- we are gong to get all the methods for the newtype dictionary
433 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
434 -- The 'tys' here come from the partial application
435 -- in the deriving clause. The last arg is the new
438 -- We must pass the superclasses; the newtype might be an instance
439 -- of them in a different way than the representation type
440 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
441 -- Then the Show instance is not done via isomorphism; it shows
443 -- The Num instance is derived via isomorphism, but the Show superclass
444 -- dictionary must the Show instance for Foo, *not* the Show dictionary
445 -- gotten from the Num dictionary. So we must build a whole new dictionary
446 -- not just use the Num one. The instance we want is something like:
447 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
450 -- There's no 'corece' needed because after the type checker newtypes
453 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
456 -- If there are no tyvars, there's no need
457 -- to abstract over the dictionaries we need
458 dict_tvs = deriv_tvs ++ tc_tvs
459 dict_args | null dict_tvs = []
460 | otherwise = rep_pred : sc_theta
462 -- Finally! Here's where we build the dictionary Id
463 mk_inst_spec dfun_name
464 = mkLocalInstance dfun overlap_flag
466 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
468 -------------------------------------------------------------------
469 -- Figuring out whether we can only do this newtype-deriving thing
471 right_arity = length tys + 1 == classArity clas
473 -- Never derive Read,Show,Typeable,Data this way
474 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
475 can_derive_via_isomorphism
476 = not (getUnique clas `elem` non_iso_classes)
477 && right_arity -- Well kinded;
478 -- eg not: newtype T ... deriving( ST )
479 -- because ST needs *2* type params
480 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
481 -- eg not: newtype T = T Int deriving( Monad )
482 && n_args_to_keep >= 0 -- Rep type has right kind:
483 -- eg not: newtype T a = T Int deriving( Monad )
484 && eta_ok -- Eta reduction works
485 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
486 -- newtype A = MkA [A]
488 -- instance Eq [A] => Eq A !!
489 -- Here's a recursive newtype that's actually OK
490 -- newtype S1 = S1 [T1 ()]
491 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
492 -- It's currently rejected. Oh well.
493 -- In fact we generate an instance decl that has method of form
494 -- meth @ instTy = meth @ repTy
495 -- (no coerce's). We'd need a coerce if we wanted to handle
496 -- recursive newtypes too
498 -- Check that eta reduction is OK
499 -- (a) the dropped-off args are identical
500 -- (b) the remaining type args mention
501 -- only the remaining type variables
502 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
503 && (tyVarsOfType rep_fn' `subVarSet` mkVarSet tyvars_to_keep)
505 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
506 (vcat [ptext SLIT("even with cunning newtype deriving:"),
507 if isRecursiveTyCon tycon then
508 ptext SLIT("the newtype is recursive")
510 if not right_arity then
511 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
513 if not (n_tyvars_to_keep >= 0) then
514 ptext SLIT("the type constructor has wrong kind")
515 else if not (n_args_to_keep >= 0) then
516 ptext SLIT("the representation type has wrong kind")
517 else if not eta_ok then
518 ptext SLIT("the eta-reduction property does not hold")
522 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
523 (vcat [non_std_why clas,
524 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
526 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
528 std_class gla_exts clas
529 = key `elem` derivableClassKeys
530 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
534 std_class_via_iso clas -- These standard classes can be derived for a newtype
535 -- using the isomorphism trick *even if no -fglasgow-exts*
536 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
537 -- Not Read/Show because they respect the type
538 -- Not Enum, becuase newtypes are never in Enum
541 new_dfun_name clas tycon -- Just a simple wrapper
542 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
543 -- The type passed to newDFunName is only used to generate
544 -- a suitable string; hence the empty type arg list
546 ------------------------------------------------------------------
547 mkDataTypeEqn :: TyCon -> Class -> TcM DerivEqn
548 mkDataTypeEqn tycon clas
549 | clas `hasKey` typeableClassKey
550 = -- The Typeable class is special in several ways
551 -- data T a b = ... deriving( Typeable )
553 -- instance Typeable2 T where ...
555 -- 1. There are no constraints in the instance
556 -- 2. There are no type variables either
557 -- 3. The actual class we want to generate isn't necessarily
558 -- Typeable; it depends on the arity of the type
559 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
560 ; dfun_name <- new_dfun_name real_clas tycon
561 ; return (dfun_name, real_clas, tycon, [], []) }
564 = do { dfun_name <- new_dfun_name clas tycon
565 ; return (dfun_name, clas, tycon, tyvars, constraints) }
567 tyvars = tyConTyVars tycon
568 constraints = extra_constraints ++ ordinary_constraints
569 extra_constraints = tyConStupidTheta tycon
570 -- "extra_constraints": see note [Data decl contexts] above
573 = [ mkClassPred clas [arg_ty]
574 | data_con <- tyConDataCons tycon,
575 arg_ty <- dataConInstOrigArgTys data_con (map mkTyVarTy (tyConTyVars tycon)),
576 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
580 ------------------------------------------------------------------
581 -- Check side conditions that dis-allow derivability for particular classes
582 -- This is *apart* from the newtype-deriving mechanism
584 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
585 checkSideConditions gla_exts tycon deriv_tvs clas tys
586 | notNull deriv_tvs || notNull tys
587 = Just ty_args_why -- e.g. deriving( Foo s )
589 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
590 [] -> Just (non_std_why clas)
591 [cond] -> cond (gla_exts, tycon)
592 other -> pprPanic "checkSideConditions" (ppr clas)
594 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
596 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
598 sideConditions :: [(Unique, Condition)]
600 = [ (eqClassKey, cond_std),
601 (ordClassKey, cond_std),
602 (readClassKey, cond_std),
603 (showClassKey, cond_std),
604 (enumClassKey, cond_std `andCond` cond_isEnumeration),
605 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
606 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
607 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
608 (dataClassKey, cond_glaExts `andCond` cond_std)
611 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
613 orCond :: Condition -> Condition -> Condition
616 Nothing -> Nothing -- c1 succeeds
617 Just x -> case c2 tc of -- c1 fails
619 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
622 andCond c1 c2 tc = case c1 tc of
623 Nothing -> c2 tc -- c1 succeeds
624 Just x -> Just x -- c1 fails
626 cond_std :: Condition
627 cond_std (gla_exts, tycon)
628 | any (not . isVanillaDataCon) data_cons = Just existential_why
629 | null data_cons = Just no_cons_why
630 | otherwise = Nothing
632 data_cons = tyConDataCons tycon
633 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
634 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
636 cond_isEnumeration :: Condition
637 cond_isEnumeration (gla_exts, tycon)
638 | isEnumerationTyCon tycon = Nothing
639 | otherwise = Just why
641 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
643 cond_isProduct :: Condition
644 cond_isProduct (gla_exts, tycon)
645 | isProductTyCon tycon = Nothing
646 | otherwise = Just why
648 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
650 cond_typeableOK :: Condition
651 -- OK for Typeable class
652 -- Currently: (a) args all of kind *
653 -- (b) 7 or fewer args
654 cond_typeableOK (gla_exts, tycon)
655 | tyConArity tycon > 7 = Just too_many
656 | not (all (isArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
657 | otherwise = Nothing
659 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
660 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
662 cond_glaExts :: Condition
663 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
664 | otherwise = Just why
666 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
669 %************************************************************************
671 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
673 %************************************************************************
675 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
676 terms, which is the final correct RHS for the corresponding original
680 Each (k,TyVarTy tv) in a solution constrains only a type
684 The (k,TyVarTy tv) pairs in a solution are canonically
685 ordered by sorting on type varible, tv, (major key) and then class, k,
690 solveDerivEqns :: OverlapFlag
692 -> TcM [Instance]-- Solns in same order as eqns.
693 -- This bunch is Absolutely minimal...
695 solveDerivEqns overlap_flag orig_eqns
696 = iterateDeriv 1 initial_solutions
698 -- The initial solutions for the equations claim that each
699 -- instance has an empty context; this solution is certainly
700 -- in canonical form.
701 initial_solutions :: [DerivSoln]
702 initial_solutions = [ [] | _ <- orig_eqns ]
704 ------------------------------------------------------------------
705 -- iterateDeriv calculates the next batch of solutions,
706 -- compares it with the current one; finishes if they are the
707 -- same, otherwise recurses with the new solutions.
708 -- It fails if any iteration fails
709 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
710 iterateDeriv n current_solns
711 | n > 20 -- Looks as if we are in an infinite loop
712 -- This can happen if we have -fallow-undecidable-instances
713 -- (See TcSimplify.tcSimplifyDeriv.)
714 = pprPanic "solveDerivEqns: probable loop"
715 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
718 inst_specs = zipWithEqual "add_solns" mk_inst_spec
719 orig_eqns current_solns
722 -- Extend the inst info from the explicit instance decls
723 -- with the current set of solutions, and simplify each RHS
724 extendLocalInstEnv inst_specs $
725 mappM gen_soln orig_eqns
726 ) `thenM` \ new_solns ->
727 if (current_solns == new_solns) then
730 iterateDeriv (n+1) new_solns
732 ------------------------------------------------------------------
733 gen_soln (_, clas, tc,tyvars,deriv_rhs)
734 = setSrcSpan (srcLocSpan (getSrcLoc tc)) $
735 do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
736 ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
737 tcSimplifyDeriv tc tyvars deriv_rhs
738 ; addErrCtxt (derivInstCtxt2 theta clas inst_tys) $
739 checkValidInstance tyvars theta clas inst_tys
740 ; return (sortLe (<=) theta) } -- Canonicalise before returning the soluction
744 ------------------------------------------------------------------
745 mk_inst_spec (dfun_name, clas, tycon, tyvars, _) theta
746 = mkLocalInstance dfun overlap_flag
748 dfun = mkDictFunId dfun_name tyvars theta clas
749 [mkTyConApp tycon (mkTyVarTys tyvars)]
751 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
752 -- Add new locally-defined instances; don't bother to check
753 -- for functional dependency errors -- that'll happen in TcInstDcls
754 extendLocalInstEnv dfuns thing_inside
755 = do { env <- getGblEnv
756 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
757 env' = env { tcg_inst_env = inst_env' }
758 ; setGblEnv env' thing_inside }
761 %************************************************************************
763 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
765 %************************************************************************
767 After all the trouble to figure out the required context for the
768 derived instance declarations, all that's left is to chug along to
769 produce them. They will then be shoved into @tcInstDecls2@, which
770 will do all its usual business.
772 There are lots of possibilities for code to generate. Here are
773 various general remarks.
778 We want derived instances of @Eq@ and @Ord@ (both v common) to be
779 ``you-couldn't-do-better-by-hand'' efficient.
782 Deriving @Show@---also pretty common--- should also be reasonable good code.
785 Deriving for the other classes isn't that common or that big a deal.
792 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
795 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
798 We {\em normally} generate code only for the non-defaulted methods;
799 there are some exceptions for @Eq@ and (especially) @Ord@...
802 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
803 constructor's numeric (@Int#@) tag. These are generated by
804 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
805 these is around is given by @hasCon2TagFun@.
807 The examples under the different sections below will make this
811 Much less often (really just for deriving @Ix@), we use a
812 @_tag2con_<tycon>@ function. See the examples.
815 We use the renamer!!! Reason: we're supposed to be
816 producing @LHsBinds Name@ for the methods, but that means
817 producing correctly-uniquified code on the fly. This is entirely
818 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
819 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
820 the renamer. What a great hack!
824 -- Generate the InstInfo for the required instance,
825 -- plus any auxiliary bindings required
826 genInst :: Instance -> TcM (InstInfo, LHsBinds RdrName)
828 = do { fix_env <- getFixityEnv
830 (tyvars,_,clas,[ty]) = instanceHead spec
831 clas_nm = className clas
832 tycon = tcTyConAppTyCon ty
833 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
835 -- Bring the right type variables into
836 -- scope, and rename the method binds
837 -- It's a bit yukky that we return *renamed* InstInfo, but
838 -- *non-renamed* auxiliary bindings
839 ; (rn_meth_binds, _fvs) <- discardWarnings $
840 bindLocalNames (map varName tyvars) $
841 rnMethodBinds clas_nm (\n -> []) [] meth_binds
843 -- Build the InstInfo
844 ; return (InstInfo { iSpec = spec,
845 iBinds = VanillaInst rn_meth_binds [] },
849 genDerivBinds clas fix_env tycon
850 | className clas `elem` typeableClassNames
851 = (gen_Typeable_binds tycon, emptyLHsBinds)
854 = case assocMaybe gen_list (getUnique clas) of
855 Just gen_fn -> gen_fn fix_env tycon
856 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
858 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
859 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
860 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
861 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
862 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
863 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
864 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
865 ,(showClassKey, no_aux_binds gen_Show_binds)
866 ,(readClassKey, no_aux_binds gen_Read_binds)
867 ,(dataClassKey, gen_Data_binds)
870 -- no_aux_binds is used for generators that don't
871 -- need to produce any auxiliary bindings
872 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
873 ignore_fix_env f fix_env tc = f tc
877 %************************************************************************
879 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
881 %************************************************************************
886 con2tag_Foo :: Foo ... -> Int#
887 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
888 maxtag_Foo :: Int -- ditto (NB: not unlifted)
891 We have a @con2tag@ function for a tycon if:
894 We're deriving @Eq@ and the tycon has nullary data constructors.
897 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
901 We have a @tag2con@ function for a tycon if:
904 We're deriving @Enum@, or @Ix@ (enum type only???)
907 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
910 genTaggeryBinds :: [InstInfo] -> TcM (LHsBinds RdrName)
911 genTaggeryBinds infos
912 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
913 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
914 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
916 all_CTs = [ (cls, tcTyConAppTyCon ty)
918 let (cls,ty) = simpleInstInfoClsTy info ]
919 all_tycons = map snd all_CTs
920 (tycons_of_interest, _) = removeDups compare all_tycons
922 do_con2tag acc_Names tycon
923 | isDataTyCon tycon &&
924 ((we_are_deriving eqClassKey tycon
925 && any isNullarySrcDataCon (tyConDataCons tycon))
926 || (we_are_deriving ordClassKey tycon
927 && not (isProductTyCon tycon))
928 || (we_are_deriving enumClassKey tycon)
929 || (we_are_deriving ixClassKey tycon))
931 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
936 do_tag2con acc_Names tycon
937 | isDataTyCon tycon &&
938 (we_are_deriving enumClassKey tycon ||
939 we_are_deriving ixClassKey tycon
940 && isEnumerationTyCon tycon)
941 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
942 : (maxtag_RDR tycon, tycon, GenMaxTag)
947 we_are_deriving clas_key tycon
948 = is_in_eqns clas_key tycon all_CTs
950 is_in_eqns clas_key tycon [] = False
951 is_in_eqns clas_key tycon ((c,t):cts)
952 = (clas_key == classKey c && tycon == t)
953 || is_in_eqns clas_key tycon cts
957 derivingThingErr clas tys tycon tyvars why
958 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
961 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
963 derivCtxt :: TyCon -> SDoc
965 = ptext SLIT("When deriving instances for") <+> quotes (ppr tycon)
967 derivInstCtxt1 clas inst_tys
968 = ptext SLIT("When deriving the instance for") <+> quotes (pprClassPred clas inst_tys)
970 derivInstCtxt2 theta clas inst_tys
971 = vcat [ptext SLIT("In the derived instance declaration"),
972 nest 2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta,
973 pprClassPred clas inst_tys])]