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 Type ( splitKindFunTys )
43 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
44 tyConStupidTheta, isProductTyCon, isDataTyCon, newTyConRhs,
45 isEnumerationTyCon, isRecursiveTyCon, TyCon, isNewTyCon
47 import TcType ( TcType, ThetaType, mkTyVarTys, mkTyConApp, tcTyConAppTyCon,
48 isUnLiftedType, mkClassPred, tyVarsOfType,
49 isSubArgTypeKind, 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/
316 Note [Newtype deriving superclasses]
317 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
319 The 'tys' here come from the partial application
320 in the deriving clause. The last arg is the new
323 We must pass the superclasses; the newtype might be an instance
324 of them in a different way than the representation type
325 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
326 Then the Show instance is not done via isomorphism; it shows
328 The Num instance is derived via isomorphism, but the Show superclass
329 dictionary must the Show instance for Foo, *not* the Show dictionary
330 gotten from the Num dictionary. So we must build a whole new dictionary
331 not just use the Num one. The instance we want is something like:
332 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
335 There may be a coercion needed which we get from the tycon for the newtype
336 when the dict is constructed in TcInstDcls.tcInstDecl2
340 makeDerivEqns :: OverlapFlag
342 -> TcM ([DerivEqn], -- Ordinary derivings
343 [InstInfo]) -- Special newtype derivings
345 makeDerivEqns overlap_flag tycl_decls
346 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
347 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
349 ------------------------------------------------------------------
350 derive_these :: [(NewOrData, Name, LHsType Name)]
351 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
352 derive_these = [ (nd, tycon, pred)
353 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
354 tcdDerivs = Just preds }) <- tycl_decls,
357 ------------------------------------------------------------------
358 mk_eqn :: (NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
359 -- We swizzle the tyvars and datacons out of the tycon
360 -- to make the rest of the equation
362 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
363 -- we allow deriving (forall a. C [a]).
365 mk_eqn (new_or_data, tycon_name, hs_deriv_ty)
366 = tcLookupTyCon tycon_name `thenM` \ tycon ->
367 setSrcSpan (srcLocSpan (getSrcLoc tycon)) $
368 addErrCtxt (derivCtxt tycon) $
369 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
370 -- the type variables for the type constructor
371 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
372 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
373 mk_eqn_help gla_exts new_or_data tycon deriv_tvs clas tys
375 ------------------------------------------------------------------
376 -- data/newtype T a = ... deriving( C t1 t2 )
377 -- leads to a call to mk_eqn_help with
378 -- tycon = T, deriv_tvs = ftv(t1,t2), clas = C, tys = [t1,t2]
380 mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys
381 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
382 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
384 = do { eqn <- mkDataTypeEqn tycon clas
385 ; returnM (Just eqn, Nothing) }
387 mk_eqn_help gla_exts NewType tycon deriv_tvs clas tys
388 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
389 = -- Go ahead and use the isomorphism
390 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
391 new_dfun_name clas tycon `thenM` \ dfun_name ->
392 returnM (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
393 iBinds = NewTypeDerived tycon rep_tys }))
394 | std_class gla_exts clas
395 = mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
397 | otherwise -- Non-standard instance
398 = bale_out (if gla_exts then
399 cant_derive_err -- Too hard
401 non_std_err) -- Just complain about being a non-std instance
403 -- Here is the plan for newtype derivings. We see
404 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
405 -- where t is a type,
406 -- ak...an is a suffix of a1..an
407 -- ak...an do not occur free in t,
408 -- (C s1 ... sm) is a *partial applications* of class C
409 -- with the last parameter missing
411 -- We generate the instances
412 -- instance C s1 .. sm (t ak...ap) => C s1 .. sm (T a1...ap)
413 -- where T a1...ap is the partial application of the LHS of the correct kind
416 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
417 -- instance Monad (ST s) => Monad (T s) where
418 -- fail = coerce ... (fail @ ST s)
419 -- (Actually we don't need the coerce, because non-rec newtypes are transparent
421 clas_tyvars = classTyVars clas
422 kind = tyVarKind (last clas_tyvars)
423 -- Kind of the thing we want to instance
424 -- e.g. argument kind of Monad, *->*
426 (arg_kinds, _) = splitKindFunTys kind
427 n_args_to_drop = length arg_kinds
428 -- Want to drop 1 arg from (T s a) and (ST s a)
429 -- to get instance Monad (ST s) => Monad (T s)
431 -- Note [newtype representation]
432 -- Need newTyConRhs *not* newTyConRep to get the representation
433 -- type, because the latter looks through all intermediate newtypes
435 -- newtype B = MkB Int
436 -- newtype A = MkA B deriving( Num )
437 -- We want the Num instance of B, *not* the Num instance of Int,
438 -- when making the Num instance of A!
439 (tc_tvs, rep_ty) = newTyConRhs tycon
440 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
442 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
443 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
444 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
446 n_args_to_keep = length rep_ty_args - n_args_to_drop
447 args_to_drop = drop n_args_to_keep rep_ty_args
448 args_to_keep = take n_args_to_keep rep_ty_args
450 rep_fn' = mkAppTys rep_fn args_to_keep
451 rep_tys = tys ++ [rep_fn']
452 rep_pred = mkClassPred clas rep_tys
453 -- rep_pred is the representation dictionary, from where
454 -- we are gong to get all the methods for the newtype dictionary
455 -- here we are figuring out what superclass dictionaries to use
456 -- see Note [Newtype deriving superclasses] above
458 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
461 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
464 -- If there are no tyvars, there's no need
465 -- to abstract over the dictionaries we need
466 dict_tvs = deriv_tvs ++ tc_tvs
467 dict_args -- | null dict_tvs = []
468 | otherwise = rep_pred : sc_theta
470 -- Finally! Here's where we build the dictionary Id
471 mk_inst_spec dfun_name
472 = mkLocalInstance dfun overlap_flag
474 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
476 -------------------------------------------------------------------
477 -- Figuring out whether we can only do this newtype-deriving thing
479 right_arity = length tys + 1 == classArity clas
481 -- Never derive Read,Show,Typeable,Data this way
482 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
483 can_derive_via_isomorphism
484 = not (getUnique clas `elem` non_iso_classes)
485 && right_arity -- Well kinded;
486 -- eg not: newtype T ... deriving( ST )
487 -- because ST needs *2* type params
488 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
489 -- eg not: newtype T = T Int deriving( Monad )
490 && n_args_to_keep >= 0 -- Rep type has right kind:
491 -- eg not: newtype T a = T Int deriving( Monad )
492 && eta_ok -- Eta reduction works
493 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
494 -- newtype A = MkA [A]
496 -- instance Eq [A] => Eq A !!
497 -- Here's a recursive newtype that's actually OK
498 -- newtype S1 = S1 [T1 ()]
499 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
500 -- It's currently rejected. Oh well.
501 -- In fact we generate an instance decl that has method of form
502 -- meth @ instTy = meth @ repTy
503 -- (no coerce's). We'd need a coerce if we wanted to handle
504 -- recursive newtypes too
506 -- Check that eta reduction is OK
507 -- (a) the dropped-off args are identical
508 -- (b) the remaining type args mention
509 -- only the remaining type variables
510 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
511 && (tyVarsOfType rep_fn' `subVarSet` mkVarSet tyvars_to_keep)
513 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
514 (vcat [ptext SLIT("even with cunning newtype deriving:"),
515 if isRecursiveTyCon tycon then
516 ptext SLIT("the newtype is recursive")
518 if not right_arity then
519 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
521 if not (n_tyvars_to_keep >= 0) then
522 ptext SLIT("the type constructor has wrong kind")
523 else if not (n_args_to_keep >= 0) then
524 ptext SLIT("the representation type has wrong kind")
525 else if not eta_ok then
526 ptext SLIT("the eta-reduction property does not hold")
530 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
531 (vcat [non_std_why clas,
532 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
534 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
536 std_class gla_exts clas
537 = key `elem` derivableClassKeys
538 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
542 std_class_via_iso clas -- These standard classes can be derived for a newtype
543 -- using the isomorphism trick *even if no -fglasgow-exts*
544 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
545 -- Not Read/Show because they respect the type
546 -- Not Enum, becuase newtypes are never in Enum
549 new_dfun_name clas tycon -- Just a simple wrapper
550 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
551 -- The type passed to newDFunName is only used to generate
552 -- a suitable string; hence the empty type arg list
554 ------------------------------------------------------------------
555 mkDataTypeEqn :: TyCon -> Class -> TcM DerivEqn
556 mkDataTypeEqn tycon clas
557 | clas `hasKey` typeableClassKey
558 = -- The Typeable class is special in several ways
559 -- data T a b = ... deriving( Typeable )
561 -- instance Typeable2 T where ...
563 -- 1. There are no constraints in the instance
564 -- 2. There are no type variables either
565 -- 3. The actual class we want to generate isn't necessarily
566 -- Typeable; it depends on the arity of the type
567 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
568 ; dfun_name <- new_dfun_name real_clas tycon
569 ; return (dfun_name, real_clas, tycon, [], []) }
572 = do { dfun_name <- new_dfun_name clas tycon
573 ; return (dfun_name, clas, tycon, tyvars, constraints) }
575 tyvars = tyConTyVars tycon
576 constraints = extra_constraints ++ ordinary_constraints
577 extra_constraints = tyConStupidTheta tycon
578 -- "extra_constraints": see note [Data decl contexts] above
581 = [ mkClassPred clas [arg_ty]
582 | data_con <- tyConDataCons tycon,
583 arg_ty <- dataConInstOrigArgTys data_con (map mkTyVarTy (tyConTyVars tycon)),
584 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
588 ------------------------------------------------------------------
589 -- Check side conditions that dis-allow derivability for particular classes
590 -- This is *apart* from the newtype-deriving mechanism
592 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
593 checkSideConditions gla_exts tycon deriv_tvs clas tys
594 | notNull deriv_tvs || notNull tys
595 = Just ty_args_why -- e.g. deriving( Foo s )
597 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
598 [] -> Just (non_std_why clas)
599 [cond] -> cond (gla_exts, tycon)
600 other -> pprPanic "checkSideConditions" (ppr clas)
602 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
604 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
606 sideConditions :: [(Unique, Condition)]
608 = [ (eqClassKey, cond_std),
609 (ordClassKey, cond_std),
610 (readClassKey, cond_std),
611 (showClassKey, cond_std),
612 (enumClassKey, cond_std `andCond` cond_isEnumeration),
613 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
614 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
615 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
616 (dataClassKey, cond_glaExts `andCond` cond_std)
619 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
621 orCond :: Condition -> Condition -> Condition
624 Nothing -> Nothing -- c1 succeeds
625 Just x -> case c2 tc of -- c1 fails
627 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
630 andCond c1 c2 tc = case c1 tc of
631 Nothing -> c2 tc -- c1 succeeds
632 Just x -> Just x -- c1 fails
634 cond_std :: Condition
635 cond_std (gla_exts, tycon)
636 | any (not . isVanillaDataCon) data_cons = Just existential_why
637 | null data_cons = Just no_cons_why
638 | otherwise = Nothing
640 data_cons = tyConDataCons tycon
641 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
642 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
644 cond_isEnumeration :: Condition
645 cond_isEnumeration (gla_exts, tycon)
646 | isEnumerationTyCon tycon = Nothing
647 | otherwise = Just why
649 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
651 cond_isProduct :: Condition
652 cond_isProduct (gla_exts, tycon)
653 | isProductTyCon tycon = Nothing
654 | otherwise = Just why
656 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
658 cond_typeableOK :: Condition
659 -- OK for Typeable class
660 -- Currently: (a) args all of kind *
661 -- (b) 7 or fewer args
662 cond_typeableOK (gla_exts, tycon)
663 | tyConArity tycon > 7 = Just too_many
664 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
665 | otherwise = Nothing
667 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
668 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
670 cond_glaExts :: Condition
671 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
672 | otherwise = Just why
674 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
677 %************************************************************************
679 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
681 %************************************************************************
683 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
684 terms, which is the final correct RHS for the corresponding original
688 Each (k,TyVarTy tv) in a solution constrains only a type
692 The (k,TyVarTy tv) pairs in a solution are canonically
693 ordered by sorting on type varible, tv, (major key) and then class, k,
698 solveDerivEqns :: OverlapFlag
700 -> TcM [Instance]-- Solns in same order as eqns.
701 -- This bunch is Absolutely minimal...
703 solveDerivEqns overlap_flag orig_eqns
704 = iterateDeriv 1 initial_solutions
706 -- The initial solutions for the equations claim that each
707 -- instance has an empty context; this solution is certainly
708 -- in canonical form.
709 initial_solutions :: [DerivSoln]
710 initial_solutions = [ [] | _ <- orig_eqns ]
712 ------------------------------------------------------------------
713 -- iterateDeriv calculates the next batch of solutions,
714 -- compares it with the current one; finishes if they are the
715 -- same, otherwise recurses with the new solutions.
716 -- It fails if any iteration fails
717 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
718 iterateDeriv n current_solns
719 | n > 20 -- Looks as if we are in an infinite loop
720 -- This can happen if we have -fallow-undecidable-instances
721 -- (See TcSimplify.tcSimplifyDeriv.)
722 = pprPanic "solveDerivEqns: probable loop"
723 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
726 inst_specs = zipWithEqual "add_solns" mk_inst_spec
727 orig_eqns current_solns
730 -- Extend the inst info from the explicit instance decls
731 -- with the current set of solutions, and simplify each RHS
732 extendLocalInstEnv inst_specs $
733 mappM gen_soln orig_eqns
734 ) `thenM` \ new_solns ->
735 if (current_solns == new_solns) then
738 iterateDeriv (n+1) new_solns
740 ------------------------------------------------------------------
741 gen_soln (_, clas, tc,tyvars,deriv_rhs)
742 = setSrcSpan (srcLocSpan (getSrcLoc tc)) $
743 do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
744 ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
745 tcSimplifyDeriv tc tyvars deriv_rhs
746 ; addErrCtxt (derivInstCtxt2 theta clas inst_tys) $
747 checkValidInstance tyvars theta clas inst_tys
748 ; return (sortLe (<=) theta) } -- Canonicalise before returning the soluction
752 ------------------------------------------------------------------
753 mk_inst_spec (dfun_name, clas, tycon, tyvars, _) theta
754 = mkLocalInstance dfun overlap_flag
756 dfun = mkDictFunId dfun_name tyvars theta clas
757 [mkTyConApp tycon (mkTyVarTys tyvars)]
759 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
760 -- Add new locally-defined instances; don't bother to check
761 -- for functional dependency errors -- that'll happen in TcInstDcls
762 extendLocalInstEnv dfuns thing_inside
763 = do { env <- getGblEnv
764 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
765 env' = env { tcg_inst_env = inst_env' }
766 ; setGblEnv env' thing_inside }
769 %************************************************************************
771 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
773 %************************************************************************
775 After all the trouble to figure out the required context for the
776 derived instance declarations, all that's left is to chug along to
777 produce them. They will then be shoved into @tcInstDecls2@, which
778 will do all its usual business.
780 There are lots of possibilities for code to generate. Here are
781 various general remarks.
786 We want derived instances of @Eq@ and @Ord@ (both v common) to be
787 ``you-couldn't-do-better-by-hand'' efficient.
790 Deriving @Show@---also pretty common--- should also be reasonable good code.
793 Deriving for the other classes isn't that common or that big a deal.
800 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
803 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
806 We {\em normally} generate code only for the non-defaulted methods;
807 there are some exceptions for @Eq@ and (especially) @Ord@...
810 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
811 constructor's numeric (@Int#@) tag. These are generated by
812 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
813 these is around is given by @hasCon2TagFun@.
815 The examples under the different sections below will make this
819 Much less often (really just for deriving @Ix@), we use a
820 @_tag2con_<tycon>@ function. See the examples.
823 We use the renamer!!! Reason: we're supposed to be
824 producing @LHsBinds Name@ for the methods, but that means
825 producing correctly-uniquified code on the fly. This is entirely
826 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
827 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
828 the renamer. What a great hack!
832 -- Generate the InstInfo for the required instance,
833 -- plus any auxiliary bindings required
834 genInst :: Instance -> TcM (InstInfo, LHsBinds RdrName)
836 = do { fix_env <- getFixityEnv
838 (tyvars,_,clas,[ty]) = instanceHead spec
839 clas_nm = className clas
840 tycon = tcTyConAppTyCon ty
841 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
843 -- Bring the right type variables into
844 -- scope, and rename the method binds
845 -- It's a bit yukky that we return *renamed* InstInfo, but
846 -- *non-renamed* auxiliary bindings
847 ; (rn_meth_binds, _fvs) <- discardWarnings $
848 bindLocalNames (map varName tyvars) $
849 rnMethodBinds clas_nm (\n -> []) [] meth_binds
851 -- Build the InstInfo
852 ; return (InstInfo { iSpec = spec,
853 iBinds = VanillaInst rn_meth_binds [] },
857 genDerivBinds clas fix_env tycon
858 | className clas `elem` typeableClassNames
859 = (gen_Typeable_binds tycon, emptyLHsBinds)
862 = case assocMaybe gen_list (getUnique clas) of
863 Just gen_fn -> gen_fn fix_env tycon
864 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
866 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
867 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
868 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
869 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
870 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
871 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
872 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
873 ,(showClassKey, no_aux_binds gen_Show_binds)
874 ,(readClassKey, no_aux_binds gen_Read_binds)
875 ,(dataClassKey, gen_Data_binds)
878 -- no_aux_binds is used for generators that don't
879 -- need to produce any auxiliary bindings
880 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
881 ignore_fix_env f fix_env tc = f tc
885 %************************************************************************
887 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
889 %************************************************************************
894 con2tag_Foo :: Foo ... -> Int#
895 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
896 maxtag_Foo :: Int -- ditto (NB: not unlifted)
899 We have a @con2tag@ function for a tycon if:
902 We're deriving @Eq@ and the tycon has nullary data constructors.
905 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
909 We have a @tag2con@ function for a tycon if:
912 We're deriving @Enum@, or @Ix@ (enum type only???)
915 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
918 genTaggeryBinds :: [InstInfo] -> TcM (LHsBinds RdrName)
919 genTaggeryBinds infos
920 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
921 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
922 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
924 all_CTs = [ (cls, tcTyConAppTyCon ty)
926 let (cls,ty) = simpleInstInfoClsTy info ]
927 all_tycons = map snd all_CTs
928 (tycons_of_interest, _) = removeDups compare all_tycons
930 do_con2tag acc_Names tycon
931 | isDataTyCon tycon &&
932 ((we_are_deriving eqClassKey tycon
933 && any isNullarySrcDataCon (tyConDataCons tycon))
934 || (we_are_deriving ordClassKey tycon
935 && not (isProductTyCon tycon))
936 || (we_are_deriving enumClassKey tycon)
937 || (we_are_deriving ixClassKey tycon))
939 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
944 do_tag2con acc_Names tycon
945 | isDataTyCon tycon &&
946 (we_are_deriving enumClassKey tycon ||
947 we_are_deriving ixClassKey tycon
948 && isEnumerationTyCon tycon)
949 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
950 : (maxtag_RDR tycon, tycon, GenMaxTag)
955 we_are_deriving clas_key tycon
956 = is_in_eqns clas_key tycon all_CTs
958 is_in_eqns clas_key tycon [] = False
959 is_in_eqns clas_key tycon ((c,t):cts)
960 = (clas_key == classKey c && tycon == t)
961 || is_in_eqns clas_key tycon cts
965 derivingThingErr clas tys tycon tyvars why
966 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
969 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
971 derivCtxt :: TyCon -> SDoc
973 = ptext SLIT("When deriving instances for") <+> quotes (ppr tycon)
975 derivInstCtxt1 clas inst_tys
976 = ptext SLIT("When deriving the instance for") <+> quotes (pprClassPred clas inst_tys)
978 derivInstCtxt2 theta clas inst_tys
979 = vcat [ptext SLIT("In the derived instance declaration"),
980 nest 2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta,
981 pprClassPred clas inst_tys])]