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,
48 import TcType ( TcType, ThetaType, mkTyVarTys, mkTyConApp, tcTyConAppTyCon,
49 isUnLiftedType, mkClassPred, tyVarsOfType,
50 isSubArgTypeKind, tcEqTypes, tcSplitAppTys, mkAppTys )
51 import Var ( TyVar, tyVarKind, varName )
52 import VarSet ( mkVarSet, subVarSet )
54 import SrcLoc ( srcLocSpan, Located(..) )
55 import Util ( zipWithEqual, sortLe, notNull )
56 import ListSetOps ( removeDups, assocMaybe )
61 %************************************************************************
63 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
65 %************************************************************************
69 data T a b = C1 (Foo a) (Bar b)
74 [NOTE: See end of these comments for what to do with
75 data (C a, D b) => T a b = ...
78 We want to come up with an instance declaration of the form
80 instance (Ping a, Pong b, ...) => Eq (T a b) where
83 It is pretty easy, albeit tedious, to fill in the code "...". The
84 trick is to figure out what the context for the instance decl is,
85 namely @Ping@, @Pong@ and friends.
87 Let's call the context reqd for the T instance of class C at types
88 (a,b, ...) C (T a b). Thus:
90 Eq (T a b) = (Ping a, Pong b, ...)
92 Now we can get a (recursive) equation from the @data@ decl:
94 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
95 u Eq (T b a) u Eq Int -- From C2
96 u Eq (T a a) -- From C3
98 Foo and Bar may have explicit instances for @Eq@, in which case we can
99 just substitute for them. Alternatively, either or both may have
100 their @Eq@ instances given by @deriving@ clauses, in which case they
101 form part of the system of equations.
103 Now all we need do is simplify and solve the equations, iterating to
104 find the least fixpoint. Notice that the order of the arguments can
105 switch around, as here in the recursive calls to T.
107 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
111 Eq (T a b) = {} -- The empty set
114 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
115 u Eq (T b a) u Eq Int -- From C2
116 u Eq (T a a) -- From C3
118 After simplification:
119 = Eq a u Ping b u {} u {} u {}
124 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
125 u Eq (T b a) u Eq Int -- From C2
126 u Eq (T a a) -- From C3
128 After simplification:
133 = Eq a u Ping b u Eq b u Ping a
135 The next iteration gives the same result, so this is the fixpoint. We
136 need to make a canonical form of the RHS to ensure convergence. We do
137 this by simplifying the RHS to a form in which
139 - the classes constrain only tyvars
140 - the list is sorted by tyvar (major key) and then class (minor key)
141 - no duplicates, of course
143 So, here are the synonyms for the ``equation'' structures:
146 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
147 -- The Name is the name for the DFun we'll build
148 -- The tyvars bind all the variables in the RHS
150 pprDerivEqn (n,c,tc,tvs,rhs)
151 = parens (hsep [ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
153 type DerivRhs = ThetaType
154 type DerivSoln = DerivRhs
158 [Data decl contexts] A note about contexts on data decls
159 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
162 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
164 We will need an instance decl like:
166 instance (Read a, RealFloat a) => Read (Complex a) where
169 The RealFloat in the context is because the read method for Complex is bound
170 to construct a Complex, and doing that requires that the argument type is
173 But this ain't true for Show, Eq, Ord, etc, since they don't construct
174 a Complex; they only take them apart.
176 Our approach: identify the offending classes, and add the data type
177 context to the instance decl. The "offending classes" are
181 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
182 pattern matching against a constructor from a data type with a context
183 gives rise to the constraints for that context -- or at least the thinned
184 version. So now all classes are "offending".
191 newtype T = T Char deriving( C [a] )
193 Notice the free 'a' in the deriving. We have to fill this out to
194 newtype T = T Char deriving( forall a. C [a] )
196 And then translate it to:
197 instance C [a] Char => C [a] T where ...
202 %************************************************************************
204 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
206 %************************************************************************
209 tcDeriving :: [LTyClDecl Name] -- All type constructors
210 -> TcM ([InstInfo], -- The generated "instance decls"
211 HsValBinds Name) -- Extra generated top-level bindings
213 tcDeriving tycl_decls
214 = recoverM (returnM ([], emptyValBindsOut)) $
215 do { -- Fish the "deriving"-related information out of the TcEnv
216 -- and make the necessary "equations".
217 overlap_flag <- getOverlapFlag
218 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns overlap_flag tycl_decls
220 ; (ordinary_inst_info, deriv_binds)
221 <- extendLocalInstEnv (map iSpec newtype_inst_info) $
222 deriveOrdinaryStuff overlap_flag ordinary_eqns
223 -- Add the newtype-derived instances to the inst env
224 -- before tacking the "ordinary" ones
226 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
228 -- If we are compiling a hs-boot file,
229 -- don't generate any derived bindings
230 ; is_boot <- tcIsHsBoot
232 return (inst_info, emptyValBindsOut)
236 -- Generate the generic to/from functions from each type declaration
237 ; gen_binds <- mkGenericBinds tycl_decls
239 -- Rename these extra bindings, discarding warnings about unused bindings etc
240 -- Set -fglasgow exts so that we can have type signatures in patterns,
241 -- which is used in the generic binds
243 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
244 { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn deriv_binds [])
245 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
246 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
248 ; return (rn_deriv `plusHsValBinds` rn_gen) }
252 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
253 (ddump_deriving inst_info rn_binds))
255 ; returnM (inst_info, rn_binds)
258 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
259 ddump_deriving inst_infos extra_binds
260 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
262 -----------------------------------------
263 deriveOrdinaryStuff overlap_flag [] -- Short cut
264 = returnM ([], emptyLHsBinds)
266 deriveOrdinaryStuff overlap_flag eqns
267 = do { -- Take the equation list and solve it, to deliver a list of
268 -- solutions, a.k.a. the contexts for the instance decls
269 -- required for the corresponding equations.
270 inst_specs <- solveDerivEqns overlap_flag eqns
272 -- Generate the InstInfo for each dfun,
273 -- plus any auxiliary bindings it needs
274 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst inst_specs
276 -- Generate any extra not-one-inst-decl-specific binds,
277 -- notably "con2tag" and/or "tag2con" functions.
278 ; extra_binds <- genTaggeryBinds inst_infos
281 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
284 -----------------------------------------
285 mkGenericBinds tycl_decls
286 = do { tcs <- mapM tcLookupTyCon
288 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
289 -- We are only interested in the data type declarations
290 ; return (unionManyBags [ mkTyConGenericBinds tc |
291 tc <- tcs, tyConHasGenerics tc ]) }
292 -- And then only in the ones whose 'has-generics' flag is on
296 %************************************************************************
298 \subsection[TcDeriv-eqns]{Forming the equations}
300 %************************************************************************
302 @makeDerivEqns@ fishes around to find the info about needed derived
303 instances. Complicating factors:
306 We can only derive @Enum@ if the data type is an enumeration
307 type (all nullary data constructors).
310 We can only derive @Ix@ if the data type is an enumeration {\em
311 or} has just one data constructor (e.g., tuples).
314 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
317 Note [Newtype deriving superclasses]
318 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
320 The 'tys' here come from the partial application
321 in the deriving clause. The last arg is the new
324 We must pass the superclasses; the newtype might be an instance
325 of them in a different way than the representation type
326 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
327 Then the Show instance is not done via isomorphism; it shows
329 The Num instance is derived via isomorphism, but the Show superclass
330 dictionary must the Show instance for Foo, *not* the Show dictionary
331 gotten from the Num dictionary. So we must build a whole new dictionary
332 not just use the Num one. The instance we want is something like:
333 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
336 There may be a coercion needed which we get from the tycon for the newtype
337 when the dict is constructed in TcInstDcls.tcInstDecl2
341 makeDerivEqns :: OverlapFlag
343 -> TcM ([DerivEqn], -- Ordinary derivings
344 [InstInfo]) -- Special newtype derivings
346 makeDerivEqns overlap_flag tycl_decls
347 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
348 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
350 ------------------------------------------------------------------
351 derive_these :: [(NewOrData, Name, LHsType Name)]
352 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
353 derive_these = [ (nd, tycon, pred)
354 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
355 tcdDerivs = Just preds }) <- tycl_decls,
358 ------------------------------------------------------------------
359 mk_eqn :: (NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
360 -- We swizzle the tyvars and datacons out of the tycon
361 -- to make the rest of the equation
363 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
364 -- we allow deriving (forall a. C [a]).
366 mk_eqn (new_or_data, tycon_name, hs_deriv_ty)
367 = tcLookupTyCon tycon_name `thenM` \ tycon ->
368 setSrcSpan (srcLocSpan (getSrcLoc tycon)) $
369 addErrCtxt (derivCtxt tycon) $
370 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
371 -- the type variables for the type constructor
372 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
373 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
374 mk_eqn_help gla_exts new_or_data tycon deriv_tvs clas tys
376 ------------------------------------------------------------------
377 -- data/newtype T a = ... deriving( C t1 t2 )
378 -- leads to a call to mk_eqn_help with
379 -- tycon = T, deriv_tvs = ftv(t1,t2), clas = C, tys = [t1,t2]
381 mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys
382 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
383 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
385 = do { eqn <- mkDataTypeEqn tycon clas
386 ; returnM (Just eqn, Nothing) }
388 mk_eqn_help gla_exts NewType tycon deriv_tvs clas tys
389 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
390 = -- Go ahead and use the isomorphism
391 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
392 new_dfun_name clas tycon `thenM` \ dfun_name ->
393 returnM (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
394 iBinds = NewTypeDerived tycon rep_tys }))
395 | std_class gla_exts clas
396 = mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
398 | otherwise -- Non-standard instance
399 = bale_out (if gla_exts then
400 cant_derive_err -- Too hard
402 non_std_err) -- Just complain about being a non-std instance
404 -- Here is the plan for newtype derivings. We see
405 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
406 -- where t is a type,
407 -- ak...an is a suffix of a1..an
408 -- ak...an do not occur free in t,
409 -- (C s1 ... sm) is a *partial applications* of class C
410 -- with the last parameter missing
412 -- We generate the instances
413 -- instance C s1 .. sm (t ak...ap) => C s1 .. sm (T a1...ap)
414 -- where T a1...ap is the partial application of the LHS of the correct kind
417 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
418 -- instance Monad (ST s) => Monad (T s) where
419 -- fail = coerce ... (fail @ ST s)
420 -- (Actually we don't need the coerce, because non-rec newtypes are transparent
422 clas_tyvars = classTyVars clas
423 kind = tyVarKind (last clas_tyvars)
424 -- Kind of the thing we want to instance
425 -- e.g. argument kind of Monad, *->*
427 (arg_kinds, _) = splitKindFunTys kind
428 n_args_to_drop = length arg_kinds
429 -- Want to drop 1 arg from (T s a) and (ST s a)
430 -- to get instance Monad (ST s) => Monad (T s)
432 -- Note [newtype representation]
433 -- Need newTyConRhs *not* newTyConRep to get the representation
434 -- type, because the latter looks through all intermediate newtypes
436 -- newtype B = MkB Int
437 -- newtype A = MkA B deriving( Num )
438 -- We want the Num instance of B, *not* the Num instance of Int,
439 -- when making the Num instance of A!
440 (tc_tvs, rep_ty) = newTyConRhs tycon
441 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
443 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
444 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
445 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
447 n_args_to_keep = length rep_ty_args - n_args_to_drop
448 args_to_drop = drop n_args_to_keep rep_ty_args
449 args_to_keep = take n_args_to_keep rep_ty_args
451 rep_fn' = mkAppTys rep_fn args_to_keep
452 rep_tys = tys ++ [rep_fn']
453 rep_pred = mkClassPred clas rep_tys
454 -- rep_pred is the representation dictionary, from where
455 -- we are gong to get all the methods for the newtype dictionary
456 -- here we are figuring out what superclass dictionaries to use
457 -- see Note [Newtype deriving superclasses] above
459 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
462 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
465 -- If there are no tyvars, there's no need
466 -- to abstract over the dictionaries we need
467 dict_tvs = deriv_tvs ++ tc_tvs
468 dict_args -- | null dict_tvs = []
469 | otherwise = rep_pred : sc_theta
471 -- Finally! Here's where we build the dictionary Id
472 mk_inst_spec dfun_name
473 = mkLocalInstance dfun overlap_flag
475 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
477 -------------------------------------------------------------------
478 -- Figuring out whether we can only do this newtype-deriving thing
480 right_arity = length tys + 1 == classArity clas
482 -- Never derive Read,Show,Typeable,Data this way
483 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
484 can_derive_via_isomorphism
485 = not (getUnique clas `elem` non_iso_classes)
486 && right_arity -- Well kinded;
487 -- eg not: newtype T ... deriving( ST )
488 -- because ST needs *2* type params
489 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
490 -- eg not: newtype T = T Int deriving( Monad )
491 && n_args_to_keep >= 0 -- Rep type has right kind:
492 -- eg not: newtype T a = T Int deriving( Monad )
493 && eta_ok -- Eta reduction works
494 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
495 -- newtype A = MkA [A]
497 -- instance Eq [A] => Eq A !!
498 -- Here's a recursive newtype that's actually OK
499 -- newtype S1 = S1 [T1 ()]
500 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
501 -- It's currently rejected. Oh well.
502 -- In fact we generate an instance decl that has method of form
503 -- meth @ instTy = meth @ repTy
504 -- (no coerce's). We'd need a coerce if we wanted to handle
505 -- recursive newtypes too
507 -- Check that eta reduction is OK
508 -- (a) the dropped-off args are identical
509 -- (b) the remaining type args mention
510 -- only the remaining type variables
511 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
512 && (tyVarsOfType rep_fn' `subVarSet` mkVarSet tyvars_to_keep)
514 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
515 (vcat [ptext SLIT("even with cunning newtype deriving:"),
516 if isRecursiveTyCon tycon then
517 ptext SLIT("the newtype is recursive")
519 if not right_arity then
520 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
522 if not (n_tyvars_to_keep >= 0) then
523 ptext SLIT("the type constructor has wrong kind")
524 else if not (n_args_to_keep >= 0) then
525 ptext SLIT("the representation type has wrong kind")
526 else if not eta_ok then
527 ptext SLIT("the eta-reduction property does not hold")
531 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
532 (vcat [non_std_why clas,
533 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
535 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
537 std_class gla_exts clas
538 = key `elem` derivableClassKeys
539 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
543 std_class_via_iso clas -- These standard classes can be derived for a newtype
544 -- using the isomorphism trick *even if no -fglasgow-exts*
545 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
546 -- Not Read/Show because they respect the type
547 -- Not Enum, becuase newtypes are never in Enum
550 new_dfun_name clas tycon -- Just a simple wrapper
551 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
552 -- The type passed to newDFunName is only used to generate
553 -- a suitable string; hence the empty type arg list
555 ------------------------------------------------------------------
556 mkDataTypeEqn :: TyCon -> Class -> TcM DerivEqn
557 mkDataTypeEqn tycon clas
558 | clas `hasKey` typeableClassKey
559 = -- The Typeable class is special in several ways
560 -- data T a b = ... deriving( Typeable )
562 -- instance Typeable2 T where ...
564 -- 1. There are no constraints in the instance
565 -- 2. There are no type variables either
566 -- 3. The actual class we want to generate isn't necessarily
567 -- Typeable; it depends on the arity of the type
568 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
569 ; dfun_name <- new_dfun_name real_clas tycon
570 ; return (dfun_name, real_clas, tycon, [], []) }
573 = do { dfun_name <- new_dfun_name clas tycon
574 ; return (dfun_name, clas, tycon, tyvars, constraints) }
576 tyvars = tyConTyVars tycon
577 constraints = extra_constraints ++ ordinary_constraints
578 extra_constraints = tyConStupidTheta tycon
579 -- "extra_constraints": see note [Data decl contexts] above
582 = [ mkClassPred clas [arg_ty]
583 | data_con <- tyConDataCons tycon,
584 arg_ty <- dataConInstOrigArgTys data_con (map mkTyVarTy (tyConTyVars tycon)),
585 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
589 ------------------------------------------------------------------
590 -- Check side conditions that dis-allow derivability for particular classes
591 -- This is *apart* from the newtype-deriving mechanism
593 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
594 checkSideConditions gla_exts tycon deriv_tvs clas tys
595 | notNull deriv_tvs || notNull tys
596 = Just ty_args_why -- e.g. deriving( Foo s )
598 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
599 [] -> Just (non_std_why clas)
600 [cond] -> cond (gla_exts, tycon)
601 other -> pprPanic "checkSideConditions" (ppr clas)
603 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
605 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
607 sideConditions :: [(Unique, Condition)]
609 = [ (eqClassKey, cond_std),
610 (ordClassKey, cond_std),
611 (readClassKey, cond_std),
612 (showClassKey, cond_std),
613 (enumClassKey, cond_std `andCond` cond_isEnumeration),
614 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
615 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
616 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
617 (dataClassKey, cond_glaExts `andCond` cond_std)
620 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
622 orCond :: Condition -> Condition -> Condition
625 Nothing -> Nothing -- c1 succeeds
626 Just x -> case c2 tc of -- c1 fails
628 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
631 andCond c1 c2 tc = case c1 tc of
632 Nothing -> c2 tc -- c1 succeeds
633 Just x -> Just x -- c1 fails
635 cond_std :: Condition
636 cond_std (gla_exts, tycon)
637 | any (not . isVanillaDataCon) data_cons = Just existential_why
638 | null data_cons = Just no_cons_why
639 | otherwise = Nothing
641 data_cons = tyConDataCons tycon
642 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
643 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
645 cond_isEnumeration :: Condition
646 cond_isEnumeration (gla_exts, tycon)
647 | isEnumerationTyCon tycon = Nothing
648 | otherwise = Just why
650 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
652 cond_isProduct :: Condition
653 cond_isProduct (gla_exts, tycon)
654 | isProductTyCon tycon = Nothing
655 | otherwise = Just why
657 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
659 cond_typeableOK :: Condition
660 -- OK for Typeable class
661 -- Currently: (a) args all of kind *
662 -- (b) 7 or fewer args
663 cond_typeableOK (gla_exts, tycon)
664 | tyConArity tycon > 7 = Just too_many
665 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
666 | otherwise = Nothing
668 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
669 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
671 cond_glaExts :: Condition
672 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
673 | otherwise = Just why
675 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
678 %************************************************************************
680 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
682 %************************************************************************
684 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
685 terms, which is the final correct RHS for the corresponding original
689 Each (k,TyVarTy tv) in a solution constrains only a type
693 The (k,TyVarTy tv) pairs in a solution are canonically
694 ordered by sorting on type varible, tv, (major key) and then class, k,
699 solveDerivEqns :: OverlapFlag
701 -> TcM [Instance]-- Solns in same order as eqns.
702 -- This bunch is Absolutely minimal...
704 solveDerivEqns overlap_flag orig_eqns
705 = iterateDeriv 1 initial_solutions
707 -- The initial solutions for the equations claim that each
708 -- instance has an empty context; this solution is certainly
709 -- in canonical form.
710 initial_solutions :: [DerivSoln]
711 initial_solutions = [ [] | _ <- orig_eqns ]
713 ------------------------------------------------------------------
714 -- iterateDeriv calculates the next batch of solutions,
715 -- compares it with the current one; finishes if they are the
716 -- same, otherwise recurses with the new solutions.
717 -- It fails if any iteration fails
718 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
719 iterateDeriv n current_solns
720 | n > 20 -- Looks as if we are in an infinite loop
721 -- This can happen if we have -fallow-undecidable-instances
722 -- (See TcSimplify.tcSimplifyDeriv.)
723 = pprPanic "solveDerivEqns: probable loop"
724 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
727 inst_specs = zipWithEqual "add_solns" mk_inst_spec
728 orig_eqns current_solns
731 -- Extend the inst info from the explicit instance decls
732 -- with the current set of solutions, and simplify each RHS
733 extendLocalInstEnv inst_specs $
734 mappM gen_soln orig_eqns
735 ) `thenM` \ new_solns ->
736 if (current_solns == new_solns) then
739 iterateDeriv (n+1) new_solns
741 ------------------------------------------------------------------
742 gen_soln (_, clas, tc,tyvars,deriv_rhs)
743 = setSrcSpan (srcLocSpan (getSrcLoc tc)) $
744 do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
745 ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
746 tcSimplifyDeriv tc tyvars deriv_rhs
747 ; addErrCtxt (derivInstCtxt2 theta clas inst_tys) $
748 checkValidInstance tyvars theta clas inst_tys
749 ; return (sortLe (<=) theta) } -- Canonicalise before returning the soluction
753 ------------------------------------------------------------------
754 mk_inst_spec (dfun_name, clas, tycon, tyvars, _) theta
755 = mkLocalInstance dfun overlap_flag
757 dfun = mkDictFunId dfun_name tyvars theta clas
758 [mkTyConApp tycon (mkTyVarTys tyvars)]
760 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
761 -- Add new locally-defined instances; don't bother to check
762 -- for functional dependency errors -- that'll happen in TcInstDcls
763 extendLocalInstEnv dfuns thing_inside
764 = do { env <- getGblEnv
765 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
766 env' = env { tcg_inst_env = inst_env' }
767 ; setGblEnv env' thing_inside }
770 %************************************************************************
772 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
774 %************************************************************************
776 After all the trouble to figure out the required context for the
777 derived instance declarations, all that's left is to chug along to
778 produce them. They will then be shoved into @tcInstDecls2@, which
779 will do all its usual business.
781 There are lots of possibilities for code to generate. Here are
782 various general remarks.
787 We want derived instances of @Eq@ and @Ord@ (both v common) to be
788 ``you-couldn't-do-better-by-hand'' efficient.
791 Deriving @Show@---also pretty common--- should also be reasonable good code.
794 Deriving for the other classes isn't that common or that big a deal.
801 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
804 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
807 We {\em normally} generate code only for the non-defaulted methods;
808 there are some exceptions for @Eq@ and (especially) @Ord@...
811 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
812 constructor's numeric (@Int#@) tag. These are generated by
813 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
814 these is around is given by @hasCon2TagFun@.
816 The examples under the different sections below will make this
820 Much less often (really just for deriving @Ix@), we use a
821 @_tag2con_<tycon>@ function. See the examples.
824 We use the renamer!!! Reason: we're supposed to be
825 producing @LHsBinds Name@ for the methods, but that means
826 producing correctly-uniquified code on the fly. This is entirely
827 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
828 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
829 the renamer. What a great hack!
833 -- Generate the InstInfo for the required instance,
834 -- plus any auxiliary bindings required
835 genInst :: Instance -> TcM (InstInfo, LHsBinds RdrName)
837 = do { fix_env <- getFixityEnv
839 (tyvars,_,clas,[ty]) = instanceHead spec
840 clas_nm = className clas
841 tycon = tcTyConAppTyCon ty
842 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
844 -- Bring the right type variables into
845 -- scope, and rename the method binds
846 -- It's a bit yukky that we return *renamed* InstInfo, but
847 -- *non-renamed* auxiliary bindings
848 ; (rn_meth_binds, _fvs) <- discardWarnings $
849 bindLocalNames (map varName tyvars) $
850 rnMethodBinds clas_nm (\n -> []) [] meth_binds
852 -- Build the InstInfo
853 ; return (InstInfo { iSpec = spec,
854 iBinds = VanillaInst rn_meth_binds [] },
858 genDerivBinds clas fix_env tycon
859 | className clas `elem` typeableClassNames
860 = (gen_Typeable_binds tycon, emptyLHsBinds)
863 = case assocMaybe gen_list (getUnique clas) of
864 Just gen_fn -> gen_fn fix_env tycon
865 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
867 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
868 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
869 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
870 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
871 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
872 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
873 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
874 ,(showClassKey, no_aux_binds gen_Show_binds)
875 ,(readClassKey, no_aux_binds gen_Read_binds)
876 ,(dataClassKey, gen_Data_binds)
879 -- no_aux_binds is used for generators that don't
880 -- need to produce any auxiliary bindings
881 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
882 ignore_fix_env f fix_env tc = f tc
886 %************************************************************************
888 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
890 %************************************************************************
895 con2tag_Foo :: Foo ... -> Int#
896 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
897 maxtag_Foo :: Int -- ditto (NB: not unlifted)
900 We have a @con2tag@ function for a tycon if:
903 We're deriving @Eq@ and the tycon has nullary data constructors.
906 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
910 We have a @tag2con@ function for a tycon if:
913 We're deriving @Enum@, or @Ix@ (enum type only???)
916 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
919 genTaggeryBinds :: [InstInfo] -> TcM (LHsBinds RdrName)
920 genTaggeryBinds infos
921 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
922 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
923 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
925 all_CTs = [ (cls, tcTyConAppTyCon ty)
927 let (cls,ty) = simpleInstInfoClsTy info ]
928 all_tycons = map snd all_CTs
929 (tycons_of_interest, _) = removeDups compare all_tycons
931 do_con2tag acc_Names tycon
932 | isDataTyCon tycon &&
933 ((we_are_deriving eqClassKey tycon
934 && any isNullarySrcDataCon (tyConDataCons tycon))
935 || (we_are_deriving ordClassKey tycon
936 && not (isProductTyCon tycon))
937 || (we_are_deriving enumClassKey tycon)
938 || (we_are_deriving ixClassKey tycon))
940 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
945 do_tag2con acc_Names tycon
946 | isDataTyCon tycon &&
947 (we_are_deriving enumClassKey tycon ||
948 we_are_deriving ixClassKey tycon
949 && isEnumerationTyCon tycon)
950 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
951 : (maxtag_RDR tycon, tycon, GenMaxTag)
956 we_are_deriving clas_key tycon
957 = is_in_eqns clas_key tycon all_CTs
959 is_in_eqns clas_key tycon [] = False
960 is_in_eqns clas_key tycon ((c,t):cts)
961 = (clas_key == classKey c && tycon == t)
962 || is_in_eqns clas_key tycon cts
966 derivingThingErr clas tys tycon tyvars why
967 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
970 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
972 derivCtxt :: TyCon -> SDoc
974 = ptext SLIT("When deriving instances for") <+> quotes (ppr tycon)
976 derivInstCtxt1 clas inst_tys
977 = ptext SLIT("When deriving the instance for") <+> quotes (pprClassPred clas inst_tys)
979 derivInstCtxt2 theta clas inst_tys
980 = vcat [ptext SLIT("In the derived instance declaration"),
981 nest 2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta,
982 pprClassPred clas inst_tys])]