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, tcLookupLocatedTyCon, 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 )
28 import TypeRep ( PredType )
30 import RnBinds ( rnMethodBinds, rnTopBinds )
31 import RnEnv ( bindLocalNames )
32 import HscTypes ( FixityEnv )
34 import Class ( className, classArity, classKey, classTyVars, classSCTheta, Class )
35 import Type ( zipOpenTvSubst, substTheta, pprThetaArrow, pprClassPred, mkTyVarTy )
36 import ErrUtils ( dumpIfSet_dyn )
37 import MkId ( mkDictFunId )
38 import DataCon ( isNullarySrcDataCon, isVanillaDataCon, dataConInstOrigArgTys )
39 import Maybes ( catMaybes )
40 import RdrName ( RdrName )
41 import Name ( Name, getSrcLoc )
42 import NameSet ( duDefs )
43 import Type ( splitKindFunTys )
44 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
45 tyConStupidTheta, isProductTyCon, isDataTyCon, isNewTyCon, newTyConRhs,
46 isEnumerationTyCon, isRecursiveTyCon, TyCon
48 import TcType ( TcType, ThetaType, mkTyVarTys, mkTyConApp, tcTyConAppTyCon,
49 isUnLiftedType, mkClassPred, tyVarsOfType, tyVarsOfTypes,
50 isSubArgTypeKind, tcEqTypes, tcSplitAppTys, mkAppTys )
51 import Var ( TyVar, tyVarKind, varName )
52 import VarSet ( mkVarSet, disjointVarSet )
54 import SrcLoc ( SrcSpan, srcLocSpan, Located(..), unLoc )
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 = (SrcSpan, InstOrigin, 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 :: DerivEqn -> SDoc
151 pprDerivEqn (l,_,n,c,tc,tvs,rhs)
152 = parens (hsep [ppr l, ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
154 type DerivRhs = ThetaType
155 type DerivSoln = DerivRhs
159 [Data decl contexts] A note about contexts on data decls
160 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
163 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
165 We will need an instance decl like:
167 instance (Read a, RealFloat a) => Read (Complex a) where
170 The RealFloat in the context is because the read method for Complex is bound
171 to construct a Complex, and doing that requires that the argument type is
174 But this ain't true for Show, Eq, Ord, etc, since they don't construct
175 a Complex; they only take them apart.
177 Our approach: identify the offending classes, and add the data type
178 context to the instance decl. The "offending classes" are
182 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
183 pattern matching against a constructor from a data type with a context
184 gives rise to the constraints for that context -- or at least the thinned
185 version. So now all classes are "offending".
192 newtype T = T Char deriving( C [a] )
194 Notice the free 'a' in the deriving. We have to fill this out to
195 newtype T = T Char deriving( forall a. C [a] )
197 And then translate it to:
198 instance C [a] Char => C [a] T where ...
203 %************************************************************************
205 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
207 %************************************************************************
210 tcDeriving :: [LTyClDecl Name] -- All type constructors
211 -> [LDerivDecl Name] -- All stand-alone deriving declarations
212 -> TcM ([InstInfo], -- The generated "instance decls"
213 HsValBinds Name) -- Extra generated top-level bindings
215 tcDeriving tycl_decls deriv_decls
216 = recoverM (returnM ([], emptyValBindsOut)) $
217 do { -- Fish the "deriving"-related information out of the TcEnv
218 -- and make the necessary "equations".
219 overlap_flag <- getOverlapFlag
220 ; (ordinary_eqns, newtype_inst_info)
221 <- makeDerivEqns overlap_flag tycl_decls deriv_decls
223 ; (ordinary_inst_info, deriv_binds)
224 <- extendLocalInstEnv (map iSpec newtype_inst_info) $
225 deriveOrdinaryStuff overlap_flag ordinary_eqns
226 -- Add the newtype-derived instances to the inst env
227 -- before tacking the "ordinary" ones
229 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
231 -- If we are compiling a hs-boot file,
232 -- don't generate any derived bindings
233 ; is_boot <- tcIsHsBoot
235 return (inst_info, emptyValBindsOut)
239 -- Generate the generic to/from functions from each type declaration
240 ; gen_binds <- mkGenericBinds tycl_decls
242 -- Rename these extra bindings, discarding warnings about unused bindings etc
243 -- Set -fglasgow exts so that we can have type signatures in patterns,
244 -- which is used in the generic binds
246 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
247 { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn deriv_binds [])
248 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
249 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
251 ; return (rn_deriv `plusHsValBinds` rn_gen) }
255 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
256 (ddump_deriving inst_info rn_binds))
258 ; returnM (inst_info, rn_binds)
261 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
262 ddump_deriving inst_infos extra_binds
263 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
265 -----------------------------------------
266 deriveOrdinaryStuff overlap_flag [] -- Short cut
267 = returnM ([], emptyLHsBinds)
269 deriveOrdinaryStuff overlap_flag eqns
270 = do { -- Take the equation list and solve it, to deliver a list of
271 -- solutions, a.k.a. the contexts for the instance decls
272 -- required for the corresponding equations.
273 inst_specs <- solveDerivEqns overlap_flag eqns
275 -- Generate the InstInfo for each dfun,
276 -- plus any auxiliary bindings it needs
277 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst inst_specs
279 -- Generate any extra not-one-inst-decl-specific binds,
280 -- notably "con2tag" and/or "tag2con" functions.
281 ; extra_binds <- genTaggeryBinds inst_infos
284 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
287 -----------------------------------------
288 mkGenericBinds tycl_decls
289 = do { tcs <- mapM tcLookupTyCon
291 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
292 -- We are only interested in the data type declarations
293 ; return (unionManyBags [ mkTyConGenericBinds tc |
294 tc <- tcs, tyConHasGenerics tc ]) }
295 -- And then only in the ones whose 'has-generics' flag is on
299 %************************************************************************
301 \subsection[TcDeriv-eqns]{Forming the equations}
303 %************************************************************************
305 @makeDerivEqns@ fishes around to find the info about needed derived
306 instances. Complicating factors:
309 We can only derive @Enum@ if the data type is an enumeration
310 type (all nullary data constructors).
313 We can only derive @Ix@ if the data type is an enumeration {\em
314 or} has just one data constructor (e.g., tuples).
317 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
320 Note [Newtype deriving superclasses]
321 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
322 The 'tys' here come from the partial application in the deriving
323 clause. The last arg is the new instance type.
325 We must pass the superclasses; the newtype might be an instance
326 of them in a different way than the representation type
327 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
328 Then the Show instance is not done via isomorphism; it shows
330 The Num instance is derived via isomorphism, but the Show superclass
331 dictionary must the Show instance for Foo, *not* the Show dictionary
332 gotten from the Num dictionary. So we must build a whole new dictionary
333 not just use the Num one. The instance we want is something like:
334 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
337 There may be a coercion needed which we get from the tycon for the newtype
338 when the dict is constructed in TcInstDcls.tcInstDecl2
342 makeDerivEqns :: OverlapFlag
345 -> TcM ([DerivEqn], -- Ordinary derivings
346 [InstInfo]) -- Special newtype derivings
348 makeDerivEqns overlap_flag tycl_decls deriv_decls
349 = do derive_these_top_level <- mapM top_level_deriv deriv_decls >>= return . catMaybes
350 (maybe_ordinaries, maybe_newtypes)
351 <- mapAndUnzipM mk_eqn (derive_these ++ derive_these_top_level)
352 return (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
354 ------------------------------------------------------------------
355 derive_these :: [(SrcSpan, InstOrigin, NewOrData, Name, LHsType Name)]
356 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
357 derive_these = [ (srcLocSpan (getSrcLoc tycon), DerivOrigin, nd, tycon, pred)
358 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
359 tcdDerivs = Just preds }) <- tycl_decls,
362 top_level_deriv :: LDerivDecl Name -> TcM (Maybe (SrcSpan, InstOrigin, NewOrData, Name, LHsType Name))
363 top_level_deriv d@(L l (DerivDecl inst ty_name)) = recoverM (returnM Nothing) $ setSrcSpan l $
364 do tycon <- tcLookupLocatedTyCon ty_name
365 let new_or_data = if isNewTyCon tycon then NewType else DataType
366 traceTc (text "Stand-alone deriving:" <+> ppr (new_or_data, unLoc ty_name, inst))
367 return $ Just (l, StandAloneDerivOrigin, new_or_data, unLoc ty_name, inst)
369 ------------------------------------------------------------------
370 -- takes (whether newtype or data, name of data type, partially applied type class)
371 mk_eqn :: (SrcSpan, InstOrigin, NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
372 -- We swizzle the tyvars and datacons out of the tycon
373 -- to make the rest of the equation
375 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
376 -- we allow deriving (forall a. C [a]).
378 mk_eqn (loc, orig, new_or_data, tycon_name, hs_deriv_ty)
379 = tcLookupTyCon tycon_name `thenM` \ tycon ->
381 addErrCtxt (derivCtxt tycon) $
382 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
383 -- the type variables for the type constructor
384 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
385 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
386 mk_eqn_help loc orig gla_exts new_or_data tycon deriv_tvs clas tys
388 ------------------------------------------------------------------
389 mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys
390 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
391 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
393 = do { eqn <- mkDataTypeEqn loc orig tycon clas
394 ; returnM (Just eqn, Nothing) }
396 mk_eqn_help loc orig gla_exts NewType tycon deriv_tvs clas tys
397 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
398 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
399 ; -- Go ahead and use the isomorphism
400 dfun_name <- new_dfun_name clas tycon
401 ; return (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
402 iBinds = NewTypeDerived ntd_info })) }
403 | std_class gla_exts clas
404 = mk_eqn_help loc orig gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
406 | otherwise -- Non-standard instance
407 = bale_out (if gla_exts then
408 cant_derive_err -- Too hard
410 non_std_err) -- Just complain about being a non-std instance
412 -- Here is the plan for newtype derivings. We see
413 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
414 -- where t is a type,
415 -- ak+1...an is a suffix of a1..an
416 -- ak+1...an do not occur free in t, nor in the s1..sm
417 -- (C s1 ... sm) is a *partial applications* of class C
418 -- with the last parameter missing
419 -- (T a1 .. ak) matches the kind of C's last argument
420 -- (and hence so does t)
422 -- We generate the instance
423 -- instance forall ({a1..ak} u fvs(s1..sm)).
424 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
425 -- where T a1...ap is the partial application of
426 -- the LHS of the correct kind and p >= k
428 -- NB: the variables below are:
429 -- tc_tvs = [a1, ..., an]
430 -- tyvars_to_keep = [a1, ..., ak]
431 -- rep_ty = t ak .. an
432 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
433 -- tys = [s1, ..., sm]
436 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
437 -- We generate the instance
438 -- instance Monad (ST s) => Monad (T s) where
440 clas_tyvars = classTyVars clas
441 kind = tyVarKind (last clas_tyvars)
442 -- Kind of the thing we want to instance
443 -- e.g. argument kind of Monad, *->*
445 (arg_kinds, _) = splitKindFunTys kind
446 n_args_to_drop = length arg_kinds
447 -- Want to drop 1 arg from (T s a) and (ST s a)
448 -- to get instance Monad (ST s) => Monad (T s)
450 -- Note [newtype representation]
451 -- Need newTyConRhs *not* newTyConRep to get the representation
452 -- type, because the latter looks through all intermediate newtypes
454 -- newtype B = MkB Int
455 -- newtype A = MkA B deriving( Num )
456 -- We want the Num instance of B, *not* the Num instance of Int,
457 -- when making the Num instance of A!
458 (tc_tvs, rep_ty) = newTyConRhs tycon
459 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
461 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
462 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
463 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
465 n_args_to_keep = length rep_ty_args - n_args_to_drop
466 args_to_drop = drop n_args_to_keep rep_ty_args
467 args_to_keep = take n_args_to_keep rep_ty_args
469 rep_fn' = mkAppTys rep_fn args_to_keep
470 rep_tys = tys ++ [rep_fn']
471 rep_pred = mkClassPred clas rep_tys
472 -- rep_pred is the representation dictionary, from where
473 -- we are gong to get all the methods for the newtype dictionary
475 -- Next we figure out what superclass dictionaries to use
476 -- See Note [Newtype deriving superclasses] above
478 inst_tys = tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)]
479 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
482 -- If there are no tyvars, there's no need
483 -- to abstract over the dictionaries we need
484 -- Example: newtype T = MkT Int deriving( C )
485 -- We get the derived instance
488 -- instance C Int => C T
489 dict_tvs = deriv_tvs ++ tyvars_to_keep
490 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
491 (dict_args, ntd_info) | null dict_tvs = ([], Just all_preds)
492 | otherwise = (all_preds, Nothing)
494 -- Finally! Here's where we build the dictionary Id
495 mk_inst_spec dfun_name = mkLocalInstance dfun overlap_flag
497 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
499 -------------------------------------------------------------------
500 -- Figuring out whether we can only do this newtype-deriving thing
502 right_arity = length tys + 1 == classArity clas
504 -- Never derive Read,Show,Typeable,Data this way
505 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
506 can_derive_via_isomorphism
507 = not (getUnique clas `elem` non_iso_classes)
508 && right_arity -- Well kinded;
509 -- eg not: newtype T ... deriving( ST )
510 -- because ST needs *2* type params
511 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
512 -- eg not: newtype T = T Int deriving( Monad )
513 && n_args_to_keep >= 0 -- Rep type has right kind:
514 -- eg not: newtype T a = T Int deriving( Monad )
515 && eta_ok -- Eta reduction works
516 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
517 -- newtype A = MkA [A]
519 -- instance Eq [A] => Eq A !!
520 -- Here's a recursive newtype that's actually OK
521 -- newtype S1 = S1 [T1 ()]
522 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
523 -- It's currently rejected. Oh well.
524 -- In fact we generate an instance decl that has method of form
525 -- meth @ instTy = meth @ repTy
526 -- (no coerce's). We'd need a coerce if we wanted to handle
527 -- recursive newtypes too
529 -- Check that eta reduction is OK
530 -- (a) the dropped-off args are identical
531 -- (b) the remaining type args do not mention any of teh dropped type variables
532 -- (c) the type class args do not mention any of teh dropped type variables
533 dropped_tvs = mkVarSet tyvars_to_drop
534 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
535 && (tyVarsOfType rep_fn' `disjointVarSet` dropped_tvs)
536 && (tyVarsOfTypes tys `disjointVarSet` dropped_tvs)
538 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
539 (vcat [ptext SLIT("even with cunning newtype deriving:"),
540 if isRecursiveTyCon tycon then
541 ptext SLIT("the newtype is recursive")
543 if not right_arity then
544 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
546 if not (n_tyvars_to_keep >= 0) then
547 ptext SLIT("the type constructor has wrong kind")
548 else if not (n_args_to_keep >= 0) then
549 ptext SLIT("the representation type has wrong kind")
550 else if not eta_ok then
551 ptext SLIT("the eta-reduction property does not hold")
555 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
556 (vcat [non_std_why clas,
557 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
559 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
561 std_class gla_exts clas
562 = key `elem` derivableClassKeys
563 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
567 std_class_via_iso clas -- These standard classes can be derived for a newtype
568 -- using the isomorphism trick *even if no -fglasgow-exts*
569 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
570 -- Not Read/Show because they respect the type
571 -- Not Enum, becuase newtypes are never in Enum
574 new_dfun_name clas tycon -- Just a simple wrapper
575 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
576 -- The type passed to newDFunName is only used to generate
577 -- a suitable string; hence the empty type arg list
579 ------------------------------------------------------------------
580 mkDataTypeEqn :: SrcSpan -> InstOrigin -> TyCon -> Class -> TcM DerivEqn
581 mkDataTypeEqn loc orig tycon clas
582 | clas `hasKey` typeableClassKey
583 = -- The Typeable class is special in several ways
584 -- data T a b = ... deriving( Typeable )
586 -- instance Typeable2 T where ...
588 -- 1. There are no constraints in the instance
589 -- 2. There are no type variables either
590 -- 3. The actual class we want to generate isn't necessarily
591 -- Typeable; it depends on the arity of the type
592 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
593 ; dfun_name <- new_dfun_name real_clas tycon
594 ; return (loc, orig, dfun_name, real_clas, tycon, [], []) }
597 = do { dfun_name <- new_dfun_name clas tycon
598 ; return (loc, orig, dfun_name, clas, tycon, tyvars, constraints) }
600 tyvars = tyConTyVars tycon
601 constraints = extra_constraints ++ ordinary_constraints
602 extra_constraints = tyConStupidTheta tycon
603 -- "extra_constraints": see note [Data decl contexts] above
606 = [ mkClassPred clas [arg_ty]
607 | data_con <- tyConDataCons tycon,
608 arg_ty <- dataConInstOrigArgTys data_con (map mkTyVarTy (tyConTyVars tycon)),
609 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
613 ------------------------------------------------------------------
614 -- Check side conditions that dis-allow derivability for particular classes
615 -- This is *apart* from the newtype-deriving mechanism
617 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
618 checkSideConditions gla_exts tycon deriv_tvs clas tys
619 | notNull deriv_tvs || notNull tys
620 = Just ty_args_why -- e.g. deriving( Foo s )
622 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
623 [] -> Just (non_std_why clas)
624 [cond] -> cond (gla_exts, tycon)
625 other -> pprPanic "checkSideConditions" (ppr clas)
627 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
629 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
631 sideConditions :: [(Unique, Condition)]
633 = [ (eqClassKey, cond_std),
634 (ordClassKey, cond_std),
635 (readClassKey, cond_std),
636 (showClassKey, cond_std),
637 (enumClassKey, cond_std `andCond` cond_isEnumeration),
638 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
639 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
640 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
641 (dataClassKey, cond_glaExts `andCond` cond_std)
644 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
646 orCond :: Condition -> Condition -> Condition
649 Nothing -> Nothing -- c1 succeeds
650 Just x -> case c2 tc of -- c1 fails
652 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
655 andCond c1 c2 tc = case c1 tc of
656 Nothing -> c2 tc -- c1 succeeds
657 Just x -> Just x -- c1 fails
659 cond_std :: Condition
660 cond_std (gla_exts, tycon)
661 | any (not . isVanillaDataCon) data_cons = Just existential_why
662 | null data_cons = Just no_cons_why
663 | otherwise = Nothing
665 data_cons = tyConDataCons tycon
666 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
667 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
669 cond_isEnumeration :: Condition
670 cond_isEnumeration (gla_exts, tycon)
671 | isEnumerationTyCon tycon = Nothing
672 | otherwise = Just why
674 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
676 cond_isProduct :: Condition
677 cond_isProduct (gla_exts, tycon)
678 | isProductTyCon tycon = Nothing
679 | otherwise = Just why
681 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
683 cond_typeableOK :: Condition
684 -- OK for Typeable class
685 -- Currently: (a) args all of kind *
686 -- (b) 7 or fewer args
687 cond_typeableOK (gla_exts, tycon)
688 | tyConArity tycon > 7 = Just too_many
689 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
690 | otherwise = Nothing
692 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
693 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
695 cond_glaExts :: Condition
696 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
697 | otherwise = Just why
699 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
702 %************************************************************************
704 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
706 %************************************************************************
708 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
709 terms, which is the final correct RHS for the corresponding original
713 Each (k,TyVarTy tv) in a solution constrains only a type
717 The (k,TyVarTy tv) pairs in a solution are canonically
718 ordered by sorting on type varible, tv, (major key) and then class, k,
723 solveDerivEqns :: OverlapFlag
725 -> TcM [Instance]-- Solns in same order as eqns.
726 -- This bunch is Absolutely minimal...
728 solveDerivEqns overlap_flag orig_eqns
729 = iterateDeriv 1 initial_solutions
731 -- The initial solutions for the equations claim that each
732 -- instance has an empty context; this solution is certainly
733 -- in canonical form.
734 initial_solutions :: [DerivSoln]
735 initial_solutions = [ [] | _ <- orig_eqns ]
737 ------------------------------------------------------------------
738 -- iterateDeriv calculates the next batch of solutions,
739 -- compares it with the current one; finishes if they are the
740 -- same, otherwise recurses with the new solutions.
741 -- It fails if any iteration fails
742 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
743 iterateDeriv n current_solns
744 | n > 20 -- Looks as if we are in an infinite loop
745 -- This can happen if we have -fallow-undecidable-instances
746 -- (See TcSimplify.tcSimplifyDeriv.)
747 = pprPanic "solveDerivEqns: probable loop"
748 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
751 inst_specs = zipWithEqual "add_solns" mk_inst_spec
752 orig_eqns current_solns
755 -- Extend the inst info from the explicit instance decls
756 -- with the current set of solutions, and simplify each RHS
757 extendLocalInstEnv inst_specs $
758 mappM gen_soln orig_eqns
759 ) `thenM` \ new_solns ->
760 if (current_solns == new_solns) then
763 iterateDeriv (n+1) new_solns
765 ------------------------------------------------------------------
766 gen_soln :: DerivEqn -> TcM [PredType]
767 gen_soln (loc, orig, _, clas, tc,tyvars,deriv_rhs)
769 do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
770 ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
771 tcSimplifyDeriv orig tc tyvars deriv_rhs
772 ; addErrCtxt (derivInstCtxt2 theta clas inst_tys) $
773 checkValidInstance tyvars theta clas inst_tys
774 ; return (sortLe (<=) theta) } -- Canonicalise before returning the soluction
778 ------------------------------------------------------------------
779 mk_inst_spec :: DerivEqn -> DerivSoln -> Instance
780 mk_inst_spec (loc, orig, dfun_name, clas, tycon, tyvars, _) theta
781 = mkLocalInstance dfun overlap_flag
783 dfun = mkDictFunId dfun_name tyvars theta clas
784 [mkTyConApp tycon (mkTyVarTys tyvars)]
786 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
787 -- Add new locally-defined instances; don't bother to check
788 -- for functional dependency errors -- that'll happen in TcInstDcls
789 extendLocalInstEnv dfuns thing_inside
790 = do { env <- getGblEnv
791 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
792 env' = env { tcg_inst_env = inst_env' }
793 ; setGblEnv env' thing_inside }
796 %************************************************************************
798 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
800 %************************************************************************
802 After all the trouble to figure out the required context for the
803 derived instance declarations, all that's left is to chug along to
804 produce them. They will then be shoved into @tcInstDecls2@, which
805 will do all its usual business.
807 There are lots of possibilities for code to generate. Here are
808 various general remarks.
813 We want derived instances of @Eq@ and @Ord@ (both v common) to be
814 ``you-couldn't-do-better-by-hand'' efficient.
817 Deriving @Show@---also pretty common--- should also be reasonable good code.
820 Deriving for the other classes isn't that common or that big a deal.
827 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
830 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
833 We {\em normally} generate code only for the non-defaulted methods;
834 there are some exceptions for @Eq@ and (especially) @Ord@...
837 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
838 constructor's numeric (@Int#@) tag. These are generated by
839 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
840 these is around is given by @hasCon2TagFun@.
842 The examples under the different sections below will make this
846 Much less often (really just for deriving @Ix@), we use a
847 @_tag2con_<tycon>@ function. See the examples.
850 We use the renamer!!! Reason: we're supposed to be
851 producing @LHsBinds Name@ for the methods, but that means
852 producing correctly-uniquified code on the fly. This is entirely
853 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
854 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
855 the renamer. What a great hack!
859 -- Generate the InstInfo for the required instance,
860 -- plus any auxiliary bindings required
861 genInst :: Instance -> TcM (InstInfo, LHsBinds RdrName)
863 = do { fix_env <- getFixityEnv
865 (tyvars,_,clas,[ty]) = instanceHead spec
866 clas_nm = className clas
867 tycon = tcTyConAppTyCon ty
868 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
870 -- Bring the right type variables into
871 -- scope, and rename the method binds
872 -- It's a bit yukky that we return *renamed* InstInfo, but
873 -- *non-renamed* auxiliary bindings
874 ; (rn_meth_binds, _fvs) <- discardWarnings $
875 bindLocalNames (map varName tyvars) $
876 rnMethodBinds clas_nm (\n -> []) [] meth_binds
878 -- Build the InstInfo
879 ; return (InstInfo { iSpec = spec,
880 iBinds = VanillaInst rn_meth_binds [] },
884 genDerivBinds clas fix_env tycon
885 | className clas `elem` typeableClassNames
886 = (gen_Typeable_binds tycon, emptyLHsBinds)
889 = case assocMaybe gen_list (getUnique clas) of
890 Just gen_fn -> gen_fn fix_env tycon
891 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
893 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
894 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
895 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
896 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
897 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
898 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
899 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
900 ,(showClassKey, no_aux_binds gen_Show_binds)
901 ,(readClassKey, no_aux_binds gen_Read_binds)
902 ,(dataClassKey, gen_Data_binds)
905 -- no_aux_binds is used for generators that don't
906 -- need to produce any auxiliary bindings
907 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
908 ignore_fix_env f fix_env tc = f tc
912 %************************************************************************
914 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
916 %************************************************************************
921 con2tag_Foo :: Foo ... -> Int#
922 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
923 maxtag_Foo :: Int -- ditto (NB: not unlifted)
926 We have a @con2tag@ function for a tycon if:
929 We're deriving @Eq@ and the tycon has nullary data constructors.
932 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
936 We have a @tag2con@ function for a tycon if:
939 We're deriving @Enum@, or @Ix@ (enum type only???)
942 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
945 genTaggeryBinds :: [InstInfo] -> TcM (LHsBinds RdrName)
946 genTaggeryBinds infos
947 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
948 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
949 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
951 all_CTs = [ (cls, tcTyConAppTyCon ty)
953 let (cls,ty) = simpleInstInfoClsTy info ]
954 all_tycons = map snd all_CTs
955 (tycons_of_interest, _) = removeDups compare all_tycons
957 do_con2tag acc_Names tycon
958 | isDataTyCon tycon &&
959 ((we_are_deriving eqClassKey tycon
960 && any isNullarySrcDataCon (tyConDataCons tycon))
961 || (we_are_deriving ordClassKey tycon
962 && not (isProductTyCon tycon))
963 || (we_are_deriving enumClassKey tycon)
964 || (we_are_deriving ixClassKey tycon))
966 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
971 do_tag2con acc_Names tycon
972 | isDataTyCon tycon &&
973 (we_are_deriving enumClassKey tycon ||
974 we_are_deriving ixClassKey tycon
975 && isEnumerationTyCon tycon)
976 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
977 : (maxtag_RDR tycon, tycon, GenMaxTag)
982 we_are_deriving clas_key tycon
983 = is_in_eqns clas_key tycon all_CTs
985 is_in_eqns clas_key tycon [] = False
986 is_in_eqns clas_key tycon ((c,t):cts)
987 = (clas_key == classKey c && tycon == t)
988 || is_in_eqns clas_key tycon cts
992 derivingThingErr clas tys tycon tyvars why
993 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
996 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
998 derivCtxt :: TyCon -> SDoc
1000 = ptext SLIT("When deriving instances for") <+> quotes (ppr tycon)
1002 derivInstCtxt1 clas inst_tys
1003 = ptext SLIT("When deriving the instance for") <+> quotes (pprClassPred clas inst_tys)
1005 derivInstCtxt2 theta clas inst_tys
1006 = vcat [ptext SLIT("In the derived instance declaration"),
1007 nest 2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta,
1008 pprClassPred clas inst_tys])]