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 -- data/newtype T a = ... deriving( C t1 t2 )
390 -- leads to a call to mk_eqn_help with
391 -- tycon = T, deriv_tvs = ftv(t1,t2), clas = C, tys = [t1,t2]
393 mk_eqn_help loc orig gla_exts DataType tycon deriv_tvs clas tys
394 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
395 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
397 = do { eqn <- mkDataTypeEqn loc orig tycon clas
398 ; returnM (Just eqn, Nothing) }
400 mk_eqn_help loc orig gla_exts NewType tycon deriv_tvs clas tys
401 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
402 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
403 ; -- Go ahead and use the isomorphism
404 dfun_name <- new_dfun_name clas tycon
405 ; return (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
406 iBinds = NewTypeDerived ntd_info })) }
407 | std_class gla_exts clas
408 = mk_eqn_help loc orig gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
410 | otherwise -- Non-standard instance
411 = bale_out (if gla_exts then
412 cant_derive_err -- Too hard
414 non_std_err) -- Just complain about being a non-std instance
416 -- Here is the plan for newtype derivings. We see
417 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
418 -- where t is a type,
419 -- ak+1...an is a suffix of a1..an
420 -- ak+1...an do not occur free in t, nor in the s1..sm
421 -- (C s1 ... sm) is a *partial applications* of class C
422 -- with the last parameter missing
423 -- (T a1 .. ak) matches the kind of C's last argument
424 -- (and hence so does t)
426 -- We generate the instance
427 -- instance forall ({a1..ak} u fvs(s1..sm)).
428 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
429 -- where T a1...ap is the partial application of
430 -- the LHS of the correct kind and p >= k
432 -- NB: the variables below are:
433 -- tc_tvs = [a1, ..., an]
434 -- tyvars_to_keep = [a1, ..., ak]
435 -- rep_ty = t ak .. an
436 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
437 -- tys = [s1, ..., sm]
440 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
441 -- We generate the instance
442 -- instance Monad (ST s) => Monad (T s) where
444 clas_tyvars = classTyVars clas
445 kind = tyVarKind (last clas_tyvars)
446 -- Kind of the thing we want to instance
447 -- e.g. argument kind of Monad, *->*
449 (arg_kinds, _) = splitKindFunTys kind
450 n_args_to_drop = length arg_kinds
451 -- Want to drop 1 arg from (T s a) and (ST s a)
452 -- to get instance Monad (ST s) => Monad (T s)
454 -- Note [newtype representation]
455 -- Need newTyConRhs *not* newTyConRep to get the representation
456 -- type, because the latter looks through all intermediate newtypes
458 -- newtype B = MkB Int
459 -- newtype A = MkA B deriving( Num )
460 -- We want the Num instance of B, *not* the Num instance of Int,
461 -- when making the Num instance of A!
462 (tc_tvs, rep_ty) = newTyConRhs tycon
463 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
465 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
466 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
467 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
469 n_args_to_keep = length rep_ty_args - n_args_to_drop
470 args_to_drop = drop n_args_to_keep rep_ty_args
471 args_to_keep = take n_args_to_keep rep_ty_args
473 rep_fn' = mkAppTys rep_fn args_to_keep
474 rep_tys = tys ++ [rep_fn']
475 rep_pred = mkClassPred clas rep_tys
476 -- rep_pred is the representation dictionary, from where
477 -- we are gong to get all the methods for the newtype dictionary
479 -- Next we figure out what superclass dictionaries to use
480 -- See Note [Newtype deriving superclasses] above
482 inst_tys = tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)]
483 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
486 -- If there are no tyvars, there's no need
487 -- to abstract over the dictionaries we need
488 -- Example: newtype T = MkT Int deriving( C )
489 -- We get the derived instance
492 -- instance C Int => C T
493 dict_tvs = deriv_tvs ++ tyvars_to_keep
494 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
495 (dict_args, ntd_info) | null dict_tvs = ([], Just all_preds)
496 | otherwise = (all_preds, Nothing)
498 -- Finally! Here's where we build the dictionary Id
499 mk_inst_spec dfun_name = mkLocalInstance dfun overlap_flag
501 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
503 -------------------------------------------------------------------
504 -- Figuring out whether we can only do this newtype-deriving thing
506 right_arity = length tys + 1 == classArity clas
508 -- Never derive Read,Show,Typeable,Data this way
509 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
510 can_derive_via_isomorphism
511 = not (getUnique clas `elem` non_iso_classes)
512 && right_arity -- Well kinded;
513 -- eg not: newtype T ... deriving( ST )
514 -- because ST needs *2* type params
515 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
516 -- eg not: newtype T = T Int deriving( Monad )
517 && n_args_to_keep >= 0 -- Rep type has right kind:
518 -- eg not: newtype T a = T Int deriving( Monad )
519 && eta_ok -- Eta reduction works
520 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
521 -- newtype A = MkA [A]
523 -- instance Eq [A] => Eq A !!
524 -- Here's a recursive newtype that's actually OK
525 -- newtype S1 = S1 [T1 ()]
526 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
527 -- It's currently rejected. Oh well.
528 -- In fact we generate an instance decl that has method of form
529 -- meth @ instTy = meth @ repTy
530 -- (no coerce's). We'd need a coerce if we wanted to handle
531 -- recursive newtypes too
533 -- Check that eta reduction is OK
534 -- (a) the dropped-off args are identical
535 -- (b) the remaining type args do not mention any of teh dropped type variables
536 -- (c) the type class args do not mention any of teh dropped type variables
537 dropped_tvs = mkVarSet tyvars_to_drop
538 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
539 && (tyVarsOfType rep_fn' `disjointVarSet` dropped_tvs)
540 && (tyVarsOfTypes tys `disjointVarSet` dropped_tvs)
542 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
543 (vcat [ptext SLIT("even with cunning newtype deriving:"),
544 if isRecursiveTyCon tycon then
545 ptext SLIT("the newtype is recursive")
547 if not right_arity then
548 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
550 if not (n_tyvars_to_keep >= 0) then
551 ptext SLIT("the type constructor has wrong kind")
552 else if not (n_args_to_keep >= 0) then
553 ptext SLIT("the representation type has wrong kind")
554 else if not eta_ok then
555 ptext SLIT("the eta-reduction property does not hold")
559 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
560 (vcat [non_std_why clas,
561 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
563 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
565 std_class gla_exts clas
566 = key `elem` derivableClassKeys
567 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
571 std_class_via_iso clas -- These standard classes can be derived for a newtype
572 -- using the isomorphism trick *even if no -fglasgow-exts*
573 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
574 -- Not Read/Show because they respect the type
575 -- Not Enum, becuase newtypes are never in Enum
578 new_dfun_name clas tycon -- Just a simple wrapper
579 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
580 -- The type passed to newDFunName is only used to generate
581 -- a suitable string; hence the empty type arg list
583 ------------------------------------------------------------------
584 mkDataTypeEqn :: SrcSpan -> InstOrigin -> TyCon -> Class -> TcM DerivEqn
585 mkDataTypeEqn loc orig tycon clas
586 | clas `hasKey` typeableClassKey
587 = -- The Typeable class is special in several ways
588 -- data T a b = ... deriving( Typeable )
590 -- instance Typeable2 T where ...
592 -- 1. There are no constraints in the instance
593 -- 2. There are no type variables either
594 -- 3. The actual class we want to generate isn't necessarily
595 -- Typeable; it depends on the arity of the type
596 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
597 ; dfun_name <- new_dfun_name real_clas tycon
598 ; return (loc, orig, dfun_name, real_clas, tycon, [], []) }
601 = do { dfun_name <- new_dfun_name clas tycon
602 ; return (loc, orig, dfun_name, clas, tycon, tyvars, constraints) }
604 tyvars = tyConTyVars tycon
605 constraints = extra_constraints ++ ordinary_constraints
606 extra_constraints = tyConStupidTheta tycon
607 -- "extra_constraints": see note [Data decl contexts] above
610 = [ mkClassPred clas [arg_ty]
611 | data_con <- tyConDataCons tycon,
612 arg_ty <- dataConInstOrigArgTys data_con (map mkTyVarTy (tyConTyVars tycon)),
613 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
617 ------------------------------------------------------------------
618 -- Check side conditions that dis-allow derivability for particular classes
619 -- This is *apart* from the newtype-deriving mechanism
621 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
622 checkSideConditions gla_exts tycon deriv_tvs clas tys
623 | notNull deriv_tvs || notNull tys
624 = Just ty_args_why -- e.g. deriving( Foo s )
626 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
627 [] -> Just (non_std_why clas)
628 [cond] -> cond (gla_exts, tycon)
629 other -> pprPanic "checkSideConditions" (ppr clas)
631 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
633 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
635 sideConditions :: [(Unique, Condition)]
637 = [ (eqClassKey, cond_std),
638 (ordClassKey, cond_std),
639 (readClassKey, cond_std),
640 (showClassKey, cond_std),
641 (enumClassKey, cond_std `andCond` cond_isEnumeration),
642 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
643 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
644 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
645 (dataClassKey, cond_glaExts `andCond` cond_std)
648 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
650 orCond :: Condition -> Condition -> Condition
653 Nothing -> Nothing -- c1 succeeds
654 Just x -> case c2 tc of -- c1 fails
656 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
659 andCond c1 c2 tc = case c1 tc of
660 Nothing -> c2 tc -- c1 succeeds
661 Just x -> Just x -- c1 fails
663 cond_std :: Condition
664 cond_std (gla_exts, tycon)
665 | any (not . isVanillaDataCon) data_cons = Just existential_why
666 | null data_cons = Just no_cons_why
667 | otherwise = Nothing
669 data_cons = tyConDataCons tycon
670 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
671 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
673 cond_isEnumeration :: Condition
674 cond_isEnumeration (gla_exts, tycon)
675 | isEnumerationTyCon tycon = Nothing
676 | otherwise = Just why
678 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
680 cond_isProduct :: Condition
681 cond_isProduct (gla_exts, tycon)
682 | isProductTyCon tycon = Nothing
683 | otherwise = Just why
685 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
687 cond_typeableOK :: Condition
688 -- OK for Typeable class
689 -- Currently: (a) args all of kind *
690 -- (b) 7 or fewer args
691 cond_typeableOK (gla_exts, tycon)
692 | tyConArity tycon > 7 = Just too_many
693 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
694 | otherwise = Nothing
696 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
697 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
699 cond_glaExts :: Condition
700 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
701 | otherwise = Just why
703 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
706 %************************************************************************
708 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
710 %************************************************************************
712 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
713 terms, which is the final correct RHS for the corresponding original
717 Each (k,TyVarTy tv) in a solution constrains only a type
721 The (k,TyVarTy tv) pairs in a solution are canonically
722 ordered by sorting on type varible, tv, (major key) and then class, k,
727 solveDerivEqns :: OverlapFlag
729 -> TcM [Instance]-- Solns in same order as eqns.
730 -- This bunch is Absolutely minimal...
732 solveDerivEqns overlap_flag orig_eqns
733 = iterateDeriv 1 initial_solutions
735 -- The initial solutions for the equations claim that each
736 -- instance has an empty context; this solution is certainly
737 -- in canonical form.
738 initial_solutions :: [DerivSoln]
739 initial_solutions = [ [] | _ <- orig_eqns ]
741 ------------------------------------------------------------------
742 -- iterateDeriv calculates the next batch of solutions,
743 -- compares it with the current one; finishes if they are the
744 -- same, otherwise recurses with the new solutions.
745 -- It fails if any iteration fails
746 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
747 iterateDeriv n current_solns
748 | n > 20 -- Looks as if we are in an infinite loop
749 -- This can happen if we have -fallow-undecidable-instances
750 -- (See TcSimplify.tcSimplifyDeriv.)
751 = pprPanic "solveDerivEqns: probable loop"
752 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
755 inst_specs = zipWithEqual "add_solns" mk_inst_spec
756 orig_eqns current_solns
759 -- Extend the inst info from the explicit instance decls
760 -- with the current set of solutions, and simplify each RHS
761 extendLocalInstEnv inst_specs $
762 mappM gen_soln orig_eqns
763 ) `thenM` \ new_solns ->
764 if (current_solns == new_solns) then
767 iterateDeriv (n+1) new_solns
769 ------------------------------------------------------------------
770 gen_soln :: DerivEqn -> TcM [PredType]
771 gen_soln (loc, orig, _, clas, tc,tyvars,deriv_rhs)
773 do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
774 ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
775 tcSimplifyDeriv orig tc tyvars deriv_rhs
776 ; addErrCtxt (derivInstCtxt2 theta clas inst_tys) $
777 checkValidInstance tyvars theta clas inst_tys
778 ; return (sortLe (<=) theta) } -- Canonicalise before returning the soluction
782 ------------------------------------------------------------------
783 mk_inst_spec :: DerivEqn -> DerivSoln -> Instance
784 mk_inst_spec (loc, orig, dfun_name, clas, tycon, tyvars, _) theta
785 = mkLocalInstance dfun overlap_flag
787 dfun = mkDictFunId dfun_name tyvars theta clas
788 [mkTyConApp tycon (mkTyVarTys tyvars)]
790 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
791 -- Add new locally-defined instances; don't bother to check
792 -- for functional dependency errors -- that'll happen in TcInstDcls
793 extendLocalInstEnv dfuns thing_inside
794 = do { env <- getGblEnv
795 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
796 env' = env { tcg_inst_env = inst_env' }
797 ; setGblEnv env' thing_inside }
800 %************************************************************************
802 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
804 %************************************************************************
806 After all the trouble to figure out the required context for the
807 derived instance declarations, all that's left is to chug along to
808 produce them. They will then be shoved into @tcInstDecls2@, which
809 will do all its usual business.
811 There are lots of possibilities for code to generate. Here are
812 various general remarks.
817 We want derived instances of @Eq@ and @Ord@ (both v common) to be
818 ``you-couldn't-do-better-by-hand'' efficient.
821 Deriving @Show@---also pretty common--- should also be reasonable good code.
824 Deriving for the other classes isn't that common or that big a deal.
831 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
834 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
837 We {\em normally} generate code only for the non-defaulted methods;
838 there are some exceptions for @Eq@ and (especially) @Ord@...
841 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
842 constructor's numeric (@Int#@) tag. These are generated by
843 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
844 these is around is given by @hasCon2TagFun@.
846 The examples under the different sections below will make this
850 Much less often (really just for deriving @Ix@), we use a
851 @_tag2con_<tycon>@ function. See the examples.
854 We use the renamer!!! Reason: we're supposed to be
855 producing @LHsBinds Name@ for the methods, but that means
856 producing correctly-uniquified code on the fly. This is entirely
857 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
858 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
859 the renamer. What a great hack!
863 -- Generate the InstInfo for the required instance,
864 -- plus any auxiliary bindings required
865 genInst :: Instance -> TcM (InstInfo, LHsBinds RdrName)
867 = do { fix_env <- getFixityEnv
869 (tyvars,_,clas,[ty]) = instanceHead spec
870 clas_nm = className clas
871 tycon = tcTyConAppTyCon ty
872 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
874 -- Bring the right type variables into
875 -- scope, and rename the method binds
876 -- It's a bit yukky that we return *renamed* InstInfo, but
877 -- *non-renamed* auxiliary bindings
878 ; (rn_meth_binds, _fvs) <- discardWarnings $
879 bindLocalNames (map varName tyvars) $
880 rnMethodBinds clas_nm (\n -> []) [] meth_binds
882 -- Build the InstInfo
883 ; return (InstInfo { iSpec = spec,
884 iBinds = VanillaInst rn_meth_binds [] },
888 genDerivBinds clas fix_env tycon
889 | className clas `elem` typeableClassNames
890 = (gen_Typeable_binds tycon, emptyLHsBinds)
893 = case assocMaybe gen_list (getUnique clas) of
894 Just gen_fn -> gen_fn fix_env tycon
895 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
897 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
898 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
899 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
900 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
901 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
902 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
903 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
904 ,(showClassKey, no_aux_binds gen_Show_binds)
905 ,(readClassKey, no_aux_binds gen_Read_binds)
906 ,(dataClassKey, gen_Data_binds)
909 -- no_aux_binds is used for generators that don't
910 -- need to produce any auxiliary bindings
911 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
912 ignore_fix_env f fix_env tc = f tc
916 %************************************************************************
918 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
920 %************************************************************************
925 con2tag_Foo :: Foo ... -> Int#
926 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
927 maxtag_Foo :: Int -- ditto (NB: not unlifted)
930 We have a @con2tag@ function for a tycon if:
933 We're deriving @Eq@ and the tycon has nullary data constructors.
936 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
940 We have a @tag2con@ function for a tycon if:
943 We're deriving @Enum@, or @Ix@ (enum type only???)
946 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
949 genTaggeryBinds :: [InstInfo] -> TcM (LHsBinds RdrName)
950 genTaggeryBinds infos
951 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
952 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
953 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
955 all_CTs = [ (cls, tcTyConAppTyCon ty)
957 let (cls,ty) = simpleInstInfoClsTy info ]
958 all_tycons = map snd all_CTs
959 (tycons_of_interest, _) = removeDups compare all_tycons
961 do_con2tag acc_Names tycon
962 | isDataTyCon tycon &&
963 ((we_are_deriving eqClassKey tycon
964 && any isNullarySrcDataCon (tyConDataCons tycon))
965 || (we_are_deriving ordClassKey tycon
966 && not (isProductTyCon tycon))
967 || (we_are_deriving enumClassKey tycon)
968 || (we_are_deriving ixClassKey tycon))
970 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
975 do_tag2con acc_Names tycon
976 | isDataTyCon tycon &&
977 (we_are_deriving enumClassKey tycon ||
978 we_are_deriving ixClassKey tycon
979 && isEnumerationTyCon tycon)
980 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
981 : (maxtag_RDR tycon, tycon, GenMaxTag)
986 we_are_deriving clas_key tycon
987 = is_in_eqns clas_key tycon all_CTs
989 is_in_eqns clas_key tycon [] = False
990 is_in_eqns clas_key tycon ((c,t):cts)
991 = (clas_key == classKey c && tycon == t)
992 || is_in_eqns clas_key tycon cts
996 derivingThingErr clas tys tycon tyvars why
997 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
1000 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
1002 derivCtxt :: TyCon -> SDoc
1004 = ptext SLIT("When deriving instances for") <+> quotes (ppr tycon)
1006 derivInstCtxt1 clas inst_tys
1007 = ptext SLIT("When deriving the instance for") <+> quotes (pprClassPred clas inst_tys)
1009 derivInstCtxt2 theta clas inst_tys
1010 = vcat [ptext SLIT("In the derived instance declaration"),
1011 nest 2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta,
1012 pprClassPred clas inst_tys])]