2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcDeriv]{Deriving}
6 Handles @deriving@ clauses on @data@ declarations.
9 module TcDeriv ( tcDeriving ) where
11 #include "HsVersions.h"
14 import DynFlags ( DynFlag(..) )
16 import Generics ( mkTyConGenericBinds )
18 import TcMType ( checkValidInstance )
19 import TcEnv ( newDFunName, pprInstInfoDetails,
20 InstInfo(..), InstBindings(..), simpleInstInfoClsTy,
21 tcLookupClass, tcLookupTyCon, tcExtendTyVarEnv
23 import TcGenDeriv -- Deriv stuff
24 import InstEnv ( Instance, OverlapFlag, mkLocalInstance, instanceHead, extendInstEnvList )
25 import Inst ( getOverlapFlag )
26 import TcHsType ( tcHsDeriv )
27 import TcSimplify ( tcSimplifyDeriv )
29 import RnBinds ( rnMethodBinds, rnTopBinds )
30 import RnEnv ( bindLocalNames )
31 import HscTypes ( FixityEnv )
33 import Class ( className, classArity, classKey, classTyVars, classSCTheta, Class )
34 import Type ( zipOpenTvSubst, substTheta, pprThetaArrow, pprClassPred, mkTyVarTy )
35 import ErrUtils ( dumpIfSet_dyn )
36 import MkId ( mkDictFunId )
37 import DataCon ( isNullarySrcDataCon, isVanillaDataCon, dataConOrigArgTys, dataConInstOrigArgTys )
38 import Maybes ( catMaybes )
39 import RdrName ( RdrName )
40 import Name ( Name, getSrcLoc )
41 import NameSet ( duDefs )
42 import Kind ( splitKindFunTys )
43 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
44 tyConStupidTheta, isProductTyCon, isDataTyCon, newTyConRhs,
45 isEnumerationTyCon, isRecursiveTyCon, TyCon
47 import TcType ( TcType, ThetaType, mkTyVarTys, mkTyConApp, tcTyConAppTyCon,
48 isUnLiftedType, mkClassPred, tyVarsOfType,
49 isArgTypeKind, tcEqTypes, tcSplitAppTys, mkAppTys )
50 import Var ( TyVar, tyVarKind, varName )
51 import VarSet ( mkVarSet, subVarSet )
53 import SrcLoc ( srcLocSpan, Located(..) )
54 import Util ( zipWithEqual, sortLe, notNull )
55 import ListSetOps ( removeDups, assocMaybe )
60 %************************************************************************
62 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
64 %************************************************************************
68 data T a b = C1 (Foo a) (Bar b)
73 [NOTE: See end of these comments for what to do with
74 data (C a, D b) => T a b = ...
77 We want to come up with an instance declaration of the form
79 instance (Ping a, Pong b, ...) => Eq (T a b) where
82 It is pretty easy, albeit tedious, to fill in the code "...". The
83 trick is to figure out what the context for the instance decl is,
84 namely @Ping@, @Pong@ and friends.
86 Let's call the context reqd for the T instance of class C at types
87 (a,b, ...) C (T a b). Thus:
89 Eq (T a b) = (Ping a, Pong b, ...)
91 Now we can get a (recursive) equation from the @data@ decl:
93 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
94 u Eq (T b a) u Eq Int -- From C2
95 u Eq (T a a) -- From C3
97 Foo and Bar may have explicit instances for @Eq@, in which case we can
98 just substitute for them. Alternatively, either or both may have
99 their @Eq@ instances given by @deriving@ clauses, in which case they
100 form part of the system of equations.
102 Now all we need do is simplify and solve the equations, iterating to
103 find the least fixpoint. Notice that the order of the arguments can
104 switch around, as here in the recursive calls to T.
106 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
110 Eq (T a b) = {} -- The empty set
113 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
114 u Eq (T b a) u Eq Int -- From C2
115 u Eq (T a a) -- From C3
117 After simplification:
118 = Eq a u Ping b u {} u {} u {}
123 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
124 u Eq (T b a) u Eq Int -- From C2
125 u Eq (T a a) -- From C3
127 After simplification:
132 = Eq a u Ping b u Eq b u Ping a
134 The next iteration gives the same result, so this is the fixpoint. We
135 need to make a canonical form of the RHS to ensure convergence. We do
136 this by simplifying the RHS to a form in which
138 - the classes constrain only tyvars
139 - the list is sorted by tyvar (major key) and then class (minor key)
140 - no duplicates, of course
142 So, here are the synonyms for the ``equation'' structures:
145 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
146 -- The Name is the name for the DFun we'll build
147 -- The tyvars bind all the variables in the RHS
149 pprDerivEqn (n,c,tc,tvs,rhs)
150 = parens (hsep [ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
152 type DerivRhs = ThetaType
153 type DerivSoln = DerivRhs
157 [Data decl contexts] A note about contexts on data decls
158 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
161 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
163 We will need an instance decl like:
165 instance (Read a, RealFloat a) => Read (Complex a) where
168 The RealFloat in the context is because the read method for Complex is bound
169 to construct a Complex, and doing that requires that the argument type is
172 But this ain't true for Show, Eq, Ord, etc, since they don't construct
173 a Complex; they only take them apart.
175 Our approach: identify the offending classes, and add the data type
176 context to the instance decl. The "offending classes" are
180 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
181 pattern matching against a constructor from a data type with a context
182 gives rise to the constraints for that context -- or at least the thinned
183 version. So now all classes are "offending".
190 newtype T = T Char deriving( C [a] )
192 Notice the free 'a' in the deriving. We have to fill this out to
193 newtype T = T Char deriving( forall a. C [a] )
195 And then translate it to:
196 instance C [a] Char => C [a] T where ...
201 %************************************************************************
203 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
205 %************************************************************************
208 tcDeriving :: [LTyClDecl Name] -- All type constructors
209 -> TcM ([InstInfo], -- The generated "instance decls"
210 HsValBinds Name) -- Extra generated top-level bindings
212 tcDeriving tycl_decls
213 = recoverM (returnM ([], emptyValBindsOut)) $
214 do { -- Fish the "deriving"-related information out of the TcEnv
215 -- and make the necessary "equations".
216 overlap_flag <- getOverlapFlag
217 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns overlap_flag tycl_decls
219 ; (ordinary_inst_info, deriv_binds)
220 <- extendLocalInstEnv (map iSpec newtype_inst_info) $
221 deriveOrdinaryStuff overlap_flag ordinary_eqns
222 -- Add the newtype-derived instances to the inst env
223 -- before tacking the "ordinary" ones
225 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
227 -- If we are compiling a hs-boot file,
228 -- don't generate any derived bindings
229 ; is_boot <- tcIsHsBoot
231 return (inst_info, emptyValBindsOut)
235 -- Generate the generic to/from functions from each type declaration
236 ; gen_binds <- mkGenericBinds tycl_decls
238 -- Rename these extra bindings, discarding warnings about unused bindings etc
239 -- Set -fglasgow exts so that we can have type signatures in patterns,
240 -- which is used in the generic binds
242 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
243 { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn deriv_binds [])
244 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
245 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
247 ; return (rn_deriv `plusHsValBinds` rn_gen) }
251 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
252 (ddump_deriving inst_info rn_binds))
254 ; returnM (inst_info, rn_binds)
257 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
258 ddump_deriving inst_infos extra_binds
259 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
261 -----------------------------------------
262 deriveOrdinaryStuff overlap_flag [] -- Short cut
263 = returnM ([], emptyLHsBinds)
265 deriveOrdinaryStuff overlap_flag eqns
266 = do { -- Take the equation list and solve it, to deliver a list of
267 -- solutions, a.k.a. the contexts for the instance decls
268 -- required for the corresponding equations.
269 inst_specs <- solveDerivEqns overlap_flag eqns
271 -- Generate the InstInfo for each dfun,
272 -- plus any auxiliary bindings it needs
273 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst inst_specs
275 -- Generate any extra not-one-inst-decl-specific binds,
276 -- notably "con2tag" and/or "tag2con" functions.
277 ; extra_binds <- genTaggeryBinds inst_infos
280 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
283 -----------------------------------------
284 mkGenericBinds tycl_decls
285 = do { tcs <- mapM tcLookupTyCon
287 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
288 -- We are only interested in the data type declarations
289 ; return (unionManyBags [ mkTyConGenericBinds tc |
290 tc <- tcs, tyConHasGenerics tc ]) }
291 -- And then only in the ones whose 'has-generics' flag is on
295 %************************************************************************
297 \subsection[TcDeriv-eqns]{Forming the equations}
299 %************************************************************************
301 @makeDerivEqns@ fishes around to find the info about needed derived
302 instances. Complicating factors:
305 We can only derive @Enum@ if the data type is an enumeration
306 type (all nullary data constructors).
309 We can only derive @Ix@ if the data type is an enumeration {\em
310 or} has just one data constructor (e.g., tuples).
313 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
317 makeDerivEqns :: OverlapFlag
319 -> TcM ([DerivEqn], -- Ordinary derivings
320 [InstInfo]) -- Special newtype derivings
322 makeDerivEqns overlap_flag tycl_decls
323 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
324 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
326 ------------------------------------------------------------------
327 derive_these :: [(NewOrData, Name, LHsType Name)]
328 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
329 derive_these = [ (nd, tycon, pred)
330 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
331 tcdDerivs = Just preds }) <- tycl_decls,
334 ------------------------------------------------------------------
335 mk_eqn :: (NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
336 -- We swizzle the tyvars and datacons out of the tycon
337 -- to make the rest of the equation
339 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
340 -- we allow deriving (forall a. C [a]).
342 mk_eqn (new_or_data, tycon_name, hs_deriv_ty)
343 = tcLookupTyCon tycon_name `thenM` \ tycon ->
344 setSrcSpan (srcLocSpan (getSrcLoc tycon)) $
345 addErrCtxt (derivCtxt tycon) $
346 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
347 -- the type variables for the type constructor
348 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
349 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
350 mk_eqn_help gla_exts new_or_data tycon deriv_tvs clas tys
352 ------------------------------------------------------------------
353 mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys
354 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
355 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
357 = do { eqn <- mkDataTypeEqn tycon clas
358 ; returnM (Just eqn, Nothing) }
360 mk_eqn_help gla_exts NewType tycon deriv_tvs clas tys
361 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
362 = -- Go ahead and use the isomorphism
363 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
364 new_dfun_name clas tycon `thenM` \ dfun_name ->
365 returnM (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
366 iBinds = NewTypeDerived rep_tys }))
367 | std_class gla_exts clas
368 = mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
370 | otherwise -- Non-standard instance
371 = bale_out (if gla_exts then
372 cant_derive_err -- Too hard
374 non_std_err) -- Just complain about being a non-std instance
376 -- Here is the plan for newtype derivings. We see
377 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
378 -- where t is a type,
379 -- ak...an is a suffix of a1..an
380 -- ak...an do not occur free in t,
381 -- (C s1 ... sm) is a *partial applications* of class C
382 -- with the last parameter missing
384 -- We generate the instances
385 -- instance C s1 .. sm (t ak...ap) => C s1 .. sm (T a1...ap)
386 -- where T a1...ap is the partial application of the LHS of the correct kind
389 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
390 -- instance Monad (ST s) => Monad (T s) where
391 -- fail = coerce ... (fail @ ST s)
392 -- (Actually we don't need the coerce, because non-rec newtypes are transparent
394 clas_tyvars = classTyVars clas
395 kind = tyVarKind (last clas_tyvars)
396 -- Kind of the thing we want to instance
397 -- e.g. argument kind of Monad, *->*
399 (arg_kinds, _) = splitKindFunTys kind
400 n_args_to_drop = length arg_kinds
401 -- Want to drop 1 arg from (T s a) and (ST s a)
402 -- to get instance Monad (ST s) => Monad (T s)
404 -- Note [newtype representation]
405 -- Need newTyConRhs *not* newTyConRep to get the representation
406 -- type, because the latter looks through all intermediate newtypes
408 -- newtype B = MkB Int
409 -- newtype A = MkA B deriving( Num )
410 -- We want the Num instance of B, *not* the Num instance of Int,
411 -- when making the Num instance of A!
412 (tc_tvs, rep_ty) = newTyConRhs tycon
413 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
415 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
416 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
417 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
419 n_args_to_keep = length rep_ty_args - n_args_to_drop
420 args_to_drop = drop n_args_to_keep rep_ty_args
421 args_to_keep = take n_args_to_keep rep_ty_args
423 rep_fn' = mkAppTys rep_fn args_to_keep
424 rep_tys = tys ++ [rep_fn']
425 rep_pred = mkClassPred clas rep_tys
426 -- rep_pred is the representation dictionary, from where
427 -- we are gong to get all the methods for the newtype dictionary
429 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
430 -- The 'tys' here come from the partial application
431 -- in the deriving clause. The last arg is the new
434 -- We must pass the superclasses; the newtype might be an instance
435 -- of them in a different way than the representation type
436 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
437 -- Then the Show instance is not done via isomprphism; it shows
439 -- The Num instance is derived via isomorphism, but the Show superclass
440 -- dictionary must the Show instance for Foo, *not* the Show dictionary
441 -- gotten from the Num dictionary. So we must build a whole new dictionary
442 -- not just use the Num one. The instance we want is something like:
443 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
446 -- There's no 'corece' needed because after the type checker newtypes
449 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
452 -- If there are no tyvars, there's no need
453 -- to abstract over the dictionaries we need
454 dict_tvs = deriv_tvs ++ tc_tvs
455 dict_args | null dict_tvs = []
456 | otherwise = rep_pred : sc_theta
458 -- Finally! Here's where we build the dictionary Id
459 mk_inst_spec dfun_name
460 = mkLocalInstance dfun overlap_flag
462 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
464 -------------------------------------------------------------------
465 -- Figuring out whether we can only do this newtype-deriving thing
467 right_arity = length tys + 1 == classArity clas
469 -- Never derive Read,Show,Typeable,Data this way
470 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
471 can_derive_via_isomorphism
472 = not (getUnique clas `elem` non_iso_classes)
473 && right_arity -- Well kinded;
474 -- eg not: newtype T ... deriving( ST )
475 -- because ST needs *2* type params
476 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
477 -- eg not: newtype T = T Int deriving( Monad )
478 && n_args_to_keep >= 0 -- Rep type has right kind:
479 -- eg not: newtype T a = T Int deriving( Monad )
480 && eta_ok -- Eta reduction works
481 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
482 -- newtype A = MkA [A]
484 -- instance Eq [A] => Eq A !!
485 -- Here's a recursive newtype that's actually OK
486 -- newtype S1 = S1 [T1 ()]
487 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
488 -- It's currently rejected. Oh well.
489 -- In fact we generate an instance decl that has method of form
490 -- meth @ instTy = meth @ repTy
491 -- (no coerce's). We'd need a coerce if we wanted to handle
492 -- recursive newtypes too
494 -- Check that eta reduction is OK
495 -- (a) the dropped-off args are identical
496 -- (b) the remaining type args mention
497 -- only the remaining type variables
498 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
499 && (tyVarsOfType rep_fn' `subVarSet` mkVarSet tyvars_to_keep)
501 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
502 (vcat [ptext SLIT("even with cunning newtype deriving:"),
503 if isRecursiveTyCon tycon then
504 ptext SLIT("the newtype is recursive")
506 if not right_arity then
507 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
509 if not (n_tyvars_to_keep >= 0) then
510 ptext SLIT("the type constructor has wrong kind")
511 else if not (n_args_to_keep >= 0) then
512 ptext SLIT("the representation type has wrong kind")
513 else if not eta_ok then
514 ptext SLIT("the eta-reduction property does not hold")
518 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
519 (vcat [non_std_why clas,
520 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
522 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
524 std_class gla_exts clas
525 = key `elem` derivableClassKeys
526 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
530 std_class_via_iso clas -- These standard classes can be derived for a newtype
531 -- using the isomorphism trick *even if no -fglasgow-exts*
532 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
533 -- Not Read/Show because they respect the type
534 -- Not Enum, becuase newtypes are never in Enum
537 new_dfun_name clas tycon -- Just a simple wrapper
538 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
539 -- The type passed to newDFunName is only used to generate
540 -- a suitable string; hence the empty type arg list
542 ------------------------------------------------------------------
543 mkDataTypeEqn :: TyCon -> Class -> TcM DerivEqn
544 mkDataTypeEqn tycon clas
545 | clas `hasKey` typeableClassKey
546 = -- The Typeable class is special in several ways
547 -- data T a b = ... deriving( Typeable )
549 -- instance Typeable2 T where ...
551 -- 1. There are no constraints in the instance
552 -- 2. There are no type variables either
553 -- 3. The actual class we want to generate isn't necessarily
554 -- Typeable; it depends on the arity of the type
555 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
556 ; dfun_name <- new_dfun_name real_clas tycon
557 ; return (dfun_name, real_clas, tycon, [], []) }
560 = do { dfun_name <- new_dfun_name clas tycon
561 ; return (dfun_name, clas, tycon, tyvars, constraints) }
563 tyvars = tyConTyVars tycon
564 constraints = extra_constraints ++ ordinary_constraints
565 extra_constraints = tyConStupidTheta tycon
566 -- "extra_constraints": see note [Data decl contexts] above
569 = [ mkClassPred clas [arg_ty]
570 | data_con <- tyConDataCons tycon,
571 arg_ty <- dataConInstOrigArgTys data_con (map mkTyVarTy (tyConTyVars tycon)),
572 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
576 ------------------------------------------------------------------
577 -- Check side conditions that dis-allow derivability for particular classes
578 -- This is *apart* from the newtype-deriving mechanism
580 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
581 checkSideConditions gla_exts tycon deriv_tvs clas tys
582 | notNull deriv_tvs || notNull tys
583 = Just ty_args_why -- e.g. deriving( Foo s )
585 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
586 [] -> Just (non_std_why clas)
587 [cond] -> cond (gla_exts, tycon)
588 other -> pprPanic "checkSideConditions" (ppr clas)
590 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
592 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
594 sideConditions :: [(Unique, Condition)]
596 = [ (eqClassKey, cond_std),
597 (ordClassKey, cond_std),
598 (readClassKey, cond_std),
599 (showClassKey, cond_std),
600 (enumClassKey, cond_std `andCond` cond_isEnumeration),
601 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
602 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
603 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
604 (dataClassKey, cond_glaExts `andCond` cond_std)
607 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
609 orCond :: Condition -> Condition -> Condition
612 Nothing -> Nothing -- c1 succeeds
613 Just x -> case c2 tc of -- c1 fails
615 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
618 andCond c1 c2 tc = case c1 tc of
619 Nothing -> c2 tc -- c1 succeeds
620 Just x -> Just x -- c1 fails
622 cond_std :: Condition
623 cond_std (gla_exts, tycon)
624 | any (not . isVanillaDataCon) data_cons = Just existential_why
625 | null data_cons = Just no_cons_why
626 | otherwise = Nothing
628 data_cons = tyConDataCons tycon
629 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
630 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
632 cond_isEnumeration :: Condition
633 cond_isEnumeration (gla_exts, tycon)
634 | isEnumerationTyCon tycon = Nothing
635 | otherwise = Just why
637 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
639 cond_isProduct :: Condition
640 cond_isProduct (gla_exts, tycon)
641 | isProductTyCon tycon = Nothing
642 | otherwise = Just why
644 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
646 cond_typeableOK :: Condition
647 -- OK for Typeable class
648 -- Currently: (a) args all of kind *
649 -- (b) 7 or fewer args
650 cond_typeableOK (gla_exts, tycon)
651 | tyConArity tycon > 7 = Just too_many
652 | not (all (isArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
653 | otherwise = Nothing
655 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
656 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
658 cond_glaExts :: Condition
659 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
660 | otherwise = Just why
662 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
665 %************************************************************************
667 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
669 %************************************************************************
671 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
672 terms, which is the final correct RHS for the corresponding original
676 Each (k,TyVarTy tv) in a solution constrains only a type
680 The (k,TyVarTy tv) pairs in a solution are canonically
681 ordered by sorting on type varible, tv, (major key) and then class, k,
686 solveDerivEqns :: OverlapFlag
688 -> TcM [Instance]-- Solns in same order as eqns.
689 -- This bunch is Absolutely minimal...
691 solveDerivEqns overlap_flag orig_eqns
692 = iterateDeriv 1 initial_solutions
694 -- The initial solutions for the equations claim that each
695 -- instance has an empty context; this solution is certainly
696 -- in canonical form.
697 initial_solutions :: [DerivSoln]
698 initial_solutions = [ [] | _ <- orig_eqns ]
700 ------------------------------------------------------------------
701 -- iterateDeriv calculates the next batch of solutions,
702 -- compares it with the current one; finishes if they are the
703 -- same, otherwise recurses with the new solutions.
704 -- It fails if any iteration fails
705 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
706 iterateDeriv n current_solns
707 | n > 20 -- Looks as if we are in an infinite loop
708 -- This can happen if we have -fallow-undecidable-instances
709 -- (See TcSimplify.tcSimplifyDeriv.)
710 = pprPanic "solveDerivEqns: probable loop"
711 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
714 inst_specs = zipWithEqual "add_solns" mk_inst_spec
715 orig_eqns current_solns
718 -- Extend the inst info from the explicit instance decls
719 -- with the current set of solutions, and simplify each RHS
720 extendLocalInstEnv inst_specs $
721 mappM gen_soln orig_eqns
722 ) `thenM` \ new_solns ->
723 if (current_solns == new_solns) then
726 iterateDeriv (n+1) new_solns
728 ------------------------------------------------------------------
729 gen_soln (_, clas, tc,tyvars,deriv_rhs)
730 = setSrcSpan (srcLocSpan (getSrcLoc tc)) $
731 do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
732 ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
733 tcSimplifyDeriv tc tyvars deriv_rhs
734 ; addErrCtxt (derivInstCtxt2 theta clas inst_tys) $
735 checkValidInstance tyvars theta clas inst_tys
736 ; return (sortLe (<=) theta) } -- Canonicalise before returning the soluction
740 ------------------------------------------------------------------
741 mk_inst_spec (dfun_name, clas, tycon, tyvars, _) theta
742 = mkLocalInstance dfun overlap_flag
744 dfun = mkDictFunId dfun_name tyvars theta clas
745 [mkTyConApp tycon (mkTyVarTys tyvars)]
747 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
748 -- Add new locally-defined instances; don't bother to check
749 -- for functional dependency errors -- that'll happen in TcInstDcls
750 extendLocalInstEnv dfuns thing_inside
751 = do { env <- getGblEnv
752 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
753 env' = env { tcg_inst_env = inst_env' }
754 ; setGblEnv env' thing_inside }
757 %************************************************************************
759 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
761 %************************************************************************
763 After all the trouble to figure out the required context for the
764 derived instance declarations, all that's left is to chug along to
765 produce them. They will then be shoved into @tcInstDecls2@, which
766 will do all its usual business.
768 There are lots of possibilities for code to generate. Here are
769 various general remarks.
774 We want derived instances of @Eq@ and @Ord@ (both v common) to be
775 ``you-couldn't-do-better-by-hand'' efficient.
778 Deriving @Show@---also pretty common--- should also be reasonable good code.
781 Deriving for the other classes isn't that common or that big a deal.
788 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
791 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
794 We {\em normally} generate code only for the non-defaulted methods;
795 there are some exceptions for @Eq@ and (especially) @Ord@...
798 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
799 constructor's numeric (@Int#@) tag. These are generated by
800 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
801 these is around is given by @hasCon2TagFun@.
803 The examples under the different sections below will make this
807 Much less often (really just for deriving @Ix@), we use a
808 @_tag2con_<tycon>@ function. See the examples.
811 We use the renamer!!! Reason: we're supposed to be
812 producing @LHsBinds Name@ for the methods, but that means
813 producing correctly-uniquified code on the fly. This is entirely
814 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
815 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
816 the renamer. What a great hack!
820 -- Generate the InstInfo for the required instance,
821 -- plus any auxiliary bindings required
822 genInst :: Instance -> TcM (InstInfo, LHsBinds RdrName)
824 = do { fix_env <- getFixityEnv
826 (tyvars,_,clas,[ty]) = instanceHead spec
827 clas_nm = className clas
828 tycon = tcTyConAppTyCon ty
829 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
831 -- Bring the right type variables into
832 -- scope, and rename the method binds
833 -- It's a bit yukky that we return *renamed* InstInfo, but
834 -- *non-renamed* auxiliary bindings
835 ; (rn_meth_binds, _fvs) <- discardWarnings $
836 bindLocalNames (map varName tyvars) $
837 rnMethodBinds clas_nm (\n -> []) [] meth_binds
839 -- Build the InstInfo
840 ; return (InstInfo { iSpec = spec,
841 iBinds = VanillaInst rn_meth_binds [] },
845 genDerivBinds clas fix_env tycon
846 | className clas `elem` typeableClassNames
847 = (gen_Typeable_binds tycon, emptyLHsBinds)
850 = case assocMaybe gen_list (getUnique clas) of
851 Just gen_fn -> gen_fn fix_env tycon
852 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
854 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
855 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
856 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
857 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
858 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
859 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
860 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
861 ,(showClassKey, no_aux_binds gen_Show_binds)
862 ,(readClassKey, no_aux_binds gen_Read_binds)
863 ,(dataClassKey, gen_Data_binds)
866 -- no_aux_binds is used for generators that don't
867 -- need to produce any auxiliary bindings
868 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
869 ignore_fix_env f fix_env tc = f tc
873 %************************************************************************
875 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
877 %************************************************************************
882 con2tag_Foo :: Foo ... -> Int#
883 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
884 maxtag_Foo :: Int -- ditto (NB: not unlifted)
887 We have a @con2tag@ function for a tycon if:
890 We're deriving @Eq@ and the tycon has nullary data constructors.
893 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
897 We have a @tag2con@ function for a tycon if:
900 We're deriving @Enum@, or @Ix@ (enum type only???)
903 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
906 genTaggeryBinds :: [InstInfo] -> TcM (LHsBinds RdrName)
907 genTaggeryBinds infos
908 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
909 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
910 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
912 all_CTs = [ (cls, tcTyConAppTyCon ty)
914 let (cls,ty) = simpleInstInfoClsTy info ]
915 all_tycons = map snd all_CTs
916 (tycons_of_interest, _) = removeDups compare all_tycons
918 do_con2tag acc_Names tycon
919 | isDataTyCon tycon &&
920 ((we_are_deriving eqClassKey tycon
921 && any isNullarySrcDataCon (tyConDataCons tycon))
922 || (we_are_deriving ordClassKey tycon
923 && not (isProductTyCon tycon))
924 || (we_are_deriving enumClassKey tycon)
925 || (we_are_deriving ixClassKey tycon))
927 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
932 do_tag2con acc_Names tycon
933 | isDataTyCon tycon &&
934 (we_are_deriving enumClassKey tycon ||
935 we_are_deriving ixClassKey tycon
936 && isEnumerationTyCon tycon)
937 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
938 : (maxtag_RDR tycon, tycon, GenMaxTag)
943 we_are_deriving clas_key tycon
944 = is_in_eqns clas_key tycon all_CTs
946 is_in_eqns clas_key tycon [] = False
947 is_in_eqns clas_key tycon ((c,t):cts)
948 = (clas_key == classKey c && tycon == t)
949 || is_in_eqns clas_key tycon cts
953 derivingThingErr clas tys tycon tyvars why
954 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
957 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
959 derivCtxt :: TyCon -> SDoc
961 = ptext SLIT("When deriving instances for") <+> quotes (ppr tycon)
963 derivInstCtxt1 clas inst_tys
964 = ptext SLIT("When deriving the instance for") <+> quotes (pprClassPred clas inst_tys)
966 derivInstCtxt2 theta clas inst_tys
967 = vcat [ptext SLIT("In the derived instance declaration"),
968 nest 2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta,
969 pprClassPred clas inst_tys])]