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 TcEnv ( newDFunName, pprInstInfoDetails,
19 InstInfo(..), InstBindings(..), simpleInstInfoClsTy,
20 tcLookupClass, tcLookupTyCon, tcExtendTyVarEnv
22 import TcGenDeriv -- Deriv stuff
23 import InstEnv ( Instance, OverlapFlag, mkLocalInstance, instanceHead, extendInstEnvList )
24 import Inst ( getOverlapFlag )
25 import TcHsType ( tcHsDeriv )
26 import TcSimplify ( tcSimplifyDeriv )
28 import RnBinds ( rnMethodBinds, rnTopBinds )
29 import RnEnv ( bindLocalNames )
30 import HscTypes ( FixityEnv )
32 import Class ( className, classArity, classKey, classTyVars, classSCTheta, Class )
33 import Type ( zipOpenTvSubst, substTheta )
34 import ErrUtils ( dumpIfSet_dyn )
35 import MkId ( mkDictFunId )
36 import DataCon ( isNullarySrcDataCon, isVanillaDataCon, dataConOrigArgTys )
37 import Maybes ( catMaybes )
38 import RdrName ( RdrName )
39 import Name ( Name, getSrcLoc )
40 import NameSet ( duDefs )
41 import Kind ( splitKindFunTys )
42 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
43 tyConStupidTheta, isProductTyCon, isDataTyCon, newTyConRhs,
44 isEnumerationTyCon, isRecursiveTyCon, TyCon
46 import TcType ( TcType, ThetaType, mkTyVarTys, mkTyConApp, tcTyConAppTyCon,
47 isUnLiftedType, mkClassPred, tyVarsOfTypes, isArgTypeKind,
48 tcEqTypes, tcSplitAppTys, mkAppTys )
49 import Var ( TyVar, tyVarKind, varName )
50 import VarSet ( mkVarSet, subVarSet )
52 import SrcLoc ( srcLocSpan, Located(..) )
53 import Util ( zipWithEqual, sortLe, notNull )
54 import ListSetOps ( removeDups, assocMaybe )
59 %************************************************************************
61 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
63 %************************************************************************
67 data T a b = C1 (Foo a) (Bar b)
72 [NOTE: See end of these comments for what to do with
73 data (C a, D b) => T a b = ...
76 We want to come up with an instance declaration of the form
78 instance (Ping a, Pong b, ...) => Eq (T a b) where
81 It is pretty easy, albeit tedious, to fill in the code "...". The
82 trick is to figure out what the context for the instance decl is,
83 namely @Ping@, @Pong@ and friends.
85 Let's call the context reqd for the T instance of class C at types
86 (a,b, ...) C (T a b). Thus:
88 Eq (T a b) = (Ping a, Pong b, ...)
90 Now we can get a (recursive) equation from the @data@ decl:
92 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
93 u Eq (T b a) u Eq Int -- From C2
94 u Eq (T a a) -- From C3
96 Foo and Bar may have explicit instances for @Eq@, in which case we can
97 just substitute for them. Alternatively, either or both may have
98 their @Eq@ instances given by @deriving@ clauses, in which case they
99 form part of the system of equations.
101 Now all we need do is simplify and solve the equations, iterating to
102 find the least fixpoint. Notice that the order of the arguments can
103 switch around, as here in the recursive calls to T.
105 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
109 Eq (T a b) = {} -- The empty set
112 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
113 u Eq (T b a) u Eq Int -- From C2
114 u Eq (T a a) -- From C3
116 After simplification:
117 = Eq a u Ping b u {} u {} u {}
122 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
123 u Eq (T b a) u Eq Int -- From C2
124 u Eq (T a a) -- From C3
126 After simplification:
131 = Eq a u Ping b u Eq b u Ping a
133 The next iteration gives the same result, so this is the fixpoint. We
134 need to make a canonical form of the RHS to ensure convergence. We do
135 this by simplifying the RHS to a form in which
137 - the classes constrain only tyvars
138 - the list is sorted by tyvar (major key) and then class (minor key)
139 - no duplicates, of course
141 So, here are the synonyms for the ``equation'' structures:
144 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
145 -- The Name is the name for the DFun we'll build
146 -- The tyvars bind all the variables in the RHS
148 pprDerivEqn (n,c,tc,tvs,rhs)
149 = parens (hsep [ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
151 type DerivRhs = ThetaType
152 type DerivSoln = DerivRhs
156 [Data decl contexts] A note about contexts on data decls
157 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
160 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
162 We will need an instance decl like:
164 instance (Read a, RealFloat a) => Read (Complex a) where
167 The RealFloat in the context is because the read method for Complex is bound
168 to construct a Complex, and doing that requires that the argument type is
171 But this ain't true for Show, Eq, Ord, etc, since they don't construct
172 a Complex; they only take them apart.
174 Our approach: identify the offending classes, and add the data type
175 context to the instance decl. The "offending classes" are
179 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
180 pattern matching against a constructor from a data type with a context
181 gives rise to the constraints for that context -- or at least the thinned
182 version. So now all classes are "offending".
189 newtype T = T Char deriving( C [a] )
191 Notice the free 'a' in the deriving. We have to fill this out to
192 newtype T = T Char deriving( forall a. C [a] )
194 And then translate it to:
195 instance C [a] Char => C [a] T where ...
200 %************************************************************************
202 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
204 %************************************************************************
207 tcDeriving :: [LTyClDecl Name] -- All type constructors
208 -> TcM ([InstInfo], -- The generated "instance decls"
209 HsValBinds Name) -- Extra generated top-level bindings
211 tcDeriving tycl_decls
212 = recoverM (returnM ([], emptyValBindsOut)) $
213 do { -- Fish the "deriving"-related information out of the TcEnv
214 -- and make the necessary "equations".
215 overlap_flag <- getOverlapFlag
216 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns overlap_flag tycl_decls
218 ; (ordinary_inst_info, deriv_binds)
219 <- extendLocalInstEnv (map iSpec newtype_inst_info) $
220 deriveOrdinaryStuff overlap_flag ordinary_eqns
221 -- Add the newtype-derived instances to the inst env
222 -- before tacking the "ordinary" ones
224 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
226 -- If we are compiling a hs-boot file,
227 -- don't generate any derived bindings
228 ; is_boot <- tcIsHsBoot
230 return (inst_info, emptyValBindsOut)
234 -- Generate the generic to/from functions from each type declaration
235 ; gen_binds <- mkGenericBinds tycl_decls
237 -- Rename these extra bindings, discarding warnings about unused bindings etc
238 -- Set -fglasgow exts so that we can have type signatures in patterns,
239 -- which is used in the generic binds
241 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
242 { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn deriv_binds [])
243 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
244 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
246 ; return (rn_deriv `plusHsValBinds` rn_gen) }
250 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
251 (ddump_deriving inst_info rn_binds))
253 ; returnM (inst_info, rn_binds)
256 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
257 ddump_deriving inst_infos extra_binds
258 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
260 -----------------------------------------
261 deriveOrdinaryStuff overlap_flag [] -- Short cut
262 = returnM ([], emptyLHsBinds)
264 deriveOrdinaryStuff overlap_flag eqns
265 = do { -- Take the equation list and solve it, to deliver a list of
266 -- solutions, a.k.a. the contexts for the instance decls
267 -- required for the corresponding equations.
268 inst_specs <- solveDerivEqns overlap_flag eqns
270 -- Generate the InstInfo for each dfun,
271 -- plus any auxiliary bindings it needs
272 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst inst_specs
274 -- Generate any extra not-one-inst-decl-specific binds,
275 -- notably "con2tag" and/or "tag2con" functions.
276 ; extra_binds <- genTaggeryBinds inst_infos
279 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
282 -----------------------------------------
283 mkGenericBinds tycl_decls
284 = do { tcs <- mapM tcLookupTyCon
286 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
287 -- We are only interested in the data type declarations
288 ; return (unionManyBags [ mkTyConGenericBinds tc |
289 tc <- tcs, tyConHasGenerics tc ]) }
290 -- And then only in the ones whose 'has-generics' flag is on
294 %************************************************************************
296 \subsection[TcDeriv-eqns]{Forming the equations}
298 %************************************************************************
300 @makeDerivEqns@ fishes around to find the info about needed derived
301 instances. Complicating factors:
304 We can only derive @Enum@ if the data type is an enumeration
305 type (all nullary data constructors).
308 We can only derive @Ix@ if the data type is an enumeration {\em
309 or} has just one data constructor (e.g., tuples).
312 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
316 makeDerivEqns :: OverlapFlag
318 -> TcM ([DerivEqn], -- Ordinary derivings
319 [InstInfo]) -- Special newtype derivings
321 makeDerivEqns overlap_flag tycl_decls
322 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
323 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
325 ------------------------------------------------------------------
326 derive_these :: [(NewOrData, Name, LHsType Name)]
327 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
328 derive_these = [ (nd, tycon, pred)
329 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
330 tcdDerivs = Just preds }) <- tycl_decls,
333 ------------------------------------------------------------------
334 mk_eqn :: (NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
335 -- We swizzle the tyvars and datacons out of the tycon
336 -- to make the rest of the equation
338 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
339 -- we allow deriving (forall a. C [a]).
341 mk_eqn (new_or_data, tycon_name, hs_deriv_ty)
342 = tcLookupTyCon tycon_name `thenM` \ tycon ->
343 setSrcSpan (srcLocSpan (getSrcLoc tycon)) $
344 addErrCtxt (derivCtxt Nothing tycon) $
345 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
346 -- the type variables for the type constructor
347 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
348 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
349 mk_eqn_help gla_exts new_or_data tycon deriv_tvs clas tys
351 ------------------------------------------------------------------
352 mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys
353 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
354 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
356 = do { eqn <- mkDataTypeEqn tycon clas
357 ; returnM (Just eqn, Nothing) }
359 mk_eqn_help gla_exts NewType tycon deriv_tvs clas tys
360 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
361 = -- Go ahead and use the isomorphism
362 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
363 new_dfun_name clas tycon `thenM` \ dfun_name ->
364 returnM (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
365 iBinds = NewTypeDerived rep_tys }))
366 | std_class gla_exts clas
367 = mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
369 | otherwise -- Non-standard instance
370 = bale_out (if gla_exts then
371 cant_derive_err -- Too hard
373 non_std_err) -- Just complain about being a non-std instance
375 -- Here is the plan for newtype derivings. We see
376 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
377 -- where t is a type,
378 -- ak...an is a suffix of a1..an
379 -- ak...an do not occur free in t,
380 -- (C s1 ... sm) is a *partial applications* of class C
381 -- with the last parameter missing
383 -- We generate the instances
384 -- instance C s1 .. sm (t ak...ap) => C s1 .. sm (T a1...ap)
385 -- where T a1...ap is the partial application of the LHS of the correct kind
388 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
389 -- instance Monad (ST s) => Monad (T s) where
390 -- fail = coerce ... (fail @ ST s)
391 -- (Actually we don't need the coerce, because non-rec newtypes are transparent
393 clas_tyvars = classTyVars clas
394 kind = tyVarKind (last clas_tyvars)
395 -- Kind of the thing we want to instance
396 -- e.g. argument kind of Monad, *->*
398 (arg_kinds, _) = splitKindFunTys kind
399 n_args_to_drop = length arg_kinds
400 -- Want to drop 1 arg from (T s a) and (ST s a)
401 -- to get instance Monad (ST s) => Monad (T s)
403 -- Note [newtype representation]
404 -- Need newTyConRhs *not* newTyConRep to get the representation
405 -- type, because the latter looks through all intermediate newtypes
407 -- newtype B = MkB Int
408 -- newtype A = MkA B deriving( Num )
409 -- We want the Num instance of B, *not* the Num instance of Int,
410 -- when making the Num instance of A!
411 (tc_tvs, rep_ty) = newTyConRhs tycon
412 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
414 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
415 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
416 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
418 n_args_to_keep = length rep_ty_args - n_args_to_drop
419 args_to_drop = drop n_args_to_keep rep_ty_args
420 args_to_keep = take n_args_to_keep rep_ty_args
422 rep_tys = tys ++ [mkAppTys rep_fn args_to_keep]
423 rep_pred = mkClassPred clas rep_tys
424 -- rep_pred is the representation dictionary, from where
425 -- we are gong to get all the methods for the newtype dictionary
427 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
428 -- The 'tys' here come from the partial application
429 -- in the deriving clause. The last arg is the new
432 -- We must pass the superclasses; the newtype might be an instance
433 -- of them in a different way than the representation type
434 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
435 -- Then the Show instance is not done via isomprphism; it shows
437 -- The Num instance is derived via isomorphism, but the Show superclass
438 -- dictionary must the Show instance for Foo, *not* the Show dictionary
439 -- gotten from the Num dictionary. So we must build a whole new dictionary
440 -- not just use the Num one. The instance we want is something like:
441 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
444 -- There's no 'corece' needed because after the type checker newtypes
447 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
450 -- If there are no tyvars, there's no need
451 -- to abstract over the dictionaries we need
452 dict_tvs = deriv_tvs ++ tc_tvs
453 dict_args | null dict_tvs = []
454 | otherwise = rep_pred : sc_theta
456 -- Finally! Here's where we build the dictionary Id
457 mk_inst_spec dfun_name
458 = mkLocalInstance dfun overlap_flag
460 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
462 -------------------------------------------------------------------
463 -- Figuring out whether we can only do this newtype-deriving thing
465 right_arity = length tys + 1 == classArity clas
467 -- Never derive Read,Show,Typeable,Data this way
468 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
469 can_derive_via_isomorphism
470 = not (getUnique clas `elem` non_iso_classes)
471 && right_arity -- Well kinded;
472 -- eg not: newtype T ... deriving( ST )
473 -- because ST needs *2* type params
474 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
475 -- eg not: newtype T = T Int deriving( Monad )
476 && n_args_to_keep >= 0 -- Rep type has right kind:
477 -- eg not: newtype T a = T Int deriving( Monad )
478 && eta_ok -- Eta reduction works
479 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
480 -- newtype A = MkA [A]
482 -- instance Eq [A] => Eq A !!
483 -- Here's a recursive newtype that's actually OK
484 -- newtype S1 = S1 [T1 ()]
485 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
486 -- It's currently rejected. Oh well.
487 -- In fact we generate an instance decl that has method of form
488 -- meth @ instTy = meth @ repTy
489 -- (no coerce's). We'd need a coerce if we wanted to handle
490 -- recursive newtypes too
492 -- Check that eta reduction is OK
493 -- (a) the dropped-off args are identical
494 -- (b) the remaining type args mention
495 -- only the remaining type variables
496 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
497 && (tyVarsOfTypes args_to_keep `subVarSet` mkVarSet tyvars_to_keep)
499 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
500 (vcat [ptext SLIT("even with cunning newtype deriving:"),
501 if isRecursiveTyCon tycon then
502 ptext SLIT("the newtype is recursive")
504 if not right_arity then
505 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
507 if not (n_tyvars_to_keep >= 0) then
508 ptext SLIT("the type constructor has wrong kind")
509 else if not (n_args_to_keep >= 0) then
510 ptext SLIT("the representation type has wrong kind")
511 else if not eta_ok then
512 ptext SLIT("the eta-reduction property does not hold")
516 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
517 (vcat [non_std_why clas,
518 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
520 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
522 std_class gla_exts clas
523 = key `elem` derivableClassKeys
524 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
528 std_class_via_iso clas -- These standard classes can be derived for a newtype
529 -- using the isomorphism trick *even if no -fglasgow-exts*
530 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
531 -- Not Read/Show because they respect the type
532 -- Not Enum, becuase newtypes are never in Enum
535 new_dfun_name clas tycon -- Just a simple wrapper
536 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
537 -- The type passed to newDFunName is only used to generate
538 -- a suitable string; hence the empty type arg list
540 ------------------------------------------------------------------
541 mkDataTypeEqn :: TyCon -> Class -> TcM DerivEqn
542 mkDataTypeEqn tycon clas
543 | clas `hasKey` typeableClassKey
544 = -- The Typeable class is special in several ways
545 -- data T a b = ... deriving( Typeable )
547 -- instance Typeable2 T where ...
549 -- 1. There are no constraints in the instance
550 -- 2. There are no type variables either
551 -- 3. The actual class we want to generate isn't necessarily
552 -- Typeable; it depends on the arity of the type
553 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
554 ; dfun_name <- new_dfun_name real_clas tycon
555 ; return (dfun_name, real_clas, tycon, [], []) }
558 = do { dfun_name <- new_dfun_name clas tycon
559 ; return (dfun_name, clas, tycon, tyvars, constraints) }
561 tyvars = tyConTyVars tycon
562 constraints = extra_constraints ++ ordinary_constraints
563 extra_constraints = tyConStupidTheta tycon
564 -- "extra_constraints": see note [Data decl contexts] above
567 = [ mkClassPred clas [arg_ty]
568 | data_con <- tyConDataCons tycon,
569 arg_ty <- dataConOrigArgTys data_con,
570 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
574 ------------------------------------------------------------------
575 -- Check side conditions that dis-allow derivability for particular classes
576 -- This is *apart* from the newtype-deriving mechanism
578 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
579 checkSideConditions gla_exts tycon deriv_tvs clas tys
580 | notNull deriv_tvs || notNull tys
581 = Just ty_args_why -- e.g. deriving( Foo s )
583 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
584 [] -> Just (non_std_why clas)
585 [cond] -> cond (gla_exts, tycon)
586 other -> pprPanic "checkSideConditions" (ppr clas)
588 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
590 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
592 sideConditions :: [(Unique, Condition)]
594 = [ (eqClassKey, cond_std),
595 (ordClassKey, cond_std),
596 (readClassKey, cond_std),
597 (showClassKey, cond_std),
598 (enumClassKey, cond_std `andCond` cond_isEnumeration),
599 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
600 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
601 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
602 (dataClassKey, cond_glaExts `andCond` cond_std)
605 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
607 orCond :: Condition -> Condition -> Condition
610 Nothing -> Nothing -- c1 succeeds
611 Just x -> case c2 tc of -- c1 fails
613 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
616 andCond c1 c2 tc = case c1 tc of
617 Nothing -> c2 tc -- c1 succeeds
618 Just x -> Just x -- c1 fails
620 cond_std :: Condition
621 cond_std (gla_exts, tycon)
622 | any (not . isVanillaDataCon) data_cons = Just existential_why
623 | null data_cons = Just no_cons_why
624 | otherwise = Nothing
626 data_cons = tyConDataCons tycon
627 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
628 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
630 cond_isEnumeration :: Condition
631 cond_isEnumeration (gla_exts, tycon)
632 | isEnumerationTyCon tycon = Nothing
633 | otherwise = Just why
635 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
637 cond_isProduct :: Condition
638 cond_isProduct (gla_exts, tycon)
639 | isProductTyCon tycon = Nothing
640 | otherwise = Just why
642 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
644 cond_typeableOK :: Condition
645 -- OK for Typeable class
646 -- Currently: (a) args all of kind *
647 -- (b) 7 or fewer args
648 cond_typeableOK (gla_exts, tycon)
649 | tyConArity tycon > 7 = Just too_many
650 | not (all (isArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
651 | otherwise = Nothing
653 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
654 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
656 cond_glaExts :: Condition
657 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
658 | otherwise = Just why
660 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
663 %************************************************************************
665 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
667 %************************************************************************
669 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
670 terms, which is the final correct RHS for the corresponding original
674 Each (k,TyVarTy tv) in a solution constrains only a type
678 The (k,TyVarTy tv) pairs in a solution are canonically
679 ordered by sorting on type varible, tv, (major key) and then class, k,
684 solveDerivEqns :: OverlapFlag
686 -> TcM [Instance]-- Solns in same order as eqns.
687 -- This bunch is Absolutely minimal...
689 solveDerivEqns overlap_flag orig_eqns
690 = iterateDeriv 1 initial_solutions
692 -- The initial solutions for the equations claim that each
693 -- instance has an empty context; this solution is certainly
694 -- in canonical form.
695 initial_solutions :: [DerivSoln]
696 initial_solutions = [ [] | _ <- orig_eqns ]
698 ------------------------------------------------------------------
699 -- iterateDeriv calculates the next batch of solutions,
700 -- compares it with the current one; finishes if they are the
701 -- same, otherwise recurses with the new solutions.
702 -- It fails if any iteration fails
703 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
704 iterateDeriv n current_solns
705 | n > 20 -- Looks as if we are in an infinite loop
706 -- This can happen if we have -fallow-undecidable-instances
707 -- (See TcSimplify.tcSimplifyDeriv.)
708 = pprPanic "solveDerivEqns: probable loop"
709 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
712 inst_specs = zipWithEqual "add_solns" mk_inst_spec
713 orig_eqns current_solns
716 -- Extend the inst info from the explicit instance decls
717 -- with the current set of solutions, and simplify each RHS
718 extendLocalInstEnv inst_specs $
719 mappM gen_soln orig_eqns
720 ) `thenM` \ new_solns ->
721 if (current_solns == new_solns) then
724 iterateDeriv (n+1) new_solns
726 ------------------------------------------------------------------
727 gen_soln (_, clas, tc,tyvars,deriv_rhs)
728 = setSrcSpan (srcLocSpan (getSrcLoc tc)) $
729 addErrCtxt (derivCtxt (Just clas) tc) $
730 tcSimplifyDeriv tyvars deriv_rhs `thenM` \ theta ->
731 returnM (sortLe (<=) theta) -- Canonicalise before returning the soluction
733 ------------------------------------------------------------------
734 mk_inst_spec (dfun_name, clas, tycon, tyvars, _) theta
735 = mkLocalInstance dfun overlap_flag
737 dfun = mkDictFunId dfun_name tyvars theta clas
738 [mkTyConApp tycon (mkTyVarTys tyvars)]
740 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
741 -- Add new locally-defined instances; don't bother to check
742 -- for functional dependency errors -- that'll happen in TcInstDcls
743 extendLocalInstEnv dfuns thing_inside
744 = do { env <- getGblEnv
745 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
746 env' = env { tcg_inst_env = inst_env' }
747 ; setGblEnv env' thing_inside }
750 %************************************************************************
752 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
754 %************************************************************************
756 After all the trouble to figure out the required context for the
757 derived instance declarations, all that's left is to chug along to
758 produce them. They will then be shoved into @tcInstDecls2@, which
759 will do all its usual business.
761 There are lots of possibilities for code to generate. Here are
762 various general remarks.
767 We want derived instances of @Eq@ and @Ord@ (both v common) to be
768 ``you-couldn't-do-better-by-hand'' efficient.
771 Deriving @Show@---also pretty common--- should also be reasonable good code.
774 Deriving for the other classes isn't that common or that big a deal.
781 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
784 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
787 We {\em normally} generate code only for the non-defaulted methods;
788 there are some exceptions for @Eq@ and (especially) @Ord@...
791 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
792 constructor's numeric (@Int#@) tag. These are generated by
793 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
794 these is around is given by @hasCon2TagFun@.
796 The examples under the different sections below will make this
800 Much less often (really just for deriving @Ix@), we use a
801 @_tag2con_<tycon>@ function. See the examples.
804 We use the renamer!!! Reason: we're supposed to be
805 producing @LHsBinds Name@ for the methods, but that means
806 producing correctly-uniquified code on the fly. This is entirely
807 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
808 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
809 the renamer. What a great hack!
813 -- Generate the InstInfo for the required instance,
814 -- plus any auxiliary bindings required
815 genInst :: Instance -> TcM (InstInfo, LHsBinds RdrName)
817 = do { fix_env <- getFixityEnv
819 (tyvars,_,clas,[ty]) = instanceHead spec
820 clas_nm = className clas
821 tycon = tcTyConAppTyCon ty
822 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
824 -- Bring the right type variables into
825 -- scope, and rename the method binds
826 -- It's a bit yukky that we return *renamed* InstInfo, but
827 -- *non-renamed* auxiliary bindings
828 ; (rn_meth_binds, _fvs) <- discardWarnings $
829 bindLocalNames (map varName tyvars) $
830 rnMethodBinds clas_nm [] meth_binds
832 -- Build the InstInfo
833 ; return (InstInfo { iSpec = spec,
834 iBinds = VanillaInst rn_meth_binds [] },
838 genDerivBinds clas fix_env tycon
839 | className clas `elem` typeableClassNames
840 = (gen_Typeable_binds tycon, emptyLHsBinds)
843 = case assocMaybe gen_list (getUnique clas) of
844 Just gen_fn -> gen_fn fix_env tycon
845 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
847 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
848 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
849 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
850 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
851 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
852 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
853 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
854 ,(showClassKey, no_aux_binds gen_Show_binds)
855 ,(readClassKey, no_aux_binds gen_Read_binds)
856 ,(dataClassKey, gen_Data_binds)
859 -- no_aux_binds is used for generators that don't
860 -- need to produce any auxiliary bindings
861 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
862 ignore_fix_env f fix_env tc = f tc
866 %************************************************************************
868 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
870 %************************************************************************
875 con2tag_Foo :: Foo ... -> Int#
876 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
877 maxtag_Foo :: Int -- ditto (NB: not unlifted)
880 We have a @con2tag@ function for a tycon if:
883 We're deriving @Eq@ and the tycon has nullary data constructors.
886 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
890 We have a @tag2con@ function for a tycon if:
893 We're deriving @Enum@, or @Ix@ (enum type only???)
896 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
899 genTaggeryBinds :: [InstInfo] -> TcM (LHsBinds RdrName)
900 genTaggeryBinds infos
901 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
902 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
903 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
905 all_CTs = [ (cls, tcTyConAppTyCon ty)
907 let (cls,ty) = simpleInstInfoClsTy info ]
908 all_tycons = map snd all_CTs
909 (tycons_of_interest, _) = removeDups compare all_tycons
911 do_con2tag acc_Names tycon
912 | isDataTyCon tycon &&
913 ((we_are_deriving eqClassKey tycon
914 && any isNullarySrcDataCon (tyConDataCons tycon))
915 || (we_are_deriving ordClassKey tycon
916 && not (isProductTyCon tycon))
917 || (we_are_deriving enumClassKey tycon)
918 || (we_are_deriving ixClassKey tycon))
920 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
925 do_tag2con acc_Names tycon
926 | isDataTyCon tycon &&
927 (we_are_deriving enumClassKey tycon ||
928 we_are_deriving ixClassKey tycon
929 && isEnumerationTyCon tycon)
930 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
931 : (maxtag_RDR tycon, tycon, GenMaxTag)
936 we_are_deriving clas_key tycon
937 = is_in_eqns clas_key tycon all_CTs
939 is_in_eqns clas_key tycon [] = False
940 is_in_eqns clas_key tycon ((c,t):cts)
941 = (clas_key == classKey c && tycon == t)
942 || is_in_eqns clas_key tycon cts
946 derivingThingErr clas tys tycon tyvars why
947 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
950 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
952 derivCtxt :: Maybe Class -> TyCon -> SDoc
953 derivCtxt maybe_cls tycon
954 = ptext SLIT("When deriving") <+> cls <+> ptext SLIT("for type") <+> quotes (ppr tycon)
956 cls = case maybe_cls of
957 Nothing -> ptext SLIT("instances")
958 Just c -> ptext SLIT("the") <+> quotes (ppr c) <+> ptext SLIT("instance")