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, dataConInstOrigArgTys )
38 import Maybes ( catMaybes )
39 import RdrName ( RdrName )
40 import Name ( Name, getSrcLoc )
41 import NameSet ( duDefs )
42 import Type ( splitKindFunTys )
43 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
44 tyConStupidTheta, isProductTyCon, isDataTyCon, newTyConRhs,
45 isEnumerationTyCon, isRecursiveTyCon, TyCon
47 import TcType ( TcType, ThetaType, mkTyVarTys, mkTyConApp, tcTyConAppTyCon,
48 isUnLiftedType, mkClassPred, tyVarsOfType, tyVarsOfTypes,
49 isSubArgTypeKind, tcEqTypes, tcSplitAppTys, mkAppTys )
50 import Var ( TyVar, tyVarKind, varName )
51 import VarSet ( mkVarSet, disjointVarSet )
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/
316 Note [Newtype deriving superclasses]
317 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
318 The 'tys' here come from the partial application in the deriving
319 clause. The last arg is the new instance type.
321 We must pass the superclasses; the newtype might be an instance
322 of them in a different way than the representation type
323 E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
324 Then the Show instance is not done via isomorphism; it shows
326 The Num instance is derived via isomorphism, but the Show superclass
327 dictionary must the Show instance for Foo, *not* the Show dictionary
328 gotten from the Num dictionary. So we must build a whole new dictionary
329 not just use the Num one. The instance we want is something like:
330 instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
333 There may be a coercion needed which we get from the tycon for the newtype
334 when the dict is constructed in TcInstDcls.tcInstDecl2
338 makeDerivEqns :: OverlapFlag
340 -> TcM ([DerivEqn], -- Ordinary derivings
341 [InstInfo]) -- Special newtype derivings
343 makeDerivEqns overlap_flag tycl_decls
344 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
345 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
347 ------------------------------------------------------------------
348 derive_these :: [(NewOrData, Name, LHsType Name)]
349 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
350 derive_these = [ (nd, tycon, pred)
351 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
352 tcdDerivs = Just preds }) <- tycl_decls,
355 ------------------------------------------------------------------
356 mk_eqn :: (NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
357 -- We swizzle the tyvars and datacons out of the tycon
358 -- to make the rest of the equation
360 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
361 -- we allow deriving (forall a. C [a]).
363 mk_eqn (new_or_data, tycon_name, hs_deriv_ty)
364 = tcLookupTyCon tycon_name `thenM` \ tycon ->
365 setSrcSpan (srcLocSpan (getSrcLoc tycon)) $
366 addErrCtxt (derivCtxt tycon) $
367 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
368 -- the type variables for the type constructor
369 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
370 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
371 mk_eqn_help gla_exts new_or_data tycon deriv_tvs clas tys
373 ------------------------------------------------------------------
374 -- data/newtype T a = ... deriving( C t1 t2 )
375 -- leads to a call to mk_eqn_help with
376 -- tycon = T, deriv_tvs = ftv(t1,t2), clas = C, tys = [t1,t2]
378 mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys
379 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
380 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
382 = do { eqn <- mkDataTypeEqn tycon clas
383 ; returnM (Just eqn, Nothing) }
385 mk_eqn_help gla_exts NewType tycon deriv_tvs clas tys
386 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
387 = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys)
388 ; -- Go ahead and use the isomorphism
389 dfun_name <- new_dfun_name clas tycon
390 ; return (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
391 iBinds = NewTypeDerived ntd_info })) }
392 | std_class gla_exts clas
393 = mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
395 | otherwise -- Non-standard instance
396 = bale_out (if gla_exts then
397 cant_derive_err -- Too hard
399 non_std_err) -- Just complain about being a non-std instance
401 -- Here is the plan for newtype derivings. We see
402 -- newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
403 -- where t is a type,
404 -- ak+1...an is a suffix of a1..an
405 -- ak+1...an do not occur free in t, nor in the s1..sm
406 -- (C s1 ... sm) is a *partial applications* of class C
407 -- with the last parameter missing
408 -- (T a1 .. ak) matches the kind of C's last argument
409 -- (and hence so does t)
411 -- We generate the instance
412 -- instance forall ({a1..ak} u fvs(s1..sm)).
413 -- C s1 .. sm t => C s1 .. sm (T a1...ak)
414 -- where T a1...ap is the partial application of
415 -- the LHS of the correct kind and p >= k
417 -- NB: the variables below are:
418 -- tc_tvs = [a1, ..., an]
419 -- tyvars_to_keep = [a1, ..., ak]
420 -- rep_ty = t ak .. an
421 -- deriv_tvs = fvs(s1..sm) \ tc_tvs
422 -- tys = [s1, ..., sm]
425 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
426 -- We generate the instance
427 -- instance Monad (ST s) => Monad (T s) where
429 clas_tyvars = classTyVars clas
430 kind = tyVarKind (last clas_tyvars)
431 -- Kind of the thing we want to instance
432 -- e.g. argument kind of Monad, *->*
434 (arg_kinds, _) = splitKindFunTys kind
435 n_args_to_drop = length arg_kinds
436 -- Want to drop 1 arg from (T s a) and (ST s a)
437 -- to get instance Monad (ST s) => Monad (T s)
439 -- Note [newtype representation]
440 -- Need newTyConRhs *not* newTyConRep to get the representation
441 -- type, because the latter looks through all intermediate newtypes
443 -- newtype B = MkB Int
444 -- newtype A = MkA B deriving( Num )
445 -- We want the Num instance of B, *not* the Num instance of Int,
446 -- when making the Num instance of A!
447 (tc_tvs, rep_ty) = newTyConRhs tycon
448 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
450 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
451 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
452 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
454 n_args_to_keep = length rep_ty_args - n_args_to_drop
455 args_to_drop = drop n_args_to_keep rep_ty_args
456 args_to_keep = take n_args_to_keep rep_ty_args
458 rep_fn' = mkAppTys rep_fn args_to_keep
459 rep_tys = tys ++ [rep_fn']
460 rep_pred = mkClassPred clas rep_tys
461 -- rep_pred is the representation dictionary, from where
462 -- we are gong to get all the methods for the newtype dictionary
464 -- Next we figure out what superclass dictionaries to use
465 -- See Note [Newtype deriving superclasses] above
467 inst_tys = tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)]
468 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
471 -- If there are no tyvars, there's no need
472 -- to abstract over the dictionaries we need
473 -- Example: newtype T = MkT Int deriving( C )
474 -- We get the derived instance
477 -- instance C Int => C T
478 dict_tvs = deriv_tvs ++ tyvars_to_keep
479 all_preds = rep_pred : sc_theta -- NB: rep_pred comes first
480 (dict_args, ntd_info) | null dict_tvs = ([], Just all_preds)
481 | otherwise = (all_preds, Nothing)
483 -- Finally! Here's where we build the dictionary Id
484 mk_inst_spec dfun_name = mkLocalInstance dfun overlap_flag
486 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
488 -------------------------------------------------------------------
489 -- Figuring out whether we can only do this newtype-deriving thing
491 right_arity = length tys + 1 == classArity clas
493 -- Never derive Read,Show,Typeable,Data this way
494 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
495 can_derive_via_isomorphism
496 = not (getUnique clas `elem` non_iso_classes)
497 && right_arity -- Well kinded;
498 -- eg not: newtype T ... deriving( ST )
499 -- because ST needs *2* type params
500 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
501 -- eg not: newtype T = T Int deriving( Monad )
502 && n_args_to_keep >= 0 -- Rep type has right kind:
503 -- eg not: newtype T a = T Int deriving( Monad )
504 && eta_ok -- Eta reduction works
505 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
506 -- newtype A = MkA [A]
508 -- instance Eq [A] => Eq A !!
509 -- Here's a recursive newtype that's actually OK
510 -- newtype S1 = S1 [T1 ()]
511 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
512 -- It's currently rejected. Oh well.
513 -- In fact we generate an instance decl that has method of form
514 -- meth @ instTy = meth @ repTy
515 -- (no coerce's). We'd need a coerce if we wanted to handle
516 -- recursive newtypes too
518 -- Check that eta reduction is OK
519 -- (a) the dropped-off args are identical
520 -- (b) the remaining type args do not mention any of teh dropped type variables
521 -- (c) the type class args do not mention any of teh dropped type variables
522 dropped_tvs = mkVarSet tyvars_to_drop
523 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
524 && (tyVarsOfType rep_fn' `disjointVarSet` dropped_tvs)
525 && (tyVarsOfTypes tys `disjointVarSet` dropped_tvs)
527 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
528 (vcat [ptext SLIT("even with cunning newtype deriving:"),
529 if isRecursiveTyCon tycon then
530 ptext SLIT("the newtype is recursive")
532 if not right_arity then
533 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
535 if not (n_tyvars_to_keep >= 0) then
536 ptext SLIT("the type constructor has wrong kind")
537 else if not (n_args_to_keep >= 0) then
538 ptext SLIT("the representation type has wrong kind")
539 else if not eta_ok then
540 ptext SLIT("the eta-reduction property does not hold")
544 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
545 (vcat [non_std_why clas,
546 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
548 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
550 std_class gla_exts clas
551 = key `elem` derivableClassKeys
552 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
556 std_class_via_iso clas -- These standard classes can be derived for a newtype
557 -- using the isomorphism trick *even if no -fglasgow-exts*
558 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
559 -- Not Read/Show because they respect the type
560 -- Not Enum, becuase newtypes are never in Enum
563 new_dfun_name clas tycon -- Just a simple wrapper
564 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
565 -- The type passed to newDFunName is only used to generate
566 -- a suitable string; hence the empty type arg list
568 ------------------------------------------------------------------
569 mkDataTypeEqn :: TyCon -> Class -> TcM DerivEqn
570 mkDataTypeEqn tycon clas
571 | clas `hasKey` typeableClassKey
572 = -- The Typeable class is special in several ways
573 -- data T a b = ... deriving( Typeable )
575 -- instance Typeable2 T where ...
577 -- 1. There are no constraints in the instance
578 -- 2. There are no type variables either
579 -- 3. The actual class we want to generate isn't necessarily
580 -- Typeable; it depends on the arity of the type
581 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
582 ; dfun_name <- new_dfun_name real_clas tycon
583 ; return (dfun_name, real_clas, tycon, [], []) }
586 = do { dfun_name <- new_dfun_name clas tycon
587 ; return (dfun_name, clas, tycon, tyvars, constraints) }
589 tyvars = tyConTyVars tycon
590 constraints = extra_constraints ++ ordinary_constraints
591 extra_constraints = tyConStupidTheta tycon
592 -- "extra_constraints": see note [Data decl contexts] above
595 = [ mkClassPred clas [arg_ty]
596 | data_con <- tyConDataCons tycon,
597 arg_ty <- dataConInstOrigArgTys data_con (map mkTyVarTy (tyConTyVars tycon)),
598 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
602 ------------------------------------------------------------------
603 -- Check side conditions that dis-allow derivability for particular classes
604 -- This is *apart* from the newtype-deriving mechanism
606 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
607 checkSideConditions gla_exts tycon deriv_tvs clas tys
608 | notNull deriv_tvs || notNull tys
609 = Just ty_args_why -- e.g. deriving( Foo s )
611 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
612 [] -> Just (non_std_why clas)
613 [cond] -> cond (gla_exts, tycon)
614 other -> pprPanic "checkSideConditions" (ppr clas)
616 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
618 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
620 sideConditions :: [(Unique, Condition)]
622 = [ (eqClassKey, cond_std),
623 (ordClassKey, cond_std),
624 (readClassKey, cond_std),
625 (showClassKey, cond_std),
626 (enumClassKey, cond_std `andCond` cond_isEnumeration),
627 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
628 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
629 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
630 (dataClassKey, cond_glaExts `andCond` cond_std)
633 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
635 orCond :: Condition -> Condition -> Condition
638 Nothing -> Nothing -- c1 succeeds
639 Just x -> case c2 tc of -- c1 fails
641 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
644 andCond c1 c2 tc = case c1 tc of
645 Nothing -> c2 tc -- c1 succeeds
646 Just x -> Just x -- c1 fails
648 cond_std :: Condition
649 cond_std (gla_exts, tycon)
650 | any (not . isVanillaDataCon) data_cons = Just existential_why
651 | null data_cons = Just no_cons_why
652 | otherwise = Nothing
654 data_cons = tyConDataCons tycon
655 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
656 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
658 cond_isEnumeration :: Condition
659 cond_isEnumeration (gla_exts, tycon)
660 | isEnumerationTyCon tycon = Nothing
661 | otherwise = Just why
663 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
665 cond_isProduct :: Condition
666 cond_isProduct (gla_exts, tycon)
667 | isProductTyCon tycon = Nothing
668 | otherwise = Just why
670 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
672 cond_typeableOK :: Condition
673 -- OK for Typeable class
674 -- Currently: (a) args all of kind *
675 -- (b) 7 or fewer args
676 cond_typeableOK (gla_exts, tycon)
677 | tyConArity tycon > 7 = Just too_many
678 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
679 | otherwise = Nothing
681 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
682 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
684 cond_glaExts :: Condition
685 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
686 | otherwise = Just why
688 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
691 %************************************************************************
693 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
695 %************************************************************************
697 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
698 terms, which is the final correct RHS for the corresponding original
702 Each (k,TyVarTy tv) in a solution constrains only a type
706 The (k,TyVarTy tv) pairs in a solution are canonically
707 ordered by sorting on type varible, tv, (major key) and then class, k,
712 solveDerivEqns :: OverlapFlag
714 -> TcM [Instance]-- Solns in same order as eqns.
715 -- This bunch is Absolutely minimal...
717 solveDerivEqns overlap_flag orig_eqns
718 = iterateDeriv 1 initial_solutions
720 -- The initial solutions for the equations claim that each
721 -- instance has an empty context; this solution is certainly
722 -- in canonical form.
723 initial_solutions :: [DerivSoln]
724 initial_solutions = [ [] | _ <- orig_eqns ]
726 ------------------------------------------------------------------
727 -- iterateDeriv calculates the next batch of solutions,
728 -- compares it with the current one; finishes if they are the
729 -- same, otherwise recurses with the new solutions.
730 -- It fails if any iteration fails
731 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
732 iterateDeriv n current_solns
733 | n > 20 -- Looks as if we are in an infinite loop
734 -- This can happen if we have -fallow-undecidable-instances
735 -- (See TcSimplify.tcSimplifyDeriv.)
736 = pprPanic "solveDerivEqns: probable loop"
737 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
740 inst_specs = zipWithEqual "add_solns" mk_inst_spec
741 orig_eqns current_solns
744 -- Extend the inst info from the explicit instance decls
745 -- with the current set of solutions, and simplify each RHS
746 extendLocalInstEnv inst_specs $
747 mappM gen_soln orig_eqns
748 ) `thenM` \ new_solns ->
749 if (current_solns == new_solns) then
752 iterateDeriv (n+1) new_solns
754 ------------------------------------------------------------------
755 gen_soln (_, clas, tc,tyvars,deriv_rhs)
756 = setSrcSpan (srcLocSpan (getSrcLoc tc)) $
757 do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
758 ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
759 tcSimplifyDeriv tc tyvars deriv_rhs
760 ; addErrCtxt (derivInstCtxt2 theta clas inst_tys) $
761 checkValidInstance tyvars theta clas inst_tys
762 ; return (sortLe (<=) theta) } -- Canonicalise before returning the soluction
766 ------------------------------------------------------------------
767 mk_inst_spec (dfun_name, clas, tycon, tyvars, _) theta
768 = mkLocalInstance dfun overlap_flag
770 dfun = mkDictFunId dfun_name tyvars theta clas
771 [mkTyConApp tycon (mkTyVarTys tyvars)]
773 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
774 -- Add new locally-defined instances; don't bother to check
775 -- for functional dependency errors -- that'll happen in TcInstDcls
776 extendLocalInstEnv dfuns thing_inside
777 = do { env <- getGblEnv
778 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
779 env' = env { tcg_inst_env = inst_env' }
780 ; setGblEnv env' thing_inside }
783 %************************************************************************
785 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
787 %************************************************************************
789 After all the trouble to figure out the required context for the
790 derived instance declarations, all that's left is to chug along to
791 produce them. They will then be shoved into @tcInstDecls2@, which
792 will do all its usual business.
794 There are lots of possibilities for code to generate. Here are
795 various general remarks.
800 We want derived instances of @Eq@ and @Ord@ (both v common) to be
801 ``you-couldn't-do-better-by-hand'' efficient.
804 Deriving @Show@---also pretty common--- should also be reasonable good code.
807 Deriving for the other classes isn't that common or that big a deal.
814 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
817 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
820 We {\em normally} generate code only for the non-defaulted methods;
821 there are some exceptions for @Eq@ and (especially) @Ord@...
824 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
825 constructor's numeric (@Int#@) tag. These are generated by
826 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
827 these is around is given by @hasCon2TagFun@.
829 The examples under the different sections below will make this
833 Much less often (really just for deriving @Ix@), we use a
834 @_tag2con_<tycon>@ function. See the examples.
837 We use the renamer!!! Reason: we're supposed to be
838 producing @LHsBinds Name@ for the methods, but that means
839 producing correctly-uniquified code on the fly. This is entirely
840 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
841 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
842 the renamer. What a great hack!
846 -- Generate the InstInfo for the required instance,
847 -- plus any auxiliary bindings required
848 genInst :: Instance -> TcM (InstInfo, LHsBinds RdrName)
850 = do { fix_env <- getFixityEnv
852 (tyvars,_,clas,[ty]) = instanceHead spec
853 clas_nm = className clas
854 tycon = tcTyConAppTyCon ty
855 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
857 -- Bring the right type variables into
858 -- scope, and rename the method binds
859 -- It's a bit yukky that we return *renamed* InstInfo, but
860 -- *non-renamed* auxiliary bindings
861 ; (rn_meth_binds, _fvs) <- discardWarnings $
862 bindLocalNames (map varName tyvars) $
863 rnMethodBinds clas_nm (\n -> []) [] meth_binds
865 -- Build the InstInfo
866 ; return (InstInfo { iSpec = spec,
867 iBinds = VanillaInst rn_meth_binds [] },
871 genDerivBinds clas fix_env tycon
872 | className clas `elem` typeableClassNames
873 = (gen_Typeable_binds tycon, emptyLHsBinds)
876 = case assocMaybe gen_list (getUnique clas) of
877 Just gen_fn -> gen_fn fix_env tycon
878 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
880 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
881 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
882 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
883 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
884 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
885 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
886 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
887 ,(showClassKey, no_aux_binds gen_Show_binds)
888 ,(readClassKey, no_aux_binds gen_Read_binds)
889 ,(dataClassKey, gen_Data_binds)
892 -- no_aux_binds is used for generators that don't
893 -- need to produce any auxiliary bindings
894 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
895 ignore_fix_env f fix_env tc = f tc
899 %************************************************************************
901 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
903 %************************************************************************
908 con2tag_Foo :: Foo ... -> Int#
909 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
910 maxtag_Foo :: Int -- ditto (NB: not unlifted)
913 We have a @con2tag@ function for a tycon if:
916 We're deriving @Eq@ and the tycon has nullary data constructors.
919 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
923 We have a @tag2con@ function for a tycon if:
926 We're deriving @Enum@, or @Ix@ (enum type only???)
929 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
932 genTaggeryBinds :: [InstInfo] -> TcM (LHsBinds RdrName)
933 genTaggeryBinds infos
934 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
935 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
936 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
938 all_CTs = [ (cls, tcTyConAppTyCon ty)
940 let (cls,ty) = simpleInstInfoClsTy info ]
941 all_tycons = map snd all_CTs
942 (tycons_of_interest, _) = removeDups compare all_tycons
944 do_con2tag acc_Names tycon
945 | isDataTyCon tycon &&
946 ((we_are_deriving eqClassKey tycon
947 && any isNullarySrcDataCon (tyConDataCons tycon))
948 || (we_are_deriving ordClassKey tycon
949 && not (isProductTyCon tycon))
950 || (we_are_deriving enumClassKey tycon)
951 || (we_are_deriving ixClassKey tycon))
953 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
958 do_tag2con acc_Names tycon
959 | isDataTyCon tycon &&
960 (we_are_deriving enumClassKey tycon ||
961 we_are_deriving ixClassKey tycon
962 && isEnumerationTyCon tycon)
963 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
964 : (maxtag_RDR tycon, tycon, GenMaxTag)
969 we_are_deriving clas_key tycon
970 = is_in_eqns clas_key tycon all_CTs
972 is_in_eqns clas_key tycon [] = False
973 is_in_eqns clas_key tycon ((c,t):cts)
974 = (clas_key == classKey c && tycon == t)
975 || is_in_eqns clas_key tycon cts
979 derivingThingErr clas tys tycon tyvars why
980 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
983 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
985 derivCtxt :: TyCon -> SDoc
987 = ptext SLIT("When deriving instances for") <+> quotes (ppr tycon)
989 derivInstCtxt1 clas inst_tys
990 = ptext SLIT("When deriving the instance for") <+> quotes (pprClassPred clas inst_tys)
992 derivInstCtxt2 theta clas inst_tys
993 = vcat [ptext SLIT("In the derived instance declaration"),
994 nest 2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta,
995 pprClassPred clas inst_tys])]