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 Type ( splitKindFunTys )
43 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
44 tyConStupidTheta, isProductTyCon, isDataTyCon, newTyConRhs,
45 isEnumerationTyCon, isRecursiveTyCon, TyCon, isNewTyCon,
48 import TcType ( TcType, ThetaType, mkTyVarTys, mkTyConApp, tcTyConAppTyCon,
49 isUnLiftedType, mkClassPred, tyVarsOfType,
50 isSubArgTypeKind, tcEqTypes, tcSplitAppTys, mkAppTys )
51 import Var ( TyVar, tyVarKind, varName )
52 import VarSet ( mkVarSet, subVarSet )
54 import SrcLoc ( srcLocSpan, Located(..) )
55 import Util ( zipWithEqual, sortLe, notNull )
56 import ListSetOps ( removeDups, assocMaybe )
61 %************************************************************************
63 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
65 %************************************************************************
69 data T a b = C1 (Foo a) (Bar b)
74 [NOTE: See end of these comments for what to do with
75 data (C a, D b) => T a b = ...
78 We want to come up with an instance declaration of the form
80 instance (Ping a, Pong b, ...) => Eq (T a b) where
83 It is pretty easy, albeit tedious, to fill in the code "...". The
84 trick is to figure out what the context for the instance decl is,
85 namely @Ping@, @Pong@ and friends.
87 Let's call the context reqd for the T instance of class C at types
88 (a,b, ...) C (T a b). Thus:
90 Eq (T a b) = (Ping a, Pong b, ...)
92 Now we can get a (recursive) equation from the @data@ decl:
94 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
95 u Eq (T b a) u Eq Int -- From C2
96 u Eq (T a a) -- From C3
98 Foo and Bar may have explicit instances for @Eq@, in which case we can
99 just substitute for them. Alternatively, either or both may have
100 their @Eq@ instances given by @deriving@ clauses, in which case they
101 form part of the system of equations.
103 Now all we need do is simplify and solve the equations, iterating to
104 find the least fixpoint. Notice that the order of the arguments can
105 switch around, as here in the recursive calls to T.
107 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
111 Eq (T a b) = {} -- The empty set
114 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
115 u Eq (T b a) u Eq Int -- From C2
116 u Eq (T a a) -- From C3
118 After simplification:
119 = Eq a u Ping b u {} u {} u {}
124 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
125 u Eq (T b a) u Eq Int -- From C2
126 u Eq (T a a) -- From C3
128 After simplification:
133 = Eq a u Ping b u Eq b u Ping a
135 The next iteration gives the same result, so this is the fixpoint. We
136 need to make a canonical form of the RHS to ensure convergence. We do
137 this by simplifying the RHS to a form in which
139 - the classes constrain only tyvars
140 - the list is sorted by tyvar (major key) and then class (minor key)
141 - no duplicates, of course
143 So, here are the synonyms for the ``equation'' structures:
146 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
147 -- The Name is the name for the DFun we'll build
148 -- The tyvars bind all the variables in the RHS
150 pprDerivEqn (n,c,tc,tvs,rhs)
151 = parens (hsep [ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
153 type DerivRhs = ThetaType
154 type DerivSoln = DerivRhs
158 [Data decl contexts] A note about contexts on data decls
159 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
162 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
164 We will need an instance decl like:
166 instance (Read a, RealFloat a) => Read (Complex a) where
169 The RealFloat in the context is because the read method for Complex is bound
170 to construct a Complex, and doing that requires that the argument type is
173 But this ain't true for Show, Eq, Ord, etc, since they don't construct
174 a Complex; they only take them apart.
176 Our approach: identify the offending classes, and add the data type
177 context to the instance decl. The "offending classes" are
181 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
182 pattern matching against a constructor from a data type with a context
183 gives rise to the constraints for that context -- or at least the thinned
184 version. So now all classes are "offending".
191 newtype T = T Char deriving( C [a] )
193 Notice the free 'a' in the deriving. We have to fill this out to
194 newtype T = T Char deriving( forall a. C [a] )
196 And then translate it to:
197 instance C [a] Char => C [a] T where ...
202 %************************************************************************
204 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
206 %************************************************************************
209 tcDeriving :: [LTyClDecl Name] -- All type constructors
210 -> TcM ([InstInfo], -- The generated "instance decls"
211 HsValBinds Name) -- Extra generated top-level bindings
213 tcDeriving tycl_decls
214 = recoverM (returnM ([], emptyValBindsOut)) $
215 do { -- Fish the "deriving"-related information out of the TcEnv
216 -- and make the necessary "equations".
217 overlap_flag <- getOverlapFlag
218 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns overlap_flag tycl_decls
220 ; (ordinary_inst_info, deriv_binds)
221 <- extendLocalInstEnv (map iSpec newtype_inst_info) $
222 deriveOrdinaryStuff overlap_flag ordinary_eqns
223 -- Add the newtype-derived instances to the inst env
224 -- before tacking the "ordinary" ones
226 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
228 -- If we are compiling a hs-boot file,
229 -- don't generate any derived bindings
230 ; is_boot <- tcIsHsBoot
232 return (inst_info, emptyValBindsOut)
236 -- Generate the generic to/from functions from each type declaration
237 ; gen_binds <- mkGenericBinds tycl_decls
239 -- Rename these extra bindings, discarding warnings about unused bindings etc
240 -- Set -fglasgow exts so that we can have type signatures in patterns,
241 -- which is used in the generic binds
243 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
244 { (rn_deriv, _dus1) <- rnTopBinds (ValBindsIn deriv_binds [])
245 ; (rn_gen, dus_gen) <- rnTopBinds (ValBindsIn gen_binds [])
246 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
248 ; return (rn_deriv `plusHsValBinds` rn_gen) }
252 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
253 (ddump_deriving inst_info rn_binds))
255 ; returnM (inst_info, rn_binds)
258 ddump_deriving :: [InstInfo] -> HsValBinds Name -> SDoc
259 ddump_deriving inst_infos extra_binds
260 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
262 -----------------------------------------
263 deriveOrdinaryStuff overlap_flag [] -- Short cut
264 = returnM ([], emptyLHsBinds)
266 deriveOrdinaryStuff overlap_flag eqns
267 = do { -- Take the equation list and solve it, to deliver a list of
268 -- solutions, a.k.a. the contexts for the instance decls
269 -- required for the corresponding equations.
270 inst_specs <- solveDerivEqns overlap_flag eqns
272 -- Generate the InstInfo for each dfun,
273 -- plus any auxiliary bindings it needs
274 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst inst_specs
276 -- Generate any extra not-one-inst-decl-specific binds,
277 -- notably "con2tag" and/or "tag2con" functions.
278 ; extra_binds <- genTaggeryBinds inst_infos
281 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
284 -----------------------------------------
285 mkGenericBinds tycl_decls
286 = do { tcs <- mapM tcLookupTyCon
288 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
289 -- We are only interested in the data type declarations
290 ; return (unionManyBags [ mkTyConGenericBinds tc |
291 tc <- tcs, tyConHasGenerics tc ]) }
292 -- And then only in the ones whose 'has-generics' flag is on
296 %************************************************************************
298 \subsection[TcDeriv-eqns]{Forming the equations}
300 %************************************************************************
302 @makeDerivEqns@ fishes around to find the info about needed derived
303 instances. Complicating factors:
306 We can only derive @Enum@ if the data type is an enumeration
307 type (all nullary data constructors).
310 We can only derive @Ix@ if the data type is an enumeration {\em
311 or} has just one data constructor (e.g., tuples).
314 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
318 makeDerivEqns :: OverlapFlag
320 -> TcM ([DerivEqn], -- Ordinary derivings
321 [InstInfo]) -- Special newtype derivings
323 makeDerivEqns overlap_flag tycl_decls
324 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
325 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
327 ------------------------------------------------------------------
328 derive_these :: [(NewOrData, Name, LHsType Name)]
329 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
330 derive_these = [ (nd, tycon, pred)
331 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
332 tcdDerivs = Just preds }) <- tycl_decls,
335 ------------------------------------------------------------------
336 mk_eqn :: (NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
337 -- We swizzle the tyvars and datacons out of the tycon
338 -- to make the rest of the equation
340 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
341 -- we allow deriving (forall a. C [a]).
343 mk_eqn (new_or_data, tycon_name, hs_deriv_ty)
344 = tcLookupTyCon tycon_name `thenM` \ tycon ->
345 setSrcSpan (srcLocSpan (getSrcLoc tycon)) $
346 addErrCtxt (derivCtxt tycon) $
347 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
348 -- the type variables for the type constructor
349 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
350 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
351 mk_eqn_help gla_exts new_or_data tycon deriv_tvs clas tys
353 ------------------------------------------------------------------
354 -- data/newtype T a = ... deriving( C t1 t2 )
355 -- leads to a call to mk_eqn_help with
356 -- tycon = T, deriv_tvs = ftv(t1,t2), clas = C, tys = [t1,t2]
358 mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys
359 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
360 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
362 = do { eqn <- mkDataTypeEqn tycon clas
363 ; returnM (Just eqn, Nothing) }
365 mk_eqn_help gla_exts NewType tycon deriv_tvs clas tys
366 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
367 = -- Go ahead and use the isomorphism
368 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
369 new_dfun_name clas tycon `thenM` \ dfun_name ->
370 returnM (Nothing, Just (InstInfo { iSpec = mk_inst_spec dfun_name,
371 iBinds = NewTypeDerived (newTyConCo tycon) rep_tys }))
372 | std_class gla_exts clas
373 = mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
375 | otherwise -- Non-standard instance
376 = bale_out (if gla_exts then
377 cant_derive_err -- Too hard
379 non_std_err) -- Just complain about being a non-std instance
381 -- Here is the plan for newtype derivings. We see
382 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
383 -- where t is a type,
384 -- ak...an is a suffix of a1..an
385 -- ak...an do not occur free in t,
386 -- (C s1 ... sm) is a *partial applications* of class C
387 -- with the last parameter missing
389 -- We generate the instances
390 -- instance C s1 .. sm (t ak...ap) => C s1 .. sm (T a1...ap)
391 -- where T a1...ap is the partial application of the LHS of the correct kind
394 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
395 -- instance Monad (ST s) => Monad (T s) where
396 -- fail = coerce ... (fail @ ST s)
397 -- (Actually we don't need the coerce, because non-rec newtypes are transparent
399 clas_tyvars = classTyVars clas
400 kind = tyVarKind (last clas_tyvars)
401 -- Kind of the thing we want to instance
402 -- e.g. argument kind of Monad, *->*
404 (arg_kinds, _) = splitKindFunTys kind
405 n_args_to_drop = length arg_kinds
406 -- Want to drop 1 arg from (T s a) and (ST s a)
407 -- to get instance Monad (ST s) => Monad (T s)
409 -- Note [newtype representation]
410 -- Need newTyConRhs *not* newTyConRep to get the representation
411 -- type, because the latter looks through all intermediate newtypes
413 -- newtype B = MkB Int
414 -- newtype A = MkA B deriving( Num )
415 -- We want the Num instance of B, *not* the Num instance of Int,
416 -- when making the Num instance of A!
417 (tc_tvs, rep_ty) = newTyConRhs tycon
418 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
420 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
421 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
422 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
424 n_args_to_keep = length rep_ty_args - n_args_to_drop
425 args_to_drop = drop n_args_to_keep rep_ty_args
426 args_to_keep = take n_args_to_keep rep_ty_args
428 rep_fn' = mkAppTys rep_fn args_to_keep
429 rep_tys = tys ++ [rep_fn']
430 rep_pred = mkClassPred clas rep_tys
431 -- rep_pred is the representation dictionary, from where
432 -- we are gong to get all the methods for the newtype dictionary
434 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
435 -- The 'tys' here come from the partial application
436 -- in the deriving clause. The last arg is the new
439 -- We must pass the superclasses; the newtype might be an instance
440 -- of them in a different way than the representation type
441 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
442 -- Then the Show instance is not done via isomorphism; it shows
444 -- The Num instance is derived via isomorphism, but the Show superclass
445 -- dictionary must the Show instance for Foo, *not* the Show dictionary
446 -- gotten from the Num dictionary. So we must build a whole new dictionary
447 -- not just use the Num one. The instance we want is something like:
448 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
451 -- There's no 'corece' needed because after the type checker newtypes
454 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
457 -- If there are no tyvars, there's no need
458 -- to abstract over the dictionaries we need
459 dict_tvs = deriv_tvs ++ tc_tvs
460 dict_args | null dict_tvs = []
461 | otherwise = rep_pred : sc_theta
463 -- Finally! Here's where we build the dictionary Id
464 mk_inst_spec dfun_name
465 = mkLocalInstance dfun overlap_flag
467 dfun = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
469 -------------------------------------------------------------------
470 -- Figuring out whether we can only do this newtype-deriving thing
472 right_arity = length tys + 1 == classArity clas
474 -- Never derive Read,Show,Typeable,Data this way
475 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
476 can_derive_via_isomorphism
477 = not (getUnique clas `elem` non_iso_classes)
478 && right_arity -- Well kinded;
479 -- eg not: newtype T ... deriving( ST )
480 -- because ST needs *2* type params
481 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
482 -- eg not: newtype T = T Int deriving( Monad )
483 && n_args_to_keep >= 0 -- Rep type has right kind:
484 -- eg not: newtype T a = T Int deriving( Monad )
485 && eta_ok -- Eta reduction works
486 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
487 -- newtype A = MkA [A]
489 -- instance Eq [A] => Eq A !!
490 -- Here's a recursive newtype that's actually OK
491 -- newtype S1 = S1 [T1 ()]
492 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
493 -- It's currently rejected. Oh well.
494 -- In fact we generate an instance decl that has method of form
495 -- meth @ instTy = meth @ repTy
496 -- (no coerce's). We'd need a coerce if we wanted to handle
497 -- recursive newtypes too
499 -- Check that eta reduction is OK
500 -- (a) the dropped-off args are identical
501 -- (b) the remaining type args mention
502 -- only the remaining type variables
503 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
504 && (tyVarsOfType rep_fn' `subVarSet` mkVarSet tyvars_to_keep)
506 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
507 (vcat [ptext SLIT("even with cunning newtype deriving:"),
508 if isRecursiveTyCon tycon then
509 ptext SLIT("the newtype is recursive")
511 if not right_arity then
512 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
514 if not (n_tyvars_to_keep >= 0) then
515 ptext SLIT("the type constructor has wrong kind")
516 else if not (n_args_to_keep >= 0) then
517 ptext SLIT("the representation type has wrong kind")
518 else if not eta_ok then
519 ptext SLIT("the eta-reduction property does not hold")
523 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
524 (vcat [non_std_why clas,
525 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
527 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
529 std_class gla_exts clas
530 = key `elem` derivableClassKeys
531 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
535 std_class_via_iso clas -- These standard classes can be derived for a newtype
536 -- using the isomorphism trick *even if no -fglasgow-exts*
537 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
538 -- Not Read/Show because they respect the type
539 -- Not Enum, becuase newtypes are never in Enum
542 new_dfun_name clas tycon -- Just a simple wrapper
543 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
544 -- The type passed to newDFunName is only used to generate
545 -- a suitable string; hence the empty type arg list
547 ------------------------------------------------------------------
548 mkDataTypeEqn :: TyCon -> Class -> TcM DerivEqn
549 mkDataTypeEqn tycon clas
550 | clas `hasKey` typeableClassKey
551 = -- The Typeable class is special in several ways
552 -- data T a b = ... deriving( Typeable )
554 -- instance Typeable2 T where ...
556 -- 1. There are no constraints in the instance
557 -- 2. There are no type variables either
558 -- 3. The actual class we want to generate isn't necessarily
559 -- Typeable; it depends on the arity of the type
560 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
561 ; dfun_name <- new_dfun_name real_clas tycon
562 ; return (dfun_name, real_clas, tycon, [], []) }
565 = do { dfun_name <- new_dfun_name clas tycon
566 ; return (dfun_name, clas, tycon, tyvars, constraints) }
568 tyvars = tyConTyVars tycon
569 constraints = extra_constraints ++ ordinary_constraints
570 extra_constraints = tyConStupidTheta tycon
571 -- "extra_constraints": see note [Data decl contexts] above
574 = [ mkClassPred clas [arg_ty]
575 | data_con <- tyConDataCons tycon,
576 arg_ty <- dataConInstOrigArgTys data_con (map mkTyVarTy (tyConTyVars tycon)),
577 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
581 ------------------------------------------------------------------
582 -- Check side conditions that dis-allow derivability for particular classes
583 -- This is *apart* from the newtype-deriving mechanism
585 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
586 checkSideConditions gla_exts tycon deriv_tvs clas tys
587 | notNull deriv_tvs || notNull tys
588 = Just ty_args_why -- e.g. deriving( Foo s )
590 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
591 [] -> Just (non_std_why clas)
592 [cond] -> cond (gla_exts, tycon)
593 other -> pprPanic "checkSideConditions" (ppr clas)
595 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
597 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
599 sideConditions :: [(Unique, Condition)]
601 = [ (eqClassKey, cond_std),
602 (ordClassKey, cond_std),
603 (readClassKey, cond_std),
604 (showClassKey, cond_std),
605 (enumClassKey, cond_std `andCond` cond_isEnumeration),
606 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
607 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
608 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
609 (dataClassKey, cond_glaExts `andCond` cond_std)
612 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
614 orCond :: Condition -> Condition -> Condition
617 Nothing -> Nothing -- c1 succeeds
618 Just x -> case c2 tc of -- c1 fails
620 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
623 andCond c1 c2 tc = case c1 tc of
624 Nothing -> c2 tc -- c1 succeeds
625 Just x -> Just x -- c1 fails
627 cond_std :: Condition
628 cond_std (gla_exts, tycon)
629 | any (not . isVanillaDataCon) data_cons = Just existential_why
630 | null data_cons = Just no_cons_why
631 | otherwise = Nothing
633 data_cons = tyConDataCons tycon
634 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
635 existential_why = quotes (ppr tycon) <+> ptext SLIT("has non-Haskell-98 constructor(s)")
637 cond_isEnumeration :: Condition
638 cond_isEnumeration (gla_exts, tycon)
639 | isEnumerationTyCon tycon = Nothing
640 | otherwise = Just why
642 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
644 cond_isProduct :: Condition
645 cond_isProduct (gla_exts, tycon)
646 | isProductTyCon tycon = Nothing
647 | otherwise = Just why
649 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
651 cond_typeableOK :: Condition
652 -- OK for Typeable class
653 -- Currently: (a) args all of kind *
654 -- (b) 7 or fewer args
655 cond_typeableOK (gla_exts, tycon)
656 | tyConArity tycon > 7 = Just too_many
657 | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
658 | otherwise = Nothing
660 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
661 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
663 cond_glaExts :: Condition
664 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
665 | otherwise = Just why
667 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
670 %************************************************************************
672 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
674 %************************************************************************
676 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
677 terms, which is the final correct RHS for the corresponding original
681 Each (k,TyVarTy tv) in a solution constrains only a type
685 The (k,TyVarTy tv) pairs in a solution are canonically
686 ordered by sorting on type varible, tv, (major key) and then class, k,
691 solveDerivEqns :: OverlapFlag
693 -> TcM [Instance]-- Solns in same order as eqns.
694 -- This bunch is Absolutely minimal...
696 solveDerivEqns overlap_flag orig_eqns
697 = iterateDeriv 1 initial_solutions
699 -- The initial solutions for the equations claim that each
700 -- instance has an empty context; this solution is certainly
701 -- in canonical form.
702 initial_solutions :: [DerivSoln]
703 initial_solutions = [ [] | _ <- orig_eqns ]
705 ------------------------------------------------------------------
706 -- iterateDeriv calculates the next batch of solutions,
707 -- compares it with the current one; finishes if they are the
708 -- same, otherwise recurses with the new solutions.
709 -- It fails if any iteration fails
710 iterateDeriv :: Int -> [DerivSoln] -> TcM [Instance]
711 iterateDeriv n current_solns
712 | n > 20 -- Looks as if we are in an infinite loop
713 -- This can happen if we have -fallow-undecidable-instances
714 -- (See TcSimplify.tcSimplifyDeriv.)
715 = pprPanic "solveDerivEqns: probable loop"
716 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
719 inst_specs = zipWithEqual "add_solns" mk_inst_spec
720 orig_eqns current_solns
723 -- Extend the inst info from the explicit instance decls
724 -- with the current set of solutions, and simplify each RHS
725 extendLocalInstEnv inst_specs $
726 mappM gen_soln orig_eqns
727 ) `thenM` \ new_solns ->
728 if (current_solns == new_solns) then
731 iterateDeriv (n+1) new_solns
733 ------------------------------------------------------------------
734 gen_soln (_, clas, tc,tyvars,deriv_rhs)
735 = setSrcSpan (srcLocSpan (getSrcLoc tc)) $
736 do { let inst_tys = [mkTyConApp tc (mkTyVarTys tyvars)]
737 ; theta <- addErrCtxt (derivInstCtxt1 clas inst_tys) $
738 tcSimplifyDeriv tc tyvars deriv_rhs
739 ; addErrCtxt (derivInstCtxt2 theta clas inst_tys) $
740 checkValidInstance tyvars theta clas inst_tys
741 ; return (sortLe (<=) theta) } -- Canonicalise before returning the soluction
745 ------------------------------------------------------------------
746 mk_inst_spec (dfun_name, clas, tycon, tyvars, _) theta
747 = mkLocalInstance dfun overlap_flag
749 dfun = mkDictFunId dfun_name tyvars theta clas
750 [mkTyConApp tycon (mkTyVarTys tyvars)]
752 extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
753 -- Add new locally-defined instances; don't bother to check
754 -- for functional dependency errors -- that'll happen in TcInstDcls
755 extendLocalInstEnv dfuns thing_inside
756 = do { env <- getGblEnv
757 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
758 env' = env { tcg_inst_env = inst_env' }
759 ; setGblEnv env' thing_inside }
762 %************************************************************************
764 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
766 %************************************************************************
768 After all the trouble to figure out the required context for the
769 derived instance declarations, all that's left is to chug along to
770 produce them. They will then be shoved into @tcInstDecls2@, which
771 will do all its usual business.
773 There are lots of possibilities for code to generate. Here are
774 various general remarks.
779 We want derived instances of @Eq@ and @Ord@ (both v common) to be
780 ``you-couldn't-do-better-by-hand'' efficient.
783 Deriving @Show@---also pretty common--- should also be reasonable good code.
786 Deriving for the other classes isn't that common or that big a deal.
793 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
796 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
799 We {\em normally} generate code only for the non-defaulted methods;
800 there are some exceptions for @Eq@ and (especially) @Ord@...
803 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
804 constructor's numeric (@Int#@) tag. These are generated by
805 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
806 these is around is given by @hasCon2TagFun@.
808 The examples under the different sections below will make this
812 Much less often (really just for deriving @Ix@), we use a
813 @_tag2con_<tycon>@ function. See the examples.
816 We use the renamer!!! Reason: we're supposed to be
817 producing @LHsBinds Name@ for the methods, but that means
818 producing correctly-uniquified code on the fly. This is entirely
819 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
820 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
821 the renamer. What a great hack!
825 -- Generate the InstInfo for the required instance,
826 -- plus any auxiliary bindings required
827 genInst :: Instance -> TcM (InstInfo, LHsBinds RdrName)
829 = do { fix_env <- getFixityEnv
831 (tyvars,_,clas,[ty]) = instanceHead spec
832 clas_nm = className clas
833 tycon = tcTyConAppTyCon ty
834 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
836 -- Bring the right type variables into
837 -- scope, and rename the method binds
838 -- It's a bit yukky that we return *renamed* InstInfo, but
839 -- *non-renamed* auxiliary bindings
840 ; (rn_meth_binds, _fvs) <- discardWarnings $
841 bindLocalNames (map varName tyvars) $
842 rnMethodBinds clas_nm (\n -> []) [] meth_binds
844 -- Build the InstInfo
845 ; return (InstInfo { iSpec = spec,
846 iBinds = VanillaInst rn_meth_binds [] },
850 genDerivBinds clas fix_env tycon
851 | className clas `elem` typeableClassNames
852 = (gen_Typeable_binds tycon, emptyLHsBinds)
855 = case assocMaybe gen_list (getUnique clas) of
856 Just gen_fn -> gen_fn fix_env tycon
857 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
859 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
860 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
861 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
862 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
863 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
864 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
865 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
866 ,(showClassKey, no_aux_binds gen_Show_binds)
867 ,(readClassKey, no_aux_binds gen_Read_binds)
868 ,(dataClassKey, gen_Data_binds)
871 -- no_aux_binds is used for generators that don't
872 -- need to produce any auxiliary bindings
873 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
874 ignore_fix_env f fix_env tc = f tc
878 %************************************************************************
880 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
882 %************************************************************************
887 con2tag_Foo :: Foo ... -> Int#
888 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
889 maxtag_Foo :: Int -- ditto (NB: not unlifted)
892 We have a @con2tag@ function for a tycon if:
895 We're deriving @Eq@ and the tycon has nullary data constructors.
898 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
902 We have a @tag2con@ function for a tycon if:
905 We're deriving @Enum@, or @Ix@ (enum type only???)
908 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
911 genTaggeryBinds :: [InstInfo] -> TcM (LHsBinds RdrName)
912 genTaggeryBinds infos
913 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
914 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
915 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
917 all_CTs = [ (cls, tcTyConAppTyCon ty)
919 let (cls,ty) = simpleInstInfoClsTy info ]
920 all_tycons = map snd all_CTs
921 (tycons_of_interest, _) = removeDups compare all_tycons
923 do_con2tag acc_Names tycon
924 | isDataTyCon tycon &&
925 ((we_are_deriving eqClassKey tycon
926 && any isNullarySrcDataCon (tyConDataCons tycon))
927 || (we_are_deriving ordClassKey tycon
928 && not (isProductTyCon tycon))
929 || (we_are_deriving enumClassKey tycon)
930 || (we_are_deriving ixClassKey tycon))
932 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
937 do_tag2con acc_Names tycon
938 | isDataTyCon tycon &&
939 (we_are_deriving enumClassKey tycon ||
940 we_are_deriving ixClassKey tycon
941 && isEnumerationTyCon tycon)
942 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
943 : (maxtag_RDR tycon, tycon, GenMaxTag)
948 we_are_deriving clas_key tycon
949 = is_in_eqns clas_key tycon all_CTs
951 is_in_eqns clas_key tycon [] = False
952 is_in_eqns clas_key tycon ((c,t):cts)
953 = (clas_key == classKey c && tycon == t)
954 || is_in_eqns clas_key tycon cts
958 derivingThingErr clas tys tycon tyvars why
959 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
962 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
964 derivCtxt :: TyCon -> SDoc
966 = ptext SLIT("When deriving instances for") <+> quotes (ppr tycon)
968 derivInstCtxt1 clas inst_tys
969 = ptext SLIT("When deriving the instance for") <+> quotes (pprClassPred clas inst_tys)
971 derivInstCtxt2 theta clas inst_tys
972 = vcat [ptext SLIT("In the derived instance declaration"),
973 nest 2 (ptext SLIT("instance") <+> sep [pprThetaArrow theta,
974 pprClassPred clas inst_tys])]