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 CmdLineOpts ( DynFlag(..) )
16 import Generics ( mkTyConGenericBinds )
18 import TcEnv ( newDFunName, pprInstInfoDetails,
19 InstInfo(..), InstBindings(..),
20 tcLookupClass, tcLookupTyCon, tcExtendTyVarEnv
22 import TcGenDeriv -- Deriv stuff
23 import InstEnv ( simpleDFunClassTyCon, extendInstEnvList )
24 import TcHsType ( tcHsDeriv )
25 import TcSimplify ( tcSimplifyDeriv )
27 import RnBinds ( rnMethodBinds, rnTopBinds )
28 import RnEnv ( bindLocalNames )
29 import HscTypes ( DFunId, FixityEnv )
31 import Class ( className, classArity, classKey, classTyVars, classSCTheta, Class )
32 import Type ( zipOpenTvSubst, substTheta )
33 import ErrUtils ( dumpIfSet_dyn )
34 import MkId ( mkDictFunId )
35 import DataCon ( isNullarySrcDataCon, isVanillaDataCon, dataConOrigArgTys )
36 import Maybes ( catMaybes )
37 import RdrName ( RdrName )
38 import Name ( Name, getSrcLoc )
39 import NameSet ( duDefs )
40 import Kind ( splitKindFunTys )
41 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
42 tyConStupidTheta, isProductTyCon, isDataTyCon, newTyConRhs,
43 isEnumerationTyCon, isRecursiveTyCon, TyCon
45 import TcType ( TcType, ThetaType, mkTyVarTys, mkTyConApp, tcTyConAppTyCon,
46 isUnLiftedType, mkClassPred, tyVarsOfTypes, isArgTypeKind,
47 tcEqTypes, tcSplitAppTys, mkAppTys, tcSplitDFunTy )
48 import Var ( TyVar, tyVarKind, idType, varName )
49 import VarSet ( mkVarSet, subVarSet )
51 import SrcLoc ( srcLocSpan, Located(..) )
52 import Util ( zipWithEqual, sortLe, notNull )
53 import ListSetOps ( removeDups, assocMaybe )
58 %************************************************************************
60 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
62 %************************************************************************
66 data T a b = C1 (Foo a) (Bar b)
71 [NOTE: See end of these comments for what to do with
72 data (C a, D b) => T a b = ...
75 We want to come up with an instance declaration of the form
77 instance (Ping a, Pong b, ...) => Eq (T a b) where
80 It is pretty easy, albeit tedious, to fill in the code "...". The
81 trick is to figure out what the context for the instance decl is,
82 namely @Ping@, @Pong@ and friends.
84 Let's call the context reqd for the T instance of class C at types
85 (a,b, ...) C (T a b). Thus:
87 Eq (T a b) = (Ping a, Pong b, ...)
89 Now we can get a (recursive) equation from the @data@ decl:
91 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
92 u Eq (T b a) u Eq Int -- From C2
93 u Eq (T a a) -- From C3
95 Foo and Bar may have explicit instances for @Eq@, in which case we can
96 just substitute for them. Alternatively, either or both may have
97 their @Eq@ instances given by @deriving@ clauses, in which case they
98 form part of the system of equations.
100 Now all we need do is simplify and solve the equations, iterating to
101 find the least fixpoint. Notice that the order of the arguments can
102 switch around, as here in the recursive calls to T.
104 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
108 Eq (T a b) = {} -- The empty set
111 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
112 u Eq (T b a) u Eq Int -- From C2
113 u Eq (T a a) -- From C3
115 After simplification:
116 = Eq a u Ping b u {} u {} u {}
121 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
122 u Eq (T b a) u Eq Int -- From C2
123 u Eq (T a a) -- From C3
125 After simplification:
130 = Eq a u Ping b u Eq b u Ping a
132 The next iteration gives the same result, so this is the fixpoint. We
133 need to make a canonical form of the RHS to ensure convergence. We do
134 this by simplifying the RHS to a form in which
136 - the classes constrain only tyvars
137 - the list is sorted by tyvar (major key) and then class (minor key)
138 - no duplicates, of course
140 So, here are the synonyms for the ``equation'' structures:
143 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
144 -- The Name is the name for the DFun we'll build
145 -- The tyvars bind all the variables in the RHS
147 pprDerivEqn (n,c,tc,tvs,rhs)
148 = parens (hsep [ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
150 type DerivRhs = ThetaType
151 type DerivSoln = DerivRhs
155 [Data decl contexts] A note about contexts on data decls
156 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
159 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
161 We will need an instance decl like:
163 instance (Read a, RealFloat a) => Read (Complex a) where
166 The RealFloat in the context is because the read method for Complex is bound
167 to construct a Complex, and doing that requires that the argument type is
170 But this ain't true for Show, Eq, Ord, etc, since they don't construct
171 a Complex; they only take them apart.
173 Our approach: identify the offending classes, and add the data type
174 context to the instance decl. The "offending classes" are
178 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
179 pattern matching against a constructor from a data type with a context
180 gives rise to the constraints for that context -- or at least the thinned
181 version. So now all classes are "offending".
188 newtype T = T Char deriving( C [a] )
190 Notice the free 'a' in the deriving. We have to fill this out to
191 newtype T = T Char deriving( forall a. C [a] )
193 And then translate it to:
194 instance C [a] Char => C [a] T where ...
199 %************************************************************************
201 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
203 %************************************************************************
206 tcDeriving :: [LTyClDecl Name] -- All type constructors
207 -> TcM ([InstInfo], -- The generated "instance decls"
208 [HsBindGroup Name]) -- Extra generated top-level bindings
210 tcDeriving tycl_decls
211 = recoverM (returnM ([], [])) $
212 do { -- Fish the "deriving"-related information out of the TcEnv
213 -- and make the necessary "equations".
214 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns tycl_decls
216 ; (ordinary_inst_info, deriv_binds)
217 <- extendLocalInstEnv (map iDFunId newtype_inst_info) $
218 deriveOrdinaryStuff ordinary_eqns
219 -- Add the newtype-derived instances to the inst env
220 -- before tacking the "ordinary" ones
222 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
224 -- If we are compiling a hs-boot file,
225 -- don't generate any derived bindings
226 ; is_boot <- tcIsHsBoot
228 return (inst_info, [])
232 -- Generate the generic to/from functions from each type declaration
233 ; gen_binds <- mkGenericBinds tycl_decls
235 -- Rename these extra bindings, discarding warnings about unused bindings etc
236 -- Set -fglasgow exts so that we can have type signatures in patterns,
237 -- which is used in the generic binds
239 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
240 { (rn_deriv, _dus1) <- rnTopBinds deriv_binds []
241 ; (rn_gen, dus_gen) <- rnTopBinds gen_binds []
242 ; keepAliveSetTc (duDefs dus_gen) -- Mark these guys to
244 ; return (rn_deriv ++ rn_gen) }
248 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
249 (ddump_deriving inst_info rn_binds))
251 ; returnM (inst_info, rn_binds)
254 ddump_deriving :: [InstInfo] -> [HsBindGroup Name] -> SDoc
255 ddump_deriving inst_infos extra_binds
256 = vcat (map pprInstInfoDetails inst_infos) $$ vcat (map ppr extra_binds)
258 -----------------------------------------
259 deriveOrdinaryStuff [] -- Short cut
260 = returnM ([], emptyLHsBinds)
262 deriveOrdinaryStuff eqns
263 = do { -- Take the equation list and solve it, to deliver a list of
264 -- solutions, a.k.a. the contexts for the instance decls
265 -- required for the corresponding equations.
266 ; new_dfuns <- solveDerivEqns eqns
268 -- Generate the InstInfo for each dfun,
269 -- plus any auxiliary bindings it needs
270 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst new_dfuns
272 -- Generate any extra not-one-inst-decl-specific binds,
273 -- notably "con2tag" and/or "tag2con" functions.
274 ; extra_binds <- genTaggeryBinds new_dfuns
277 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
280 -----------------------------------------
281 mkGenericBinds tycl_decls
282 = do { tcs <- mapM tcLookupTyCon
284 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
285 -- We are only interested in the data type declarations
286 ; return (unionManyBags [ mkTyConGenericBinds tc |
287 tc <- tcs, tyConHasGenerics tc ]) }
288 -- And then only in the ones whose 'has-generics' flag is on
292 %************************************************************************
294 \subsection[TcDeriv-eqns]{Forming the equations}
296 %************************************************************************
298 @makeDerivEqns@ fishes around to find the info about needed derived
299 instances. Complicating factors:
302 We can only derive @Enum@ if the data type is an enumeration
303 type (all nullary data constructors).
306 We can only derive @Ix@ if the data type is an enumeration {\em
307 or} has just one data constructor (e.g., tuples).
310 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
314 makeDerivEqns :: [LTyClDecl Name]
315 -> TcM ([DerivEqn], -- Ordinary derivings
316 [InstInfo]) -- Special newtype derivings
318 makeDerivEqns tycl_decls
319 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
320 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
322 ------------------------------------------------------------------
323 derive_these :: [(NewOrData, Name, LHsType Name)]
324 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
325 derive_these = [ (nd, tycon, pred)
326 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
327 tcdDerivs = Just preds }) <- tycl_decls,
330 ------------------------------------------------------------------
331 mk_eqn :: (NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
332 -- We swizzle the tyvars and datacons out of the tycon
333 -- to make the rest of the equation
335 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
336 -- we allow deriving (forall a. C [a]).
338 mk_eqn (new_or_data, tycon_name, hs_deriv_ty)
339 = tcLookupTyCon tycon_name `thenM` \ tycon ->
340 setSrcSpan (srcLocSpan (getSrcLoc tycon)) $
341 addErrCtxt (derivCtxt Nothing tycon) $
342 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
343 -- the type variables for the type constructor
344 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
345 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
346 mk_eqn_help gla_exts new_or_data tycon deriv_tvs clas tys
348 ------------------------------------------------------------------
349 mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys
350 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
351 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
353 = do { eqn <- mkDataTypeEqn tycon clas
354 ; returnM (Just eqn, Nothing) }
356 mk_eqn_help gla_exts NewType tycon deriv_tvs clas tys
357 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
358 = -- Go ahead and use the isomorphism
359 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
360 new_dfun_name clas tycon `thenM` \ dfun_name ->
361 returnM (Nothing, Just (InstInfo { iDFunId = mk_dfun dfun_name,
362 iBinds = NewTypeDerived rep_tys }))
363 | std_class gla_exts clas
364 = mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
366 | otherwise -- Non-standard instance
367 = bale_out (if gla_exts then
368 cant_derive_err -- Too hard
370 non_std_err) -- Just complain about being a non-std instance
372 -- Here is the plan for newtype derivings. We see
373 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
374 -- where t is a type,
375 -- ak...an is a suffix of a1..an
376 -- ak...an do not occur free in t,
377 -- (C s1 ... sm) is a *partial applications* of class C
378 -- with the last parameter missing
380 -- We generate the instances
381 -- instance C s1 .. sm (t ak...ap) => C s1 .. sm (T a1...ap)
382 -- where T a1...ap is the partial application of the LHS of the correct kind
385 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
386 -- instance Monad (ST s) => Monad (T s) where
387 -- fail = coerce ... (fail @ ST s)
388 -- (Actually we don't need the coerce, because non-rec newtypes are transparent
390 clas_tyvars = classTyVars clas
391 kind = tyVarKind (last clas_tyvars)
392 -- Kind of the thing we want to instance
393 -- e.g. argument kind of Monad, *->*
395 (arg_kinds, _) = splitKindFunTys kind
396 n_args_to_drop = length arg_kinds
397 -- Want to drop 1 arg from (T s a) and (ST s a)
398 -- to get instance Monad (ST s) => Monad (T s)
400 -- Note [newtype representation]
401 -- Need newTyConRhs *not* newTyConRep to get the representation
402 -- type, because the latter looks through all intermediate newtypes
404 -- newtype B = MkB Int
405 -- newtype A = MkA B deriving( Num )
406 -- We want the Num instance of B, *not* the Num instance of Int,
407 -- when making the Num instance of A!
408 (tc_tvs, rep_ty) = newTyConRhs tycon
409 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
411 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
412 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
413 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
415 n_args_to_keep = length rep_ty_args - n_args_to_drop
416 args_to_drop = drop n_args_to_keep rep_ty_args
417 args_to_keep = take n_args_to_keep rep_ty_args
419 rep_tys = tys ++ [mkAppTys rep_fn args_to_keep]
420 rep_pred = mkClassPred clas rep_tys
421 -- rep_pred is the representation dictionary, from where
422 -- we are gong to get all the methods for the newtype dictionary
424 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
425 -- The 'tys' here come from the partial application
426 -- in the deriving clause. The last arg is the new
429 -- We must pass the superclasses; the newtype might be an instance
430 -- of them in a different way than the representation type
431 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
432 -- Then the Show instance is not done via isomprphism; it shows
434 -- The Num instance is derived via isomorphism, but the Show superclass
435 -- dictionary must the Show instance for Foo, *not* the Show dictionary
436 -- gotten from the Num dictionary. So we must build a whole new dictionary
437 -- not just use the Num one. The instance we want is something like:
438 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
441 -- There's no 'corece' needed because after the type checker newtypes
444 sc_theta = substTheta (zipOpenTvSubst clas_tyvars inst_tys)
447 -- If there are no tyvars, there's no need
448 -- to abstract over the dictionaries we need
449 dict_tvs = deriv_tvs ++ tc_tvs
450 dict_args | null dict_tvs = []
451 | otherwise = rep_pred : sc_theta
453 -- Finally! Here's where we build the dictionary Id
454 mk_dfun dfun_name = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
456 -------------------------------------------------------------------
457 -- Figuring out whether we can only do this newtype-deriving thing
459 right_arity = length tys + 1 == classArity clas
461 -- Never derive Read,Show,Typeable,Data this way
462 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
463 can_derive_via_isomorphism
464 = not (getUnique clas `elem` non_iso_classes)
465 && right_arity -- Well kinded;
466 -- eg not: newtype T ... deriving( ST )
467 -- because ST needs *2* type params
468 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
469 -- eg not: newtype T = T Int deriving( Monad )
470 && n_args_to_keep >= 0 -- Rep type has right kind:
471 -- eg not: newtype T a = T Int deriving( Monad )
472 && eta_ok -- Eta reduction works
473 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
474 -- newtype A = MkA [A]
476 -- instance Eq [A] => Eq A !!
477 -- Here's a recursive newtype that's actually OK
478 -- newtype S1 = S1 [T1 ()]
479 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
480 -- It's currently rejected. Oh well.
481 -- In fact we generate an instance decl that has method of form
482 -- meth @ instTy = meth @ repTy
483 -- (no coerce's). We'd need a coerce if we wanted to handle
484 -- recursive newtypes too
486 -- Check that eta reduction is OK
487 -- (a) the dropped-off args are identical
488 -- (b) the remaining type args mention
489 -- only the remaining type variables
490 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
491 && (tyVarsOfTypes args_to_keep `subVarSet` mkVarSet tyvars_to_keep)
493 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
494 (vcat [ptext SLIT("even with cunning newtype deriving:"),
495 if isRecursiveTyCon tycon then
496 ptext SLIT("the newtype is recursive")
498 if not right_arity then
499 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
501 if not (n_tyvars_to_keep >= 0) then
502 ptext SLIT("the type constructor has wrong kind")
503 else if not (n_args_to_keep >= 0) then
504 ptext SLIT("the representation type has wrong kind")
505 else if not eta_ok then
506 ptext SLIT("the eta-reduction property does not hold")
510 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
511 (vcat [non_std_why clas,
512 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
514 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
516 std_class gla_exts clas
517 = key `elem` derivableClassKeys
518 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
522 std_class_via_iso clas -- These standard classes can be derived for a newtype
523 -- using the isomorphism trick *even if no -fglasgow-exts*
524 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
525 -- Not Read/Show because they respect the type
526 -- Not Enum, becuase newtypes are never in Enum
529 new_dfun_name clas tycon -- Just a simple wrapper
530 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
531 -- The type passed to newDFunName is only used to generate
532 -- a suitable string; hence the empty type arg list
534 ------------------------------------------------------------------
535 mkDataTypeEqn :: TyCon -> Class -> TcM DerivEqn
536 mkDataTypeEqn tycon clas
537 | clas `hasKey` typeableClassKey
538 = -- The Typeable class is special in several ways
539 -- data T a b = ... deriving( Typeable )
541 -- instance Typeable2 T where ...
543 -- 1. There are no constraints in the instance
544 -- 2. There are no type variables either
545 -- 3. The actual class we want to generate isn't necessarily
546 -- Typeable; it depends on the arity of the type
547 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
548 ; dfun_name <- new_dfun_name real_clas tycon
549 ; return (dfun_name, real_clas, tycon, [], []) }
552 = do { dfun_name <- new_dfun_name clas tycon
553 ; return (dfun_name, clas, tycon, tyvars, constraints) }
555 tyvars = tyConTyVars tycon
556 constraints = extra_constraints ++ ordinary_constraints
557 extra_constraints = tyConStupidTheta tycon
558 -- "extra_constraints": see note [Data decl contexts] above
561 = [ mkClassPred clas [arg_ty]
562 | data_con <- tyConDataCons tycon,
563 arg_ty <- dataConOrigArgTys data_con,
564 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
568 ------------------------------------------------------------------
569 -- Check side conditions that dis-allow derivability for particular classes
570 -- This is *apart* from the newtype-deriving mechanism
572 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
573 checkSideConditions gla_exts tycon deriv_tvs clas tys
574 | notNull deriv_tvs || notNull tys
575 = Just ty_args_why -- e.g. deriving( Foo s )
577 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
578 [] -> Just (non_std_why clas)
579 [cond] -> cond (gla_exts, tycon)
580 other -> pprPanic "checkSideConditions" (ppr clas)
582 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
584 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
586 sideConditions :: [(Unique, Condition)]
588 = [ (eqClassKey, cond_std),
589 (ordClassKey, cond_std),
590 (readClassKey, cond_std),
591 (showClassKey, cond_std),
592 (enumClassKey, cond_std `andCond` cond_isEnumeration),
593 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
594 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
595 (typeableClassKey, cond_glaExts `andCond` cond_typeableOK),
596 (dataClassKey, cond_glaExts `andCond` cond_std)
599 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
601 orCond :: Condition -> Condition -> Condition
604 Nothing -> Nothing -- c1 succeeds
605 Just x -> case c2 tc of -- c1 fails
607 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
610 andCond c1 c2 tc = case c1 tc of
611 Nothing -> c2 tc -- c1 succeeds
612 Just x -> Just x -- c1 fails
614 cond_std :: Condition
615 cond_std (gla_exts, tycon)
616 | any (not . isVanillaDataCon) data_cons = Just existential_why
617 | null data_cons = Just no_cons_why
618 | otherwise = Nothing
620 data_cons = tyConDataCons tycon
621 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
622 existential_why = quotes (ppr tycon) <+> ptext SLIT("has existentially-quantified constructor(s)")
624 cond_isEnumeration :: Condition
625 cond_isEnumeration (gla_exts, tycon)
626 | isEnumerationTyCon tycon = Nothing
627 | otherwise = Just why
629 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
631 cond_isProduct :: Condition
632 cond_isProduct (gla_exts, tycon)
633 | isProductTyCon tycon = Nothing
634 | otherwise = Just why
636 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
638 cond_typeableOK :: Condition
639 -- OK for Typeable class
640 -- Currently: (a) args all of kind *
641 -- (b) 7 or fewer args
642 cond_typeableOK (gla_exts, tycon)
643 | tyConArity tycon > 7 = Just too_many
644 | not (all (isArgTypeKind . tyVarKind) (tyConTyVars tycon)) = Just bad_kind
645 | otherwise = Nothing
647 too_many = quotes (ppr tycon) <+> ptext SLIT("has too many arguments")
648 bad_kind = quotes (ppr tycon) <+> ptext SLIT("has arguments of kind other than `*'")
650 cond_glaExts :: Condition
651 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
652 | otherwise = Just why
654 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
657 %************************************************************************
659 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
661 %************************************************************************
663 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
664 terms, which is the final correct RHS for the corresponding original
668 Each (k,TyVarTy tv) in a solution constrains only a type
672 The (k,TyVarTy tv) pairs in a solution are canonically
673 ordered by sorting on type varible, tv, (major key) and then class, k,
678 solveDerivEqns :: [DerivEqn]
679 -> TcM [DFunId] -- Solns in same order as eqns.
680 -- This bunch is Absolutely minimal...
682 solveDerivEqns orig_eqns
683 = iterateDeriv 1 initial_solutions
685 -- The initial solutions for the equations claim that each
686 -- instance has an empty context; this solution is certainly
687 -- in canonical form.
688 initial_solutions :: [DerivSoln]
689 initial_solutions = [ [] | _ <- orig_eqns ]
691 ------------------------------------------------------------------
692 -- iterateDeriv calculates the next batch of solutions,
693 -- compares it with the current one; finishes if they are the
694 -- same, otherwise recurses with the new solutions.
695 -- It fails if any iteration fails
696 iterateDeriv :: Int -> [DerivSoln] ->TcM [DFunId]
697 iterateDeriv n current_solns
698 | n > 20 -- Looks as if we are in an infinite loop
699 -- This can happen if we have -fallow-undecidable-instances
700 -- (See TcSimplify.tcSimplifyDeriv.)
701 = pprPanic "solveDerivEqns: probable loop"
702 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
705 dfuns = zipWithEqual "add_solns" mk_deriv_dfun orig_eqns current_solns
708 -- Extend the inst info from the explicit instance decls
709 -- with the current set of solutions, and simplify each RHS
710 extendLocalInstEnv dfuns $
711 mappM gen_soln orig_eqns
712 ) `thenM` \ new_solns ->
713 if (current_solns == new_solns) then
716 iterateDeriv (n+1) new_solns
718 ------------------------------------------------------------------
720 gen_soln (_, clas, tc,tyvars,deriv_rhs)
721 = setSrcSpan (srcLocSpan (getSrcLoc tc)) $
722 addErrCtxt (derivCtxt (Just clas) tc) $
723 tcSimplifyDeriv tyvars deriv_rhs `thenM` \ theta ->
724 returnM (sortLe (<=) theta) -- Canonicalise before returning the soluction
726 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
727 = mkDictFunId dfun_name tyvars theta
728 clas [mkTyConApp tycon (mkTyVarTys tyvars)]
730 extendLocalInstEnv :: [DFunId] -> TcM a -> TcM a
731 -- Add new locall-defined instances; don't bother to check
732 -- for functional dependency errors -- that'll happen in TcInstDcls
733 extendLocalInstEnv dfuns thing_inside
734 = do { env <- getGblEnv
735 ; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
736 env' = env { tcg_inst_env = inst_env' }
737 ; setGblEnv env' thing_inside }
740 %************************************************************************
742 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
744 %************************************************************************
746 After all the trouble to figure out the required context for the
747 derived instance declarations, all that's left is to chug along to
748 produce them. They will then be shoved into @tcInstDecls2@, which
749 will do all its usual business.
751 There are lots of possibilities for code to generate. Here are
752 various general remarks.
757 We want derived instances of @Eq@ and @Ord@ (both v common) to be
758 ``you-couldn't-do-better-by-hand'' efficient.
761 Deriving @Show@---also pretty common--- should also be reasonable good code.
764 Deriving for the other classes isn't that common or that big a deal.
771 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
774 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
777 We {\em normally} generate code only for the non-defaulted methods;
778 there are some exceptions for @Eq@ and (especially) @Ord@...
781 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
782 constructor's numeric (@Int#@) tag. These are generated by
783 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
784 these is around is given by @hasCon2TagFun@.
786 The examples under the different sections below will make this
790 Much less often (really just for deriving @Ix@), we use a
791 @_tag2con_<tycon>@ function. See the examples.
794 We use the renamer!!! Reason: we're supposed to be
795 producing @LHsBinds Name@ for the methods, but that means
796 producing correctly-uniquified code on the fly. This is entirely
797 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
798 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
799 the renamer. What a great hack!
803 -- Generate the InstInfo for the required instance,
804 -- plus any auxiliary bindings required
805 genInst :: DFunId -> TcM (InstInfo, LHsBinds RdrName)
807 = getFixityEnv `thenM` \ fix_env ->
809 (tyvars,_,clas,[ty]) = tcSplitDFunTy (idType dfun)
810 clas_nm = className clas
811 tycon = tcTyConAppTyCon ty
812 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
814 -- Bring the right type variables into
815 -- scope, and rename the method binds
816 bindLocalNames (map varName tyvars) $
817 rnMethodBinds clas_nm [] meth_binds `thenM` \ (rn_meth_binds, _fvs) ->
819 -- Build the InstInfo
820 returnM (InstInfo { iDFunId = dfun, iBinds = VanillaInst rn_meth_binds [] },
823 genDerivBinds clas fix_env tycon
824 | className clas `elem` typeableClassNames
825 = (gen_Typeable_binds tycon, emptyLHsBinds)
828 = case assocMaybe gen_list (getUnique clas) of
829 Just gen_fn -> gen_fn fix_env tycon
830 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
832 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
833 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
834 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
835 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
836 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
837 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
838 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
839 ,(showClassKey, no_aux_binds gen_Show_binds)
840 ,(readClassKey, no_aux_binds gen_Read_binds)
841 ,(dataClassKey, gen_Data_binds)
844 -- no_aux_binds is used for generators that don't
845 -- need to produce any auxiliary bindings
846 no_aux_binds f fix_env tc = (f fix_env tc, emptyLHsBinds)
847 ignore_fix_env f fix_env tc = f tc
851 %************************************************************************
853 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
855 %************************************************************************
860 con2tag_Foo :: Foo ... -> Int#
861 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
862 maxtag_Foo :: Int -- ditto (NB: not unlifted)
865 We have a @con2tag@ function for a tycon if:
868 We're deriving @Eq@ and the tycon has nullary data constructors.
871 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
875 We have a @tag2con@ function for a tycon if:
878 We're deriving @Enum@, or @Ix@ (enum type only???)
881 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
884 genTaggeryBinds :: [DFunId] -> TcM (LHsBinds RdrName)
885 genTaggeryBinds dfuns
886 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
887 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
888 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
890 all_CTs = map simpleDFunClassTyCon dfuns
891 all_tycons = map snd all_CTs
892 (tycons_of_interest, _) = removeDups compare all_tycons
894 do_con2tag acc_Names tycon
895 | isDataTyCon tycon &&
896 ((we_are_deriving eqClassKey tycon
897 && any isNullarySrcDataCon (tyConDataCons tycon))
898 || (we_are_deriving ordClassKey tycon
899 && not (isProductTyCon tycon))
900 || (we_are_deriving enumClassKey tycon)
901 || (we_are_deriving ixClassKey tycon))
903 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
908 do_tag2con acc_Names tycon
909 | isDataTyCon tycon &&
910 (we_are_deriving enumClassKey tycon ||
911 we_are_deriving ixClassKey tycon
912 && isEnumerationTyCon tycon)
913 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
914 : (maxtag_RDR tycon, tycon, GenMaxTag)
919 we_are_deriving clas_key tycon
920 = is_in_eqns clas_key tycon all_CTs
922 is_in_eqns clas_key tycon [] = False
923 is_in_eqns clas_key tycon ((c,t):cts)
924 = (clas_key == classKey c && tycon == t)
925 || is_in_eqns clas_key tycon cts
929 derivingThingErr clas tys tycon tyvars why
930 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
933 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
935 derivCtxt :: Maybe Class -> TyCon -> SDoc
936 derivCtxt maybe_cls tycon
937 = ptext SLIT("When deriving") <+> cls <+> ptext SLIT("for type") <+> quotes (ppr tycon)
939 cls = case maybe_cls of
940 Nothing -> ptext SLIT("instances")
941 Just c -> ptext SLIT("the") <+> quotes (ppr c) <+> ptext SLIT("instance")