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, extendInstEnv )
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 Subst ( mkTyVarSubst, substTheta )
33 import ErrUtils ( dumpIfSet_dyn )
34 import MkId ( mkDictFunId )
35 import DataCon ( isNullaryDataCon, isExistentialDataCon, dataConOrigArgTys )
36 import Maybes ( catMaybes )
37 import RdrName ( RdrName )
38 import Name ( Name, getSrcLoc )
39 import NameSet ( NameSet, emptyNameSet, duDefs )
40 import Kind ( splitKindFunTys )
41 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
42 tyConTheta, 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, sortLt, 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
209 NameSet) -- Binders to keep alive
211 tcDeriving tycl_decls
212 = recoverM (returnM ([], [], emptyNameSet)) $
213 do { -- Fish the "deriving"-related information out of the TcEnv
214 -- and make the necessary "equations".
215 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns tycl_decls
217 ; (ordinary_inst_info, deriv_binds)
218 <- extendLocalInstEnv (map iDFunId newtype_inst_info) $
219 deriveOrdinaryStuff ordinary_eqns
220 -- Add the newtype-derived instances to the inst env
221 -- before tacking the "ordinary" ones
223 -- Generate the generic to/from functions from each type declaration
224 ; gen_binds <- mkGenericBinds tycl_decls
225 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
227 -- Rename these extra bindings, discarding warnings about unused bindings etc
228 -- Set -fglasgow exts so that we can have type signatures in patterns,
229 -- which is used in the generic binds
230 ; (rn_binds, gen_bndrs)
231 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
232 { (rn_deriv, _dus1) <- rnTopBinds deriv_binds []
233 ; (rn_gen, dus_gen) <- rnTopBinds gen_binds []
234 ; return (rn_deriv ++ rn_gen, duDefs dus_gen) }
238 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
239 (ddump_deriving inst_info rn_binds))
241 ; returnM (inst_info, rn_binds, gen_bndrs)
244 ddump_deriving :: [InstInfo] -> [HsBindGroup Name] -> SDoc
245 ddump_deriving inst_infos extra_binds
246 = vcat (map pprInstInfoDetails inst_infos) $$ vcat (map ppr extra_binds)
248 -----------------------------------------
249 deriveOrdinaryStuff [] -- Short cut
250 = returnM ([], emptyBag)
252 deriveOrdinaryStuff eqns
253 = do { -- Take the equation list and solve it, to deliver a list of
254 -- solutions, a.k.a. the contexts for the instance decls
255 -- required for the corresponding equations.
256 ; new_dfuns <- solveDerivEqns eqns
258 -- Generate the InstInfo for each dfun,
259 -- plus any auxiliary bindings it needs
260 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst new_dfuns
262 -- Generate any extra not-one-inst-decl-specific binds,
263 -- notably "con2tag" and/or "tag2con" functions.
264 ; extra_binds <- genTaggeryBinds new_dfuns
267 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
270 -----------------------------------------
271 mkGenericBinds tycl_decls
272 = do { tcs <- mapM tcLookupTyCon
274 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
275 -- We are only interested in the data type declarations
276 ; return (unionManyBags [ mkTyConGenericBinds tc |
277 tc <- tcs, tyConHasGenerics tc ]) }
278 -- And then only in the ones whose 'has-generics' flag is on
282 %************************************************************************
284 \subsection[TcDeriv-eqns]{Forming the equations}
286 %************************************************************************
288 @makeDerivEqns@ fishes around to find the info about needed derived
289 instances. Complicating factors:
292 We can only derive @Enum@ if the data type is an enumeration
293 type (all nullary data constructors).
296 We can only derive @Ix@ if the data type is an enumeration {\em
297 or} has just one data constructor (e.g., tuples).
300 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
304 makeDerivEqns :: [LTyClDecl Name]
305 -> TcM ([DerivEqn], -- Ordinary derivings
306 [InstInfo]) -- Special newtype derivings
308 makeDerivEqns tycl_decls
309 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
310 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
312 ------------------------------------------------------------------
313 derive_these :: [(NewOrData, Name, LHsType Name)]
314 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
315 derive_these = [ (nd, tycon, pred)
316 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
317 tcdDerivs = Just preds }) <- tycl_decls,
320 ------------------------------------------------------------------
321 mk_eqn :: (NewOrData, Name, LHsType Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
322 -- We swizzle the tyvars and datacons out of the tycon
323 -- to make the rest of the equation
325 -- The "deriv_ty" is a LHsType to take account of the fact that for newtype derivign
326 -- we allow deriving (forall a. C [a]).
328 mk_eqn (new_or_data, tycon_name, hs_deriv_ty)
329 = tcLookupTyCon tycon_name `thenM` \ tycon ->
330 addSrcSpan (srcLocSpan (getSrcLoc tycon)) $
331 addErrCtxt (derivCtxt Nothing tycon) $
332 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
333 -- the type variables for the type constructor
334 tcHsDeriv hs_deriv_ty `thenM` \ (deriv_tvs, clas, tys) ->
335 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
336 mk_eqn_help gla_exts new_or_data tycon deriv_tvs clas tys
338 ------------------------------------------------------------------
339 mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys
340 | Just err <- checkSideConditions gla_exts tycon deriv_tvs clas tys
341 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
343 = do { eqn <- mkDataTypeEqn tycon clas
344 ; returnM (Just eqn, Nothing) }
346 mk_eqn_help gla_exts NewType tycon deriv_tvs clas tys
347 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
348 = -- Go ahead and use the isomorphism
349 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
350 new_dfun_name clas tycon `thenM` \ dfun_name ->
351 returnM (Nothing, Just (InstInfo { iDFunId = mk_dfun dfun_name,
352 iBinds = NewTypeDerived rep_tys }))
353 | std_class gla_exts clas
354 = mk_eqn_help gla_exts DataType tycon deriv_tvs clas tys -- Go via bale-out route
356 | otherwise -- Non-standard instance
357 = bale_out (if gla_exts then
358 cant_derive_err -- Too hard
360 non_std_err) -- Just complain about being a non-std instance
362 -- Here is the plan for newtype derivings. We see
363 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
364 -- where t is a type,
365 -- ak...an is a suffix of a1..an
366 -- ak...an do not occur free in t,
367 -- (C s1 ... sm) is a *partial applications* of class C
368 -- with the last parameter missing
370 -- We generate the instances
371 -- instance C s1 .. sm (t ak...ap) => C s1 .. sm (T a1...ap)
372 -- where T a1...ap is the partial application of the LHS of the correct kind
375 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
376 -- instance Monad (ST s) => Monad (T s) where
377 -- fail = coerce ... (fail @ ST s)
378 -- (Actually we don't need the coerce, because non-rec newtypes are transparent
380 clas_tyvars = classTyVars clas
381 kind = tyVarKind (last clas_tyvars)
382 -- Kind of the thing we want to instance
383 -- e.g. argument kind of Monad, *->*
385 (arg_kinds, _) = splitKindFunTys kind
386 n_args_to_drop = length arg_kinds
387 -- Want to drop 1 arg from (T s a) and (ST s a)
388 -- to get instance Monad (ST s) => Monad (T s)
390 -- Note [newtype representation]
391 -- Need newTyConRhs *not* newTyConRep to get the representation
392 -- type, because the latter looks through all intermediate newtypes
394 -- newtype B = MkB Int
395 -- newtype A = MkA B deriving( Num )
396 -- We want the Num instance of B, *not* the Num instance of Int,
397 -- when making the Num instance of A!
398 (tc_tvs, rep_ty) = newTyConRhs tycon
399 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
401 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
402 tyvars_to_drop = drop n_tyvars_to_keep tc_tvs
403 tyvars_to_keep = take n_tyvars_to_keep tc_tvs
405 n_args_to_keep = length rep_ty_args - n_args_to_drop
406 args_to_drop = drop n_args_to_keep rep_ty_args
407 args_to_keep = take n_args_to_keep rep_ty_args
409 rep_tys = tys ++ [mkAppTys rep_fn args_to_keep]
410 rep_pred = mkClassPred clas rep_tys
411 -- rep_pred is the representation dictionary, from where
412 -- we are gong to get all the methods for the newtype dictionary
414 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
415 -- The 'tys' here come from the partial application
416 -- in the deriving clause. The last arg is the new
419 -- We must pass the superclasses; the newtype might be an instance
420 -- of them in a different way than the representation type
421 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
422 -- Then the Show instance is not done via isomprphism; it shows
424 -- The Num instance is derived via isomorphism, but the Show superclass
425 -- dictionary must the Show instance for Foo, *not* the Show dictionary
426 -- gotten from the Num dictionary. So we must build a whole new dictionary
427 -- not just use the Num one. The instance we want is something like:
428 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
431 -- There's no 'corece' needed because after the type checker newtypes
434 sc_theta = substTheta (mkTyVarSubst clas_tyvars inst_tys)
437 -- If there are no tyvars, there's no need
438 -- to abstract over the dictionaries we need
439 dict_tvs = deriv_tvs ++ tc_tvs
440 dict_args | null dict_tvs = []
441 | otherwise = rep_pred : sc_theta
443 -- Finally! Here's where we build the dictionary Id
444 mk_dfun dfun_name = mkDictFunId dfun_name dict_tvs dict_args clas inst_tys
446 -------------------------------------------------------------------
447 -- Figuring out whether we can only do this newtype-deriving thing
449 right_arity = length tys + 1 == classArity clas
451 -- Never derive Read,Show,Typeable,Data this way
452 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
453 can_derive_via_isomorphism
454 = not (getUnique clas `elem` non_iso_classes)
455 && right_arity -- Well kinded;
456 -- eg not: newtype T ... deriving( ST )
457 -- because ST needs *2* type params
458 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
459 -- eg not: newtype T = T Int deriving( Monad )
460 && n_args_to_keep >= 0 -- Rep type has right kind:
461 -- eg not: newtype T a = T Int deriving( Monad )
462 && eta_ok -- Eta reduction works
463 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
464 -- newtype A = MkA [A]
466 -- instance Eq [A] => Eq A !!
467 -- Here's a recursive newtype that's actually OK
468 -- newtype S1 = S1 [T1 ()]
469 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
470 -- It's currently rejected. Oh well.
471 -- In fact we generate an instance decl that has method of form
472 -- meth @ instTy = meth @ repTy
473 -- (no coerce's). We'd need a coerce if we wanted to handle
474 -- recursive newtypes too
476 -- Check that eta reduction is OK
477 -- (a) the dropped-off args are identical
478 -- (b) the remaining type args mention
479 -- only the remaining type variables
480 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
481 && (tyVarsOfTypes args_to_keep `subVarSet` mkVarSet tyvars_to_keep)
483 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
484 (vcat [ptext SLIT("even with cunning newtype deriving:"),
485 if isRecursiveTyCon tycon then
486 ptext SLIT("the newtype is recursive")
488 if not right_arity then
489 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
491 if not (n_tyvars_to_keep >= 0) then
492 ptext SLIT("the type constructor has wrong kind")
493 else if not (n_args_to_keep >= 0) then
494 ptext SLIT("the representation type has wrong kind")
495 else if not eta_ok then
496 ptext SLIT("the eta-reduction property does not hold")
500 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
501 (vcat [non_std_why clas,
502 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
504 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
506 std_class gla_exts clas
507 = key `elem` derivableClassKeys
508 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
512 std_class_via_iso clas -- These standard classes can be derived for a newtype
513 -- using the isomorphism trick *even if no -fglasgow-exts*
514 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
515 -- Not Read/Show because they respect the type
516 -- Not Enum, becuase newtypes are never in Enum
519 new_dfun_name clas tycon -- Just a simple wrapper
520 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
521 -- The type passed to newDFunName is only used to generate
522 -- a suitable string; hence the empty type arg list
524 ------------------------------------------------------------------
525 mkDataTypeEqn :: TyCon -> Class -> TcM DerivEqn
526 mkDataTypeEqn tycon clas
527 | clas `hasKey` typeableClassKey
528 = -- The Typeable class is special in several ways
529 -- data T a b = ... deriving( Typeable )
531 -- instance Typeable2 T where ...
532 -- 1. There are no constraints in the instance
533 -- 2. There are no type variables either
534 -- 2. The actual class we want to generate isn't necessarily
535 -- Typeable; it depends on the arity of the type
536 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
537 ; dfun_name <- new_dfun_name real_clas tycon
538 ; return (dfun_name, real_clas, tycon, [], []) }
541 = do { dfun_name <- new_dfun_name clas tycon
542 ; return (dfun_name, clas, tycon, tyvars, constraints) }
544 tyvars = tyConTyVars tycon
545 constraints = extra_constraints ++ ordinary_constraints
546 extra_constraints = tyConTheta tycon
547 -- "extra_constraints": see note [Data decl contexts] above
550 = [ mkClassPred clas [arg_ty]
551 | data_con <- tyConDataCons tycon,
552 arg_ty <- dataConOrigArgTys data_con,
553 -- Use the same type variables
554 -- as the type constructor,
555 -- hence no need to instantiate
556 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
560 ------------------------------------------------------------------
561 -- Check side conditions that dis-allow derivability for particular classes
562 -- This is *apart* from the newtype-deriving mechanism
564 checkSideConditions :: Bool -> TyCon -> [TyVar] -> Class -> [TcType] -> Maybe SDoc
565 checkSideConditions gla_exts tycon deriv_tvs clas tys
566 | notNull deriv_tvs || notNull tys
567 = Just ty_args_why -- e.g. deriving( Foo s )
569 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
570 [] -> Just (non_std_why clas)
571 [cond] -> cond (gla_exts, tycon)
572 other -> pprPanic "checkSideConditions" (ppr clas)
574 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
576 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
578 sideConditions :: [(Unique, Condition)]
580 = [ (eqClassKey, cond_std),
581 (ordClassKey, cond_std),
582 (readClassKey, cond_std),
583 (showClassKey, cond_std),
584 (enumClassKey, cond_std `andCond` cond_isEnumeration),
585 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
586 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
587 (typeableClassKey, cond_glaExts `andCond` cond_allTypeKind),
588 (dataClassKey, cond_glaExts `andCond` cond_std)
591 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
593 orCond :: Condition -> Condition -> Condition
596 Nothing -> Nothing -- c1 succeeds
597 Just x -> case c2 tc of -- c1 fails
599 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
602 andCond c1 c2 tc = case c1 tc of
603 Nothing -> c2 tc -- c1 succeeds
604 Just x -> Just x -- c1 fails
606 cond_std :: Condition
607 cond_std (gla_exts, tycon)
608 | any isExistentialDataCon data_cons = Just existential_why
609 | null data_cons = Just no_cons_why
610 | otherwise = Nothing
612 data_cons = tyConDataCons tycon
613 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
614 existential_why = quotes (ppr tycon) <+> ptext SLIT("has existentially-quantified constructor(s)")
616 cond_isEnumeration :: Condition
617 cond_isEnumeration (gla_exts, tycon)
618 | isEnumerationTyCon tycon = Nothing
619 | otherwise = Just why
621 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
623 cond_isProduct :: Condition
624 cond_isProduct (gla_exts, tycon)
625 | isProductTyCon tycon = Nothing
626 | otherwise = Just why
628 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
630 cond_allTypeKind :: Condition
631 cond_allTypeKind (gla_exts, tycon)
632 | all (isArgTypeKind . tyVarKind) (tyConTyVars tycon) = Nothing
633 | otherwise = Just why
635 why = quotes (ppr tycon) <+> ptext SLIT("is parameterised over arguments of kind other than `*'")
637 cond_glaExts :: Condition
638 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
639 | otherwise = Just why
641 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
644 %************************************************************************
646 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
648 %************************************************************************
650 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
651 terms, which is the final correct RHS for the corresponding original
655 Each (k,TyVarTy tv) in a solution constrains only a type
659 The (k,TyVarTy tv) pairs in a solution are canonically
660 ordered by sorting on type varible, tv, (major key) and then class, k,
665 solveDerivEqns :: [DerivEqn]
666 -> TcM [DFunId] -- Solns in same order as eqns.
667 -- This bunch is Absolutely minimal...
669 solveDerivEqns orig_eqns
670 = iterateDeriv 1 initial_solutions
672 -- The initial solutions for the equations claim that each
673 -- instance has an empty context; this solution is certainly
674 -- in canonical form.
675 initial_solutions :: [DerivSoln]
676 initial_solutions = [ [] | _ <- orig_eqns ]
678 ------------------------------------------------------------------
679 -- iterateDeriv calculates the next batch of solutions,
680 -- compares it with the current one; finishes if they are the
681 -- same, otherwise recurses with the new solutions.
682 -- It fails if any iteration fails
683 iterateDeriv :: Int -> [DerivSoln] ->TcM [DFunId]
684 iterateDeriv n current_solns
685 | n > 20 -- Looks as if we are in an infinite loop
686 -- This can happen if we have -fallow-undecidable-instances
687 -- (See TcSimplify.tcSimplifyDeriv.)
688 = pprPanic "solveDerivEqns: probable loop"
689 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
692 dfuns = zipWithEqual "add_solns" mk_deriv_dfun orig_eqns current_solns
695 -- Extend the inst info from the explicit instance decls
696 -- with the current set of solutions, and simplify each RHS
697 extendLocalInstEnv dfuns $
698 mappM gen_soln orig_eqns
699 ) `thenM` \ new_solns ->
700 if (current_solns == new_solns) then
703 iterateDeriv (n+1) new_solns
705 ------------------------------------------------------------------
707 gen_soln (_, clas, tc,tyvars,deriv_rhs)
708 = addSrcSpan (srcLocSpan (getSrcLoc tc)) $
709 addErrCtxt (derivCtxt (Just clas) tc) $
710 tcSimplifyDeriv tyvars deriv_rhs `thenM` \ theta ->
711 returnM (sortLt (<) theta) -- Canonicalise before returning the soluction
713 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
714 = mkDictFunId dfun_name tyvars theta
715 clas [mkTyConApp tycon (mkTyVarTys tyvars)]
717 extendLocalInstEnv :: [DFunId] -> TcM a -> TcM a
718 -- Add new locall-defined instances; don't bother to check
719 -- for functional dependency errors -- that'll happen in TcInstDcls
720 extendLocalInstEnv dfuns thing_inside
721 = do { env <- getGblEnv
722 ; let inst_env' = foldl extendInstEnv (tcg_inst_env env) dfuns
723 env' = env { tcg_inst_env = inst_env' }
724 ; setGblEnv env' thing_inside }
727 %************************************************************************
729 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
731 %************************************************************************
733 After all the trouble to figure out the required context for the
734 derived instance declarations, all that's left is to chug along to
735 produce them. They will then be shoved into @tcInstDecls2@, which
736 will do all its usual business.
738 There are lots of possibilities for code to generate. Here are
739 various general remarks.
744 We want derived instances of @Eq@ and @Ord@ (both v common) to be
745 ``you-couldn't-do-better-by-hand'' efficient.
748 Deriving @Show@---also pretty common--- should also be reasonable good code.
751 Deriving for the other classes isn't that common or that big a deal.
758 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
761 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
764 We {\em normally} generate code only for the non-defaulted methods;
765 there are some exceptions for @Eq@ and (especially) @Ord@...
768 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
769 constructor's numeric (@Int#@) tag. These are generated by
770 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
771 these is around is given by @hasCon2TagFun@.
773 The examples under the different sections below will make this
777 Much less often (really just for deriving @Ix@), we use a
778 @_tag2con_<tycon>@ function. See the examples.
781 We use the renamer!!! Reason: we're supposed to be
782 producing @LHsBinds Name@ for the methods, but that means
783 producing correctly-uniquified code on the fly. This is entirely
784 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
785 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
786 the renamer. What a great hack!
790 -- Generate the InstInfo for the required instance,
791 -- plus any auxiliary bindings required
792 genInst :: DFunId -> TcM (InstInfo, LHsBinds RdrName)
794 = getFixityEnv `thenM` \ fix_env ->
796 (tyvars,_,clas,[ty]) = tcSplitDFunTy (idType dfun)
797 clas_nm = className clas
798 tycon = tcTyConAppTyCon ty
799 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
801 -- Bring the right type variables into
802 -- scope, and rename the method binds
803 bindLocalNames (map varName tyvars) $
804 rnMethodBinds clas_nm [] meth_binds `thenM` \ (rn_meth_binds, _fvs) ->
806 -- Build the InstInfo
807 returnM (InstInfo { iDFunId = dfun, iBinds = VanillaInst rn_meth_binds [] },
810 genDerivBinds clas fix_env tycon
811 | className clas `elem` typeableClassNames
812 = (gen_Typeable_binds tycon, emptyBag)
815 = case assocMaybe gen_list (getUnique clas) of
816 Just gen_fn -> gen_fn fix_env tycon
817 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
819 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
820 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
821 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
822 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
823 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
824 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
825 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
826 ,(showClassKey, no_aux_binds gen_Show_binds)
827 ,(readClassKey, no_aux_binds gen_Read_binds)
828 ,(dataClassKey, gen_Data_binds)
831 -- no_aux_binds is used for generators that don't
832 -- need to produce any auxiliary bindings
833 no_aux_binds f fix_env tc = (f fix_env tc, emptyBag)
834 ignore_fix_env f fix_env tc = f tc
838 %************************************************************************
840 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
842 %************************************************************************
847 con2tag_Foo :: Foo ... -> Int#
848 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
849 maxtag_Foo :: Int -- ditto (NB: not unlifted)
852 We have a @con2tag@ function for a tycon if:
855 We're deriving @Eq@ and the tycon has nullary data constructors.
858 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
862 We have a @tag2con@ function for a tycon if:
865 We're deriving @Enum@, or @Ix@ (enum type only???)
868 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
871 genTaggeryBinds :: [DFunId] -> TcM (LHsBinds RdrName)
872 genTaggeryBinds dfuns
873 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
874 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
875 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
877 all_CTs = map simpleDFunClassTyCon dfuns
878 all_tycons = map snd all_CTs
879 (tycons_of_interest, _) = removeDups compare all_tycons
881 do_con2tag acc_Names tycon
882 | isDataTyCon tycon &&
883 ((we_are_deriving eqClassKey tycon
884 && any isNullaryDataCon (tyConDataCons tycon))
885 || (we_are_deriving ordClassKey tycon
886 && not (isProductTyCon tycon))
887 || (we_are_deriving enumClassKey tycon)
888 || (we_are_deriving ixClassKey tycon))
890 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
895 do_tag2con acc_Names tycon
896 | isDataTyCon tycon &&
897 (we_are_deriving enumClassKey tycon ||
898 we_are_deriving ixClassKey tycon
899 && isEnumerationTyCon tycon)
900 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
901 : (maxtag_RDR tycon, tycon, GenMaxTag)
906 we_are_deriving clas_key tycon
907 = is_in_eqns clas_key tycon all_CTs
909 is_in_eqns clas_key tycon [] = False
910 is_in_eqns clas_key tycon ((c,t):cts)
911 = (clas_key == classKey c && tycon == t)
912 || is_in_eqns clas_key tycon cts
916 derivingThingErr clas tys tycon tyvars why
917 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
920 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
922 derivCtxt :: Maybe Class -> TyCon -> SDoc
923 derivCtxt maybe_cls tycon
924 = ptext SLIT("When deriving") <+> cls <+> ptext SLIT("for type") <+> quotes (ppr tycon)
926 cls = case maybe_cls of
927 Nothing -> ptext SLIT("instances")
928 Just c -> ptext SLIT("the") <+> quotes (ppr c) <+> ptext SLIT("instance")