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"
13 import HsSyn ( HsBinds(..), TyClDecl(..), MonoBinds(..),
15 import RdrHsSyn ( RdrNameMonoBinds )
16 import RnHsSyn ( RenamedHsBinds, RenamedTyClDecl, RenamedHsPred )
17 import CmdLineOpts ( DynFlag(..) )
19 import Generics ( mkTyConGenericBinds )
21 import TcEnv ( newDFunName,
22 InstInfo(..), pprInstInfo, InstBindings(..),
23 pprInstInfoDetails, tcLookupTyCon, tcExtendTyVarEnv
25 import TcGenDeriv -- Deriv stuff
26 import InstEnv ( simpleDFunClassTyCon, extendInstEnv )
27 import TcHsType ( tcHsPred )
28 import TcSimplify ( tcSimplifyDeriv )
30 import RnBinds ( rnMethodBinds, rnTopMonoBinds )
31 import RnEnv ( bindLocalNames )
32 import TcRnMonad ( thenM, returnM, mapAndUnzipM )
33 import HscTypes ( DFunId, FixityEnv, typeEnvTyCons )
35 import BasicTypes ( NewOrData(..) )
36 import Class ( className, classArity, classKey, classTyVars, classSCTheta, Class )
37 import Subst ( mkTyVarSubst, substTheta )
38 import ErrUtils ( dumpIfSet_dyn )
39 import MkId ( mkDictFunId )
40 import DataCon ( dataConOrigArgTys, isNullaryDataCon, isExistentialDataCon )
41 import Maybes ( catMaybes )
42 import Name ( Name, getSrcLoc )
43 import NameSet ( NameSet, emptyNameSet, duDefs )
44 import Unique ( Unique, getUnique )
46 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
47 tyConTheta, isProductTyCon, isDataTyCon,
48 isEnumerationTyCon, isRecursiveTyCon, TyCon
50 import TcType ( TcType, ThetaType, mkTyVarTy, mkTyVarTys, mkTyConApp,
51 getClassPredTys_maybe, tcTyConAppTyCon,
52 isUnLiftedType, mkClassPred, tyVarsOfTypes, tcSplitFunTys, isTypeKind,
53 tcEqTypes, tcSplitAppTys, mkAppTys, tcSplitDFunTy )
54 import Var ( TyVar, tyVarKind, idType, varName )
55 import VarSet ( mkVarSet, subVarSet )
57 import Util ( zipWithEqual, sortLt, notNull )
58 import ListSetOps ( removeDups, assoc )
62 %************************************************************************
64 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
66 %************************************************************************
70 data T a b = C1 (Foo a) (Bar b)
75 [NOTE: See end of these comments for what to do with
76 data (C a, D b) => T a b = ...
79 We want to come up with an instance declaration of the form
81 instance (Ping a, Pong b, ...) => Eq (T a b) where
84 It is pretty easy, albeit tedious, to fill in the code "...". The
85 trick is to figure out what the context for the instance decl is,
86 namely @Ping@, @Pong@ and friends.
88 Let's call the context reqd for the T instance of class C at types
89 (a,b, ...) C (T a b). Thus:
91 Eq (T a b) = (Ping a, Pong b, ...)
93 Now we can get a (recursive) equation from the @data@ decl:
95 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
96 u Eq (T b a) u Eq Int -- From C2
97 u Eq (T a a) -- From C3
99 Foo and Bar may have explicit instances for @Eq@, in which case we can
100 just substitute for them. Alternatively, either or both may have
101 their @Eq@ instances given by @deriving@ clauses, in which case they
102 form part of the system of equations.
104 Now all we need do is simplify and solve the equations, iterating to
105 find the least fixpoint. Notice that the order of the arguments can
106 switch around, as here in the recursive calls to T.
108 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
112 Eq (T a b) = {} -- The empty set
115 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
116 u Eq (T b a) u Eq Int -- From C2
117 u Eq (T a a) -- From C3
119 After simplification:
120 = Eq a u Ping b u {} u {} u {}
125 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
126 u Eq (T b a) u Eq Int -- From C2
127 u Eq (T a a) -- From C3
129 After simplification:
134 = Eq a u Ping b u Eq b u Ping a
136 The next iteration gives the same result, so this is the fixpoint. We
137 need to make a canonical form of the RHS to ensure convergence. We do
138 this by simplifying the RHS to a form in which
140 - the classes constrain only tyvars
141 - the list is sorted by tyvar (major key) and then class (minor key)
142 - no duplicates, of course
144 So, here are the synonyms for the ``equation'' structures:
147 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
148 -- The Name is the name for the DFun we'll build
149 -- The tyvars bind all the variables in the RHS
151 pprDerivEqn (n,c,tc,tvs,rhs)
152 = parens (hsep [ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
154 type DerivRhs = ThetaType
155 type DerivSoln = DerivRhs
159 [Data decl contexts] A note about contexts on data decls
160 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
163 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
165 We will need an instance decl like:
167 instance (Read a, RealFloat a) => Read (Complex a) where
170 The RealFloat in the context is because the read method for Complex is bound
171 to construct a Complex, and doing that requires that the argument type is
174 But this ain't true for Show, Eq, Ord, etc, since they don't construct
175 a Complex; they only take them apart.
177 Our approach: identify the offending classes, and add the data type
178 context to the instance decl. The "offending classes" are
182 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
183 pattern matching against a constructor from a data type with a context
184 gives rise to the constraints for that context -- or at least the thinned
185 version. So now all classes are "offending".
189 %************************************************************************
191 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
193 %************************************************************************
196 tcDeriving :: [RenamedTyClDecl] -- All type constructors
197 -> TcM ([InstInfo], -- The generated "instance decls"
198 RenamedHsBinds, -- Extra generated top-level bindings
199 NameSet) -- Binders to keep alive
201 tcDeriving tycl_decls
202 = recoverM (returnM ([], EmptyBinds, emptyNameSet)) $
203 do { -- Fish the "deriving"-related information out of the TcEnv
204 -- and make the necessary "equations".
205 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns tycl_decls
207 ; (ordinary_inst_info, deriv_binds)
208 <- extendLocalInstEnv (map iDFunId newtype_inst_info) $
209 deriveOrdinaryStuff ordinary_eqns
210 -- Add the newtype-derived instances to the inst env
211 -- before tacking the "ordinary" ones
213 -- Generate the generic to/from functions from each type declaration
214 ; gen_binds <- mkGenericBinds tycl_decls
215 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
217 -- Rename these extra bindings, discarding warnings about unused bindings etc
218 ; (rn_binds, gen_bndrs)
219 <- discardWarnings $ do
220 { (rn_deriv, _dus1) <- rnTopMonoBinds deriv_binds []
221 ; (rn_gen, dus_gen) <- rnTopMonoBinds gen_binds []
222 ; return (rn_deriv `ThenBinds` rn_gen, duDefs dus_gen) }
226 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
227 (ddump_deriving inst_info rn_binds))
229 ; returnM (inst_info, rn_binds, gen_bndrs)
232 ddump_deriving :: [InstInfo] -> RenamedHsBinds -> SDoc
233 ddump_deriving inst_infos extra_binds
234 = vcat (map pprInstInfoDetails inst_infos) $$ ppr extra_binds
236 -----------------------------------------
237 deriveOrdinaryStuff [] -- Short cut
238 = returnM ([], EmptyMonoBinds)
240 deriveOrdinaryStuff eqns
241 = do { -- Take the equation list and solve it, to deliver a list of
242 -- solutions, a.k.a. the contexts for the instance decls
243 -- required for the corresponding equations.
244 ; new_dfuns <- solveDerivEqns eqns
246 -- Generate the InstInfo for each dfun,
247 -- plus any auxiliary bindings it needs
248 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst new_dfuns
250 -- Generate any extra not-one-inst-decl-specific binds,
251 -- notably "con2tag" and/or "tag2con" functions.
252 ; extra_binds <- genTaggeryBinds new_dfuns
255 ; returnM (inst_infos, andMonoBindList (extra_binds : aux_binds_s)) }
257 -----------------------------------------
258 mkGenericBinds tycl_decls
259 = do { tcs <- mapM tcLookupTyCon [tc_name | TyData { tcdName = tc_name } <- tycl_decls]
260 -- We are only interested in the data type declarations
261 ; return (andMonoBindList [mkTyConGenericBinds tc | tc <- tcs, tyConHasGenerics tc]) }
262 -- And then only in the ones whose 'has-generics' flag is on
266 %************************************************************************
268 \subsection[TcDeriv-eqns]{Forming the equations}
270 %************************************************************************
272 @makeDerivEqns@ fishes around to find the info about needed derived
273 instances. Complicating factors:
276 We can only derive @Enum@ if the data type is an enumeration
277 type (all nullary data constructors).
280 We can only derive @Ix@ if the data type is an enumeration {\em
281 or} has just one data constructor (e.g., tuples).
284 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
288 makeDerivEqns :: [RenamedTyClDecl]
289 -> TcM ([DerivEqn], -- Ordinary derivings
290 [InstInfo]) -- Special newtype derivings
292 makeDerivEqns tycl_decls
293 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
294 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
296 ------------------------------------------------------------------
297 derive_these :: [(NewOrData, Name, RenamedHsPred)]
298 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
299 -- NB: only source-language decls have deriving, no imported ones do
300 derive_these = [ (nd, tycon, pred)
301 | TyData {tcdND = nd, tcdName = tycon, tcdDerivs = Just preds} <- tycl_decls,
304 ------------------------------------------------------------------
305 mk_eqn :: (NewOrData, Name, RenamedHsPred) -> TcM (Maybe DerivEqn, Maybe InstInfo)
306 -- We swizzle the tyvars and datacons out of the tycon
307 -- to make the rest of the equation
309 mk_eqn (new_or_data, tycon_name, pred)
310 = tcLookupTyCon tycon_name `thenM` \ tycon ->
311 addSrcLoc (getSrcLoc tycon) $
312 addErrCtxt (derivCtxt Nothing tycon) $
313 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
314 -- the type variables for the type constructor
315 tcHsPred pred `thenM` \ pred' ->
316 case getClassPredTys_maybe pred' of
317 Nothing -> bale_out (malformedPredErr tycon pred)
318 Just (clas, tys) -> doptM Opt_GlasgowExts `thenM` \ gla_exts ->
319 mk_eqn_help gla_exts new_or_data tycon clas tys
321 ------------------------------------------------------------------
322 mk_eqn_help gla_exts DataType tycon clas tys
323 | Just err <- checkSideConditions gla_exts clas tycon tys
324 = bale_out (derivingThingErr clas tys tycon tyvars err)
326 = new_dfun_name clas tycon `thenM` \ dfun_name ->
327 returnM (Just (dfun_name, clas, tycon, tyvars, constraints), Nothing)
329 tyvars = tyConTyVars tycon
330 constraints = extra_constraints ++ ordinary_constraints
331 -- "extra_constraints": see note [Data decl contexts] above
332 extra_constraints = tyConTheta tycon
335 | clas `hasKey` typeableClassKey -- For the Typeable class, the constraints
336 -- don't involve the constructor ags, only
338 -- e.g. data T a b = ...
340 -- instance (Typeable a, Typable b)
341 -- => Typeable (T a b) where
342 = [mkClassPred clas [mkTyVarTy tv] | tv <- tyvars]
344 = [ mkClassPred clas [arg_ty]
345 | data_con <- tyConDataCons tycon,
346 arg_ty <- dataConOrigArgTys data_con,
347 -- Use the same type variables
348 -- as the type constructor,
349 -- hence no need to instantiate
350 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
353 mk_eqn_help gla_exts NewType tycon clas tys
354 | can_derive_via_isomorphism && (gla_exts || standard_class gla_exts clas)
355 = -- Go ahead and use the isomorphism
356 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
357 new_dfun_name clas tycon `thenM` \ dfun_name ->
358 returnM (Nothing, Just (InstInfo { iDFunId = mk_dfun dfun_name,
359 iBinds = NewTypeDerived rep_tys }))
360 | standard_class gla_exts clas
361 = mk_eqn_help gla_exts DataType tycon clas tys -- Go via bale-out route
363 | otherwise -- Non-standard instance
364 = bale_out (if gla_exts then
365 cant_derive_err -- Too hard
367 non_std_err) -- Just complain about being a non-std instance
369 -- Here is the plan for newtype derivings. We see
370 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
371 -- where aj...an do not occur free in t, and the (C s1 ... sm) is a
372 -- *partial applications* of class C with the last parameter missing
374 -- We generate the instances
375 -- instance C s1 .. sm (t ak...aj) => C s1 .. sm (T a1...aj)
376 -- where T a1...aj is the partial application of the LHS of the correct kind
378 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
379 -- instance Monad (ST s) => Monad (T s) where
380 -- fail = coerce ... (fail @ ST s)
382 clas_tyvars = classTyVars clas
383 kind = tyVarKind (last clas_tyvars)
384 -- Kind of the thing we want to instance
385 -- e.g. argument kind of Monad, *->*
387 (arg_kinds, _) = tcSplitFunTys kind
388 n_args_to_drop = length arg_kinds
389 -- Want to drop 1 arg from (T s a) and (ST s a)
390 -- to get instance Monad (ST s) => Monad (T s)
392 -- Note [newtype representation]
393 -- We must not use newTyConRep to get the representation
394 -- type, because that looks through all intermediate newtypes
395 -- To get the RHS of *this* newtype, just look at the data
396 -- constructor. For example
397 -- newtype B = MkB Int
398 -- newtype A = MkA B deriving( Num )
399 -- We want the Num instance of B, *not* the Num instance of Int,
400 -- when making the Num instance of A!
401 tyvars = tyConTyVars tycon
402 rep_ty = head (dataConOrigArgTys (head (tyConDataCons tycon)))
403 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
405 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
406 tyvars_to_drop = drop n_tyvars_to_keep tyvars
407 tyvars_to_keep = take n_tyvars_to_keep tyvars
409 n_args_to_keep = length rep_ty_args - n_args_to_drop
410 args_to_drop = drop n_args_to_keep rep_ty_args
411 args_to_keep = take n_args_to_keep rep_ty_args
413 rep_tys = tys ++ [mkAppTys rep_fn args_to_keep]
414 rep_pred = mkClassPred clas rep_tys
415 -- rep_pred is the representation dictionary, from where
416 -- we are gong to get all the methods for the newtype dictionary
418 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
419 -- The 'tys' here come from the partial application
420 -- in the deriving clause. The last arg is the new
423 -- We must pass the superclasses; the newtype might be an instance
424 -- of them in a different way than the representation type
425 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
426 -- Then the Show instance is not done via isomprphism; it shows
428 -- The Num instance is derived via isomorphism, but the Show superclass
429 -- dictionary must the Show instance for Foo, *not* the Show dictionary
430 -- gotten from the Num dictionary. So we must build a whole new dictionary
431 -- not just use the Num one. The instance we want is something like:
432 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
435 -- There's no 'corece' needed because after the type checker newtypes
438 sc_theta = substTheta (mkTyVarSubst clas_tyvars inst_tys)
441 -- If there are no tyvars, there's no need
442 -- to abstract over the dictionaries we need
443 dict_args | null tyvars = []
444 | otherwise = rep_pred : sc_theta
446 -- Finally! Here's where we build the dictionary Id
447 mk_dfun dfun_name = mkDictFunId dfun_name tyvars dict_args clas inst_tys
449 -------------------------------------------------------------------
450 -- Figuring out whether we can only do this newtype-deriving thing
452 right_arity = length tys + 1 == classArity clas
454 -- Never derive Read,Show,Typeable,Data this way
455 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
456 can_derive_via_isomorphism
457 = not (getUnique clas `elem` non_iso_classes)
458 && right_arity -- Well kinded;
459 -- eg not: newtype T ... deriving( ST )
460 -- because ST needs *2* type params
461 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
462 -- eg not: newtype T = T Int deriving( Monad )
463 && n_args_to_keep >= 0 -- Rep type has right kind:
464 -- eg not: newtype T a = T Int deriving( Monad )
465 && eta_ok -- Eta reduction works
466 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
467 -- newtype A = MkA [A]
469 -- instance Eq [A] => Eq A !!
471 -- Here's a recursive newtype that's actually OK
472 -- newtype S1 = S1 [T1 ()]
473 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
474 -- It's currently rejected. Oh well.
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)
505 standard_class gla_exts clas = key `elem` derivableClassKeys
506 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
513 new_dfun_name clas tycon -- Just a simple wrapper
514 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
515 -- The type passed to newDFunName is only used to generate
516 -- a suitable string; hence the empty type arg list
518 ------------------------------------------------------------------
519 -- Check side conditions that dis-allow derivability for particular classes
520 -- This is *apart* from the newtype-deriving mechanism
522 checkSideConditions :: Bool -> Class -> TyCon -> [TcType] -> Maybe SDoc
523 checkSideConditions gla_exts clas tycon tys
525 = Just ty_args_why -- e.g. deriving( Foo s )
527 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
528 [] -> Just (non_std_why clas)
529 [cond] -> cond (gla_exts, tycon)
530 other -> pprPanic "checkSideConditions" (ppr clas)
532 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
534 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
536 sideConditions :: [(Unique, Condition)]
538 = [ (eqClassKey, cond_std),
539 (ordClassKey, cond_std),
540 (readClassKey, cond_std),
541 (showClassKey, cond_std),
542 (enumClassKey, cond_std `andCond` cond_isEnumeration),
543 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
544 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
545 (typeableClassKey, cond_glaExts `andCond` cond_allTypeKind),
546 (dataClassKey, cond_glaExts `andCond` cond_std)
549 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
551 orCond :: Condition -> Condition -> Condition
554 Nothing -> Nothing -- c1 succeeds
555 Just x -> case c2 tc of -- c1 fails
557 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
560 andCond c1 c2 tc = case c1 tc of
561 Nothing -> c2 tc -- c1 succeeds
562 Just x -> Just x -- c1 fails
564 cond_std :: Condition
565 cond_std (gla_exts, tycon)
566 | any isExistentialDataCon data_cons = Just existential_why
567 | null data_cons = Just no_cons_why
568 | otherwise = Nothing
570 data_cons = tyConDataCons tycon
571 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
572 existential_why = quotes (ppr tycon) <+> ptext SLIT("has existentially-quantified constructor(s)")
574 cond_isEnumeration :: Condition
575 cond_isEnumeration (gla_exts, tycon)
576 | isEnumerationTyCon tycon = Nothing
577 | otherwise = Just why
579 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
581 cond_isProduct :: Condition
582 cond_isProduct (gla_exts, tycon)
583 | isProductTyCon tycon = Nothing
584 | otherwise = Just why
586 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
588 cond_allTypeKind :: Condition
589 cond_allTypeKind (gla_exts, tycon)
590 | all (isTypeKind . tyVarKind) (tyConTyVars tycon) = Nothing
591 | otherwise = Just why
593 why = quotes (ppr tycon) <+> ptext SLIT("is parameterised over arguments of kind other than `*'")
595 cond_glaExts :: Condition
596 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
597 | otherwise = Just why
599 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
602 %************************************************************************
604 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
606 %************************************************************************
608 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
609 terms, which is the final correct RHS for the corresponding original
613 Each (k,TyVarTy tv) in a solution constrains only a type
617 The (k,TyVarTy tv) pairs in a solution are canonically
618 ordered by sorting on type varible, tv, (major key) and then class, k,
623 solveDerivEqns :: [DerivEqn]
624 -> TcM [DFunId] -- Solns in same order as eqns.
625 -- This bunch is Absolutely minimal...
627 solveDerivEqns orig_eqns
628 = iterateDeriv 1 initial_solutions
630 -- The initial solutions for the equations claim that each
631 -- instance has an empty context; this solution is certainly
632 -- in canonical form.
633 initial_solutions :: [DerivSoln]
634 initial_solutions = [ [] | _ <- orig_eqns ]
636 ------------------------------------------------------------------
637 -- iterateDeriv calculates the next batch of solutions,
638 -- compares it with the current one; finishes if they are the
639 -- same, otherwise recurses with the new solutions.
640 -- It fails if any iteration fails
641 iterateDeriv :: Int -> [DerivSoln] ->TcM [DFunId]
642 iterateDeriv n current_solns
643 | n > 20 -- Looks as if we are in an infinite loop
644 -- This can happen if we have -fallow-undecidable-instances
645 -- (See TcSimplify.tcSimplifyDeriv.)
646 = pprPanic "solveDerivEqns: probable loop"
647 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
650 dfuns = zipWithEqual "add_solns" mk_deriv_dfun orig_eqns current_solns
653 -- Extend the inst info from the explicit instance decls
654 -- with the current set of solutions, and simplify each RHS
655 extendLocalInstEnv dfuns $
656 mappM gen_soln orig_eqns
657 ) `thenM` \ new_solns ->
658 if (current_solns == new_solns) then
661 iterateDeriv (n+1) new_solns
663 ------------------------------------------------------------------
665 gen_soln (_, clas, tc,tyvars,deriv_rhs)
666 = addSrcLoc (getSrcLoc tc) $
667 addErrCtxt (derivCtxt (Just clas) tc) $
668 tcSimplifyDeriv tyvars deriv_rhs `thenM` \ theta ->
669 returnM (sortLt (<) theta) -- Canonicalise before returning the soluction
671 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
672 = mkDictFunId dfun_name tyvars theta
673 clas [mkTyConApp tycon (mkTyVarTys tyvars)]
675 extendLocalInstEnv :: [DFunId] -> TcM a -> TcM a
676 -- Add new locall-defined instances; don't bother to check
677 -- for functional dependency errors -- that'll happen in TcInstDcls
678 extendLocalInstEnv dfuns thing_inside
679 = do { env <- getGblEnv
680 ; let inst_env' = foldl extendInstEnv (tcg_inst_env env) dfuns
681 env' = env { tcg_inst_env = inst_env' }
682 ; setGblEnv env' thing_inside }
685 %************************************************************************
687 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
689 %************************************************************************
691 After all the trouble to figure out the required context for the
692 derived instance declarations, all that's left is to chug along to
693 produce them. They will then be shoved into @tcInstDecls2@, which
694 will do all its usual business.
696 There are lots of possibilities for code to generate. Here are
697 various general remarks.
702 We want derived instances of @Eq@ and @Ord@ (both v common) to be
703 ``you-couldn't-do-better-by-hand'' efficient.
706 Deriving @Show@---also pretty common--- should also be reasonable good code.
709 Deriving for the other classes isn't that common or that big a deal.
716 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
719 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
722 We {\em normally} generate code only for the non-defaulted methods;
723 there are some exceptions for @Eq@ and (especially) @Ord@...
726 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
727 constructor's numeric (@Int#@) tag. These are generated by
728 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
729 these is around is given by @hasCon2TagFun@.
731 The examples under the different sections below will make this
735 Much less often (really just for deriving @Ix@), we use a
736 @_tag2con_<tycon>@ function. See the examples.
739 We use the renamer!!! Reason: we're supposed to be
740 producing @RenamedMonoBinds@ for the methods, but that means
741 producing correctly-uniquified code on the fly. This is entirely
742 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
743 So, instead, we produce @RdrNameMonoBinds@ then heave 'em through
744 the renamer. What a great hack!
748 -- Generate the InstInfo for the required instance,
749 -- plus any auxiliary bindings required
750 genInst :: DFunId -> TcM (InstInfo, RdrNameMonoBinds)
752 = getFixityEnv `thenM` \ fix_env ->
754 (tyvars,_,clas,[ty]) = tcSplitDFunTy (idType dfun)
755 clas_nm = className clas
756 tycon = tcTyConAppTyCon ty
757 (meth_binds, aux_binds) = assoc "gen_bind:bad derived class"
758 gen_list (getUnique clas) fix_env tycon
760 -- Bring the right type variables into
761 -- scope, and rename the method binds
762 bindLocalNames (map varName tyvars) $
763 rnMethodBinds clas_nm [] meth_binds `thenM` \ (rn_meth_binds, _fvs) ->
765 -- Build the InstInfo
766 returnM (InstInfo { iDFunId = dfun, iBinds = VanillaInst rn_meth_binds [] },
769 gen_list :: [(Unique, FixityEnv -> TyCon -> (RdrNameMonoBinds, RdrNameMonoBinds))]
770 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
771 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
772 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
773 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
774 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
775 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
776 ,(showClassKey, no_aux_binds gen_Show_binds)
777 ,(readClassKey, no_aux_binds gen_Read_binds)
778 ,(dataClassKey, gen_Data_binds)
781 -- no_aux_binds is used for generators that don't
782 -- need to produce any auxiliary bindings
783 no_aux_binds f fix_env tc = (f fix_env tc, EmptyMonoBinds)
784 ignore_fix_env f fix_env tc = f tc
788 %************************************************************************
790 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
792 %************************************************************************
797 con2tag_Foo :: Foo ... -> Int#
798 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
799 maxtag_Foo :: Int -- ditto (NB: not unlifted)
802 We have a @con2tag@ function for a tycon if:
805 We're deriving @Eq@ and the tycon has nullary data constructors.
808 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
812 We have a @tag2con@ function for a tycon if:
815 We're deriving @Enum@, or @Ix@ (enum type only???)
818 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
821 genTaggeryBinds :: [DFunId] -> TcM RdrNameMonoBinds
822 genTaggeryBinds dfuns
823 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
824 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
825 ; return (andMonoBindList (map gen_tag_n_con_monobind nm_alist_etc)) }
827 all_CTs = map simpleDFunClassTyCon dfuns
828 all_tycons = map snd all_CTs
829 (tycons_of_interest, _) = removeDups compare all_tycons
831 do_con2tag acc_Names tycon
832 | isDataTyCon tycon &&
833 ((we_are_deriving eqClassKey tycon
834 && any isNullaryDataCon (tyConDataCons tycon))
835 || (we_are_deriving ordClassKey tycon
836 && not (isProductTyCon tycon))
837 || (we_are_deriving enumClassKey tycon)
838 || (we_are_deriving ixClassKey tycon))
840 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
845 do_tag2con acc_Names tycon
846 | isDataTyCon tycon &&
847 (we_are_deriving enumClassKey tycon ||
848 we_are_deriving ixClassKey tycon
849 && isEnumerationTyCon tycon)
850 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
851 : (maxtag_RDR tycon, tycon, GenMaxTag)
856 we_are_deriving clas_key tycon
857 = is_in_eqns clas_key tycon all_CTs
859 is_in_eqns clas_key tycon [] = False
860 is_in_eqns clas_key tycon ((c,t):cts)
861 = (clas_key == classKey c && tycon == t)
862 || is_in_eqns clas_key tycon cts
866 derivingThingErr clas tys tycon tyvars why
867 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
870 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
872 malformedPredErr tycon pred = ptext SLIT("Illegal deriving item") <+> ppr pred
874 derivCtxt :: Maybe Class -> TyCon -> SDoc
875 derivCtxt maybe_cls tycon
876 = ptext SLIT("When deriving") <+> cls <+> ptext SLIT("for type") <+> quotes (ppr tycon)
878 cls = case maybe_cls of
879 Nothing -> ptext SLIT("instances")
880 Just c -> ptext SLIT("the") <+> quotes (ppr c) <+> ptext SLIT("instance")