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,
19 InstInfo(..), InstBindings(..),
20 pprInstInfoDetails, tcLookupTyCon, tcExtendTyVarEnv
22 import TcGenDeriv -- Deriv stuff
23 import InstEnv ( simpleDFunClassTyCon, extendInstEnv )
24 import TcHsType ( tcHsPred )
25 import TcSimplify ( tcSimplifyDeriv )
27 import RnBinds ( rnMethodBinds, rnTopBinds )
28 import RnEnv ( bindLocalNames )
29 import TcRnMonad ( thenM, returnM, mapAndUnzipM )
30 import HscTypes ( DFunId, FixityEnv )
32 import BasicTypes ( NewOrData(..) )
33 import Class ( className, classArity, classKey, classTyVars, classSCTheta, Class )
34 import Subst ( mkTyVarSubst, substTheta )
35 import ErrUtils ( dumpIfSet_dyn )
36 import MkId ( mkDictFunId )
37 import DataCon ( dataConOrigArgTys, isNullaryDataCon, isExistentialDataCon )
38 import Maybes ( catMaybes )
39 import RdrName ( RdrName )
40 import Name ( Name, getSrcLoc )
41 import NameSet ( NameSet, emptyNameSet, duDefs )
42 import Unique ( Unique, getUnique )
44 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
45 tyConTheta, isProductTyCon, isDataTyCon,
46 isEnumerationTyCon, isRecursiveTyCon, TyCon
48 import TcType ( TcType, ThetaType, mkTyVarTy, mkTyVarTys, mkTyConApp,
49 getClassPredTys_maybe, tcTyConAppTyCon,
50 isUnLiftedType, mkClassPred, tyVarsOfTypes, tcSplitFunTys, isTypeKind,
51 tcEqTypes, tcSplitAppTys, mkAppTys, tcSplitDFunTy )
52 import Var ( TyVar, tyVarKind, idType, varName )
53 import VarSet ( mkVarSet, subVarSet )
55 import SrcLoc ( srcLocSpan, Located(..) )
56 import Util ( zipWithEqual, sortLt, notNull )
57 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 :: [LTyClDecl Name] -- All type constructors
197 -> TcM ([InstInfo], -- The generated "instance decls"
198 [HsBindGroup Name], -- Extra generated top-level bindings
199 NameSet) -- Binders to keep alive
201 tcDeriving tycl_decls
202 = recoverM (returnM ([], [], 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 -- Set -fglasgow exts so that we can have type signatures in patterns,
219 -- which is used in the generic binds
220 ; (rn_binds, gen_bndrs)
221 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
222 { (rn_deriv, _dus1) <- rnTopBinds deriv_binds []
223 ; (rn_gen, dus_gen) <- rnTopBinds gen_binds []
224 ; return (rn_deriv ++ rn_gen, duDefs dus_gen) }
228 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
229 (ddump_deriving inst_info rn_binds))
231 ; returnM (inst_info, rn_binds, gen_bndrs)
234 ddump_deriving :: [InstInfo] -> [HsBindGroup Name] -> SDoc
235 ddump_deriving inst_infos extra_binds
236 = vcat (map pprInstInfoDetails inst_infos) $$ vcat (map ppr extra_binds)
238 -----------------------------------------
239 deriveOrdinaryStuff [] -- Short cut
240 = returnM ([], emptyBag)
242 deriveOrdinaryStuff eqns
243 = do { -- Take the equation list and solve it, to deliver a list of
244 -- solutions, a.k.a. the contexts for the instance decls
245 -- required for the corresponding equations.
246 ; new_dfuns <- solveDerivEqns eqns
248 -- Generate the InstInfo for each dfun,
249 -- plus any auxiliary bindings it needs
250 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst new_dfuns
252 -- Generate any extra not-one-inst-decl-specific binds,
253 -- notably "con2tag" and/or "tag2con" functions.
254 ; extra_binds <- genTaggeryBinds new_dfuns
257 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
260 -----------------------------------------
261 mkGenericBinds tycl_decls
262 = do { tcs <- mapM tcLookupTyCon
264 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
265 -- We are only interested in the data type declarations
266 ; return (unionManyBags [ mkTyConGenericBinds tc |
267 tc <- tcs, tyConHasGenerics tc ]) }
268 -- And then only in the ones whose 'has-generics' flag is on
272 %************************************************************************
274 \subsection[TcDeriv-eqns]{Forming the equations}
276 %************************************************************************
278 @makeDerivEqns@ fishes around to find the info about needed derived
279 instances. Complicating factors:
282 We can only derive @Enum@ if the data type is an enumeration
283 type (all nullary data constructors).
286 We can only derive @Ix@ if the data type is an enumeration {\em
287 or} has just one data constructor (e.g., tuples).
290 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
294 makeDerivEqns :: [LTyClDecl Name]
295 -> TcM ([DerivEqn], -- Ordinary derivings
296 [InstInfo]) -- Special newtype derivings
298 makeDerivEqns tycl_decls
299 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
300 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
302 ------------------------------------------------------------------
303 derive_these :: [(NewOrData, Name, LHsPred Name)]
304 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
305 -- NB: only source-language decls have deriving, no imported ones do
306 derive_these = [ (nd, tycon, pred)
307 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
308 tcdDerivs = Just (L _ preds) }) <- tycl_decls,
311 ------------------------------------------------------------------
312 mk_eqn :: (NewOrData, Name, LHsPred Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
313 -- We swizzle the tyvars and datacons out of the tycon
314 -- to make the rest of the equation
316 mk_eqn (new_or_data, tycon_name, pred)
317 = tcLookupTyCon tycon_name `thenM` \ tycon ->
318 addSrcSpan (srcLocSpan (getSrcLoc tycon)) $
319 addErrCtxt (derivCtxt Nothing tycon) $
320 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
321 -- the type variables for the type constructor
322 tcHsPred pred `thenM` \ pred' ->
323 case getClassPredTys_maybe pred' of
324 Nothing -> bale_out (malformedPredErr tycon pred)
325 Just (clas, tys) -> doptM Opt_GlasgowExts `thenM` \ gla_exts ->
326 mk_eqn_help gla_exts new_or_data tycon clas tys
328 ------------------------------------------------------------------
329 mk_eqn_help gla_exts DataType tycon clas tys
330 | Just err <- checkSideConditions gla_exts clas tycon tys
331 = bale_out (derivingThingErr clas tys tycon tyvars err)
333 = new_dfun_name clas tycon `thenM` \ dfun_name ->
334 returnM (Just (dfun_name, clas, tycon, tyvars, constraints), Nothing)
336 tyvars = tyConTyVars tycon
337 constraints = extra_constraints ++ ordinary_constraints
338 -- "extra_constraints": see note [Data decl contexts] above
339 extra_constraints = tyConTheta tycon
342 | clas `hasKey` typeableClassKey -- For the Typeable class, the constraints
343 -- don't involve the constructor ags, only
345 -- e.g. data T a b = ...
347 -- instance (Typeable a, Typable b)
348 -- => Typeable (T a b) where
349 = [mkClassPred clas [mkTyVarTy tv] | tv <- tyvars]
351 = [ mkClassPred clas [arg_ty]
352 | data_con <- tyConDataCons tycon,
353 arg_ty <- dataConOrigArgTys data_con,
354 -- Use the same type variables
355 -- as the type constructor,
356 -- hence no need to instantiate
357 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
360 mk_eqn_help gla_exts NewType tycon clas tys
361 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
362 = -- Go ahead and use the isomorphism
363 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
364 new_dfun_name clas tycon `thenM` \ dfun_name ->
365 returnM (Nothing, Just (InstInfo { iDFunId = mk_dfun dfun_name,
366 iBinds = NewTypeDerived rep_tys }))
367 | std_class gla_exts clas
368 = mk_eqn_help gla_exts DataType tycon clas tys -- Go via bale-out route
370 | otherwise -- Non-standard instance
371 = bale_out (if gla_exts then
372 cant_derive_err -- Too hard
374 non_std_err) -- Just complain about being a non-std instance
376 -- Here is the plan for newtype derivings. We see
377 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
378 -- where aj...an do not occur free in t, and the (C s1 ... sm) is a
379 -- *partial applications* of class C with the last parameter missing
381 -- We generate the instances
382 -- instance C s1 .. sm (t ak...aj) => C s1 .. sm (T a1...aj)
383 -- where T a1...aj 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)
389 clas_tyvars = classTyVars clas
390 kind = tyVarKind (last clas_tyvars)
391 -- Kind of the thing we want to instance
392 -- e.g. argument kind of Monad, *->*
394 (arg_kinds, _) = tcSplitFunTys kind
395 n_args_to_drop = length arg_kinds
396 -- Want to drop 1 arg from (T s a) and (ST s a)
397 -- to get instance Monad (ST s) => Monad (T s)
399 -- Note [newtype representation]
400 -- We must not use newTyConRep to get the representation
401 -- type, because that looks through all intermediate newtypes
402 -- To get the RHS of *this* newtype, just look at the data
403 -- constructor. For example
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 tyvars = tyConTyVars tycon
409 rep_ty = head (dataConOrigArgTys (head (tyConDataCons tycon)))
410 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
412 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
413 tyvars_to_drop = drop n_tyvars_to_keep tyvars
414 tyvars_to_keep = take n_tyvars_to_keep tyvars
416 n_args_to_keep = length rep_ty_args - n_args_to_drop
417 args_to_drop = drop n_args_to_keep rep_ty_args
418 args_to_keep = take n_args_to_keep rep_ty_args
420 rep_tys = tys ++ [mkAppTys rep_fn args_to_keep]
421 rep_pred = mkClassPred clas rep_tys
422 -- rep_pred is the representation dictionary, from where
423 -- we are gong to get all the methods for the newtype dictionary
425 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
426 -- The 'tys' here come from the partial application
427 -- in the deriving clause. The last arg is the new
430 -- We must pass the superclasses; the newtype might be an instance
431 -- of them in a different way than the representation type
432 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
433 -- Then the Show instance is not done via isomprphism; it shows
435 -- The Num instance is derived via isomorphism, but the Show superclass
436 -- dictionary must the Show instance for Foo, *not* the Show dictionary
437 -- gotten from the Num dictionary. So we must build a whole new dictionary
438 -- not just use the Num one. The instance we want is something like:
439 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
442 -- There's no 'corece' needed because after the type checker newtypes
445 sc_theta = substTheta (mkTyVarSubst clas_tyvars inst_tys)
448 -- If there are no tyvars, there's no need
449 -- to abstract over the dictionaries we need
450 dict_args | null tyvars = []
451 | otherwise = rep_pred : sc_theta
453 -- Finally! Here's where we build the dictionary Id
454 mk_dfun dfun_name = mkDictFunId dfun_name tyvars 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 !!
478 -- Here's a recursive newtype that's actually OK
479 -- newtype S1 = S1 [T1 ()]
480 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
481 -- It's currently rejected. Oh well.
483 -- Check that eta reduction is OK
484 -- (a) the dropped-off args are identical
485 -- (b) the remaining type args mention
486 -- only the remaining type variables
487 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
488 && (tyVarsOfTypes args_to_keep `subVarSet` mkVarSet tyvars_to_keep)
490 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
491 (vcat [ptext SLIT("even with cunning newtype deriving:"),
492 if isRecursiveTyCon tycon then
493 ptext SLIT("the newtype is recursive")
495 if not right_arity then
496 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
498 if not (n_tyvars_to_keep >= 0) then
499 ptext SLIT("the type constructor has wrong kind")
500 else if not (n_args_to_keep >= 0) then
501 ptext SLIT("the representation type has wrong kind")
502 else if not eta_ok then
503 ptext SLIT("the eta-reduction property does not hold")
507 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
508 (vcat [non_std_why clas,
509 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
511 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
513 std_class gla_exts clas
514 = key `elem` derivableClassKeys
515 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
519 std_class_via_iso clas -- These standard classes can be derived for a newtype
520 -- using the isomorphism trick *even if no -fglasgow-exts*
521 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
522 -- Not Read/Show because they respect the type
523 -- Not Enum, becuase newtypes are never in Enum
526 new_dfun_name clas tycon -- Just a simple wrapper
527 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
528 -- The type passed to newDFunName is only used to generate
529 -- a suitable string; hence the empty type arg list
531 ------------------------------------------------------------------
532 -- Check side conditions that dis-allow derivability for particular classes
533 -- This is *apart* from the newtype-deriving mechanism
535 checkSideConditions :: Bool -> Class -> TyCon -> [TcType] -> Maybe SDoc
536 checkSideConditions gla_exts clas tycon tys
538 = Just ty_args_why -- e.g. deriving( Foo s )
540 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
541 [] -> Just (non_std_why clas)
542 [cond] -> cond (gla_exts, tycon)
543 other -> pprPanic "checkSideConditions" (ppr clas)
545 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
547 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
549 sideConditions :: [(Unique, Condition)]
551 = [ (eqClassKey, cond_std),
552 (ordClassKey, cond_std),
553 (readClassKey, cond_std),
554 (showClassKey, cond_std),
555 (enumClassKey, cond_std `andCond` cond_isEnumeration),
556 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
557 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
558 (typeableClassKey, cond_glaExts `andCond` cond_allTypeKind),
559 (dataClassKey, cond_glaExts `andCond` cond_std)
562 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
564 orCond :: Condition -> Condition -> Condition
567 Nothing -> Nothing -- c1 succeeds
568 Just x -> case c2 tc of -- c1 fails
570 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
573 andCond c1 c2 tc = case c1 tc of
574 Nothing -> c2 tc -- c1 succeeds
575 Just x -> Just x -- c1 fails
577 cond_std :: Condition
578 cond_std (gla_exts, tycon)
579 | any isExistentialDataCon data_cons = Just existential_why
580 | null data_cons = Just no_cons_why
581 | otherwise = Nothing
583 data_cons = tyConDataCons tycon
584 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
585 existential_why = quotes (ppr tycon) <+> ptext SLIT("has existentially-quantified constructor(s)")
587 cond_isEnumeration :: Condition
588 cond_isEnumeration (gla_exts, tycon)
589 | isEnumerationTyCon tycon = Nothing
590 | otherwise = Just why
592 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
594 cond_isProduct :: Condition
595 cond_isProduct (gla_exts, tycon)
596 | isProductTyCon tycon = Nothing
597 | otherwise = Just why
599 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
601 cond_allTypeKind :: Condition
602 cond_allTypeKind (gla_exts, tycon)
603 | all (isTypeKind . tyVarKind) (tyConTyVars tycon) = Nothing
604 | otherwise = Just why
606 why = quotes (ppr tycon) <+> ptext SLIT("is parameterised over arguments of kind other than `*'")
608 cond_glaExts :: Condition
609 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
610 | otherwise = Just why
612 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
615 %************************************************************************
617 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
619 %************************************************************************
621 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
622 terms, which is the final correct RHS for the corresponding original
626 Each (k,TyVarTy tv) in a solution constrains only a type
630 The (k,TyVarTy tv) pairs in a solution are canonically
631 ordered by sorting on type varible, tv, (major key) and then class, k,
636 solveDerivEqns :: [DerivEqn]
637 -> TcM [DFunId] -- Solns in same order as eqns.
638 -- This bunch is Absolutely minimal...
640 solveDerivEqns orig_eqns
641 = iterateDeriv 1 initial_solutions
643 -- The initial solutions for the equations claim that each
644 -- instance has an empty context; this solution is certainly
645 -- in canonical form.
646 initial_solutions :: [DerivSoln]
647 initial_solutions = [ [] | _ <- orig_eqns ]
649 ------------------------------------------------------------------
650 -- iterateDeriv calculates the next batch of solutions,
651 -- compares it with the current one; finishes if they are the
652 -- same, otherwise recurses with the new solutions.
653 -- It fails if any iteration fails
654 iterateDeriv :: Int -> [DerivSoln] ->TcM [DFunId]
655 iterateDeriv n current_solns
656 | n > 20 -- Looks as if we are in an infinite loop
657 -- This can happen if we have -fallow-undecidable-instances
658 -- (See TcSimplify.tcSimplifyDeriv.)
659 = pprPanic "solveDerivEqns: probable loop"
660 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
663 dfuns = zipWithEqual "add_solns" mk_deriv_dfun orig_eqns current_solns
666 -- Extend the inst info from the explicit instance decls
667 -- with the current set of solutions, and simplify each RHS
668 extendLocalInstEnv dfuns $
669 mappM gen_soln orig_eqns
670 ) `thenM` \ new_solns ->
671 if (current_solns == new_solns) then
674 iterateDeriv (n+1) new_solns
676 ------------------------------------------------------------------
678 gen_soln (_, clas, tc,tyvars,deriv_rhs)
679 = addSrcSpan (srcLocSpan (getSrcLoc tc)) $
680 addErrCtxt (derivCtxt (Just clas) tc) $
681 tcSimplifyDeriv tyvars deriv_rhs `thenM` \ theta ->
682 returnM (sortLt (<) theta) -- Canonicalise before returning the soluction
684 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
685 = mkDictFunId dfun_name tyvars theta
686 clas [mkTyConApp tycon (mkTyVarTys tyvars)]
688 extendLocalInstEnv :: [DFunId] -> TcM a -> TcM a
689 -- Add new locall-defined instances; don't bother to check
690 -- for functional dependency errors -- that'll happen in TcInstDcls
691 extendLocalInstEnv dfuns thing_inside
692 = do { env <- getGblEnv
693 ; let inst_env' = foldl extendInstEnv (tcg_inst_env env) dfuns
694 env' = env { tcg_inst_env = inst_env' }
695 ; setGblEnv env' thing_inside }
698 %************************************************************************
700 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
702 %************************************************************************
704 After all the trouble to figure out the required context for the
705 derived instance declarations, all that's left is to chug along to
706 produce them. They will then be shoved into @tcInstDecls2@, which
707 will do all its usual business.
709 There are lots of possibilities for code to generate. Here are
710 various general remarks.
715 We want derived instances of @Eq@ and @Ord@ (both v common) to be
716 ``you-couldn't-do-better-by-hand'' efficient.
719 Deriving @Show@---also pretty common--- should also be reasonable good code.
722 Deriving for the other classes isn't that common or that big a deal.
729 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
732 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
735 We {\em normally} generate code only for the non-defaulted methods;
736 there are some exceptions for @Eq@ and (especially) @Ord@...
739 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
740 constructor's numeric (@Int#@) tag. These are generated by
741 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
742 these is around is given by @hasCon2TagFun@.
744 The examples under the different sections below will make this
748 Much less often (really just for deriving @Ix@), we use a
749 @_tag2con_<tycon>@ function. See the examples.
752 We use the renamer!!! Reason: we're supposed to be
753 producing @LHsBinds Name@ for the methods, but that means
754 producing correctly-uniquified code on the fly. This is entirely
755 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
756 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
757 the renamer. What a great hack!
761 -- Generate the InstInfo for the required instance,
762 -- plus any auxiliary bindings required
763 genInst :: DFunId -> TcM (InstInfo, LHsBinds RdrName)
765 = getFixityEnv `thenM` \ fix_env ->
767 (tyvars,_,clas,[ty]) = tcSplitDFunTy (idType dfun)
768 clas_nm = className clas
769 tycon = tcTyConAppTyCon ty
770 (meth_binds, aux_binds) = assoc "gen_bind:bad derived class"
771 gen_list (getUnique clas) fix_env tycon
773 -- Bring the right type variables into
774 -- scope, and rename the method binds
775 bindLocalNames (map varName tyvars) $
776 rnMethodBinds clas_nm [] meth_binds `thenM` \ (rn_meth_binds, _fvs) ->
778 -- Build the InstInfo
779 returnM (InstInfo { iDFunId = dfun, iBinds = VanillaInst rn_meth_binds [] },
782 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
783 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
784 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
785 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
786 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
787 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
788 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
789 ,(showClassKey, no_aux_binds gen_Show_binds)
790 ,(readClassKey, no_aux_binds gen_Read_binds)
791 ,(dataClassKey, gen_Data_binds)
794 -- no_aux_binds is used for generators that don't
795 -- need to produce any auxiliary bindings
796 no_aux_binds f fix_env tc = (f fix_env tc, emptyBag)
797 ignore_fix_env f fix_env tc = f tc
801 %************************************************************************
803 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
805 %************************************************************************
810 con2tag_Foo :: Foo ... -> Int#
811 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
812 maxtag_Foo :: Int -- ditto (NB: not unlifted)
815 We have a @con2tag@ function for a tycon if:
818 We're deriving @Eq@ and the tycon has nullary data constructors.
821 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
825 We have a @tag2con@ function for a tycon if:
828 We're deriving @Enum@, or @Ix@ (enum type only???)
831 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
834 genTaggeryBinds :: [DFunId] -> TcM (LHsBinds RdrName)
835 genTaggeryBinds dfuns
836 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
837 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
838 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
840 all_CTs = map simpleDFunClassTyCon dfuns
841 all_tycons = map snd all_CTs
842 (tycons_of_interest, _) = removeDups compare all_tycons
844 do_con2tag acc_Names tycon
845 | isDataTyCon tycon &&
846 ((we_are_deriving eqClassKey tycon
847 && any isNullaryDataCon (tyConDataCons tycon))
848 || (we_are_deriving ordClassKey tycon
849 && not (isProductTyCon tycon))
850 || (we_are_deriving enumClassKey tycon)
851 || (we_are_deriving ixClassKey tycon))
853 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
858 do_tag2con acc_Names tycon
859 | isDataTyCon tycon &&
860 (we_are_deriving enumClassKey tycon ||
861 we_are_deriving ixClassKey tycon
862 && isEnumerationTyCon tycon)
863 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
864 : (maxtag_RDR tycon, tycon, GenMaxTag)
869 we_are_deriving clas_key tycon
870 = is_in_eqns clas_key tycon all_CTs
872 is_in_eqns clas_key tycon [] = False
873 is_in_eqns clas_key tycon ((c,t):cts)
874 = (clas_key == classKey c && tycon == t)
875 || is_in_eqns clas_key tycon cts
879 derivingThingErr clas tys tycon tyvars why
880 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
883 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
885 malformedPredErr tycon pred = ptext SLIT("Illegal deriving item") <+> ppr pred
887 derivCtxt :: Maybe Class -> TyCon -> SDoc
888 derivCtxt maybe_cls tycon
889 = ptext SLIT("When deriving") <+> cls <+> ptext SLIT("for type") <+> quotes (ppr tycon)
891 cls = case maybe_cls of
892 Nothing -> ptext SLIT("instances")
893 Just c -> ptext SLIT("the") <+> quotes (ppr c) <+> ptext SLIT("instance")