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 Class ( className, classArity, classKey, classTyVars, classSCTheta, Class )
33 import Subst ( mkTyVarSubst, substTheta )
34 import ErrUtils ( dumpIfSet_dyn )
35 import MkId ( mkDictFunId )
36 import DataCon ( dataConOrigArgTys, isNullaryDataCon, isExistentialDataCon )
37 import Maybes ( catMaybes )
38 import RdrName ( RdrName )
39 import Name ( Name, getSrcLoc )
40 import NameSet ( NameSet, emptyNameSet, duDefs )
41 import Unique ( Unique, getUnique )
42 import Kind ( splitKindFunTys )
43 import TyCon ( tyConTyVars, tyConDataCons, tyConArity, tyConHasGenerics,
44 tyConTheta, isProductTyCon, isDataTyCon,
45 isEnumerationTyCon, isRecursiveTyCon, TyCon
47 import TcType ( TcType, ThetaType, mkTyVarTy, mkTyVarTys, mkTyConApp,
48 getClassPredTys_maybe, tcTyConAppTyCon,
49 isUnLiftedType, mkClassPred, tyVarsOfTypes, isArgTypeKind,
50 tcEqTypes, tcSplitAppTys, mkAppTys, tcSplitDFunTy )
51 import Var ( TyVar, tyVarKind, idType, varName )
52 import VarSet ( mkVarSet, subVarSet )
54 import SrcLoc ( srcLocSpan, Located(..) )
55 import Util ( zipWithEqual, sortLt, notNull )
56 import ListSetOps ( removeDups, assoc )
61 %************************************************************************
63 \subsection[TcDeriv-intro]{Introduction to how we do deriving}
65 %************************************************************************
69 data T a b = C1 (Foo a) (Bar b)
74 [NOTE: See end of these comments for what to do with
75 data (C a, D b) => T a b = ...
78 We want to come up with an instance declaration of the form
80 instance (Ping a, Pong b, ...) => Eq (T a b) where
83 It is pretty easy, albeit tedious, to fill in the code "...". The
84 trick is to figure out what the context for the instance decl is,
85 namely @Ping@, @Pong@ and friends.
87 Let's call the context reqd for the T instance of class C at types
88 (a,b, ...) C (T a b). Thus:
90 Eq (T a b) = (Ping a, Pong b, ...)
92 Now we can get a (recursive) equation from the @data@ decl:
94 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
95 u Eq (T b a) u Eq Int -- From C2
96 u Eq (T a a) -- From C3
98 Foo and Bar may have explicit instances for @Eq@, in which case we can
99 just substitute for them. Alternatively, either or both may have
100 their @Eq@ instances given by @deriving@ clauses, in which case they
101 form part of the system of equations.
103 Now all we need do is simplify and solve the equations, iterating to
104 find the least fixpoint. Notice that the order of the arguments can
105 switch around, as here in the recursive calls to T.
107 Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
111 Eq (T a b) = {} -- The empty set
114 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
115 u Eq (T b a) u Eq Int -- From C2
116 u Eq (T a a) -- From C3
118 After simplification:
119 = Eq a u Ping b u {} u {} u {}
124 Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
125 u Eq (T b a) u Eq Int -- From C2
126 u Eq (T a a) -- From C3
128 After simplification:
133 = Eq a u Ping b u Eq b u Ping a
135 The next iteration gives the same result, so this is the fixpoint. We
136 need to make a canonical form of the RHS to ensure convergence. We do
137 this by simplifying the RHS to a form in which
139 - the classes constrain only tyvars
140 - the list is sorted by tyvar (major key) and then class (minor key)
141 - no duplicates, of course
143 So, here are the synonyms for the ``equation'' structures:
146 type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
147 -- The Name is the name for the DFun we'll build
148 -- The tyvars bind all the variables in the RHS
150 pprDerivEqn (n,c,tc,tvs,rhs)
151 = parens (hsep [ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
153 type DerivRhs = ThetaType
154 type DerivSoln = DerivRhs
158 [Data decl contexts] A note about contexts on data decls
159 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
162 data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
164 We will need an instance decl like:
166 instance (Read a, RealFloat a) => Read (Complex a) where
169 The RealFloat in the context is because the read method for Complex is bound
170 to construct a Complex, and doing that requires that the argument type is
173 But this ain't true for Show, Eq, Ord, etc, since they don't construct
174 a Complex; they only take them apart.
176 Our approach: identify the offending classes, and add the data type
177 context to the instance decl. The "offending classes" are
181 FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
182 pattern matching against a constructor from a data type with a context
183 gives rise to the constraints for that context -- or at least the thinned
184 version. So now all classes are "offending".
188 %************************************************************************
190 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
192 %************************************************************************
195 tcDeriving :: [LTyClDecl Name] -- All type constructors
196 -> TcM ([InstInfo], -- The generated "instance decls"
197 [HsBindGroup Name], -- Extra generated top-level bindings
198 NameSet) -- Binders to keep alive
200 tcDeriving tycl_decls
201 = recoverM (returnM ([], [], emptyNameSet)) $
202 do { -- Fish the "deriving"-related information out of the TcEnv
203 -- and make the necessary "equations".
204 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns tycl_decls
206 ; (ordinary_inst_info, deriv_binds)
207 <- extendLocalInstEnv (map iDFunId newtype_inst_info) $
208 deriveOrdinaryStuff ordinary_eqns
209 -- Add the newtype-derived instances to the inst env
210 -- before tacking the "ordinary" ones
212 -- Generate the generic to/from functions from each type declaration
213 ; gen_binds <- mkGenericBinds tycl_decls
214 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
216 -- Rename these extra bindings, discarding warnings about unused bindings etc
217 -- Set -fglasgow exts so that we can have type signatures in patterns,
218 -- which is used in the generic binds
219 ; (rn_binds, gen_bndrs)
220 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
221 { (rn_deriv, _dus1) <- rnTopBinds deriv_binds []
222 ; (rn_gen, dus_gen) <- rnTopBinds gen_binds []
223 ; return (rn_deriv ++ rn_gen, duDefs dus_gen) }
227 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
228 (ddump_deriving inst_info rn_binds))
230 ; returnM (inst_info, rn_binds, gen_bndrs)
233 ddump_deriving :: [InstInfo] -> [HsBindGroup Name] -> SDoc
234 ddump_deriving inst_infos extra_binds
235 = vcat (map pprInstInfoDetails inst_infos) $$ vcat (map ppr extra_binds)
237 -----------------------------------------
238 deriveOrdinaryStuff [] -- Short cut
239 = returnM ([], emptyBag)
241 deriveOrdinaryStuff eqns
242 = do { -- Take the equation list and solve it, to deliver a list of
243 -- solutions, a.k.a. the contexts for the instance decls
244 -- required for the corresponding equations.
245 ; new_dfuns <- solveDerivEqns eqns
247 -- Generate the InstInfo for each dfun,
248 -- plus any auxiliary bindings it needs
249 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst new_dfuns
251 -- Generate any extra not-one-inst-decl-specific binds,
252 -- notably "con2tag" and/or "tag2con" functions.
253 ; extra_binds <- genTaggeryBinds new_dfuns
256 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
259 -----------------------------------------
260 mkGenericBinds tycl_decls
261 = do { tcs <- mapM tcLookupTyCon
263 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
264 -- We are only interested in the data type declarations
265 ; return (unionManyBags [ mkTyConGenericBinds tc |
266 tc <- tcs, tyConHasGenerics tc ]) }
267 -- And then only in the ones whose 'has-generics' flag is on
271 %************************************************************************
273 \subsection[TcDeriv-eqns]{Forming the equations}
275 %************************************************************************
277 @makeDerivEqns@ fishes around to find the info about needed derived
278 instances. Complicating factors:
281 We can only derive @Enum@ if the data type is an enumeration
282 type (all nullary data constructors).
285 We can only derive @Ix@ if the data type is an enumeration {\em
286 or} has just one data constructor (e.g., tuples).
289 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
293 makeDerivEqns :: [LTyClDecl Name]
294 -> TcM ([DerivEqn], -- Ordinary derivings
295 [InstInfo]) -- Special newtype derivings
297 makeDerivEqns tycl_decls
298 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
299 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
301 ------------------------------------------------------------------
302 derive_these :: [(NewOrData, Name, LHsPred Name)]
303 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
304 -- NB: only source-language decls have deriving, no imported ones do
305 derive_these = [ (nd, tycon, pred)
306 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
307 tcdDerivs = Just (L _ preds) }) <- tycl_decls,
310 ------------------------------------------------------------------
311 mk_eqn :: (NewOrData, Name, LHsPred Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
312 -- We swizzle the tyvars and datacons out of the tycon
313 -- to make the rest of the equation
315 mk_eqn (new_or_data, tycon_name, pred)
316 = tcLookupTyCon tycon_name `thenM` \ tycon ->
317 addSrcSpan (srcLocSpan (getSrcLoc tycon)) $
318 addErrCtxt (derivCtxt Nothing tycon) $
319 tcExtendTyVarEnv (tyConTyVars tycon) $ -- Deriving preds may (now) mention
320 -- the type variables for the type constructor
321 tcHsPred pred `thenM` \ pred' ->
322 case getClassPredTys_maybe pred' of
323 Nothing -> bale_out (malformedPredErr tycon pred)
324 Just (clas, tys) -> doptM Opt_GlasgowExts `thenM` \ gla_exts ->
325 mk_eqn_help gla_exts new_or_data tycon clas tys
327 ------------------------------------------------------------------
328 mk_eqn_help gla_exts DataType tycon clas tys
329 | Just err <- checkSideConditions gla_exts clas tycon tys
330 = bale_out (derivingThingErr clas tys tycon tyvars err)
332 = new_dfun_name clas tycon `thenM` \ dfun_name ->
333 returnM (Just (dfun_name, clas, tycon, tyvars, constraints), Nothing)
335 tyvars = tyConTyVars tycon
336 constraints = extra_constraints ++ ordinary_constraints
337 -- "extra_constraints": see note [Data decl contexts] above
338 extra_constraints = tyConTheta tycon
341 | clas `hasKey` typeableClassKey -- For the Typeable class, the constraints
342 -- don't involve the constructor ags, only
344 -- e.g. data T a b = ...
346 -- instance (Typeable a, Typable b)
347 -- => Typeable (T a b) where
348 = [mkClassPred clas [mkTyVarTy tv] | tv <- tyvars]
350 = [ mkClassPred clas [arg_ty]
351 | data_con <- tyConDataCons tycon,
352 arg_ty <- dataConOrigArgTys data_con,
353 -- Use the same type variables
354 -- as the type constructor,
355 -- hence no need to instantiate
356 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
359 mk_eqn_help gla_exts NewType tycon clas tys
360 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
361 = -- Go ahead and use the isomorphism
362 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
363 new_dfun_name clas tycon `thenM` \ dfun_name ->
364 returnM (Nothing, Just (InstInfo { iDFunId = mk_dfun dfun_name,
365 iBinds = NewTypeDerived rep_tys }))
366 | std_class gla_exts clas
367 = mk_eqn_help gla_exts DataType tycon clas tys -- Go via bale-out route
369 | otherwise -- Non-standard instance
370 = bale_out (if gla_exts then
371 cant_derive_err -- Too hard
373 non_std_err) -- Just complain about being a non-std instance
375 -- Here is the plan for newtype derivings. We see
376 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
377 -- where aj...an do not occur free in t, and the (C s1 ... sm) is a
378 -- *partial applications* of class C with the last parameter missing
380 -- We generate the instances
381 -- instance C s1 .. sm (t ak...aj) => C s1 .. sm (T a1...aj)
382 -- where T a1...aj is the partial application of the LHS of the correct kind
384 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
385 -- instance Monad (ST s) => Monad (T s) where
386 -- fail = coerce ... (fail @ ST s)
388 clas_tyvars = classTyVars clas
389 kind = tyVarKind (last clas_tyvars)
390 -- Kind of the thing we want to instance
391 -- e.g. argument kind of Monad, *->*
393 (arg_kinds, _) = splitKindFunTys kind
394 n_args_to_drop = length arg_kinds
395 -- Want to drop 1 arg from (T s a) and (ST s a)
396 -- to get instance Monad (ST s) => Monad (T s)
398 -- Note [newtype representation]
399 -- We must not use newTyConRep to get the representation
400 -- type, because that looks through all intermediate newtypes
401 -- To get the RHS of *this* newtype, just look at the data
402 -- constructor. For example
403 -- newtype B = MkB Int
404 -- newtype A = MkA B deriving( Num )
405 -- We want the Num instance of B, *not* the Num instance of Int,
406 -- when making the Num instance of A!
407 tyvars = tyConTyVars tycon
408 rep_ty = head (dataConOrigArgTys (head (tyConDataCons tycon)))
409 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
411 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
412 tyvars_to_drop = drop n_tyvars_to_keep tyvars
413 tyvars_to_keep = take n_tyvars_to_keep tyvars
415 n_args_to_keep = length rep_ty_args - n_args_to_drop
416 args_to_drop = drop n_args_to_keep rep_ty_args
417 args_to_keep = take n_args_to_keep rep_ty_args
419 rep_tys = tys ++ [mkAppTys rep_fn args_to_keep]
420 rep_pred = mkClassPred clas rep_tys
421 -- rep_pred is the representation dictionary, from where
422 -- we are gong to get all the methods for the newtype dictionary
424 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
425 -- The 'tys' here come from the partial application
426 -- in the deriving clause. The last arg is the new
429 -- We must pass the superclasses; the newtype might be an instance
430 -- of them in a different way than the representation type
431 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
432 -- Then the Show instance is not done via isomprphism; it shows
434 -- The Num instance is derived via isomorphism, but the Show superclass
435 -- dictionary must the Show instance for Foo, *not* the Show dictionary
436 -- gotten from the Num dictionary. So we must build a whole new dictionary
437 -- not just use the Num one. The instance we want is something like:
438 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
441 -- There's no 'corece' needed because after the type checker newtypes
444 sc_theta = substTheta (mkTyVarSubst clas_tyvars inst_tys)
447 -- If there are no tyvars, there's no need
448 -- to abstract over the dictionaries we need
449 dict_args | null tyvars = []
450 | otherwise = rep_pred : sc_theta
452 -- Finally! Here's where we build the dictionary Id
453 mk_dfun dfun_name = mkDictFunId dfun_name tyvars dict_args clas inst_tys
455 -------------------------------------------------------------------
456 -- Figuring out whether we can only do this newtype-deriving thing
458 right_arity = length tys + 1 == classArity clas
460 -- Never derive Read,Show,Typeable,Data this way
461 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
462 can_derive_via_isomorphism
463 = not (getUnique clas `elem` non_iso_classes)
464 && right_arity -- Well kinded;
465 -- eg not: newtype T ... deriving( ST )
466 -- because ST needs *2* type params
467 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
468 -- eg not: newtype T = T Int deriving( Monad )
469 && n_args_to_keep >= 0 -- Rep type has right kind:
470 -- eg not: newtype T a = T Int deriving( Monad )
471 && eta_ok -- Eta reduction works
472 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
473 -- newtype A = MkA [A]
475 -- instance Eq [A] => Eq A !!
477 -- Here's a recursive newtype that's actually OK
478 -- newtype S1 = S1 [T1 ()]
479 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
480 -- It's currently rejected. Oh well.
482 -- Check that eta reduction is OK
483 -- (a) the dropped-off args are identical
484 -- (b) the remaining type args mention
485 -- only the remaining type variables
486 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
487 && (tyVarsOfTypes args_to_keep `subVarSet` mkVarSet tyvars_to_keep)
489 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
490 (vcat [ptext SLIT("even with cunning newtype deriving:"),
491 if isRecursiveTyCon tycon then
492 ptext SLIT("the newtype is recursive")
494 if not right_arity then
495 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
497 if not (n_tyvars_to_keep >= 0) then
498 ptext SLIT("the type constructor has wrong kind")
499 else if not (n_args_to_keep >= 0) then
500 ptext SLIT("the representation type has wrong kind")
501 else if not eta_ok then
502 ptext SLIT("the eta-reduction property does not hold")
506 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
507 (vcat [non_std_why clas,
508 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
510 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
512 std_class gla_exts clas
513 = key `elem` derivableClassKeys
514 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
518 std_class_via_iso clas -- These standard classes can be derived for a newtype
519 -- using the isomorphism trick *even if no -fglasgow-exts*
520 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
521 -- Not Read/Show because they respect the type
522 -- Not Enum, becuase newtypes are never in Enum
525 new_dfun_name clas tycon -- Just a simple wrapper
526 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
527 -- The type passed to newDFunName is only used to generate
528 -- a suitable string; hence the empty type arg list
530 ------------------------------------------------------------------
531 -- Check side conditions that dis-allow derivability for particular classes
532 -- This is *apart* from the newtype-deriving mechanism
534 checkSideConditions :: Bool -> Class -> TyCon -> [TcType] -> Maybe SDoc
535 checkSideConditions gla_exts clas tycon tys
537 = Just ty_args_why -- e.g. deriving( Foo s )
539 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
540 [] -> Just (non_std_why clas)
541 [cond] -> cond (gla_exts, tycon)
542 other -> pprPanic "checkSideConditions" (ppr clas)
544 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
546 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
548 sideConditions :: [(Unique, Condition)]
550 = [ (eqClassKey, cond_std),
551 (ordClassKey, cond_std),
552 (readClassKey, cond_std),
553 (showClassKey, cond_std),
554 (enumClassKey, cond_std `andCond` cond_isEnumeration),
555 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
556 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
557 (typeableClassKey, cond_glaExts `andCond` cond_allTypeKind),
558 (dataClassKey, cond_glaExts `andCond` cond_std)
561 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
563 orCond :: Condition -> Condition -> Condition
566 Nothing -> Nothing -- c1 succeeds
567 Just x -> case c2 tc of -- c1 fails
569 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
572 andCond c1 c2 tc = case c1 tc of
573 Nothing -> c2 tc -- c1 succeeds
574 Just x -> Just x -- c1 fails
576 cond_std :: Condition
577 cond_std (gla_exts, tycon)
578 | any isExistentialDataCon data_cons = Just existential_why
579 | null data_cons = Just no_cons_why
580 | otherwise = Nothing
582 data_cons = tyConDataCons tycon
583 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
584 existential_why = quotes (ppr tycon) <+> ptext SLIT("has existentially-quantified constructor(s)")
586 cond_isEnumeration :: Condition
587 cond_isEnumeration (gla_exts, tycon)
588 | isEnumerationTyCon tycon = Nothing
589 | otherwise = Just why
591 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
593 cond_isProduct :: Condition
594 cond_isProduct (gla_exts, tycon)
595 | isProductTyCon tycon = Nothing
596 | otherwise = Just why
598 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
600 cond_allTypeKind :: Condition
601 cond_allTypeKind (gla_exts, tycon)
602 | all (isArgTypeKind . tyVarKind) (tyConTyVars tycon) = Nothing
603 | otherwise = Just why
605 why = quotes (ppr tycon) <+> ptext SLIT("is parameterised over arguments of kind other than `*'")
607 cond_glaExts :: Condition
608 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
609 | otherwise = Just why
611 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
614 %************************************************************************
616 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
618 %************************************************************************
620 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
621 terms, which is the final correct RHS for the corresponding original
625 Each (k,TyVarTy tv) in a solution constrains only a type
629 The (k,TyVarTy tv) pairs in a solution are canonically
630 ordered by sorting on type varible, tv, (major key) and then class, k,
635 solveDerivEqns :: [DerivEqn]
636 -> TcM [DFunId] -- Solns in same order as eqns.
637 -- This bunch is Absolutely minimal...
639 solveDerivEqns orig_eqns
640 = iterateDeriv 1 initial_solutions
642 -- The initial solutions for the equations claim that each
643 -- instance has an empty context; this solution is certainly
644 -- in canonical form.
645 initial_solutions :: [DerivSoln]
646 initial_solutions = [ [] | _ <- orig_eqns ]
648 ------------------------------------------------------------------
649 -- iterateDeriv calculates the next batch of solutions,
650 -- compares it with the current one; finishes if they are the
651 -- same, otherwise recurses with the new solutions.
652 -- It fails if any iteration fails
653 iterateDeriv :: Int -> [DerivSoln] ->TcM [DFunId]
654 iterateDeriv n current_solns
655 | n > 20 -- Looks as if we are in an infinite loop
656 -- This can happen if we have -fallow-undecidable-instances
657 -- (See TcSimplify.tcSimplifyDeriv.)
658 = pprPanic "solveDerivEqns: probable loop"
659 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
662 dfuns = zipWithEqual "add_solns" mk_deriv_dfun orig_eqns current_solns
665 -- Extend the inst info from the explicit instance decls
666 -- with the current set of solutions, and simplify each RHS
667 extendLocalInstEnv dfuns $
668 mappM gen_soln orig_eqns
669 ) `thenM` \ new_solns ->
670 if (current_solns == new_solns) then
673 iterateDeriv (n+1) new_solns
675 ------------------------------------------------------------------
677 gen_soln (_, clas, tc,tyvars,deriv_rhs)
678 = addSrcSpan (srcLocSpan (getSrcLoc tc)) $
679 addErrCtxt (derivCtxt (Just clas) tc) $
680 tcSimplifyDeriv tyvars deriv_rhs `thenM` \ theta ->
681 returnM (sortLt (<) theta) -- Canonicalise before returning the soluction
683 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
684 = mkDictFunId dfun_name tyvars theta
685 clas [mkTyConApp tycon (mkTyVarTys tyvars)]
687 extendLocalInstEnv :: [DFunId] -> TcM a -> TcM a
688 -- Add new locall-defined instances; don't bother to check
689 -- for functional dependency errors -- that'll happen in TcInstDcls
690 extendLocalInstEnv dfuns thing_inside
691 = do { env <- getGblEnv
692 ; let inst_env' = foldl extendInstEnv (tcg_inst_env env) dfuns
693 env' = env { tcg_inst_env = inst_env' }
694 ; setGblEnv env' thing_inside }
697 %************************************************************************
699 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
701 %************************************************************************
703 After all the trouble to figure out the required context for the
704 derived instance declarations, all that's left is to chug along to
705 produce them. They will then be shoved into @tcInstDecls2@, which
706 will do all its usual business.
708 There are lots of possibilities for code to generate. Here are
709 various general remarks.
714 We want derived instances of @Eq@ and @Ord@ (both v common) to be
715 ``you-couldn't-do-better-by-hand'' efficient.
718 Deriving @Show@---also pretty common--- should also be reasonable good code.
721 Deriving for the other classes isn't that common or that big a deal.
728 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
731 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
734 We {\em normally} generate code only for the non-defaulted methods;
735 there are some exceptions for @Eq@ and (especially) @Ord@...
738 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
739 constructor's numeric (@Int#@) tag. These are generated by
740 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
741 these is around is given by @hasCon2TagFun@.
743 The examples under the different sections below will make this
747 Much less often (really just for deriving @Ix@), we use a
748 @_tag2con_<tycon>@ function. See the examples.
751 We use the renamer!!! Reason: we're supposed to be
752 producing @LHsBinds Name@ for the methods, but that means
753 producing correctly-uniquified code on the fly. This is entirely
754 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
755 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
756 the renamer. What a great hack!
760 -- Generate the InstInfo for the required instance,
761 -- plus any auxiliary bindings required
762 genInst :: DFunId -> TcM (InstInfo, LHsBinds RdrName)
764 = getFixityEnv `thenM` \ fix_env ->
766 (tyvars,_,clas,[ty]) = tcSplitDFunTy (idType dfun)
767 clas_nm = className clas
768 tycon = tcTyConAppTyCon ty
769 (meth_binds, aux_binds) = assoc "gen_bind:bad derived class"
770 gen_list (getUnique clas) fix_env tycon
772 -- Bring the right type variables into
773 -- scope, and rename the method binds
774 bindLocalNames (map varName tyvars) $
775 rnMethodBinds clas_nm [] meth_binds `thenM` \ (rn_meth_binds, _fvs) ->
777 -- Build the InstInfo
778 returnM (InstInfo { iDFunId = dfun, iBinds = VanillaInst rn_meth_binds [] },
781 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
782 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
783 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
784 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
785 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
786 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
787 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
788 ,(showClassKey, no_aux_binds gen_Show_binds)
789 ,(readClassKey, no_aux_binds gen_Read_binds)
790 ,(dataClassKey, gen_Data_binds)
793 -- no_aux_binds is used for generators that don't
794 -- need to produce any auxiliary bindings
795 no_aux_binds f fix_env tc = (f fix_env tc, emptyBag)
796 ignore_fix_env f fix_env tc = f tc
800 %************************************************************************
802 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
804 %************************************************************************
809 con2tag_Foo :: Foo ... -> Int#
810 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
811 maxtag_Foo :: Int -- ditto (NB: not unlifted)
814 We have a @con2tag@ function for a tycon if:
817 We're deriving @Eq@ and the tycon has nullary data constructors.
820 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
824 We have a @tag2con@ function for a tycon if:
827 We're deriving @Enum@, or @Ix@ (enum type only???)
830 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
833 genTaggeryBinds :: [DFunId] -> TcM (LHsBinds RdrName)
834 genTaggeryBinds dfuns
835 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
836 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
837 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
839 all_CTs = map simpleDFunClassTyCon dfuns
840 all_tycons = map snd all_CTs
841 (tycons_of_interest, _) = removeDups compare all_tycons
843 do_con2tag acc_Names tycon
844 | isDataTyCon tycon &&
845 ((we_are_deriving eqClassKey tycon
846 && any isNullaryDataCon (tyConDataCons tycon))
847 || (we_are_deriving ordClassKey tycon
848 && not (isProductTyCon tycon))
849 || (we_are_deriving enumClassKey tycon)
850 || (we_are_deriving ixClassKey tycon))
852 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
857 do_tag2con acc_Names tycon
858 | isDataTyCon tycon &&
859 (we_are_deriving enumClassKey tycon ||
860 we_are_deriving ixClassKey tycon
861 && isEnumerationTyCon tycon)
862 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
863 : (maxtag_RDR tycon, tycon, GenMaxTag)
868 we_are_deriving clas_key tycon
869 = is_in_eqns clas_key tycon all_CTs
871 is_in_eqns clas_key tycon [] = False
872 is_in_eqns clas_key tycon ((c,t):cts)
873 = (clas_key == classKey c && tycon == t)
874 || is_in_eqns clas_key tycon cts
878 derivingThingErr clas tys tycon tyvars why
879 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
882 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
884 malformedPredErr tycon pred = ptext SLIT("Illegal deriving item") <+> ppr pred
886 derivCtxt :: Maybe Class -> TyCon -> SDoc
887 derivCtxt maybe_cls tycon
888 = ptext SLIT("When deriving") <+> cls <+> ptext SLIT("for type") <+> quotes (ppr tycon)
890 cls = case maybe_cls of
891 Nothing -> ptext SLIT("instances")
892 Just c -> ptext SLIT("the") <+> quotes (ppr c) <+> ptext SLIT("instance")