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 ( 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 ( isNullaryDataCon, isExistentialDataCon, dataConOrigArgTys )
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, newTyConRhs,
45 isEnumerationTyCon, isRecursiveTyCon, TyCon
47 import TcType ( TcType, ThetaType, 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, assocMaybe )
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".
191 newtype T = T Char deriving( C [a] )
193 Notice the free 'a' in the deriving. We have to fill this out to
194 newtype T = T Char deriving( forall a. C [a] )
196 And then translate it to:
197 instance C [a] Char => C [a] T where ...
202 %************************************************************************
204 \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
206 %************************************************************************
209 tcDeriving :: [LTyClDecl Name] -- All type constructors
210 -> TcM ([InstInfo], -- The generated "instance decls"
211 [HsBindGroup Name], -- Extra generated top-level bindings
212 NameSet) -- Binders to keep alive
214 tcDeriving tycl_decls
215 = recoverM (returnM ([], [], emptyNameSet)) $
216 do { -- Fish the "deriving"-related information out of the TcEnv
217 -- and make the necessary "equations".
218 ; (ordinary_eqns, newtype_inst_info) <- makeDerivEqns tycl_decls
220 ; (ordinary_inst_info, deriv_binds)
221 <- extendLocalInstEnv (map iDFunId newtype_inst_info) $
222 deriveOrdinaryStuff ordinary_eqns
223 -- Add the newtype-derived instances to the inst env
224 -- before tacking the "ordinary" ones
226 -- Generate the generic to/from functions from each type declaration
227 ; gen_binds <- mkGenericBinds tycl_decls
228 ; let inst_info = newtype_inst_info ++ ordinary_inst_info
230 -- Rename these extra bindings, discarding warnings about unused bindings etc
231 -- Set -fglasgow exts so that we can have type signatures in patterns,
232 -- which is used in the generic binds
233 ; (rn_binds, gen_bndrs)
234 <- discardWarnings $ setOptM Opt_GlasgowExts $ do
235 { (rn_deriv, _dus1) <- rnTopBinds deriv_binds []
236 ; (rn_gen, dus_gen) <- rnTopBinds gen_binds []
237 ; return (rn_deriv ++ rn_gen, duDefs dus_gen) }
241 ; ioToTcRn (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
242 (ddump_deriving inst_info rn_binds))
244 ; returnM (inst_info, rn_binds, gen_bndrs)
247 ddump_deriving :: [InstInfo] -> [HsBindGroup Name] -> SDoc
248 ddump_deriving inst_infos extra_binds
249 = vcat (map pprInstInfoDetails inst_infos) $$ vcat (map ppr extra_binds)
251 -----------------------------------------
252 deriveOrdinaryStuff [] -- Short cut
253 = returnM ([], emptyBag)
255 deriveOrdinaryStuff eqns
256 = do { -- Take the equation list and solve it, to deliver a list of
257 -- solutions, a.k.a. the contexts for the instance decls
258 -- required for the corresponding equations.
259 ; new_dfuns <- solveDerivEqns eqns
261 -- Generate the InstInfo for each dfun,
262 -- plus any auxiliary bindings it needs
263 ; (inst_infos, aux_binds_s) <- mapAndUnzipM genInst new_dfuns
265 -- Generate any extra not-one-inst-decl-specific binds,
266 -- notably "con2tag" and/or "tag2con" functions.
267 ; extra_binds <- genTaggeryBinds new_dfuns
270 ; returnM (inst_infos, unionManyBags (extra_binds : aux_binds_s))
273 -----------------------------------------
274 mkGenericBinds tycl_decls
275 = do { tcs <- mapM tcLookupTyCon
277 L _ (TyData { tcdLName = L _ tc_name }) <- tycl_decls]
278 -- We are only interested in the data type declarations
279 ; return (unionManyBags [ mkTyConGenericBinds tc |
280 tc <- tcs, tyConHasGenerics tc ]) }
281 -- And then only in the ones whose 'has-generics' flag is on
285 %************************************************************************
287 \subsection[TcDeriv-eqns]{Forming the equations}
289 %************************************************************************
291 @makeDerivEqns@ fishes around to find the info about needed derived
292 instances. Complicating factors:
295 We can only derive @Enum@ if the data type is an enumeration
296 type (all nullary data constructors).
299 We can only derive @Ix@ if the data type is an enumeration {\em
300 or} has just one data constructor (e.g., tuples).
303 [See Appendix~E in the Haskell~1.2 report.] This code here deals w/
307 makeDerivEqns :: [LTyClDecl Name]
308 -> TcM ([DerivEqn], -- Ordinary derivings
309 [InstInfo]) -- Special newtype derivings
311 makeDerivEqns tycl_decls
312 = mapAndUnzipM mk_eqn derive_these `thenM` \ (maybe_ordinaries, maybe_newtypes) ->
313 returnM (catMaybes maybe_ordinaries, catMaybes maybe_newtypes)
315 ------------------------------------------------------------------
316 derive_these :: [(NewOrData, Name, LHsPred Name)]
317 -- Find the (nd, TyCon, Pred) pairs that must be `derived'
318 derive_these = [ (nd, tycon, pred)
319 | L _ (TyData { tcdND = nd, tcdLName = L _ tycon,
320 tcdDerivs = Just (L _ preds) }) <- tycl_decls,
323 ------------------------------------------------------------------
324 mk_eqn :: (NewOrData, Name, LHsPred Name) -> TcM (Maybe DerivEqn, Maybe InstInfo)
325 -- We swizzle the tyvars and datacons out of the tycon
326 -- to make the rest of the equation
328 mk_eqn (new_or_data, tycon_name, pred)
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 tcHsPred pred `thenM` \ pred' ->
335 case getClassPredTys_maybe pred' of
336 Nothing -> bale_out (malformedPredErr tycon pred)
337 Just (clas, tys) -> doptM Opt_GlasgowExts `thenM` \ gla_exts ->
338 mk_eqn_help gla_exts new_or_data tycon clas tys
340 ------------------------------------------------------------------
341 mk_eqn_help gla_exts DataType tycon clas tys
342 | Just err <- checkSideConditions gla_exts clas tycon tys
343 = bale_out (derivingThingErr clas tys tycon (tyConTyVars tycon) err)
345 = do { eqn <- mkDataTypeEqn tycon clas
346 ; returnM (Just eqn, Nothing) }
348 mk_eqn_help gla_exts NewType tycon clas tys
349 | can_derive_via_isomorphism && (gla_exts || std_class_via_iso clas)
350 = -- Go ahead and use the isomorphism
351 traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) `thenM_`
352 new_dfun_name clas tycon `thenM` \ dfun_name ->
353 returnM (Nothing, Just (InstInfo { iDFunId = mk_dfun dfun_name,
354 iBinds = NewTypeDerived rep_tys }))
355 | std_class gla_exts clas
356 = mk_eqn_help gla_exts DataType tycon clas tys -- Go via bale-out route
358 | otherwise -- Non-standard instance
359 = bale_out (if gla_exts then
360 cant_derive_err -- Too hard
362 non_std_err) -- Just complain about being a non-std instance
364 -- Here is the plan for newtype derivings. We see
365 -- newtype T a1...an = T (t ak...an) deriving (.., C s1 .. sm, ...)
366 -- where t is a type,
367 -- ak...an is a suffix of a1..an
368 -- ak...an do not occur free in t,
369 -- (C s1 ... sm) is a *partial applications* of class C
370 -- with the last parameter missing
372 -- We generate the instances
373 -- instance C s1 .. sm (t ak...ap) => C s1 .. sm (T a1...ap)
374 -- where T a1...ap is the partial application of the LHS of the correct kind
377 -- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
378 -- instance Monad (ST s) => Monad (T s) where
379 -- fail = coerce ... (fail @ ST s)
380 -- (Actually we don't need the coerce, because non-rec newtypes are transparent
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, _) = splitKindFunTys 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, rep_ty) = newTyConRhs tycon
402 (rep_fn, rep_ty_args) = tcSplitAppTys rep_ty
404 n_tyvars_to_keep = tyConArity tycon - n_args_to_drop
405 tyvars_to_drop = drop n_tyvars_to_keep tyvars
406 tyvars_to_keep = take n_tyvars_to_keep tyvars
408 n_args_to_keep = length rep_ty_args - n_args_to_drop
409 args_to_drop = drop n_args_to_keep rep_ty_args
410 args_to_keep = take n_args_to_keep rep_ty_args
412 rep_tys = tys ++ [mkAppTys rep_fn args_to_keep]
413 rep_pred = mkClassPred clas rep_tys
414 -- rep_pred is the representation dictionary, from where
415 -- we are gong to get all the methods for the newtype dictionary
417 inst_tys = (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars_to_keep)])
418 -- The 'tys' here come from the partial application
419 -- in the deriving clause. The last arg is the new
422 -- We must pass the superclasses; the newtype might be an instance
423 -- of them in a different way than the representation type
424 -- E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
425 -- Then the Show instance is not done via isomprphism; it shows
427 -- The Num instance is derived via isomorphism, but the Show superclass
428 -- dictionary must the Show instance for Foo, *not* the Show dictionary
429 -- gotten from the Num dictionary. So we must build a whole new dictionary
430 -- not just use the Num one. The instance we want is something like:
431 -- instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
434 -- There's no 'corece' needed because after the type checker newtypes
437 sc_theta = substTheta (mkTyVarSubst clas_tyvars inst_tys)
440 -- If there are no tyvars, there's no need
441 -- to abstract over the dictionaries we need
442 dict_args | null tyvars = []
443 | otherwise = rep_pred : sc_theta
445 -- Finally! Here's where we build the dictionary Id
446 mk_dfun dfun_name = mkDictFunId dfun_name tyvars dict_args clas inst_tys
448 -------------------------------------------------------------------
449 -- Figuring out whether we can only do this newtype-deriving thing
451 right_arity = length tys + 1 == classArity clas
453 -- Never derive Read,Show,Typeable,Data this way
454 non_iso_classes = [readClassKey, showClassKey, typeableClassKey, dataClassKey]
455 can_derive_via_isomorphism
456 = not (getUnique clas `elem` non_iso_classes)
457 && right_arity -- Well kinded;
458 -- eg not: newtype T ... deriving( ST )
459 -- because ST needs *2* type params
460 && n_tyvars_to_keep >= 0 -- Type constructor has right kind:
461 -- eg not: newtype T = T Int deriving( Monad )
462 && n_args_to_keep >= 0 -- Rep type has right kind:
463 -- eg not: newtype T a = T Int deriving( Monad )
464 && eta_ok -- Eta reduction works
465 && not (isRecursiveTyCon tycon) -- Does not work for recursive tycons:
466 -- newtype A = MkA [A]
468 -- instance Eq [A] => Eq A !!
469 -- Here's a recursive newtype that's actually OK
470 -- newtype S1 = S1 [T1 ()]
471 -- newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
472 -- It's currently rejected. Oh well.
473 -- In fact we generate an instance decl that has method of form
474 -- meth @ instTy = meth @ repTy
475 -- (no coerce's). We'd need a coerce if we wanted to handle
476 -- recursive newtypes too
478 -- Check that eta reduction is OK
479 -- (a) the dropped-off args are identical
480 -- (b) the remaining type args mention
481 -- only the remaining type variables
482 eta_ok = (args_to_drop `tcEqTypes` mkTyVarTys tyvars_to_drop)
483 && (tyVarsOfTypes args_to_keep `subVarSet` mkVarSet tyvars_to_keep)
485 cant_derive_err = derivingThingErr clas tys tycon tyvars_to_keep
486 (vcat [ptext SLIT("even with cunning newtype deriving:"),
487 if isRecursiveTyCon tycon then
488 ptext SLIT("the newtype is recursive")
490 if not right_arity then
491 quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("does not have arity 1")
493 if not (n_tyvars_to_keep >= 0) then
494 ptext SLIT("the type constructor has wrong kind")
495 else if not (n_args_to_keep >= 0) then
496 ptext SLIT("the representation type has wrong kind")
497 else if not eta_ok then
498 ptext SLIT("the eta-reduction property does not hold")
502 non_std_err = derivingThingErr clas tys tycon tyvars_to_keep
503 (vcat [non_std_why clas,
504 ptext SLIT("Try -fglasgow-exts for GHC's newtype-deriving extension")])
506 bale_out err = addErrTc err `thenM_` returnM (Nothing, Nothing)
508 std_class gla_exts clas
509 = key `elem` derivableClassKeys
510 || (gla_exts && (key == typeableClassKey || key == dataClassKey))
514 std_class_via_iso clas -- These standard classes can be derived for a newtype
515 -- using the isomorphism trick *even if no -fglasgow-exts*
516 = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
517 -- Not Read/Show because they respect the type
518 -- Not Enum, becuase newtypes are never in Enum
521 new_dfun_name clas tycon -- Just a simple wrapper
522 = newDFunName clas [mkTyConApp tycon []] (getSrcLoc tycon)
523 -- The type passed to newDFunName is only used to generate
524 -- a suitable string; hence the empty type arg list
526 ------------------------------------------------------------------
527 mkDataTypeEqn :: TyCon -> Class -> TcM DerivEqn
528 mkDataTypeEqn tycon clas
529 | clas `hasKey` typeableClassKey
530 = -- The Typeable class is special in several ways
531 -- data T a b = ... deriving( Typeable )
533 -- instance Typeable2 T where ...
534 -- 1. There are no constraints in the instance
535 -- 2. There are no type variables either
536 -- 2. The actual class we want to generate isn't necessarily
537 -- Typeable; it depends on the arity of the type
538 do { real_clas <- tcLookupClass (typeableClassNames !! tyConArity tycon)
539 ; dfun_name <- new_dfun_name real_clas tycon
540 ; return (dfun_name, real_clas, tycon, [], []) }
543 = do { dfun_name <- new_dfun_name clas tycon
544 ; return (dfun_name, clas, tycon, tyvars, constraints) }
546 tyvars = tyConTyVars tycon
547 constraints = extra_constraints ++ ordinary_constraints
548 extra_constraints = tyConTheta tycon
549 -- "extra_constraints": see note [Data decl contexts] above
552 = [ mkClassPred clas [arg_ty]
553 | data_con <- tyConDataCons tycon,
554 arg_ty <- dataConOrigArgTys data_con,
555 -- Use the same type variables
556 -- as the type constructor,
557 -- hence no need to instantiate
558 not (isUnLiftedType arg_ty) -- No constraints for unlifted types?
562 ------------------------------------------------------------------
563 -- Check side conditions that dis-allow derivability for particular classes
564 -- This is *apart* from the newtype-deriving mechanism
566 checkSideConditions :: Bool -> Class -> TyCon -> [TcType] -> Maybe SDoc
567 checkSideConditions gla_exts clas tycon tys
569 = Just ty_args_why -- e.g. deriving( Foo s )
571 = case [cond | (key,cond) <- sideConditions, key == getUnique clas] of
572 [] -> Just (non_std_why clas)
573 [cond] -> cond (gla_exts, tycon)
574 other -> pprPanic "checkSideConditions" (ppr clas)
576 ty_args_why = quotes (ppr (mkClassPred clas tys)) <+> ptext SLIT("is not a class")
578 non_std_why clas = quotes (ppr clas) <+> ptext SLIT("is not a derivable class")
580 sideConditions :: [(Unique, Condition)]
582 = [ (eqClassKey, cond_std),
583 (ordClassKey, cond_std),
584 (readClassKey, cond_std),
585 (showClassKey, cond_std),
586 (enumClassKey, cond_std `andCond` cond_isEnumeration),
587 (ixClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
588 (boundedClassKey, cond_std `andCond` (cond_isEnumeration `orCond` cond_isProduct)),
589 (typeableClassKey, cond_glaExts `andCond` cond_allTypeKind),
590 (dataClassKey, cond_glaExts `andCond` cond_std)
593 type Condition = (Bool, TyCon) -> Maybe SDoc -- Nothing => OK
595 orCond :: Condition -> Condition -> Condition
598 Nothing -> Nothing -- c1 succeeds
599 Just x -> case c2 tc of -- c1 fails
601 Just y -> Just (x $$ ptext SLIT(" and") $$ y)
604 andCond c1 c2 tc = case c1 tc of
605 Nothing -> c2 tc -- c1 succeeds
606 Just x -> Just x -- c1 fails
608 cond_std :: Condition
609 cond_std (gla_exts, tycon)
610 | any isExistentialDataCon data_cons = Just existential_why
611 | null data_cons = Just no_cons_why
612 | otherwise = Nothing
614 data_cons = tyConDataCons tycon
615 no_cons_why = quotes (ppr tycon) <+> ptext SLIT("has no data constructors")
616 existential_why = quotes (ppr tycon) <+> ptext SLIT("has existentially-quantified constructor(s)")
618 cond_isEnumeration :: Condition
619 cond_isEnumeration (gla_exts, tycon)
620 | isEnumerationTyCon tycon = Nothing
621 | otherwise = Just why
623 why = quotes (ppr tycon) <+> ptext SLIT("has non-nullary constructors")
625 cond_isProduct :: Condition
626 cond_isProduct (gla_exts, tycon)
627 | isProductTyCon tycon = Nothing
628 | otherwise = Just why
630 why = quotes (ppr tycon) <+> ptext SLIT("has more than one constructor")
632 cond_allTypeKind :: Condition
633 cond_allTypeKind (gla_exts, tycon)
634 | all (isArgTypeKind . tyVarKind) (tyConTyVars tycon) = Nothing
635 | otherwise = Just why
637 why = quotes (ppr tycon) <+> ptext SLIT("is parameterised over arguments of kind other than `*'")
639 cond_glaExts :: Condition
640 cond_glaExts (gla_exts, tycon) | gla_exts = Nothing
641 | otherwise = Just why
643 why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
646 %************************************************************************
648 \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
650 %************************************************************************
652 A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
653 terms, which is the final correct RHS for the corresponding original
657 Each (k,TyVarTy tv) in a solution constrains only a type
661 The (k,TyVarTy tv) pairs in a solution are canonically
662 ordered by sorting on type varible, tv, (major key) and then class, k,
667 solveDerivEqns :: [DerivEqn]
668 -> TcM [DFunId] -- Solns in same order as eqns.
669 -- This bunch is Absolutely minimal...
671 solveDerivEqns orig_eqns
672 = iterateDeriv 1 initial_solutions
674 -- The initial solutions for the equations claim that each
675 -- instance has an empty context; this solution is certainly
676 -- in canonical form.
677 initial_solutions :: [DerivSoln]
678 initial_solutions = [ [] | _ <- orig_eqns ]
680 ------------------------------------------------------------------
681 -- iterateDeriv calculates the next batch of solutions,
682 -- compares it with the current one; finishes if they are the
683 -- same, otherwise recurses with the new solutions.
684 -- It fails if any iteration fails
685 iterateDeriv :: Int -> [DerivSoln] ->TcM [DFunId]
686 iterateDeriv n current_solns
687 | n > 20 -- Looks as if we are in an infinite loop
688 -- This can happen if we have -fallow-undecidable-instances
689 -- (See TcSimplify.tcSimplifyDeriv.)
690 = pprPanic "solveDerivEqns: probable loop"
691 (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
694 dfuns = zipWithEqual "add_solns" mk_deriv_dfun orig_eqns current_solns
697 -- Extend the inst info from the explicit instance decls
698 -- with the current set of solutions, and simplify each RHS
699 extendLocalInstEnv dfuns $
700 mappM gen_soln orig_eqns
701 ) `thenM` \ new_solns ->
702 if (current_solns == new_solns) then
705 iterateDeriv (n+1) new_solns
707 ------------------------------------------------------------------
709 gen_soln (_, clas, tc,tyvars,deriv_rhs)
710 = addSrcSpan (srcLocSpan (getSrcLoc tc)) $
711 addErrCtxt (derivCtxt (Just clas) tc) $
712 tcSimplifyDeriv tyvars deriv_rhs `thenM` \ theta ->
713 returnM (sortLt (<) theta) -- Canonicalise before returning the soluction
715 mk_deriv_dfun (dfun_name, clas, tycon, tyvars, _) theta
716 = mkDictFunId dfun_name tyvars theta
717 clas [mkTyConApp tycon (mkTyVarTys tyvars)]
719 extendLocalInstEnv :: [DFunId] -> TcM a -> TcM a
720 -- Add new locall-defined instances; don't bother to check
721 -- for functional dependency errors -- that'll happen in TcInstDcls
722 extendLocalInstEnv dfuns thing_inside
723 = do { env <- getGblEnv
724 ; let inst_env' = foldl extendInstEnv (tcg_inst_env env) dfuns
725 env' = env { tcg_inst_env = inst_env' }
726 ; setGblEnv env' thing_inside }
729 %************************************************************************
731 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}
733 %************************************************************************
735 After all the trouble to figure out the required context for the
736 derived instance declarations, all that's left is to chug along to
737 produce them. They will then be shoved into @tcInstDecls2@, which
738 will do all its usual business.
740 There are lots of possibilities for code to generate. Here are
741 various general remarks.
746 We want derived instances of @Eq@ and @Ord@ (both v common) to be
747 ``you-couldn't-do-better-by-hand'' efficient.
750 Deriving @Show@---also pretty common--- should also be reasonable good code.
753 Deriving for the other classes isn't that common or that big a deal.
760 Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
763 Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
766 We {\em normally} generate code only for the non-defaulted methods;
767 there are some exceptions for @Eq@ and (especially) @Ord@...
770 Sometimes we use a @_con2tag_<tycon>@ function, which returns a data
771 constructor's numeric (@Int#@) tag. These are generated by
772 @gen_tag_n_con_binds@, and the heuristic for deciding if one of
773 these is around is given by @hasCon2TagFun@.
775 The examples under the different sections below will make this
779 Much less often (really just for deriving @Ix@), we use a
780 @_tag2con_<tycon>@ function. See the examples.
783 We use the renamer!!! Reason: we're supposed to be
784 producing @LHsBinds Name@ for the methods, but that means
785 producing correctly-uniquified code on the fly. This is entirely
786 possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
787 So, instead, we produce @MonoBinds RdrName@ then heave 'em through
788 the renamer. What a great hack!
792 -- Generate the InstInfo for the required instance,
793 -- plus any auxiliary bindings required
794 genInst :: DFunId -> TcM (InstInfo, LHsBinds RdrName)
796 = getFixityEnv `thenM` \ fix_env ->
798 (tyvars,_,clas,[ty]) = tcSplitDFunTy (idType dfun)
799 clas_nm = className clas
800 tycon = tcTyConAppTyCon ty
801 (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
803 -- Bring the right type variables into
804 -- scope, and rename the method binds
805 bindLocalNames (map varName tyvars) $
806 rnMethodBinds clas_nm [] meth_binds `thenM` \ (rn_meth_binds, _fvs) ->
808 -- Build the InstInfo
809 returnM (InstInfo { iDFunId = dfun, iBinds = VanillaInst rn_meth_binds [] },
812 genDerivBinds clas fix_env tycon
813 | className clas `elem` typeableClassNames
814 = (gen_Typeable_binds tycon, emptyBag)
817 = case assocMaybe gen_list (getUnique clas) of
818 Just gen_fn -> gen_fn fix_env tycon
819 Nothing -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
821 gen_list :: [(Unique, FixityEnv -> TyCon -> (LHsBinds RdrName, LHsBinds RdrName))]
822 gen_list = [(eqClassKey, no_aux_binds (ignore_fix_env gen_Eq_binds))
823 ,(ordClassKey, no_aux_binds (ignore_fix_env gen_Ord_binds))
824 ,(enumClassKey, no_aux_binds (ignore_fix_env gen_Enum_binds))
825 ,(boundedClassKey, no_aux_binds (ignore_fix_env gen_Bounded_binds))
826 ,(ixClassKey, no_aux_binds (ignore_fix_env gen_Ix_binds))
827 ,(typeableClassKey,no_aux_binds (ignore_fix_env gen_Typeable_binds))
828 ,(showClassKey, no_aux_binds gen_Show_binds)
829 ,(readClassKey, no_aux_binds gen_Read_binds)
830 ,(dataClassKey, gen_Data_binds)
833 -- no_aux_binds is used for generators that don't
834 -- need to produce any auxiliary bindings
835 no_aux_binds f fix_env tc = (f fix_env tc, emptyBag)
836 ignore_fix_env f fix_env tc = f tc
840 %************************************************************************
842 \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
844 %************************************************************************
849 con2tag_Foo :: Foo ... -> Int#
850 tag2con_Foo :: Int -> Foo ... -- easier if Int, not Int#
851 maxtag_Foo :: Int -- ditto (NB: not unlifted)
854 We have a @con2tag@ function for a tycon if:
857 We're deriving @Eq@ and the tycon has nullary data constructors.
860 Or: we're deriving @Ord@ (unless single-constructor), @Enum@, @Ix@
864 We have a @tag2con@ function for a tycon if:
867 We're deriving @Enum@, or @Ix@ (enum type only???)
870 If we have a @tag2con@ function, we also generate a @maxtag@ constant.
873 genTaggeryBinds :: [DFunId] -> TcM (LHsBinds RdrName)
874 genTaggeryBinds dfuns
875 = do { names_so_far <- foldlM do_con2tag [] tycons_of_interest
876 ; nm_alist_etc <- foldlM do_tag2con names_so_far tycons_of_interest
877 ; return (listToBag (map gen_tag_n_con_monobind nm_alist_etc)) }
879 all_CTs = map simpleDFunClassTyCon dfuns
880 all_tycons = map snd all_CTs
881 (tycons_of_interest, _) = removeDups compare all_tycons
883 do_con2tag acc_Names tycon
884 | isDataTyCon tycon &&
885 ((we_are_deriving eqClassKey tycon
886 && any isNullaryDataCon (tyConDataCons tycon))
887 || (we_are_deriving ordClassKey tycon
888 && not (isProductTyCon tycon))
889 || (we_are_deriving enumClassKey tycon)
890 || (we_are_deriving ixClassKey tycon))
892 = returnM ((con2tag_RDR tycon, tycon, GenCon2Tag)
897 do_tag2con acc_Names tycon
898 | isDataTyCon tycon &&
899 (we_are_deriving enumClassKey tycon ||
900 we_are_deriving ixClassKey tycon
901 && isEnumerationTyCon tycon)
902 = returnM ( (tag2con_RDR tycon, tycon, GenTag2Con)
903 : (maxtag_RDR tycon, tycon, GenMaxTag)
908 we_are_deriving clas_key tycon
909 = is_in_eqns clas_key tycon all_CTs
911 is_in_eqns clas_key tycon [] = False
912 is_in_eqns clas_key tycon ((c,t):cts)
913 = (clas_key == classKey c && tycon == t)
914 || is_in_eqns clas_key tycon cts
918 derivingThingErr clas tys tycon tyvars why
919 = sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
922 pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])
924 malformedPredErr tycon pred = ptext SLIT("Illegal deriving item") <+> ppr pred
926 derivCtxt :: Maybe Class -> TyCon -> SDoc
927 derivCtxt maybe_cls tycon
928 = ptext SLIT("When deriving") <+> cls <+> ptext SLIT("for type") <+> quotes (ppr tycon)
930 cls = case maybe_cls of
931 Nothing -> ptext SLIT("instances")
932 Just c -> ptext SLIT("the") <+> quotes (ppr c) <+> ptext SLIT("instance")