X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcInstDcls.lhs;h=d4d8d2fbc543e6133eee500eb3955fdcca77a9e6;hp=571cd7010430599119781e722344ef252dc1549e;hb=792449f555bb4dfa8e718079f6d42dc9babe938a;hpb=d2ce0f52d42edf32bb9f13796e6ba6edba8bd516 diff --git a/compiler/typecheck/TcInstDcls.lhs b/compiler/typecheck/TcInstDcls.lhs index 571cd70..d4d8d2f 100644 --- a/compiler/typecheck/TcInstDcls.lhs +++ b/compiler/typecheck/TcInstDcls.lhs @@ -12,9 +12,11 @@ import HsSyn import TcBinds import TcTyClsDecls import TcClassDcl +import TcPat( addInlinePrags ) import TcRnMonad import TcMType import TcType +import BuildTyCl import Inst import InstEnv import FamInst @@ -22,19 +24,20 @@ import FamInstEnv import TcDeriv import TcEnv import RnSource ( addTcgDUs ) -import TcSimplify( simplifySuperClass ) import TcHsType import TcUnify +import MkCore ( nO_METHOD_BINDING_ERROR_ID ) import Type import Coercion import TyCon import DataCon import Class import Var -import VarSet ( emptyVarSet ) +import Pair +import VarSet import CoreUtils ( mkPiTypes ) import CoreUnfold ( mkDFunUnfolding ) -import CoreSyn ( Expr(Var) ) +import CoreSyn ( Expr(Var), DFunArg(..), CoreExpr ) import Id import MkId import Name @@ -181,13 +184,14 @@ Instead we use a cunning trick. Note [Single-method classes] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If the class has just one method (or, more accurately, just one element -of {superclasses + methods}), then we still use the *same* strategy +of {superclasses + methods}), then we use a different strategy. class C a where op :: a -> a instance C a => C [a] where op = -We translate the class decl into a newtype, which just gives -a top-level axiom: +We translate the class decl into a newtype, which just gives a +top-level axiom. The "constructor" MkC expands to a cast, as does the +class-op selector. axiom Co:C a :: C a ~ (a->a) @@ -197,44 +201,82 @@ a top-level axiom: MkC :: forall a. (a->a) -> C a MkC = /\a.\op. op |> (sym Co:C a) - df :: forall a. C a => C [a] - {-# NOINLINE df DFun[ $cop_list ] #-} - df = /\a. \d. MkC ($cop_list a d) +The clever RULE stuff doesn't work now, because ($df a d) isn't +a constructor application, so exprIsConApp_maybe won't return +Just . - $cop_list :: forall a. C a => [a] -> [a] - $cop_list = +Instead, we simply rely on the fact that casts are cheap: -The "constructor" MkC expands to a cast, as does the class-op selector. -The RULE works just like for multi-field dictionaries: + $df :: forall a. C a => C [a] + {-# INLINE df #-} -- NB: INLINE this + $df = /\a. \d. MkC [a] ($cop_list a d) + = $cop_list |> forall a. C a -> (sym (Co:C [a])) - * (df a d) returns (Just (MkC,..,[$cop_list a d])) - to exprIsConApp_Maybe + $cop_list :: forall a. C a => [a] -> [a] + $cop_list = - * The RULE for op picks the right result +So if we see + (op ($df a d)) +we'll inline 'op' and '$df', since both are simply casts, and +good things happen. -This is a bit of a hack, because (df a d) isn't *really* a constructor -application. But it works just fine in this case, exprIsConApp_maybe -is otherwise used only when we hit a case expression which will have -a real data constructor in it. +Why do we use this different strategy? Because otherwise we +end up with non-inlined dictionaries that look like + $df = $cop |> blah +which adds an extra indirection to every use, which seems stupid. See +Trac #4138 for an example (although the regression reported there +wasn't due to the indirction). -The biggest reason for doing it this way, apart from uniformity, is -that we want to be very careful when we have +There is an awkward wrinkle though: we want to be very +careful when we have instance C a => C [a] where {-# INLINE op #-} op = ... then we'll get an INLINE pragma on $cop_list but it's important that $cop_list only inlines when it's applied to *two* arguments (the -dictionary and the list argument +dictionary and the list argument). So we nust not eta-expand $df +above. We ensure that this doesn't happen by putting an INLINE +pragma on the dfun itself; after all, it ends up being just a cast. + +There is one more dark corner to the INLINE story, even more deeply +buried. Consider this (Trac #3772): + + class DeepSeq a => C a where + gen :: Int -> a + + instance C a => C [a] where + gen n = ... + + class DeepSeq a where + deepSeq :: a -> b -> b -The danger is that we'll get something like - op_list :: C a => [a] -> [a] - op_list = /\a.\d. $cop_list a d -and then we'll eta expand, and then we'll inline TOO EARLY. This happened in -Trac #3772 and I spent far too long fiddling around trying to fix it. -Look at the test for Trac #3772. + instance DeepSeq a => DeepSeq [a] where + {-# INLINE deepSeq #-} + deepSeq xs b = foldr deepSeq b xs - (Note: re-reading the above, I can't see how using the - uniform story solves the problem.) +That gives rise to these defns: + + $cdeepSeq :: DeepSeq a -> [a] -> b -> b + -- User INLINE( 3 args )! + $cdeepSeq a (d:DS a) b (x:[a]) (y:b) = ... + + $fDeepSeq[] :: DeepSeq a -> DeepSeq [a] + -- DFun (with auto INLINE pragma) + $fDeepSeq[] a d = $cdeepSeq a d |> blah + + $cp1 a d :: C a => DeepSep [a] + -- We don't want to eta-expand this, lest + -- $cdeepSeq gets inlined in it! + $cp1 a d = $fDeepSep[] a (scsel a d) + + $fC[] :: C a => C [a] + -- Ordinary DFun + $fC[] a d = MkC ($cp1 a d) ($cgen a d) + +Here $cp1 is the code that generates the superclass for C [a]. The +issue is this: we must not eta-expand $cp1 either, or else $fDeepSeq[] +and then $cdeepSeq will inline there, which is definitely wrong. Like +on the dfun, we solve this by adding an INLINE pragma to $cp1. Note [Subtle interaction of recursion and overlap] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -271,13 +313,12 @@ See the overlapping instances for RegexContext, and the fact that they call 'nullFail' just like the example above. The DoCon package also does the same thing; it shows up in module Fraction.hs -Conclusion: when typechecking the methods in a C [a] instance, we want -to have C [a] available. That is why we have the strange local -definition for 'this' in the definition of op1_i in the example above. -We can typecheck the defintion of local_op1, and when doing tcSimplifyCheck -we supply 'this' as a given dictionary. Only needed, though, if there -are some type variables involved; otherwise there can be no overlap and -none of this arises. +Conclusion: when typechecking the methods in a C [a] instance, we want to +treat the 'a' as an *existential* type variable, in the sense described +by Note [Binding when looking up instances]. That is why isOverlappableTyVar +responds True to an InstSkol, which is the kind of skolem we use in +tcInstDecl2. + Note [Tricky type variable scoping] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -331,40 +372,41 @@ tcInstDecls1 tycl_decls inst_decls deriv_decls ; let { (local_info, at_tycons_s) = unzip local_info_tycons ; at_idx_tycons = concat at_tycons_s ++ idx_tycons - ; clas_decls = filter (isClassDecl.unLoc) tycl_decls - ; implicit_things = concatMap implicitTyThings at_idx_tycons - ; aux_binds = mkRecSelBinds at_idx_tycons - } + ; implicit_things = concatMap implicitTyConThings at_idx_tycons + ; aux_binds = mkRecSelBinds at_idx_tycons } -- (2) Add the tycons of indexed types and their implicit -- tythings to the global environment - ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do { + ; tcExtendGlobalEnv (map ATyCon at_idx_tycons ++ implicit_things) $ do { - -- (3) Instances from generic class declarations - ; generic_inst_info <- getGenericInstances clas_decls -- Next, construct the instance environment so far, consisting -- of -- (a) local instance decls - -- (b) generic instances - -- (c) local family instance decls + -- (b) local family instance decls ; addInsts local_info $ - addInsts generic_inst_info $ addFamInsts at_idx_tycons $ do { - -- (4) Compute instances from "deriving" clauses; + -- (3) Compute instances from "deriving" clauses; -- This stuff computes a context for the derived instance -- decl, so it needs to know about all the instances possible -- NB: class instance declarations can contain derivings as -- part of associated data type declarations - failIfErrsM -- If the addInsts stuff gave any errors, don't - -- try the deriving stuff, becuase that may give - -- more errors still - ; (deriv_inst_info, deriv_binds, deriv_dus) + failIfErrsM -- If the addInsts stuff gave any errors, don't + -- try the deriving stuff, because that may give + -- more errors still + ; (deriv_inst_info, deriv_binds, deriv_dus, deriv_tys, deriv_ty_insts) <- tcDeriving tycl_decls inst_decls deriv_decls - ; gbl_env <- addInsts deriv_inst_info getGblEnv + + -- Extend the global environment also with the generated datatypes for + -- the generic representation + ; let all_tycons = map ATyCon (deriv_tys ++ deriv_ty_insts) + ; gbl_env <- tcExtendGlobalEnv all_tycons $ + tcExtendGlobalEnv (concatMap implicitTyThings all_tycons) $ + addFamInsts deriv_ty_insts $ + addInsts deriv_inst_info getGblEnv ; return ( addTcgDUs gbl_env deriv_dus, - generic_inst_info ++ deriv_inst_info ++ local_info, + deriv_inst_info ++ local_info, aux_binds `plusHsValBinds` deriv_binds) }}} @@ -372,18 +414,14 @@ addInsts :: [InstInfo Name] -> TcM a -> TcM a addInsts infos thing_inside = tcExtendLocalInstEnv (map iSpec infos) thing_inside -addFamInsts :: [TyThing] -> TcM a -> TcM a +addFamInsts :: [TyCon] -> TcM a -> TcM a addFamInsts tycons thing_inside - = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside - where - mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon - mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts" - (ppr tything) + = tcExtendLocalFamInstEnv (map mkLocalFamInst tycons) thing_inside \end{code} \begin{code} tcLocalInstDecl1 :: LInstDecl Name - -> TcM (InstInfo Name, [TyThing]) + -> TcM (InstInfo Name, [TyCon]) -- A source-file instance declaration -- Type-check all the stuff before the "where" -- @@ -396,10 +434,8 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags)) badBootDeclErr - ; (tyvars, theta, tau) <- tcHsInstHead poly_ty - - -- Now, check the validity of the instance. - ; (clas, inst_tys) <- checkValidInstance poly_ty tyvars theta tau + ; (tyvars, theta, clas, inst_tys) <- tcHsInstHead poly_ty + ; checkValidInstance poly_ty tyvars theta clas inst_tys -- Next, process any associated types. ; idx_tycons <- recoverM (return []) $ @@ -419,8 +455,7 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys ispec = mkLocalInstance dfun overlap_flag - ; return (InstInfo { iSpec = ispec, - iBinds = VanillaInst binds uprags False }, + ; return (InstInfo { iSpec = ispec, iBinds = VanillaInst binds uprags False }, idx_tycons) } where @@ -430,7 +465,7 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) checkValidAndMissingATs :: Class -> ([TyVar], [TcType]) -- instance types -> [(LTyClDecl Name, -- source form of AT - TyThing)] -- Core form of AT + TyCon)] -- Core form of AT -> TcM () checkValidAndMissingATs clas inst_tys ats = do { -- Issue a warning for each class AT that is not defined in this @@ -448,12 +483,11 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) ; mapM_ (checkIndexes clas inst_tys) ats } - checkIndexes clas inst_tys (hsAT, ATyCon tycon) + checkIndexes clas inst_tys (hsAT, tycon) -- !!!TODO: check that this does the Right Thing for indexed synonyms, too! = checkIndexes' clas inst_tys hsAT (tyConTyVars tycon, snd . fromJust . tyConFamInst_maybe $ tycon) - checkIndexes _ _ _ = panic "checkIndexes" checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys) = let atName = tcdName . unLoc $ hsAT @@ -513,8 +547,8 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) | isTyVarTy ty = return () | otherwise = addErrTc $ mustBeVarArgErr ty checkIndex ty (Just instTy) - | ty `tcEqType` instTy = return () - | otherwise = addErrTc $ wrongATArgErr ty instTy + | ty `eqType` instTy = return () + | otherwise = addErrTc $ wrongATArgErr ty instTy listToNameSet = addListToNameSet emptyNameSet @@ -527,7 +561,182 @@ tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats)) tv1 `sameLexeme` tv2 = nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2) in - extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement + TcType.extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement +\end{code} + + +%************************************************************************ +%* * + Type checking family instances +%* * +%************************************************************************ + +Family instances are somewhat of a hybrid. They are processed together with +class instance heads, but can contain data constructors and hence they share a +lot of kinding and type checking code with ordinary algebraic data types (and +GADTs). + +\begin{code} +tcFamInstDecl :: TopLevelFlag -> LTyClDecl Name -> TcM TyCon +tcFamInstDecl top_lvl (L loc decl) + = -- Prime error recovery, set source location + setSrcSpan loc $ + tcAddDeclCtxt decl $ + do { -- type family instances require -XTypeFamilies + -- and can't (currently) be in an hs-boot file + ; type_families <- xoptM Opt_TypeFamilies + ; is_boot <- tcIsHsBoot -- Are we compiling an hs-boot file? + ; checkTc type_families $ badFamInstDecl (tcdLName decl) + ; checkTc (not is_boot) $ badBootFamInstDeclErr + + -- Perform kind and type checking + ; tc <- tcFamInstDecl1 decl + ; checkValidTyCon tc -- Remember to check validity; + -- no recursion to worry about here + + -- Check that toplevel type instances are not for associated types. + ; when (isTopLevel top_lvl && isAssocFamily tc) + (addErr $ assocInClassErr (tcdName decl)) + + ; return tc } + +isAssocFamily :: TyCon -> Bool -- Is an assocaited type +isAssocFamily tycon + = case tyConFamInst_maybe tycon of + Nothing -> panic "isAssocFamily: no family?!?" + Just (fam, _) -> isTyConAssoc fam + +assocInClassErr :: Name -> SDoc +assocInClassErr name + = ptext (sLit "Associated type") <+> quotes (ppr name) <+> + ptext (sLit "must be inside a class instance") + + + +tcFamInstDecl1 :: TyClDecl Name -> TcM TyCon + + -- "type instance" +tcFamInstDecl1 (decl@TySynonym {tcdLName = L loc tc_name}) + = kcIdxTyPats decl $ \k_tvs k_typats resKind family -> + do { -- check that the family declaration is for a synonym + checkTc (isFamilyTyCon family) (notFamily family) + ; checkTc (isSynTyCon family) (wrongKindOfFamily family) + + ; -- (1) kind check the right-hand side of the type equation + ; k_rhs <- kcCheckLHsType (tcdSynRhs decl) (EK resKind EkUnk) + -- ToDo: the ExpKind could be better + + -- we need the exact same number of type parameters as the family + -- declaration + ; let famArity = tyConArity family + ; checkTc (length k_typats == famArity) $ + wrongNumberOfParmsErr famArity + + -- (2) type check type equation + ; tcTyVarBndrs k_tvs $ \t_tvs -> do { -- turn kinded into proper tyvars + ; t_typats <- mapM tcHsKindedType k_typats + ; t_rhs <- tcHsKindedType k_rhs + + -- (3) check the well-formedness of the instance + ; checkValidTypeInst t_typats t_rhs + + -- (4) construct representation tycon + ; rep_tc_name <- newFamInstTyConName tc_name t_typats loc + ; buildSynTyCon rep_tc_name t_tvs (SynonymTyCon t_rhs) + (typeKind t_rhs) + NoParentTyCon (Just (family, t_typats)) + }} + + -- "newtype instance" and "data instance" +tcFamInstDecl1 (decl@TyData {tcdND = new_or_data, tcdLName = L loc tc_name, + tcdCons = cons}) + = kcIdxTyPats decl $ \k_tvs k_typats resKind fam_tycon -> + do { -- check that the family declaration is for the right kind + checkTc (isFamilyTyCon fam_tycon) (notFamily fam_tycon) + ; checkTc (isAlgTyCon fam_tycon) (wrongKindOfFamily fam_tycon) + + ; -- (1) kind check the data declaration as usual + ; k_decl <- kcDataDecl decl k_tvs + ; let k_ctxt = tcdCtxt k_decl + k_cons = tcdCons k_decl + + -- result kind must be '*' (otherwise, we have too few patterns) + ; checkTc (isLiftedTypeKind resKind) $ tooFewParmsErr (tyConArity fam_tycon) + + -- (2) type check indexed data type declaration + ; tcTyVarBndrs k_tvs $ \t_tvs -> do { -- turn kinded into proper tyvars + + -- kind check the type indexes and the context + ; t_typats <- mapM tcHsKindedType k_typats + ; stupid_theta <- tcHsKindedContext k_ctxt + + -- (3) Check that + -- (a) left-hand side contains no type family applications + -- (vanilla synonyms are fine, though, and we checked for + -- foralls earlier) + ; mapM_ checkTyFamFreeness t_typats + + ; dataDeclChecks tc_name new_or_data stupid_theta k_cons + + -- (4) construct representation tycon + ; rep_tc_name <- newFamInstTyConName tc_name t_typats loc + ; let ex_ok = True -- Existentials ok for type families! + ; fixM (\ rep_tycon -> do + { let orig_res_ty = mkTyConApp fam_tycon t_typats + ; data_cons <- tcConDecls ex_ok rep_tycon + (t_tvs, orig_res_ty) k_cons + ; tc_rhs <- + case new_or_data of + DataType -> return (mkDataTyConRhs data_cons) + NewType -> ASSERT( not (null data_cons) ) + mkNewTyConRhs rep_tc_name rep_tycon (head data_cons) + ; buildAlgTyCon rep_tc_name t_tvs stupid_theta tc_rhs Recursive + h98_syntax NoParentTyCon (Just (fam_tycon, t_typats)) + -- We always assume that indexed types are recursive. Why? + -- (1) Due to their open nature, we can never be sure that a + -- further instance might not introduce a new recursive + -- dependency. (2) They are always valid loop breakers as + -- they involve a coercion. + }) + }} + where + h98_syntax = case cons of -- All constructors have same shape + L _ (ConDecl { con_res = ResTyGADT _ }) : _ -> False + _ -> True + +tcFamInstDecl1 d = pprPanic "tcFamInstDecl1" (ppr d) + +-- Kind checking of indexed types +-- - + +-- Kind check type patterns and kind annotate the embedded type variables. +-- +-- * Here we check that a type instance matches its kind signature, but we do +-- not check whether there is a pattern for each type index; the latter +-- check is only required for type synonym instances. + +kcIdxTyPats :: TyClDecl Name + -> ([LHsTyVarBndr Name] -> [LHsType Name] -> Kind -> TyCon -> TcM a) + -- ^^kinded tvs ^^kinded ty pats ^^res kind + -> TcM a +kcIdxTyPats decl thing_inside + = kcHsTyVars (tcdTyVars decl) $ \tvs -> + do { let tc_name = tcdLName decl + ; fam_tycon <- tcLookupLocatedTyCon tc_name + ; let { (kinds, resKind) = splitKindFunTys (tyConKind fam_tycon) + ; hs_typats = fromJust $ tcdTyPats decl } + + -- we may not have more parameters than the kind indicates + ; checkTc (length kinds >= length hs_typats) $ + tooManyParmsErr (tcdLName decl) + + -- type functions can have a higher-kinded result + ; let resultKind = mkArrowKinds (drop (length hs_typats) kinds) resKind + ; typats <- zipWithM kcCheckLHsType hs_typats + [ EK kind (EkArg (ppr tc_name) n) + | (kind,n) <- kinds `zip` [1..]] + ; thing_inside tvs typats resultKind fam_tycon + } \end{code} @@ -560,16 +769,6 @@ tcInstDecls2 tycl_decls inst_decls -- Done ; return (dm_binds `unionBags` unionManyBags inst_binds_s) } - -tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id) -tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds }) - = recoverM (return emptyLHsBinds) $ - setSrcSpan loc $ - addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ - tc_inst_decl2 dfun_id ibinds - where - dfun_id = instanceDFunId ispec - loc = getSrcSpan dfun_id \end{code} See Note [Default methods and instances] @@ -586,102 +785,120 @@ So right here in tcInstDecl2 we must re-extend the type envt with the default method Ids replete with their INLINE pragmas. Urk. \begin{code} -tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id) --- Returns a binding for the dfun -tc_inst_decl2 dfun_id inst_binds - = do { let rigid_info = InstSkol - inst_ty = idType dfun_id - loc = getSrcSpan dfun_id - - -- Instantiate the instance decl with skolem constants - ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty - -- These inst_tyvars' scope over the 'where' part - -- Those tyvars are inside the dfun_id's type, which is a bit - -- bizarre, but OK so long as you realise it! - ; let - (clas, inst_tys') = tcSplitDFunHead inst_head' - (class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas - -- Instantiate the super-class context with inst_tys - sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta - - -- Create dictionary Ids from the specified instance contexts. - ; dfun_ev_vars <- newEvVars dfun_theta' - ; self_dict <- newSelfDict clas inst_tys' - -- Default-method Ids may be mentioned in synthesised RHSs, - -- but they'll already be in the environment. - - -- Cook up a binding for "self = df d1 .. dn", - -- to use in each method binding - -- Why? See Note [Subtle interaction of recursion and overlap] - ; let self_ev_bind = EvBind self_dict $ - EvDFunApp dfun_id (mkTyVarTys inst_tyvars') dfun_ev_vars +tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id) + -- Returns a binding for the dfun +tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds }) + = recoverM (return emptyLHsBinds) $ + setSrcSpan loc $ + addErrCtxt (instDeclCtxt2 (idType dfun_id)) $ + do { -- Instantiate the instance decl with skolem constants + ; (inst_tyvars, dfun_theta, inst_head) <- tcSkolDFunType (idType dfun_id) + -- We instantiate the dfun_id with superSkolems. + -- See Note [Subtle interaction of recursion and overlap] + -- and Note [Binding when looking up instances] + ; let (clas, inst_tys) = tcSplitDFunHead inst_head + (class_tyvars, sc_theta, _, op_items) = classBigSig clas + sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys) sc_theta + n_ty_args = length inst_tyvars + n_silent = dfunNSilent dfun_id + (silent_theta, orig_theta) = splitAt n_silent dfun_theta + + ; silent_ev_vars <- mapM newSilentGiven silent_theta + ; orig_ev_vars <- newEvVars orig_theta + ; let dfun_ev_vars = silent_ev_vars ++ orig_ev_vars + + ; (sc_dicts, sc_args) + <- mapAndUnzipM (tcSuperClass n_ty_args dfun_ev_vars) sc_theta' + + -- Check that any superclasses gotten from a silent arguemnt + -- can be deduced from the originally-specified dfun arguments + ; ct_loc <- getCtLoc ScOrigin + ; _ <- checkConstraints skol_info inst_tyvars orig_ev_vars $ + emitFlats $ listToBag $ + [ mkEvVarX sc ct_loc | sc <- sc_dicts, isSilentEvVar sc ] -- Deal with 'SPECIALISE instance' pragmas -- See Note [SPECIALISE instance pragmas] - ; spec_info <- tcSpecInstPrags dfun_id inst_binds + ; spec_info@(spec_inst_prags,_) <- tcSpecInstPrags dfun_id ibinds -- Typecheck the methods ; (meth_ids, meth_binds) - <- tcExtendTyVarEnv inst_tyvars' $ - tcInstanceMethods dfun_id clas inst_tyvars' dfun_ev_vars - inst_tys' self_ev_bind spec_info - op_items inst_binds - - -- Figure out bindings for the superclass context - ; let tc_sc = tcSuperClass inst_tyvars' dfun_ev_vars self_ev_bind - (sc_eqs, sc_dicts) = splitAt (classSCNEqs clas) sc_theta' - ; (sc_dict_ids, sc_binds) <- ASSERT( equalLength sc_sels sc_dicts ) - ASSERT( all isEqPred sc_eqs ) - mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts) - - -- NOT FINISHED! - ; (_eq_sc_binds, sc_eq_vars) <- checkConstraints InstSkol emptyVarSet - inst_tyvars' dfun_ev_vars $ - emitWanteds ScOrigin sc_eqs + <- tcExtendTyVarEnv inst_tyvars $ + -- The inst_tyvars scope over the 'where' part + -- Those tyvars are inside the dfun_id's type, which is a bit + -- bizarre, but OK so long as you realise it! + tcInstanceMethods dfun_id clas inst_tyvars dfun_ev_vars + inst_tys spec_info + op_items ibinds -- Create the result bindings - ; let dict_constr = classDataCon clas - dict_bind = mkVarBind self_dict dict_rhs - dict_rhs = foldl mk_app inst_constr dict_and_meth_ids - dict_and_meth_ids = sc_dict_ids ++ meth_ids - inst_constr = L loc $ wrapId (mkWpEvVarApps sc_eq_vars - <.> mkWpTyApps inst_tys') - (dataConWrapId dict_constr) + ; self_dict <- newEvVar (ClassP clas inst_tys) + ; let class_tc = classTyCon clas + [dict_constr] = tyConDataCons class_tc + dict_bind = mkVarBind self_dict dict_rhs + dict_rhs = foldl mk_app inst_constr $ + map HsVar sc_dicts ++ map (wrapId arg_wrapper) meth_ids + inst_constr = L loc $ wrapId (mkWpTyApps inst_tys) + (dataConWrapId dict_constr) -- We don't produce a binding for the dict_constr; instead we -- rely on the simplifier to unfold this saturated application -- We do this rather than generate an HsCon directly, because -- it means that the special cases (e.g. dictionary with only one - -- member) are dealt with by the common MkId.mkDataConWrapId code rather - -- than needing to be repeated here. + -- member) are dealt with by the common MkId.mkDataConWrapId + -- code rather than needing to be repeated here. - mk_app :: LHsExpr Id -> Id -> LHsExpr Id - mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id))) - arg_wrapper = mkWpEvVarApps dfun_ev_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars') + mk_app :: LHsExpr Id -> HsExpr Id -> LHsExpr Id + mk_app fun arg = L loc (HsApp fun (L loc arg)) + + arg_wrapper = mkWpEvVarApps dfun_ev_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars) -- Do not inline the dfun; instead give it a magic DFunFunfolding -- See Note [ClassOp/DFun selection] -- See also note [Single-method classes] - dfun_id_w_fun = dfun_id - `setIdUnfolding` mkDFunUnfolding inst_ty (map Var dict_and_meth_ids) - -- Not right for equality superclasses - `setInlinePragma` dfunInlinePragma - - (spec_inst_prags, _) = spec_info - main_bind = AbsBinds { abs_tvs = inst_tyvars' + dfun_id_w_fun + | isNewTyCon class_tc + = dfun_id `setInlinePragma` alwaysInlinePragma { inl_sat = Just 0 } + | otherwise + = dfun_id `setIdUnfolding` mkDFunUnfolding dfun_ty (sc_args ++ meth_args) + `setInlinePragma` dfunInlinePragma + meth_args = map (DFunPolyArg . Var) meth_ids + + main_bind = AbsBinds { abs_tvs = inst_tyvars , abs_ev_vars = dfun_ev_vars - , abs_exports = [(inst_tyvars', dfun_id_w_fun, self_dict, + , abs_exports = [(inst_tyvars, dfun_id_w_fun, self_dict, SpecPrags spec_inst_prags)] , abs_ev_binds = emptyTcEvBinds , abs_binds = unitBag dict_bind } - ; return (unitBag (L loc main_bind) `unionBags` - listToBag meth_binds `unionBags` - listToBag sc_binds) + ; return (unitBag (L loc main_bind) `unionBags` + listToBag meth_binds) } + where + skol_info = InstSkol + dfun_ty = idType dfun_id + dfun_id = instanceDFunId ispec + loc = getSrcSpan dfun_id ------------------------------ -tcSpecInstPrags :: DFunId -> InstBindings Name +tcSuperClass :: Int -> [EvVar] -> PredType -> TcM (EvVar, DFunArg CoreExpr) +-- All superclasses should be either +-- (a) be one of the arguments to the dfun, of +-- (b) be a constant, soluble at top level +tcSuperClass n_ty_args ev_vars pred + | Just (ev, i) <- find n_ty_args ev_vars + = return (ev, DFunLamArg i) + | otherwise + = ASSERT2( isEmptyVarSet (tyVarsOfPred pred), ppr pred) -- Constant! + do { sc_dict <- emitWanted ScOrigin pred + ; return (sc_dict, DFunConstArg (Var sc_dict)) } + where + find _ [] = Nothing + find i (ev:evs) | pred `eqPred` evVarPred ev = Just (ev, i) + | otherwise = find (i+1) evs + +------------------------------ +tcSpecInstPrags :: DFunId -> InstBindings Name -> TcM ([Located TcSpecPrag], PragFun) tcSpecInstPrags _ (NewTypeDerived {}) = return ([], \_ -> []) @@ -690,52 +907,79 @@ tcSpecInstPrags dfun_id (VanillaInst binds uprags _) filter isSpecInstLSig uprags -- The filter removes the pragmas for methods ; return (spec_inst_prags, mkPragFun uprags binds) } - ------------------------------- -tcSuperClass :: [TyVar] -> [EvVar] - -> EvBind - -> (Id, PredType) -> TcM (Id, LHsBind Id) --- Build a top level decl like --- sc_op = /\a \d. let this = ... in --- let sc = ... in --- sc --- The "this" part is just-in-case (discarded if not used) --- See Note [Recursive superclasses] -tcSuperClass tyvars dicts - self_ev_bind@(EvBind self_dict _) - (sc_sel, sc_pred) - = do { (ev_binds, wanted, sc_dict) - <- newImplication InstSkol emptyVarSet tyvars dicts $ - emitWanted ScOrigin sc_pred - - ; simplifySuperClass self_dict wanted - -- We include self_dict in the 'givens'; the simplifier - -- is clever enough to stop sc_pred geting bound by just - -- selecting from self_dict!! - - ; uniq <- newUnique - ; let sc_op_ty = mkForAllTys tyvars $ mkPiTypes dicts (varType sc_dict) - sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq - (getName sc_sel) - sc_op_id = mkLocalId sc_op_name sc_op_ty - sc_op_bind = VarBind { var_id = sc_op_id, var_inline = False - , var_rhs = L noSrcSpan $ wrapId sc_wrapper sc_dict } - sc_wrapper = mkWpTyLams tyvars - <.> mkWpLams dicts - <.> mkWpLet (EvBinds (unitBag self_ev_bind)) - <.> mkWpLet ev_binds - - ; return (sc_op_id, noLoc sc_op_bind) } \end{code} -Note [Recursive superclasses] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -See Trac #1470 for why we would *like* to add "self_dict" to the -available instances here. But we can't do so because then the superclases -get satisfied by selection from self_dict, and that leads to an immediate -loop. What we need is to add self_dict to Avails without adding its -superclasses, and we currently have no way to do that. - +Note [Silent Superclass Arguments] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider the following (extreme) situation: + class C a => D a where ... + instance D [a] => D [a] where ... +Although this looks wrong (assume D [a] to prove D [a]), it is only a +more extreme case of what happens with recursive dictionaries. + +To implement the dfun we must generate code for the superclass C [a], +which we can get by superclass selection from the supplied argument! +So we’d generate: + dfun :: forall a. D [a] -> D [a] + dfun = \d::D [a] -> MkD (scsel d) .. + +However this means that if we later encounter a situation where +we have a [Wanted] dw::D [a] we could solve it thus: + dw := dfun dw +Although recursive, this binding would pass the TcSMonadisGoodRecEv +check because it appears as guarded. But in reality, it will make a +bottom superclass. The trouble is that isGoodRecEv can't "see" the +superclass-selection inside dfun. + +Our solution to this problem is to change the way ‘dfuns’ are created +for instances, so that we pass as first arguments to the dfun some +``silent superclass arguments’’, which are the immediate superclasses +of the dictionary we are trying to construct. In our example: + dfun :: forall a. (C [a], D [a] -> D [a] + dfun = \(dc::C [a]) (dd::D [a]) -> DOrd dc ... + +This gives us: + + ----------------------------------------------------------- + DFun Superclass Invariant + ~~~~~~~~~~~~~~~~~~~~~~~~ + In the body of a DFun, every superclass argument to the + returned dictionary is + either * one of the arguments of the DFun, + or * constant, bound at top level + ----------------------------------------------------------- + +This means that no superclass is hidden inside a dfun application, so +the counting argument in isGoodRecEv (more dfun calls than superclass +selections) works correctly. + +The extra arguments required to satisfy the DFun Superclass Invariant +always come first, and are called the "silent" arguments. DFun types +are built (only) by MkId.mkDictFunId, so that is where we decide +what silent arguments are to be added. + +This net effect is that it is safe to treat a dfun application as +wrapping a dictionary constructor around its arguments (in particular, +a dfun never picks superclasses from the arguments under the dictionary +constructor). + +In our example, if we had [Wanted] dw :: D [a] we would get via the instance: + dw := dfun d1 d2 + [Wanted] (d1 :: C [a]) + [Wanted] (d2 :: D [a]) + [Derived] (d :: D [a]) + [Derived] (scd :: C [a]) scd := scsel d + [Derived] (scd2 :: C [a]) scd2 := scsel d2 + +And now, though we *can* solve: + d2 := dw +we will get an isGoodRecEv failure when we try to solve: + d1 := scsel d + or + d1 := scsel d2 + +Test case SCLoop tests this fix. + Note [SPECIALISE instance pragmas] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider @@ -783,11 +1027,12 @@ tcSpecInst :: Id -> Sig Name -> TcM TcSpecPrag tcSpecInst dfun_id prag@(SpecInstSig hs_ty) = addErrCtxt (spec_ctxt prag) $ do { let name = idName dfun_id - ; (tyvars, theta, tau) <- tcHsInstHead hs_ty - ; let spec_ty = mkSigmaTy tyvars theta tau - ; co_fn <- tcSubType (SpecPragOrigin name) (SigSkol SpecInstCtxt) - (idType dfun_id) spec_ty - ; return (SpecPrag co_fn defaultInlinePragma) } + ; (tyvars, theta, clas, tys) <- tcHsInstHead hs_ty + ; let (_, spec_dfun_ty) = mkDictFunTy tyvars theta clas tys + + ; co_fn <- tcSubType (SpecPragOrigin name) SpecInstCtxt + (idType dfun_id) spec_dfun_ty + ; return (SpecPrag dfun_id co_fn defaultInlinePragma) } where spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag) @@ -812,15 +1057,14 @@ tcInstanceMethod tcInstanceMethods :: DFunId -> Class -> [TcTyVar] -> [EvVar] -> [TcType] - -> EvBind -- "This" and its binding - -> ([Located TcSpecPrag], PragFun) + -> ([Located TcSpecPrag], PragFun) -> [(Id, DefMeth)] -> InstBindings Name -> TcM ([Id], [LHsBind Id]) -- The returned inst_meth_ids all have types starting -- forall tvs. theta => ... tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys - self_dict_ev (spec_inst_prags, prag_fn) + (spec_inst_prags, prag_fn) op_items (VanillaInst binds _ standalone_deriv) = mapAndUnzipM tc_item op_items where @@ -834,27 +1078,30 @@ tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys ---------------------- tc_body :: Id -> Bool -> LHsBind Name -> TcM (TcId, LHsBind Id) tc_body sel_id generated_code rn_bind - = add_meth_ctxt generated_code rn_bind $ + = add_meth_ctxt sel_id generated_code rn_bind $ do { (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars inst_tys sel_id - ; (meth_id1, spec_prags) <- tcPrags NonRecursive False True - meth_id (prag_fn (idName sel_id)) - + ; let prags = prag_fn (idName sel_id) + ; meth_id1 <- addInlinePrags meth_id prags + ; spec_prags <- tcSpecPrags meth_id1 prags ; bind <- tcInstanceMethodBody InstSkol tyvars dfun_ev_vars - mb_dict_ev - meth_id1 local_meth_id - meth_sig_fn - (SpecPrags (spec_inst_prags ++ spec_prags)) + meth_id1 local_meth_id meth_sig_fn + (mk_meth_spec_prags meth_id1 spec_prags) rn_bind ; return (meth_id1, bind) } ---------------------- tc_default :: Id -> DefMeth -> TcM (TcId, LHsBind Id) + + tc_default sel_id (GenDefMeth dm_name) + = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id dm_name + ; tc_body sel_id False {- Not generated code? -} meth_bind } +{- tc_default sel_id GenDefMeth -- Derivable type classes stuff = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id ; tc_body sel_id False {- Not generated code? -} meth_bind } - +-} tc_default sel_id NoDefMeth -- No default method at all = do { warnMissingMethod sel_id ; (meth_id, _) <- mkMethIds clas tyvars dfun_ev_vars @@ -873,30 +1120,33 @@ tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys = do { -- Build the typechecked version directly, -- without calling typecheck_method; -- see Note [Default methods in instances] - -- Generate /\as.\ds. let this = df as ds - -- in $dm inst_tys this + -- Generate /\as.\ds. let self = df as ds + -- in $dm inst_tys self -- The 'let' is necessary only because HsSyn doesn't allow -- you to apply a function to a dictionary *expression*. + ; self_dict <- newEvVar (ClassP clas inst_tys) + ; let self_ev_bind = EvBind self_dict $ + EvDFunApp dfun_id (mkTyVarTys tyvars) dfun_ev_vars + ; (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars inst_tys sel_id ; dm_id <- tcLookupId dm_name ; let dm_inline_prag = idInlinePragma dm_id - EvBind self_dict _ = self_dict_ev rhs = HsWrap (mkWpEvVarApps [self_dict] <.> mkWpTyApps inst_tys) $ HsVar dm_id meth_bind = L loc $ VarBind { var_id = local_meth_id , var_rhs = L loc rhs - , var_inline = False } + , var_inline = False } meth_id1 = meth_id `setInlinePragma` dm_inline_prag -- Copy the inline pragma (if any) from the default -- method to this version. Note [INLINE and default methods] bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars , abs_exports = [( tyvars, meth_id1, local_meth_id - , SpecPrags spec_inst_prags)] - , abs_ev_binds = EvBinds (unitBag self_dict_ev) + , mk_meth_spec_prags meth_id1 [])] + , abs_ev_binds = EvBinds (unitBag self_ev_bind) , abs_binds = unitBag meth_bind } -- Default methods in an instance declaration can't have their own -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but @@ -906,6 +1156,18 @@ tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys ; return (meth_id1, L loc bind) } ---------------------- + mk_meth_spec_prags :: Id -> [LTcSpecPrag] -> TcSpecPrags + -- Adapt the SPECIALISE pragmas to work for this method Id + -- There are two sources: + -- * spec_inst_prags: {-# SPECIALISE instance :: #-} + -- These ones have the dfun inside, but [perhaps surprisingly] + -- the correct wrapper + -- * spec_prags_for_me: {-# SPECIALISE op :: #-} + mk_meth_spec_prags meth_id spec_prags_for_me + = SpecPrags (spec_prags_for_me ++ + [ L loc (SpecPrag meth_id wrap inl) + | L loc (SpecPrag _ wrap inl) <- spec_inst_prags]) + loc = getSrcSpan dfun_id meth_sig_fn _ = Just ([],loc) -- The 'Just' says "yes, there's a type sig" -- But there are no scoped type variables from local_method_id @@ -915,22 +1177,16 @@ tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys -- instance C [c] where { op = } -- In , 'c' is scope but 'b' is not! - mb_dict_ev = if null tyvars then Nothing else Just self_dict_ev - -- Only need the self_dict stuff if there are type - -- variables involved; otherwise overlap is not possible - -- See Note [Subtle interaction of recursion and overlap] - -- in TcInstDcls - - -- For instance decls that come from standalone deriving clauses + -- For instance decls that come from standalone deriving clauses -- we want to print out the full source code if there's an error -- because otherwise the user won't see the code at all - add_meth_ctxt generated_code rn_bind thing - | generated_code = addLandmarkErrCtxt (derivBindCtxt clas inst_tys rn_bind) thing + add_meth_ctxt sel_id generated_code rn_bind thing + | generated_code = addLandmarkErrCtxt (derivBindCtxt sel_id clas inst_tys rn_bind) thing | otherwise = thing tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys - _ _ op_items (NewTypeDerived coi _) + _ op_items (NewTypeDerived coi _) -- Running example: -- class Show b => Foo a b where @@ -958,7 +1214,7 @@ tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys -- by the constraint solver, since the may be -- user-specified. - = do { rep_d_stuff <- checkConstraints InstSkol emptyVarSet tyvars dfun_ev_vars $ + = do { rep_d_stuff <- checkConstraints InstSkol tyvars dfun_ev_vars $ emitWanted ScOrigin rep_pred ; mapAndUnzipM (tc_item rep_d_stuff) op_items } @@ -967,13 +1223,12 @@ tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys inst_tvs = fst (tcSplitForAllTys (idType dfun_id)) Just (init_inst_tys, _) = snocView inst_tys - rep_ty = fst (coercionKind co) -- [p] + rep_ty = pFst (coercionKind co) -- [p] rep_pred = mkClassPred clas (init_inst_tys ++ [rep_ty]) -- co : [p] ~ T p - co = substTyWith inst_tvs (mkTyVarTys tyvars) $ - case coi of { IdCo ty -> ty ; - ACo co -> mkSymCoercion co } + co = substCoWithTys inst_tvs (mkTyVarTys tyvars) $ + mkSymCo coi ---------------- tc_item :: (TcEvBinds, EvVar) -> (Id, DefMeth) -> TcM (TcId, LHsBind TcId) @@ -997,7 +1252,8 @@ tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys ---------------- mk_op_wrapper :: Id -> EvVar -> HsWrapper mk_op_wrapper sel_id rep_d - = WpCast (substTyWith sel_tvs (init_inst_tys ++ [co]) local_meth_ty) + = WpCast (liftCoSubstWith sel_tvs (map mkReflCo init_inst_tys ++ [co]) + local_meth_ty) <.> WpEvApp (EvId rep_d) <.> mkWpTyApps (init_inst_tys ++ [rep_ty]) where @@ -1026,11 +1282,15 @@ mkMethIds clas tyvars dfun_ev_vars inst_tys sel_id wrapId :: HsWrapper -> id -> HsExpr id wrapId wrapper id = mkHsWrap wrapper (HsVar id) -derivBindCtxt :: Class -> [Type ] -> LHsBind Name -> SDoc -derivBindCtxt clas tys bind - = vcat [ ptext (sLit "When typechecking a standalone-derived method for") - <+> quotes (pprClassPred clas tys) <> colon - , nest 2 $ pprSetDepth AllTheWay $ ppr bind ] +derivBindCtxt :: Id -> Class -> [Type ] -> LHsBind Name -> SDoc +derivBindCtxt sel_id clas tys _bind + = vcat [ ptext (sLit "When typechecking the code for ") <+> quotes (ppr sel_id) + , nest 2 (ptext (sLit "in a standalone derived instance for") + <+> quotes (pprClassPred clas tys) <> colon) + , nest 2 $ ptext (sLit "To see the code I am typechecking, use -ddump-deriv") ] + +-- Too voluminous +-- , nest 2 $ pprSetDepth AllTheWay $ ppr bind ] warnMissingMethod :: Id -> TcM () warnMissingMethod sel_id @@ -1161,7 +1421,7 @@ instDeclCtxt2 :: Type -> SDoc instDeclCtxt2 dfun_ty = inst_decl_ctxt (ppr (mkClassPred cls tys)) where - (_,cls,tys) = tcSplitDFunTy dfun_ty + (_,_,cls,tys) = tcSplitDFunTy dfun_ty inst_decl_ctxt :: SDoc -> SDoc inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc @@ -1183,4 +1443,37 @@ wrongATArgErr ty instTy = , ptext (sLit "Found") <+> quotes (ppr ty) <+> ptext (sLit "but expected") <+> quotes (ppr instTy) ] + +tooManyParmsErr :: Located Name -> SDoc +tooManyParmsErr tc_name + = ptext (sLit "Family instance has too many parameters:") <+> + quotes (ppr tc_name) + +tooFewParmsErr :: Arity -> SDoc +tooFewParmsErr arity + = ptext (sLit "Family instance has too few parameters; expected") <+> + ppr arity + +wrongNumberOfParmsErr :: Arity -> SDoc +wrongNumberOfParmsErr exp_arity + = ptext (sLit "Number of parameters must match family declaration; expected") + <+> ppr exp_arity + +badBootFamInstDeclErr :: SDoc +badBootFamInstDeclErr + = ptext (sLit "Illegal family instance in hs-boot file") + +notFamily :: TyCon -> SDoc +notFamily tycon + = vcat [ ptext (sLit "Illegal family instance for") <+> quotes (ppr tycon) + , nest 2 $ parens (ppr tycon <+> ptext (sLit "is not an indexed type family"))] + +wrongKindOfFamily :: TyCon -> SDoc +wrongKindOfFamily family + = ptext (sLit "Wrong category of family instance; declaration was for a") + <+> kindOfFamily + where + kindOfFamily | isSynTyCon family = ptext (sLit "type synonym") + | isAlgTyCon family = ptext (sLit "data type") + | otherwise = pprPanic "wrongKindOfFamily" (ppr family) \end{code}