X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcDeriv.lhs;h=5053a0668e0527078adedf763f4c793d0db60c34;hb=1fab5eeaf45798ee7832497d6518883be451bfca;hp=44ea1fc4caa2e69adc898878b2f8ac117f867220;hpb=5f8d93baa07271687825458e01c187081bcb1ddc;p=ghc-hetmet.git diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs index 44ea1fc..5053a06 100644 --- a/compiler/typecheck/TcDeriv.lhs +++ b/compiler/typecheck/TcDeriv.lhs @@ -459,19 +459,34 @@ deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name, (arg_kinds, _) = splitKindFunTys kind n_args_to_drop = length arg_kinds n_args_to_keep = tyConArity tc - n_args_to_drop - inst_ty = mkTyConApp tc (take n_args_to_keep tc_args) - inst_ty_kind = typeKind inst_ty - + args_to_drop = drop n_args_to_keep tc_args + inst_ty = mkTyConApp tc (take n_args_to_keep tc_args) + inst_ty_kind = typeKind inst_ty + dropped_tvs = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop) + univ_tvs = (mkVarSet tvs `extendVarSetList` deriv_tvs) + `minusVarSet` dropped_tvs + -- Check that the result really is well-kinded ; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind)) (derivingKindErr tc cls cls_tys kind) + ; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && -- (a) + tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b) + (derivingEtaErr cls cls_tys inst_ty) + -- Check that + -- (a) The data type can be eta-reduced; eg reject: + -- data instance T a a = ... deriving( Monad ) + -- (b) The type class args do not mention any of the dropped type + -- variables + -- newtype T a s = ... deriving( ST s ) + -- Type families can't be partially applied - -- e.g. newtype instance T Int a = ... deriving( Monad ) + -- e.g. newtype instance T Int a = MkT [a] deriving( Monad ) + -- Note [Deriving, type families, and partial applications] ; checkTc (not (isOpenTyCon tc) || n_args_to_drop == 0) (typeFamilyPapErr tc cls cls_tys inst_ty) - ; mkEqnHelp DerivOrigin (tvs++deriv_tvs) cls cls_tys inst_ty Nothing } } + ; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } } where -- Tiresomely we must figure out the "lhs", which is awkward for type families -- E.g. data T a b = .. deriving( Eq ) @@ -490,8 +505,37 @@ deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name, deriveTyData _other = panic "derivTyData" -- Caller ensures that only TyData can happen +\end{code} ------------------------------------------------------------------- +Note [Deriving, type families, and partial applications] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +When there are no type families, it's quite easy: + + newtype S a = MkS [a] + -- :CoS :: S ~ [] -- Eta-reduced + + instance Eq [a] => Eq (S a) -- by coercion sym (Eq (coMkS a)) : Eq [a] ~ Eq (S a) + instance Monad [] => Monad S -- by coercion sym (Monad coMkS) : Monad [] ~ Monad S + +When type familes are involved it's trickier: + + data family T a b + newtype instance T Int a = MkT [a] deriving( Eq, Monad ) + -- :RT is the representation type for (T Int a) + -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced + -- :Co:R1T :: :RT ~ [] -- Eta-reduced + + instance Eq [a] => Eq (T Int a) -- easy by coercion + instance Monad [] => Monad (T Int) -- only if we can eta reduce??? + +The "???" bit is that we don't build the :CoF thing in eta-reduced form +Henc the current typeFamilyPapErr, even though the instance makes sense. +After all, we can write it out + instance Monad [] => Monad (T Int) -- only if we can eta reduce??? + return x = MkT [x] + ... etc ... + +\begin{code} mkEqnHelp :: InstOrigin -> [TyVar] -> Class -> [Type] -> Type -> Maybe ThetaType -- Just => context supplied (standalone deriving) -- Nothing => context inferred (deriving on data decl) @@ -673,19 +717,17 @@ mk_typeable_eqn orig tvs cls tycon tc_args rep_tc _rep_tc_args mtheta -- family tycon (with indexes) in error messages. data DerivStatus = CanDerive - | NonDerivableClass - | DerivableClassError SDoc + | DerivableClassError SDoc -- Standard class, but can't do it + | NonDerivableClass -- Non-standard class checkSideConditions :: Bool -> Class -> [TcType] -> TyCon -> DerivStatus checkSideConditions mayDeriveDataTypeable cls cls_tys rep_tc - | notNull cls_tys - = DerivableClassError ty_args_why -- e.g. deriving( Foo s ) - | otherwise - = case sideConditions cls of - Nothing -> NonDerivableClass - Just cond -> case (cond (mayDeriveDataTypeable, rep_tc)) of - Nothing -> CanDerive - Just err -> DerivableClassError err + | Just cond <- sideConditions cls + = case (cond (mayDeriveDataTypeable, rep_tc)) of + Just err -> DerivableClassError err -- Class-specific error + Nothing | null cls_tys -> CanDerive + | otherwise -> DerivableClassError ty_args_why -- e.g. deriving( Eq s ) + | otherwise = NonDerivableClass -- Not a standard class where ty_args_why = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class") @@ -850,6 +892,7 @@ mkNewTypeEqn :: InstOrigin -> Bool -> Bool -> [Var] -> Class -> TcRn EarlyDerivSpec mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving tvs cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta +-- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ... | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls) = do { traceTc (text "newtype deriving:" <+> ppr tycon <+> ppr rep_tys) ; dfun_name <- new_dfun_name cls tycon @@ -934,7 +977,7 @@ mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving tvs -- See Note [Newtype deriving superclasses] above cls_tyvars = classTyVars cls - dfun_tvs = tyVarsOfTypes tc_args + dfun_tvs = tyVarsOfTypes inst_tys inst_ty = mkTyConApp tycon tc_args inst_tys = cls_tys ++ [inst_ty] sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys) @@ -977,19 +1020,14 @@ mkNewTypeEqn orig mayDeriveDataTypeable newtype_deriving tvs -- recursive newtypes too -- Check that eta reduction is OK - eta_ok = (nt_eta_arity <= length rep_tc_args) - -- (a) the newtype can be eta-reduced to match the number + eta_ok = nt_eta_arity <= length rep_tc_args + -- The newtype can be eta-reduced to match the number -- of type argument actually supplied -- newtype T a b = MkT (S [a] b) deriving( Monad ) -- Here the 'b' must be the same in the rep type (S [a] b) -- And the [a] must not mention 'b'. That's all handled -- by nt_eta_rity. - && (tyVarsOfTypes cls_tys `subVarSet` dfun_tvs) - -- (c) the type class args do not mention any of the dropped type - -- variables - -- newtype T a b = ... deriving( Monad b ) - cant_derive_err = vcat [ptext (sLit "even with cunning newtype deriving:"), if isRecursiveTyCon tycon then ptext (sLit "the newtype may be recursive") @@ -1246,6 +1284,12 @@ derivingKindErr tc cls cls_tys cls_kind 2 (ptext (sLit "Class") <+> quotes (ppr cls) <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind)) +derivingEtaErr :: Class -> [Type] -> Type -> Message +derivingEtaErr cls cls_tys inst_ty + = sep [ptext (sLit "Cannot eta-reduce to an instance of form"), + nest 2 (ptext (sLit "instance (...) =>") + <+> pprClassPred cls (cls_tys ++ [inst_ty]))] + typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message typeFamilyPapErr tc cls cls_tys inst_ty = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty]))) @@ -1270,7 +1314,9 @@ standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for" derivInstCtxt :: Class -> [Type] -> Message derivInstCtxt clas inst_tys - = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys) + = vcat [ptext (sLit "Alternative fix: use a standalone 'deriving instance' declaration"), + nest 2 (ptext (sLit "instead, so you can specify the instance context yourself")), + ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)] badDerivedPred :: PredType -> Message badDerivedPred pred