PS: You can use deriving( Typeable )even when there's an existential. It's just deriving( Data ) you have to do by hand. | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Akos Korosmezey | Sent: 21 April 2004 13:23 | To: glasgow-haskell-users@haskell.org | Subject: Typeable and 'forall' in data constructors | | I am tying to write a Term class with function application: | | data (Typeable a) => Term a = | Const a | | LVar Int | | forall b. Typeable b => App (Term (b -> a)) (Term b) | | Lam (Term a) | | Because 'forall' is present, ghc refuses to derive Typeable and Data for | Term. I tried to implement them: | | instance (Typeable a) => Typeable (Term a) where | typeOf w = mkAppTy (mkTyCon "Term.Term") [typeOf (undefined :: a)] | | instance (Typeable a) => Data (Term a) where | toConstr (Const _) = mkConstr 1 "Const" Prefix | toConstr (LVar _) = mkConstr 3 "LVar" Prefix | toConstr (App _ _) = mkConstr 4 "App" Prefix | toConstr (Lam _) = mkConstr 5 "Lam" Prefix | | But ghc 6.2.1 returns with error on the line 'toConstr (App _ _)...': | parse error on input `b'. How can this be fixed? | Thank you | | Akos Korosmezey | | | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users