Jim You've found a strange corner case. Here's a cut-down example, which fails in exactly the same way. class Baz v x where foo :: x -> x -- Notice that v is not mentioned foo y = y instance Baz Int Int But it succeeds if you put the instance explicitly: instance Baz Int Int where foo y = y GHC does not actually macro-expand the instance decl. Instead, it defines a default method function, thus $dmfoo :: Baz v x => x -> x $dmfoo y = y Notice that this is an ambiguous type: you can't call $dmfoo without triggering an error. And when you write an instance decl, it calls the default method: instance Baz Int Int where foo = $dmfoo I'd never thought of that. You might think that we should just *infer* the type of the default method (here forall a. a->a), but in the presence of higher rank types etc we can't necessarily do that. So I'm not sure what to do. I've added it as an expected-failure test (tc199). I have not added a SourceForge bug because the original program seems so implausible. Meanwhile, the workaround is to define your own default method, thus: dmfoo :: x -> x dmfoo y = y instance Baz Int Int where foo = dmfoo Simom | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Jim Apple | Sent: 04 December 2005 17:24 | To: glasgow-haskell-users@haskell.org | Subject: Data.Typeable and default instances | | > {-# OPTIONS -fglasgow-exts #-} | > | > import Maybe | > import Data.Typeable | > | > data Nil = Nil deriving (Eq,Typeable,Show) | > | > class (Typeable t) => List a t where | > init :: (t -> b) -> (forall y . (List a y) => y -> b) | > init f z = fromJust $ do x <- cast z | > return $ f x | > | > instance List a Nil where | | Could not deduce (List a1 y) | from the context (List a Nil, Typeable Nil, List a y) | arising from use of `Main.$dminit' at Main.hs:21:0 | Probable fix: add (List a1 y) to the class or instance method | `Main.init' | In the definition of `init': init = Main.$dminit | In the definition for method `Main.init' | In the instance declaration for `List a Nil' | | but copying and pasting the code from init to the instance declaration | works fine. | | Jim | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users