It's intentional. Consider this data type declaration data B b = MkT (A a b) Should that be accepted? The only thing it could possibly mean would be data B b = MkT (forall a. A a b) and I suppose that might possibly be useful. But in this case you're also saying that "a" determines "b". But you can hardly say *forall* a, if the choice of a determines the (fixed by context) b. These functional dependencies are tricky things. Martin Sulzmann and I wrote a paper about some of the trickiness (http://research.microsoft.com/%7Esimonpj/Papers/fd-chr/index.htm), and we're working on a journal version. My wild guess is that you are trying to convert an O-O idiom into Haskell, and that's led you to a rather strange program. Maybe there's another way to do it. Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Frank | Sent: 08 August 2005 18:17 | To: haskell-cafe@haskell.org | Subject: [Haskell-cafe] Functional dependenices in class declarations | | In ghc 6.4 (fine work - many gratulations to all who did it!) it is | permitted | to have type parameters which are reachable (7.4.3.1 context of type | signatures). | I tried the following: | | module TypeDependencies where | | class A a b | a -> b | | class (A a b) => B b where | op :: a -> a | | x :: A a b => a -> a | x = id | | Which compiles fine for the function x, but does complain | "not in scope: type variable 'a' " for the class (A a b) => B b. | | Is this an intentional restriction that 'reachable' type variables are only | permitted in type definitions, but not in classes? or do I misunderstand | something? | | Any help appreciated! | Andrew Frank | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe