Well it might not be ambiguous. Consider instance C Int b where... Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] | On Behalf Of Ross Paterson | Sent: 11 December 2006 12:41 | To: glasgow-haskell-users@haskell.org | Subject: Re: [Haskell] GHC Error question | | On Mon, Dec 11, 2006 at 12:16:06PM +0000, Simon Peyton-Jones wrote: | > [...] Just to summarise, the difficulty is this: | > I have a dictionary of type (C a b1) | > I need a dictionary of type (C a b2) | > There is no functional dependency between C's parameters | > | > PS: the complete program is this: | > class C a b where | > op :: a -> a | > | > f :: C a b => a -> a | > f x = op x | | That raises a point I'd wondered about. GHC requires only that each | type variable in the context be reachable from the type via a chain | of assertions: | | http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html... | | What's the rationale for that, rather than calling types like the above | type of f ambiguous? | | The examples given in the User's Guide involve functional dependencies, | albeit obscured by superclasses | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users