On Mon, May 21, 2007 at 12:39:20PM +0100, Simon Peyton-Jones wrote:
| > | > instance G a b where | > | > data E a = EC b -- this line - the b is not in scope. | > | > wrap = EC | > | > unwrap (EC b) = b | > | | > | I get "Not in scope: type variable `b'". | > | > That's a bug. b should be in scope
I was wrong. It's not a bug. E is supposed to be a type *function*, so it can't mention anything on the RHS that is not bound on the LHS. We'll improve the documentation.
Ok, thanks for the clarification. One final question then, how could I rewrite the following in associated types: class OneStep a b | a -> b instance OneStep (Cons v t) t class TwoStep a b | a -> b instance (OneStep a b, OneStep b c) => TwoStep a c If the fundep and b is dropped then I get: class OneStep a data OS a :: * instance OneStep (Cons v t) data OS (Cons v t) = t class TwoStep a data TS a :: * instance (OneStep a, OneStep b) => TwoStep a This last one can't be right, as I can't express the fact that the OS in "OneStep a" provides the link with "OneStep b". So is there a way to achieve this sort of chaining with associated types? Sure, the fundep version requires undecidable instances, but I've been writing quite a lot of code lately that requires that! Many thanks, Matthew -- Matthew Sackman http://www.wellquite.org/