On Wed, Nov 01, 2006 at 03:15:38PM -0600, Brian Smith wrote:
How an instance is defined (explicitly or derived) should have nothing to do with how it is imported/exported in a module.
In particular, I think having features like : import M1 hiding (instance C T) and module M hiding (instance C T) would eliminate the need for special-case handling of derived instances (if two imported modules happen to derive the same instances, you can just hide the instances from one of them). Instance hiding is an important feature in its own right.
selective importing and exporting of instances is a really tricky technical issue for a lot of reasons, and a bad idea for many others. For a quick example, imagine a Set created with some operations using one Ord instance and other operations using another. the globalness of instance declarations is a great tool. Fortunately, newtype deriving (hopefully extended as I have mentioned on this thread) and rank-n polymorphism make it pretty much unneeded. John -- John Meacham - ⑆repetae.net⑆john⑈