On Mon, 3 Dec 2007, Yitzchak Gale wrote:
David Benbennick wrote:
I propose to add a Bounded instance to IntSet.hs.
I am opposed to this proposal, unless someone comes up with some important use cases.
Unfortunately, there is still no way to control export of instances.
You can put instances into a separate module (and get GHC warnings about that :-) But it is not sensible to have different instances for the same type and class, because they will collide sooner or later.
So libraries should avoid defining instances unless there is a compelling reason to do so. We need to be reasonably certain that the usefulness of the instance will overwhelm any unforeseen namespace pollution problems that it may cause.
I think there is no much sense in defining instances privately in code that uses a class definition from a library, because the custom instance in turn may break other modules. In the past it happened for me at each GHC upgrade, that instances that I defined privately (like Show for FiniteMap) collide with new instances defined in the imported standard module.