exporting instances: was Using associated data types to create unpacked data structures

re ...

As I understand it the generated code is not exported from the translation unit so there are no collisions at link time. We could do the same if we could force the generated type class instance to not be exported from the module.

I have encountered several occasions when I wished control over what instances get imported or exported from a module. I have wondered and perhaps someone can explain: what are the issues in explicit control of instance export and import? (apart from defining an appropriate syntax) John