Simon Marlow
Consider that at the moment, an import declaration can be considered completely independently of the rest of the module. Imports do not affect each other. Under your proposal, all imports have to be considered together to determine whether any are ambiguous. We can't do Hugs-style one-at-a-time import chasing, for example.
Ah, now I see what you are getting at. Program P imports modules M and N. How do the compiler know where M is located? It must presume the default namespace. But then later, module N might re-export a different namespace which means that M is now ambiguous. Conversely, let's say M is not available in the default namespace. So we must wait until module N has been imported, to see if it brings a namespace into scope that does contain a module M. Very nasty.
I accept that the GhcPackages proposal has a serious shortcoming that your proposal addresses, namely that there's no way to name a package in just one place if you're using package-qualified imports. I don't yet know of a good way to fix this, but I'm fairly sure that generalising the module system with namespaces is not a good power/weight tradeoff.
OK, I accept that my proposal for first-class namespaces is rather too heavy a solution. (But I still think the problem is a real one, and we should keep thinking about other ways to solve it.) Regards, Malcolm