I don't get the impression they're saying, "we release this new library and all the users of the existing libraries automatically switch to it". I think what they're saying is, "this library can be used by existing applications, but they still need to choose to use it". That's how I interpreted the "drop-in replacement" phrase.

Technically speaking Nulan can support that... you would change library1 and library2 to just import library3:

  # library1
  (import library3)
  
  # library2
  (import library3)
  
  # library3
  (def foo ...)
  (def bar ...)

---

"I think the only place we really disagree now is that you're content to neglect the "RubyGem" parts of these challenges for the purposes this discussion, while I consider them the hardest parts. I think if you do try to design a package manager, you may very well get most of a namespace system out of the deal, or even a whole programming language."

They are the hardest parts, I just don't think it has to do with namespace support, which that entire topic was devoted to. Nor am I particularly interested in designing a package manager.

So my suggestion still stands: if you feel this strongly about package managers, make up your own semi-contrived challenges that try to demonstrate what a package manager should be capable of doing. I'm sure it will include all kinds of stuff that was left out of the namespace challenges, because package managers are much more complex and involved than namespaces.

Then you can point to those challenges and I can say, "Nulan doesn't support that stuff" rather than trying to insist that Nulan is failing the namespace challenges because you're interpreting them far larger than what they actually said. I think it's useful to have "package manager challenges", but I think it's also useful to have "namespace challenges". I don't see why we need to throw out the namespace challenges just because you view it as "not as hard" as package managers.

---

"The challenge we're talking about puts the words "different, incompatible versions" in italics. If direct substitution were unimportant, the challenge would not even need to use the word "version"; it could just say there were two separate libraries with name collisions. And yet that's precisely the scenario of the previous challenge."

Like I said, I don't get the impression they're talking about package managers. I think they used the phrase "different incompatible versions" because that's a situation that can actually occur in real life. They're using it as an example, that's all.