[Evbn]

It scales in the sense that all types are extensible: you can make A implement B by adding the appropriate definitions anywhere, not just in the file that defines B. Rubyists call this monkey patching and they love it (but Ruby monkey patching can do wackier things beyond adding new methods)

[quatrevingts]

But I can only make A implement B in the file that defines A, at least according to the comments here:

https://news.ycombinator.com/item?id=5443344