Yeah, turns out it was pretty darn easy once I understood what the hell I was doing. First I tried grafting it onto Arubic/Python, which did work, but it was klunky and incomplete.
Then I tried writing an Arubic interpreter in Arc. Then I tried extending the ar compiler to turn it into Arubic... and then I finally realized that I could just... write it as a library in Arc, which only took me a few hours and works far better than my other attempts. :P
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Moral of the story: Arc is good. Arc is very good. I guess I'm still too used to other languages, where if you wanted to do something like add in classes/inheritance/closures/lambdas/whatever, you'd basically need to create a new language, and write a new interpreter/compiler for it. But Arc is flexible enough that you can do a lot with it, and ar makes Arc a hecka lot more flexible than it already was.
(ar still needs something like defcall, though, because it's just so useful when creating new data types, but don't worry, I'll work on getting defcall into ar)