Is there any other language that implements this? I'll freely admit that I'm no expert on Lisp, so I've tried digging through the code, and I sorta get what the concept is, but I don't really understand it. Any other languages have the same idea?
Dylan does, and my language Slate (http://www.slatelanguage.org) does, as well as its related younger siblings, Atomo (http://atomo-lang.org) and Atomy (http://atomy-lang.org). And all of these implement them in the context of an object model, rather than carefully avoiding CLOS dependencies as in CL.
I just added conditions & restarts today to Fancy (http://fancy-lang.org/). It's currently in a branch but it works =).
Thanks for doing that. More implementations means there's a wider spectrum of places for developers to learn these tools and maybe even improve on them.
I believe Mesa did. On the other hand, Stroustrup claims (in the Design and Evolution of C++, I think) that the Cedar system written in Mesa basically never used the ability to restart as a reason why C++ doesn't have continuable exceptions. I've never been able to track down more information about that than his claim though.