1) pg is rumored to have actually implemented Arc several times, on various Lisps. I would assume he went for the more portable approach, one which would work on both CL and Scheme.
2) There "should be" no real difference between continuations and closures anyway. For example, in arc2c, everything is converted to CPS anyway.
The portability thing was my initial thought on why he chose to eschew first class continuations for a closure-based CPS architecture. For some reason I decided to read over his "Lisp in Web Applications" essay again and I noticed that the mechanism he was using in Arc was described in perfect detail in his essay, and that got me thinking--was he just using this method because that's what's been in Arc all along? Are there other advantages outside of portability to different implementation languages that can be derived from using closures vs native continuations? Now that Arc is finally available for human consumption and running on mzscheme is there any barrier to porting the srv.arc application to a native continuations-based architecture?
These are just the thoughts I have wondering around in my head as I read through the Arc source. I think Arc is valuable for two reasons: 1) as a language to get things done, 2) as a crash course in language and library design. The second attribute of Arc is the one I find most interesting as it's rare that we get to look into the design of a language and its libraries at such an early and still uncluttered state. Add to that the fact that it's written in Scheme and you have a perfect case study for stimulating conversation.
Anyway, just my thoughts. I appreciate the response, and I'm hoping to get a few more, perhaps even one from Paul himself.
