"I started Jarc by writing an s-expression reader. Then I implemented the 9 Arc primitives in Java and then I started writing eval in Java. Then I incrementally added the missing global functions used in arc.arc - that took a while, but wasn't particularly difficult."
That's how I planned to start the implementation, but I've also considered writing a compiler that translates the arc code into MSIL using Reflection.Emit. That would allow me to do tail-call optimization, and possibly CPS.
So what do you have currently for your Arc/Java interface, and what are your thoughts on difficulty of implementation, ease of use, etc. associated with that choice? Are there other options that you considered and discarded, or haven't gotten around to implementing yet? I was thinking of trying to treat assemblies, namespaces, classes, and objects like hash tables, and letting (obj 'name) access the member, but I don't know how well that will work with mostly static typing.
I'm not sure what to think about fractional numbers. I don't think I've ever had a good reason for using them, so I won't bother implementing them myself until I do.