You know, if you used a input-port instead of a string, then next-char and bump-j would just be peekc and readc. I don't know if that would make you're life much easier, but it's an idea.
Right, a port encapsulates some state (where you're reading from, what your position is in the input), and then Arc uses a Scheme parameter (stdin) so that the input port doesn't need to be passed to every function that calls some function that calls some function that calls read or readc. So like you say I could use an input-port if that did what I needed, or I could implement my own thing if that turned out to be necessary.
