minischeme[0] has a pretty good parser built into the SECD machine.

I've been hacking on it here and there and almost have it r4rs compliant -- replaced the lexer with re2c and was going to do the parser in lemon but the design they already have is just too simple to mess with.

[0]https://github.com/catseye/minischeme --note: the bog stock original is floating around on the internets if you google hard enough.