You'll need car, cdr, cons, atom, and eq. You'll also need ratom and patom for input and output. That's 7 primitive functions. To be able to define recursi

You'll need car, cdr, cons, atom, and eq. You'll also need ratom and patom for input and output. That's 7 primitive functions. To be able to define recursive functions, you'll also need cond, lambda, setq (or define), quote, and function. That's five special forms. Then you have to write read, eval, apply, and print in your implementation language. That gives you a minimal interpreter. These four functions aren't primitives, but you have to write them and you might as well expose them to the user. Eval and apply call a few other functions which are generally useful, such as assoc, evlis and pairlis. This excludes arithmetic functions and a whole lot of other stuff you'd need for your interpreter to be useful.