A calculator using reverse polish notation, written in Common Lisp

reverse-polish-calc.lisp

#!/usr/bin/sbcl --script

(loop for n = (read t)
      for stack = (list n) then (cons n stack)
      until (member n '(exit quit))
      do (unless (numberp n)
           (cond
             ((endp (nthcdr 2 stack))
              (format t "Not enough numbers in stack~%")
              (pop stack))
             (t
              (let ((fun (pop stack))
                    (op2 (pop stack))
                    (op1 (pop stack)))
                (push (funcall fun op1 op2) stack)))))
         (format t "(~{~A~^ ~})~%" (reverse stack)))

Using "do" instead of "loop":

#!/usr/bin/sbcl --script

(do ((stack '())
     (n (read t) (read t)))
    ((member n '(exit quit)) nil)
  (push n stack)
  (unless (numberp n)
    (cond ((endp (nthcdr 2 stack))
           (format t "Not enough numbers in stack~%")
           (pop stack))
          (t
           (let ((fun (pop stack))
                 (op2 (pop stack))
                 (op1 (pop stack)))
             (push (funcall fun op1 op2) stack)))))
  (format t "(~{~A~^ ~})~%" (reverse stack)))