gistfile1.lisp

;; returns t if the number is prime, nil otherwise                                 
(defun is-prime (n prev-primes)
  (not (find-if
        (lambda (p)
          (eq (mod n p) 0))
        prev-primes)))

;; returns all primes between start and end - 1 inclusive                          
;; prev-primes should be a list of primes before start, if start > 2               
(defun primes-between (start end &optional prev-primes)
  ;; check for the end condition first                                             
  (if (eq start end)
      ;; this is it, return the primes found so far                                
      prev-primes
    (progn
      ;; check if start is prime                                                   
      (if (is-prime start prev-primes)
          ;; it is! so push it onto previous primes                                
          (push start prev-primes))
      ;; recurse                                                                   
      (primes-between (+ 1 start) end prev-primes))))

;; test -- all primes between 2 and 100                                            
(format 't "~a"
        (primes-between 2 100))