Weasel code in Common Lisp

weasel.lisp

(defconstant target "ME THINKS IT IS LIKE A WEASEL")

(defun rand-letter ()
  (let ((r (random 27)))
    (if (= r 26)
	#\space
	(code-char (+ (char-code #\A) r)))))

(defun rand-string (n)
  (coerce (loop for i from 1 to n
		collect (rand-letter))
	  'string))

(defun chance-mutate (s)
  (let ((l (coerce s 'list)))
    (coerce
     (loop for i in l
	   collect (let ((r (random 100)))     
		     (if (and (>= r 42)
			      (<= r 52))
			 (rand-letter)
			 i)))
     'string)))

(defun create-population (parent n)
  (loop for i from 1 to n
	collect (chance-mutate parent)))

(defun score (s target-s)
  (reduce #'+
	  (loop for i in (coerce s 'list)
		for j in (coerce target-s 'list)
		collect (abs (- (char-code i)
				(char-code j))))
	  :start 0))

(defun find-survivor (pop target-s)
  (second
   (first
    (sort
     (loop for i in pop
	   collect (list
		    (score i target-s)
		    i))
     #'(lambda (p q) (< (first p) (first q)))))))

(defun generations (start
		    stop-n
		    target-s
		    &key (population 1000))
  (do ((p start (find-survivor
		 (create-population p population)
		 target-s))
       (i 1 (+ i 1)))
      ((or (> i stop-n)
	   (string= p target-s))
       (return p))
    (format t
	    "Gen ~A - ~A~%"
	    i
	    p)))

(defun weasels (target-s &key (n 200))
  (generations (rand-string (length target-s))
	       n
	       target-s))