cl:reduce で文字列として join する。

join.lisp

(defun join (lis &optional (sep " "))
  (reduce #'(lambda (x y)
	      (concatenate 'string
                           (princ-to-string x) sep (princ-to-string y)))
	  lis))

#|
CL-USER> (defun join (lis &optional (sep " "))
  (reduce #'(lambda (x y)
	      (concatenate 'string
                           (princ-to-string x) sep (princ-to-string y)))
	  lis))

JOIN
CL-USER> (join '(a b c d) "===")
"A===B===C===D"
CL-USER> (join '(a b c d) "=")
"A=B=C=D"
CL-USER> (join '(1 2 3 4) ",")
"1,2,3,4"
CL-USER> 
|#