Common Lisp function that converts a list into a string of concatenated characters.

scratch.lisp

(defun numlist-to-string (lst)
  (when lst
    (concatenate 'string 
                 (write-to-string (car lst)) (numlist-to-string (cdr lst)))))
 
; Convert a list of numbers into a number
; (list 1 2 3 4) => 1234
(parse-integer (numlist-to-string (list 1 2 3 4)))

This should be the standard way:

(coerce some-list-of-chars 'string)