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)))

I know it's old, but I guess the "better" version would be:

(defun list-to-string (lst)
    (format nil "~{~A~}" lst))

This ensures that the function always returns the string (although sometimes it might be empty), and works for arbitrary things to concatenate them.

I guess it would also be faster, since there would be no recursion.

This should be the standard way:

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