Function: JOIN-STRINGS

Source

(defun join-strings (list)
  (let* ((length (reduce #'+ list :key #'length))
         (result (make-string length)))
    (loop
       for string in list
       for start = 0 then end
       for end = (+ start (length string))
       while string
       do (replace result string :start1 start :end1 end)
       finally (return result))))