This is the core functionality from https://github.com/bluegraybox/examples/blob/master/indent-parser/parse-indents.lisp

parse-indents.lisp

(defun parse-text-line (line)
  ;; converts "    text" to (:content "text" :indent 4)
  (let ((content (string-left-trim '(#\Space #\Tab) line)))
    (list :content content :indent (- (length line) (length content)))))

(defun split-list (criterion data-list)
  ;; break a list into two lists; the second begins with the first element that matches the criterion.
  (if data-list
      (if (funcall criterion (first data-list))
          (list () data-list)
          (let ((me (pop data-list)))
            (let ((chunks (split-list criterion data-list)))
              (push me (first chunks))
              chunks)))
      (list () ())))

(defun build-nodes (lines)
  (if lines
      (let* ((me (pop lines))
             ;; split off our children from the rest of the lines.
             ;; the first line with an indent <= ours is a sibling, not a child
             (chunks (split-list (lambda (x) (<= (getf x :indent) (getf me :indent))) lines)))
        (let ((children (build-nodes (pop chunks)))
              (siblings (build-nodes (pop chunks))))
          (let ((node (list :content (getf me :content))))
            (if children (setf (getf node :children) children))
            ;; (format t "~a~%" (getf node :content))
            (push node siblings))))
      nil))

Here's the full code for the Lisp version of the indent parser. There's also a sequential Erlang version and a concurrent Erlang version.