(defun validate-post (title title-supplied-p
                      path path-supplied-p)
  (let ((errors '()))
    (when title-supplied-p
      (unless (post-title-p title)
        (setf errors (acons 'title "Invalid title" errors))))
    (when path-supplied-p
      (unless (post-path-p path)
        (setf errors (acons 'path "Invalid path" errors))))
    errors))

(defun perform-update-post (post title title-supplied-p path path-supplied-p)
  (setf (slot-value post 'title) title
        (slot-value post 'path) path)
  (mito:save-dao post)
  (values t nil))

(defun update-post (post &key (title nil title-supplied-p)
                           (path nil path-supplied-p))
  (let ((errors (validate-post title title-supplied-p
                               path path-supplied-p)))
    (if errors (values nil errors)
        (handler-case (perform-update-post post title title-supplied-p path path-supplied-p)
          (error (e) (values nil (list 'db-error error)))))))