(define-condition validation-error (error)
  ((errors :reader errors :initarg :errors))
  (:default-initargs :errors (error "Must provide ERRORS.")))

(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 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)))
    (when errors (error 'validation-error :errors errors))
    (setf (slot-value post 'title) title
          (slot-value post 'path) path)
    (mito:save-dao post)))