elisp - Emacs Lisp : How to use interactive (for conditional arguments)?

Question

我想根据第一个问题的答案向用户提出第二个问题。

(defun something (a b)
  (interactive
   (list
    (read-number "First num: ")
    (read-number "Second num: ")))
  (message "a is %s and b is %s" a b))

所以我需要一种方法来测试条目值:

(defun something-else (a &optional b)
  (interactive
   (list
    (read-number "First num: ")
    (if (< a 2)
        (read-number "Second num: "))))
  (message "a is %s" a))

但是

if: Symbol's value as variable is void: a

问题:我怎样才能以真正互动的方式使用 interactive？

Answer 1

(defun xtest (a &optional b)
  (interactive
   (let (a b)
     (setq a (read-number "First: "))
     (if (< a 2)
         (setq b (read-number "Second: ")))
     (list a b)))

  (message "a is %s b is %s" a b))