Elisp 定义阿克曼函数

Question

我正在阅读SICP并参考它
Exercise 1.10. The following procedure computes a mathematical function called Ackermann's function.

(define (A x y)
  (cond ((= y 0) 0)
        ((= x 0) (* 2 y))
        ((= y 1) 2)
        (else (A (- x 1)
                 (A x (- y 1))))))

用elisp重写

(defun A (x y)
  (cond ((= y 0) 0)
        ((= x 0) (* 2 y))
        ((= y 1) 2)
        (else (A (- x 1)
                 (A x (- y 1))))))

(A 1 10)

报错：
A: Symbol’s value as variable is void: else

参考 elisp 的控制结构，因此在各种替代方案中丢失。

能否请您提供提示以使用 cond 和 else

定义该函数

Answer 1

在 Emacs Lisp 中，您使用 t 而不是 else。在这种情况下，语言参考 是开始研究的好地方...https://www.gnu.org/software/emacs/manual/html_node/elisp/Conditionals.html:

Often we want to execute the last clause whenever none of the previous clauses was successful. To do this, we use t as the condition of the last clause, like this: (t body-forms). The form t evaluates to t, which is never nil, so this clause never fails, provided the cond gets to it at all. For example:
(setq a 5)
(cond ((eq a 'hack) 'foo)
      (t "default"))
⇒ "default"

Elisp 定义阿克曼函数

Elisp to define a Ackermann's function

elisp