lisp - Common Lisp 中的本地状态

Question

Common Lisp 中的新手问题:

如何让我的程序每次调用都返回具有自己本地绑定(bind)的不同程序对象？目前，我使用 let 创建本地状态，但是两个函数调用共享相同的本地状态。这是代码，

(defun make-acc ()
  (let ((balance 100))
    (defun withdraw (amount)
      (setf balance (- balance amount))
      (print balance))
    (defun deposit (amount)
      (setf balance (+ balance amount))
      (print balance))
    (lambda (m)
      (cond ((equal m 'withdraw)
              (lambda (x) (withdraw x)))
            ((equal m 'deposit)
              (lambda (x) (deposit x)))))))

;; test

(setf peter-acc (make-acc))

(setf paul-acc (make-acc))

(funcall (funcall peter-acc 'withdraw) 10)
;; Give 90

(funcall (funcall paul-acc 'withdraw) 10)
;; Expect 90 but give 80

我应该以其他方式吗？我的写作方式有问题吗？有人可以帮我解决这个疑问吗？提前致谢。

Answer 1

请注意，即使在 defun 之后-is-global 问题得到了处理，你需要的机器比你需要做的事情要少得多。例如:

(defun make-account (balance)
  (lambda (op amount)
    (ecase op
      ((withdraw)
       (decf balance amount))
      ((deposit)
       (incf balance amount)))))

(defun account-operation (account op &rest args-to-op)
  (apply account op args-to-op))

然后

> (setf joe-acct (make-account 10))
#<Closure 1 subfunction of make-account 4060010B54>

> (setf mary-acct (make-account 100))
#<Closure 1 subfunction of make-account 4060010C94>

> (account-operation joe-acct 'withdraw 10)
0

> (account-operation mary-acct 'deposit 10)
110

显然 account-operation只是一种方便。