变量的局部状态 [英] Local state of a variable

问题描述

我试图完全理解对象及其变量的局部状态

I am trying to fully understand Objects and local states of their variables

这段代码似乎对多次调用的同一个过程产生了不同的结果，这意味着局部变量发生了变化:

This code seems to produce different results for the same procedure called multiple times, meaning the local variable changes:

(define new-withdraw
  (let ((balance 100))
    (lambda (amount)
      (if (>= balance amount)
          (begin (set! balance (- balance amount))
                 balance)
          "Insufficient funds"))))

对于这个其他代码，它产生相同的结果，这意味着它为每个过程调用创建一个新的局部变量:

For this other code, it produces the same result, which means it creates a new local variable for every procedure call:

(define (make-account)
  (let ((balance 100))
    (define (withdraw amount)
      (if (>= balance amount)
          (begin (set! balance (- balance amount))
                 balance)
          "Insufficient funds"))
    (define (deposit amount)
      (set! balance (+ balance amount))
      balance)
    (define (dispatch m)
      (cond ((eq? m 'withdraw) withdraw)
            ((eq? m 'deposit) deposit)
            (else (error "Unknown request -- MAKE-ACCOUNT"
                         m))))
    dispatch))

我的问题是:

• 尽管使用 let 创建了局部变量，但为什么它们的行为有所不同?

• Why do they behave differently despite creating a local variable using let?

有没有办法让第二个代码像第一个一样工作，而无需将 balance 作为 make-account 的参数传递?

Is there a way one can make the second code work as the first one without passing balance as a parameter of make-account?

谢谢

推荐答案

测试代码 1:

> (new-withdraw 0)
100
> (new-withdraw 50)
50
> (new-withdraw 10)
40

测试代码 2:

> (define ac (make-account))
> ((ac 'withdraw) 0)
100
> ((ac 'withdraw) 50)
50
> ((ac 'withdraw) 10)
40

所以两个代码都保持它们的本地状态.代码 1 和代码 2 之间的区别在于代码 1 仅适用于一个帐户，而代码 2 在每次调用时创建一个新帐户" - 对过程的调用返回您需要绑定到变量的调度过程，并且然后如上图使用.

So both codes keep their local state. The difference between code 1 and code 2 is that code 1 works for one account only, whereas code 2 "creates a new account" on each call - the call to the procedure returns the dispatching procedure that you need to bind to a variable, and then use as shown above.

因此你会觉得本地状态丢失了；不是，您可能每次都创建一个新帐户.

Therefore you get the impression that the local state is lost; it's not, you were probably creating a new account every time.

这篇关于变量的局部状态的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！