reactive programming 2.1 Functions and state

1.  Stateful Objects 有状态的对象

  One normally describes the world as a set of objects, some of which have

  state that changes over the course of time.

  An object has a state if its behavior is influenced by its history.

   一个对象是有状态的，如果它的行为受历史的影响

Example: a bank account has a state, because the answer to the question

“can I withdraw 100 CHF ?”

may vary over the course of the lifetime of the account

一个账户是有状态的，因为对问题：我能取100元吗？的回答在不同时候可能是不一样的

2.带有var的对象不一定是有状态的

def cons[T](hd: T, tl: => Stream[T]) = new Stream[T] {
  def head = hd
  private var tlOpt: Option[Stream[T]] = None
  def tail: T = tlOpt match {
    case Some(x) => x
    case None => tlOpt = Some(tl); tail
  }
}

每次调用tail和head返回的结果是一样的

3.不带有var的对象不一定是没有状态的

class BankAccountProxy(ba: BankAccount) {
  def deposit(amount: Int): Unit = ba.deposit(amount)
  def withdraw(amount: Int): Int = ba.withdraw(amount)
}