选项[T]的要点是什么？

问题描述

我无法理解Scala中的 code>不使用

• 文档：方法类型签名会告诉您是否总是返回一个对象。

单向可组合性
>

后者需要更长时间才能充分理解，并且它不适合简单的示例，因为它仅显示其优势复杂的代码。所以，我会在下面举一个例子，但我很清楚，除了那些已经获得它的人以外，它几乎不会有任何意义。

  for {
 person<  -  getUsers 
 email<  -  person.getEmail //假设getEmail返回Option [String] 
} yield（person，email）
  

I am not able to understand the point of Option[T] class in Scala. I mean, I am not able to see any advanages of None over null.

For example, consider the code:

object Main{
  class Person(name: String, var age: int){
    def display = println(name+" "+age)
  }

  def getPerson1: Person = {
    // returns a Person instance or null
  }

  def getPerson2: Option[Person] = {
    // returns either Some[Person] or None
  }

  def main(argv: Array[String]): Unit = {
    val p = getPerson1
    if (p!=null) p.display

    getPerson2 match{
      case Some(person) => person.display
      case None => /* Do nothing */
    }
  }
}

Now suppose, the method getPerson1 returns null, then the call made to display on first line of main is bound to fail with NPE. Similarly if getPerson2 returns None, the display call will again fail with some similar error.

If so, then why does Scala complicate things by introducing a new value wrapper (Option[T]) instead of following a simple approach used in Java?

UPDATE:

I have edited my code as per @Mitch's suggestion. I am still not able to see any particular advantage of Option[T]. I have to test for the exceptional null or None in both cases. :(

If I have understood correctly from @Michael's reply, is the only advantage of Option[T] is that it explicitly tells the programmer that this method could return None? Is this the only reason behind this design choice?

解决方案

You'll get the point of Option better if you force yourself to never, ever, use get. That's because get is the equivalent of "ok, send me back to null-land".

So, take that example of yours. How would you call display without using get? Here are some alternatives:

getPerson2 foreach (_.display)
for (person <- getPerson2) person.display
getPerson2 match {
  case Some(person) => person.display
  case _ =>
}
getPerson2.getOrElse(Person("Unknown", 0)).display

None of this alternatives will let you call display on something that does not exist.

As for why get exists, Scala doesn't tell you how your code should be written. It may gently prod you, but if you want to fall back to no safety net, it's your choice.

---

You nailed it here:

Except for the "only". But let me restate that in another way: the main advantage of Option[T] over T is type safety. It ensures you won't be sending a T method to an object that may not exist, as the compiler won't let you.

You said you have to test for nullability in both cases, but if you forget -- or don't know -- you have to check for null, will the compiler tell you? Or will your users?

Of course, because of its interoperability with Java, Scala allows nulls just as Java does. So if you use Java libraries, if you use badly written Scala libraries, or if you use badly written personal Scala libraries, you'll still have to deal with null pointers.

Other two important advantages of Option I can think of are:

• Documentation: a method type signature will tell you whether an object is always returned or not.

• Monadic composability.

The latter one takes much longer to fully appreciate, and it's not well suited to simple examples, as it only shows its strength on complex code. So, I'll give an example below, but I'm well aware it will hardly mean anything except for the people who get it already.

for {
  person <- getUsers
  email <- person.getEmail // Assuming getEmail returns Option[String]
} yield (person, email)

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