Scala 选项：映射与模式匹配
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原文地址: http://stackoverflow.com/questions/21923600/
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Scala Option: map vs Pattern Matching
提问by Prasanna
While dealing with Optionin Scala what are the things I should be considering to decide whether to map or patten match? For example, if I have Option[MyClass], I can deal with it the following ways:
Option在 Scala 中处理时，我应该考虑哪些事情来决定是映射还是模式匹配？例如，如果我有Option[MyClass]，我可以通过以下方式处理它：
def getList(myOptionInstance: Option[MyClass]): List[String] =
myOptionInstance map (...) getOrElse(List.empty[String])
or
或者
def getList(myOptionInstance: Option[MyClass]): List[String] = myOptionInstance match {
case Some(mySomeInstance) => .....
case None => List.empty[String]
}
When will I choose one over the other?
我什么时候会选择一个？
回答by vptheron
I second @rarry: foldis the preferred way to deal with this.
我第二次@rarry：fold是处理这个问题的首选方式。
Some prefer pattern matching because it's "cool" (whatever it means) and sometimes easier to read.
有些人更喜欢模式匹配，因为它“很酷”（不管它意味着什么），有时更容易阅读。
I try to avoid using getOrElsebecause it does not force you to use the same type for the default value as the type wrapped in your Option:
我尽量避免使用，getOrElse因为它不会强制您对默认值使用与您的Option.
def getOrElse[B >: A](default: ? B): B
So you can write:
所以你可以写：
val v = Some(42).getOrElse("FortyTwo")
Here vhas type Any. It's very easy to see the problem with such a stupid example but sometimes it's not as obvious and can lead to issues.
这里v有类型Any。用这样一个愚蠢的例子很容易看出问题，但有时它并不那么明显，可能会导致问题。
While fold:
虽然fold：
def fold[B](ifEmpty: ? B)(f: (A) ? B): B
It forces you to return the same type for both branches.
它强制您为两个分支返回相同的类型。
scala> Some(42).fold("fortyTwo")(v => v)
<console>:8: error: type mismatch;
found : Int
required: String
Some(42).fold("fortyTwo")(v => v)
回答by mauhiz
Pattern matching is :
模式匹配是：
- slightly more efficient
- not anal about subtypes (in this case @rarry had to add a type hint)
- easier to read
- endorsed by Martin Oderksy: https://stackoverflow.com/a/5332657/578101
- 稍微有效率
- 不是关于子类型的肛门（在这种情况下@rarry 必须添加类型提示）
- 更容易阅读
- Martin Oderksy 认可：https://stackoverflow.com/a/5332657/578101
回答by rarry
I would go for this:
我会这样做：
myOptionInstance.fold(Nil: List[String])(...)
