Scala模式匹配

模式匹配是根据模式检查值的机制。一个成功的匹配可以将值分解成其组成部分。它Java switch语句的一个更强大的版本，也可用于替代一系列if/else语句。

语法

一个匹配表达式有一个值，match关键字和至少一个case语句。

import scala.util.Random
val x: Int = Random.nextInt(10)
x match {
  case 0 => "zero"
  case 1 => "one"
  case 2 => "two"
  case _ => "many"
}

上面的val x是一个在0到10之间的随机整数。x是match操作符的左操作数，右边是包含4个cases的表达式。最后一个case _是一个”catch all”，代表了任意大于2的数字。Cases可被称为选择。

匹配表达式有一个值。

def matchTest(x: Int): String = x match {
  case 1 => "one"
  case 2 => "two"
  case _ => "many"
}
matchTest(3)  // many
matchTest(1)  // one

这是一个字符串匹配表达式，所有的cases都返回字符串，因此，matchTest函数返回一个字符串。

Case类匹配

Case类对于模式匹配特别有用。

abstract class Notification

case class Email(sender: String, title: String, body: String) extends Notification

case class SMS(caller: String, message: String) extends Notification

case class VoiceRecording(contactName: String, link: String) extends Notification

Notification是一个抽象父类，有三个具体的Notification类型，分别是Email，SMS和VoiceRecording。现在可以对这些Case类进行模式匹配。

def showNotification(notification: Notification): String = {
  notification match {
    case Email(email, title, _) =>
      s"You got an email from $email with title: $title"
    case SMS(number, message) =>
      s"You got an SMS from $number! Message: $message"
    case VoiceRecording(name, link) =>
      s"you received a Voice Recording from $name! Click the link to hear it: $link"
  }
}
val someSms = SMS("12345", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")

println(showNotification(someSms))  // prints You got an SMS from 12345! Message: Are you there?

println(showNotification(someVoiceRecording))  // you received a Voice Recording from Tom! Click the link to hear it: voicerecording.org/id/123

showNotification函数有一个Notification类型的参数，内部对Notification类型进行了匹配（分别匹配Email，SMS和VoiceRecording）。在case Email(email, title, _)中字段email和title用于返回值，但是body使用_进行了忽略。

模式守护

模式守护是简单的布尔表达式，用于让情况更加具体。只要在模式后面添加if <boolean expression>。

def showImportantNotification(notification: Notification, importantPeopleInfo: Seq[String]): String = {
  notification match {
    case Email(email, _, _) if importantPeopleInfo.contains(email) =>
      "You got an email from special someone!"
    case SMS(number, _) if importantPeopleInfo.contains(number) =>
      "You got an SMS from special someone!"
    case other =>
      showNotification(other) // nothing special, delegate to our original showNotification function
  }
}

val importantPeopleInfo = Seq("867-5309", "jenny@gmail.com")

val someSms = SMS("867-5309", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")
val importantEmail = Email("jenny@gmail.com", "Drinks tonight?", "I'm free after 5!")
val importantSms = SMS("867-5309", "I'm here! Where are you?")

println(showImportantNotification(someSms, importantPeopleInfo))
println(showImportantNotification(someVoiceRecording, importantPeopleInfo))
println(showImportantNotification(importantEmail, importantPeopleInfo))
println(showImportantNotification(importantSms, importantPeopleInfo))

在case Email(email, _, _) if importantPeopleInfo.contains(email)中，只有email在important people列表中才能匹配模式。

只匹配类型

可以按照如下方式匹配类型：

abstract class Device
case class Phone(model: String) extends Device{
  def screenOff = "Turning screen off"
}
case class Computer(model: String) extends Device {
  def screenSaverOn = "Turning screen saver on..."
}

def goIdle(device: Device) = device match {
  case p: Phone => p.screenOff
  case c: Computer => c.screenSaverOn
}

def goIdle会根据Device类型不同有不同的行为。当case需要调用模式的方法时非常有用。习惯使用类型的第一个字符作为case的标识符（这个例子中是p和c）。

密封类

特征和类可以标记为sealed，意思是所有子类型都必须在同一个文件中声明。确保所有子类型是已知的。

sealed abstract class Furniture
case class Couch() extends Furniture
case class Chair() extends Furniture

def findPlaceToSit(piece: Furniture): String = piece match {
  case a: Couch => "Lie on the couch"
  case b: Chair => "Sit on the chair"
}

因为我们不需要”catch all” case，所以这对模式匹配非常有用。

Notes

Scala的模式匹配语句对于通过Case类表达的代数类型匹配非常有用。Scala也允许Case类独立定义模式，在extractor objects中使用unapply方法。