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。
