diff --git a/_zh-cn/tour/pattern-matching.md b/_zh-cn/tour/pattern-matching.md index 96d9d849b2..8f03480ac6 100644 --- a/_zh-cn/tour/pattern-matching.md +++ b/_zh-cn/tour/pattern-matching.md @@ -1,6 +1,6 @@ --- layout: tour -title: Pattern Matching +title: 模式匹配 discourse: false @@ -13,3 +13,142 @@ language: zh-cn next-page: singleton-objects previous-page: case-classes --- + +模式匹配是检查某个值(value)是否匹配某一个模式的机制,一个成功的匹配同时会将匹配值解构为其组成部分。它是Java中的`switch`语句的升级版,同样可以用于替代一系列的 if/else 语句。 + +## 语法 +一个模式匹配语句包括一个待匹配的值,`match`关键字,以及至少一个`case`语句。 + +```tut +import scala.util.Random + +val x: Int = Random.nextInt(10) + +x match { + case 0 => "zero" + case 1 => "one" + case 2 => "two" + case _ => "many" +} +``` +上述代码中的`val x`是一个0到10之间的随机整数,将它放在`match`运算符的左侧对其进行模式匹配,`match`的右侧是包含4条`case`的表达式,其中最后一个`case _`表示匹配其余所有情况,在这里即是`x`大于2的情况。 + +`match`表达式具有一个结果值 +```tut +def matchTest(x: Int): String = x match { + case 1 => "one" + case 2 => "two" + case _ => "many" +} +matchTest(3) // many +matchTest(1) // one +``` +这个`match`表达式是String类型的,因为所有的情况(case)均返回String,所以`matchTest`函数的返回值是String类型。 + +## 案例类(case classes)的匹配 + +案例类非常适合用于模式匹配。 + +```tut +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` 是一个虚基类,它有三个具体的子类`Email`, `SMS`和`VoiceRecording`,我们可以在这些案例类(Case Class)上像这样使用模式匹配: + +``` +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`函数接受一个抽象类`Notification`对象作为输入参数,然后匹配其具体类型。(也就是判断它是一个`Email`,`SMS`,还是`VoiceRecording`)。在`case Email(email, title, _)`中,对象的`email`和`title`属性在返回值中被使用,而`body`属性则被忽略,故使用`_`代替。 + +## 模式守卫(Pattern gaurds) +为了让匹配更加具体,可以使用模式守卫,也就是在模式后面加上`if `。 +``` + +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,除了要求`notification`是`Email`类型外,还需要`email`在重要人物列表`importantPeopleInfo`中,才会匹配到该模式。 + + +## 仅匹配类型 +也可以仅匹配类型,如下所示: +```tut +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 +} +``` +当不同类型对象需要调用不同方法时,仅匹配类型的模式非常有用,如上代码中`goIdle`函数对不同类型的`Device`有着不同的表现。一般使用类型的首字母作为`case`的标识符,例如上述代码中的`p`和`c`,这是一种惯例。 + +## 密封类 + +特质(trait)和类(class)可以用`sealed`标记为密封的,这意味着其所有子类都必须与之定义在相同文件中,从而保证所有子类型都是已知的。 + +```tut +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" +} +``` +这对于模式匹配很有用,因为我们不再需要一个匹配其他任意情况的`case`。 + +## 备注 + +Scala的模式匹配语句对于使用[案例类(case classes)](case-classes.html)表示的类型非常有用,同时也可以利用[提取器对象(extractor objects)](extractor-objects.html)中的`unapply`方法来定义非案例类对象的匹配。 +