Scala 多类型模式匹配

Question

我想知道如何使用多种类型的模式匹配。我有：

abstract class MyAbstract

case class MyFirst extends MyAbstract
case class MySecond extends MyAbstract
case class MyThird extends MyAbstract // shouldn't be matched and shouldn't call doSomething()

val x: MyAbstract = MyFirst

x match { 
 case a: MyFirst => doSomething()
 case b: MySecond => doSomething()
 case _ => doSomethingElse()
}

所以我想写这样的东西：

x match {
 case a @ (MyFirst | MySecond) => doSomething()
 case _ => doSomethingElse()
}

我在一些教程中看到了类似的结构，但它给了我错误：

pattern type is incompatible with expected type;
[error]  found   : object MyFirst
[error]  required: MyAbstract

那么有没有办法在 on case 子句中定义几个不同的类型？我认为它会使代码更漂亮。好像我会有 5 个一样，我会写 5 次相同的代码（调用 doSomething()）。

提前致谢！

Answer 1

您的案例类缺少括号。 不推荐使用没有参数列表的案例类。

试试这个：

abstract class MyAbstract
case class MyFirst() extends MyAbstract
case class MySecond() extends MyAbstract

val x: MyAbstract = MyFirst()


x match {
   case aOrB @ (MyFirst() | MySecond()) => doSomething(aOrB)
   case _ => doSomethingElse()
}

如果您的案例类有太多参数并且不喜欢编写冗长的Foo(_,_,..) 模式，那么也许：

x match {
   case aOrB @ (_:MyFirst | _:MySecond) => doSomething(aOrB)
   case _ => doSomethingElse()
}

或者只是：

x match {
   case _:MyFirst | _:MySecond => doSomething(x) // just use x instead of aOrB
   case _ => doSomethingElse(x)
}

但也许您只是想要单例案例对象？

abstract class MyAbstract
case object MyFirst extends MyAbstract
case object MySecond extends MyAbstract

val x: MyAbstract = MyFirst

x match {
   case aOrB @ (MyFirst | MySecond) => doSomething()
   case _ => doSomethingElse()
}