分部函数匹配的对象引用是什么

Question

以这个函数为例：

def receive = {
  case "test" => log.info("received test")
  case _      => log.info("received unknown message")
}

在什么对象上匹配？在箭头的右侧，如何引用正在匹配的对象？

Answer 1

你可以用一个if-guard来做到这一点：

def receive: String => Unit = {
  case str if str == "test" => println(str)
  case _ => println("other")
}
Option("test").map(receive) // prints "test"
Option("foo").map(receive) // prints "other"

请注意，如果你有一个想要引用的对象，那么像 foo: Foo(s)不起作用（foo: Foo会，但随后您将失去对 Foo 值的引用s ）。在这种情况下，您需要使用 @ 运算符：

case class Foo(s: String)
def receive: Foo => Unit = {
  case foo@Foo(s) => println(foo.s) // could've referred to just "s" too
  case _ => println("other")
}
Option(Foo("test")).map(receive) // prints "test"

Answer 2

如果您希望大小写与任何内容匹配，并引用它，请使用变量名而不是下划线

def receive = {
  case "test" => log.info("received test")
  case other  => log.info("received unknown message: " + other)
}