部分函数匹配的对象的引用是什么？

Question

以这个函数为例：

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

匹配的对象是什么？在箭头的右侧，我如何引用匹配的对象？

Answer 1

你可以用 if-guard:

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，如果您有一个要引用的对象，那么例如foo: Foo(s) 不会工作（foo: Foo 会，但是你失去了对 Foo 值 s 的引用）。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

如果您希望大小写匹配任何内容并引用它，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)
}

部分函数匹配的对象的引用是什么？

What is the reference of the object that a partial function matches on?

scala

pattern-matching

partialfunction