scala - 如何键入匿名PartialFunction

这有效:

scala> List(1, "aa") collect {  case n : Int => n+2 }
res52: List[Int] = List(3)

这很漂亮:

scala> var f:PartialFunction[Any, Int] = { case n : Int => n+2 }
f: PartialFunction[Any,Int] = <function1>

scala> var g:PartialFunction[Any, String] = { case n : String => n + " plus two " }
g: PartialFunction[Any,String] = <function1>

scala> List(1, "aa") collect (f orElse g)
res51: List[Any] = List(3, "aa plus two ")

但是，如果我尝试一起做这两个，那就不行了:

scala> List(1, "aa") collect { case n : Int => n+2 } orElse { case n : String => n + " plus two " }
<console>:8: error: missing parameter type for expanded function
The argument types of an anonymous function must be fully known. (SLS 8.5)
Expected type was: PartialFunction[?,?]
              List(1, "aa") collect { case n : Int => n+2 } orElse { case n : String => n + " plus two " }

我不完全理解为什么推理失败，但是我可以猜测。重要的问题:如何解决？

最佳答案

您需要告诉编译器匿名PartialFunction的参数类型。您可以通过注释其类型直接执行此操作:

List(1, "aa") collect ({
  { case n : Int => n+2 }: PartialFunction[Any, _]
} orElse {
  { case n : String => n + " plus two " }: PartialFunction[Any, _]
})

请注意，必须将表达式括在collect右边的括号中。

如果您不喜欢那么冗长的代码，并且不介意让试图理解您的代码的人感到沮丧，则可以在PartialFunction上定义一个输入函数为Any的身份函数:

def pfa[T](f: PartialFunction[Any, T]): PartialFunction[Any, T] = f

List(1, "aa") collect (
  pfa { case n : Int => n+2 }
  orElse pfa { case n : String => n + " plus two " }
)

您甚至可以拿出一个适当的怪异名称，并假装这是一种Scala语言功能:

def @:[T](f: PartialFunction[Any, T]): PartialFunction[Any, T] = f

scala> @:{case x: Int => x + 3}
res29: PartialFunction[Any,Int] = <function1>