[英]How do I type an anonymous PartialFunction
这有效:
scala> List(1, "aa") collect { case n : Int => n+2 }
res52: List[Int] = List(3)
这很好用:
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 ")
但如果我试着将两者结合在一起,那么:
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 " }
我不明白为什么推理会失败,但我可以猜到。 重要的问题:我该如何解决?
您需要告诉编译器您的匿名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,必须将表达式括在括号中的collect
右侧。
如果您不喜欢罗嗦,并且您不介意试图理解您的代码的人感到沮丧,您可以在PartialFunction
上定义一个具有输入类型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 " }
)
您甚至可以提出一个非常奇怪的名称并假装它是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>
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.