List.collect上的匹配错误

Question

第二次实施有什么问题？

作品:

scala> List(1,2,3).collect{ case i: Int if i % 2 == 0 => i }
res1: List[Int] = List(2)

不:

scala> val evens = PartialFunction[Any, Int]{
     | case i: Int if i % 2 == 0 => i
     | }
evens: PartialFunction[Any,Int] = <function1>

scala> List(1,2,3).collect{evens}
scala.MatchError: 1 (of class java.lang.Integer)
    at $anonfun$1.apply(<console>:7)
    at $anonfun$1.apply(<console>:7)
    at scala.PartialFunction$$anonfun$apply$1.applyOrElse(PartialFunction.scala:242)
    at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:33)
    at scala.collection.TraversableLike$$anonfun$collect$1.apply(TraversableLike.scala:278)
    at scala.collection.immutable.List.foreach(List.scala:318)
    ...

Answer 1

你应该这样创建evens PartialFunction:

val evens: PartialFunction[Any, Int] = {
  case i: Int if i % 2 == 0 => i
}

随着PartialFunction[Any, Int]{ ... }你调用一个apply对象的方法PartialFunction.它定义如下:

def apply[A, B](f: (A) ? B): PartialFunction[A, B]

所以{ case i: Int if i % 2 == 0 => i }用作函数,而不是a PartialFunction和evens任何参数定义:

scala> val evens = PartialFunction[Any, Int]{
     |   case i: Int if i % 2 == 0 => i
     | }

scala> evens.isDefinedAt(1)
res1: Boolean = true

scala> val evens: PartialFunction[Any, Int] = {
     |   case i: Int if i % 2 == 0 => i
     | }

scala> evens.isDefinedAt(1)
res5: Boolean = false