匿名部分函数语法

Question

>我之前问过这个问题：将 Partial Function 与常规函数相结合

然后意识到，我实际上并没有问对。所以，这是另一个尝试。

如果我这样做：

 val foo = PartialFunction[Int, String] { case 1 => "foo" }
 val bar = foo orElse { case x => x.toString }

它不编译：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[?,?]

但这工作正常：

   val x: Seq[String] = List(1,2,3).collect { case x => x.toString }

问题是有什么区别？在这两种情况下，参数的类型是相同的：PartialFunction[Int, String] 。传入的值实际上是相同的。为什么一种情况有效，而另一种情况无效？

Answer 1

您需要指定bar的类型，因为编译器无法推断它。这将编译：

val foo = PartialFunction[Int, String] { case 1 => "foo" }
val bar : (Int => String) = foo orElse { case x => x.toString }

Answer 2

在List(1,2,3).collect{case x => x.toString}的情况下，编译器能够根据List的类型推断分部函数的输入类型。

final override def collect[B, That](pf: PartialFunction[A, B])(implicit bf: CanBuildFrom[List[A], B, That])

根据类型参数，编译器可以推断您正在传递正确类型的分部函数。这就是为什么List(1,2,3).collect{case x:String => x.toString}不编译，也不List(1,2,3).collect{case x:Int => x.toString; case x: String => x.toString}.

由于List是协变的，编译器能够推断偏函数{case x => x.toString}是Int上的偏函数。您会注意到List(1,2,3).collect{case x => x.length}无法编译，因为编译器推断您正在Int的Int或子类上运行。

还要记住，{case x => x.toString}只是句法糖。如果我们执行以下操作，那么您的示例将按预期工作

val f = new PartialFunction[Int, String](){
  override def isDefinedAt(x: Int): Boolean = true
  override def apply(v1: Int): String = v1.toString
}
val foo = PartialFunction[Int, String] { case 1 => "foo" }
val bar = foo orElse f //This compiles fine.
List(1,2,3).collect{f} // This works as well.

因此，从我的角度来看，唯一合乎逻辑的答案是，能够为{case x => x.toString}生成PartialFunction实例的语法糖在编译时没有足够的信息，无法在orElse情况下将其充分键入为PartialFunction[Int, String]。

Answer 3

您可以使用库 Extractor.scala。

import com.thoughtworks.Extractor._
// Define a PartialFunction
val pf: PartialFunction[Int, String] = {
  case 1 => "matched by PartialFunction"
}
// Define an optional function
val f: Int => Option[String] = { i =>
  if (i == 2) {
    Some("matched by optional function")
  } else {
    None
  }
}
// Convert an optional function to a PartialFunction
val pf2: PartialFunction[Int, String] = f.unlift
util.Random.nextInt(4) match {
  case pf.extract(m) => // Convert a PartialFunction to a pattern
    println(m)
  case f.extract(m) => // Convert an optional function to a pattern
    println(m)
  case pf2.extract(m) => // Convert a PartialFunction to a pattern
    throw new AssertionError("This case should never occur because it has the same condition as `f.extract`.")
  case _ =>
    println("Not matched")
}