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我只是在研究 scala，因为大多数人似乎都喜欢它。所以我想我会写 FizzBu​​zz（因为它比 HelloWorld 有趣得多），但我有点卡住了，因为无论我在其中添加什么，如果我PartialFunction用匹配/大小写语法定义我的 s，他们的isDefinedAt方法似乎总是返回 true . 谁能向我解释我缺少什么（或者这是一个错误？）。我正在使用 Scala 2.10.0 版本。

代码：

object Main extends App {
  def applyAndJoin[A, B](f: PartialFunction[A, B],
    g: PartialFunction[A, B])(h: (B, B) => B): PartialFunction[A, B] = {
    PartialFunction[A, B] {
      case a if (f isDefinedAt a) && (g isDefinedAt a) => h(f(a), g(a))
    } orElse f orElse g
  }

  val fizz = PartialFunction[Int, String] { case a if a % 3 == 0 => "Fizz" }
  val buzz = PartialFunction[Int, String] { case a if a % 5 == 0 => "Buzz" }
  val makeString = PartialFunction[Int, String] { case a => a.toString }
  val fizzBuzz = applyAndJoin(fizz, buzz)((a, b) => a + b) orElse makeString
  1 to 100 collect fizzBuzz foreach println //this fails because scala thinks
                                            //that fizz is defined at 1
}

堆栈跟踪：

Exception in thread "main" scala.MatchError: 1 (of class java.lang.Integer)
    at Main$$anonfun$1.apply(Main.scala:9)
    at Main$$anonfun$1.apply(Main.scala:9)
    at scala.PartialFunction$$anonfun$apply$1.applyOrElse(PartialFunction.scala:242)
    at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:33)
    at Main$$anonfun$applyAndJoin$1.apply(Main.scala:5)
    at scala.PartialFunction$$anonfun$apply$1.applyOrElse(PartialFunction.scala:242)
    at scala.PartialFunction$OrElse.apply(PartialFunction.scala:162)
    at scala.collection.TraversableLike$$anonfun$collect$1.apply(TraversableLike.scala:278)
    at scala.collection.immutable.Range.foreach(Range.scala:142)
    at scala.collection.TraversableLike$class.collect(TraversableLike.scala:278)
    at scala.collection.AbstractTraversable.collect(Traversable.scala:105)
    at Main$delayedInit$body.apply(Main.scala:13)
    at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
    at scala.App$$anonfun$main$1.apply(App.scala:71)
    at scala.App$$anonfun$main$1.apply(App.scala:71)
    at scala.collection.immutable.List.foreach(List.scala:309)
    at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:32)
    at scala.App$class.main(App.scala:71)
    at Main$.main(Main.scala:1)
    at Main.main(Main.scala)
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1 回答 1

7

您的 PartialFunction 定义没有按照您的想法进行，您应该编写

val fizz: PartialFunction[Int, String] = { case a if a % 3 == 0 => "Fizz" }
val buzz: PartialFunction[Int, String] = { case a if a % 5 == 0 => "Buzz" }
val makeString: PartialFunction[Int, String] = { case a => a.toString }

通过这种方式，您定义了一个类型的变量，PartialFunction[Int, String]其值为匿名定义{ case a if ...}

相反，您正在定义一个变量

val fizz = ...

使用对PartialFunction对象apply()方法的调用，在 scala 2.10 中定义为

def apply[A, B](f: A => B): PartialFunction[A, B] = { case x => f(x) }

在这种情况下，结果函数fizz是为 every 定义的x，但是当f(x)调用“wrapped”时，会发生错误。

于 2013-01-26T13:43:16.537 回答