Scala:无情况下`for`模式匹配中的奇怪行为 [英] Scala: strange behavior in `for` pattern matching for None case

问题描述

for 循环模式匹配中的奇怪行为:

Strange behavior in for cycle pattern matching:

scala> val a = Seq(Some(1), None)
a: Seq[Option[Int]] = List(Some(1), None)

scala> for (Some(x) <- a) { println(x) }
1

scala> for (None <- a) { println("none") }
none
none

为什么在第二个示例中产生了两个输出 'none'?也许这个例子是合成的，不实用，但这种行为是不可预期的.这是错误还是功能?

Why in second example two output 'none' produced? Maybe this example is synthetic and not practical, but such behavior is not expectable. Is this bug or feature?

推荐答案

你知道吗，这是一个错误:

What do you know, it is a bug:

https://issues.scala-lang.org/browse/SI-9324

scala> val vs = Seq(Some(1), None)
vs: Seq[Option[Int]] = List(Some(1), None)

scala> for (n @ None <- vs) println(n)
None

不明确的规范:

http://www.scala-lang.org/files/archive/spec/2.11/06-expressions.html#for-comprehensions-and-for-loops

比较没有出现错误的中游分配:

Compare midstream assignment, which does not exhibit the bug:

scala> for (v <- vs; None = v) println(v)
scala.MatchError: Some(1) (of class scala.Some)
  at $anonfun$1.apply(<console>:9)
  at $anonfun$1.apply(<console>:9)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:245)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:245)
  at scala.collection.immutable.List.foreach(List.scala:381)
  at scala.collection.TraversableLike$class.map(TraversableLike.scala:245)
  at scala.collection.immutable.List.map(List.scala:285)
  ... 33 elided

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