在Scala中，双冒号（或冒号）::是什么意思？ [英] What does double colon (or colon-colon) :: mean in Scala?

问题描述

我在我的Scala项目的 sbt 版本中遇到问题（重复项： META-INF / MANIFEST.MF ）和以下几行代码解决了该问题：

I was having an issue with the sbt build of my Scala project (duplicate entry: META-INF/MANIFEST.MF) and the following lines solved the problem:

assemblyMergeStrategy in assembly := {
  case PathList("META-INF", xs @ _*) =>
    (xs map {_.toLowerCase}) match {
      case ("manifest.mf" :: Nil) | ("index.list" :: Nil) | ("dependencies" :: Nil) => MergeStrategy.discard
      case _ => MergeStrategy.last
    }
}

我现在试图理解双冒号在什么意思以上情况。我在 Quora中找到了答案，但这无济于事。

I am now trying to understand what the double colon means in the above context. I found an answer in Quora, but this did not help.

此外，我是Scala的新手，实际上并没有帮助。

Moreover, I am a newbie to Scala which does not really help.

编辑：

看到答案，我仍然不明白为什么原因，因为我们正在创建列表，因此以下内容不起作用（重复 manifest.mf 项不会被丢弃）：

Seeing the answers, I still don't understand why since we are creating a list the following does not work (duplicate manifest.mf entries are not discarded):

case List("manifest.mf", "index.list", "dependencies") => MergeStrategy.discard

以下都不是：（error：'）'预期，但'=> ）：

Neither does the following: (error: ')' expected but '=>' found.):

case List("manifest.mf") | List(("index.list") | List("dependencies") => MergeStrategy.discard

编辑2：

去掉多余的括号即可完成以下工作：

Removing the extra bracket made the following work:

case List("manifest.mf") | List("index.list") | List("dependencies") => MergeStrategy.discard

对于那些对此主题感兴趣的人... 在Scala中创建列表的5种方法。

And for those interested into the topic... 5 ways to create lists in Scala.

推荐答案

通常， manifest.mf :: Nil 与`List（ manifest.mf）相同。
只是一种制作或

In general, "manifest.mf" :: Nil is the same with `List("manifest.mf"). It is just a way to make or pattern match a list.

对于Scala中的模式匹配（也可以搜索 unapply ）如何工作，有很好的资源，主要用于 case 语句。

There are good resources for how pattern matching (also search unapply) work in Scala, mainly used in case statements.

此处t表示：如果该列表具有一个元素，并且是（ manifest.mf ， index.list 之一， 依赖性），然后选择 MergeStrategy.discard

Here it says: If that list has one element, and that is one of (manifest.mf,index.list, dependencies) then pick MergeStrategy.discard

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