Scala @ 运算符
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Scala @ operator
提问by lowercase
What does Scala's @ operator do?
Scala 的@ 运算符有什么作用？
For example, in the blog post Formal Language Processing in Scala, Part 2there is a something like this
例如，在博客文章Scala 中的形式语言处理，第 2 部分中有这样的内容
case x @ Some(Nil) => x
回答by Daniel C. Sobral
It enables one to bind a matched pattern to a variable. Consider the following, for instance:
它使人们能够将匹配的模式绑定到变量。例如，请考虑以下情况：
val o: Option[Int] = Some(2)
You can easily extract the content:
您可以轻松提取内容：
o match {
case Some(x) => println(x)
case None =>
}
But what if you wanted not the contentof Some, but the option itself? That would be accomplished with this:
但是，如果你想没有内容的Some，但选择本身？这将通过以下方式实现：
o match {
case x @ Some(_) => println(x)
case None =>
}
Note that @can be used at anylevel, not just at the top level of the matching.
请注意，@可以在任何级别使用，而不仅仅是在匹配的顶级。
回答by retronym
@can be used to bind a name to a successfully matched pattern, or subpattern. Patterns can be used in pattern matching, the left hand side of the <-in for comprehensions, and in destructuring assigments.
@可用于将名称绑定到成功匹配的模式或子模式。模式可用于模式匹配、in 的左侧<-用于理解，以及解构赋值。
scala> val d@(c@Some(a), Some(b)) = (Some(1), Some(2))
d: (Some[Int], Some[Int]) = (Some(1),Some(2))
c: Some[Int] = Some(1)
a: Int = 1
b: Int = 2
scala> (Some(1), Some(2)) match { case d@(c@Some(a), Some(b)) => println(a, b, c, d) }
(1,2,Some(1),(Some(1),Some(2)))
scala> for (x@Some(y) <- Seq(None, Some(1))) println(x, y)
(Some(1),1)
scala> val List(x, xs @ _*) = List(1, 2, 3)
x: Int = 1
xs: Seq[Int] = List(2, 3)
回答by sepp2k
When pattern matching variable @ patternbinds variableto the value matched by patternif the pattern matches. In this case that means that the value of xwill be Some(Nil)in that case-clause.
当模式匹配variable @ pattern结合可变由匹配的值模式，如果模式匹配。在这种情况下，这意味着 的值x将Some(Nil)在 case-clause 中。
回答by Mitch Blevins
Allows you to match the top-level of a pattern. Example:
允许您匹配模式的顶级。例子：
case x @ "three" => assert(x.equals("three"))
case x @ Some("three") => assert(x.get.equals("three")))
case x @ List("one", "two", "three") => for (element <- x) { println(element) }
回答by oxbow_lakes
It sets the value of xto the pattern which matches. In your example, xwould therefore be Some(Nil)(as you could determine from a call to println)
它将 的值设置为x匹配的模式。在您的示例中，x因此将是Some(Nil)（正如您可以从对println的调用中确定的那样）
