What does Scala's @ operator do?
For example, in the blog post Formal Language Processing in Scala, Part 2 there is a something like this
case x @ Some(Nil) => x
It enables one to bind a matched pattern to a variable. Consider the following, for instance:
val o: Option[Int] = Some(2)
You can easily extract the content:
o match {
case Some(x) => println(x)
case None =>
}
But what if you wanted not the content of Some
, but the option itself? That would be accomplished with this:
o match {
case x @ Some(_) => println(x)
case None =>
}
Note that @
can be used at any level, not just at the top level of the matching.
@
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.
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)
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