1 scala中”_“的用法总结
参考链接：
what-are-all-the-uses-of-an-underscore-in-scala
http://www.slideshare.net/normation/scala-dreaded
import scala._ // Wild card -- all of Scala is imported
import scala.{ Predef => _, _ } // Exception, everything except Predef
def f[M[_]] // Higher kinded type parameter
def f(m: M[_]) // Existential type
_ + _ // Anonymous function placeholder parameter
m _ // Eta expansion of method into method value
m(_) // Partial function application
_ => 5 // Discarded parameter
case _ => // Wild card pattern -- matches anything
val (a, _) = (1, 2) // same thing
for (_ <- 1 to 10) // same thing
f(xs: _*) // Sequence xs is passed as multiple parameters to f(ys: T*)
case Seq(xs @ _*) // Identifier xs is bound to the whole matched sequence
var i: Int = _ // Initialization to the default value
def abc_<>! // An underscore must separate alphanumerics from symbols on identifiers
t._2 // Part of a method name, such as tuple getters
Existential types
def foo(l: List[Option[_]]) = ...
Higher kinded type parameters
case class A[K[_],T](a: K[T])
Ignored variables
val _ = 5
Ignored parameters
List(1, 2, 3) foreach { _ => println("Hi") }
Ignored names of self types
trait MySeq { _: Seq[_] => }
Wildcard patterns
Some(5) match { case Some(_) => println("Yes") }
Wildcard imports
import java.util._
Hiding imports
import java.util.{ArrayList => _, _}
Joining letters to punctuation
def bang_!(x: Int) = 5
Assignment operators
def foo_=(x: Int) { ... }
Placeholder syntax
List(1, 2, 3) map (_ + 2)
Partially applied functions
List(1, 2, 3) foreach println _
Converting call-by-name parameters to functions
def toFunction(callByName: => Int): () => Int = callByName _
def sum(xs: Int*) = (0 /: xs) ((x, y) => x + y)
Then
sum(1, 2, 3, 4)
sum(List(1, 2, 3, 4): _*)
