update: 2017-07-15

A. 语法

method vs function

// method
scala> def m(x: Int, y: Int): Int = x + y
res1: m: (x: Int, y: Int)Int

// function
scala> val f = (x: Int, y: Int) => x + y
res2: f: (Int, Int) => Int = <function2>

// method to function
scala> m _
res3: (Int, Int) => Int = <function2>

// curried
scala> f.curried
res4: Int => (Int => Int) = <function1>

scala> (m _).curried
res5: Int => (Int => Int) = <function1>

传参时，每个_对应一个参数：

// (1) wrong
List(1, 2, 3).map(x => x + 1)

List(1, 2, 3).map(_ => _ + 1) // won't compile
// = List(1, 2, 3).map(x => {y => y + 1})

// (2) right
List(1, 2, 3, 4).reduce((x, y) => x + y)
// is equivalent to
List(1, 2, 3, 4).reduce(_ + _)

参考：In Scala, what is the difference between using the _ and using a named identifier?

Implicit

B. 用例

遍历Option[Array[A]]类型

val oas: Option[Array[Int]] = Some(Array(1, 2, 3))

val res =
 for {
   as <- oas.toArray // or use .toSeq
   a <- as
 } yield a + 1

这里之所以要使用.toArray变换，是为了使外层的Option.flatMap转成Array.flatMap，从而和内层的Array.map类型兼容。

参考：For comprehension over Option array

函数用元组做为参数列表

scala> def f(i : Int, j : Int) = i + j
scala> val args = (0, 1)

scala> val tf = (f _).tupled
tf: ((Int, Int)) => Int = <function1>

scala> tf(args)
res7: Int = 1

参考：How to apply a function to a tuple?