显式类型的call-by-name函数参数

Question

给出方法

def twice(s: => Int): Int = s + s

我可以通过以下方式获得Function1类型(=> Int) => Int：

val valTwiceA: (=> Int) => Int = twice
// or
val valTwiceC: (=> Int) => Int = {(byName/*:(=>Int)*/) => twice(byName)}

我想无法明确指定byName的类型。

1. 我是对的吗？
2. 明确指定byName类型的正确方法是什么？ (byName: (=>Int))会产生错误“此处不允许使用名称参数类型”


代码模板：

// scalaVersion "2.10.4" or "2.11.1"
import org.scalatest.FreeSpec

class ByNameTest extends FreeSpec {
  def twice(s: => Int): Int = s + s
  val valTwiceA: (=> Int) => Int = twice
  val valTwiceB: (=> Int) => Int = {byName => twice(byName)}
  val valTwiceC: (=> Int) => Int = {(byName/*:(=>Int)*/) => twice(byName)}

  "twice" in {
    val it = Iterator from 1
    assert(valTwiceA(it.next) === 3)
    assert(valTwiceB(it.next) === 7)
    assert(valTwiceC(it.next) === 11)
  }
}