[英]Scala: methods in trait can't use methods from companion object
當我嘗試編譯以下內容時,我得到:“未找到:值 cons”和“未找到:值為空”的 take 和 drop 方法定義。
不知何故,特征沒有“看到”伴隨對象?
我正在使用 IntelliJ IDEA,以防萬一。
import scala.annotation.tailrec
object Run extends App {
sealed trait StreamRed[+A] {
def headOption: Option[A] = this match {
case Empty => None
case Cons(h,t) => Some(h())
}
def toList: List[A] = {
@tailrec
def toListRec(stream: StreamRed[A], accumulated: List[A]): List[A] = this match {
case Cons(h,t) => toListRec(t(), h()::accumulated)
case _ => accumulated
}
toListRec(this, List()).reverse
}
def take(n: Int): StreamRed[A] = this match {
case Cons(h, t) if n > 1 => cons(h(), t().take(n - 1))
case Cons(h, _) if n == 1 => cons(h(), empty)
case _ => empty
}
@tailrec
def drop(n: Int): StreamRed[A] = this match {
case Cons(_,t) if n > 0 => t().drop(n-1)
case _ => empty
}
}
case object Empty extends StreamRed[Nothing]
case class Cons[+A](h: () => A, t: () => StreamRed[A]) extends StreamRed[A]
object StreamRed {
def cons[A](hd: => A, tl: => StreamRed[A]): StreamRed[A] = {
lazy val head = hd
lazy val tail = tl
Cons(() => head, () => tail)
}
def empty[A]: StreamRed[A] = Empty
def apply[A](as: A*): StreamRed[A] =
if (as.isEmpty) empty else cons(as.head, apply(as.tail: _*))
}
}
從訪問修飾符不是問題的意義上說,同伴可以看到彼此的成員。
class A {
private def foo: Unit = ()
A.bar
}
object A {
private def bar: Unit = ()
(new A).foo
}
可以,相反
class A {
private def foo: Unit = ()
B.bar
}
object B {
private def bar: Unit = ()
(new A).foo
}
(但如果你用private[this]
替換private
,前者也不起作用。)
但這並不意味着名稱空間是自動導入的。
class A {
private def foo: Unit = ()
import A._
bar
}
object A {
private def bar: Unit = ()
val a = new A
import a._
foo
}
可以,相反
class A {
private def foo: Unit = ()
bar
}
object A {
private def bar: Unit = ()
foo
}
無論如何,方法必須知道它的this
。
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