为流实现take时出错

Question

我试图解决Scala函数编程中的练习2。问题如下：

练习2：编写返回前n个元素的函数take流。def-take（n:Int）：流[A]

我的解决方案如下：

    import Stream._
    trait Stream[+A]{
            def uncons:Option[(A,Stream[A])]
            def isEmpty:Boolean = uncons.isEmpty
            def toList:List[A] = {
                    val listBuffer = new collection.mutable.ListBuffer[A]
                    @annotation.tailrec
                    def go(str:Stream[A]):List[A] = str uncons match {
                            case Some((a,tail)) => listBuffer += a;go(tail)
                            case _ => listBuffer.toList
                    }
                    go(this)
            }
            def take(n:Int):Stream[A] = uncons match {
                    case Some((hd,tl)) if (n > 0) => cons(hd,tl.take(n-1))
                    case _ => Stream()
            }
    }
   object Stream{
        def empty[A]:Stream[A] = new Stream[A]{def uncons = None}
        def cons[A](hd: => A,tl: => Stream[A]):Stream[A] = new Stream[A]{
                lazy val uncons = Some((hd,tl))
        }
        def apply[A](as: A*):Stream[A] = {
                if(as.isEmpty) empty else
                        cons(as.head,apply(as.tail: _ *))
        }
}

我将其存储为Stream2.scala，然后从REPL执行以下操作：

:load Stream2.scala

当REPL试图加载我的脚本时，它会弹出以下错误：

scala>：加载Stream2.scala

Loading Stream2.scala...
import Stream._
<console>:24: error: type mismatch;
 found   : Stream[A]
 required: scala.collection.immutable.Stream[?]
            case Some((hd,tl)) if (n > 0) => cons(hd,tl.take(n-1))
                                                                ^
<console>:25: error: type mismatch;
 found   : scala.collection.immutable.Stream[Nothing]
 required: Stream[A]
            case _ => Stream()
                                ^
<console>:11: error: object creation impossible, since method tailDefined in class Stream of type => Boolean is not defined
        def empty[A]:Stream[A] = new Stream[A]{def uncons = None}
                                     ^
<console>:12: error: object creation impossible, since method tailDefined in class Stream of type => Boolean is not defined
        def cons[A](hd: => A,tl: => Stream[A]):Stream[A] = new Stream[A]{

有人能指出这里可能出了什么问题吗？

Answer 1

只需将import语句放在Stream特性下即可。它不起作用，因为Scala编译器要导入scala.collection.immutable.Stream，但不能导入配套对象。而且，正如评论中令人难过的那样，在控制台中使用:paste，但将其粘贴为孔代码，否则它将不会成为您的特征的伴随对象