我试图从Functional Programming in Scala解决练习2。问题如下:
练习2:写一个返回前n个元素的函数 流。 def取(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尝试加载我的脚本时,它会barfs并出现以下错误:
阶> :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]{
有人可以指出这里可能出现的问题吗?
答案 0 :(得分:1)
只需在Stream特征下放置import语句即可。它不起作用导致您导入scala.collection.immutable.Stream
的Scala编译器,而不是您的伴随对象。并且,因为评论中的悲伤在控制台中使用:paste
,但是将其粘贴为漏洞代码,否则它将不是您的特征的伴随对象