斯卡拉流方法TakeWhile

Question

我通过曼宁的工作“函数式编程Scala中”并与流的问题，这是文件：

package chapter05 


sealed trait Stream[+A]{ 
case object Empty extends Stream[Nothing] 
case class Cons[+A](h:() => A, t:() => Stream[A]) extends Stream[A] 

def headOption: A = this match { 
case Empty => throw new Exception("optional") 
case Cons(h, t) => h() 
} 

def toList: List[A] = this match { 
    case Cons(h, t) => h() :: t().toList 
    case Empty => Nil 
} 


    def takeWhile1(p: A => Boolean): Stream[A] = 
    this match { 
    case Cons(h, t) if (p(h())) => Stream.cons(h(), t().takeWhile1(p)) 
    case _ => Empty 
} 


object Stream { 

    def cons[A](hd: => A, tl : => Stream[A]): Stream[A] = { 
    lazy val head = hd 
    lazy val tail = tl 
    Cons(() => head,() => tail) 
    } 
    def empty[A]: Stream[A] = Empty 

    def apply[A](as: A*): Stream[A] = 
    if (as.isEmpty) empty else cons(as.head, apply(as.tail:_*)) 
    } 
} 

应用和takeWhile不进行编译，我不知道为什么，它的逻辑似乎是好的（从书中取材）。

Answer 1

你在代码中有一些问题，因为它是，修改后的版本：

case object Empty extends Stream[Nothing] 
case class Cons[+A](h:() => A, t:() => Stream[A]) extends Stream[A] 
sealed trait Stream[+A]{ 

    def headOption: A = this match { 
    case Empty => throw new Exception("optional") 
    case Cons(h, t) => h() 
    } 

    def toList: List[A] = this match { 
    case Cons(h, t) => h() :: t().toList 
    case Empty => Nil 
    } 


    def takeWhile1(p: A => Boolean): Stream[A] = this match { 
    case Cons(h, t) if (p(h())) => Stream.cons(h(), t().takeWhile1(p)) 
    case _ => Empty 
    } 
} // Missed this 


object Stream { 

    def cons[A](hd: => A, tl : => Stream[A]): Stream[A] = { 
    lazy val head = hd 
    lazy val tail = tl 
    Cons(() => head,() => tail) 
    } 

    def empty[A]: Stream[A] = Empty 

    def apply[A](as: A*): Stream[A] = 
    if (as.isEmpty) empty else cons(as.head, apply(as.tail:_*)) 
    // there was one too many } 
} 

请注意，在代码中的注释。

第一个问题是性格内部的缺陷和空缺，我认为这不合理，它会在伴侣对象或顶层内部有意义。

第二个问题，如果你已经正确缩进了代码，你会很容易发现平衡括号的问题。