我正在通过" Scala中的功能编程"书。第5.2章从以下代码开始:
sealed trait Stream[+A]
case object Empty extends Stream[Nothing]
case class Cons[+A](h: () => A, t: () => Stream[A]) extends Stream[A]
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: _*))
}
然后给出以下定义:
def headOption: Option[A] = this match {
case Empty => None
case Cons(h, t) => Some(h())
}
我认为它应该是伴侣对象的一部分。为了不立即得到编译错误,我添加" headOption [A]"。但是当我将结果包含在"对象流"我收到错误"模式类型与预期类型不兼容; found:myPackage.Empty.type required:myPackage.Stream.type"与"空"下划线。
我在这里有点失落。我做错了什么?
答案 0 :(得分:2)
它应该是trait Stream[+A]
。
此处this
引用Stream
(特征流不是伴侣对象)。 Stream
可以是Empty
,也可以是Cons
。因此,this
上的模式匹配是有意义的。
headOption
给出了流的第一个元素(如果可用)。
case object Empty extends Stream[Nothing]
case class Cons[+A](h: () => A, t: () => Stream[A]) extends Stream[A]
sealed trait Stream[+A] {
def headOption: Option[A] = this match {
case Empty => None
case Cons(h, t) => Some(h())
}
}
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: _*))
}
Scala REPL
scala> :paste
// Entering paste mode (ctrl-D to finish)
case object Empty extends Stream[Nothing]
case class Cons[+A](h: () => A, t: () => Stream[A]) extends Stream[A]
sealed trait Stream[+A] {
def headOption: Option[A] = this match {
case Empty => None
case Cons(h, t) => Some(h())
}
}
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: _*))
}
// Exiting paste mode, now interpreting.
defined object Empty
defined class Cons
defined trait Stream
defined object Stream
scala> Stream.cons(1, Empty).headOption
res0: Option[Int] = Some(1)