递归期间的多态类型错误 - 如何解决？

Question

我是 Scala 的新手，正在学习这本书 "FP in Scala"。现在我正在为正在重新创建的 Stream 数据类型编写展开函数。问题是，类型检查器告诉我递归的多态类型似乎是错误的。

这是 Stream trait 及其静态对象，包括展开函数：

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

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

  def unfold[A, S](z: S)(f: S => Option[(A, S)]): StreamTrait[A] = {
      f(z) match {
        case None => StreamTrait.empty
        case Some(tuple) =>
          StreamTrait.cons(tuple._1, unfold[A, S](tuple._2)(f))
      }
  }
}

输出为：

polymorphic expression cannot be instantiated to expected type; found : [A(in method unfold)](f: ((A(in method constantUnfold), A(in method constantUnfold))) => Option[(A(in method unfold), (A(in method constantUnfold), A(in method constantUnfold)))])StreamTrait[A(in method unfold)] required: StreamTrait[A(in method constantUnfold)] def constantUnfold[A](a: A): StreamTrait[A] = unfold(a, identity(a));

Answer 1

如果将 StreamTrait.empty 替换为 Empty 则编译正常。

不知道它是否有效...

Answer 2

似乎我定义了另一个函数，该函数使用错误的参数调用此展开方法...愚蠢的我，无论如何谢谢:)

递归期间的多态类型错误 - 如何解决？

Polymorphic type error during recursion - how to solve?

scala

scala-streams