Scala：实现通用折叠

Question

I don't understand why thid fold doesn't compile. Could anyone give me a clue?

sealed trait ListG[A] {

  def fold[A,B](end: B, f: (A,B) => B): B = this match {
    case End() => end
    case Cons(hd,tl) => f(hd, tl.fold(end,f))
  }
}

Error:(20, 28) type mismatch; found : hd.type (with underlying type A) required: A case Cons(hd,tl) => f(hd, tl.fold(end,f)) ^ final case class EndA extends ListG[A] final case class Cons[A](hd:A, tl:ListG[A]) extends ListG[A]

Answer 1

你的阴影类型参数A的ListG当你定义一个额外的类型参数A对fold功能。

Answer 2

Adding type ascription appears to fix the problem.

case Cons(hd:A, tl) => ...
            ^^

There is a warning about type erasure but it does compile and appears to run.