Question

任何人都可以告诉我在Scala中是否可以使用以下语法编写某些内容？

  @annotation.tailrec
  def traverse[E,A,B](es: List[A])(f: A => Either[E, B]): Either[E, List[B]] = {
    def go(es: List[A], rs: Either[E, List[B]]): Either[E, List[B]] = {
      es match {
        case Nil => rs
        case x::xs => for {
          Right(b) <- f(x);
          Right(ls) <- rs
        } yield go(xs, Right(b::ls))
      }
    }

    go(es, Right(List()))
  }

我不断收到以下语法异常

Error:(47, 12) constructor cannot be instantiated to expected type;
 found   : A$A400.this.Right[A]
 required: List[?B3] where type ?B3 <: B (this is a GADT skolem)
          Right(ls) <- rs
          ^

Answer 1

说实话，我并不完全确定该功能的目的是什么，但是，猜测f是什么，这里有什么东西可以做你想要的？

@annotation.tailrec
  def f[A, B, E](e: A): Either[E, B] = ???

  def go[A, B, E](es: List[A], rs: Either[E, List[B]]): Either[E, List[B]] = {
    es match {
      case Nil => rs
      case x :: xs => (f(x), rs) match {
        case (Right(b), Right(ls)) => go(xs, Right(b :: ls))
      }
    }

    go(es, Right(List()))
  }

用于理解类型检查

1 个答案: