Json 无法解码Circe中的集合

我正在尝试解码这段Json：

{
  "id" : "e07cff6a-bbf7-4bc9-b2ec-ff2ea8e46288",
  "paper" : {
    "title" : "Example Title",
    "authors" : [
      "1bf5e911-8878-4e06-ba8e-8159aadb052c"
    ]
  }
}

然而，当它到达集合部分时，它失败了。错误消息没有帮助

DecodingFailure([A]Set[A], List())

这是我的文件编码员：

  implicit val paperIdDecoder: Decoder[PaperId] = Decoder.decodeString.emap[PaperId] { str ⇒
    Either.catchNonFatal(PaperId(str)).leftMap(_.getMessage)
  }

  implicit val paperAuthorDecoder: Decoder[PaperAuthor] = Decoder.decodeString.emap[PaperAuthor] { str ⇒
    Either.catchNonFatal(PaperAuthor(str)).leftMap(_.getMessage)
  }

  implicit val paperDecoder: Decoder[Paper] = {
    for {
      title <- Decoder.decodeString
      authors <- Decoder.decodeSet[PaperAuthor]
    } yield Paper(title, authors)
  }

  implicit val paperViewDecoder: Decoder[PublishedPaperView] = for {
    id <- Decoder[PaperId]
    paper <- Decoder[Paper]
  } yield PublishedPaperView(id, paper) 

---

虽然错误描述远未明确，但您的问题与解码器的一元API的错误使用有关：请记住，理解的a是map/flatMap的语法糖

从

io.circe.Decoder

  /**
   * Monadically bind a function over this [[Decoder]].
   */
  final def flatMap[B](f: A => Decoder[B]): Decoder[B] = new Decoder[B] {
    final def apply(c: HCursor): Decoder.Result[B] = self(c).flatMap(a => f(a)(c))

    override def tryDecode(c: ACursor): Decoder.Result[B] = {
      self.tryDecode(c).flatMap(a => f(a).tryDecode(c))
    }

    override def decodeAccumulating(c: HCursor): AccumulatingDecoder.Result[B] =
      self.decodeAccumulating(c).andThen(result => f(result).decodeAccumulating(c))
  }

查看这段代码，您可以看到，当您平面映射一个解码器时，您会看到一个新的解码器在相同的光标上运行：光标是解析操作的当前位置

在以下代码中：

implicit val paperDecoder: Decoder[Paper] = {
    for {
      title <- Decoder.decodeString
      authors <- Decoder.decodeSet[PaperAuthor]
    } yield Paper(title, authors)
  }

解码器：解码器[纸张]={
为了{
标题
为了{
身份证件
implicit val paperDecoder: Decoder[Paper] = {
    for {
      title <- Decoder.decodeString
      authors <- Decoder.decodeSet[PaperAuthor]
    } yield Paper(title, authors)
  }

  implicit val paperDecoder: Decoder[Paper] = Decoder.instance(cursor => Xor.right(Paper("",Set.empty)))

  implicit val paperViewDecoder: Decoder[PublishedPaperView] = Decoder.instance(
    cursor =>
      for {
        id <- cursor.downField("id").as[PaperId]
        paper <- cursor.downField("paper").as[Paper]
     } yield PublishedPaperView(id, paper)
  )