Question

尝试使用Play的Json库反序列化json字符串时出现以下错误。我理解它无法解决重载方法，但我无法理解为什么？

Error:(45, 50) overloaded method value apply with alternatives:
  [B](f: B => (String, String, com.model.ESource.Value, com.model.Address, java.time.ZonedDateTime, java.time.ZonedDateTime))(implicit fu: play.api.libs.functional.ContravariantFunctor[play.api.libs.json.Reads])play.api.libs.json.Reads[B] <and>
  [B](f: (String, String, com.model.ESource.Value, com.model.Address, java.time.ZonedDateTime, java.time.ZonedDateTime) => B)(implicit fu: play.api.libs.functional.Functor[play.api.libs.json.Reads])play.api.libs.json.Reads[B]
 cannot be applied to (com.model.Event.type)
      (JsPath \ "startTime").read[ZonedDateTime] and
                                                     ^

上述异常是由（我使用Play的JSON库 - import play.api.libs）引起的：

case class Event(name: String,
                 sourceId: String,
                 sourceType: ESource.ESource,
                 address: Address,
                 startTime: ZonedDateTime,
                 endTime: ZonedDateTime) {
  val id = Array( name, startTime, endTime ).mkString("-")

  def toJsonString(): String = Json.toJson(this)(Event.jsonWrites).toString()

  def fromJsonString(jsonString: String): Event = {
    val jv = Json.parse(jsonString)
    Json.fromJson[Event](jv)(Event.jsonReads).get
  }
}

object Event {
  val jsonWrites: Writes[Event] = (
    (JsPath \ "name").write[String] and
      (JsPath \ "sourceId").write[String] and
      (JsPath \ "sourceType").write[ESource.ESource] and
      (JsPath \ "address").write[Address](Address.jsonWrites) and
      (JsPath \ "startTime").write[ZonedDateTime] and
      (JsPath \ "endTime").write[ZonedDateTime]
    )(unlift(Event.unapply))

  val jsonReads: Reads[Event] = (
    (JsPath \ "name").read[String] and
      (JsPath \ "sourceId").read[String] and
      (JsPath \ "sourceType").read[ESource.ESource] and
      (JsPath \ "address").read[Address](Address.jsonReads) and
      (JsPath \ "startTime").read[ZonedDateTime] and
      (JsPath \ "endTime").read[ZonedDateTime]
    )(Event)
}

我甚至不确定语法实际意味着什么？我看到=>用于匿名函数，其中右边是参数，左边是函数表达式。但是我不确定异常中的B是什么意思以及如何解释2个方法签名？

Answer 1

在jsonReads中，(Event)必须为(Event.apply _)。读取组合器的函数必须与其签名匹配，在本例中为Event.apply _。

Event本身指的是伴随对象，其类型为Event.type。因此错误消息片段cannot be applied to (com.model.Event.type)。

解析重载方法时出现Scala错误

1 个答案: