Scala error while resolving overloaded methods

Question

Im getting the following error while trying to use the Play `s Json library for de-serializing json string. I understand it is failing to resolve to an overloaded method but Im failing to understand why?

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
                                                     ^

the above exception is caused by ( Im using Play's JSON library - import play.api.libs ):

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)
}

Im not even sure what the syntax actually means? I see that => is used for anonymous functions where the right side are arguments and left would be the function expression. But Im not sure what the B in the exception refers to and how the 2 method signatures are to be interpreted?

Answer 1

In jsonReads, (Event) needs to be (Event.apply _). The function that the reads combinators are being applied to must match their signature, which in this case is Event.apply _.

Event by itself is referring to the companion object, which has type Event.type. Hence the error message snippet cannot be applied to (com.model.Event.type).