找不到参数编码器的隐式值:io.circe.Encoder[com.sweetsoft.SapHealth]

Question

我有以下代码，无法编译：

import java.time.Instant
import io.circe.{Decoder, Encoder}
import io.circe.generic.auto._
import io.circe.syntax._
trait SapHealth {}
case class SapHealthRejected(reason: String) extends SapHealth
case class SapHealthAccepted(sapId: String, requestedAt: Long) extends SapHealth

object SapHealth {
  private val build: SapHealth = SapHealthAccepted(SapmockActor.system.name, Instant.now().getEpochSecond)
  val create: String = build.asJson.noSpaces
  implicit val encodeFieldType: Encoder[SapHealthAccepted] =
    Encoder.forProduct2("sap-id", "requested_at")(SapHealthAccepted.unapply(_).get)
  implicit val decodeFieldType: Decoder[SapHealthAccepted] =
    Decoder.forProduct2("sap-id", "requested_at")(SapHealthAccepted.apply)
}

编译器抱怨：

could not find implicit value for parameter encoder: io.circe.Encoder[com.sweetsoft.SapHealth]
[error]   val create: String = build.asJson.noSpaces

我错过了什么？

Answer 1

您专门将build向上转换为SapHealth，但您没有为SapHealth提供Encoder实例(仅SapHealthAccepted(，并且 circe-generic 无法派生一个，因为您尚未密封特征层次结构。

最直接的解决方案是添加sealed：

import io.circe.{Decoder, Encoder}
import io.circe.generic.auto._
import io.circe.syntax._
sealed trait SapHealth {}
case class SapHealthRejected(reason: String) extends SapHealth
case class SapHealthAccepted(sapId: String, requestedAt: Long) extends SapHealth
object SapHealth {
  implicit val encodeFieldType: Encoder[SapHealthAccepted] =
    Encoder.forProduct2("sap-id", "requested_at")(SapHealthAccepted.unapply(_).get)
  implicit val decodeFieldType: Decoder[SapHealthAccepted] =
    Decoder.forProduct2("sap-id", "requested_at")(SapHealthAccepted.apply)
  private val build: SapHealth = SapHealthAccepted("foo", 123L)
  val create: String = build.asJson.noSpaces
}

请注意，您还需要重新排列定义以避免由于初始化顺序而遇到空指针异常(如果将create放在encodeFieldType之前，派生的SapHealth编码器将在初始化之前尝试使用encodeFieldType(。通过上面的重新排列，这工作得很好：

scala> SapHealth.create
res2: String = {"SapHealthAccepted":{"sap-id":"foo","requested_at":123}}

请注意，派生的SapHealth编码器使用的是自定义SapHealthAccepted编码器，我认为这是您想要的。

Answer 2

应

密封SapHealth特征。

Answer 3

对于案例类，您可以使用 deriveEncoder/deriveDecoder。但是在复杂类的情况下，您应该(正如我认为的那样(为参数的类指定它。

https://circe.github.io/circe/codecs/semiauto-derivation.html

import io.circe._, io.circe.generic.semiauto._
case class Foo(a: Int, b: String, c: Boolean)
implicit val fooDecoder: Decoder[Foo] = deriveDecoder[Foo]
implicit val fooEncoder: Encoder[Foo] = deriveEncoder[Foo]