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

特性SapHealth应该被封印。

Answer 2

您专门 up-cast build 到 SapHealth，但您没有为 SapHealth 提供 Encoder 实例（仅 SapHealthAccepted), circe-generic 无法推导出一个，因为你还没有密封特征层次结构。

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 运行 由于初始化顺序而变成 null-pointer 异常（如果您将 create 放在 encodeFieldType 之前，派生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，效果很好：

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

请注意，派生的 SapHealth 编码器正在使用您的自定义 SapHealthAccepted 编码器，我认为这就是您想要的。

Answer 3

只是为了 class 你可以使用 deriveEncoder/deriveDecoder。但是在复杂的情况下 class 你应该（我认为）为参数的 classes.

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]