Circe和Scala的枚举类型 [英] Circe and Scala's Enumeration type

问题描述

我想把头围在Circe上.

I'm trying to wrap my head around Circe.

所以，这是我得到的模型:

So, here's the model I've been given:

object Gender extends Enumeration {
     type Gender = Value
     val Male, Female, Unisex, Unknown = Value
}

case class Product(id: String, gender: Gender.Value)

我想

a)将此简单示例编码为JSON字符串

a) encode this simple example to a JSON string

        val product = Product(id = "1234", gender = Gender.Female)

b)将生成的JSON映射回Product案例类.

b) map the resulting JSON back onto the Product case class.

我自己的尝试并没有使我走得很远:

My own attempt didn't get me very far:

  object JsonProtocol {
      implicit val productDecoder: Decoder[Product] = deriveDecoder
      implicit val productEncoder: Encoder[Product] = deriveEncoder
  }

导致编译时错误

   Error:(52, 49) could not find Lazy implicit value of type io.circe.generic.decoding.DerivedDecoder[A]
   implicit val productDecoder: Decoder[Product] = deriveDecoder
                                            ^

我不知道为什么会抛出该异常以及解决方案的外观.也许是Enumeration类型的用法?但是，我只是在猜测.

I've no idea why this exception is thrown and what the solution could look like. Maybe it's the usage of the Enumeration type? But, I'm only guessing.

推荐答案

尝试使用以下方法为枚举定义自己的编码器和解码器:

Try defining your own encoders and decoders for the enum using:

Decoder.enumDecoder[E <: Enumeration](enum: E)
Encoder.enumEncoder[E <: Enumeration](enum: E)

类似:

object JsonProtocol {
  implicit val genderDecoder: Decoder[Gender.Value] = Decoder.enumDecoder(Gender)
  implicit val genderEncoder: Encoder[Gender.Value] = Encoder.enumEncoder(Gender)
  implicit val productDecoder: Decoder[Product] = deriveDecoder
  implicit val productEncoder: Encoder[Product] = deriveEncoder
}

这些是必需的，因为据我所知，自动/半自动派生器仅适用于sealed trait s和case classes的层次结构.之所以会看到该错误，是因为Product的派生编解码器将隐式要求使用编码器/解码器来表示其每个参数的类型. String的编码器/解码器是Circe的标准部分，但您可能需要为自己的枚举创建一个编码器/解码器.

These are needed because the automatic/semiautomatic derivers only work for hierarchies of sealed traits and case classes as far as I know. The reason you see that error is because the derived codecs for Product will implicitly require encoders/decoders for the types of each of it's parameters. An encoder/decoder for String is a standard part of Circe, but you'll probably need to create ones for your own enumerations.

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