Question

我想为值类派生Encoder实例。使用semiauto机制，我无法派生嵌套类。

图像以下案例类结构

{
  case class Recipient(email: Recipient.Email, name: Recipient.Name) 

  object Recipient {
     case class Email(value: String) extends AnyVal
     case class Name(value: String) extends AnyVal
  }
}

在菊石壳中（也添加收件人案例类）

load.ivy("io.circe" %% "circe-core" % "0.6.1")
load.ivy("io.circe" %% "circe-generic" % "0.6.1")

import io.circe._
import io.circe.generic.semiauto._
import io.circe.syntax._

现在在

中按预期导出Email个结果的解码器

Recipient.Email("ab@cd.com").asJson(deriveEncoder[Recipient.Email]) 
Json = {
  "value" : "ab@cd.com"
}

导出Encoder[Recipient]无法正常工作

deriveDecoder[Recipient]

could not find Lazy implicit value of type 
io.circe.generic.decoding.DerivedDecoder[$sess.cmd5.Recipient]

我想要做的是派生一个返回包装类型的Encoder[Recipient.Email]。如果我明确地派生出编解码器，这个小部分就可以工作。

import shapeless.Unwrapped

implicit def encodeAnyVal[W <: AnyVal, U](
        implicit unwrapped: Unwrapped.Aux[W, U],
                 encoderUnwrapped: Encoder[U]): Encoder[W] = {
        Encoder.instance[W](v => encoderUnwrapped(unwrapped.unwrap(v)))
}

Recipient.Email("ab@cd.com").asJson(encodeAnyVal[Recipient.Email, String])
res11: Json = "ab@cd.com"

我仍然无法获得Encoder[Recipient]

implicit val emailEncoder: Encoder[Recipient.Email] = encodeAnyVal[Recipient.Email, String] 
implicit val nameEncoder: Encoder[Recipient.Name] = encodeAnyVal[Recipient.Name, String]

deriveDecoder[Recipient] 
cmd14.sc:1: could not find Lazy implicit value of type io.circe.generic.decoding.DerivedDecoder[$sess.cmd5.Recipient]

有没有人做过类似的事情？

提前致谢，缪奇

Answer 1

您应该添加隐式见证实例而不是类型绑定。最终会得到这样的结论：

implicit def encodeAnyVal[W, U](
        implicit ev: W <:< Anyval,
                 unwrapped: Unwrapped.Aux[W, U],
                 encoderUnwrapped: Encoder[U]): Encoder[W] = {
        Encoder.instance[W](v => encoderUnwrapped(unwrapped.unwrap(v)))
}

使用Circe的AnyVal类型的通用推导

1 个答案: