How can I define some generic decoder/encoder with circe?

Question

Suppose I have multiple types of ids defined as follows.

sealed trait Id {
  val value: String
}
case class IdA(value: String) extends Id
case class IdB(value: String) extends Id
case class IdC(value: String) extends Id

These classes should be decoded from and encoded to the following JSON.

 {
    "id: "some-id"
 }

How can I define some generic decoder/encoder for the id,

case class A(id: IdA, name: String, count: Int)
case class B(id: IdB, name: String, count: Int)
case class C(id: IdC, name: String, count: Int)

so that the classes above can be decoded from and encoded to the JSON below?

{
   "id" : "some-id",
   "name": "some-name",
   "count": 2
}

if possible, I want the id field to be flexible for both decoder and encoder that is id maybe "id-x" in one case and "id-y" in another.

You can create a custom `Codec` for the trait `Id` defining how it should work. — Luis Miguel Mejía Suárez, Feb 14 '21 at 13:16
You can check out [the official documentation about custom codecs](https://circe.github.io/circe/codecs/custom-codecs.html) — francoisr, Feb 15 '21 at 07:20
I feel like you are mixing few questions into one. Which json should decode into which classes? And which classes should encode into which jsons? — Tomer Shetah, Feb 15 '21 at 10:39

score 1 · Accepted Answer · answered Feb 15 '21 at 13:30

I have solved it by simply defining the following decoder and encoder.

import cats.implicits._
import io.circe.{Decoder, Encoder}
import shapeless._

 implicit def encoderValueClass[T <: Id, V](implicit
      g: Lazy[Generic.Aux[T, V :: HNil]],
      e: Encoder[V]
  ): Encoder[T] = Encoder.instance { value =>
    e(g.value.to(value).head)
  }

  implicit def idDecoder[T <: Id, V](implicit
      g: Lazy[Generic.Aux[T, V :: HNil]],
      d: Decoder[V]
  ): Decoder[T] = Decoder.instance { cursor =>
    d(cursor).map { value =>
      g.value.from(value :: HNil)
    }
  }

score 0 · Answer 2 · answered Mar 03 '23 at 12:48

0

This answer https://stackoverflow.com/a/75627162/21293767 with Circe decoder for generic case class with default parameters using generics with case class MyClass[T] might help

answered Mar 03 '23 at 12:48

Alberto Serna

1
3

How can I define some generic decoder/encoder with circe?

2 Answers2