Custom Codecs

Custom encoders/decoders

If you want to write your own codec instead of using automatic or semi-automatic derivation, you can do so in a couple of ways.

Firstly, you can write a new Encoder[A] and Decoder[A] from scratch

class Thing()

implicit val encodeFoo: Encoder[Thing] = new Encoder[Thing] {
  final def apply(a: Thing): Json = ??? // your implementation goes here

implicit val decodeFoo: Decoder[Thing] = new Decoder[Thing] {
  final def apply(c: HCursor): Decoder.Result[Thing] = Left(DecodingFailure("Not implemented yet", c.history))

But in many cases you might find it more convenient to piggyback on top of the decoders that are already available. For example, a codec for java.time.Instant might look like this:

import cats.syntax.either._

import java.time.Instant

implicit val encodeInstant: Encoder[Instant] = Encoder.encodeString.contramap[Instant](_.toString)

implicit val decodeInstant: Decoder[Instant] = Decoder.decodeString.emap { str =>
  Either.catchNonFatal(Instant.parse(str)).leftMap(t => "Instant")

Custom key types

If you need to encode/decode Map[K, V] where K is not String (or Symbol, Int, Long, etc), you need to provide a KeyEncoder and/or KeyDecoder for your custom key type.

For example:

import io.circe._, import io.circe.syntax._

case class Foo(value: String)

implicit val fooKeyEncoder = new KeyEncoder[Foo] {
  override def apply(foo: Foo): String = foo.value

val map = Map[Foo, Int](
  Foo("hello") -> 123,
  Foo("world") -> 456

val json = map.asJson

implicit val fooKeyDecoder = new KeyDecoder[Foo] {
  override def apply(key: String): Option[Foo] = Some(Foo(key))
}[Map[Foo, Int]]

What would be returned as a result of decoding and traversing the returned Map:

json.hcursor.downField("hello").as[Int] should be(res0)

Custom key mappings via annotations

It’s often necessary to work with keys in your JSON objects that aren’t idiomatic case class member names in Scala. While the standard generic derivation doesn’t support this use case, the experimental circe-generic-extras module does provide two ways to transform your case class member names during encoding and decoding.

In many cases the transformation is as simple as going from camel case to snake case, in which case all you need is a custom implicit configuration:

import io.circe.generic.extras._, io.circe.syntax._
val config: Configuration = Configuration.default.withSnakeCaseMemberNames

@ConfiguredJsonCodec case class User(firstName: String, lastName: String)

User("Foo", "McBar").asJson.noSpaces shouldBe res0

In other cases you may need more complex mappings. These can be provided as a function:

val config: Configuration = Configuration.default.copy(
  transformMemberNames = {
    case "i" => "my-int"
    case other => other

@ConfiguredJsonCodec case class Bar(i: Int, s: String)

Bar(13, "Qux").asJson.noSpaces shouldBe res0

Since this is a common use case, we also support for mapping member names via an annotation:

val config: Configuration = Configuration.default

@ConfiguredJsonCodec case class Bar(@JsonKey("my-int") i: Int, s: String)

Bar(13, "Qux").asJson.noSpaces shouldBe res0