[SOLVED] Is that possible to make semiauto decoders consider default values for case class fields?

Is that possible to make semiauto decoders consider default values for case class fields?

The following code will fail with:

Left(DecodingFailure(Attempt to decode value on failed cursor, List(DownField(isActive))))

I thought circe would consider the default value for the case class field isActive

case class Person(
  id: Option[Int] = None,
  name: String,
  isActive: Boolean = true
)

implicit val personJsonDecoder: Decoder[Person] = deriveDecoder

val rawJson = """
{
  "name": "Geovanny Junio"
}
"""

val r = for {
  j <- parse(rawJson)
  p <- j.as[Person]
} yield p

println(r)

Solution

Yes, but you'll need circe-generic-extras:

import io.circe.Decoder
import io.circe.generic.extras.Configuration
import io.circe.generic.extras.semiauto.deriveDecoder

case class Person(
  id: Option[Int] = None,
  name: String,
  isActive: Boolean = true
)

object Person {
  implicit val personConfig: Configuration =
    Configuration.default.withDefaults
  implicit val personJsonDecoder: Decoder[Person] = deriveDecoder
}

And then:

scala> io.circe.jawn.decode[Person]("""{"name": "Geovanny Junio"}""")
res0: Either[io.circe.Error,Person] = Right(Person(None,Geovanny Junio,true))

I've been intending to add this functionality to circe-derivation, but haven't had time, so circe-generic-extras is the only way to make it work for now (short of writing your own decoder).