是否可以使半自动解码器考虑案例类字段的默认值？

Question

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)

Answer 1

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).