Decode config to case class with pureconfig with default values

Question

Assuming I have the following config:

{
  “default-value”:5,
  “some-seq”: [
    {“some-other-value”:100},
    {“foo”:”bar”},
    {“some-other-value”:1500}
  ]
}

I want it to be decoded to case classes:

case class Config(defaultValue: Int, someSeq: Seq[SomeInteger])
case class SomeInteger(someOtherValue: Int)

So that it creates Config(5, Seq(SomeInteger(100), SomeInteger(5), SomeInteger(1500))) (Second one is 5 since there is no some-other-value key in the second object of the list) Is there a way to do so?

Answer 1

You can add a type parameter to SomeInteger and Config to specify what the type of someOtherValue should be. Then you create a ConfigReader[Config[Option[Int]]] and use the map method to apply the default:

case class Config[A](defaultValue: Int, someSeq: Seq[SomeInteger[A]])
object Config {
  private def applyDefault(config: Config[Option[Int]]): Config[Int] =
    config.copy(
      someSeq = config.someSeq.map(i =>
        i.copy(
          someOtherValue = i.someOtherValue.getOrElse(config.defaultValue))))

  implicit val reader: ConfigReader[Config[Int]] =
    deriveReader[Config[Option[Int]]]
      .map(applyDefault)
}
case class SomeInteger[A](someOtherValue: A)
object SomeInteger {
  implicit val reader: ConfigReader[SomeInteger[Option[Int]]] =
    deriveReader[SomeInteger[Option[Int]]]
}

Unfortunately this means you need to write Config[Int] everywhere instead of just Config. But this can easily be fixed by renaming Config to e. g. GenConfig and add a type alias: type Config = GenConfig[Int].