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?

Sorry, forgot to add quotes around the variable names in config and trailing commas. Was writing really fast, now edited to look like a normal application.conf file. It is HOCON — D Artem, Jun 12 '21 at 08:06

Matthias Berndt · Accepted Answer · 2021-06-16T19:25:57.690

2

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].

edited Jun 16 '21 at 19:25

answered Jun 12 '21 at 21:39

Matthias Berndt

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Thanks a lot, that worked! I have a question - is there any more convenient way to do so? In my case, I have a bit more complex config, and only one field should be processed this way. Is there a way to introduce smth like custom reading for specific class? – D Artem Jun 14 '21 at 10:16
Yes, there is a way to customize reading for a specific class: write a custom `ConfigReader` like above. Regarding the “more complex config” thing, I don't see the problem. If you declare it like `case class MoreComplexConfig(config: GenConfig[Int])`, then things should just work. You don't need a type parameter for `MoreComplexConfig` – looks convenient to me. – Matthias Berndt Jun 14 '21 at 15:48
Thanks a lot! I was trying to write something like this for several hours, and finally gave up . Now everything works perfectly – D Artem Jun 16 '21 at 10:43
In that case you may want to mark my answer as accepted. – Matthias Berndt Jun 16 '21 at 16:08
Thanks, it was my first question on stackoverflow, didn't know how to mark as accepted – D Artem Jun 17 '21 at 09:42

Decode config to case class with pureconfig with default values

1 Answers1