Scala 3 Derivation

Scala 3 introduced native support for type class derivation - a way to automatically generate type class instances for enums, enum cases, case classes, case objects and sealed traits that only have the aforementioned types as children. Instances are generated on demand by adding a derives clause on data types.

PureConfig has some limited support for derivation of ConfigReader instances. In order to enable it, the relevant extensions must be imported:

import pureconfig._
import pureconfig.generic.derivation.default._

After that, you can derive ConfigReader instances for your config class using a derives clause:

sealed trait AnimalConf derives ConfigReader
case class DogConf(age: Int) extends AnimalConf
case class BirdConf(canFly: Boolean) extends AnimalConf

ConfigSource.string("{ type: dog-conf, age: 4 }").load[AnimalConf]
// val res0: pureconfig.ConfigReader.Result[AnimalConf] = Right(DogConf(4))

Readers for enumerations of objects can also be derived by using EnumConfigReader instead:

import pureconfig.generic.derivation.EnumConfigReader

enum Season derives EnumConfigReader {
  case Spring, Summer, Autumn, Winter

case class MyConf(list: List[Season]) derives ConfigReader

ConfigSource.string("{ list: [spring, summer, autumn, winter] }").load[MyConf]
// val res1: pureconfig.ConfigReader.Result[MyConf] = Right(MyConf(List(Spring, Summer, Autumn, Winter)))


There is currently no way to customize case class and sealed trait derivation - any ProductHint and CoproductHint instances in scope are ignored.

ConfigWriter derivation is not supported at the moment.

Overall, consider this to be in alpha/beta stage. This implementation is not as mature as the Shapeless or Magnolia-based derivation libraries available in Scala 2 yet so please report any bugs that you find!