configchain.scala

//scalastyle:off

trait ConfigurationProvider {
  def apply(key: String): Option[String]
}

object NullConfigurationProvider extends ConfigurationProvider {
  override def apply(key: String): Option[String] = None
}

abstract class ChainedConfigurationProvider(next: ConfigurationProvider = NullConfigurationProvider) extends ConfigurationProvider {

  override def apply(key: String): Option[String] = {
    get(key) match {
      case None => next(key)
      case Some(value) => Some(value)
    }
  }

  def get(key: String): Option[String]
}

class MapConfigurationProvider(data: Map[String, String], next: ConfigurationProvider = NullConfigurationProvider) extends ChainedConfigurationProvider(next) {
  override def get(key: String): Option[String] = data.get(key)
}

var defaults = Map("a" -> "default", "b" -> "default")
var overridden = Map("a" -> "override")

var defaultCP = new MapConfigurationProvider(defaults)
var overridenCP = new MapConfigurationProvider(overridden, defaultCP)

// Some(override)
println(overridenCP("a"))
// Some(default)
println(overridenCP("b"))
// None
println(overridenCP("c"))

configlist.scala

trait ConfigurationProvider {
  def apply(key: String): Option[String]
}

class MapConfigurationProvider(data: Map[String, String]) extends ConfigurationProvider{
  override def apply(key: String): Option[String] = data.get(key)
}

class Configuration(configurations: Seq[ConfigurationProvider]) extends ConfigurationProvider {
  override def apply(key: String): Option[String] = {
    configurations.find(_(key).isDefined) match {
      case Some(cp) => cp(key)
      case None => None
    }
  }
}

var defaults = Map("a" -> "default", "b" -> "default")
var overridden = Map("a" -> "override")

var defaultCP = new MapConfigurationProvider(defaults)
var overridenCP = new MapConfigurationProvider(overridden)

var cp = new Configuration(Seq(overridenCP, defaultCP))

// Some(override)
println(cp("a"))
// Some(default)
println(cp("b"))
// None
println(cp("c"))