Validating Typesafe Configuration Files

InvariantConfig.scala

package cakesolutions.example

import cakesolutions.config._
import scala.concurrent.duration.FiniteDuration
import scala.util.Try
import shapeless._

object InvariantConfig {
  case object NameShouldBeNonEmptyAndLowerCase extends Exception
  case object ShouldNotBeNegative extends Exception

  sealed abstract case class HttpConfig(host: String, port: Int)
  sealed abstract case class Settings(name: String, heartbeat: FiniteDuration, http: HttpConfig)

  // Following allows Shapeless to create instances of our sealed abstract case classes
  private implicit val genHttpConfig: Generic[HttpConfig] = new Generic[HttpConfig] {
    type Repr = String :: Int :: HNil

    def to(t: HttpConfig): Repr =
      t.host :: t.port :: HNil

    def from(r: Repr): HttpConfig =
      new HttpConfig(r(0), r(1)) {}
  }
  private implicit val genSettings: Generic[Settings] = new Generic[Settings] {
    type Repr = String :: FiniteDuration :: HttpConfig :: HNil

    def to(t: Settings): Repr =
      t.name :: t.heartbeat :: t.http :: HNil

    def from(r: Repr): Settings =
      new Settings(r(0), r(1), r(2)) {}
  }

  // The following code (due to the build implementation) may throw runtime class cast exceptions
  def apply(): Try[Settings] = {
    validateConfig("application.conf") { implicit config =>
      build[Settings]((
        validate[String]("name", NameShouldBeNonEmptyAndLowerCase)(_.matches("[a-z0-9_-]+")),
        unchecked[FiniteDuration](required("http.heartbeat", "NOT_SET")),
        via("http") { implicit config =>
          build[HttpConfig]((
            unchecked[String]("host"),
            validate[Int]("port", ShouldNotBeNegative)(_ >= 0)
          ))
        }
      ))
    }
  }
}