Scala Type Safe Configuration with Type Level Programming

demo.scala

import SharedStreamsCfg._

object SharedStreamsCfg {
  abstract class HasConfig
  case object Y extends HasConfig
  case object N extends HasConfig
  type Loaded = Y.type
  type NotLoaded = N.type

  def apply()= new SharedStreamsCfg[NotLoaded, NotLoaded, NotLoaded] ("", 0,0)
}

case class SharedStreamsCfg[
  HasName <: HasConfig,
  HasAge <: HasConfig,
  HasDob <: HasConfig](name: String, age: Int, dob: Long) {

  def withName(name: String) = SharedStreamsCfg[Loaded, HasAge, HasDob](name, this.age, this.dob)
  def withAge(age: Int) = SharedStreamsCfg[HasName, Loaded,HasDob](this.name, age, this.dob)
  def withDob(dob: Long) = SharedStreamsCfg[HasName, HasAge, Loaded](this.name, this.age, dob)
}

object AppDemo {
  
  // Only works with fully loaded config
  def runApp(cfg: SharedStreamsCfg[Loaded, Loaded, Loaded]) = {
    println(s"Can only use fully loaded config $cfg")
  }

  val cfg = SharedStreamsCfg() //SharedStreamsCfg[NotLoaded, NotLoaded, NotLoaded]

  // Compile time error
  // runApp(cfg)

  val withName = cfg.withName("Oliver") //SharedStreamsCfg[Loaded, NotLoaded, NotLoaded]

  // Compile time error
  // runApp(withName)

  val withNameAndAge = withName.withAge(69) //SharedStreamsCfg[Loaded, Loaded, NotLoaded]

  // Compile time error
  // runApp(withNameAndAge)

  val fullyConfigured = withNameAndAge.withDob(313189942000l) //SharedStreamsCfg[Loaded, Loaded, Loaded]

  // Compiles!
  runApp(fullyConfigured)
}