[dwohnitmok]

Yeah the `Key` and `DB` example, while somewhat differing in semantics, can be emulated by Java.

Here's an example that extends it slightly and can't be emulated by Java.

  // I forget if I need to curry the second argument in a separate argument list
  // I'm not next to a computer with scalac at the moment, so I'll just use separate argument lists
  type DB = (k: Key) => k.IsVerified => Option[k.Value]

  trait Key {
    type Value
    val keyValue: Value
    case class IsVerified(underlyingBool: Boolean)
  }

  val myDB: DB =
    key => isVerified => x match
      case IsVerified(bool) =>
        if bool then
          // return the actual value
          doSomething()
        else
          None

  def verifyKey(key: Key): key.IsVerified =
    if isValidKey(key) then
      key.IsVerified(true)
    else
      key.IsVerified(false)

  val key0: Key = ...
  val key1: Key = ...

  val key0Verification: key0.IsVerified = verifyKey(key0)

  // Fails to compile
  // You're trying to cheat!
  // You only verified key0 and are trying to use its verification to bypass 
  // verifying key1
  myDB(key1)(key0Verification)