ErrorExample.scala

  object ErrorStates {
    // Type alias to string so it compiles
    type Input = String
    type Output = String

    sealed trait ErrorState[Context] {
      val err: Option[Throwable]
      val ctx: Context
    }

    // Some examples of error types that might crop up in the real world
    // and an excuse to exercise the option in ErrorState
    case class KnownErrorState(ctx: Input)
      extends ErrorState[Input]  {
      
      val err = None
    }

    case class UnknownErrorState(e: Throwable, ctx: Input)
      extends ErrorState[Input] {
      
      val err = Some(e)
    }
  }
  
  
  object ErrorExample {
    import ErrorStates._
    
    class FooException extends Throwable
    
    def doSomethingThatCouldThrow(): Try[Output] = ???

    def doThing(input: Input): Either[ErrorState[Input], Output] = {
      doSomethingThatCouldThrow match {
        case Success(result) => Right(result)
        case Failure(_: FooException) => Left(KnownErrorState(input))
        case Failure(e) => Left(UnknownErrorState(e, input))
      }
    }
  }