Accumulative Validator

ValidatorImpl.scala

object ValidatorImpl {

  sealed trait Validated[+E, +S]

  object Validated {
    final case class Passed[S](value: S)        extends Validated[Nothing, S]
    final case class Failed[E](errors: List[E]) extends Validated[E, Nothing]
  }

  trait Validator[+E, S] { previous =>

    import Validator.PredicateMeta

    protected def input: S

    protected def failedPredicates[E1 >: E]: List[PredicateMeta[E1, S]] = List.empty

    def satisfying[E1 >: E](f: S => Boolean, failureReason: E1): Validator[E1, S] = new Validator[E1, S] {
      override def input: S = previous.input

      override def failedPredicates[E2 >: E1]: List[PredicateMeta[E2, S]] =
        if (f(input)) previous.failedPredicates 
        else PredicateMeta[E2, S](f, failureReason) :: previous.failedPredicates
    }

    def validated: Validated[E, S] = failedPredicates match {
      case Nil => Validated.Passed(input)
      case _   => Validated.Failed(failedPredicates.map[E](_.failureReason).reverse)
    }
  }

  object Validator {
    def apply[E, S](s: S): Validator[E, S] = new Validator[E, S] {
      override def input: S = s
    }

    protected[Validator] final case class PredicateMeta[+E, S](f: S => Boolean, failureReason: E)
  }

  def main(args: Array[String]): Unit = {

    final case class Person(name: String, age: Int)

    sealed trait PersonValidationError { def reason: String }

    object PersonValidationError {
      case object EmptyName extends PersonValidationError { override def reason: String = "Person has an empty name" }
      case object NonAdult  extends PersonValidationError { override def reason: String = "Person is non adult" }
    }

    val person = Person("", 14)

    val validatedPerson = 
      Validator(person)
        .satisfying(_.name.nonEmpty, PersonValidationError.EmptyName)
        .satisfying(_.age >= 18, PersonValidationError.NonAdult)
        .validated

    validatedPerson match {
      case Validated.Passed(value)  => println(s"passed $value")
      case Validated.Failed(errors) => println(s"failed: $errors") // List(EmptyName, NonAdult)
    }
  }
}