blog-201706.scala

blog-201706.scala

sealed trait Field {
  type Value
  def fold[F[_]](f: Field.Fold[F]): F[Value]
}

object Field {
  case object Name extends Field {
    override type Value = String
    override def fold[F[_]](f: Field.Fold[F]): F[Value] = f.name(this)
  }

  case object Age extends Field {
    override type Value = Int
    override def fold[F[_]](f: Field.Fold[F]): F[Value] = f.age(this)
  }

  final case class Fold[F[_]](name: Field.Name.type => F[Field.Name.Value],
                              age : Field.Age .type => F[Field.Age .Value]) {
    def apply(f: Field): F[f.Value] =
      f.fold(this)
  }
}

object Examples {

  type GetValue[Value] = Value
  val getValue = Field.Fold[GetValue](
    name = _ => "Hello",
    age  = _ => 99)


  type Print[Value] = Value => Unit
  val printValue = Field.Fold[Print](
    name = _ => n => println(s"My name is $n."),
    age  = _ => i => println(s"I am $i years old."))


  type Validate[Value] = Value => Either[String, Value]
  val validateName: Validate[String] =
    name => {
      val name2 = name.trim
      if (name2.isEmpty) Left("Name can't be empty") else Right(name2)
    }
  val validateAge: Validate[Int] =
    a => if (a >= 0 && a <= 150) Right(a) else Left("Your age is bullshit")
  val validate = Field.Fold[Validate](
    name = _ => validateName,
    age  = _ => validateAge)


  val f = Field.Name
  val v = getValue(f)
  printValue(f)(v)
}