f.scala

package p

/** Decodes strings into Ts */
trait Decoder[T] {
  def decode(s: String): Either[String, T]
}

object Decoder {
  def apply[T](f: String => Either[String, T]): Decoder[T] =
    new Decoder[T] { def decode(s: String) = f(s) }
}


final class PosInt private (val x: Int)

object PosInt extends PosInt0 {
  def apply(x: Int): Either[String, PosInt] =
    if (x > 0) Right(new PosInt(x)) else Left("not a positive int")
}

// Not in PosInt so that the private constructor isn't reachable and apply must be used instead
abstract class PosInt0 { self: PosInt.type =>
  def apply(x: Int): Either[String, PosInt]

  // final so it can't be redefined in PosInt.type
  final implicit val posIntDecoder: Decoder[PosInt] =
    Decoder(s => try apply(s.toInt) catch { case _: java.lang.NumberFormatException => Left("not an int") })
}