NewtypeRefinementTypeInScala3.scala

trait Newtype[A] {
  opaque type Type = A
  inline def apply(a: A): Type = a

  given newtypeCanEqual: CanEqual[Type, Type] = CanEqual.derived

  extension (typ: Type) {
    def value: A = typ
  }
}

trait Refined[A] {
  import compiletime.*

  opaque type Type = A

  given newRefinedCanEqual: CanEqual[Type, Type] = CanEqual.derived

  inline def apply(a: A): Type =
    inline if predicate(a) then a else error("Invalid value: " + codeOf(a))

  def from(a: A): Either[String, Type] =
    if predicate(a) then Right(a) else Left("Invalid value: " + a)

  def predicate(a: A): Boolean

  extension (typ: Type) {
    def value: A = typ
  }
}

type Name = Name.Type
object Name extends Newtype[String]

type PositiveInt = PositiveInt.Type
object PositiveInt extends Refined[Int] {
  inline override def predicate(a: Int): Boolean = a > 0
}

type MyInt = MyInt.Type
object MyInt extends Newtype[PositiveInt]

def hello(name: Name): Unit = println(s"Hello ${name.value}")

@main def run(): Unit = {
  val n = 1
  val n2 = 0
  println(PositiveInt.from(n))  // Right(1)
  println(PositiveInt.from(n2)) // Left(Invalid value: 0)
  hello(Name("Kevin")) // Hello Kevin
  println(PositiveInt(1) == PositiveInt(1)) // true
  println(PositiveInt(1) == PositiveInt(2)) // false
  println(PositiveInt(1)) // 1
//    println(PositiveInt(0)) // compile-time error
  println(MyInt(PositiveInt(1)) == MyInt(PositiveInt(1))) // true
  println(MyInt(PositiveInt(1))) // 1
}