nested opaque types demo

seethru.scala

package object demo {

  opaque type Collection[A] = Array[A]
  
  object Collection {
    opaque type Pointer[A] = Int
    
    object Pointer {
      def first(c: Collection[A]): Option[Pointer[A]] =
        if (c.length > 0) Some(0) else None
      def next(c: Collection[A], p: Pointer[A]): (A, Option[Pointer[A]]) =
        (c(p), if (c.length - 1 <= p) None else Some(p + 1))
    }
  } 
}

trait Module[C[_]] {
  type Pointer[I <: Singleton]
  def isEmpty[A](c: C[A]): Boolean
  def first[A](c: C[A]): Option[Pointer[c.type]]
  def next[A](c: C[A])(p: Pointer[c.type]): Option[Pointer[c.type]]
  def get[A](c: C[A])(p: Pointer[c.type]): A
}

object ArrayModule[Array[_]] {
  
  opaque type Pointer[I <: Singleton] = Int

  object Pointer {
    // code that treats Pointer as an Int must go in here.
    def first[A](c: Array[A]): Option[Pointer[c.type]] =
      if (c.length == 0) None else Some(0)
    def next[A](c: Array[A])(p: Pointer[c.type]): Option[Pointer[c.type]] =
      if (c.length > p + 1) Some(p + 1) else None
    def get[A](c: C[A])(p: Pointer[c.type]): A =
      c(p)
  }

  def isEmpty[A](c: Array[A]): Boolean =
    c.isEmpty
  def first[A](c: Array[A]): Option[Pointer[c.type]] =
    Pointer.first(c)
  def next[A](c: Array[A])(p: Pointer[c.type]): Option[Pointer[c.type]] =
    Pointer.next(c)(p)
  def get[A](c: Array[A])(p: Pointer[c.type]): A =
    Pointer.get(c)(p)
}