// *** Iterators / Iteratees *** //

// Mutable variant:
trait Iterator[E] {
  def next: E
  def hasNext: Boolean
}

// Immutable variant:
trait Iterator[E] {
  def next: Option[(E, Iterator[E])]
}

// Free variant:
case class Get[E, A](f: Option[E] => A)
trait Iteratee[E, A] = Free[({type λ[x] = Get[E, x]})#λ, A]

// Why would you ever use Iteratee when you have State with Iterator?


// *** Consumers / Lists *** /

// Mutable variant, for use with Reader.
trait Consumer[A] {
  def feed(a: A): Unit
  def eof: Unit
}

// Immutable variant, for use with State.
trait Consumer[-A] {
  def feed(a: A): Consumer[A]
  def eof: Consumer[A]
}

// Equivalent little DSL, for use with Free.
trait Producer[S, +A]
case class Produce[S, A](s: S, a: A)

type IntProducer[A] = Producer[Int, A]

// Note that Free[IntProducer, Unit] is equivalent to List[Int]

// The question is:
//   Why would we ever use List[Int] when State[Consumer[Int], A] works just fine?