iterv1.scala

import IterV._

implicit lazy val StreamEnumerator: Enumerator[Stream] = new Enumerator[Stream] {
  def apply[E, A](e: Stream[E], i: IterV[E, A]): IterV[E, A] = e match {
    case Stream() => i
    case x #:: xs => i.fold(done = (_, _) => i, cont = k => apply(xs, k(El(x))))
  }
}

head(Stream(3, 4, 6)).run assert_=== Some(3)

iterv2.scala

head(Stream(1, 2, 3)).run assert_=== Some(1)

peek(Stream(1, 2, 3)).run assert_=== Some(1)

length(Stream(1, 2, 3)).run assert_=== 3

drop(2)(Stream(1, 2, 3)).run assert_=== ()

collect[Int, List].apply(Stream(1, 2, 3)).run assert_=== List(1, 2, 3)

groupBy[Int, List](_ === _).apply(Stream(1, 1, 2, 1, 1, 2)).run assert_=== List(1, 1)

takeWhile[Int, List](_ % 2 === 0).apply(Stream(2, 4, 5, 6)).run assert_=== List(2, 4)

reversed[Int, List](ListReducer)(Stream(1, 2, 3)).run assert_=== List(3, 2, 1)

repeat[Int, List[Int], List](groupBy[Int, List](_ === _)).apply(Stream(1, 1, 2, 1, 1, 2)).run assert_=== List(List(1, 1), List(2), List(1, 1), List(2))

peekDoneOr[Int, List[Int]](5.pure[List], i => takeWhile(_ lt 3))(Stream(1, 2, 3)).run assert_=== List(1, 2)

iterv3.scala

(drop[Int](2) >|> length).apply(Stream(1, 2, 3)).run assert_=== 1

(drop[Int](1) >|> collect[Int, List]).apply(Stream(1, 2, 3)).run assert_=== List(2, 3)

(head[Int] >>= (o => head.map(_ |+| o))).apply(Stream(1, 5, 3)).run assert_=== Some(6)