loop and label implementation of the Essence of the Iterator Pattern

gistfile1.scala

import scalaz._
import Scalaz._

val l = List(10, 20, 30, 40)

// loop from The Essence of the Iterator Pattern
// accumulates elements effectfully, but modifies elements purely and independently of accumulation

def loop[T[_]:Traverse, A, B](f: A => B, t: T[A]) =
  t.traverse[({type λ[x] = State[Int,x]})#λ, (B, Int)](a =>
    state((i: Int) => (i+1, (f(a), i)))) ! 0

loop((a: Int) => 2 * a, l) should equal(List[(Int, Int)] = List((20,0), (40,1), (60,2), (80,3)))

// label from The Essence of the Iterator Pattern
// modifies elements purely but dependent on the state evolving this state independently of the elements

def label[T[_]:Traverse, A, B](t: T[A]) =
  t.traverse[({type λ[x] = State[Int,x]})#λ, Int](a =>
    state((i: Int) => (i+1, i))) ! 0

label(l) should equal(List(0,1,2,3))

Now need to work on the higher level abstractions *collect* & *disperse* ..