Folder.scala

import scalaz.Monad
import scalaz.syntax.monad._

/**
 * Non stack-using Foldable substitute
 */
trait Folder[F[_], A] {
  def foldLefterM[G[_], B](z: B)(f: (B, A) => G[B])(implicit M: Monad[G]): G[B]
}

object Folder {
  implicit def FolderList[A](ls: List[A]) =
    new Folder[List, A] {
      def foldLefterM[G[_], B](z: B)(f: (B, A) => G[B])(implicit M: Monad[G]): G[B] =
        ls.foldLeft(z.point[G]) {
          case (mg, a) => mg >>= { b => f(b, a) }
        }
    }
}