scalaz_unfold.scala

object StreamT extends StreamTInstances {

// 略

def unfoldM[M[_],A,B](start: B)(f: B => M[Option[(A,B)]])(implicit M: Functor[M]): StreamT[M,A] =
    StreamT[M,A](M.map(f(start)) {
      case Some((a, b)) => Yield(a, unfoldM(b)(f))
      case None => Done
    })

  def unfold[A,B](b: B)(f: B => Option[(A,B)]): StreamT[Id,A] = unfoldM[Id,A,B](b)(f)
}