scalaz-stream scan

scan.scala

  def scan[F[_], A, B](b: B, p: Process[F, A])(f: (B, A) => B): Process[F, B] = {
    import Process._

    def scan0(b: B, p: Process[F, A]) = scan(b, p)(f)

    p match {
      case h @ Halt(e) => h

      case await : Await[F, Any, A] => 
        Process.await[F, Any, B](await.req)( 
          (a: Any) => scan0(b, await.recv(a)), 
          scan0(b, await.fallback1), 
          scan0(b, await.cleanup1)
        )

      case emit : Emit[F, A] =>
        val b2 = emit.head.scanLeft(b)(f)

        Emit(b2, scan0(b2.last, emit.tail))
    }
  }