foldIncremental.scala

// this seems like a useful FS2 Stream function, is it already built in?
// Folds a function into a stream and gives you a stream of the intermediate
// accumulator values, i.e.:
//   > val ones = Stream.iterate(1)(x => x) 
//   > ones.foldIncremental(0)(_ + _) take 3 toList
//   ==> List[Int] = List(1, 2, 3)

implicit class StreamOps[F[_], O](private val self: Stream[F, O]) {
  def foldIncremental[O2](z: O2)(f: (O2, O) => O2): Stream[F, O2] =
    self.mapAccumulate(z) {
      case (z, o) =>
        val o2 = f(z, o)
        (o2, o2)
    } map { _._1 }
}