Stream.scala

import Stream._

  /** A possibly finite stream that repeatedly applies a given function to a start value.
   *
   *  @param start the start value of the stream
   *  @param f     the function that's repeatedly applied
   *  @return      the stream returning the possibly finite sequence of values `start, f(start), f(f(start)), ...`
   */

  def iterate[A](f: A => A, a: A): Stream[A] = unfold((x: A) => Some((x, f(x))), a)

  def unfold[A, B](start: B)(f: B => Option[Tuple2[A,B]]): Stream[A] = f(start) match {
    case Some((elem, next)) => elem #:: unfold(next)(f)
    case None => empty
  }

  // Could be integrated here -> https://lampsvn.epfl.ch/trac/scala/browser/scala/tags/R_2_9_1_final/src//library/scala/collection/immutable/Stream.scala#L647

missingfaktor commented on Nov 2, 2011

Why not just use Stream.iterate ?
scala> Stream.iterate(2)(2 *)
res29: scala.collection.immutable.Stream[Int] = Stream(2, ?)

scala> res29 take 10 toList
res30: List[Int] = List(2, 4, 8, 16, 32, 64, 128, 256, 512, 1024)
Owner
aloiscochard commented on Nov 2, 2011

@missingfaktor because Stream.iterate only support infinite sequence, with my version you can stop by returning None
missingfaktor commented on Nov 2, 2011

Oh, I see. It's more similar to unfold from Scalaz.
scala> 5.unfold [Stream, Int](x => if%28x < 100%29 Some%28x * 2, x + 1%29 else None)
res41: Stream[Int] = Stream(10, ?)

scala> res41.toList
res42: List[Int] = List(10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100, 102, 104, 106, 108, 110, 112, 114, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134, 136, 138, 140, 142, 144, 146, 148, 150, 152, 154, 156, 158, 160, 162, 164, 166, 168, 170, 172, 174, 176, 178, 180, 182, 184, 186, 188, 190, 192, 194, 196, 198)
Owner
aloiscochard commented on Nov 3, 2011

Ok, I'm reinventing scalaz here :-D ... Make sense, I had this idea while learning haskell!

But I still believe it should be in scala-core as presented here.

Thanks for pointing this out mate.
missingfaktor commented on Nov 3, 2011

Welcome, mate! Yes, I agree unfold is pretty basic, and deserves a place in the standard library.
Owner
aloiscochard commented on Nov 3, 2011

I've just added unfold implementation which could be integrated in std lib