Question

In Scala, is there a way to get the currently evaluated items in a Stream? For example in the Stream

val s: Stream[Int] = Stream.cons(1, Stream.cons(2, Stream.cons(3, s.map(_+1))))

the method should return only List(1,2,3).

Answer 1

In 2.8, there is a protected method called tailDefined that will return false when you get to the point in the stream that has not yet been evaluated.

This isn’t too useful (unless you want to write your own Stream class) except that Cons itself makes the method public. I’m not sure why it’s protected in Stream and not in Cons–I would think one or the other might be a bug. But for now, at least, you can write a method like so (writing a functional equivalent is left as an exercise to the reader):

def streamEvalLen[T](s: Stream[T]) = {
  if (s.isEmpty) 0
  else {
    var i = 1
    var t = s
    while (t match {
      case c: Stream.Cons[_] => c.tailDefined
      case _ => false
    }) {
      i += 1
      t = t.tail
    }
    i
  }
}

Here you can see it in action:

scala> val s = Stream.iterate(0)(_+1)
s: scala.collection.immutable.Stream[Int] = Stream(0, ?)

scala> streamEvalLen(s)
res0: Int = 1

scala> s.take(3).toList
res1: List[Int] = List(0, 1, 2)

scala> s
res2: scala.collection.immutable.Stream[Int] = Stream(0, 1, 2, ?)

scala> streamEvalLen(s)
res3: Int = 3