Use-cases for Streams in Scala

Question

In Scala there is a Stream class that is very much like an iterator. The topic Difference between Iterator and Stream in Scala? offers some insights into the similarities and differences between the two.

Seeing how to use a stream is pretty simple but I don't have very many common use-cases where I would use a stream instead of other artifacts.

The ideas I have right now:

• If you need to make use of an infinite series. But this does not seem like a common use-case to me so it doesn't fit my criteria. (Please correct me if it is common and I just have a blind spot)
• If you have a series of data where each element needs to be computed but that you may want to reuse several times. This is weak because I could just load it into a list which is conceptually easier to follow for a large subset of the developer population.
• Perhaps there is a large set of data or a computationally expensive series and there is a high probability that the items you need will not require visiting all of the elements. But in this case an Iterator would be a good match unless you need to do several searches, in that case you could use a list as well even if it would be slightly less efficient.
• There is a complex series of data that needs to be reused. Again a list could be used here. Although in this case both cases would be equally difficult to use and a Stream would be a better fit since not all elements need to be loaded. But again not that common... or is it?

So have I missed any big uses? Or is it a developer preference for the most part?

Thanks

Answer 1

The main difference between a Stream and an Iterator is that the latter is mutable and "one-shot", so to speak, while the former is not. Iterator has a better memory footprint than Stream, but the fact that it is mutable can be inconvenient.

Take this classic prime number generator, for instance:

def primeStream(s: Stream[Int]): Stream[Int] =
  Stream.cons(s.head, primeStream(s.tail filter { _ % s.head != 0 }))
val primes = primeStream(Stream.from(2))

It can be easily be written with an Iterator as well, but an Iterator won't keep the primes computed so far.

So, one important aspect of a Stream is that you can pass it to other functions without having it duplicated first, or having to generate it again and again.

As for expensive computations/infinite lists, these things can be done with Iterator as well. Infinite lists are actually quite useful -- you just don't know it because you didn't have it, so you have seen algorithms that are more complex than strictly necessary just to deal with enforced finite sizes.