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    <p>First, they are all <em>non-strict</em>. That has a particular mathematical meaning related to functions, but, basically, means they are computed on-demand instead of in advance.</p> <p><code>Stream</code> is a lazy list indeed. In fact, in Scala, a <code>Stream</code> is a <code>List</code> whose <code>tail</code> is a <code>lazy val</code>. Once computed, a value stays computed and is reused. Or, as you say, the values are cached.</p> <p>An <code>Iterator</code> can only be used once because it is a <em>traversal pointer</em> into a collection, and not a collection in itself. What makes it special in Scala is the fact that you can apply transformation such as <code>map</code> and <code>filter</code> and simply get a new <code>Iterator</code> which will only apply these transformations when you ask for the next element.</p> <p>Scala used to provide iterators which could be reset, but that is very hard to support in a general manner, and they didn't make version 2.8.0.</p> <p>Views are meant to be viewed much like a database view. It is a series of transformation which one applies to a collection to produce a "virtual" collection. As you said, all transformations are re-applied each time you need to fetch elements from it.</p> <p>Both <code>Iterator</code> and views have excellent memory characteristics. <code>Stream</code> is nice, but, in Scala, its main benefit is writing infinite sequences (particularly sequences recursively defined). One <em>can</em> avoid keeping all of the <code>Stream</code> in memory, though, by making sure you don't keep a reference to its <code>head</code> (for example, by using <code>def</code> instead of <code>val</code> to define the <code>Stream</code>).</p> <p>Because of the penalties incurred by views, one should usually <code>force</code> it after applying the transformations, or keep it as a view if only few elements are expected to ever be fetched, compared to the total size of the view.</p>
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    1. USDaniel C. Sobral
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    1. CO`Iterator` is also pretty handy for probing the infinite, and I generally prefer them over streams where possible. The real benefit in streams is that previously accessed values are cached, which is a serious boon when trying to implement something like the fibonacci sequence - which is defined in terms of previous values.
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      1. USKevin Wright
    2. COFibonacci is a less than perfect example as it only needs the last 2 previous values, and keeping the whole stream is a waste. The Ackermann function is probably the canonical example.
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      1. USJürgen Strobel
    3. CO@JürgenStrobel Ackermann would result in lousy performance, since indexed access of streams is O(n). But I agree wrt fibonacci.
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      1. USDaniel C. Sobral
 

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