First, they are all non-strict. That has a particular mathematical meaning related to functions, but, basically, means they are computed on-demand instead of in advance.
Stream is a lazy list indeed. In fact, in Scala, a
Stream is a
tail is a
lazy val. Once computed, a value stays computed and is reused.
Or, as you say, the values are cached.
Iterator can only be used once because it is a traversal pointer 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
simply get a new
Iterator which will only apply these transformations when
you ask for the next element.
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.
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.
Iterator and views have excellent memory characteristics.
nice, but, in Scala, its main benefit is writing infinite sequences
(particularly sequences recursively defined). One can avoid keeping all of
Stream in memory, though, by making sure you don’t keep a reference to
head (for example, by using
def instead of
val to define the
Because of the penalties incurred by views, one should usually
force 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.
This answer was originally submitted in response to this question on Stack Overflow.