What are views for collections and when would you want to use them?

Question

In Scala, for many (all?) types of collections you can create views.

What exactly is a view and for which purposes are views useful?

Answer 1

See Views from Scala 2.8 Collections API.

Scala collections are by default strict in all their transformers, except for Stream, which implements all its transformer methods lazily. However, there is a systematic way to turn every collection into a lazy one and vice versa, which is based on collection views. A view is a special kind of collection that represents some base collection, but implements all transformers lazily.

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There are two reasons why you might want to consider using views. The first is performance. You have seen that by switching a collection to a view the construction of intermediate results can be avoided. These savings can be quite important.

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The second use case applies to views over mutable sequences. Many transformer functions on such views provide a window into the original sequence that can then be used to update selectively some elements of that sequence.