# FlattenSequence A sequence consisting of all the elements contained in each segment contained in some `Base` sequence. ``` @frozen struct FlattenSequence where Base : Sequence, Base.Element : Sequence ``` ## Overview The elements of this view are a concatenation of the elements of each sequence in the base. The `joined` method is always lazy, but does not implicitly confer laziness on algorithms applied to its result. In other words, for ordinary sequences `s`: - `s.joined()` does not create new storage - `s.joined().map(f)` maps eagerly and returns a new array - `s.lazy.joined().map(f)` maps lazily and returns a `LazyMapSequence` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)