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# Supporting Types

Use wrappers, indices, and iterators in operations like slicing, flattening, and
reversing a collection.

## Topics

### Slices

[`struct Slice`](/documentation/Swift/Slice)

A view into a subsequence of elements of another collection.

### Range Expressions

[`struct PartialRangeUpTo`](/documentation/Swift/PartialRangeUpTo)

A partial half-open interval up to, but not including, an upper bound.

[`struct PartialRangeThrough`](/documentation/Swift/PartialRangeThrough)

A partial interval up to, and including, an upper bound.

[`struct PartialRangeFrom`](/documentation/Swift/PartialRangeFrom)

A partial interval extending upward from a lower bound.

[`protocol RangeExpression`](/documentation/Swift/RangeExpression)

A type that can be used to slice a collection.

[`enum UnboundedRange_`](/documentation/Swift/UnboundedRange_)

A range expression that represents the entire range of a collection.

### Type-Erasing Wrappers

[`struct AnySequence`](/documentation/Swift/AnySequence)

A type-erased sequence.

[`struct AnyCollection`](/documentation/Swift/AnyCollection)

A type-erased wrapper over any collection with indices that
support forward traversal.

[`struct AnyBidirectionalCollection`](/documentation/Swift/AnyBidirectionalCollection)

A type-erased wrapper over any collection with indices that
support bidirectional traversal.

[`struct AnyRandomAccessCollection`](/documentation/Swift/AnyRandomAccessCollection)

A type-erased wrapper over any collection with indices that
support random access traversal.

[`struct AnyIterator`](/documentation/Swift/AnyIterator)

A type-erased iterator of `Element`.

[`struct AnyIndex`](/documentation/Swift/AnyIndex)

A wrapper over an underlying index that hides the specific underlying type.

[`struct AnyHashable`](/documentation/Swift/AnyHashable)

A type-erased hashable value.

### Lazy Wrappers

Use these lazy wrappers to defer any filtering or transformation of collection elements
until elements are accessed.

[`struct LazySequence`](/documentation/Swift/LazySequence)

A sequence containing the same elements as a `Base` sequence, but
on which some operations such as `map` and `filter` are
implemented lazily.

[`struct LazyMapSequence`](/documentation/Swift/LazyMapSequence)

A `Sequence` whose elements consist of those in a `Base`
`Sequence` passed through a transform function returning `Element`.
These elements are computed lazily, each time they’re read, by
calling the transform function on a base element.

[`struct LazyFilterSequence`](/documentation/Swift/LazyFilterSequence)

A sequence whose elements consist of the elements of some base
sequence that also satisfy a given predicate.

[`struct LazyPrefixWhileSequence`](/documentation/Swift/LazyPrefixWhileSequence)

A sequence whose elements consist of the initial consecutive elements of
some base sequence that satisfy a given predicate.

[`struct LazyDropWhileSequence`](/documentation/Swift/LazyDropWhileSequence)

A sequence whose elements consist of the elements that follow the initial
consecutive elements of some base sequence that satisfy a given predicate.

[`typealias LazyCollection`](/documentation/Swift/LazyCollection)

A collection containing the same elements as a `Base` collection,
but on which some operations such as `map` and `filter` are
implemented lazily.

[`typealias LazyDropWhileCollection`](/documentation/Swift/LazyDropWhileCollection)

A lazy wrapper that includes the elements of an underlying
collection after any initial consecutive elements that satisfy a
predicate.

[`typealias LazyFilterCollection`](/documentation/Swift/LazyFilterCollection)

A lazy `Collection` wrapper that includes the elements of an
underlying collection that satisfy a predicate.

[`typealias LazyMapCollection`](/documentation/Swift/LazyMapCollection)

A `Collection` whose elements consist of those in a `Base`
`Collection` passed through a transform function returning `Element`.
These elements are computed lazily, each time they’re read, by
calling the transform function on a base element.

[`typealias LazyPrefixWhileCollection`](/documentation/Swift/LazyPrefixWhileCollection)

A lazy collection wrapper that includes the initial consecutive
elements of an underlying collection that satisfy a predicate.

### Wrappers for Algorithms

Many collection operations are performed by wrapping a collection in another type,
instead of copying the collection’s contents.

[`struct CollectionDifference`](/documentation/Swift/CollectionDifference)

A collection of insertions and removals that describe the difference
between two ordered collection states.

[`struct DropFirstSequence`](/documentation/Swift/DropFirstSequence)

A sequence that lazily consumes and drops `n` elements from an underlying
`Base` iterator before possibly returning the first available element.

[`struct DropWhileSequence`](/documentation/Swift/DropWhileSequence)

A sequence that lazily consumes and drops `n` elements from an underlying
`Base` iterator before possibly returning the first available element.

[`struct EnumeratedSequence`](/documentation/Swift/EnumeratedSequence)

An enumeration of the elements of a sequence or collection.

[`typealias FlattenCollection`](/documentation/Swift/FlattenCollection)

[`struct FlattenSequence`](/documentation/Swift/FlattenSequence)

A sequence consisting of all the elements contained in each segment
contained in some `Base` sequence.

[`struct JoinedSequence`](/documentation/Swift/JoinedSequence)

A sequence that presents the elements of a base sequence of sequences
concatenated using a given separator.

[`struct PrefixSequence`](/documentation/Swift/PrefixSequence)

A sequence that only consumes up to `n` elements from an underlying
`Base` iterator.

[`struct Repeated`](/documentation/Swift/Repeated)

A collection whose elements are all identical.

[`struct ReversedCollection`](/documentation/Swift/ReversedCollection)

A collection that presents the elements of its base collection
in reverse order.

[`struct StrideTo`](/documentation/Swift/StrideTo)

A sequence of values formed by striding over a half-open interval.

[`struct StrideThrough`](/documentation/Swift/StrideThrough)

A sequence of values formed by striding over a closed interval.

[`struct UnfoldSequence`](/documentation/Swift/UnfoldSequence)

A sequence whose elements are produced via repeated applications of a
closure to some mutable state.

[`struct Zip2Sequence`](/documentation/Swift/Zip2Sequence)

A sequence of pairs built out of two underlying sequences.

### Collections of Indices

[`struct DefaultIndices`](/documentation/Swift/DefaultIndices)

A collection of indices for an arbitrary collection

### Indices and Iterators

Index and iterator types for other sequence and collection types in the standard
library.

[`struct IteratorSequence`](/documentation/Swift/IteratorSequence)

A sequence built around an iterator of type `Base`.

[`struct IndexingIterator`](/documentation/Swift/IndexingIterator)

A type that iterates over a collection using its indices.

[`typealias EnumeratedIterator`](/documentation/Swift/EnumeratedIterator)

[`typealias SetIterator`](/documentation/Swift/SetIterator)

[`struct StrideThroughIterator`](/documentation/Swift/StrideThroughIterator)

An iterator for a `StrideThrough` instance.

[`struct StrideToIterator`](/documentation/Swift/StrideToIterator)

An iterator for a `StrideTo` instance.

### Deprecated

[`typealias DictionaryIndex`](/documentation/Swift/DictionaryIndex)

[`typealias SetIndex`](/documentation/Swift/SetIndex)

[`typealias CountableClosedRange`](/documentation/Swift/CountableClosedRange)

[`typealias CountablePartialRangeFrom`](/documentation/Swift/CountablePartialRangeFrom)

[`typealias CountableRange`](/documentation/Swift/CountableRange)


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