## Introduction
Add a new property `cycle` to CollectionType that returns an infinite
SequenceType that yields the elements of the collection in a loop.
## Motivation
It's sometimes useful to be able to have an infinite sequence. For example,
`CollectionOfOne(x).cycle` could be used to have an infinite sequence of a
single element (similar to Repeat but without a count). A common use for
infinite sequences is zipping with a finite sequence. As far as I'm aware, the
stdlib does not currently provide any way to create such an infinite sequence.
## Proposed solution
Extend CollectionType with a new property `cycle` that yields a type that
conforms to SequenceType. This sequence yields each element of the collection
in an infinite loop.
## Detailed design
2 new types would be added:
struct CycleSequence<Base : CollectionType> : LazySequenceType { ... }
struct CycleGenerator<Base : CollectionType> : GeneratorType { ... }
CollectionType would be extended with a property:
extension CollectionType {
public var cycle: CycleSequence<Self> { get }
}
This is an extension of CollectionType instead of SequenceType because it
requires a multi-pass sequence (and SequenceType does not provide that
guarantee). The returned type conforms to SequenceType instead of
CollectionType because there is no possible `endIndex` that satisfies the
requirement of being reachable from `startIndex` by zero or more applications
of `successor()`.
Because the default eager versions of map and filter will execute forever on an
infinite sequence, CycleSequence conforms to LazySequenceType instead of
SequenceType in order to provide lazy versions of those functions.
Additionally, it will provide implementations of the eager versions that simply
trigger a fatalError(), as the alternative is an infinite loop that consumes
more and more memory.
## Impact on existing code
None
-Kevin Ballard
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