[swift-evolution] [Pitch] New collection-based 'repeat' API

[Xiaodi Wu]

Does this gain something by being part of the standard library as opposed
to being built on top of it?

On Mon, May 1, 2017 at 21:34 Karl Wagner via swift-evolution <
swift-evolution at swift.org> wrote:

> Currently, we have the Repeated<T> type, which presents a single element
> as though it were a Collection.
>
> > for i in repeatElement(1, count: 3) { print(i) }
> 1
> 1
> 1
>
> > for i in repeatElement([1, 2, 3], count: 3) { print(i) }
> [1, 2, 3]
> [1, 2, 3]
> [1, 2, 3]
>
>
> However, we lack the ability for Collections to repeat their contents in a
> single list; basically, some kind of “flatMap” to repeatElement’s “map”. So
> I’d like to pitch a new API for repeated values.
>
> - We would add a RepeatCollection<C: Collection> type, which loops over
> its base Collection a certain number of times (or until a maximum ‘count’).
>   Implementation might look something like this (
> https://gist.github.com/karwa/5228974a0b4dfd000a916f0aac2721c6), except
> that we’d add some optimised map(), filter() and contains() functions which
> apply the algorithm once to the base and multiply the result.
>
> - We would add 3 new functions to all Collections:
>
> /// Repeats the collection *itself* N times.
> ///
> func repeated(_ times: Int) -> RepeatCollection<CollectionOfOne<Self>>
>
> /// Repeats the collection’s *contents* N times.
> ///
> func repeatElements(_ times: Int) -> RepeatCollection<Self>
>
> /// Loops the collection’s contents to present a Collection of length N.
> ///
> func repeatElements(count: Int) -> RepeatCollection<Self>
>
>
> - We would replace the existing Repeated<T> type with a typealias to
> RepeatCollection<CollectionOfOne<T>>
> - The existing, top-level repeatElement(T, Int) function *could* stay,
> but could also be replaced with an incantation on CollectionOfOne. I’m
> fairly ambivalent about this point - it’d be nice to see the function go,
> but the replacement also isn’t obvious.
>
> Example usage of the new API:
>
> // Equivalent to repeatElement(1, count: 3)
>
> > for i in CollectionOfOne(1).repeatElements(3).forEach { print(i) }
> 1
> 1
> 1
>
> // Equivalent to repeatElement([1, 2, 3], count: 3)
>
> > for i in [1, 2, 3].repeated(3).forEach { print(i) }
> [1, 2, 3]
> [1, 2, 3]
> [1, 2, 3]
>
> // New, flat repetition
>
> > for i in [1, 2, 3].repeatElements(3) { print(i) }
> 1
> 2
> 3
> 1
> 2
> 3
> 1
> 2
> 3
>
> // New, flat repetition
>
> > for i in [1, 2, 3].repeatElements(count: 4) { print(i) }
> 1
> 2
> 3
> 1
>
> // Additional benefit: you can now repeat slices!
>
> > String(“yellow”.characters.dropFirst().dropLast().repeat(times: 3))
> “elloelloello"
>
>
> Thoughts?
>
> - Karl
>
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