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

[Ole Begemann]

For reference, here are some links to previous discussions on related 
topics:

1) Kevin Ballard in December 2015: Proposal: CollectionType.cycle 
property for an infinite sequence 
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151228/004635.html>

2) Ben Cohen in February 2016: Sequence/Collection Enhancements 
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170213/032120.html> 
(one of the enhancements that Ben proposed to consider for adding to the 
standard library is a cycle method for Collection and/or Sequence. I 
don't think there is a more specific proposal for this yet, but at least 
we know it's on the core team's radar.

On 02.05.2017 04:34, Karl Wagner via swift-evolution 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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