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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