[swift-evolution] [Pitch] Improve `init(repeating:count)`
Max Moiseev
moiseev at apple.com
Thu Aug 17 13:04:05 CDT 2017
> On Aug 17, 2017, at 10:05 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
>
> Also, for those of you here who haven't heard my previous rant on the subject, I dislike using map for generating values that don't depend on transforming a domain to a range. (It has been argued that `_ in` is mapping from `Void`, but I still dislike it immensely)
Can you please elaborate why (or maybe point me at the rant)? Does the same argument apply to `for _ in 1..<N { … }`?
But back to the discussion. I quickly looked at the sil being generated, and measured the performance of:
public class C {
init() {}
}
@inline(never)
func foo(n: Int) -> [C] {
var result: [C] = []
result.reserveCapacity(n)
for _ in 1...n {
result.append(C())
}
return result
}
@inline(never)
func bar(n: Int) -> [C] {
return (1...n).map { _ in C() }
}
Version using map wins in both the size of sil and performance. It is also easier to implement right (fewer moving parts, I guess). If one forgets to call `reserveCapacity` on `result` in the first case, performance drops even further.
Max
>
> Here are the ways that I have approached this:
>
> // Ugh
> [UIView(), UIView(), UIView(), UIView(), UIView()]
>
> // No:
> let viewsA = Array(repeating: UIView(), count: 5)
> // You end up with 5 of the same view
>
> // Two statements that really should be one
> var views: [UIView] = []
> for _ in 1 ... 5 { views.append(UIView()) }
>
> // Wrong use of `map`, because it's mapping over `Void`
> let viewsA = (1 ... 5).map({ _ in UIView() })
>
> // You can introduce `collect` specifically for the `_ in` in case, matching other languages:
> public func collect<T>(_ generator: () throws -> T) rethrows -> [T]
>
> // I think these are just ugly
> let viewsA__ = sequence(first: UIView(), next: { _ in UIView() }).lazy.prefix(5)
> let viewsB__ = sequence(state: 5, next: { defer { $0 -= 1 }; $0 == 0 ? nil : UIView() }
>
> // You can build an iterator
>
> let labeler = AnyIterator({ return UILabel() })
> let labels4 = Array(labeler.prefix(5))
>
> // which lets you create multiple "slices" off the iterator
>
> let randoms = AnyIterator({ return Int(arc4random_uniform(100) )})
>
> print(Array(randoms.prefix(5)))
> print(Array(randoms.prefix(5)))
>
>
> // A little complex and I don't like making this `Int` based, because it pulls the semantics away from sequences/collections
>
> extension Int {
> func of<T>(_ generator: @autoclosure () -> T) -> [T] {
> assert(self >= 0, "cannot generate negative-count collection")
> return (0 ..< self).map({ _ in generator() })
> }
> }
>
> 5.of(UIView())
>
> // Even worse
> protocol Constructable {
> init()
> }
>
> extension UIView: Constructable {}
>
> extension Int {
> func of<T: Constructable>(_ generator: @autoclosure () -> T) -> UnfoldSequence<T, Int> {
> assert(self > 0, "cannot generate negative-count collection")
> return sequence(state: self, next: { (state: inout Int) -> T? in
> defer { state -= 1 }; return state == 0 ? nil : T.init() })
> }
> }
>
> print(Array(5.of(UIView())))
>
> // or
>
> extension Int {
> func of<T>(_ generator: @escaping @autoclosure () -> T) -> LazyMapRandomAccessCollection<(CountableRange<Int>), T> {
> assert(self > 0, "cannot generate negative-count collection")
> return (0 ..< self).lazy.map({ _ in generator() })
> }
> }
>
> // where some people preferred calling this `elements`, for example `5.elements(of: UIView())`
>
> // You can go Array:
>
> extension Array {
> /// Creates a new array containing the specified number of values created by repeating a generating closure.
> ///
> /// - Parameters:
> /// - count: The number of times to apply the closure. `count` must be zero or greater.
> /// - generator: The closure to execute
> public init(count: Int, repeating: () -> Element) {
> precondition(count >= 0, "")
> self.init((1 ... count).map({ _ in repeating() }))
> }
> }
>
> let labels = Array(count: 4) { UILabel() }
>
> // Or similarly
>
> extension Array {
> init(repeat count: Int, byCalling generator: @autoclosure () -> Element) {
> self = (1 ... count).map({ _ in generator() })
> }
> }
>
> // From Oliver H:
>
> extension Array {
> convenience init(count: Int, repeating: () -> Elements) {
> self = Array( (0..<count).map { _ in repeating() } )
> }
> }
>
> let views: [UIView] = Array(count: 5) { UIView(frame: .zero) }
>
> // This one is from Soroush K. I think it's clever but I wouldn't really use it
>
> func * <T>(generator: @autoclosure () -> T, n: Int) -> [T] {
> (0..<n).map({ _ in generator() }
> }
>
> UIView() * 5
>
>
>> On Aug 17, 2017, at 10:36 AM, Tony Allevato via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>>
>>
>> On Thu, Aug 17, 2017 at 9:20 AM Ross O'Brien <narrativium+swift at gmail.com <mailto:narrativium%2Bswift at gmail.com>> wrote:
>> (0..<3).map{ _ in UIView() } - map already returns an Array.
>>
>> Array((0..<3).map{ _ in UIView() }) is redundant.
>>
>> Ah, right, thanks for pointing that out. I couldn't remember off the top of my head whether it returned an array or some kind of special sequence type that would need to be converted over.
>>
>> In that case, I still think the map version wins—it's very clear that a repeated *operation* is occurring, whereas the originally proposed @autoclosure version hides that very important semantic information.
>>
>>
>> I've fallen foul before, of trying to create an array of six buttons and getting an array of one button six times; I think that should be easier. But if each button corresponds to an existing value or needs to be initialised based on its index in that array, map transposing values or indices into buttons is already covered.
>>
>> On Thu, Aug 17, 2017 at 5:03 PM, Tony Allevato via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> Couldn't this be rewritten more simply today as:
>>
>> Array((0..<3).map { index in MyView(forIndex: index) })
>>
>> And the version that doesn't need the index could be written:
>>
>> Array((0..<3).map { _ in UIView() })
>>
>> The AnyIterator approach posted above is also nice—I wouldn't have thought of that one. But I suppose that only works in the case where you don't need the index.
>>
>> So the question is, should we increase the API surface of Array for something that (IMO) is already fairly straightforward to do? The nice thing about the approaches above is that they're composable. Array doesn't have to make any assumptions about closures or index arguments to those closures; it just takes a sequence and we already have primitives to construct sequences of new objects using .map. Array(repeating:count:) and repeatedElement(_:count:) are nice when the value being repeated is fixed, but I think once you start getting into questions like "what if I need a different thing each time?" or "what if I need to involve the index as part of the elements?" then it's better suited to compose the features already there to build something up than to add new APIs that try to cover each of these special cases.
>>
>>
>> On Thu, Aug 17, 2017 at 8:40 AM Christopher Kornher via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> We might as well add the index to the call so elements can be created from other lists, etc.
>>
>> Array(repeatedlyCalling: { (index:Int) in MyView( forIndex:index ) }, count: 3) // This might by syntactically correct...
>>
>>
>>
>>> On Aug 17, 2017, at 9:24 AM, Rob Mayoff via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>
>>> On Thu, Aug 17, 2017 at 7:04 AM, Robert Bennett via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> Alternatively, instead of replacing the current definition with an autoclosure version, we could leave the current version in place and add a version taking a function. This could be especially useful when you’ve defined a function like `makeMySpecialKindOfButton() -> UIButton` that does a lot of customization to an object before returning it.
>>>
>>> Array(repeating: { return UIView() }, count: 3)
>>> and
>>> Array(repeating: makeMySpecialKindOfButton, count: 3)
>>>
>>> This is still source-breaking, because an array of closures is legal. We need a different name to avoid breaking source:
>>>
>>> Array(repeatedlyCalling: { UIView() }, count: 3)
>>>
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>>
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