[swift-evolution] [Review] SE-0094: Add sequence(initial:next:) and sequence(state:next:) to the stdlib

[Erica Sadun]

> On May 19, 2016, at 6:52 PM, Kevin Ballard via swift-evolution <swift-evolution at swift.org> wrote:
> 
> After having given this some thought, it seems apparent that `sequence(state:next:)` is equivalent to `AnyIterator({ ... })` where the closure captures a single mutable variable. The microbenchmark performance may differ slightly, as the AnyIterator version will allocate a box on the heap to hold the captured variable (assuming it can't get inlined entirely), whereas UnfoldSequence won't. But the functionality is the same.
>  
> Thus the question: do we want to keep `sequence(state:next:)` or is it too close to AnyIterator in functionality? Arguments in favor of `sequence(state:next:)`:
>  
> * It's equivalent to unfold and the dual of reduce, so people who've used functional programming languages may expect it to exist.
> * It allows you to create ad-hoc stateful sequences without polluting the current scope with a variable that exists solely to be captured.
> * If the cost of a small heap allocation is significant for your code, it may be more performant than AnyIterator.
>  
> Personally, the most important reason here for me is not having to pollute the current closure with a variable. And this could actually be solved another way, by allowing the use of `var` in a capture list, which would let you say something like `AnyGenerator({ [var state=foo] in ... })`.
>  
> Given all this, at this point I'm actually leaning towards saying`sequence(state:next:)` doesn't pull its own weight and we should just go with `sequence(initial:next:)`.
>  
> -Kevin Ballard

Adding on, to the best of my understanding the biggest win in the stateful variation is to be able to create a sequence from a starting state without declaring any external variables, as in the perfectly wrong and evil example I showed Kevin:

enum Finger: Int { case Thumb = 1, Pointer, Middle, Ring, Pinky }

extension Finger {
    static func members() -> AnySequence<Finger> {
        return sequence(Thumb.rawValue, next: {
            (inout idx: Int) in
            defer { idx += 1 }
            return Finger(rawValue: idx)
        })
    }
}

for finger in Finger.members() { print(finger) }

-- E

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