> On May 19, 2016, at 6:52 PM, Kevin Ballard via swift-evolution
> <[email protected]> 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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