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

[Patrick Smith]

I think that is a little confusing and has potential to be ‘abused’. I think it’s more confusing that a `for(;;)` loop for instance, and that got removed. I think var + AnyIterator is more explicit, and can become the canonical way to do this.

Hopefully AnyIterator can be optimized to the same performance.


> On 20 May 2016, at 10:57 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> 
>> On May 19, 2016, at 6:52 PM, Kevin Ballard via swift-evolution <swift-evolution at swift.org <mailto: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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