[swift-evolution] [swift-evolution-announce] [Accepted with Revision] SE-0094: Add sequence(initial:next:) and sequence(state:next:) to the stdlib

[Kevin Ballard]

On Thu, May 19, 2016, at 05:52 PM, Kevin Ballard wrote (on a
different thread):
> 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.
 
For the record, I just realized that the type signature of
UnfoldSequence<T> actually requires that we do heap allocation as well,
because this type can only be used for the stateful version if we erase
the state type by capturing it in a closure.
 
As part of implementing this, I'm going to go ahead and modify the type
signature to UnfoldSequence<T, State>, with `state(first:next:)`
returning the type UnfoldSequence<T, T?>. I think it's better to diverge
slightly from the proposal than it is to introduce unnecessary (albeit
small) performance cost. I hope there are no objections.
 
-Kevin Ballard
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