[swift-dev] Implementation of Existential Collection (AnySequence and co.)

Luke Larson llarson at apple.com
Sat Apr 8 16:11:04 CDT 2017

David Abrahams is the code owner for the Swift standard library.


> On Apr 8, 2017, at 1:45 PM, Pavol Vaskovic via swift-dev <swift-dev at swift.org> wrote:
> Jordan Rose says in the comment on SR-4499 that Dmitri no longer works on Swift. Who owns that part of code now?
> Also, who is able to answer my original questions from this thread?
> --Pavol
> On Wed, 5 Apr 2017 at 18:56, Pavol Vaskovic <pali at pali.sk <mailto:pali at pali.sk>> wrote:
> On Wednesday, 5 April 2017 at 16:04, rintaro ishizaki wrote:
> > Just FYI
> >
> > Implementation of Type Erasers
> > https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20160905/002852.html <https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20160905/002852.html>
> Thanks for that link!
> This looks like a followup to Chris’s question, so I’m pinging Dmitri to chime in on this.
> Bug report: https://bugs.swift.org/browse/SR-4499 <https://bugs.swift.org/browse/SR-4499>
> For a reproducer, see the benchmark for `run_SuffixAnySequence` - that is utterly dominated by wending elements from the underlying sequence using generic witness table. There is no specialization occurring at all. As I’ve reasoned in my original e-mail, this is understandable - semantics of _AnyIteratorBox used in AnyIterator’s `next` method require virtual dispatch.
> Closure based approach to type erasure should’t have such a semantic barrier for optimization - the limit would probably just be currently conservative optimization?
> Best regards
> Pavol Vaskovic
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