[swift-evolution] Prosposal: LazyCollectionType.prefix() to return a lazy collection

Dmitri Gribenko gribozavr at gmail.com
Sun Feb 21 20:55:06 CST 2016

On Sun, Feb 14, 2016 at 4:12 AM, Dmitri Gribenko <gribozavr at gmail.com> wrote:
> On Sun, Feb 14, 2016 at 2:51 AM, Mark Aron Szulyovszky via
> swift-evolution <swift-evolution at swift.org> wrote:
>> Problem:
>> .prefix() works eagerly even on LazyCollectionType. This means access is not
>> O(1).
>> Implications:
>> There's no way to only get the first N elements of a lazy sequence without a
>> nasty for loop. If prefix() used on a lazy sequence, it'll trigger
>> computation on the whole array, just to return the first N elements.
>> Proposal:
>> It would be more more useful if lazy.<insert methods here>.prefix() returned
>> LazyPrefixCollection<Generator.Element>
>> instead of
>> Slice<LazyCustomCollection<Range<Generator.Element>>>.
> Thanks, Mark!
> I think this would be a good addition for forward and bidirectional
> collections.  You'd need two separate implementations for forward and
> bidirectional wrappers.
> For random-access collections, the eager implementation is optimal
> (finding the end index runs in O(1) and Slice<> does not introduce any
> constant factor compared to non-trivial wrappers), and we should keep
> that behavior by adding an overload that would return a
> LazyCollection<Slice<Base>>.

Hi Mark,

I think you can continue with writing a formal proposal in the
swift-evolution repository, presenting the full API that you are


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