[swift-evolution] Fixing the confusion between non-mutating algorithms and single-pass sequences

David Waite david at alkaline-solutions.com
Wed Jul 20 16:41:32 CDT 2016

> On Jul 20, 2016, at 2:57 PM, Jonathan Hull via swift-evolution <swift-evolution at swift.org> wrote:
>> > True, people might try to get the iterator a second time, but we can
>> > make the iteratorProvider method optional (and trying to get an
>> > iterator from an iterator which is spent would return nil)
>> > and then they are forced to deal with the case where it was
>> > single-pass.
>> Now you can't loop over the same array multiple times.
> I must be missing something.  Isn’t that the point?
> I mean, your version is called “IterableOnce”.  Why do you want to iterate on IterableOnce more than once?  The point (at least in my mind) is to provide a common interface for things that we want to iterate over a single time.  If you want to iterate multiple times, use collection’s interface where you are guaranteed multi-pass.
> That said, you actually can loop multiple times for collections by getting a new iterator from the provider (which could point to the same array storage).  The optional just forces you to check for the single-pass case.
> I have a feeling like I am missing your true meaning here though...
> Thanks,
> Jon

Right - an iterator instance is single pass, but a method that returns an iterator may be able to make multiple iterators which iterate over the same sequence of values.

I still think ideally you can operate on iterators as streams of data directly, rather than requiring a base protocol. I understand how this could cause lots of duplication to have two ‘sequence’ implementations, however.


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