[swift-evolution] Fixing the confusion between non-mutating algorithms and single-pass sequences
dabrahams at apple.com
Wed Jul 20 17:26:37 CDT 2016
on Wed Jul 20 2016, Jonathan Hull <jhull-AT-gbis.com> wrote:
>> >>> Basically, I added back in a super-minimal protocol to fill the
>> >>> structural gap left by Sequence. I call it “IteratorProvider” and it
>> >>> only has a single function which vends an iterator. Collection
>> >>> adheres to this, and Iterator adheres to it by returning itself. All
>> >>> of the other methods from Sequence remain on Iterator. Thus anyone
>> >>> with API that only needs a single pass would take a IteratorProvider
>> >>> and then work on the iterator it provides.
>> >> That leaves us back where we are now: people will see that
>> >> IteratorProvider is a simple, universal protocol for both single-and
>> >> multi-pass sequences, write algorithm libraries that depend on
>> >> multi-pass-ness, and test them with the most prevalent examples, which
>> >> happen to be multi pass.
>> > Let me make a quick counter-argument, because I thought about it a
>> > bit, and I don’t think it does have the same problem (especially with
>> > careful/better naming).
>> > The difference is that the ONLY method on IteratorProvider is the one
>> > to get an iterator. There is no map, filter, sort, first, count, etc…
>> > just a way to get a single-pass iterator. This changes the mindset
>> > when using it. You are aware that you are getting a single-pass
>> > iterator.
>> Maybe. What's to stop people from extending IteratorProvider?
> Nothing. But that is true of any protocol. I am ok with individual's
> extensions. They would have to use that single method to build up
> from anyway, so presumably they would have to consider the single pass
> case in their extensions...
>> > 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?
No. Arrays are multipass.
> I mean, your version is called “IterableOnce”. Why do you want to
> iterate on IterableOnce more than once?
Because it happens to be multipass.
> 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.
for ... in uses Iterators.
> 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.
Oh, I'm sorry; I didn't realize you were saying that only single-pass
IteratorProviders would ever return nil from their methods.
> I have a feeling like I am missing your true meaning here though...
Probably a communication failure on my end.
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