[swift-evolution] [Pitch] Remove destructive consumption from Sequence

Matthew Johnson matthew at anandabits.com
Fri Jul 1 20:45:09 CDT 2016


> On Jul 1, 2016, at 6:32 PM, Dave Abrahams <dabrahams at apple.com> wrote:
> 
> 
> on Fri Jul 01 2016, Matthew Johnson <matthew-AT-anandabits.com <http://matthew-at-anandabits.com/>> wrote:
> 
>>> On Jul 1, 2016, at 11:51 AM, Dave Abrahams <dabrahams at apple.com <mailto:dabrahams at apple.com>> wrote:
>>> 
>>> 
>>> on Fri Jul 01 2016, Matthew Johnson <matthew-AT-anandabits.com <http://matthew-at-anandabits.com/> <http://matthew-at-anandabits.com/ <http://matthew-at-anandabits.com/>>> wrote:
>>> 
>> 
>>>>> On Jun 30, 2016, at 12:26 PM, Dave Abrahams <dabrahams at apple.com <mailto:dabrahams at apple.com>>
>>>> wrote:
>>>>> 
>>>>> 
>>>>> on Wed Jun 29 2016, Haravikk <swift-evolution-AT-haravikk.me <http://swift-evolution-at-haravikk.me/>> wrote:
>>>>> 
>>>> 
>>>>>>> On 29 Jun 2016, at 00:10, Matthew Johnson via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>>> 
>>>>>>> Swift is a language that embraces value semantics.  Many common
>>>>>>> iterators *can* be implemented with value semantics.  Just because we
>>>>>>> can’t implement *all* iterators with value semantics doesn’t mean we
>>>>>>> should require them to have reference semantics.  It just means you
>>>>>>> can’t *assume* value semantics when working with iterators in generic
>>>>>>> code unless / until we have a way to specify a value semantics
>>>>>>> constraint.  That’s not necessarily a bad thing especially when it
>>>>>>> leaves the door open to interesting future possibilities.
>>>>>>> 
>>>>>>> -Matthew
>>>>>> 
>>>>>> I'm kind of undecided about this personally. I think one of the
>>>>>> problems with Swift is that the only indication that you have a
>>>>>> reference type is that you can declare it as a constant, yet still
>>>>>> call mutating methods upon it, this isn't a very positive way of
>>>>>> identifying it however. This may be more of a GUI/IDE issue though, in
>>>>>> that something being a class isn't always that obvious at a glance.
>>>>>> 
>>>>>> I wonder, could we somehow force iterators stored in variables to be
>>>>>> passed via inout? This would make it pretty clear that you're using
>>>>>> the same iterator and not a copy in all cases, encouraging you to
>>>>>> obtain another if you really do need to perform multiple passes.
>>>>> 
>>>>> I'm going to push single-pass iteration on the stack briefly and talk
>>>>> about the topic that's been under discussion here: infinite multipass
>>>>> sequences.
>>>>> 
>>>>> ## Fitting “Infinite Multipass” Into the Model
>>>>> 
>>>>> It remains to be decided whether it's worth doing, but if it's to
>>>>> happen
>>>> 
>>>> I definitely think it’s worth doing.  
>>> 
>>> Opinions are nice, but rationales are better.  How will we understand
>>> *why* it's worth doing?
>> 
>> I agree.  
>> 
>> The rationale has been discussed quite a bit already in this thread.
>> The current protocols do not provide the semantics many people are
>> assuming in their code, leading to a lot of code that is incorrect
>> despite the fact that it usually works in practice.
>> 
>> This is especially frequent in the case of the finite assumption.
>> This assumption is so common it seems very wise to me to encode it as
>> a semantic requirement in a protocol.
>> 
>> IMO these are problem worth addressing, especially now that we have a
>> good handle on what a solution would look like.
>> 
>>> 
>>>> I really appreciate the attention that the library team has given to
>>>> this.
>>>> 
>>>>> , the standard library team thinks the right design is roughly
>>>>> this:
>>>>> 
>>>>> /// A multipass sequence that may be infinite
>>>>> protocol Collection {
>>>>> 
>>>>>  // Only eager algorithms that can terminate available here
>>>>>  func index(where predicate: (Element)->Bool) -> Index
>>>>> 
>>>>>  // all lazy algorithms available here
>>>>>  var lazy: ...
>>>>> 
>>>>>  var startIndex: Index
>>>>>  var endIndex: Index // possibly not reachable from startIndex
>>>>> 
>>>>>  associatedtype SubSequence : Collection
>>>>>  // do we need an associated FiniteSubsequence, e.g. for prefixes?
>>>>> }
>>>>> 
>>>>> protocol FiniteCollection : Collection {
>>>>> 
>>>>>  // All eager algorithms available here
>>>>>  func map(...) ->
>>>>>  var count: ...
>>>>> }
>>>>> 
>>>>> protocol BidirectionalCollection : Collection { ... }
>>>>> 
>>>>> protocol RandomAccessCollection : BidirectionalCollection { … }
>>>> 
>>>> Does this design entirely break with the relationship between
>>>> collections and iterators (dropping `makeIterator` as a protocol
>>>> requirement)?  If so, would for..in (over collections) be built on top
>>>> of indices and use `formIndex(after:)`?  Would it require a finite
>>>> collection (unless we add `until` to the language and then allow
>>>> `for..in..until` to work with infinite collections)?
>>> 
>>> All of these points are up for discussion.  
>> 
>> Cool.  I think the collection for..in has some nice advantages, but
>> since it sounds like we’ll probably keep Iterator around it might be
>> best to take the approach of making them both work.
>> 
>> You already know that I would prefer to see the current for..in built
>> on finite sequences and allow for..in..unitl to be used with infinite
>> sequences if we add that in the future.  :)
>> 
>>> John McCall pointed out to
>>> me that an index-based for..in would make it possible to implement
>>> 
>>> for inout x in y { mutate(&x) }
>> 
>> That would be very nice!
>> 
>> I think it might also increase performance.  I don’t know exactly how
>> for..in is implemented today, but the implementation of
>> IndexingIterator compares position to endIndex.  If for..in is also
>> comparing checking the optional for nil that’s an extra comparison.
>> We shouldn't need to actually construct the optional in the first
>> place using an index-based for..in.  Maybe optimizations like this
>> already exist?  But even if they do, it seems like they wouldn’t be
>> possible in some cases where the type of the sequence isn’t statically
>> known.
>> 
>>> 
>>>> Would we still retain `IndexingIterator`even if we break the
>>>> relationship in the protocol requirements?
>>> 
>>> Yes: it should be possible to implement Collection algorithms in terms
>>> of Iterator algorithms, and IndexingIterator provides the means.  That
>>> said, I think the makeIterator requirement does little harm, especially
>>> when it can be defaulted for Collections.
>> 
>> I like this answer.
>> 
>>> 
>>>> Would it still be possible to do things like zip a multi-pass sequence
>>>> with a single-pass sequence (assuming we keep single-pass sequences or
>>>> add them back eventually)?  This seems like a use case worth
>>>> supporting in some way.
>>> 
>>> Yes.  If you can create an Iterator from a Collection, and you can zip
>>> Iterators, you can do this.
>> 
>> Yes, of course.  I’m glad we would keep this relationship in tact.
>> 
>>> 
>>>> One subtle change I think this implies is that things like
>>>> `LazyFilterSequence` can implement `makeIterator` with constant
>>>> complexity, deferring the O(N) complexity to the first call to `next`.
>>> 
>>> I don't believe that's a difference, though I could be wrong.
>> 
>> You’re right, I was wrong.  `LazyFilterSequence` just constructs an
>> iterator and returns it.  `LazyFilterCollection` has to loop until it
>> finds the first item matching the predicate in its `startIndex`
>> implementation.  The part I was missing is that `IndexingIterator`
>> gets the `startIndex` in its initializer.
>> 
>>> 
>>>> `startIndex` for `LazyFilterCollection` currently has O(N) complexity.
>>>> The complexity of a complete iteration doesn’t change and probably
>>>> isn’t a big deal, but it’s worth noting.
>>> 
>>> Filtered collection views always require a bit of hand-waving around
>>> performance guarantees; I don't think that changes.
>>> 
>>>> I’ve been looking at some code that wraps a sequence and considering
>>>> how it would be impacted.  With iterators it looks like this:
>>>> 
>>>> guard let element = base.next()
>>>> else { return nil }
>>>> 
>>>> With collections and indices it would look something like this:
>>>> 
>>>> base.formIndex(after: &index)
>>>> guard index != baseEndIndex
>>>>  else { return endIndex }
>>>> 
>>>> let element = base[index]
>>>> 
>>>> That’s not too bad but it is more verbose.  
>>> 
>>> Sequence today is a single-pass thing.  If you are wrapping Sequence
>>> today presumably you'd wrap an Iterator tomorrow, and you wouldn't have
>>> to deal with indices.
>>> 
>>>> If we’re going to push people towards collections and indices we
>>>> should try to make common patterns like “update the iteration state
>>>> and return the next element if there is one" simpler.  
>>> 
>>> That's IndexingIterator.
>> 
>> Cool, I wrote this thinking that was going away.
>> 
>>> 
>>>> This could be accomplished with an extension method along these lines:
>>>> 
>>>> guard let element = base.formIndex(after: &index,
>>>> .returningOptionalElement)
>>>>   else { return endIndex }
>>>> 
>>>> With an implementation something like:
>>>> 
>>>> enum FormIndexResult {
>>>>   .returningOptionalElement
>>>> }
>>>> extension Collection {
>>>>   func formIndex(after i: inout Self.Index, _ result:
>>>> FormIndexResult) -> Self.Element?
>>>> }
>>>> 
>>>> This would provide similar functionality to `IndexingIterator` without
>>>> coupling the storage of `elements` and `position` (which is important
>>>> if you’re wrapping a collection and need to wrap the collection and
>>>> its indices independently).
>>> 
>>> I'm afraid I don't understand.  Could you be more explicit about what
>>> you have in mind?
>> 
>> The idea was to provide functionality similar to `IndexingIterator` in
>> the sense the following code would provide equivalent functionality to
>> `iterator.next()` but expressed in terms of a collection and an index:
>> 
>> let optionalElement = myCollection.formIndex(after: &myIndex, . returningOptionalElement)
>> 
>> vs
>> 
>> let optionalElement = myIterator.next()
>> 
>> The single case enum is just there to provide a label that
>> differentiates the overload.
>> 
>> If we’re keeping IndexingIterator this probably isn’t necessary.  I
>> still have a use case for it but it is rather obscure.
> 
> I can imagine wanting a design like the above for cases where
> implementing the endIndex requires adding an extra bit of state, e.g. in
> 
>  struct LazyPrefix<Base: Collection> : Collection {
>    init(_ base: Base, where: (C.Element)->Bool)
>    ...
>  }
> 
> you don't want to traverse the base collection eagerly just to come up
> with an endIndex, so you store an optional Base.Index in
> LazyPrefix.Index, which is nil for the endIndex.  In these cases, index
> comparison is less efficient than it might otherwise be.
> 
> But my answer for these cases is simple: simply use a specialized
> Iterator that will be more efficient than IndexingIterator.  Is there a
> reason that doesn't work for your case?

I would index a custom iterator in my case, but I am talking about code that would live inside the implementation of `index(after:)` in the `Collection` conformance of a wrapper `Collection` that bears some resemblance to `LazyFlattenCollection`.  In this case you are receiving your `Index` which wraps `Base.Index`.

Like I said, this is a pretty obscure case and I would never suggest including it on the grounds of a use case that is only relevant to `index(after:)` implementations in wrapper collections. :)  I brought it because I thought you may have been suggesting a more drastic change that removed the collection iterators.

> 
>>> On second thought, I believe it is important to have a way to support
>>> existing “partially formed” multipass sequences that don't expose
>>> copying or equality for their iteration states.  
>> 
>> Can you provide examples of these?  I’m having difficulty thinking of
>> one.
> 
> NSSet is an example.
> 
>>> Iterator is the right way to do that.  So I think we need to keep
>>> Iterator around.
>> 
>> I don’t have any objection to keeping it either. :) Hopefully we’d
>> still be able to improve the design in the future if / when new
>> enabling language features come along.
>> 
>>> 
>>>> In the meantime, people would be able to implement their own protocol
>>>> for single pass sequences.  What they would lose is for..in as well as
>>>> the standard library algorithms.  I’m not sure how many people this
>>>> would impact or how big the impact would be for them.  We have seen a
>>>> couple of examples in this discussion, but probably not enough to
>>>> asses the overall impact.
>>>> 
>>>> One thing you don’t mention here is a distinction between finite and
>>>> infinite single-pass sequences (iterators).  I don’t know if the
>>>> finite / infinite distinction is as important here, but wanted to
>>>> point it out.  Obviously if we remove support single-pass sequences
>>>> now we could defer that discussion until we’re ready to bring back
>>>> support for them.
>>> 
>>> There are a few possible answers I can think of:
>>> 
>>> 1. Do the “obvious” thing and create a separate protocol for finite
>>>  single-pass sequences
>>> 
>>> 2. Decide that the combination of infinite and single-pass is rare
>>>  enough (/dev/urandom, temperature sensor) that it's better to just
>>>  ask people handling them to be careful and not, e.g., try to “count”
>>>  them.
>>> 
>>> 3. Decide that everything on a single-pass sequence is lazy. Since you
>>>  can only take a single pass anyway, people won't observe their
>>>  closures being called more often than necessary, which was the main
>>>  motivator for making map, filter, et. al eager on collections without
>>>  an explicit .lazy.
>>> 
>>> Implications of #3:
>>> 
>>> * Any “partially-formed” multipass sequences (modeling only Iterator)
>>> would be free to expose an accurate underestimatedCount, thereby
>>> optimizing the process of copying into an array. The lazy filter
>>> Iterator adaptor would have an underestimatedCount of 0.
>>> 
>>> * All algorithms that require multiple passes, such as sorted(), would
>>> be unavailable on Iterator.  You'd have to construct an Array (or
>>> other MutableCollection) and sort that in-place.  Of course,
>>> constructing an Array from an Iterator could still go on forever if
>>> the Iterator turned out to be infinite, which means, at some level #3
>>> is just a refinement of #2 that makes it less error-prone.
>> 
>> Do you lean towards any of these?
> 
> Yes, #3.  
> 
> We can always make the few operations that have to be eager—such as
> Array construction from an Iterator—explicit with a label or something:
> 
>      Array(claimingFiniteness: someIterator)

This makes sense.  Finite single-pass iterators can always be added in the future if compelling use cases emerge. We’re not taking anything away.  

All of the code I have looked at that makes a finite assumption would be converted to require `Collection` in the new model.

> 
> -- 
> Dave

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