[swift-evolution] [Draft] Rename Sequence.elementsEqual

Xiaodi Wu xiaodi.wu at gmail.com
Mon Oct 16 09:20:07 CDT 2017

On Mon, Oct 16, 2017 at 05:48 Jonathan Hull <jhull at gbis.com> wrote:

> On Oct 15, 2017, at 9:58 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> On Sun, Oct 15, 2017 at 8:51 PM, Jonathan Hull <jhull at gbis.com> wrote:
>> On Oct 14, 2017, at 10:48 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>  That ordering can be arbitrary, but it shouldn’t leak internal
>>>> representation such that the method used to create identical things affects
>>>> the outcome of generic methods because of differences in internal
>>>> representation.
>>>>>  It would be better to say that the iteration order is well-defined.
>>>>> That will almost always mean documented, and usually predictable though
>>>>> obviously e.g. RNGs and iterating in random order will not be predictable
>>>>> by design.
>>>>>> That's actually more semantically constrained than what Swift calls a
>>>>>> `Collection` (which requires conforming types to be multi-pass and(?)
>>>>>> finite). By contrast, Swift's `SpongeBob` protocol explicitly permits
>>>>>> conforming single-pass, infinite, and/or unordered types.
>>>>>> I think you’re talking about Sequence here, I’ve lost track of your
>>>>>> nonsense by now. Yes, the current Swift protocol named Sequence allows
>>>>>> unordered types. You seem to keep asserting that but not actually
>>>>>> addressing my argument, which is *that allowing Sequences to be
>>>>>> unordered with the current API is undesired and actively harmful, and
>>>>>> should* *therefore** be changed*.
>>>>> What is harmful about it?
>>>>> After thinking about it, I think the harmful bit is that unordered
>>>>> sequences are leaking internal representation (In your example, this is
>>>>> causing people to be surprised when two sets with identical elements are
>>>>> generating different sequences/orderings based on how they were created).
>>>>> You are correct when you say that this problem is even true for for-in.
>>>> I would not say it is a problem. Rather, by definition, iteration
>>>> involves retrieving one element after another; if you're allowed to do that
>>>> with Set, then the elements of a Set are observably ordered in some way.
>>>> Since it's not an OrderedSet--i.e., order doesn't matter--then the only
>>>> sensible conclusion is that the order of elements obtained in a for...in
>>>> loop must be arbitrary. If you think this is harmful, then you must believe
>>>> that one should be prohibited from iterating over an instance of Set.
>>>> Otherwise, Set is inescapably a Sequence by the Swift definition of
>>>> Sequence. All extension methods on Sequence like drop(while:) are really
>>>> just conveniences for common things that you can do with iterated access;
>>>> to my mind, they're essentially just alternative ways of spelling various
>>>> for...in loops.
>>>> I think an argument could be made that you shouldn’t be able to iterate
>>>> over a set without first defining an ordering on it (even if that ordering
>>>> is somewhat arbitrary).  Maybe we have something like a “Sequenc(e)able”
>>>> protocol which defines things which can be turned into a sequence when
>>>> combined with some sort of ordering.  One possible ordering could be the
>>>> internal representation (At least in that case we are calling it out
>>>> specifically).  If I had to say
>>>> “setA.arbitraryOrder.elementsEqual(setB.arbitraryOrder)” I would definitely
>>>> be less surprised when it returns false even though setA == setB.
>>> Well, that's a totally different direction, then; you're arguing that
>>> `Set` and `Dictionary` should not conform to `Sequence` altogether. That's
>>> fine (it's also a direction that some of us explored off-list a while ago),
>>> but at this point in Swift's evolution, realistically, it's not within the
>>> realm of possible changes.
>>> I am actually suggesting something slightly different.  Basically, Set
>>> and Dictionary’s conformance to Collection would have a different
>>> implementation.  They would conform to another protocol declaring that they
>>> are unordered. That protocol would fill in part of the conformance to
>>> sequence/collection using a default ordering, which is mostly arbitrary,
>>> but guaranteed to produce the same ordering for the same list of elements
>>> (even across collection types).  This would be safer, but a tiny bit slower
>>> than what we have now (We could also potentially develop a way for
>>> collections like set to amortize the cost). For those who need to recover
>>> speed, the new protocol would also define a property which quickly returns
>>> a sequence/iterator using the internal ordering (I arbitrarily called it
>>> .arbitraryOrder).
>>> I believe it would not be source breaking.
>> That is indeed something slightly different.
>> In an ideal world--and my initial understanding of what you were
>> suggesting--Set and Dictionary would each have a member like `collection`,
>> which would expose the underlying data as a `SetCollection` or
>> `DictionaryCollection` that in turn would conform to `Collection`;
>> meanwhile, Set and Dictionary themselves would not offer methods such as
>> `prefix`, or indexing by subscript, which are not compatible with being
>> unordered. For those who want a particular ordering, there'd be something
>> like `collection(ordered areInIncreasingOrder: (T, T) -> Bool) ->
>> {Set|Dictionary}Collection`.
>> What you suggest here instead would be minimally source-breaking.
>> However, I'm unsure of where these guarantees provide benefit to justify
>> the performance cost. Certainly not for `first` or `dropFirst(_:)`, which
>> still yields an arbitrary result which doesn't make sense for something
>> _unordered_. We *could* have an underscored customization point named
>> something like `_customOrderingPass` that is only invoked from
>> `elementsEqual` or other such methods to pre-rearrange the internal
>> ordering of unordered collections in some deterministic way before
>> comparison. Is that what you have in mind?
>> Something like that.  Whatever we do, there will be a tradeoff between
>> speed, correctness, and ergonomics.
>> My suggestion trades speed for correctness, and provides a way to recover
>> speed through additional typing (which is slightly less ergonomic).
> You haven't convinced me that this is at all improved in "correctness." It
> trades one arbitrary iteration order for another on a type that tries to
> model an unordered collection.
>> We could do something like you suggest. I don’t think the method would
>> need to be underscored… the ordering pass could just be a method on the
>> protocol which defines it as unordered.  Then we could provide a special
>> conformance for things where order really matters based on adherence to
>> that protocol.  That might be an acceptable tradeoff.  It would give us
>> speed at the cost of having the correct implementation being less ergonomic
>> and more error prone (you have to remember to check that it is unordered
>> and call the ordering method when it mattered).
>> I’d still be a bit worried that people would make incorrect generic
>> algorithms based on expecting an order from unordered things, but at least
>> it would be possible for them check and handle it correctly.  I think I
>> could get behind that tradeoff/compromise, given where we are in the swift
>> process and Swift's obsession with speed (though I still slightly prefer
>> the safer default).  At least the standard library would handle all the
>> things correctly, and that is what will affect the majority of programmers.
> What is an example of such an "incorrect" generic algorithm that would be
> made correct by such a scheme?
> To start with, the one you gave as an example at the beginning of this
> discussion: Two sets with identical elements which have different internal
> storage and thus give different orderings as sequences.  You yourself have
> argued that the confusion around this is enough of a problem that we need
> to make a source-breaking change (renaming it) to warn people that the
> results of the ‘elementsEqual’ algorithm are undefined for sets and
> dictionaries.

No, I am arguing that the confusion about ‘elementsEqual’ is foremost a
problem with its name; the result of this operation is not at all undefined
for two sets but actually clearly defined: it returns true if two sets have
the same elements in the same iteration order, which is a publicly
observable behavior of sets (likewise dictionaries).

I don’t see why a non-source-breaking change is suddenly off-limits.
> But more than that, any generic algorithm which is assuming that the
> sequence is coming from an ordered source (i.e. many things using
> first/last).  Some uses of first are ok because the programmer actually
> means ‘any’, but anywhere where they actually mean first/last may be
> problematic.

Such as...?

Currently, there is no way to test for ordered-ness, so there is no way for
> even a careful programmer to mitigate this problem.  By adding a protocol
> which states that something is unordered, we can either branch on it, or
> create a separate version of an algorithm for things which conform.

It is clearly the case that Swift’s protocol hierarchy fits sets and
collections imperfectly; however, it is in the nature of modeling that
imperfections are present. The question is not whether it is possible to
incur performance, API surface area, and other trade-offs to make the model
more faithful, but rather whether this usefully solves any problem. What is
the problem being mitigated? As I write above, Swift’s Set and Dictionary
types meet the semantic requirements for Collection and moonlight as
ordered collections. What is a generic algorithm on an ordered collection
that is  “not OK” for Set and Dictionary? (“elementsEqual”, as I’ve said,
is not such an example.)
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