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

[Xiaodi Wu]

On Sat, Oct 14, 2017 at 2:28 AM, Kevin Nattinger <swift at nattinger.net>
wrote:

>
> On Oct 13, 2017, at 8:28 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
>
>
> On Fri, Oct 13, 2017 at 12:03 PM, Kevin Nattinger <swift at nattinger.net>
> wrote:
>
>> On Oct 13, 2017, at 6:52 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>> You’re welcome to bikeshed the entire API surface area of sequences and
>> collections, but you won’t be the first to explore this area. A number of
>> us looked into this area in the past few years and did not reach a
>> measurable improved result.
>>
>>
>> I don’t need or want to bikeshed the entire sequence and collection
>> surface area, I just want to fix one clear and GLARING issue:
>>
>> A Set is NOT a sequence.
>>
>> Note that this goes for dictionaries and any other unordered “sequences"
>> as well.
>>
>> That was in an early draft of my original email, but I dropped it because
>> I was afraid people would just stop reading and dismiss the idea
>> out-of-hand without considering the problem or arguments. Apparently I
>> should have at least put it at the bottom, so sorry if the root issue was
>> unclear.
>>
>> Sequences can be ordered or unordered,
>>
>>
>> You seem to be confusing the English word “sequence” with the (current)
>> Swift protocol “Sequence." A sequence is, by definition, ordered. Not
>> enforcing that in a protocol does not override the English language, and as
>> this entire thread demonstrates, causes issues further on down the line.
>>
>
> We are discussing the Swift protocol `Sequence`. It really doesn't matter
> at all what the English word "sequence" means, and any difference between
> the English word and the Swift term is emphatically *not* the root cause of
> the issue. Here's why:
>
> * A Swift `Sequence` is, to put it simplistically, a thing that can be
> iterated over in a `for...in` loop. If it would make you happy, for the
> rest of the discussion, let's suppose we called the protocol `ForLoopable`
> instead of `Sequence`.
>
>
> ForLoopable is so ugly. Since we’re just iterating over the elements, how
> about, oh, say, `Iterable`? Hey, that looks familiar.
>

I'm not trying to bikeshed the name of `Sequence`. I'm picking an
intentionally unwieldy name for the purposes of discussing the semantics of
this particular protocol. The point is that the underlying issue has
nothing to do with the name; it can be `Iterable` or it can be `SpongeBob`
for all I care.


>
>
> * `Set` should conform to `ForLoopable`. (This I state as a premise; if
> you disagree with the notion that we should be able to iterate over the
> elements of an instance of `Set` with a `for...in loop`, then it's clearly
> a whole other discussion and not a question of what the English word
> "sequence" means.)
>
>
> Obviously, `Set: Iterable`. I don’t think I’ve said anything to suggest
> you shouldn’t be able to iterate over unordered collections.
>
>
> * If a type `T` conforms to `ForLoopable` and an instance `t` of that type
> has at least one element, then *something* has to be the first element in a
> `for element in t { ... }` loop. Put another way, every instance of a type
> that conforms to `ForLoopable` must have at least one publicly observable
> order (although, intriguingly, I'm not sure it has to be a repeatable one).
> It is possible, therefore, to have a semantic answer to the question of
> which element is `first` or (if finite) `last`; one can also
> `drop(while:)`, etc., and perform lexicographical comparisons.
>
>
> As a side effect of Swift being a procedural language each iteration
> happens to occur in some order, yes, but that order is meaningless and
> reflects nothing about the Set itself.  In fact, I’d say that *`first`,
> `last`, etc. are not even defined on the original Set per se, only on the
> specific order that a particular iteration resulted in*. And that order
> is not necessarily predictable, nor necessarily stable, as you yourself
> said.
>
> Consider an Iterable that gives a different order every time it’s
> iterated.
> Should calling `.first` or `last` give a different object every time?
> That’s absurd.
> Should an object lexicographically compare not equal to itself? Even more
> absurd.
>

What's your basis for saying that such behavior is absurd? It is explicitly
permitted for instances of types conforming to `SpongeBob` to be
single-pass and/or infinite. For a single-pass `SpongeBob`, `first` will
certainly return a different value every time it is invoked.

A random number generator fulfills all the semantic requirements of
conforming to `SpongeBob`, and in fact I do just that in NumericAnnex
<https://github.com/xwu/NumericAnnex/blob/master/Sources/PRNG.swift#L53>.
`first` gives a different value every time, and a randomly generated
`SpongeBob` would unsurprisingly compare lexicographically not equal to
itself.

On the other hand, if I have a collection of objects that I want iterated
> in a particular order, I can use a container that iterates in a specific,
> known, well-defined way, and use that to construct the sequence of
> objects.  That’s clearly an Iterable collection, but the guarantee is
> stronger than that. Since it iterates objects in a specific sequence, the
> logical way to express that would be `Sequence: Iterable`. Again, we’ve
> seen that before.
>
> Now, since a Sequence is guaranteed to iterate the same every time,
> suddenly our `first`, `last`, `drop*`, etc. methods have a meaning inherent
> to the collection itself, rather than a specific iteration.
>

What you call a "Sequence" here would have to be multi-pass, finite, and
ordered. 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. As long
as it is possible to iterate over a `SpongeBob`, it is meaningful to ask
what element is first obtained upon iteration or to drop the first element
obtained upon iteration. And as long as iteration is not required to be
repeatable (and it isn't), it is perfectly acceptable for these algorithms
to return a different result every time.

`first` is the first object in the Sequence. It doesn’t matter how the
> sequence came to be in that order; it doesn’t matter whether or not the
> sequence has already been iterated or how many times. `first` is the first
> object that is, was, and always will be presented by the Sequence’s
> Iterator. (Until the collection is mutated, obviously).
>
> *To summarize,*
> A Set has no intrinsic order. You can iterate over it, and a specific
> iteration of a set has an order, but that order is not tied to the Set
> itself beyond including all and only the items therein. Therefore, the Set
> itself has no intrinsic `first`, `last`, lexicographical comparison, etc.;
> only its iterations do, and they are not themselves Sets.
> A Sequence does have an intrinsic order. The order of iteration reflects
> the order inherent to the Sequence. Therefore, a Sequence has a `first`,
> `last`, lexicographical comparison, etc.
>
> Just in case it’s not obvious, `Set` here is pretty much interchangeable
> with any other unordered iterable.
>

What you want to call a "Sequence" is what Swift calls a `Collection`, with
additional restrictions. What you want to be called "Iterable" is what
Swift calls `Sequence` (or now, `SpongeBob`). Clearly, shuffling names will
not make these protocols support any more functionality, so that can be put
aside.

Now, with that out of the way, why do you think that only `Collection`
types should have `first` and `last`? These helper properties and methods
are simply convenient ways to spell things that can be done with
`for...in`--the whole point of supplying them is to allow people to work
with these types in a more functional style.

public protocol Iterable {
>>>>   associatedtype Iterator: IteratorProtocol
>>>>   func map<T>(...) -> [T] // Iterable where .Iterator.Element == T
>>>>   func filter(...) -> [Iterator.Element] // Iterable where
>>>> .Iterator.Element == Self.Iterator.Element
>>>>   func forEach(...)
>>>>   func makeIterator() -> Iterator
>>>>   var underestimatedCount: Int { get }
>>>> }
>>>>
>>>> public protocol Sequence: Iterable { // Maybe OrderedSequence just to
>>>> make the well-defined-order requirement explicit
>>>>   associatedtype SubSequence
>>>>   func dropFirst(...)   -> SubSequence   // Sequence where
>>>> .Iterator.Element == Self.Iterator.Element
>>>>   func dropLast(...)    -> SubSequence   //    " "
>>>>   func drop(while...)   -> SubSequence   //    " "
>>>>   func prefix(...)      -> SubSequence   //    " "
>>>>   func prefix(while...) -> SubSequence   //    " "
>>>>   func suffix(...)      -> SubSequence   //    " "
>>>>   func split(...where...)  -> [SubSequence] // Iterable where
>>>> .Iterator.Element == (Sequence where .Iterator.Element ==
>>>> Self.Iterator.Element)
>>>> }
>>>>
>>>
> And just to be explicit,
> struct Set: Iterable {…}
> struct Dictionary: Iterable {…}
> struct Array: Sequence {…}
> etc.
>
> Hopefully at some point:
> struct OrderedSet: Sequence {…}
>
>
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