> On Oct 13, 2017, at 8:28 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote: > > > > On Fri, Oct 13, 2017 at 12:03 PM, Kevin Nattinger <sw...@nattinger.net > <mailto:sw...@nattinger.net>> wrote: >> On Oct 13, 2017, at 6:52 AM, Xiaodi Wu <xiaodi...@gmail.com >> <mailto:xiaodi...@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. > > * `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. 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. `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. >> 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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