on Wed Jun 29 2016, Haravikk <swift-evolution-AT-haravikk.me> wrote:
>> On 29 Jun 2016, at 00:10, Matthew Johnson via swift-evolution >> <[email protected]> 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, 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 { ... } Q: Why should there be indices on an infinite multipass sequence? A: Because the operations on indices apply equally well whether the sequence is finite or not. Find the index of a value in the sequence, slice the sequence, find again, etc. Q: Why is there an endIndex on an infinite seque? A: So you can write algorithms such as index(where:) once. Q: Why not allow endIndex to have a different type from startIndex? A: It appears to offer insufficient benefit for the associated complexity in typical usage. A classic use case that argues for a different endIndex type is the null-terminated C string. But you can't index one of those safely without actually counting the length, and once you've done that you can make the endIndex an Int. ## Single Pass Iteration The refinement relationship between Sequence and Collection is problematic, because it means either: a) algorithms such as map on single-pass sequences claim to be nonmutating even though it's a lie (status quo) b) those algorithms can't be used on immutable (“let bound”) multipass sequences. IMO that would be totally unacceptable. If we drop the refinement, we can have a saner world. We also don't need to separate Sequence and Iterator anymore. We can simply drop Sequence altogether, and the protocol for single-pass iteration becomes Iterator. ### Mutation and Reference Semantics Everything in Swift is copiable via `let copy = thing` (let's please not argue over the definition of copy for classes; this is the one built into the lowest level of the language—I refer to the other one, that requires allocation, as “clone”). Anything you do with a sequence that's truly single-pass mutates the sequence *and of its copies*. Therefore, such a type *fundamentally* has reference semantics. One day we may be able to model single-pass sequences with “move-only” value types, which cannot be copied. You can find move-only types in languages like Rust and C++, but they are not supported by Swift today. So it seems reasonable that all Iterators in Swift today should be modeled as classes. The fact that Swift doesn't have a mutation model for classes, though, means that mutating methods on a class constrained protocol can't be labeled as such. So consuming operations on a class-constrained Iterator protocol would not be labeled as mutating. The standard library team is currently trying to evaluate the tradeoffs in this area. One possibility under consideration is simply dropping support for single-pass sequences until Swift can support move-only value types and/or gets a mutation model for class instances. It would be very interesting to know about any real-world models of single-pass sequences that people are using in Swift, since we don't supply any in the standard library. -- Dave _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
