<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi all,<div class=""><br class=""></div><div class=""><b class=""><font size="4" class="">Introduction</font></b></div><div class=""><b class=""><br class=""></b></div><div class="">The “Complete Generics” goal for Swift 3 has been fairly ill-defined thus fair, with just this short blurb in the list of goals:</div><div class=""><br class=""></div><div class=""><ul style="box-sizing: border-box; padding: 0px 0px 0px 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""><li style="box-sizing: border-box;" class=""><strong style="box-sizing: border-box;" class="">Complete generics</strong>: Generics are used pervasively in a number of Swift libraries, especially the standard library. However, there are a number of generics features the standard library requires to fully realize its vision, including recursive protocol constraints, the ability to make a constrained extension conform to a new protocol (i.e., an array of <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">Equatable</code> elements is <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 14px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">Equatable</code>), and so on. Swift 3.0 should provide those generics features needed by the standard library, because they affect the standard library's ABI.</li></ul><div class="">This message expands upon the notion of “completing generics”. It is not a plan for Swift 3, nor an official core team communication, but it collects the results of numerous discussions among the core team and Swift developers, both of the compiler and the standard library. I hope to achieve several things:</div><div class=""><br class=""></div><div class=""><ul class="MailOutline"><li class="">Communicate a vision for Swift generics, building on the <a href="https://github.com/apple/swift/blob/master/docs/Generics.rst" class="">original generics design document</a>, so we have something concrete and comprehensive to discuss.</li><li class="">Establish some terminology that the Swift developers have been using for these features, so our discussions can be more productive (“oh, you’re proposing what we refer to as ‘conditional conformances’; go look over at this thread”).</li><li class="">Engage more of the community in discussions of specific generics features, so we can coalesce around designs for public review. And maybe even get some of them implemented.</li></ul><div class=""><br class=""></div></div><div class="">A message like this can easily turn into a <a href="http://www.urbandictionary.com/define.php?term=centithread" class="">centithread</a>. To separate concerns in our discussion, I ask that replies to this specific thread be limited to discussions of the vision as a whole: how the pieces fit together, what pieces are missing, whether this is the right long-term vision for Swift, and so on. For discussions of specific language features, e.g., to work out the syntax and semantics of conditional conformances or discuss the implementation in compiler or use in the standard library, please start a new thread based on the feature names I’m using.</div><div class=""><br class=""></div></div><div class="">This message covers a lot of ground; I’ve attempted a rough categorization of the various features, and kept the descriptions brief to limit the overall length. Most of these aren’t my ideas, and any syntax I’m providing is simply a way to express these ideas in code and is subject to change. Not all of these features will happen, either soon or ever, but they are intended to be a fairly complete whole that should mesh together. I’ve put a * next to features that I think are important in the nearer term vs. being interesting “some day”. Mostly, the *’s reflect features that will have a significant impact on the Swift standard library’s design and implementation.</div><div class=""><br class=""></div><div class="">Enough with the disclaimers; it’s time to talk features.</div><div class=""><br class=""></div><div class=""><b class=""><font size="4" class="">Removing unnecessary restrictions</font></b></div><div class=""><b class=""><br class=""></b></div><div class="">There are a number of restrictions to the use of generics that fall out of the implementation in the Swift compiler. Removal of these restrictions is a matter of implementation only; one need not introduce new syntax or semantics to realize them. I’m listing them for two reasons: first, it’s an acknowledgment that these features are intended to exist in the model we have today, and, second, we’d love help with the implementation of these features.</div><div class=""><b class=""><br class=""></b></div><div class=""><b class=""><br class=""></b></div><div class=""><span style="font-size: 14px;" class=""><i class="">*Recursive protocol constraints</i><br class=""></span></div><div class=""><br class=""></div><div class="">Currently, an associated type cannot be required to conform to its enclosing protocol (or any protocol that inherits that protocol). For example, in the standard library SubSequence type of a Sequence should itself be a Sequence:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">protocol Sequence {</font></div><div class=""><font face="Menlo" class=""> associatedtype Iterator : IteratorProtocol</font></div><div class=""><font face="Menlo" class=""> …</font></div><div class=""><font face="Menlo" class=""> </font><span style="font-family: Menlo;" class="">associatedtype</span><font face="Menlo" class=""> SubSequence <b class="">: Sequence </b><i class="">// currently ill-formed, but should be possible</i></font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class="">The compiler currently rejects this protocol, which is unfortunate: it effectively pushes the SubSequence-must-be-a-Sequence requirement into every consumer of SubSequence, and does not communicate the intent of this abstraction well.</div><div class=""><br class=""></div><div class=""><i style="font-size: 14px;" class="">Nested generics</i></div><div class=""><br class=""></div><div class="">Currently, a generic type cannot be nested within another generic type, e.g.</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">struct X<T> {</font></div><div class=""><font face="Menlo" class=""> struct Y<U> { } <i class="">// currently ill-formed, but should be possible</i></font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class="">There isn’t much to say about this: the compiler simply needs to be improved to handle nested generics throughout.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><span style="font-size: 14px;" class=""><i class="">Concrete same-type requirements</i><br class=""></span></div><div class=""><br class=""></div><div class=""><div class="">Currently, a constrained extension cannot use a same-type constraint to make a type parameter equivalent to a concrete type. For example:</div></div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">extension Array <b class="">where Element == String</b> {</font></div><div class=""><font face="Menlo" class=""> func makeSentence() -> String {</font></div><div class=""><font face="Menlo" class=""> // uppercase first string, concatenate with spaces, add a period, whatever</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class="">This is a highly-requested feature that fits into the existing syntax and semantics. Note that one could imagine introducing new syntax, e.g., extending “Array<String>”, which gets into new-feature territory: see the section on “Parameterized extensions”.</div><div class=""><br class=""></div><div class=""><b class=""><font size="4" class="">Parameterizing other declarations</font></b></div><div class=""><br class=""></div><div class="">There are a number of Swift declarations that currently cannot have generic parameters; some of those have fairly natural extensions to generic forms that maintain their current syntax and semantics, but become more powerful when made generic.</div><div class=""><br class=""></div><i style="font-size: 14px;" class="">Generic typealiases<br class=""></i><br class=""><div class="">Typealiases could be allowed to carry generic parameters. They would still be aliases (i.e., they would not introduce new types). For example:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">typealias StringDictionary<Value> = Dictionary<String, Value></font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">var d1 = StringDictionary<Int>()</font></div><div class=""><font face="Menlo" class="">var d2: </font><span style="font-family: Menlo;" class="">Dictionary<String, Int> = d1 // okay: d1 and d2 have the same type, Dictionary<String, Int></span></div></blockquote><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><i style="font-size: 14px;" class="">Generic subscripts</i><br class=""><br class="">Subscripts could be allowed to have generic parameters. For example, we could introduce a generic subscript on a Collection that allows us to pull out the values at an arbitrary set of indices:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">extension Collection {</font></div><div class=""><font face="Menlo" class=""> subscript<b class=""><Indices: Sequence where Indices.Iterator.Element == Index></b>(indices: Indices) -> [Iterator.Element] {</font></div><div class=""><font face="Menlo" class=""> get {</font></div><div class=""><font face="Menlo" class=""> var result = [Iterator.Element]()</font></div><div class=""><font face="Menlo" class=""> for index in indices {</font></div><div class=""><font face="Menlo" class=""> result.append(self[index])</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class=""> return result</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class=""> set {</font></div><div class=""><font face="Menlo" class=""> for (index, value) in zip(indices, newValue) {</font></div><div class=""><font face="Menlo" class=""> self[index] = value</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><span style="font-family: Menlo;" class=""> }</span></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class=""><br class=""><i style="font-size: 14px;" class="">Generic constants</i></div><div class=""><br class=""></div><div class="">let constants could be allowed to have generic parameters, such that they produce differently-typed values depending on how they are used. For example, this is particularly useful for named literal values, e.g.,</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">let π<T : FloatLiteralConvertible>: T = 3.141592653589793238462643383279502884197169399</font></div></blockquote><div class=""><br class=""></div><div class="">The Clang importer could make particularly good use of this when importing macros.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><i style="font-size: 14px;" class="">Parameterized extensions</i></div><div class=""><br class=""></div><div class="">Extensions themselves could be parameterized, which would allow some structural pattern matching on types. For example, this would permit one to extend an array of optional values, e.g.,</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">extension<b class=""><T></b> Array <b class="">where Element == T?</b> {</font></div><div class=""><font face="Menlo" class=""> var someValues: [T] {</font></div><div class=""><font face="Menlo" class=""> var result = [T]()</font></div><div class=""><font face="Menlo" class=""> for opt in self {</font></div><div class=""><font face="Menlo" class=""> if let value = opt { result.append(value) }</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class=""> return result</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class=""><div class="">We can generalize this to a protocol extensions:</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><div class=""><font face="Menlo" class="">extension<b class=""><T></b> Sequence <b class="">where Element == T?</b> {</font></div></div></div><div class=""><div class=""><font face="Menlo" class=""> var someValues: [T] {</font></div></div><div class=""><div class=""><font face="Menlo" class=""> var result = [T]()</font></div></div><div class=""><div class=""><font face="Menlo" class=""> for opt in self {</font></div></div><div class=""><div class=""><font face="Menlo" class=""> if let value = opt { result.append(value) }</font></div></div><div class=""><div class=""><font face="Menlo" class=""> }</font></div></div><div class=""><div class=""><font face="Menlo" class=""> return result</font></div></div><div class=""><div class=""><font face="Menlo" class=""> }</font></div></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div></blockquote><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><div class="">Note that when one is extending nominal types, we could simplify the syntax somewhat to make the same-type constraint implicit in the syntax:</div><div class=""><br class=""></div><div class=""><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">extension<b class=""><T></b> Array<b class=""><T?></b> {</font></div><div class=""><font face="Menlo" class=""> var someValues: [T] {</font></div><div class=""><font face="Menlo" class=""> var result = [T]()</font></div><div class=""><font face="Menlo" class=""> for opt in self {</font></div><div class=""><font face="Menlo" class=""> if let value = opt { result.append(value) }</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class=""> return result</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote></div></div><div class=""><br class=""></div><div class="">When we’re working with concrete types, we can use that syntax to improve the extension of concrete versions of generic types (per “Concrete same-type requirements”, above), e.g.,</div><div class=""><br class=""></div><div class=""><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">extension Array<b class=""><String></b> {</font></div><div class=""><font face="Menlo" class=""> func makeSentence() -> String {</font></div><div class=""><font face="Menlo" class=""> // uppercase first string, concatenate with spaces, add a period, whatever</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><br class=""></div><div class=""><br class=""></div></blockquote></div><div class=""><font size="4" class=""><b class="">Minor extensions</b><br class=""></font><br class=""></div><div class="">There are a number of minor extensions we can make to the generics system that don’t fundamentally change what one can express in Swift, but which can improve its expressivity.</div><div class=""><br class=""></div><div class=""><span style="font-size: 14px;" class=""><i class="">*Arbitrary requirements in protocols<br class=""></i></span></div><div class=""><br class=""></div><div class="">Currently, a new protocol can inherit from other protocols, introduce new associated types, and add new conformance constraints to associated types (by redeclaring an associated type from an inherited protocol). However, one cannot express more general constraints. Building on the example from “Recursive protocol constraints”, we really want the element type of a Sequence’s SubSequence to be the same as the element type of the Sequence, e.g.,</div><div class=""><br class=""></div><div class=""><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">protocol Sequence {</font></div><div class=""><font face="Menlo" class=""> associatedtype Iterator : IteratorProtocol</font></div><div class=""><span style="font-family: Menlo;" class=""> …</span></div><div class=""><font face="Menlo" class=""> </font><span style="font-family: Menlo;" class="">associatedtype</span><font face="Menlo" class=""> SubSequence :<b class=""> </b>Sequence<b class=""> where SubSequence.Iterator.Element == Iterator.Element</b></font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote></div><div class=""><br class=""></div><div class="">Hanging the where clause off the associated type is protocol not ideal, but that’s a discussion for another thread.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><span style="font-size: 14px;" class=""><i class="">*Typealiases in protocols and protocol extensions<br class=""></i></span></div><div class=""><br class=""></div><div class=""><div class="">Now that associated types have their own keyword (thanks!), it’s reasonable to bring back “typealias”. Again with the Sequence protocol:</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><div class=""><font face="Menlo" class="">protocol Sequence {</font></div></div></div><div class=""><div class=""><div class=""><font face="Menlo" class=""> associatedtype Iterator : IteratorProtocol</font></div></div></div><div class=""><font face="Menlo" class=""> typealias Element = Iterator.Element // rejoice! now we can refer to SomeSequence.Element rather than SomeSequence.Iterator.Element</font></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div></blockquote><div class=""><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><span style="font-size: 14px;" class=""><i class="">Default generic arguments </i></span></div></div><div class=""><br class=""></div><div class="">Generic parameters could be given the ability to provide default arguments, which would be used in cases where the type argument is not specified and type inference could not determine the type argument. For example:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">public final class Promise<Value, Reason=Error> { … }</font></div></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">func getRandomPromise() -> Promise<Int, ErrorProtocol> { … }</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">var p1: Promise<Int> = …</font></div><div class=""><font face="Menlo" class="">var p2: Promise<Int, Error> = p1 <i class="">// okay: p1 and p2 have the same type Promise<Int, Error></i></font></div><div class=""><font face="Menlo" class="">var p3: Promise = getRandomPromise() <i class="">// p3 has type </i></font><span style="font-family: Menlo;" class=""><i class="">Promise<Int, ErrorProtocol> due to type inference</i></span></div></blockquote><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><i style="font-size: 14px;" class="">Generalized “class” constraints</i></div><div class=""><br class=""></div><div class="">The “class” constraint can currently only be used for defining protocols. We could generalize it to associated type and type parameter declarations, e.g.,</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">protocol P {</font></div></div><div class=""><div class=""><font face="Menlo" class=""> associatedtype A : class</font></div></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div><div class=""><div class=""><font face="Menlo" class=""><br class=""></font></div></div><div class=""><div class=""><font face="Menlo" class="">func foo<T : class>(t: T) { }</font></div></div></blockquote><div class=""><br class=""></div><div class="">As part of this, the magical <font face="Menlo" class="">AnyObject</font> protocol could be replaced with an existential with a class bound, so that it becomes a typealias:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">typealias AnyObject = protocol<class></font></div></blockquote><div class=""><br class=""></div><div class="">See the “Existentials” section, particularly “Generalized existentials”, for more information.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><span style="font-size: 14px;" class=""><i class="">*Allowing subclasses to override requirements satisfied by defaults</i></span></div><div class=""><span style="font-size: 14px;" class=""><i class=""><br class=""></i></span></div><div class="">When a superclass conforms to a protocol and has one of the protocol’s requirements satisfied by a member of a protocol extension, that member currently cannot be overridden by a subclass. For example:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">protocol P {</font></div><div class=""><font face="Menlo" class=""> func foo()</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">extension P {</font></div><div class=""><font face="Menlo" class=""> func foo() { print(“P”) }</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">class C : P {</font></div><div class=""><font face="Menlo" class=""> // gets the protocol extension’s </font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">class D : C {</font></div><div class=""><font face="Menlo" class=""> /*override not allowed!*/ func foo() { print(“D”) }</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">let p: P = D()</font></div><div class=""><font face="Menlo" class="">p.foo() // gotcha: prints “P” rather than “D”!</font></div></blockquote><div class=""><br class=""></div><div class="">D.foo should be required to specify “override” and should be called dynamically.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><b class=""><font size="4" class="">Major extensions to the generics model</font></b></div><div class=""><b class=""><br class=""></b></div><div class="">Unlike the minor extensions, major extensions to the generics model provide more expressivity in the Swift generics system and, generally, have a much more significant design and implementation cost.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><i style="font-size: 14px;" class="">*Conditional conformances<br class=""></i></div><div class=""><br class=""></div><div class="">Conditional conformances express the notion that a generic type will conform to a particular protocol only under certain circumstances. For example, Array is Equatable only when its elements are Equatable:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">extension Array <b class="">: Equatable where Element : Equatable</b> { }</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">func ==<T : Equatable>(lhs: Array<T>, rhs: Array<T>) -> Bool { … }</font></div></blockquote><div class=""><br class=""></div><div class="">Conditional conformances are a potentially very powerful feature. One important aspect of this feature is how deal with or avoid overlapping conformances. For example, imagine an adaptor over a Sequence that has conditional conformances to Collection and MutableCollection:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">struct SequenceAdaptor<S: Sequence> : Sequence { }</font></div><div class=""><font face="Menlo" class="">extension SequenceAdaptor : Collection where S: Collection { … }</font></div><div class=""><font face="Menlo" class="">extension SequenceAdaptor : MutableCollection where S: MutableCollection { }</font></div></blockquote><div class=""><br class=""></div><div class="">This should almost certainly be permitted, but we need to cope with or reject “overlapping” conformances:</div><div class=""><br class=""></div><div class=""><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><font face="Menlo" class="">extension SequenceAdaptor : Collection where S: SomeOtherProtocolSimilarToCollection { } <i class="">// trouble: two ways for SequenceAdaptor to conform to Collection</i></font></blockquote></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class="">See the section on “Private conformances” for more about the issues with having the same type conform to the same protocol multiple times.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><span style="font-size: 14px;" class=""><i class="">Variadic generics</i></span></div><div class=""><br class=""></div><div class=""><div class="">Currently, a generic parameter list contains a fixed number of generic parameters. If one has a type that could generalize to any number of generic parameters, the only real way to deal with it today involves creating a set of types. For example, consider the standard library’s “zip” function. It returns one of these when provided with two arguments to zip together:</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><div class=""><font face="Menlo" class="">public struct Zip2Sequence<Sequence1 : Sequence,</font></div><div class=""><font face="Menlo" class=""> Sequence2 : Sequence> : Sequence { … }</font></div></div></div><div class=""><div class=""><font face="Menlo" class=""><br class=""></font></div></div><div class=""><div class=""><font face="Menlo" class="">public func zip<Sequence1 : Sequence, Sequence2 : Sequence>(</font></div><div class=""><font face="Menlo" class=""> sequence1: Sequence1, _ sequence2: Sequence2)</font></div><div class=""><span style="font-family: Menlo;" class=""> -> Zip2Sequence<Sequence1, Sequence2> { … }</span></div></div></blockquote><div class=""><div class=""><br class=""></div><div class="">Supporting three arguments would require copy-paste of those of those:</div><div class=""><br class=""></div><div class=""><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">public struct Zip3Sequence<Sequence1 : Sequence,</font></div><div class=""><font face="Menlo" class=""> Sequence2 : Sequence,</font></div><span style="font-family: Menlo;" class=""> Sequence3 : Sequence</span><span style="font-family: Menlo;" class="">> : Sequence { … }</span><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><div class=""><font face="Menlo" class="">public func zip<Sequence1 : Sequence, Sequence2 : Sequence</font><span style="font-family: Menlo;" class="">, Sequence3 : Sequence</span><span style="font-family: Menlo;" class="">>(</span></div><div class=""><span style="font-family: Menlo;" class=""> sequence1: Sequence1, _ sequence2: Sequence2, _ sequence3: sequence3)</span></div><div class=""><font face="Menlo" class=""> -> Zip3Sequence<Sequence1, Sequence2, Sequence3> { … }</font></div></div></blockquote><div class=""></div></div><div class=""><br class=""></div><div class="">Variadic generics would allow us to abstract over a set of generic parameters. The syntax below is hopelessly influenced by <a href="http://www.jot.fm/issues/issue_2008_02/article2/" class="">C++11 variadic templates</a> (sorry), where putting an ellipsis (“…”) to the left of a declaration makes it a “parameter pack” containing zero or more parameters and putting an ellipsis to the right of a type/expression/etc. expands the parameter packs within that type/expression into separate arguments. The important part is that we be able to meaningfully abstract over zero or more generic parameters, e.g.:</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">public struct ZipIterator<... <b class="">Iterators</b> : IteratorProtocol</font><span style="font-family: Menlo;" class="">> : Iterator { <i class="">// zero or more type parameters, each of which conforms to IteratorProtocol</i></span></div></div><div class=""><span style="font-family: Menlo;" class=""> public typealias Element = (<b class="">Iterators.Element...</b>) <i class="">// a tuple containing the element types of each iterator in Iterators</i></span></div><div class=""><span style="font-family: Menlo;" class=""><br class=""></span></div><div class=""><span style="font-family: Menlo;" class=""> var (<b class="">...iterators</b>): (<b class="">Iterators...</b>) <i class="">// zero or more stored properties, one for each type in Iterators</i></span><span style="font-family: Menlo;" class=""> </span></div><font face="Menlo" class=""> var reachedEnd: Bool = false</font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""><br class=""></font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> public mutating func next() -> Element? {</font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> if reachedEnd { return nil }</font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""><br class=""></font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> guard let values = (<b class="">iterators.next()...</b>) { <i class="">// call “next” on each of the iterators, put the results into a tuple named “values"</i></font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> reachedEnd = true</font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> return nil</font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> }</font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""><br class=""></font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> return values</font></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> }<br class=""></font><div class=""><span style="font-family: Menlo;" class="">}</span></div><div class=""><span style="font-family: Menlo;" class=""><br class=""></span></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><div class=""><div class=""><font face="Menlo" class="">public struct ZipSequence<<b class="">...Sequences</b> : Sequence</font><span style="font-family: Menlo;" class="">> : Sequence {</span></div><div class=""><span style="font-family: Menlo;" class=""> public typealias Iterator = ZipIterator<<b class="">Sequences.Iterator...</b>> <i class="">// get the zip iterator with the iterator types of our Sequences</i></span></div><div class=""><span style="font-family: Menlo;" class=""><i class=""><br class=""></i></span></div><div class=""><div class=""><font face="Menlo" class=""> var (...</font><b class=""><font face="Menlo" class="">sequences</font></b><font face="Menlo" class="">): (<b class="">Sequences</b></font><b style="font-family: Menlo;" class="">...</b><font face="Menlo" class="">) </font><i style="font-family: Menlo;" class="">// zero or more stored properties, one for each type in Sequences</i><span style="font-family: Menlo;" class=""> </span></div><div class=""><span style="font-family: Menlo;" class=""><br class=""></span></div><font face="Menlo" class=""></font></div><div class=""><span style="font-family: Menlo;" class=""> <i class="">// details ...</i></span></div><div class=""><span style="font-family: Menlo;" class="">}</span></div></div></div></div><div class=""><div class=""><div class=""><br class=""></div></div></div></blockquote><div class=""><div class="">Such a design could also work for function parameters, so we can pack together multiple function arguments with different types, e.g.,</div><div class=""><br class=""></div><div class=""><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">public func zip<<b class="">... Sequences : SequenceType</b>>(<b class="">... sequences: Sequences...</b>) </font></div><div class=""><span style="font-family: Menlo;" class=""> -> ZipSequence<<b class="">Sequences...</b>> {</span></div><div class=""><span style="font-family: Menlo;" class=""> return ZipSequence(<b class="">sequences...</b>)</span></div><div class=""><span style="font-family: Menlo;" class="">}</span></div></blockquote><div class=""></div></div><div class=""><br class=""></div><div class="">Finally, this could tie into the discussions about a tuple “splat” operator. For example:</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">func apply<... Args, Result>(fn: (Args...) -> Result, <i class="">// function taking some number of arguments and producing Result</i></font></div><div class=""><font face="Menlo" class=""> args: (Args...)) -> Result { <i class="">// tuple of arguments</i></font></div></div><div class=""><div class=""><font face="Menlo" class=""> return fn(<b class="">args...</b>) // expand the arguments in the tuple “args” into separate arguments</font></div><div class=""><font face="Menlo" class="">}</font></div></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class=""><br class=""></font></div></blockquote><div class=""><div class=""><span style="font-size: 14px;" class=""><i class="">Extensions of structural types</i></span></div></div><div class=""><br class=""></div><div class=""><div class="">Currently, only nominal types (classes, structs, enums, protocols) can be extended. One could imagine extending structural types—particularly tuple types—to allow them to, e.g., conform to protocols. For example, pulling together variadic generics, parameterized extensions, and conditional conformances, one could express “a tuple type is Equatable if all of its element types are Equatable”:</div></div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">extension<...Elements : Equatable> <b class="">(Elements...)</b> : Equatable { <i class="">// extending the tuple type “(Elements…)” to be Equatable</i></font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class="">There are some natural bounds here: one would need to have actual structural types. One would not be able to extend every type:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">extension<T> T { <i class="">// error: neither a structural nor a nominal type</i></font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class="">And before you think you’re cleverly making it possible to have a conditional conformance that makes every type T that conforms to protocol P also conform to protocol Q, see the section "Conditional conformances via protocol extensions”, below:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">extension<T : P> T : Q { <i class="">// error: neither a structural nor a nominal type</i></font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><b class=""><font size="4" class="">Syntactic improvements</font></b></div><div class=""><b class=""><br class=""></b></div><div class="">There are a number of potential improvements we could make to the generics syntax. Such a list could go on for a very long time, so I’ll only highlight some obvious ones that have been discussed by the Swift developers.</div><div class=""><br class=""></div><div class=""><div class=""><span style="font-size: 14px;" class=""><i class="">*Default implementations in protocols</i></span></div></div><div class=""><br class=""></div><div class=""><div class="">Currently, protocol members can never have implementations. We could allow one to provide such implementations to be used as the default if a conforming type does not supply an implementation, e.g.,</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">protocol Bag {</font></div></div><div class=""><font face="Menlo" class=""> associatedtype Element : Equatable</font></div><div class=""><font face="Menlo" class=""> func contains(element: Element) -> Bool</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class=""> func containsAll<S: Sequence where Sequence.Iterator.Element == Element>(elements: S) -> Bool {</font></div><div class=""><font face="Menlo" class=""> for x in elements {</font></div><div class=""><font face="Menlo" class=""> if contains(x) { return true }</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class=""> return false</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><div class=""><span style="font-family: Menlo;" class="">}</span></div></div><div class=""><div class=""><font face="Menlo" class=""><br class=""></font></div></div><div class=""><div class=""><font face="Menlo" class="">struct IntBag : Bag {</font></div><div class=""><font face="Menlo" class=""> typealias Element = Int</font></div><div class=""><font face="Menlo" class=""> func contains(element: Int) -> Bool { ... }</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class=""> // okay: containsAll requirement is satisfied by Bag’s default implementation</font></div><div class=""><font face="Menlo" class="">}</font></div></div><div class=""><font face="Menlo" class=""><br class=""></font></div></blockquote><div class=""><div class="">One can get this effect with protocol extensions today, hence the classification of this feature as a (mostly) syntactic improvement:</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><div class=""><font face="Menlo" class="">protocol Bag {</font></div></div></div><div class=""><div class=""><div class=""><font face="Menlo" class=""> associatedtype Element : Equatable</font></div></div></div><div class=""><div class=""><div class=""><font face="Menlo" class=""> func contains(element: Element) -> Bool</font></div></div></div><div class=""><div class=""><div class=""><font face="Menlo" class=""><br class=""></font></div></div></div><div class=""><div class=""><div class=""><font face="Menlo" class=""> func containsAll<S: Sequence where Sequence.Iterator.Element == Element>(elements: S) -> Bool</font></div></div></div><div class=""><div class=""><div class=""><span style="font-family: Menlo;" class="">}</span></div></div></div><div class=""><span style="font-family: Menlo;" class=""><br class=""></span></div><div class=""><span style="font-family: Menlo;" class="">extension Bag {</span></div><div class=""><div class=""><font face="Menlo" class=""> func containsAll<S: Sequence where Sequence.Iterator.Element == Element>(elements: S) -> Bool {</font></div><div class=""><font face="Menlo" class=""> for x in elements {</font></div><div class=""><font face="Menlo" class=""> if contains(x) { return true }</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class=""> return false</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><div class=""></div></div></div><div class=""><span style="font-family: Menlo;" class="">}</span></div></blockquote><div class=""><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><span style="font-size: 14px;" class=""><i class="">*Moving the where clause outside of the angle brackets<br class=""></i></span></div></div><div class=""><br class=""></div><div class="">The “where” clause of generic functions comes very early in the declaration, although it is generally of much less concern to the client than the function parameters and result type that follow it. This is one of the things that contributes to “angle bracket blindness”. For example, consider the containsAll signature above:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><span style="font-family: Menlo;" class="">func containsAll<S: Sequence where Sequence.Iterator.Element == Element>(elements: S) -> Bool</span></div></blockquote><div class=""><br class=""></div><div class="">One could move the “where” clause to the end of the signature, so that the most important parts—name, generic parameter, parameters, result type—precede it:</div><div class=""><br class=""></div><div class=""><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><span style="font-family: Menlo;" class="">func containsAll<S: Sequence>(elements: S) -> Bool </span></blockquote><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><span style="font-family: Menlo;" class=""> where Sequence.Iterator.Element == Element</span></blockquote></div><div class=""><span style="font-family: Menlo;" class=""><br class=""></span></div><div class=""><span style="font-family: Menlo;" class=""><br class=""></span></div><div class=""><span style="font-size: 14px;" class=""><i class="">*Renaming “protocol<…>” to “Any<…>”.</i></span></div><div class=""><br class=""></div><div class="">The “protocol<…>” syntax is a bit of an oddity in Swift. It is used to compose protocols together, mostly to create values of existential type, e.g.,</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">var x: protocol<NSCoding, NSCopying></font></div></blockquote><div class=""><br class=""></div><div class="">It’s weird that it’s a type name that starts with a lowercase letter, and most Swift developers probably never deal with this feature unless they happen to look at the definition of Any:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">typealias Any = protocol<></font></div></blockquote><div class=""><br class=""></div><div class="">“Any” might be a better name for this functionality. “Any” without brackets could be a keyword for “any type”, and “Any” followed by brackets could take the role of “protocol<>” today:</div><div class=""><br class=""></div><div class=""><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><font face="Menlo" class="">var x: Any<NSCoding, NSCopying></font></blockquote></div><div class=""><br class=""></div><div class="">That reads much better: “Any type that conforms to NSCoding and NSCopying”. See the section "Generalized existentials” for additional features in this space.</div><div class=""><br class=""></div><div class=""><b class=""><font size="4" class="">Maybe</font></b></div><div class=""><b class=""><br class=""></b></div><div class="">There are a number of features that get discussed from time-to-time, while they could fit into Swift’s generics system, it’s not clear that they belong in Swift at all. The important question for any feature in this category is not “can it be done” or “are there cool things we can express”, but “how can everyday Swift developers benefit from the addition of such a feature?”. Without strong motivating examples, none of these “maybes” will move further along.</div><div class=""><b class=""><br class=""></b></div><div class=""><div class=""><span style="font-size: 14px;" class=""><i class="">Dynamic dispatch for members of protocol extensions</i></span></div><div class=""><br class=""></div><div class="">Only the requirements of protocols currently use dynamic dispatch, which can lead to surprises:</div></div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">protocol P {</font></div><div class=""><font face="Menlo" class=""> func foo()</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">extension P {</font></div><div class=""><font face="Menlo" class=""> func foo() { print(“P.foo()”)</font></div><div class=""><font face="Menlo" class=""> func bar() { print(“P.bar()”)</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">struct X : P {</font></div><div class=""><font face="Menlo" class=""> func foo() { print(“X.foo()”)</font></div><div class=""><font face="Menlo" class=""> func bar() { print(“X.bar()”)</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">let x = X()</font></div><div class=""><font face="Menlo" class="">x.foo() // X.foo()</font></div><div class=""><font face="Menlo" class="">x.bar() // X.bar()</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">let p: P = X()</font></div><div class=""><div class=""><font face="Menlo" class="">p.foo() // X.foo()</font></div></div><div class=""><div class=""><font face="Menlo" class="">p.bar() // P.bar()</font></div></div></blockquote><div class=""><br class=""></div><div class="">Swift could adopt a model where members of protocol extensions are dynamically dispatched.</div><div class=""><br class=""></div><div class=""><div class=""><span style="font-size: 14px;" class=""><i class="">Named generic parameters<br class=""></i></span></div></div><div class=""><br class=""></div><div class="">When specifying generic arguments for a generic type, the arguments are always positional: Dictionary<String, Int> is a Dictionary whose Key type is String and whose Value type is Int, by convention. One could permit the arguments to be labeled, e.g.,</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">var d: Dictionary<<b class="">Key:</b> String, <b class="">Value:</b> Int></font></div></blockquote><div class=""><br class=""></div><div class="">Such a feature makes more sense if Swift gains default generic arguments, because generic argument labels would allow one to skip defaulted arguments.</div><div class=""><br class=""></div><div class=""><div class=""><span style="font-size: 14px;" class=""><i class="">Generic value parameters<br class=""></i></span></div><div class=""><br class=""></div><div class="">Currently, Swift’s generic parameters are always types. One could imagine allowing generic parameters that are values, e.g.,</div><div class=""><br class=""></div><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">struct MultiArray<T,<b class=""> let Dimensions: Int</b>> { <i class="">// specify the number of dimensions to the array</i></font></div><div class=""><font face="Menlo" class=""> subscript (indices: Int...) -> T {</font></div><div class=""><font face="Menlo" class=""> get {</font></div><div class=""><font face="Menlo" class=""> require(indices.count == <b class="">Dimensions</b>)</font></div><div class=""><font face="Menlo" class=""> // ...</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class="">A suitably general feature might allow us to express fixed-length array or vector types as a standard library component, and perhaps also allow one to implement a useful dimensional analysis library. Tackling this feature potentially means determining what it is for an expression to be a “constant expression” and diving into dependent-typing, hence the “maybe”.</div><div class=""><br class=""></div></div><div class=""><span style="font-size: 14px;" class=""><i class="">Higher-kinded types<br class=""></i></span></div><div class=""><br class=""></div><div class="">Higher-kinded types allow one to express the relationship between two different specializations of the same nominal type within a protocol. For example, if we think of the Self type in a protocol as really being “Self<T>”, it allows us to talk about the relationship between “Self<T>” and “Self<U>” for some other type U. For example, it could allow the “map” operation on a collection to return a collection of the same kind but with a different operation, e.g.,</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">let intArray: Array<Int> = …</font></div><div class=""><font face="Menlo" class="">intArray.map { String($0) } <i class="">// produces Array<String></i></font></div><div class=""><font face="Menlo" class="">let intSet: Set<Int> = …</font></div><div class=""><font face="Menlo" class="">intSet.map { String($0) } <i class="">// produces Set<String></i></font></div></blockquote><div class=""><br class=""></div><div class=""><br class=""></div><div class="">Potential syntax borrowed from <a href="https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151214/002736.html" class="">one thread on higher-kinded types</a> uses ~= as a “similarity” constraint to describe a Functor protocol:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">protocol Functor {</font></div><div class=""><font face="Menlo" class=""> associatedtype A</font></div><div class=""><font face="Menlo" class=""> func fmap<FB where <b class="">FB ~= Self</b>>(f: A -> FB.A) -> FB</font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><div class=""><span style="font-size: 14px;" class=""><i class="">Specifying type arguments for uses of generic functions</i></span></div></div><div class=""><br class=""></div><div class="">The type arguments of a generic function are always determined via type inference. For example, given:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">func f<T>(t: T)</font></div></blockquote><div class=""><br class=""></div><div class="">one cannot directly specify T: either one calls “f” (and T is determined via the argument’s type) or one uses “f” in a context where it is given a particular function type (e.g., “let x: (Int) -> Void = f” would infer T = Int). We could permit explicit specialization here, e.g.,</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">let x = f<Int> // x has type (Int) -> Void</font></div></blockquote><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><b class=""><font size="4" class="">Unlikely</font></b></div><div class=""><br class=""></div><div class="">Features in this category have been requested at various times, but they don’t fit well with Swift’s generics system because they cause some part of the model to become overly complicated, have unacceptable implementation limitations, or overlap significantly with existing features.</div><div class=""><br class=""></div><div class=""><div class=""><span style="font-size: 14px;" class=""><i class="">Generic protocols</i></span></div><div class=""><br class=""></div><div class="">One of the most commonly requested features is the ability to parameterize protocols themselves. For example, a protocol that indicates that the Self type can be constructed from some specified type T:</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">protocol ConstructibleFromValue<b class=""><T></b> {</font></div></div><div class=""><font face="Menlo" class=""> init(_ value: T)</font></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div></blockquote><div class=""><div class=""><br class=""></div><div class="">Implicit in this feature is the ability for a given type to conform to the protocol in two different ways. A “Real” type might be constructible from both Float and Double, e.g.,</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">struct Real { … }</font></div></div><div class=""><div class=""><font face="Menlo" class="">extension Real : ConstructibleFrom<Float> {</font></div></div><div class=""><div class=""><font face="Menlo" class=""> init(_ value: Float) { … }</font></div></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div><div class=""><div class=""><div class=""><font face="Menlo" class="">extension Real : ConstructibleFrom<Double> {</font></div></div></div><div class=""><div class=""><div class=""><font face="Menlo" class=""> init(_ value: Double) { … }</font></div></div></div><div class=""><div class=""><div class=""><font face="Menlo" class="">}</font></div></div></div></blockquote><div class=""><div class=""><br class=""></div><div class="">Most of the requests for this feature actually want a different feature. They tend to use a parameterized Sequence as an example, e.g.,</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">protocol Sequence<Element> { … }</font></div></div><div class=""><div class=""><font face="Menlo" class=""><br class=""></font></div></div><div class=""><div class=""><font face="Menlo" class="">func foo(strings: Sequence<String>) { /// works on any sequence containing Strings</font></div></div><div class=""><div class=""><font face="Menlo" class=""> // ...</font></div></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div></blockquote><div class=""><div class=""><br class=""></div><div class="">The actual requested feature here <span class="Apple-tab-span" style="white-space:pre"> </span>is the ability to say “Any type that conforms to Sequence whose Element type is String”, which is covered by the section on “Generalized existentials”, below.</div><div class=""><br class=""></div><div class="">More importantly, modeling Sequence with generic parameters rather than associated types is tantalizing but wrong: you don’t want a type conforming to Sequence in multiple ways, or (among other things) your for..in loops stop working, and you lose the ability to dynamically cast down to an existential “Sequence” without binding the Element type (again, see “Generalized existentials”). Use cases similar to the ConstructibleFromValue protocol above seem too few to justify the potential for confusion between associated types and generic parameters of protocols; we’re better off not having the latter.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><span style="font-size: 14px;" class=""><i class="">Private conformances </i></span></div><div class=""><br class=""></div><div class="">Right now, a protocol conformance can be no less visible than the minimum of the conforming type’s access and the protocol’s access. Therefore, a public type conforming to a public protocol must provide the conformance publicly. One could imagine removing that restriction, so that one could introduce a private conformance:</div></div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">public protocol P { }</font></div><div class=""><font face="Menlo" class="">public struct X { }</font></div></div><div class=""><div class=""><font face="Menlo" class="">extension X : <b class="">internal P</b> { … } // X conforms to P, but only within this module</font></div></div></blockquote><div class=""><div class=""><br class=""></div><div class="">The main problem with private conformances is the interaction with dynamic casting. If I have this code:</div><div class=""><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">func foo(value: Any) {</font></div></div><div class=""><div class=""><font face="Menlo" class=""> if let x = value as? P { print(“P”) }</font></div></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div><div class=""><div class=""><font face="Menlo" class=""><br class=""></font></div></div><div class=""><div class=""><font face="Menlo" class="">foo(X())</font></div></div></blockquote><div class=""><div class=""><br class=""></div><div class="">Under what circumstances should it print “P”? If foo() is defined within the same module as the conformance of X to P? If the call is defined within the same module as the conformance of X to P? Never? Either of the first two answers requires significant complications in the dynamic casting infrastructure to take into account the module in which a particular dynamic cast occurred (the first option) or where an existential was formed (the second option), while the third answer breaks the link between the static and dynamic type systems—none of which is an acceptable result.</div><div class=""><br class=""></div><span style="font-size: 14px;" class=""><i class="">Conditional conformances via protocol extensions<br class=""></i></span></div><div class=""><br class=""></div><div class=""><div class="">We often get requests to make a protocol conform to another protocol. This is, effectively, the expansion of the notion of “Conditional conformances” to protocol extensions. For example:</div></div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">protocol P {</font></div><div class=""><font face="Menlo" class=""> func foo()</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">protocol Q {</font></div><div class=""><font face="Menlo" class=""> func bar()</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">extension <b class="">Q : P</b> { <i class="">// every type that conforms to Q also conforms to P</i></font></div><div class=""><font face="Menlo" class=""> func foo() { <i class="">// implement “foo” requirement in terms of “bar"</i></font></div><div class=""><font face="Menlo" class=""> bar()</font></div><div class=""><font face="Menlo" class=""> }</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">func f<T: P>(t: T) { … }</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">struct X : Q {</font></div><div class=""><font face="Menlo" class=""> func bar() { … }</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">f(X()) // okay: X conforms to P through the conformance of Q to P</font></div></blockquote><div class=""><br class=""></div><div class="">This is an extremely powerful feature: is allows one to map the abstractions of one domain into another domain (e.g., every Matrix is a Graph). However, similar to private conformances, it puts a major burden on the dynamic-casting runtime to chase down arbitrarily long and potentially cyclic chains of conformances, which makes efficient implementation nearly impossible.</div><div class=""><br class=""></div><div class=""><div class=""><b class=""><font size="4" class="">Potential removals</font></b></div><div class=""><b class=""><br class=""></b></div><div class="">The generics system doesn’t seem like a good candidate for a reduction in scope; most of its features do get used fairly pervasively in the standard library, and few feel overly anachronistic. However...</div><div class=""><br class=""></div><div class=""><span style="font-size: 14px;" class=""><i class="">Associated type inference</i></span></div></div><div class=""><br class=""></div><div class="">Associated type inference is the process by which we infer the type bindings for associated types from other requirements. For example:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">protocol IteratorProtocol {</font></div><div class=""><font face="Menlo" class=""> associatedtype Element</font></div><div class=""><font face="Menlo" class=""> mutating func next() -> Element?</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">struct IntIterator : IteratorProtocol {</font></div><div class=""><font face="Menlo" class=""> mutating func next() -> Int? { … } // use this to infer Element = Int</font></div><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><br class=""></div><div class="">Associated type inference is a useful feature. It’s used throughout the standard library, and it helps keep associated types less visible to types that simply want to conform to a protocol. On the other hand, associated type inference is the only place in Swift where we have a <b class="">global</b> type inference problem: it has historically been a major source of bugs, and implementing it fully and correctly requires a drastically different architecture to the type checker. Is the value of this feature worth keeping global type inference in the Swift language, when we have deliberatively avoided global type inference elsewhere in the language?</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><b class=""><font size="4" class="">Existentials</font></b><br class=""><br class="">Existentials aren’t really generics per se, but the two systems are closely intertwined due to their mutable dependence on protocols.<br class=""><br class=""><span style="font-size: 14px;" class=""><i class="">*Generalized existentials</i></span></div><div class=""><br class=""></div><div class="">The restrictions on existential types came from an implementation limitation, but it is reasonable to allow a value of protocol type even when the protocol has Self constraints or associated types. For example, consider IteratorProtocol again and how it could be used as an existential:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><font face="Menlo" class="">protocol IteratorProtocol {</font></div></div><div class=""><div class=""><font face="Menlo" class=""> associatedtype Element</font></div></div><div class=""><div class=""><font face="Menlo" class=""> mutating func next() -> Element?</font></div></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">let it: IteratorProtocol = …</font></div><div class=""><font face="Menlo" class="">it.next() // if this is permitted, it could return an “Any?”, i.e., the existential that wraps the actual element</font></div></blockquote><div class=""><font face="Menlo" class=""><br class=""></font></div><div class="">Additionally, it is reasonable to want to constrain the associated types of an existential, e.g., “a Sequence whose element type is String” could be expressed by putting a where clause into “protocol<…>” or “Any<…>” (per “Renaming protocol<…> to Any<…>”):</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">let strings: Any<Sequence<b class=""> where .Iterator.Element == String</b>> = [“a”, “b”, “c”]</font></div></blockquote><div class=""><br class=""></div><div class="">The leading “.” indicates that we’re talking about the dynamic type, i.e., the “Self” type that’s conforming to the Sequence protocol. There’s no reason why we cannot support arbitrary “where” clauses within the “Any<…>”. This very-general syntax is a bit unwieldy, but common cases can easily be wrapped up in a generic typealias (see the section “Generic typealiases” above):</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">typealias AnySequence<Element> = <b class="">Any<Sequence where .Iterator.Element == Element></b></font></div><div class=""><font face="Menlo" class="">let strings: AnySequence<String> = [“a”, “b”, “c”]</font></div></blockquote><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><i style="font-size: 14px;" class="">Opening existentials</i></div><div class=""><br class=""></div><div class="">Generalized existentials as described above will still have trouble with protocol requirements that involve Self or associated types in function parameters. For example, let’s try to use Equatable as an existential:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">protocol Equatable {</font></div><div class=""><font face="Menlo" class=""> func ==(lhs: Self, rhs: Self) -> Bool</font></div><div class=""><font face="Menlo" class=""> func !=(lhs: Self, rhs: Self) -> Bool</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">let e1: Equatable = …</font></div><div class=""><font face="Menlo" class="">let e2: Equatable = …</font></div><div class=""><font face="Menlo" class="">if e1 == e2 { … } <i class="">// <b class="">error</b>:</i> e1 and e2 don’t necessarily have the same dynamic type</font></div></blockquote><div class=""><br class=""></div><div class="">One explicit way to allow such operations in a type-safe manner is to introduce an “open existential” operation of some sort, which extracts and gives a name to the dynamic type stored inside an existential. For example:</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span> </div><div class=""><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><font face="Menlo" class="">if let storedInE1 = e1 openas T { // T is a the type of storedInE1, a copy of the value stored in e1</font></blockquote><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> if let storedInE2 = e2 as? T { // is e2 also a T?</font></blockquote><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> if storedInE1 == storedInE2 { … } // okay: storedInT1 and storedInE2 are both of type T, which we know is Equatable</font></blockquote><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><font face="Menlo" class=""> }</font></blockquote><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><font face="Menlo" class="">}</font></blockquote></div><div class=""><br class=""></div><div class="">Thoughts?</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>- Doug</div><div class=""><br class=""></div></body></html>