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From: Douglas Gregor via swift-evolution <swift-evolution-m3FHrko0VLzYtjvyW6yDsg <at> public.gmane.org>
Subject: [Manifesto] Completing Generics
Newsgroups: gmane.comp.lang.swift.evolution
Date: Thursday 3rd March 2016 01:22:57 UTC (over 2 years ago)
Hi all,


The “Complete Generics” goal for Swift 3 has been fairly ill-defined
thus fair, with just this short blurb in the list of goals:

Complete generics: 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
Equatable elements is Equatable), and so on. Swift 3.0 should provide those
generics features needed by the standard library, because they affect the
standard library's ABI.
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:

Communicate a vision for Swift generics, building on the original generics
design document <https://github.com/apple/swift/blob/master/docs/Generics.rst>,
so we have something concrete and comprehensive to discuss.
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”).
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.

A message like this can easily turn into a centithread <http://www.urbandictionary.com/define.php?term=centithread>.
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.

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.

Enough with the disclaimers; it’s time to talk features.

Removing unnecessary restrictions

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.

*Recursive protocol constraints

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:

protocol Sequence {
  associatedtype Iterator : IteratorProtocol
  associatedtype SubSequence : Sequence   // currently ill-formed, but
should be possible

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.

Nested generics

Currently, a generic type cannot be nested within another generic type,

struct X {
  struct Y { }  // currently ill-formed, but should be possible

There isn’t much to say about this: the compiler simply needs to be
improved to handle nested generics throughout.

Concrete same-type requirements

Currently, a constrained extension cannot use a same-type constraint to
make a type parameter equivalent to a concrete type. For example:

extension Array where Element == String {
  func makeSentence() -> String {
    // uppercase first string, concatenate with spaces, add a period,

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”, which gets into new-feature territory: see
the section on “Parameterized extensions”.

Parameterizing other declarations

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.

Generic typealiases

Typealiases could be allowed to carry generic parameters. They would still
be aliases (i.e., they would not introduce new types). For example:

typealias StringDictionary = Dictionary

var d1 = StringDictionary()
var d2: Dictionary = d1 // okay: d1 and d2 have the same type,

Generic subscripts

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:

extension Collection {
  subscript(indices: Indices) -> [Iterator.Element] {
    get {
      var result = [Iterator.Element]()
      for index in indices {

      return result

    set {
      for (index, value) in zip(indices, newValue) {
        self[index] = value

Generic constants

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.,

let π: T =

The Clang importer could make particularly good use of this when importing

Parameterized extensions

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.,

extension Array where Element == T? {
  var someValues: [T] {
    var result = [T]()
    for opt in self {
      if let value = opt { result.append(value) }
   return result

We can generalize this to a protocol extensions:

extension Sequence where Element == T? {
  var someValues: [T] {
    var result = [T]()
    for opt in self {
      if let value = opt { result.append(value) }
   return result

Note that when one is extending nominal types, we could simplify the syntax
somewhat to make the same-type constraint implicit in the syntax:

extension Array {
  var someValues: [T] {
    var result = [T]()
    for opt in self {
      if let value = opt { result.append(value) }
   return result

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.,

extension Array {
  func makeSentence() -> String {
    // uppercase first string, concatenate with spaces, add a period,

Minor extensions

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.

*Arbitrary requirements in protocols

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.,

protocol Sequence {
  associatedtype Iterator : IteratorProtocol
  associatedtype SubSequence : Sequence where SubSequence.Iterator.Element
== Iterator.Element

Hanging the where clause off the associated type is protocol not ideal, but
that’s a discussion for another thread.

*Typealiases in protocols and protocol extensions

Now that associated types have their own keyword (thanks!), it’s
reasonable to bring back “typealias”. Again with the Sequence protocol:

protocol Sequence {
  associatedtype Iterator : IteratorProtocol
  typealias Element = Iterator.Element   // rejoice! now we can refer to
SomeSequence.Element rather than SomeSequence.Iterator.Element

Default generic arguments 

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:

public final class Promise { … }

func getRandomPromise() -> Promise { … }

var p1: Promise = …
var p2: Promise = p1     // okay: p1 and p2 have the same type
var p3: Promise = getRandomPromise() // p3 has type Promise due to type inference

Generalized “class” constraints

The “class” constraint can currently only be used for defining
protocols. We could generalize it to associated type and type parameter
declarations, e.g.,

protocol P {
  associatedtype A : class

func foo(t: T) { }

As part of this, the magical AnyObject protocol could be replaced with an
existential with a class bound, so that it becomes a typealias:

typealias AnyObject = protocol

See the “Existentials” section, particularly “Generalized
existentials”, for more information.

*Allowing subclasses to override requirements satisfied by defaults

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:

protocol P {
  func foo()

extension P {
  func foo() { print(“P”) }

class C : P {
  // gets the protocol extension’s 

class D : C {
  /*override not allowed!*/ func foo() { print(“D”) }

let p: P = D()
p.foo() // gotcha: prints “P” rather than “D”!

D.foo should be required to specify “override” and should be called

Major extensions to the generics model

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.

*Conditional conformances

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:

extension Array : Equatable where Element : Equatable { }

func ==(lhs: Array, rhs: Array) -> Bool { … }

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:

struct SequenceAdaptor : Sequence { }
extension SequenceAdaptor : Collection where S: Collection { … }
extension SequenceAdaptor : MutableCollection where S: MutableCollection {

This should almost certainly be permitted, but we need to cope with or
reject “overlapping” conformances:

extension SequenceAdaptor : Collection where S:
SomeOtherProtocolSimilarToCollection { } // trouble: two ways for
SequenceAdaptor to conform to Collection

See the section on “Private conformances” for more about the issues
with having the same type conform to the same protocol multiple times.

Variadic generics

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:

public struct Zip2Sequence : Sequence { … }

public func zip(
              sequence1: Sequence1, _ sequence2: Sequence2)
            -> Zip2Sequence { … }

Supporting three arguments would require copy-paste of those of those:

public struct Zip3Sequence : Sequence { … }

public func zip(
              sequence1: Sequence1, _ sequence2: Sequence2, _ sequence3:
            -> Zip3Sequence { … }

Variadic generics would allow us to abstract over a set of generic
parameters. The syntax below is hopelessly influenced by C++11 variadic
templates <http://www.jot.fm/issues/issue_2008_02/article2/>
(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.:

public struct ZipIterator<... Iterators : IteratorProtocol> : Iterator { 
// zero or more type parameters, each of which conforms to IteratorProtocol
  public typealias Element = (Iterators.Element...)                      
// a tuple containing the element types of each iterator in Iterators

  var (...iterators): (Iterators...)    // zero or more stored properties,
one for each type in Iterators 
  var reachedEnd: Bool = false

  public mutating func next() -> Element? {
    if reachedEnd { return nil }

    guard let values = (iterators.next()...) {   // call “next” on each
of the iterators, put the results into a tuple named “values"
      reachedEnd = true
      return nil

    return values

public struct ZipSequence<...Sequences : Sequence> : Sequence {
  public typealias Iterator = ZipIterator   // get
the zip iterator with the iterator types of our Sequences

  var (...sequences): (Sequences...)    // zero or more stored properties,
one for each type in Sequences 

  // details ...

Such a design could also work for function parameters, so we can pack
together multiple function arguments with different types, e.g.,

public func zip<... Sequences : SequenceType>(... sequences: Sequences...) 
            -> ZipSequence {
  return ZipSequence(sequences...)

Finally, this could tie into the discussions about a tuple “splat”
operator. For example:

func apply<... Args, Result>(fn: (Args...) -> Result,    // function taking
some number of arguments and producing Result
                           args: (Args...)) -> Result {  // tuple of
  return fn(args...)                                     // expand the
arguments in the tuple “args” into separate arguments

Extensions of structural types

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”:

extension<...Elements : Equatable> (Elements...) : Equatable {   //
extending the tuple type “(Elements…)” to be Equatable

There are some natural bounds here: one would need to have actual
structural types. One would not be able to extend every type:

extension T { // error: neither a structural nor a nominal type

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:

extension T : Q { // error: neither a structural nor a nominal type

Syntactic improvements

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

*Default implementations in protocols

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.,

protocol Bag {
  associatedtype Element : Equatable
  func contains(element: Element) -> Bool

  func containsAll(elements: S) -> Bool {
    for x in elements {
      if contains(x) { return true }
    return false

struct IntBag : Bag {
  typealias Element = Int
  func contains(element: Int) -> Bool { ... }

  // okay: containsAll requirement is satisfied by Bag’s default

One can get this effect with protocol extensions today, hence the
classification of this feature as a (mostly) syntactic improvement:

protocol Bag {
  associatedtype Element : Equatable
  func contains(element: Element) -> Bool

  func containsAll(elements: S) -> Bool

extension Bag {
  func containsAll(elements: S) -> Bool {
    for x in elements {
      if contains(x) { return true }
    return false

*Moving the where clause outside of the angle brackets

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:

func containsAll(elements: S) -> Bool

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:

func containsAll(elements: S) -> Bool 
       where Sequence.Iterator.Element == Element

*Renaming “protocol<…>” to “Any<…>”.

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,

var x: protocol

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:

typealias Any = protocol<>

“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:

var x: Any

That reads much better: “Any type that conforms to NSCoding and
NSCopying”. See the section "Generalized existentials” for additional
features in this space.


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.

Dynamic dispatch for members of protocol extensions

Only the requirements of protocols currently use dynamic dispatch, which
can lead to surprises:

protocol P {
  func foo()

extension P {
  func foo() { print(“P.foo()”)
  func bar() { print(“P.bar()”)

struct X : P {
  func foo() { print(“X.foo()”)
  func bar() { print(“X.bar()”)

let x = X()
x.foo() // X.foo()
x.bar() // X.bar()

let p: P = X()
p.foo() // X.foo()
p.bar() // P.bar()

Swift could adopt a model where members of protocol extensions are
dynamically dispatched.

Named generic parameters

When specifying generic arguments for a generic type, the arguments are
always positional: Dictionary 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.,

var d: Dictionary

Such a feature makes more sense if Swift gains default generic arguments,
because generic argument labels would allow one to skip defaulted

Generic value parameters

Currently, Swift’s generic parameters are always types. One could imagine
allowing generic parameters that are values, e.g.,

struct MultiArray { // specify the number of
dimensions to the array
  subscript (indices: Int...) -> T {
    get {
      require(indices.count == Dimensions)
      // ...

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

Higher-kinded types

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”, it allows us to talk about the relationship between
“Self” and “Self” 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.,

let intArray: Array = …
intArray.map { String($0) } // produces Array
let intSet: Set = …
intSet.map { String($0) }   // produces Set

Potential syntax borrowed from one thread on higher-kinded types <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151214/002736.html>
uses ~= as a “similarity” constraint to describe a Functor protocol:

protocol Functor {
  associatedtype A
  func fmap(f: A -> FB.A) -> FB

Specifying type arguments for uses of generic functions

The type arguments of a generic function are always determined via type
inference. For example, given:

func f(t: T)

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,

let x = f // x has type (Int) -> Void


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

Generic protocols

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:

protocol ConstructibleFromValue {
  init(_ value: T)

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.,

struct Real { … }
extension Real : ConstructibleFrom {
  init(_ value: Float) { … }
extension Real : ConstructibleFrom {
  init(_ value: Double) { … }

Most of the requests for this feature actually want a different feature.
They tend to use a parameterized Sequence as an example, e.g.,

protocol Sequence { … }

func foo(strings: Sequence) {  /// works on any sequence containing
  // ...

The actual requested feature here 	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.

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.

Private conformances 

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:

public protocol P { }
public struct X { }
extension X : internal P { … } // X conforms to P, but only within this

The main problem with private conformances is the interaction with dynamic
casting. If I have this code:

func foo(value: Any) {
  if let x = value as? P { print(“P”) }


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.

Conditional conformances via protocol extensions

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:

protocol P {
  func foo()

protocol Q {
  func bar()

extension Q : P { // every type that conforms to Q also conforms to P
  func foo() {    // implement “foo” requirement in terms of “bar"

func f(t: T) { … }

struct X : Q {
  func bar() { … }

f(X()) // okay: X conforms to P through the conformance of Q to P

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

Potential removals

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...

Associated type inference

Associated type inference is the process by which we infer the type
bindings for associated types from other requirements. For example:

protocol IteratorProtocol {
  associatedtype Element
  mutating func next() -> Element?

struct IntIterator : IteratorProtocol {
  mutating func next() -> Int? { … }  // use this to infer Element = Int

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 global 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?


Existentials aren’t really generics per se, but the two systems are
closely intertwined due to their mutable dependence on protocols.

*Generalized existentials

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:

protocol IteratorProtocol {
  associatedtype Element
  mutating func next() -> Element?

let it: IteratorProtocol = …
it.next()   // if this is permitted, it could return an “Any?”, i.e.,
the existential that wraps the actual element

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<…>”):

let strings: Any = [“a”,
“b”, “c”]

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):

typealias AnySequence = Any
let strings: AnySequence = [“a”, “b”, “c”]

Opening existentials

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:

protocol Equatable {
  func ==(lhs: Self, rhs: Self) -> Bool
  func !=(lhs: Self, rhs: Self) -> Bool

let e1: Equatable = …
let e2: Equatable = …
if e1 == e2 { … } // error: e1 and e2 don’t necessarily have the same
dynamic type

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

if let storedInE1 = e1 openas T {     // T is a the type of storedInE1, a
copy of the value stored in e1
  if let storedInE2 = e2 as? T {      // is e2 also a T?
    if storedInE1 == storedInE2 { … } // okay: storedInT1 and storedInE2
are both of type T, which we know is Equatable


	- Doug
CD: 4ms