[swift-evolution] [proposal] Generic type aliases

Adrian Zubarev adrian.zubarev at devandartist.com
Mon Mar 21 14:38:57 CDT 2016

Ok I read all the previous posts and I’m totally fine with this feature as it’s mentioned in its base form. Looking forward to see generic typealias allowing type constraints.

There is one thing still on my mind: how do we use a generic typealias?

typealias SomeTuple<T> = (T, T)

func foo<U>(tuple: SomeTuple<U>) { /*…*/ }

or only for Ints

func foo(tuple: SomeTuple<Int>) { /*…*/ }

What about global generic typealias?

class Boo<T> {

	var tuple: SomeTuple<T>

	init(tuple: SomeTuple<T>) {

		self.tuple = tuple // what is T here?

Lets say generic typealias already has the possibility to specify type constraints at this point.
Here is a quick minimal sample code where I would want to use a global generic typealias and keep the class as non-generic:


protocol ChildProtocol: class {
	func foo()

protocol Delegate: class {
	// easy to use because MyClass is non-generic
	func parentDidSomething(class: ParentClass)

// for any type 
// typealias ViewWithProtocol<T: ChildProtocol> = T

// or better allowing only UIView as base class + ChildProtocol
typealias ViewWithProtocol<T: UIView where T: ChildProtocol> = T

class ParentClass {

	var delegate: MyDelegate?

	var child: ViewWithProtocol<UIView> // any UIView that extends with ChildProtocol can be stored here
	init(child: ViewWithProtocol<UIView>) {
		self.child = child

	// it’s easy to access the property
	func doSomeWork() {
		self.child.tag = 42 // since it’s a UIView which has the `tag` property

One additional cool feature of generic typealias would be the possibility to access default implementation functions which are not specified inside the protocol body itself:


protocol SomeProtocol {}

extension SomeProtocol where Self: BaseClass {
	func print(text: String) {

class BaseClass {}
class SubClass: BaseClass {
	var text = "hello world"

typealias BaseClassWithProtocol<T: BaseClass where T: SomeProtocol> = T

func printSomethingWith(instance: BaseClassWithProtocol<SubClass>) {

	instance.print(instance.text) // should print "hello world"

+1 for this feature

Adrian Zubarev
Sent with Airmail

Am 21. März 2016 bei 11:16:06, Adrian Zubarev (adrian.zubarev at devandartist.com) schrieb:

Will this feature allow something like this?

protocol SomeProtocol: class { /* some functions here */ }
typealias ProtoView<T: UIView where T: SomeProtocol> = T

I recently came across a design issue where I needed this type to be global instead of the generic top level of a class:
class A<T: UIView where T: SomeProtocol> { /*...*/ }

I couldn’t do something like this in my project and was forced to extend UIView with some kind of an extra backdoor protocol.  
Adrian Zubarev
Sent with Airmail

Am 10. März 2016 bei 05:47:50, Chris Lattner via swift-evolution (swift-evolution at swift.org) schrieb:

Hi All,

I’ve started prototyping generic type aliases in master, but we’d like to run this through the evolution process for discussion.  Comments and discussion are welcome.  Here’s the start of the email thread for the actual formal proposal:


This proposal aims to add generic typealiases to Swift.

Swift-evolution thread: <you are here>


Generic typealiases are a somewhat obvious generalization of the existing Swift model for type aliases, which allow you to provide a name for an existing nominal generic type, or to provide a name for a non-nominal type (e.g. tuples, functions, etc) with generic parameters.

Proposed solution

The solution solution is straight-forward: allow type aliases to introduce type parameters, which are in scope for their definition.  This allows one to express things like:

    typealias StringDictionary<T> = Dictionary<String, T>
    typealias IntFunction<T> = (T) -> Int
    typealias MatchingTriple<T> = (T, T, T)
    typealias BackwardTriple<T1,T2,T3> = (T3, T2, T1)

This is consistent with the rest of Swift’s approach to generics, and slots directly into the model.

Detailed design

This is a minimal proposal for introducing type aliases into Swift, and intentionally chooses to keep them limited to being “aliases”.  As such, additional constraints are not allowed in this base proposal, e.g. you can’t write:

    typealias StringDictionary<T where T : Hashable> = Dictionary<String, T>

Otherwise, generic type aliases follow the model of type aliases and the precedent of the other generic declarations in Swift.  For example, they allow the usual access control features that type aliases support.  Similarly, like non-generic type aliases, generic type aliases cannot be “resilient”.

Impact on existing code

This is a new feature, so there is no impact on existing code.

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