[swift-evolution] Proposal: Change of syntax for class protocols

[Adrian Zubarev]

Lets say (and hope) in some future we’ll be able to write functions like this:

func foo<T: class>(_: T) { /* do something */ }
func foo<T: enum>(_: T) { /* do something */ }
func foo<T: struct>(_: T) { /* do something */ }

// or we might want to have more than one generic types

func foo<A: class, B: enum, C: struct>(_: A, _: B, _: C) {
    // do something better
}

// try to use `where` clause with `class A`, `struct C`. It looks ugly to me.
So ins’t it better to stick to the original syntax and write code like this!?

protocol MagicType {} // can be applied to any type
protocol ClassType: class {} // only for classes
protocol StructType: struct {} only for structs
protocol ValueType: struct, enum {}  
protocol MixedType: struct, class {}
This is why in my eyes the current syntax for protocols is just perfect.



— 
Regards Adrian

Am 7. Dezember 2015 bei 23:29:34, Felix Gabel via swift-evolution (swift-evolution at swift.org) schrieb:

Great to hear about being able to apply this to type parameters and associated types in the future. But I still propose to rethink the syntax decision. 
 
class protocol FooType {}
typealias class Bar: FooType
func foo<class T>()

This is more consistent with for example the declaration of a class or property

Construct name: Type

On 07 Dec 2015, at 23:04, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:


On Dec 7, 2015, at 8:58 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:


On Dec 7, 2015, at 8:00 AM, Matthew Cheok via swift-evolution <swift-evolution at swift.org> wrote:

Currently, we declare class protocols with the following syntax:

protocol TestProtocol: class, OtherProtocol {}

This is odd for a few reasons:
1) The keyword class exists in the middle of the declaration
2) The keyword class follows the colon and looks a lot like inheritance
3) The keyword class occupies a somewhat arbitrary first position after the colon (otherwise we have an error)

We also have another use of the class keyword as a modifier when declaring class methods:

class func doSomething() {}

I’m suggesting a change of syntax that rectifies the above issues:

class protocol TestProtocol: OtherProtocol {}

Would love to hear other thoughts on this.

The constraint syntax is used because, in the fullness of time, it should also be applicable to type parameters and associated types:

protocol Foo { typealias T: class }

func foo<T: class>(x: T)

Right. This is exactly the reason why we have the syntax

protocol X : class { … }

and why I’m against changing the current syntax.

- Doug


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