[swift-users] Constraining the conforming type of a protocol

[Slava Pestov]

What you’re trying to do should be equivalent to this:

protocol Toggling : Equatable {
  …
}

It’s a bug that placing the constraint on ‘Self’ does not have the same effect though; do you mind filing a JIRA?

Slava

> On Jul 29, 2017, at 12:24 PM, Ray Fix via swift-users <swift-users at swift.org> wrote:
> 
> Hi,
> 
> I had a question about defining protocols. Originally I wrote:
> 
> protocol Toggling where Self: Equatable {
>   static var all: [Self] { get }
>   func toggled() -> Self
>   mutating func toggle()
> }
> 
> extension Toggling {
> 
>   func toggled() -> Self {
>     let current = Self.all.index(of: self) ?? 0
>     let next = (current + 1) % Self.all.count
>     return Self.all[next]
>   }
> 
>   mutating func toggle() {
>     self = toggled()
>   }
> }
> 
> This resulted in a bunch of errors.  
> 
> Playground execution failed:
>                          ^
> error: Toggler.playground:7:28: error: cannot invoke 'index' with an argument list of type '(of: Self)'
>     let current = Self.all.index(of: self) ?? 0
>                            ^
> 
> Toggler.playground:7:28: note: expected an argument list of type '(of: Self.Element)'
>     let current = Self.all.index(of: self) ?? 0
> 
> This approach worked:
> 
> 
> protocol Toggling {
>   static var all: [Self] { get }
>   func toggled() -> Self
>   mutating func toggle()
> }
> 
> extension Toggling where Self: Equatable {
> 
>   func toggled() -> Self {
>     let current = Self.all.index(of: self) ?? 0
>     let next = (current + 1) % Self.all.count
>     return Self.all[next]
>   }
> 
>   mutating func toggle() {
>     self = toggled()
>   }
> }
> 
> This version is probably better anyway but I am wondering if the first approach should have shown an error at the point of trying to attach the constraint to the protocol declaration.  Any insights on this?
> 
> Thank you,
> Ray  Fix
> 
> 
> 
> 
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