[swift-evolution] [Pitch] Tweak `Self` and introduce `Current`

[thislooksfun]

I like this idea, however, if I understand the proposal correctly, I think that the naming would make more sense the other way around. `Self` is, at least in my head, tied directly and statically to the enclosing type, where as `Current` sounds more dynamic, and could change from place-to-place.

-thislooksfun (tlf)

> On Jan 7, 2017, at 4:38 AM, Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hi Swift community,
> 
> I’d like to talk to about current Self keyword. If I’m not totally mistaken then the current Self has a few meanings:
> 
> Refer to the current type, or refer to the dynamic type for non-final classes inside containing type (SE–0068 - not yet implemented).
> For non-final class types use Self as return type on the conforming super type (or return an instance of receiver Self).
> Let me visualize the behaviors quickly in some short code snippet:
> 
> protocol Foo {
>     func foo(_ f: Self) -> Self
> }
> 
> class A : Foo {
>     // forced to use `A` as parameter type and `Self` as return type
>     func foo(_ f: A) -> Self { return self }
>     // Returning `A()` would cause an error: Cannot convert return expression of type 'A' to return type 'Self'
>     func bar() -> A { return A() /* or self */ }
>     func zoo() -> Self { return /* only */ self }
> }
> 
> class B : A {
>     // Both is fine `B` or `Self` as the return type
>     // If `B` is used you can return a different instance like `B()`
>     // `Self` does only allow `self` to be used here
>     override func foo(_ f: A) -> B { return self }
> }
> 
> struct D : Foo {
>     // No `Self` allowed here at all
>     func foo(_ f: D) -> D { return self /* or D() */ }
> }
> The behavior of Self is a little magical, because it sometimes refers to the current type it is used in, or it has a contract of using self.
> 
> I propose of introducing a new keyword called Current to solve a few problems here.
> 
> Self on parameter types would be disallowed for protocol members, because conformances to that protocol already disallow that (see A above). Instead one would use Current and get the correct meaning.
> 
> protocol Boo {
>     func boo(_ b: Current) -> Self
> }
> 
> procotol Loo {
>     func loo() -> Current
> }
> 
> class X : Boo, Loo {
>     func boo(_ b: X) -> Self { return self }
>     func loo() -> X { return self /* or X() */ }
> }
> 
> final class Y : Boo {
>     func boo(_ b: X) -> Y { return self /* or Y */ }
> }
> Using Self inside the containing type would always mean as one would refer to the dynamic type, like the magical syntax function type(of:) does.
> 
> Current can only refer to the current containing type.
> 
> On classes Self has always the contract of returning self.
> 
> Self could be discouraged in favor of Current on value types, as a shorthand to refer to the containing type.
> 
> Generics could benefit from Current too:
> 
> extension AReallyLongNonFinalClassName {
>     func casted<T : Current>() -> T { ... }
> }
> // `Self` wouldn't here, because it would refer to the dynamic type
> #1 Would affect a lot of protocols which implies that it would affect ABI.
> 
> These are the first ideas I had in my mind. We can polish it further if it receives positive and constructive feedback.
> 
> Best regards,
> 
> 
> 
> 
> --
> Adrian Zubarev
> Sent with Airmail
> 
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