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

[Xiaodi Wu]

`Self` _always_ refers to the dynamic type of `self`. It just happens to be
that in the case of structs the dynamic type is the same as the static
type. The idea of having a shorthand for the containing type (spelled #Self
or StaticSelf) was discussed during consideration of SE-0068. The accepted
version of the proposal rejects that idea, having adopted the position that
"You will continue to specify full type names for any other use. Joe Groff
writes, 'I don't think it's all that onerous to have to write ClassName.foo
if that's really what you specifically mean.'"


On Sat, Jan 7, 2017 at 11:14 AM, thislooksfun via swift-evolution <
swift-evolution at swift.org> wrote:

> 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:
>
>    1. Refer to the *current* type, or refer to the dynamic type for
>    non-final classes inside containing type (SE–0068 - not yet implemented).
>    2. 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.
>
>    1.
>
>    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 */ }
>    }
>
>    2.
>
>    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.
>    3.
>
>    Current can only refer to the current containing type.
>    4.
>
>    On classes Self has always the contract of returning self.
>    5.
>
>    Self could be discouraged in favor of Current on value types, as a
>    shorthand to refer to the containing type.
>    6.
>
>    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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