[swift-evolution] [RFC] #Self

Thorsten Seitz tseitz42 at icloud.com
Wed May 11 07:33:01 CDT 2016

Am 10. Mai 2016 um 20:11 schrieb Matthew Johnson <matthew at anandabits.com>:

Sent from my iPad

On May 10, 2016, at 12:59 PM, Thorsten Seitz <tseitz42 at icloud.com> wrote:

Am 10.05.2016 um 18:41 schrieb Timothy Wood via swift-evolution <swift-evolution at swift.org>:

On May 10, 2016, at 9:28 AM, Matthew Johnson <matthew at anandabits.com> wrote:
Yep, understood. It's perfectly clear to me but I understand why Chris is concerned about it having potential to confuse people. It is a pretty subtle difference especially since Self and #Self are the same in some contexts. In any case, I would be content to live with any name that wins out.

Ah, OK -- it sounds like we just differ on what would be least confusing =)

The other proposed name of #StaticSelf, seems like it would be very clear (if a bit redundant and longer than needed, once you’ve come across it once or twice). I could certainly live with #StaticSelf.

In that case StaticSelf would be sufficient IMHO. The # should only be needed to distinguish between Self and #Self.


Self, #Self
Self, StaticSelf
DynamicSelf, StaticSelf

As far as I understand #Self should be the type of the implementor (ImplementorSelf?) or conforming type (ConformingSelf?).
How would this work with default methods?

protocol A {
func f() -> #Self

extension A {
func f() -> #Self { return init() } // what type has #Self here?

The conforming type. C in your example. If we have 'class D: C' and it overrides 'f' the override would have a return type of C, not D. The returned instance could be of type D since it is a subtype of C. We could also explore allowing overrides to have a covariant return type, it just wouldn't be visible when accessed via the protocol through a generic constraint or an existential (those would only guarantee C, the type that declared the conformance.

Thanks, that makes sense.
So within a default method like in extension A above the (concrete) type of #Self is still unknown and I only know that it will conform to A. That's fine. As soon as a non protocol type like a class conforms to the protocol #Self gets fixed to that type and because we have no multiple inheritance for non protocols there is no possibility to create conflicts.

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