[swift-evolution] [Pitch] Rename `x.dynamicType` to `x.Self`
Dave Abrahams
dabrahams at apple.com
Thu Apr 14 14:29:13 CDT 2016
on Wed Apr 13 2016, Joe Groff <swift-evolution at swift.org> wrote:
> It's been pitched before, but I don't think we've had a dedicated
> thread to this idea. Erica has proposed making `Self` generally
> available within methods in types to refer to the dynamic type of the
> current receiver. One could think of `Self` as a special associated
> type member that exists in every type for this purpose. This also
> happens to be what you get when ask for the `dynamicType` member of a
> value. We could unify these concepts and get rid of the clunky
> `dynamicType` keyword, replacing it with `x.Self`.
>
> There's another benefit to this syntax change. Looking to the future,
> one of the many features Doug pitched in his generics manifesto was to
> generalize protocol existentials, lifting our current restrictions on
> protocols "with Self or associated types" and allowing them to be used
> as dynamic types in addition to static generic constraints. Once you
> do this, you often want to "open" the type of the existential, so that
> you can refer to its Self and associated types in the types of other
> values. I think a natural way would be to let you directly use Self
> and associated type members of existentials as types themselves, for
> example:
>
> let a: Equatable = /*...*/
> let b: Equatable = /*...*/
>
> // This is not allowed, since Equatable requires two values
> with the same static type, but
> // a and b may have different dynamic types.
> a == b
>
> // However, we can dynamically cast one to the other's dynamic
> type:
> if let bAsA = b as? a.Self {
> return a == bAsA
> }
This doesn't entirely work, I think:
class Z : Equatable {} // ...
class A : Z {}
class B : Z {}
let a: Equatable = A()
let b: Equatable = B()
if let bAsA = b as? a.Self { ... } // test fails.
Of course, one could decide we don't care about classes conforming to
protocols with Self requirements.
> let x: RangeReplaceableCollection = /*...*/
> let y: Collection = /*...*/
>
> // If y has the same dynamic Element type as x, append it to x
> var z: x.Self = x
> if let yAsX = y as? Any<Collection where Element == x.Element>
I don't think x.Element can work. Do you mean x.Self.Element?
>
> { z.append(yAsX) }
>
> `x.Self` then becomes just the first step in this direction.
>
> -Joe
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--
Dave
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