I do not understand why we can’t do
type(self)
or even
#type(self)
Personally, I find .Self business to be quite confusing and idiosyncratic.
Having a generic #type() directive would be a general improvement for the
language design and it would also open up future possibilities such as
extension A where A.B == #type(A.C.property1) {
}
and other potentially useful things.
> On 14 Apr 2016, at 03:41, Joe Groff via swift-evolution
> <[email protected]> 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
> }
>
> 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> {
> z.append(yAsX)
> }
>
> `x.Self` then becomes just the first step in this direction.
>
> -Joe
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