[swift-evolution] [Pitch] Rename `x.dynamicType` to `x.Self`

[Eugene Gubin]

I like this idea. I think, if we could use `self` to access instance of
self, we should be allowed to use `Self` to access type of self.

2016-04-14 8:46 GMT+04:00 William Dillon via swift-evolution <
swift-evolution at swift.org>:

> This would be great!
>
> I use a nearly identical pattern in my networking framework that would be
> nice to streamline:
>
> public func ==(lhs: NetworkAddress, rhs: NetworkAddress) -> Bool {
>     // Only addresses of the same protocol are considered equal.
>     guard lhs.dynamicType == rhs.dynamicType else {
>         return false
>     }
>
>
>     if lhs.dynamicType == IPv4NetworkAddress.self {
>         return (lhs as! IPv4NetworkAddress) == (rhs as! IPv4NetworkAddress
> )
>     }
>
>     if lhs.dynamicType == IPv6NetworkAddress.self {
>         return (lhs as! IPv6NetworkAddress) == (rhs as! IPv6NetworkAddress
> )
>     }
>
>
>     return false
> }
>
> - Will
>
> On Apr 13, 2016, at 6:41 PM, Joe Groff via swift-evolution <
> 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
> }
>
> 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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