[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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