[swift-evolution] [Pitch] Refactor Metatypes

[Brent Royal-Gordon]

> On Sep 29, 2016, at 8:14 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I'm confused by this explanation.Today, `type(of:)` is the new `.dynamicType`. Is this proposal suggesting a silent change so that it now returns the static type? If so, why (particularly when you explain that this is often *not* what you would want)?

I'm short-handing the names to talk about the return values. In other words, I assume that, if we have both `type(of:)` and `subtype(of:)`, their signatures would be:

	func type<T>(of: T) -> Type<T>
	func subtype<T>(of: T) -> Subtype<T>

And I'm saying that, given these names, `type(of:)` is confusing and near-useless, whereas `subtype(of:)` is what you almost always want.

We *could*, of course, have a function called `type(of:)` which returned `Subtype<T>` and had the semantics I'm referring to as `subtype(of:)`. A name is just a name.

> I'm also somewhat puzzled about the proposed design. This proposal explains that Subtype<T> should be a supertype of Type<T> and its subtypes. Why is a supertype named Subtype?

Because a type's name should describe the *instances*; that's why you don't put "Class" at the end of all of your class names. (It's also why we're proposing `Type<T>` instead of `Metatype<T>`.)

Every instance of `Subtype<T>` is the type instance for a subtype of `T`. For instance, in this hierarchy:

	NSObject
	NSResponder: NSObject
	NSView: NSResponder

`Type<NSResponder>` is a `Subtype<NSObject>`, but not a `Subtype<NSView>`.

Thus, this reads correctly:

	let aType: Subtype<NSResponder> = NSView.self

Whereas this does not:

	let aType: Supertype<NSResponder> = NSView.self

-- 
Brent Royal-Gordon
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