[swift-evolution] [Pitch] Refactor Metatypes

[Brent Royal-Gordon]

> On Sep 29, 2016, at 3:24 PM, Russ Bishop via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Why would we not have type(of:) and subtype(of:)? Why would I want the Subtype<T> instead of the specific Type<T>?

Let's turn this around. Suppose you write:

	let obj: NSObject = …
	let ty = type(of: obj)

What is `ty`? Well, it's a `Type<NSObject>`, and there's only one of those: `NSObject.self`. So there's only one possible instance that could be assigned to that variable.

This is true in general: If `type(of:)` returns `Type<T>`, then it can only have one possible return value. In other words, the return value of `type(of:)` would always be the *static* type of the variable, not its dynamic type. There may be some narrow cases where that'd be useful, but 99% of the time, you want `subtype(of:)` because you're trying to discover which of many dynamic subtypes of the static type you're actually dealing with. So most uses of `type(of:)` would probably be mistaken attempts to perform `subtype(of:)` instead.

> What is the rationale for losing the meta type relationships by having Type<U> not be a subtype of Type<T>?

The relationships aren't lost; they're just expressed through `Subtype`, not `Type`.

Again, turn this around. `Subtype` is the normal thing that you'll want to use most of the time. `Type` is the weird thing whose existence is hard to explain. (One version of this proposal used `Type` for `Subtype` and `ExactType` for `Type` in order to imply that subtype is usually what you want, but some of the contributors weren't happy with that.)

So, `Type` is the weird thing. Why does it exist? Two reasons:

1. `Subtype<T>` only includes *inheritable* type members of `T`. `Type<T>` also includes *non-inheritable* members, particularly non-required initializers.

2. It allows precise type matches: `subty is Subtype<NSObject>` would match for any subtype of `NSObject`, whereas `subty is Type<NSObject>` would only match for `NSObject` itself.

-- 
Brent Royal-Gordon
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