> On Sep 29, 2016, at 3:24 PM, Russ Bishop via swift-evolution
> <[email protected]> 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
Architechies
_______________________________________________
swift-evolution mailing list
[email protected]
https://lists.swift.org/mailman/listinfo/swift-evolution
