> On Feb 8, 2017, at 12:09 PM, Daryle Walker via swift-evolution
> <[email protected]> wrote:
>
> I’ve been trying to get the maximum of a list of counts. I started by using
> “myCollection.map { $0.myCountProperty }.reduce(0, max)”. Then I thought I
> should really use the value closest to negative infinity as the base. The
> problem is how to get that value. The current hard-coded “0” works because
> the count type ultimately aliases “Int”, but shouldn’t have to count on that.
> I put in “MyCountType.min”, where “MyCountType” is hard coded from the docs
> of “myCountProperty”, but I shouldn’t have to do that either.
>
> There is a “type(of:)” global function, which I did use. But not only did I
> have to make up an expression to go in there, its return value, some sort of
> meta-type stuff I don’t understand, can’t be used on the right side of a
> “typealias” construct. I ultimately used “let lowestCount = type(of:
> anElement.myCountProperty).min” to get what I needed.
>
> Some queries/requests:
>
> 1. Is the expression within “type(of:)” evaluated?
Yes. Eg,
class Base {}
class Derived : Base {}
func foo() -> Base { return Derived() }
type(of: foo()) // this is Derived.self, not Base.self
> 2. I think we need the equivalent of "std::declval” from C++. Especially if
> the answer to (1) is yes.
> 3. Why isn’t the return type(?) of “type(of:)” compile-time enough to be used
> as a type-alias?
type(of:) is not a real function because it’s return type depends on the input
type in a funny way. If the input type T is an existential, then type(of:)
returns an existential metatype; otherwise, it returns a concrete metatype. So
it’s almost as if the signature is (T) -> (T.Type), but not quite.
Slava
> 4. Should we add something like the C++ type-traits collection of generic
> structures to give use type information as type-aliases or type-level
> instances?
>
> —
> Daryle Walker
> Mac, Internet, and Video Game Junkie
> darylew AT mac DOT com
>
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