Seems like ->StaticSelf can actually only means returns the class for which
protocol conformance was applied first(in hierarchy). I.e. some `->BaseSelf`.
But in this case we need a method to get this 'base' class from protocol..
something like(just pseudo code!):
func makeWithZero<T: Makable >(x: Int) -> Makable(T).FirstConformedClass {
return T.make(value: 0)
}
or
func makeWithZero<T: Makable >(x: Int) -> type<T.make(value:)> {
return T.make(value: 0)
}
I don't know if all of this makes sense at all ;-)
On 13.05.2016 23:12, Joe Groff via swift-evolution wrote:
On May 13, 2016, at 9:06 AM, Matthew Johnson <[email protected]> wrote:
On May 13, 2016, at 10:55 AM, Joe Groff <[email protected]> wrote:
On May 13, 2016, at 8:18 AM, Matthew Johnson <[email protected]> wrote:
When I write a class Base with non-final methods that return instances of Base
I can choose whether to state the return type as Self (covariant) or Base
(invariant, under this proposal StaticSelf would also be an alternative way to
state this). If I choose to specify Base as the return type derived classes
*may* override the method but are not required to. Further, if they *do*
override the method they are allowed to choose whether their implementation
returns Base or Derived.
`StaticSelf` requirements by themselves don't even save you from covariance. If
Base conforms to a protocol (with Self == Base), Derived inherits that
conformance and also conforms (with Self == Derived). If `StaticSelf` always
refers to the conforming type, then it must also be bindable to Base and
Derived, so a base class must still use a covariant-returning method to satisfy
the `StaticSelf` requirement.
We are specifying that `StaticSelf` refers to the type that explicitly declares
conformance. If a class inherits conformance it refers to the base class which
explicitly declared the conformance it is inheriting.
That makes `StaticSelf` tricky to use in generic code. This would be invalid:
protocol Makable {
static func make(value: Int) -> StaticSelf
}
func makeWithZero<T: Fooable>(x: Int) -> T {
return T.make(value: 0) // ERROR: T.StaticSelf may be a supertype of T
so isn't convertible to T
}
`StaticSelf` in this model is effectively an associated type of the protocol,
with a `Self: StaticSelf` constraint (if that were supported).
-Joe
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