I like the idea as associatedtype is playing the role of generic type and
in extension we conforms to the protocol with some specific generic type as
associated type.
I mean the first idea probably could be
protocol From<T> {
init(_ value: T)
}
but "protocols do not allow generic parameters; use associated types
instead", so it seems natural to express concrete type as associated type
for protocol in generic syntax <Type>
Probably alternative syntax could look like:
extension Int : From where .FromType = Float { }
Also, it seems like this proposal could help to solve a problem with the
same name of associated type in different protocols:
protocol One {
associatedtype Element
func foo(t: Element)
}
protocol Two {
associatedtype Element
func bar(t: Element)
}
struct OneTwo : One, Two {
func foo(t: Int) {}
func bar(t: String) {}
}
// type 'OneTwo' does not conform to protocol 'Two'
// candidate has non-matching type '(t: String) -> ()' [with Element = Element]
So, as I understand, will be possible
struct OneTwo : One, Two<String> {
func foo(t: Int) {} // OneTwo.Element will be Int
func bar(t: String) {}
}
On 08.06.2016 22:07, Антон Жилин via swift-evolution wrote:
==Motivation==
protocol From {
associatedtype FromType
init(_ value: FromType)
}
The problem is, one type cannot implement multiple From "conversions".
==Proposed solution==
Allow specifying all associated types using generic syntax.
extension Int : From<Float> { }
extension Int : From<Double> { }
This is only allowed in conformance declarations.
==Future directions==
We can replace all *Convertible protocols with From and Into, which will be
defined similarly to Rust.
- Anton
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