Yeah I wrote that proposal. I eventually stripped it down to just disallow all
capturing, but it was still not selected by the core team for review ¯\_(ツ)_/¯
As for capturing semantics, once you start working through use-cases, it’s
becomes clear that it's going to require generalised existentials. Otherwise,
how would you use a Generic<T>.P?
struct Generic<T> {
protocol P { func f() -> T }
var object: P // uh-oh! ‘Generic protocol can only be used as a generic
parameter constraint'
}
So, you would need to add a generic parameter to actually use P from within
Generic<T>, which of course limits you to a single concrete type of P:
struct Generic<T, TypeOfP> where TypeOfP: Self.P { // Could this even
work? What if P captures TypeOfP?
protocol P { /* … */ }
var object: TypeOfP
}
Which is just yucky.
Ideally, the type of ‘object’ should be ‘Any<P where P.T == T>’, to express
that it can be any conforming type with the appropriate constraints. You
wouldn’t need to write that all out; we could infer that capturing is
equivalent to a same-type constraint (or perhaps one of these “generalised
supertype constraints” that were pitched recently). But we can’t express those
kinds of existentials everywhere in the type-system today, so most examples of
capturing fall down pretty quickly.
- Karl
> On 25. Dec 2017, at 03:56, Slava Pestov via swift-evolution
> <[email protected]> wrote:
>
> There was a proposal to allow protocols to be nested inside types at one
> point but it didn’t move forward.
>
> Basically, if the outer type is a non-generic class, struct or enum, there’s
> no conceptual difficulty at all.
>
> If the outer type is a generic type or another protocol, you have a problem
> where the inner protocol can reference generic parameters or associated types
> of the outer type. This would either have to be banned, or we would need to
> come up with coherent semantics for it:
>
> struct Generic<T> {
> protocol P {
> func f() -> T
> }
> }
>
> struct Conforms : Generic<Int>.P {
> func f() -> Int { … } // Like this?
> }
>
> let c = Conforms()
> c is Generic<String>.P // is this false? Ie, are Generic<Int>.P and
> Generic<String>.P different protocols?
>
> Slava
>
>> On Dec 24, 2017, at 6:53 PM, Kelvin Ma via swift-evolution
>> <[email protected]> wrote:
>>
>> is there a reason why it’s not allowed to nest a protocol declaration inside
>> another type?
>> _______________________________________________
>> swift-evolution mailing list
>> [email protected]
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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