> On 28. Dec 2017, at 12:34, Adrian Zubarev <[email protected]>
> wrote:
>
> I disagree with some of your points. Do begin with, I don’t think we should
> disallow capturing the generic type parameter, because I think there might be
> a good way to prevent parameterization of nested protocols.
>
> To me this only feels like a natural consequence of nesting protocols
> anyways. To achieve this we have to provide an explicit associated type which
> will have a default type that refers to the captured outer generic type
> parameter. At this point we discover another issue that we cannot
> disambiguate generic type parameter like associated types yet and would be
> forced to name the associated type of the protocol differently.
>
> struct Generic<T> {
> protocol P {
> associatedtype R = T
> func f() -> R
> }
> }
> As you can see I didn’t include the variable in this example, because
> existential are orthogonal this issue. David Hart and I still want to write a
> proposal to allow the where clause on typealiases - maybe after the forum
> officially launches.
>
> Above I said that there is an issue and provided an example that would solve
> that issue with todays syntax, but I’d rather expand this idea. Consider this
> syntax of a generic type and a protocol with an associated type.
>
> protocol Proto {
> associatedtype Element
> }
>
> Proto.Element // This is an error like this, but it's still allowed in a
> generic context
>
> func function<P : Proto>(_: P) where P.Element == Int {}
>
> protocol OtherProto : Proto where Element == Int {}
>
> struct Test<Element> {}
>
> extension Test where Element == Int {}
>
> Test.Element // Can/should we allow this?
> If we could allow this in Swift then the above example with the nested
> protocol could be disambiguated nicely.
>
> struct Generic<T> {
> protocol P {
> associatedtype T = Generic.T
> func f() -> T
> }
> }
> Remember that Generic.T is only the default for P.T if you don’t set it
> yourself but when you conform or use that that protocol (in a generic
> context) you can still set it differntly.
>
> This consequence disallows protocol parameterization through nesting in a
> generic types, but still behaves very similar to nested generic types:
>
> struct Test<T> {
> struct NonGeneric {
> var t: T
> }
>
> struct Generic<R> {
> var t: T
> var r: R
> }
> }
>
> _ = Test<String>.NonGeneric(t: "😎")
> _ = Test<String>.Generic<Int>(t: "🤓", r: 42)
> ——
>
>
Yeah, that’s all well and good. I don’t think we should parameterise protocols
either; it feels like Swift hasn’t been designed with those in mind, and that’s
fine. You would get in to all kinds of horrible conflicts if you tried to
conform to both Generic<Int>.P and Generic<String>.P, because most functions
would have the same signature but possibly very different implementations. You
would likely end up having to separate the conformances by using a wrapper
struct — in which case, why not just make them the same protocol and have the
existing duplicate-conformance rules take care of it?
An earlier version of the proposal included something like you describe.
Basically, Generic<Int>.P and Generic<String>.P would be the same protocol.
They would have an associated type to represent the parameter from Generic<T>,
and within Generic<T>, all references to P would be implicitly constrained so
that P.T == Self.T. You would write conformances to “Generic.P” with a
constraint for T, as you do today.
> And for the existential variable inside Genric it really should be something
> like this (when the where clause is allowed and if we can refer differently
> to generic type parameters as well):
>
> struct Generic<T> {
> …
> typealias PConstrainedByT = P where T == Self.T
> var object: PConstrainedByT
> }
>
If we have that ability, then we can totally do capturing. Forgive me, but I
understand that as pretty-much the same as generalised existentials (without
local type binding).
If I can write the type of object as an existential of (generic protocol +
constraints) via a typealias, then surely I must also be able to do it
directly? So I could also write:
struct Generic<T> {
var object: P where T == Self.T
}
Anyway, I thought that was not on the table, and in any case I’m convinced that
it should be a separate proposal. This gets to the heart of the interaction
between generic types and protocols, and we all know it’s not always a smooth
transition (hello AnyCollection, AnyHashable, etc...). We can cover the common
cases (i.e. the Apple frameworks) without requiring capturing - especially
since it’s apparently not too difficult to implement - and build from there.
- Karl
>
>
>
> Am 27. Dezember 2017 um 19:53:36, Karl Wagner via swift-evolution
> ([email protected] <mailto:[email protected]>) schrieb:
>
>> 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] <mailto:[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] <mailto:[email protected]>> wrote:
>>>>
>>>> is there a reason why it’s not allowed to nest a protocol declaration
>>>> inside another type?
>>>> _______________________________________________
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>>>> [email protected] <mailto:[email protected]>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
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