> Am 12.08.2016 um 04:39 schrieb Douglas Gregor via swift-evolution
> <swift-evolution@swift.org>:
>
>
>
> Sent from my iPhone
>
>> On Aug 4, 2016, at 2:36 AM, Haravikk via swift-evolution
>> <swift-evolution@swift.org> wrote:
>>
>>
>>>> On 4 Aug 2016, at 03:19, Brent Royal-Gordon via swift-evolution
>>>> <swift-evolution@swift.org> wrote:
>>>>
>>>> On Aug 3, 2016, at 10:17 AM, Manav Gabhawala via swift-evolution
>>>> <swift-evolution@swift.org> wrote:
>>>>
>>>> I was wondering why this would put any more of a burden on the runtime
>>>> than simple inheritance of protocols. The way this could be
>>>> implemented is to augment the ConformanceTable for nominal types by
>>>> looking up its protocol extension’s inheritance clauses. I can
>>>> definitely see this impacting compile time but I don’t see why runtime
>>>> performance will be any different than simple inheritance. Further,
>>>> cyclic chains can be detected and broken (compiler error) during the
>>>> second pass of semantic analysis.
>>>
>>> My understanding—which may be incorrect, by the way—is that the issue is
>>> mainly with protocol extensions adding conformances, not specifically with
>>> those conformances being conditional, and that it specifically has to do
>>> with `is` and `as?` checks across module boundaries.
>>>
>>> Suppose you have these declarations in module M:
>>>
>>> public protocol AProtocol {…}
>>> public protocol BProtocol: AProtocol {…}
>>> public protocol CProtocol {…}
>>>
>>> // Public or otherwise doesn't matter here.
>>> public struct Foo: BProtocol {…}
>>>
>>> Foo essentially has a flat list of the protocols it conforms to attached to
>>> it. Notionally, you can think of that list as looking like:
>>>
>>> Foo.self.conformsTo = [BProtocol.self, AProtocol.self]
>>>
>>> And when you write `foo is CProtocol`, that eventually translates into:
>>>
>>> foo.dynamicType.conformsTo.contains(CProtocol.self)
>>>
>>> For a `Foo`, since the `conformsTo` list doesn't include `CProtocol.self`,
>>> it returns `false`.
>>>
>>> Now imagine that you write a new module, N, and in it you say:
>>>
>>> extension Foo: CProtocol {…}
>>>
>>> You have now retroactively conformed `Foo` to `CProtocol`. Swift needs to
>>> reach into module M and add `CProtocol.self` to the `Foo.self.conformsTo`
>>> list. This is perfectly doable for a concrete type—it's one flat list,
>>> after all.
>>>
>>> Instead, though, imagine that module N extended `AProtocol` to add a
>>> conformance:
>>>
>>> extension AProtocol: CProtocol {…}
>>>
>>> There are two ways to handle this. One is to find all types conforming to
>>> `AProtocol`, recursively, and add `CProtocol.self` to their conformance
>>> list. The other is to scrap the flat list of conformances and instead make
>>> `is` and `as?` recursively search each protocol. Either way, you have
>>> replaced a fast, flat operation with a slow, recursive one.
>>>
>>> Conditional conformance adds another wrinkle to this, of course—you must
>>> not only recursively search the list, but also evaluate the condition to
>>> see if it applies in this case. But the general problem of having to
>>> replace a fast search with a slow search applies either way.
>>
>> Great explanation! This switch from flat to recursively searched though
>> seems like it would only occur when the extension is in an external module
>> though; for internal modules would it not still be possible to determine the
>> flat list for each type? In that case extending a type from another module
>> could be either disallowed, or produce a warning to indicate the performance
>> implication?
>>
>> The feature would still be very useful even just for internal use after all.
>> Also it seems useful on a relatively small number of types, and the number
>> of external modules that need/want to do this must narrow that even further,
>> so external extensions may be quite niche, i.e- not worth losing the feature
>> for internal use if that is indeed easier?
>
> Swift doesn't really have any features that stop working across modules.
> We're okay with the programmer having to think more and be more explicit
> across module boundaries (since it is API design at that point), but it'd
> take a very strong argument to have different runtime semantics across module
> boundaries.
>
> FWIW, I'm planning to write a complete proposal for conditional conformances
> and will start posting drafts once it is far enough along to be useful. It
> won't have support for protocols conforming to other protocols, though.
>
> - Doug
>
Hi Doug,
When can we expect your proposal?
Best regards
Maximilian
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