> On Feb 19, 2017, at 1:14 AM, Adrian Zubarev <adrian.zuba...@devandartist.com>
> wrote:
<snip>
> Here is a list of hidden and semi-hidden protocols from the standard library
> that could be closed. Formatted version:
> https://gist.github.com/DevAndArtist/168c800d784829be536c407311953ab7
> <https://gist.github.com/DevAndArtist/168c800d784829be536c407311953ab7>The
> majority of this list seems to more likely be non-resilient, appropriate to
> be hidden rather than being closed. Several of these I believe only exist due
> to features which are not yet in the Swift language, such as conditional
> conformances and generalized existentials.
The types in the standard library which I think are relevant to a
closed-protocol/union-type discussion would be:
CVarArg - interface of types appropriate to withVaList. “Closed” nature already
enforced by compiler, e.g.;
class FooBar : CVarArg {} //error: type 'FooBar' does not conform to protocol
‘CVarArg’
AFAICT, there is no reason that a third party type *couldn’t* implement
CVarArg, other than the requirements of the protocol being considered private
to the standard library (and possibly non-resilient). A valid protocol
implementation returns the data (as an [Int] array) that is to be appended to
the va_list.
MirrorPath - conceptually very similar to the SubscriptParameter, supporting
String, Int and IntMax types. While the protocol is not closed,
Mirror.descendent will preconditionFail if a different type other than String,
Int, or IntMax is passed in.
-DW
>
>
>
>
>
> --
> Adrian Zubarev
> Sent with Airmail
>
> Am 19. Februar 2017 um 07:59:45, David Waite via swift-evolution
> (swift-evolution@swift.org <mailto:swift-evolution@swift.org>) schrieb:
>
>> I am unsure if this feature is a good idea. Does someone have a real-world
>> use for this which isn’t just hiding strong implementation coupling behind a
>> protocol?
>>
>> When I consume a protocol, it is under the assumption that the protocol is
>> documented such that I would be able to work against *any* implementation of
>> the protocol. With a closed protocol, I would have to assume that there are
>> significant side effects, either undocumented or difficult for a third party
>> to duplicate. To my experience, that sounds brittle.
>>
>> Assuming you aren’t switching on the implementing type of a protocol (which
>> itself can be a sign that your design isn’t properly using polymorphism),
>> one could get this design by creating a struct with the interface desired,
>> and passing invocations through to an internal protocol reference.
>>
>> -DW
>>
>> > On Feb 18, 2017, at 1:41 PM, Matthew Johnson via swift-evolution
>> > <swift-evolution@swift.org> wrote:
>> >
>> > Now that we’re in phase 2 I’d like to officially propose we introduce
>> > `open` protocols and require conformances to `public` protocols be inside
>> > the declaring module. Let’s use this thread for feedback on the official
>> > proposal. After a healthy round of discussion I’ll open a PR to submit it
>> > for review.
>> >
>> >
>> > # Feature name
>> >
>> > * Proposal: [SE-NNNN](NNNN-open-public-protocols.md)
>> > * Authors: [Matthew Johnson](https://github.com/anandabits)
>> > * Review Manager: TBD
>> > * Status: **Awaiting review**
>> >
>> > ## Introduction
>> >
>> > This proposal introduces `open protocol` and changes the meaning of
>> > `public protocol` to match the meaning of `public class` (in this case,
>> > conformances are only allowed inside the declaring module).
>> >
>> > The pitch thread leading up to this proposal was: [consistent public
>> > access
>> > modifiers](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170206/031653.html)
>> >
>> > ## Motivation
>> >
>> > A general principle the Swift community has adopted for access control is
>> > that defaults should reserve maximum flexibility for a library. The
>> > ensures that any capabilities beyond mere visibility are not available
>> > unless the author of the library has explicitly declared their intent that
>> > the capabilities be made available. Finally, when it is possible to switch
>> > from one semantic to another without breaking clients (but not vice-versa)
>> > we should prefer the more forgiving (i.e. fixable) semantic as the (soft)
>> > default.
>> >
>> > `public` is considered a "soft default" in the sense that it is the first
>> > access modifier a user will reach for when exposing a declaration outside
>> > of the module. In the case of protocols the current meaning of `public`
>> > does not meet the principle of preserving maximum flexibility for the
>> > author of the library. It allows users of the library to conform to the
>> > protocol.
>> >
>> > There are good reasons a library may not wish to allow users to add
>> > conformances to a protocol. For example, it may not wish to expose the
>> > conforming concrete types. While similar behavior could be accomplished
>> > with an enum if cases could be private, that requires an implementation to
>> > use switch statements rather than polymorphism.
>> >
>> > Even if all the conforming types are also public there are cases where
>> > polymorphism is the preferred implementation. For example, if the set of
>> > conforming types is not expected to be fixed (despite all being inside the
>> > library) the authors may not want to have to maintain switch statements
>> > every time they need to add or remove a confroming type which would be
>> > necessary if an enum were used instead. Polymorphism allows us to avoid
>> > this, giving us the ability to add and remove conforming types within the
>> > implementation of the library without the burden of maintaining switch
>> > statements.
>> >
>> > Aligning the access modifiers for protocols and classes allows us to
>> > specify both conformable and non-conformable protocols, provides a soft
>> > default that is consistent with the principle of (soft) defaults reserving
>> > maximum flexibility for the library, and increases the overall consistency
>> > of the language by aligning the semantics of access control for protocols
>> > and classes.
>> >
>> > The standard library currently has at least one protocol (`MirrorPath`)
>> > that is documented as disallowing client conformances. If this proposal is
>> > adopted it is likely that `MirrorPath` would be declared `public protocol`
>> > and not `open protocol`.
>> >
>> > Jordan Rose has indicated that the Apple frameworks also include a number
>> > of protocols documented with the intent that users do not add
>> > conformances. Perhaps an importer annotation would allow the compiler to
>> > enforce these semantics in Swift code as well.
>> >
>> > ## Proposed solution
>> >
>> > The proposed solution is to change the meaning of `public protocol` to
>> > disallow conformances outside the declaring module and introduce `open
>> > protocol` to allow conformances outside the decalring module (equivalent
>> > to the current meaning of `public protocol`).
>> >
>> > ## Detailed design
>> >
>> > The detailed design is relatively straightforward but there are three
>> > important wrinkles to consider.
>> >
>> > ### User refinement of public protocols
>> >
>> > Consider the following example:
>> >
>> > ```swift
>> > // Library module:
>> > public protocol P {}
>> > public class C: P {}
>> >
>> > // User module:
>> > protocol User: P {}
>> > extension C: User {}
>> > ```
>> >
>> > The user module is allowed to add a refinement to `P` because this does
>> > not have any impact on the impelementation of the library or its possible
>> > evolution. It simply allows the user to write code that is generic over a
>> > subset of the conforming types provided by the library.
>> >
>> > ### Public protocols with open conforming classes
>> >
>> > Consider the following example:
>> >
>> > ```swift
>> > public protocol P P{}
>> > open class C: P {}
>> > ```
>> >
>> > Users of this module will be able to add subclasses of `C` that have a
>> > conformance to `P`. This is allowed becuase the client of the module did
>> > not need to explicitly declare a conformance and the module has explicitly
>> > stated its intent to allow subclasses of `C` with the `open` access
>> > modifier.
>> >
>> > ### Open protocols that refine public protocols
>> >
>> > Consider the following example:
>> >
>> > ```swift
>> > // library module:
>> > public protocol P {}
>> > open protocol Q: P {}
>> > open protocol R: P {}
>> >
>> > // user module:
>> > struct S: P {} // error `P` is not `open`
>> > struct T: Q {} // ok
>> > struct U: R {} // ok
>> > ```
>> >
>> > The user module is allowed to introudce a conformance to `P`, but only
>> > indirectly by also conforming to `Q`. The meaning we have ascribed to the
>> > keywords implies that this should be allowed and it offers libraries a
>> > very wide design space from which to choose. The library is able to have
>> > types that conform directly to `P`, while placing additional requirements
>> > on user types if necessary.
>> >
>> > ## Source compatibility
>> >
>> > This proposal breaks source compatibility, but in a way that allows for a
>> > simple mechanical migration. A multi-release stratgegy will be used to
>> > roll out this proposal to provide maximum possible source compatibility
>> > from one release to the next.
>> >
>> > 1. In Swift 4, introduce the `open` keyword and the `@nonopen` attribute
>> > (which can be applied to `public protocol` to give it the new semantics of
>> > `public`).
>> > 2. In Swift 4 (or 4.1 if necessary) start warning for `public protocol`
>> > with no annotation.
>> > 3. In the subsequent release `public protocol` without annotation becomes
>> > an error.
>> > 4. In the subsequent relase `public protocol` without annotation takes on
>> > the new semantics.
>> > 5. `@nonopen` becomes a warning, and evenutally an erro as soon as we are
>> > comfortable making those changes.
>> >
>> > ## Effect on ABI stability
>> >
>> > I would appreciate it if others can offer input regarding this section. I
>> > believe this proposal has ABI consequences, but it's possible that it
>> > could be an additivie ABI change where the ABI for conformable protocols
>> > remains the same and we add ABI for non-conformable protocols later. If
>> > that is possible, the primary impact would be the ABI of any standard
>> > library protocols that would prefer to be non-conformable.
>> >
>> > ## Effect on API resilience
>> >
>> > This proposal would may impact one or more protocols in the standard
>> > library, such as `MirrorPath`, which would likely choose to remain
>> > `public` rather than adopt `open`.
>> >
>> > ## Alternatives considered
>> >
>> > The primary alternatives are to either make no change, or to add something
>> > like `closed protocol`. The issues motivating the current proposal as a
>> > better alternative than either of these options are covered in the
>> > motivation section.
>> >
>> > _______________________________________________
>> > swift-evolution mailing list
>> > swift-evolution@swift.org
>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>> _______________________________________________
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