[swift-evolution] [Pitch] consistent public access modifiers

Sat Feb 11 12:40:47 CST 2017

On Sat, Feb 11, 2017 at 6:41 AM, Matthew Johnson <matthew at anandabits.com>
wrote:

>
>
> Sent from my iPad
>
> On Feb 10, 2017, at 9:48 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> On Wed, Feb 8, 2017 at 5:05 PM, Matthew Johnson via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> I’ve been thinking a lot about our public access modifier story lately in
>> the context of both protocols and enums.  I believe we should move further
>> in the direction we took when introducing the `open` keyword.  I have
>> identified what I think is a promising direction and am interested in
>> feedback from the community.  If community feedback is positive I will
>> flesh this out into a more complete proposal draft.
>>
>>
>> Background and Motivation:
>>
>> In Swift 3 we had an extended debate regarding whether or not to allow
>> inheritance of public classes by default or to require an annotation for
>> classes that could be subclassed outside the module.  The decision we
>> reached was to avoid having a default at all, and instead make `open` an
>> access modifier.  The result is library authors are required to consider
>> the behavior they wish for each class.  Both behaviors are equally
>> convenient (neither is penalized by requiring an additional boilerplate-y
>> annotation).
>>
>> A recent thread (https://lists.swift.org/piper
>> mail/swift-evolution/Week-of-Mon-20170206/031566.html) discussed a
>> similar tradeoff regarding whether public enums should commit to a fixed
>> set of cases by default or not.  The current behavior is that they *do*
>> commit to a fixed set of cases and there is no option (afaik) to modify
>> that behavior.  The Library Evolution document (
>> https://github.com/apple/swift/blob/master/docs/LibraryEvol
>> ution.rst#enums) suggests a desire to change this before locking down
>> ABI such that public enums *do not* make this commitment by default, and
>> are required to opt-in to this behavior using an `@closed` annotation.
>>
>> In the previous discussion I stated a strong preference that closed enums
>> *not* be penalized with an additional annotation.  This is because I feel
>> pretty strongly that it is a design smell to: 1) expose cases publicly if
>> consumers of the API are not expected to switch on them and 2) require
>> users to handle unknown future cases if they are likely to switch over the
>> cases in correct use of the API.
>>
>> The conclusion I came to in that thread is that we should adopt the same
>> strategy as we did with classes: there should not be a default.
>>
>> There have also been several discussions both on the list and via Twitter
>> regarding whether or not we should allow closed protocols.  In a recent
>> Twitter discussion Joe Groff suggested that we don’t need them because we
>> should use an enum when there is a fixed set of conforming types.  There
>> are at least two  reasons why I still think we *should* add support for
>> closed protocols.
>>
>> As noted above (and in the previous thread in more detail), if the set of
>> types (cases) isn’t intended to be fixed (i.e. the library may add new
>> types in the future) an enum is likely not a good choice.  Using a closed
>> protocol discourages the user from switching and prevents the user from
>> adding conformances that are not desired.
>>
>> Another use case supported by closed protocols is a design where users
>> are not allowed to conform directly to a protocol, but instead are required
>> to conform to one of several protocols which refine the closed protocol.
>> Enums are not a substitute for this use case.  The only option is to resort
>> to documentation and runtime checks.
>>
>>
>> Proposal:
>>
>> This proposal introduces the new access modifier `closed` as well as
>> clarifying the meaning of `public` and expanding the use of `open`.  This
>> provides consistent capabilities and semantics across enums, classes and
>> protocols.
>>
>> `open` is the most permissive modifier.  The symbol is visible outside
>> the module and both users and future versions of the library are allowed to
>> add new cases, subclasses or conformances.  (Note: this proposal does not
>> introduce user-extensible `open` enums, but provides the syntax that would
>> be used if they are added to the language)
>>
>> `public` makes the symbol visible without allowing the user to add new
>> cases, subclasses or conformances.  The library reserves the right to add
>> new cases, subclasses or conformances in a future version.
>>
>> `closed` is the most restrictive modifier.  The symbol is visible
>> publicly with the commitment that future versions of the library are *also*
>> prohibited from adding new cases, subclasses or conformances.
>> Additionally, all cases, subclasses or conformances must be visible outside
>> the module.
>>
>> Note: the `closed` modifier only applies to *direct* subclasses or
>> conformances.  A subclass of a `closed` class need not be `closed`, in fact
>> it may be `open` if the design of the library requires that.  A class that
>> conforms to a `closed` protocol also need not be `closed`.  It may also be
>> `open`.  Finally, a protocol that refines a `closed` protocol need not be
>> `closed`.  It may also be `open`.
>>
>> This proposal is consistent with the principle that libraries should
>> opt-in to all public API contracts without taking a position on what that
>> contract should be.  It does this in a way that offers semantically
>> consistent choices for API contract across classes, enums and protocols.
>> The result is that the language allows us to choose the best tool for the
>> job without restricting the designs we might consider because some kinds of
>> types are limited with respect to the `open`, `public` and `closed`
>> semantics a design might require.
>>
>>
>> Source compatibility:
>>
>> This proposal affects both public enums and public protocols.  The
>> current behavior of enums is equivalent to a `closed` enum under this
>> proposal and the current behavior of protocols is equivalent to an `open`
>> protocol under this proposal.  Both changes allow for a simple mechanical
>> migration, but that may not be sufficient given the source compatibility
>> promise made for Swift 4.  We may need to identify a multi-release strategy
>> for adopting this proposal.
>>
>> Brent Royal-Gordon suggested such a strategy in a discussion regarding
>> closed protocols on Twitter:
>>
>> * In Swift 4: all unannotated public protocols receive a warning,
>> possibly with a fix-it to change the annotation to `open`.
>> * Also in Swift 4: an annotation is introduced to opt-in to the new
>> `public` behavior.  Brent suggested `@closed`, but as this proposal
>> distinguishes `public` and `closed` we would need to identify something
>> else.  I will use `@annotation` as a placeholder.
>> * Also In Swift 4: the `closed` modifier is introduced.
>>
>> * In Swift 5 the warning becomes a compiler error.  `public protocol` is
>> not allowed.  Users must use `@annotation public protocol`.
>> * In Swift 6 `public protocol` is allowed again, now with the new
>> semantics.  `@annotation public protocol` is also allowed, now with a
>> warning and a fix-it to remove the warning.
>> * In Swift 7 `@annotation public protocol` is no longer allowed.
>>
>> A similar mult-release strategy would work for migrating public enums.
>>
>
> A different line of feedback here:
>
> As per previous reply, I now think if we clarify the mental model of the
> access modifier hierarchy you're proposing and adopt or reject with that
> clarity, we'll be fine whether we go with `closed` or with `@closed`. But I
> don't think the source compatibility strategy you list is the most simple
> or the most easy to understand for end users.
>
>
> I'm pretty neutral on what kind of source compatibility strategy we would
> adopt.  I am happy to defer to the community and core team.
>
>
> - I'll leave aside closed protocols, which as per Jordan Rose's feedback
> may or may not have sufficient interestingness.
>
>
> Jordan supported allowing protocols to have the same choice of contract
> that classes do today. `public protocol` has the same meaning as `open
> class` today so if we want consistency we need a breaking change.
>

Sure; I was specifically considering the phased introduction of `closed`.
It's been a while since I've thought about how to phase in a change
regarding public protocols and open protocols.

That said, others make good points about _conforming to_ protocols by a
type vs. _refining_ protocols by another protocol, and whether either or
both of these is more akin to subclassing a class.

- With respect to enums, I don't think we need such a drastic whiplash in
> terms of what will compile in future versions. Instead, we could take a
> more pragmatic approach:
>
> 1. In Swift 4, remove the warning (or is it error?) about `default` cases
> in switch statements over public enums. Simultaneously, add `closed` or
> `@closed` (whatever is the approved spelling) and start annotating standard
> library APIs. The annotation will be purely future-proofing and have no
> functional effect (i.e. the compiler will do nothing differently for a
> `closed enum` or `@closed public enum` (as the case may be) versus a plain
> `public enum`).
> 2. In Swift 4.1, _warn_ if switch statements over public enums don't have
> a `default` statement: offer a fix-it to insert `default: fatalError()`
> and, if the enum is in the same project, offer a fix-it to insert `closed`
> or `@closed`.
>
>
> Why do you say "if the enum is in the same project, offer a fix-it to
> insert `closed`?  If the enum is in the same project we can perform an
> exhaustive switch regardless of its public API contract (except for `open`
> enums if we decide to add those).
>

Hmm, well now I'm not in favor of my own suggestion. A public enum, though
it may gain or lose cases in future versions, can be exhaustively switched
over in the present whether it's same-module or third-party. No warning or
error should issue on attempting to switch over a public enum without a
default case.

> 3. In Swift 5, upgrade the warning to an error for non-exhaustiveness if a
> switch statement over a public enum doesn't have a `default` statement.
> Now, new syntax to extend an `open enum` can be introduced and the compiler
> can treat closed and public enums differently.
>
>
> If the community and core team support this strategy I will also.  It
> seems reasonable and speeds up the transition by using the point release.
> That's a great idea!
>
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