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

Xiaodi Wu xiaodi.wu at gmail.com
Fri Feb 10 21:48:43 CST 2017

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/LibraryEvolution.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'll leave aside closed protocols, which as per Jordan Rose's feedback
may or may not have sufficient interestingness.
- 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
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.
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