[swift-evolution] [Draft] Harmonize access modifiers for extensions

Adrian Zubarev adrian.zubarev at devandartist.com
Sun Jul 17 01:50:22 CDT 2016

I’m struggling to understand your proposal, can you provide some specific code samples how it works now and what will change. The example from the draft doesn’t help my understanding. :/

Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 04:40:45, Xiaodi Wu via swift-evolution (swift-evolution at swift.org) schrieb:

On Sat, Jul 16, 2016 at 7:56 PM, Jose Cheyo Jimenez <cheyo at masters3d.com> wrote:
I think you can simplify this proposal by just saying something like this and give a couple of examples that are easy to follow:

Disallow explicit public access modifier on non-protocol-conforming type extensions.

It took me a while to process what you're trying to say here, but this is a good idea and would go along well with the first draft's proposed solution. I will spell it out. (If we say that you can use an explicit modifier only to lower the access level of members, then `public` as an explicit modifier could be entirely disallowed.)

I think if you only focus on that breaking change then the proposal will have a good chance of getting accepted and fixing the immediate issue of public. There is a reason why protocol conforming extensions do not allow explicitly saying public 
`public extension Type: Protocol {}` // public not allowed 

Actually, no modifiers are allowed in that scenario, IIUC.

In essence we will be asking for the same behavior for types. 

Allowing methods declared inside extensions to have a higher declared visibility is not a breaking change and can be introduced later. 

It is a breaking change in that I am proposing that the rules be harmonized so that the implicit default access level will be notionally `internal` (there are follow-on benefits to this change). That cannot be changed later.
Nobody wants private extensions or implicit internal extensions to go away. :)

I know that there are people who don't want it to go away. That was why the first draft proposed keeping them, but it sounds like it would make for an illogical system. I know that Jordan and John have both indicated that they don't think it's worth keeping around but don't seem to feel too strongly about it, and I think I feel the same way (leaning towards not keeping them, but don't feel very strongly). I will definitely feature this concern (using extensions as access modifier groups) prominently in the proposal and hope for a robust discussion to see how it plays out with the community and core team.

On Jul 16, 2016, at 4:22 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:

On Sat, Jul 16, 2016 at 6:10 PM, David Hart <david at hartbit.com> wrote:
This proposal really confuses me. Two comments:

1) With the proposal, we loose the ability to use access modifiers on extensions as a way of grouping members by access. That's a huge loss for me.

You lose the ability to group public members only. That part is intentional, so that only methods declared with `public func` are public.
2) If we adopt the proposal, I now have no idea what explicit access modifiers on extensions do.

I propose keeping explicit access modifiers because previous comments on this list have said that it's useful for grouping members by access. You can continue to use extensions to group fileprivate members of an internal type, or internal members of a public type.

More generally, I don't understand this proposal as it's trying to apply the same access modifier rules on extensions as for types but extensions are not types. They are just a declaration for extending types which already have an access level.

On 16 Jul 2016, at 20:04, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:

With the impending withdrawal of SE-0119 and the closing window for (most) source-breaking changes, I thought I'd draft up a proposal to address some of the key points raised in that discussion.

The proposed changes are deliberately limited in scope to rationalizing access modifier rules without adding any new facilities (such as conformances of lower visibility than the type), which might be more appropriate for the Swift 4 timeline.

I hope this will prove satisfactory to the community :)

Harmonize access modifiers for extensions

Proposal: SE-XXXX
Author: Xiaodi Wu
Status: Awaiting review
Review manager: TBD

During discussion of SE-0119, the community articulated the view that access modifiers for extensions were and should continue to be subject to the same rules as access modifiers for types. Unfortunately, it is not factually true today; this proposal aims to make it so.

Swift-evolution threads:

[Proposal] Revising access modifiers on extensions
[More to be added here]

Consider the following:

public struct foo {
  func frobnicate() { } // implicitly internal
public extension foo { }

public struct bar { }
public extension bar {
  func frobnicate() { } // implicitly public, according to SE-0025
According to SE-0025, a method moved from the body of a public struct into a public extension becomes public without modification. This is surprising behavior contrary to Swift's general rule of not exposing public API by default.

Furthermore, SE-0025 now permits the owner of a type to design access for members as though the type will have a higher access level than it currently does. For example, users will be able to design public methods inside an internaltype before "flipping the switch" and making that type public. The same approach is prohibited by SE-0025 for extensions, although conceptually it need not be.

Proposed solution

The proposed solution is to change access modifier rules for extensions with the following effect: if any method (or computed property) declared within the body of a type at file scope is moved without modification into the body of an extension in the same file, the move will not change its accessibility.

In code:

struct foo {
  // Any method declared here...
extension foo {
  // ...should have the same visibility when moved here.
This implies that public API commitments will need to be annotated as public at declaration sites inside an extension just as it must be at declaration sites inside types.

Detailed design

Declarations inside the extension will, like declarations inside types, have a default access level of internal.
The compiler should not warn when a broader level of access control is used for a method (or computed property, etc.) declared within an extension with more restrictive access. This allows the owner of the extension to design the access level they would use for a method if the type or extension were to be made more widely accessible.
An extension declared without an explicit access modifier will have the same access level as the type being extended.
An extension declared without protocol conformance may optionally use an explicit access modifier to provide an upper bound for the visibility of its members.
Alternatives considered

One alternative, still open for consideration, is to eliminate #4 and disallow explicit access modifiers on extensions. As an advantage, this would clarify the mental model that extensions are not their own entities, as they cannot be referred to by name and have no runtime representation. As a disadvantage, extensions cease to be an access modifier grouping construct, which some users really like.

Thanks to all discussants on the list, especially Adrian Zubarev and Jose Cheyo Jimenez.

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