[Draft] Harmonize access modifiers for extensions

xwu · July 18, 2016, 4:45pm

My understanding is that “being able to change access levels by
simply flipping a switch on the type itself” is a positive goal of Swift.

I agree.

Therefore, it emphatically *should* be allowed to set the default

visibility for an extension the same as (or higher than!) the type it
extends. For example, a private type can be given a private extension, so
later making the type internal will preserve the privacy of the extension
members.

As discussed in SE-0119, there is no such thing as a private extension as a
first-class entity. The default access modifier is *only* modifying the
members declared within; therefore, using the "flipping a switch" analogy,
it is not a switch but rather the default lightbulb; the only relevant
switch is the access modifier of the original type.

If any change is to be made in this area, it should simply be that “the

default access level of extension members is `internal`”, which means
public members of an extension must be explicitly marked public.

That requires no change, as it is already slated to be the rule after
SE-0025.

The access modifier on an extension would serve as an upper bound on
visibility for its members,

That approach was my first instinct as well, but it was rejected in SE-0119.

so the default (implicit) modifier for extensions would be “public”.
Members of a public extension would be implicitly internal, as noted above.

The visibility of a type is an upper bound on its members’ visibility as
well, so in a sense we would be making all access modifiers, including
those on member declarations, act as upper bounds. This allows designs like
the following:

[internal] struct makePublicInTheFuture { ... }
[public] extension makePublicInTheFuture {
  private var _p
  fileprivate var _f
  [internal] var _i
  public var x
  ...
}

Now if the struct’s visibility is changed, each extension member will be
accessible up to the maximum availability of the type, the extension, and
itself.

That seems like the most consistent approach to me.

It took me a while to understand what Jordan and John were saying in the
review process for SE-0119, but I think I now understand. They and others
reject the upper bound approach because, unlike types, extensions are not
first-class entities. Therefore, because they do not exist, extensions
should not be able to set bounds.

However, I am still not convinced that any change is required at all. The
only “problem case” now is `public extension` making its member public by
default. I am not a framework designer, so I do not know how useful or
non-useful this may be.

But if any change is to be made, I think “extension members default to
internal” is the simplest.

If you look back, that was indeed my first draft of this proposal. However,
feedback has made it clear that only two options are potentially
acceptable: access modifiers on extensions can either be eliminated, or
they can continue to be a shorthand, but they cannot be harmonized to be
upper bounds.

All access modifier act as upper bounds, extensions are implicitly
`public`, and other declarations (including types) are implicitly
`internal`.

Then programmers can set access levels to “where they should be” in
preparation for “flipping the switch” on a type or extension’s visibility
in the future, and public APIs must be explicitly marked `public` at the
declaration site.

How does this sound?

It sounded great to me, but it was shot down in SE-0119 and it was rejected
again on the first draft of this proposal.

···

On Mon, Jul 18, 2016 at 11:25 AM, Nevin Brackett-Rozinsky < nevin.brackettrozinsky@gmail.com> wrote:

Nevin

On Mon, Jul 18, 2016 at 11:32 AM, Xiaodi Wu via swift-evolution < > swift-evolution@swift.org> wrote:
On Mon, Jul 18, 2016 at 6:09 AM, Adrian Zubarev via swift-evolution < >> swift-evolution@swift.org> wrote:

+1 :)

Let’s analyze this: public > internal > fileprivate >= private

An extension may optionally be marked with an explicit access modifier
that specifies the default scope [see SE–0025]. However, such an explicit
modifier *must not match* (or exceed) the original type’s access level.

public struct A {}

// I assume that we go with "must not match" here!

// can't be public anymore -> no more
// <<implicitly>> public extension members
// -> breaking change -> I can live with that
extension A {}

// no default access modifier -> extension
// follows the access rule by the extended type A
// -> here every extension member is internal by default
// -> can be overridden to public member wise
extension A {}

// default access modifier acts as the upper bound
// inside an extended public type A
// -> every extension member are fileprivate
// -> extension member can be explicitly set to private
// -> these will be only visible inside this extension scope
fileprivate extension A {}

// at file scope `private` acts like `fileprivate`
// (if `private` is allowed at filescope) - haven't read the extended SE-0025 yet
// -> I assume any member that are explicitly set to private
// will only be visible inside this extension scope
private extension A {}

Let’s check internal types:

internal struct B {}

// "must not match" does not work here anymore
// do we use "must not exceed" here???

// I assume the second.

// doens't break anything
// works as before
// no default access modifier for internal types
// equals `internal extension A {}`
// members are default internal
// -> can be overridden to `fileprivate` or scope level `private`
extension B {}

// same as for `public extension A`
fileprivate extension B {}

// same as for `public extension A`
private extension B {}

// that sounds fine right?

// let's check if we'd go with "must not match" instead:

// we cannot extend internal types with internal members
// anymore -> ups, that would be really strange
extension B {}

Adrian, I propose only that _explicit_ access modifiers on extensions
must not match the access level of the original type. This proposal does
not touch what happens inside extensions without explicit access modifiers.
I had to re-read SE-0025 a few times to fully understand its meaning.
According to SE-0025, "extensions with explicit access modifiers continue
to override the default `internal` access." This implies that members
declared inside an extension without an explicit access modifier will have
default `internal` access. Likewise, "the compiler will continue to warn on
overly broad access within an extension with an explicit access modifier."
This implies that, in the absence of an explicit access modifier, the
compiler will not warn about overly broad access inside types, just as it
will not for members inside types.

// same as for `public extension A`
fileprivate extension B {}

// same as for `public extension A`
private extension B {}

Just for the record we also check fileprivate and private:

fileprivate struct C {}

// "must not exceed" assumed

// no default access modifier means all
// extension member will folow the upper bound by
// the extended type -> fileprivate by default

(See discussion above.)

// -> members can be set to be `private` and only
// visible inside this extension scope
// -> equivalent to `fileprivate extension B {}`
// and `private extension C {}`
extension C {}

// "must not match" -> would break like it breaks the
// internal access model

See above. Nothing breaks.

``swift // at file scope acts likefileprivate` private struct D {}

// “must not exceed” assumed

// same as for fileprivate extension D {}

// “must not match” -> would break ```

Great compromise here!

This rule would preserve the possibility of using extensions as grouping
constructs. At the same time, it would (1) remove the possibility of
writing public extension to default the access level of members to public;

We still can group internal and fileprivate with this, but it’s okay I
guess.

Let’s re-check default protocol implementation:

public protocol G {
    func foo()
}

// currently we have 3 different ways to make them public
// #1
extension G {
    public func foo() { /* implement */ }
}

// #2
public extension G {
    func foo() { /* implement */ }
}

// #3
public extension G {
    public func foo() { /* implement */ }
}

// with "must not match" for `public` only #1 will work
// but everyone will need to repeat `public`
// no laziness for `public` anymore - hurray
extension G {
    public func foo() { /* implement */ }
}

// "must not exceed" doesn't solve the problem of `public` at all

The last topic is conformance to protocols:

public protocol P {}
internal protocol PI {}
fileprivate protocol PF {}
private protocol PP {}

public type Y {}

// "must not exceed" results in this, which is what it looks right now
extension Y : P {}

// just fine here
// we still can grant `PI` members visibility up to `public`
// the lower bound for these is `internal`
extension Y : PI {}

// same as `PI` but the lower bound is `fileprivate` now
extension Y : PF {}

// same as `PI` but the lower bound is `private` now
extension Y : PP {}

// this does not work atm.
// but should be allowed in general where we could grant visibility up to `internal`
internal extension Y : PI, PF, PP {}

fileprivate extension Y : PF, PP {}

There are a few more combinations I don’t want to type out here.

As I mentioned previously, since explicit access modifiers are not
currently permitted for conformance to protocols, any such change is
additive and I do not include it within the scope of this proposal.
My conclusion it this:

   -

   “must not match” does solve a few problems with public but only
   allows explicit internal, fileprivate and private usage, which is
   kinda odd. This is a new exceptional rule that must be documented.
   -

   “must not exceed” does not solve anything if it does not follow the
   typical public with default internal rule.
   1. With this exception it’s no more a default access modifier and
      totally useless on extensions, except if >>no access modifier<< would mean
      the upper bound is implicitly internal where you can’t grant
      visibility up to public and forced to use public extension if you
      wish to achieve this.
      2. With the exception in (1) we would need to allow access
      modifier on extension with protocol conformance to achieve the same result
      everywhere.
      3. With all that we’ll have to use #3 for default protocol
      implementations to make them public.

That said we’re end up with the same upper- lower bound access control
model on extension I proposed, even if my proposal title and some of my
writing there caused a lot of confusion.

--
Adrian Zubarev
Sent with Airmail

Am 18. Juli 2016 um 11:14:09, David Hart (david@hartbit.com) schrieb:

This compromise solution looks very good to me. Thanks Xiaodi for the
effort put into working through our whining to come to the best solution
IMHO.

On 18 Jul 2016, at 09:50, Xiaodi Wu via swift-evolution < >>> swift-evolution@swift.org> wrote:

All righty, thanks for all of your feedback. I've worked on revising the
proposal this evening, re-reading previous documents and messages and
re-analyzing what people meant. I think Jose is absolutely right in the
end, and the proposal has turned out like he suggested. Here is the current
draft below:

Harmonize access modifiers for extensions

   - Proposal: SE-XXXX
   <https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md>
   - Author: Xiaodi Wu <https://github.com/xwu>
   - Status: Awaiting review
   - Review manager: TBD

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#introduction>
Introduction

During discussion of SE-0119
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0119-extensions-access-modifiers.md>,
some voiced concern that writing public extension increases the default
access level for members declared within that extension, whereas writing public
class or public struct does not do the same.

This behavior is explained as follows: since extensions have no runtime
representation and are not first-class entities, access modifiers on
extensions serve as a shorthand to set the default access level for
members. Certain members of the community have indicated that such behavior
makes extensions a natural grouping construct.

A general principle of Swift, recently strengthened by proposals such as
SE-0117
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0117-non-public-subclassable-by-default.md>,
has been that public API commitments should require explicit opt-in. Given
the different behavior of classes and structs, the fact that extensions
allow public methods to be declared without spelling out public at the
declaration site has been called "confusing" or "odd."

The aim of this proposal is to, in as conservative a manner as possible,
require explicit use of public for public methods declared inside any
extension.

Swift-evolution threads:

   - [Proposal] Revising access modifiers on extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/022144.html>
   - [Review] SE-0119: Remove access modifiers from extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024224.html>
   - [Draft] Harmonize access modifiers for extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024522.html>

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#motivation>
Motivation

Consider the following:

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

public struct bar { }
public extension bar {
  func frobnicate() { } // public
}

This outcome is explained by rules regarding access modifiers
specifically on extensions Swift 2
<The Swift Programming Language: Redirect,
which is slated for preservation in Swift 3 as detailed in SE-0025
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0025-scoped-access-level.md>\.
However, it is arguably surprising that, of two declarations spelled
identically, one leads to a public API commitment while the other does not.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#proposed-solution>Proposed
solution

The proposed solution is to amend access modifier rules to eliminate the
possibility of defaulting the access level of members declared inside an
extension to public.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#detailed-design>Detailed
design

Amend access modifier rules as follows:

An extension may optionally be marked with an explicit access modifier
that specifies the default scope [see SE-0025]. However, such an explicit
modifier *must not match (or exceed) the original type's access level*.

This rule would preserve the possibility of using extensions as grouping
constructs. At the same time, it would (1) remove the possibility of
writing public extension to default the access level of members to
public; and (2) clarify the notion that an access modifier on an
extension is a shorthand and not a way to create a first-class entity by
disallowing repeating of the original type's access level.

*Explicit* access modifiers will continue to set the maximum allowed
access within an extension, as clarified in SE-0025.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#alternatives-considered>Alternatives
considered

One alternative is to eliminate explicit access modifiers on extensions
altogether. As an advantage, this would further clarify the mental model
that extensions are not their own first-class entities. As a disadvantage,
extensions cease to be an access modifier grouping construct, which some
users really like.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#acknowledgments>
Acknowledgments

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

On Sun, Jul 17, 2016 at 11:08 AM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:
I understand how it works.

By aligning access modifier rules inside extensions with those inside
types, all other modifiers would continue to work as it does now
(implicitly internal members would be limited by the upper bound). The only
change in this respect is removing the ability to have public API without
writing `public func`.

On Sun, Jul 17, 2016 at 11:01 Adrian Zubarev via swift-evolution < >>>> swift-evolution@swift.org> wrote:
I tackled it as an upper bound but highly rejected by the community.
That’s exactly what my proposal was all about. An upper boundary would be
more elegant, but I still see arguments ‘because it’s not a type’.

I could live without access modifiers on extensions in general.

The default access modifier rule permits public methods to be written
without public func

You meant this?

public extension SomeType {
    // I don't need to write public
    func foo() {}
    var computed: Type {}
}

This applies to all access modifiers which are not optional (like
internal):

public SomeType
fileprivate extension SomeType {
    // I don't need to repeat fileprivate
    func foo() {}
    var computed: Type {}
}

// which is more likely `fileprivate` because it's on file scope
private extension SomeType {
    // even if the inner access modifier would pretend to be private
    // since the extension is on filescope, everything will be `fileprivate`
    func foo() {}
    var computed: Type {}
}

--
Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 17:50:31, Xiaodi Wu (xiaodi.wu@gmail.com) schrieb:

The proposal is that the access modifier for an extension will either
be removed entirely or remain as an upper bound, losing its function as a
default access modifier. The default access modifier rule permits public
methods to be written without `public func`; this is a proposal to remove
that feature because it is a source of confusion.
On Sun, Jul 17, 2016 at 10:43 Adrian Zubarev via swift-evolution < >>>>> swift-evolution@swift.org> wrote:
I still don’t catch to point here. There is no implicit public
there. It’s explicit set by the default access modifier of extensions. It’s
how they work and how they should remain (at least as long the community
want default access modifier to exist on extensions). Disallowing
setting public on extensions when you extend a public type makes no
sense. If you want your member to be internal like it’s in types,
then remove the access modifier from extension and all member will follow
the type access modifier.

--
Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 17:37:02, Xiaodi Wu (xiaodi.wu@gmail.com)
schrieb:

That's a good point. I will incorporate these into a revised draft.
Only two things will change:
public struct Foo {
  // implicitly internal
  func frobnicate1() { }
}
public extension Foo {
  // currently implicitly public
  //
  // depending on which alternative is adopted,
  // the proposal will either prohibit `public extension`
  // or this method will be implicitly internal
  func frobnicate2() { }
}
internal struct Bar {
  // permitted by SE-0025 without a warning
  // this method can only be accessed within module anyway
  // because `internal struct` bounds access of its members
  public func frobnicate1() { }
}
extension Bar {
  // not permitted by SE-0025
  //
  // after proposal, this will also be permitted without a warning
  // and this method will also be accessible only within module
  public func frobnicate2() { }
}
On Sun, Jul 17, 2016 at 1:50 AM, Adrian Zubarev via swift-evolution < >>>>>> swift-evolution@swift.org> wrote:

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@swift.org) schrieb:

On Sat, Jul 16, 2016 at 7:56 PM, Jose Cheyo Jimenez < >>>>>>> cheyo@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@swift.org> wrote:

On Sat, Jul 16, 2016 at 6:10 PM, David Hart <david@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@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
   <https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md>
   - Author: Xiaodi Wu <https://github.com/xwu>
   - Status: Awaiting review
   - Review manager: TBD

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#introduction>
Introduction

During discussion of SE-0119
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0119-extensions-access-modifiers>,
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
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/022144.html>
   - [More to be added here]

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#motivation>
Motivation

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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#proposed-solution>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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#detailed-design>Detailed
design

   1. Declarations inside the extension will, like declarations
   inside types, have a default access level of internal.
   2. 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.
   3. An extension declared without an explicit access modifier
   will have the same access level as the type being extended.
   4. An extension declared without protocol conformance may
   optionally use an explicit access modifier to provide an upper bound for
   the visibility of its members.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#alternatives-considered>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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#acknowledgments>
Acknowledgments

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

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Nevin · July 18, 2016, 4:25pm

My understanding is that “being able to change access levels by
simply flipping a switch on the type itself” is a positive goal of Swift.

Therefore, it emphatically *should* be allowed to set the default
visibility for an extension the same as (or higher than!) the type it
extends. For example, a private type can be given a private extension, so
later making the type internal will preserve the privacy of the extension
members.

If any change is to be made in this area, it should simply be that “the
default access level of extension members is `internal`”, which means
public members of an extension must be explicitly marked public.

The access modifier on an extension would serve as an upper bound on
visibility for its members, so the default (implicit) modifier for
extensions would be “public”. Members of a public extension would be
implicitly internal, as noted above.

The visibility of a type is an upper bound on its members’ visibility as
well, so in a sense we would be making all access modifiers, including
those on member declarations, act as upper bounds. This allows designs like
the following:

[internal] struct makePublicInTheFuture { ... }
[public] extension makePublicInTheFuture {
  private var _p
  fileprivate var _f
  [internal] var _i
  public var x
  ...
}

Now if the struct’s visibility is changed, each extension member will be
accessible up to the maximum availability of the type, the extension, and
itself.

That seems like the most consistent approach to me.

However, I am still not convinced that any change is required at all. The
only “problem case” now is `public extension` making its member public by
default. I am not a framework designer, so I do not know how useful or
non-useful this may be.

But if any change is to be made, I think “extension members default to
internal” is the simplest.

All access modifier act as upper bounds, extensions are implicitly
`public`, and other declarations (including types) are implicitly
`internal`.

Then programmers can set access levels to “where they should be” in
preparation for “flipping the switch” on a type or extension’s visibility
in the future, and public APIs must be explicitly marked `public` at the
declaration site.

How does this sound?

Nevin

···

On Mon, Jul 18, 2016 at 11:32 AM, Xiaodi Wu via swift-evolution < swift-evolution@swift.org> wrote:

On Mon, Jul 18, 2016 at 6:09 AM, Adrian Zubarev via swift-evolution < > swift-evolution@swift.org> wrote:

+1 :)

Let’s analyze this: public > internal > fileprivate >= private

An extension may optionally be marked with an explicit access modifier
that specifies the default scope [see SE–0025]. However, such an explicit
modifier *must not match* (or exceed) the original type’s access level.

public struct A {}

// I assume that we go with "must not match" here!

// can't be public anymore -> no more
// <<implicitly>> public extension members
// -> breaking change -> I can live with that
extension A {}

// no default access modifier -> extension
// follows the access rule by the extended type A
// -> here every extension member is internal by default
// -> can be overridden to public member wise
extension A {}

// default access modifier acts as the upper bound
// inside an extended public type A
// -> every extension member are fileprivate
// -> extension member can be explicitly set to private
// -> these will be only visible inside this extension scope
fileprivate extension A {}

// at file scope `private` acts like `fileprivate`
// (if `private` is allowed at filescope) - haven't read the extended SE-0025 yet
// -> I assume any member that are explicitly set to private
// will only be visible inside this extension scope
private extension A {}

Let’s check internal types:

internal struct B {}

// "must not match" does not work here anymore
// do we use "must not exceed" here???

// I assume the second.

// doens't break anything
// works as before
// no default access modifier for internal types
// equals `internal extension A {}`
// members are default internal
// -> can be overridden to `fileprivate` or scope level `private`
extension B {}

// same as for `public extension A`
fileprivate extension B {}

// same as for `public extension A`
private extension B {}

// that sounds fine right?

// let's check if we'd go with "must not match" instead:

// we cannot extend internal types with internal members
// anymore -> ups, that would be really strange
extension B {}

Adrian, I propose only that _explicit_ access modifiers on extensions must
not match the access level of the original type. This proposal does not
touch what happens inside extensions without explicit access modifiers. I
had to re-read SE-0025 a few times to fully understand its meaning.
According to SE-0025, "extensions with explicit access modifiers continue
to override the default `internal` access." This implies that members
declared inside an extension without an explicit access modifier will have
default `internal` access. Likewise, "the compiler will continue to warn on
overly broad access within an extension with an explicit access modifier."
This implies that, in the absence of an explicit access modifier, the
compiler will not warn about overly broad access inside types, just as it
will not for members inside types.

// same as for `public extension A`
fileprivate extension B {}

// same as for `public extension A`
private extension B {}

Just for the record we also check fileprivate and private:

fileprivate struct C {}

// "must not exceed" assumed

// no default access modifier means all
// extension member will folow the upper bound by
// the extended type -> fileprivate by default

(See discussion above.)

// -> members can be set to be `private` and only
// visible inside this extension scope
// -> equivalent to `fileprivate extension B {}`
// and `private extension C {}`
extension C {}

// "must not match" -> would break like it breaks the
// internal access model

See above. Nothing breaks.

``swift // at file scope acts likefileprivate` private struct D {}

// “must not exceed” assumed

// same as for fileprivate extension D {}

// “must not match” -> would break ```

Great compromise here!

This rule would preserve the possibility of using extensions as grouping
constructs. At the same time, it would (1) remove the possibility of
writing public extension to default the access level of members to public;

We still can group internal and fileprivate with this, but it’s okay I
guess.

Let’s re-check default protocol implementation:

public protocol G {
    func foo()
}

// currently we have 3 different ways to make them public
// #1
extension G {
    public func foo() { /* implement */ }
}

// #2
public extension G {
    func foo() { /* implement */ }
}

// #3
public extension G {
    public func foo() { /* implement */ }
}

// with "must not match" for `public` only #1 will work
// but everyone will need to repeat `public`
// no laziness for `public` anymore - hurray
extension G {
    public func foo() { /* implement */ }
}

// "must not exceed" doesn't solve the problem of `public` at all

The last topic is conformance to protocols:

public protocol P {}
internal protocol PI {}
fileprivate protocol PF {}
private protocol PP {}

public type Y {}

// "must not exceed" results in this, which is what it looks right now
extension Y : P {}

// just fine here
// we still can grant `PI` members visibility up to `public`
// the lower bound for these is `internal`
extension Y : PI {}

// same as `PI` but the lower bound is `fileprivate` now
extension Y : PF {}

// same as `PI` but the lower bound is `private` now
extension Y : PP {}

// this does not work atm.
// but should be allowed in general where we could grant visibility up to `internal`
internal extension Y : PI, PF, PP {}

fileprivate extension Y : PF, PP {}

There are a few more combinations I don’t want to type out here.

As I mentioned previously, since explicit access modifiers are not
currently permitted for conformance to protocols, any such change is
additive and I do not include it within the scope of this proposal.
My conclusion it this:

   -

   “must not match” does solve a few problems with public but only
   allows explicit internal, fileprivate and private usage, which is
   kinda odd. This is a new exceptional rule that must be documented.
   -

   “must not exceed” does not solve anything if it does not follow the
   typical public with default internal rule.
   1. With this exception it’s no more a default access modifier and
      totally useless on extensions, except if >>no access modifier<< would mean
      the upper bound is implicitly internal where you can’t grant
      visibility up to public and forced to use public extension if you
      wish to achieve this.
      2. With the exception in (1) we would need to allow access
      modifier on extension with protocol conformance to achieve the same result
      everywhere.
      3. With all that we’ll have to use #3 for default protocol
      implementations to make them public.

That said we’re end up with the same upper- lower bound access control
model on extension I proposed, even if my proposal title and some of my
writing there caused a lot of confusion.

--
Adrian Zubarev
Sent with Airmail

Am 18. Juli 2016 um 11:14:09, David Hart (david@hartbit.com) schrieb:

This compromise solution looks very good to me. Thanks Xiaodi for the
effort put into working through our whining to come to the best solution
IMHO.

On 18 Jul 2016, at 09:50, Xiaodi Wu via swift-evolution < >> swift-evolution@swift.org> wrote:

All righty, thanks for all of your feedback. I've worked on revising the
proposal this evening, re-reading previous documents and messages and
re-analyzing what people meant. I think Jose is absolutely right in the
end, and the proposal has turned out like he suggested. Here is the current
draft below:

Harmonize access modifiers for extensions

   - Proposal: SE-XXXX
   <https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md>
   - Author: Xiaodi Wu <https://github.com/xwu>
   - Status: Awaiting review
   - Review manager: TBD

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#introduction>
Introduction

During discussion of SE-0119
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0119-extensions-access-modifiers.md>,
some voiced concern that writing public extension increases the default
access level for members declared within that extension, whereas writing public
class or public struct does not do the same.

This behavior is explained as follows: since extensions have no runtime
representation and are not first-class entities, access modifiers on
extensions serve as a shorthand to set the default access level for
members. Certain members of the community have indicated that such behavior
makes extensions a natural grouping construct.

A general principle of Swift, recently strengthened by proposals such as
SE-0117
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0117-non-public-subclassable-by-default.md>,
has been that public API commitments should require explicit opt-in. Given
the different behavior of classes and structs, the fact that extensions
allow public methods to be declared without spelling out public at the
declaration site has been called "confusing" or "odd."

The aim of this proposal is to, in as conservative a manner as possible,
require explicit use of public for public methods declared inside any
extension.

Swift-evolution threads:

   - [Proposal] Revising access modifiers on extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/022144.html>
   - [Review] SE-0119: Remove access modifiers from extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024224.html>
   - [Draft] Harmonize access modifiers for extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024522.html>

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#motivation>
Motivation

Consider the following:

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

public struct bar { }
public extension bar {
  func frobnicate() { } // public
}

This outcome is explained by rules regarding access modifiers
specifically on extensions Swift 2
<The Swift Programming Language: Redirect,
which is slated for preservation in Swift 3 as detailed in SE-0025
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0025-scoped-access-level.md>\.
However, it is arguably surprising that, of two declarations spelled
identically, one leads to a public API commitment while the other does not.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#proposed-solution>Proposed
solution

The proposed solution is to amend access modifier rules to eliminate the
possibility of defaulting the access level of members declared inside an
extension to public.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#detailed-design>Detailed
design

Amend access modifier rules as follows:

An extension may optionally be marked with an explicit access modifier
that specifies the default scope [see SE-0025]. However, such an explicit
modifier *must not match (or exceed) the original type's access level*.

This rule would preserve the possibility of using extensions as grouping
constructs. At the same time, it would (1) remove the possibility of
writing public extension to default the access level of members to public;
and (2) clarify the notion that an access modifier on an extension is a
shorthand and not a way to create a first-class entity by disallowing
repeating of the original type's access level.

*Explicit* access modifiers will continue to set the maximum allowed
access within an extension, as clarified in SE-0025.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#alternatives-considered>Alternatives
considered

One alternative is to eliminate explicit access modifiers on extensions
altogether. As an advantage, this would further clarify the mental model
that extensions are not their own first-class entities. As a disadvantage,
extensions cease to be an access modifier grouping construct, which some
users really like.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#acknowledgments>
Acknowledgments

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

On Sun, Jul 17, 2016 at 11:08 AM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:
I understand how it works.

By aligning access modifier rules inside extensions with those inside
types, all other modifiers would continue to work as it does now
(implicitly internal members would be limited by the upper bound). The only
change in this respect is removing the ability to have public API without
writing `public func`.

On Sun, Jul 17, 2016 at 11:01 Adrian Zubarev via swift-evolution < >>> swift-evolution@swift.org> wrote:
I tackled it as an upper bound but highly rejected by the community.
That’s exactly what my proposal was all about. An upper boundary would be
more elegant, but I still see arguments ‘because it’s not a type’.

I could live without access modifiers on extensions in general.

The default access modifier rule permits public methods to be written
without public func

You meant this?

public extension SomeType {
    // I don't need to write public
    func foo() {}
    var computed: Type {}
}

This applies to all access modifiers which are not optional (like
internal):

public SomeType
fileprivate extension SomeType {
    // I don't need to repeat fileprivate
    func foo() {}
    var computed: Type {}
}

// which is more likely `fileprivate` because it's on file scope
private extension SomeType {
    // even if the inner access modifier would pretend to be private
    // since the extension is on filescope, everything will be `fileprivate`
    func foo() {}
    var computed: Type {}
}

--
Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 17:50:31, Xiaodi Wu (xiaodi.wu@gmail.com) schrieb:

The proposal is that the access modifier for an extension will either
be removed entirely or remain as an upper bound, losing its function as a
default access modifier. The default access modifier rule permits public
methods to be written without `public func`; this is a proposal to remove
that feature because it is a source of confusion.
On Sun, Jul 17, 2016 at 10:43 Adrian Zubarev via swift-evolution < >>>> swift-evolution@swift.org> wrote:
I still don’t catch to point here. There is no implicit public there.
It’s explicit set by the default access modifier of extensions. It’s how
they work and how they should remain (at least as long the community want default
access modifier to exist on extensions). Disallowing setting public
on extensions when you extend a public type makes no sense. If you want
your member to be internal like it’s in types, then remove the access
modifier from extension and all member will follow the type access modifier.

--
Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 17:37:02, Xiaodi Wu (xiaodi.wu@gmail.com) schrieb:

That's a good point. I will incorporate these into a revised draft.
Only two things will change:
public struct Foo {
  // implicitly internal
  func frobnicate1() { }
}
public extension Foo {
  // currently implicitly public
  //
  // depending on which alternative is adopted,
  // the proposal will either prohibit `public extension`
  // or this method will be implicitly internal
  func frobnicate2() { }
}
internal struct Bar {
  // permitted by SE-0025 without a warning
  // this method can only be accessed within module anyway
  // because `internal struct` bounds access of its members
  public func frobnicate1() { }
}
extension Bar {
  // not permitted by SE-0025
  //
  // after proposal, this will also be permitted without a warning
  // and this method will also be accessible only within module
  public func frobnicate2() { }
}
On Sun, Jul 17, 2016 at 1:50 AM, Adrian Zubarev via swift-evolution < >>>>> swift-evolution@swift.org> wrote:

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@swift.org) schrieb:

On Sat, Jul 16, 2016 at 7:56 PM, Jose Cheyo Jimenez < >>>>>> cheyo@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@swift.org> wrote:

On Sat, Jul 16, 2016 at 6:10 PM, David Hart <david@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@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
   <https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md>
   - Author: Xiaodi Wu <https://github.com/xwu>
   - Status: Awaiting review
   - Review manager: TBD

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#introduction>
Introduction

During discussion of SE-0119
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0119-extensions-access-modifiers>,
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
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/022144.html>
   - [More to be added here]

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#motivation>
Motivation

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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#proposed-solution>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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#detailed-design>Detailed
design

   1. Declarations inside the extension will, like declarations
   inside types, have a default access level of internal.
   2. 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.
   3. An extension declared without an explicit access modifier
   will have the same access level as the type being extended.
   4. An extension declared without protocol conformance may
   optionally use an explicit access modifier to provide an upper bound for
   the visibility of its members.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#alternatives-considered>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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#acknowledgments>
Acknowledgments

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

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Nevin · July 18, 2016, 4:44pm

Now if the struct’s visibility is changed, each extension member will be
accessible up to the maximum availability of the type, the extension, and
itself.

…I of course meant “minimum” here.

Nevin

···

On Mon, Jul 18, 2016 at 12:25 PM, Nevin Brackett-Rozinsky < nevin.brackettrozinsky@gmail.com> wrote:

My understanding is that “being able to change access levels by
simply flipping a switch on the type itself” is a positive goal of Swift.

Therefore, it emphatically *should* be allowed to set the default
visibility for an extension the same as (or higher than!) the type it
extends. For example, a private type can be given a private extension, so
later making the type internal will preserve the privacy of the extension
members.

If any change is to be made in this area, it should simply be that “the
default access level of extension members is `internal`”, which means
public members of an extension must be explicitly marked public.

The access modifier on an extension would serve as an upper bound on
visibility for its members, so the default (implicit) modifier for
extensions would be “public”. Members of a public extension would be
implicitly internal, as noted above.

The visibility of a type is an upper bound on its members’ visibility as
well, so in a sense we would be making all access modifiers, including
those on member declarations, act as upper bounds. This allows designs like
the following:

[internal] struct makePublicInTheFuture { ... }
[public] extension makePublicInTheFuture {
  private var _p
  fileprivate var _f
  [internal] var _i
  public var x
  ...
}

Now if the struct’s visibility is changed, each extension member will be
accessible up to the maximum availability of the type, the extension, and
itself.

That seems like the most consistent approach to me.

However, I am still not convinced that any change is required at all. The
only “problem case” now is `public extension` making its member public by
default. I am not a framework designer, so I do not know how useful or
non-useful this may be.

But if any change is to be made, I think “extension members default to
internal” is the simplest.

All access modifier act as upper bounds, extensions are implicitly
`public`, and other declarations (including types) are implicitly
`internal`.

Then programmers can set access levels to “where they should be” in
preparation for “flipping the switch” on a type or extension’s visibility
in the future, and public APIs must be explicitly marked `public` at the
declaration site.

How does this sound?

Nevin

On Mon, Jul 18, 2016 at 11:32 AM, Xiaodi Wu via swift-evolution < > swift-evolution@swift.org> wrote:
On Mon, Jul 18, 2016 at 6:09 AM, Adrian Zubarev via swift-evolution < >> swift-evolution@swift.org> wrote:

+1 :)

Let’s analyze this: public > internal > fileprivate >= private

An extension may optionally be marked with an explicit access modifier
that specifies the default scope [see SE–0025]. However, such an explicit
modifier *must not match* (or exceed) the original type’s access level.

public struct A {}

// I assume that we go with "must not match" here!

// can't be public anymore -> no more
// <<implicitly>> public extension members
// -> breaking change -> I can live with that
extension A {}

// no default access modifier -> extension
// follows the access rule by the extended type A
// -> here every extension member is internal by default
// -> can be overridden to public member wise
extension A {}

// default access modifier acts as the upper bound
// inside an extended public type A
// -> every extension member are fileprivate
// -> extension member can be explicitly set to private
// -> these will be only visible inside this extension scope
fileprivate extension A {}

// at file scope `private` acts like `fileprivate`
// (if `private` is allowed at filescope) - haven't read the extended SE-0025 yet
// -> I assume any member that are explicitly set to private
// will only be visible inside this extension scope
private extension A {}

Let’s check internal types:

internal struct B {}

// "must not match" does not work here anymore
// do we use "must not exceed" here???

// I assume the second.

// doens't break anything
// works as before
// no default access modifier for internal types
// equals `internal extension A {}`
// members are default internal
// -> can be overridden to `fileprivate` or scope level `private`
extension B {}

// same as for `public extension A`
fileprivate extension B {}

// same as for `public extension A`
private extension B {}

// that sounds fine right?

// let's check if we'd go with "must not match" instead:

// we cannot extend internal types with internal members
// anymore -> ups, that would be really strange
extension B {}

Adrian, I propose only that _explicit_ access modifiers on extensions
must not match the access level of the original type. This proposal does
not touch what happens inside extensions without explicit access modifiers.
I had to re-read SE-0025 a few times to fully understand its meaning.
According to SE-0025, "extensions with explicit access modifiers continue
to override the default `internal` access." This implies that members
declared inside an extension without an explicit access modifier will have
default `internal` access. Likewise, "the compiler will continue to warn on
overly broad access within an extension with an explicit access modifier."
This implies that, in the absence of an explicit access modifier, the
compiler will not warn about overly broad access inside types, just as it
will not for members inside types.

// same as for `public extension A`
fileprivate extension B {}

// same as for `public extension A`
private extension B {}

Just for the record we also check fileprivate and private:

fileprivate struct C {}

// "must not exceed" assumed

// no default access modifier means all
// extension member will folow the upper bound by
// the extended type -> fileprivate by default

(See discussion above.)

// -> members can be set to be `private` and only
// visible inside this extension scope
// -> equivalent to `fileprivate extension B {}`
// and `private extension C {}`
extension C {}

// "must not match" -> would break like it breaks the
// internal access model

See above. Nothing breaks.

``swift // at file scope acts likefileprivate` private struct D {}

// “must not exceed” assumed

// same as for fileprivate extension D {}

// “must not match” -> would break ```

Great compromise here!

This rule would preserve the possibility of using extensions as grouping
constructs. At the same time, it would (1) remove the possibility of
writing public extension to default the access level of members to public;

We still can group internal and fileprivate with this, but it’s okay I
guess.

Let’s re-check default protocol implementation:

public protocol G {
    func foo()
}

// currently we have 3 different ways to make them public
// #1
extension G {
    public func foo() { /* implement */ }
}

// #2
public extension G {
    func foo() { /* implement */ }
}

// #3
public extension G {
    public func foo() { /* implement */ }
}

// with "must not match" for `public` only #1 will work
// but everyone will need to repeat `public`
// no laziness for `public` anymore - hurray
extension G {
    public func foo() { /* implement */ }
}

// "must not exceed" doesn't solve the problem of `public` at all

The last topic is conformance to protocols:

public protocol P {}
internal protocol PI {}
fileprivate protocol PF {}
private protocol PP {}

public type Y {}

// "must not exceed" results in this, which is what it looks right now
extension Y : P {}

// just fine here
// we still can grant `PI` members visibility up to `public`
// the lower bound for these is `internal`
extension Y : PI {}

// same as `PI` but the lower bound is `fileprivate` now
extension Y : PF {}

// same as `PI` but the lower bound is `private` now
extension Y : PP {}

// this does not work atm.
// but should be allowed in general where we could grant visibility up to `internal`
internal extension Y : PI, PF, PP {}

fileprivate extension Y : PF, PP {}

There are a few more combinations I don’t want to type out here.

As I mentioned previously, since explicit access modifiers are not
currently permitted for conformance to protocols, any such change is
additive and I do not include it within the scope of this proposal.
My conclusion it this:

   -

   “must not match” does solve a few problems with public but only
   allows explicit internal, fileprivate and private usage, which is
   kinda odd. This is a new exceptional rule that must be documented.
   -

   “must not exceed” does not solve anything if it does not follow the
   typical public with default internal rule.
   1. With this exception it’s no more a default access modifier and
      totally useless on extensions, except if >>no access modifier<< would mean
      the upper bound is implicitly internal where you can’t grant
      visibility up to public and forced to use public extension if you
      wish to achieve this.
      2. With the exception in (1) we would need to allow access
      modifier on extension with protocol conformance to achieve the same result
      everywhere.
      3. With all that we’ll have to use #3 for default protocol
      implementations to make them public.

That said we’re end up with the same upper- lower bound access control
model on extension I proposed, even if my proposal title and some of my
writing there caused a lot of confusion.

--
Adrian Zubarev
Sent with Airmail

Am 18. Juli 2016 um 11:14:09, David Hart (david@hartbit.com) schrieb:

This compromise solution looks very good to me. Thanks Xiaodi for the
effort put into working through our whining to come to the best solution
IMHO.

On 18 Jul 2016, at 09:50, Xiaodi Wu via swift-evolution < >>> swift-evolution@swift.org> wrote:

All righty, thanks for all of your feedback. I've worked on revising the
proposal this evening, re-reading previous documents and messages and
re-analyzing what people meant. I think Jose is absolutely right in the
end, and the proposal has turned out like he suggested. Here is the current
draft below:

Harmonize access modifiers for extensions

   - Proposal: SE-XXXX
   <https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md>
   - Author: Xiaodi Wu <https://github.com/xwu>
   - Status: Awaiting review
   - Review manager: TBD

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#introduction>
Introduction

During discussion of SE-0119
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0119-extensions-access-modifiers.md>,
some voiced concern that writing public extension increases the default
access level for members declared within that extension, whereas writing public
class or public struct does not do the same.

This behavior is explained as follows: since extensions have no runtime
representation and are not first-class entities, access modifiers on
extensions serve as a shorthand to set the default access level for
members. Certain members of the community have indicated that such behavior
makes extensions a natural grouping construct.

A general principle of Swift, recently strengthened by proposals such as
SE-0117
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0117-non-public-subclassable-by-default.md>,
has been that public API commitments should require explicit opt-in. Given
the different behavior of classes and structs, the fact that extensions
allow public methods to be declared without spelling out public at the
declaration site has been called "confusing" or "odd."

The aim of this proposal is to, in as conservative a manner as possible,
require explicit use of public for public methods declared inside any
extension.

Swift-evolution threads:

   - [Proposal] Revising access modifiers on extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/022144.html>
   - [Review] SE-0119: Remove access modifiers from extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024224.html>
   - [Draft] Harmonize access modifiers for extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024522.html>

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#motivation>
Motivation

Consider the following:

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

public struct bar { }
public extension bar {
  func frobnicate() { } // public
}

This outcome is explained by rules regarding access modifiers
specifically on extensions Swift 2
<The Swift Programming Language: Redirect,
which is slated for preservation in Swift 3 as detailed in SE-0025
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0025-scoped-access-level.md>\.
However, it is arguably surprising that, of two declarations spelled
identically, one leads to a public API commitment while the other does not.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#proposed-solution>Proposed
solution

The proposed solution is to amend access modifier rules to eliminate the
possibility of defaulting the access level of members declared inside an
extension to public.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#detailed-design>Detailed
design

Amend access modifier rules as follows:

An extension may optionally be marked with an explicit access modifier
that specifies the default scope [see SE-0025]. However, such an explicit
modifier *must not match (or exceed) the original type's access level*.

This rule would preserve the possibility of using extensions as grouping
constructs. At the same time, it would (1) remove the possibility of
writing public extension to default the access level of members to
public; and (2) clarify the notion that an access modifier on an
extension is a shorthand and not a way to create a first-class entity by
disallowing repeating of the original type's access level.

*Explicit* access modifiers will continue to set the maximum allowed
access within an extension, as clarified in SE-0025.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#alternatives-considered>Alternatives
considered

One alternative is to eliminate explicit access modifiers on extensions
altogether. As an advantage, this would further clarify the mental model
that extensions are not their own first-class entities. As a disadvantage,
extensions cease to be an access modifier grouping construct, which some
users really like.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#acknowledgments>
Acknowledgments

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

On Sun, Jul 17, 2016 at 11:08 AM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:
I understand how it works.

By aligning access modifier rules inside extensions with those inside
types, all other modifiers would continue to work as it does now
(implicitly internal members would be limited by the upper bound). The only
change in this respect is removing the ability to have public API without
writing `public func`.

On Sun, Jul 17, 2016 at 11:01 Adrian Zubarev via swift-evolution < >>>> swift-evolution@swift.org> wrote:
I tackled it as an upper bound but highly rejected by the community.
That’s exactly what my proposal was all about. An upper boundary would be
more elegant, but I still see arguments ‘because it’s not a type’.

I could live without access modifiers on extensions in general.

The default access modifier rule permits public methods to be written
without public func

You meant this?

public extension SomeType {
    // I don't need to write public
    func foo() {}
    var computed: Type {}
}

This applies to all access modifiers which are not optional (like
internal):

public SomeType
fileprivate extension SomeType {
    // I don't need to repeat fileprivate
    func foo() {}
    var computed: Type {}
}

// which is more likely `fileprivate` because it's on file scope
private extension SomeType {
    // even if the inner access modifier would pretend to be private
    // since the extension is on filescope, everything will be `fileprivate`
    func foo() {}
    var computed: Type {}
}

--
Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 17:50:31, Xiaodi Wu (xiaodi.wu@gmail.com) schrieb:

The proposal is that the access modifier for an extension will either
be removed entirely or remain as an upper bound, losing its function as a
default access modifier. The default access modifier rule permits public
methods to be written without `public func`; this is a proposal to remove
that feature because it is a source of confusion.
On Sun, Jul 17, 2016 at 10:43 Adrian Zubarev via swift-evolution < >>>>> swift-evolution@swift.org> wrote:
I still don’t catch to point here. There is no implicit public
there. It’s explicit set by the default access modifier of extensions. It’s
how they work and how they should remain (at least as long the community
want default access modifier to exist on extensions). Disallowing
setting public on extensions when you extend a public type makes no
sense. If you want your member to be internal like it’s in types,
then remove the access modifier from extension and all member will follow
the type access modifier.

--
Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 17:37:02, Xiaodi Wu (xiaodi.wu@gmail.com)
schrieb:

That's a good point. I will incorporate these into a revised draft.
Only two things will change:
public struct Foo {
  // implicitly internal
  func frobnicate1() { }
}
public extension Foo {
  // currently implicitly public
  //
  // depending on which alternative is adopted,
  // the proposal will either prohibit `public extension`
  // or this method will be implicitly internal
  func frobnicate2() { }
}
internal struct Bar {
  // permitted by SE-0025 without a warning
  // this method can only be accessed within module anyway
  // because `internal struct` bounds access of its members
  public func frobnicate1() { }
}
extension Bar {
  // not permitted by SE-0025
  //
  // after proposal, this will also be permitted without a warning
  // and this method will also be accessible only within module
  public func frobnicate2() { }
}
On Sun, Jul 17, 2016 at 1:50 AM, Adrian Zubarev via swift-evolution < >>>>>> swift-evolution@swift.org> wrote:

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@swift.org) schrieb:

On Sat, Jul 16, 2016 at 7:56 PM, Jose Cheyo Jimenez < >>>>>>> cheyo@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@swift.org> wrote:

On Sat, Jul 16, 2016 at 6:10 PM, David Hart <david@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@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
   <https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md>
   - Author: Xiaodi Wu <https://github.com/xwu>
   - Status: Awaiting review
   - Review manager: TBD

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#introduction>
Introduction

During discussion of SE-0119
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0119-extensions-access-modifiers>,
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
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/022144.html>
   - [More to be added here]

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#motivation>
Motivation

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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#proposed-solution>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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#detailed-design>Detailed
design

   1. Declarations inside the extension will, like declarations
   inside types, have a default access level of internal.
   2. 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.
   3. An extension declared without an explicit access modifier
   will have the same access level as the type being extended.
   4. An extension declared without protocol conformance may
   optionally use an explicit access modifier to provide an upper bound for
   the visibility of its members.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#alternatives-considered>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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#acknowledgments>
Acknowledgments

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

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Nevin · July 18, 2016, 6:24pm

I remember it differently. The opposition to SE-0119 “Remove access
modifiers from extensions” was primarily because the proposal itself was
malformed and, to quote you from the review thread, “doesn’t do what it
says on the tin (aka in the title)”.

In that thread Jordan Rose said “the proposal is unclear” and he personally
is against making “access modifiers on extensions set a maximum level of
access for members, like they do for types” because he doesn’t think
“people should think of extensions as first-class entities.”

However, the core team as a whole never weighed in on the matter, and Chris
Lattner included “Revising access modifiers on extensions” in the list of
“open Swift 3 design topics”.

If we had a proposal titled “Make all access modifiers act as upper bounds”
which explained that (since the follow-on of SE-0025) they already behave
this way on both types and members, and all that remains is to bring the
same behavior to extensions, I think it would gain substantial traction.

Notably, this approach does not involve thinking of extensions as
“first-class entities”, but simply as a grouping and extending mechanism.

The benefits include consistency and simplicity of the model, as well as
ensuring that public APIs must be marked `public` at the declaration site,
while still allowing “private extension” to work as expected.

If you and I both agree that this is the sensible approach, and it has
never actually been reviewed on its merits, and it addresses an open Swift
3 design topic, then I think we should push it forward.

So far the only opposition that has been voiced to this approach was a
claim that it encouraged “thinking of extensions as first-class entities”.
I disagree with that assessment, and I do not think we should let such an
opinion preemptively derail the cogent and straightforward answer.

All that said, I am still not convinced there is anything wrong with the
way things work now.

Nevin

···

On Mon, Jul 18, 2016 at 12:45 PM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:

On Mon, Jul 18, 2016 at 11:25 AM, Nevin Brackett-Rozinsky < > nevin.brackettrozinsky@gmail.com> wrote:

My understanding is that “being able to change access levels by
simply flipping a switch on the type itself” is a positive goal of Swift.

I agree.

Therefore, it emphatically *should* be allowed to set the default

visibility for an extension the same as (or higher than!) the type it
extends. For example, a private type can be given a private extension, so
later making the type internal will preserve the privacy of the extension
members.

As discussed in SE-0119, there is no such thing as a private extension as
a first-class entity. The default access modifier is *only* modifying the
members declared within; therefore, using the "flipping a switch" analogy,
it is not a switch but rather the default lightbulb; the only relevant
switch is the access modifier of the original type.

If any change is to be made in this area, it should simply be that “the

default access level of extension members is `internal`”, which means
public members of an extension must be explicitly marked public.

That requires no change, as it is already slated to be the rule after
SE-0025.

The access modifier on an extension would serve as an upper bound on
visibility for its members,

That approach was my first instinct as well, but it was rejected in
SE-0119.

so the default (implicit) modifier for extensions would be “public”.
Members of a public extension would be implicitly internal, as noted above.

The visibility of a type is an upper bound on its members’ visibility as
well, so in a sense we would be making all access modifiers, including
those on member declarations, act as upper bounds. This allows designs like
the following:

[internal] struct makePublicInTheFuture { ... }
[public] extension makePublicInTheFuture {
  private var _p
  fileprivate var _f
  [internal] var _i
  public var x
  ...
}

Now if the struct’s visibility is changed, each extension member will be
accessible up to the maximum availability of the type, the extension, and
itself.

That seems like the most consistent approach to me.

It took me a while to understand what Jordan and John were saying in the
review process for SE-0119, but I think I now understand. They and others
reject the upper bound approach because, unlike types, extensions are not
first-class entities. Therefore, because they do not exist, extensions
should not be able to set bounds.

However, I am still not convinced that any change is required at all. The
only “problem case” now is `public extension` making its member public by
default. I am not a framework designer, so I do not know how useful or
non-useful this may be.

But if any change is to be made, I think “extension members default to
internal” is the simplest.

If you look back, that was indeed my first draft of this proposal.
However, feedback has made it clear that only two options are potentially
acceptable: access modifiers on extensions can either be eliminated, or
they can continue to be a shorthand, but they cannot be harmonized to be
upper bounds.

All access modifier act as upper bounds, extensions are implicitly
`public`, and other declarations (including types) are implicitly
`internal`.

Then programmers can set access levels to “where they should be” in
preparation for “flipping the switch” on a type or extension’s visibility
in the future, and public APIs must be explicitly marked `public` at the
declaration site.

How does this sound?

It sounded great to me, but it was shot down in SE-0119 and it was
rejected again on the first draft of this proposal.
Nevin

On Mon, Jul 18, 2016 at 11:32 AM, Xiaodi Wu via swift-evolution < >> swift-evolution@swift.org> wrote:
On Mon, Jul 18, 2016 at 6:09 AM, Adrian Zubarev via swift-evolution < >>> swift-evolution@swift.org> wrote:

+1 :)

Let’s analyze this: public > internal > fileprivate >= private

An extension may optionally be marked with an explicit access modifier
that specifies the default scope [see SE–0025]. However, such an explicit
modifier *must not match* (or exceed) the original type’s access level.

public struct A {}

// I assume that we go with "must not match" here!

// can't be public anymore -> no more
// <<implicitly>> public extension members
// -> breaking change -> I can live with that
extension A {}

// no default access modifier -> extension
// follows the access rule by the extended type A
// -> here every extension member is internal by default
// -> can be overridden to public member wise
extension A {}

// default access modifier acts as the upper bound
// inside an extended public type A
// -> every extension member are fileprivate
// -> extension member can be explicitly set to private
// -> these will be only visible inside this extension scope
fileprivate extension A {}

// at file scope `private` acts like `fileprivate`
// (if `private` is allowed at filescope) - haven't read the extended SE-0025 yet
// -> I assume any member that are explicitly set to private
// will only be visible inside this extension scope
private extension A {}

Let’s check internal types:

internal struct B {}

// "must not match" does not work here anymore
// do we use "must not exceed" here???

// I assume the second.

// doens't break anything
// works as before
// no default access modifier for internal types
// equals `internal extension A {}`
// members are default internal
// -> can be overridden to `fileprivate` or scope level `private`
extension B {}

// same as for `public extension A`
fileprivate extension B {}

// same as for `public extension A`
private extension B {}

// that sounds fine right?

// let's check if we'd go with "must not match" instead:

// we cannot extend internal types with internal members
// anymore -> ups, that would be really strange
extension B {}

Adrian, I propose only that _explicit_ access modifiers on extensions
must not match the access level of the original type. This proposal does
not touch what happens inside extensions without explicit access modifiers.
I had to re-read SE-0025 a few times to fully understand its meaning.
According to SE-0025, "extensions with explicit access modifiers continue
to override the default `internal` access." This implies that members
declared inside an extension without an explicit access modifier will have
default `internal` access. Likewise, "the compiler will continue to warn on
overly broad access within an extension with an explicit access modifier."
This implies that, in the absence of an explicit access modifier, the
compiler will not warn about overly broad access inside types, just as it
will not for members inside types.

// same as for `public extension A`
fileprivate extension B {}

// same as for `public extension A`
private extension B {}

Just for the record we also check fileprivate and private:

fileprivate struct C {}

// "must not exceed" assumed

// no default access modifier means all
// extension member will folow the upper bound by
// the extended type -> fileprivate by default

(See discussion above.)

// -> members can be set to be `private` and only
// visible inside this extension scope
// -> equivalent to `fileprivate extension B {}`
// and `private extension C {}`
extension C {}

// "must not match" -> would break like it breaks the
// internal access model

See above. Nothing breaks.

``swift // at file scope acts likefileprivate` private struct D {}

// “must not exceed” assumed

// same as for fileprivate extension D {}

// “must not match” -> would break ```

Great compromise here!

This rule would preserve the possibility of using extensions as
grouping constructs. At the same time, it would (1) remove the possibility
of writing public extension to default the access level of members to
public;

We still can group internal and fileprivate with this, but it’s okay I
guess.

Let’s re-check default protocol implementation:

public protocol G {
    func foo()
}

// currently we have 3 different ways to make them public
// #1
extension G {
    public func foo() { /* implement */ }
}

// #2
public extension G {
    func foo() { /* implement */ }
}

// #3
public extension G {
    public func foo() { /* implement */ }
}

// with "must not match" for `public` only #1 will work
// but everyone will need to repeat `public`
// no laziness for `public` anymore - hurray
extension G {
    public func foo() { /* implement */ }
}

// "must not exceed" doesn't solve the problem of `public` at all

The last topic is conformance to protocols:

public protocol P {}
internal protocol PI {}
fileprivate protocol PF {}
private protocol PP {}

public type Y {}

// "must not exceed" results in this, which is what it looks right now
extension Y : P {}

// just fine here
// we still can grant `PI` members visibility up to `public`
// the lower bound for these is `internal`
extension Y : PI {}

// same as `PI` but the lower bound is `fileprivate` now
extension Y : PF {}

// same as `PI` but the lower bound is `private` now
extension Y : PP {}

// this does not work atm.
// but should be allowed in general where we could grant visibility up to `internal`
internal extension Y : PI, PF, PP {}

fileprivate extension Y : PF, PP {}

There are a few more combinations I don’t want to type out here.

As I mentioned previously, since explicit access modifiers are not
currently permitted for conformance to protocols, any such change is
additive and I do not include it within the scope of this proposal.
My conclusion it this:

   -

   “must not match” does solve a few problems with public but only
   allows explicit internal, fileprivate and private usage, which is
   kinda odd. This is a new exceptional rule that must be documented.
   -

   “must not exceed” does not solve anything if it does not follow the
   typical public with default internal rule.
   1. With this exception it’s no more a default access modifier and
      totally useless on extensions, except if >>no access modifier<< would mean
      the upper bound is implicitly internal where you can’t grant
      visibility up to public and forced to use public extension if
      you wish to achieve this.
      2. With the exception in (1) we would need to allow access
      modifier on extension with protocol conformance to achieve the same result
      everywhere.
      3. With all that we’ll have to use #3 for default protocol
      implementations to make them public.

That said we’re end up with the same upper- lower bound access control
model on extension I proposed, even if my proposal title and some of my
writing there caused a lot of confusion.

--
Adrian Zubarev
Sent with Airmail

Am 18. Juli 2016 um 11:14:09, David Hart (david@hartbit.com) schrieb:

This compromise solution looks very good to me. Thanks Xiaodi for the
effort put into working through our whining to come to the best solution
IMHO.

On 18 Jul 2016, at 09:50, Xiaodi Wu via swift-evolution < >>>> swift-evolution@swift.org> wrote:

All righty, thanks for all of your feedback. I've worked on revising
the proposal this evening, re-reading previous documents and messages and
re-analyzing what people meant. I think Jose is absolutely right in the
end, and the proposal has turned out like he suggested. Here is the current
draft below:

Harmonize access modifiers for extensions

   - Proposal: SE-XXXX
   <https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md>
   - Author: Xiaodi Wu <https://github.com/xwu>
   - Status: Awaiting review
   - Review manager: TBD

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#introduction>
Introduction

During discussion of SE-0119
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0119-extensions-access-modifiers.md>,
some voiced concern that writing public extension increases the
default access level for members declared within that extension, whereas
writing public class or public struct does not do the same.

This behavior is explained as follows: since extensions have no runtime
representation and are not first-class entities, access modifiers on
extensions serve as a shorthand to set the default access level for
members. Certain members of the community have indicated that such behavior
makes extensions a natural grouping construct.

A general principle of Swift, recently strengthened by proposals such
as SE-0117
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0117-non-public-subclassable-by-default.md>,
has been that public API commitments should require explicit opt-in. Given
the different behavior of classes and structs, the fact that extensions
allow public methods to be declared without spelling out public at the
declaration site has been called "confusing" or "odd."

The aim of this proposal is to, in as conservative a manner as
possible, require explicit use of public for public methods declared
inside any extension.

Swift-evolution threads:

   - [Proposal] Revising access modifiers on extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/022144.html>
   - [Review] SE-0119: Remove access modifiers from extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024224.html>
   - [Draft] Harmonize access modifiers for extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024522.html>

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#motivation>
Motivation

Consider the following:

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

public struct bar { }
public extension bar {
  func frobnicate() { } // public
}

This outcome is explained by rules regarding access modifiers
specifically on extensions Swift 2
<The Swift Programming Language: Redirect,
which is slated for preservation in Swift 3 as detailed in SE-0025
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0025-scoped-access-level.md>\.
However, it is arguably surprising that, of two declarations spelled
identically, one leads to a public API commitment while the other does not.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#proposed-solution>Proposed
solution

The proposed solution is to amend access modifier rules to eliminate
the possibility of defaulting the access level of members declared inside
an extension to public.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#detailed-design>Detailed
design

Amend access modifier rules as follows:

An extension may optionally be marked with an explicit access modifier
that specifies the default scope [see SE-0025]. However, such an explicit
modifier *must not match (or exceed) the original type's access level*.

This rule would preserve the possibility of using extensions as
grouping constructs. At the same time, it would (1) remove the possibility
of writing public extension to default the access level of members to
public; and (2) clarify the notion that an access modifier on an
extension is a shorthand and not a way to create a first-class entity by
disallowing repeating of the original type's access level.

*Explicit* access modifiers will continue to set the maximum allowed
access within an extension, as clarified in SE-0025.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#alternatives-considered>Alternatives
considered

One alternative is to eliminate explicit access modifiers on extensions
altogether. As an advantage, this would further clarify the mental model
that extensions are not their own first-class entities. As a disadvantage,
extensions cease to be an access modifier grouping construct, which some
users really like.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#acknowledgments>
Acknowledgments

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

On Sun, Jul 17, 2016 at 11:08 AM, Xiaodi Wu <xiaodi.wu@gmail.com> >>>> wrote:
I understand how it works.

By aligning access modifier rules inside extensions with those inside
types, all other modifiers would continue to work as it does now
(implicitly internal members would be limited by the upper bound). The only
change in this respect is removing the ability to have public API without
writing `public func`.

On Sun, Jul 17, 2016 at 11:01 Adrian Zubarev via swift-evolution < >>>>> swift-evolution@swift.org> wrote:
I tackled it as an upper bound but highly rejected by the community.
That’s exactly what my proposal was all about. An upper boundary would be
more elegant, but I still see arguments ‘because it’s not a type’.

I could live without access modifiers on extensions in general.

The default access modifier rule permits public methods to be written
without public func

You meant this?

public extension SomeType {
    // I don't need to write public
    func foo() {}
    var computed: Type {}
}

This applies to all access modifiers which are not optional (like
internal):

public SomeType
fileprivate extension SomeType {
    // I don't need to repeat fileprivate
    func foo() {}
    var computed: Type {}
}

// which is more likely `fileprivate` because it's on file scope
private extension SomeType {
    // even if the inner access modifier would pretend to be private
    // since the extension is on filescope, everything will be `fileprivate`
    func foo() {}
    var computed: Type {}
}

--
Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 17:50:31, Xiaodi Wu (xiaodi.wu@gmail.com)
schrieb:

The proposal is that the access modifier for an extension will either
be removed entirely or remain as an upper bound, losing its function as a
default access modifier. The default access modifier rule permits public
methods to be written without `public func`; this is a proposal to remove
that feature because it is a source of confusion.
On Sun, Jul 17, 2016 at 10:43 Adrian Zubarev via swift-evolution < >>>>>> swift-evolution@swift.org> wrote:
I still don’t catch to point here. There is no implicit public
there. It’s explicit set by the default access modifier of extensions. It’s
how they work and how they should remain (at least as long the community
want default access modifier to exist on extensions). Disallowing
setting public on extensions when you extend a public type makes no
sense. If you want your member to be internal like it’s in types,
then remove the access modifier from extension and all member will follow
the type access modifier.

--
Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 17:37:02, Xiaodi Wu (xiaodi.wu@gmail.com)
schrieb:

That's a good point. I will incorporate these into a revised draft.
Only two things will change:
public struct Foo {
  // implicitly internal
  func frobnicate1() { }
}
public extension Foo {
  // currently implicitly public
  //
  // depending on which alternative is adopted,
  // the proposal will either prohibit `public extension`
  // or this method will be implicitly internal
  func frobnicate2() { }
}
internal struct Bar {
  // permitted by SE-0025 without a warning
  // this method can only be accessed within module anyway
  // because `internal struct` bounds access of its members
  public func frobnicate1() { }
}
extension Bar {
  // not permitted by SE-0025
  //
  // after proposal, this will also be permitted without a warning
  // and this method will also be accessible only within module
  public func frobnicate2() { }
}
On Sun, Jul 17, 2016 at 1:50 AM, Adrian Zubarev via swift-evolution >>>>>>> <swift-evolution@swift.org> wrote:

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@swift.org) schrieb:

On Sat, Jul 16, 2016 at 7:56 PM, Jose Cheyo Jimenez < >>>>>>>> cheyo@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@swift.org> wrote:

On Sat, Jul 16, 2016 at 6:10 PM, David Hart <david@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@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
   <https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md>
   - Author: Xiaodi Wu <https://github.com/xwu>
   - Status: Awaiting review
   - Review manager: TBD

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#introduction>
Introduction

During discussion of SE-0119
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0119-extensions-access-modifiers>,
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
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/022144.html>
   - [More to be added here]

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#motivation>
Motivation

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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#proposed-solution>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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#detailed-design>Detailed
design

   1. Declarations inside the extension will, like declarations
   inside types, have a default access level of internal.
   2. 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.
   3. An extension declared without an explicit access modifier
   will have the same access level as the type being extended.
   4. An extension declared without protocol conformance may
   optionally use an explicit access modifier to provide an upper bound for
   the visibility of its members.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#alternatives-considered>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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#acknowledgments>
Acknowledgments

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

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xwu · July 18, 2016, 6:41pm

I remember it differently. The opposition to SE-0119 “Remove access
modifiers from extensions” was primarily because the proposal itself was
malformed and, to quote you from the review thread, “doesn’t do what it
says on the tin (aka in the title)”.

In that thread Jordan Rose said “the proposal is unclear” and he
personally is against making “access modifiers on extensions set a maximum
level of access for members, like they do for types” because he doesn’t
think “people should think of extensions as first-class entities.”

However, the core team as a whole never weighed in on the matter, and
Chris Lattner included “Revising access modifiers on extensions” in the
list of “open Swift 3 design topics”.

If we had a proposal titled “Make all access modifiers act as upper
bounds” which explained that (since the follow-on of SE-0025) they already
behave this way on both types and members, and all that remains is to bring
the same behavior to extensions, I think it would gain substantial traction.

Notably, this approach does not involve thinking of extensions as
“first-class entities”, but simply as a grouping and extending mechanism.

The benefits include consistency and simplicity of the model, as well as
ensuring that public APIs must be marked `public` at the declaration site,
while still allowing “private extension” to work as expected.

If you and I both agree that this is the sensible approach, and it has
never actually been reviewed on its merits, and it addresses an open Swift
3 design topic, then I think we should push it forward.

So far the only opposition that has been voiced to this approach was a
claim that it encouraged “thinking of extensions as first-class entities”.
I disagree with that assessment, and I do not think we should let such an
opinion preemptively derail the cogent and straightforward answer.

Sure; with the PR already in the queue, let's bring this up during review
(assuming there will be one). I, for one, think it is a cogent argument.

···

On Mon, Jul 18, 2016 at 1:24 PM, Nevin Brackett-Rozinsky < nevin.brackettrozinsky@gmail.com> wrote:

All that said, I am still not convinced there is anything wrong with the
way things work now.

Nevin

On Mon, Jul 18, 2016 at 12:45 PM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:
On Mon, Jul 18, 2016 at 11:25 AM, Nevin Brackett-Rozinsky < >> nevin.brackettrozinsky@gmail.com> wrote:

My understanding is that “being able to change access levels by
simply flipping a switch on the type itself” is a positive goal of Swift.

I agree.

Therefore, it emphatically *should* be allowed to set the default

visibility for an extension the same as (or higher than!) the type it
extends. For example, a private type can be given a private extension, so
later making the type internal will preserve the privacy of the extension
members.

As discussed in SE-0119, there is no such thing as a private extension as
a first-class entity. The default access modifier is *only* modifying the
members declared within; therefore, using the "flipping a switch" analogy,
it is not a switch but rather the default lightbulb; the only relevant
switch is the access modifier of the original type.

If any change is to be made in this area, it should simply be that “the

default access level of extension members is `internal`”, which means
public members of an extension must be explicitly marked public.

That requires no change, as it is already slated to be the rule after
SE-0025.

The access modifier on an extension would serve as an upper bound on
visibility for its members,

That approach was my first instinct as well, but it was rejected in
SE-0119.

so the default (implicit) modifier for extensions would be “public”.
Members of a public extension would be implicitly internal, as noted above.

The visibility of a type is an upper bound on its members’ visibility as
well, so in a sense we would be making all access modifiers, including
those on member declarations, act as upper bounds. This allows designs like
the following:

[internal] struct makePublicInTheFuture { ... }
[public] extension makePublicInTheFuture {
  private var _p
  fileprivate var _f
  [internal] var _i
  public var x
  ...
}

Now if the struct’s visibility is changed, each extension member will be
accessible up to the maximum availability of the type, the extension, and
itself.

That seems like the most consistent approach to me.

It took me a while to understand what Jordan and John were saying in the
review process for SE-0119, but I think I now understand. They and others
reject the upper bound approach because, unlike types, extensions are not
first-class entities. Therefore, because they do not exist, extensions
should not be able to set bounds.

However, I am still not convinced that any change is required at all.
The only “problem case” now is `public extension` making its member public
by default. I am not a framework designer, so I do not know how useful or
non-useful this may be.

But if any change is to be made, I think “extension members default to
internal” is the simplest.

If you look back, that was indeed my first draft of this proposal.
However, feedback has made it clear that only two options are potentially
acceptable: access modifiers on extensions can either be eliminated, or
they can continue to be a shorthand, but they cannot be harmonized to be
upper bounds.

All access modifier act as upper bounds, extensions are implicitly
`public`, and other declarations (including types) are implicitly
`internal`.

Then programmers can set access levels to “where they should be” in
preparation for “flipping the switch” on a type or extension’s visibility
in the future, and public APIs must be explicitly marked `public` at the
declaration site.

How does this sound?

It sounded great to me, but it was shot down in SE-0119 and it was
rejected again on the first draft of this proposal.
Nevin

On Mon, Jul 18, 2016 at 11:32 AM, Xiaodi Wu via swift-evolution < >>> swift-evolution@swift.org> wrote:
On Mon, Jul 18, 2016 at 6:09 AM, Adrian Zubarev via swift-evolution < >>>> swift-evolution@swift.org> wrote:

+1 :)

Let’s analyze this: public > internal > fileprivate >= private

An extension may optionally be marked with an explicit access modifier
that specifies the default scope [see SE–0025]. However, such an explicit
modifier *must not match* (or exceed) the original type’s access
level.

public struct A {}

// I assume that we go with "must not match" here!

// can't be public anymore -> no more
// <<implicitly>> public extension members
// -> breaking change -> I can live with that
extension A {}

// no default access modifier -> extension
// follows the access rule by the extended type A
// -> here every extension member is internal by default
// -> can be overridden to public member wise
extension A {}

// default access modifier acts as the upper bound
// inside an extended public type A
// -> every extension member are fileprivate
// -> extension member can be explicitly set to private
// -> these will be only visible inside this extension scope
fileprivate extension A {}

// at file scope `private` acts like `fileprivate`
// (if `private` is allowed at filescope) - haven't read the extended SE-0025 yet
// -> I assume any member that are explicitly set to private
// will only be visible inside this extension scope
private extension A {}

Let’s check internal types:

internal struct B {}

// "must not match" does not work here anymore
// do we use "must not exceed" here???

// I assume the second.

// doens't break anything
// works as before
// no default access modifier for internal types
// equals `internal extension A {}`
// members are default internal
// -> can be overridden to `fileprivate` or scope level `private`
extension B {}

// same as for `public extension A`
fileprivate extension B {}

// same as for `public extension A`
private extension B {}

// that sounds fine right?

// let's check if we'd go with "must not match" instead:

// we cannot extend internal types with internal members
// anymore -> ups, that would be really strange
extension B {}

Adrian, I propose only that _explicit_ access modifiers on extensions
must not match the access level of the original type. This proposal does
not touch what happens inside extensions without explicit access modifiers.
I had to re-read SE-0025 a few times to fully understand its meaning.
According to SE-0025, "extensions with explicit access modifiers continue
to override the default `internal` access." This implies that members
declared inside an extension without an explicit access modifier will have
default `internal` access. Likewise, "the compiler will continue to warn on
overly broad access within an extension with an explicit access modifier."
This implies that, in the absence of an explicit access modifier, the
compiler will not warn about overly broad access inside types, just as it
will not for members inside types.

// same as for `public extension A`
fileprivate extension B {}

// same as for `public extension A`
private extension B {}

Just for the record we also check fileprivate and private:

fileprivate struct C {}

// "must not exceed" assumed

// no default access modifier means all
// extension member will folow the upper bound by
// the extended type -> fileprivate by default

(See discussion above.)

// -> members can be set to be `private` and only
// visible inside this extension scope
// -> equivalent to `fileprivate extension B {}`
// and `private extension C {}`
extension C {}

// "must not match" -> would break like it breaks the
// internal access model

See above. Nothing breaks.

``swift // at file scope acts likefileprivate` private struct D {}

// “must not exceed” assumed

// same as for fileprivate extension D {}

// “must not match” -> would break ```

Great compromise here!

This rule would preserve the possibility of using extensions as
grouping constructs. At the same time, it would (1) remove the possibility
of writing public extension to default the access level of members to
public;

We still can group internal and fileprivate with this, but it’s okay
I guess.

Let’s re-check default protocol implementation:

public protocol G {
    func foo()
}

// currently we have 3 different ways to make them public
// #1
extension G {
    public func foo() { /* implement */ }
}

// #2
public extension G {
    func foo() { /* implement */ }
}

// #3
public extension G {
    public func foo() { /* implement */ }
}

// with "must not match" for `public` only #1 will work
// but everyone will need to repeat `public`
// no laziness for `public` anymore - hurray
extension G {
    public func foo() { /* implement */ }
}

// "must not exceed" doesn't solve the problem of `public` at all

The last topic is conformance to protocols:

public protocol P {}
internal protocol PI {}
fileprivate protocol PF {}
private protocol PP {}

public type Y {}

// "must not exceed" results in this, which is what it looks right now
extension Y : P {}

// just fine here
// we still can grant `PI` members visibility up to `public`
// the lower bound for these is `internal`
extension Y : PI {}

// same as `PI` but the lower bound is `fileprivate` now
extension Y : PF {}

// same as `PI` but the lower bound is `private` now
extension Y : PP {}

// this does not work atm.
// but should be allowed in general where we could grant visibility up to `internal`
internal extension Y : PI, PF, PP {}

fileprivate extension Y : PF, PP {}

There are a few more combinations I don’t want to type out here.

As I mentioned previously, since explicit access modifiers are not
currently permitted for conformance to protocols, any such change is
additive and I do not include it within the scope of this proposal.
My conclusion it this:

   -

   “must not match” does solve a few problems with public but only
   allows explicit internal, fileprivate and private usage, which is
   kinda odd. This is a new exceptional rule that must be documented.
   -

   “must not exceed” does not solve anything if it does not follow
   the typical public with default internal rule.
   1. With this exception it’s no more a default access modifier and
      totally useless on extensions, except if >>no access modifier<< would mean
      the upper bound is implicitly internal where you can’t grant
      visibility up to public and forced to use public extension if
      you wish to achieve this.
      2. With the exception in (1) we would need to allow access
      modifier on extension with protocol conformance to achieve the same result
      everywhere.
      3. With all that we’ll have to use #3 for default protocol
      implementations to make them public.

That said we’re end up with the same upper- lower bound access control
model on extension I proposed, even if my proposal title and some of my
writing there caused a lot of confusion.

--
Adrian Zubarev
Sent with Airmail

Am 18. Juli 2016 um 11:14:09, David Hart (david@hartbit.com) schrieb:

This compromise solution looks very good to me. Thanks Xiaodi for the
effort put into working through our whining to come to the best solution
IMHO.

On 18 Jul 2016, at 09:50, Xiaodi Wu via swift-evolution < >>>>> swift-evolution@swift.org> wrote:

All righty, thanks for all of your feedback. I've worked on revising
the proposal this evening, re-reading previous documents and messages and
re-analyzing what people meant. I think Jose is absolutely right in the
end, and the proposal has turned out like he suggested. Here is the current
draft below:

Harmonize access modifiers for extensions

   - Proposal: SE-XXXX
   <https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md>
   - Author: Xiaodi Wu <https://github.com/xwu>
   - Status: Awaiting review
   - Review manager: TBD

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#introduction>
Introduction

During discussion of SE-0119
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0119-extensions-access-modifiers.md>,
some voiced concern that writing public extension increases the
default access level for members declared within that extension, whereas
writing public class or public struct does not do the same.

This behavior is explained as follows: since extensions have no
runtime representation and are not first-class entities, access modifiers
on extensions serve as a shorthand to set the default access level for
members. Certain members of the community have indicated that such behavior
makes extensions a natural grouping construct.

A general principle of Swift, recently strengthened by proposals such
as SE-0117
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0117-non-public-subclassable-by-default.md>,
has been that public API commitments should require explicit opt-in. Given
the different behavior of classes and structs, the fact that extensions
allow public methods to be declared without spelling out public at
the declaration site has been called "confusing" or "odd."

The aim of this proposal is to, in as conservative a manner as
possible, require explicit use of public for public methods declared
inside any extension.

Swift-evolution threads:

   - [Proposal] Revising access modifiers on extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/022144.html>
   - [Review] SE-0119: Remove access modifiers from extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024224.html>
   - [Draft] Harmonize access modifiers for extensions
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024522.html>

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#motivation>
Motivation

Consider the following:

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

public struct bar { }
public extension bar {
  func frobnicate() { } // public
}

This outcome is explained by rules regarding access modifiers
specifically on extensions Swift 2
<The Swift Programming Language: Redirect,
which is slated for preservation in Swift 3 as detailed in SE-0025
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0025-scoped-access-level.md>\.
However, it is arguably surprising that, of two declarations spelled
identically, one leads to a public API commitment while the other does not.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#proposed-solution>Proposed
solution

The proposed solution is to amend access modifier rules to eliminate
the possibility of defaulting the access level of members declared inside
an extension to public.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#detailed-design>Detailed
design

Amend access modifier rules as follows:

An extension may optionally be marked with an explicit access modifier
that specifies the default scope [see SE-0025]. However, such an explicit
modifier *must not match (or exceed) the original type's access level*
.

This rule would preserve the possibility of using extensions as
grouping constructs. At the same time, it would (1) remove the possibility
of writing public extension to default the access level of members to
public; and (2) clarify the notion that an access modifier on an
extension is a shorthand and not a way to create a first-class entity by
disallowing repeating of the original type's access level.

*Explicit* access modifiers will continue to set the maximum allowed
access within an extension, as clarified in SE-0025.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#alternatives-considered>Alternatives
considered

One alternative is to eliminate explicit access modifiers on
extensions altogether. As an advantage, this would further clarify the
mental model that extensions are not their own first-class entities. As a
disadvantage, extensions cease to be an access modifier grouping construct,
which some users really like.

<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md#acknowledgments>
Acknowledgments

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

On Sun, Jul 17, 2016 at 11:08 AM, Xiaodi Wu <xiaodi.wu@gmail.com> >>>>> wrote:
I understand how it works.

By aligning access modifier rules inside extensions with those inside
types, all other modifiers would continue to work as it does now
(implicitly internal members would be limited by the upper bound). The only
change in this respect is removing the ability to have public API without
writing `public func`.

On Sun, Jul 17, 2016 at 11:01 Adrian Zubarev via swift-evolution < >>>>>> swift-evolution@swift.org> wrote:
I tackled it as an upper bound but highly rejected by the community.
That’s exactly what my proposal was all about. An upper boundary would be
more elegant, but I still see arguments ‘because it’s not a type’.

I could live without access modifiers on extensions in general.

The default access modifier rule permits public methods to be
written without public func

You meant this?

public extension SomeType {
    // I don't need to write public
    func foo() {}
    var computed: Type {}
}

This applies to all access modifiers which are not optional (like
internal):

public SomeType
fileprivate extension SomeType {
    // I don't need to repeat fileprivate
    func foo() {}
    var computed: Type {}
}

// which is more likely `fileprivate` because it's on file scope
private extension SomeType {
    // even if the inner access modifier would pretend to be private
    // since the extension is on filescope, everything will be `fileprivate`
    func foo() {}
    var computed: Type {}
}

--
Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 17:50:31, Xiaodi Wu (xiaodi.wu@gmail.com)
schrieb:

The proposal is that the access modifier for an extension will
either be removed entirely or remain as an upper bound, losing its function
as a default access modifier. The default access modifier rule permits
public methods to be written without `public func`; this is a proposal to
remove that feature because it is a source of confusion.
On Sun, Jul 17, 2016 at 10:43 Adrian Zubarev via swift-evolution < >>>>>>> swift-evolution@swift.org> wrote:
I still don’t catch to point here. There is no implicit public
there. It’s explicit set by the default access modifier of extensions. It’s
how they work and how they should remain (at least as long the community
want default access modifier to exist on extensions). Disallowing
setting public on extensions when you extend a public type makes
no sense. If you want your member to be internal like it’s in
types, then remove the access modifier from extension and all member will
follow the type access modifier.

--
Adrian Zubarev
Sent with Airmail

Am 17. Juli 2016 um 17:37:02, Xiaodi Wu (xiaodi.wu@gmail.com)
schrieb:

That's a good point. I will incorporate these into a revised draft.
Only two things will change:
public struct Foo {
  // implicitly internal
  func frobnicate1() { }
}
public extension Foo {
  // currently implicitly public
  //
  // depending on which alternative is adopted,
  // the proposal will either prohibit `public extension`
  // or this method will be implicitly internal
  func frobnicate2() { }
}
internal struct Bar {
  // permitted by SE-0025 without a warning
  // this method can only be accessed within module anyway
  // because `internal struct` bounds access of its members
  public func frobnicate1() { }
}
extension Bar {
  // not permitted by SE-0025
  //
  // after proposal, this will also be permitted without a warning
  // and this method will also be accessible only within module
  public func frobnicate2() { }
}
On Sun, Jul 17, 2016 at 1:50 AM, Adrian Zubarev via swift-evolution >>>>>>>> <swift-evolution@swift.org> wrote:

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@swift.org) schrieb:

On Sat, Jul 16, 2016 at 7:56 PM, Jose Cheyo Jimenez < >>>>>>>>> cheyo@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@swift.org> wrote:

On Sat, Jul 16, 2016 at 6:10 PM, David Hart <david@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@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
   <https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/XXXX-harmonize-access-modifiers.md>
   - Author: Xiaodi Wu <https://github.com/xwu>
   - Status: Awaiting review
   - Review manager: TBD

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#introduction>
Introduction

During discussion of SE-0119
<https://github.com/xwu/swift-evolution/blob/harmonize-access-modifiers/proposals/0119-extensions-access-modifiers>,
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
   <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/022144.html>
   - [More to be added here]

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#motivation>
Motivation

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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#proposed-solution>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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#detailed-design>Detailed
design

   1. Declarations inside the extension will, like declarations
   inside types, have a default access level of internal.
   2. 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.
   3. An extension declared without an explicit access modifier
   will have the same access level as the type being extended.
   4. An extension declared without protocol conformance may
   optionally use an explicit access modifier to provide an upper bound for
   the visibility of its members.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#alternatives-considered>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.

<https://github.com/xwu/swift-evolution/tree/harmonize-access-modifiers#acknowledgments>
Acknowledgments

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

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