[swift-evolution] [Draft] Fix Private Access Levels

Matthew Johnson matthew at anandabits.com
Tue Feb 21 08:48:48 CST 2017

Sent from my iPhone

> On Feb 21, 2017, at 7:51 AM, Vladimir.S via swift-evolution <swift-evolution at swift.org> wrote:
> FWIW, I do think "Solution 2: Rename the scoped access level to scoped" is the best solution. It has(like current 'private') a value of protecting (even from yourself) with compiler's help access to very internal details of type, which must not be accessed anywhere else for ex. by mistake/autocompletion, even in extension/in subclass/in the same file.
> Even having at some moment submodules in Swift(where we can use 'internal' to provide details for extensions in other files, but at the same time hide details outside of submodule), IMO we need 'scoped' access level inside submodule for the same reasons.
> Also, 'scoped' can be extended in future, like scoped(subtype), scoped(extension), scoped(futureScope) or other variants (for example to represent some kind of 'protected' if we'll decide to have it).
> I'd suggest add this to the Solution 2 proposal: 'scoped' access modifier can not be used (how to describe this better...) in global scope i.e. outside of type declaration, to remove one point of ambiguity.
> So, there should be no possibility to mark type/func/var as scoped like here:
> // File.swift
> scoped class MyClass {} // not allowed, only private allowed here
> private class MyClass {} // ok, "private to file"
> And IMO it is weird to change access modifiers in Swift3, provide new scoped access, and then in Swift4, not just rename but remove that access level. New access level was massively discussed, it was not accepted fast or without considerations of other directions. Many found current 'private' helpful in code organization, they already built their rules/styles with using of this access level. Someone who don't like it - just don't use it.
> Currently we can refine "base" access modifiers(revert to public/internal/private) and keep new access level('scoped'). With the perspective to have more granulated scoped levels. This is the right direction I believe.
> (personally I still think we additionally need "the same file and subclasses/extensions in other files inside the module" access level, which has a value of distributing coupled code by a number of files, to not keep all such code in the same huge file, which can be accessed by a number of devs in team. Matthew, didn't you considered this direction also?)

Now that the submodules discussion is active I'm going to share my thoughts on it, including how I see `scoped` playing a role, as soon as I have a chance to write them up.  It will take a day or two to do this.  I'm pretty busy right now and want to relate my ideas to other ideas that have been shared thus far.  Stay tuned!

>> On 21.02.2017 9:58, David Hart via swift-evolution wrote:
>> Hello list,
>> Matthew Johnson and I have been putting our proposals together towards a
>> joint “let’s fix private access levels” proposal. As the community seems
>> quite divided on the issue, we offer two solutions in our proposal to let
>> the community debate and to let the core team make the final decision.
>> I’d like to concentrate this round of feedback on the quality of the
>> proposal, and not on the merits of Solution 1 or 2. thoughts?
>> https://github.com/hartbit/swift-evolution/blob/fix-private-access-levels/proposals/XXXX-fix-private-access-levels.md
>> David.
>>  Fix Private Access Levels
>>  * Proposal: SE-XXXX
>>    <https://github.com/hartbit/swift-evolution/blob/fix-private-access-levels/proposals>
>>  * Authors: David Hart <http://github.com/hartbit>, Matthew Johnson
>>    <https://github.com/anandabits>
>>  * Review Manager: TBD
>>  * Status: TBD
>>    <https://github.com/hartbit/swift-evolution/tree/fix-private-access-levels#introduction>Introduction
>> This proposal presents the problems the came with the the access level
>> modifications in SE-0025
>> <https://github.com/apple/swift-evolution/blob/master/proposals/0025-scoped-access-level.md> and
>> presents two community driven solutions to fix them. As a consensus will
>> not easily emerge, this proposal will allow a last round of voting and let
>> the core team decide. Once that is done, this proposal will be ammended to
>> describe the chosen solution.
>>    <https://github.com/hartbit/swift-evolution/tree/fix-private-access-levels#motivation>Motivation
>> Since the release of Swift 3, the access level change of SE-0025 was met
>> with dissatisfaction by a substantial proportion of the general Swift
>> community. Before offering solutions, lets discuss how and why it can be
>> viewed as /actiely harmful/, the new requirement for syntax/API changes.
>>      <https://github.com/hartbit/swift-evolution/tree/fix-private-access-levels#criticisms-of-se-0025>Criticisms
>>      of SE-0025
>> There are two primary criticism that have been offered.
>> The first is that |private| is a "soft default" access modifier for
>> restricting access within a file. Scoped access is not a good behavior for
>> a "soft default" because it is extremely common to use several extensions
>> within a file. A "soft default" (and therefore |private|) should work well
>> with this idiom. It is fair to say that changing the behavior
>> of |private| such that it does not work well with extensions meets the
>> criteria of actively harmful in the sense that it subtly encourages overuse
>> of scoped access control and discourages the more reasonable default by
>> giving it the awkward name |fileprivate|.
>> The second is that Swift's system of access control is too complex. Many
>> people feel like restricting access control to scopes less than a file is
>> of dubious value and therefore wish to simplify Swift's access control
>> story by removing scoped access. However, there are many others who like
>> the ability to have the compiler verify tighter access levels and believe
>> it helps make it easier to reason about code which is protecting invariants.
>>    <https://github.com/hartbit/swift-evolution/tree/fix-private-access-levels#detailed-design>Detailed
>>    design
>> Both authors agree that the |private| keyword should be reverted back to
>> its Swift 2 file-based meaning, resolving the first criticism. But the
>> authors disagree on what should be done about the scoped access level and
>> the following solutions represent the two main opinions in the community:
>>      <https://github.com/hartbit/swift-evolution/tree/fix-private-access-levels#solution-1-remove-the-scoped-access-level>Solution
>>      1: Remove the scoped access level
>> Compared to a file-based access level, the scoped-based access level adds
>> meaningful information by hiding implementation details which do not
>> concern other types or extensions in the same file. But is that distinction
>> between |private| and |fileprivate| actively used by the larger community
>> of Swift developers? And if it were used pervasively, would it be worth the
>> cognitive load and complexity of keeping two very similar access levels in
>> the language? This solution argues that answer to both questions is no and
>> that the scoped access level should be removed to resolve the complexity
>> criticism.
>> This solution has the added advantage of leaving the most design
>> breathing-room for future discussions about access levels in regards to
>> submodules.
>>      <https://github.com/hartbit/swift-evolution/tree/fix-private-access-levels#solution-2-rename-the-scoped-access-level-to-scoped>Solution
>>      2: Rename the scoped access level to |scoped|
>> It is difficult to make the case that a feature which a nontrivial number
>> of Swift users find valuable and which is easy for teams to avoid is
>> actively harmful. It seems like something that falls more into the category
>> of a debate over style (which could be addressed by a linter). Should we
>> remove a feature whose utility is a question of style, but is not actively
>> harmful in the sense of causing programmer error? The second solution
>> argues against it and proposes renaming it to |scoped|.
>> The |scoped| keyword is a good choice not only because the community has
>> been calling this feature “scoped access control” all along, but also
>> because the principle underlying all of Swift’s access levels is the idea
>> of a scope.
>>    <https://github.com/hartbit/swift-evolution/tree/fix-private-access-levels#source-compatibility>Source
>>    compatibility
>> In Swift 3 compatibility mode, the compiler will continue to
>> treat |private| and |fileprivate| as was previously the case.
>> In Swift 4 mode, the compiler will deprecate the |fileprivate| keyword and
>> revert the semantics of the |private| access level to be file based. The
>> migrator will rename all uses of |fileprivate| to |private|. In solution 2,
>> the migrator will also rename all uses of |private| to |scoped|.
>> With solution 1 (and with solution 2 if the migrator is not run), cases
>> where a type had |private| declarations with the same signature in
>> different scopes will produce a compiler error. For example, the following
>> piece of code compiles in Swift 3 compatibilty mode but generates
>> a |Invalid redeclaration of 'foo()'| error in Swift 4 mode.
>> struct Foo {
>>    private func bar() {}
>> }
>> extension Foo {
>>    private func bar() {}
>> }
>>    <https://github.com/hartbit/swift-evolution/tree/fix-private-access-levels#alternatives-considered>Alternatives
>>    Considered
>> 1. Deprecate |fileprivate| and modify the semantics of |private| to
>>    include same-type extension scopes in the same file.
>> 2. Deprecate |fileprivate| and modify the semantics of |private| to
>>    include same-type extension scopes in the same module.
>> The alternatives are potentially interesting but completely remove the file
>> access level while making the new |private|access level more complicated to
>> explain and understand.
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