[swift-evolution] access control proposal

Ilya ilya at harmonicsense.com
Mon Dec 7 13:00:09 CST 2015

Yes, that's a great idea!

Ilya Belenkiy
On Mon, Dec 7, 2015 at 1:35 PM Matthew Johnson <matthew at anandabits.com>

> I think what you are really trying to accomplish is to restrict access to
> a member to the scope in which it is declared, which just happens to be a
> type declaration.
> By looking at it in terms of scope the feature would be come more
> generally applicable.  For example, you could have a 'classified' member in
> an extension which is only visible inside the body of the extension.  This
> would be useful if the extension contains helper methods as part of its
> implementation.
> I would absolutely support extending Swift's scope-based access control to
> allow for more granular control of visibility within a file by extending it
> to syntactic scopes.  It seems like a pretty natural enhancement to the
> existing feature.
> Sent from my iPad
> On Dec 7, 2015, at 11:02 AM, Ilya <ilya at harmonicsense.com> wrote:
> If it is not allowed in extensions or subclasses (and it shouldn't), the
> model is still very simple.
> I understand the current model, but I think that there is a lot of value
> in getting the compiler's help in ensuring that class internals cannot be
> broken by anything outside the class.
> The current model doesn't solve this problem well. Putting everything in
> separate files seems like a very unfortunate workaround.
> --
> Ilya Belenkiy
> On Mon, Dec 7, 2015 at 9:41 AM Matthew Johnson <matthew at anandabits.com>
> wrote:
>> I was not sure how I felt about Swift's access control when it was first
>> announced but it didn't take long for me to believe it strikes a pretty
>> good balance between complexity and ability to hide implementation details.
>> It follows a very straightforward scope-driven strategy.  If it is
>> extended further I would like to see any enhancements follow the same
>> scope-driven strategy rather than being type-driven as is seen in other
>> languages.  Proposals along these lines might be interesting to consider
>> and might be able to accomplish what you are hoping to accomplish while
>> affording additional flexibility.
>> In a language where distributing the implementation of a type across
>> several extensions sometimes in different files a type-driven access
>> control model doesn't make a lot of sense to me.  I think it has a lot of
>> potential to be confusing.
>> What would it mean to limit access to a member to the type itself in
>> Swift?  Would an extensions in the same (or different) files be able to see
>> the member?  What about extensions in a different module?  And what about
>> subclasses? Subclasses probably wouldn't have visibility to them, but any
>> discussion of something like this will probably lead to discussion of
>> protected as well.
>> Sent from my iPad
>> On Dec 7, 2015, at 7:45 AM, Ilya via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> Also, one problem with the current approach is that it's all or nothing.
>> The classes in one file see everything from the other classes, and it may
>> be useful to limit that and share only some additional APIs with those
>> related classes, but not all of the implementation details / internal
>> state, while still hiding those same APIs from unrelated classes.
>> Currently, there is no way of doing it, even when splitting code into
>> separate files (the class state must be all in one place and can't be in an
>> extension).
>> --
>> Ilya Belenkiy
>> On Mon, Dec 7, 2015 at 8:17 AM Ilya <ilya at harmonicsense.com> wrote:
>>> My main proposal was not to change the semantics of private (although I
>>> would like that) but to introduce a way to keep class implementation
>>> details inaccessible to anything outside the class. I think that it should
>>> still be possible and useful to have the current level of sharing (I would
>>> just name it differently, given what private usually means in other
>>> languages).
>>> Just as it is convenient to group several related classes that can refer
>>> to each other's implementation details, it is very convenient to have
>>> several classes that don't do this but are still related in other important
>>> way, for example related APIs or classes that are based on the same
>>> implementation idea (like array based data structures).
>>> Having all code in one place rather than jumping through many small
>>> files is a very nice thing to have because it really helps to keep
>>> everything consistent. The more manual work people have to do for
>>> consistency, the more likelihood that it won't be done.
>>> --
>>> Ilya Belenkiy
>>> On Mon, Dec 7, 2015 at 12:21 AM John McCall <rjmccall at apple.com> wrote:
>>>> > On Dec 5, 2015, at 8:39 PM, Ilya via swift-evolution <
>>>> swift-evolution at swift.org> wrote:
>>>> >
>>>> > I think the it would help a great deal to have an access level
>>>> modifier that is really private and visible only inside the class itself.
>>>> Right now, the only way to hide implementation details for a class is to
>>>> hide the class code in a separate file, which is very inconvenient for
>>>> several reasons:
>>>> >
>>>> > 1) the meaning of the code changes depending on which file the class
>>>> is in. It's very easy to accidentally expose class internal details and
>>>> then call class elements that are meant to be used only inside the class.
>>>> Having a keyword for class internals will allow the compiler to ensure that
>>>> only the public API for the class is used from the outside world. The user
>>>> can check types on his own, but it's better that the compiler does it
>>>> automatically. Similarly, the user can check that only the proper APIs are
>>>> called, but it's better that the compiler does it automatically.
>>>> >
>>>> > 2) accessibility by file structure may cause some really short files.
>>>> >
>>>> > 3) It's impossible to group related classes in one file but still
>>>> hide implementation details inside each class
>>>> >
>>>> > I think that it the best solution is to make private keyword do what
>>>> it states -- keep the class element private to the class. But if it's
>>>> really important to have a separate keyword for backward compatibility, it
>>>> would be the next best thing.
>>>> But on the flip side, with your proposed semantics for private, it
>>>> would be impossible to write a group of related types, functions, and
>>>> extensions that do need to refer to each other’s internal details without
>>>> exposing those details to the entire module.  That’s not really acceptable.
>>>> The Swift language rule encourages you to put independent definitions
>>>> in different files.  That definitely means that, if you want to enforce
>>>> very tight separation of concerns, you’ll end up with more and smaller
>>>> files.  You haven’t explained why that’s really a *problem*, though.
>>>> John.
>>> _______________________________________________
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