[swift-evolution] [Discussion] A Problem With SE-0025?

[Matthew Johnson]

> On Jun 29, 2016, at 5:51 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> On Wed, Jun 29, 2016 at 5:41 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
> On Wed, Jun 29, 2016 at 5:36 PM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
> 
>> On Jun 29, 2016, at 4:57 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> 
>> On Wed, Jun 29, 2016 at 4:46 PM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>> 
>>> On Jun 29, 2016, at 4:19 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>> 
>>> On Wed, Jun 29, 2016 at 4:16 PM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>>> 
>>>> On Jun 29, 2016, at 4:12 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> On Wed, Jun 29, 2016 at 4:07 PM, Jordan Rose <jordan_rose at apple.com <mailto:jordan_rose at apple.com>> wrote:
>>>> 
>>>>> On Jun 29, 2016, at 14:03, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>>>> 
>>>>> On Wed, Jun 29, 2016 at 3:15 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>wrote:
>>>>> 
>>>>> 
>>>>> > On Jun 29, 2016, at 13:13, Jose Cheyo Jimenez <cheyo at masters3d.com <mailto:cheyo at masters3d.com>> wrote:
>>>>> >
>>>>> > I know this might be have been brought up before but
>>>>> >
>>>>> > why not just disallow the “private" keyword for top level types, extensions etc.
>>>>> >
>>>>> > A fixit could change top level `private` to `fileprivate`.
>>>>> >
>>>>> > I think this is a little less confusing since effectively this is what is happening in the background.
>>>>> 
>>>>> That doesn’t fix anything for inner types, so it’s a lot less important than the rest of the amendment.
>>>>> 
>>>>> There actually is an answer to this, which is that the core team expects 'private' to be the common keyword, and therefore it’s better if you can use it at the top level and ignore ‘fileprivate’ altogether in most programs.
>>>>> 
>>>>> On second thought, wouldn't all of this be inapplicable if `private` literally meant visibility *only* within the current declaration, and neither outside it nor inside any nested types, etc.?
>>>> 
>>>> Yes, but that's not very useful:
>>>> 
>>>> public struct Foo {
>>>>   private var value: Int = 0
>>>>   public func test() {
>>>>     print(value) // error
>>>>   }
>>>> }
>>>> 
>>>> I suppose you could say that nested types are different from nested functions, but then we start getting complexity in a different direction. And it still doesn't fix the default access within a private type.
>>>> 
>>>> Let me offer a principled rule: if I write `private var foo`, then `foo` is invisible at such places within the declaration where writing `private var bar` at the same place would cause `bar` to be visible where `foo` is not or vice versa.
>>> 
>>> This violates the principle behind all of Swift’s access control rules.  That principle is that access control is strictly based on a hierarchy of lexical scopes.  This is a really great principle and is what makes Swift’s access control better than any other I know of (IMO of course).
>>> 
>>> But however you slice it, some principle of Swift's access control rules is violated by `private`. If `foo` is visible in a place where I cannot write `private var bar` to mean the same visibility, then the access level of `foo` is unutterable in that location, which is unprecedented as well.
>> 
>> I don’t think utterability was a conscious principle to the degree that scope based access control was.  If that was the case the issue would surely have been identified during review.  It wasn’t until Robert started the implementation that anyone (AFAIK) notices that the proposal introduces unutterable visibility in some cases.  Utterability just isn’t something people were thinking about until then.
>> 
>> But you are right that unutterability is unprecedented and I think everyone agrees that it poses problems which is why Jordan and Robert have amended the proposal to make the visibility members of private types without explicit access control utterable.
>> 
>> The solution we want is to preserve *both* of these principles, not change which one were violating. :)
>> 
>> If a private member must be visible within a nested type, then that access level necessarily becomes unutterable within the nested type unless we introduce another keyword, which is out of scope without a new proposal. There is no squaring the circle to be had. The amendment, to my understanding, simply hacks around this issue to make `private` nonetheless useful by allowing `fileprivate` things inside `private` things, but in so doing we're enshrining which of these principles we're violating, not finding a solution that avoids violating them.
> 
> Do you mean a third principle which says something like “a member shall not have a higher access level than its parent”.  If so, you are correct that Jordan’s amendment does violate that and another proposal would be necessary to give it a name that does not exist today.  I don’ think that's going to happen for Swift 3.
> 
> No, not exactly what I mean, but that is a concern. Jordan's proposal introduces a dichotomy between the actual access level (i.e. the answer to the question, which code can see this member?) and the access modifier which is used in conjunction with the member (which would become `fileprivate`). So yes, I can utter something, but it doesn't change the fact that the question "which code can actually see this member?" still has no utterable answer.
> 
> 
> !!!
> This just hit me like a ton of bricks. Jordan's proposal is repurposing `fileprivate` to mean this previously unutterable scope and divorcing it from meaning "private to file”.

Yes, with Jordan’s amendment it basically means “private to the first ancestor scope which is marked private, or the file if there are no private ancestor scopes”. 

>  
> 
>>>> Jordan
>>>> 
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