[swift-evolution] [swift-evolution-announce] [Review] SE-0169: Improve Interaction Between private Declarations and Extensions

Xiaodi Wu xiaodi.wu at gmail.com
Sat Apr 8 15:12:09 CDT 2017

On Fri, Apr 7, 2017 at 11:34 PM, John McCall via swift-evolution <
swift-evolution at swift.org> wrote:

> > On Apr 7, 2017, at 8:12 PM, Jakub Suder via swift-evolution <
> swift-evolution at swift.org> wrote:
> >
> > > What is your evaluation of the proposal?
> >
> > If this is the last option we have to change the status quo, any
> renaming is off the table, no further changes after Swift 4, and it's
> either this or being stuck with 'fileprivate' until the end of time, then
> +1 from me. It will increase the convenience of access control for people
> like me who see types and their extensions as parts of the same entity,
> just spread visually across neighboring blocks. In almost any other
> language these two would indeed be one entity, since most languages don't
> have any way of dividing types into pieces this way.
> >
> > However, IMHO any of these would be a better solution:
> I'd like to respond briefly to this to clarify the Core Team's decisions
> about what solutions are under consideration, both now and in the future.
> By doing this, I don't mean to pressure you towards any particular stance.
> The Core Team asked for this to be proposed because we wanted to know how
> the community felt about it; we are not specifically trying to get it
> approved, at least as a group.
> > 1) Rename 'private' to something else ('scoped'?) and rename
> 'fileprivate' back to 'private'
> The Core Team has rejected making such a major change in the
> interpretation of 'private'.  'private' will be tied to scopes, now and
> forever.  The only question is whether extensions of the same type within a
> file should be considered part of the same scope for the purposes of
> 'private'.  Swift 4 is the deadline for making that change; if it, too, is
> rejected, 'private' will be purely lexical forever.
> > 2) Rename 'fileprivate' to something more friendly (I liked the 'local'
> suggestion that Vladimir made today)
> The Core Team is willing to consider adding a new keyword to replace
> 'fileprivate', but not in Swift 4.
> Speaking just for myself, I don't think we'd accept such a change purely
> for aesthetics;

If I recall correctly, Chris in post-review discussions communicated that a
new keyword to replace `fileprivate` would be considered if `fileprivate`
turned out to be commonly used enough to be aesthetically problematic?

I bring this up because, in replying to BJ Homer, it occurs to me that
there is an interesting solution. If--as I understand--successive access
modifiers being supersets of preceding ones is a non-negotiable part of the
design, then type-based access modifiers cannot be accommodated in Swift
because of the existence of extensions. Ergo, the word "protected" cannot
ever be used to mean what it does in C++, etc. It is a perfectly nice word
that by its dictionary meaning is plausibly intermediate between "private"
and "internal," which we can repurpose to mean protected by the end of
file! By co-opting this word, it has the key virtue of definitively
demonstrating that Swift cannot and will not support access modifiers that
are tied to types.

it would have to be because 'fileprivate' seemed inappropriate for some new
> generalization, e.g. if we added sub-file submodules and wanted
> 'fileprivate' to allow access only within the submodule.  That is assuming
> a lot about how a future submodule feature would work, and we aren't going
> to design that feature specifically with a goal of replacing this keyword,
> and frankly we don't know when we're going to take that on at all.  I would
> caution people against assuming that 'fileprivate' will be renamed.
> > 3) Postpone this until we add submodules, but with the assumption that
> it will be possible to make some source-breaking changes at that point
> The Core Team is not willing to change the basic design of 'private' in
> future releases of Swift.  If some feature — perhaps submodules — demands
> that we define its interaction with 'private', the design of that
> interaction will have to feel natural and consistent with the at-that-point
> extant design of 'private'.  For example, it would not be acceptable if,
> say, adding a submodule declaration to a file suddenly changed the
> interpretation of 'private'.
> An option (4) that you didn't list but which I should cover for
> completeness would be to add new keywords in the future with new
> interpretations.  This is something that the Core Team is willing to
> consider.  However, speaking just for myself again, I find it unlikely that
> we would add new access control features just to express
> increasingly-precise refinements; it would have to be something that felt
> necessary because of some real inadequacy in the existing access-control
> levels as applied to some other new feature (e.g. submodules).
> John.
> > The thing I don't like about this proposal (or status quo) - apart from
> the fact that it will make people who like the current strict private
> unhappy - is that 'private' even right now means kind of two different
> things:
> >
> > - for a property or a method defined in a class/struct, it means
> "available only inside this type"
> > - for a private global variable/constant/function, or a private type or
> extension, it means "available in this file" i.e. the same as 'fileprivate'
> >
> > So if we're worried that this proposal makes the meaning of 'private'
> unclear - it already is unclear. Wouldn't it be much more clear if private
> global variables, functions and classes were required to use the access
> level that means "available in this file", since that's how they actually
> work? (as long as this level is not named 'fileprivate' :)
> >
> > And the other access level, the "available only inside this type"
> (private / scoped), could only be used for things that are actually
> contained inside a type, and wouldn't have two different meanings depending
> on whether it's at the root of the file or nested inside something else.
> >
> > I really believe that even though this is kind of painful for everyone,
> it's worth spending time to figure out a solution that satisfies most of us
> and makes the language clearer and friendlier in the long term, even if it
> means breaking compatibility again. Do we want to be stuck with an
> imperfect solution 10 years from now, because we didn't want to do this
> last breaking change now?
> >
> >
> > > Is the problem being addressed significant enough to warrant a change
> to Swift?
> >
> > I think almost everyone here agrees it is significant.
> >
> >
> > > Does this proposal fit well with the feel and direction of Swift?
> >
> > This is a difficult question. IMHO it would definitely fit much better
> with the direction of Swift if we bit the bullet and did whatever we agree
> will make the language simpler and better long term, regardless how many
> changed lines in git this will cause when Swift 4 is released.
> >
> >
> > > If you have used other languages or libraries with a similar feature,
> how do you feel that this proposal compares to those?
> >
> > I've used many languages, but I think in none of them it was a common
> thing (or even a possibility) to split types into several blocks like we do
> in Swift with extensions, so the main issue we're talking about didn't
> exist there. In Ruby you can kind of do the same thing with modules - if
> you have a private method in a module, you can access it from methods in
> the main type, even if they're defined in a different file. (But Ruby isn't
> very strict about access control in general, e.g. it allows you to call
> private methods on any object via #send).
> >
> >
> > > How much effort did you put into your review? A glance, a quick
> reading, or an in-depth study?
> >
> > I've read or skimmed through most of the messages in the recent threads
> about this and the last proposal.
> >
> > Disclaimer: I have very little experience with Swift 3 (but plenty with
> Swift 2.x).
> >
> > Kuba
> >
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