[swift-evolution] Type-based ‘private’ access within a file

BJ Homer bjhomer at gmail.com
Fri Apr 7 08:41:53 CDT 2017


SE-0159 was rejected because it was determined that some developers are actively using strongly-scoped access control. This proposal removes that strong scoping, so I do not see how we can reasonably reject that proposal but accept this one.

The entire reason we're having this discussion is that "fileprivate" is such an awkward term for something that's so common in the language. I think the main thing we need to fix is the naming of that keyword.

I continue to believe that the best solution is to revert "private" to mean file scope as in Swift 2, and introduce a new "scoped" keyword for those developers who are specifically desiring the scoped functionality. This was rejected during the discussion because the migration would be too disruptive, but it is only disruptive if the migrator rewrites "private"->"scoped". I assert that most developers would not *want* that migration to happen; most developers use "private" because they want the default less-than-internal access control. The few developers who are using specifically scoped control can modify their code manually. Under this model, scoped access control is still available for those who need it, and most users can once again use "private" in cases where it is the natural default. 

This proposal proposes that "fileprivate" would become a marker to call out cases where exceptional across-type access is happening. In practice, I don't believe that will happen, simply because there are many existing cases of "fileprivate" out there, and this proposal does not suggest migrating them.

I also disagree that it's useful to call out "fileprivate" as an exceptional case. It's slightly useful, I'll acknowledge, but it would be *more* useful to call out the exceptional cases where scope-only control is being used.

So I disagree with the proposal. But I give it only -0.5 because even with all of that, this is a better definition for "private" than the current one.


> On Apr 3, 2017, at 12:34 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
> Hello Swift Community,
> In rejecting SE-0159, the core team described a potential direction we would like to investigate for “private” access control that admits a limited form of type-based access control within files. The core team is seeking some discussion here and a motivated volunteer to put together a proposal along these lines for review in the Swift 4 time-frame (i.e., very soon). To be clear, the core team it’s sure this is the right direction to go… but it appears promising and we would *love* to be able to settle the access-control issue.
> The design, specifically, is that a “private” member declared within a type “X” or an extension thereof would be accessible from:
> 	* An extension of “X” in the same file
> 	* The definition of “X”, if it occurs in the same file
> 	* A nested type (or extension thereof) of one of the above that occurs in the same file
> This design has a number of apparent benefits:
> 	+ “private” becomes the right default for “less than whole module” visibility, and aligns well with Swift coding style that divides a type’s definition into a number of extensions.
> 	+ “fileprivate” remains for existing use cases, but now it’s use it more rare, which has several advantages:
> 		+ It fits well with the "progressive disclosure” philosophy behind Swift: you can use public/internal/private for a while before encountering and having to learn about “fileprivate”   (note: we thought this was going to be true of SE-0025, but we were clearly wrong)
> 		+ When “fileprivate” occurs, it means there’s some interesting coupling between different types in the same file. That makes fileprivate a useful alert to the reader rather than, potentially, something that we routinely use and overlook so that we can separate implementations into extensions.
> 	+ “private” is more closely aligned with other programming languages that use type-based access control, which can help programmers just coming to Swift. When they reach for “private”, they’re likely to get something similar to what they expect—with a little Swift twist due to Swift’s heavy use of extensions.
> 	+ Loosening the access restrictions on “private” is unlikely to break existing code.
> There are likely some drawbacks:
> 	- Developers using patterns that depend on the existing lexically-scoped access control of “private” may find this new interpretation of “private” to be insufficiently strict
> 	- Swift’s access control would go from “entirely lexical” to “partly lexical and partly type-based”, which can be viewed as being more complicated
> Thoughts? Volunteer?
> 	- Doug
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