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

Matthew Johnson matthew at anandabits.com
Wed Apr 5 20:34:27 CDT 2017

> On Apr 5, 2017, at 7:11 PM, Colin Barrett via swift-evolution <swift-evolution at swift.org> wrote:
> I'm broadly in favor of this.
> On Mon, Apr 3, 2017 at 2:35 PM Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> Hello Swift Community,
> In rejecting SE-0159 <https://github.com/apple/swift-evolution/blob/master/proposals/0159-fix-private-access-levels.md>, 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 <https://github.com/apple/swift-evolution/blob/master/proposals/0025-scoped-access-level.md>, 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
> I think it might not as complicated as it may first appear. I haven't fully convinced myself of this, but I believe that it would be lexically scoped modulo inlining extensions.
> To put it somewhat more formally (and awkwardly), if one imagines a textual transformation which blindly inlines any extension in a single file into their type definitions (should those definitions occur within that file) then accessing a "private" member in the original file would compile if and only if the access would be lexically scoped if the above postulated transformation were to be applied to it.
> This is a pleasing property (to me anyway) as it means that it is unlikely (impossible?) for moving code from one extension (or definition) to another to cause a compilation failure.
> Take this with a grain of salt of course, as there's probably an edge case I'm not thinking of :P (This is why we write proofs!)

This is a very interesting perspective Colin.  This idea seems to align well with the intent for extensions that Chris stated earlier.  This way of thinking about it does make the proposal much cleaner conceptually.  If we’re going to adopt this perspective I think we should go all the way - same file extensions really should be inline semantically.  This means same file extensions would also have the ability to include stored properties which is something many people have asked for.  

I wonder how David and the core team would feel about modifying the proposal to adopt this “inline same file extensions" perspective.  IMO this  motivates the proposal better by acknowledging that most developers in the community use same file extensions for purely organization purposes.  The implicit desire is that they be transparent boundaries that carry no semantic weight at all.  I think a proposal with this kind of motivation is more likely to be well received by the broader community than a change that is only focused on access control in a very specific context.  Instead of “why do they keep changing access control” the response might be “yay, extensions finally work the way I always wanted them to”.


> Cheers,
> -Colin
> Thoughts? Volunteer?
> 	- Doug
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