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

[Chris Lattner]

On Apr 3, 2017, at 11:34 AM, Douglas Gregor via swift-evolution <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

Another way to explain this is as a relaxation of the Swift 3 access control, to would allow private members in a type to also be accessible in extensions to that type, so long as they are in the same file.

While I typically try to avoid chiming in on early discussions like this, I pretty strongly believe that this is a good solution for the reasons you mention:

 - fileprivate should really become much more rare, which makes it more meaningful and significant where it occurs.  This was the original idea and intent behind SE-0025.

 - Similarly, this simplifies access control for most people.  Most people will now only care about private/internal/public.  fileprivate will become an expert feature used in specific cases to solve a specific class of problems.  Progressive disclosure of complexity is important.

 - This design is true to the existing design of Swift: we want to encourage the implementation of types to be freely broken into extensions.  This alignment with extension oriented programming was the one important virtue of the Swift 1/2 access control design that Swift 3 lost.


From a pragmatic perspective, I feel like this is a great solution that really does solve the problems we have with current access control, all without breaking source compatibility.  This is also a major progression from where we are, and doesn’t appear to cut off any future directions (e.g. submodules) since those are cross-file concepts that live between internal/public or between fileprivate/internal.

Just MHO, but I think that the rhetorical attempts to paint this as being similar to “protected” are unsound.

-Chris

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