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

Zach Waldowski zach at waldowski.me
Mon Apr 3 14:27:25 CDT 2017

Depending on the actual implications of the loosening of `private`, I'd
then advocate for deprecating `fileprivate` in the Swift 4 timeframe and
removing when it makes sense. Is that an option?

I am personally against muddying the implementation of access control
further (in addition to its existing muddying in the syntactical sense,
but let's avoid rehashing that now), and would like it if someone from
the Core Team could speak to the complexities involved there. I wouldn't
want access control changes to, for instance, hurt compile times or
stymie ABI efforts.


  Zachary Waldowski

  zach at waldowski.me

On Mon, Apr 3, 2017, at 02:34 PM, Douglas Gregor via swift-evolution wrote:
> Hello Swift Community,


> In rejecting SE-0159[1], 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[2], 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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  1. https://github.com/apple/swift-evolution/blob/master/proposals/0159-fix-private-access-levels.md
  2. https://github.com/apple/swift-evolution/blob/master/proposals/0025-scoped-access-level.md
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