Type-based ‘private’ access within a file


(Goffredo Marocchi) #1

From: Goffredo Marocchi <panajev@gmail.com>
Date: 3 April 2017 at 22:39:25 BST
To: Douglas Gregor <dgregor@apple.com>
Subject: Re: [swift-evolution] Type-based ‘private’ access within a file

+1 this brings some of the best points brought on both side of the private and fileprivate arguments while also thinking of users approaching Swift from other major programming language and fitting in with Swift's goal of progressive disclosure.

I like this idea very much (off-topic: almost as much as bringing back argument labels in function/closure types ;)).

Sent from my iPhone

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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Sent from my iPhone
Begin forwarded message:

On 3 Apr 2017, at 19:34, Douglas Gregor via swift-evolution <swift-evolution@swift.org> wrote: