[swift-evolution] [Review] SE-0025 Scoped Access Level

Patrick Pijnappel patrickpijnappel at gmail.com
Mon Feb 29 06:42:37 CST 2016


*Fortifies extension-based approach*
The change fits well with Swift's style of splitting types' subsystems and
protocol conformances into their own extensions. This modularity is an
important tool in managing complexity and assuring quality/safety (key
Swift values), and the language should encourage this pattern. It'll become
only more relevant if extensions within the same module will in the future
be allowed to add stored properties, a change which would fit very well
with this one.

*Why not file split & use private*
Splitting each extension into a separate file is impractical because if
properly done, the size of the individual extensions is usually quite small.

*Self-documents extension API*
It makes API per extension more self-documenting – it immediately tells you
what the extension exposes to the rest of the file and what is an
implementation detail.

On Mon, Feb 29, 2016 at 9:45 PM, Brent Royal-Gordon via swift-evolution <
swift-evolution at swift.org> wrote:

> >
> https://github.com/apple/swift-evolution/blob/master/proposals/0025-scoped-access-level.md
> >       • What is your evaluation of the proposal?
> I don't think it's a good idea.
> I think the source of our disagreement is expressed in this paragraph from
> the proposal:
> > It forces a one class per file structure, which is very limiting.
> Putting related APIs and/or related implementations in the same file helps
> ensure consistency and reduces the time to find a particular API or
> implementation. This does not mean that the classes in the same file need
> to share otherwise hidden APIs, but there is no way to express it with the
> current access levels.
> This reflects a view that grouping APIs into files is a purely stylistic
> choice. You can move APIs around freely and organize them however you like.
> If Swift gets in the way of your preferred arrangement, Swift should be
> changed to allow it. The most important thing is that you be free to
> express yourself artistically through the medium of the file system.
> I believe that view is mistaken. Swift has a very opinionated view of how
> code should be organized into files; they are as semantically meaningful as
> declaration blocks.
> The purpose of a file is to implement one concern—essentially, one logical
> piece of the module's functionality with its own self-contained
> implementation details. A concern is not necessarily fully represented by a
> single type; a type may include several concerns, and several types may
> implement one concern. (It's not a coincidence that a file can contain
> several types and a type can be split across several files.) The precise
> boundaries of a concern are a little nebulous, especially when you build
> convenience APIs which don't depend on anything private, but it's usually
> roughly clear what they are.
> When you want to use `local`, that usually means you're not organizing
> your code the way Swift thinks you should. It's no wonder Swift seems to
> not be expressive enough: You're fighting the language.
> Now, there are two exceptions to this general rule, but I think they're
> both best handled in other ways.
> The first: Sometimes Swift will not allow you to move certain things to
> separate files; for instance, only one file can declare stored properties
> on a type, and so stored properties must either be made more visible than
> they should be, or several concerns must be mixed into a single file. I
> think this is best handled by allowing extensions to declare stored
> properties and other such one-file-only constructs, rather than by
> complicating access control.
> The second: Sometimes a particular concern has an especially complicated,
> self-contained "sub-concern" which has implementation details of its own.
> You would like to keep the sub-concerns implementation details private from
> the containing concern, but the sub-concern is *itself* an implementation
> detail of the containing concern, so you *also* want to keep the
> sub-concern private from other, unrelated concerns. In these cases, some
> sort of more nuanced access control would be better—but even then, I don't
> think `local` is actually a very good way to do it.
> There's nothing about a declaration block that makes it a natural choice
> for scoping declarations. `local` hides the declaration from containing and
> sibling declaration blocks and exposes it to nested declaration blocks. But
> if concerns often transcend type boundaries, surely sub-concerns do as
> well, so `local` will often be either too limiting or not limiting enough.
> To properly handle this problem, we would be better off coming up with
> some way to limit the scope of `internal` to only particular files which
> need to interface with that file's concern. `internal` would expose the API
> to your file's "clients"—by default all files in the module, but
> potentially narrowed down to a particular subset—while `private` would
> remain as something truly limited to a single file.
> However, that approach is rather complicated, bordering on the horror of
> C++ `friend` declarations. Ultimately, I just don't think it's too large of
> a burden to say, "You have a three-level namespace, and anything that
> crosses files goes into `internal`; if something in `internal` is only
> meant to be used in a particular file, show some discipline."
> >       • Is the problem being addressed significant enough to warrant a
> change to Swift?
> As I often say, the problem is arguably significant enough, but I don't
> think the solution is the right one.
> >       • Does this proposal fit well with the feel and direction of Swift?
> I don't think so. Swift has strong, opinionated ideas about how code
> should be organized; this proposal doesn't follow that pattern.
> >       • If you have used other languages or libraries with a similar
> feature, how do you feel that this proposal compares to those?
> I've used languages with file-based scoping, and languages with arbitrary
> lexical scoping, but not languages with both.
> >       • How much effort did you put into your review? A glance, a quick
> reading, or an in-depth study?
> In addition to reading the present proposal, I also participated fairly
> extensively in discussions about it and read previously-posted reviews.
> --
> Brent Royal-Gordon
> Architechies
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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