[swift-evolution] Simplifying Access Using 'Hidden'

Michel Fortin michel.fortin at michelf.ca
Wed Feb 15 09:37:37 CST 2017

> Le 15 févr. 2017 à 9:28, Vladimir.S via swift-evolution <swift-evolution at swift.org> a écrit :
> On 15.02.2017 14:29, Joanna Carter via swift-evolution wrote:
>>> The beauty of Swift 2's access modifiers was that they were based
>>> around files and modules, explicitly rejecting types and scopes as
>>> units for determining visibility. It seems at base there's a group of
>>> people who reject that decision altogether. Hence, new `private`,
>>> proposals around `protected`, `friend`, `hidden`, `extensible`, etc.
>> I suppose, for those coming from an Objective-C only background, any
>> extra visibilities are seen as a bonus.
>> For those coming from a Pascal background, file-based visibility seems
>> more natural.
>> But for those of us who have used languages like C++ and C#, losing the
>> visibilities that we have been used to can seriously affect how we think
>> and design stuff.
> Was surprised that you expressed exactly what I'm thinking about this subject.
> I do believe even in Swift we need a way to say "only code that *knows* what it does should see this" and to not force us to write(and manage) the the code in same file.
> Yes, some implementation details that shouldn't be used by "users" of the type, but can be used by "extenders" of our type. At least in the same module.
> While reading, the question was raised(if was discussed, let me know pls): what if we extend the meaning of 'private' to 'visible to current type/scope and subtypes and extensions in the *same module*' ?
> This will help us to split types by files, help to better model access policy for fields/methods(i.e. only scope that can know about details can see them) while 'private' still will be hidden outside of the module even for subclasses/extensions.
> So, we'll have such situation:
> "modifier" -> "can be accessed from"
> -------------------------------
> public -> outside of the mondule
> internal -> inside the module only
> fileprivate -> the same file only
> private -> the same type/scope, or subtype, or extension in the same module. I.e. some kind of 'internal' but scope-oriented.
> Actually I still believe we need current 'private' and additionally some 'protected' with meaning "access from subtype, or extension" probably with this limitation: "only in the same module". But suggested extended 'private' will be also OK for me.
> Thoughts?

Here's an idea in three points:

1. Return to the Swift 2 definition of `private`.
2. Introduce `scoped` to limit the visibility to within the same type, subtype, or extension.
3. Allow mixing the two.

So you have:

	private         // current file
	private scoped  // current file, in the same type, subtype, or ext.
	internal        // current module
	internal scoped // current module, in the same type, subtype, or ext.
	public          // external modules
	public scoped   // maybe we want that one?

Since `internal` is implied, using `scoped` alone would be equivalent to `internal scoped`.

Swift 3's private could be mapped to `private scoped` to achieve (almost) the same results and thus preserve source compatibility.

Michel Fortin

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