[swift-evolution] private & fileprivate

Xiaodi Wu xiaodi.wu at gmail.com
Sat Oct 8 15:24:56 CDT 2016

On Sat, Oct 8, 2016 at 2:15 PM, Matthew Johnson <matthew at anandabits.com>

> On Oct 8, 2016, at 2:01 PM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
> On Sat, Oct 8, 2016 at 12:02 PM, Karl via swift-evolution <
> swift-evolution at swift.org> wrote:
>> On 8 Oct 2016, at 16:47, Braeden Profile <jhaezhyr12 at gmail.com> wrote:
>> On Oct 8, 2016, at 6:58 AM, Karl via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> I was thinking that the domains themselves could be associated with a
>> domain, so you could create alternate domains which are also
>> publicly-visible, but distinct from the default, “public” domain.
>> For example, if you have a bunch of methods which should be visible to
>> subclasses, but you don’t want them to clutter your regular interface.
>> Perhaps they have names which are confusingly-similar to the public API. I
>> believe that is what “protected” is typically used for.
>> Yes, but “protected" was specifically put down by the core team, seeing
>> that any code from outside the library should see the class as one
>> well-designed whole, not something with complicated, visible implementation
>> details.  If your class-internal methods are confusing (and aren’t
>> necessary for normal use), they shouldn’t be made public in any way.
>> Subclasses would too easily confuse the distinction between your
>> implementation methods and your public ones.
>> For what it’s worth, I was only confused by “private” and “fileprivate”
>> for a minute or two until I looked up the actual proposal.  I haven’t had
>> trouble with it, and it does actually provide more flexibility for code
>> access at the file level than we had before.  Even if the syntax is clunky.
>> I’m not saying that (file)private is confusing - it’s very clear about
>> what it does. But it is limiting; anything that wants access to those
>> semi-private details needs to live in the same file. That’s clearly not
>> scalable. Enormous files many thousands of lines long are easy for the
>> compiler to digest, but less easy for humans to understand and navigate. In
>> fact, I believe this whole “file-based” access control originally came out
>> of the compiler’s implementation details.
> I'm interested in more information about this. What sorts of code have you
> been writing where a file would have to be thousands of lines long in order
> to accommodate `fileprivate`? Many entire modules are only thousands of
> lines long--is there a reason this code couldn't be refactored into a
> module of its own? As mentioned by Matthew, isn't this calling for some
> notion of submodules?
> There are rather significant optimization barriers at module boundaries
> right now.  These do not exist for the standard library due to its tight
> relationship with the compiler, but for the moment it is a non-trivial
> concern for 3rd party code that is performance sensitive.  This is
> effectively a language pressure in the direction of larger modules, at
> least for some domains.  (Hopefully this pressure will eventually be
> alleviated - there has been at least some talk in that direction)

Great point. IMO, it'd be important to identify how much of the desire for
additional varieties of access modifiers represent workarounds to compiler
limitations, because we'd be better off making optimization across module
boundaries work better if that's the root of the issue. Certainly, also,
submodules are a discussion that, having made a brief appearance at the end
of Swift 3 evolution, would be ripe for Swift 4 phase 2.

As for this topic about additional access modifiers, I'd like to understand
what current use cases motivate them that arise because the current syntax
is insufficiently expressive, rather than compiler shortcomings or the lack
of other features that are on the horizon. The latter sorts of motivation,
IMO, would be more definitively addressed by tackling the root problem;
adding additional syntax in order to work around compiler limitations seems
as best inelegant.

> What it would basically come down to is that the interface of the object
>> would be separated in to blocks based on your access privileges. When
>> viewing the interface, it wouldn’t look much different to an extension:
>> *access(public)* class TabController {
>>    var tabs : [Tab] { get }
>>    func closeTab(at: Int)
>> }
>> *access(TabBarStuff)* extension TabController {
>>     func close(tab: Tab)
>> }
>> I definitely want something between internal and fileprivate, at least. I
>> don’t see any reason at all why objects shouldn’t be allowed to present
>> optional “slices” of their interface to appropriate clients. In fact, that
>> is what access control is all about. I just want to generalise it to allow
>> for user-defined visibility scopes (as well as the default ones for public,
>> module, file and scope). That leads to the question of what visibility
>> those user-defined scopes would have; and if you leave them entirely open
>> to adopt any scope (except themselves), then you end up with the ability to
>> slice your API for different use-cases. Or we could be boring and limit
>> them to the module they are defined in.
>> The whole reason I’m bringing this up is because I don’t like the “file”
>> part of fileprivate. How I split my files up is a readability decision.
>> Karl
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