[swift-evolution] [swift-evolution-announce] [Review] SE-0159: Fix Private Access Levels

[David Hart]

> On 23 Mar 2017, at 21:49, Drew Crawford <drew at sealedabstract.com> wrote:
> 
>> I'm curious to hear what issue your client had with you using many frameworks that static linking doesn't solve.
> 
> The issue here is the number of frameworks that the user drags and drops into Xcode.  Most libraries ship as a single framework, see this page <https://www.mapbox.com/ios-sdk/> for a typical example of what the installation documentation for this process generally looks like.
> 
> From a user POV, there is no difference between dragging 4 frameworks and dragging 4 .a libraries.  Actually, the .a case is worse in Swift because in addition to object code (.a) we have .swiftmodule, .swiftdoc, and .modulemap.  So that's a lot of files to drag.

But with static linking, your user wouldn’t have to drag 4 .a libraries but only one, which contains all libraries statically linked together.

> I have been involved in generating solutions for this problem in other areas (see the atbin standard <http://anarchytools.org/docs/atbin.html> for example) but none of them are supported by Xcode.
> 
> Finally, static linking has nothing to do with visibility or the problems of exposing these frameworks as public API.
> 
>> I don't see why submodules could not profit from WMO: the module is still compiled all together. Submodules are simply a scoping/hiding mechanism.
> 
> Then you will be surprised to hear that this is a subject of some debate.  Relevant thread. <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160620/021749.html>
> 
>> That looks like a very corner case. I haven't yet found myself in the case where I needed multiple versions of a code base in a same product (binary, framework, application)
> 
> 
> There are 253 pages <http://stackoverflow.com/search?q=duplicate+symbols> of search results for "duplicate symbol" on StackOverflow.  Compare with 48 pages <http://stackoverflow.com/search?q=fileprivate> on fileprivate.  It is quite clear which is the more complicated feature.

Those stats are misleading: an enormous majority of the search results concerning “duplicate symbol” concern incorrect build settings, not a wish to use different versions of the same code.

>> It would be very strange to me if they were independent libraries: what would different them from modules then?
> 
> The organization of object code on the filesystem does not necessarily have any relationship to "submodules the philosophical construct".
> 
>> But at the same time, we can't write and review proposals with no regard for future proposals coming down the road or we end up with a clunky language.
> 
> 
> I'm not aware of evidence any submodule proposal is actually coming.  For example here <https://github.com/apple/swift/blob/master/docs/Modules.rst#id3> is the only authoritative statement of the feature, "slated" for Swift 1.0 😆, a giant warning that we do not even have a design, and the doc mostly consists of questions for how the feature should work.

Well the community has definitely shown a LOT of interest in the topic during the last year. I’d be surprised if a proposal wouldn’t be put through review as soon as it’s “in scope”.

> A scoped access modifier on the other hand is a feature that was designed, is implemented, and is now widely used. What you are suggesting is we should throw it away because at any moment a bird could appear in the bush.

No. I’m suggesting we throw it away because I argue that it brings more confusion and complexity to the language than it solves problems. Again, I’m not saying you and other people don’t use it: we all have our own development experiences. But that hasn’t stoped us from removing features in the past. The submodule argument was just to mention that it would solve some of the same problems you are solving with scoped-private today.

> But we've waited for that bird for 3 releases.  Rather, if that bird were to appear, we could then study whether or not it solves all the problems of the bird in our hand, or whether it does not.  But that hypothetical is quite far from the present circumstance.
> On March 23, 2017 at 2:02:25 PM, David Hart (david at hartbit.com <mailto:david at hartbit.com>) wrote:
> 
>> 
>> 
>> On 23 Mar 2017, at 16:49, Drew Crawford <drew at sealedabstract.com <mailto:drew at sealedabstract.com>> wrote:
>> 
>>> 
>>> 
>>> 
>>> On March 23, 2017 at 2:22:20 AM, David Hart (david at hartbit.com <mailto:david at hartbit.com>) wrote:
>>> 
>>>> > We will get static linking at some point in the near future.
>>> 
>>> Static linking does not fix this issue. Just change "framework" to ".a".
>>> 
>> I'm curious to hear what issue your client had with you using many frameworks that static linking doesn't solve.
>> 
>>>> > If we wait until we get submodules, we won't be able to revisit. This is probably our last chance to "remove" a feature. Submodules can always add features down the way.
>>> 
>>> Maybe submodules will solve this issue, maybe not.  But submodules are *much* more complex than scoped access:
>>> 
>>> * Performance.  This is hot code we compile with WMO.  Moving it into a submodule could reduce visibility for optimization in a way that causes a performance regression.  In particular, we know that specialization of T is a performance requirement, it isn't clear whether that would be preserved.  Does WMO provide the same visibility across submodules?  Nobody knows.
>>> 
>> I don't see why submodules could not profit from WMO: the module is still compiled all together. Submodules are simply a scoping/hiding mechanism.
>>> * Namespacing.  It's possible that one program may ship 3-4 versions of this code because each dependency has a slightly different version under our current samizdat process.  It is not clear whether submodules would avoid the "duplicate symbols" issue from C/ObjC.  Xiaodi seems quite concerned about a related "duplicate functions" problem involved with private today, doubling down on that is not a good idea.
>>> 
>> That looks like a very corner case. I haven't yet found myself in the case where I needed multiple versions of a code base in a same product (binary, framework, application)
>>> * It is not clear whether submodules are from an objectcode point of view merged into the parent library or kept as individual libraries
>>> 
>> It would be very strange to me if they were independent libraries: what would different them from modules then? No other language I've used works that way.
>>> * It is not clear from a .swiftmodule point of view whether submodules are merged into the parent module or distributed as .swiftmodules / .swiftdocs
>>> 
>>> * Not clear how much ABI impact there is from submodules at a time when we are supposed to be trying to stabilize it
>>> 
>>> I would love to believe that a proposal on submodules will come through having solutions to all these issues and many more, then we will implement it and all sing kumbayah.  But we are a long distance from that, and it may never happen at all, certainly we cannot evaluate proposals that haven't been written.  Meanwhile we have a solution in the hand.
>>> 
>> But at the same time, we can't write and review proposals with no regard for future proposals coming down the road or we end up with a clunky language.

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