[swift-evolution] A case for postponing ABI stability

[James Berry]

> On Jan 27, 2017, at 8:59 AM, Charlie Monroe via swift-evolution <swift-evolution at swift.org> wrote:
>> On Jan 27, 2017, at 5:43 PM, Tino Heth <2th at gmx.de <mailto:2th at gmx.de>> wrote:
>> 
>>>> - runtime libraries for Swift 4
>>>> - all system frameworks will need to contain two variants - one compatible with Swift 4 and one with Swift 5. This is IMHO absolutely unmaintainable in the long run. For how long would you need to keep several versions of the framework around? What happens when Swift 6 comes along with another breaking changes? Would each system framework have 3 versions embedded?
>>> 
>>> That's right. If the OS frameworks use Swift then either (1) you have to clone the framework stack for each Swift version, or (2) you have only one copy of the frameworks but frameworks and apps can't share their Swift objects or publish Swift API.
>>> 
>>> The framework structure that Apple inherited from NeXT supports framework versioning, but *no frameworks use it*. It doesn't scale. 
>> 
>> 
>> sure, it's preferable to have a single version that works with all apps — but if it's technically possible to have one clone installed with the OS, isn't that better than one version for each app?
>> Managing several versions shouldn't be that hard (have a look at https://nixos.org/nix/ <https://nixos.org/nix/>).
> 
> It would mean for Apple (and others who'd distribute compiled frameworks) to maintain several code bases of the same framework given that they would need to maintain backward compatibility and hence wouldn't be able to use new language features, etc. It's IMHO not that much about the technical constraint of having multiple binaries within the framework bundle as much as maintaining the code in a way that would compile under all Swift versions you'd like to support.

Rather than Apple have to commit in perpetuity to ship all relevant versions of the frameworks, one could imagine more of an app-thinning/install-time optimization: “thinned” versions of apps would be built and signed without the shared system frameworks, but with dependency information recorded. At install-time, the app would be installed, and the working set of required shared frameworks on the device would be updated with any needed dependent frameworks. Thus the only the set of frameworks required by installed apps would be present on device.

This would require more app store, install-time, and perhaps dynamic linking, infrastructure, but would seem to solve the problem in a way that wouldn’t require ongoing development resources be applied to old versions.

James

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