[swift-evolution] A case for postponing ABI stability

Dave Abrahams dabrahams at apple.com
Thu Jan 26 15:10:14 CST 2017

on Thu Jan 26 2017, David Hart <swift-evolution at swift.org> wrote:

> Thanks Michael for the manifesto. It definitely made quite a few things clearer for me.
> Concerning the topic of when ABI stability should happen, I still have
> a strong feelings that Swift 4 might not be the best time for
> it. Concerning Data Layout, Type Metadata, Mangling, the Calling
> Convention and the Runtime, I don’t know enough about them to
> comment. I’m really centring my discussion on the Standard Library.
> If we look back at the evolution of the Standard Library for Swift 3,
> they were many changes. And I’m personally very happy with the
> thoughtful design that went into those. But they are still a few
> gotchas, which is to be expected when so many changes are made at
> once. But we only discover them once the thousands of Swift developers
> start using those APIs.
> I just worry that all the big changes that will come for Swift 4 won’t
> have time to mature. Furthermore, it seems like several extra compiler
> features which won’t happen in Swift 4 are really necessary to
> simplify the Standard Library surface area. I’m specifically thinking
> of type constraints on Existentials which would allow us to get rid of
> all the Any* structs and replace them with typedefs. But I’m sure
> there are more examples like those which are just waiting for the
> generics to become powerful enough to express APIs more elegantly.
> Perhaps someone from the Standard Library team can chime in to give us
> their opinion on this topic.

I have had exactly the same worry for quite some time.  We're still
waiting for many basic components of the generics system, and, if our
experience with protocol extensions is any guide, before we have those
features in hand, it will be impossible to anticipate the design changes
we'd want to make to the standard library... and that cuts against the
grain of *source* (to say nothing of ABI) stability.

So far I've been unable to form a mental model for what source and/or
ABI stability actually means for our ability to make changes to the
standard library in the future.  It's possible that we discover a
workable path forward, but it's equally possible that we find ourselves
painted into a corner.


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