[swift-evolution] [Late Pitch] Deprecations, Moves, and Renames

Ben Rimmington me at benrimmington.com
Wed Aug 10 09:38:18 CDT 2016

> On 10 Aug 2016, at 00:36, Dave Abrahams wrote:
>> on Tue Aug 09 2016, Ben Rimmington wrote:
>>> On 9 Aug 2016, at 20:09, Dave Abrahams wrote:
>>> Hi Everybody,
>>> With another round of apologies for taking late action, we propose to
>>> make some deprecations, moves, and renames.  The background for these
>>> moves is as follows:
>>> We've always known that when Swift reached ABI stability (now slated for
>>> Swift 4), we would be committed to supporting many of the standard
>>> library's design decisions for years to come.  What we only realized
>>> very recently is that, although Swift 3.0 is *not* shipping with a
>>> stable ABI, the promise that Swift 3.0 code will work with Swift 4.0
>>> code creates similar implications when it comes to certain protocols,
>>> today.  Especially where these protocols show up in refinement
>>> hierarchies, we can't keep Swift 3 code working in the future without
>>> carrying them forward into future libraries.
>>> The proposed changes are as follows:
>>> * Move `CustomPlaygroundQuickLookable` to the PlaygroundSupport module.
>>> This component is really only useful for playgrounds, and doesn't
>>> belong in the standard library.
>> I didn't think it was possible to `import PlaygroundSupport` unless the 
>> current file is within a playground. If so, how can corelibs-foundation 
>> or third-party modules add `CustomPlaygroundQuickLookable`
>> conformance?
> Ahhh... yeah, I didn't realize that, and we don't have time to change
> that restriction for Swift 3.  It's not a problem for
> corelibs-foundation but it would be for 3rd-party libraries.  
> OK, Dmitri and I just figured out how we can change the home of these
> types for Swift 4 without breaking Swift 3 code (mirrors to the rescue!)
> So I'm going to withdraw this part of the proposal.

Will the following need to be reverted?



>>> * Deprecate the Indexable protocols with a message indicating that they
>>> will be gone in Swift 4.  These protocols are implementation details
>>> of the standard library designed to work around language limitations
>>> that we expect to be gone in Swift 4.  There's no reason for anyone to
>>> ever touch these; users should always use a corresponding Collection
>>> protocol (e.g. instead of MutableIndexable, use MutableCollection).
>>> * Deprecate the ExpressibleByStringInterpolation protocol with a
>>> message indicating that its design is expected to change.  We know
>>> this protocol to be mis-designed
>>> (https://bugs.swift.org/browse/SR-1260) and limited
>>> (https://bugs.swift.org/browse/SR-2303), but there's no time to fix it
>>> for Swift 3.  If we knew what the new design should look like, we
>>> might be able to calculate that the current API is supportable in a
>>> forward-compatible way (as we do for Comparable).  Unfortunately, we
>>> do not.
>>> * Rename Streamable to TextOutputStreamable and add a deprecated
>>> Streamable typealias for it.  Now that OutputStream been renamed to
>>> TextOutputStream, we should also move Streamable out of the way.

In apple/swift#4131, these lines have overrun the 80 column limit:



TextFormatting.rst also needs `Streamable` => `TextOutputStreamable`:


>>> Deprecation is being proposed instead of underscoring or renaming
>>> because it allows existing code to keep working (with warnings).  At
>>> this late stage, it would be bad to actually break anything.
>> If the SE-0104 (protocol-oriented integers) proposal has been deferred, 
>> should any protocols (e.g. SignedNumber) be deprecated?

Will the SE-0104 proposal be accepted for Swift 4.0?

-- Ben

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